Wood and Bio Mass Heat

Combustion values of wet and dry wood - Btu/pounds, kJ/kg and kcal/kg

In the laboratory it is possible to get a heat value of 8660 Btu/lb from wood fuel. This "high heat value" is obtained only with perfectly dry wood with 0% moisture content and in an atmosphere of pure oxygen. For laboratory use this is a useful number handy for theoretical problem analysis. But for the practical world it is unrealistic.

When wood is alive and fresh it consists primarily of water, i.e. most of the weight is actually water. After being cut to length and stacked for a year or two the average moisture content generally drops to approximately 20%.

In the combustion process water is evaporated and the temperature is raised to flue gas temperature.

Condition of WoodApproximate Combustion Values
btu/lbkJ/kgkcal/kg
Wet 4000 9300 2220
Dry 7000 16300 3890
  • 6.3 lbs dry wood = 1 Boiler Horse Power
  • 11.6 lbs wet wood = 1 Boiler Horse Power

Moisture Content and Usable Energy

Moisture Content and Usable Energy
Moisture Content
%
Energy by Volume Unit
%
Energy per Weight Unit
%
0 (oven dry) 100 100
20 (air-dry) 97 81
50 (green) 92 62
100 (wet) 85 42

Note that

  • by volume wet wood has about 85% of the energy of oven-dry wood
  • by weight wet wood has less than half -  42% - of the energy of oven-dry wood

One weight unit of wood has enough energy to evaporate 6 weight units of water.

Related Topics

  • Combustion - Boiler house topics - fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency

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