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May-2022

Economic viability of biomass to liquid via Fisher-Tropsch

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

Lorenzo Micucci
Siirtec Nigi SpA

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Article Summary

Global 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 (CO2) emissions from burning fuels of fossil origin must be reduced to achieve carbon neutrality by 2050. The transportation sector is a major source of CO2 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 SiO2, TiO2, or Al2O3.

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 H2/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 H2/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 syncrude components are different for different FT technologies and catalysts.

An FT operated at high temperature yields a syncrude containing light gases, LPG, naphtha, distillate, and aqueous products. Residue/wax, distillate, and naphtha are the major components yielded by a low-temperature FT plant. For both cases, an upgrade or a syncrude refining is needed to produce a more valuable product slate.

The waste energy related to the generation of syngas with the heat produced in the FT synthesis is generally recovered as steam and converted into electric power for internal use and export. Thus, electric power is typically a by-product of the current FT processes. The energy adds to the product slate and contributes a source of revenue.

Economics

The economics of an FT plant are strongly affected by the cost of the carbon-bearing feedstock, the cost of CO2 emissions, the product pricing, and the facility's capital cost.

The cost of feedstock is a sizable component of operating costs, yet its price cannot be controlled because it is source dependent and on the distance from the production and harvesting (or collection and separation of biowaste) sites to the FT facility: the greater the distance, the higher the feedstock transportation costs. The latter is part of the key to biomass price at the fence of the FT plant and, ultimately, to its profitability.

It is worth noting that the SAFs produced by an FT plant are rich in alkanes and may command a price premium depending upon the end users. For example, an extra price is paid for FT-naphtha when used in the petrochemical industry because it gives a higher yield of ethylene than that derived from petroleum. Russian refineries typically blend diesel with an additive to adjust the cetane number. As FT-diesels have a cetane number of 73, compared to 51 in diesel that meets the EN-590 standard, it can command an extra premium on account of the additive savings it delivers when FT-diesel is blended in the diesel pool.

Analysis of the existing FT plant shows that these facilities are capital expensive. Indeed, the capital expenditure for industrial running natural gas FT plants, which benefit from the most favourable technical and economic conditions, ranges from $100,000 bbl/d to $146,000 bbl/d. The capital-intensive character of these industrial installations calls for large-scale production to achieve the economy of scale. In fact, today’s commercial plant capacity spans from 15,000 bbl/d to 146,000 bbl/d. For the case study below, the total cost of the investment was estimated at 174,320 $/bbl/d.


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