The effect of microalgae as a binder on the characteristics of agglomerated coal fines

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.