Decarbonisation Technology May 2022 Issue

When the Fisher-Tropsch process is coupled with a biomass gasification facility, sustainable liquid fuels can be produced for aviation and marine propulsion Economic viability of biomass to liquid via Fisher-Tropsch

Lorenzo Micucci Siirtec Nigi SpA

G lobal warming prompts limiting the Earth's average temperature rise to less than 2°C. To achieve this goal, greenhouse gas emissions must be reduced. Notably, the anthropogenic carbon dioxide (CO 2 ) emissions from burning fuels of fossil origin must be reduced to achieve carbon neutrality by 2050. The transportation sector is a major source of CO 2 emissions to the atmosphere, and aviation fuels are one of the more difficult transport modes to decarbonise. Despite the efforts being made to find alternative fuels for aircraft and vessels propulsion, liquid fuels remain the most practical solution. Producing synthetic liquid fuels from biomass via Fisher-Tropsch technology (BTL-FT) is a way to decarbonise the transport sector. This article discusses the fundamentals of this technology and spotlights the conditions under which the economic viability of BTL-FT investment is assured. Fischer-Tropsch process The Fischer-Tropsch synthesis process (FT) involves the non-selective polymerisation of carbon monoxide (CO) under reductive conditions. The polymerisation is catalysed by most Group VIII metals, notably iron or cobalt- based catalysts, typically supported on SiO 2 , TiO 2 , or Al 2 O 3 . Due to the lack of selectivity, a wide variety of side reactions occur; hence, the synthesis products include alkanes and alkenes with a very broad composition, along with oxygen- containing compounds, mainly alcohols, carbonyl compounds, acids, and esters. The product distribution depends on the H 2 /CO in the syngas, the catalyst employed, the reactor design, and

the operating conditions, most notably the operating temperature. Cobalt-based catalysts give a higher yield of middle distillate products with much less oxygenated relative to the use of iron-based catalysts. They show higher selectivity for paraffinic derivatives at low temperatures; hence, they can be used to produce sustainable aviation fuel (SAF). At high temperatures, however, an undesired quantity of methane forms. Thus, this type of catalyst is not suitable for high- temperature FT processes. Iron-based catalysts are relatively inexpensive, tolerate flexible operation conditions, and are suitable for synthesis with low H 2 /CO ratio syngas – typically derived from low-quality feedstock such as biomass – although it produces a significant quantity of non-paraffinic derivatives as byproducts. As the FT reactions are highly exothermic, the accuracy of the reactor temperature control significantly impacts the products (paraffins and/ or olefins). In principle, syngas can be produced from any carbonaceous feedstock, including biomass and organic wastes. The FT process architecture may be either an open loop or a closed loop, depending on the feedstock to be processed. In an open-loop scheme, the light ends are separated from the cooled reactor outlet and used to generate electric power for the FT process and export to the grid. In a closed loop, part of the light ends can be recycled back for further conversion to synthetic liquid fuel, while the remaining part is used for power generation. The product from an FT plant is a synthetic crude analogous to crude oil of fossil origin, albeit