Field Productivities of Napier Grass for Production of Sugars and Ethanol

Napier grass (Pennisetum purpureum (L) Schum) is being developed as a bioenergy crop for production in the southeastern United States. Important criteria for selecting a feedstock are cost, consistent biomass production, composition, and process related quality. In this study, we considered the effects of fertilizer application and cutting regimes on production yield, chemical composition, and process yields. Napier grass was grown for 4 years in field plots (Shellman, GA) with three treatments, which were selected to maximize production yields and replicated in four subplots. The treatments varied by cuttings (one or two) and number of fertilizer applications (either all at once or split in two). It was observed that multiple seasonal cuts negatively impacted production yield by 21% over the 4 year period. Samples from years 2 and 4 were analyzed for composition. Glucan, xylan, and acid insoluble lignin differed among the samples. Carbohydrate yields were 11.1-25.7 Mg/ha. Samples were pretreated with low moisture ammonia hydroxide (110 °C, 2 days) and evaluated for conversion to sugars, using commercial cellulases, as well as to ethanol using separate hydrolysis and fermentation with Scheffersomyces stipitis. Glucose and xylose yields were 317-379 kg/Mg and 141-164 kg/Mg, respectively, for years 2 and 4. Ethanol yields were 268-313 L/Mg or 61.3-79.3% of maximum. This equates to annual ethanol yields of 5.9-12.8 m³/ha. Ethanol yields for year 4 were over 50% higher for the crop harvested in winter versus the crops harvested in summer and winter. The ethanol production per hectare from winter harvested Napier grass is comparable to that of grain-based biofuel crops.