Specific and lower heating value of firewood: features of peat, wood, birch firewood

The combustion heat of wood is an important parameter that shows the total amount of heat released during the combustion of 1 kg of fuel. This is how specific heat is determined, although there is also an indicator of lower and higher heat. What it is and what fuel is most effective for a private home is described in detail in the material presented.

What is heat of combustion

Heat is defined as the amount of thermal energy that can be obtained by burning a particular fuel. The indicator is important - it is quite clear that the greater the heat per unit volume, the more effective firewood, peat or other fuel is.

For a more accurate determination, an indicator such as the specific heat of combustion of firewood is used, that is, the amount of heat that can be obtained as a result of the complete combustion of 1 kg of firewood. This value is measured in MJ/kg. So, for dry firewood (birch, pine) the heat is 15 MJ/kg.

A similar value is true for other hardwoods:

• oak;
• beech;
• ash.

But although the specific heat of combustion of birch firewood is indeed 15 MJ/kg, in practice it is slightly lower - 12-13 MJ/kg. The fact is that the “extra” 2-3 MJ are spent on the evaporation of water, which is contained in fairly large quantities in wood, especially in fresh wood.

The actual indicator is called the lower heating value of wood (also known as working value).That is, the lowest is always somewhat less compared to the specific one. But if the wood is dry, these parameters are approximately equal. If we take into account the heat that is released as a result of condensation of water released from wood, we obtain the highest calorific value. It is always higher than the lower heating value of wood.

Heat of combustion of different fuels

As already mentioned, the heat of combustion of wood is approximately the same and is approximately 15 MJ/kg. Although if you take perfectly dry wood, the figure rises to 16-20 MJ/kg. As for other types of fuel, the indicator will be as follows:

• charcoal 31 MJ/kg;
• anthracite coal 31 MJ/kg;
• specific heat of combustion of peat 8.1 MJ/kg;
• pellets (special granules from wood waste) 17-18 MJ/kg;
• pine needle briquettes 20-28 MJ/kg.

Based on these data, we can come to the conclusion that pellets and fuel brackets are more energy efficient, so firewood as such is not needed. Indeed, the specific heat of combustion of wood is 15-20 MJ/kg, which is comparable to pellets (17-18) and much less compared to briquettes (20-28).

But it is important to understand that both of these types of fuel have their disadvantages:

• are more expensive;
• not always on sale;
• burn quickly and do not provide uniform heat production.

Thus, the main indicator that you should focus on is the specific heat of combustion of dry firewood. But since in practice they often have a large proportion of moisture, it is worth reducing it by 2-3 MJ/kg. Moreover, regardless of the wood, all firewood has approximately the same heat. It is much larger than peat, but inferior to pellets and pine needle briquettes.