Ash-Forming Matter in Torrefied Birch Wood: Changes in Chemical Association

Torrefaction is heat treatment of biomass at relatively low temperatures (similar to 240-300 degrees C) in the absence of air. The torrefied fuel offers advantages to traditional biomass, such as higher energy density, better grindability, and reduced biological decay. These factors could, for example, lead to increased use of biomass in pulverized coal boilers. Ash-forming elements are present in biomass as water-soluble salts, ion-exchangeable elements, included or excluded minerals, and covalently bound sulfur and chlorine. In this work, we have studied the change in the chemical association of ash-forming elements in birch wood as a function of the extent of torrefaction. The birch wood was torrefied at 240, 255, 270, or 280 degrees C at ECN, The Netherlands. The raw and torrefied birch wood samples were studied using three different techniques: chemical fractionation, potentiometric titration, and methylene blue sorption. Chemical fractionation was performed on the original wood sample, and the samples of wood were torrefied at either 240 or 280 degrees C. These results give a first understanding of the changes in the association of ash-forming elements during torrefaction. The most significant changes can be seen in the distribution of calcium, magnesium, and manganese, with some change in water solubility seen in potassium. These changes may, in part, be due to the destruction of carboxylic acid groups, which were measured by both potentiometric titration and methylene blue sorption. In addition to some changes in water and acid solubility of phosphorus, a clear decrease in the concentration of both chloride and sulfur was measured. The results provide new data about chemical changes with regards to the inorganic elements during torrefaction. The decrease in the chloride content should be investigated further with high-chloride-containing fuels. If a significant level of chloride is removed by torrefaction, this would be a significant additional benefit for the combustion of torrefied biomass.