Pressurized Steam Torrefaction of Biomass: Focus on Solid, Liquid, and Gas Phase Distributions

Torrefaction is a thermal pretreatment of biomass feedstocks aimed at their conversion into a commodity solid fuel with more uniform and standard properties. Pressurized torrefaction--a novel concept--deserves consideration because of its possible advantages with respect to the atmospheric process, in particular the establishment of favorable conditions for generation of valuable condensed products. The objectives of this research are (i) an investigation on the feasibility of the pressurized, steam-assisted, batch torrefaction of some agro-industrial biomass residues and (ii) the conversion of these latter into solid and liquid products to be used in energy production or chemical processes, with improved characteristics with respect to the raw biomass. The results reported in this article prove that the operation under pressure allows maintaining high water vapor pressure in the system, enhancing the solid biomass conversion to liquid products. The recovery of valuable liquid compounds from the solid residue proved to be further boosted by a subsequent stage of solvent extraction. The condensed liquid fraction resulting from torrefaction turns out to be much higher (i.e., up to 3 times) under pressurized condition than under the atmospheric conditions. The influence of the water-to-fuel feed ratio on the distribution between solid and liquid fractions is also noticeable. Thermodynamic computations demonstrate a decrease of the heat duty required for the torrefaction at the highest pressure (P = 40 atm) due to the presence of liquid water in the final system.