Improving energy from oilseed rape straw
Researchers at the Institute of Food Research in the UK are looking at how to turn straw from oilseed rape into biofuel. Preliminary findings are pointing at ways the process could be made more efficient, as well as how the straw itself could be improved.
Straw from crops such as wheat, barley, oats and oilseed rape is seen as a potential source of biomass for second generation biofuel production. Currently the UK produces around 12 million tonnes of straw. Although much is used for animal bedding, mushroom compost and energy generation, there still exists a vast surplus.
Straw contains a mix of sugars that could be used as a source of biofuels that do not compete with food production but instead represent a sustainable way of utilising waste. However, the sugars are in a form that makes them inaccessible to the enzymes that release them for conversion into biofuels, so pre-treatments are needed. The pre-treatments make the complex carbohydrates more accessible to enzymes that convert them to glucose, in a process called saccharification. This is then fermented by yeast into ethanol.
Professor Keith Waldron and his team have been looking at the steps needed to unlock the sugars tied up in the tough straw structure. In particular, they have looked at the pre-treatment stage, focusing on steam explosion, which involves ‘pressure-cooking’ the biomass, to drive a number of chemical reactions. A rapid pressure-release then causes the material to be ripped open, to further improve accessibility.
They varied the temperature and duration of steam explosion and then used a variety of physical and biochemical techniques to characterise what effects varying the pre-treatments had on the different types of sugars before and after saccharification.
The amount of cellulose converted to glucose increased with the severity of the pretreatment. Saccharification efficiency is also associated with the loss of specific sugars, and subsequent formation of sugar breakdown products.
In a further study the scientists discovered the key factors that determine the efficiency of saccharification. One particular compound, uronic acid, limited the rate at which enzymes worked. The final sugar yield was closely related to the removal of xylan, a common component of plant cell walls. The abundance of lignin, a ‘woody’ cell wall component, was positively related to the amount of available sugars.
These findings will help improve the efficiency by which straw can be converted to biofuels. For example, adding enzymes that more effectively remove xylan should improve yield. Controlling the level of lignin in the material should also help.
It may even be possible to improve the straw itself, for example to reduce the uronic acid content in the biomass, as suggested by these findings. In the main, oilseed rape has been bred to improve oilseed yield and disease resistance, without paying much attention to the straw. The IFR is working with colleagues at the University of York and the John Innes Centre to see whether there are ways of breeding more “biofuel-ready” varieties of oilseed rape, with the same yields of oilseed but with more amenable straw. In addition, a full understanding of the polysaccharides and other compounds made available during pretreatment may mean other valuable co-products, like platform chemicals, may be viably produced from the surplus straw.