EXPERIMENTAL INVESTIGATION OF THE 2-PHASE THEORY IN A FLUIDIZED-BED COMBUSTOR

Though the two-phase theory of fluidization is well-accepted, no direc t experimental measurements of the different gas concentrations predic ted to occur in bubble and particulate phases could be found in the li terature. For the first time, theoretical predictions of these differe nt gas concentrations have been validated experimentally, using a comb ined oxygen/bubble probe. Based on the two-phase theory, a mathematica l model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminat ed when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (Delta T) was the only adjustable parameter in the model . A value of 20 degrees C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch mass es. Model predictions of the oxygen concentrations were not sensitive to Delta T during the first half of burnoff, when mass transfer contro lled the combustion rate, so the mass transfer processes were predicte d correctly by the model effectively with no adjustable parameter. The Delta T value of 20 degrees C was significantly lower than experiment al measurements of maximum burning char particle temperatures, reporte d to be 70 degrees C for the small-diameter bed particles used in this work The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kine tics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phas e, with experimental char particle temperature measurements biased tow ards the higher bubble phase values. It was inferred: (i) that the max imum values of Delta T measured experimentally are too high for calcul ation of the char particle combustion rate during the kinetic-controll ed latter stage of burnoff; and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate ph ase deduced from these particle temperature measurements may have been underestimated.