Quantification and Minimization of Methane Emissions in the Biogas Sector

Methane is a valuable energy carrier but also a very potent greenhouse gas (GHG). The reduction of greenhouse gas emission mitigation effect within the energy sector by possible methane emission from the biogas production process is one major point of discussion within the biogas sector.

Although the number of studies concerning the GHG emissions from biogas plants increased during the last decade, it is still not possible to compare these results and to conclude the overall emissions produced in the biogas sector. Due to a lack of reliable harmonized methods to determine methane emissions and the absence of agreements concerning regular examinations of potential emission sources at biogas plants, the available information lead to a confusing debate and rather reduces acceptance than helps the industry. Consequently, a very important key point for better date is a reliable quantification of the methane emissions and a resulting implementation of abatement measures to minimize the emissions.

In principle, there are two different approaches used to determine the methane emission rate of a biogas plant. In the first approach (on-site method), the single sources are identified and the emission rate from these sources is subsequently quantified. A challenge for the determination of methane emission rates from single components is the large variety of different sources that can occur at a biogas plant as well as the identification of the entire emission sources. To quantify the several sources, different methods have to be used. There can be continuous point sources like unknown leakages on digester roofs, gas leading and exhaust pipes or gas utilization devices, areal sources like the emissions from open storage tanks as well as temporal sources like the emissions from pressure relief valves. The point sources are usually examined with dynamic chamber methods [1, 2], for the areal sources, static chambers are used [2, 3]. An approach for long-term monitoring of pressure relief valves is described in [4].

The other approach means the determination of the overall emissions from the biogas plant including all single emission sources at the same time from a proper distance of the plant (remote sensing method). One example of the remote sensing approach is the use of open-path Tunable diode absorption spectroscopy (OP-TDLAS) in combination with a measurement of the meteorological conditions and inverse dispersion modelling, e.g. in [5-7]. Other options in use are DIAL (Differential Absorption Lidar) system [8], or the Dynamic Tracer Dispersion Method (TDM) (e.g., [9, 10]). A challenge for these methods is the dependence on the prevailing weather conditions and topology/location of the plant.

Currently, a harmonization concerning methane emission rate determination from biogas plants of the different methods is in process (within the project "MetHarmo", funded by 9th ERA-Net Bioenergy call). From that, a better comparability of the different methods to determine methane emissions will be reached. This will lead in a further step to a better understanding of the GHG emissions from biogas plants and an approach to quantify the GHG emissions of the whole biogas sector.

The results of measurement campaign lead to a number of potential mitigation measures to minimize methane emissions: frequent maintenance of the CHP, and a monitoring of the CHP emissions, frequent leakage control surveys, the avoidance of PRV releases by optimizing the gas management or an exhaust treatment at the upgrading facilities, avoidance or emissions from the digestate storage. Within the intended presentation, the different emission sources occurring at a biogas plant are described and the methodologies used for their quantification are introduced and compared. The intended presentation will also give an overview of the possible mitigation measures to reduce methane emissions at biogas plants and the perspectives concerning emission monitoring within the biogas sector.