Greenhouse Gas (GHG) calculator

In this example, about two-thirds of the GHG emissions are released in the production of the plant feedstocks, which are grown specifically for use in the biogas plant. Slightly more than half of the warming potential is due to the release of nitrous oxide from nitrogen fertilization; again, about half of this amount of CO2 equivalent is emitted from the production of synthetic nitrogen fertilizer. Thus, the most effective way to reduce GHG emissions from these sources is to improve nitrogen use efficiency and use as little synthetic fertilizer as possible.

The largest single source of GHG emissions is methane slip, which is conservatively estimated here at 1.5% of total methane produced. At a methane slip of 1.0%, which can be easily achieved by modern, regularly maintained engines, this would reduce total CO2 equivalent emissions by nearly 10% in this example profile. In contrast, complete elimination of methane in the exhaust gas of reciprocating engines is only possible with very great technical effort.

Compared to the marginal electricity mix, the specific GHG emissions for electricity feed-in from this biogas plant are about 80 % lower. The specific GHG emissions of the heat actually used are about 10 % lower than for heat from natural gas at the given high heat utilization rate of about 70 %.

Here, the ratio of GHG emissions from substrate provision on the one hand and biogas production and use on the other is roughly reversed compared to the first example. The values output result from fully accounting for the GHG emissions from clover-grass cultivation to biogas production. It could be argued that in the course of crop rotation on the organic farm, clover grass would be grown even without utilization in the biogas plant, and thus the GHG emissions released by this are only partially (or not at all) to be credited to biogas production.

In any case, this example brings the biogas plant to the forefront as a source of GHG emissions. The most effective measure here would be to equip the digestate storage facility with a gas-tight cover and gas collection: this could prevent more than a quarter of the GHG emissions altogether. In this constellation, special attention must be paid to the management of the digestate, as it contains a lot of nitrogen and therefore there is a high risk of nitrogen losses during storage and application.

Compared to the marginal electricity mix, the specific GHG emissions for electricity feed-in from this biogas plant are almost 80% lower. The specific GHG emissions for heat are arithmetically very high for this plant in its current state, since only about one fifth of the heat is used; if the heat utilization rate were increased to 70 %, these could be reduced by about 80 %.