Combustible carryover predictions for a wood-waste boiler

Abstract

An analytical model of a spreader-stoker type wood-waste fired boiler is used to predict the flight times and mass reductions of combustible particles entrained by the flow of furnace gases. The predicted mass ratio, initial char mass to final char mass at the furnace exit, is coupled with literature values for wood waste particle size distribution in order to predict furnace exit particle mass loading and size distribution. Only the combustible fraction of carryover is considered in these calculations, and it is assumed that the entrained combustible particle size distribution is the same as the entrainable fraction of the fuel. A simple analytical expression for the efficiency of a multicyclone-type particulate collector is applied to the predicted furnace exit loading and size distribution in order to predict stack particle loading and size distribution of the combustible fraction. This procedure is repeated for various boiler operating and fuel conditions. The influence of steam generation rate, excess air, and initial fuel moisture on stack mass loading and particle size distribution is shown to be relatively minor. It is further shown that the influence of the initial fuel size distribution has a very pronounced effect on the predicted results both for the furnace exit and for the stack.