Supercritical Direct-Methane-to-Methanol Coupled with Gas-to-Wire for Low-Emission Offshore Processing of CO2-Rich Natural Gas: Techno-Economic and Thermodynamic Analyses

Abstract

A greater H/C ratio and energy demand are factors that boost natural gas conversion into electricity. The Brazilian offshore pre-salt basin has large reserves of CO2-rich associated gas. Selling this gas requires high-depth long-distance subsea pipelines, making gas-to-pipe costly; in particular, gas-to-wire instead of gas-to-pipe is more practical since it is easier to transmit electricity via long subsea distances. This research proposes and investigates an innovative low-emission gas-to-wire alternative consisting of installing supercritical direct-methane-to-methanol upstream to gas-to-wire, which is embedded in an exhaust-gas recycle loop that reduces the subsequent carbon capture costs. The process exports methanol and electricity from remote offshore oil-and-gas fields with available CO2-rich natural gas, while capturing CO2. Techno-economic, thermodynamic and lost work analyses assess the alternative. Supercritical direct-methane-to-methanol is conducted in supercritical water with air. This route is chosen because supercritical water readily dissolves methanol and CO2, helping to preserve methanol via stabilization against further oxidation by gaseous air. Besides being novel, this process has intensification since it implements exhaust-gas recycle for –flue-gas reduction, CO2 abatement via post-combustion capture with aqueous monoethanolamine, CO2 dehydration with triethylene glycol and CO2 densification for enhanced oil recovery. The process is fed with 6.5 MMS m3/d of CO2-rich natural gas (CO2 > 40%mol) exporting methanol (2.2 t/h), electricity (457.1 MW) and dense CO2 for enhanced oil recovery, with an investment of 1544 MMUSD, 452 MMUSD/y in manufacturing costs and 820 MMUSD/y in revenues, reaching 1021 MMUSD net present value (50 years) and a 10 year payback time. The Second Law analysis reveals overall thermodynamic efficiency of 28%. The lost work analysis unveils the gas-combined-cycle sub-system as the major lost work sink (76% lost work share), followed by the post-combustion capture plant (14% lost work share), being the units that prominently require improvements for better economic and environmental performance. This work demonstrates that the newly proposed process is techno-economically feasible, environmentally friendly, thermodynamically efficient and competitive with the gas-to-wire processes in the literature.