Operation Performance Improvement And Application Of The CO₂-Reducing Electromethanogenesis System

The burning of fossil fuels emits a large amount of CO₂,leading to a serious greenhouse effect,and renewable energy sources such as solar or wind energy have been widely developed.However,renewable energy is unstable and requires supporting energy storage equipment to increase the grid connection rate and maintain the stable operation of the grid.Therefore,mankind urgently needs to develop carbon-neutral energy storage technology to promote the process of peaking carbon dioxide emissions and carbon neutrality.Electromethanogenesis(EMG)is a new type of green energy conversion system.It uses methanogenic biofilm as a biocatalyst to achieve CO₂ reduction under mild reaction conditions and convert the excess renewable energy into chemical energy of methane.This technology has the advantages of low cost,high conversion efficiency,high product selectivity,and was environmentally friendly.However,at present,EMG has technical bottlenecks such as low current density,low methane production rate,time-consuming start-up process of the methanogenic biocathode,poor operational stability and power density that not reaching the application level of energy storage,which make it difficult to put into practical use.In order to solve the above-mentioned problems,this paper carried out research on the start-up and operation of the methanogenic biocathode using electrochemical technology.At the same time,the scale-up EMG system was modeled and predicted based on the artificial neural network and the adaptive fuzzy inference system,and the main influencing factors of the cathodic methane production performance were analyzed.The results achieved are as follows:(1)Three electrochemical techniques of galvanostatic(GS),applying constant voltage(ACV),potentiostatic(PS)for the start-up and operation of the biocathode were studied.It was found that the methane production rate of the biocathode enriched by the ACV method was145%and 238%higher than that of the GS method and the PS method,respectively,and its charge transfer resistance was only 45%of the PS method and 71%of the GS method.It was revealed that the supply of sufficient reducing equivalents(e^-or H₂)is more important than changing the electrode potential to adjust the composition of the microbial community for the enrichment of high-efficiency methanogenic biofilms.(2)Developed an intermittent step-up voltage(ISUV)strategy that quickly enriched the granular activated carbon biocathode with high specific surface area.The start-up of the biocathode enriched by ISUV strategy was shortened to 15 days,the current density reached23.39 A m^-2,and its methane production rate(65.66 L m^-2 d^-1)was 11.7 times higher than the intermittent constant voltage strategy(IACV);The stability and recovery speed of EMG under fluctuating and intermittent power input was increased by 56%and 500%,respectively.It was found that the improvement in operation performance and stability was due to the enrichment of compact biofilms containing nanowires in the ISUV strategy,which had high coverage,high population abundance and diversity.This research provided ideas for solving the problems of poor EMG operation stability and low methane production rate.(3)Developed an 18 L scale-up semi-continuous flow double-chamber EMG operating under the ISUV strategy and achieved a power density of 173.28 W m^-2.Based on artificial neural network(ANN)and adaptive neuro-fuzzy inference system(ANFIS),a predictive operation performance model of dual-chamber EMG was constructed.It was found that the prediction accuracy R² of ANFIS on the four output variables of average current,methane production rate,cathode methanogenesis coulombic efficiency,and power-to-methane conversion efficiency were all above 0.9971,which was higher than the ANN model.Sensitivity analysis showed that the operating period had the largest weight on the average current(41%);the applied voltage had the largest weight on the methane production rate(41%).This research provided theoretical basis and instruction of key technology for the scaling-up of EMG.(4)For the first time,the electrochemical oxidation-methanogenesis(EO-M)coupling technology was proposed,and a system was constructed to degrade the antibiotic ciprofloxacin with low energy consumption while reducing CO₂ to produce methane.The methanogenesis rate of 86.4 L m^-2 d^-1 and the CCE-CH₄ of 71.76%were obtained.At the same time,99.99%of ciprofloxacin and 90.53%of chemical oxygen demand were removed;the energy consumption of degrading ciprofloxacin was 3.03 Wh L^-1 lower than that of a single electrochemical oxidation system.Through the analysis of cathode biofilm morphology and microbial community,it was found that the flagella-containing methanogenic bacteria Methanobacterium movens was enriched on the cathode,which might promote the direct electron transfer of biocathode mediated by the flagella,thereby reducing the cathode overpotential.This research provided a new idea to solve the problem of high energy consumption in traditional electrochemical oxidation process,which was caused by the difficulty of H₂ recovery and the low potential of cathode.