United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 2771.1
Research and Development
EPA-600/S7-81 -091 Dec. 1981
Project Summary
Control of Emissions from
Residential Wood Burning by
Combustion Modification
John M. Allen and Marcus W. Cooke
An exploratory study is described of
factors contributing to atmospheric
emissions from residential wood-fired
combustion equipment. Three com-
mercial appliances were operated
with both normal and modified de-
signs, providing different burning
modes: up-draft with a grate, up-draft
with a hearth, cross-draft, down-
draft, and a high-turbulence mode
utilizing a forced-draft blower. Fuels
were naturally dried commercial oak
cordwood, commercial green pine
cordwood, oven-dried fir brands, and
naturally dried oak cut into reproduc-
ible triangles. Continuous measure-
ments of stack gases included Oa,
CO2, CO, NO, SOa, and total hydro-
carbons (FID) as an indication of the
total organic species in the stack gases
during batchtype operation. Several
combustion modification techniques
were identified which have an appre-
ciable effect on emission factors and,
therefore, can be developed and applied
to reduce emissions in consumer use.
The more promising design modifi-
cations include: preventing heating of
the wood inventory within the stove,
focusing the air supply into the primary
burning area with high turbulence,
and increasing the temperatures in the
secondary burning regions of the
appliances.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The study reported in this document
was initiated to explore the combustion
modification techniques that might be
applied beneficially to wood stoves, and
more specifically to identify those tech-
niques which show promise of providing
a significant improvement in emissions
when adequately developed and applied.
The focus of the effort is on naturally
drafted, hand-fired radiant stoves, as
these constitute the majority of units
used extensively for residential heating.
The combustion in wood stoves, and
the resulting emissions, can be modified
in many ways. In this experimental
program several modifications were
made in stove operation in attempts to
correlate specific changes in emissions
with each of several specific combustion
modifications. Synergistic effects of
several simultaneous modifications
were not specifically sought. However,
each set of tests conducted to demon-
strate the effects of a specific modifi-
cation necessarily depends on the other
design and operating conditions in use
at the time. The effects of a specific
modification under different design and
operating conditions can only be as-
sumed, and the synergistic effects
determined by inference.
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Table 1. Summary of Wood Combustion Research Reviewed
Performing Laboratory
Project Focus
Monsanto Research Corporation
California Air Resources Board
Bowdoin College
Canadian Combustion Research Laboratory
Auburn University
Argonne National Laboratory
Tennessee Valley A uthority
Virginia Polytechnic Institute
National Bureau of Standards
Thermocore, Inc.
New York University (Plattsburgh)
Vermont Environmental Control Agency
Institute of Man and Resources
U.S. EPA. Industrial Environmental Research
Laboratory (RTF), Combustion Research
Branch
Del Green Associates
OMNI Environmental
Extensive analysis of residential wood stove emissions
Residential and laboratory tests on free standing stoves
Measured paniculate emissions at low burn rates using
a dilution tunnel
Continuous monitoring of gas emissions including
efficiency measurements of different stove designs
and fireplaces at low rates of burning
Measured stove performance and safety, focusing
on efficiency measurements
Measuring emission factors for residential
wood stoves
Evaluation of several wood stoves including
particulate and gas emission and efficiency
calculations
Studies of catalytic-bed after-burners and
staged combustion
Basic study of detailed characterization of
emissions and their formation
Measurement of air supply configuration effects
in stove design
Studies on thermostatic control development
and stove design focused on particulates
Ambient emission studies focusing on particulates
in the atmosphere
Paniculate and limited gas measurements
for 10 central systems using wood for
residential heating
Continuous measurement of gas emissions and
comparison of stack sampling and dilution
tunnel sampling during several phases of burning
cordwood and pellets used as fuels
Just starting an emission measurement program
using six stoves to determine reasonable standards
for emissions, to develop a simplified testing
procedure, and to evaluate the effects of fuel moisture
An emission evaluation to establish baseline data on
several new, commercially available stove designs
The modifications conducted in this
laboratory program were of three types:
stove design, fuel properties, and oper-
ator techniques. The uses of non-wood
fuel and combustion additives were not
considered. The use of processed wood,
such as pellets, was also not considered,
as this fuel can most effectively be
burned in equipment specifically de-
signed for its use.
Wood Combust/on
The combustion of wood is generally
recognized as involving three processes
or phases: moisture evaporation,
pyrolysis with subsequent space burning,
and surface char burning. These
processes occur successively at any
local particle of wood, but in real com-
bustion systems there is appreciable
overlap resulting in all three processes
occurring simultaneously within a
combustion chamber loaded with wood.
This overlap is especially significant
when a reservoir of fuel is supplied
within the combustion space for pro-
longed burning, as often occurs in
stoves.
Types of Wood Combustors
Several options of air flow and fuel
patterns within the stove have been
adopted in commercialized designs.
This study investigated up-draft airflow
using a grate, up-draft using an
impervious hearth, cross-draft, and
down-draft through a grate. Also studied
was a novel residential combustor
design using forced-draft, highly
turbulent combustion with additional
design characteristics promoting low
emissions.
I
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Fuel Characteristics
Several types of wood were burned
including commercially available air-
dried oak cordwood, commercially
available green pine, oven dried fir, and
air-dried oak cut into reproducible
triangles. The effects of piece size and
dryness on emissions were found to be
significant: extremely dry wood and
small pieces resulted in high emissions
of CO and hydrocarbons.
Operator Techniques
Stove operation at high burning rates
was shown to result in reduced
emissions of CO and organics as
compared to low burning rates, and
large inventories of wood in the com-
bustion chamber reservoir also increase
these emissions.
Current Research Summaries
This study i ncl uded a review of current
research programs related to wood
stove emissions. Table 1 summarizes
the programs and organizations
performing wood combustion research
including the emission measurements
discussed in this study. There may be
other studies unknown to the authors.
Conclusions
This laboratory program included an
investigation of the effects of several
independent variables on the atmo-
spheric emissions averaged over com-
plete burning cycles. Emissions were
measured for CO and a total of organic
species as indicated by total hydrocarbons
(THC) measured by flame ionization
detection. For a few tests, emissions of
polycyclic aromatic hydrocarbons (PAH)
and benzo(a)pyrene (BaP) were also
measured by integrated gas chromatog-
raphy and mass spectroscopy.
When burning air-dried oak at mod-
erate rates, >4.5 kg/hr (>10 Ib/hr),
small differences were observed between
up-draft burning with a grate and side
draft burning modes in Table 2. The up-
draft mode burning on a hearth resulted
in lower emissions. When the side-draft
stove was converted to the down-draft
burning mode, the emissions were
appreciably lower. Emissions from the
novel high-turbulence wood burner
were lower than those of conventional
radiant wood stoves.
In the few tests involving measure-
ments of specific PAH compounds from
five different conditions of burning, the
total PAH emissions varied from 11 x
10 3g/kg wood for high-turbulence
Table 2. Atmospheric Emissions as Measured by Flame Ionization Detection
Emissions
g emitted per kg wood as fired*
Burning Mode
Up-Draft on Hearth
Up-Draft with a Grate
Side-Draft
Down-Draft
High - Turbulence
CO
33 to 37
200 to 400
34 to 210
4 to 110
-25
THC
8 to 19
32 to 42
24 to 220
5 to 56
~5
*
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Table 3. Summary of Emission Control Techniques
Factors Adversely
Affecting Emissions
Approaches for
Possible Improvements
Principal
Modifications
Required
Applicable
to Existing
Stoves?
Premature Pyrolysis in Wood
Magazine Within Stove
Pyrolysis Rate in Primary
Combustion Area Exceeds
Local Air Supply Preventing
Complete Combustion
Control of Emissions in
Primary Burning Area
Control of Emissions in
Secondary Combustion Zone
Add-on Systems Affecting
Emissions Reduction
1. Preventing heating of wood inventory
2. Feed wood frequently in small amounts
3. Use large wood pieces, low surface-to-
volume ratio
4. Burn moderate moisture content wood to
retard pyrolysis
5. Burn devolatilized wood, charcoal
1. Maintain high rate of primary air supply,
with ensuing high burning rates
2. Focus air supply into limited burning
area to prevent widespread burning
3. Maintain high turbulence in active
combustion re'gion
4. Limit quantity of fuel in active burning
area; i.e. approach fuel-controlled
burning
5. Maintain high temperatures in active
burning area
6. Avoid sharp and/or frequent reductions
in air supply rate
1. Provide high level of turbulence in
burning area to promote mixing
2. Maintain high temperatures in burning area
3. Provide long gas residence time at the
high temperatures
4. Duct pyrolysis products from magazine
into burning area
5. Provide down-draft combustion, with bed
area reduction to accomodate low burning
rates
1. Maintain high temperatures
2. Use heated secondary air
3. Increase combustible content of primary
combustion products
4. Provide anxilliary combustion using an
ignition source and/or supplementary fuel
1. Use catalytic afterburner
2. Use separately fueled afterburner
3. Add heat storage capacity to the system,
permitting other modifications to be
acceptable for consumer utilization
Design
Operation
Fuel
Fuel
Fuel
Operation
Design
Design
Design
Design
Operation
Design
Design
Design
Design
Design
Design
Design
Design
Design
(Operation)
Design
Design
Design
No
Yes
Yes
Yes
Yes
Yes
Retrofit
No
No
Retrofit
Yes
No
Retrofit
No
No
No
Retrofit
No
No
No
Yes
Yes
Yes
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John M. Allen and Marcus W. Cooke are with Battelle-Columbus Laboratories,
505 King Avenue. Columbus, OH 43201.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Control of Emissions from Residential Wood
Burning by Combustion Modification,"(Order No. PBS 1-217 655; Cost: $9.50,
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
. S. GOVERNMENT PRINTING OFFICE: 1982/559-092/3370
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