Mechanical pretreatments of different agri-based feedstock in full-scale biogas plants under real operational conditions

Abstract

Mechanical pretreatments are commonly used devices at full-scale to reduce the size of feedstock and to increase methane production, improving mixing, heat, and mass transfer into the anaerobic digesters of biogas plants. Nevertheless, physical pretreatments require considerable energy. This work aimed to investigate how the energy consumption in different mechanical pretreatments that effectively run under real operational conditions at full-scale level affects physical modifications and anaerobic degradability of agri-based feedstock. Four mechanical pretreatment were investigated: knife milling, hammer milling, extrusion, shredding + hydrodynamic cavitation. After pretreatments, the specific methane yield increased in the range from 1% to 13% and the maximum methane production rate increased in the range from 4% to 48%. Each pretreatment leads to a positive energy balance (0.14–27 kJ kJ−1 used). The increasing of energy consumption indicates a more intense pretreatment with an incremental reduction of particles with diameter bigger than 5 mm. On the basis of the calculated regression analysis, a specific combination of additional specific surface area and additional energy in the day of maximum methane production should be expected with mechanical pretreatment that run effectively at full-scale level.