Predictions of Intrinsic Ash Emissions From Wood-Waste Boilers

Abstract

A primary cause of particulate emissions from wood-waste fired boilers is the suspension burning of solid fuel particles. These small particles are entrained by the upward flow of furnace gas and partially burn during their passage through the furnace. Because of the high moisture content of the wood fuel the furnace temperatures are low and, consequently, particle burning rate is low. As a result, the carryover of particulate matter consists of both combustible char particles and noncombustible intrinsic ash and tramp solids which are released as the wood material is consumed. The physical processes responsible for particle combustion and entrainment have been previously studied with a computer model specifically developed for wood-waste fuel. In the present work this model is used, along with three fuel size distributions, to predict the mass loading of intrinsic ash at the stack of a spreader stoker boiler as a function of operating conditions. The results indicate that the amount of intrinsic ash in the stack flow increases with increased combustion rate of the particles, which confirms physical intuition. The particle combustion rate increases with either increased fuel firing rate or decreased fuel moisture and excess air. These emission results for intrinsic ash, along with previous results for combustible carryover, are compared with recent data obtained from several operating wood-waste fired boilers.