Biowaste recovery, methanisation steps on the gas

There is a lot of bio-waste in our daily lives: food scraps, but also dead leaves, animal excrements, etc. Until recently, this waste was mainly incinerated or piled up at waste collection centres, producing, in one case, carbon dioxide from combustion and, in the other, biogas (including biomethane) from fermentation. Both of these gases being major contributors to global warming. However, since January, 1st, 2024, and the application of regulations on the separate collection of bio-waste, the management and treatment of this waste has raised a number of questions.

For some organic waste rich in lignin (a molecule found in wood) and not very biodegradable, thermochemical treatments - which basically consist of burning the material to produce heat - remain the current management method. For fermentable organic waste, on the other hand, composting or methanisation offer much more interesting ways of recovery.

Composting is based on the degradation of organic waste in a humid environment and in the presence of oxygen. Inversely, as far as methanisation is concerned, the fermentation of waste in an "anaerobic" environment (deprived of oxygen) encourages the formation of biogas, essentially made up of biomethane and carbon dioxide. These two methods make it possible to return fertilising residues to the soil, in the form of compost or digestate (the residues from methanisation), and to capitalise on the biogas, in the case of methanisation, to make an energy vector.

Biowaste on wheels

Biogas from methanisation has a wide range of uses: from simple heat transfer to purified biomethane, free of carbon dioxide, which can be injected into the natural gas network. With some processing, biomethane can also be used as a fuel. In Paris, some buses are demonstrating that they can run on biomethane from landfill sites. Khaled Loubar, a researcher at the IMT Atlantique campus in Nantes, is working specifically on the use of biogas in thermal machines. He is looking to improve the efficiency of current engines, but is also studying new combustion processes, including multi-fuel engines such as dual-fuel.

Khaled Loubar, researcher at IMT Atlantique

The advantage of these engines is that they are much more efficient than spark-ignition engines (conventional petrol engines). They consume less gas to produce the same amount of energy, and are also less sensitive to variability in gas composition. "Below a certain percentage of methane, conventional spark-ignition engines either do not work or just stop," explains the researcher. "Conversely, dual-fuel technology is adaptable, even with gas with a fairly low level of methane."

In the laboratory, Khaled Loubar and his team have succeeded in converting diesel engines to run on a diesel-biogas mixture containing up to 80% biogas, with a minimum of modifications. This prototype paves the way for the development of a demonstrator which, as part of the COGEPRO project, will be deployed on the industrial site of a brewery. They will exploit the biogas produced by the treatment of process water.

So, it's a gas?

While the use of biogas offers many prospects for the production of low-carbon energy, the challenge remains to ensure the efficient and competitive production of this energy vector. In the same IMT Atlantique laboratory, researcher Yves Andrès is exploring different processes for treating biomass waste. By combining several areas of expertise (microbiology, energetics, process engineering, etc.), the two researchers and their team are studying the most suitable recovery processes, taking into account the waste or residues available in a given area and the energy balance of the chosen solution. Their work focuses in particular on integrating methanisation into these recovery processes, and on exploiting the biogas produced.

The choice of methanisation is based above all on a positive or zero energy balance. IMT Atlantique scientists use complex calculation and modelling tools to study the viability and sizing of the equipment. The latter depend on the biogas production capacity and, therefore, primarily on the nature and availability of the inputs, i.e. the waste that will be fed into the methaniser. "Our aim is to estimate the minimum input required so that the plant does not have to supply more energy than it produces," explains Yves Andrès. If the energy balance is favourable, modelling the future quantities of biogas recovered will enable us to determine the size of the facilities, so that they can produce in a functional way: with a good yield and a stable quality over time.

In this way, scientists are helping to set up facilities on different scales. From micro-methanisation on the scale of an eco-neighbourhood or a small farm, so that it is self-sufficient in energy, to biological reactors for very large volumes. "In particular, we helped a company managing a landfill site to renew its waste-to-energy facilities," explains Khaled Loubar. "In order to size the new equipment, we typically had to model the quantities of gas that the centre would produce over 15 or 20 years!"

From waste to dedicated crops, the deviations of productivity

Of course, as with any industry, the problem for the operator is not just knowing his production capacity, but also how to increase it. So methanisation is not limited to available waste? The answer is no. Firstly, because in the case of agricultural installations, the continuous supply of reactors sometimes means using "external" inputs, which are not subject to seasonality, for example. Secondly, because mixing different inputs has a direct impact on the composition and quality of the biogas. "For example, a waste product can provide too much carbon and not enough nitrogen during its degradation. In that case, it makes sense to supplement it with an input that is richer in nitrogen, to balance it out and improve the biogas production yield," explains Yves Andrès. In some cases, this means using crops or raw materials that are not necessarily waste.

However, the researcher warns of certain abuses. A farmer can significantly increase the yield from his methaniser if he feeds it with biomass that produces a lot of energy, such as starches, flours, sugars and so on. "That's not the approach we're taking with our work," stresses Yves Andrès. "But in Germany, for example, milk producers who were initially producing methane from cow slurry realised that they were gaining much more by producing only maize for methane production. So they stopped producing milk".

In France, the use of intermediate crops for energy purposes (CIVE) is tolerated because they do not compete with food crops. On the contrary, the practice can be used to rehabilitate poor, over-exploited soils or soils suffering from saline intrusion, for example. CIVE then acts as a "remediation" crop, feeding methanisers, while restoring the agronomic quality of the soil. "Our aim is not for this type of crop to be permanent. That's why in our laboratory we don't necessarily focus on the best yield for the reactors, but essentially on the recovery of waste and residues", adds the researcher.

Catalytic methanation: bringing carbon dioxide into the virtuous cycle

One way of increasing methane production on farms without affecting inputs is to use catalytic methanation. Unlike methanisation, which is a natural process optimised by an industrial process, methanation produces methane in an entirely industrial way. Scientists at IMT Atlantique in Nantes have explored this solution with one of their industrial partners.

"We proposed a Power-to-Gas system that consists of producing dihydrogen by electrolysis of water, using solar panels installed on the infrastructure", explains Khaled Loubar. "The carbon dioxide released during methanisation is reacted with this hydrogen to produce synthetic methane by catalytic methanation." Although the methane produced in this way does not qualify as biomethane, catalytic methanation is an additional way of extracting value from the biogas produced by methanisation. The process still presents a number of scientific and technological hurdles, but researchers at IMT Atlantique are working on it to continue to broaden the range of ways in which it can be used.