Elsevier

Combustion and Flame

Volume 234, December 2021, 111591
Combustion and Flame

Effect of oxygen on the burning rate of wood

https://doi.org/10.1016/j.combustflame.2021.111591Get rights and content

Abstract

The large-scale adoption of wood as a construction material for tall buildings could pave the way for sustainable construction. Its adoption, however, is hindered by a limited understanding of wood's behaviour in a fire. In particular, the effect of oxygen and heat flux on the burning (including pyrolysis) and ignition behaviour of wood is poorly understood. We addressed this gap by studying the effect of oxygen concentration and heat flux on the burning and ignition behaviour of particleboard experimentally and computationally. Particleboard was chosen as a proxy for all woody construction materials. We conducted over 60 experiments in an FPA on samples of particleboard spanning different oxygen concentrations (0–21%), heat fluxes (10–70 kW/m2), sample densities (600–800 kg/m2), and sample thicknesses (6–25 mm). Only the heat flux and oxygen concentration significantly affected the charring rate, time-to-flaming ignition, and burning mode (pyrolysis, smouldering, flaming). To explore this effect further, we used a multi-physics model of particleboard charring developed in Gpyro. Combining the computational and experimental results, we showed that particleboard undergoes only pyrolysis in oxygen concentrations below 4%, smouldering between 4 and 15%, and flaming above 15% at a heat flux of 30 kW/m2. These oxygen concentration thresholds were found to decrease as the heat flux increases. We also showed that smouldering and flaming increases the charring rate by 25 and 37%, respectively. This means that the rate of loss of a section of structural wood, quantified by the charring rate, in a fire due to smouldering is similar to that of flaming combustion. In addition, we noted the existence of a triple point for the ignition of wood at which a slight change in environmental conditions can lead to either smouldering, flaming, or only pyrolysis. In summary, this paper quantified for the first time the contributions of the three modes of burning to the charring rate of wood and highlights the importance of smouldering for timber construction.

Section snippets

Introduction to wood fires

Engineered wood is becoming a popular construction material due to its strength, lightweight, and sustainability [1]. It is made by gluing wood fibres (Medium Density Particleboard), chips (Low-Density Particleboard), or planks (Cross Laminated Timber) together in various assemblies. However, a limited understanding of the fire behaviour of engineered wood hinders its uptake [1]. Specifically for tall buildings, wooden products are largely prohibited or too costly due to the strict fire safety

Experimental method

The experiments were conducted using a Fire Propagation Apparatus (FPA) following the ASTM E 2058 standard. In an FPA, the fuel sample is exposed to a radiative heat flux, and a pilot ignition source placed above its free surface. In this study, the range of radiative heat imposed varied from 10 to 70 kW/m2 and was maintained constant for the whole duration of the experiment. The sample mass was continuously measured and the exhaust gases were analysed for composition, temperature, and flow

Model validation and analysis

The model was developed at the microscale and mesoscale against several experiments based on our previous work on natural wood [23,29]. Engineered wood differs from natural wood due to the presence of resin (affects kinetic model and heat of reactions), absence of grain direction (thermal conductivity), density gradient (density), and porosity gradient (porosity, pore diameter). We found the resin to have an insignificant influence on the chemical kinetics as our chosen kinetic model performs

Conclusions

In this paper, we experimentally and computationally characterised the influence of oxygen concentration and other variables on the burning behaviour of particleboard. Heat flux had a significant influence, while density and sample thickness had no effect on the charring rate and time-to-flaming ignition. Oxygen concentration controls the mode of burning with pyrolysis dominating below 4% [O2]a, smouldering between 4 and 15% [O2]a, and flaming above 15%. These thresholds agree with the

Declaration of Competing Interest

None.

Acknowledgment

The authors thank EPSRC, Arup, and BRE Trust for financial support and the University of Edinburgh for the experimental facilities used. Furthermore, we thank members of Imperial Hazelab for insightful discussions and helpful comments.

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