Technical Report

Methane emissions from biogas plants

Methods for measurement, results and effect on greenhouse gas balance of electricity produced

December 2017

Executive Summary

Methane is a potent greenhouse gas with a global warming potential much higher than carbon dioxide. Fugitive methane emissions from a renewable energy production system are not conducive to the ambition of reducing Greenhouse Gas (GHG) emissions. The biogas industry is growing and innovative technologies are associated with the rising numbers of facilities in operation. With new technologies it is essential to ensure minimum fugitive emissions; this leads to new challenges regarding emission monitoring, quantification and reduction. Within the biogas sector methane emission quantification is becoming a significant topic for the scientific community but is still under development for the industry sector. The methods used and the interpretation and evaluation of the results obtained is not as yet standardised. This report addresses methods used for evaluation, presents selected results of measurements, proposes mitigation measures and puts methane emissions in a context of a standard greenhouse gas balance in order to evaluate the impact of these emissions on the sustainability of the biogas system.

Methods

Currently several methods are in use and a variety of data sets have been provided from different international teams. The methods used can be distinguished into two major approaches. The single source method aims at an identification, quantification and summation of every emission source. The overall plant measurement aims at the quantification of the plant emissions in total and is effected by remote sensing. The approaches have different advantages and limitations and are therefore applicable for different purposes.

An additional complexity is that the methods applied by industry and by the scientific community can vary in general approach, execution, data analysis and interpretation; this can lead to non-comparable results. An important task for the future is therefore method harmonization including for documentation and reporting of the results. Factors influencing the results involve: the limitations of the methods used; the duration of measurement (in order to cover time variability of specific emission sources); the completeness of plant components measured and potential sources included but not belonging to the biogas facility (such as barns); and the operational mode of the plant. For a representative emission factor, which covers the average emissions during operation, all aspects need to be sufficiently well considered for a sound result.

Results of measurements

The parameters with the largest influence on the quantity of methane emissions can be distinguished by structural (the technologies deployed) and operational (plant management) means. The most important sources included: open storage of the digestate; the combined heat and power (CHP) engine; leaks; and the pressure release valve (PRV). Large quantities of uncontrolled methane emissions have been reported caused by single large leaks or long lasting pressure relief events. It is very difficult to give general, average numbers for emissions from components or complete biogas plants. Firstly, the results given in literature have large differences due to the variations within the methodologies applied. Even emissions from the CHP engine show a substantial variability, although the methods for quantification are well defined and engine construction and operation should lead to similar emissions. Secondly, the plants are highly individualized and any generalisation needs to include a classification considering the plant design and plant operation in order to obtain a general emission factor for the sector. Thirdly, methane emissions need to be seen in context with other factors influencing GHG emissions and sustainability of the bioenergy installation. Looking at the methane emission in isolation will not allow assessment of the full impact of the system on the GHG emissions or sustainability in relation to renewable energy production or waste treatment.

The results available show a large variability regarding the amount of emissions from biogas plants. There are not sufficient data for a general assessment of the sector, but trends indicate which components should be monitored and which measures are useful to minimize the amount of released methane.

Reduction measures

The application of specific monitoring and maintenance and/or the application of specific technologies can reduce emissions. A crucial part of any operation should be a monitoring plan and in particular frequent monitoring of any potential emission sources on site. Some of the potentially larger sources (CHP, PRV and large leaks) are dependent on operation and time and therefore need to be routinely monitored. In case of high emissions, they can be substantially reduced by operational measures.

Reduction measures can include the following:

• Emissions from digestate storage should be minimized since they are one of the major sources. Either the digestate tank should be covered (gas tight with gas utilisation) or the degradation of the substrate should minimize the possibility of emissions. As soon as the digestate leaves the process its emission potential needs to be minimized. In case the digestate is used to condition substrate for better handling or to support hydrolysis in a pre-treatment step, this should happen within encapsulated units and any gas produced during this step should be treated. Any aerobic post-treatment should include a sufficient oxygen supply in order to avoid methanogenic activity. The monitoring of oxygen supply (or methanogenic activity) within the process is recommended.

• The exhaust of the CHP can contain high methane concentrations due to incomplete combustion. Frequent control and documentation of motor settings and frequent maintenance and control of methane concentrations can help to minimize these emissions. Further reduction can be achieved by means of post combustion of the exhaust gas, but this is an expensive solution. There are no catalysts for methane emission reduction available at the market for lean-burn engines. However, Selective Catalytic Reduction (SCR) is also discussed as an option for optimising the emissions from CHP since it allows the unit to operate with lower lambda (air fuel ratio) leading to lower methane emissions.

• In the case of biogas upgrading technology, depending on the applied type of technology, the concentration of methane in the off gas varies due to varying separation efficiency. In case of significant emissions caused by the off-gas, a post treatment is recommended. Frequent function control and monitoring of the performance of such devices is necessary.

• The biogas containing components should be frequently monitored to identify leakages. This includes surveys with leakage detection systems such as methane cameras and handheld lasers. Such a survey should be carried out every 1 to 3 years, depending on the status (age and number of leaks found) of the plant. Monitoring for elevated methane concentrations within the off-gas streams from air inflated double membrane roofs should be included in routine measures.

• Plant management should aim at avoidance of PRV releases (and flaring events) in order to minimize emissions and losses in general. This includes the automatic operation of the flare linked to the filling level of the gas storage. A stationary flare is required, which is operational in parallel to the CHP and kicks in before the PRV opens. The filling level of the gas storage should be well below 80% during normal operation (in order to compensate weather and operation induced changes); a value of around 50% is recommended. The level indicators need to be capable of delivering precise measurements in any range of filling level. Connected membrane gas storage systems need to be adjusted to each other in order to allow controlled filling levels and pressure conditions in all vessels under all process conditions. Accordingly gas transfer between several gas storage systems needs to be controllable in order to avoid unbalanced filling levels as well as pressure ratios, which might lead to PRV release in one vessel although other vessels have idle or spare capacity. In case flare operation is not set to avoid PRV events, a monitoring system for PRV operation is recommended to record the number and duration of release events. The gas management system can also include the adjustment of feeding during shutdown of the gas utilisation or periods of reduced load of the CHP. Adequate dimensions of pipes, blowers in the gas pipes and controllable air pressure in the air inflated roofs are measures to achieve well balanced filling levels in all gas storages.

GHG balance

When putting the methane emissions into a context of a GHG balance of the bioenergy system, it becomes apparent that beside the fugitive methane emissions other important factors (in decreasing order) include: the substrate used; the heat utilization; and the parasitic energy demand. In case of a clear GHG reduction target the plant design needs to be chosen carefully, since some components (such as CHP unit, open digestate storage) cause inevitably certain emissions once in operation.

By using the data and methodology adopted by the European Commission, and assuming 30% of the Fossil Fuel Comparator (FFC) for electricity as a targeted limit for the operation, it was shown that energy crop based plants will experience difficulties in reaching this reduction target without specific measures (such as heat utilization or exhaust treatment at the CHP) since the energy crops come with a GHG burden associated with the production of the crop. Manure based plants come with a large credit due to avoided emissions from raw manure storage. Consequently, manure digestion reduces emissions significantly and this effect is also to be seen in co-digestion systems.

Outlook

The major task for the future is an improvement of precision, reproducibility and representativeness of the methods used for emission quantification. A method harmonization or at least a defined protocol will be necessary to compare results from different measurements. An important aspect of the documentation is the definition of the status of the plant and how highly time variant emissions (such as PRV release events) are included in a long-term reference time period. Only comparable results in combination with a sufficient number of plants analysed will lead to a better understanding of the emissions from the whole sector and a reliable data base for emission inventory. A general task for the future is to raise awareness within plant operators and plant manufactures of this issue. Only if the industry is sensitive to the subject, can emissions be further reduced.

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