Mineralogy and Petrology of Chars Produced by South African Caking Coals and Density-Separated Fractions during Pyrolysis and Their Effects on Caking Propensity

Abstract

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 °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.

Pyrolysis of Tetralin Liquefaction Derived Residues from Lighter Density Fractions of Waste Coals Taken from Waste Coal Disposal Sites in South Africa

“…The ash yields were higher for the liquefaction residues due to a decrease in organic matter, resulting in a relative decrease in the available volatile matter and fixed carbon. The mineralogical results of the 1.3 g/cm 3 float fraction and its char samples when using automated X-ray-based QEMSCAN indicate that kaolinite and fusinite, which are contained in these samples, did not transform or decompose at 550 °C to form meta-kaolinite during pyrolysis . Vitrinite and possibly carboxylic acid salts contained in these float fraction particles softened and formed agglomerates during the pyrolysis experiments .…”

“…Minor proportions (<0.5%) of carboxylic acid salts may contribute to catalyzing the pyrolysis reactions in South African coals. , The minor proportions of carboxylic acid salts, inherent minerals, and significant proportions of extraneous minerals contained in these coals do not contribute to the pyrolysis reactions at 450–550 °C during the South African coals pyrolysis under an inert atmosphere. However, significant fractions of carboxylic acid salts contained in the overseas lignites may catalyze pyrolysis and gasification reactions …”

“…Also, a high proportion of amorphous material (organic carbon) is contained in these samples. Other coal researchers 15,35,33 found the presence of these similar coal minerals in South African coals. The ash yields were higher for the liquefaction residues due to a decrease in organic matter, resulting in a relative decrease in the available volatile matter and fixed carbon.…”

“…The mineralogical results of the 1.3 g/cm 3 float fraction and its char samples when using automated Xray-based QEMSCAN indicate that kaolinite and fusinite, which are contained in these samples, did not transform or decompose at 550 °C to form meta-kaolinite during pyrolysis. 33 Vitrinite and possibly carboxylic acid salts contained in these float fraction particles softened and formed agglomerates during the pyrolysis experiments. 33 The kaolinite particles contained in the heavy density (>1.9 g/cm 3 ) fraction undergo a partial transformation to produce meta-kaolinite and hydration water vapor.…”

“…yields for the liquefaction residues than those of the lighter density fractions. 28,31 The XRD analysis 33 was followed to qualify and quantify the concentrations of crystalline and amorphous phases contained in the lighter density fractions and their corresponding liquefaction residues, and the mineralogical results are presented in Table 2. From the XRD results, it is clear that the most predominate minerals contained in these samples are quartz and kaolinite with a smaller quantity of dolomite, calcite, and microcline.…”