Sweet Sorghum as a Biofuel Crop

Abstract

Sweet sorghum is a C4 grass which is traditionally cultivated for making syrup from the sugars in the stalks. Sweet and grain sorghum are in the same species, Sorghum bicolor (L.) Moench. In optimum conditions, sweet sorghum can grow 4.5 meters tall and produce 45 to 110 Mg of fresh weight biomass per hectare with less N and water than maize. Ethanol can be produced from sweet sorghum stalks by extracting the juice and fermenting the sugars with yeast. Bagasse remaining after extraction can be fed to livestock or converted to useable energy by burning to produce steam for generating electricity, anaerobic digestion to make methane, or reacting with oxygen at high temperature to produce synthetic gas. Sweet sorghum can be grown in most climates. Some cultivars will grow as far north and south as the 45° latitude. It is normally grown as an annual crop. But, in warm climates, a single seed planting can be managed for two or three years by leaving lower stalks and roots at harvest to produce new tillers for the next growing cycle. Rotating sweet sorghum in alternate years with a non-grass crop is an effective management tool for reducing insect, weed, and disease pests. Nitrogen fertilizer can often be reduced when sweet sorghum is planted after a legume crop such as soybean. Most of the sugar for syrup and ethanol from sweet sorghum is currently produced with open pollinating cultivars. Recently, hybrid sweet sorghums with increased sugar content and biomass yield have been developed in India, China, and the United States.