Modeling of anaerobic digestion and its coupling with catalytic methanation

With a growing share of energy being produced from renewable sources, new challenges emerge on how to deal with this new energy paradigm, where production is highly affected by conditions humans cannot manipulate. In the EU, wind and solar energy capacities have rapidly increased in recent years. Solar energy production peaks during sunny days and ceases at night, while wind energy generation fluctuates with wind speed. This variability in electricity production will have to be accounted for in the future energy grid, which will need new energy storage solutions and grid management strategies. In this scenario, the development of multi-energy platforms will be required, as one source can compensate for or store the energy produced by another. In addition, it will enable to explore synergies from these different systems. Integrating methanation and anaerobic digestion, for instance, represents an alternative to convert renewable electricity into gas in a power-to-gas approach. Moreover, it enables the energetic valorization of CO2 from biogas. However, these two reactors are operationally different in terms of their characteristic time and thermal behavior. For this reason, mathematical models can play a valuable role in defining their adequate management. In such context, at first, a model for a biogas digester was developed. This model contains the kinetics from a modified version of the Anaerobic Digestion N°1 (ADM1), one of the most widespread models in this domain, along with an advanced thermal description based on empirical correlations for the heat transfer terms. In addition, the biogas storage in an air-inflated double-membrane gasholder was also included in this work. The digester model was validated using literature data for both lab-scale and large-scale digesters. It was also used in a study case to evaluate the influence of geographical location of the biogas plant on its heat losses. To simulate an entire biomethane plant, the process units composing the biogas cleaning and upgrading stages were also modeled. Integrating these models with the digester 16 allowed for the evaluation of the entire system’s functioning. In addition, an existing model for catalytic methanation developed in a previous PhD thesis carried out in LaTEP was included in this simulation environment. This allowed simulating the whole system: biomethane and catalytic methanation plants. Simulations of this multi-energy platform showed that this solution would be able to store excess energy from the grid, as the start-up of the methanation reactors could started up within a few minutes. Besides, it was shown that the production in these reactors would be affected by seasonal variations in weather conditions. Finally, insights into the process safety of this system and potential opportunities for heat integration were discussed.