Freeboard ignition of premixed hydrocarbon gas in a fluidized bed

Abstract

Conditions under which overbed as opposed to in-bed combustion were defined for a bench scale (76 mm i.d.) 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 miniexplosions was used to define a critical bed temperature, T cr , above which combustion occurred within the bed. It was found that T cr 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.