Decarbonisation Technology - February 2023

route. These efforts led to the development of Sasol-2 and Sasol-3 plants in the 1980s, as well as the completion of the 22.5 kbpd Mossel Bay GtL plant in 1992 by Petro SA (the first plant at that time to use natural gas for the syngas production), the commercialisation of Sasol’s Slurry Phase Distillate FT process, and the development of the 34 kbpd Oryx-GtL plant in Qatar in 2007. In the 1970s, Shell started its synthetic fuels research at its Amsterdam labs and later developed its own GtL technology: the Shell Middle Distillates Synthesis process. This was commercialised in the 1990s with the 12.5 kbpd GtL facility in Bintulu and, in the early 2010s, the world-scale 140 kbpd Pearl GtL plant in Qatar. Gas to liquids through RWGS Historically, syngas production started using coal as the feedstock, then later included natural gas. While coal and gas were expedient in their time, the green energy transition has focused further development on applying the FT process with renewable routes to syngas. Gasification of biomass is one possible option. Another is firstly to create syngas from green hydrogen with CO from the reduction of captured CO 2 through the reverse water gas shift (RWGS) endothermic reaction: CO 2 + H 2 ↔ CO + H 2 O , Δ Η = 41.166 KJ/kmol The chemistry of both RWGS and the GtL processes is supported by many years of R&D. But the integration of these two processes into a commercially viable solution on an industrial scale presents huge challenges and inevitably requires strong policy support for two

prerequisites: green hydrogen production and CO 2 capture. The generic set-up of a power to liquids (PtL) plant is shown in Figure 1 . It consists of the electrolytic green hydrogen production section, the RWGS reaction section to produce syngas with a suitable stoichiometric H 2 :CO ratio, the FT synthesis section, and the final hydro- conversion and separation section. Process description and particulars In this study, a generic pilot-scale 30 bpd GtL plant is simulated, comprising the RWGS, FT synthesis, and separation sections. Pilot plants with similar capacities are currently being developed in Bilbao, Spain (Repsol/ Aramco/Petronor) and in Punta Arenas, Chilean Patagonia (Haru Oni Project, Porsche/Enel/ Exxon Mobil/Siemens Energy). The system molar feed ratio of H 2 :CO 2 stands at 3.8:1. A 512 kg/h stream of captured CO2 is used as feed to the RWGS reactor, and it is assumed that there is sufficient RES power for total green hydrogen production of 89 kg/h. These streams are assumed to be available at ambient temperature (20°C) and pressure (1 bar). The streams are mixed and compressed to a pressure of 25 bar. The compression results in preheating the feed, which is further heated to 908°C before being fed to the RWGS reactor. A Gibbs reactor is selected for the RWGS section, while a conversion reactor is selected for the FT synthesis section. The model is based on the pure components hydrogen, CO, CO 2, and H 2 O and, in the synthesis section, on hydrocarbon species of C 1 up to C 30 (mainly paraffins), while the WGS side-reaction of

Heat

Water

Electrolysis

Oxygen

SynOil

Separation

RES surplus power

Hydrogen

Wax hydrocracking

Syngas (CO, H) cleaning

FT synthesis

RWGS

Captured CO

Heavy waxes

Heat

Figure 1 Simplified model diagram of a power to liquids plant