From Laboratory to System. Studies of Superheated Steam Drying of Biofuel

Replacement of fossil fuels by more sustainable fuels such as biofuels has become increasingly important during the last decade. This is mainly due to economical and environmental reasons. This thesis describes a study, both theoretical and experimental, of the fundamentals of superheated steam drying of biofuels and an investigation of biofuel systems where a super heated steam dryer is integrated with heat and power production. The fundamental studies show that it is possible and enlightening to describe the equilibrium moisture content of wood in moist air and in superheated steam in an analogous way. The equilibrium moisture content depends on the activity of the superheated steam, the temperature and the material. The experimental results show that the release of organic compounds depends on the temperature of the superheated steam, the drying time and the age of the chips. The formation rate is strongly dependent on temperature. It has been shown that life cycle assessment (LCA) is a valuable tool in comparing different alternatives for heat and power production and for environmental optimisation of biofuel systems. Different evaluation methods show that Case 2 (dried biofuel) is an improvement over Case 1 (wet biofuel and coal). Of major importance is the large reduction of fossil CO2 of Case 2 compared to Case 1. The environmental impact of the organic compounds from the exhaust gas during the drying is estimated to be small. In this study a new approach to dynamic situations is taken where the environmental impact is expressed as cumulative formation of ecopoints (or equivalent). Drying until the limit at a low temperature (until the constant rate is ended) is preferred. The economical optimum is different from the environmental optimum. Pareto-curves may support the decision making process when combined economic and environmental optima are required