Our planet urgently needs a change of energy model, we must advance in the development and implementation of renewable energy sources since the excessive consumption of fossil fuels for energy continues to release tons of CO2 and other greenhouse gases into the atmosphere, which has brought us to this point in terms of global warming.
Although some countries have made great progress in the implementation of photovoltaic and wind power systems and in the electrification of automobiles, there is still a long way to go, especially since trucks, airplanes and ships still need fossil fuels because they have not developed electrical systems that can supply their energy and autonomy requirements.
In this sense, we need alternative sources to produce hydrocarbon-based fuels without having to add more pollutants to the environment, and it seems that the solution lies in a chemical process that is almost 100 years old and by which synthetic fuels could be produced from raw materials that are even available in the air, this process is called Fischer-Tropsch.
Fuels can be produced from feedstock that is normally discarded. Source: @emiliomoron, contains public domain image.
Fischer-Tropsch Process
This process consists of the transformation of a mixture composed of carbon monoxide and hydrogen into liquid hydrocarbons by means of a catalyzed reaction. This process, patented in 1925 by Franz Fischer and Hans Tropsch, allows obtaining different products such as naphthas, olefins and methanol, among others.
The Fischer-Tropsch reaction is carried out under moderate conditions, with temperatures between 200 and 300 °C and pressures between 10 and 40 bar, in fixed or fluidized bed reactors, and the catalysts used are generally based on iron or cobalt.
General scheme of a Fischer-Tropsch fluidized bed reactor. Image elaborated in powerpoint.
Reaction Mechanism
The Fischer-Tropsch process proceeds through a series of chemical reactions in which a variety of hydrocarbons can be obtained that ideally have the formula CnH2n+2. In its most useful form hydrocarbons are produced by the following reaction:
Where, CnH2n+2 represents a product composed mainly of linear paraffinic hydrocarbons of variable chain length, with n typically with values between 10 and 20, which are suitable for obtaining hydrocarbons, although competitive reactions can lead to the formation of alkenes, alcohols and other oxygenated products.
Logically the above reaction takes place in several stages, where several intermediates are produced. Building a hydrocarbon chain can be seen as the repetition of a sequence in which H atoms are added to the C-O bond of carbon monoxide, thus, to produce an –CH2- group from CO and H2 the following steps are necessary:
- Dissociative adsorption of CO and 2H2.
- Cleavage of the C-O bond.
- Transfer of 2H to oxygen to produce H2O.
- Transfer of 2H to carbon to produce CH2.
- Desorption of the products.
Proposed synthesis mechanism. Image elaborated in Powerpoint.
Operating conditions
In general, the process conditions are established according to the hydrocarbon synthesis to be obtained, C1-C15 hydrocarbons, olefins and oxygenates are operated in reactors with Fe catalysts and at high temperatures, while linear hydrocarbons with longer chains can use Fe or Co catalysts and lower temperatures. The operating conditions of both processes are summarized in the following image.
Fischer-Tropsch processes can be classified according to process temperature. Image prepared in Powerpoint.
Catalysts
For Fischer-Tropsch synthesis the most used catalysts are transition metals such as cobalt, iron, nickel and ruthenium, using porous metal oxides such as alumina and zeolites as support. The catalyst based on supported metallic cobalt is the most widely used catalyst for obtaining heavy kerosenes at low temperatures.
temperatures.
| Compound | Content (%Weight) |
|---|---|
| Cobalt | 15-30 |
| Promoter (noble metal) | 0.05-0.1 |
| Promoter (oxide) | 1-10 |
| Alumina (support) | % remaining |
These catalysts are prepared in most cases by the techniques of impregnation of the support with solutions of the metal salt or co-precipitation, with subsequent calcination at a programmed temperature. The activity of the catalyst is enhanced by the use of promoters, although other factors such as the pore size distribution of the support and the synthesis conditions affect the catalytic activity, which is why much of the current research is focused on the development of more active catalysts.
Use of raw materials
Reformed gas
The carbon monoxide used in Fisher-Tropsh synthesis can use low thermal value hydrocarbons from other processes that would be burned or discarded anyway, an important reaction is methane reforming that converts methane to CO and H2. This technology is known as Gas-to-Liquids.
Representation of the conversion of CO2 to methane on the surface of the catalyst. Image elaborated in powerpoint.
Biomass and residue
The advantage of the Fischer-Tropsch process is that any organic compound with high energy is basically suitable to be used as feedstock for the process, so in addition to coal, which was the original feedstock for the process, biogas, agricultural or household waste can be used. In 2014, airlines such as British Airways opted for the production of Fischer-Tropsch fuels from household waste, and began building facilities in London to produce jet fuel from garbage.
Carbon dioxide reuse
Although carbon dioxide is not a typical feedstock for Fischer-Tropsch synthesis, recent developments have made it possible to employ this gas, which can be taken from power plant flue gas exhaust or any other fixed source of CO2 generation. The hydrogen and carbon dioxide are then reacted over a cobalt-based catalyst, producing methane.
I recently came across the news that researchers at the Swiss laboratory ETH Zürich have achieved a breakthrough, designing a method that mimics photosynthesis, whereby atmospheric CO2, water and sunlight are absorbed, converting this raw material into organic compounds.
The process developed by this team of researchers consists of taking CO2 and water from the atmosphere and storing them and then putting them in contact with a basaltic rock that absorbs the unwanted carbon; in a next step, concentrated sunlight is used to heat a cerium oxide based material, which, when heated to a certain temperature reacts with the CO2 and water, this reaction produces CO from the CO2 and Hydrogen from the water, generating oxygen as the only by-product of the process, which can be safely vented to the atmosphere.
The mixture of CO and hydrogen is then used as feedstock to obtain liquid hydrocarbons, such as kerosene or gasoline, through the Fischer-Tropsch process.
General scheme of operation of the prototype. Image created in PowerPoint.
As we can see, this process has made it possible to obtain fuels by taking advantage of carbon monoxide derived from other processes or from the atmosphere itself. This type of approach is necessary, since most of the solutions thought to mitigate climate change go through the electrification of the transportation system, but replacing the total vehicle fleet of the world is something that may take longer than we would like, so with a fuel that is derived from biomass or CO2 from burning fossil fuels, would have a zero carbon balance would be a short-term solution.
For now let's hope that this type of process is developed enough to make it economically viable and industrially efficient to produce fuels in magnitudes similar to the fossil fuels produced today.
Well friends, I hope to have shared interesting information about this interesting way of obtaining hydrocarbons, see you next time!
References
Wikipedia.com. Proceso Fischer-Tropsc.
S.C. Araujo-Ferrer, A. De Almeida, A. Zabala y A. Granados, (2013). Use of catalysts in Fischer-Tropsc process. Revista Mexicana de Ingeniería Química.
Stecker, T. and Pyper, J., (2014). Garbage Fuel Will Power British Airways Planes. Scientific American
Schäppi, R., Rutz, D., Dähler, F. (2021). Drop-in Fuels from Sunlight and Air. Nature