Future shock

Will cellulosic ethanol truly be the panacea promised by the biofuel industry: a plentiful, environmentally-friendly fuel that doesn’t endanger food supplies? Probably not, say three Danish chemical engineers, who argue that there are much more effective and environmentally-beneficial ways to utilise plant biomass than transforming it into ethanol.

Their reasoning is based on the realisation that one important factor has been ignored by all the many studies exploring the environmental costs and benefits of biofuels: that there is only so much plant biomass to go around. Furthermore, the demands placed on biomass are set to rise as plants are increasingly used to produce liquid biofuels, biogas, heat, electricity and chemicals, as well as still being required for food.

To make their case, the three chemical engineers, led by Henrik Wenzel at the University of Southern Denmark, compared the theoretical maximum amount of plant biomass available around the world for energy use with the amount that would be needed to replace mankind’s yearly demand for fossil fuels. Looking ahead to 2030, they found that, depending on which estimates for biomass availability they used, plant biomass could only ever meet 17–62% of the world’s projected fossil fuel needs.

Things get even worse if economics are introduced, with Wenzel and his colleagues finding that only 10–22% of the world’s fossil fuel needs in 2030 could cost effectively be met by plant biomass.

What this all means is that the supply of plant biomass will be constrained in the future and so mankind needs to make sure that it is making the most effective use of this limited resource. The question then is whether producing ethanol, even cellulosic ethanol, represents the most effective use.

To find out, the chemical engineers imagined a future scenario in which mankind’s transport and heat and power needs could be met by various different sources. For transport, these sources were petrol, ethanol, biogas, compressed natural gas or electricity, while for heat and power they were coal and natural gas, ethanol, biogas or willow.

Then, taking advantage of findings from three of their earlier studies, they conducted life cycle studies to determine which of these combinations of sources were best at reducing both fossil fuel use and greenhouse gas emissions.

They based their ethanol analysis on a cellulosic ethanol production process developed by Dong Energy, a Danish energy group, and assumed that whole maize plants were used as the feedstock. This cellulosic ethanol process involves integrating the ethanol plant with a coal-fired power station, which can also generate electricity by burning solid waste material created during ethanol production. Hence, this single cellulosic ethanol process is able to meet both transport and heat and power needs.

Nevertheless, producing cellulosic ethanol this way turns out not to be a particularly efficient use of plant biomass. Wenzel and his colleagues found that most of the other combinations, even some of those that utilised petrol for transport, achieved the same or better reductions in fossil fuel use and greenhouse gas emissions.

For despite the undoubted efficiency of this ethanol production process, it still takes take a great deal more energy to produce cellulosic ethanol than it does to produce electricity by burning willow or biogas by the anaerobic digestion of maize plants. From this analysis, the most effective use of biomass involved burning willow to produce heat and electricity and then using electric-powered vehicles for transport. Wenzel and his team recently reported these findings in the journal Environmental Science & Technology.

‘In the short term, there is in general no justification for the known liquid biofuels, as the conversion losses are much too large,’ concludes Wenzel. ‘There are so many places where we can exchange the biomass for oil or gas with much higher replacements of fossil fuels and much better cost/efficiency as well.’

But Bruce Dale, a professor of chemical engineering at Michigan State University who conducts similar life cycle studies of biofuels, points out the inherent difficulty of trying to predict the future. ‘Ultimately, the future is not fixed, it is ours to envision and create,’ he told BioFPR. ‘I believe that hard facts and creative thinking can achieve a very large role for sustainable cellulosic biofuels.’

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.