Wood - Combustion Heat Values

Type of wood - whether it is hardwood or softwood - burned in the combustion process is important for the heat value and the energy efficiency.

Hardwoods have less resin and burn slower and longer. Softwoods burn quickly. In addition the seasoned length influences on the fuel efficiency. Seasoning the wood refers to the allowed drying time before combustion.

Wood need to be dried at least 4 to 6 months before use.

Densities and heat values of some common wood species are indicated in the table below. Note that the volume of a stack of firewood varies considerably on whether or not it is split and how it is stacked. The moisture content also play a role - the values below are based on a average moisture content of 20%.

For full table - rotate the screen!

• 1 ft (foot) = 0.3048 m
• 1 lb = 0.4536 kg
• 1 lb/ft³ = 16.018 kg/m³
• 1 Btu (British thermal unit) = 1,055.06 J = 107.6 kpm = 2.931x10^-4 kWh = 0.252 kcal = 778.16 ft lb_f = 1.055x10¹⁰ ergs = 252 cal = 0.293 watt hour

Note that in the table above 1 net cord volume = 85 ft³ is used to convert between the "Density" and "Weight of Cord" column (1 stacked cord volume = 128 ft³). Be aware that the densities used for the wood species varies significantly. The densities used above is for natural dried wood where the average moisture content is approximately 20%.

Heat values of cords with dry wood can be estimated by adding 10% to the green wood cords values.

Recoverable heat values are calculated with a stove efficiency of approximately 65%.

How to Calculate the Combustion Heat in MJ/kg from the Table above

1. calculate "Density of Dry Wood" in kg/m³ by multiplying lb/ft³ with 16.018
2. calculate "Weight of Dry Wood" in kg/cord by multiplying lb/cord with 0.4536
3. calculate "Recoverable Heat Value of Cord (Dry Wood)" in MJ/cord by multiplying Millions Btu/cord with 1055.06
4. calculate "Recoverable Heat Value per kg (Dry Wood)" in MJ/kg by dividing 3 with 2

Example - Red Oak

1. "Density of Dry Wood" : 44.2 (lb/ft³) 16.018 = = 708 (kg/m³)
2. "Weight of Dry Wood" :  3760 (lb/cord) 0.4536 = 1705.5 (kg/cord)
3. "Recoverable Heat Value of Cord (Dry Wood)" : 24.0 (Millions Btu/cord) 1055.06 = 25304 (MJ/cord)
4. "Recoverable Heat Value per kg (Dry Wood)" : 25304 (MJ/cord) / 1705.5 (kg/cord) = 14.8 (MJ/kg)

The Combustion Process of Burning Wood

1. Wood heats up to approximately 212 ^oF (100 ^oC) evaporating the moisture in it. There is no heating from the wood at this point
2. Wood solids starts to break down converting the fuel gases (near 575 ^oF, 300 ^oC)
3. From 575 ^oF to 1100 ^oF (300 - 600 ^oC ) the main energy in the wood is released when fuel vapors containing 40% to 60% of the energy burn
4. After the burning of fuel vapors and the moisture is evaporated, only charcoal remains burning at temperatures higher than 1100^o F

• T_C = 5 / 9 (T_F - 32)