Bioenergy with carbon emissions capture and utilisation towards GHG neutrality: Power-to-Gas storage via hydrothermal gasification

The low efficiency of renewable electricity storage has been considered as a bottleneck of the scalable and low-carbon Power-to-Gas energy transformation concept. This paper investigates the combination of CO2 biofixation using Spirulina platensis microalgae and catalytic hydrothermal gasification of wet organic feedstock for the storage of fluctuating electricity and direct utilisation of waste CO2. The presented method enables wet microalgae biomass conversion into H2 and C1-C2 rich fuel gas stream using hydrothermal conversion that is valorised further to methane. For bridging the gap between theoretical investigations and the application of this approach, experiments were carried out at elevated temperatures (632.9-717.0 °C) based on a central composite design of the experiment. Biogas upgrading was evaluated by ASPEN Plus flowsheeting software. The results show that the proposed storage cycle outperforms the state-of-the-art biological and chemical-based Sabatier methanations with an overall round-trip efficiency of 42.3%. The optimised thermo-chemical process enables to achieve simultaneously high H2 (9.05 mol kg−1) and CH4 (7.91 mol kg−1) yields with an enhanced 71.23% carbon conversion ratio. Moreover, the environmental and cost evaluations of the currently proposed bio-synthetic process indicate low associated CO2 equivalent emission (99.4 ± 12.6 g CO2,eq kWh−1) with 144.9 €MWh-1 normalised total annual natural gas production cost. Ideally the proposed storage cycle requires less H2 from external sources, effective CO2 utilisation becomes available through the biofixation and hydrothermal conversion of the wet organic feedstock and closed carbon emission cycle can be accomplished.