Using the Solid Digestate from a Wet Anaerobic Digestion Process as an Energy Resource

Abstract

The wet anaerobic digestion process is a widely used method to produce biogas from biomass. To avoid the risks involved with using the digestion waste as a fertilizer, this work investigates the possibilities to use the solid digestate as an energy resource to produce heat and electricity, which could save some energy currently consumed by the plant and, therefore, may increase the overall efficiency of a biogas plant. Simulations were conducted based on real data from the Växtkraft biogas plant in Västerås, Sweden as a case study. Results show that it is necessary to dry the solid digestate before combustion and include flue-gas condensation to recover enough heat for the drying process. When a steam turbine cycle is integrated, the generated electricity could cover 13–18 % of the total electricity consumption of the plant, depending on the degree of dryness. In addition, reducing the digestion period can increase the carbon content (ultimate analysis), the heating value, and the mass flow of the solid digestate. As a result, the production of electricity and heat is augmented in the steam turbine cycle. However, the production of biogas is reduced. Therefore, a comprehensive economic evaluation is suggested to optimize a biogas plant that uses the solid digestate from a wet anaerobic digestion process as an energy resource.