Flue gas treatment by Power-to-Gas integration for methane and ammonia synthesis - energy and environmental analysis

The present paper aims at assessing the carbon and energy footprint of an innovative process for carbon dioxide recycling, with flue gas as feedstock of nitrogen and carbon dioxide. Nitrogen is converted into ammonia through the Haber-Bosch process and carbon dioxide into methane via Sabatier reaction using hydrogen produced by renewable electricity excess. Carbon and energy footprint analysis of the process was assessed based on experimental data related to hydrogen production by electrolysis, methane synthesis via Sabatier reaction, energy consumption and energy output of the process units for flue gas separation, carbon dioxide methanation and ammonia synthesis. A Life Cycle Assessment method is applied, based on the experimental and computational data, both in case of renewable electricity excess and electricity from the grid. Results show that in case of renewable electricity excess, for a functional unit of 1 kg of treated flue gas, the specific carbon footprint is 0.7819 kgCO2eq and energy footprint is 50.73 MJ, which correspond to 4.012 kg and 260.3 MJ per 1 kg of produced hydrogen. In case of electricity from the grid, the specific carbon footprint is 1.550 kgCO2eq and energy footprint is 59.12 MJ per flue gas mass unit. If the carbon footprint is positive, the process indirectly leads to avoided emissions, ranging from 0.673 to 0.844 kgCO2eq kg−1fluegas, thus proving the sustainability of the proposed pathway.