Torrefaction of Demineralized Wood with Flue Gas: Kinetics, Product Distribution, and Thermal Conversion

Flue gas torrefaction is an emerging biomass pretreatment technology that utilizes industrial flue gas as a reactive medium to replace inert atmospheres. However, the intrinsic complexity of biomass and the catalytic interference of ash hinder mechanistic elucidation. This study investigated the torrefaction behavior of demineralized poplar wood under N2, CO2, dry flue gas (DFG), and wet flue gas (WFG) at 300 °C for 5–20 min. Thermogravimetric analysis combined with kinetic modeling (FWO, KAS, and CR methods) revealed that the apparent activation energy (Eα) varied non-monotonically with atmosphere oxidizability. Under N2, the average Eα was 177 kJ/mol following the three-dimensional diffusion model (D5). CO2 gave the highest average Eα (314 kJ/mol) with the Avrami–Erofeev nucleation model (A1/4). DFG and WFG significantly reduced the average Eα to 133 and 128 kJ/mol, respectively, both following the A1/3 model. Consistently, WFG yields the lowest char and the highest gas yield. XPS and FTIR analyses indicated that flue gas atmospheres, especially WFG, promoted deeper deoxygenation and aromatization of biochar. Tar composition underwent a noticeable transition from ketones to aldehydes and saccharides under flue gas conditions, with the most remarkable variation observed under WFG. Gaseous products were dominated by CO2 under N2 and by CO under CO2, while DFG and WFG produced moderate and stable gas compositions. These findings demonstrate that flue gas torrefaction, particularly under WFG, effectively enhances biomass effectively upgrades biomass quality by regulating pyrolysis kinetics and product distribution, and demineralized biomass is a suitable intermediate model for mechanistic investigation.