Carbon dioxide management via exergy-based sustainability assessment: Carbon Capture and Storage versus conversion to methanol

Carbon Capture and Storage and Carbon Capture and Utilization refer to carbon dioxide management technologies for its removal from flue-gases, followed by carbon recycling or storage, aiming at limiting global warming. For large-scale deployment, geological storage is the most promising alternative but imposes an economic penalty to the emitting process, while the utilization monetizes carbon dioxide contributing to compensate for the large capture costs. The exergy concept builds a suitable framework to measure useful power according to the Second Law of Thermodynamics, such that maximizing exergy efficiency necessarily promotes sustainability. This work applies a novel framework for exergy assessment of processes with chemical reactions, which is employed to evaluate the performance of two methanol production routes from carbon dioxide from power plant flue-gas: the direct hydrogenation and the indirect conversion through natural gas bi-reforming for synthesis gas production. Exergy efficiency of the direct route is about 66.3%, against 55.8% for the indirect one, indicating the lower sustainability of the latter. Carbon capture and storage had the worst Exergy efficiency, even lower than the emission scenario, accounting for 44.8% against 53.5%. Exergy metrics pinpoint low scalability as the main drawback of the utilization technologies, despite high exergy and capture efficiency.