A New Multistage Biofuel Drying System Integrated into an Industrial CHP-power Plant: Description of Process and Performance Calculations

The drying of biofuels should be as effluent-free and energy-efficient as possible to ensure safe and economical operation. This paper presents a new multistage drying system (MSDS), that provides significant benefits when compared to conventional single dryer systems. Most promising, this new application is installed in integrated pulp and paper mills. The MSDS, simultaneously using secondary process energy, and back-pressure and extraction steams 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 even 100 % of the drying air can be utilized as combustion air. The use of MSDS also enables an increase in power boiler capacity, which enhances the production of power and heat in combined heat and power (CHP) plants. For example, if the solid capacity increase of a biofuel boiler is 10 %, the plant would produce about 2.4 % more net heat and 16.4 % more net power at the generator's terminals, minus the power demand of the boiler - MSDS process. Additionally, the improvement in CHP can be attained with decreased emissions of unburned organic compounds and CO from combustion 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 condensate from the drying operation. Equations in this article follow SIunits, unless separately mentioned.