Characterizing rate inhibition in steam/hydrogen gasification via analysis of adsorbed hydrogen

Analysis of hydrogen adsorbed onto Saran and coal char is conducted via post-gasification temperature programmed desorption to 1770 K to characterize hydrogen inhibition in steam gasification. Exposure of annealed, outgassed chars to H2O/H-2/Ar mixtures at 1120 K and pressures to 3.1 MPa results in rapid adsorption of hydrogen on the char over the initial 0.5% of char conversion. Steam gasification rate simultaneously declines as a result of adsorbing hydrogen blocking a portion of reactive surface sites. Between 0.5 and 40% char conversion, adsorbed hydrogen concentration is constant at similar to 2 x 10(-3) mmol H-2 m(-2) at all conditions; steam gasification rate is also constant with conversion but depends strongly on hydrogen partial pressure through the reverse oxygen exchange [C(O) + H-2 --> C-r + H2O] pathway. Thus, dissociative adsorption is the observed mode of inhibition when quantities of hydrogen are limited, and reverse oxygen exchange dictates rate dependence on hydrogen at elevated pressures where the char surface is essentially saturated in hydrogen. (C) 1998 Elsevier Science Ltd. All rights reserved.