Methanisation reduces the volume of organic waste and prevents the dispersion of the biogas (produced by fermentation) into the atmosphere, including methane, a gas with a strong greenhouse effect. Furthemore, the biogas is used to produce heat, electricity and even biomethane for injection into the natural gas network. Yves Andrès and Khaled Loubar, researchers at IMT Atlantique, tell us more about this technology, which is seen as a lever for the energy transition.
How to explain so many methanisation installations in recent years?
Yves Andres: Methanisation is the transformation of organic matter by bacteria which, in an anaerobic environment (deprived of oxygen) and after a number of stages, produce biogas. When organic waste is left unmanaged, it naturally emits methane and carbon dioxide as it decomposes. There are many sources of organic waste, including local authorities, the food and agriculture industries, etc. In addition to the issue of waste management, methane has 21 times more impact on the greenhouse effect than carbon dioxide. Fermenting so much waste therefore poses a real risk to global warming and the environment!
Khaled Loubar: This is why some agricultural sites are directly equipped with biogas plants, to prevent livestock manure from being left in the air, and to control the methanisation process. The same applies to landfill sites. Today, all these centres are equipped with biogas capture networks to prevent these emissions from being released into the atmosphere.
Yves Andrès and Khaled Loubar, researchers in the DSEE department
Is it possible to use the biogas produced by methanisation?
YA : For a very long time, this naturally occurring process has been used to treat organic matter, but also to create an energy vector. At the very least, the biogas recovered is flared, dispersing heat and carbon dioxide into the atmosphere. But the trend today is clearly to exploit the potential of this resource as much as possible.
How is it enhanced?
YA : There are three levels of biogas recovery. The first is to use it as a fuel for boilers, in other words to simply produce heat. The second is to use it in cogeneration engines, to recover the heat and produce electricity. The heat is used to heat water or buildings - particularly in farms - but also to keep the biogas plant at temperature.
As the process works between 30 and 40°C, a little extra energy is needed under our latitudes. The heat produced by the cogeneration engine is one possible source, in which case the system is self-powered. The co-generated electricity is also used on site, or fed back into the grid. The third level of recovery involves purifying the biogas to increase the methane concentration and make biomethane.
What are the steps involved in obtaining biomethane?
YA: When the process is properly controlled, the biogas recovered contains 40 to 60% methane. The rest is made of carbon dioxide and a few corrosive pollutants, such as sulfurated hydrogen. In engineered landfill sites, biogas can also contain siloxanes, organic molecules made of carbon and silica which, when burnt, form silica crystals that degrade engines. Biogas is therefore always treated to eliminate these pollutants, and sometimes purified to remove the carbon dioxide and obtain biomethane.
KL: Apart from the microbiological aspects, which we leave to more specialised laboratories, we have the expertise to work on all the stages of the methanisation process, including this separation phase. One of the IMT Atlantique teams is therefore working on carbon dioxide capture, in order to purify the biogas and produce biomethane, using fewer solvents. Once separated, the carbon dioxide can be recovered by manufacturers for various uses, and the biomethane intended for injection into the natural gas network is conditioned, with compression, odorisation, etc.
Does the variability of organic waste have an impact on the gas produced?
YA: Methane is a very simple molecule, one carbon atom and four hydrogen atoms (CH4). Whether it's produced by synthetic methanation, biologically, or in a cow's rumen, its composition does not change. Only the purity of the finished product differs. On the other hand, supply can be a problem. Agricultural biomass in particular is produced on a seasonal basis, and anaerobic digestion reactors have to operate year-round. As organic matter is fermentable, it degrades and is difficult to store.
Another solution is to gasify dry biomass or solid recovered fuels. It is important to select the best solution in the light of the energy balance: it is clearly not a good idea to dry cow manure, but why not other valuable raw materials with high potential?
The alternative is to use codigestion, i.e. to mix the inputs, the raw materials that feed the reactor. Then, we carry out laboratory tests to analyse the methanogenic potential of different organic substrates, with the aim of providing a mix that guarantees good biogas production yield.
What are the different uses of biomethane?
YA : Some of the biogas is of course used in thermal engines. It is quite common in Paris to see buses using biomethane fuel from landfill sites. But methanisation plants are moving much more towards injection, encouraged by operators like GRDF to get more green gas into their networks. By 2030, renewable energies should account for 32% of energy consumption in France, and biogas will play an increasingly important role in the energy mix.
For farms, methanisation is becoming an additional source of income. However, each stage in the process that needs to be added in order to recover the biogas and obtain biomethane that can be injected into the grid, has an energy and economic cost. The more farms want purified gas, the more they have to produce to break even.
Obviously, this raises a lot of questions, because to achieve financial equilibrium, you need very large biogas plants, and therefore a lot of biomass to feed them. This potentially means bringing in biomass from other sites, at a significant energy cost, or even encouraging the cultivation of biomass intended solely for energy production, as seen in Germany. That is why it is vital to carry out energy balances on a case-by-case basis - which are part of the technical tools we master in the laboratory - to establish whether the energy and economic balance is favourable.
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This article is republished from the I'MTech blog.
Read the original article: « Quèsaco la méthanisation ? »
by Pierre-Hervé VAILLANT