Reaction mechanism investigation on Plasma-Assisted steam reforming of toluene based on emission spectrum analysis and kinetic simulation

Plasma-enhanced reforming has been widely studied as an efficient technology for online tar removal. By combining reforming experiments with kinetic modeling and in-situ optical emission spectrometry (OES) analysis, interaction mechanism and coupling rules of plasma/thermal reactions during plasma-assisted steam reforming of toluene were well explored in this study. A chemistry model was innovatively developed for plasma-assisted steam reforming of toluene, and predicted experimental results under different operations conditions with superb agreements. The rate of production and sensitivity analysis implied that OH, H and N-2* mainly contributed to primary toluene conversion, while the electronic attachment dissociation of H2O, H addition onto benzene ring and benzyl oxidation by OH were the most enhancing-sensitive reactions. Resultantly, toluene conversion was greatly increased with steam addition before Vf(H2O) = 10 %, but after which Vf(H2O) increase deteriorated the reforming performance due to the inhibition of E-N-2 collision by high Vf(H2O). As temperature increased, plasma and thermal synergistically promoted reforming performance before 300 degrees C, at which > 60 % toluene was converted into small molecules such as CO, H-2 and C2H2, with yields of 23.6 %, 9.4 % and 16.3 % correspondingly at 12 kJ/L, but further temperature increase would not benefit due to the reduced discharge field strength at higher temperatures, being confirmed by the decreased relative intensity of OH, CN feature bands on OES. It was initially proposed that non-thermal plasma governed the reforming process at T < 300 degrees C, and thermal reactions predominated at T > 500 degrees C after a transition region (300 similar to 500 degrees C). Increase of input energy caused higher discharge field strength, promoted the electronic collision ionization or dissociation of N-2 and H2O, thus enhanced toluene conversion and gas yields. A detailed reaction pathway has also been clarified.