Insights into the burning behaviour of wood in the cone calorimeter

Sammanfattning: Climate change and its accompanying environmental issues have caused the building industry to use more environmentally friendly building materials. Wood have always been a buildingmaterial but due to the renewed interest in imparting sustainability and renewability, its usage has increased over the recent years. With a rising interest in wood, it is of great importance to enhance the knowledge of its burning behaviour in order to predict and prevent fire hazards. Fire development is often characterized in terms of heat release rate (HRR) as a function oftime. Therefore, HRR is considered one of the most important variables in the evaluation of material fire hazards. This study aims to generate greater knowledge of the HRR curve of wood when exposed to heating in the cone calorimeter and how the curve can be described quantitatively. Furthermore, it was attempted to comprehend the properties and functions of char and its effects on HRR during combustion. The study is based on laboratory tests carried out with a cone calorimeter and a Scanning Electron Microscope (SEM). The cone calorimeter was set to generate a heat flux of 35 kWm-2. Spruce wood samples of three thicknesses were analysed, namely 10, 20 and 30mm. The samples were assembled with one of three types of material on the rear side of the samples, which were Kaowool, steel plates and aluminium foil wrapped around wood. The different materials were used as they are greatly dissimilar in their thermal properties. Wood with both low and normal moisture content was also analysed. Char was analysed with SEM. The results show that there are four major points of interest in the HRR curve of wood. The first point is the initial peak heat release rate (PHRR) that occurs when the sample surface ignites causing great production of heat which increases the HRR. The second point of interest is the vast decrease in HRR soon after the first PHRR, this is due to char formation, which acts as a protective barrier preventing the exchange of volatile gases and oxygen. The third point of interest is a second PHRR close to the end of the combustion that occurs as a response to sample burn through, which means that the heat gradient reaches the rear side of the sample. The second PHRR is highly dependent on the boundary condition defined by the rear material, which determines the heat losses at the rear side of the specimen, and consequently the temperature of the specimen. The higher is the specimen temperature, the higher is the pyrolysis rate, and therefore also the higher the second PHRR. Moreover, high moisture content delays the time of occurrence of the second PHRR as more water needs to undergo phase change, which requires a high amount of energy. The final point of interest is the final decrease in HRR as a result of fuel depletion leading to the sample smouldering or the fire being extinguished. Char, formed by mainly lignin and some cellulose in wood, affects the overall HRR. The SEM analysis showed that the char cracks grew wider during the second PHRR. It is, however, observed that char cracking has no significance in the time of occurrence of the second PHRR as this is based on sample burn through, and it was difficult to determine to what extent char cracking affected the intensity of the PHRR. This systematic study is considered adequate to justify the research questions and aim of this study. It has also created new questions for further study in the area as well as provided a deeper understanding of the fundamental burning behaviour of wood.