United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-061 July 1984
oEPA Project Summary
Control of Wood Stove Emissions
Using Improved Secondary
Combustion
J.M. Allen and W.H. Piispanen
Self-initiating secondary combustion
in wood stoves is encouraged by
designs that introduce additional heated
air and turbulence to the primary
combustion products. This can be very
effective in reducing carbon monoxide
(CO) and hydrocarbon emissions at
high burning rates. At low burning rates
the effectiveness is limited by low
temperatures, inadequate mixing, and
thermal quenching by the primary air
which bypasses the wood. Two stoves
were operated in the laboratory with
simultaneous on-line chemical analysis
of the gases entering the secondary
combustion zone and the gases leaving
the stove. Stove modifications providing
increased temperatures and improved
mixing in the secondary combustion
zone in a small box stove resulted in
minor improvements in secondary
burning. The continued burning of CO
in the secondary zone was not greatly
affected. In a large side-draft stove,
with effective secondary burning at
high burning rates, the secondary
burning at low rates was not effective at
any air flow distribution available to the
operator.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
Increased use of wood as a residential
heating fuel has resulted from the rising
prices of other fuels and public skepticism
of the reliability of normal fuel supplies.
Government agencies, equipment manu-
facturers, and many environmentalists
are supporting the use of wood as an
alternate fuel. With increased use is also
the widespread impression that wood
burning is environmentally clean. Unfor-
tunately, technology characterizing the
objectionable emissions from wood
stoves is limited, and few experiments
have been performed to try to effectively
reduce emissions. Often, residential
stove manufacturers and operators are
adopting stove designs and operating
practices that increase emissions. This
accentuates the environmental signifi-
cance of the increasing use of wood as
residential heating fuel.
There are at least five ways to reduce
objectionable emissions from residential
wood stoves: (1) prevent or reduce
emissions formed in the fuel magazine in
the stove; (2) prevent or reduce emissions
formed in the primary combustion zone;
(3) destroy emissions in the primary
combustion zone; (4) destroy emissions
in a secondary combustion zone; and (5)
add systems or devices that will reduce
emissions.
Of the five, the first three can be
accomplished by burning well seasoned
wood at a relatively high rate. Recently
introduced stoves with catalytic afterbur-
ners employ Method 5, so it is not
discussed here. Method 4 was the focus
of this study; to investigate the effects of
various modifications of commercial
stoves as a way to improve the effective-
ness of secondary combustion.
Secondary combustion is the combus-
tion of fuel materials that are not
completely burned in the primary com-
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bustion zone; i.e., in the immediate
vicinity of the wood. These materials can
result from quenching the primary
combustion products or from pyrolysis of
wood without complete combustion in
the primary combustion zone. Secondary
combustion can be achieved by mixing
the gases from the wood and from the
primary combustion with suitable oxygen
at a temperature sufficient to ignite the
mixture or sustain burning. Sometimes
secondary combustion is an extension or
continuation of primary combustion, and
specific secondary ignition may not be
required. In some stoves, secondary
combustion takes placedirectlyabovethe
burning wood. In other stoves, the
secondary zone is separated from the
primary zone by a baffle or barrier; the
primary and secondary air are supplied at
different locations. For secondary com-
bustion to take place the gas composition
must be within the flammability limits of
the gases. Adding secondary air can
sometimes dilute the combustion gases
below these limits or cool the mixture
below ignition temperatures.
In actual practice, secondary combus-
tion is hindered by: (1) limited mixing rate
and turbulence due to low air velocities
available in natural draft stoves, and (2)
low temperatures in the secondary zone
due to excess air and wall quenching.
As a result, in many conventional
stoves secondary combustion is often not
sustained for any significant length of
time. Just after a charge of new wood has
been added, the temperature of the
secondary air and the combustion pro-
ducts may be too low to initiate or
maintain secondary combustion. In mid-
period burning, the gases from pyrolysis
tend to be hotter and contain more
combustible matter, thus satisfying the
conditions for continued burning. Later in
the burning cycle, when gas evolution is
reduced as the wood becomes char, there
may be insufficient combustible gas to
sustain secondary burning.
Procedure
Two basic stove designs were investi-
gated in an experimental laboratory
study: the Nordic, a small box stove with a
horizontal baffle; and the Defiant, a
larger side-draft stove with a vertical
baffle. Figure 1 shows schematic diagrams
of both stoves. The Nordic was modified
extensively during this study, but the
Defiant was used only in the form
received from the manufacturer.
Each stove was mounted on a platform
that permitted weight loss (burning rate)
measurement during operation. The
Front View
Side View
Small Box Stove (Nordic)
C
4
W
t' ™~* I^I* ~. • " •""' ^^ ™T a
_ — _ ^ —. ••-• ^
o
A
W
w V
Front View
Side View
Large Side-Draft Stove (Defiant/
Legend
Alternative secondary
air inlets
s 1 upper inlet
s2 No. 2 inlet
s3 No. 3 inlet
s4 Bottom inlet
Figure 1. Schematic diagram of stoves.
P primary air inlet
S secondary air inlet
C outlet to chimney
F baffle
W wood
A primary combustion gas sample
B secondary combust/on gas
sample
2 secondary combustion zone
stoves were equipped with shielded
aspirated thermocouples (at the inlet and
outlet of the secondary combustion zone)
to measure gas temperatures and with-
draw a representative gas sample for
analysis by on-line instrumentation to
determine gas composition. Each gas
sample was supplied to both a heated line
leading to a heated Total Hydrocarbon
(THC) Analyzer and an unheated line
leading to a cooling/trapping system
which in turn led to inorganic gas
analyzers. The gas analyses, weights.
and temperatures were machine-pro-
cessed with an on-line computer (to
determine emission factors and other
engineering calculations) and recorded
on magnetic tape.
The computer program for data reduc-
tion developed by Battelle provided con-
tinuous 1-minute parameter checks and
emission calculations. All data were
stored in engineering terms for subse-
quent computer graphic presentations.
A total of 32 test burns were performed
in the box stove. The first 10 were back-
-------
ground tests to check supporting equip-
ment and determine operating charac-
teristics. The remaining 22 included: 6
baseline tests that established typical
and also some atypical characteristics of
the unmodified stove; and 16 tests to
examine the effect of various modifica-
tions on the stove flue gas emissions. Six
tests were also conducted in a large-side
draft stove to demonstrate effects of
secondary air control in a large stove.
The wood for these tests was all
triangular split, seasoned white oak. Two
different lots were used: one contained
about 10 percent moisture, and the
second, about 18 percent (determined by
average weight loss after drying in a
105°C laboratory oven). For each, the
appropriate moisture value was entered
in the computer data reduction program.
Design Modifications
Several modifications were made in a
Nordic box stove, and the emission
factors calculated and averaged. The
modifications were:
• The stove body was sealed, elimina-
ting extensive air leaks, and making
the stove airtight.
• A diverter plate was installed on the
inside of the door to direct the
primary air flow down toward the
wood, and the secondary air up
toward the secondary combustion
chamber. (Both primary and secon-
dary air normally enter through
dampered holes in the door.)
• An independent secondary air dis-
tributor with multiple outlets was
installed at the inlet to the secondary
combustion chamber, to control the
rate and distribution of secondary air,
and improve mixing.
• A bypass of the secondary combus-
tion chamber was installed by the
manufacturer, permitting primary
combustion products to pass directly
out of the stove.
• The exterior of the secondary cham-
ber was insulated to reduce heat loss
and thus increase gas temperatures.
• The interior of the primary combus-
tion chamber was insulated to
reduce heat loss.
• The hearth was lined with firebrick,
spaced to permit primary air to flow
up between the bricks, into the
burning wood.
• The interior of the secondary com-
bustion chamber was insulated with
firebrick to reduce the flow passage
area, thus increasing gas velocity
and improving mixing.
Effects of Design
Modifications on Small Box
Stove
Table 1 shows representative emission
factors calculated from emissions mea-
sured during the experimental operation
of the small Nordic box stove.
The conventional baffled box stove was
first operated as purchased and subse-
quently operated with modifications
intended to improve the effectiveness of
secondary combustion. Dispersed and
uncontrolled secondary air infiltration (by
leakage) resulted in stove emission
factors that were at least as low as those
obtained with any of several restricted
and controlled secondary air introduction
systems. However, the burning rate was
not easily controlled until the stove was
made airtight. The reduction in emission
factors obtained in the secondary com-
bustion chamber by the controlled
introduction of secondary air was gener-
ally greater than that obtained in the
same upper chamber of the stove with the
uncontrolled, secondary air supply. These
observations are consistent with the
generally recognized characteristics of
airtight stoves.
A mild degree of turbulence and mixing
was introduced into the inlet of the
secondary combustion zone by injecting
secondary air into the stove through
small jets. Only when the jets were
directed counter to the primary flue gas
stream was there evidence of turbulence-
induced continued combustion. At best,
the emission factors for CO and hydrocar-
bons were reduced 40 percent.
Thermal insulation was applied to the
small box stove in several places to
attempt to increase significantly the
temperature of the combustion gases at
the point where secondary air was
supplied. Reductions of up to 46 percent
of the emission factors of CO and
hydrocarbons were then obtained in the
secondary combustion. Extensive insula-
tion was apparently not sufficient to
maintain the gases at high enough
temperatures to complete the combustion
process during low rates of burning. Only
when the burning rate was increased
appreciably and gas temperatures in the
secondary combustion chamber were
above 425°C were reductions in emission
factors more significant.
Under all operating conditions, CO
continued to oxidize slowly during the gas
transit through the secondary combus-
tion zone, whether the hydrocarbons
were oxidized significantly or not. Although
the lower temperatures may have impeded
the oxidation of CO, it was not completely
quenched. Secondary (or continued)
combustion of the hydrocarbons was less
evident and more erratic than CO
combustion. The technique used to
analyze hydrocarbons (flame ionization
detection) is known to be inexact for the
many gases released from wood, but this
possible discrepancy is not considered to
be significant. At these low burning rates,
combustion of split wood in a stove
becomes inherently variable, and the
emission factors may not be reproduced
exactly with repeated tests.
Effects of Operating
Techniques on Side-Draft
Stove
In previous tests, the Defiant side-draft
stove achieved very distinct and effective
secondary combustion at high burning
rates (i.e., 9 kg/hr). This stove incorporates
several features that promote secondary
combustion. More than a ten-fold reduc-
tion in CO and hydrocarbon concentrations
in the flue gas was obtained with active
secondary burning at the high burning
rates.
Table 1.
Emission Factors for Modified Box Stove
Average Emission Factors, g/kg
l/l/nnrf Carbon Monoxide Total Hydrocarbons
Test
No.
3
10
12
16
20
. Burn Leaving
Rate, Primary Leaving
kg/hr Chamber Stove
Unsealed stove with uncontrolled air infiltration
3.0 76 35
Secondary air supplied in controlled manner and location
2.7 100 64
External insulation of secondary combustion chamber
2.3 143 114
Both chambers insulated, primary chamber internally
2.0 84 45
Firebrick installed in secondary combustion chamber
2.6 85 65
Leaving
Primary
Chamber
6
31
43
17
24
Leaving
Stove
6
18
43
19
17
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Lower burning rates, which are more
typical of residential use, were used in
this program. Air control settings on the
unaltered stove were changed to deter-
mine their effects at these lower rates.
Distinct secondary burning was not easily
identified, either by gas analysis or by gas
temperature readings. Several test runs
using thoroughly dried split oak were
conducted with the Defiant stove.
Table 2 summarizes the emission
factors obtained in these tests. The first
four tests were run with similar settings
of the air-flow controls and in the same
manner. The emission factors were
consistently reduced by the continued
burning in the secondary combustion
zone. However, the reductions were
relatively small, and the emission factors
varied considerably.
In Test 5, the secondary air supply was
shut, and in Test 6, the primary air supply
was shut. Leakage air was available in
both tests, because neither flow control
system was airtight. With the secondary
air supply damper shut, the primary
combustion products were relatively hot,
and the hydrocarbon emissions were
relatively low. When the primary air
supply damper was shut, the primary
combustion product temperatures were
lower, and the hydrocarbon emissions
were significantly higher. In neither
extreme of air distribution was there a
large effect on emission factors which
could be related to effective burning in
the secondary combustion zone.
As in the smaller box stove, the lowgas
temperatures associated with low burning
rates apparently precluded complete burn-
ing of the flue gases. In the larger stove,
effective distribution of heated secondary
air by tuyeres did not result in large-scale
reductions in emission factors at low
burning rates.
Advanced Stove Design
The Tennessee Valley Authority (TVA)
recently tested several wood stoves
incorporating advanced technologies for
efficient and clean burning of wood. A
new European-built stove evaluated in
that program is designed to optimize non-
catalytic secondary combustion. This
advanced design introduces heated
secondary air through several small holes
into the primary combustion products as
they leave the primary combustion
chamber through a ceramic-lined pas-
sageway. TVA found that this design
provided equal or higher combustion
efficiency compared to a typical airtight
stove at medium and high burning rates
(above 2.5 kg/hr), but not as high as with
a catalytic stove. Emission factors for CO,
Table 2.
Emission Factor Averages for Different Air Supply Control Settings on a Large Side-
Draft Stove
Average Emission Factors, g/kg Wood Burned
Carbon Monoxide
Total Hydrocarbons
Test
No.
4
5
6
ury Wood
Burn Rate.
kg/hr
2.0
2.5
2.0
Air Supply"
Primary
1/3
1/3
Shut
Secondary
Full
Shut
Full
Primary
Zone
167
153
171
Secondary
Zone
135
114
122
Primary
Zone
41
21
75
Secondary
Zone
31
13
53
"Fraction of full open damper.
particulates, and polycyclic aromatic
hydrocarbons were found to be lower
than for the conventional airtight stove.
However, the advanced secondary com-
bustion stove required more operator
attention to maintain the effective
secondary combustion at the medium
burning rates. Effective secondary com-
bustion in the stove could not be obtained
at low burning rates, so the stove was not
tested at those rates.
TVA's observations of effective secon-
dary combustion at high burning rates,
but not at low burning rates, agree with
observations from this program and those
from a previous Battelle study.
Conclusions
This program has demonstrated that
secondary (or continued) combustion
supported by an independent secondary
air supply is very difficult to attain at low
burning rates in wood stoves. An almost
universal characteristic of wood stoves at
low burning rates is the abundance of
excess air in the combustion products
leaving the primary combustion zone. At
these low burning rates, the pyrolysis and
immediate partial burning of the wood
result in some unburned fuel gases (CO
and hydrocarbons), which leave the wood
with the combustion products. The
combustible concentration range of these
fuel gases is limited by both lowtempera-
ture and the concentration of inert
compounds (residual nitrogen and reac-
tion products of completed burning).
Requirements of a minimum temperature
and an acceptable mixture of fuel, air, and
the diluent inerts are seldom met.
J. M. Allen and W. H. Piispanen are with Battelle-Columbus Laboratories,
Columbus. OH 43201.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Control of Wood Stove Emissions Using Improved
Secondary Combustion," (Order No. PB 84-199 033; Cost: $8.50. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
AUSGPO: 1984-759-102/10612
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