What is the Fischer-Tropsch Process?

Introduction

Occasionally when reading about renewable energy sources and alternative fuels you see the oddly-named Fischer-Tropsch process touted as a possible replacement for fossil fuel-derived oil products like diesel and jet fuel. This sounds great, but it's not the whole solution. It's not a completely new or greener fuel; in fact it's virtually the same as conventional refined crude oil fuels, just from a different source. While we should be looking for carbon-neutral energy sources, in the short term we're going to need something to bridge the gap while we reduce our dependence on oil.

The Fischer-Tropsch process involves the conversion of a mixture of gases to liquid fuel, potentially allowing the use of waste biomass for fuel production.

Let's investigate further:

The Inventors

Franz Fischer and Hans Tropsch
Franz Fischer and Hans Tropsch

History

The process was developed by two German scientists, Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s. At the time there was a shortage of oil in Germany, forcing research into alternative ways of producing liquid fuels. However, coal was plentiful, and Fischer and Tropsch discovered a way to convert coal to a mixture of gases (syngas) and then to liquid hydrocarbon fuel i.e. gasoline, diesel and kerosene (jet fuel).

The process is expensive, energy-intensive and produces relatively small amounts of fuel compared with traditional crude oil refining, so interest in the technology outside of research waned. However, recently it has been re-investigated as the price of oil rises and concerns about remaining oil reserves increase.

The United States Department of Defense has a particular interest in Fischer-Tropsch fuels as if the country can produce more of its oil requirements itself reliance on foreign oil will be reduced. The world's largest exporters of crude oil are found in the Middle East, a region noted for political instability, so other countries have a strong interest in becoming self-sufficient in their fuel requirements. The US Air Force has tested a 50:50 blend of FT:conventional fuel in its aircraft successfully, and has a commitment to obtain 50% of its aviation fuel from alternative sources by 2016.

Fischer-Tropsch can also be economical in countries which have no oil of their own but reserves of other fossil fuels. The South African company Sasol uses coal and natural gas as a feedstock for the Fischer-Tropsch process which it uses to produce most of the country's diesel fuel.

The Sasol Fischer-Tropsch plant in South Africa
The Sasol Fischer-Tropsch plant in South Africa

What is the process?

The Fischer-Tropsch process involves the conversion of syngas, a mixture of hydrogen and carbon monoxide, into liquid fuels.

Syngas is produced from coal, natural gas or biomass either by incomplete burning of the fuel or by a process called gasification which heats the starting fuel in the presence of a controlled amount of oxygen and steam to give the best syngas mixture.

The syngas is passed at high temperature and pressure over a catalyst which speeds up the reaction of the gases together to form larger products. The catalyst used is often iron, although others that have been used include cobalt and nickel. In the diagram below iron is used because the syngas has come from coal which contains a lot of impurities such as sulphur. Iron is more resistant to "catalyst poisoning" which makes more sensitive catalysts such as cobalt less effective.

A number of products are formed in the reaction chamber, the most important of which are alkanes, which are chains of carbon atoms with hydrogen attached. The longer the chain the heavier the fuel. The lightest product is methane, longer chains give liquid fuels such as gasoline and kerosene, and the longest chains give paraffins and waxes. Obviously the medium length products are the most useful, and the reaction conditions are carefully controlled to ensure as much of these as possible are produced.

The products are separated, cleaned and may be processed further to increase yields of desirable products, then are ready to use. The whole process is summarised in the diagram below.

Fischer-Tropsch - from coal to liquid fuel

What's the catch?

Sounds great, doesn't it? Well, not quite.

As I said it's a very energy-intensive process and although it may seem an attractive way to reduce overall carbon emissions by using waste biomass to generate fuel, it uses a lot of energy and results in an expensive fuel. This means it's not commercially viable with oil prices at a reasonable level. If in the future crude prices rose dramatically it could become more of an option.

The fuels produced are very pure which may sound like a good thing but actually brings its own set of problems. Fuels refined from crude oil contain a small number of aromatic compounds (carbon compounds with a benzene ring), branched-chain alkanes and other impurities containing nitrogen and oxygen. These have lubricant effects in engines and help to swell rubber seals in fuel systems to prevent leakage. This is why currently FT fuels are used in blends or have to have small amounts of impurities added to ensure efficient operation of today's engines.

There is more work to be done...

Fischer-Tropsch diesel on the left, conventional on the right. Note the difference in colour due to impurities in the conventional fuel
Fischer-Tropsch diesel on the left, conventional on the right. Note the difference in colour due to impurities in the conventional fuel

In the future

Research into improving Fischer-Tropsch efficiency and commercial reliability is ongoing. Much of the work is funded in the US, including research into improving the lubricant and rubber-swelling properties of pure FT fuels.

The US Navy has researched using electrolysis to split seawater into hydrogen and oxygen, then using a modified FT process with the carbon dioxide already dissolved in the water to produce short chain alkanes.

Investigation is underway into the use of solar power to convert waste carbon dioxide into carbon monoxide, which can then be used in the FT process. This would have the dual effect of using energy from a renewable source whilst reducing carbon dioxide emissions from other industrial processes.

There is potential for the Fischer-Tropsch process to be useful in the future, but currently it is prohibitively expensive and is not widely used on a commercial scale.

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