Thinking outside the box

Transforming plant biomass into electricity will get a vehicle much further along the road than transforming the same biomass into ethanol, and release far less greenhouse gases.

These are the main conclusions of a new study conducted by a team of three US scientists led by Elliott Campbell from the University of California, Merced. The study will shortly be published in Science.

A similar conclusion was reached last year by a team of chemical engineers led by Henrik Wenzel from the University of Southern Denmark. They showed that the most effective way to generate energy from willow was to burn it to produce heat and electricity (see Future shock). Campbell and his colleagues have now expanded on this finding, showing that it also applies to corn and switchgrass and a range of different conventional and electric vehicles.

Using a biofuel life cycle model and fuel consumption data from the US Environmental Protection Agency, the scientists analysed the energy efficiencies and greenhouse gas reductions obtained by converting corn and switchgrass into cellulosic ethanol and electricity. In doing this, they took account of both the energy used to produce the ethanol and electricity, and the energy used to produce the vehicles.

They found that, on average, converting a set amount of biomass into electricity would take an electric car 81% further than a conventional car powered by ethanol produced from the same amount of biomass. It would also more than double the greenhouse gas savings.

According to the scientists, electricity’s superior performance is mainly due to battery-powered electric engines being much more efficient than conventional internal combustion engines. ‘Even the best ethanol-producing technologies with hybrid vehicles aren’t enough to overcome this,’ says Campbell.

Nevertheless, the scientists caution that this is hardly the final word on the subject. For a start, their study only considered two criteria, energy efficiency and greenhouse gas savings, but many other factors need to be taken into account. ‘We also need to compare these options for other issues such as water consumption, air pollution and economic costs,’ says Campbell.

Furthermore, the scientists only compared electricity with cellulosic ethanol, rather than any other liquid fuels that can be produced from biomass, such as synthetic fuels. Indeed, the next generation of synthetic fuels could end up turning the tables back towards liquid biofuels.

This was demonstrated by Edgar Hertwich and Xiangping Zhang at the Norwegian University of Science and Technology in Trondheim. In a paper recently published in Environmental Science & Technology, Hertwich and Zhang outlined their novel plan for converting plant biomass into methanol by heating the biomass with concentrated sunlight.

The advantage of using sunlight to heat biomass is obvious, because it means that the heat doesn’t have to come from fossil fuels or from other biomass. As such, the process generates hardly any carbon dioxide and is able to convert a high proportion of the carbon in the biomass directly into methanol.

The process begins with a solar power tower, whereby a large array of hundreds or thousands of mirrors focuses sunlight onto a receiver at the top of a tower. Via an intermediary substance such as salt, this concentrated sunlight can be used to heat plant biomass in the presence of steam to produce syngas. Generating methanol from syngas is then fairly straightforward.

Although the process is currently only theoretical, Hertwich and Zhang argue that it is technologically feasible. Indeed, such solar power towers are already being used to generate electricity in Spain and the US.

According to Hertwich and Zhang’s calculations, their process should convert 90% of the carbon in plant biomass into methanol. This compares with just 30% if plant biomass is used as both the feedstock and the heat source or if the biomass is replaced by coal.

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.