Washing sorghum biomass with water to improve its quality for combustion

Sorghum [Sorghum bicolor (L.) Moench] is ideally suited as an energy feedstock given its versatility as a single source of starch, sugar, and lignocellulose. Alkali oxides, halides and ash content can be problematic when feedstock is intended for combustion applications because these elements can lead to slagging and fouling of boilers. Solutions for counteracting the alkali content of herbaceous biomass include chemical addition to neutralize alkali, blending high-alkali biomass with low-alkali sources to achieve an acceptable ash-fusion temperature in the resulting mixture, and removal of the alkali content through washing techniques. The objective of this experiment was to investigate the effect of washing sorghum biomass with water to improve its quality for combustion. Biomass separated in 1-kg replicates were washed with 7.6, 15.2 and 22.8 L of water and analyzed for composition. Ash content was reduced by up to 20% while lignin content increased up to 53% with washing. Higher heating values significantly increased by washing and were inversely correlated with ash and positively correlated with lignin content. Nitrogen, chlorine, calcium, magnesium and potassium were reduced by washing. In contrast, carbon, hydrogen, silicon, aluminum, titanium, iron, sodium, sulfur (as % of ash), and phosphorus significantly increased by washing. When calculated, total alkali kg/GJ decreased with washing to levels close to those known to be acceptable for combustion. Further experiments should be performed to understand the economics of washing sorghum biomass at a commercial scale as well as the potential value of leachates within a closed-loop system of sorghum feedstock production.