TCC 918 Cone Calorimeter – Reaction of Fire to Wood

Introduction

Modern building regulations and safety standards require that building materials are not only structurally safe, but also fire safe. Wood, a proven building material, offers many benefits, but also carries risks as it can be ignited by small sources of heat such as sparks, releasing smoke gases.

As smoke inhalation is the most common cause of death in fires and dense smoke makes orientation and escape difficult, it is essential to analyze the fire behavior and smoke emission of wood.

Extensive fire testing and certification are required to confirm the suitability of wood as a building material from this point of view.

Measurement Conditions

To investigate the reaction to fire, spruce wood specimens (100 x 100 x 17 mm³) were tested in the NETZSCH TCC 918 Cone Calorimeter. This device records the heat release rate (HRR), mass loss, and the DensityThe mass density is defined as the ratio between mass and volume. density and composition of the resulting flue gases.

The wood samples were positioned on a horizontal sample holder mounted on a load cell to continuously record the mass loss during the measurement. An electrical heating cone heated the specimens from above and initiated the PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.pyrolysis of the wood. Once sufficient PyrolysisPyrolysis is the thermal decomposition of organic compounds in an inert atmosphere.pyrolysis gases had been released, ignition was provided by a spark igniter. The resulting combustion gases flowed through the heating cone and were collected by an exhaust system.

The mass flow, the temperature of the flue gas and the concentrations of O₂, CO₂ and CO were measured continuously in the exhaust system. In addition, the smoke DensityThe mass density is defined as the ratio between mass and volume. density was determined by the transmission of laser light. The gas analyzer (Siemens Oxymat/Ultramat) was calibrated prior to the measurements and the^C-factor1 was checked using a methane burner. The measurement conditions are summarized in Table 1.

After heating the heating cone, the shutter was closed and the prepared sample holder was positioned on the base plate. The measurement was started by automatic opening of the shutter and the released gases were ignited by the automatic ignition system. Figure 2 shows the sample preparation and measurement setup.

^1TheC-factor is a key calibration parameter in cone calorimetry, defined in accordance with ISO 5660-1. It serves as a constant for the accurate determination of the heat release rate (HRR) by establishing the relationship between the signal from the oxygen analyzer and the actual heat energy released.

Table 1: Measurement conditions

Measurement Results

Figure 3 shows the mass loss of the three wood specimens over time during combustion. Immediately after ignition, a rapid mass loss occurs due to the combustion of volatile components such as water and highly combustible organic substances. After the flame is extinguished, a slow glowing process begins, resulting in a smaller, continuous mass loss.

Figure 4 displays the course of the heat release rate (HRR⁾² of the specimens. Immediately after ignition, the HRR of all samples rises sharply and reaches a maximum at approximately 170^kW/m2. As the highly flammable components are consumed, the HRR drops significantly, indicating less intense combustion. This also indicates that the volatiles have largely been consumed and that the combustion of the solid residues (charcoal) is dominant. A further increase in HRR just before the flame is extinguished is typical of wood and is caused by the breaking up of the charcoal layer, releasing more volatile components which are then burned. After about 20 minutes, the values stabilize at a lower level. This indicates that most of the combustible material has been used up, leaving mainly charred residues. These residues continue to burn slowly and evenly, resulting in a sustained but low heat release.

² The Heat Release Rate (HRR) is a measure of the amount of heat released per unit time during the combustion of a material(https://analyzing-testing.NETZSCH.com/en/products/fire-testing/tcc-918)

Another key aspect of the analysis is smoke generation, which is determined by measuring transmission. A decrease in transmission indicates an increase in smoke DensityThe mass density is defined as the ratio between mass and volume. density. Figure 5 illustrates the smoke measurements of the samples and highlights the correlation between smoke production and heat release. Initially, there is a pronounced maximum in the smoke production rate (SPR), indicating rapid ignition and the release of large quantities of combustible gases and particles. However, this initial peak quickly decreases, which is characteristic of the combustion of volatile components that quickly lead to smoke formation.

The results provide valuable insights into the complex combustion processes of wood, particularly in terms of mass loss, heat release and smoke formation.

The differences between the samples are minor and can be explained by natural variations in the wood, such as differences in structure, moisture or DensityThe mass density is defined as the ratio between mass and volume. density.

Summary

In summary, wood is a valuable and versatile building material with a natural appearance, sustainability and mechanical strength. The fire resistance of wood is improved by the formation of a char layer which insulates the internal structure of the wood and slows down combustion. This char layer contributes to the dimensional stability and strength of timber components, allowing timber buildings to remain structurally stable in fires for longer than many other materials.

The low Thermal ConductivityThermal conductivity (λ with the unit W/(m•K)) describes the transport of energy – in the form of heat – through a body of mass as the result of a temperature gradient (see fig. 1). According to the second law of thermodynamics, heat always flows in the direction of the lower temperature.thermal conductivity of wood reduces heat dissipation, which supports the dimensional stability and strength of components. As a result of these properties, the structure of timber buildings remains intact for longer in the event of a fire, which explains the saying among firefighters that ‘wood burns safely’. However, it is vital that the fire resistance of wood be further investigated and optimized to ensure the safety and longevity of timber structures in modern construction.