The release of chlorine (Cl) and sulfur (S) during biomass torrefaction and pyrolysis has been investigated via experiments in two laboratory-scale reactors: a rotating reactor and a fixed bed reactor. Six biomasses with different chemical compositions covering a wide range of ash content and ash-forming elements were torrefied/pyrolyzed in the temperature range of 150−500 °C. The relative release of chlorine and sulfur was calculated based on mass balance and analysis of the biomass before and after torrefaction. In selected cases, measurement of methyl chloride (CH 3 Cl) in the gas from straw torrefaction has furthermore been conducted. The release of chlorine from straw was first observed at 250 °C and peaked with about 60−70% at 350 °C. Analysis of the released gas showed that most of the chlorine was released as methyl chloride. Increasing the straw content in the reactor resulted in a lower fractional release of Cl, probably due to more reactive sites in contact with gas phase Cl species leading to secondary binding of Cl to the solid product. Almost complete release of chlorine was observed for woody biomass at 350 °C. This result is in agreement with previous studies reporting that biomasses with a lower chlorine content release a higher fraction of chlorine during the pyrolysis process. A significant sulfur release (about 60%) was observed from the six biomasses investigated at 350 °C. The initial sulfur content in the biomass did not influence the fraction of sulfur release during torrefaction.
Torrefaction is heat treatment of biomass at relatively low temperatures (∼240–300 °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 °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 °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.
Torrefaction is the thermal pretreatment of biomass at temperatures of 200−300 °C in an inert atmosphere with the objectives of improving resistance to biodegradation, reducing hydrophilicity, improving grindability and increasing energy density. In this work, we studied the effect of torrefaction temperature (240−280 °C) on the chemistry of birch wood. The samples were from a pilot plant at ECN, and in that way, they were representative of industrially produced samples. We have measured the concentration of hemicellulose and cellulose; changes in the extractives content and composition; and in the lignin structure. We used acid methanolysis and acid hydrolysis for hemicellulose and cellulose analysis, respectively; Klason lignin method, 13 C CP-MAS NMR, Dipolar Dephasing NMR, and Py-GC-MS analysis for lignin characterization; and acetone extraction, HPSEC, GC-FID, and GC-MS analysis for extractives characterization. The results provide a more complete picture of the chemical changes in wood by torrefaction.
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