Box 4.2 Bioenergy-effi ciency
The use of sisal, a plant native to East Africa, is a good example of how the bioenergy-efficiency concept can be put into practice. Traditionally sisal is used to make fibre and twine, with 2-4 percent of the total plant being used and the rest discarded to decompose. But sisal waste is now being used as a value-added product to generate biogas in various areas of East Africa. Using the whole sisal plant now doubles carbon emission savings by eliminating decomposition of sisal waste.
Sources: UNIDO Available at: www.unido.org/index.php?id=6464 Figure 4.5 Biorefi nery, general concept
More efficient use of biomass is also needed, including the optimal the use of waste and residues. Specifically, energy recovery from municipal organic waste and residues from agriculture and forestry hold significant, yet largely untapped energy potential. With little or no environmental impact, recovery of these materials yields many co-benefits, including a cut in carbon emissions otherwise released through traditional disposal or combustion.
However, not everything that looks like waste is unused. Assessments of potential competing waste
uses, such as soil fertiliser, as well as longer-term availability of the waste stream should be made prior to developing a biofuel plant.
Bioenergy offers many ways to combine uses, for example by using biomass first to produce material and then recovering the energy content of the resulting waste (cascading use). The forestry sector has been maximising the use of wood products by creating value with its residue waste stream – providing biomaterials for both fibre and fuel. Often these residues can be pelletised and burnt in cogeneration plants to supply heat and power.
Finally, consideration should be given to the most efficient end-use of biomass. For example, stationary use of biomass to generate heat and/or electricity is typically more energy-efficient than converting biomass to a liquid fuel. Of course, economic efficiency may lead to a different conclusion, and future trends with fossil fuels becoming more difficult to extract may change the equation of environmental benefits.