Unraveling the Excess Air/Coal Fineness Enigma

Abstract

In the use of powered coal as a boiler fuel, the factors involved in heat loss from unburnt carbon in the ash are but partially understood. More finely pulverized coal particles will result in lower carbon-in-ash losses. On the other hand, the finer grind of coal requires a considerably higher energy input to the pulverizing mill. Thus, an optimum balance must be achieved between fuel savings and grinding costs to arrive at the maximum economic savings. If these were the only considerations, the solution would be simple. The problem, however, is compounded by a concurrent reduction in mill capacity as the fineness is increased. In addition, there are marked interactions between coal particle size and excess air as they effect carbon burning rate with its concomitant effect upon carbon-in-ash heat loss. Finally, variability in the excess air will alter the energy supplied to the induced-draft and forced-draft air fans.

To generate a greater understanding of the impact of the aforesaid factors, upon potential fuel savings, a computer simulation was developed for the powdered coal boiler. The current paper presents results of exploratory studies using the simulator.