Which biomass?
One of the main costs in biofuel production, in terms of both economics and environment, is the biomass feedstock. The choice of feedstock is central to the controversy surrounding biofuels today, with current technologies associated with the use of food as fuel and large scale changes in land usage. For biofuels to have any meaningful impact on energy, biomass feedstocks must be widely available at low cost and without negative environmental impact. Lignocellulose – the non-food component of plants, fits this description. Future technologies are based on harnessing the energy locked within lignocellulose, converting the carbohydrate components to biofuel.
Lignocellulose biomass includes:
- · Agricultural waste such as straw, corn stover and bagasse.
- · Industrial waste such as sawdust and paper pulp.
- · Woody biomass from forestry.
- · Municipal solid waste including food and garden waste and paper products.
- · Specific non-food energy crops such as switchgrass.
Conversion to biofuel first requires hydrolysis of the biomass to yield a fermentable susbtrate. This step often involves a combination of physical, chemical and enzymatic treatments. Complete hydrolysis of the polysaccharide polymers yields glucose from cellulose and a complex mixture of sugars from hemicellulose including pentoses (xylose and arabinose) and hexoses (glucose, mannose, galactose and rhamnose). The second step is to ferment the hydrolysate to biofuel. In order for a lignocellulose process to be efficient, it is desirable for the microbes to utilise all of the carbohydrates. It is here that solventogenic clostridia have the advantage, enabling complete conversion of lignocellulosic sugars to butanol and co-products.