Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching

Inorganic constituents of ash in biomass fuels are responsible for equipment failure and operating difficulties in thermochemical energy conversion facilities. Alkali metals, in the presence of chlorine and sulfur, are the leading contributors to this problem. Banagrass, a herbaceous species being considered for use as a dedicated energy crop, contains high levels of potassium and chlorine. Some inorganic elements are water soluble and the opportunity exists to remove them by mechanical dewatering and leaching as part of the feedstock preparation process. Laboratory-scale equipment, representative of processes employed in the commercial extraction of sugar from cane, was used to prepare banagrass fuel treatments that included two degrees of comminution (coarse and fine) and two dewatering schemes (mechanical dewatering only, and a multi-step process consisting of initial mechanical dewatering followed by a water rinse and second dewatering). The treatment that included fine comminution and multi-step dewatering resulted in a fuel with substantial reductions in ash (45%), K (90%), Cl (98%), S (55%), Na (68%), P (72%) and Mg (68%). The coarse comminution and multi-step dewatering scheme also resulted in reductions, but generally with 10-20% more of the initial constituent mass retained in the fuel. These two treatments produced fuels containing 0.11 and 0.23 kg (Na2O + K2O) GJ(-1), respectively, with corresponding ash fusion temperature estimates of 1250 and 1075 degrees C. By comparison, bagasse, the fibrous by-product of sugar cane, contains 0.06 kg (Na2O + K2O) GJ(-1) and has an estimated ash fusion temperature of roughly 1500 degrees C. Banagrass subjected to the most severe treatment, fine comminution with multi-step dewatering, should produce a boiler fuel with characteristics similar to those of bagasse. (C) 1997 Published by Elsevier Science Ltd.