The effect of microalgae as a binder on the characteristics of agglomerated coal fines JA Meyer orcid.org/ 0000-0002-3398-4463 Dissertation accepted in fulfilment of the requirements for the degree Master of Science in Engineering Sciences with Chemical Engineering at the North West University Supervisor: Prof CA Strydom Co-supervisor: Prof JR Bunt Graduation: June 2021 Student number: 24297070 JA Meyer Declaration I, Jacobus Arnoldus Meyer, hereby declare that the dissertation titled: The effect of microalgae as a binder on the characteristics of agglomerated coal fines, submitted in fulfilment of the requirements for the degree of Master of Engineering in Chemical Engineering is my own work, except where specified in the text, and has not been submitted at any other tertiary institution. JA Meyer I JA Meyer Acknowledgements I would like to acknowledge the following people who supported or assisted me during this study and played a role in the completion of my dissertation: • The NRF, the SARChI Coal Research Chair, and the North-West University for the financial support of this study • My supervisors, Prof Christien Strydom and Prof John Bunt, for their guidance and support throughout this study. • My colleague, Dr Romanus Uwaoma, for all his help and advice. • Dr Henry Matjie, for his input and arrangements for the XRD and XRF analyses of the LTA samples. • Dr David French at UNSW (Australia) for XRD and XRF analyses of the LTA samples. • Dr Roelf Venter for conducting GC-MS analyses on the tar samples. • Nicolaas Engelbrecht, HySA Infrastructure CoC, for his assistance with calibration gases and gas sample analyses (GC). • Willem Swanepoel at Bureau Veritas for conducting proximate and ultimate analyses on the various samples. • Mr Nico Lemmer for his assistance with the particle size distribution, calorific value determinations, and other laboratory assistance. • Clement Mgano for his advice and laboratory assistance. • Mr Jan Kroeze, Adrian Bock, Warren Brauns, and Elias Mofokeng for their technical assistance in the workshop. • Tannie Anriëtte Pretorius at the NWU Natural Science Library for her research assistance. • My friend and colleague, Chantalé Henning, for her help and advice during our studies. • My friends, Ettiene Wiese and Neels Lombard, for their advice and encouragement during our studies. • My parents, Thania and Allan Rowan, for their unwavering support and motivation throughout my studies. II JA Meyer Table of contents Declaration I Acknowledgments II List of tables V List of figures VI List of abbreviations VII Abstract VIII 1 Introduction 1 1.1 Background and motivation 1 1.2 Problem statement 2 1.3 Aim and objectives 3 1.4 Scope of this study 4 1.5 Chapter outline 5 Chapter references 6 2 Literature review 8 2.1 Coal overview 8 2.1.1 Properties and classification of coal 8 2.1.2 Coal processes 11 2.1.3 Coal gasification 12 2.1.3.1 Chemical reactions and mechanisms of gasification 13 2.1.3.2 Characteristics of coal affecting gasification 15 2.2 Coal pyrolysis and devolatilisation overview 17 2.2.1 Bench-scale pyrolysis techniques 20 2.2.2 Fischer Assay pyrolysis 20 2.2.3 The utilisation of coal pyrolysis products 22 2.2.4 Factors affecting coal pyrolysis 24 2.2.4.1 Properties of coal 24 2.2.4.2 Operating conditions 27 2.3 Coal discards and fines 30 2.3.1 Agglomeration of coal fines 31 2.3.1.1 Factors affecting coal agglomeration 31 2.4 Co-pyrolysis of microalgae and coal 34 2.4.1 Microalgae overview 34 2.4.1.1 Method for algae cultivation (InnoVenton) 36 III JA Meyer 2.4.2 Overview of the properties and characteristics of coal-algae 36 2.4.3 Agglomeration using a microalgae binder 37 2.4.4 Co-pyrolysis of microalgae and coal 38 2.4.5 Combustion and gasification of coal-algae 39 Chapter references 40 3 Characterisation of fine coal discards, microalgae, and coal-algae agglomerates 45 3.1 Introduction 45 3.2 Experimental section 47 3.3 Results and discussion 49 3.3.1 Proximate- and ultimate analyses 49 3.3.2 Compressive strength (CS) and water resistance index (WRI) 50 3.3.3 Calorific value 51 3.3.4 Thermogravimetric analyses (TGA) 52 3.4 Conclusions 55 Chapter references 57 4 Pyrolysis product yield and composition of fine coal discards agglomerated by a microalgae binder 59 4.1 Introduction 60 4.2 Experimental section 61 4.3 Results and discussion 66 4.3.1 Pyrolysis product distribution 66 4.3.2 Char analyses 69 4.3.3 Tar analyses 71 4.3.4 Gas analyses 72 4.4 Conclusions 74 Chapter references 76 5 Conclusions and recommendations 79 5.1 Conclusions 79 5.2 Recommendations for future work 85 IV JA Meyer List of tables Table 2-1: Major properties of coal characterisation 10 Table 2-2: Some of the most important processes of coal 11 Table 2-3: The basic reactions of coal gasification 13 Table 2-4: The basic reaction mechanisms of coal gasification 14 Table 2-5: Temperature regions in coal pyrolysis 18 Table 2-6: Most important bench-scale reactors used for characterizing the pyrolytic behaviour of solid fuels 20 Table 2-7: Weight loss (%) of macerals when heated to 800°C 25 Table 2-8: Decomposition reactions of mineral matter 26 Table 2-9: Product yield from low- and high temperature pyrolysis 28 Table 2-10: Product composition from low- and high temperature pyrolysis 28 Table 2-11: Lipid content of various algae species 35 Table 2-12: Chemical properties obtained from coal-algae blends 37 Table 2-13: Physical properties obtained from coal-algae blended pellets 37 Table 3-1: ISO methods used for proximate analyses of the samples 47 Table 3-2: Proximate- and ultimate analyses of coal fines and microalgae 50 Table 3-3: Compressive strength values (CS) of the various samples with the addition of 12% water prior to agglomeration 50 Table 3-4: Calorific value results obtained from agglomerated samples using a bomb calorimeter and calculated higher heating value (HHV) (MJ/kg) 52 Table 3-5: A comparison of weight losses observed during pyrolysis of the different samples used in this study with other biomass sources 54 Table 4-1: Proximate- and ultimate analysis results of coal fines and microalgae 61 Table 4-2: ASTM methods used for proximate analysis of the samples 65 Table 4-3: GC-MS oven temperature profile method 65 Table 4-4: Proximate- and ultimate analysis results of the coal and coal-algae chars 70 Table 4-5: Qualitative GC-MS results obtained from the pyrolysis tar samples 71 V JA Meyer List of figures Figure 1-1: Flow diagram of the scope of this study 4 Figure 2-1: Experimental automated Fischer Assay setup 22 Figure 2-2: Cultivation of microalgae through the patented hybrid system (InnoVenton) 39 Figure 3-1: TGA and DTG plots of coal fines, algae, and blends at a heating rate of 5 °C/min to 920°C 53 Figure 4-1: Experimental automated Fischer Assay setup 63 Figure 4-2: Temperature profiles indicating the temperature lag between the oven and retort bed for temperatures up to 520°C, 720°C and 920°C 64 Figure 4-3: a) Char yields, b) tar yields, c) gas yields, and d) water yields for the coal and coal-algae blends obtained from Fischer Assay experiments at 520°C, 720°C and 920°C 67 Figure 4-4: A comparison of the tar and gas fractions obtained from the volatile evolved during Fischer Assay experiments 68 Figure 4-5: Gaseous composition of H2, CH4, CO and CO2 evolved from Fischer Assay experiments 73 VI JA Meyer List of abbreviations AFT Ash fusion temperature AMD Acid mine drainage ASTM American Society for Testing and Materials a.d.b. Air-dried basis BTX Benzene, toluene, and xylene CA Coal-algae blends Cf Carbon-free sites CS Compressive strength CV Calorific value DTG Derived thermogram d.a.f. Dry, ash-free basis d.m.m.f. Dry, mineral matter-free basis FR Fuel ratio GC Gas chromatography GC-MS Gas chromatography-mass spectrometry GHG Greenhouse gas HHV Higher heating value ISO International Organization for Standardization m.a.f.b. Moisture, ash-free basis PAHs Polyaromatic hydrocarbons PBRs Photobioreactors PUFAs Polyunsaturated fatty acids ROM Run-of-mine SNG Synthetic natural gas TGA Thermogravimetric analysis Ti Initial devolatilisation temperature Tf Final devolatilisation temperature WRI Water resistant index XRD X-ray diffraction XRF X-ray fluorescence VII JA Meyer Abstract The global coal demand is increasing at an alarming rate, while reserves are depleting. This creates the need for more coal to be produced or to introduce an alternative approach that is more sustainable. The processing of coal creates residues such as dust and fines that are discarded in large stockpiles. It has been estimated that South Africa produces approximately 65 Mt of fine coal discards annually, with approximately 10 Mt that are classified as ultra-fines (-150 µm). Estimates suggest that these fines have accumulated to more than 1 Gt in South Africa. Coal fines and discards possess major disposal and environmental challenges. Studies have therefore been carried out on the beneficiation of these fines. Various studies have been done on the agglomeration of coal fines with or without the addition of binding agents. The agglomerates were evaluated based on their physical, chemical and thermal behaviour using various techniques. The use of biomass materials as an agglomerating agent has been studied extensively, including their effects on the physical, mechanical, chemical and thermal behaviour of coal when co-fired during different processes. It has been found that biomass and coal co-firing exhibit good characteristics during agglomeration, combustion, gasification and pyrolysis. In this study, microalgae biomass was used as an agglomerating agent, and as a co-pyrolysis agent during pyrolysis experiments – executed using a modified Fischer Assay setup. Coal-algae samples were prepared with different algae concentrations (5, 10, 20 wt.%). The effects of algae on fine coal discards were evaluated, and their potential as a coal substitute for pyrolysis and gasification was investigated. The coal-algae blends were agglomerated and characterised by compressive strength, water resistance, calorific value and thermogravimetric analyses at 920°C. In addition, the pyrolysis product distribution and composition were evaluated at 520°C, 720°C and 920°C. The various products obtained from the pyrolysis experiments were successfully separated and set for characterisation. The char, tar and gas fractions were characterised using proximate and ultimate analyses, gas chromatography-mass spectrometry and gas chromatography, respectively. The results showed that the compressive strength values of the agglomerates increased with the addition and concentration of algae to the fine coal discards. However, the water resistance indices could not be determined as the agglomerates disintegrated completely when submerged into water. Calorific values of the various samples showed no significant changes on the measured calorific values. Thermogravimetric analyses confirmed the high volatile matter content of algae and indicated that a large amount of mass loss occurred at lower temperatures compared to volatiles driven off during coal devolatilisation. VIII JA Meyer The Fischer Assay experiments showed that temperature had a large influence on the product yields and composition during pyrolysis. The amount of chars produced decreased as a function of pyrolysis temperature and addition of algae and it was observed that the fuel ratio of the chars increased at higher pyrolysis temperatures. The highest amount of tars was produced from the samples containing 10 and 20 wt.% algae at 720°C; however, at 920°C, the least amount of tars was produced. The composition of the tars varied as a function of pyrolysis temperature and algae concentration. More aliphatic compounds were present in the coal-algae samples and the tars produced at higher pyrolysis temperatures. However, the amount of benzenes, phenols and PAHs was lower in the tars derived from the coal-algae samples. The amount of gas produced from the Fischer Assay experiments was determined by difference and it was clear that the amount of gas produced increased significantly as a function of increasing algae concentration and pyrolysis temperature. From the results obtained during this study, it was evident that the addition of microalgae biomass has a significant influence on the agglomeration and pyrolytic behaviour of fine coal discards. While some limitations exist, coal-algae agglomeration and co-pyrolysis exhibit the potential to be used as an alternative to coal. Keywords: Coal fine discards, microalgae, coal-algae, agglomeration, co-pyrolysis, pyrolysis, gasification, Fischer Assay, pyrolysis product distribution IX CHAPTER 1 INTRODUCTION This chapter provides an overview of this study and includes the relevant information pertaining to the problem statement, aim and objectives, scope of this study, and chapter outline. 1.1 Background and motivation Coal is the most widespread fossil fuel currently in the world, as it is responsible for providing 30% of the global energy needs and contributes to 40% of the world’s electricity generation1. Coal is primarily used as a feedstock for liquid fuels, and for gasification purposes2-3. Other important uses of coal include its use as a chemical reductant for steel manufacturing, and within the iron-, alumina refining-, paper-, and cement manufacturing industries1. The global coal reserves are depleting, as the energy requirements and global coal demands are increasing at an alarming rate. The world coal trade reached 1 169 Mt in 2018 – being shipped over great distances to reach overseas markets. It is a significant amount of coal and it accounts for only 21% of the total coal consumed globally, as most of the coal used within most countries has been produced locally4. The production and mechanical processing of coal hold many environmental challenges, as it creates significant residues such as dust and fines. Around 65 Mt of these fines (<500 µm) are generated annually in South Africa, and have accumulated to an estimate of 1 Gt5-6. These fines present major disposal and processing challenges for the industry, with various environmental impacts7-10. These drawbacks include air pollution, water pollution, soil degradation, and acid mine drainage (AMD). In addition, fines are susceptible to spontaneous combustion, making it a hazard when stored. These fines have the potential to be an important resource with viable processing9- 11. This creates the need for new technologies to be implemented in processing coal fines to obtain a useful product. Coal fines have the potential to be an important resource with viable processing12. Coal fines agglomeration has been studied extensively over the last century10. The process of agglomeration is carried out by applying pressure on a coal sample with or without a binder, to produce a solid in the form of a pellet or briquette. Coal agglomerates should, however, possess good physical-, chemical-, and thermal characteristics to conform to the required capabilities of a potential substitute for run-of-mine (ROM) coal, in addition to utilising large amounts of unused discarded coal. These solids have the potential to be used for various domestic and industrial applications, such as coal combustion and gasification processes. Various types of binders have been studied where some are by-products produced from some processes, such as lignosulphonate (from the production of wood pulp), oil composites (such as light- and heavy hydrocarbons or non-hydrocarbon oils), and low-grade fuels13. Others, such as biomass materials are waste materials from agriculture, municipal sources, or crops grown JA Meyer Introduction specially for the use as biofuels14. The use of biomass materials may reduce greenhouse gas emissions, as biomass materials are regarded as carbon-neutral. This is the assumed principle, as biomass that is grown in a regenerative fashion will produce no net CO emmissions142 . There are, however, some limitations when using biomass and it relates to its supply and composition, which can be reduced when co-fired with coal14. The study focuses on the agglomeration and pyrolytic behaviour of coal-algae composites and should provide insight into the relevant characteristics of microalgae biomass co-fired with fine coal discards based on their physical-, chemical-, and thermal properties taking different processes into account. In addition, this study will provide an alternative approach for evaluating fine coal discards and microalgae as a biomass feedstock substitute. 1.2 Problem statement In South Africa, coal processing is necessary for the removal of non-carbon material and results in significant amounts of discarded coal, some of which are considered ultra-fines (-150 µm)5-7, 11. These fines are considered uneconomical, difficult to process, environmentally hazardous and are susceptible to spontaneous combustion11. These coal fines or discards have the potential to be an important asset with viable processing11, 15. Therefore, agglomeration of coal discards may produce a product that can be advantageous to the industry – adding value to these waste materials. The drawbacks with current agglomerating methods include economic feasibility, quality, hydrophobicity and/or mechanical strength16. These elements should reduce challenges relating to transportation, handling, storage, and beneficiation of these carbonaceous fines17. Briquettes or pellets obtained without a binder (or binding agent) present a lack in their physical properties, which include compressive strength and hydrophobicity. Most binders studied did not meet these physical properties and viability, and therefore a binder should be implemented to produce a solid within the ideal parameters. The focus has, however, shifted to investigating the use of biomass as an addition or alternative to fossil fuels12, 14, 18-20, because biomass is regarded as carbon neutral and is readily available21- 23. The use of microalgae biomass with coal has been studied over the past decade and has proven to be a potentially valuable addition to coal within various processes, including agglomeration, co-pyrolysis, gasification, combustion, etc.12, 14, 18, 24-26. Gaqa et al.12 investigated the physical properties of Coalgae® (coal-algae blends) and found that the application of microalgae as a binder added to the compressive strength and water resistibility12. Microalgae have potential as a binder, but more studies should be done to evaluate its effect on gasification reactivity, pyrolysis product yield and composition before it can be applied within the industry. 2 JA Meyer Introduction 1.3 Aim and objectives The aim of this study is to co-pyrolysis discarded coal fines with microalgae biomass and to obtain a usable solid product through agglomeration processes. In addition, the aims are to investigate its effect on the pyrolysis product distribution using Fischer Assay experiments and characterise the coal-algae blends based on their physical, chemical, and thermal behaviour. The following objectives should be achieved after completion of this study: • Characterise the coal and algae samples based on their chemical compositions using proximate- and ultimate analyses; • Prepare various coal-algae blends by blending microalgae with fine coal discards; • Characterise the coal-algae blends by means of thermogravimetric analyses (TGA); • Produce solids in the form of pellets via agglomeration of the coal-algae blends; • Characterise the agglomerated samples using compressive strength, water resistibility, and calorific value determinations; • Evaluate the effect of algae on the pyrolysis product distribution of coal at 520°C, 720°C and 920°C via Fischer Assay experiments; and • Analyse the different char, tar and gas fractions using proximate- and ultimate analyses, qualitative gas chromatography-mass spectrometry (GC-MS), and gas chromatography (GC) techniques. 3 JA Meyer Introduction 1.4 Scope of this study Figure 1-1 summarises the outline of this study and includes the experimental procedures, materials used, various products obtained, and the characterisation methods used in this study. Figure 1-1 Flow diagram of the scope of this study 4 JA Meyer Introduction Comments: Some analyses and results were not included in this study, as they will be used for another article. These include: XRF analyses on the coal and algae samples, along with XRD analyses, CO2 gasification reactivities, FT-IR of tars derived from Fischer Assay pyrolysis. A reactivity and kinetic gasification study of the coal-algae composites will also be conducted. It should be noted that the focus of this study was on the effect of algae as a binder to fine coal discards, and the pyrolysis product distribution and composition at different temperatures using a Fischer Assay setup. The focus of this study does not include any economic feasibility or viability of microalgae as an agglomerant, co-pyrolysis agent, or replacement for coal. 1.5 Chapter outline This dissertation will include the following chapters: Chapter 1: An introductory chapter that covers the background and motivation, as well as the problem statement, aims and objectives. Chapter 2: A literature survey that focuses on the relevant topics pertaining to coal properties, processes, pyrolysis, coal discards and agglomeration. Chapter 3: The first article that consists of information regarding physical, chemical and thermal characteristics of fine coal discards agglomerated by a microalgae binder. Chapter 4: The second article that involves an evaluation of the pyrolysis product distribution of coal-algae blends, and characterisation of the various fractions obtained from Fischer Assay experiments at 520°C, 720°C and 920°C. Chapter 5: This chapter provides the final conclusions and recommendations based on the outcomes of this research study. 5 JA Meyer Introduction CHAPTER REFERENCES 1. WCA Uses of Coal. http://www.worldcoal.org/coal/uses-coal (accessed 12 February 2020). 2. Hancox, P. J.; Götz, A. E., South Africa's coalfields—A 2014 perspective. International Journal of Coal Geology 2014, 132, 170-254. 3. Collot, A.-G., Matching gasification technologies to coal properties. International Journal of Coal Geology 2006, 65 (3-4), 191-212. 4. WCA Coal market & pricing. http://www.worldcoal.org/coal/coal-market-pricing (accessed 14 February 2020). 5. Ratshomo, K.; Nembahe, R. South African Coal Sector Report; Department of Energy: Pretoria, 2017. 6. DoE Coal Resources - discards. http://www.energy.gov.za/files/coal_frame.html (accessed 2020/05/13). 7. Langenhoven, H., The state of coal mining in South Africa. Africa, M. C. S., Ed. Minerals Council: Johannesburg, 2019; p 21. 8. Wagner, N., The characterization of weathered discard coals and their behaviour during combustion. Fuel 2008, 87 (8-9), 1687-1697. 9. Klima, M. S.; Arnold, B. J.; Bethell, P. J., Challenges in Fine Coal Processing, Dewatering, and Disposal. SME: 2012. 10. Orr, F., Coal Waste Impoundments: Risks, Responses and Alternatives. Committee on Coal Waste Impoundments, National Research Council, National Academy Press, Washington, DC 2002. 11. Muzenda, E. In Potential uses of South African coal fines: a review, International Conference on Mechanical, Electronics and Mechatronics Engineering: 2014. 12. Gaqa, S.; Watts, P., The agglomeration of coal fines using wet microalgae biomass. Journal of Energy in Southern Africa 2018, 29 (2), 43-50. 13. Özer, M.; Basha, O. M.; Morsi, B., Coal-agglomeration processes: A review. International Journal of Coal Preparation and Utilization 2017, 37 (3), 131-167. 14. Fernando, R., Cofiring high ratios of biomass with coal. IEA clean coal Centre 2012, 300, 194. 15. Bunt, J. R.; Neomagus, H. W.; Botha, A. A.; Waanders, F. B., Reactivity study of fine discard coal agglomerates. Journal of analytical and applied pyrolysis 2015, 113, 723-728. 16. Lowry, H. H., Chemistry of coal utilization: supplementary volume. 1963. 17. Tsai, S. C., Fundamentals of coal beneficiation and utilization. Elsevier Amsterdam: 1982; Vol. 2. 18. Wu, Z.; Yang, W.; Li, Y.; Yang, B., Co-pyrolysis behavior of microalgae biomass and low- quality coal: Products distributions, char-surface morphology, and synergistic effects. Bioresource technology 2018, 255, 238-245. 19. Pang, S., Fuel flexible gas production: Biomass, coal and bio-solid wastes. In Fuel Flexible Energy Generation, Elsevier: 2016; pp 241-269. 20. Wu, Z.; Wang, S.; Zhao, J.; Chen, L.; Meng, H., Thermochemical behavior and char morphology analysis of blended bituminous coal and lignocellulosic biomass model compound co-pyrolysis: effects of cellulose and carboxymethylcellulose sodium. Fuel 2016, 171, 65-73. 21. Mafu, L. D.; Neomagus, H. W.; Everson, R. C.; Strydom, C. A.; Carrier, M.; Okolo, G. N.; Bunt, J. R., Chemical and structural characterization of char development during lignocellulosic biomass pyrolysis. Bioresource technology 2017, 243, 941-948. 22. Chen, P.; Min, M.; Chen, Y., Review of the biological and engineering aspects of algae to fuels approach. 2009; 2 (4). Cited times 24, 64. 23. Sajjadi, B.; Chen, W.-Y.; Raman, A. A. A.; Ibrahim, S., Microalgae lipid and biomass for biofuel production: A comprehensive review on lipid enhancement strategies and their effects on fatty acid composition. Renewable and Sustainable Energy Reviews 2018, 97, 200-232. 6 JA Meyer Introduction 24. Qadi, N. M.; Hidayat, A.; Takahashi, F.; Yoshikawa, K., Co-gasification kinetics of coal char and algae char under CO2 atmosphere. Biofuels 2017, 8 (2), 281-289. 25. MAGIDA, N. E.; ZEELIE, B.; DUGMORE, G., AN EVALUATION OF THE GREENHOUSE GAS REDUCTION POTENTIAL THROUGH THE CO-FIRING OF COAL AND MICROALGAE BIOMASS. WIT Transactions on Ecology and the Environment 2017, 224, 257-265. 26. Li, W.; Li, W.; Liu, H., The resource utilization of algae—Preparing coal slurry with algae. Fuel 2010, 89 (5), 965-970. 7 CHAPTER 2 LITERATURE REVIEW This chapter provides a review of the relevant topics pertaining to coal pyrolysis and char gasification. The focus will, however, be shifted to account for the addition of microalgae biomass as an agglomerating agent to discarded coal fines. The topics include, but are not limited to: a coal overview and its various processes, discarded coal fines and agglomeration, coal pyrolysis, algae, and coal-algae. An evaluation of the properties that may affect these processes will also be discussed. 2.1 Coal overview The global coal demand is increasing at an alarming rate. The coal trade in the world reached 1169 Mt in 2018 – being shipped over great distances to reach overseas markets. It is a significant amount of coal and it accounts for only 21% of the total coal consumed globally, as most of the coal used within most countries has been produced locally1. For more than a century and a half, coal has been regarded as an essential commodity in South Africa’s economy, as well as the primary source of energy within the electricity generation sector and as a feedstock for liquid fuels2. Coal is mainly used for electricity generation, liquid fuel production, combustion, gasification, and within the steel-, iron-, alumina refining-, paper-, and cement manufacturing industries3. The leading coal consumers in South Africa are predominantly Eskom (coal-fired power generation) and Sasol (gasification), consuming approximately 90 Mt and 30 Mt, respectively4-5. Around 30% of liquid fuel produced and 70% of South Africa’s energy needs are provided for using coal, as coal-fired power stations contribute to 83% of South Africa’s power supply – dominated by Eskom5. Other means for electricity generation include nuclear, gas, pumped storage, hydro and wind5-6. The beneficiation of coal accounts for more than 65 Mt of coal discards annually in South Africa, and has been estimated to have accumulated to more than 1 Gt5, 7 – discussed in more detail in section 2.3: Coal discards and fines. 2.1.1 Properties and classification of coal Since coal is not considered a homogeneous substance, it makes it difficult to understand the varying characteristics as well as their effect on coal processes. Therefore, coal is classified based on a wide range of its properties and characteristics8. The most common description of coal is its rank, which is the measurement of the degree of coalification during metamorphosis from peat to the graphite-like carbonaceous material known as coal. The rank of coal can be JA Meyer Literature review deducted from a proximate analysis, which quantifies, among others, the fixed carbon and volatile matter contents, and by the calorific value and petrographic analysis of the coal specified by the ASTM Standard for Classification of Coal by Rank8. The calorific value of coal increases with decreasing moisture present in coal, ranging from lignite to subbituminous coal to low-volatile bituminous coal. However, a decrease in volatile matter may contribute to lower calorific values. The various physical and chemical properties may vary significantly with different stages of coal rank, ranging from low-ranking coals to high-ranking coals8. A brief description of each of the distinctive coal ranks, adapted from Schobert et al. (1987)9, is discussed below: Lignite Lignite is the lowest rank of coal that contains a significant amount of moisture and volatile matter and is generally referred to as brown coal in Europe, Asia and Australia. It is considered a low- quality coal based on its low heating value. The high volatile content of lignite coal makes it easily ignitable. When dried and exposed to air, lignite coal has the tendency to disintegrate or slack, i.e. to disintegrate upon exposure to weather, when exposed to air, and wetted or dried and may spontaneously combust during storage or transport. Subbituminous coal An intermediate rank of coal that has matured to a point where no plant-like textures are apparent. The black coal has, however, the same tendencies toward slacking and spontaneous combustion as lignite coal. This type of coal burns clearer and is of better quality than lignite coal based on its intermediate heating values. Bituminous coal Bituminous coal is the most recognised and used coal globally. This glossy black coal has a high heating value, with lower moisture and volatile matter contents. This is the only rank of coal that exhibits agglomerating or caking potential, making it promising to produce coke, an important material in the production of iron and steel. Higher ranks of bituminous coals have low-volatile matter contents and the greatest heating value compared to the other ranks of coal. When burned, higher ranking bituminous coal and anthracite are regarded as smokeless fuels. Anthracite Regarded as the highest rank of coal, containing significantly low volatile matter content. When anthracite is burned it produces a hot, smokeless and uniform flame. Based on its combustion properties and limited availability, anthracite can be used to produce premium fuels. This jet-black material is regarded as the hardest and most dense coal compared to all other coal ranks. Anthracite, however, does not produce coke when heated in an inert environment (pyrolysis). 9 JA Meyer Literature review There are however, alternative methods for coal classification, as rank is sometimes referred to as being imprecise8. Hensel (1981)10 published coal data on a moisture, ash-free basis (m.a.f.b.), which eliminates the need to know the inherent moisture of coal accurately. Another method that is often satisfactory for specifying coal rank is the determination of both aromaticity and macromolecular cluster size, as well as vitrinite reflectance. With increasing rank, the degree of aromaticity and vitrinite reflectance also increases. Knowledge of vitrinite reflectance can also provide insight into other properties such as calorific value, volatile matter content, and the gas and tar/oil yield of coal10-11. There are various specific criteria through which coals are graded and valued – based on the requirements for each coal process8. These criteria are related to the coal rank, type or petrographic properties, and inorganic constituents. Various quantitative and qualitative characterisation techniques exist that can be used to provide valuable insight into the various properties of coal. Table 2-1 summarises the analytical techniques associated with the seven major properties of coal: Table 2-1 Major properties of coal characterisation, adapted from Smith et al. (1994)8 Properties Comments Chemical properties8 • Proximate analysis Overall composition of coal, i.e. moisture, volatile matter, ash and fixed carbon content • Ultimate analysis Elemental composition of coal, i.e. C, H, N & O (excluding ash elements) • Atomic ratios H/C and O/C ratios • Elemental analysis Elements resulting from coal and ash • Sulphur species Chemically bonded sulphur (in coal): organic, sulphide and sulphate Physical properties8 • Density True density measured by helium displacement • Specific gravity Apparent density – using a non-penetrable fluid • Pore structure Including the specification and nature of porosity, and pore structure of macro-, micro- and transitional pores • Surface area Determined through N2 or CO2 adsorption • Reflectivity Valuable in petrographic analysis Mechanical properties8 • Strength Compressive strength of coal measured in kN • Elasticity Recovering after deformation • Hardness/abrasiveness Indentation of hardness and abrasiveness of coal • Friability Ability to withstand degradation when handled and tendency toward breakage: measured by means of tumbler test and drop shatter test • Grindability Work needed to pulverise a coal sample (against a standard) – measured by Hardgrove grindability index • Dustiness index Dust produced through handling of coal Thermal properties8, 12 10 JA Meyer Literature review • Calorific value Energy output of coal combustion in oxygen – measured in MJ/kg • Heat capacity Amount of heat required to increase the temperature for one-unit amount of coal • Thermal conductivity Heat transfer rate through unit area, thickness and temperature difference • Plasticity/agglutinating Plastic behaviour of coal upon heating and caking – Gieseler plastometer test • Agglomerating index Nature of residue from 1 g sample when heated at 950°C – Roga index • Free-swelling index Increase in volume of a coal sample when heated without restriction – indication of caking and plastic properties Electrical properties8 • Electrical resistance Electrical resistance of coal – coal is regarded as a semiconductor • Dielectric constant Electrostatic polarizability related to pi-electrons on aromatic rings • Magnetic susceptibility Diamagnetic, ferromagnetic and paramagnetic characteristics of coal Ash properties8 • Elemental analysis Major elements in ash, i.e. oxides such as SiO2, Al2O3 and Fe2O3 • Mineralogy analysis Analysis of the mineral content in coal ash • Trace element analysis Analysis of trace elements found in coal • Ash fusibility Temperature pertaining to the different stages of ash fusing and flow Petrographic properties8 • Maceral composition Indication of the maceral components in a coal sample, contributing to the reactivity of coal during a conversion process • Vitrinite reflectance Useful in maceral analysis and rank calculations 2.1.2 Coal processes Most of the coal consumed globally is used for power generation by means of direct combustion of pulverised coal in large-scale furnaces. This may change, as new technologies are being incorporated for power generation by means of greener or more carbon neutral initiatives, as well as the widespread utilisation of processes such as gasification, pyrolysis and liquefaction. Various other processes exist and can be classified based on the process type or end-product, coal type, particle size or temperature. Some of the most important processes are mentioned below:13 Table 2-2 Some of the most important processes of coal, adapted from Smoot and Smith (1985)13 Direct combustion • Pulverised coal combustion Small particles • Fluidised bed Medium-sized particles • Fixed bed (e.g. stokers) Larger particles Gasification • Entrained bed Small particles • Fluidised bed Medium-sized particles • Fixed or moving bed Larger particles • Other 11 JA Meyer Literature review Carbonisation and coking (pyrolysis) • Low temperature (480-700°C) • Medium temperature (750-900°C) • High temperature (900-1050°C) Liquefaction • Pyrolysis • Extraction Other • Magnetohydrodynamics (MHD) power generation • Fuel cell The table above only summarises the most important coal processes. Their commercial use, coal types used, and availability vary significantly with each process. The focus of this study is pyrolysis (§ 2.2) and gasification (next section) and therefore these other industrial coal processes will not be described and discussed. 2.1.3 Coal gasification The main processes of coal include coal combustion and gasification. This section of the review will be focused on coal gasification, which includes an overview of what coal gasification is, the products produced from different operating conditions, and the chemical reactions and mechanisms entailed. Coal gasification involves two primary steps: initially, the rapid pyrolysis of coal to produce char, tar and gases, followed by subsequent gasification of the produced char14. The second step, however, is rather slow as it controls the conversion process. The controlling factors that influence the gasification process are mainly the char’s intrinsic reactivity, catalytic effects of inorganic constituents, and pore structure14. Coal gasification, in general, is the thermal process that converts carbon-based material to energy when the feedstock is reacted with air, oxygen or steam without burning it. This process produces synthesis gas or ‘syngas’15-16. At Sasol, processes such as coal gasification, which initially go through drying and devolatilisation stages in the upper zone of the reactor, are followed by gasification and combustion in the bottom section of the gasifier (Lurgi gasifier), producing synthesis gas (a CO-H2 mixture) that is converted to paraffinic liquid-fuels and other chemical feedstocks by the Fischer-Tropsch synthesis over iron-based catalysts4, 17. A wide range of chemical products are obtained as by-products of the main coal processes, such as benzene, phenol, creosote oil and naphthalene that is produced from refined coal tar3. Other valuable products or gases are also produced from the gasification of coal – suitable for different uses depending on the operating conditions, as well as the type of gasifier used. Different heating value gases can be produced, e.g. low-, medium- and high-heating value gas. Low-heating value gas is produced using air and steam and can be used as an industrial fuel and as a fuel for power 12 JA Meyer Literature review generation. Medium-heating value gas is produced from a reaction with oxygen and steam, producing gases consisting predominantly of CO and H2 (syngas) that are used as a fuel or a chemical feedstock – for the production of chemicals such as ammonia, methanol and liquid fuels derived from methanol16. The advantages of coal gasification over combustion related to pollutant production can be summarised as follows: Although the physical and chemical processes are similar, the formation of nitrogen- and sulphur-containing pollutants differs in the two processes. Under reducing conditions, sulphur is converted to H2S, rather than SO2, and nitrogen is converted to NH3, while almost no NOx is formed. This is the basis that coal gasification is characterised as a cleaner coal technology when compared to combustion13, 16. 2.1.3.1 Chemical reactions and mechanisms of gasification Coal gasification refers to the reaction of coal with steam, air, oxygen, carbon dioxide or a mixture of these gases to produce gaseous products, such as H2, CH4, CO, CO2 and small amounts of sulphur- and nitrogen-baring gases, leaving ash as a residue behind14, 16, 18. The basic reactions of coal gasification are given below: Table 2-3 The basic reactions of coal gasification, adapted from Smooth and Smith (1985)13, Vamvuka (1999)16, Tsai (1982)18, and Lowry (1963)11 Devolatilisation ΔH (KJ/mol) 1. 4CnHm → mCH4 + (4n-m)C Char combustion (sequential oxidation of carbon) 2. C + ½O2 → CO -123.0 to -110.5 3. C + O2 → CO2 -406.0 Char gasification 4. C + H2O → CO + H2 +118.5 to +131.4 5. C + CO2 → 2CO +170.7 to +172.0 6. C + 2H2 → CH4 -74.8 Gas-phase reactions 7. CO + ½O2 → CO2 (CO oxidation) -283.1 to -282.0 8. H2 + ½O2 → H2O (oxidation of volatile-matter-hydrogen) -241.6 9. H2O + CO → H2 + CO2 (water- gas shift reaction) -42.3 10. CO + 3H2 → CH4 + H2O (methanation) -206.0 A variety of reactions can occur and are outlined in the table above. However, these equations do not represent the inherent mechanistic complexities of the representing processes16. There are active carbon-free sites (Cf) throughout the carbon structure resulting from lattice imperfections or discontinuities. These active sites consist of unpaired electrons that influence the reactive gas constituents’ chemisorption to form surface complexes. The specific reaction rate and order are determined through the rate at which these complexes form and are removed, as 13 JA Meyer Literature review well as the amount and exposure of active carbon-free sites8, 16, 18. Therefore, the mechanisms of the above-mentioned basic reactions can be portrayed as follows: Table 2-4 The basic reaction mechanisms of coal gasification, adapted from Vamvuka (1999)16, Lowry (1963)11, Smith et al. (1994)8, and Tsai (1982)18 Reaction mechanism Description Char-oxygen reactions 2 & 3 Cf + O2 → C(O) + O Irreversible chemisorption of oxygen on active sites Cf + O → C(O) Irreversible chemisorption of oxygen atom on active sites C(O) → CO + Cf Decomposition of surface oxides Cf → Cinactive Annealing Char-steam reaction 4 Cf + H2O ↔ C(O) + H2 Reversible oxygen exchange between active sites and steam C(O) ↔ CO + nCf Decomposition of surface oxides C(O) + CO ↔ CO2 + Cf Reversible oxygen exchange between surface oxides and CO Cf → Cinactive Annealing Char-CO2 reaction 5 C(O) → CO + Cf Decomposition of surface oxides Cf → Cinactive Annealing Cf + CO2 ↔ C(O) + CO Reversible oxygen exchange between surface oxides and CO Char-hydrogen reaction 6 It is assumed that the reaction may proceed via successive hydrogen addition reactions at the edges of carbon-crystal-lattices, as shown in the figure below19. H H H H H H + H2 + H2 + 2CH4 H H H 3 C CH3 + H2 These reaction mechanisms, along with the thermodynamic properties of the different processes, control the rate and efficiency of coal gasification, as well as the composition of the gaseous products. Gasification, like all other chemical reactions, will always tend to a state of equilibrium, either favouring the one side, or the other. Therefore, after a certain amount of time, depending on the temperature and pressure, there will be a fixed relationship between the quantities of starting materials and end-products16. 14 JA Meyer Literature review 2.1.3.2 Characteristics of coal affecting gasification The characteristics of coal may be the most important factors that influence the processes of gasification. Low-rank coals are preferred over caking bituminous coal as the feedstock for commercial gasification processes. The main properties of these low-rank coals include reactivity, moisture-, volatile- and oxygen content, as well as the caking properties, ash mineralogy/characteristics and sulphur content16. Reactivity Gasification of low-rank coal chars has shown to be more than 100 times more reactive than high- rank coal chars. The reason may be ascribed to the amount of active sites present in these coals; higher porosity, enhancing the reactive gas accessibility; and higher Ca-containing minerals present, which promote the catalytic effect of minerals on the char gasification process 16, 20. Due to these properties of low-rank coals, the consumption of oxygen and steam is reduced, and as a result increases carbon conversion – important for gasifiers that pertain to short resident times, such as entrained bed type gasifiers16. In addition, lower operating temperatures can be applied to more reactive chars, which are favourable for fixed-bed gasification units16, 21. Moisture Low-rank coals usually contain a higher moisture content that limits the performance of the gasification process and subsequently increases the amount of feed required for the gasifier, compared to high-rank coals16. The increase in feed may generate greater amounts of solids, liquids and gases, which require larger equipment to be used16. Fixed-bed gasifiers can run efficiently with coals that contain moisture contents of up to 35%, provided that the ash content does not exceed 10%, while entrained and fluidised-bed gasifiers can only operate at moisture contents (generally) lower than 5%. Oxygen content High oxygen contents in low-rank coals may have similar effects on the gasification process than high moisture contents. Because low-rank coals are partly oxidised, the energy obtained from the gases produced is lower. Therefore, the amount of feed required should be increased to obtain energies similar to that of high-rank coals16. Volatile content The composition of volatiles produced from low-rank coals may differ significantly from volatiles produced from high-rank coals during gasification. It is therefore important to note that the separation processes of these by-products are also different. Fixed-bed gasifiers produce a high amount of organic tars and oils, including phenols and other compounds. However, these compounds are cracked to hydrogen and carbon in fluidised and entrained-bed systems that 15 JA Meyer Literature review operate at temperatures above 800°C. This eliminates the need for expensive and complex treatment facilities16. Caking properties As mentioned earlier, some bituminous coals exhibit caking and agglomerating properties – they tend to swell and agglomerate when heated between 350°C and 550°C16. However, the cakes produced during this process limit the constant gas flow through the reactor and decrease thermal efficiencies. When gasified in entrained-bed systems, particle interactions are minimised, and gasification can continue without any problems. When compared to fluidised or fixed-bed systems, bituminous coals must undergo treatment to reduce their caking behaviour prior to gasification. Low-rank coals, on the other hand, do not have to undergo any treatment and can be used in any type of gasifier16. Ash characteristics Important ash characteristics within the gasifier include the catalytic effects on the gasification rate, the relationship between temperature and viscosity, and corrosive properties of some components present in ash (such as calcium and iron)16, 21. Studies have shown that the high reactivity of low-rank coals may be as a result of exchangeable metal cations present on their surface16, 22-23. The catalytic effects resulting from the different metal cations vary with gasification atmosphere: in an oxidising atmosphere, the presence of sodium, potassium, and calcium improves the reactivity of char, while in a reducing environment, sodium’s catalytic effect is much greater at carbon burn-offs below 45% and iron at higher burn-offs. Under steam environments, potassium and calcium have shown to be better catalysts than iron and sodium13, 16. Although the ash fusion temperature is important when considering slagging in fixed and entrained-bed systems, the relationship between temperature and viscosity must also be considered, since the slag flow characteristics are critical. High ash fusion temperatures may require the addition of fluxes to effectively remove slag from the system11, 16. The temperature-viscosity relationship differs between low- and high-rank coals and should therefore require different operating conditions, such as gasification temperature, to maintain constant slagging or non-slagging conditions. In addition, the presence of calcium and iron in ash may contribute to a corrosive environment. Low-rank coals are usually richer in calcium-containing minerals (such as CaO) and lower in iron compared to high-ranking coals16, 21. Sulphur content High-rank coals usually contain a higher sulphur content compared to low-rank coals, which creates the need to implement removal processes to meet environmental requirements. Consequently, the production costs are increases as acid gas scrubbing systems must be put in place. 16 JA Meyer Literature review 2.2 Coal pyrolysis and devolatilisation overview Pyrolysis is the process of thermal decomposition of carbonaceous material24. Pyrolysis of coal occurs when coal is heated in an inert environment to produce char (or coke), liquids (water and tar) and gases25. Following combustion, pyrolysis or carbonisation represents the second largest use of coal as the major product is predominantly coke or char that is mainly used as a fuel or within the metallurgical industry as a reducing agent25. This process is mainly characterised by carbonisation and devolatilisation11, 13, 25. Temperature is regarded as the most important variable during pyrolysis – moisture present in coal will evolve relatively early as the temperature begins to rise. While the temperature continues to increase, gases and tar-forming substances are released13, 25. At temperatures below 350°C, the primary reactions are dominated by vaporisation, but from 320°C, linkages between carbon and elements such as oxygen, nitrogen and sulphur (including carbon-carbon linkages) break the molecules up into fragments8, 26. Therefore, pyrolysis temperatures may contribute to internal transformations, as aliphatic carbon-carbon linkages are the first to break, while aromatic carbon- carbon linkages do not break readily in this temperature range as they are stabilised by resonance, followed by carbon-hydrogen linkages breaking when temperatures approach 600°C 8, 24, 27 with H 272 production . With regard to depolymerisation reactions, the dissociation energies for different carbon-carbon chemical bonds vary, where carbon-carbon bonds at aliphatic bridges between aromatic systems are weaker than that of other carbon-carbon bonds8. Therefore, it is expected that the first stage of pyrolysis would contribute to the cracking of weak bonds relating to aliphatic hydrocarbon bridges and resulting in alkyl aromatics, followed by alkyl radicals and aromatic ring structures8, 28. The presence of more aromatic systems is related to increased stabilisation through resonance of the pi-electrons8. Oxygen-containing functional groups are important in coal pyrolysis, as compounds differ in linkage strength8. For example, aliphatic ether linkages are among the weakest and are most likely to break first during pyrolysis. When considering thermal and catalytic reactions, the relative order of reactivity of oxygen-containing functional groups is ethers>aldehyde>ketone>phenols>furans8, 29. The oxygen is mainly removed as water and carbon oxides during thermal reactions due to decarboxylation and dehydration reactions8. During pyrolysis, the stability of oxygen-containing functional groups decreases in order: OH>CO>OCH3>CO2H29. The breakup of the coal macromolecular network (depolymerisation) and the formation of resulting products depend on the relative rates at which bond breaking (cleavage), cross-linking and mass transport occur30. Studies have shown that the main pyrolysis step at which tar is evolved is related to the cleavage of alkyl-aryl-ether bridges31. The dominant reaction during this cleavage is carbon-oxygen scission. However, the reaction pathways of the decomposition of 17 JA Meyer Literature review these bridges play a vital role in the formation of carbon monoxide through the detachment of the hydrogen atom from methyl groups8, 32. In addition, the formation of carbon monoxide is predominantly from the cleavage of the carbon-oxygen bond of ethers reacting with phenoxy radicals – similar to phenolic reactions8, 32. Methane and carbon monoxide are formed at higher temperatures through the cleavage of methyl groups originating from aromatic rings, the cleavage of bi-aryl ethers, and the rearrangement of aryloxy-radicals31. During further cleavage of methylene bridges, more carbon monoxide is formed as methyl and methylene radicals react with phenolic oxygen. In vitrinite and inertinite rich coals, these reactions give rise to the formation of benzene and tar31. Results indicate that CO2 yields have a linear relationship with carboxylic groups present in coal33. Coal pyrolysis can be divided into five different temperature regions as outlined in Table 2-5. The temperature ranges correspond with the major processes that contribute to the evolution of the various products during pyrolysis. Table 2-5 Temperature regions in coal pyrolysis, adapted from Ladner (1988)25 Region Temperature range Reactions Products Main uses (°C) < 350 Dominated by Moisture and Fundamental vaporisation volatile organics studies Low temp. pyrolysis 400-750 Primary Gas, tar and Smokeless fuels degradation and liquor and chemicals devolatilisation34 Medium temp. 750-900 Secondary Gas, tar, liquor, Smokeless fuels, pyrolysis reactions and additional chemicals, and hydrogen metallurgical coke High temp. pyrolysis 900-1100 Secondary Gas, tar, liquor, Smokeless fuels, reactions and additional chemicals, and hydrogen metallurgical coke Plasma >1650 Tar cracking Acetylene Uneconomic Below 350°C, no significant structural changes occur12. Weight loss of water is dominant initially; however, studies have found that hydrocarbons, including aromatic hydrocarbons (benzene and toluene), and HCl are also released from coal particles25. Adsorbed gases (H2O and N2) are also removed in this temperature region27. At 400°C, coal starts to undergo pyrolysis and the main process occurring at this temperature is the removal of phenolic carboxyl functional groups that are linked to poor thermal stability27. Within the 400-750°C temperature range, a variation of the major product yields of tar, char and gases is observed. Gas yield increases, while char yield decreases as the temperature rises. When compared to high temperature pyrolysis, the yields of gas increase significantly, while lights oils and chars have corresponding reductions. The yield of 18 JA Meyer Literature review char slightly increases25. At around 390 to 400°C, the initial softening of coal takes place, and aromatisation and condensation reactions start to rapidly increase while H/C ratios start to decrease. Residues generated at around 450°C could be regarded as precursors for semi-coke12. During primary devolatilisation reactions, weaker bonds start to break and fragmentation occurs (as discussed above). During this stage, low molecular weight fragments vaporise and escape the coal particle as tar forming vapour. This process produces low molecular weight oils and tars8, 35. Fragments with high molecular weights do not escape the coal particle. These compounds are known as metaplast, and their nature determines the softening behaviour of coal35. Low molecular weight oil in general comprises mostly paraffins, olefins and aromatics; the tar fraction derived from hydroxyl substituted aromatics and tar pitch25. Hydrocarbons, such as methane, are the major component of the gas produced25 and peak at around 500°C indicating the cracking of organic volatiles27. From 750°C, the carbon content of coal remains relatively constant, while hydrogen and oxygen contents decrease in high-volatile coal with hydrogen released being the main gaseous product in this temperature region25. When the temperature exceeds 800°C, the coal sample starts to carbonise and convert to char/coke27. During this stage, the coal structure is destroyed and CO2 generation is augmented, leaving behind mostly carbonaceous material27. High temperature pyrolysis, between 900 and 1100°C, is one of the most important temperatures at which coke, used in iron and steel manufacturing industries, is produced25. The majority of products obtained from high temperature pyrolysis are coke and gas, which account for more than 90% of the products25. The coals used during this process are in general assessed based on their volatile matter, dilatation, caking properties and maceral composition. Gases produced are dominated by hydrogen, followed by hydrocarbons, where hydrogen accounts for around 50%; low molecular weight oils by benzene (~70%) and toluene (~10%); and tar by tar pitch (~60%) as well as aromatics (~25%)25. At pyrolysis temperatures exceeding 1 650°C, the formation of acetylene and unsaturated hydrocarbons is thermodynamically favoured, with the quench gas released during this process rich in acetylene25. Gases contain relatively few saturated hydrocarbons and the tar produced is negligible as most of the volatiles produced are cracked to form gaseous components. The volatile matter present in coal is especially important in this temperature region, as it is linearly related to the acetylene yield. When argon in the system is replaced by hydrogen, the conversion may increase by up to 80%. These experiments are performed through a technique known as argon plasma pyrolysis and produces an important by-product, e.g. carbon black. The production of acetylene from coal is uneconomic because of the high cost of energy required25. 19 JA Meyer Literature review 2.2.1 Bench-scale pyrolysis techniques There are several bench-scale reactors that are frequently used to characterise the pyrolytic behaviour of solid fuels. These reactors include fixed- and fluidised-bed reactors, entrained-flow (‘drop-tube’) reactors, and versatile wire-mesh (‘heated-grid’) reactors. Table 2-6 summarises the most important bench-scale reactors that are used to characterise the pyrolytic behaviour of solid fuels. The table outlines the various operating conditions and main uses generally associated with each bench-scale reactor. Table 2-6 Most important bench-scale reactors used for characterizing the pyrolytic behaviour of solid fuels36-37 Reactor Operating Heating Pressure Main uses and sample sizes temperature rate Wire-mesh Up to 1 to Vacuum to Characterisation of recovered 2 000°C 20 000°C/s 160 bars volatiles and/or tars. Limited to small particle sizes. Sample sizes of 5-15 mg. Fixed-bed (‘hot- Up to 10°C/min Atmospheric Investigating the hydropyrolysis rod’) 1 000°C to 10°C/s to 150 bars behaviour of coals, as well as the production of BTX from the hydropyrolysis of middle rank coals. Sample sizes of 50-1000 mg. Fluidised-bed Up to 900°C Flash Atmospheric Examining product distributions. heating Sample sizes of 1-15 g. rate Entrained-flow Up to >104 °C/s Atmospheric Used to simulate coal pyrolysis (‘drop-tube’) 2 200°C to 50 bars38 and combustion processes under conditions related to pulverised-fuel firing conditions. Fischer Assay Up to 520°C Up to 10 Atmospheric Discussed in § 2.2.2 °C/s39 Fischer Assay pyrolysis will be discussed in the next section as it is the method that will be utilized in this study, followed by factors affecting the pyrolysis products which include temperature, heating rate, pressure and characteristics of coal, including coal type/rank will be discussed in § 2.2.413, 25, 27, 40. 20 JA Meyer Literature review 2.2.2 Fischer Assay pyrolysis In 1920, Franz Fischer and Hans Shrader formulated a method for quantitatively determining the yields of tar, char, gas and water from low-temperature pyrolysis of coal, also known as the Fischer Assay method41. This method was formulated by the ISO 647 standard in 197442. The process is executed using an aluminium retort and heating the coal up to 530°C using a gas- generated flame43-45. During this process, coal is converted to char as condensable volatiles evolve43. The condensable volatiles consist of tar/oil and water, while the latter originates from the original moisture content present in coal as well as from the decomposition reactions of organic and inorganic constituents present in coal43. The water fraction is effectively removed using a Dean and Stark distillation method for azeotropic distillation46. There are some limitations to the original Fischer Assay method: (1) the aluminium retorts are limited to their operating conditions based on the aluminium’s thermal properties; (2) only applicable to medium-ranking coals (as a result of low operating conditions); (3) the use of a gas- generated flame makes it difficult to maintain consistent heating rates; (4) it is regarded as a performance test and not a quantitative test; (5) repeatability varies as the temperature is controlled and regulated manually by the analyst; and (6) difficulties to regulate the coal-bed temperature as the thermocouple measures the wall temperature39, 45, 47. However, some modified versions of the Fischer Assay exist. The use of electrical heating mechanisms instead of gas burners was introduced by Hubbard (1965)48, and termed the method as an automated modified Fischer Assay. His improvements are still the basis for methods used today for oil shale characterisation45, 48. Roets et al. (2014)39 introduced a new automated duplicate-sample Fischer Assay setup for quantitatively determining the pyrolysis product yields more accurately than the standard ISO 647 method. The experiments indicated lower error percentage values for the derived tar, water and gases produced, with a slight increase for chars produced – while still below the maximum accepted differences stated by ISO 64739, 42. A schematic representation of the automated Fischer Assay setup introduced by Roets et al. (2014)39 is illustrated in Figure 2-1. 21 JA Meyer Literature review Thermocouple Oven Argon gas (in) Stainless steel reactor Sample Gas wash phase Gas sampling bag Tar trap Ice bath Toluene Figure 2-1 Experimental automated Fischer Assay setup The new automated Fischer Assay setup introduced stainless steel retorts, replacing the aluminium retorts of the original Fischer Assay setup so as to increase pyrolysis temperature39. Aluminium melts around 660°C, while stainless steel melts well above 1 000°C depending on the type of stainless steel used49. While working conditions are lower than that of the melting points, pyrolysis experiments can be safely conducted up to 1 000°C using stainless-steel retorts. However, stainless steel has a significantly lower thermal conductivity than aluminium – around ten times lower49 – limiting the heat transfer between the oven and the coal bed, affecting the heating time and rate39. It is therefore important to monitor the coal-bed temperature with a thermocouple placed within the retort. In the study by Roets et al. (2014), the differences in heating time and -rate affected the final tar yield, but experiments were found to be more repeatable than that of experiments outlined by the ISO 647 standard39. Results indicated that the method is acceptable for both quantitative and qualitative pyrolysis research39. 2.2.3 The utilisation of coal pyrolysis products The main products obtained and studied from coal pyrolysis are tar, char and gas. The most important constituents are the yields of tar, CO2, CO, H2 and some hydrocarbon gases (e.g. CH4, C3H2, C 502H4 etc.) . Coke or char, obtained from pyrolysis or devolatilisation, is mainly used as a fuel or metallurgical reducing agent17, 25. This section provides an overview of some of the most important products obtained from the pyrolysis of coal. The non-fuel uses of coal include11, 17: 22 JA Meyer Literature review • High temperature carbonisation of bituminous- or subbituminous coal to produce metallurgical coke, used as a chemical reducing agent in the production of steel; • Manufacturing of carbon materials such as activated carbon, carbon molecular sieves, and carbon for producing chemicals such as phosphorus (precursor for phosphoric acid); • Production of specialty carbon materials such as graphite, fullerene and diamond; • Aromatic feedstock compounds; • Aromatic and phenolic chemicals produced from tars obtained from pyrolysis or gasification; • Pitch binders, carbon fibres and activated carbon fibres produced from tar pitch; and • Manufacturing of polymer composites. Coal is an important resource when considered as a feedstock for carbon-based materials, aromatic chemicals, and specialty chemicals17. Studies in advanced polymer materials, utilising aromatic and polyaromatic units, have created opportunities to develop organic chemicals from coal and tars from the carbonisation of coal17. Liquid products obtained after processing can be used for a variety of chemicals – historically, the origin of the organic chemical industry. Gases produced from carbonisation of coal are mainly used as a fuel (or fuel precursor)25. Another approach is to convert coals to liquids/tars through pyrolysis, carbonisation, extraction or liquefaction, followed by conversion into higher value products17. These processes can lead to the successful production of aromatic chemicals such as benzene, toluene and xylene (BTX), as well as phenol, naphthalene, phenanthrene, pyrene, biphenyl and their derivatives11, 17. Most of the one- to four-ring aromatics and polar compounds can be used to obtain valuable chemicals. For example, phenols can be used to produce phenolic resins or as a monomer precursor for aromatic polymers and plastics11, 17. Benzene, which is extracted from BTX, can be converted into cumene that is used to produce synthetic phenol and acetone11, 17. Other important chemicals produced from coal derivatives include synthetic ammonia, coal-to-methanol technologies, synthetic natural gas (SNG) and olefins11, 17. Hydrogen and hydrogen donor-based components are also being used to develop thermally stable jet fuel11, 17. Other studies have concluded that constituents derived from pyrolysis, such as aromatic, hydroaromatic and naphthenic compounds, proved to have thermally stable properties useful for producing superior jet fuels51. The utilisation of chars obtained from pyrolysis has several important applications. The use of low-temperature chars for domestic applications has been widely recognised worldwide11. Chars are also used as blast furnace fuel, as a reducing agent for the production of elements such as phosphorus and copper (high-temperatures coke), and for producing electrodes, which include the process of carbonisation of coal at low temperatures, e.g. between 300 and 550°C11. Other uses of chars include the use of chars as the primary step for gasification, as a substitute for 23 JA Meyer Literature review charcoal in the production of carbon disulphide, and as one of the steps for the production of ethylene and aromatics from lignite coal carbonized to a maximum of 550°C – following an 800°C cracking step to produce benzene and ethylene11. Carbonisation can also be applied to other carbonaceous materials such as biomass, coal fines, lignite or peat, for the production of useful chars11. 2.2.4 Factors affecting coal pyrolysis The varying characteristics of coal pyrolysis have been one of most important focuses of coal research – observing the various stages of devolatilisation and factors affecting the products obtained from pyrolysis. These factors include the various properties of coals as well as operating conditions during pyrolysis, and their effect on the evolution of the four products and their derivatives13, 25, 27, 52. 2.2.4.1 Properties of coal Coal rank/type and petrography The behaviour of coal during carbonisation is largely dependent on two parameters: its rank, measured by the carbon content, and its maceral composition25. It is important to note that low-rank coals are associated with high gas yields, as well as low tar yields8, 13, 33, 35. High-volatile bituminous coals produce high tar yields and moderate gas yields, while high-rank coals produce moderate to low tar yields and low gas yields8, 13, 33, 36. When considering the release of oxygen-containing gases (CO and H2O), it correlates with the carbon and oxygen contents in the coal – as the carbon content increases with higher-ranking coals, the oxygen contents decrease and lowers the yields of gases associated33. However, CO2 evolution correlates rather poorly with carbon content and is considered to be derived (mostly) from carboxylic groups, which are pre-dominantly found in lower-ranking coals33. The release of CH4 during pyrolysis increases with carbon content, i.e. in higher-ranking coals. H2 evolution is not only dependent on the hydrogen content in coal, but also upon the oxygen content, as hydrogen found in coals with high oxygen contents correlates well with the evolution of H 332O . It was also found that the yield of tars has a linear correlation to the amount of volatile matter in coal, and char yield based on the carbon content of coal8, 33. It was found that tar yields decrease with maceral composition in the order of liptinite>vitrinite>inertinite at 400 to 450°C, while vitrinite and inertinite formed approximately the same amounts of tar at 500°C, whereas tar formation from liptinite ceased8, 31. Table 2-7 provides an overview of the weight loss of the different macerals at 800°C. 24 JA Meyer Literature review Table 2-7 Weight loss (%) of macerals when heated to 800°C, adapted from Ladner (1988)25 Carbon content (%) Maceral Weight loss (%) Liptinite 50.8 83.5 Vitrinite 32.0 Inertinite 19.3 Liptinite 41.2 85.7 Vitrinite 27.9 Inertinite 18.8 Liptinite 24.0 88.4 Vitrinite 23.3 Inertinite 16.4 The highest amount of volatiles are predominantly found in liptinites, while inertinites contain the least amount of volatiles25. Other effects that play an important role on the yield of tar include metaplast formation, cross-linking reactions and resulting plastic behaviour during coal devolatilisation8. Effect of mineral matter The amount and composition of inorganic material, or mineral matter, have shown significant effects on coal reactivity during pyrolysis and gasification53. However, the variations in maceral composition, thermoplastic properties and char morphology limit studies based on the effects of mineral matter on coal pyrolysis. Different studies have reported that mineral matter affects the processes related to pyrolysis and gasification. These effects are based on final product distribution, coal reactivity and process limitations arising from slag. Smoot and Smith13 have reported some of the most important effects of mineral matter on the reactivity of a char: • Thermal effects – where the amount of ash changes the thermal behaviour of char resulting in heat capacity and phase changes; • Radiation effects – resulting from the difference in radiative properties between ash and char, where ash is the only source providing radiative heat transfer after carbon consumption; • Particle size effects – where the fragmentation properties of char are affected by the quantity and nature of mineral matter; • Catalytic effects – where some minerals present in char have an increased effect on the reactivity of coal/char; and • Hindrance effects – where the mineral matter present in coal forms a barrier through which reactive gases should pass before reacting with carbon. 25 JA Meyer Literature review Liu et al.53 reported that the addition of inorganic matter such as CaO, K2CO3 and Al2O3 to demineralised coal had catalytic effects on the reactivity of coal pyrolysis. Others have also reported catalytic effects occurring from organically associated elements, e.g. Ca, Mg, K, Na and Fe8, 13-14, 54-55. Most of these effects were studied through demineralisation (using acids such as HCl or HF53) and the addition of specific mineral species, and to record individual effects8, 14. The decomposition reactions of mineral matter can contribute to significant amounts of CO2 and H2O released during coal pyrolysis (Table 2-8). Table 2-8 Decomposition reactions of mineral matter, from Gräbner and Lester (2016)56 Species Decomposition or phase transformation reaction Mass loss Temperature Muscovite K2O3∙3Al2O3∙6SiO2∙2H2O → K2O∙3Al2O3∙6SiO2 + 2H2O -4.5% 450-700°C Kaolinite Al2O3∙2SiO2∙2H2O → Al2O3∙2SiO2 + 2H2O -14.0% 400-600°C Quartz SiO2 → SiO2 0.0% None Hematite Fe2O3 → Fe2O3 0.0% None Pyrite 2FeS2 + 7.5O2 → Fe2O3 + 4SO3 -33.5%a Oxidation Calcite CaCO3 → CaO + CO2 -40.0% 920°Cb 580°C + Siderite 2FeCO3 + 0.5O2 → Fe2O3 + 2CO2 -31.1% oxidation Albite Na2O∙Al2O3∙6SiO2 → Na2O∙Al2O3∙6SiO2 0.0% None Orthoclase K2O∙Al2O3∙6SiO2 → K2O∙Al2O3∙6SiO2 0.0% None 780°C & Dolomite CaCO3∙MgCO3 → CaO∙MgO + 2CO2 -47.7% 920°C Ankerite 2CaCO3∙FeCO3 + 0.5O2 → 2CaO + Fe2O3 + 4CO2 -37.1% 700°C a Without SO3 capture in ash (otherwise +37.1%), b does not apply to proximate analysis conditions Particle size It has been shown by Tian et al. (2016)34 that coal particle size plays an important role during pyrolysis of bituminous coals. When compared, larger coal particle sizes revealed higher mass losses and higher pyrolysis reactivity34. In addition, larger coal particle sizes also affected devolatilisation temperature – increased the initial devolatilisation temperature (Ti) and decreased the final devolatilisation temperature (T )34f . It was also found that ash yield and volatile matter content increased with larger coal particle sizes34. Particle size also plays an important role in the evolution of gaseous and tar products. Particle sizes of -74 µm compared to sizes of 840 to 1 400 26 JA Meyer Literature review µm show increased CO2 yields; however, at around 750°C, CO2 yields for the latter were higher and can be attributed to calcite decomposition34, 57. As for CO yields, there are no distinct correlations observed, while CH4 yields increased with increasing particle sizes34. The same trend can be observed for aliphatic hydrocarbons and light aromatics, i.e. their yields increase as the particle size increases34. These effects on the product distribution and devolatilisation temperatures and rates may be ascribed to the formation of metaplast and the adverse impact from volatile and metaplast interactions34. The results are, however, in contrast with work done by Smoot and Smith (1985)13 and Anthony and Howard (1976)37. They observed that the product distribution and rate of devolatilisation did not vary significantly with change in particle size (74-1000 µm). In addition, no significant effect on mass loss was observed during pyrolysis for particle sizes of up to 400 µm13, 37. However, with larger particle sizes (>1 500 µm), the effects may differ from that of pulverised coal as products produced near the centre of the particle should migrate to the outside to escape. This may lead to cracking, condensation or polymerisation occurring, while some carbon deposition may also take place13. With increasing particle size, the migration is greater, and therefore lower volatile yields are observed13, 36. It is therefore evident that coal particle size has a significant effect on the products produced. 2.2.4.2 Operating conditions Temperature and heating rate In general, the increase in heating rate increases the temperature at which various processes take place8. At low heating rates (~1 °C/min), occluded CO2 and CH4 are driven off at 200°C, and further internal condensation of the macromolecular structure of low-rank coals takes place as the temperature rises, initiating CO2 and water evolution8. Between 200 and 500°C, CH4 starts to evolve along with olefins, H2 starts to evolve at 400 to 500°C with a critical point at 700°C, where H2 along with CO evolution is accelerated. In the range of 500 to 700°C, the yields of gases such as H 82, CO, CH4 and N2 increase as temperature rises, while hydrocarbons decrease . The production of tars is initiated at temperatures around 300 to 400°C with the maximum yield occurring at approximately 500 to 550°C, dependent on the particle size and heating rate8. In addition, the composition of tars varies with temperature: Low-temperature tar mainly comprises olefins, paraffin hydrocarbons and cyclic hydroaromatic structure, while high-temperature tars are found to mostly comprise aromatic hydrocarbons8. Ladner (1988)25 has compared results related to pyrolysis product yields from low- and high temperature pyrolysis (Table 2-9), and the composition of oils and tars from low- and high temperature pyrolysis (Table 2-10). 27 JA Meyer Literature review Table 2-9 Product yield from low- and high temperature pyrolysis, adapted from Ladner (1988)25 Low temperature (wt.% dry High temperature (wt.% dry Product coal) coal) Gas 7.6 17.2 Liquor 13.0 2.5 Light oils 1.4 0.8 Tar 8.0 4.5 Char 70.0 75.0 Table 2-10 Product composition from low- and high temperature pyrolysis, adapted from Ladner (1988)25 Low temperature pyrolysis (400-750°C) Gas wt.% Light oil wt.% Tar wt.% Hydrogen 10 Paraffins 46 BTX 1.5 Hydrocarbons 65 Olefins 16 Phenols 1.5 Carbon monoxide 5 Cyclo-paraffins 8 Cresols 4.5 Carbon dioxide 9 Cyclo-olefins 9 Xylenols 7.0 Others 11 Aromatics 16 Other phenols 16.0 Others 5 Tar bases 2.0 Naphtha 3.5 Other aromatics 38.0 Pitch 26.0 High temperature pyrolysis (900-1100°C) Gas wt.% Light oil wt.% Tar wt.% Hydrogen 50 Benzene 72 BTX 0.6 Hydrocarbons 34 Toluene 13 Phenols 1.6 Carbon monoxide 8 Xylene 4 Cresols Carbon dioxide 3 Alicyclics 5 Xylenols 0.5 Others 5 Aliphatics 6 Other phenols 1.0 Naphthalene 8.9 Anthracene 1.0 Other aromatics 24.6 Tar bases 1.8 Pitch 60.0 When comparing the results of high-temperature (900-1100°C) pyrolysis and low-temperature (400-750°C) pyrolysis, it was found that the yields of liquor, light oils, and tar were higher for low- temperature pyrolysis, whereas gas and char yields were lower. Based on the results obtained 28 JA Meyer Literature review from the table above, it can be assumed that lower molecular species evolve during high temperature pyrolysis. The tar produced during high temperature pyrolysis is more aromatic in nature, with the presence of unsubstituted polycyclic aromatic hydrocarbons25. The heating rate during pyrolysis experiments has significant effects on the devolatilisation profile performance as well as on gaseous products released34. When the heating rate is increased, the temperatures in the main devolatilisation stage (Ti, Tf, and Tmax) shift toward a higher temperature range34, 36. This phenomenon may be attributed to heat and mass loss limitations as a result of the poor thermal conductivity of coal34, 58. It is therefore important to note that devolatilisation temperatures rise as the heating rate rises. Pressure In an inert environment, the effect of pressure corresponds to some major effects during coal devolatilisation25, 59. The main effect of increasing pressure can relate to an increase in char yield, followed by a corresponding decrease in the production of tar as well as a significant decrease in volatiles produced8, 25, 36, 59. This may be attributed to lower molecular mass compounds having sufficient vapour pressures to change phase, consequently the opportunity to promote depolymerisation cross-linking reactions of heavier tar precursors into chars8. An increase in some gaseous hydrocarbons can be observed, particularly methane. Some minor variations occur with coals of different ranks, for example water formation in lignite coal is suppressed while enhanced for bituminous coals25, 59. Atmosphere The purpose of a purge gas (or carrier gas) is usually to remove volatiles from coal during pyrolysis. Usually, an inert medium such as nitrogen, argon or helium is used as the purge gas during pyrolysis. However, some reactive gases are also known to be used, e.g. carbon dioxide, hydrogen (also known as hydropyrolysis) and/or steam25, 60. Hydropyrolysis is usually used when methane is the desired gas; this is implemented by using hydrogen as the carrier gas at elevated temperatures and pressures25. When compared to nitrogen used, the yield of the char is higher than that obtained while using hydrogen; this may be explained by the higher thermal conductivity of hydrogen, or by the reactions between hydrogen and carbon composites. For example, by the two basic reactions (all in the gaseous phase) occurring when hydrogen is present, with or without the addition of water/steam60: C + 2H2 → CH4 (1) 2C + H2 + H2O → CO + CH4 (2) 29 JA Meyer Literature review Since there is moisture present in coal, the latter will always occur during hydropyrolysis60. Other differences that occur when using an inert medium (pyrolysis) compared to hydrogen (hydropyrolysis), include higher carbon dioxide yields, lower methane and other CH gaseous yields, and lower light CH liquids yields25. 2.3 Coal discards and fines Coal mining in South Africa has remained relatively constant over the past two decades, producing around 260 Mt/a where around 60 Mt/a of fine coal discards are generated, i.e. particles of <500 µm, of which 10 Mt/a are classified as ultra-fines (<150 µm)61-63. Fines have negative environmental effects, such as acid mine drainage, air pollution and spontaneous combustion63. These fines usually have qualities equivalent to that of run-of-mine (ROM) coal; the average calorific value of these ultra-fines are 23 MJ/kg, making it a viable option for combustion – as the minimum heating value needed for power generation at most Eskom power plants is 16 MJ/kg63- 64. Unprocessed fines are usually low-quality fines that are (generally) sold to power stations to be used as a feedstock. These fines are discarded due to their poor quality, pertaining (mostly) to low calorific value, high ash and sulphur content. They have calorific values ranging between 2 and 14.6 MJ/kg and ash contents of around 45%63. It is also important to note that the inorganic components vary greatly within the mined coal seams, as impurities of the roof and floor form part of the feed. As the feed moves through the preparation plant, some of the mineral species are found more prominently in the tailings. Some of these minerals can be separated by means of density separation, where other more finely disseminated particles require significant particle size reduction before the minerals may be liberated from the coal. Minerals will also contribute to the ions present in a solution and contribute to the pH, the charge of the coal and mineral particle surfaces, and the processes of reagents for flotation, dewatering and thickening65. Coal macerals are grouped in three classes, namely vitrinite, liptinite and inertinite25, 65. Vitrinite in general is predominantly found in coal, but concentrations of inertinite macerals such as fusinite in fines would increase, as these macerals are less hydrophobic and result in lower flotation yields. This may play a role in some coals, but not all. The friability of macerals such as fusinite is also greater and could generate more fines than expected65. Hydrophobicity is also rank dependent, as high-ranking coals are more hydrophobic, while lower-ranking coals have higher oxygen contents and may contain greater concentrations of humic acids. The friability of high- rank coals is in general higher and will result in more fines present65. Coal fines are unfortunately an unavoidable waste and by-product produced from coal processing but may possess the potential to be utilised when converted back to a useable product – in the case of this study through agglomeration that will be discussed in the next section. 30 JA Meyer Literature review 2.3.1 Agglomeration of coal fines Coal fines are considered uneconomic to utilise, a hazard to the environment and difficult to process; therefore, technologies should be incorporated to beneficiate coal fines. When considering coal fine utilisation, it is important to assess several factors contributing to the quality of agglomerated coal fines. Coal fines utilisation and agglomeration have been studied extensively over the last century66. The process of agglomeration is carried out by applying pressure on a coal sample with or without a binding agent, to produce a solid in the form of a pellet or briquette. Briquetting has widely been used to agglomerate bituminous coal and anthracite. Peat and other fuels have also been briquetted, however, only in insignificant amounts. The objective of briquetting is to convert an inferior fuel to a fuel of superior quality. For example, brown coals are briquetted from a rather friable material with a high moisture content to a more durable hard product with a higher calorific value11. The size and shape of the solid depends on its application demand. These solids hold the potential to be used for various domestic and industrial applications. Different factors must be considered when applying the agglomeration process. These factors include physical, chemical, mechanical, and thermal characteristics. Coal pellets or briquettes are not strong and water resistant enough without a binder present when considering the effects of transport and handling, as well as weakening of the solid when exposed to wet conditions. The binder, therefore, must add to its compressive strength and water resistibility11. An understanding of the agglomeration process should consist of knowledge pertaining to cohesive forces between particles, particle surface characteristics, adhesive properties of binders (or bridging liquids), and the rheological behaviour of agglomerates and particulate masses11, 67. These factors will be discussed in the next section. 2.3.1.1 Factors affecting coal agglomeration Coal agglomeration relies on the wettability difference between the coal particles and that of mineral matter by a bridging agent or an agglomerant, such as light – and heavy hydrocarbons or non-hydrocarbon oils67. Agglomeration of coal materials is heavily dependent on coal petrology, coal structure, type, rank, surface oxidation, and the type and concentration of the agglomerant67. Coal particles should have hydrophobic properties prior to agglomeration, however, if not, surface-modification agents are often added. These agents, in some instances, may decrease the amount of agglomerant needed in the process, as well as render the ash components more hydrophilic or coal macerals more hydrophobic67. Some of the most important factors affecting coal agglomeration are discussed below. 31 JA Meyer Literature review Coal rank The rank of coal correlates with its hydrophobicity, as its rank ranges from lignite to anthracite the hydrophobicity increases65, 67. However, the hydrophobicity of anthracite may slightly decrease when its specific surface area increases68. Higher ranking coal exhibiting strong hydrophobic properties has better agglomerating performance compared to lower-ranking coals and minerals (minerals such as pyrite, silicates, clay, etc.) with weak hydrophobic properties69. Coal oxidation Coal oxidation affects the particle surface characteristics, as well as its ability for agglomeration. In general, there are three stages of coal oxidation: (1) formation of acidic coal-oxygen complexes resulting from surface oxidation, (2) humic acid and hydroxy-carboxylic acid formation, and (3) breakup of humic acids to form water soluble acids70. Oxygen adsorption onto coal surfaces may lead to the release of CO, CO2, and H2O, forming carboxylic and phenolic functional groups that result in the development of polar sites and, in turn, render the surface hydrophilic71-72. The presence of these oxygen-containing functional groups suppresses non-polar agglomerating agents such as the oils mentioned above, and produces high negative charges on the particle surfaces and increases electrostatic repulsion of negatively charges compounds67. Coal oxidation, therefore, contributes to the formation of carboxylic and phenolic functional groups (including ether linkages) that reduce the natural hydrophobicity of coals resulting in weaker agglomerating properties67. Coal petrography and composition On the microscopic scale, the existence of coal’s cross-linked network of polymeric macromolecules affects the insolubility and swelling properties of coal; however, on the macroscopic scale, coal consists of a mixture of organic macerals – predominantly vitrinite, liptinite and inertinite25, 65, 67, 73. When considering these three macerals, the dominant lithotypes are durain (inertinite- and liptinite-rich), fusain (inertinite-rich), clarain (vitrinite- and liptinite-rich), and vitrain (vitrinite-rich). Humic coals contain a significant amount of vitrinite macerals, which affects the coking, flotation and combustion behaviour of the coal74. It is known that the wettability of every coal maceral differs, and the quantification or measurement remains difficult; for example, fusain and vitrain differ based on their hydrophobic properties, oxygen-containing functional groups, elemental composition, and exhibit different degrees of floatability67, 74. The hydrophobicity of different coal macerals has been reported by various authors, and decrease in the order: liptinite>vitrinite>inertinite8, 31, 67. It was found that the oxidation rate of coal increased with vitrinite content and decreased with higher ranking coal75. 32 JA Meyer Literature review Furthermore, the aromaticity of coal has been reported to influence the agglomerating potential of coal. When more aromatic structures are present in the macromolecular network of coal, the agglomeration properties improved significantly76. Coal particle size In general, particle size reduction of coal during the crushing process improves the liberation of mineral matter from coal, and, in turn, leads to the disintegration and liberation of impurities65, 77. Fine coal particles are agglomerated with more ease compared to coarser coal particles, as finer particles fill the spaces generated between larger particles increasing the bulk density of the agglomerated products. It has been shown that the bulk density of coal agglomerates is highly dependent on the moisture content, particle size distribution, and the inter- and intraporosity of coal particles78. These properties also play an important role in the agglomerate particle packing, as well as the capillary forces generated between particles. Coarser coal particles are associated with weak capillary forces that produce weak agglomerates with lower bulk densities compared to that of finer coal particles, as the interparticle bond strengths are related to the capillary forces generated by the bridging liquid at particle surfaces67. Bridging liquids The most used bridging liquid during coal agglomeration has been oil due to its low cost, availability and clean coal yield67. Clean coal yield can be attributed to the decrease in moisture content and impurities such as ash and sulphur contents11. The effect and nature of various types of oil have been investigated in order to address their effect on the agglomerant. Labuschagne (1986)79 concluded that the behaviour of the agglomerated products depends on the chemical, physical and structural properties of the bridging liquids. It should also be noted that there is a strong relationship between the hydrophobicity of coal and the performance of oil agglomeration. The nature and concentration of bridging liquids do, however, also play an important role67. Other factors Other factors that have shown to have an effect on the agglomeration performance are: (1) pulp density and pH; (2) additives to increase coal hydrophobicity; (3) surfactants used to improve oil adsorption to the coal-particle surface; (4) positive electrolytes to destabilise wetting films on hydrophobic coal particles and suppress negatively charged coal particles; and (5) the effects of agitation intensity and time67. 33 JA Meyer Literature review 2.4 Co-pyrolysis of microalgae and coal South Africa is dependent on coal for most of its energy demands; it is therefore necessary to develop and implement clean coal technologies that would reduce environmental impacts such as greenhouse gas (GHG) emissions. The use of renewable biomass material for co-firing processes with coal has shown GHG reduction potential80-81. New initiatives using microalgae along with coal, or discarded coal, have been studied over the past few years. These studies include the use of algae as an agglomerating agent, combustion additive, gasification additive, co-pyrolytic additive, and solely for the production of biofuels, biochar, bio-syngas and other chemicals80-88. The focus of this section will be on the co-pyrolysis of microalgae biomass with coal, and an evaluation of the products resulting from the pyrolysis and gasification of coal-algae. 2.4.1 Microalgae overview Microalgae are microscopic algae that are usually found in freshwater, brackish water, or marine systems89-90. These species are unicellular and may exist individually, or in groups, known as colonies. Microalgae are classified based on their pigmentation, cellular structure and lifecycle. The various groups of microalgae include green algae (Chlorophyceae), blue-green algae (Cyanophyseae), diatoms (Bacillariophyceae), and golden algae (Chrysophyseae). Other groups include yellow-green algae, red algae, brown algae, dinoflagellates and Pico-plankton90-91. The most abundant species of algae are green algae, blue-green algae and diatoms, and are of great importance. These algae store energy in the form of starches and triacylglycerols92. The biochemical composition of microalgae comprises four principle groups of molecules, namely lipids, proteins, carbohydrates and nucleic acids that vary in proportions based on different algae classes. Lipids are the most energy-rich compounds (37.6 kJ/g), followed by proteins and carbohydrates with energies of 16.7 kJ/g and 15.7 kJ/g, respectively91. Microalgae contain mostly polar and nonpolar lipids known as triglycerides, phospholipids and glycolipids. Polar lipids are known as structural lipids (e.g. phospholipids and glycolipids), while nonpolar lipids are storage lipids (e.g. triglycerides)89, 91. Table 2-11 outlines the lipid contents of some important algae species93. 34 JA Meyer Literature review Table 2-11 Lipid content of various algae species, adapted from Mata et al. (2010)93 Algae species Lipid content (dry wt.%) Batryococcus braunii 25-75 Chlorella sp. 10-48 Crypthecodinium cohnii 20-51 Dunaliella tertiolecta 17-71 Isochrysis sp. 25-33 Monallanthus salina 20-22 Nanochloris sp. 20-34 Nannochloropsis 31-68 Nitzschia sp. 45-47 Phaedactylum tricomulum 20-30 Scenedesmus sp. 19-21 Algae synthesise fatty acids which are the building blocks for lipid formation – the most common synthesised fatty acids have carbon chain lengths ranging from C16 to C 9018 . The lipid content contributes to the high oil yield of algae, dominated by polyunsaturated fatty acids (PUFAs)91. The high oil yield of algae makes it a promising source of renewable biomass. The main advantages of algae as a feedstock for biofuel include89-91: • Photosynthetic processes are more efficient than other biomass sources; • High growth rates; • Higher oil production per area than any other biomass/plant material; • Benefits wastewater bioremediation by utilising nitrogen and phosphorus as primary growth nutrients; • It can produce large quantities of neutral oils, e.g. between 20 and 50% of the cell’s dry weight; • Land occupation is significantly smaller compared to that of agricultural land; and • Approximately half of the atmospheric oxygen is produced from microalgae, making it a viable source for green technologies. 35 JA Meyer Literature review 2.4.1.1 Method for algae cultivation (InnoVenton) InnoVenton, a research institute from the Nelson Mandela University (NMU), cultivated algae in a patented hybrid system that consists of traditional raceway systems and photobioreactors (PBRs) of the bubble column type94-95. Figure 2-2 shows this hybrid system. Figure 2-2 Cultivation of microalgae through the patented hybrid system (InnoVenton)94-95 Algae are seeded in the system from separate seed colonies (PBRs). They are grown in a nutrient medium (similar to fertiliser mix), which is a modified version of Bolds Basal Medium (BBM)95. CO2-enriched air is the carbon source for algae growth, which is distributed through the system using the bubble columns. In addition, this controls the pH in the optimal range (pH ±8.5). The algae growth medium moves through the raceway assisted with a paddlewheel. Natural light allows photosynthesis, and the oxygen evolved during this process is removed by agitation. When the concentration has reached a level that equates to the inflection point on the growth curve (i.e. when the growth rate starts to decrease), the solution is pumped into a gravity settling tank. A significant quantity of the algae settles overnight and is removed. The remainder goes back to seed the next generation95. The concentrated algae solution is mainly used for research purposes and for the addition to coal fines for producing Coalgae®87, 95-96. 2.4.2 Overview of the properties and characteristics of coal-algae The properties of algae include high volatile matter contents and low fixed carbon contents87-88. Gaqa and Watts (2018)87 studied the agglomerating effect of microalgae biomass on ultra-fine coal (<150 µm) and evaluated the physical and chemical characteristics (proximate, ultimate, compressive strength, water resistance and impact resistance analysis). The algae used was of the Scenedesmus species obtained from InnoVenton at Nelson Mandela University, Port- Elizabeth. Results obtained from this study are presented in Table 2-12. 36 JA Meyer Literature review Table 2-12 Chemical properties obtained from coal-algae blends adapted from Gaqa and Watts (2018)87 Volatile Sample Fixed carbon Ash HHV (MJ/kg) matter Raw coal 20.0 41.4 38.6 16.4 Algae 78.0 14.6 7.4 17.3 CA 90:10 (mixed & centrifuged) 25.4 33.9 35.4 16.6 CA 80:20 (mixed & centrifuged) 34.0 25.5 35.1 15.1 CA 90:10 (centrifuged after 24h) 25.7 33.4 35.8 16.4 CA 80:20 (centrifuged after 24h) 34.8 27.8 32.9 15.8 CA = coal-algae blends, HHV = higher heating value The proximate analysis indicated an increase in volatile matter with the addition of algae, as biomass materials generally have a higher volatile matter content than coal. Ash yield and fixed- carbon content decreased with the addition of algae. A decrease in higher heating value (HHV) is also observed in the blended samples. The coal used is of poor quality, with similar calorific values as the microalgae, and therefore the effect may not be so accurately observed. 2.4.3 Agglomeration using a microalgae binder Compressive strength tests done by Gaqa and Watts (2018)87 indicated that a higher algae-coal ratio showed better strength characteristics. Different moisture contents were tested, i.e. 17% and 22%, to determine whether the moisture contents of the samples had any effect on the produced pellets. Table 2-13 summarises the compressive strength values and water resistance indices as observed by the investigators83. 37 JA Meyer Literature review Table 2-13 Physical properties obtained from coal-algae blended pellets adapted from Gaqa and Watts (2018)87 CS (17% CS (22% WRI (17% WRI (22% Sample moisture) moisture) moisture) moisture) Raw coal 0.6 0.3 0.0 0.0 CA 90:10 (mixed & centrifuged) 0.7 1.2 0.0 75.6 CA 90:10 (mixed & centrifuged) 1.3 1.1 88.9 81.3 CA 90:10 (centrifuged after 24h) 0.7 0.5 75.5 80.5 CA 80:20 (centrifuged after 24h) 1.4 1.0 65.5 86.7 CS (Compressive strength) measured in kg/mm2, WRI (Water resistance index) = 100 - % water after 30 minutes immersed in water Results indicated that the amounts of algae added, moisture contents and curing duration influenced the quality of the pellets. Compressive strength values increased with the addition of algae to coal fines; however, there is no noticeable trend observed when comparing the moisture content, curing time, and/or algae concentration to the compressive strength values of the coal- algae blends. It is, therefore, important to note that there are several variables that should be considered when producing pellets using micro-algae as a binder. 2.4.4 Co-pyrolysis of microalgae and coal It is important to understand the properties and characteristics of coal that contribute to the pyrolysis processes and product evolution. When considering low-temperature pyrolysis, it is known that a lower H/C atomic ratio of low-rank coals is associated with lower tar yield. However, biomass has higher H/C atomic ratios and can therefore promote tar yield and improve the quality of char through co-pyrolysis with coal85, 97-98. Microalgae, an aquatic biomass, has the advantages of rapid growth rates, no land occupation, and high fixation rates of CO2, over other biomass materials. When compared to terrestrial crops, microalgae contain lower amounts of lignin, which, in turn, enhances the thermo-chemical conversion process of co-pyrolysis with coal85. During microalgae pyrolysis, large amounts of alkyl radicals are generated, and the addition or elimination of hydrogen radicals from theses alkyl radicals results in alkane and α-olefin formation, respectively85. And consequently, encouraging the production of pyrolytic oil that is predominantly higher in hydrocarbon content and lower in oxygen content. Therefore, co-pyrolysis of coal and microalgae can promote the quality and yield of tar under milder conditions85. Studies conducted on the co-pyrolysis of microalgae and coal have had different conclusions on the volatile matter yields. Yang et al. (2013)99 investigated the co-pyrolysis of Dunaliella tertiolecta with coal using 38 JA Meyer Literature review thermogravimetric analysis and found that there exists a synergistic effect between algae and coal in the temperature range between 200°C and 500°C, which promoted the volatile product yields. The same conclusions were made by Qian et al. (2013)100, while Chen et al. (2012)101 found that microalgae added during co-pyrolysis promoted the formation of char. Furthermore, Kirtania and Bhattacharya (2013)102 indicated that no chemical interactions occur during co- pyrolysis of Chlorococcum humicola and lignite (or brown coal). Baloyi and Dugmore (2019)86 examined the pyrolytic behaviour occurring during co-pyrolysis of Scenedesmus microalgae and coal under mild inert conditions. Pyrolysis experiments of the coal-algae composites were performed at 450°C in a batch reactor. A higher yield of volatile content in the coal-algae was observed compared to the raw coal. Pyrolysis results indicated a decrease in char yield accompanied by an increase in gas and tar yield in the coal-algae sample. In addition, a significant degree of deoxygenation, dehydrogenation and denitrification was observed in the coal-algae composite. However, these findings may be more relevant to the algae in this temperature range and the full effect of co-pyrolysis would not be observed. Due to the large variety of microalgae, whether there exists a synergistic effect from co-pyrolysis with coal on the volatile yield and kinetic characteristics, it is still not conclusive and effective for predicting product yields85. 2.4.5 Combustion and gasification of coal-algae Coal combustion produces greenhouse gases such as CO2, CO, SO2, NOx (nitrogen oxides), N2O, and particulate material80. The effect of algae on the reduction potential of GHG emissions of coal was studied by Magida et al.80. The results indicated that there was a significant reduction in gases such as CO2, SO2 and NOx concentrations when combusted and analysed using a Lancom 4 portable flue gas analyser80. This can be supported by the fact that biomass material, when combusted, is considered carbon neutral – the CO2 produced from the process forms part of the active cycle, where CO2 produced from fossil fuels forms part of the greenhouse gases80- 81, 103. Kadam (2002)104 also suggested that CO2 produced from power plants should be used as a CO2 source for microalgae cultivation, and, in turn, co-firing the microalgae with coal for power generation. The gasification reactivity of coal-algae slurries compared to coal-water slurries was studied by Li et al. (2010)88. The results indicated that the coal-algae slurries displayed a significant reactivity improvement compared to that of the coal-water slurries. In addition, the coal-algae slurries were more stable88. 39 JA Meyer Literature review CHAPTER REFERENCES 1. WCA Coal market & pricing. http://www.worldcoal.org/coal/coal-market-pricing (accessed 14 February 2020). 2. Hancox, P. J.; Götz, A. E., South Africa's coalfields—A 2014 perspective. International Journal of Coal Geology 2014, 132, 170-254. 3. WCA Uses of Coal. http://www.worldcoal.org/coal/uses-coal (accessed 12 February 2020). 4. Van Dyk, J.; Keyser, M.; Coertzen, M., Syngas production from South African coal sources using Sasol–Lurgi gasifiers. International Journal of Coal Geology 2006, 65 (3-4), 243-253. 5. Ratshomo, K.; Nembahe, R. South African Coal Sector Report; Department of Energy: Pretoria, 2017. 6. Ratshomo, K.; Nembahe, R. The South African Energy Sector Report; Pretoria, 2019. 7. DoE Coal Resources - discards. http://www.energy.gov.za/files/coal_frame.html (accessed 2020/05/13). 8. 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School of Chemical and Minerals Engineering, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa Abstract The beneficiation of coal in South Africa accounts for more than 65 Mt of coal discards annually, of which 10 Mt are classified as ultra-fines (<150 µm). These fines pose significant disposal problems and have negative environmental effects, such as acid mine drainage, air pollution and spontaneous combustion. It has been estimated that South Africa currently has accumulated more than 1000 Mt of discarded coal fines. Coal fine agglomerates produced without the addition of a binder generally possess poor compressive strength and water resistance characteristics. The objective of this study is to agglomerate coal fines using microalgae biomass (e.g. 20%, 10%, and 5% microalgae in the coal-algae blends) and to examine the effects on its physical, chemical and thermal behaviour (devolatilisation). It was found that the agglomerates produced from the coal-algae blends exhibited greater compressive strengths compared to those produced from coal fines alone; however, the water-resistance indices were impaired with the addition of microalgae. In addition, the mass loss during high temperature pyrolysis was evaluated using thermogravimetric analyses. Thermogravimetric analyses indicated that volatiles evolved at lower temperatures with the addition of microalgae, indicating that complete carbonisation occurred earlier. The amount of char remaining after algae pyrolysis is significantly lower (~30%) than that of the coal and coal-algae samples (>70%). The results indicate that the addition of microalgae biomass to fine coal discards has the potential to produce a solid with increased physical and thermal properties that is essential for agglomeration and thermal processing. Keywords: Coal fines; microalgae; agglomeration; compressive strength; calorific value 3.1 Introduction Coal is the most widespread fossil fuel currently in the world, as it is the primary source of energy for power generation and feedstock for liquid fuels1. Coal is mainly used for electricity generation, liquid fuel production, combustion, gasification, and within the steel-, iron-, and cement manufacturing industries2. The global coal reserves are depleting, but energy requirements are increasing, which creates the need for more coal to be produced. The global coal demand is increasing at an alarming rate. The coal trade in the world reached 1169 Mt in 2018 – being JA Meyer Chapter 3 shipped over great distances to reach overseas markets. It is a significant amount of coal and it accounts for only 21% of the total coal consumed globally, as most of the coal used within most countries has been produced locally4. For more than a century and a half, coal has been regarded as an essential commodity in South Africa’s economy. The leading coal consumers in South Africa are predominantly Eskom (coal- fired power generation) and Sasol (gasification), consuming approximately 90 Mt and 30 Mt, respectively5-6. Around 30% of liquid fuel produced and 70% of South Africa’s energy needs are provided using coal, as coal-fired power stations contribute to more than 90% of South Africa’s power supply – dominated by Eskom6. The production and beneficiation of coal create significant residues such as dust and fines that present major disposal and processing challenges for the industry, with various environmental impacts such as acid mine drainage, air pollution and spontaneous combustion7. This accounts for more than 65 Mt of fine coal discards (<500 µm) produced annually in South Africa, and has been estimated to have accumulated to more than 1 Gt6, 8. Coal fines have the potential to be an important resource with viable processing9. This creates the need for new technologies to be implemented in processing coal fines to obtain a useful product. Coal fines agglomeration has been studied extensively over the last century10. The process of agglomeration is carried out by applying pressure on a coal sample with or without a binder, to produce a solid in the form of a pellet or briquette. The size and shape of the solid depends on its application demand. These solids hold the potential to be used for various domestic and industrial applications, such as coal combustion and gasification processes. Different factors must be considered when applying the agglomeration process. These factors include physical, chemical, mechanical and thermal characteristics11. Coal pellets or briquettes are not strong and water resistant enough, without a binder present, when considering transport and handling, as well as weakening of the solid when exposed to wet conditions12. The binder, therefore, must add to its compressive strength and water resistibility. The purpose of this study is to investigate the effect that microalgae have on coal discards when agglomerated. Its effect on physical, chemical and thermal parameters will be investigated9, 13. 3.2 Experimental section General: A filter cake coal sample was used in this study, originating from a Highveld inertinite- rich coal source. These fines were screened to -250 µm. The microalgae biomass used as the agglomerating agent is a common Scenedesmus microalgae obtained from InnoVenton, a 46 JA Meyer Chapter 3 research institute based at Nelson Mandela University in Port-Elizabeth, South Africa. The microalgae were harvested and concentrated by settling as described previously9. Sample preparation The filter cake sample was air-dried for three days to drive off most of the moisture. The cone- and-quartering method was applied to the sample to obtain a good representative sample. The coal discarded filter cake sample will be referred to as coal fines. A proximate analysis was conducted on the coal to determine the amounts of algae needed, as the weight ratio is determined on a dry, ash-free (d.a.f.) basis. Characterization The coal and microalgae samples were characterised by proximate analysis according to the ISO standard methods listed in Table 3-1, and by ultimate analysis according to the ISO standard method: ISO 12902 – CHN. Table 3-1 ISO methods used for proximate analysis of the samples Moisture content (%) BV-TPM-010 based on ISO11722: 1999 Ash content (%) BV-TPM-011 based on ISO 1171: 2010 Volatile matter content (%) BV-TPM-012 based on ISO 562: 2010 Total sulphur (%) BV-TPM-013 based on ISO 19579: 2006 Fixed carbon content (%) Calculated by difference The results from proximate analysis were used to calculate the higher heating value (HHV) of each sample from the following equations, where the fixed carbon content is given by FC, the volatile matter content by VM, and ash content by ASH: 𝐻𝐻𝑉 (𝑐𝑜𝑎𝑙) = 0.3536𝐹𝐶 + 0.1559𝑉𝑀 − 0.0078𝐴𝑆𝐻 Parikh et al. (2015)14 (1) 𝐻𝐻𝑉 (𝑏𝑖𝑜𝑚𝑎𝑠𝑠 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙) = 0.2521𝐹𝐶 + 0.1905𝑉𝑀 Yin (2011)15 (2) The calorific values of the various agglomerated samples were determined using a bomb calorimeter with an IKA® C 5000 controller, IKA® C 5003 measurement cell, IKA® C 5010 decomposition vessel and an IKA® KV 600 digital chiller. The controller is connected to a Shimadzu AY220 balance to determine the weight of the sample. Coal-algae preparation It was necessary to determine the amount of dry algae present in the algae slurry prior to the preparation of the coal-algae blends. A dry-weight determination was performed by recording the weight of three crucibles, followed by weighing algae slurry samples that were placed in the 47 JA Meyer Chapter 3 crucibles. The crucibles were then placed in an oven for an hour and cooled in a desiccator to prevent the dried algae from coming into contact with moisture. The amount of dry algae was noted by the difference in weight. Three coal-algae blends were prepared on a dry-algae basis, e.g. CA5 (Coal with 5% algae addition), CA10 (coal with 10% algae addition), and CA20 (coal with 20% algae addition). The blends were stirred for 18 hours (the curing stage) using an overhead stirrer, followed by centrifuging the blends for 10 minutes at 4 000 rpm (HERMLE 2383). The different blends were spread in pans and oven dried at 100°C overnight. The samples were stored in air-tight vacuum sealed bags to keep moisture levels constant. The coal-algae samples were milled and screened to -250 µm and set aside for agglomeration and characterisation. Agglomeration of samples Agglomeration is the process of applying pressure on a coal sample, with or without a binder, to produce a solid in the form of a pellet or briquette. Different weight fractions of distilled water were tested during the agglomeration process, and the 12 wt.% addition showed the best results. Therefore, 12 wt.% distilled water was added to the materials prior to agglomeration. An LRXplus instrument from LOYD Instruments (Ametek) was used to press the various pellets, weighing 1 gram each into a Specac PT No. 3000 10 mm diameter die set. The compression rate of the instrument was set to 15 mm/min until a load limit of 1.5 kN was reached and maintained for 15 seconds. Compressive strength (CS) and water-resistant index (WRI) The compressive strength of the agglomerates obtained from the various samples was determined using an LRXplus instrument from LOYD Instruments (Ametek) and recorded at the breaking point in MPa. In addition, the agglomerates were placed in water for 30 minutes in order to calculate their water resistance index (WRI) using the following equation16: 𝑊𝑅𝐼 = 100 − % 𝑤𝑎𝑡𝑒𝑟 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑎𝑓𝑡𝑒𝑟 30 𝑚𝑖𝑛𝑢𝑡𝑒𝑠 𝑖𝑚𝑚𝑒𝑟𝑠𝑖𝑜𝑛 (3) Thermogravimetric analysis (TGA) The thermo-decomposition of organic material, i.e. coal and algae, was investigated by means of thermogravimetric analysis (TGA). The results indicate the relationship between weight loss and temperature due to decomposition, oxidation and dehydration reactions in a TGA plot and its derivative with respect to temperature in a DTG plot. 15 mg of the coal, algae and coal-algae blends were subject to thermogravimetric thermal analysis using an SDT Q600 Thermogravimetric Analyzer (TGA) from TA Instruments and supplied by Advanced Laboratory Solutions (ALS). The samples were heated from 40°C to 920°C at a constant heating rate of 5 °C/min in a nitrogen atmosphere with a flow rate of 100 ml/min. 48 JA Meyer Chapter 3 3.3 Results and discussion 3.3.1 Proximate- and ultimate analyses The results obtained from the proximate- and ultimate analyses of the coal and algae samples are presented in Table 3-2. The proximate analysis results of the coal indicate a significantly high ash content with a relatively high volatile content and low fixed carbon content. Coal discards in general have high ash contents of up to 45%, as impurities from the roof and floor of the mined seam form part of the feed7. In this case, the ash content of the fine coal discard sample is somewhat lower at 29%. The results from the proximate analysis of the coal sample confirm that its rank is in the range of medium volatile bituminous coals17. Biomass materials generally have lower fixed-carbon contents and ash yields, while having higher volatile matter contents than coal. The relatively high volatile content in algae indicates that, under pyrolysis, a large amount of tars and oils may be produced, and that weight loss would be imminent at low temperatures. The low fixed carbon content is an indicator that a small amount of char would be produced from the pyrolysis of algae. However, when compared to other biomass types, such as pine chips (and sawdust18), sugarcane bagasse, bamboo, as well as algae from lake blooms, a higher ash yield and lower volatile and fixed carbon contents are observed15, 18. Ultimate analyses of the samples reveal their C, H, N, S, and O (by difference) contents, and their respective H/C (degree of aromaticity) and O/C (degree of polarity) molar ratios. The hydrogen and carbon percentages (by Seyler’s classification model), as well as the H/C and O/C molar ratios indicate that the coal is in the range of subbituminous coals. The higher H/C molar ratio of algae indicates that there are lower concentrations of aromatic structures present, the lower H/C molar ratio observed in the coal sample indicates that the coal is more aromatic in nature, projecting higher concentrations of poly-aromatic hydrocarbons (PAH’s)19. A relatively high O/C molar ratio is observed in the algae sample. This suggests that there are some polar compounds present in the algae, making it more hydrophilic in nature19-21. However, a low O/C molar ratio is observed in the coal sample, indicating that the coal may be more hydrophobic19. The nitrogen content of the algae is significantly high and may be related to the availability of nutrients, e.g. nitrogen, phosphorus and/or silicates (in diatoms), present in the culture medium. These nutrients are important during growing stages and key to the primary production of microalgae22. Most nitrogen present in algae is as a result of the various proteins present22-23. Sulphur contents in biomass materials are usually lower than that present in coal; however, the algae used in this study have a slightly higher sulphur content than for the coal sample24. The results from proximate and ultimate analyses of the microalgae agree with the results obtained by Gaqa and Watts (2018)9, as the same type and origin of microalgae were used. 49 JA Meyer Chapter 3 Table 3-2 Proximate- and ultimate analyses results of coal fines and microalgae Coal Microalgae Proximate analyses (a.d.b.) % Inherent moisture content 3.8 6.7 % Ash content 28.9 16.4 % Volatile matter content 23.9 67.4 % Fixed carbon (by calculation) 43.4 9.5 Ultimate analyses (d.m.m.f.) % Carbon content 78.7 56.6 % Hydrogen content 4.7 7.7 % Nitrogen content 2.0 9.7 % Total sulphur 0.74 1.17 % Oxygen content (calculation) 14.6 26.0 H/C molar ratio 0.71 1.62 O/C molar ratio 0.14 0.34 Fuel ratio 1.82 0.14 a.d.b = air-dried basis; d.m.m.f. = dry-mineral-matter free basis; Fuel ratio is expressed by FR = fixed carbon/volatile matter content25 3.3.2 Compressive strength (CS) and water resistance index (WRI) Compression strength testing of agglomerates is an important method for determining the ultimate compressive load that a solid can withstand. This will give an indication of the friability of the solid and to what degree it will withstand degradation when handled, as well as its tendency towards breakage. The water resistance indices of agglomerates give an indication of their potential to endure a wet environment and retain its structure during stockpiling. Table 3-3 shows the compressive strength results obtained for the agglomerated samples. Table 3-3 Compressive strength values (CS) of the various samples with the addition of 12% water prior to agglomeration Coal fines CA5 CA10 CA20 CS (MPa) 4.2 4.5 5.1 6.0 The results indicate that the compressive strength of the agglomerated samples increase with the addition of algae. A linear trend of increasing compressive strength was observed, indicating that the concentration of the agglomerant does have a significant effect on the agglomerating process. The CS of coal fines had an average value of 4.2 MPa, whereas the blended samples, e.g. CA5, 50 JA Meyer Chapter 3 CA10 and CA20, had values of 4.5 MPa, 5.1 MPa, and 6.0 MPa, respectively. The CA20 blend shows a 43% increase in CS when compared to the coal fines sample. This effect may be attributed to the small particle sizes of the algae material, or to some other factors affecting the agglomeration process. These factors may include the polarity of algae, or chemical transformation that occurred during the blending and curing stages. The compressive strength of run-of-mine (ROM) coal is ca. 14 MPa26. It is of great importance that the compressive strength of briquettes/agglomerates is several times larger than 150 N (around 1.9 MPa for the sample sizes used in this study) to be suitable within various processes27. Richards (1990)16 also suggests that it would be favourable that a compressive strength above 350 kPa be maintained. The coal agglomerates present good compressive strength without the addition of the algae and this may be attributed to coal rank, specific surface area, minerals present, degree of coal oxidation, coal petrology and maceral composition, particle size, etc.28 The water resistance indices of all agglomerated samples could not be determined, as they were found to mostly disintegrate when submerged in water. The blended samples did, however, disintegrate faster compared to the coal sample, which maintained its structure for a brief period. This occurrence indicates that the algae exhibited hydrophilic properties that are promoted by its higher polarity than observed for the coal sample. Algae in principle absorbs and retains water29, as a result from the presence of carbohydrates such as polysaccharides, lipids such as phospholipids, and lignocellulose22, 30-32. One mechanism for the attraction of water molecules to biomass materials such as algae can be attributed the presence of a double layer phospholipids in the cell membrane, where the non-polar ends face each other while the polar hydrophilic ends point outwards33. The presence of lignin, one of the major components present in biomass, also contributes to the polarity and hydrophilicity of biomass materials34. The results are, however, in contrast with the work done by Gaqa and Watts (2018)9, who found that the agglomeration of fine coal with a microalgae binder added to the hydrophobicity of the agglomerant, improving its water resistance. 3.3.3 Calorific value Bomb calorimetry is a standard experimental method for establishing the energy density of solid and liquid fuels. The heating value, also known as the calorific value (CV), determines the total energy released as heat when the samples undergo complete combustion in an oxygen atmosphere. The calorific values, recorded in MJ/kg, of the agglomerated samples were determined and are presented in Table 3-4. The higher heating values (HHV), calculated from proximate analysis, are also included in this table for comparison. 51 JA Meyer Chapter 3 Table 3-4 Calorific value results obtained from agglomerated samples using a bomb calorimeter and calculated higher heating value (HHV) (MJ/kg) Coal fines CA5 CA10 CA20 Algae Calorific values a 20.6 20.4 20.5 20.3 15.6 HHV b 18.8 18.6 18.3 17.8 15.2 a Determined on the agglomerated samples; b calculated from proximate analyses on an air-dried basis Each sample’s measurement is an average of five experiments. The calorific value of the agglomerated coal fines is 20.6 MJ/kg, indicating that the coal is medium rank C Bituminous. Coal power stations require high grade coal in the range of 21-23 MJ/kg6, 35. There are no significant differences observed between the calorific values of the coal fines and coal-algae blends, although a lower value was measured for the algae sample. This may be a result of the low additive concentrations. The HHV calculated for the algae sample correlates well with the experimental values. This indicates that the higher heating value prediction model proposed by Yin (2011)15 for biomass materials is adequate for calculating heating values for algae. 3.3.4 Thermogravimetric analyses (TGA) The TGA and DTG (derived thermogram) curves of the five samples are shown in Figure 3-1. The pyrolytic behaviour of the various samples was evaluated from 40°C to 920°C in an inert environment. The first stage is associated with dehydration, which starts occurring early at low temperatures and continues up to 135°C. The weight loss is due to the moisture removed from the sample. The second stage is the main devolatilisation stage, during which the bulk weight loss is observed, and results in a maximum peak in the DTG plot. Coal’s main devolatilisation stage ranges between 400 and 750°C, whereas for biomass material it ranges at significantly lower temperatures. The last stage is associated with slow weight loss and carbonisation of the remaining material, producing char. During this stage, the original coal structure is destroyed, leaving mostly carbonaceous material behind36. 52 JA Meyer Chapter 3 100 0.55 90 0.45 80 Coal fines 0.35 70 CA5 CA10 60 CA20 0.25 Algae Coal fines Deriv. Weight 50 CA5 Deriv. Weight 0.15 CA10 Deriv. Weight 40 CA20 Deriv. Weight 0.05 30 20 -0.05 0 100 200 300 400 500 600 700 800 900 Temperature (°C) Figure 3-1 TGA and DTG plots of coal fines, algae, and blends at a heating rate of 5°C/min to 920°C From the DTG plot, it is evident that algae has a maximum rate of weight loss at 300°C, while coal loses its weight the fastest at around 440°C. In this range, i.e. 400 to 500 °C, low molecular weight molecules escape the coal particles as tar vapour and are assigned to the production of low molecular tars and oils37. At 330°C, no peaks are observed for the coal fines sample, where only small peaks are observed in the blended samples. The blended samples show peaks at temperatures higher than that of the algae sample in this range. This can be assigned to volatiles evolving from the algae, but are hindered by the surrounding coal particles resulting in higher evolving temperatures. These peaks can be assigned to weight loss from gas and tar production. No distinct DTG peak shifts are observed at temperatures below 600°C for all samples tested; however, at 650 to 700°C, the blended samples reveal maximum rate of weight loss at lower temperatures than for the coal sample. The maximum rate of weight loss of the coal sample in this temperature range is observed at 677°C compared to the CA20 sample at 652°C. This may be an indication of an effect on the reactivity of the coal sample caused by the addition of algae, as the temperatures at the peak maximum weight loss in this range decreased in the order: coal fines>CA5>CA10>CA20, with no peak observed for the algae sample. In this range, weight loss is associated with gas evolution, mostly H2 with some hydrocarbon gases38. From 750°C onwards, the TGA curve remains relatively flat, with a slow decreasing tendency for all samples, and indicates that carbonisation may occur. The pyrolytic behaviour of coal-algae composites at 450°C was also evaluated by Baloyi and Dugmore (2019)39. However, at this low temperature range, no distinctive conclusions can be made regarding the influence of algae on coal pyrolysis 53 Weigtht (%) Deriv. Weight (%/°C) JA Meyer Chapter 3 processes. Table 3-5 shows a comparison of the weight loss during pyrolysis of the different samples with other biomass materials. Table 3-5 A comparison of weight losses observed during pyrolysis of the different samples used in this study with other biomass sources20, 40 Temperature at Maximum rate of Remaining solid maximum weight loss weight loss (wt.%/°C) residue at 900 °C (wt. (Tmax, °C) %) Corncob 40 346 - 10.0 Rice husk 40 367 - 12.6 Eucalyptus 40 367 - 13.3 Sawdust 40 367 - 13.6 Palm shell 40 377 - 20.8 Coconut shell 40 355 - 22.1 Spirulina Sp. 20 324 - 26.0 Coal fines a 436 0.0976 76.3* CA5 a 437 0.0940 75.6* CA10 a 437 0.0970 74.5* CA20 a 435 0.0961 72.7* Algae (Scenedesmus) a 300 0.5208 29.8* a Experimental data from this study; * remaining solid residue at a temperature of 920°C From Table 3-5, it follows that the temperature at which the maximum rate of weight loss of Scenedesmus microalgae occurs is lower than that of other biomass materials, as well as Spirulina Sp. algae. This indicates that pyrolysis of Scenedesmus algae can occur at a lower temperature than for the other biomass materials. The maximum weight loss of algae is much greater than that of coal, which indicates that the amount of volatile matter from algae is greater and more tar/oil and gas may be produced than in the case of coal. The amount of biochar produced from the pyrolysis of Scenedesmus microalgae is higher than the other biomass sources, including Spirulina Sp. algae. 54 JA Meyer Chapter 3 3.4 Conclusions The conclusions for the blending and agglomeration of discarded coal fines with microalgae biomass are listed as follows: • The proximate analyses of the coal fines and microalgae samples indicate a high ash yield in the coal fines, with a significantly lower yield present in the microalgae sample. The microalgae ash yield is relatively lower than for other biomass materials. High volatile matter contents are observed in the microalgae sample with low fixed carbon contents. Low sulphur contents are observed in both the coal and microalgae sample; • Atomic H/C and O/C molar ratios can be derived from the ultimate analyses of the coal and microalgae samples. The results from the H/C molar ratios may give an indication of low concentrations of aromatic structures present in the microalgae sample, with higher amounts of aromatic structures and PAHs present in the coal sample. The relatively high O/C molar ratios suggest that more polar compounds are present in the microalgae sample, indicating that the sample is more hydrophilic in nature. A low O/C value is observed for the coal sample, making it more hydrophobic; • An increase in compressive strength is observed with the addition of microalgae to coal fines, while negatively impairing the water-resistance of the compacted fuel; • The addition of microalgae biomass to the coal fines did not show any significant influence on the measured calorific value; and • From thermogravimetric analyses, it was observed that the coal fines’ main devolatilisation stage ranges between 400 and 750°C, whereas microalgae were observed between 200 and 500°C. The peaks at maximum rate of weight loss of the coal fines and microalgae were observed at 436 and 300°C, respectively. Additionally, weight loss peak shifts were observed between 650 and 700°C – decreasing in order: coal fines>CA5>CA10>CA20. The residual char left after pyrolysis of the coal-algae blends did not change significantly and would be sufficient for further utilisation, e.g. investigating the thermal behaviour of the produced chars. It can be concluded that the addition of a biomass material such as microalgae to discarded coal fines influences the degree of polarity and degree of aromaticity. It is evident that the microalgae adds to the physical characteristics such as compressive strength; however, it impairs the water- resistance of the agglomerates. The topic of the next section involves pyrolysis experiments carried out at 520°C, 720°C and 920°C using a modified Fischer Assay setup in order to evaluate the various product yields, distribution and product characterisation. In addition, an evaluation of the CO2 gasification reactivities of the produced chars will be performed. 55 JA Meyer Chapter 3 Acknowledgments The information presented in this paper is based on the research financially supported by the National Research Foundation (NRF) and South African Research Chairs Initiatives (SARChI) of the Department of Science and Technology (Coal Research chair Grant No. 86880). Any opinions, conclusions or recommendations expressed are that of the author(s) and no liability is accepted on the NRF’s regard. 56 JA Meyer Chapter 3 CHAPTER REFERENCES 1. Hancox, P. J.; Götz, A. E., South Africa's coalfields—A 2014 perspective. International Journal of Coal Geology 2014, 132, 170-254. 2. 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Journal of Energy in Southern Africa 2019, 30 (3), 44-51. 40. Choobuathong, N., Effects of chemical composition of biomass on pyrolysis and combustion. Department of Chemical Technology, Chulalongkorn University, Bangkok 2007. 58 CHAPTER 4 PYROLYSIS PRODUCT YIELD AND COMPOSITION OF FINE COAL DISCARDS AGGLOMERATED BY A MICROALGAE BINDER JA Meyer, CA Strydom, JR Bunt, RC Uwaoma Centre of Excellence in Carbon-Based Fuels. School of Chemical and Minerals Engineering, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa. Abstract The processing of coal results in the production of dust and fines that render major disposal problems when discarded and stockpiled. The beneficiation of these fines has been the focus of numerous research studies aiming to ameliorate the negative environmental impacts and provide a useful product. The co-processing of coal with biomass material is a major focus area. The co- pyrolysis of biomass material with coal will provide a better way to utilise coal discards in a more sustainable manner. This study examines the pyrolysis products derived from Fischer Assay pyrolysis of fine coal discards agglomerated with the addition of microalgae biomass at 520°C, 720°C and 920°C. The process produces coal char, volatiles in the form of tar and gas, and water that is removed using a Dean-Stark distillation setup. The various char, tar and gas fractions were characterised using proximate and ultimate analyses, gas chromatography-mass spectrometry, and gas chromatography, respectively. The results indicated that the evolution of volatile matter was affected by pyrolysis temperature and algae concentration, and subsequently affected the amounts and compounds present in the produced tars and gases. The wt.% chars decreased, while the total evolved volatiles increased, with higher algae concentrations and higher pyrolysis temperatures. The study revealed that the addition of microalgae biomass influences the pyrolytic behaviour of coal under different pyrolysis conditions. Keywords: Fine coal discards; microalgae; co-pyrolysis; pyrolysis; Fischer Assay 4.1 Introduction South Africa is among the top producers and exporters of coal globally. The world’s energy needs are primarily dependent on the processing of coal1. However, coal processing and beneficiation create significant residues such as dust and fines that are harmful to the environment2-4. Stockpiling of these fines creates significant environmental issues such as acid mine drainage (AMD), ground water pollution, air pollution, and is susceptible to spontaneous combustion2. The utilisation of these discarded carbonaceous materials has the potential to be an important asset when processed properly. To account for large amounts of greenhouse gas emissions produced JA Meyer Chapter 4 during coal processing, the influence of co-firing coal with biomass materials has been studied extensively. The advantages of biomass over coal focuses mainly on its carbon neutral, and renewable or somewhat clean energy aspects. The utilisation of biomass in thermo-processing has shown promising results and is studied extensively for the production of biofuels5-7. Raw coal is converted into energy and chemicals by liquefaction, combustion, gasification, and fast or slow pyrolysis8-9. The second largest use of coal, after combustion, is pyrolysis that produces char or coke that is used within the metallurgical industries or as a fuel10. This process is mainly described as carbonisation and/or devolatilisation of coal9-11. The products derived from coal pyrolysis are coal char, tar/oils and gas. The main uses of coal char include its use as a chemical reducing agent in the production of steel through high temperature pyrolysis of bituminous and subbituminous coal, and for the production of syngas and synfuels (or synthetic fuels) from char gasification10-12. Coal tar is a by-product from the production of coal char and gas, and can be used as a raw material for industries relating to the production of synthetic fibres such as graphite, pharmaceuticals, construction of roads, and for the manufacturing of paints and dyes13-14. Compounds such as phenols, naphthalenes and anthracenes are also extracted and refined from coal tar and used for the production of various oils, cement additives, and an important component for the production of gasoline and diesel by catalytic hydrogenation reactions13, 15. Gases derived from coal pyrolysis comprise mostly H2, CH4, CO, and CO2, with some C2-C6 hydrocarbon gases present10. Gases such as CO and H2 are usually referred to as syngas and are used as a fuel, or as a feedstock for the production of liquid fuels or chemicals such as methanol, methanol derivatives and ammonia16. Some of the most important factors that influence the product distribution and pyrolytic behaviour of coal revolve around coal’s properties and pyrolysis conditions. These include coal rank and petrography, mineral matter, particle size, pyrolysis temperature, heating rate, pressure and atmosphere9-10, 17-21. Biomass processing and characterisation have been a major focus for the production of biofuels 6, 22. The use of biomass has been recognised as a renewable energy source that has the potential to replace fossil fuels and reduce greenhouse gas emissions5. Biomass is regarded as carbon neutral on the basis of its ability to absorb CO2 from the atmosphere while growing through photosynthesis, and in addition contains less sulphur than coal, reducing SO2 emissions6. The sole use of biomass exhibits some drawbacks, including its seasonal supply, varying cost, needed infrastructure, composition, etc.23 It should be noted that the composition of biomass is very different compared to coal – having high moisture and volatile matter contents, and low heating values23-24. Co-pyrolysis of biomass with coal may eliminate some of these drawbacks. Recently, the use of biomass with coal has attracted much attention. Studies have evaluated the effect of co-pyrolysis of biomass materials with coal, suggesting a possible greener initiative. The use of biomass material with coal discards presents major advantages that involve reduced 60 JA Meyer Chapter 4 environmental effects, no mining needed, reducing the need for stockpiling, and increase the utilisation of discarded coal23, 25-26. The addition of algae biomass to coal has recently attracted some interest, which includes its use as an agglomerating agent26, the production of bio-oil and bio-char from low temperature pyrolysis5, the pyrolytic behaviour of coal-algae composites at low temperature pyrolysis27, other pyrolysis studies28-32, and gasification and combustion studies25, 33-36. Various studies have claimed that co-firing and co-pyrolysis of coal with algae has shown synergistic effects27, 32, 37. The purpose of this study is to evaluate the pyrolysis product distribution and composition of coal- algae composites with varying algae concentrations during Fischer Assay pyrolysis at 520 °C, 720 °C and 920 °C. 4.2 Experimental section General: The coal sample used in this study is of a Highveld inertinite-rich coal source and is classified as fine coal discards. These fines were screened to -250 µm. The microalgae biomass type is that of Scenedesmus sp., originated from InnoVenton, a research institute based at Nelson Mandela University. The microalgae were used as an agglomerating agent and co-fired with fine coal discards. The proximate- and ultimate analyses results of the samples prior to Fischer Assay experiments are shown in Table 4-1, as also presented in Chapter 3. Table 4-1 Proximate- and ultimate analyses results of coal fines and microalgae Coal Microalgae Proximate analysis (a.d.b.) % Inherent moisture content 3.8 6.7 % Ash content 28.9 16.4 % Volatile matter content 23.9 67.4 % Fixed carbon (by difference) 43.4 9.5 Ultimate analyses (d.m.m.f.) % Carbon content 78.7 56.6 % Hydrogen content 4.7 7.7 % Nitrogen content 2.0 9.7 % Total sulphur 0.74 1.17 % Oxygen content (difference) 14.6 26.0 H/C molar ratio 0.71 1.62 O/C molar ratio 0.14 0.34 Fuel ratio 1.82 0.14 a.d.b = air-dried basis; d.m.m.f. = dry-mineral-matter free basis; Fuel ratio calculated from the fixed carbon content divided by the volatile matter content 61 JA Meyer Chapter 4 Sample preparation The fine coal discards were blended with the microalgae on a dry basis with 0%, 5%, 10%, and 20% (wt.) microalgae, as reported in the previous chapter. The coal and coal-algae blends were mixed with 12 wt.% water and agglomerated using an LRXplus instrument from LOYD Instruments (Ametek) with a Specac PT No. 3000 10 mm die-set. The rate of compression and load limit were set to 15 mm/min and 1.5 kN, respectively. The various samples were set to dry and cure overnight at 100°C. The weight of the agglomerates was 1 g (±0.05 g). Pyrolysis experiments Pyrolysis experiments were performed on the agglomerated samples, using an automated Fischer-Assay setup (Figure 4-1) from NWU38. This modified setup was developed in order to execute Fischer Assay experiments above the ISO 647 specified temperature39. The modifications made included replacing the aluminium retorts, which are limited to around 660°C, with stainless-steel retorts that can withstand and operate at temperatures up to 1 000°C, introducing a tar trap and gas sampling bags for the capture of non-condensable gases, a programmable temperature control system, and oven and sample bed temperature measurements38. Argon or nitrogen can be used to create an inert environment to prevent combustion of the samples; in this case, nitrogen was used. Thermocouples were fitted within the retorts to measure the bed temperature, and within the oven chamber to measure the temperature outside the retort. Because of a heat transfer lag occurring between the oven and the inside of the retorts, optimal settings were selected to get a ramp rate of 10 °C/min in the oven, and around 5 °C/min in the retorts. The various temperature profiles for 520°C, 720°C and 920°C are presented in Figure 4-2. The Fischer Assay experiments were carried out at 520°C, 720°C and 920°C with the oven’s ramp rate set at 10 °C/min, while the retorts’ ramp rate averaged at around 5 °C/min. The volatiles evolved during the heating of the samples flowed and bubbled through a tar trap and gas washing phase in order to separate the gas fractions from the tarry substances. Toluene was used as the solvent in the tar trap and washing phase, while maintaining a low temperature using an ice bath. The gases were captured in 10 L Tedlar gas sampling bags. The water fraction, e.g. moisture and pyrolytic water, was isolated from the tar and toluene mixture using a Dean-Stark distillation setup. This was followed by rotary evaporation of the tar/toluene mixture at 60°C at a pressure of 77 mbar using a Büchi Rotavapor R-100 with Büchi Vacuum pump V-100. The gas yields were determined by difference according to the ISO 647 method. The pyrolysis product distribution of the various fractions (char, tar, gas and water) was evaluated based on their weight. 62 JA Meyer Chapter 4 Thermocouple Oven Argon gas (in) Stainless steel reactor Sample Gas wash phase Gas sampling bag Tar trap Ice bath Toluene Figure 4-1 Experimental automated Fischer Assay setup 63 JA Meyer Chapter 4 520°C 600 500 400 Oven temp. (°C) 300 Retort temp. (°C) 200 100 0 0 20 40 60 80 100 120 Time (min) 720°C 800 700 600 500 Oven temp. (°C) 400 Retort temp. (°C) 300 200 100 0 0 20 40 60 80 100 120 140 160 Time (min) 920°C 1000 900 800 700 600 Oven temp. (°C) 500 Retort temp. (°C) 400 300 200 100 0 0 50 100 150 Time (min) Figure 4-2 Temperature profiles indicating the temperature lag between the oven and retort bed for set temperatures up to 520°C, 720°C and 920°C 64 Temperature (°C) Temperature (°C) Temperature (°C) JA Meyer Chapter 4 Pyrolysis product analyses The residual char produced from pyrolysis was weighed and characterised which includes proximate- and ultimate analyses. The standard methods for proximate analysis, indicated in Table 4-2, and the ISO 12902 – CHO method for ultimate analysis, were used. Table 4-2 ISO methods used for proximate analysis of the samples Moisture content (%) BV-TPM-010 based on ISO11722: 1999 Ash content (%) BV-TPM-011 based on ISO 1171: 2010 Volatile matter content (%) BV-TPM-012 based on ISO 562: 2010 Total sulphur (%) BV-TPM-013 based on ISO 19579: 2006 Fixed carbon content (%) Calculated by difference Tars obtained from the Fischer Assay experiments, after Dean-Stark distillation separation and rotary evaporation, were kept in poly-top vials and stored in a cool environment prior to analyses. Qualitative gas chromatography-mass spectroscopy (GC-MS) analyses were conducted on the various tar samples using an Agilent Technologies 7890A model gas chromatograph along with an Agilent Technologies 5975C model mass spectrometer. 500 µg of the tar samples were dissolved using 2 mL dichloromethane (solvent) in PTFE vials. A VF-5ht Ultimetal column from Agilent Technologies with a column length of 30 m, internal diameter of 250 µm, and film thickness of 0.1 µm was used. The method was set up having a split ratio of 200 with an initial oven temperature of 60°C that was held for 5 minutes. Helium was used as the carrier gas with a flow rate of 1.2 mL/min. Table 4-3 provides the oven temperature profile method. Table 4-3 GC-MS oven temperature profile method Ramp Rate (°C/min) Temp. (°C) Hold time (min) Run time (min) Initial - 45 8.0 8.0 Ramp 1 2 100 0.0 35.5 Ramp 2 7 162 5.0 49.4 Ramp 3 7 300 0.0 69.1 Ramp 4 20 350 0.0 71.6 The components present in the tars were categorised in various molecular family groups, namely aliphatics, aromatics, benzenes, phenols, naphthalenes and polyaromatic hydrocarbons (PAHs). Compounds were selected based on the likelihood of their occurrence, e.g. the qualitative analysis (>85%). 65 JA Meyer Chapter 4 The composition of the gaseous products was analysed using an SRI 8610C gas chromatograph (GC). The GC was fitted with a sample loop of 1 mL. Separation between the gas components was achieved using a Molecular Sieve 13X column (6 ft. length), using argon as carrier gas. At the start of each analysis, the sample loop was switched to the ‘inject’ position, which caused the sample to flow through the separation column, and ultimately detected by a thermal conductivity detector (TCD). During the first six minutes of the analysis, the sample gas components eluted and detected in the order of H2, N2, CH4 and CO. At 6.3 minutes, the sample loop was switched to the ‘load’ position, which reversed the flow of argon through the column and ensured that CO2 eluted to the TCD. The GC column oven temperature was started at 50°C when sample injection took place and stayed constant at this temperature for three minutes. After three minutes, the oven temperature was ramped up at 25 °C/min until 200°C was reached, after a total time of nine minutes. A further one minute at 200°C ensured that the CO2 peak was eluted completely from the column and detected by the TCD. The calibration graphs for all the gaseous components were such that a detection error below 1% was guaranteed. 4.3 Results and discussion 4.3.1 Pyrolysis product distribution The wt.% char, tar, gas and water fractions produced from Fischer Assay experiments at 520°C, 720°C and 920°C are presented in Figure 4-3. The various fractions were separated and quantified by their weight (%). It is evident that the char, tar, gas and water yields are dependent on the pyrolysis temperature, as well as algae concentration. 66 JA Meyer Chapter 4 a) b) 8.00 85.00 6.00 80.00 75.00 4.00 70.00 2.00 65.00 0.00 520 °C 720 °C 920 °C 520 °C 720 °C 920 °C c) d) 25.00 6.00 20.00 4.00 15.00 10.00 2.00 5.00 0.00 520 °C 720 °C 920 °C 520 °C 720 °C 920 °C Figure 4-3 a) Char yields, b) tar yields, c) gas yields, and d) water yields for the coal and coal-algae blends obtained from Fischer Assay experiments at 520°C, 720°C and 920°C The wt.% chars produced from Fischer Assay experiments indicate a decreasing trend as a function of temperature. As the pyrolysis temperature increases, more volatiles and pyrolytic water start to evolve (devolatilisation). The same trend is observed with the addition of algae. Algae, as all biomass materials, have a significantly higher volatile matter content than coal, and during pyrolysis, result in more degradation than the coal. This can be as a result of more low- molecular weight compounds present in the algae, while coal has more macromolecular weight compounds and therefore produces a higher char yield40. The chars produced at 920°C correlate well with the thermogravimetric results as presented in § 3.3.4. There is, however, a small difference observed that may be the result of the changing gaseous environment in the Fischer Assay retort, as there is not a constant nitrogen flow forcing the evolved gases out of the chamber as in the TGA’s case. This occurrence may lead to a change in the reaction environment, allowing some other reactions to take place and decrease the char yield. The tars produced did not show any definite trend with regard to temperature (Table 4-3b); however, the CA10 and CA20 sample produced the most tar at 720°C. Similar results were obtained by Uwaoma et al. (2019)40 during Fischer Assay experiments. The addition of algae to 67 Wt.%, a.d.b. Wt.%, a.d.b. Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 Wt.%, a.d.b. Wt.%, a.d.b. Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 Coal Coal CA5 CA5 CA10 CA10 CA20 CA20 JA Meyer Chapter 4 the coal did, however, have an influence on the amount of tars produced – an increase in tar yield is observed when the concentration of algae increased. This is a result of the higher volatile matter content and composition of the microalgae biomass5, 16, 27. 100 80 60 40 20 0 Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 520 °C 720 °C 920 °C % Tar % Gas Figure 4-4 A comparison of the tar and gas fractions obtained from the volatile evolved during Fischer Assay experiments From the total evolved volatiles, gas is the non-condensable and low molecular weight fraction that generally increases with a decrease in coal rank and at elevated temperatures41. A similar trend can be seen from the gas yield results presented in Figure 4-3c. The results indicate that gas evolution is a function of temperature and algae concentration, as also found by Baloyi and Dugmore (2019)27. A linear trend is observed from the amount of gases produced when increasing the algae concentration from 0 to 20%, along with varying the pyrolysis temperature, e.g. 520°C, 720°C and 920°C. The CA20 sample showed the most promising results compared to the coal sample, as the gas yields produced at 520°C, 720°C and 920°C increased from 9.5 to 11.9 wt.%, 14.6 to 15.4 wt.%, and 16.2 to 20.9 wt.%, respectively. It is evident that most of the evolved volatiles remain in the gas phase, while only a fraction condensed into the tar fraction (Figure 4- 4). It can be noted that the ratio of tar-to-gas is substantially lower at 920°C and is the result of some aromatic compounds present in the tar that is cracked at high temperatures, as well as other depolymerisation reactions occurring in the coal’s macromolecular network19, 42-44. The water yields indicated the total water content of the sample, which includes the surface and pore moisture of the coal that is driven off at 100°C, the inherent moisture content that is associated with the carbon matrix driven off at 130 to 200°C, and pyrolytic water associated with clay minerals that undergo dehydroxylating reactions from 300°C40, 45. At 520°C, a slight increase 68 Wt.% JA Meyer Chapter 4 in water content is observed with the addition of microalgae and may be attributed to the algae’s higher inherent moisture content. The average water content increases with temperature, indicating that more pyrolytic water was generated at elevated temperatures. It is expected that more pyrolytic water would be generated in the coal sample as a result of hydrolysis reactions of the coal matrix, and dehydration reactions of minerals above 700°C46. This occurrence can also be associated with the higher ash contents present in the coal sample (Table 4-2). A decrease in char yield was observed when comparing raw coal to coal-algae (10%), while an increase in oil/tar, gas and water yield was observed with the addition of algae. Similar results were obtained by Baloyi and Dugmore (2019)27. However, no trends could be observed as there were no temperature variations used in their study, and only one coal-algae concentration was used to compare the product distribution with that of raw coal. Chaiwong et al. (2013)5 have studied the bio-oil and bio-char production from algae during slow pyrolysis at 450 to 600°C, but found no significant changes in char, liquid and gas yield as a function of temperature. There was, however, a slight deviation at 550°C, where the gas yield slightly decreased while liquids produced slightly increased. 4.3.2 Char analyses Results obtained from the proximate- and ultimate analyses of the charred products obtained from Fischer Assay experiments of the coal and coal-algae samples are presented in Table 4-4. When compared to the results obtained from the coal and algae samples pre-pyrolysis, it can be observed that there are considerable differences with respect to the char analyses. The ash content and fixed carbon content of the various chars are significantly higher than for the original materials. This is as a result of the high amount of volatiles evolved during pyrolysis, and when increasing the pyrolysis temperature, an increase in volatile evolution is expected. 69 JA Meyer Chapter 4 Table 4-4 Proximate- and ultimate analyses results of the coal and coal-algae chars 520°C 720°C 920°C Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 Proximate analysis (a.d.b.) % Inherent moisture content 1.9 1.9 2.0 2.0 2.6 2.5 2.6 2.7 3.3 3.2 2.9 4.0 % Ash content 33.0 33.8 33.7 33.5 37.6 37.9 37.5 38.2 38.5 38.7 38.1 38.8 % Volatile matter content 14.3 12.8 12.2 14.1 4.5 4.3 4.7 4.5 1.7 1.6 1.6 1.6 % Fixed carbon (by 50.8 51.5 52.1 50.4 55.3 55.3 55.2 54.6 56.5 56.5 57.4 55.6 calculation) Ultimate analyses (d.m.m.f.) % Carbon content 82.8 84.1 84.3 81.8 93.5 93.8 94.1 93.9 96.5 97.2 97.3 98.0 % Hydrogen content 3.1 2.2 2.8 3.1 0.8 0.2 0.3 0.8 -0.3 -0.4 -0.3 -0.2 % Nitrogen content 2.5 2.3 2.6 2.9 2.0 2.0 2.2 2.4 1.2 1.3 1.4 1.4 % Total sulphur 1.2 1.2 1.0 1.0 1.3 1.2 1.1 1.0 1.4 1.3 1.2 1.2 % Oxygen content 10.5 10.3 9.3 11.2 2.3 2.9 2.3 1.9 1.2 0.6 0.5 -0.4 (calculation) H/C molar ratio 0.45 0.31 0.39 0.45 0.10 0.03 0.04 0.10 -0.04 -0.05 -0.04 -0.02 O/C molar ratio 0.10 0.09 0.08 0.10 0.02 0.02 0.02 0.02 0.01 0.00 0.00 0.00 Fuel ratio 3.55 4.02 4.27 3.57 12.29 12.86 11.74 12.13 33.24 35.31 35.88 34.75 a.d.b. = air-dried basis; d.m.m.f. = dry, mineral matter-free basis; Fuel ratio calculated from the fixed carbon content divided by the volatile matter content The slight increase in inherent moisture contents of the chars produced at higher temperatures, e.g. 720°C and 920°C, and higher algae contents may be associated with the hygroscopic behaviour of chars. As the evolution of volatile matter increases, the porosity of the resulting chars increases. An increase in moisture adsorption is presumably considered to be as a result of a higher porosity in coals47, and in chars that are produced at elevated temperatures48. As mentioned, an increase in pyrolysis temperature will give rise to devolatilisation to a greater extent. In addition, it is expected that the added biomass material will increase the volatile matter content of the coal sample and in turn will increase the amount of volatiles driven off during pyrolysis. However, from the proximate analyses results, this is not clear. The fixed carbon content of the produced chars also increases as the pyrolysis temperature is increased. The effect of temperature and algae concentration on the chars’ elemental composition can also be seen in Table 4-3 on a dry, ash-free basis. It can be seen that the carbon contents of the coal sample (pre-pyrolysis) increase from below 80% to above 82% in the chars produced at 520°C, and increasing to more than 96% in the chars produced at 920°C. The initial increases in carbon 70 JA Meyer Chapter 4 content ranging between 9.7% and 12.1% is observed when the temperature was increased from 520°C to 720°C, whereas the increases observed between 720°C and 920°C were smaller, ranging between 3.0% and 4.5%. This is in agreement with literature, where devolatilisation was studied at low- to high temperatures38, 49-50. Furthermore, the values for oxygen and hydrogen contents decrease as a function of temperature. The decrease in oxygen and hydrogen content is mainly associated with evolution of oxygen- and hydrogen containing gases such as CO and CO2, and C1-C6 hydrocarbon gases10, 51. Significantly lower O/C and H/C values were obtained after pyrolysis, which is characteristic of carbonisation. It can further be seen that the fuel ratio increases to an extensive degree as a result of the pyrolysis process and as a function of the increased pyrolysis temperatures. 4.3.3 Tar analyses The determination of the complex coal tar composition can be most effectively characterised by GC-MS analyses13. It has been identified that coal tar can consist of between 129 and 160 compounds in the volatile region13, 52-53. GC-MS analysis is, however, limited and is unable to detect high boiling point components (>300°C)54. As a result, it is difficult to quantify the numerous compounds in tar produced from pyrolysis53. The results were therefore qualitatively determined and simplified so that only the variations in compound family groups are observed, e.g. increase, decrease, or stay relatively constant, as a function of temperature and/or algae concentration (Table 4-5). The total amount of components in the tars ranged from 129 to over 200 compounds and these are dependent on the pyrolysis temperatures. The amount of components produced as detected via GC-MS analyses increased as the pyrolysis temperature was increased. 71 JA Meyer Chapter 4 Table 4-5 Qualitative GC-MS results obtained from the pyrolysis tar samples Aliphatics Aromatics Benzenes Phenols Naphthalenes PAHs 520°C Coal ++ + + ++++ + ++ CA5 ++ + + ++ + ++ CA10 ++ + <+ +++ + ++ CA20 +++ + <+ +++ <+ + 720°C Coal ++ + ++ +++ + ++ CA5 ++ + + ++ + ++ CA10 +++ + + ++ + + CA20 ++++ + <+ ++ + + 920°C Coal ++ + + +++ + +++ CA5 +++ + <+ ++ + +++ CA10 ++++ + <+ ++ + ++ CA20 ++++ + + +++ + ++ Where + is a low absolute area value of a specific compound group; ranging to ++++ indicating a significant increase in the absolute area value of a specific compound group. Area value is an indication of amount. Compounds that show aromaticity were not categorised under benzenes, phenols, naphthalenes, and PAHs, and their respective derivatives were rather characterised as aromatics (or other aromatics). These include indenes, indanes, biphenyls, furans, etc. Benzene derivatives such as phenols were separately categorised, as were PAHs such as naphthalenes. Para-cresol was found in all samples and grouped under phenols. The results indicate that the amount of aliphatic compounds evolved increased with increasing pyrolysis temperature in the coal-algae samples; however, no changes were observed in the coal sample as a function of temperature. The amount of aliphatics evolved increased with increasing algae concentration in all pyrolysis temperature ranges. This is a result of the low aromatic nature of biomass materials and their tendency to contain more aliphatic compounds than coal. This occurrence can also be supported by the higher H/C content of algae. Tar compounds containing benzenes and naphthalenes, and their derivatives, did not show any change relating to pyrolysis temperatures and algae addition. It seems that the tars derived from the coal sample formed more phenolic compounds than the coal-algae samples at all temperatures and a similar trend can be seen for PAHs. The content of PAHs in the tars shows an increasing trend with increasing pyrolysis temperatures. Pretorius et al. (2017)49 found similar results on rank-dependent coals. 72 JA Meyer Chapter 4 These changes may be due to some catalytic effects occurring as a result of the minerals present in coal38, and complex tar forming reactions during pyrolysis53. 4.3.4 Gas analyses Gas chromatography (GC) analyses were conducted on the captured gas fractions of the various Fischer Assay experiments. The prominent gaseous species derived from coal pyrolysis consist of CO, CO2, CH4 and H2, as reported in this investigation. The results reported were normalised on a nitrogen and C2-C6 hydrocarbon-free basis, as the pyrolysis experiments were conducted in nitrogen, and the gas chromatograph was not calibrated for C2-C6 hydrocarbon gases, which consist of less than 4 wt.% of the total evolved gases (determined by difference). The composition of the gases captured from the pyrolysis experiments of the various samples at 520°C, 720°C and 920°C is presented in Figure 4-5 and is based on the amount of gas produced from the pyrolysis product distribution. 10 8 6 4 2 0 Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 Coal CA5 CA10 CA20 520 °C 720 °C 920 °C H2 CH4 CO CO2 Figure 4-5 Gaseous composition of H2, CH4, CO and CO2 evolved from Fischer Assay experiments The H2 gas yield of the samples at 520°C did not show any change with the addition of algae, however, this increased at 720°C and 920°C as the concentration of algae increased. It should be noted that the amount of H2 produced increased significantly from 520°C to 720°C. At pyrolysis temperatures of 720°C and 920°C, it is evident that more H2 is produced with the addition of algae to coal. This can be due to aliphatic carbon-hydrogen linkages breaking up when the temperature approaches 600°C55, or the cracking of tar compounds at elevated temperatures56. It is also possible that some minerals may assist or catalyse H2 transfer in the coal structure at high 73 Wt.% gas JA Meyer Chapter 4 temperatures and decrease H2 production57-58; however, this was not the case in this study. The coal-algae samples produced the most H2 at 720°C and 920°C, peaking with the CA20 sample at 920°C. No noteworthy CH4 trends were observed during the pyrolysis experiments executed at 520°C and 720°C, only a slight increase in CH4 evolution was observed at 920°C with the addition of algae. CH4 and CO2 start to evolve at temperatures as low as 200°C, and as temperature rises internal condensation of the macromolecular structure of low-rank coals takes place, initiating CO2 and H2O evolution19. It can be seen from the results obtained that CO2 evolution increases slightly with increased pyrolysis temperatures. At 920°C, the CO2 peak indicates that algae have a significant effect on CO2 production, which may be attributed to the presence of carboxylic groups present in vitrinite- and inertinite-rich coals, as well as mineral decomposition reactions of calcite and dolomite59. At high temperatures, high amounts of CO2, H2 and CO are produced5. However, the latter is not true for this study, but CO evolution increases with the addition and increasing concentration of algae at 520°C and 920°C. The evolution of these gases suggests that deoxygenation, demethylation and dehydrogenation reactions take place during the pyrolysis process, as reported by Li et al. (2012)60. 4.4 Conclusions The conclusions for the pyrolysis product yields and evaluation of discard coal fines co-fired with a microalgae biomass are listed below: • From the pyrolysis product distribution, it can be observed that the weight percentages of the chars produced decreased as a function of increasing pyrolysis temperature and the addition of microalgae biomass. The results correlate well with literature, as the amounts of chars produced from biomass materials are lower than that of coal, as a result of a higher volatile matter content. From the char analyses, it is observed that the fuel ratio increases significantly as a result of the decreasing volatile matter content. H/C and O/C molar ratios also decrease as a function of pyrolysis temperature. • Tars produced from the CA10 and CA20 coal-algae samples from Fischer Assay experiments showed the highest yields at 720°C. Qualitative GC-MS analyses were performed on the tar fractions and it was observed that samples with higher algae concentrations produced more aliphatic compounds than coal. At higher pyrolysis temperatures the coal-algae sample produced more aliphatic compounds, while no changes in amounts of aliphatic compounds were observed for the coal sample. More phenolic compounds were produced from the coal than coal-algae samples at all temperatures. A similar trend can be seen with the formation of PAHs; however, at 920°C, more PAHs were produced than at lower pyrolysis temperatures. 74 JA Meyer Chapter 4 • An increase in gas yields was observed during Fischer Assay experiments as a function of temperature and algae addition. The gas composition comprised mostly H2, CH4, CO, and CO2. The gaseous species produced at 520°C comprised CH4>CO>H2>CO2; however, at 720°C and 920°C, H2 evolution increased considerably. H2 and CO evolution increased as a function of increasing pyrolysis temperatures and increased algae concentrations. The production of these gases indicates deoxygenation, demethylation and dehydrogenation reactions occurring during pyrolysis. It can be concluded that the addition of a biomass material such as microalgae to fine coal discards influences the pyrolysis product distribution at 520°C, 720°C and 920°C. In addition, it also affects the product composition as the pyrolysis temperature and algae concentration are varied. Acknowledgments The information presented in this paper is based on the research financially supported by the National Research Foundation (NRF) and South African Research Chairs Initiatives (SARChI) of the Department of Science and Technology (Coal Research chair Grant No. 86880). Any opinions, conclusions or recommendations expressed are that of the author(s) and no liability is accepted on the NRF’s regard. 75 JA Meyer Chapter 4 CHAPTER REFERENCES 1. 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Journal of analytical and applied pyrolysis 2016, 121, 41-49. 58. Yu, Q.-Z.; Brage, C.; Nordgreen, T.; Sjöström, K., Effects of Chinese dolomites on tar cracking in gasification of birch. Fuel 2009, 88 (10), 1922-1926. 59. Gräbner, M.; Lester, E., Proximate and ultimate analysis correction for kaolinite-rich Chinese coals using mineral liberation analysis. Fuel 2016, 186, 190-198. 60. Li, R.; Zhong, Z.; Jin, B.; Zheng, A., Selection of temperature for bio-oil production from pyrolysis of algae from lake blooms. Energy & fuels 2012, 26 (5), 2996-3002. 78 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS This chapter provides the concluding remarks made throughout this study regarding the physical, chemical and thermal characteristics of coal, algae and coal-algae composites that were evaluated through various processes such as sample characterisation, agglomeration, pyrolysis product distribution and characterisation, and the relationships between the derived products and the raw coal. Additionally, future work and recommendations will be discussed in § 5.2. The conclusions discussed in § 5.1 are based on the objectives as stipulated in § 1.3. 5.1 Conclusions Characterisation of the coal and algae samples based on their chemical compositions using proximate- and ultimate analyses The coal and algae samples were characterised by proximate and ultimate analyses in order to determine their properties. A large difference was observed in fixed carbon and volatile matter contents of the coal and algae samples, ranging from 43.4 to 9.5% and 23.9 to 67.4%, respectively. The low fixed carbon content and high volatile matter content of algae were expected and are significant of biomass materials. Furthermore, the fuel ratio (FR) was calculated from the fixed carbon content divided by the volatile matter content. FRs of 1.82 and 0.14 were obtained for the coal and algae sample, respectively. The results show that the FRs for the algae are significantly lower than the coal sample, indicating that the FR will decrease when the coal is blended with algae. An ash content of 16.4% was obtained in the algae sample, lower than that of the coal sample containing an ash content of 28.9%. Coal discards in general can have ash contents of up to 45%; however, a lower content was obtained for the coal used within this study. From ultimate analyses, the C, H, N, S and O (by difference) contents were determined. It was found that the coal and algae samples contained 78.7% and 56.6% carbon, 4.7% and 7.7% hydrogen, 2.0% and 9.7% nitrogen, and 14.6% and 26.0% oxygen, respectively. Low sulphur contents were observed in both samples, consisting of 0.74% in the coal sample and 1.17% in the algae sample. The H/C and O/C molar ratios that were derived from the elemental analyses indicated that high concentrations of aromatic structures, including PAHs, may be expected in coal samples, however, lower in the algae sample. The O/C values suggest that the algae sample may be more polar in nature than the coal sample, and as a result more hydrophilic in nature. These results may give insight into the composition of the tars and gases derived from pyrolysis experiments, and the interaction of the coal-algae agglomerates with water during water resistance evaluations. JA Meyer Conclusions and recommendations Preparation of the various coal-algae blends by mixing microalgae with fine coal discards The coal and algae samples were pre-prepared prior to the preparation of the coal-algae blends. The fine coal discards were air-dried, and the cone-and-quartering method was used to obtain a good representative sample. The coal was screened to -250 µm. The proximate analysis results of the coal were used to determine the amount of algae needed, as the weight ratio is determined on a dry, ash-free basis. A dry-weight determination of the algae slurry was also done in order to calculate the amount of dry algae needed for the coal-algae blends. Three coal-algae blends were prepared containing 5, 10, and 20% algae. However, the live algae used and was left in the slurry phase as part of the curing phase. The coal-algae slurries were stirred overnight using an overhead stirrer and centrifuged afterwards. The samples were dried at 100°C and milled and screened to -250 µm. The process of blending the fine coal discards with algae was executed successfully. Characterisation of coal-algae blends by thermogravimetric analyses (TGA) The pyrolytic behaviour, observed as weight loss were evaluated from 40°C to 920°C in nitrogen by thermogravimetric analyses, and the rate of weight loss was indicated by DTG curves. The results indicated that weight loss occurred at three temperature regions that correspond with devolatilisation and/or carbonisation. Only a small weight loss was observed below 135°C and is associated with moisture removed during dehydration of the samples. It was observed that the main devolatilisation stage of algae was significantly lower than that of coal – where the algae’s maximum weight loss and rate of weight loss (derived from the DTG curve) occurred at 300°C, compared to the 400 to 500°C of coal. The coal-algae samples exhibited similar peaks at 300°C, while no peaks were observed in the temperature range for coal. This indicated that volatiles (tars and gases) evolved relatively early in the algae and coal-algae samples during pyrolysis, increasing as the concentration of added algae was increased. The coal and coal-algae samples showed weight losses that are consistent with coal’s main devolatilisation stage that ranges between 400°C and 750°C. In this range, no further weight loss for algae is observed from 500°C, indicating that the algae char has been produced at a significantly lower temperature compared to coal. At 650°C to 700°C, the temperature of the maximum rate of weight loss decreased in order: coal>CA5>CA10>CA20, while there was no peak observed in the algae sample. The weight loss in this temperature range is mainly associated with gas evolution, such as H2 and some hydrocarbons. From the TGA results, it is evident that no significant changes in the weight of coal and coal-algae composites occurred after 750°C. The high volatile matter content of the algae sample was confirmed via thermogravimetric analysis, as found by proximate analysis. The results from algae weight loss during pyrolysis 80 JA Meyer Conclusions and recommendations compared to other biomass materials indicate that the temperature at maximum weight loss for algae is lower than other biomass materials. In addition, the remaining residue of algae (used in this study) after pyrolysis is also greater than the other biomass materials, as reported. The amount of char produced from the coal-algae blends after pyrolysis experiments did not change significantly (<4% in the CA20 sample) with the addition of algae. Production of solids in the form of pellets via agglomeration of the coal-algae blends Coal and coal-algae agglomerates, with 10 mm diameters, were successfully produced by mixing 1 gram of the various samples with 12% distilled water and compressing them under a load limit of 1.9 kN for 15 seconds. The weight % of added water was selected as it showed the most promising results with regard to compressive strength. The agglomerates were dried at 100°C in an oven for 24 hours. Agglomerates were produced of each sample to add to a total of 50 g. These agglomerates were used throughout the study. Characterisation of the agglomerated samples using compressive strength, water resistibility and calorific value A linear correlation of increasing compressive strength was observed with the concentration of algae, ranging from 0% in the coal sample to 5, 10 and 20% in the coal-algae samples. The linear trend that was observed indicated that there were no synergistic effects on the basis of algae concentration. The compressive strength increased from 4.2 MPa, in the coal sample, to 4.5 MPa, 5.1 MPa and 6.0 MPa in the CA5, CA10 and CA20 samples, respectively. The CA20 sample, containing the highest amount of algae, showed a 43% increase in compressive strength when compared to the coal sample. The coal agglomerates presented good compressive strengths without the addition of algae; however, with the addition of algae, significantly higher compressive strength values were obtained. The good agglomeration potential of the coal may be attributed to coal rank, petrography and maceral composition, specific surface area, mineral composition, degree of coal oxidation, particle size, etc. However, more studies have to be conducted to evaluate the effect of these factors on coal discards. All agglomerates achieved compressive strength values several times larger than the minimum values suitable for various processes. The water resistance indices of all samples could not be determined, as the agglomerates were found to mostly disintegrate when submerged in water. The blended samples did, however, disintegrate faster compared to the coal sample, which maintained its structure for a brief period. It may be ascertained that the addition of algae does not add to the water resistance and hydrophobicity of the agglomerates produced in this study. 81 JA Meyer Conclusions and recommendations The average calorific values obtained from the coal, coal-algae blends, and algae (included in the characterisation) agglomerates were determined using a bomb calorimeter. The fine coal agglomerates displayed calorific values of 20.6 MJ/kg, exhibiting values close to that required by coal firing stations using high grade coal (ranging between 21-23 MJ/kg). The agglomerated algae sample hd a calorific value of 15.6 MJ/kg, somewhat lower than coal. However, no significant changes in calorific value were observed in the coal-algae agglomerates when compared to the coal agglomerate. This may be as a result of the low concentrations of algae added to the blends. The results from the proximate analysis of the coal and algae were used to determine the higher heating value (HHV) of the samples and compared to the calorific values of the of coal, algae and coal-algae blends. The results show a definite correlation between the calorific values obtained and higher heating values calculated. Evaluation of the effect of algae on the pyrolysis product distribution of coal at various temperatures via Fischer Assay experiments Char, tar, gas and water fractions were successfully produced from Fischer Assay experiments of the different samples at 520°C, 720°C and 920°C. The chars produced from Fischer Assay experiments indicated a decreasing trend as a function of temperature, as expected. As the pyrolysis temperature was increased, more volatiles and pyrolytic water evolved resulting from devolatilisation and mineral decomposition reactions. The same trend was observed with the addition of algae. Algae, like all biomass materials, have a significantly higher volatile matter content than coal and, during pyrolysis, produce lower amounts of char. The weight percentages of chars produced from Fischer Assay experiments at 920°C correlate well with the results obtained through thermogravimetric analyses. However, the slight deviation observed in the samples may be an indication of cross-linking reactions taking place, i.e. when the produced gases react within the sample in the Fischer Assay retorts, while an inert environment was maintained during thermogravimetric analyses. No distinctive trends were observed in tar yields with respect to pyrolysis temperature. The addition of algae influenced the evolution volatiles, and it can be observed that the amounts of tars produced from the coal-algae blends were higher than tar produced from coal alone. The CA10 and CA20 samples produced the most amount of tars at 720°C. It is also expected that, at higher temperatures, more gases would be produced from further decomposition of volatile matter, while reducing the amount of tars produced. When comparing the amount of tars and gases produced from the total evolved volatiles, it was found that the amounts of gases produced were substantially higher, while only a small amount of tars was produced in all samples. It is evident that most of the evolved volatiles remained in the gaseous phase, while only a small fraction condensed into the tar fraction. 82 JA Meyer Conclusions and recommendations The results indicated that pyrolysis temperature, as well as algae addition, had a noticeable influence on the amount of gas produced during the various pyrolysis experiments. It was found that the amount of gas produced increased as the pyrolysis temperature increased. The CA20 sample showed the most promising results with regard to gas production when compared to the coal sample, as the gases produced at 520°C, 720°C and 920°C increased from 9.5 to 11.9 wt.%, 14.6 to 15.4 wt.%, and 16.2 to 20.9 wt.%, respectively. Gas evolution was enhanced as a function of increasing algae concentration. Gases produced from all coal-algae blends were higher than the amount of gas produced from the coal sample at all pyrolysis temperatures. It is evident that more gaseous species were produced at elevated pyrolysis temperatures and higher algae concentrations. Analyses of the different char, tar and gas fractions using proximate- and ultimate analyses, qualitative gas chromatography-mass spectrometry (GC-MS), and gas chromatography (GC) techniques Varying the pyrolysis temperature and adding algae to coal fines did not only affect the product distribution, but also their composition. It was necessary to analyse and compare the composition of the various products obtained from Fischer Assay experiments. Char analyses: A small increase in moisture content was observed in the chars produced at higher temperatures, which may be the result of the samples’ polarity and/or porosity that increased their moisture absorbance potential. The increase in pyrolysis temperature showed that devolatilisation occurred to a greater extent. However, no differences were observed in the coal- algae char samples, as most of the volatiles from algae evolved at temperatures below 500°C (supported by the thermogravimetric analysis of algae). The fuel ratios, derived from the fixed carbon content and volatile matter content of the different samples, were found to decrease excessively after pyrolysis, as most of the volatiles are driven off during devolatilisation. The highest fuel ratios were obtained from the chars produced at 920°C. However, no distinctive influence of the algae was observed in the blended samples. From the ultimate analysis results, it can be seen that the carbon contents increased from below 80% (coal sample pre-pyrolysis) to more than 82%, 93%, and 96% in the chars produced at 520°C, 720°C and 920°C, respectively. A higher incremental increase in carbon contents was observed in the chars produced between 520°C and 720°C, compared to 720°C and 920°C. Hydrogen and oxygen contents decreased significantly when the pyrolysis temperature was increased, resulting in lower H/C and O/C molar ratios. No significant changes were observed between the coal and coal-algae chars regarding the elemental compositions. 83 JA Meyer Conclusions and recommendations Tar analyses: GC-MS analyses indicated that the amount of components present in the produced tars increased as the pyrolysis temperature increased. The results indicated the evolution of aliphatic compounds from the coal-algae samples, increased with increasing pyrolysis temperature. No changes in aliphatic evolution were observed in the coal samples with increasing pyrolysis temperature. In addition, the tar samples produced at all temperatures showed increasing aliphatic content as the concentration of algae increased. This occurrence may result from the low aromatic nature of algae, or biomass materials in general, and their tendency to produce more aliphatic compounds during pyrolysis than coal. The phenomenon can also be described by the higher aromatic nature of coal, producing more aromatic compound during pyrolysis. This information is supported by the respective H/C molar ratios of coal and algae, as it is an indication of the degree of aromaticity. However, no distinct changes were observed in the tars containing benzenes, naphthalenes, and their derivatives as a function of pyrolysis temperature and algae concentration. Changes were observed in the amounts of phenolic compounds, as the coal produced more phenols than the coal-algae blends. Similarly, PAHs were more prominent in the coal and lower percentage algae containing samples. The production of PAHs also increased as a function of increasing pyrolysis temperature. Gas analyses: The gases produced were effectively separated and captured from the total evolved volatiles using a tar trap and gas washing phase. The gases consisted mostly of H2, CH4, CO and CO2, with a small fraction consisting of C2-C6 hydrocarbons. The composition of the gases produced at 520°C consisted of CH4>CO>H2>CO2. The amount of H2 produced increased significantly at 720°C and 920°C. It is evident that the evolution of H2 and CO increased as a function of increasing pyrolysis temperature and algae concentration. The production of these gases during pyrolysis indicates that deoxygenation, demethylation and dehydrogenation reactions take place. Final remarks It can be concluded that the addition of algae to fine coal discards adds to the various properties of the agglomerated samples and affects the pyrolysis product distribution and composition of the agglomerates to a great extent at various temperatures. The objectives mentioned in Chapter 1 were successfully executed throughout this study. 84 JA Meyer Conclusions and recommendations 5.2 Recommendations for future work The recommendations for future work are based on the findings of this study and limitations experienced during this study: • More studies have to be done to evaluate the agglomerating potential of coal discards and the influenced of coal properties such as coal rank, petrology and maceral composition, specific surface area, mineral composition, degree of coal oxidation, particle size, etc. • The effect of polar compounds present in the macromolecular network of algae and coal- algae composites on the hydrophobicity may be evaluated to better understand how water resistance is influenced by polarity. • Quantitative GC-MS analysis may be performed to effectively determine the amounts of different compounds present in the tars produced during pyrolysis. • GC analysis should incorporate the amounts of SOx and NOx gases present in the gas sample, as it will give insight into the production of harmful gas emissions. • An inert environment in the Fischer Assay retorts with a constant nitrogen flow may be maintained so that no other reactions can take place. The resulting gas analyses may be inaccurate, as the gas fraction will mostly contain nitrogen and will tend to increase the error value of the gas composition. Therefore, a new method should be incorporated that will not inhibit the gas analyses. • An extensive evaluation should be done to determine the economic viability and feasibility of algae used for co-pyrolysis processes. An economic investigation was not planned as part of this investigation. 85