A laboratory from the University of Pau and Pays de l’Adour is developing a dynamic model which makes it possible to optimize the valorization of CO2 present in biogas in order to produce methane. It will be able to control all the parameters of the methanization unit, as well as all the operating variables of the methanation process.
The biogas produced by methanization from agricultural waste mainly contains methane. Another gas is also present inside: CO2, the proportion of which varies between 30 and 50% depending on the nature of the biomass. Currently, most operating units release this CO2 into the atmosphere. As part of a project called Optimeth, the Thermal Energy and Process Laboratory (LaTEP) of the University of Pau and Pays de l’Adour is developing a dynamic model whose aim is to optimize the valorization of this CO2 by methane.
“We are developing a digital model to control all the parameters that make it possible to produce methane from CO2 from biogas, explains Sabine Sochard, lecturer at ENSGTI (National School of Industrial Technology Engineering). It will be able to predict, over a determined period, all the operating variables of the system so as to optimize a criterion, which can for example be the cost of producing this methane, while imposing constraints, such as that consisting of responding to a demand for methane for an end consumer. This tool will allow us to carry out control and planning. »
To produce methane from CO2, it is necessary to carry out the Sabatier reaction, which consists of adding hydrogen. And for this operation to be completely “green”, this hydrogen must be produced from a renewable source, in this case solar panels or wind turbines, which supply electricity to a electrolyzer of water.
Weather conditions are one of the main parameters to take into account, because they act on several levels. First of all, on the production of renewable energies, which is by nature intermittent. For example, faced with a lack of sunshine and the risk of no longer having enough hydrogen, the model will be able to calculate the optimal hydrogen storage volume so that this gas never runs out in order to implement Sabatier’s reaction”, add Sabine Sochard.
Avoiding a runaway of the catalytic reactor in the methane reactor
Weather variations, and in particular the outside temperature, also affect the methanizer, inside which there are anaerobic bacteria responsible for producing biogas. For them to grow optimally, the temperature of the unit must remain constant, often at values between 35 and 40 degrees. To maintain it at this level, the digital model must then activate or not a heating system for the methanization unit.
Temperature management is also a variable to take into consideration, but this time inside the methane tank, that is to say the enclosure where the Sabatier’s reaction and which has the particularity of being exothermic. To avoid a runaway of the catalytic reactor, it is important to adjust the flow rate of the coolant which allows the reactor to be cooled. This operation will be carried out automatically by the digital model. The latter will also take into account the nature of the agricultural waste introduced into the methanizer, which determines the share of CO2 present in the biogas.
To make this new tool work, researchers are trying to represent all the physical and chemical phenomena that occur at the level of the methanization unit and that of methanation. It differs from models using artificial intelligence algorithms which attempt to make predictions from input data. “Our tool is what we call a knowledge model, specifies Sabine Sochard. For example, for methanization, we rely on an already existing model called ADM1 (Anaerobic Digestion Model No. 1) and which traces all the methanization reaction patterns and their kinetics. We also model all heat flows exchanged with the outside world, as well as the nature of agricultural waste qwhich determines the type of macromolecules found in the digester. »
The scientists teamed up with the University of Turin, which specializes in methanization, and tested this model on different types of biomass, including corn silage. And they have already validated the dynamic model of the methane tanker, which implements the Sabatier reaction. The main challenge now is to combine these two models to optimize the entire process for producing methane from CO2 from biogas.