Optimization of the Bark Drying Process in Combined Heat and Power Production of Pulp and Paper Mill

Solid biofuels, such as bark, forest residues, sawdust, and wood chips are important mill fuels in the forest industry. They are combusted in combined heat and power plants (CHP-plants) to generate heat and electricity for pulp and paper making processes. Biofuels usually have high moisture content. The high moisture content decreases the effective heating value of fuel and makes the operation of the boiler more difficult. Until now, biofuels have not been dried before combustion in Finland; however, the forest industry is currently highly interested in biofuel drying. The designer of the drying process should be able to determine the final fuel moisture, drying temperature, and degree of energy efficiency of the dryer, thereby maximizing earnings from the drying. This paper presents a calculation model for answering these questions. Earnings are determined using net present value. The calculation model is created for a continuous cross-flow dryer, in which air is aspirated through a fixed bark bed. The highest possible drying temperature depends on the heat source used for the heating of the drying air. The most suitable heat sources in pulp and paper mills are secondary heat flows, backpressure steam, and extraction steam. Using current energy prices, and the economic lifetime of 10 years with an interest rate of 5%, the optimally designed dryer uses only secondary heat for the heating of drying air, consists of two drying stages and dries the fuel/bark flow to final moisture, which is in the order of 0.3kg/kgdm.