Exergy-based evaluation of methanol production from natural gas with CO2 utilization

Energy and exergy analyses were carried out for a medium-capacity methanol plant based on a low-pressure synthesis process for natural gas. The process comprises a pretreatment of natural gas, a steam-methane reforming unit for generation of synthesis gas, a methanol synthesis, a distillation of crude methanol, and an integrated steam cycle for waste heat recovery. Carbon dioxide from carbon capture is used for gas conditioning by adjusting the syngas module for methanol synthesis through counterbalancing of the excess hydrogen. A sensitivity analysis was performed to identify favorable operation parameters for the tubular steam reformer. The energetic and exergetic efficiencies for the overall system were found to be 35.9% and 37.7%, respectively. The specific energy requirements (energy intensities) are 19.6 GJth/tCH3OH and 0.8 GJel/tCH3OH, while the specific methane consumption was calculated to be 0.54 ton CH4 per ton CH3OH. Compared to a stand-alone plant, the utilization of carbon dioxide increases the methanol yield by 22%. The exergy analysis shows that the highest inefficiencies occur in the reforming unit, the steam cycle, and the synthesis unit. In particular, the steam reformer, the synthesis reactor, and several heat exchangers show a high potential for thermodynamic improvement.