The Amount of Energy in Wood Fuel

There are a number of different values used for the energy content of wood fuels. This discussion will attempt to show the relationship between these different values.

In a laboratory, it is possible to get about 8660 Btu/lb of wood fuel. This number is often presented as the number to use in determining outputs and efficiencies of appliances. This "high heat value" is obtained only with perfectly dry wood (0% moisture content) and only in an atmosphere of pure oxygen in a "bomb calorimeter." For laboratory use, this is a useful number and is handy for theoretical problem analysis. But for the practical world, it is unrealistic.

When wood is alive 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 20% or so. Another way of saying this is that 1.25 pounds of well seasoned wood contains 0.25 pound of water and 1.0 pound of wood fibres. You can easily see that our piece of seasoned wood has 8660 Btu per 1.25 (total) pounds or 6930 Btu/pound of actual total weight.

We now confront the problem of having 0.25 pound of water in our sample piece. It will be necessary to evaporate this water and raise its temperature to that of the flue gas temperature. There are also two other sources of water that we should consider that will also have to be heated. They are: 1) the moisture in the humidity content of the air used for combustion and excess air which is quite variable dependent on the relative humidity; and 2) the moisture produced as a by product of the combustion process. Hydrogen atoms in the wood combine with the oxygen atoms in the draft air to form water vapor. This is one of the major chemical reactions that occurs in combustion to give off energy. In wood heating, it is generally second to the carbon-to-carbon monoxide /dioxide reactions in producing energy.

The energy used in vaporizing and heating the water/water vapor exists in the water vapor as "latent heat." In principle all latent heat can be recovered to produce more usable output. Since this possibility exists, many researchers use a "high heat value (HHV)" for wood energy content that does not take latent heat into consideration. Therefore, they use the 8660 or for 20% moisture wood, 6930 figure in their calculations. In real terms, it would be necessary to have the flue gases exhaust at the temperature of the initial incoming draft air which may be near 0° Fahrenheit.

Another approach to the situation is to account for latent heat effects. This is the so-called European system approach that was the only analysis is use (except in laboratories) up to about 1978. We at JUCA favor this approach since it comes much closer to reproducing the actual conditions of consumer use of a product. The latent heat put into the water vapors from all three sources are removed from the calculations as being not recoverable for all practical purposes.

Text Font Face
.
Text Size
.
Background
Color
.
(for printing)

Since about 1050 Btu are necessary to boil or evaporate a pound of water, and 1 Btu additional is necessary to raise the pound's temperature 1°F, it is possible to determine the latent heat fairly easily by knowing the total weight of water vapor given off by the fire. We had the 0.25 pound of moisture content. Add about 0.54 pound of water vapor as products of combustion. If we assume low humidity conditions that contribution is small. We now have 0.79 pounds of water vapor that started at say 60°F average temperature and was heated to say 400°F. The latent heat is then 0.79 times (1050 plus 340 temp rise) or 1098 Btu per 1.25 pound piece, or 880 Btu/pound. Therefore, the "low heat value (LHV)" of wood fuel is less than the high heat value (HHV) by this amount. The result is that the available energy in seasoned (20% moisture content) wood used in an actual usage environment (400°F flue gases) is about 6050 Btu/pound. We feel that this is the most realistic number to use for domestic wood burning as it is the number that would apply if the user weighed his wood as part of determining efficiency of his appliance.

Some charts you may run across use a figure described as an output per cord or pound of wood. This is always based on some assumption about the efficiency of the device being used. Often 50% or 40% is assumed, so that if the actual device had substantially different efficiency the figures would be wrong. Even the LHV must be slightly compensated for if the flue gas temperature is not as assumed, but these changes are relatively small and generally will not materially affect comparison results.

There are also differences in types of wood. Softwoods usually have a lot of resin content that has high energy content so their total energy content is usually higher than for hardwoods (often by about 5%). The softwoods tend to burn up faster than hardwoods and have other characteristics that reduce their attractiveness as fuel. The fact that their average density is usually lower than hardwoods means that you get less weight of wood in a cord and the extra 5% of volatile fuel will not make up for this.

For comparison sake, using LHV gives results about 8% higher that the same results using HHV. Thus, an 80% device (LHV) is 74% efficient using HHV. A 45% HHV reading is equivalent to about 49% LHV.

.

It is also useful to note how these concepts apply to un-seasoned (green) wood fuel. If only seasoned a short time, 50% moisture is a realistic figure. Then a two-pound piece has one pound of wood fibers (worth 8660 Btu). There will be 1.54 pounds of water to vaporize and heat up (taking away 2200 Btu). The two-pound piece has a net available energy content of 6460 Btu or 3230 Btu/pound. This is only HALF of the available energy present when burning seasoned wood. Green wood consumes the bulk of its energy just to keep itself going, and is obviously subject to easily going out.

A freshly cut tree has even higher moisture content, often above 60%. Similar calculations show that this fresh wood has only 2000 Btu/pound of energy available. This explains why it is so difficult to burn freshly cut trees.


The JUCA Home Page is at: juca