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Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Mineralogy and Petrology of Chars Produced by South African Caking Coals and Density-Separated Fractions during Pyrolysis and Their Effects on Caking Propensity Mosele M. Tsemane,† Ratale H. Matjie,*,† John R. Bunt,† Hein W. J. P. Neomagus,† Christien A. Strydom,‡ Frans B. Waanders,† Chris Van Alphen,§ and Romanus Uwaoma‡
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Centre of Excellence in Carbon-based Fuels, School of Chemical and Minerals Engineering, North-West University, Potchefstroom 2520, South Africa ‡ Centre of Excellence in Carbon-based Fuels, School of Physical and Chemical Sciences, North-West University, Potchefstroom 2520, South Africa § Sustainability Division, Research, Testing and Development, Eskom, Brackenfell 7560, South Africa 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 1.9 g/cm3 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. atmospheres.47 Pyrolysis controls the char quality and the pyrolysis mechanisms depending on a number of coal properties that have been studied extensively.5−8 The coal properties of important gasification and combustion technologies include coal rank, coal petrographic, and mineralogical composition; caking propensity, and operating parameters (particle size, temperature, heating rate, pressure, and operating atmosphere).5−9 In addition, during pyrolysis, the plasticity of the caking coal mass is mostly seen at temperatures ranging between 400 and 500 °C.60,61 The caking propensity of caking coal with high proportions of vitrinite- and liptinite-containing functional groups could initiate caking, which is an inevitable technical problem that occurs in fixed-bed coal gasification.10−12 Caking coal particles can soften, swell, and coalesce (forming large cakes) in the fixed-bed gasifier during the gasification process.13−15 This subsequently reduces gas flow and causes
1. INTRODUCTION Pyrolysis of caking coal and noncaking coal is the useful intermediate stage for conversion of organic matter, which takes place in the pyrolysis zone of South African fixed-bed gasifiers during coal thermochemical processes.1−3 This intermediate stage highly depends on the organic fraction of the coal (macerals) that is more reactive at low temperatures, i.e., less than 500 °C, while other inorganic matters are only reactive at temperatures higher than 700 °C. It has been reported that highly aliphatic liptinite and vitrinite macerals in the caking coal contribute significantly to the formation of metaplast (liquidmolten vitrinite), and in the plastic temperature range (350− 550 °C) to the development of fluidity and swelling, hence caking. The included kaolinite, submicron minerals associated within macerals (for example, pyrite (FeS2), siderite (FeCO3), calcite (CaCO3), and dolomite (CaMg(CO3)2)) and organically associated inorganic elements in the coal react with each other to form artifact minerals at temperatures less than 500 °C.4,5 The polluting gases, i.e., H2S, COS, and SO2, are released from the sulfur-bearing compounds (organic sulfur and sulfur-bearing minerals) during the pyrolysis of coal under inert © XXXX American Chemical Society
Received: April 23, 2019 Revised: June 27, 2019 Published: July 8, 2019 A
DOI: 10.1021/acs.energyfuels.9b01275 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Experimental setup of pyrolysis experiments.49,58,59
past to determine the proportions of minerals and macerals in the caking coal and density-separated fractions,57 will be used in the present investigation. Chars derived from the caking feed coal and its density-separated fractions were not previously characterized by this advanced analytical technique.57 The selected samples (feed coal,
