Torrefaction of Acacia nilotica: Oxygen Distribution and Carbon Densification Mechanism Based on In-Depth Analyses of Solid, Liquid, and Gaseous Products
2020
Singh, Satyansh | Chakraborty, Jyoti Prasad | Mondal, Monoj Kumar
Torrefaction has been proven as a promising pretreatment process that can effectively reduce the moisture content and increase the energy density and higher heating value (HHV) of raw biomass and convert it into high-grade solid biofuel. To reveal the overall efficacy, mechanism, oxygen distribution, and carbon densification during torrefaction, in-depth analyses of solid residues, liquid condensates, and gaseous products became reasonably significant. Therefore, the present work aimed to establish the oxygen distribution and carbon densification mechanism during torrefaction by investigating the characteristics of solid residues, liquid condensates, and gaseous products by performing torrefaction between 225 and 300 °C and retention time (RT) (15–60 min). Results revealed that oxygen distribution and carbon densification are mainly responsible for the upgradation of raw biomass. The oxygen removal efficiency and carbon densification increased by 39.6 and 51.8%, respectively, at 300 °C and 30 min of RT. The densification of carbon in torrefied biomass resulted in an increase of HHV by 41.6% as compared to raw biomass. The oxygens distributed in the form of CO₂ (52.4 wt %) and CO (47.5 wt %) in gaseous products and oxygen-containing liquid condensates were mainly water (43.9 wt %) followed by phenols (18.0 wt %), ketones (15.6 wt %), furans (13.1 wt %), aldehydes (4.4 wt %), and acids (4.4 wt %) at 300 °C and 30 min of RT. The physical property like water content improved due to dehydration reaction, while viscosity and pH improved due to decrease in water content and acidic compounds, respectively, present in the liquid condensate. The ¹³C NMR spectroscopy suggested that the total aromatic carbon and aromaticity of the liquid condensate increased with temperature during the process.
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