Flexible Energy from Biogas: Use of Secondary Digesters for Heat Storage—Results of Fermentation Tests

Abstract

A process for a more efficient and flexible use of the produced heat in biogas plants is tested under real operational conditions. The process uses a mesophilic digester and a thermophilic secondary digester, and the heat demand of the plant is covered by using the heat stored in the secondary digester. The efficient heat use is achieved by a controlled variation of temperature in combination with a suitable process monitoring, without affecting the biogas production or disturbing the biological process. The concept was previously tested in ideal laboratory conditions. Here, the validation tests, in preparation for a planned large-scale trial, comprising one-year semicontinuous fermentation tests in the laboratory (close to real operational conditions) are presented. These real conditions are characterized by a high organic load of the reactors as well as by a more manifold substrate mix. During the experimental phase, it is determined that the maximum temperature, at high variation rates between −4 and +2 K per day, reaches 54 °C, allowing a stable process. These experiments confirm that the adaptability to temperature changes depends on the boundary conditions of the fermentation and case-specific experiments are required before significant changes are made to the temperature management of anaerobic digestion systems.