Ethanol Conversion Efficiency of Biomass From Woody Invasive Shrubs Compared with Switchgrass.

Switchgrass biomass is viewed as a high potential source of cellulosic ethanol that is renewable, reduces the carbon footprint, and does not directly jeopardize food stocks.  While Switchgrass is a C4 plant that grows well in warm climates with high radiation loads, it may not represent the optimal biomass source in the cool, moist climate of the northeastern Midwest.  Our objective was to evaluate the potential of invasive shrubs to produce cellulosic ethanol in comparison with Switchgrass in southwest Michigan.  Biomass samples from 1, 2, and 3-year old common buckthorn, glossy buckthorn, and autumn olive plants were compared with the annual growth of Switchgrass.  Frequent shrub cutting minimizes the risk of seed production and re-growth studies indicated that 98% of cut plants were capable of re-establishment, regardless of plant age.  Grinding and soaking biomass samples in 80% ethanol followed by aggressive acid hydrolysis and separation of monosaccharide derivatives via gas chromatography allowed quantification of extractives, lignin, monosaccharide, and ash fractions.  Older woody biomass produced progressively larger proportions of lignin (up to 24%) compared with 10% of the total biomass fraction from Switchgrass.  Ethanol soluble extractives, mainly proteins and lipids that do not contribute ethanol, decreased from 23% of the total biomass to 19% with increasing plant age.  Switchgrass produced the highest fermentable sugar content at 69% compared with 60% from one-year old or 62% three-year old woody biomass.  Glucose was the primary monosaccharide sugar of the cellulose and hemicellulose fraction, ranging between 75 and 81%, regardless of species or age.  Of the invasive species evaluated, autumn olive exhibited the optimum biomass conversion efficiency.  Compared with Switchgrass, three-year old Autumn olive biomass produced 33% less extractives, 10% less cellulose and hemicellulose, but more than twice as much lignin.  While lignin is not easily convertible to ethanol it has value as an energy source in the fermentation process.