OCR Text |
Show 4 their volatility or propensity to associate with different sizes of major ash constituents (1, 12). In this study, a carefully controlled bench-scale coal conversion system was used to investigate the impacts of varied gasification conditions, such as reducing and oxidizing atmospheres, gas-phase composition, pressure, and temperature, on the transformation and partitioning of toxic metals. All of the experiments were performed on an Illinois No.6 coal. Of particular interest is the fate of As, Cr, Cd, Hg, Ni, Se, and Ph in a simulated gasification system. EXPERIMENTAL An Illinois No.6 coal from the Penn State Coal Sample Bank was acquired and analyzed using conventional and advanced inorganic analyses. Conventional analyses included ASTM proximate/ultimate; major element oxide composition of the coal ash; and atomic absorption, inductively coupled argon plasma analysis. Advanced analyses included computer-controlled scanning electron microscopy (CCSEM) (13), wavelength-dispersive x-ray spectrometry, and a chemical leaching technique called chemical fractionation. CCSEM provides the particle-size and composition distribution for the mineral grains in coal which is important for deriving the mineral associations of trace metals. Mineral grains that are identified using CCSEM can be analyzed with the more sensitive wavelengthdispersive x-ray spectrometer (WDS) to determine trace metal abundance (14). Chemical fractionation analysis (15) was performed for the seven trace metals and consists of three solvent extractions performed on the coal with concentrations of the metals removed being determined for each extraction. A water extraction is first performed which generally removes soluble salts. The second extraction uses ammonium acetate which removes salts of carboxylic acids. The final extraction uses hydrochloric acid which removes elements primarily associated with carbonates and coordination complexes. |