Rotary Drying of Wood Waste Fuels with Boiler Exhaust Gases: Simulation, Field Studies, Economics

Two computer simulation models of a boiler/dryer system fueled by wood waste were developed, each general enough to be used for different boiler/dryer configurations and to predict boiler performance expected after rotary dryer installation. Model accuracy was tested by comparing predictions with field data from three different boiler/dryer systems currently in operation in the forest products industry. In each case, the models predicted fuel consumption within, on the average, 2 percent of the observed value. The simulation models were then used to examine the effects of various operating parameters on boiler performance. On the basis of these studies, the rotary dryer is expected to reduce fuel consumption and decrease the boiler’s sensitivity to fluctuations in fuel moisture content (MC), excess air after combustion, and boiler exhaust temperature. An economic analysis of retrofitting a rotary dryer to a boiler operating at moderate conditions indicated cash-flow payback periods based on hogged fuel savings to be 3.6 years for a used dryer and 5.2 years for a new dryer. The analysis assumes that the fuel was dried from 50 to 30 percent MC, wet basis. Payback periods based on Btu-equivalent savings of natural gas and No. 6 heating oil were estimated to be, respectively, 1.2 and 1.3 years for a used dryer and 2.1 and 2.2 years for a new dryer.

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