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Symposium on the Geology of Rocky Mountain Coal, October 2-4,
INORGANIC CONSTITUENTS IN LOW-RANK COAL - DIRECT COAL ANALYSIS OR ASH ANALYSIS?
Abstract. The inorganic constituents in low-rank coals are distributed among ion-exchange sites of organic acid groups and clay minerals, coordination sites, oxides, and discrete mineral phases such as calcite, quartz, pyrite, and clay minerals. The unique ways in which these inorganics are bound within the coal can affect their behavior during ashing. The standard ASTM ashing procedure provides a means of concentrating inorganic constituents of the coals at 750°C. The ash cannot represent the inorganics present in the coal because partial volatilization of some species may occur and others combine to form secondary phases. Determination of inorganics directly in coals and selective extraction of the inorganics based on their bonding within the coal matrix together provide a much more accurate assessment of coal composition. The inorganics can be reported in three major categories: organically-bound cations; carbonate/coordinated/oxide; and insoluble mineral phases. A predicted ash content is calculated by summing (on an oxide basis) the inorganics determined in dry coal. Comparison of ash contents obtained by direct coal analysis and by ashing analysis for a Beulah, ND lignite indicates that the total ash- forming constituents determined from the direct coal analysis is 10.39%, whereas the ASTM ash content is 9.5%. The differences between these determinations are explained by 43% volatilization of sodium and a 25% loss of sulfur upon ashing. Further examination reveals that 90-95% of the total sodium in this coal is bound as cations to organic acid groups within the coal, and is thus more prone to volatilize than if it were associated with a mineral phase. The volatility of sodium in 30 low-rank coals shows a linear relationship between the amount of sodium volatilized and the total sodium in the coal with a correlation coefficient of 0.97. In these coals tested up to 67% of the total sodium was volatilized.
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