In the race to achieve fuel security and to reduce our reliance on fossil fuels, the US has rapidly increased the volume of bioethanol it produces, from 6.2 billion litres/year in 2000 to 50 billion in 2010. Ethanol has the advantage that it can be mixed with petrol so cars need no conversion. However, most of this growth in ethanol has been from first generation corn ethanol, produced through fermentation. First generation biofuels have the major drawback that they are energy intensive to produce, which can counteract any reduction in green house gas emissions. Also, there is a raging food vs fuel argument surrounding biofuels made from food crops.
To overcome some of these problems, research is taking place into second generation biofuels, made from materials such as forestry wastes, grasses, wastepaper etc. These can be converted into liquid fuel, normally using enzymes. But pre-treatment steps are needed to make the enzymes more effective, and options include grinding, adding acid, steaming, or treatment with fungi. Many of these, however, haven’t made it out of the lab. A paper available this month from Biotechnology for Biofuels predicted trade-offs between different pre-treatment steps for commercial ethanol production from grass straw.
For the same amount of straw put in, using dilute acid, dilute alkali or hot water as pre-treatments produced similar quantities of ethanol. Steam explosion pre-treatment was slightly less effective so yielded less ethanol.
Costs of the plant were highest for alkali pre-treatment, and similar for the other three options, with steam being the lowest. The ethanol production costs varied by a few cents per litre ($0.84 per litre for dilute acid, $0.89 for dilute alkali, $0.81 for hot water and $0.86 for steam explosion). Water use also varied – the thirstiest treatment actually being alkali not water.
They concluded that ethanol price and energy use were highly dependent on the pre-treatment technology, demonstrating the importance of addressing the tradeoffs in costs and environmental impacts of different aspects of the pre-treatment.
New technologies are set to make bioethanol more efficient and less energy intensive to produce, which in turn reduces its environmental impact. Biofuels made with current technologies may give biofuels a bad name, but as technologies making second generation biofuels viable move beyond the lab, their environmental impact will reduce. And there don’t seem to be many viable options available for feeding our cars, so research such as this can shape our future fuel supply.
Deepak Kumar and Ganti S Murthy (2011) Impact of pretreatment and downstream processing technologies on economics and energy use in cellulosic ethanol production
Biotechnology for Biofuels, 4:27 doi:10.1186/1754-6834-4-27