Biogas production process

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The smart combination of renewable energy sources and technologies will be central to developing the energy systems of the future and achieving carbon-neutrality by 2050. Renewable gases will be part of that mix and increasingly stand out as key enablers of the acceleration of the EU energy transition.

Biogas and biomethane prevent emissions across the whole value chain, with a three-fold emissions mitigation effect. Firstly, they avoid emissions that would otherwise occur naturally: organic residues are taken to the controlled environment of biogas plants, preventing the emissions produced by the decomposition of the organic matter from being released into the atmosphere. Secondly, the biogas and biomethane produced displace fossil fuels as energy sources. Thirdly, the use of the digestate obtained in the biogas production process as biofertiliser helps return organic carbon back into the soil and reduces demand for the carbon-intensive production of mineral fertilisers.

The production of biogas and biomethane also has a positive impact on non-energy sectors. They are generated from different types of organic residues, turning waste into a valuable resource, which is the core principle of an efficient circular economy. Food waste or wastewater can be recovered from our cities and used to produce renewable energy, which helps develop a local bioeconomy. In the countryside, residues from animal farming or biomass from agriculture can be optimised and converted into energy, while digestate can be used as an organic fertiliser. This creates additional business models in the farming sector, making it more cost competitive, and promotes sustainable farming.

Biomethane –  purified biogas –  is a renewable alternative to natural gas. Its multiple applications include heat and power supply for our buildings and industries, and renewable fuel production for the transport sector, especially heavy-vehicles and vessels. The latest studies show that biomethane is an effective way to abate GHG emissions from transport, which represent 25% of the total emissions in the EU.[1]

Biogas and biomethane are already available and they are also cost-competitive, if we consider all positive externalities generated by the production of these renewable gases. Europe is the largest producer of biogas and biomethane in the world today, and it will be essential to scale up production of these renewable gases in order to meet renewable energy demand by 2030 and achieve climate targets in 2050.

[1] JRC Science for policy report: JEC Well-to-Tank report v5

Turning molecules into energy

Biogas is produced from the decomposition of organic materials. These residues are placed in a biogas digester in the absence of oxygen. With the help of a range of bacteria, organic matter breaks down, releasing a blend of gases: 45 – 85 vol% methane (CH4) and 25 – 50 vol% carbon dioxide (CO2). This natural process, also called anaerobic digestion (AD), has long been used to leaven bread and brew beer. The output is a renewable gas which can be used for multiple applications.

If we upgrade biogas, we obtain biomethane. This purified form of raw biogas can be used as a natural gas substitute: CO2, H2O, H2S and other impurities are removed during biomethane production, leaving a high-caloric, pure gas.

Renewable, flexible, enablers of decarbonisation

Biogas and biomethane are renewable gases which help abate emissions across the whole value chain. Their use is essential if we are to accelerate the reduction of GHG emissions in multiple sectors, including buildings, transport and agriculture.

The deployment of biomethane to replace fossil fuels does not require the investment of additional resources to develop new infrastructure. The existing gas infrastructure is biomethane-ready. This is key to ramping up decarbonisation and providing affordable renewable energy for consumers.

In addition, biomethane can be easily stored and produced at a constant pace, helping balance energy supply from intermittent energy sources of renewable origin, such as solar or wind. It can also be traded and produced within Europe, ensuring the EU’s security of supply, and avoiding dependence on external providers.

Renewable heat and power

Combined heat and power engines (CHP) are a common valorisation route for biogas in Europe. The idea behind CHP is that the co-generation of electrical and thermal energy is more efficient than generating them separately. Depending on the design of the biogas plants, part of the heat from the CHP may be used to support the plant’s fermentation process – for example, if the biogas reactors require heat to maintain the correct temperature. The electricity produced is mainly fed into the electricity grid, while any surplus heat is available for local heating applications.

Clean transport

In the transport sector, biomethane is used as a biofuel in the form of a CNG or LNG substitute, called bio-CNG or bio-LNG. Biomethane in transport is a high performer in terms of the reduction of GHG emissions, if we consider the full carbon footprint of the vehicles (Well-to-Wheel). Liquefied biomethane can be used, for example, in heavy-duty transport and the maritime sector, both of which are difficult to electrify.

 

Allies of the circular economy

Waste recycling

Biogas is produced principally from organic residues. These can be energy crops, agricultural waste, manure, biowaste from domestic households or industrial and commercial organic waste. Additionally, biogas can be extracted from wastewater streams or landfills.

Sustainable farming

Digestate is the remaining part of the degraded biomass after biogas production: it is stable organic matter rich in various nutrients (N, P, K). Depending on the feedstock used for biogas production, the digestate may be directly usable as organic fertiliser, in the same way raw animal slurries are spread on fields in agriculture. It can also be further upgraded to recover high quality mineral nutrients. Use of digestate as organic fertiliser offers multiple advantages: it allows the reuse of nutrients and substitutes mineral fertiliser of fossil origin. Compared to raw manure, digestate is also sanitised, as the biogas production process neutralises most of the pathogens contained in the original feedstock, such as bacteria and crop diseases. If unfit for agricultural purposes, digestate can be further processed and used as a raw material in industrial processes.

Closing the carbon loop

Carbon dioxide is a by-product of the purification of biogas to biomethane. The carbon dioxide stream can be valorised in the food industry or can be used to maximize photosynthesis potential in greenhouses. This is the last step of the so called ‘short carbon cycle’, a process which starts with the use of the carbon contained in organic residues to produce biogas, which is partly composed of carbon molecules. The ‘short carbon cycle’ continues with the re-use of the carbon contained in the digestate: spreading the digestate as organic fertiliser puts the carbon back into the soil. Completing the whole carbon cycle by valorising the carbon dioxide after producing biomethane ensures the removal of the carbon from the atmosphere.

 

Driving rural development

In many rural areas, agriculture is one of the main economic activities. Agriculture is also a major contributor to the production of renewable energy, including biogas. Combining agricultural activities with renewable energy production through biogas has a threefold extra benefit: it helps farmers to manage their waste and residues efficiently, it reduces emissions from agriculture and it improves soil quality and biodiversity in farmlands. In these healthy ecosystems, plants absorb carbon dioxide from the atmosphere acting as carbon sinks, digestate used as organic fertilizer returns nutrients into the soil and methane emissions from livestock are taken into the controlled environment of a biogas plant, instead of being released into the atmosphere.

The promotion of sustainable and efficient agricultural practices is an important driver of rural development. Sustainable bioenergy production using the Biogasdoneright methodology, for example, can restore soil quality, soil health and soil fertility. This approach is a strong enabler of sustainable bioenergy production:

Adopting cover crops

Cover crops[3] are part of a farming system in which an additional second culture is grown before or after the harvest of the main crop on the same agricultural land. The cover crop prevents soil erosion and compacting, and promotes biological, chemical, and physical activity in the soil. As a result, soil quality and fertility are enhanced, and soils are more resistant to floods and draughts. Cover crops are not normal winter crops or grassland but are sown specifically to protect bare soil in winter and early spring after the harvesting of summer crops. Apart from protecting the soil and its nutrients, cover crops can be of economic significance when used for renewable energy production such as the generation of biogas and biomethane[4].

Returning recycled carbon back into the soil

The spreading of digestate on the land can recarbonise the soil. Digestate serves as an organic fertiliser, giving the soil both nutrients and recycled carbon.

The circular bio-based economy offers opportunities for the transition to a climate-neutral economy, as well as the creation of new jobs in primary energy production and the replacement of fossil-based energy and materials. The existing potential of the bio-based economy for farmers and their cooperatives is largely untapped. Bioenergy production from agricultural waste streams and cover crops creates additional business models in the farming sector, making agriculture more cost competitive.

[1] European Union, Renewable energy in EU agriculture EPRS | European Parliamentary Research Service

[2] Eurostat – SHARES (Renewables)

[3] Panagos et al. have assessed the beneficial effect of cover crops in preventing soil erosion. They conclude that extending cover crops to 35% of European arable land would reduce the risk of soil erosion by 40%.

Panagos et al. (2015), Estimating the soil erosion cover-management factor at the European scale

[4] Navigant estimates that with the help of sequential crops, European biomethane production could reach 41 bcm.

Definitions

Anaerobic Digestion (AD)
Is a biological process in which microorganisms break down biodegradable material in the absence of oxygen creating two important products: biogas and digestate. AD makes the best use of organic materials by producing biogas for the generation of renewable heat, electricity, fuel and fertilizer while closing the nutrients cycle and reducing greenhouse gas emissions.
Thermal gasification (Gasification)
Is a physico-chemical oxygen depleted process in which the carbon containing components of the biomass break down to syngas instead of being completely combusted. It is a complementary technology to anaerobic digestion and greatly amplifies the potential of renewable energy in the form of heat, electricity and vehicle fuel.
Feedstock
AD can process almost any biogenic material including solid and liquid manure, energy crops, catch crops, agricultural waste and residues, industrial food and beverage waste, sewage sludge and the organic fraction of municipal solid waste. Gasification can theoretically process any carbon containing material and is a complementary technology to Anaerobic Digestion (AD), since it can treat high-solids feedstock with low anaerobic biodegradability; these include lignocellulosic feedstocks such as wood chips, and non-recyclable waste fractions of biomass origin currently landfilled or incinerated for energy recovery.
Biogas
The primary product of AD is a methane-rich renewable gas composed of 45 – 85 vol% methane and 25 – 50 vol% carbon dioxide.
Digestate
Remaining part of organic matter treated by AD, rich in nutrients and nitrogen, commonly used as an organic fertilizer in agriculture.
Syngas
The primary product of gasification is a mixture of carbon monoxide and hydrogen, with traces of methane and carbon dioxide. It may be used directly for electricity generation, or further transformed to increase its share of methane.
Biomethanation
Besides methane formed spontaneously during gasification, syngas can be transformed into methane through two catalyst aided reactions: the water-shift reaction (hydrogen and carbon dioxide formed from carbon monoxide and water) and the Sabatier reaction (methane formed from carbon dioxide and hydrogen).
Biomethane
When carbon dioxide and trace gases in biogas are removed, a methane rich renewable natural gas substitute is left in the form of biomethane. Biomethane can be injected into the gas grid, used as a vehicle fuel or used for combined heat and electricity generation.