Wood and Bio Mass Heat

Combustion value of wet and dry wood in btu/pounds, kJ/kg and kcal/kg

In a laboratory it is possible to get the heat value 8,660 Btu/lb from wood fuel. This "high heat value" is obtained only with perfectly dry wood with 0% moisture content and only 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 its 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%.

The water must be evaporated and temperature raised to the flue gas temperature.

• 6.3 lbs dry wood = 1 Boiler Horse Power
• 11.6 lbs wet wood = 1 Boiler Horse Power

Moisture Content and Usable Energy

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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