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Wood and Bio Mass - Combustion Heat

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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.

Wood and Bio Mass - Combustion Heat
Condition of WoodApproximate Combustion Values
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

Wood and Bio Mass - Combustion Heat vs. Moisture Content
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.

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Combustion processes and their efficiency. Boiler house and chimney topics. Properties of fuels like oil, gas, coal and wood and more. Safety valves and tanks.

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