Biogas upgrading by cryogenic techniques

The scarcity of fossil fuels and the worldwide pollution have led the scientific community to seek renewable energy alternatives. In particular, biogas has become a potential alternative fuel to be employed instead of traditional energies. Biogas is mainly composed by methane (CH4) and carbon dioxide (CO2). To obtain pure biomethane, a proper biogas upgrading to remove CO2 and other minority compounds is needed. For this purpose, upgrading processes have been developed, such as water or chemical scrubbing, membrane separation, pressure swing adsorption, and cryogenic techniques. Cryogenic techniques represent a good option to be optimized because these techniques yield high-purity products, ranging between 95 and 99%. Therefore, we present here a review on cryogenic techniques. In spite of many advantages, the high-energy penalty makes cryogenic techniques commercially inapplicable actually. Several authors have proposed novel configurations to reduce the energy consumption. Cryogenic packed-bed technology was recently tested in a coal-fired plant with an energy consumption of 1.8 MJ/kg CO2. Economic analyses were carried out for anti-sublimation CO2 capture, giving a cost of 34.5 euro/ton CO2. Among the different alternatives of cryogenic hybrid systems, cryogenic membrane processes stand out due to a 54.4% of capital cost savings.