Troubleshooting a 100 kW Biogas Plant Fed with Silages and Bovine Manure

A malfunctioning thermophilic, 100kWe biogas plant was analyzed to engineer possible solutions. The plant featured two single-stage digesters and a covered storage, and was fed with corn silage and liquid manure (50% cow, and 50% cattle). Two feeding lines were present, a tank with shredding-pump for manure, and a silage feeding hopper with digestate mixing system (without shredding). The digesters showed reduced biogas production, non-optimal temperatures and clogging. Considering the characteristics of the system only different compromises between technical needs and cost could be pursued. Option 1 is represented by the optimization of the feeding systems and of the digesters. The silage would be loaded in manure tank, with a new mixer and shredding pump. Infrared (IR) images of the digesters showed that the insulation was effective. The digesters would be improved by upgrading the internal heat exchanger, or by installing an external unit. The mixing could be improved by an hydraulic system, with wall nozzles and recirculation of digestate with a dedicated shredding pump. The two-stage shredding could help reducing the clogging problems. The mesophilic regime would be preferred, but the volume of the digesters (12 days HRT) would not be adequate. Option 2 is represented by transforming the storage in a mesophilic digester by installing external insulation, mixers and external heating system, optimizing the recovery of biogas. The availability of thermal energy could be a limit. Option 3 is represented by a new concrete, mesophilic digester, replacing the existing units: this option requires significant investment but would improve performance and simplicity.