Posted by Albert Berenguel - 11 July, 2023
The treatment of organic waste in biogas digesters not only cuts emissions but also generates green energy from waste. But the storage and production infrastructure for the biogas must be constructed and maintained correctly to prevent uncontrolled methane -which ranks as the second-largest contributor to climate change after carbon dioxide- from escaping into the atmosphere.
What is methane?
Methane (CH4) is a colourless and odourless gas that is, in general, very stable. However, mixtures of methane and air, with a methane content between 4% and 16% by volume, are explosive; in coal mines, methane or “firedamp” is much feared by miners.
Methane is generated naturally from the anaerobic decomposition of organic material in natural wetlands, flooded rice fields or organic waste in landfills. Other contributors to methane generation are the fermentation of animal waste and other emissions from livestock production and the burning of biomass (including forest fires, charcoal combustion, and firewood burning).
Researchers at the NASA Earth Observatory have found that the amount of methane in the Earth’s atmosphere continues to rise. Concentrations of methane in the atmosphere now are about 2.5 times higher than in the 1850s.
In the EU, uncontrolled methane emissions are responsible for approximately 12% of total greenhouse gas emissions. In December 2021, as part of the EU Green Deal, the EU Commission adopted a strategy to mitigate methane emissions from agriculture, waste management and other sectors to achieve the target of climate neutrality by 2050.
Wetlands in Delta de l'Ebre (Catalunya)
What is Biogas?
Biogas is a mixture of methane, carbon dioxide (CO2), hydrogen sulphide (H2S), water vapour and other trace gases that is produced industrially by anaerobic digestion of a wide range of biomass types including agricultural waste, manure, municipal waste, and primary or secondary crops.
Biogas production has enormous economic benefits, which, according to the European Biogas Association has made it the third fastest-growing renewable energy source in the world, after photovoltaic solar and wind power.
The construction and operation of industrial biogas plants have a primary objective: to prevent methane losses while simultaneously transforming waste into valuable fertilizer, enabling nutrient recycling and the generation of renewable energy.
Biogas plant
Biogas production facilities control methane emissions
Methane emissions can be effectively eliminated by treating agricultural residues, manure or biowaste, into the controlled and enclosed environment found within an anaerobic digestion plant.
When biomass is left untreated and stored (e.g., manure on farms), it leads to uncontrolled methane release. Conversely, rather than being released into the atmosphere, in a biogas production facility methane is captured and used.
Moreover, the reduction of methane emissions at anaerobic digestion facilities brings remarkable environmental advantages, as it serves as a renewable energy source that diminishes reliance on fossil fuels.
However, the same biogas plants can allow undesired accidental leakages and diffuse discharges of methane. To prevent that from happening, and to optimise the economic value of the biogas, maintenance and repair of biogas infrastructure is absolutely necessary. As part of the EU Methane strategy, which was published in October 2020, there is an obligation to improve leak detection and repair (LDAR) on all fossil gas infrastructure, production, transport and use, which will be regulated.
It can be challenging to locate the source of methane emissions from a biogas plant. However, the most common locations are:
- Open storage and buffer feedstock tanks where prolonged storage allows fermentation mechanisms to start.
- Digestate storage: if the breakdown of organic matter is not fully completed within the digester, the process of methane generation can continue. Non-gas-tight tanks allow undesired emissions, while in covered tanks, residual biogas can also be collected.
- Valves, flanges and connections of all kinds, such as joints between concrete elements, between concrete and metal elements or the connection between the concrete walls and the membrane dome can be critical spots.
Storage of agricultural waste in open facilities allows methane to escape into the atmosphere.
Even in sealed tanks such as digesters, gas can leak through pores and cracks in the concrete used for their construction.
Concrete structures are not gastight
Based on its composition, a concrete element with a certain minimum thickness should prevent substances such as water or gas from passing from one side to the other. However, all materials are permeable to gases and vapours to some extent, and concrete is not an exception.
Concrete has a strongly connected pore system. In addition, the presence of cracks due to structural movements, drying shrinkage or surface defects during placement, allows gases, vapours and even water to find an easy diffusion path to permeate through structures built with this material.
Therefore, to ensure tightness to methane, it is necessary to apply waterproofing membranes with a highly dense microstructure, high chemical resistance and good crack-bridging properties.
Additionally, movement joints and connections need to be sealed with mastics which exhibit long-term elastic behaviour and durability and also offer high chemical resistance.
Methane barriers and recommended evaluation criteria
To evaluate the capacity of a material to act as a methane barrier, we can refer to DIN 53380 “Testing of plastics – Determination of gas transmission rate – Part 1: Volumetric method for testing of plastic films”, the criteria developed by DLG e.V. (Deutsche Landwirtschafts-Gesellschaft e.V. – German Agriculture Society) and the „Sicherheitsregeln für landwirtschaftliche Biogasanlagen (TI 4)“ (Safety rules for agricultural biogas plants. Both include the following definitions of permeability:
| cm3 / m2 · d · bar | Comments |
| < 10 | Material dense to gas |
| < 400 | Very good |
| 400 to < 700 | Good |
| 700 to < 1000 | Weak |
| > 1 000 | Not acceptable |
To give an idea of what those figures mean, it’s worth noting the permeability to methane of flexible membranes, such as those made from HDPE or LDPE, that are commonly used in low-pressure storage biogas holders. Such membranes provide permeability values between 200 and 500 cm3 / m2 · d · bar.
In the table below are the results and recommended application thicknesses for two of our materials that have recently been tested as methane barriers:
MasterSeal M 689 is a highly elastic, solvent-free, ultra-fast curing, spray-applied polyurea from MBCC that offers high elasticity and crack bridging capacity combined with high chemical resistance.
MasterSeal 7000 CR is a waterproofing and concrete protection system from MBCC with a unique combination of application and performance properties that are especially suited for use in the most aggressive environments, such as wastewater treatment plants and biogas facilities, where hydrogen sulphide and biogenic sulphuric acid can be present.
| MasterSeal M 689 | MasterSeal 7000 CR | |
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| MasterSeal M 689 in a digester at a wastewater treatment plant in Spain. | Sealed concrete surface with MasterSeal 7000 CR in a digester in Denmark. | |
| Permeability | 195 cm3 / m2 · d · bar | 5.97 cm3 / m2 · d · bar |
| Application thickness | 2.1 mm | 1.5 mm |
| Consumption | 2.1 kg /m2 for 2 mm thickness. |
MS P 770: 1 x 0.35 kg/m2 MS M 790: 2 x 0,4 - 0.5 kg/m2 |
In summary, the use of MasterSeal M 689 or MasterSeal 7000 CR ensures high methane tightness when applied to concrete domes, sealing cracks and defects to prevent gas leakages. This protects the value of the resource and prevents a gas with a strong greenhouse effect from escaping into the atmosphere.
Additionally, MasterSeal 7000 CR offers high specific chemical resistance to hydrogen sulphide (H2S) and to biogenic sulphuric acids and by-products produced during the digestion of biomass, which makes it especially suitable for biogas digesters as well as biomethane containers.
References:
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Nasa Earth Observatory. https://earthobservatory.nasa.gov/images/146978/methane-emissions-continue-to-rise
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EU Methane Strategy. https://ec.europa.eu/energy/topics/oil-gas-and-coal/methane-emissions_en
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EBA: European Biogas Association Statistical report 2020.
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DLG e.V. (Deutsche Landwirtschafts-Gesellschaft e.V.) „Sicherheitsregeln für landwirtschaftliche Biogasanlagen (TI 4)“
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IEA (2021), Driving Down Methane Leaks from the Oil and Gas Industry, IEA, Paris https://www.iea.org/reports/driving-down-methane-leaks-from-the-oil-and-gas-industry
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DIN 53380: Testing of plastics – Determination of gas transmission rate – Part 1: Volumetric method for testing of plastic films.
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ISO 15105-1: Plastics - Film and Sheeting - Determination of Gas-Transmission Rate - Part 1: Differential-Pressure Method
