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-

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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
105C 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 425C 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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