Switchgrass - a viable biofuel source
Switchgrass has been researched as a bioenergy crop since the mid-1980s, because it is a native perennial warm season grass with the ability to produce moderate to high yields on marginal farmlands. It is now being considered for use in several bioenergy conversion processes, including cellulosic ethanol production, biogas, and direct combustion for thermal energy applications. The main agronomic advantages of switchgrass as a bioenergy crop are its stand longevity, drought and flooding tolerance, relatively low herbicide and fertilizer input requirements, ease of management, hardiness in poor soil and climate conditions, and widespread adaptability in temperate climates. In some warm humid southern zones such as Alabama it has the ability to produce up to 25 oven-dry tonnes per hectare (ODT/ha).
A summary of switchgrass yields across 13 research trial sites in the United States found the top two cultivars in each trial to yield 9.4 to 22.9 t/ha, with an average yield of 14.6 ODT/ha. However, these yields were recorded on small plot trials, and commercial field sites could be expected to be at least 20% lower than these results. In the United States, switchgrass yields appear to be highest in warm humid regions with long growing seasons such as the US Southeast and lowest in the dry short season areas of the Northern Great Plains.
The energy inputs required to grow switchgrass are favorable when compared with annual seed bearing crops such as corn, soybean, or canola, which can require relatively high energy inputs for field operations, crop drying, and fertilization. Whole plant herbaceous perennial C4 grass feedstocks are desirable biomass energy feedstocks, as they require fewer fossil energy inputs to grow and effectively capture solar energy because of their C4 photosynthetic system and perennial nature. One study cites that it takes from 0.97 to 1.34 GJ to produce 1 tonne of switchgrass, compared with 1.99 to 2.66 GJ to produce 1 tonne of corn. Another study found that switchgrass uses 0.8 GJ/ODT of fossil energy compared to grain corn’s 2.9 GJ/ODT. Given that switchgrass contains approximately 18.8 GJ/ODT of biomass, the energy output-to-input ratio for the crop can be up to 20:1. This highly favorable ratio is attributable to its relatively high energy output per hectare and low energy inputs for production.
Considerable effort is presently being expended in developing switchgrass as a cellulosic ethanol crop in the USA. In George W. Bush’s 2006 State of the Union Address, he proposed using switchgrass for ethanol; since then, over $100 million USD has been invested into researching switchgrass as a potential biofuel source. Switchgrass has the potential to produce up to 380 liters of ethanol per tonne harvested. However, current technology for herbaceous biomass conversion to ethanol is about 340 liters per tonne. In contrast, corn ethanol yields about 400 liters per tonne.[ The main advantage of using switchgrass over corn as an ethanol feedstock is that its cost of production is generally about 1/2 that of grain corn and more biomass energy per hectare can be captured in the field. Thus, switchgrass cellulosic ethanol should give a higher of ethanol per hectare at lower cost. However, this will depend on whether the cost of constructing and operating cellulosic ethanol plants can be reduced considerably. The switchgrass ethanol industry energy balance is also considered to be substantially better than that of corn ethanol. During the bioconversion process, the lignin fraction of switchgrass can be burned to provide sufficient steam and electricity to operate the biorefinery. Studies have found that for every unit of energy input needed to create a biofuel from switchgrass, four units of energy are yielded. In contrast, corn ethanol yields about 1.28 units of energy per unit of energy input. A recent study from the Great Plains indicated that for ethanol production from switchgrass, this figure is 5.4, or alternatively, that 540% more energy was contained in the ethanol produced than was used in growing the switchgrass and converting it to liquid fuel. However, there remain commercialization barriers to the development of cellulosic ethanol technology. Projections in the early 1990s for commercialization of cellulosic ethanol by the year 2000have not been met. The commercialization of cellulosic ethanol is thus proving to be a significant challenge, despite noteworthy research efforts.
Thermal energy applications for switchgrass appear to be closer to near-term scale-up than cellulosic ethanol for industrial or small-scale applications. For example, switchgrass can be pressed into fuel pellets that are subsequently burned in pellet stoves used to heat homes (which typically burn corn or wood pellets). Switchgrass has been widely tested as a substitute for coal in power generation. The most widely-studied project to date has been the Chariton Valley Project in Iowa. The Show-Me-Energy Cooperative (SMEC) in Missouri is using switchgrass and other warm-season grasses along with wood residues as feedstocks for pellets used for the firing of a coal-fired power plant. In Eastern Canada, switchgrass is being used on a pilot scale as a feedstock for commercial heating applications. Combustion studies have been undertaken and it appears to be well-suited as a commercial boiler fuel. Research is also being undertaken to develop switchgrass as a pellet fuel because of lack of surplus wood residues in Eastern Canada, as a slowdown in the forest products industry in 2009 is now resulting in wood pellet shortages throughout Eastern North America. Generally speaking, the direct firing of switchgrass for thermal applications can provide the highest net energy gain and energy output-to-input ratio of all switchgrass bioconversion processes. Research has found that switchgrass, when pelletized and used as a solid biofuel, is a good candidate for displacing fossil fuels. Switchgrass pellets were identified to have a 14.6:1 energy output-to-input ratio, which is substantially better than that for liquid biofuel options from farmland. As a greenhouse gas mitigation strategy, switchgrass pellets were found to be an effective means to use farmland to mitigate greenhouse gases. Using farmland to produce switchgrass pellets could mitigate 7.6-13 tonnes per hectare of CO2. In contrast, switchgrass cellulosic ethanol and corn ethanol were found to mitigate 5.2 and 1.5 tonnes of CO2 per hectare, respectively.
Historically, the major constraint to the development of grasses for thermal energy applications has been the difficulty associated with burning grasses in conventional boilers, as biomass quality problems can be of particular concern in combustion applications. These technical problems now appear to have been largely resolved through crop management practices such as fall mowing and spring harvesting that allow for leaching to occur, which leads to fewer aerosol-forming compounds (such as K and Cl) and N in the grass. This reduces clinker formation and corrosion and enables switchgrass to be a clean combustion fuel source for use in smaller combustion appliances. Fall harvested grasses likely have more application for larger commercial and industrial boilers. Switchgrass is also currently being used to heat small industrial and farm buildings in Germany and China through a process used to make a low quality natural gas substitute.
Facebook Comments: