Size- and temperature-dependent oxidative pyrolysis and auto-ignition of spherical beech and spruce wood

“The first components of the wooden matrix to decompose upon heating are the hemicelluloses, followed by cellulose and lignin [38].”

“The results of these experiments can be analysed and interpreted using the Frank-Kamenetskii theory (FK analysis), calculating the kinetic constants and extrapolating the ignition temperature of small basket experiments (cm range) to bigger sizes [20, 24].”

“The higher thermal stability of spruce might also be caused by more stable variants of cellulose and lignin in softwood species [40, 41].”

“But biomass, including solid wood, usually exhibits a lot of physical and chemical variations, which are hard to implement in models and simulations, but influence pyrolysis, ignition, and combustion on all scales [17–19].”

“In comparison to the mannan in spruce, xylan can show exothermal reactions even under pyrolytic conditions, which could be a cause for internal overheating of beech at lower temperatures [39].”

“Combining this two effects might cause cracking, and the rapid access of oxygen-rich air another causes for internal overheating and ignition [45].”

“The lowest recorded temperatures for heterogeneous auto-ignition of wood are in the range of 240 °C [5].”

“Additionally, the wood species and the chemical composition have an influence on the pyrolysis and oxidation reactions taking place [17, 30, 31].”

“Especially at lower furnace temperatures, where chemistry has a greater influence on thermal decay than heat and mass transfer, the wood species itself strongly influences auto-ignition behaviour [36, 37].”

“While heating up wood or biomass, pyrolysis also happens in an oxidising atmosphere, as gaseous pyrolysis products streaming away from the material prohibit the contact of oxygen rich air with the surface, building up an oxygen-free diffusion zone [10].”

“Current studies mostly focus on size differences at centimetre to metre scales or use a kinetic approach to the topic through microscale measurements (e.g. thermal analysis) and attempt to simulate the behaviour of wood on a larger scale [15, 16].”

“180 to 240 °C, torrefaction [8, 9].”

“For the auto-ignition of gases, the lower flammability limit of volatiles and a sufficiently high temperature must be reached to trigger homogeneous reactions between pyrolysis products and oxygen [2].”

“Due to its multi-step decomposition, which produces highly reactive charcoal not only under non-oxidative but also under oxidative conditions, especially at lower heating rates, wood is one of the few materials that exhibit this kind of phenomenon [6, 7].”

“600 to 700 °C support this conclusion [3, 25].”

“600 to 700 °C support this conclusion [3, 25].”

“180 to 240 °C, torrefaction [8, 9].”

“Values for the thermal conductivity were taken from Vay et al. [35].”

“For the larger samples, density is one influencing factor, as a higher density will result in lower heating rates under isothermal, pyrolytic conditions [42].”

“The higher thermal stability of spruce might also be caused by more stable variants of cellulose and lignin in softwood species [40, 41].”

“Similar plateaus were observed by various authors in oxidative and non-oxidative pyrolysis experiments [6, 47].”

“Many authors describe the influence of the size and geometry of wood on the auto-ignition temperature, and this is often done in the form of either small wood particles in bulk [20] or wood-based materials, with homogenous density profiles (e.g. pellets) [21, 22].”

“Especially at lower furnace temperatures, where chemistry has a greater influence on thermal decay than heat and mass transfer, the wood species itself strongly influences auto-ignition behaviour [36, 37].”

“When reaching 360 °C, various changes in the wood structure occur under pyrolytic conditions, and precursors of charcoal might have a big impact, for example, by effecting the chemisorption behaviour [12, 46].”

“Influences of the volume and the conditions at pyrolysis (e.g. heating rate and maximum temperature) are well-known for particles in the range of micrometres (e.g. TGA) and in bulk (e.g. FK-Analysis), but only few studies look at solid wood in the range of centimetres [20, 27, 28].”

“Daouk et al. used a makro-TG to study the influence of oxygen concentration on temperatures arising during pyrolysis at temperatures of 400 to 600 °C isothermally and under dynamic heating to 800 °C [6, 29].”

“When reaching 360 °C, various changes in the wood structure occur under pyrolytic conditions, and precursors of charcoal might have a big impact, for example, by effecting the chemisorption behaviour [12, 46].”

“On the porous surface of the material, heterogeneous reactions between oxygen and carbon in combination with internal self-heating can induce a combustion process [3, 4].”

“But the oxidative pyrolysis, self-heating, and auto-ignition of wood particles of all sizes depend on many aspects, above all the surface area and the surface to volume ratio [26].”

“Thermal expansion during pyrolysis, as described in Caposciutti et al. [43], and changing thermal conductivity during conversion to charcoal might also have an impact [44].”

“Influences of the volume and the conditions at pyrolysis (e.g. heating rate and maximum temperature) are well-known for particles in the range of micrometres (e.g. TGA) and in bulk (e.g. FK-Analysis), but only few studies look at solid wood in the range of centimetres [20, 27, 28].”

“Transition from smouldering and glowing combustion to flaming and the other way around is possible and poses a major safety concern [14].”

“This has the huge advantage that different shapes and large homogeneous volumes can be produced by filling the baskets with the material, which is impossible with solid wood, as its properties change with the dimension and cutting direction [23].”

“Many authors describe the influence of the size and geometry of wood on the auto-ignition temperature, and this is often done in the form of either small wood particles in bulk [20] or wood-based materials, with homogenous density profiles (e.g. pellets) [21, 22].”

“Additionally, the wood species and the chemical composition have an influence on the pyrolysis and oxidation reactions taking place [17, 30, 31].”

“Current studies mostly focus on size differences at centimetre to metre scales or use a kinetic approach to the topic through microscale measurements (e.g. thermal analysis) and attempt to simulate the behaviour of wood on a larger scale [15, 16].”

“Due to its multi-step decomposition, which produces highly reactive charcoal not only under non-oxidative but also under oxidative conditions, especially at lower heating rates, wood is one of the few materials that exhibit this kind of phenomenon [6, 7].”

“On the porous charcoal, with a high internal surface, chemisorption of oxygen and direct oxidation of carbon to carbon monoxide and carbon dioxide can initiate a glow-ing or smouldering combustion process [11–13].”

“Isothermal phases and the decomposition of beech and spruce wood were examined by Preimesberger et al. (2024), who found that the decomposition of hemicelluloses and cellulose peaks at around 360 °C. Vermesi et al. (2017) studied spontaneous ignition of wood subjected to transient irradiation and…”