Tests of a mechanism for H(sub 2)S release during coal pyrolysis

摘要

We have used a temperature programmed, gas evolution technique to compare H(sub 2)S from coal and from pyrite in the presence of minerals or polymers. Pyrite decomposition in coal with H(sub 2)S release can be observed directly only if carbonate minerals, particularly iron-containing carbonates, are absent. Two distinct chemical mechanisms are required to model conversion of pyrite in coal to H(sub 2)S and pyrrhotite. Initially a reaction at pyrite grain surfaces (shrinking core model) occurs that is controlled by the rate of iron movement toward crystallite centers and by hydrogen-donor availability. Tar evolution (as indicated by methane-plus-ethane) also requires H-donors. Organic free radicals compete so efficiently for this scarce commodity that the rate of pyrite decomposition slows. At a 10 K/min heating rate, the rate of H(sub 2)S release by the H-donor mechanism reaches a maximum at 700 K and then decreases. Unimolecular decomposition of coal pyrite to FeS and S(sub 2) then occurs sharply at 830 K. Coal pyrolysis products effectively capture S(sub 2), and the rate of H(sub 2)S release matches that of sulfur release from pure pyrite in a vacuum (0.07 mg- S/cm(sup 2)/min at 773 K). The high temperature H(sub 2)S evolution peak from coal is often distorted by inorganic sulfide hydrolysis and reversible H(sub 2)S capture, both of which depend on a rapidly changing (H(sub 2)S):(H(sub 2)O) ratio. Water release from clays affects H(sub 2)S evolution. By monitoring CO(sub 2), H(sub 2)O, and CH(sub 4)-plus-C(sub 2)H(sub 6) as well as H(sub 2)S, we can provide a consistent picture for a particularly difficult case, the Illinois (number sign)6 coal from the set of Argonne coals. Modified Illinois (number sign)6 coal (acid-leached or pyrite-free), spiked samples, and polymers were pyrolyzed with inert or reactive gas sweeps to provide supporting data. 10 refs., 4 figs., 1 tab.