Methanol production via direct carbon dioxide hydrogenation using hydrogen from photocatalytic water splitting: Process development and techno-economic analysis

Abstract

In this work, we demonstrate the process and economic viability of a new production route for methanol utilizing captured CO2 and H2 produced from renewable, photocatalytic water splitting. Initial assessments of H2 production from this approach revealed a low-cost potential through inexpensive feedstock, but with one major drawback. Simultaneous production of H2 and O2 increases the risk of forming an explosive mixture. In our previous work, we identified membrane-based processes to safely recover the H2 product. To reduce the H2 capture costs, we developed a conceptual process comprised of integrated facilities to produce methanol while solving the flammability constraints. The first unit supplies captured CO2, which is used as a raw material and a diluent to recover H2 from the photocatalytic reactors. The tertiary mixture (H2/O2/CO2) is safely processed in an optimized membrane-based separation plant to produce a 3:1 H2 and CO2 mixture. This binary mixture is then used as feedstock for methanol production via direct CO2 hydrogenation. Based on a detailed economic analysis, the break-even value of the methanol produced is higher than the current market price. The main drivers for the high cost are the separation plant and the feedstock costs of CO2. The best-case scenario (for the selected parameters), where the plant lifetime is 30 years with 0.060 $/kWh electricity price and 0.020 $/kg CO2 cost, has a break-even value for methanol at 0.96 $/kg. Based on a target solar-to-hydrogen efficiency of 10% in the photocatalytic reactor, the gross energy efficiency of the process was 6.25%. As a final element, we compared our process with alternative renewable methanol synthesis route from the literature to highlight the differences in electrochemical and thermochemical approaches.