Biogas upgrading by 2-steps methanation of its CO2-Thermodynamics analysis

A thermodynamic analysis of the methanation of CO2 within biogas sets the limits to the achievable perfor-mances, directing process and catalyst development. Practical indications of thermodynamics are elaborated. The large presence of CH4 in the biogas does not hinder the achievement of an almost complete CO2 methanation, even with CH4/CO2 = 3. The smaller the temperature, the higher the CH4 concentration in the dry biomethane, and the lower the residual H2. Results comply with some grid specification even operating at 1 bar, below 400 degrees C, where active commercial catalysts are available. CO in the product is never a concern, in this temperature range. There is no advantage in operating above approx. 400 degrees C. The H2 slip can be further reduced increasing the pressure, but the improvement is most effective with a few bars; 15 bar is already quite good. H2 in excess of the stoichiometric is not useful. The process can be completely autothermal, but adiabatic operation must be avoided. Significant improvements in biomethane purity can be achieved with two methanation steps, with steam condensation in between. Residual H2 can be reduced from 7% (single stage, 1 bar, 300 degrees C), to 1.8% (two stages, at 300 degrees C, and 1 and 15bars, respectively). Partial steam condensation allows to limit coking in the second step, without a remarkable increase of residual H2. The second step cannot be autothermal above 150 degrees C, but the first step provides heat in excess, at 300 degrees C, to support also the second step.