Halide removal from coal‐derived syngas: review and thermodynamic considerations

Abstract

Halide species are one of the most abundant and damaging impurities in syngas derived from coal. Aside from environmental concerns, halides can cause irreversible damage to downstream processes such as particulate filters, catalysts, gas separation membranes, fuel cells and turbines. The current trend towards high‐temperature, dry gas cleaning is making solid sorbents increasingly favourable, and sorbent development must take into consideration the chemical, physical, efficiency and economic aspects of sorbent performance. This article reviews the origins of halides in coals, cleaning requirements and efforts to develop solid sorbents for halide removal for gasification processes. Most research effort to date has concerned (1) the performance and optimization of basic sorbents such as calcium and sodium carbonate and (2) determining the tolerances of catalysts, filters, fuel cells and other plant components to halide impurities. In addition, thermodynamic calculations have been used to compare the performance of a large number of potential sorbents examined using criteria such as conversion, speciation, volatility and regenerability. This exercise confirms alkali and alkaline earth carbonates exhibit the greatest halide conversion at the temperatures demanded by various downstream processes, but these species are not regenerable at high temperatures in the same manner as desulfurization sorbents. Although not previously investigated, scope exists for the investigation of barium carbonate for halide sorption. Halide removal is best performed preceding sulfur removal due to the irreversible inhibition of regenerable desulfurization sorbents by halide species. Copyright © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

Alkali, boron and phosphorous removal from coal‐derived syngas: review and thermodynamic considerations

“…Previous publications in this series identified suitable materials for the sorption of sulfur and halide species, the latter of which has strong links to the present work through the abundance of alkali–halide salts in coal and biomass. It was shown previously that alkali chlorides, in the presence of syngas and gasifier slag or ash, form alkali silicates with the release of HCl.…”

“…SiC is commonly used in high‐temperature filtration and is particularly prone to corrosion by alkali halides at a corrosion at a rate of up to 100 mm per year in molten NaCl at 1000 °C . In the presence of moisture, SiC is actively corroded in a process that also releases HCl, which is then available to further corrode other plant components, as discussed previously …”

“…These strategies must also consider the need for efficiency, which dictates that alkali removal should occur at the maximum possible temperature. The previous publications in this series suggested that sulfur and halide removal should occur at ~ 600 and ~ 700 °C, respectively . Because the sorption of salt‐based alkali releases HCl, alkali removal is best performed preceding, or simultaneously to, halide removal.…”

“…A previous publication illustrated the dramatic effect of gasifier slag on the speciation of alkali halides. Reaction between slag and the chloride salts can result in the formation of alkali silicate phases, with the release of HCl.…”

“…The amount of alkali in the gas phase under oxidising conditions is independent of the sulfur content, whereas under gasification conditions, more alkali is present in the gas phase (relative to oxidising conditions). This suggests that H 2 S, the most abundant sulfur species in syngas, prevents the oxidation of alkalis.…”