To increase the sustainability of biofuels, there is currently a drive to turn away from deriving them from food crops, such as corn and sugarcane. Bioethanol derived from the waste products of agriculture and the food chain is more attractive as this avoids competition with food crops, reduces food waste and lowers the carbon footprint. Achieving this on a commercial scale needs to overcome a number of hurdles, which the Biorefinery Centre on the Norwich Research Park working on.
Sugars are the starting point for the production of bioethanol, and are readily obtainable in large quantities from food crops such as sugar beet, corn and wheat. In agri-food waste however the sugars are effectively locked away in the structure of the plant material – mostly in the form of lignocellulose. Lignocellulose gives plant cells walls their rigidity and resistance, but this makes them harder to convert into biofuels. For most agri-food waste a pre-treatment is needed to break open these structures, reducing the overall economic viability of the process.
However waste paper, particularly shredded paper that cannot be recycled, has effectively been pre-treated, with much of the lignocellulosic structure broken down.
Now researchers at IFR have for the first time produced high concentrations of bioethanol from waste paper that match the yields obtained from first generation biofuels.
Achieving this saw the team overcome a number of obstacles. Paper absorbs water and becomes difficult to mix. A specialised pilot bioreactor able to mix the material needed to be used. Adding the paper in batches also allowed digestion to occur, preventing the material from becoming too thick.
Ethanol conversion is a two-step process. Enzymes are used to break down the complex carbohydrates (saccharification) to simple sugars that yeast ferments into ethanol. Semi-simultaneous saccharification and fermentation was used. After an initial enzyme treatment, further saccharification feeds sugars into yeast fermentation simultaneously. This, along with the mixing and the batch addition of paper waste keeps the bioreactor working steadily and a final ethanol yield of 11.6% – as high as that in current commercial biofuel production and higher than any other reported yields from paper or paper pulp waste streams.
The researchers believe that there is considerable room to improve on this figure, by optimising batch addition regimes and the initial enzyme concentrations (which are low to reduce input costs). Different yeast strains may convert sugars to ethanol more efficiently, for example heat-tolerant yeasts may be better suited the exact conditions in this set-up. The researchers are working with the National Collection of Yeast Cultures, a BBSRC-supported National Capability based at IFR, to investigate this.
These initial findings relate to pilot scale experiments, and to be viable the system must work on the industrial scale. Scaling up must be economically viable, taking into account things such as the energy needed for the crucial agitation of the paper material. However, with over 12 million tonnes of paper waste being generated annually in the UK alone, there is great potential to divert this into a new sustainable source of fuel or higher value chemicals.