Mindralogy And Petrology Of Chars Produced By South African Caking Coals And Density-Separated Fractions During Pyrolysis And Their Effects On Caking Propensity

Thermochemical processes use low-rank bituminous coal to produce steam and synthetic gas. Caking coal particles can soften, swell, and coalesce in the South African boilers and fixed-bed gasifiers during pyrolysis and have low syngas quality and low production efficiency. Understanding the mineralogy, chemistry, and petrography of coal would allow a better understanding of the root causes of the caking propensity of coal particles during pyrolysis. The main objective of this study was to investigate the effects of mineral transformation and maceral decomposition on the caking propensity of coal particles during low-temperature pyrolysis. Float-sink tests were conducted on the caking coal to produce different density-separated fractions for pyrolysis tests under experimental conditions (550 degrees C, 0.87, 30 bar, and 15 min) and characterization studies using mineralogical and chemical analyses. QEMSCAN results showed that the <1.3 g/cm(3) float fractions contained high proportion of vitrinite, low contents of kaolinite, and reactive semifusinite compared to the sink fractions (mineral-rich) analyzed. QEMSCAN and chemical results for the chars also revealed that vitrinite-rich layers in the float fractions softened, swelled, melted, and coalesced (100% caking of particles) while kaolinite-rich layers and fusinite in these fractions remained unaltered. Kaolinite in the kaolinite-rich layers in the >1.9 g/cm(3) sink fraction (contains high proportions of kaolinite, fusinite, and quartz) partially dehydroxylated, fragmented, and resulted in the noncaking of particles. Mineralogical, chemical, and petrography data obtained from this study could be implemented in the blending strategies of the feed coal and density-separated fractions during feedstock preparation for utilization in thermochemical processes. The study provides a framework from which the more generic impact of variations in factors including vitrinite components (organically associated inorganic elements and functional groups) and reactive semifusinite that could possibly be responsible for the coal caking mechanisms in the South African boilers and fixed-bed gasifiers might be investigated in future studies.