The effect of volatiles on the combustion of char in a fluidised bed

The effect of propane on the combustion of char particles in an air-fluidised bed of sand was studied: the objective was to understand how volatiles from coal-simulated by the propane-influence the combustion of char derived from the coal. Each combustion experiment gave the burn-out time of a batch of char particles injected into a hot bed of sand which was fluidised either (a) by air or (b) by a mixture of air and propane. Comparison of experiments (a) and (b) showed that propane reduces the char combustion rate because the propane consumes oxygen within the fluidised bed. The reduction of the char combustion rate, due to the propane, increases with temperature because the propane burns more readily in the bed as the bed temperature increases. It follows that, for highly reactive char, the char combustion rate in the presence of propane may actually decrease as temperature increases: this surprising result suggests that it may not necessarily be beneficial to raise bed temperature to improve carbon combustion efficiency. The propane combustion causes variations of oxygen concentration within the bed. Consequently, the char combustion rate depends upon the circulation pattern of particles. It appears that the char particles are not well mixed: under typical conditions, the char particles are at the top surface of the bed for 30% of the time.