Wood and Combustion Heat Values

Firewood and combustion heat values - Pine, Elm, Hickory and many more species

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:

• 1 ft (foot) = 0.3048 m
• 1 lb = 0.4536 kg
• 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 cord = 85 ft³ is used to convert between the "Density" and "Weight of Cord" column. 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 the green wood cords values with approximately 10%.

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

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 burning fuel vapors and evaporated the moisture, only charcoal remains burning at temperatures higher than 1100^o F

• T_C = 5/9(T_F - 32)

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