Fed-Batch Ex-Situ Biomethanation: laboratory study and virtual modelling

To reduce carbon emissions, renewable energy sources are gradually becoming more and more common and new ways to exploit them are constantly researched. Among the different technologies, the concept of Power to Fuel allows the production of a fuel that is easy to use and to store, using only green electricity. In this work a Simulink model of a fed-batch ex-situ bio-methanation reactor is created, with the goal of assessing the impact that different variables have on the methane production and predict the results that can be obtained with a specific system. Before the construction of the model, a laboratory scale reactor was studied, and data were collected to then verify the correct working of the model. In the Introduction chapter, the reasons behind the transition towards green energy sources are explained, along with the consequences of this transformation. The concept of Power to Fuel is described and the main advantages of the technology are reported. The processes of green hydrogen production and of carbon capture are analysed, being the fundamental bricks for all the Power to Fuel transformations. The chapter related to the description of the bio-methanation technologies starts with the explanation of the difference between the in-situ and the ex-situ processes. Then the microorganisms that populate the system are introduced and classified, along with the different types of reactors and their main characteristics. As the most important parameter influencing the methane production rate, the gas-liquid mass transfer mechanism is described and possible ways to increase it are mentioned. At the end of the chapter the typical conditions present inside a bio-methanation reactor are discussed. The third chapter is about the experimental data collected from the laboratory scale reactor and starts with a comparison of the results obtained by different researchers with the ex-situ bio-methanation process. Then the laboratory setup and the functioning of the reactor are described, before the analysis of the main data collected and their variations in time. At the start of the fourth chapter, previous virtual models of biological reactors are discussed, in particular the Anaerobic Digestion Model N. 1, developed in 2002. Then the equations constituting the model are reported, along with the values chosen for the different parameters. To find the values of the gas-liquid mass transfer rate and microorganisms concentration parameters, that have a big impact and are difficult to quantify in the laboratory, the pressure curves simulated are matched with some of the experimental data collected. The pressure curves plotted with different values of these two parameters are then compared, along with the variations of the resulting Methane Evolution Rate and methane presence values. At last, the influence of the pressure inside the reactor is studied. From the analysis it’s clear that the gas-liquid mass transfer rate has a major impact on the methane production and allows higher performances without modifying the biological population. Also the value of the concentration of hydrogenotrophic methanogens is fundamental to predict the behaviour of the reactor. Depending on this parameter, the increase of the kla or of the internal pressure has a different impact, which can be worth the increase or not. Considering the performance required, it’s possible to use the virtual model to understand the characteristics needed from the reactor and to evaluate if a certain modification is beneficial enough.