As a clean and renewable energy, biogas is an important alternative to fossil fuels. However, the high carbon dioxide (CO2) content in biogas limits its value as a fuel. 'Biogas upgrading' is an advanced process which removes CO2 from biogas, thereby converting biogas to biomethane, which has a higher commercial value. Microbial technologies offer a sustainable and cost-effective way to upgrade biogas, removing CO2 using hydrogen (H2) as electron donor, generated by surplus electricity from renewable wind or solar energy. Hydrogenotrophic methanogens can be applied to convert CO2 with H2 to methane (CH4), or alternatively, homoacetogens can convert both CO2 and H2 into value-added chemicals. Here, we comprehensively review the current state of biogas generation and utilization, and describe the advances in biological, H2-dependent biogas upgrading technologies, with particular attention to key challenges associated with the processes, e.g., metabolic limitations, low H2 transfer rate, and finite CO2 conversion rate. We also highlight several new strategies for overcoming technical barriers to achieve efficient CO2 conversion, including process optimization to eliminate metabolic limitation, novel reactor designs to improve H2 transfer rate and utilization efficiency, and employing advanced genetic engineering tools to generate more efficient microorganisms. The insights offered in this review will promote further exploration into microbial, H2-driven biogas upgrading, towards addressing the global energy crisis and climate change associated with use of fossil fuels.
Keywords: Biogas upgrading; Biomethane; Carbon dioxide; Genetic engineering; Homoacetogens; Methanogens.
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