Transformation of Trace Metals in Coal Gasification

Trace metals pose a potential problem to emerging coal gasification electric power-generating systems. Some of the trace metals in coal are considered air toxics when released into the atmosphere and can also cause the degradation of fuel cell efficiency due to contamination. The fate of trace metals during coal conversion in integrated gasification combined cycle and integrated gasification fuel cell systems is closely tied to how the trace metals are associated in the coal and gasification conditions. Bench-scale gasification experiments were performed using Illinois No.6 coal to determine the partitioning of Hg, Se, As, Ni, Cd, Ph, and Cr into gas, liquid, and solid phases as a function of gasification conditions and coal composition. Entrained ash was generated in a pressurized drop-tube furnace and collected using a multi cyclone and impinger sampling train. Coal analysis revealed arsenic, Hg, Ni, Ph, and Se to be primarily associated with pyrite. Cr was associated primarily with clay minerals, and Cd appeared to have either a sulfide or an organic association. Cr , Ph, and Ni are enriched in the ash particulate fraction (collected in multi cyclones), while Cd, As, Se, and Hg are depleted in the particulate and are more enriched in the very fine particulate or vapor-phase fraction (collected in the final filter and impingers). Oxygen contents were varied to represent both combustion and gasification systems. Most of the work was conducted at lower oxygen/carbon (O/C) ratios. Lower O/C ratios resulted in more Hg being driven to the vapor phase. Under constant O/C ratios, Hg, Se, and Cd showed increasing volatility with increasing temperature in the reaction zone.