Biological Hydrogen Methanation (BHM): experimental lab tests

Nowadays in Europe there is a growing attention on renewable energy sources (RES), especially solar and wind power. The increasing development of RES has put emphasis on the problem of surplus power production, due to the randomness of natural phenomena. At the same time, plenty of European Countries, such as Italy, are continuously encouraging the usage of biofuels, e.g., biomethane. The aim of this thesis is to understand whether it is possible to produce biofuels in sustainable ways. Downstream of the Anaerobic Digestion, performed in Wastewater Treatment Plants, biogas is naturally produced by microorganisms. Therefore, this work focuses on the possibility to convert biogas or gas mixtures (carbon dioxide and hydrogen in ratio of 1:4) into biomethane by Hydrogenotrophic methanogens. These microorganisms use hydrogen and carbon dioxide for their metabolic activity, producing methane and water as reaction products. The process is known as Biological Hydrogen Methanation (BHM). In literature there are many scientific papers that could help answering the former question. Indeed, the BHM has been studied for some years in the Northern Europe and researchers are constantly trying to find the best solution, in terms of productivity, gas quality, energy consumption and costs. As a result of the previous laboratory-scale and pilot-scale trials, discussed in the thesis, trickle bed reactors (TBR) and continuously stirred reactors (CSTR) seem to be the best configuration. This is evidenced by methane evolution rate (MER) and methane concentration in the final product. MER represents the net production of methane (CH4) per unit volume of reactor and it is measured as LCH4/(LReactord), thus it is an indicator of the system efficiency. Both Peillex et al., 1988 and Voelklein et al., 2019 show great results performing a BHM with CSTR and working at mesophilic conditions; the former produces 289.8 LCH4/(LReactord) with a CH4 concentration of 97%; the latter reports a MER of 3.7 LCH4/(LReactord) with 96% of methane. Burkhardt et al., 2015 shows that a TBR in thermophilic conditions can produce 1.5 LCH4/(LReactord) with 98% of CH4. The research challenge is to determine the feasibility of BHM performed with a TBR. Therefore, a start-up trial is necessary to understand the behaviour of microorganisms and to estimate the MER. The start-up is performed in a CSTR working in Fed-Batch conditions. The two trials have the same reactants: primary sludge and H2/CO2 gas mixture. The former, coming from a local Wastewater Treatment Plant, is used as source of biomass; the latter, in ratio of 1:4 is the substrate. Mathematical models based on Monod’s theory can be used to design the experimental phase; the model is useful to estimate the growth of microorganisms, which is related to the consumption of substrate inoculated in the reactor.