Wood combustion

Wood consists of connected atoms to each other in strings of units long, that called molecules, which in case of wood molecules of cellulose, hemicellulose and lignin. The temperature of material is related to the violence of the atomic motion in the molecules. With the raise of the temperature, more vibrations to the atoms occurs. In the end vibration get stronger making the atoms come apart, as a result the molecules begins to separate [81]. Wood pyrolysis is a term called on the decomposition process of wood molecules caused by raise in temperature. The first wood molecule decompose is hemicellulose at temperature range of 250 to 500 °C, after that comes cellulose between 275 to 350 °C, then lignin with temperature of 250 to 500 °C. The thermal stability of lignin is a reflection of its high molecule weight and heavily crosslinked structure [82].

When wood pyrolyzed it create a char layer of carbon and ash that works as barrier for the under lying wood from direct outside heating. Because of wood shrinkage caused when it changes to char, the char layer cracks, giving an opportunity to the produced gazes to escape from the surface layer to the under lying wood [81]. That’s the reason that makes the charred surface of wood abstained temperate of 800 °C, while the main pyrolysis of wood is range between 225 to 500 °C [82].

2.7.1.2 Char

When wood pyrolysis forms two kinds of gaseous products: gazes that escape of the solid residue and gazes that don’t. Gazes that escapes are sneak from cellulose and lignin in the early-stage paralysis process, where its wealthy of oxygen and hydrogen. The gazes that do not escape from solid residue, pyrolysis keep to leak and form a secondary char.

Char formation has significant influence on wood decomposition because the char layer works as shield that slower the virgin wood decomposition process. The char does not increase strength of wood; however, it increases its thermal resistance of the virgin wood under the pyrolysis front. As a result, it causes a reduction in heat release rate and convert to a mass transport barrier for volatiles lunched from fuel and oxygen in the air [82]. The formed char has different structure and amount from wood to another. Usually, the amount of char varies from 20 to 30 °C

of the wood weight. Char basically is carbon with small quantity of hydrogen and oxygen. Carbon structure melt only at high temperature 4000 °C [81].

2.7.1.3 Smoke

Smoke is mostly carbon and ash, in general, smoke forms when big molecules in the pyrolysis gazes reacts with air but are not in complete combustion (insufficient oxygen to burn the wood completely), it condenses to tiny micrometre size droplets [81]. These tiny droplets or particles are invisible to the eye but when they come together will be clear and take form of what called smoke.

Wood can produce 25-100 m²kg^-1 of smoke in well ventilated conditions [82].

In indoor fire cases, most of people die because of smoke inhalation rather than flames.

Smoke not only decrease visibility but diminish the chances of deliverance from a flaming building because it can be extremely toxic, irritating and lethal. Carbon monoxide, carbon dioxide and water are primary products obtained of wood combustion. However, the presence of heteroelements such as nitrogen or halogens can release hydrogen cyanide and hydrogen halides [82].

2.7.1.4 Solution to overcome the wood combustion problem

Based on wood combustion process (Fig.2.3), decomposition of wood molecules (cellulose, hemicellulose and lignin) is the reason of wood pyrolysis. the best solution to overcome the problem of wood pyrolysis is to modify the wood molecules with fire retardants. Many chemicals could be used as pre-treatments to improve the fire resistance of the wood and as a result improve fire resistance of the wood-cement composite. Sodium silicate is known as a binder and fire retardant that can improve wood properties such as the mechanical properties, dimensional stability, and fire resistance[68], [83].

Fire retardants have different effects on different materials because each material has a unique response to fire based on a number of factors. For example, the material’s ignition ease, rate of burn, and flame spread over the surface are factors to consider. Additionally, the rate at which the flames infiltrate into a wall or barrier, the speed at which heat is released, and the amount of smoke and toxic gas generated all have an effect on the fire resistance of the material [84].

However, it’s important to understand the operation of fire-retardant chemicals, the differences between fire retardants, and to decide which one is better to use depending on the situation.

Most fire retardants for wood work by improving the pyrolysis of cellulose through the fire

combustion pathway that leads to char formation. Fire retardants stabilizes the wood molecules against decomposition by slowing down pyrolysis reaction. It can create isolating surface layer that reduce temperature rise, the release of pyrolysis gazes and the oxygen penetration on the surface. These factors can be achieved using intumescent fire retardants surface treatments which can expand when the temperature rises, as result form in thick layer that works as shield that will protect wood surface from flames. The typical fire retardants coating for wood are compounds that contain halogens, phosphorus, nitrogen, boron compounds [85].

Figure 2. 3:Schematic diagram represents wood combustion process [85].

2.7.2 Pre-treatment Fire retardants for wood