AGRICULTURAL BIOGAS PLANTS – ENERGY BALANCE

Greenhouse gas (GHG) emissions from agriculture could be reduced by producing biogas from animal manure. Biogas production would reduce methane emissions from stored manure and provide climate-neutral methane gas, which could be used for energy purposes, improving the GHG balance. One problem for farmers could be the investment costs for biogas plants, which are currently high. This study compared energy production in two dairy manure-based, farm-scale biogas plants in Norway and the energy consumption in producing this energy. The results for the plants, which had no heat exchanger and minimal insulation on the biogas reactor, clearly demonstrated the necessity of using a co-substrate with dairy manure. Using only manure during January at the Åna plant, SW Norway, resulted in only 10% net energy production. The best result obtained when using co-substrate was 73% (fish ensilage; October). Maximum biogas yield of 0.355 m3 CH4/kg VS was recorded by having 31% of total VS provided by fish ensilage with 31 days retention time. Using food waste to supply 29% of total VS gave maximum biogas yield of 0.268 m3 CH4/kg VS with 23 days retention time. There were also differences between the plants, which were of different designs. The lowest heat consumption was observed for the Åna plant with its one reactor (in October) and the highest heat consumption for the Tomb plant with its two reactors running in parallel (in January/February). Reactor design probably resulted in higher heat consumption for all periods in the Tomb plant, as biogas storage capacity can be too small when only the headspace of the reactor(s) is used for storage. External biogas storage capacity is recommended.