The carbon footprint of power-to-synthetic natural gas by photovoltaic solar powered electrochemical reduction of CO2

Abstract

The search for more sustainable production and consumption patterns entails the integration of emerging edge-cutting technologies. Holistic studies are needed in order to accurately evaluate properly the environmental competitiveness of the suggested solutions. Among those alternatives, it has been suggested the utilisation of CO2 for the production of synthetic natural gas, the so-called Power-to-Gas (PtG) technology. In this work, we use the PtG technology to analyse the environmental rationality in terms of the carbon footprint (CF) of a Photovoltaic (PV) solar powered Electrochemical Reduction (ER) process for the utilisation of CO2 as carbon source for the production of CH4. This synthetic natural gas is ready to be injected into the transmission and distribution network. The raw materials for the process are a source of CO2 (mixed with different ratios of N2), H2O and electricity from PV solar. The separated products are CH4, C2H4, H2/CO, O2 and HCOOH. The reaction, separation/purification and compression stages needed to deliver commercially distributable products are included. Mass and energy balances were used to create a black-box model. The input to the model is the faradaic efficiency and cathodic potential of the best cathodesperforming at lab-scale (over 60% faradaic efficiency towards CH4). It was assumed that cathodes were long-lasting. The output of the model is the distribution of products (related to 1 kg of pure CH4) and the energy consumption at each of the aforementioned stages. The overall CF is then calculated as a function of the CF PV solar reference and the total energy consumption. The effect on the distribution of each stage to the total energy consumption of both the purity of the CO2 stream and the conversion of CO2 in the reactor was analysed. The results show that the principal contributor to the total energy consumption is the ER of CH4 across all CO2concentrations and conversions. When a CO2 conversion of 50% is chosen together with an inlet stream with a N2:CO2 ratio of 24, the electricity consumption of the process is between 2.6 and 6.2 times the minimum obtained for a reference ER reactor including the separation and compression of gaseous products (18.5 kWh kg−1 of CH4). The use of PV solar energywith low CF (14⋅10−3 kg kWh−1) allows the current lab-scale performers to even the CF associated with the average world production of natural gas when the valorisation of C2H4 is included (∼1.0 kg kg−1 of CH4).