Moisture sorption characteristics and modeling of energy sorghum (Sorghum bicolor (L.) Moench)

Abstract

Long-term low cost storage of whole-stalk lignocellulosic energy sorghum biomass (specialized forage varieties of Sorghum bicolor (L.) Moench) is essential for the feedstock's successful role as a dedicated energy crop for ethanol production. As an alternative to expensive ensiling methods, aerobic storage of S. bicolor (L.) Moench biomass in traditional rectangular bale formats could alleviate feedstock supply costs if material deterioration in storage could be minimized. Moisture desorption and adsorption isotherms for S. bicolor (L.) Moench were created at 15 °C, 20 °C, 30 °C, and 40 °C with water activities from 0.1 to 0.9 using the dynamic dew-point method. Sorption isotherms were modeled using four temperature dependent and three temperature independent equations. The relationship between equilibrium moisture content and water activity was found to decrease with increasing temperatures. GAB (Guggenheim–Anderson–de Boer) monolayer moisture content and the moisture content at which microbial activity becomes limited were found to range from 5.6% db to 10.4% dry basis (db) and 12.0% db to 18.4% db, respectively. The net isosteric heat of sorption was calculated using the Clausius–Clapeyron equation and determined to be higher for desorption than adsorption with both trends decreasing exponentially at increasing levels of moisture content. The differential entropy of S. bicolor (L.) Moench was shown to exhibit a log normal relationship with moisture; peaking near the monolayer moisture content. The results of the study indicate that aerobic storage of energy sorghum biomass may be similar to other herbaceous feedstocks should extensive drying occur before entering storage.