Producing diesel and jet fuel in a low-carbon world

For the past month I’ve had a Nature Geoscience discussion article sitting in my blog ideas folder, largely forgotten. When I first came across the paper back in late November, it was under embargo, which indicated that the journal regarded the subject as being of potentially major interest. Going by the relatively low level of online reaction since the piece was published, it would appear that this assumption was in some way flawed.

If so that is unfortunate, as the subject of Harvard geochemist Dan Schrag‘s commentary is surely deserving of wide public discussion. It concerns the production of diesel oil and aviation fuel (kerosene) in a carbon-constrained world.

Fifty percent of the oil currently extracted from the ground is refined into petrol for use in cars an other light road vehicles; the other half is turned into diesel and kerosene. Before long we should be in a position to replace petrol with electric motors, hydrogen fuel cells, and ethanol derived from biomass. But there are as yet no viable substitutes for diesel oil and kerosene when it comes to powering larger road vehicles and aeroplanes. Synthetic versions of these fossil fuels are therefore the only realistic option.

Biodiesel production will be severely limited by the amount of arable land required for crops such as soybeans, and the cost of producing the fuel from algae remains prohibitively high. Is there an alternative? Schrag thinks there is, and looks to an old technology for the solution.

For more than 80 years chemical engineers have known how to produce synthetic diesel oil from coal. In the Fischer-Tropsch process, gasification of coal yields carbon monoxide and hydrogen, together with large amounts of carbon dioxide and methane. When carbon monoxide and hydrogen (syngas) are combined with an iron or cobalt catalyst, the result is liquid hydrocarbons which, depending on pressure, temperature and catalyst composition, include diesel and kerosene.

The problem is what to do with the carbon dioxide and methane. The latter is potentially useful, but not so carbon dioxide, which must be got rid of. Not only is carbon dioxide a greenhouse gas, it is toxic to the Fischer-Tropsch catalyst. Given that the separation of carbon dioxide is required, could the process not become the basis for industrial-scale production of synthetic diesel and kerosene?

In principle it could, but current Fischer-Tropsch plants produce twice as much carbon dioxide as does the extraction, refining, distribution and combustion of traditional petroleum.

Schrag argues that the necessary removal of carbon dioxide in the Fischer-Tropsch process presents an opportunity rather than an argument against the production of synthetic liquid fuels from coal. The challenge will be to compress and store the carbon dioxide in a cost-effective manner, and the question is whether the US$10–20 per tonne quoted by Schrag is credible. If this estimate is accurate, then as long as Fischer-Tropsch facilities are located near suitable and safe carbon dioxide storage reservoirs, we could have a means of generating diesel and jet fuel with a carbon footprint 5–12% lower than with traditional petroleum.

That may not sound like much of an improvement, but combine 60% coal with 40% biomass and the carbon footprint could fall to near zero, with a biomass to fuel conversion efficiency of almost 100%. Another possibility is a pure-biomass Fischer-Tropsch process, which could in theory be carbon negative. This might work on smaller scales, for example using municipal waste or other low-cost biomass as the feedstock.

Schrag acknowledges that state-subsidies for Fischer-Tropsch facilities could lead to the proliferation of synthetic fuel plants that use coal alone, and without carbon capture and storage. But that is a problem of regulation and carbon price rather than technology. The question is whether we can afford to ignore the option. Schrag believes that combining biomass with coal in synthetic fuel plants could turn an old technology from black to green.

Further reading

Dan Schrag, “Coal as a low-carbon fuel?”, Nature Geoscience 2, 818 (2009)