The larger the fuel particle is, the longer is the combustion process. Imagine a handful of sawdust quickly burning if it is thrown into a hot fire.
There is a good contact between fuel and air, since the small particles quickly dry, give off gases and burn, resulting in a high combustion
If instead you throw a log into a hot fire, it will take a long time before it is burnt out. It can be compared to a roast that is put in the oven. Although it has roasted for an hour in the oven, it is still aw in the middle. The size of the fuel, therefore, is of great importance to the speed of combustion.
The moisture content in fuel reduces the energy content expressed by the calorific value, Hn,v (see Chapter 4), since part of the energy will be used for evaporation of the water.
Dry wood has a high calorific value, and the heat from the combustion should be drawn away from the combustion chamber in order to prevent overheating and consequent damage to material.
Wet wood has a low calorific value per kg total weight, and the combustion chamber should be insulated so as to avoid reduction in boiler efficiency and
enable a continuous combustion process.
This is typically accomplished by using refractory linings round the walls of the chamber so as to conserve the heat which is generated. The boiler chamber will therefore normally be designed for burning wood within a certain moisture interval.
A moisture content in wood above 55-60% of the total weight will make it very difficult to maintain the combustion process.