Mode of occurrence and behaviour of mineral matter in fine coal rejects during thermal treatment

Abstract

The use of coal as a primary energy source remains of interest globally due to its contribution to economic development and the affordable and reliable energy. However, the depletion of coal reserves and increased energy demand highlight the need to valorise coal wastes such as finecoal rejects (FCR). In addition, conventional disposal of FCR poses significant threats to human health and the environment due to emissions of H2S, SO2, CO2, heavy metals/metallic inorganic elements such as cadmium and arsenic, as well as volatile organic compounds (VOCs) arising from pyrite-associated spontaneous combustion and the interaction of organic sulphur and calcium within the FCRs. Direct utilisation of FCR in conventional boilers and gasifiers with highash yield, high moisture content, and fine particle size may create downstream problems such as slagging, fouling, and erosion during utilisation. This is especially true if the transformational behaviour of the mineral matter (ash forming part of coal) is not fully understood. Discerning the FCR mineral matter has the potential to allow for the utilisation of FCR in conventional boilers and gasifiers as a co-feed to traditional feedstocks. This can extend coal energy security, minimise coal waste handling costs and alleviate/mitigate environmental challenges associated with FCR disposal. These benefits may be achieved by better understanding the mode of occurrence of mineral matter and its behaviour during thermal treatment. This research investigates the potential for utilisation of South African FCR in gasification and pyrolysis processes and aims to address environmental concerns and reduce disposal costs associated with this coal waste. Previous studies have focused primarily on unoxidised FCRs, particularly steam-station coals (labelled STEM in this study), and neglecting the partially oxidisedFCRs, such as those from thermal-export plants (labelled TEP in this study) and their beneficiated fractions. This investigation comprehensively characterised and compared two types of South African FCRs, one from TEP and the other from STEM. The FCRs exhibited properties similar to those of feed coals for gasification and combustion, with notable differences in the mineral matter and vitrinite proportions and the alkali index, which is higher in the TEP sample. Thermogravimetric-gasification experiments demonstrated higher reaction rates and carbon conversion for the TEP sample, indicating its superior reactivity compared to the STEM sample. Raman and X-ray diffraction analyses revealed the presence of aromatic and aliphatic hydrocarbons and mineral matter in the FCRs, contributing positively to coal gasification reactivity. An exploration of the mode of occurrence of mineral matter in FCR was also conducted by employing density-separation, chemical fractionation, and demineralisation techniques. This approach divided FCR mineral matter into different modes of occurrence, including extraneous minerals and inherent mineral matter (ion-exchangeable cations, salts of carboxylic acids and submicron minerals). The influence of extraneous minerals was studied by blending FCR and its density-separated, chemically fractionated, and demineralised residues with a composite of reactive oxides (MgO and Fe2O3) and a hydrated oxide mineral (Ca(OH)2). The results indicated an enhanced activation energy during pyrolysis, resulting in increased carbon conversion. Cleat minerals and extraneous minerals catalysed pyrolysis reactions, facilitating the cracking of heavy tars to lighter fractions. However, the presence of oxygen in the reactive oxides hindered deoxygenation reactions, affecting pyrolysis efficiency. The study also investigated the effects of carboxylate salts on FCR pyrolysis, particularly ferrous acetate, magnesium acetate and calcium acetate as Fe2+/Mg/Ca-acetate composite. The results revealed the catalytic potential dependency on FCR carbon conversion on inherent mineral matter and its association with the dominant maceral groups. Adding carboxylate salts led to enhanced decomposition rates, carbon conversion, and the formation of sulphur-bearing phases and nanocarbonates, which influenced tar yield and gas composition. Overall, this research provides valuable insights into the reactivity and mode of occurrence of mineral matter in FCR, offering a foundation for their utilisation in gasification and pyrolysis applications. Understanding these properties makes it possible to mitigate disposal costs, health hazards, air pollution, and fines volume accumulation of FCR, thereby promoting their sustainable utilisation in the energy sector.