Biological routes to ethanol depend on fermenting sugar solution, with yeast or other microorganisms, then distilling the ‘beer’ to separate the ethanol. Until now the sugar has come from cane or beet, or from hydrolysing starch from grains, potatoes or other starchy vegetables. This is so-called first generation ethanol and it has served to demonstrate that ethanol is a viable fuel, giving the petroleum industry plenty of experience in transport, storage and blending. Motor manufacturers too have made sure their vehicles are tuned to work well on current blends, and capable of adapting to higher levels of ethanol as it becomes available.
Now though the market is close to the limit of production of first generation ethanol. Using agricultural land to grow fuel crops is having an effect on global food prices and driving land use change around the world, exacerbating the very problem biofuels are designed to reduce. According to US Department of Energy studies, cellulosic ethanol could reduce greenhouse gas emissions (GHG) by 85% compared to petrol. The US renewable fuels standard (RFS2) mandates that 36 billion gallons of renewable fuels be blended into transportation fuels by 2022. Of that, at least 16 billion gallons is required to be cellulosic biofuels.
Cellulose is the most abundant organic compound on earth, and together with hemicellulose and lignin it forms the structure of all plants; cellulose makes up about 40-60% by weight of plant matter, and hemicellulose another 20-40%. Both are natural polymers of sugar, with cellulose based on sugar molecules with six carbon atoms (C6 sugars) primarily glucose and hemicellulose on C5 sugars such as arabinose and xylose.