Synthetic fuels become hot stuff

Synthetic fuels produced using thermochemical processes tend to be the wall flower of the biofuel sector, staying in the background while bioethanol and biodiesel produced via fermentation grab all the headlines. But in their unassuming way, synthetic fuels could well end up quietly capturing the majority of the biofuel market, at least in Europe. ‘We do feel that when you're looking at second-generation [biofuels], you're probably going to be looking at a thermochemical route before a fermentation one,' predicts Jeremy Tomkinson, CEO of the UK National Non-Food Crops Centre (NNFCC) in York.

The origins of synthetic fuels go back to Germany in the 1920s, where the original thermochemical processes were developed. These are based around the Fischer-Tropsch reaction, where a mixture of carbon monoxide and hydrogen known as syngas is passed over a metal catalyst, often nickel, cobalt or iron. This produces a range of hydrocarbons, which can then be fractionated to produce fuels such as diesel and kerosene. One of the main advantages of this process is that the syngas can be obtained from various different sources, including coal (by heating it with steam and oxygen), natural gas and plant material (both by partial combustion).

Another advantage is that the resultant fuel has exactly the same properties as conventional diesel and kerosene, able to be used as a direct substitute, but is much cleaner. ‘Using a thermochemical procedure, you actually make a diesel fuel that has no aromatics and no sulphur, so it burns very, very clean with less than 10g/km of carbon dioxide emissions,' says Tomkinson. This compares to carbon dioxide emissions of 160g/km for petrochemical diesel and 80g/km for rape biodiesel. The reason for the fuel's cleanliness is that chemical compounds such as carbon dioxide and hydrogen sulphide are removed from the syngas prior to the Fischer-Tropsch reaction to ensure that they don't contaminate the catalysts.

A third advantage is that synthetic fuels are already produced on a commercial basis, although only using coal and gas as feedstocks. So the trick now is to apply the same technology to plant material and build biomass-to-liquid plants, but this won't be a straight-forward process. ‘I can't underestimate that there are very serious challenges and problems to circumvent,' says Tomkinson. But it's worth trying to circumvent these problems because biomass-derived synthetic fuels offer a number of unique advantages.

For a start, the thermochemical processes should be able to handle a wide variety of plant feedstocks, especially waste material such as straw and wood chippings, whereas current enzymatic and fermentation processes tend only to work well with very specific feedstocks. This is a particular advantage in Europe, which lacks enormous supplies of a single biofuel feedstock such as corn (maize).

Furthermore, the same thermochemical processes that produce synthetic fuels can also produce chemicals, heat and power. ‘These biomass thermochemical plants are in excess of 85% efficient, if you combine heat, power, fuels and chemicals,' claims Tomkinson.

A number of companies are now developing such plants. These include existing synthetic fuel producers that are beginning to extend their technology to biomass and organic waste material, such as the US companies Rentech and Syntroleum. As well as companies focused specifically on producing biomass- and waste-derived synthetic fuels, such as the US company BioGold Fuels Corporation and the German company Choren.

Adding further impetus, several European car manufacturers, including Volkswagen, Renault and DaimlerChrysler, are also starting to get behind synthetic fuels. Together with the technology company Bosch, the oil companies Royal Dutch Shell and Chevron, and the South African synthetic fuel company Sasol, they recently formed the Alliance for Synthetic Fuels in Europe to promote synthetic fuels and support research in this area.

A final advantage of the thermochemical processes used to produce synthetic fuels is their flexibility and this is demonstrated by the US company Range Fuels, which is developing a thermochemical process for producing ethanol. It has long been known that methanol can be obtained from syngas, but now Range Fuels has developed a catalyst that can produce a range of other alcohols, including ethanol. In November 2007, it received a $76 million grant from the US Department of Energy to build a plant based on its technology, which it plans to complete this year.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.