Methanol-fired Allam cycle: combining oxy-fuel power cycles with synthetic e-fuels

Direct-fired supercritical CO2 cycles combine oxy-combustion with supercritical CO2 to achieve high thermal efficiencies in power plants with carbon capture. The Allam cycle is a transcritical CO2 oxy-fuel cycle, named after its inventor. It achieves the highest thermal efficiencies compared to other oxy-fuel cycles, with a relatively simple configuration. Currently, most of the research and the commercial development of this cycle focuses on the use of natural gas or coal-derived syngas as a fuel, and oxygen obtained in cryogenic air separation units. Only recently, a few studies have investigated the integration of water electrolysis units that provide the oxygen for combustion. Instead, it appears that currently there are not studies about the use of biofuels or e-fuels in the Allam cycle. However, their use would allow to achieve net negative CO2 emissions and, possibly, to mitigate the issues of CO2 storage. This paper proposes and analyzes for the first time a methanol-fired Allam cycle. Methanol is produced from CO2 hydrogenation using hydrogen from an electrolyzer (e-methanol). Oxygen for methanol combustion is produced in the electrolysis unit. The methanol oxy-combustion generates a stream of flue gases composed uniquely by water vapour and CO2, being supercritical CO2 used as a combustion diluent. After turbine expansion and recuperation the water vapour is condensed and used as feed to the electrolyzer. Instead, the CO2 is partially recycled to the compressor and partially used for methanol production. The aim of the study is to evaluate the performance of the integrated system, and possibly its improvement by identifying any synergy between the Allam cycle and the methanol synthesis. Distinctive features of turbine expansion deriving from the use of methanol as a fuel are highlighted. The results show the optimum operating conditions of the overall system and the energetic and environmental performance, also in comparison with the natural gas fired Allam cycle.