Biowaste can be found in many everyday places, including leftover food, dead leaves, and animal droppings. Until recently, this waste was mainly incinerated or piled up in landfills, in the first case producing carbon dioxide resulting from combustion and in the second, biogas (including biomethane) from fermentation. Both gases greatly contribute to current global warming. However, since January 1st, 2024, the management and treatment of this waste has raised many questions with the implementation in France of regulations on the separate collection of biowaste.
The current management method for certain types of lignin-rich (a molecule present in wood) organic waste that is not very biodegradable, remains thermochemical treatments, which roughly involves burning the material to produce heat. For fermentable organic waste, on the other hand, composting and methanization provide far better recovery methods.
Composting is the degradation of organic waste in humid environments in the presence of oxygen. Methanization, on the other hand, is the fermentation of waste in an “anaerobic” environment (without oxygen) that promotes the formation of biogas, essentially consisting of biomethane and carbon dioxide. These two processes make it possible to replenish the soil with fertilizing matter in the form of compost or digestate (waste remaining after methanization) and, in the case of methanization, use biogas as an energy carrier.
Bio-waste on wheels
Biogas from methanization can be used in a wide range of areas: from a simple heat transfer to purified biomethane, stripped of carbon dioxide, to be injected into natural gas grids. After treatment, biomethane can also be used as a transport fuel. Some Parisian buses have demonstrated this by running on biomethane produced at landfills. Khaled Loubar, a researcher on IMT Atlantique’s Nantes campus, works specifically on the use of biogas in thermal machines. He is looking to improve the efficiency of current engines, while also studying new combustion processes, including multi-fuel engines such as dual-fuel systems.
The advantage of these engines is that they are much more efficient than spark-ignition engines (conventional gas-powered engines). They consume less gas to produce the same amount of energy and are less sensitive to variations in gas composition. “When methane is below a certain percentage, conventional spark-ignition engines either do not work or simply stop,” the researcher explains. “On the other hand, dual-fuel technology will adapt, even when the gas has relatively low methane levels.”
In the laboratory, Khaled Loubar and his team have successfully converted diesel engines to run on a diesel-biogas mixture containing up to 80% biogas by making minimal modifications. This prototype paves the way for developing a demonstrator which, as part of the COGEPRO project, will be installed at the industrial site of a brewery. It will run on biogas from process wastewater treatment.
Finding the right fit
Although biogas offers many prospects in the production of low-carbon energy, the challenge remains the efficient and competitive production of this energy carrier. At the same IMT Atlantique laboratory, researcher Yves Andrès is exploring different processes for treating biomass waste. The two researchers and their team are combining several areas of expertise (e.g. microbiology, energy and process engineering) to study the most suitable recovery sectors, based on the waste or residues available in a given region and the energy balance of the chosen solution. Their work focuses in particular on integrating methanization into these recovery sectors, and uses for the biogas produced.
The choice of methanization is above all based on a positive or zero energy balance. The IMT Atlantique scientists have complex calculation and modeling tools to study the viability and sizing of equipment. The sizing is dependent on the biogas production capacity and is therefore primarily linked to the nature and availability of the input materials and waste that will feed the methanizer. “Our goal is to estimate the minimum amount of input materials required to avoid supplying the facility with more energy than it produces,” Yves Andrès explains. If the energy balance is favorable, modeling the future quantities of biogas recovered makes it possible to determine the size of the facilities for functional production; with good yield and stable quality over time.
The scientists are therefore helping to establish facilities with various scales. This can range from micro-methanization facilities ensuring energy-self-sufficiency for an eco-district or small farm to high-volume bioreactors. “We helped a company managing a technical landfill center renovate its energy recovery facilities,” Khaled Loubar says. “To determine the right size for the new equipment, we typically had to model the quantities of gas the center would produce over the next 15 to 20 years!”
From waste to dedicated crops: productivity deviance
That said, as in any industry, the challenges facing operators are not limited to knowing but also to increasing their production capacity. Does that mean methanization is not simply limited to the available waste? The answer is no. On the one hand, as in the case of agricultural facilities, it is sometimes necessary to use “external” input materials that are not subject to seasonal fluctuations to ensure the continuous supply of the reactors. On the other hand, mixing various input materials directly affects biogas composition and quality. “For example, waste can produce too much carbon and not enough nitrogen when it decomposes. It is then interesting to add an additional nitrogen-rich input material to balance out and improve the biogas production yield,” Yves Andrès says. In some cases, this involves using crops or raw materials that are not necessarily waste.
Yet the researcher warns against certain excessive practices. Operators can significantly increase a methanizer’s yield by feeding it with high-energy biomass, with starches, flours and sugars… “We do not take this approach in our work,” Yves Andrès stresses. “But in Germany, for example, milk producers who initially started methanization with cow slurry realized that they could greatly increase their profits if they switched to only producing corn for methanization. They therefore stopped producing milk.”
In France, the use of energy-producing intermediate crops (CIVE) is tolerated because it does not compete with food crops. The practice makes it possible to replenish poor, overworked soils, or those affected by saltwater intrusion, for example. The CIVE then serve as “remediation” crops, which feed methanizers, while also restoring agronomic quality of the soil. “Our goal is not for this kind of crop to be produced long-term. This is why we do not necessarily focus on the best possible yield for the reactors at our laboratory, but on the recovery of waste and residues,” the researcher adds.
