Decarbonisation with CO2 utilisation: review of the GtL route
A re-examination of the synthetic fuels path with CO2 used as a feedstock in the context of decarbonisation.
Hellenic Petroleum, Part of HelleniQ Energy Holdings S.A.
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Climate change has triggered a rapid expansion of renewable power generation in the electricity sector and a massive turn to the development, scaling-up, and implementation of decarbonisation technologies in the industrial sector.
Among the proposed technologies, carbon capture is considered one of the first necessary steps. It is expected to play a significant role in the EU industrial sector, especially for cement production facilities and oil refineries with a dedicated hydrogen production unit – a steam methane reformer (SMR) – as they both need to cut their costly Scope 1 CO₂ emissions. Options for the safe and cheap transportation and storage of CO₂ are being examined and appear to make sense on an industrial cluster level.
In the meantime, the same CO2 emitters (oil majors and mid-sized oil companies in the EU) have played and continue to play a significant role in the development of large-scale wind farms and solar parks, along with other traditional and non-traditional players (such as power utilities, funds). This leads to an excess of renewable power generation, which stimulates initiatives to re-examine the utilisation of CO2, especially as refineries are committed to reducing CO2 emissions and providing alternative fuels for road, maritime, and aviation sectors. This re-examination may provide useful insights and feasible alternatives that supplement the primary option of developing CO2 sinks, either through land use, land use change, and forestry or underground storage in depleted oil field formations.
Brief look at history of GtL technology
One of these utilisation routes is the well–established gas to liquids (GtL) process. The origin of GtL technology dates back to the 1910s in Germany, where scientists discovered that mixtures of hydrocarbons, oxygenated compounds, and water could be produced, under certain conditions, from the catalytic reaction of syngas (carbon monoxide with hydrogen). In 1913, Friedrich Bergius patented a process for hydrogenating brown coal to synthetic fuels. Later, in the 1920s, two other chemists – Franz Fischer and Hans Tropsch – developed the Fischer-Tropsch (FT) process to convert syngas to synthetic fuels. After extensive research on different catalysts, pressures, temperatures, and reactor designs, the process was commercialised by Ruhrchemie in 1932 and nine industrial-scale plants were commissioned in Germany. These plants were essential for producing synthetic fuels in the 1933-1945 WWII period, with an estimated peak of ~600 ktpa (~14.0 kbpd). The partnership with several German technology companies, including Lurgi, constituted a key enabler for this development. Later on, Sasol acquired the rights to the FT process for the exploitation of coal reserves in South Africa to produce synthetic liquid fuels via the development of the 5.0 kbpd Sasol-1 plant in Sasolburg in 1955.
The impact of sanctions on oil imports in South Africa during the Apartheid era and then the world oil crisis in the early 1970s acted as catalysts for existing and new players to further develop, improve, and optimise the 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 CO2 through the reverse water gas shift (RWGS) endothermic reaction:
CO2 + H2 « CO + H2O , ΔΗ = 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 CO2 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 H2: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 H2:CO2 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, CO2, and H2O and, in the synthesis section, on hydrocarbon species of C1 up to C30 (mainly paraffins), while the WGS side-reaction of CO with water (i.e. CO + H2O « CO2 + H2) is considered inevitable in the FT reactor, but with limited effect.
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