Mineralogy of the Residues from an Underground Coal Gasification Test
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MINERALOGY OF THE
RESIDUES FROM AN UNDERGROUND
COAL GASIFICATION TEST
GREGORY J. McCARTHYa, ROBERT J. STEVENSONb and ROBERT L. OLIVERC a. Departments of Chemistry and Geology, North Dakota State University, Fargo ND 58105 b. Energy and Mineral Research Center, University of North Dakota, Grand Forks, ND 58202 c. Western Research Institute, P.O. Box 3395, Laramie, Wyoming 82071 Received 24 October, 1988: refereed ABSTRACT Detailed mineralogical characterization by X-ray diffraction, scanning electron microscopy and electron microprobe analysis has been performed on a suite of residues and on three samples of unaltered overburden excavated from a 1983 underground coal gasification (UCG) test conducted at a mine site near Centralia, Washington. The residues were classified into nine zones based on their appearance and location in the burn cavity. Because they were isochemical with the overburden, six of these zones could be identified as altered overburden and proved to be particularly useful for correlations of mineralogy with the temperature and redox atmosphere prevailing in these materials during the UCG process. A previously unrecognized, moderate temperature, reaction between reduced iron phases and clay minerals to form an Al-rich hercynite-spinel solid solution (Al-spinel), (Fe,Mg)AI204, was identified. This reaction is a precursor to the well-known formation of Fe-cordierite, (Mg,Fe) 2 AI 4 Si 5 OI8 from SiO2 and the Al-spinel at higher temperatures. The mineralogies of altered overburden were analogous to high-temperature, low pressure, metamorphic rocks of the sanidinite facies, and to thermally altered rocks from natural coal combustion. Modeling of the mineral phase assemblages was successfully performed using a phase diagram for the system (FeO+MgO)-AI 2 0 3 -SiO 2 constructed from literature diagrams. Melting relations were also modeled with this diagram. Deviations from the assemblages predicted by this diagram were attributed to oxidizing conditions.
INTRODUCTION Studies by the Gas Research Institute have indicated that underground coal gasification (UCG) is one of the most efficient and economic means of producing substitute natural gas from coal. UCG leaves residues consisting typically of fused or sintered mixtures of coal ash and altered overburden that has fallen into the burn cavity. Thus, the process results directly in the disposal of a coal conversion ash that needs to be studied for any environmental impacts. The excavation of residues from the Tono I UCG test near Centralia, Washington [1,2], has provided a unique opportunity to correlate field descriptions of the actual cavity, the nature of the cavity projected from drilling, and materials characterization. Residues and unaltered overburden were excavated by the Western Research Institute. Initial characterization of these residues has been performed by Ktihnel and Scarlett [31 and Oliver et al. [4]. The purposes of the present study were to expand on this initial characterization and to attempt to apply the framework and database of
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