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1.6 Wood Residue Combustion In Boilers
1.6.1 General 1-6
The burning of wood residue in boilers is mostly confined to those industries where it is
available as a byproduct. It is burned both to obtain heat energy and to alleviate possible solid residue
disposal problems. In boilers, wood residue is normally burned in the form of hogged wood, bark,
sawdust, shavings, chips, mill rejects, sanderdust, or wood trim. Heating values for this residue range
from about 4,500 British thermal units/pound (Btu/lb) of fuel on a wet, as-fired basis, to about 8,000
Btu/lb for dry wood. The moisture content of as-fired wood is typically near 50 weight percent for the
pulp, paper and lumber industries and is typically 10 to 15 percent for the furniture industry. However,
moisture contents may vary from 5 to 75 weight percent depending on the residue type and storage
operations. Generally, bark is the major type of residue burned in pulp mills; either a mixture of wood
and bark residue or wood residue alone is burned most frequently in the lumber, furniture, and plywood
industries.
1.6.2 Firing Practices 5, 7, 8
Various boiler firing configurations are used for burning wood residue. One common type of
boiler used in smaller operations is the Dutch oven. This unit is widely used because it can burn fuels
with very high moisture content. Fuel is fed into the oven through an opening in the top of a
refractory-lined furnace. The fuel accumulates in a cone-shaped pile on a flat or sloping grate.
Combustion is accomplished in two stages: (1) drying and gasification, and (2) combustion of gaseous
products. The first stage takes place in the primary furnace, which is separated from the secondary
furnace chamber by a bridge wall. Combustion is completed in the secondary chamber before gases enter
the boiler section. The large mass of refractory helps to stabilize combustion rates but also causes a slow
response to fluctuating steam demand.
In another boiler type, the fuel cell oven, fuel is dropped onto suspended fixed grates and is fired
in a pile. Unlike the Dutch oven, the refractory-lined fuel cell also uses combustion air preheating and
positioning of secondary and tertiary air injection ports to improve boiler efficiency. Because of their
overall design and operating similarities, however, fuel cell and Dutch oven boilers have many
comparable emission characteristics.
The firing method most commonly employed for wood-fired boilers with a steam generation rate
larger than 100,000 lb/hr is the spreader stoker. In this boiler type, wood enters the furnace through a
fuel chute and is spread either pneumatically or mechanically across the furnace, where small pieces of
the fuel burn while in suspension. Simultaneously, larger pieces of fuel are spread in a thin, even bed on
a stationary or moving grate. The burning is accomplished in three stages in a single chamber:
(1) moisture evaporation; (2) distillation and burning of volatile matter; and (3) burning of fixed carbon.
This type of boiler has a fast response to load changes, has improved combustion control, and can be
operated with multiple fuels. Natural gas, oil, and/or coal, are often fired in spreader stoker boilers as
auxiliary fuels. The fossil fuels are fired to maintain constant steam production when the wood residue
moisture content or mass rate fluctuates and/or to provide more steam than can be generated from the
residue supply alone. Although spreader stokers are the most common stokers among larger wood-fired
boilers, overfeed and underfeed stokers are also utilized for smaller units.
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External Combustion Sources
1.6-1
Another boiler type sometimes used for wood combustion is the suspension-fired boiler. This
boiler differs from a spreader stoker in that small-sized fuel (normally less than 2 mm and normally low
moisture) is blown into the boiler and combusted by supporting it in air rather than on fixed grates.
Rapid changes in combustion rate and, therefore, steam generation rate are possible because the finely
divided fuel particles burn very quickly.
A later innovation in wood firing is the fluidized bed combustion (FBC) boiler. A fluidized bed
consists of inert particles through which air is blown so that the bed behaves as a fluid. Wood residue
enters in the space above the bed and burns both in suspension and in the bed. Because of the large
thermal mass represented by the hot inert bed particles, fluidized beds can handle fuels with moisture
contents up to near 70 percent (total basis). Fluidized beds can also handle dirty fuels (up to 30 percent
inert material). Wood fuel is pyrolyzed faster in a fluidized bed than on a grate due to its immediate
contact with hot bed material. As a result, combustion is rapid and results in nearly complete combustion
of the organic matter, thereby minimizing the emissions of unburned organic compounds.
1.6.3 Emissions And Controls 7-12
The major emission of concern from wood boilers is particulate matter (PM). These emissions
depend primarily on the composition of the residue fuel burned, and the particle control device. Oxides
of nitrogen (NO x ) may also be emitted in significant quantities when certain types of wood residue are
combusted or when operating conditions are poor.
1.6.3.1 Criteria Pollutants
The composition of wood residue and the characteristics of the resulting emissions depend
largely on the industry from which the wood residue originates. Pulping operations, for example,
produce great quantities of bark that may contain more than 70 weight percent moisture, sand, and other
non-combustibles. As a result, bark boilers in pulp mills may emit considerable amounts of particulate
matter to the atmosphere unless they are controlled. On the other hand, some operations, such as
furniture manufacturing, generate a clean, dry wood residue (2 to 20 weight percent moisture) which
produces relatively low particulate emission levels when properly burned. Still other operations, such as
sawmills, burn a varying mixture of bark and wood residue that results in PM emissions somewhere
between these two extremes. Additionally, NO x emissions from wet bark and wood boilers are typically
lower (approximately one-half) in comparison to NO x emissions from dry wood-fired boilers.
Furnace operating conditions are particularly important when firing wood residue. For example,
because of the high moisture content that may be present in wood residue, a larger than usual area of
refractory surface is often necessary to dry the fuel before combustion. In addition, sufficient secondary
air must be supplied over the fuel bed to burn the volatiles that account for most of the combustible
material in the residue. When proper drying conditions do not exist, or when secondary combustion is
incomplete, the combustion temperature is lowered, and increased PM, CO, and organic compound
emissions may result from any boiler type. Significant variations in fuel moisture content can cause
short-term emissions to fluctuate.
1.6.3.2 Greenhouse Gases 13-18
Carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) emissions are all produced
during wood residue combustion. Nearly all of the fuel carbon (99 percent) in wood residue is converted
to CO 2 during the combustion process. This conversion is relatively independent of firing configuration.
Although the formation of CO acts to reduce CO 2 emissions, the amount of CO produced is insignificant
compared to the amount of CO 2 produced. The majority of the fuel carbon not converted to CO 2 , due to
incomplete combustion, is entrained in the bottom ash. CO 2 emitted from this source is generally not
1.6-2
EMISSION FACTORS
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counted as greenhouse gas emissions because it is considered part of the short-term CO 2 cycle of the
biosphere.
Formation of N 2 O during the combustion process is governed by a complex series of reactions
and its formation is dependent upon many factors. Formation of N 2 O is minimized when combustion
temperatures are kept high (above 1475 o F) and excess air is kept to a minimum (less than 1 percent).
Methane emissions are highest during periods of low-temperature combustion or incomplete
combustion, such as the start-up or shut-down cycle for boilers. Typically, conditions that favor
formation of N 2 O also favor emissions of CH 4 .
1.6.4 Controls
Currently, the four most common control devices used to reduce PM emissions from wood-fired
boilers are mechanical collectors, wet scrubbers, electrostatic precipitators (ESPs), and fabric filters. The
use of multitube cyclone (or multiclone) mechanical collectors provides particulate control for many
wood-fired boilers. Often, two multiclones are used in series, allowing the first collector to remove the
bulk of the dust and the second to remove smaller particles. The efficiency of this arrangement varies
from 25 to 65 percent. The most widely used wet scrubbers for wood-fired boilers are venturi scrubbers.
With gas-side pressure drops exceeding 15 inches of water, particulate collection efficiencies of
85 percent or greater have been reported for venturi scrubbers operating on wood-fired boilers.
ESPs are employed when collection efficiencies above 90 percent are required. When applied to
wood-fired boilers, ESPs are often used downstream of mechanical collector precleaners which remove
larger-sized particles. Collection efficiencies of 90 to 99 percent for PM have been observed for ESPs
operating on wood-fired boilers.
A variation of the ESP is the electrostatic gravel bed filter. In this device, PM in flue gases is
removed by impaction with gravel media inside a packed bed; collection is augmented by an electrically
charged grid within the bed. Particulate collection efficiencies are typically over 80 percent.
Fabric filters (i. e., baghouses) have had limited applications to wood-fired boilers. The principal
drawback to fabric filtration, as perceived by potential users, is a fire danger arising from the collection
of combustible carbonaceous fly ash. Steps can be taken to reduce this hazard, including the installation
of a mechanical collector upstream of the fabric filter to remove large burning particles of fly ash (i. e.,
"sparklers"). Despite complications, fabric filters are generally preferred for boilers firing salt-laden
wood. This fuel produces fine particulates with a high salt content having a quenching effect, thereby
reducing fire hazards. Particle collection efficiencies are typically 80% or higher.
For stoker and FBC boilers, overfire air ports may be used to lower NO x emissions by staging the
combustion process. In those areas of the U. S. where NO x emissions must be reduced to their lowest
levels, the application of selective noncatalytic reduction (SNCR) to residue wood-fired boilers has been
accomplished; the application of selective catalytic reduction (SCR) is being contemplated. Both
systems are postcombustion NO x reduction techniques in which ammonia (or urea) is injected into the
flue gas to selectively reduce NO x to nitrogen and water. In one application of SNCR to an industrial
wood-fired boiler, NO x reduction efficiencies varied between 35 and 75 percent as the ammonia-to-NO x
ratio increased from 0.4 to 3.2.
Emission factors and emission factor ratings for wood residue boilers are summarized in
Tables 1.6-1, 1.6-2, 1.6-3, 1.6-4. The factors are presented on an energy basis (pound of pollutant per
million Btu of heat input). Factors for wet wood represent facilities that burn wood residue with a
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External Combustion Sources
1.6-3
moisture content of 20 percent or greater. Factors for dry wood represent wood residue with less than
20 percent moisture content. Cumulative particle size distribution data and associated emission factors
are presented in Table 1.6-5. Uncontrolled and controlled size-specific emission factors are plotted in
Figure 1.6-1.
1.6.5 Updates Since the Fifth Edition
The Fifth Edition was released in January 1995. Revisions to this section since that date are
summarized below. For further detail, consult the background report for this section. This and other
documents can be found on the CHIEF Web Site at http://www.epa.gov/ttn/chief/, or by calling the Info
CHIEF Help Desk at (919)541-1000.
Supplement A, February 1996
Significant figures were added to some PM and PM-10 emission factors.
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In the table with NO x and CO emission factors, text was added in the footnotes to clarify
meaning.
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Supplement B, October 1996
SO x , CH 4 , N 2 O, CO 2 , speciated organics, and trace elements emission factors were
corrected.
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Several HAP emission factors were updated.
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Supplement D, February 1998
Table 1.6-1, the PM-10 and one PM emission factors were revised to present two
significant figures and the PM-10 emission factor for wood-fired boilers with mechanical
collectors without flyash reinjection was revised to 2.6 lb/ton to reflect that these values
are based on wood with 50% moisture. A typographical error in the wet scrubber
emission factor for PM-10 was corrected.
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Table 1.6-2, the SO x emission factors for all boiler categories were revised to
0.075 lb/ton to reflect that these factors are based on wood with 50% moisture.
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Tables 1.6-4 and 1.6-5 were re-titled to reflect that the speciated organic and trace
element analysis presented in these tables are compiled from wood-fired boilers
equipped with a variety of PM control technologies.
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Supplement D, August 1998
Table 1.6-4, the emission factor for trichlorotrifluoroethane was removed. The phenol
emission factor was corrected to 1.47E-04; the phenanthrene factor was corrected to
5.02E-05; the chrysene factor was corrected to 4.52E-07; and, the polychlorinated
dibenzo-p-furans factor was corrected to 2.9E-08.
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1.6-4
EMISSION FACTORS
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Supplement E, February 1999
In the footnotes of tables 1.6-1, 2, 3, 4, 5, 6, 7, some text was removed that described
how to adjust the factors when burning wood with moisture and thermal content
significantly different from 50% or 4500 Btu/lb, respectively. The EPA is revising
Section 1.6 and, in the interim, consistent with EPA’s recommendations regarding proper
use of AP-42, the EPA encourages users of the wood combustion emission factors to
account for the specific assumptions included in the factors and to convert the factors to
a thermal content basis (i.e., lb/MMBtu) to estimate emissions when burning wood that
differs significantly from 4500 Btu/lb or 50% moisture.
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July 2001
All emission factors were revised and new factors were added. In some cases separate
factors were developed for wet wood (greater than or equal to 20 percent moisture
content) and dry wood (less than 20 percent moisture).
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Separate PM and NOx emission factors are provided for dry wood combustion.
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All emission factors have been converted to units of lb/MMBtu.
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PM emission factors are specified by fuel type and control device type but not by boiler
type.
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NOx, SOx and CO emission factors are specified by fuel type and not by boiler type.
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Additional toxic emission factors have been added.
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The general quality rating for PM factors are higher than before.
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TOC and CO2 emission factors are specified by all wood types and not by boiler type.
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New Source Classification Codes (SCC) were assigned for dry wood.
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March 2002
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The VOC and TOC emission factors in Table 1.6-3 were calculated incorrectly. This has
been corrected. The correct factors are 0.013 and 0.039, respectively.
September 2003
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The VOC emission factor in Table 1.6-3 was calculated incorrectly. This has been
corrected. The correct factor is 0.017.
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1.6-5

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