Fe/Mg Silicate Mining Residues as Solid Oxygen Carriers for Chemical Looping Combustion of Torrefaction Volatiles

Torrefaction of woody biomass is a thermal treatment often used to increase biomass energy density prior to its thermochemical conversion. The feasibility of integrated torrefaction, where the energy required for torrefaction is provided from the burning of its self-released volatiles, while the resulting carbon dioxide is subsequently captured in a permanent sink, is an original avenue, which was investigated in this study from the point of view of mining residue valorization. The use of abundant, cheap, and comminuted minerals as solid oxygen carriers for chemical looping combustion (CLC) and as carbon sinks for mineral carbonation in combination with torrefaction of woody biomass was studied. The mining residue consisted of iron-rich (17.3 wt %) post-nickel recovery chrysotile/lizardite silicate minerals. Air pre-calcination of the mineral at 700 degrees C helped boost the extra-framework iron fraction up to 66%, mainly in the form of hematite, as evidenced from Mossbauer spectroscopy. Such a calcined oxygen carrier was then tested in airless torrefaction-CLC tied configuration under gas recirculation over the temperature range of 500-700 degrees C to burn the volatiles released during the torrefaction of birch at 260-300 degrees C. Up to 96% of the carbon released from the torrefied birch as volatiles was converted in the CLC bed into CO2, highlighting the ability of the residue for burning torrefaction volatiles. The residue was finally tested in a torrefaction-CLC-carbonation configuration under gas recirculation for burning the torrefaction volatiles and capturing the produced carbon dioxide for various torrefaction, CLC, and carbonation temperatures. Up to 20% of produced CO2 was sequestrated using this concept at the optimal carbonation bed temperature of 50 degrees C.