The present work explores the effects of the parallel reactions on the oxidation reactivity of plant chars formed at various heat treatment temperatures by using a multi-component parallel reaction model in conjunction with the non-linear kinetic analysis technique. Four plant species were selected. A plant char was prepared in a programme-controlled horizontal tube furnace under the atmosphere of N₂ at a purity of 99.999%, and its heat treatment temperature was set at 450, 520 and 800 ℃, respectively. The aerial oxidation characteristics of plant chars were analyzed by using a simultaneous thermo-gravimetric analyzer and differential scanning calorimeter, and the kinetic data of the parallel reactions were then retrieved by fitting both the mass change and mass loss rate data obtained during the oxidation measurements. It was confirmed that for the oxidation of the chars formed at moderate heat treatment temperatures (i.e. 450 and 520 ℃), the variation patterns of mass loss rate and the heat flow rate curves are contributed by the parallel oxidation reactions of lignin residue, amorphous carbon and the other reactive substances such as crude fat and protein, etc. When the heat treatment temperature reaches 800 ℃, the reactive substances stored in the char produced are mainly amorphous carbon, and the char oxidation can be simplified to an one-step reaction. Compared with the other two reaction components, lignin residue has the lowest activation energy for oxidation with the range between 86 and 147 kJ·mol^-1, and its reaction temperatures vary between 300 and 480 ℃. The activation energy for the amorphous carbon fluctuates between 174 and 208 kJ·mol^-1 with its reaction temperatures altering between 370 and 520 ℃. The other reactive substances undergo oxidation with the activation energy between 214 and 225 kJ·mol^-1 and the corresponding reaction temperatures are between 420 and 510 ℃. It is obvious that the oxidation reactivity of plant chars mainly relies on the performance of lignin residue. With the increase in the heat treatment temperature for making a char, the oxidation reactivity of the plant char essentially reduces, which is essentially attributed to the decrease in the content of lignin residue and the increase in the activation energies for the other two components to oxidize. The established understanding lays the foundation for developing more effective methodologies for making use of the energy stored in plant chars and paves the way for identifying the role of plant chars in the spread of a wildland fire.

Key words: plant char, oxidation, kinetic analysis, reaction temperature, reactivity