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Biomass torrefaction in slot-rectangular spouted beds Wang, Ziliang


Biomass, a nearly carbon-neutral energy resource, can reduce greenhouse gas emission and replace fossil fuels, but it is characterized by heterogeneity, high moisture content, low bulk density, low calorific value, pliability and hygroscopic nature, all of which challenge its utilization. Torrefaction, a thermal pretreatment method, is capable of modifying the physical-chemical properties of biomass and enhancing its calorific value. Slot-rectangular spouted beds (SRSBs) can effectively handle biomass particles and offer a promising way to overcome the scale-up challenge of conventional spouted beds. This study explored the potential application of SRSBs to biomass torrefaction. Solids mixing in a dual-compartment slot-rectangular spouted bed (DSRSB) was first studied to address the scale-up issue, while also providing fundamental information needed for the design and operation of a DSRSB reactor. SRSB and DSRSB reactors were developed for torrefying sawdust with the semi-batch operation. Hydrodynamics of SRSB and DSRSB, torrefaction performance, torrefied product properties and torrefied product pyrolysis were subsequently investigated. Temperature, biomass feed rate, sawdust particle size and oxygen concentration influenced torrefaction performance and torrefied product properties. Temperature was found to be the most important factor. Biomass torrefaction performed better in the DSRSB than in the SRSB. Higher temperature, lower biomass feed rate, larger sawdust particle size and greater oxygen concentration all led to increased weight loss and decreased energy yield of sawdust, and produced torrefied sawdust with higher HHV, greater atomic carbon content, lower atomic hydrogen and oxygen contents, less volatile matter, greater fixed carbon and less hemicellulose. Higher temperature and greater oxygen concentration were very helpful to produce more torrefied sawdust captured by a cyclone. Performance of oxidative torrefaction was similar to that of non-oxidative torrefaction. The effect of oxygen concentration was more significant at a higher temperature. Torrefied sawdust underwent size reduction during torrefaction, with smoother and cleaner surfaces compared to raw sawdust. The activation energy for non-oxidatively torrefied sawdust was higher than for oxidatively torrefied sawdust, which was in turn greater than that of raw sawdust. Sawdust particle size affected the pressure drop across the reactor.

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Attribution-NonCommercial-NoDerivatives 4.0 International