Flameless burning of wood: parameters of macrokinetics of pyrolysis and thermo-oxidative decomposition

Abstract

Introduction. Many materials are subject to flameless, smoldering combustion: coal, cotton, peat, carbonizing ­polymers, etc. The fire hazard of smoldering burning of organic materials is that low-calorie ignition sources are sufficient to initiate the combustion process, the process is hidden, making it difficult to detect, and can spontaneously turn into a fiery one. Purpose and objectives. The purpose of this work was to determine the macrokinetics of pyrolysis and thermo­oxidative decomposition of wood of different types of conifers and deciduous species by thermal analysis. Methods. Samples were investigated by thermal analysis in an inert and air environment. For this, we used the automated modular system Du Pont-9900, including the TGA-951 thermobalance, and the DSK-910 diffe­rential scanning calorimeter. Results. It was found that the pyrolysis of the main components of wood (hemicellulose and cellulose) proceeds according to the nucleation and growth mechanism of nuclei according to the case law R ( n = 1) with activa­tion energies close in order of magnitude for different species (98–136 kJ/mol for hemicelluloses and 203–233 kJ/mol for cellulose). At the stages of thermooxidative decomposition of wood components and heterogeneous oxidation of the carbonized product, diffusion of the D3 (D4) type in spherical geometry becomes the mechanism controlling the process. The effective activation energy of the decomposition of hemicelluloses is reduced to 90.9–95.8 kJ/mol, and of cellulose to 138.3–160.9 kJ/mol. The reaction of heterogeneous oxidation of carbonized products makes a significant contribution to the flameless, smoldering combustion of the material. It is diffusion-controlled and is characterized by high values of activation energy (up to 285 kJ/mol). Conclusion. The results of the work make it possible to evaluate the macrokinetic parameters of pyrolysis and thermooxidative decomposition of wood of different species during flameless combustion. The obtained data can be used as the main parameters for modeling the heterogeneous combustion of wood of different species in buildings.