Pyrolysis and combustion of corncobs in a fluidized bed: Measurement and analysis of behavior

A study of corncob combustion in a fluid bed, for investigation of kinetic behavior, provided reproducible data on the variation of mass, volume, and density with time that was interpreted quantitatively using boundary layer diffusion (BLD) theory, and qualitatively using heat transfer and pyrolysis kinetics. The core of a corncob is an inner cylinder of pith surrounded by two annular cylinders: an inner cob, of about 80% of the total weight, and an outer hull. A total of 120 corncob segments, roughly of length equal to their diameter, and each about 13 g were used. They were immersed in a fluid sand bed at 650°C for periods up to 160 s, using a cage to withdraw the particles after immersion. The cobs were initially cylindrical but they became spherical during reaction so that combustion theory for spheres could be used in the analysis. In the first 10 s, about 10% of the corncob mass burned off, as loss of a fraction of the outer (lower density) hull, and with an increase in mean density by about 20%. In the next 80 s, as heat penetrated into the cob, there was, simultaneously: pyrolysis of the cob, governed jointly by the rate of heat penetration and the pyrolysis kinetics; and reduction of volume by combustion of the hull, quantitatively predicted by BLD theory. The char residue after pyrolysis was about 20% of the original mass, and was spherical, of low density. Its rate of combustion was also quantitatively described by BLD combustion theory. These results showed that the behavior of corncobs in combustion was reproducible, predictable, and amenable to mathematical analysis that could be applied to model descriptions of fluid bed combustion of these materials.