Conversion of H2 and CO2 to CH4 and acetate in fed-batch biogas reactors by mixed biogas community: a novel route for the power-to-gas concept

Pressing deterioration of the global climate by human activities demands the large-scale replacement of fossil fuels with renewable energy carriers [1]. The most rapidly developing and spreading renewable technologies worldwide include the conversion of wind energy and direct solar energy (photovoltaics) to electricity. In view of the discontinuous electricity production by these technologies, coupled with fluctuating utilization, severe electricity storage problems arise, which are already apparent in countries where the implementation of renewables is well advanced. A likely solution of this emerging setback is conversion of electricity to alternative energy carriers [2] or chemicals [3]. Hydrogen (H2) can be generated via electrolysis of water, a well-known and efficient process [4]; however, technologies to store and transport H2 are underdeveloped at present. Methane (CH4) is an obvious next candidate. CH4 can be transported and stored conveniently in the existing natural gas grid and can be used in all applications where fossil natural gas is employed today. Biogenic CH4 production takes place during anaerobic degradation of organic matter in biogas reactors, swamps, ruminants, termites, etc. [2]. The last step of these complex microbiological metabolic pathways is biogas formation by methanogens. These microbes are strict anaerobes belonging in the kingdom Archaea. Some methanogens split acetate and release a mixture of CH4 and CO2 (acetotrophic methanogens), while others form CH4 by reducing CO2 with H2 (hydrogenotrophic methanogens) and there are methanogens which are able to carry out both reactions.

An additional advantage of the biological conversion of electricity to CH4 is offered by coupling the process with CO2 mitigation. CO2 can be transformed by catalytic reduction using chemical reactions [5, 6], photosynthesis [7], bioelectrochemical processes [8-10], or methanogenesis [2].

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Three main ingredients should be present to form biogenic CH4 from CO2: hydrogenotrophic methanogens, CO2, and a suitable reductant. Recent metagenomic studies have revealed that hydrogenotrophic methanogens predominate among Archaea in most biogas microbial communities [11-17].

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CO2 can originate from flue gas [18] or from the biogas itself [19-22]. In the latter approach, a significant upgrading of the produced biogas has been achieved. In some cases, the anaerobic degradation of the biomass has provided the electron source [18, 23]; in other studies, H2 gas has been employed [19, 20, 22, 24]. These experiments have revealed that the poor solubility of H2 limits the yield and rate of CH4 formation. In these configurations, H2 is injected into a methanogenic reactor filled with a microbial consortium.

In the present study, fed-batch fermentation systems with the daily dispensing of H2 gas were employed in order to partially overcome the H2 solubility problem. Several operational conditions (see “Methods” section) were tested under mesophilic conditions and efficient CH4 productivity was attained. Moreover, at the appropriate combination of CO2 and H2, the simultaneous formation of acetate and CH4 as main products was observed.

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