Biomass from the agriculture sector can be used to produce biofuels – solid, liquid and gaseous. Biofuels substitute fossil fuels for energy delivery. If biomass is grown in a sustainable cycle to produce biofuels, such agriculture practices mitigate GHG emissions due to fossil fuel not being combusted. Biofuels can be derived from biomass sources such as corn, sugar cane, sorghum, soybean, crop residues, oil palm (Elaeis guineensis), switch grass, Miscanthus, bioengineered algae, and Jatropha curcas seeds, trees, and grasses. First generation biofuel crops (such as sugarcane and maize) from which sap or grain ethanol are obtained are already being used. In addition, second generation cellulosic ethanol crops (e.g., Miscanthus) appear promising.
Agricultural crops and residues are the major sources of feed stocks for energy to displace fossil fuels. A wide range of materials such as grain, crop residue, cellulosic crops (e.g., switch grass, sugar canes and various tree species) are used for the production of biofuel (Paustian et al., 2004; Eidman 2005). These products are processed further to generate liquid fuels such as ethanol or diesel fuel (Richter, 2004). These fuels release CO2 when burned, but this CO2 is of recent atmospheric origin (via photosynthesis) and displaces CO2 which otherwise would have come from fossil carbon. The net benefit to atmospheric CO2, however, depends on energy used in growing and processing the bioenergy feed stock (Spatari et al., 2005).
A significant barrier to production of biofuels from grain is the competitive need of the grain for food and feed. Systems to utilise cellulosic biomass are not yet commercially viable, although much research and subsidies are being implemented to stimulate its use. Even if research at the laboratory scale is promising, challenges exist in scaling up the infrastructure to provide a feasible supply chain for cellulosic bioenergy (Richard, 2010).
- Some of the biofuel production such as Jatropha and oil-palms can be grown in dry land and fallow area, through commercial experiences.
- About 70-88 million biogas plants can be run with fresh/dry biomass residues.
- The substrate such as cattle waste and biomass used for this technology are easily available. Their availability to biogas plants can meet the requirement of 12-30 million families.
- A larger area of land will be required to satisfy global biofuel demand. Projected growth of biofuel crops until 2030 may require over 30 million hectares of land (IEA, 2009). However, Field et al. (2008) suggested a need for 1,500 million hectares of land under cultivation of biofuel crops. Melillo et al. (2009)’s calculations show biofuel crops would require 1,600-2,000 million hectares by the year 2100 assuming most fuel demand would be met by biofuels by this time. It is practically impossible to spare such a large area of cropland to grow biofuel plants.
- The land requirement for biofuel crops would compete with that for food and feed crops, causing food prices to increase.
- In many cases for current ethanol production from grain, the fossil fuel associated with use of chemical fertilisers, tractor power and so on, results in an unacceptably small net reduction in fossil fuel use (e.g., Scharlemann and Laurance, 2008).
- Production systems with suitable enzymes for utilising cellulosic feedstocks have not yet become commercially viable.
- The resources for biogas generation are not properly managed to generate its maximum biogas potential.
- The lack of availability and the structural operation of biogas digesters are not able to generate and develop family-size biogas plants.
The use of husks as a fuel appears to be a promising mitigation option. Husk could be used for direct burning, in biomass gasifier, as briquettes or as solid char. Its relative cost is around US$4 per t CO2e saved and the reduction potential ranges from 0.9-1.2t CO2e ha-1 (depending on the level of biomass production). Rice husk can easily be collected at milling facilities, so that this source of renewable energy seems even more promising than utilisation of straw (Junginger, 2000;Wassmann and Pathak, 2007).
The potential for mitigation is huge, particularly if cellulosic biomass sources can be commercialised. However, the economics are such that biofuels have required help from legislation and subsidies to penetrate the market, at least in parts of the US where currently a proportion of gasoline must be ethanol at certain times of the year more to mitigate air pollution from ozone than to mitigate GHG emissions (e.g.,Regalbuto, 2009) and there is a legislative mandate for 16 billion gallons of cellulosic ethanol by 2022 (Robertson et al., 2008). Similarly, Europe has a mandate that 10% of all transport fuels be from renewable sources by 2020 (Robertson et al., 2008).
See the section about socio-economic development above.
Eidman, V.R., (2005): Agriculture as a producer of energy. In Agriculture as a producer and consumer of energy. (ed. J.L. Outlaw, K.J. Collins and J.A.Duffield), pp.30-67, Cambridge, MA: CABI Publishing.
Field, C.B., Campbell, J.E., Lobell, D.B. (2008): Biomass energy: the scale of the potential resource. Trends in Ecology and Evolution, 23:65-72.
IEA (2009): The impact of the financial and economic crisis on global energy investment. IEA Background paper for the G-8 Energy Ministers Meeting in Rome, 24-25 May 2009; available at: www.g8energy2009.it/pdf/IEA_ Paper_for per cent20G8-Impact_of_the_crisis_on_energy_investment.pdf.
Junginger, M., (2000): Setting up fuel supply strategies for large-scale bio-energy projects using agricultural and forest residues. A methodology for developing countries. NW&S Report number: NW&S-E-2000-16 Utrecht, The Netherlands, p. 59, (ISBN 90- 73958-58-X).
Melillo, Jerry M., John M. Reilly, David W. Kicklighter, Angelo C. Gurgel, Timothy W. Cronin, Sergey Paltsev, Benjamin S. Felzer, Xiaodong Wang, Andrei P. Sokolov, and C. Adam, (2009): Indirect Emissions from Biofuels: How Important? Science 326: 1397-1399.
Paustian, K., B.A. Babcock, J. Hatfield, R. Lal, B.A. McCarl, S. McLaughlin, A. Mosier, C. Rice, G.P. Robertson, N.J. Rosenberg, C. Rosenzweig, W.H. Schlesinger and D. Zilberman (2004): Agricultural Mitigation of Greenhouse Gases: Science and Policy Options, Council on Agricultural Science and Technology (CAST) Report, R141 2004, ISBN 1-887383-26-3, 120 pp.
Regalbuto, J.R. (2009). Cellulosic biofuels – got gasoline? Science 325:822-824.
Richard, T.L. (2010). Challenges in scaling up biofuels infrastructure. 329:793-796.
Richter, B. (2004): Using ethanol as an energy source. Science 305, 340.
Robertson, G.P., Dale, V.H., Doering, O.C., Hamburg, S.P., Melillo, J.M., Wander, M.M., Parton, W.J., Adler, P.R., Barney, J.N., Cruse, R.M., Duke, C.S., Fearnside, P.M., Follett, R.F., Gibbs, H.K., Goldemberg, J., Mladenoff, D.J., Ojima, D., Palmer, M.W., Sharpley, A., Wallace, L., Weathers, K.C., Wiens, J.A., and Wilhelm, W.W. (2008). Sustainable biofuels redux. Science 322:49-50.
Scharlemann, J.P.W., and Laurance, W.F. (2008): How green are biofuels? Science 319:43-44.
Spatari, S., Y. Zhang and H.L. Maclean, (2005): Life cycle assessment of switchgrass- and corn stoverderived ethanol-fueled automobiles. Environmental Science and Technology 39, 9750-9758.
Wassmann R and Pathak H. (2007): Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: II. Cost- benefit assessment for different technologies, regions and scales. Agricultural Systems 94:826-840.