The simulated and measured performance of rotary dryers used to dry wood waste fuels using boiler exhaust gases

Graduation date: 1983This study evaluates the feasibility of retrofitting a rotary\ud dryer to a hog fuel boiler, using the boiler exhaust gases as the\ud drying medium.\ud Two simulation models were developed. As substantiated by experimental\ud measurement of existing boiler/dryer systems, each model\ud accurately predicts system performance given site-specific parameters\ud such as boiler steam demand, fuel moisture content, boiler exhaust\ud temperature and combustion excess air. Both models predict system\ud performance in terms of boiler efficiency, fuel consumption and dryer\ud exit fuel moisture content.\ud Three rotary dryers/hog fuel boilers currently in operation in\ud the forest products industry were analyzed. The data obtained were\ud used to validate the accuracy of the simulation models and to establish\ud the performance of boiler/dryer systems under field conditions.\ud A wide range of operating conditions were encountered. Of particular\ud interest, the boiler exhaust temperatures observed ranged from 340 -\ud 500°F, indicating that significant drying could be realized at moderate\ud stack temperatures, as substantiated by experimental moisture\ud content data.\ud The simulation models were used to evaluate a general boiler/ -\ud dryer system's sensitivity to variation in operating conditions. The\ud sensitivity analyses indicated that under moderate conditions (400°F\ud boiler exhaust, etc.) the installation of a rotary dryer results in a\ud 7% increase in boiler efficiency and a 7% decrease in fuel consumption.\ud Both the field data and sensitivity analyses indicated that a\ud greater increase in boiler efficiency could be realized at higher\ud stack temperatures, approximately a 12% increase in boiler efficiency\ud for a stack temperature of 600°F.\ud The economic analysis was based on capital cost estimates for\ud retrofitting a rotary dryer to an existing boiler, and considered\ud federal and State of Oregon investment and energy tax credits. Payback\ud periods for both cash flow and income bases were determined\ud based on 7% fuel savings, and were contrasted to equivalent BTU savings\ud of natural gas and No. 6 heating oil.\ud The cash flow basis payback periods based on hog fuel savings\ud due to dryer installation ranged from 4.7 years for a used dryer to\ud 7.8 years for a new dryer. The payback periods for equivalent BTU\ud savings of gas and oil ranged from 2.5 to 3.6 years for gas and from\ud 2.7 to 3.8 years for oil.\ud In summary, this study concludes that retrofitting a rotary dryer\ud to an existing boiler is an economically feasible option to the\ud forest products industry