The Danish national effort to minimise methane emissions from biogas plants

In total, 69 biogas plants representing 59 % of Danish biogas production participated in a national effort to reduce methane (CH₄) emission. Measurements in terms of total plant CH₄ emissions, quantification of emissions from point sources, leak surveys and conceptual design plans to mitigate emissions were performed. Plant-level CH₄ emission rates varied between 1.3 and 81.2 kg CH₄ h^-1, and CH₄ losses expressed in percentages of production varied between 0.3 and 40.6 %. Agricultural plants generally had lower CH₄ loss rates compared to wastewater treatment plants. Biogas plants with a smaller gas production emitted a larger fraction of their production compared to larger plants, which was partly explained by the absence of gas collection from digestate storage tanks at smaller plants. A very commonly observed source of emission was pressure relief valves, where this source of leakage was observed at 53 % of the plants. A national emission factor (sum of CH₄ emissions/sum of CH₄ productions) was determined at 2.5 % for the Danish biogas production, whereof it was 2.1 % for agricultural biogas production and 6.7 % for biogas production at wastewater treatment plants. Measurements of total CH₄ emissions at six plants performed before and after implementation of mitigating actions showed that emissions were reduced by 46 % by carrying out relatively minor technical fixes and adjustments. An economic evaluation showed that, in some cases, mitigating actions could be economically beneficial for the biogas plant (positive net present value over a 10 year time frame), due to an increase in revenue.