Power-to-gas through CO2 methanation: Assessment of the carbon balance regarding EU directives

The transformation of carbon dioxide (CO2) into fuels is receiving a growing attention from engineers, scientists and policy makers. For instance, CO2 and hydrogen produced with renewable electricity can be combined to provide methane (power-to-gas). This strategy could store temporary surpluses of renewable electricity, but the lack of accurate carbon balance makes it difficult to establish the regulatory status of the final product, particularly when CO2 is derived from fossil resources. This article describes an adapted methodology assessing the carbon balance of power-to-gas. The emissions of greenhouse gases per unit of energy have been calculated for different scenarios, varying the source of electricity and the origin of carbon dioxide. The methodology allows accounting for the energy contained in the initial fuel that provides the carbon dioxide. When the CO2 is derived from an industrial process (e. g. cement industry), the energy content of the final product has been estimated and accounted for. The results demonstrate that power-to-gas should use electricity surpluses as much as possible. Biogenic and atmospheric CO2 are the most interesting sources, but the utilization of fossil CO2 makes sense in given situations. Results also suggest that current EU directives on renewable energy are not perfectly suited to power-to-gas. (C) 2017 Elsevier Ltd. All rights reserved.