Turning wood and agricultural waste into biofuels is one step closer to being a truly green process, according to a recently published study in the journal Science. James Dumesic of the University of Wisconsin-Madison and colleagues report a new method of extracting energy from any type of organic material, and they achieve this without using large amounts of acid or costly chemicals.
Biofuels are obtained from the carbon present in plants. In contrast to fossil fuels, which take millions of years to form, plants are can be replenished quickly. They are also seen to be carbon neutral, because the carbon they release when burnt has been offset by the amount they absorbed whilst growing.
First-generation bioethanol, a common type of biofuel, is made from the starch and sugars present in corn or sugarcane. These two components can be readily transformed into fuel by microbes. But given that these crops can be used for food too, the United Nations found that using them for making fuel is having a negative effect on the world’s food supply.
To avoid this problem, second-generation biofuels are being sought from plants that can be grown on land not suited for food production. But the problem with these crops is that most of their carbon is trapped in cellulose, a polymer which is resistant to fermentation and that has to be broken down into smaller sugar units that are soluble in water.
Producing biofuels is a four-step process: acquiring the biomass, breaking it down to individual sugars, fermenting the sugars using microbes into desired biofuel and separating all the components to make the fuel usable. Each step can be improved to make the process more effective. Dumesic’s method targets the second step, which has so far proved to be harder to optimise.
So far the process of deconstructing cellulose has required toxic chemicals and a lot of energy. Dumesic’s team make that process easier by using the chemical gamma-valerolactone (GVL), which can be easily recovered at the end, making the process a lot greener.
GVL loosens the cellulose, but it doesn’t completely break it down into the component sugars. That is achieved with small quantities of acid. The second critical part in the new method is to use liquid carbon dioxide at the end of the process to separate and recycle the GVL for the precious syrup. Sugars can then be used for the microbial fermentation that converts the biomass to biofuel.
Other attempts to optimise the second step have involved the development of enzymes, which are proteins that do a specific task, in this case breaking down cellulose. This has been an efficient process too, and many industrial enzymes exist today. But the cost of production of enzymes can be high. In contrast, GVL is cheap.
“Our approach is complementary to existing methods for biomass conversion,” Jeremy Luterbacher, the study’s lead author, said. “We find that GVL acts as the ideal solvent to convert plants to sugars without the use of concentrated acids, enzymes or other expensive alternatives.”
Luterbacher and his colleagues predict that, because it can be recycled at low energy costs, the minimum selling price of a gallon of bioethanol can be brought down from US$3.40 to US$3.23, which may not seem like a lot but on an industrial scale 5% reduction is a significant saving.
“Apart from biofuels, the sugars can also be converted into other molecules usually obtained from fossil fuel sources” Luterbacher said. GVL itself can be made from the sugars too, making the whole process greener and an attractive alternative to currently used methods.