Stage-resolved gas-solid thermochemical coupling in tobacco biomass pyrolysis: Online speciation and structural evolution across characteristic temperature ranges

Tobacco thermal conversion exhibits distinct stage-dependent characteristics that underpin the aerosol generation in both conventional cigarettes and heated tobacco products (HTPs). However, the correspondence between volatile release and solid-phase structural evolution across characteristic temperature ranges remains insufficiently elucidated, particularly regarding the mechanistic origins of aerosol constituents from the thermochemical transformation of the tobacco matrix. Six tobacco types (sun-cured red, cigar, sun-cured yellow, oriental, flue-cured, and burley tobaccos) were investigated. Five characteristic temperature ranges (CTRs) were delineated from the DTG curves. Heart-cut EGA-GC/MS and in situ DRIFTS were employed for online detection of volatile products and monitoring of solid-phase structural evolution, respectively, enabling comprehensive gas-solid coupling analysis. The 220–400 ℃ CTR constituted the primary mass-loss stage, with mass losses of 38.21%-42.26%, and exhibited the most concentrated volatile release and the most intensive solid-phase structural transformations. Above 400 ℃, volatile release gradually diminished, while solid-phase reactions shifted toward condensation of conjugated structures, aromatization, and carbonization. Gas-solid coupling analysis of sun-cured yellow tobacco revealed that carbohydrate oxygen-containing structures underwent dehydration, chain cleavage, cyclization, and skeletal rearrangement; aromatic oxygen-containing structures associated with lignin underwent ether-bond cleavage, deoxygenation, and side-chain transformation; and the evolution of carbon- and nitrogen-containing conjugated structures was associated with the migration of nicotine and other nitrogen-containing components, structural cleavage, cyclization and reorganization of the conjugated framework, as well as Maillard reactions. These findings establish the stage-dependent gas-solid correspondence during tobacco thermal conversion, providing mechanistic insights for optimizing HTP formulations, selecting raw materials for low-tar cigarettes, and regulating combustion-zone dynamics.