Evaluation of the structure and fuel properties of lignocelluloses through carbon dioxide torrefaction

•Pine and poplar were torrefied in a tubular reactor under carbon dioxide.•Increase in temperature resulted in a removal of cellulose and hemicelluloses.•HHV of product increased to 25.6 MJ/kg when torrefied at 340 °C.•The combustion activity of the torrefied sample reached a level of lignite.

Two typical lignocelluloses, pine and poplar, were torrefied in a tubular reactor under carbon dioxide medium at temperatures ranging from 220 to 340 °C. The effect of torrefaction temperature on the structure and properties of the chars obtained was investigated to evaluate their fuel applications. As the temperature raised from 220 to 340 °C, the mass yield decreased from 96.7% to 48.7% for pine, and from 97.1% to 43.8% for poplar. The higher heating value of the torrefied pine increased to 25.6 MJ/kg for the sample torrefied at 340 °C. An increase in reaction temperature resulted in a decrease in the contents of cellulose and hemicelluloses but notable increase of the content of lignin, especially for the sample subjected to the strongest torrefaction. The variation of C, H, and O contents of the samples after torrefaction suggested that a high temperature promoted the dehydration and deacetylation reactions. Thermal analysis indicated that the combustion activity of the sample was improved to a level of lignite after torrefaction. The hydrophobicity of lignocelluloses was notably increased with increased torrefaction severity, suggesting the prepared samples are good feedstocks for further processing.

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