Denmark has a political objective to increase the production of biogas with a view to reducing dependence on fossil fuels and reducing the emission of greenhouse gases. It has been decided that 50% of the livestock manure in Denmark must be used for energy purposes. Most of this energy production will be produced in biogas plants. In 2023, the total Danish biogas production accounted for more than 40% of the total gas consumption in Denmark.

Degassing of biomass in biogas plants is suitable for reducing the emission of greenhouse gases, as the degassing produces biogas, which can replace the use of fossil fuels for electricity and heating. The degassing also affects the emission of greenhouse gases that take place during the storage and application of manure. The production and utilization of biogas can, however, lead to unwanted emissions of the greenhouse gas methane, for example via leaks at biogas plants, which reduces the positive climate effect of the degassing.

There are several different greenhouse gases in the atmosphere, the most important of which are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and halocarbons such as HFCs, PFCs and SF6.

The climate impact is not the same for each individual greenhouse gas. The climate effect of the greenhouse gas nitrous oxide in the atmosphere is, for example, significantly higher than the effect of the greenhouse gas CO2. In order to be able to calculate the total climate effect of the various gases in the atmosphere, the climate effect of the various greenhouse gases is converted into CO2 equivalents. The conversion factor depends on the lifetime of the individual gases in the atmosphere (IPCC, 2007), but in practice an average lifetime of 100 years is normally used. The conversion factors are continuously changed based on IPCC assessment reports. The below shows the most recent calculations of the weighting between the various greenhouse gases.

Greenhouse gas              CO2 equivalent

Carbon dioxide (CO2)        1

Methane (CH4)                 25

Nitrous oxide (N2O)          298

 
Since methane has a very large potential as a greenhouse gas, it is therefore very important to avoid emissions of methane in connection with biogas production. The positive climate effects of biogas production can very easily be put out of control if there is an accidental release of methane.

Agriculture's emission of methane is primarily related to the formation of methane in the livestock's digestive system as well as the emission of methane when storing livestock manure. Degassing of biomass is one of the technologies that has the potential to reduce the emission of methane from both animal manure and waste.

In connection with the degassing of slurry and other organic biomasses, methane (biogas) is produced, the energy potential of which can be used for electricity and heat production or sold via the gas grid. Degassing can therefore be used to substitute energy that would otherwise have been produced by burning fossil energy sources such as coal, oil or natural gas. This substitution therefore reduces the CO2 production that would otherwise be emitted by burning the fossil fuels.

In addition to the substitution of fossil energy sources, the degassing of livestock manure also affects the emission of methane that takes place in connection with the storage of livestock manure and its subsequent use as fertilizer in plant production. The degassing reduces the manure's content of exchangeable organic (volatile solids) compounds, which limits the emission of methane from, for example, manure storage, as well as the emission of nitrous oxide from the spread manure.

In addition to this, degassing can lead to unwanted emissions of methane from biogas plants. This happens, for example, in the form of emissions of unburned methane from the gas engines that convert the biogas into electricity and heat energy, or from emissions from the upgrading plants used to purify the biogas. The emission of methane from biogas engines depends largely on the setting and type of the gas engines but is normally estimated to be between 1.5 and 3% of the amount of methane produced at the biogas plant. In an assessment of the effects of biogas projects, the Danish Ministry of Environment has estimated that the emissions of unburnt hydrocarbons from gas engines at biogas plants amount to 0.30 kg CO2 equivalents per Nm3 methane. This loss corresponds to approx. 1.8% of the biogas plant's total methane production.

A new report on methane emissions from biogas engines in Denmark has just been published in December 2023. Measurements have been made (on 158 measurement series) of both methane and NOx emissions. The measurements show that it is not possible to have low emissions of methane and NOx at the same time. The results show that approx. 90% of all engine installations have a methane loss of 1-3%.

In addition to the methane loss that occurs when unburned hydrocarbons are emitted from biogas engines (methane emission from gas engines) or from the upgrading plants, there can also be a loss of methane via leaks at the biogas plant. Methane can be emitted from different sources at the biogas plant. Sources of methane emissions could be storage tanks for untreated biomass and digestate as well as leaks in biogas and upgrading plants.

Basically, all new biogas plants in Denmark upgrade gas to the natural gas grid. The Danish Biogas Industry has concluded an agreement with the Danish Ministry of Climate, Energy and Utilities to roll out a voluntary measurement scheme. The scheme has pointed out sources of risk of methane leaks and measures to deal with such risks. Among the identified sources, pre-digesters, and post-digesters without gas collection are the most important. When the scheme was first launched, the easily accessible focus points were found, such as vacuum valves, pipe penetrations, etc. as well as the technology-dependent sources, including upgrading technologies. In recent years, considerable efforts have been made to reduce methane leaks from the plants. Until 2018, the methane loss was studied at a large number of biogas plants with a total production of about 150 million Nm3 CH4, corresponding to about half of the biogas production in 2018.

The current voluntary investigations at Danish biogas plants show where leaks can typically be found at the biogas plants. There is photographic equipment with which it is possible to find leaks, and there is measuring equipment which makes it possible to measure the concentrations of methane in connection with the leaks, but there are currently no methods to quantify the losses as this requires that a measurement is also made of the quantity emitted at the spot.

Methods have been developed to quantify the total emission from biogas plants based on either the trace gas method or camera recordings via drones or helicopters. The Danish Energy Agency is now offering a study that will extend over the next 1.5 years to try to determine a reliable method for measuring the total methane emission from a biogas plant. So far, the methods are subject to some uncertainty.

Since 2018, considerable efforts have been made to reduce the leaks, which are typically found in connection with:

• Pressure vacuum valves
• Service hatches
• Cable entries
• Looking glass
• Foam valves
• Flanges
• Shaft bushings
• Joints in concrete cover
• CO2 return from upgrading
• Compressor output
• Gas blowers
• Engine compartment

Today, Denmark uses different types of methane upgrading plants. Amine and water scrubbing plants constitute the majority (86%) of the plants, whereas a smaller number of plants uses membranes (14%).

The methane loss varies between the different technologies, and water scrubbers have the highest loss of about 1%. However, technologies, such as regenerative thermal oxidation (RTO), exist that can oxidise methane, and 5 plants use this technology today. RTO plants can reduce CH4 emissions by 99.5%. The total loss from upgrading plants in Denmark is estimated by Danish Gas Center to be about 0.3%. The Danish Energy Agency has entered into an agreement to reduce methane emissions from Danish biogas plants via a dedicated effort headed by Ramboll in partnership with Force, Technical University of Denmark, and the Danish Technological Institute. Based on current knowledge, a total loss of 1% is estimated for the biogas plant and the upgrading plant. However, plants that are not part of the voluntary scheme might have a higher loss. Also, water scrubber plants would not be able to comply with an emission limit of max. 1%, unless an RTO plant is installed, as a loss is always anticipated from the biogas plant itself in addition to the loss from the upgrading. In a similar way, plants with engine installations (Combined Heat and Power, CHP) do not have the opportunity to immediately comply with the requirement for emissions of less than 1% CH4, as an amount corresponding to this passes unburned through even very efficient engines.

The target of the Danish Biogas Industry for the loss of a maximum of 1% are extremely difficult to achieve.

Note!
Establishment and operation of biogas plants is considered an environmentally and climate-friendly solution, but it is crucial that the methane emissions from these are limited as much as possible and are under control. Minimal emissions are very difficult to avoid, but larger releases ruin the climate account and can make biogas production less appropriate. It is therefore paramount when establishing a biogas plant that the right components are chosen and that these are subsequently checked and serviced to avoid emissions.