9 - Hydropyrolysis and hydrogasification

Abstract

This chapter develops analyses for coal hydropyrolysis and hydrogasification at moderate temperatures and elevated H2 pressures, including primary hydropyrolysis, tar hydroconversion, and char hydrogasification. In hydropyrolysis, the central challenge is to explain why tar yields diminish for progressively faster heating rates, opposite to the tendency in primary devolatilization. The reason is that only the slowest heating rates provide enough time for hydrogenation to appreciably enhance the tar yields before the coal components are converted by spontaneous devolatilization channels. At the fastest heating rates, there is too little time for hydrogenation to enhance tars yields before the spontaneous channels take over, so tar yields diminish monotonically for progressively greater pressures. Under elevated H2 pressures, primary tars hydrocrack into valuable aromatic oils, which completely disrupts the progression from tar into PAH into soot. Oils yields from any coal at any hydrogasification conditions can be accurately predicted by supplementing the normal tar decomposition mechanism with steps for tar monomer hydrogenation into oils, and for hydrocracking large tar macromolecules into monomers. As much as half the ultimate oils yield is rapidly produced from primary tar monomers, whereas the rest forms more slowly from the hydrocracking reaction of larger tar. Char hydrogasification kinetics are accurately depicted by a single half-order surface reaction within the CBK/G framework. Surprisingly, the assigned hydrogasification reactivity values lie within 50% of a mean value for coals of all but the highest rank. The fundamentally different rank independence suggests that hydrogasification may not share any common adsorbed intermediates or primary reaction sites with the surface chemistry for gasification by steam and CO2.