From biogas to biomethane. Biological conversion of H2 and CO2 to CH4

Abstract

Anaerobic digestion of biomass produces biogas with 70-60% CH4 and 30-40% CO2. However, biogas with more than 90% CH4 has higher heating value, can be injected into the natural gas grid or can be used as alternative vehicle fuel. Biogas upgrading aims to increase the CH4 concentration in biogas. In this context, hydrogen assisted biological biogas upgrading has emerged as an attractive method for biogas upgrading. In this process, H2 produced by water electrolysis using off-peak electricity surplus from wind power is coupled with the CO2 contained in the biogas to convert them to CH4 via hydrogenotrophic methanogenesis (Power-to-Gas). Currently, it can be defined in two concepts namely ex-situ and in-situ depending on where the H2 is provided with respect to the anaerobic digestion. In both cases, the H2 gas-liquid mass transfer and the H2 intermittency are the challenges of the process. The aim of the present thesis is to study, develop and optimize the biological biogas upgrading process. To reach the objective, five series of experiments were performed for ex-situ and in-situ processes at thermophilic and mesophilic conditions, employing different reactor configurations (MBRs, TBFs and up-flow reactors) and feedings (H2 + CO2, H2 + biogas and H2 + sewage sludge) at lab and pilot scale. In order to improve the H2 gas-liquid mass transfer, different H2 diffusion systems (hollow-fiber and ceramic membranes, 3-phase system of TBFs and 2 stainless steel diffusers combined with 2 inert alumina ceramic sponges) were evaluated. The effects of gas recirculation (in MBRs) and directional and counter-directional H2 injection to the trickling media (in TBFs) on biomethanation efficiency were studied. Different H2 stop-feeding periods of 1, 2 and 3 weeks and the subsequent H2 reinjection were experienced in order to evaluate the dynamicity of the ex-situ process under H2 intermittent provision. Concerning about the effect of H2 on biogas microbiome, microbial community analysis were carried out. The results obtained in the present thesis demonstrated the feasibility of H2-mediated biological biogas upgrading in both ex-situ and in-situ processes. The results verified that membrane modules can be employed to transfer H2 efficiently, allowing the biological conversion to take place satisfactorily. The ex-situ systems transformed 95% of H2 fed at the maximum loading rates of 40.2 LH2/LR·d (hollow-fiber MBR) and 30.0 LH2/LR·d (ceramic MBR) reaching CH4 production rates of 8.84 LCH4/LR·d and 6.60 LCH4/LR·d, CH4 contents of 76% and 81% and 〖k_L a〗_(H_2 ) values of 430 h-1 and 268 h-1, respectively. Ceramic membranes were proposed to address and solve the long-term bioconversion stability challenge of hollow-fiber membranes at 55 o C. TBF reactors resulted in attractive configurations with promising results for the biomethanation process. The investigated systems, by means of a single-pass gas flow, upgraded biogas with 97% H2 utilization efficiency at H2 loading rate of 7.2 LH2/LR·d, reaching a CH4 production rate of 1.74 LCH4/LR·d and CH4 content of 95%, fulfilling the specifications to be used as substitute to natural gas. The results demonstrated that the injection of the influent gas mixture in counter-flow to the trickling media greatly reduced acetate production compared to the injection with the directional flow of the liquid media. In the in-situ experiment, H2 injection resulted in a 42% increase in CH4 production in comparison with the conventional anaerobic digestion of sewage sludge and 73% CH4 content was achieved while the biodegradation potential was not compromised. Gas recirculation was shown to improve the H2 gas-liquid mass transfer significantly improving the performance of the reactors. Moreover, gas recirculation seemed to have a positive effect on the in-situ biomethanation when the OLR increased. The feasibility of the system recovery to reach the initial conditions of CH4 (production, content and yield) during the intermittent provision of H2 was demonstrated, regardless of the length of the H2 lack. The repetition of the H2 intermittent provision was shown to have a positive effect on the system recovery time, since the reactors recovered faster as more H2 stop/start periods were applied. The selection-effect of H2 on community composition was revealed by microbial analysis. Methanothermobacter, Methanoculleus, Methanospirillum, Methanolinea and Methanobacterium were the hydrogenotrophic archaea genus present.