From biogas to biomethane: Evaluating the role of microalgae in sustainable energy production - A review

Methane, predominantly derived from fossil resources, is a highly utilised energy source due to its high energy density. However, the rapid consumption of fossil fuels has precipitated anthropogenic climate change and resource depletion, compelling the necessity for a transition towards sustainable and climate-compatible energy sources. This review examines biogas upgrading technologies, encompassing physical, chemical, and biological methods, with a particular focus on photosynthetic CO2 fixation to enhance methane purity and meet biomethane purity standards. Biological processes, in particular those utilising microalgae, demonstrate considerable potential for CO2 sequestration and have the capacity to reduce emissions while simultaneously generating valuable byproducts. The review investigates key process parameters, including pH, liquid-to-gas (L/G) ratio, temperature, and gas retention time (GRT), along with the removal efficiency of CO2. The mass transfer of CO2 and the role of microorganisms are also examined. Furthermore, biological, and chemical/physical upgrading technologies are economically and ecologically compared.