Heat Energy from Wood
Craig McKinley
Oklahoma State University
Printed in The Cedar Leader Apr-Jun 2007
Not long ago, I received an inquiry asking about the heating value
of wood. The easy answer to that inquiry is that all species of wood
have a heating value of about 8600 Btu’s per dry pound. (Btu stands
for British thermal unit and is defined as the amount of heat
required to raise the temperature of one pound of water by one
degree Fahrenheit). However, there is more to the story than that.
First, the 8600 Btu value is useful only for absolutely dry wood. In
most cases, we can get wood to about 17 percent moisture (based upon
the total weight of the wood plus water) by air drying. That means
that only 83 percent of a pound of wood is actually “wood”. When we
burn that pound of wood, we immediately lose the 17 percent in water
vapor that literally goes “up in smoke”.
Several publications are available that indicate how many Btu’s per
pound of wood are available at various moisture contents. As
expected, as moisture content goes up, the number of Btu’s goes
down. For example, a cord of wood (80 cubic feet) of eastern
redcedar at 0 percent moisture content contains about 19.3 million
Btu’s. At 17 percent moisture, that value drops to 18.7 million
Btu’s. At 50 percent moisture, only 16.6 million Btu’s are
available.
But there is even more to the story, The values given above assume a
perfect combustion system with no heat lost in the process. In
reality, there is a loss of heat energy in converting the water into
steam, hot gases that go up the stack, and convection losses in the
equipment (why the stove gets hot). The real question becomes how
many of the 8600 Btu’s per dry pound are actually recoverable. The
U.S. Department of Agriculture Forest Products Laboratory in General
Technical Report FPL 29 (1979) demonstrates a technique for
estimating the recoverable heat energy in wood and bark. Using the
formulas presented the actual, recoverable energy from a pound of
dry redcedar is only about 6,600 Btu or about 76 percent. That
recovery value decreases as the moisture content increases. At 20
percent moisture content, there are only about 5000 Btu’s recovered.
So, the next time someone asks you how much heating value is in a
pound of wood, the best answer is “It depends”. Have a good day.
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