FREEBOARD IGNITION OF PREMIXED HYDROCARBON-GAS IN A FLUIDIZED-BED

Conditions under which overbed as opposed to in-bed combustion were defined for a bench scale fluidized bed, burning air premixed with propane. Without mixing effects, freeboard ignition of the fuel results in pressure pulses which were measured by a pressure transducer. The recording of these mini-explosions was used to define a critical bed temperature, Tcr, above which combustion occurred within the bed. It was found that Tcr increased with decreasing bed height, increasing excess fluidizing velocity and increasing particle size. Addition of oxidation catalyst pre-empted freeboard combustion entirely. The results indicate that gradual in-bed conversion increases with bed temperature to the point at which unconverted fuel, released to the freeboard, drops below the flammability limit thereby pre-empting overbed ignition.