Enhancement of the use of wood fuels in heat and power production in integrated pulp and paper mills

The drying of moist biofuels such as wood-based biomasses should be as effluent-free and as energy-efficient as possible in order to ensure the safe and economical operation of industrial CHP plants. This work presents a multistage drying system (MSDS), which provides significant benefits in comparison with earlier conventional single dryer systems. This new application is installed most promisingly in integrated pulp and paper mills. The MSDS, which simultaneously uses secondary process energy, as well as backpressure and extraction steam as the drying energy, enables a smaller volume flow of drying air than single dryer systems. Depending on the structure of the system, up to 100 % of the exhaust drying air can be utilized as combustion air. The use of MSDSs enables an increase in CHP plant boiler capacity, which in turn boosts the production of power and heat in combined heat and power (CHP) plants. Additionally, the improvement in CHP can be achieved with reduced organic emissions from moist biomass drying. Also, the amount of unburned organic compounds and of CO in flue gases from combustion is reduced as a result of the improved quality of the biofuels. When compared to direct steam drying, the MSDS also better minimizes, or even eliminates, the formation of organically loaded condensates in the drying operation.