30 Aug 2012, Dr Madhugiri Nageswara-Rao, Dr Charles Kwit and Prof C Neal Stewart, BioSpectrum
World fossil fuel production and consumption continues to increase, however, fewer petroleum-based resources remain in the ground. With many years of over-consumption and projections of the world's population growth, reaching 10 billion by 2100, there is a great concern regarding the dwindling non-renewable fossil fuel reserves. According to the US Energy Information Administration, the worldwide petroleum consumption in 1990 was approximately 66,524,000 barrels per day, as compared to 87,269,000 barrels per day in 2011.
The peak in oil consumption is approaching at a time when the world is facing many challenges, such as rising temperatures, declining water tables along with other environmentally damaging trends.
There will be a progressive scarcity of fossil fuel products if immediate and innovative measures are not taken for its sustainable use. There is also a need to develop sustainable and renewable alternatives. In fact, bioenergy derived from sources such as starch from corns, sugar from cane, and oils from plants, will play an essential role in reaching targets and replace petroleum-based transportation fuels as a cost-effective and environmentally sustainable alternative and in reducing long-term CO2 emissions.
Although plant-derived biofuels are renewable resources and often carbon-neutral, they have been criticized because of concomitant land use change, detrimental effects to biological diversity and increases in the food prices. These drawbacks have led to tremendous amount of recent research taking place in second-generation biofuel development that primarily focus on mining energy from lignocellulosic biomass sources, where stems, leaves and husks (dedicated feedstocks and agricultural wastes) are used for the production of biofuels.
Unlike the easily processed sugars and oils in first-generation biofuels, lignocellulosic, biomass consists of matter composed of difficult-to-digest material found in cell walls of plants such as grasses, crop residue, and woody forest waste. These second generation biofuels have advantage as they are not restricted to current agricultural land, thus removing competition between the uses of land for food or fuel production.
Of special interest is the role of dedicated bioenergy feedstocks to the production of the second generation biofuels. It is envisaged that new crops, such as switchgrass, could reliably be produced to offset petroleum. Switchgrass is a C4 perennial warm season species of the North American tallgrass prairie. Its widespread environmental adaptation, ability to grow from seed on marginal lands, low nutrient requirements and production costs, high water-use efficiency, capacity to protect soil from erosion, ease of harvesting, potential for carbon storage, and winter hardiness among others, makes it a promising dedicated bioenergy feedstock for biofuel production. If the aboveground biomass is harvested once, at the end of the growing season, typical ethanol yield has been calculated to be 4353 liters per acre.
Unlike the maize grain, which must be replanted every year, switchgrass is planted once every 10-to-12 years. It is a perennial grass that re-sprouts each spring and summer from the underground parts of the plant. A detailed study involving multi-location field trails examined the net energy output, greenhouse gas emissions, biomass yields, agricultural inputs and estimated cellulosic ethanol production from switchgrass grown and managed for biomass fuel.
The study concluded that switchgrass produced 540 percent more energy than needed to grow, harvest and process it into cellulosic ethanol and the average greenhouse gas emissions from this cellulosic ethanol production were 94 percent lower than estimated greenhouse gas emissions from gasoline. Considering the biofuel crisis, the US Department of Energy has provided partial funding for the construction of cellulosic biorefineries across the US.
With the rapid development in emerging economies such as China and India, there are ethical concerns over the production of food as compared to biofuel crops, due to constraints such as limited land and the need for quality food for large populations. These challenges call for international collaborations on bioenergy research. Certain collaborative efforts have been made in this regard, like US-China Collaborative biofuel research conferences have been held at Knoxville, Tennessee, US in 2009; in Beijing, China, during in 2010 and at West Lafayette, Indiana, US, in 2011. Many collaborative initiatives and issues for implementing the lignocellulosic biofuel platforms have been developed and discussed.
Salt and drought tolerant grasses such as switchgrass have huge potential as bioenergy feedstocks as they use marginal land and meet the recommended sustainable benchmarks. The adaptability of ten switchgrass cultivars to arid and semi-arid environments, such as the Loess Plateau, China, have been investigated and has led to the optimization of choices for biomass plants.
While wild type switchgrass has many promising features and is also being improved by conventional plant breeding, biotechnology will likely be necessary to make dedicated bioenergy feedstocks economically viable. The reference whole genome sequence of switchgrass is being assembled by the Joint Genome Institute, US. This will increase the possibility of genetically engineering or manipulating many economically important genes related to biofuel production, the most urgent being the need to engineer improved ability to breakdown cell walls into sugars.
Grasses such as switchgrass cover at least 20 percent of the global land area and are known to be well-adapted to diverse environments, thus making them potential sources for second generation lignocellulosic biofuels. By understanding their basic biology, cell wall biosynthesis, eco-evolutionary relationships and population variability, it is possible to replace non-renewable fossil fuels and contribute to global environmental and economical sustainability.