Variability in biomass yield, chemical composition, and ethanol potential of individual and mixed herbaceous biomass species grown in North Dakota
Highlights
► Wheatgrass species had higher yields than switchgrass during establishment on dryland. ► Variability is lower for carbohydrate content than yield for herbaceous perennials. ► Yield is more important than composition in choosing species for ethanol potential.
Introduction
The increasing demand for energy, the instability and uncertainty in petroleum resources, and environmental concerns associated with the use of fossil fuels have made the development of alternative fuels a priority in the United States. Many renewable energy resources (solar, wind, biomass, geothermal, etc.) have been identified to mitigate fossil fuel CO2 emissions, but ethanol produced by fermenting 6-carbon or 5-carbon sugars is the main alternative for liquid transportation fuels (Goldemberg, 2007). These simple sugars can be obtained from biomass, starchy grains, and sugar crop feedstocks. Presently, ethanol is produced from corn (Zea mays L.) in the US, sugarcane (Saccharum officinale L.) in Brazil, and sugarbeet (Beta vulgaris L.) in France (Mussatto et al., 2010). However, increasing the production of ethanol will require the use of other available feedstocks such as cellulosic materials.
Cellulosic materials including wood, perennial grasses, and agricultural residues are estimated to be available at sustainable harvest rates of 0.856–1.009 billion Mg/year in the US by 2022 (DOE, 2011). The 2007 Energy Independence and Security Act mandates that by 2022, 60.5–79.5 billion liters [16–21 billion gallons] per year (BLY) of biofuel production be derived from cellulosic biomass (EIA, 2008). Currently, there is no large scale production of perennial grasses in the US for biofuel, but in order to achieve the potential biomass production, 40–60 million ha of US cropland, idle cropland, and pasture will be needed for the production of perennial grasses (DOE, 2011).
Perennial grasses offer many advantages compared to other forms of cellulosic biomass, because they can be grown in diverse environments with comparatively low energy and chemical inputs (Downing et al., 1995). Relatively high biomass yields can be obtained even when perennial grasses are planted on marginal soils because of their lower nutrient demand than food crops; they can also provide important soil and water conservation benefits to erosion-prone land (Adler et al., 2009, Downing et al., 1995). Even with biennial or annual harvesting, perennial grasses improve long-term sustainability on lands by reducing erosion and adding soil organic matter (McLauchlan et al., 2006). The availability of 4 million ha of Conservation Reserve Program (CRP) and erodible lands in North Dakota has given the state strong potential for the production of herbaceous forage in the US (Milbrandt, 2005).
Perennial grasses, like other forms of biomass, are composed of cellulose, hemicellulose, lignin, extractives, and inorganic substances (ash). The primary components (cellulose, hemicellulose, and lignin) are vital resources in sugar platform (fermentation of biomass sugars) or thermochemical platform (e.g. gasification followed by biological or chemical processing) technologies. Cellulose is a polymer of β-d-glucose units joined together by glycosidic bonds to form a long, rigid structure. As cellulose comprises 30–50% of most harvestable biomass material, it is considered the most abundant organic material on earth (McKendry, 2002). Hemicellulose, which represents 20–35% of plant biomass, is not chemically homogenous like cellulose but is made of branched structures of a variety of pentoses (xylose and arabinose) and hexoses (mannose, glucose, and galactose) with xylose constituting the highest proportion of hemicellulose in perennial grasses.
Ethanol or other fermentation products from biomass are derived through bioconversion of hydrolyzed sugar. Therefore, higher proportions of cellulose and hemicellulose in biomass will lead to higher ethanol potential. Glucose monomers from cellulose hydrolysis are the most readily fermentable but all hexoses and pentoses must be used to maximize yields and reduce costs. Biomass composition changes during plant growth and growing conditions influence relative proportions of structural carbohydrates and lignin. The rate of polysaccharide formation during photosynthesis and the amount of inorganic substances absorbed from the soil are among the many factors that influence the heterogeneity and complex chemical composition of biomass species (Adler et al., 2009, Michalet et al., 2006).
Mass-basis ethanol yields (L Mg−1) may be a general indicator of biomass quality but area-basis ethanol yields (L ha−1) will be important to processors as they will impact the area required to supply a processing plant of a given size. Mass-basis ethanol yields are proportional to the amount of polymeric sugars (glucan, xylan, arabinan, and galactan) present in the feedstock. Identifying individual and conglomerate herbaceous perennial species with the highest ethanol yield potential on both a mass and area basis will be beneficial to both agricultural producers and ethanol processors.
Much work has been done examining composition of a variety of herbaceous perennials as forage crops but most of these studies focus on crude protein and fiber contents which are less valuable in evaluating biorefinery feedstocks (Gierus et al., 2012, Licitra et al., 1997, Ritchie et al., 2006, Walter et al., 2012). Xue et al. (2011) evaluated cellulose and hemicellulose content of mixed species biomass stands harvested in 2007 in North Dakota, but that study did not evaluate the biomass composition based on individual carbohydrate content (glucan, xylan, arabinan, and galactan). Such data is required for more accurate evaluation of fermentation potential. The primary objective of this work is to compare biomass yields and analyze variability in the chemical composition among combinations of herbaceous perennial biomass species in order to predict ethanol production potential on a mass and area basis for each species combination and environment.
Section snippets
Biomass sample collection
Biomass samples were obtained from a ten-year study through the Central Grassland Research Extension Center (CGREC) in Streeter, ND to determine yields and impacts of growing mixtures of herbaceous perennial species (Nyren et al., 2007). Samples were taken from plots in Williston [48.16°N and 103.64°W] (2008, 2009, and 2010) and Minot [48.24°N and 101.38°W] (2008), ND. Plots in Williston were irrigated weekly to give approximately 635 mm (including precipitation) of water per growing season
Biomass yield
Biomass yields from Williston irrigated plots and Minot non-irrigated plots for the years 2008, 2009, and 2010 are shown in Table 3. Switchgrass cultivars incorporated as single or conglomerate species showed higher dry-matter biomass yields in Williston irrigated plots. Even though there were substantial variations among harvest years (e.g. 16.9 Mg ha−1 in 2008 and 12.4 Mg ha−1 in 2010), Sunburst switchgrass had the highest average yields of 14.4 Mg ha−1. Switchgrass plot yields have been shown to
Conclusion
Maximizing feedstock availability for cellulosic ethanol or other biomass-based industries will require cultivation of species with high biomass yields and high structural carbohydrate content. This study demonstrated that the variability in carbohydrate yields among different selected herbaceous perennial species is much less than the variability in biomass yields in different environments over several years. That is, if these different perennial biomass species are grown under different
Acknowledgments
The authors would like to acknowledge help from Nurun Nahar and Bishnu Karki during biomass compositional analysis. This study was funded through grants from the North Dakota Natural Resources Trust (award) and USDA – NIFA (Agreement #2010-34622-20794).
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