Comparison with bench-scale experiments has been used to evaluate strategies by which fuel conversion could be improved in a pilot-scale air blown fluidised bed gasifier. Bench-scale data clearly showed that lower overall conversions observed in the pilot scale gasifier do not correspond to levels of reactivity expected from the properties of the original feedstock. Increasing particle sizes were found to affect conversions adversely. Conversions also depended on the reactivity of the gaseous medium (steam > CO2) and increased with reactive gas pressure. Relative combustion reactivities of residual chars diminished rapidly with time-at-temperature : at 1000 degrees C, char reactivities declined sharply between 0 and 10 s holding timer The adverse effect of increasing pressure and of secondary char deposition on char reactivity were experimentally significant but small. In low temperature gasifiers, the combined effect of longer times required for consuming larger char particles and the rapid deactivation of chars by (similar to 10 s) exposure to gasification temperatures tends to lead to appreciable levels of residual char removal. Our findings indicate that coal/char reactivities decrease appreciably during the first 10 s at peak temperature-underlining some the difficulties of establishing reactor simulations, which include fundamental kinetic models of coal gasification. The kinetics of char deactivation as a function of time-at-temperature and particle size would appear to play an important role in determining eventual gasification conversions as well as the gasification kinetics itself. (C) 2000 Elsevier Science Ltd. All rights reserved.