To chew or to burn?

By Richard Van Noorden
June 29, 2007

There's a positive buzz of research and bold investment surrounding second generation biofuels. Turning woodchip, prairie grasses, corn stover and general biomass waste into fuel to drive our cars ticks all the energy boxes. It reduces dependence on imported oil, cuts carbon emissions from fossil fuels, and crucially doesn't deplete valuable food resources. Yet while politicians get excited and companies try to lower engineering costs for commercial development, scientists are still asking which are the best - and cheapest - ways of unlocking the energy stored in the cellulose, lignin and hemicellulose bound up in plant waste.

Burning the lot is one option, producing heat and electricity. But cars don't traditionally run on electricity. Lignocellulosic biomass has to be converted to liquid fuel to make inroads on petrol and take advantage of that industry's existing infrastructure.

In the familiar biochemical route, adapted from the mature US grain fermentation industry, chemicals and enzymes break up tough lignocellulose into sugars which are then fermented, usually to ethanol. There is another option: adapting thermochemical processes, first developed for coal, to biomass. High temperatures and pressures convert lignocellulose to carbon monoxide and hydrogen (syngas), which is transformed via the Fischer-Tropsch reaction into methanol, ethanol, or diesel, or kept as hydrogen. Variations on these themes range from pyrolysing biomass into liquid oils, to producing biobutanol.

Which is best? A new meta-analysis has concluded that the current costs of biochemical and thermochemical ways of turning waste biomass into liquid fuels are surprisingly similar. Only combining the two can make cellulosic biofuels economically competitive, say researchers. At first glance it's not obvious which options have most to offer. 'I get tired of claims that one costs more than another - when no-one can point to a meaningful comparative study,' Robert Brown, who works on renewable fuels at Iowa State University, US, told Chemistry World. Thermochemical and biochemical routes have different optimum plant sizes, feedstock costs, byproducts (which may add value), output fuels, and relationships to existing industry: so working out the resources each uses to produce the equivalent of a gallon of petrol is not easy.