“A higher content of water in wood means a larger portion of the heat supplied to wood for ignition is taken away by water in the form of latent heat of evaporation and this means the wood remains in the second stage or smouldering stage for a longer time and produces more smoke.”

 

Hydrogen, carbon and oxygen are the basic components of wood, which are in the form of polymeric materials such as cellulose. Of the many components that make up wood, hydrogen and oxygen act as the fuel during the burning of wood. Generally there are four stages to fire: incipient, smouldering, flame and heat. Once the wood is somehow ignited and as the temperature reaches 150 degrees C, decomposition of cellulose starts to take place.

Even though there is no flame or visible smoke, a number of combustible particles are produced. This is called the incipient stage. Visible smoke is produced as more and more combustible particles are produced until they become visible. Compounds of carbon, oxygen and hydrogen are the major components of smoke. However, there is no major development of heat or flame.

At the smouldering stage, the fire progresses and flame starts at around 500 degree C. And during flame stage, there is an increase in heat and a reduction in visible smoke.  And finally the heat stage is marked by the production of large amounts of heat, flame, smoke and toxic gases.

Wood being a hygroscopic material, it tends to absorb moisture from the atmosphere and therefore  constitutes about 8 percent of water. And depending on the humidity of the atmosphere this figure can vary. This wood is commonly called dry wood.

However, subjecting wood to direct rain can considerably alter the percentage of water it holds. When water is heated to above 100 degree C, it  forms steam. The heat required for the conversion of water to steam is called latent heat of vapourization (evaporation) , which is about 540 calories per gram.

When wood is ignited, the water contained in the wood also gets heated up during the incipient and smouldering stages. The evaporation of water contained in wood takes a portion of the heat in the form of latent heat of evaporation. As the percentage of the water contained in the wood increases a larger portion of the heat supplied to wood for ignition is taken by the water as latent heat of evaporation and hence a longer time is taken to reach the third and fourth stages of fire. Therefore the second stage or smouldering continues for a longer time and hence more smoke is produced.

References


[1] https://en.wikipedia.org/wiki/Smoke

[2] https://en.wikipedia.org/wiki/Hygroscopy

[3] http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/wet-wood/article2278330.ece

[4] https://en.wikipedia.org/wiki/Wood