INTERNAL CONSISTENCY OF COAL-GASIFICATION REACTIVITIES DETERMINED IN BENCH-SCALE REACTORS - EFFECT OF PYROLYSIS CONDITIONS ON CHAR REACTIVITIES UNDER HIGH-PRESSURE CO2

Relationships between char preparation conditions and CO2-gasification conversions have been examined using a wire-mesh, a fluidised-bed and a 'hot-rod' (fixed-bed) reactor. Conversions from the direct gasifica tion of untreated coal have been compared with those from the gasifica tion of chars prepared: (i) in-situ under helium in each reactor, befo re switching over to CO2; and (ii) from gasification in all three reac tors of a char sample, pre-prepared in the fixed bed reactor. Conversi ons from direct gasification were slightly higher than experiments whe re samples were first pyrolysed in situ and gasified under CO2 in the same reactor. In some cases, conversions were within experimental erro r. Under these conditions and within several percentage points, the py rolysis and gasification steps may be considered as additive. Lower ov erall conversions observed in the hot-rod reactor have been attributed to the combined effect of: (i) formation of a low reactivity char-owi ng to tar-solids contact leading to secondary char deposition; and (ii ) poor contact between char and the reactive gas. in the wire-mesh and the fluidised-bed reactors, direct CO2-gasification conversions were considerably larger than those determined using the common sample of c har, pre-prepared in the hot-rod (fixed-bed) reactor. This sharp drop in overall conversion shows that care must be taken in interpreting re activity data from 'two-reactor, two-step' procedures where char prepa ration conditions are not identical to those of the gasification step. A broadly linear relationship has been observed between conversions f rom a pilot plant reactor and direct CO2-gasification in the bench-sca le high-pressure wire-mesh reactor. Despite differences between operat ing parameters in the two reactors, the results suggest that data from the wire-mesh reactor may be successfully used to compare relative co al reactivities under pilot-plant,gasification conditions. (C) 1998 El sevier Science Ltd. All rights reserved.