The Danish national effort to minimise methane emissions from biogas plants

•Plant-level methane losses at 69 biogas plants varied between 0.3 and 40.6%.•Pressure relief valves were an often seen source of emission (53% of plants).•A national emission factor was determined at 2.5% for the Danish biogas production.•Leak search and emission quantification can lead to reduced methane emission.•Mitigation actions can sometimes be fully financed through increase in energy sales.