Wood is usually converted into energy
by burning. Combustion commences by evaporating the water present
in the wood structure. Then combustible and noncombustible components
are driven off at temperatures from 100˚ to 600˚ C.
Table 9-1 gives the proximate analysis of wood and bark,
showingthat about 75 to 85% of the wood can be volatilized. Carbon
is oxidized in the final stage of combustion.
Part of the oxygen in these reactions
comes from the oxygen already present in the wood, and the rest
that is needed comes from the air. The minimum total amount of
oxygen and hence air needed from these two sources can be theoretically
calculated, but in practice more air than the theoretical amount
is admitted to ensure complete combustion. This additional air
is called excess air and the amount is carefully controlled
in modern combustion systems.
Higher heating value (HHV) is a laboratory
measurement of the stored chemical energy in a fuel and is expressed
in British thermal units per pound (Btu/lb), joules per kilogram
(J/kg), or megajoules per kilogram (MJ/kg). Calculations in this
chapter are in Imperial units. See Appendix 1 for conversions
between energy measures.
Table 9-2 presents average higher heating
values for northwestern species. Wilson et al. (1987) have provided
an extensive compilation of ultimate and proximate analysis and
higher heating values for northwestern species. Note that HHV
is measured for oven-dry wood (zero percent moisture content).
Differences between species, between wood
and bark, and variations within a species reflect differences
in chemical composition, especially extractives. In engineering
calculations, it is common to use an average HHV of 9,000 Btu/dry
lb for the more resinous conifers and 8,300 Btu/dry lb for the
less resinous hardwoods.
Since a pound of fuelwood is generally
not received in the oven-dry condition, HHV does not correctly
represent the available potential heat. For example, a pound of
wood fuel may contain 0.6 pound of oven-dry wood and 0.4 pound
of water. In the wood fuel literature, moisture content is usually
reported on a wet basis (Chapter 1), hence the moisture content
(MCw) in this example is 40%. Since HHV assumes that
this pound is entirely oven-dry wood, HHV must be reduced to reflect
the actual amount of oven-dry wood present (Wd). The following
equation represents the wood and water components in percent form:
GHV does not represent the amount of heat available for use because
it includes all the heat that can be extracted from the products
of combustion, and there are several sources of loss that must
be accounted for in determining the recoverable heat. Recoverable
heat (RH) is important in comparing costs of fuels and fuel
systems and represents the heat energy that, for example, produces
steam for industrial purposes. The literature on these losses
can sometimes be confusing because authors differ on which losses
to consider and how some of them are calculated.
Losses due to the moisture content of
wood are often referred to as the stack heat loss. A
pound of wet fuel has a certain fraction that is oven-dry wood
plus another fraction that is its water content. In the example
above, there was 0.6 pound of oven-dry wood (Wd) and 0.4 pound
of water (MCw = 40%). Formulas for eight forms of
heat loss, numbered H1 through H8, are discussed below (Bethel
1977; Ince 1979).
H1. Heat used to raise the temperature
of water in the wood to the boiling point. In Btus this
H1 = (212 - T1) *
MCw / 100
where T1 = ambient temperature.
H2. Heat required to vaporize
the water in the wood.
In Btus this is
H2 = 970 * MCw
H3. Heat required to separate
the bound water (water below fiber saturation point) from the
In Btus this is
H3 = 136 * MCb
where MCb = fiber saturation
point (see Chapter 1) if MCw ≥ fsp
or MCb = MCw if
MCw < fsp.
Usually it is assumed that fsp
= 30% MCod or 23% MCw.
H4. Heat required to raise the
temperature of the vaporized water to the temperature of the
exhaust gases. In Btus this is
H4 = 0.46 * (T2
- 212) * MCw / 100
where T2 = temperature of exhaust
The average specific heat of
water over the range from boiling point to exhaust temperature
is about 0.46.
Losses H1, H2, and H4 are often combined
and simplified into a single equation (Ince 1979):
MCw / 100 *
[970 + (212 - T1) + 0.46 * (T2 - 212)].
H5. Heat required to evaporate
water that forms when the hydrogen component of wood is combusted.
Since wood is approximately
6% hydrogen (Table 9-1), a pound of dry wood contains about
0.06 pound of hydrogen. During combustion, one of the chemical
2H2 + O2
®2 H2O + 61,100 Btu/lb of H2.
By weight, water is one part
hydrogen and eight parts oxygen. Therefore, 0.06 pound of hydrogen
combines with 0.48 pound of oxygen to yield 0.54 pound of water,
which also escapes via the stack gases. This water incurs losses
involved with raising its temperature to the boiling point,
evaporating it, and then raising its temperature to that of
the exhaust gases. Therefore,
H5 = 0.54 Wd (212 - T1) + 0.54
Wd (970) + 0.54 Wd [0.46 (T2 - 212)]
= 0.54 * (1 - MCw/
100 )[970 + (212 - T1) + 0.46 * (T2 - 212)].
H6. Heat from combustion other
than water vapor: dry gases. In addition to the minimum
air needed for combustion reactions, there is generally some
"excess" air that enters the furnace, is heated, and
exits with the stack gases.
Table 9-1 shows that oven-dry
wood is about 52% carbon. The reaction to combust this is
C + O2 ®CO2 + 14,100 Btu/lb of C.
The atomic weights of carbon
and oxygen are 12 and 16 respectively, hence burning 12 pounds
of carbon requires 32 pounds of O2, which yields
44 pounds of CO2. Therefore, this process requires
32/12 or 2.7 pounds of O2 per pound of carbon. Since
one pound of oven-dry wood contains 0.52 pound of carbon, the
O2 demand is 0.52 times 2.7, or 1.4 pound per pound
of oven-dry wood. It was previously calculated that 0.48 pound
of O2 is required to burn the 0.06 pound of hydrogen
in a pound of oven-dry wood, hence the total O2 demand
is 1.88 pounds per pound of oven-dry wood. Since a pound of
wood contains 0.40 pound of oxygen (Table 9-1), the net oxygen
required from air is 1.48 pounds.
Because air is 23.2% oxygen,
it takes 4.3 pounds of air to provide one pound of oxygen. Therefore
the minimum air required to burn one pound of oven-dry
wood is 1.48 times 4.3, or 6.4 pounds. In addition to this theoretical
minimum air supply to sustain combustion, some excess air is
always admitted. Excess air is usually expressed as a percentage
of the theoretical minimum air. Thus 20% excess air means that
the total air admitted to burn one pound of oven-dry wood is
6.4 times 1.20, or 7.7 pounds.
The quantity of heat that escapes
in stack gases due to dry gases and excess air (EA) is calculated
H6 = (T2 - T1)(1 - MCw
/ 100) [1.44 EA /100 + 1.56].
H7. Heat required to raise the
temperature of wood to the combustion temperature. In Btus
H7 = Wd (T3 - T1)[0.226 + 0.000322
(T1 + T3 - 64)]
where T3 = combustion temperature.
H8. Other heat losses. Otherlosses
occur from radiation, conduction, and convection of heat, incomplete
combustion and so forth, and have been estimated as 3 to 4 %
(Corder et al. 1970; Miller and Hansen 1951). This can be estimated
H8 = 0.04 GHV.
This percentage of loss may be
appropriate for a well-operated system burning the type of fuel
for which it was designed but could be higher with an improperly
operated system or fuels that have improper size particles or
Recoverable Heat and Combustion Efficiency
The recoverable heat (RH) is obtained
by subtracting the sum of these eight losses from the gross
RH = GHV - (H1 + H2 + H3 + H4 +
H5 + H6 + H7 + H8).
Combustion efficiency (CE) is the ratio
of recoverable heat to available potential heat:
CE (%) = (RH / GHV) *
With current technology, combustion efficiency of wood fuels ranges from about
80% for dry fuels to
about 60% for wet fuels. Unfortunately, many
wood fuels are received in a relatively wet condition. See Example
Lower Heating Value
The terms lower heating value
are used in combustion engineering and reflect
the net heat available in a fuel after the various losses associated
with wood moisture content and water formed during combustion
are subtracted from HHV (Harris et al. 1986).
Heating Values per Unit Volume
Calculating Btus per Cubic Foot of Solid
Suppose it is desired to estimate
the heat value per cubic foot of a conifer under the following
HHV = 9,000 Btu/oven-dry
MCod = 67% (MCw
SGg = 0.40.
Chapter 1 presents the definitions of moisture content
and specific gravity. Table 1-2 indicates that a cubic foot
of this wood weighs nearly 41.9 pounds. Of this, 40% is water,
hence there are 16.8 pounds of water and 25.1 pounds of oven-dry
wood in this cubic foot. The weight of oven-dry wood per cubic
foot can also be obtained by looking up the value in Table 1-2
at the intersection of SGg = 0.40 and MCod
= 0. The difference between the 25.0 in the table and the above
calculation is rounding.
|Higher heating value (Btu/lb)
|Moisture content (% MCw)
|Combustion temperature, T3 (°F)
|Ambient temperature, T1 (°F)
|Fiber saturation point, wet basis (%)
|Exhaust temperature, T2 (°F)
|Excess air (%)
|Other losses (%)
Recoverable heat (RH) is 3,733
Btu/lb oven-dry wood in the fuel. Combustion efficiency (CE)
Using the data from Example 1:
25.1 * 9,000 = 225,900 Btu
25.1 * 5,400 = 135,540 Btu
25.1 * 3,949 = 99,120 Btu.
Calculating Btus per Cord
Select the appropriate cubic
feet per cord from Table 3-1 or 3-2 and multiply by HHV, GHV,
or RH per cubic foot calculated above.
If a standard cord of the above species
contained 80 cubic feet of wood, then
HHV/cord = 18,072,000 Btu
GHV/cord = 10,843,200 Btu
RH/cord = 7,929,600 Btu.
Calculating Btus of Chips and Hog Fuel
When the chips or hog fuel are in weight
units, first deduct weight that corresponds to the moisture
content and then apply HHV, GHV, or RH per pound of dry wood
in the fuel.
When the chips or hog fuel are in volumetricunits,
use the residue to solid wood expansion factors in Table 7-1
or 7-2 to convert the chip or hog fuel volume to solid wood
equivalent. Then apply HHV, GHV, or RH per cubic foot calculated
For example, if a unit (200 cubic feet)
of hog fuel is obtained for the species conditions outlined
earlier, first convert the 200 cubic feet of hog fuel to a solid
equivalent. Assuming an expansion factor of 2.5 cubic feet of
hog fuel per cubic foot of solid wood, a unit becomes
200 / 2.5 = 80 ft3
of solid wood.
Multiply this by either of the three heating values per cubic foot as desired.
Working with Mixtures of Wood and Bark
This involves obtaining a weighted average
higher heating value, a weighted average moisture content, and
a weighted average specific gravity. See Example 2.
Suppose western redcedar hog fuel has 40% bark
and that wood and bark have an MCod of 60 and 100%
respectively. Also, Tables 1-1 and 7-5 indicate that cedar wood
and bark have an SGg of 0.31 and 0.37 respectively. Weighted
0.4 * 8,700 + (1 - 0.4) * 9,700 =
9,300 Btu / oven-dry lb.
0.4 * 100 + (1 - 0.4) * 60 = 76%.
0.4 * 0.37 + (1 - 0.4) * 31 = 0.34.
Use these values
to calculate GHV, HV, and other measures in the preceding sections.
Heating Values of Conventional Fuels
Table 9-3 presents heating values of
fossil fuels that often compete with wood.