Energy Consumption Relationships for Mesophilic and Thermophilic Digestion of Animal Manures

Abstract

Listen

Translate article

ABSTRACT THE energy consumption for operation of methane fermentation plants for animal manures is critical to the economics of the system. Energy used in the operation of the plant is not available for use or sale elsewhere. An evaluation of the energy use of these plants when operated at 35 °C and 60 °C has been performed for all four major waste types (beef, swine, dairy and poultry) using systems analysis and a dynamic mathematical model. Studying two options for effluent heat recovery (no heat recovery and 50% heat recovery) and simulation of typical animal operations for a year's period have shown that the thermophilic operation uses a significantly less percentage of the total energy than a comparable mesophilic plant when 50% of the effluent heat is recovered. When effluent heat is not recovered, thermophilic digestion for beef and swine waste uses approximately 3% more energy when compared to mesophilic digestion. Dairy and poultry thermophilic digestion, however, still uses less energy than mesophilic digestion. This is surprising, but the apparent reason for this result is the much smaller-compact size of the thermophilic plant and the heavier loading permitted at the elevated temperature. The net energy output (GJ/YR) of both plants (mesophilic and thermophilic) is approximately the same for each individual waste type. Therefore, based on net energy production alone (economics not considered), mesophilic and thermophilic digestion appear as equals.