EFFECTS OF DESIGN FACTORS ON EMISSIONS FROM NON-CATALYTIC
RESIDENTIAL WOOD COMBUSTION APPLIANCES
PREPARED BY:
OMNI ENVIRONMENTAL SERVICES, INC. UNDER SUBCONTRACT
WITH RADIAN CORPORATION
PREPARED FOR: EMISSIONS STANDARDS AND ENGINEERING DIVISION
EPA CONTRACT NO. 68-02-3816
U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR QUALITY PLANNING AND STANDARDS
RESEARCH TRIANGLE PARK, NORTH CAROLINA .27711
MAY 1986
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DISCLAIMER
This report has been reviewed by the Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency, and approved for publication
as received from the Radian Corporation. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names of commerical products
constitute endorsement or recommendation for use.
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WOODSTOVE DESIGN FACTORS
SUMMARY
The combustion of wood in naturally drafted non-catalytic
Residential Wood Combustion (RWC) stoves involves highly complex
chemical processes which are sensitive to a wide variety of
influences. Key elements required for efficient combustion in-
clude high combustion zone temperatures, adequate air (oxygen)/
good air and fuel mixing, adequate residence time, and appro-
priate air/fuel ratios. The batch process of wood combustion in
the naturally drafted cordwood stove presents special problems in
that the entire fuel charge is involved in various and changing
stages of a complex combustion process throughout the fuel load
burning cycle. Ideal conditions vary during each stage, making
complete and efficient combustion of the entire fuel charge in a
single stove configuration very difficult.
A variety of existing stove technologies are examined for
their effectiveness in emissions reduction. Theoretical consi-
derations are followed by review of supporting data and observa-
tions. The existing data base is sparse, and positive conclu-
sions cannot be made. However, the following trends are
observed:
1. Emissions are lower from stoves with smaller fireboxes
than similar stove designs with larger fireboxes.
2. Low firebox heights appear to contribute to lower
emissions.
3. Air inlets above the fuel load in the firebox promote
good air fuel mixing and reduce potential emissions.
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WOODSTOVE DESIGN FACTORS
4. Air entering the firebox near or up through the coal bed
("underfire air") results in higher emissions.
5. Exhaust exits located low in the firebox appear to
cause high emissions.
6. Preheated secondary air/ or more properly termed, addi-
tional combustion air, introduced at high temperature
locations appears to reduce emissions.
7. Thermostatic air supply controls on non-catalytic wood-
stoves often cause air-starved conditions at high fuel
load and firebox temperatures as the thermostat cycles
closed. These conditions can significantly increase
emissions.
8. Pellet-fueled stoves utilizing mechanically assisted
drafts have demonstrated emission rates below the most
efficient catalytic stoves. Densified fuel in natural
draft cordwood stoves also exhibit significant emission
reductions.
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WOODSTOVE DESIGN FACTORS
TABLE OF CONTENTS
I. Introduction 1
II. Woodstove Combustion Fundamentals 4
III. Woodstove Design - Natural Draft Cordwood 9
A. Baseline Stoves 10
B. Firebox Geometry 11
1. Firebox Size 11
2. Firebox Shape 19
3. Pathway of Combustion Products 21
a. Baffling 24
b. Combustion Chambers
- Primary & Secondary 26
c. Downdraft Combustion 27
C. Combustion Air - Distribution,
Conditioning/ and Control 29
1. Primary Air 29
2. Onderfire Air 30
3. Secondary Air 32
4. High Minimum Burn Rates 33
5. Controls 34
D. Heat Exchange Systems 36
E. Construction Materials 39
IV. Other Non-Catalytic Technologies 43
A. Pellet Fuel 43
B. Densified Fuel 46
C. Mechanically Drafted Cordwood Stoves 46
D. Microprocessor Control 47
References
Appendix A - Stove Descriptions
Appendix B - Stove Test Data
Appendix C - Graphical Data Presentation
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WOODSTOVE DESIGN FACTORS
I. INTRODUCTION
The United States has experienced a resurgence in the popularity
of the woodstove or residential wood combustion (RWC) appliance
over the past 15 years. This growth in the use of wood fuel to
meet home heating demands is shown in Figure 1 and represents an
annual consumption of approximately 45 million cords (1, 2).
While wood still ranks a distant third as a household energy
source (Table 1), the volume and nature of woodstove emissions
make it a source of significant environmental concern.
TABLE 1 (3)
ENERGY SOURCE FOR
HOME HEATING
(percent)
Natural Gas 49.5
Electricty 24.8
Wood 9.1
Oil 7.0
Other 9.9
A 1984 study by Arthur D. Little for the U.S. Consumer
Products Safety Commission shows that there are more than 27
million wood heating devices in U.S. homes (Figure 2). Of these/
approximately 13 million are freestanding and fireplace insert
woodstove appliances. Most estimates of wood use indicate that
the use of this fuel in U.S. homes will continue to increase
through this century. While the sale of woodstoves has leveled
off at approximately 1.2 million units per year (Figure 3),
reports by "Wood n' Energy", an industry trade journal, indicate
that 63% of the purchases are for first stoves, 14% are second or
Page - 1
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Figure 1
NATIONAL ESTIMATES OF WOOD FUEL USE (1860 - 1980) (1)
3.0 -,
1860
1880
1900
1920
Year
1940
1960
1980
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Figure 2
WOOD HEATERS IN USE (A)
1984 Data
s
«
v
WOOD HEATERS IN USE
15 Million
FIREPLACES
75 Million
STOVES
4 Million
INSERTS
r 1.2 Million
FREESTANDING
FIREPt ACES/STOVES
Figure 3
SALES OF RESIDENTIAL WOOD STOVES
1200
2000-
U*001
MOO-1
wuoo-
o
I
«oo-H
400-
JOO-
Estimated Wood Stove
Sales & Imports
Tl
73 13 14 7J
77 "7« 7» "$0
62 -03 «4
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WOODSTOVE DESIGN FACTORS
more purchases/ and 23% represent replacement stoves (4).
Of more importance as an environmental concern are the air
pollutant emissions generated by this source category. Wood-
stoves produce large amounts of particulates (smoke) and carbon
monoxide at levels that present serious concerns in many urban
areas. Recent estimates indicate that woodstoves are currently
producing approximately 2.5 million tons of particulates (5).
These emissions constitute a major contributing cause of air
quality violations and interfere with the achievement and mainte-
nance of national ambient air quality standards in several areas.
These conditions have been well documented in Missoula, Montana;
Portland and Medford, Oregon; Reno, Nevada; Boise, Idaho;
Spokane/ Washington? Juneau, Alaska; Denver, Colorado; and
Albuquerque/ New Mexico. High air quality impacts from RWC have
been identified in the urban air of most eastern, northern/ and
western states.
The physical and chemical characteristics of woodstove
emissions further accentuate this source problem. The particu-
lates are predominantly submicron organic condensate materials.
Several of the organic compounds have demonstrated carcinogenic
and mutagenic properties. A recent study showed stove emissions
to be highly acidic (pH 2.5 to 3.8), with high persistance in
acidity due to organic acids (6).
These documented conditions and urban problems have caused
three states (Oregon, Colorado/ and Montana) to adopt specific
stove emissions regulations. Several other states are in the
process of developing strategies to address these problems.
This report provides a detailed discussion of the technology of
Page - 2
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WOODSTOVE DESIGN FACTORS
non-catalyst, natural draft woodstoves. The theory of woodstove
combustion, the chemistry and physics of combustion, and the
state of RWC woodstove design for improved efficiency and low
emissions are addressed.
The report presents a synopsis of woodstove combustion fun-
damentals and discussions on the effects of combustion chamber
design on the emissions and efficiency characteristics of non-
0
catalytic woodstoves. All reported values, unless otherwise
noted, were obtained at the laboratories of OMNI Environmental
Services, Inc., using the Oregon Department of Environmental
Quality "Standard .Method for Measuring the Emissions and
Efficiences of Residential Woodstoves", June 8, 1984 edition. It
is important to note that other fuel loading densities and confi-
gurations, efficiency algorithms, particulate measuring systems,
laboratory altitude, or stove operating practices will result in
potentially significant differences in reported efficiency and
emission values. Care should be taken when comparing data .gener-
ated using different procedures or methods unless equivalency has
been demonstrated and all correction coefficients are applied.
Page - 3
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WOODSTOVE DESIGN FACTORS
II. WOODSTOVE COMBUSTION FUNDAMENTALS
The burning of wood fuel under conditions found in the fire-
boxes of RWC woodstoves involves a complex combustion process
which pyrolyzes and oxidizes volatile, semi-volatile, and solid
components of the wood material. The engineering rule of thumb
is that the four conditions or elements of combustion must be
optimized for this combustion process to take place efficiently:
time, temperature, turbulence, and the air/fuel mixture ratio.
Time is required to allow thorough air and fuel mixing, for
energy-releasing chemical reactions, and for heat transfer to
occur. If the residence time of the fuel and oxygen at high
temperatures is too short, combustion will be incomplete and heat
transfer from the gases to the medium being heated will be inef-
ficient. If residence time is too long, gas velocities in the
combustion chamber will be low, and the driving mechanism for
mixing of the air and fuel gases will be weak. This also leads
to incomplete and inefficient combustion.
Temperature is important since the rates of the chemical
reactions, which are the essence of the combustion process,
increase exponentially with temperature. Generally, high combus-
tion temperatures ensure complete combustion. However, in indus-
trial gas, oil, and coal-fired systems, and in internal combus-
tion engines, excessively high combustion temperatures can also
cause nitrogen oxides pollution. Conventional residential wood
combustion typically occurs at relatively low combustion
temperatures because overall-air/fuel ratios are generally very
Page - 4
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WOODSTOVE DESIGN FACTORS
air-rich/ and because the combustion chamber also serves as the
heat exchanger.
There is an important "feedback" mechanism at work in
combustion chambers. Hot gases which are burning, or have been
nearly completely burned, must mix with and transfer heat to
fresh fuel and air so that the combustion process is sustained.
Other ways to accomplish this internal heat transfer are by
radiation from refractory combustion chamber walls to the fuel
and air, and by contact with hot coals. When the heat transfer
rate to the fresh mixtures is enough to maintain ignition tem-
peratures, a "stabilized" flame is produced.
Turbulence is an important factor because the fuel gases and
air must mix, and as just indicated, the burning and burnt mater-
ials must mix with the fresh fuel and air. Of the two types of
mixing, the first type of mixing is termed "air/fuel mixing" and
the second type is termed "age mixing" because of the old and
young cbaractistics of the mixing constituents. With a close
look at either of these types of mixing, it is observed that
different special scales are involved. The swirling and fluctua-
ting motions seen in a gaseous combustion flame illustrate this.
These turbulent gases contain different size scales, from vor-
tices almost as big as the whole combustion chamber, to eddies
down to millimeter (mm) size. As the size scale of turbulent
eddies decreases, the gases are mixed more and more intimately.
To have chemical reaction of the gaseous mixtures, oxygen mole-
cules must collide with fuel molecules and hot gas or one of the
other ignition sources must be available to provide the heat and
Page - 5
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WOODSTOVE DESIGN FACTORS
reactive species for ignition. The turbulence is very important
because it promotes molecular contact. Nearly all industrial
combustion chambers/ and many residential combustors are "mixing
controlled." That is, the rate at which the mixing processes
occur which bring the fuel gases, air, and hot gases into inti-
mate contact, controls the overall rate of burning: "if it's
mixed, it's burned."
Mixing can be produced by mechanical blowers, by
piston/valve action in reciprocating engines, by flow pulsations,
and by natural draft. Natural draft occurs because a slight
suction or negative pressure is produced in a firebox due to the
lower density (buoyancy) of the hot combustion gases. Natural
draft is the weakest driving force of the available mixing
schemes, and provides the most difficult process control
challenges. The current challenge in wood combustor design is to
use the small amount of available natural draft as effectively as
possible, or to incorporate a mechanical draft system which is
reliable and compatible with wood combustion aesthetics and
objectives.
Air/fuel ratio is important to efficient combustion for
several reasons. Generally the overall air/fuel ratio is in-
creased above the chemically correct (stoichiometric) value for
fuels which are more difficult to burn. For industrial combus-
tors, the excess air required to burn natural gas is about 5%, to
burn oil about 10 to 15%, and to burn pulverized coal about 20 to
25%. That is, just enough excess air is added to make sure that
all of the fuel molecules find oxygen molecules with which to
react. Usually this ideal amount of excess air can be identified
Page - 6
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WOODSTOVE DESIGN FACTORS
by looking for the "knee" in the curve of carbon monoxide exhaust
emissions versus air/fuel ratio. For industrial wood burners the
recommended amount of excess air is not quite as well defined/
though well engineered systems appear to operate best on 50 to
100% excess air.
Too much excess air is detrimental to efficiency/ since as
noted above, excessive air leads to low flame temperatures and
inefficient oxidation of the fuel; that is, the combustion
efficiency is affected. Combustion efficiency is a measure of
the completeness of the combustion process or the conversion of
the chemical energy of the fuel to the heat of the burned gases
in the firebox.
Too much excess air also affects thermal efficiency, or
overall efficiency, which is the percentage of the fuel chemical
energy which is actually transferred to the medium being heated.
If the excess air amount is too high, the increased flow rate
carries a higher proportion of the liberated chemical energy up
the exhaust stack, thereby decreasing the thermal efficiency.
Furthermore, less time is available for heat transfer because of
the reduced residence time, and the heat transfer potential is
reduced because the temperature of the burnt gases is diluted by
the excess air. Thus, the rule of thumb is to use only as much
excess air as is necessary, unless excess air must be used to
prevent overheating of the chamber materials.
Although all industrial and residential combustors operate
overall air-rich, local pockets of fuel-richness unavoidably
exist. These pockets are identified with carbon monoxide and
Page -' 7
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WOODSTOVE DESIGN FACTORS
aerosol formation. Flames in these regions usually exhibit a
yellow color due to thermal radiation from soot aerosol. If the
flame is well mixed, and thus well aerated throughout, its gases
will appear blue in color. Sometimes this blue color is diffi-
cult to see, however, because it is overcome by red-to-orange
thermal radiation from the hot soot aerosol.
Even though conventional residential wood combustors operate
in most cases very air-rich, much of the excess air never
becomes intimately mixed with the fuel gases, and thus, because
of poor mixing, the flame is yellow in appearance, containing
soot and unburned tar aerosols. Even if the excess air does
become mixed with the other gases, the resulting air-rich mixture
is frequently too cool to react completely.
In order for fuel to burn in a woodstove at all, all wood-
stove designs to some extent have to provide conditions for the
four elements of combustion discussed here. The extent to which
each of these elements is optimized dictates the combustion effi-
ciency and emission characteristics of a stove. The following
sections describe various design factors which have been found to
affect the combustion process, and involve some or all of the
basic combustion elements in naturally drafted, non-catalytic
woodstoves.
Page - 8
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WOODSTOVE DESIGN FACTORS
III. WOODSTOVE DESIGN - NATURAL DRAFT CORDWOOD
"Traditional" or first/second generation woodstove designs
represent toe vast majority of stoves in use and a substantial
portion of stoves currently on the market. This category of
stove is "naturally drafted", in that combustion air is drawn
into the firebox by the low pressure created in the stove by the
column of heated buoyant gases rising in the chimney. A vast
array of styles/ shapes/ configurations/ gas flow patterns/ air
distribution systems/ construction materials and sizes of stoves
are available. Manufacturer claims of high efficiency or desir-
able burning characteristics are common/ while little documenta-
tion beyond personal testimony is often available. This section
assesses these design considerations relative to the effects of
stove design on combustion efficiency and pollutant emissions.
Documentation used in this report is presented under two
categories:
1) Data collected through the certification test proce-
dures required by the Oregon DEQ Standard Method for
Measuring the Emissions and Efficiency of Woodstpves
(June 8, 1984). These data represent averages measured
over entire burn cycle tests (i.e./ the burning of an
entire fuel load); and/
2) Data collected through non-certification testing. These
data generally represent discreet burn segments or
partial burn cycle results. References are measurements
made with calibrated instrumentation or reflect
PAGE 9
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WOODSTOVE DESIGN FACTORS
qualified information collected by observation of
phenomena during controlled test conditions.
Descriptions and specifications of all stoves referenced in
the text are provided in Appendix A. Appendix A also lists the
alpha character stove code (A through FF) which is used in the
text to reference each model tested. Stove data are referenced
by the stove code and run number (e.g., Bl-1). Appendix B con-
tains a table of certification test data listed by stove code and
run numbers. Appendix C contains graphical presentation of se-
lected stove test data.
Where comments in the following discussions are based on
partial test cycles or observations in the laboratory, reference
is made to this effect.
A. BASELINE STOVES
The relative performance of newer technology stoves pre-
sented in this report is based on comparisons to conventional or
baseline type appliances. While a wide variety of conventional
box-type stove models are in use and currently on the market,
many are of a similar size and technology. These stoves typ-
ically have a 2.0 to 3.5 cubic foot firebox, have manual air
controls located on firebox sides or doors, and have limited
internal baffling. Construction materials are typically plate
steel or cast iron with or without fire-brick lining in the
firebox (Figure 4).
Laboratory testing was conducted on conventional woodstoves
with medium and large fireboxes to provide baseline data for the
development of woodstove regulations. These data are used to
PAGE 10
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Baffle
Air Inlet
FIGURE 4
BASELINE (BOX TYPE) WOQDSTOVE
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WOODSTOVE DESIGN FACTORS
define the baseline performance capabilities of conventional
stoves for comparison to newer technology stoves. These compari-
sons provide relative measurement of the emission reductions
obtained with the newer woodstove technology. Stoves Z, AA, CC,
DDf and EE are typical of baseline stove technology. Emissions
from stoves in this group ranged from 9 to 48 grams particulate
per hour.
B. FIREBOX GEOMETRY
Firebox geometry describes the size and shape of the firebox
and the pathway of combustion gases in a stove from the air inlet
to the flue collar exit. Firebox geometry represents one of the
largest variables in woodstove design and also represents one of
the most significant factors in the resulting efficiency and
emissions characteristics of a stove. Firebox size refers to the
measure of a stove's volume, while firebox shape describes the
relationship of wall, door, floor, and top dimensions. The
pathway of combustion products in the firebox, through the stove
and up the flue, is a function of firebox geometry, air inlet
location and baffling. .
1. Firebox Size - The capacity of a woodstove to hold fuel is
determined by the size, or usable volume, of the firebox. Many
manufacturers report that the U.S. market requires large stoves
which enable consumers to load their stoves with large amounts of
wood. The ability to maintain long burn duration without reload-
ing or adjusting air settings is commonly regarded as a major
marketing feature. As a result, most stoves sold in the U.S.
market have firebox volumes of about 2 to 6 cubic feet (57 to 170
PAGE 11
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WOODSTOVE DESIGN FACTORS
liters) based on test data from OMNI Environmental and observa-
tions at national trade shows. Several manufacturers from Europe,
where smaller stoves used as room heaters are the rule, have
reported increasing the size of stove models intended for the
U.S. market. The capacity for an "overnight burn" is perceived
by many manufacturers as a key design element.
Firebox size can also limit the maximum potential heat
output from a stove. If more wood can be placed in a stove and
if air supplies are sized accordingly larger, a greater mass of
wood can.be burned in a given amount of time. Larger stoves, in
«
addition to having greater fuel capacities, have greater total
surface area for greater heat exchange potential. A high heat-
ing capacity has been cited by some manufacturers as being neces-
sary to deliver enough heat to older, poorly insulated houses in
colder climates.
Another variable, often not considered by manufacturers and
consumers, is the true space heating capability of a stove. Most
stoves are marketed and sold on the ability to heat an entire
house. The installation of a stove in a single room, with little
or no forced or convective air circulation, often reduces the
heating effectiveness of an appliance to only a portion of the
total house. Practical limits of heating an entire house with
one stove do exist due to maximum "comfort range" temperatures in
the room with the stove. Adjacent or distant rooms in these
cases are usually significantly cooler. This can be important
in terms of pollutant emissions, as stoves capable of supplying
total heat requirements for an entire house would be at lower and
PAGE 12
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WOODSTOVE DESIGN FACTORS
less efficient burn rates to maintain comfort range temperatures
in the room with the stove.
The effect of firebox and fuel load size on particulate and
carbon monoxide emissions was investigated by Burnet and Tiegs in
1983. A model was being sought with which to predict emissions
without conducting extensive actual laboratory testing. A number
of performance parameters were investigated in efforts to find a
single prediction factor.
Data from existing stove tests, all conducted using the
Oregon test procedure, were reviewed. Several stove types with
baffled and non-baffled fireboxes, and with and without sec-
ondary air and secondary combustion chamber designs were com-
pared. Five stoves, each with a different firebox volume, were
selected, representing a variety of stove types (Stoves Z, AA,
BB, CC, DD, and E). Firebox sizes ranged from 0.8 to 3.5 cubic
feet (23 to 100 liters), representing stove sizes from very small
to relatively large.
The effect of burn rate on particulate emissions from each
of the five stoves is readily apparent in Figure 5. The smaller
firebox sizes show consistently lower emissions across the range
of heat outputs. When combustion efficiences from the five
stoves were examined relative to burn rate, a similar relation-
ship was observed (Figure 6). The larger stoves show poorer
combustion efficiency, which indicates higher release rates of
carbon monoxide and unburned organic species across the range of
burn rates.
With small firebox stoves demonstrating higher combustion
PAGE 13
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60
50
o>
o»
E
o
2
O
H 30
ui
UJ
!5 20
o
H
cc
Q. 10
FIREBOX
(ft3)
a 0.8
A 1.4
*2.0
2.4
3.5
STOVE
CODE
BB
CC
DD
Z
AA
6 8 10 12 14
BURN RATE (Ibs/hour)
Figure 5
The effects of burn rate on particulate
emission factors for RWC stoves with various
firebox sizes.
16
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65
3 4 5 6
BURN RATE (Ibs/hr)
Figure 6
The effects of burn rate on combustion
efficiency for RWC stoves with various
firebox sizes.
8
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WOODSTOVE DESIGN FACTORS
efficiences and lower emissions, the factors controlling these
parameters were investigated. Firebox temperatures were believed
to be a critical factor, with higher temperatures yielding high
combustion efficiences. Standardized firebox temperature data
were not available. Flue temperature was therefore used as an
indicator of firebox temperature. While the heat transfer effi-
ciency of the stove will affect flue temperatures, an indication
of firebox temperature is still possible. Figure 7 shows that
the smaller stoves (0.8 and 1.4 cubic foot) demonstrate in-
creasing combustion efficiency with decreasing size and in-
creasing burn rates as indicated by flue temperatures. The
larger stoves, however, are less sensitive to firebox size ef-
fects, and their results for combustion efficiency versus burn
rate lie on an almost universal curve.
Particulate emissions were compared with flue temperatures
(Figure 8). As the combustion efficiences indicate, lower emis-
sions were exhibited by the smaller stoves across the range of
flue temperatures. The larger stoves showed a relatively close
grouping. These data indicate that both combustion efficiences
and particulate emissions were improved with smaller stoves at
any given flue temperature. All stoves showed improved perfor-
mance at higher flue temperatures and higher burn rates.
From the data noted above, it was apparent that burn rate
and combustion temperatures were strong factors in determining
particulate emissions from a stove of any size. The importance
of firebox size appears to be a function of two parameters: the
proportion of the combustion chamber that is maintained at
elevated temperatures at any given time and the size of the fuel
PAGE 14
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95
90
85
UJ
iZ
u.
UJ
O
K
CD
S
O
o
80
75
70
65
o
I
_L
I
FIREBOX
(ft3)
a 0.8
1 1.4
[*2.0
2.4
3.5
STOVE
CODE
BB
CC
DD
Z
AA
IOO
200 300 400 500 600
FLUE GAS TEMPERATURES (°F)
700
Figure 7
The effect of firebox temperature (as indicated by flue
temperature) on combustion efficiency in RWC stoves with
various firebox sizes.
-------
50
40
E
o
30
CO
CO
CO
2 20
UJ
10
FIREBOX
(ft3)
a 0.8
A 1.4
*2.0
2.4
3.5
0.8
STOVE
CODE
BB
CC
DD
Z
AA
IOO
200 300 400 500 600
AVERAGE FLUE TEMPERATURE (°F)
700
Figure 8
The effects of firebox temperature (as indicated by flue
temperature) on emission factors in RWC stoves with various
sizes.
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WOODSTOVE DESIGN FACTORS
load. A smaller stove will maintain flame temperatures through-
out a larger fraction of the firebox than will a large stove at
the same heat output rate. As the heat output rate is increased,
the proportion of the firebox at "flame temperature" increases
for all stoves, but will always be higher for small stoves
until some point is reached for each stove at which the entire
firebox is at flame temperature.
For equivalent burn rates, mixing of gases and air appears
to be more efficient in a small firebox. This is indicated by the
lower emissions and higher combustion efficiencies measured.
Flow velocities and turbulence would be greater in these cases
due to the higher temperatures and smaller spatial dimensions.
Smaller fireboxes also theoretically allow more efficient
air/fuel mixing, due to smaller combustion chamber volumes.
Smaller dimensions reduce the distance combustion air must travel
to reach all areas in the firebox.
The size of the fuel charge appears to be the primary criti-
cal factor, with smaller loads generating lower emissions in the
same stove at the same burn rate. The batch process involved in
fueling a stove requires an entire fuel charge to be placed in
the firebox at once. As the fuel load is heated, gasification of
the wood occurs. The larger the fuel load, the more wood is
subjected to gasification, resulting in greater quantities of
fuel gas being released over a given time. At a fixed heat
output level, more fuel gas will be released from a large fuel
charge than from a small charge. Lower mixing intensities and
more cool spots in larger stoves will result in higher emissions,
PAGE 15
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WOODSTOVE DESIGN FACTORS
especially under low fire conditions. This effect is illustrated
in Figures C-l through C-5 in Appendix C. These graphs plot
emission rates from various woodstoves with and without firebox
size modification as a function of heat output.
To provide a basis for predicting emissons from stove
design criteria, a method of normalizing data from all non-
catalytic stoves was still required. In light of the available
information, a measure of relative burn intensity or an index of
the vigorousness of a fire in each stove type was developed. The
index is based on the percent of the fuel load burned per hour.
This approach permits direct comparison of different stove sizes
as a function of their relative fuel loading capacities. It
should be noted that since the Oregon test procedure calls for a
fuel loading density of 7 pounds of fuel per usable cubic foot of
firebox volume, each stove is loaded according to its specific
size for certification testing.
The percent of fuel load burned per hour provides an indica-
tion of the fraction of the fuel load that is involved in combus-
tion at a given time. An example is presented in Table 2.
PAGE 16
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WOODSTOVE DESIGN FACTORS
TABLE 2: PERCENT OF FUEL LOAD BORNEO FOR DIFFERENT
FIREBOX SIZES OPERATING AT THE SAME BORN RATE
STOVE 1 STOVE 2 STOVE 3
Firebox Volume
(Cubic Feet) 123
Fuel Load
(@ 7 Ib/Cubic Ft.) 7 Ibs. 14 Ibs. 21 Ibs.
Burn Rate (7 Ib./hr.) 7 7 7
Percent Fuel Load
Burned/Hour 100 50 33
PAGE 17
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WOODSTO.VE DESIGN FACTORS
If each stove in Table 2 burns at 7 Ibs. per hour, at the
end of one hour the 2 cubic foot stove (Stove 2) burns half of
its fuel load, and the 3 cubic foot stove (Stove 3) burning one
third of its fuel load, while the 1 cubic foot stove (Stove 1)
has consumed the entire fuel charge.
Emission Factors (gram/kg) for each of the five firebox size
stoves are plotted as a function of the percent of fuel load
burned per hour (Figure 9). The curve shows that emissions in-
crease dramatically in each stove regardless of firebox size at
burn rates less than about 40% of fuel load burned per hour.
A significant aspect of these results is that the data base
for determining the effects of firebox size on particulate emis-
sions was 19 test runs on 5 different stove types and sizes.
Test data are now available on a larger number of stoves although
all recently tested stoves are within the mid- to lower-size
range of the original test group (i.e., 2.5 to 1.2 cubic feet).
All non-catalytic stoves undergoing certification testing at the
OMNI lab through August, 1985, have had a firebox volume of less
than 2.5 cubic feet.
The original 19 data points represented designs typical of
stove technology existing in early 1980*s models. All but the
smallest stove were plate steel models. The smallest stove was a
cast iron model with an inlet air control in the door. Data from
new stoves tested in 1984 and 1985 tend to represent more recent
stove designs, some of which were specifically developed to meet
woodstove emissions standards. These "third generation" stoves
also show significant increases in emissions at burn rates of
less than 40 percent (40%) per hour; however, other factors
PAGE 18
-------
60
50
o>
^
irt
E
o
40
30
CO
CO
LJ
20
10
FIREBOX
(ft3)
00.8
A 1.4
*2.0
2.4
3.5
STOVE
CODE
BB
CC
DD
2
AA
20 40 60 80 IOO I20
% FUEL LOAD BURNED PER HOUR
Figure 9
Emission factors as a function of the percent of fuel load
burned per hour for second generation RWC stoves with various
firebox sizes.
-------
WOODSTOVE DESIGN FACTORS
appear to be acting to reduce emissions below the baseline,
especially the mid-ranges of heat output (Figure 10). Firebox
size, based on volume alone, appears to be the best first line
screening indicator of possible stove performance potential.
Other factors which are discussed in other sections of this
report, such as air supply systems and firebox shape, appear to
be capable of further emissions reductions.
2. Firebox Shape - Firebox shape relates to the configuration of
the walls, floor and top of the firebox. To a certain degree,
the firebox shape controls the route of combustion air and com-
*
bustion products through the stove, and is therefore closely tied
with primary air supply and flue exit locations. The most common
firebox shape found on all types of woodstoves is an elongated
box, designed to accomodate fuel logs either parallel or perpen-
dicular to the fuel loading door. Width is generally the longest
horizontal dimension. Height of the firebox varies, but it is
not uncommon for stoves to have roughly equal dimensions for
height, width and depth.
A small fraction of stove designs incorporate barrel or
tubular shapes. One manufacturer of a round stove postulated
that a round firebox increased combustion temperatures, since
"reflected heat from firebox walls was focused on the middle of
*
the firebox." This is not a documented phenomonon. One round
stove design tested with a firebox volume of about 2.2 cubic foot
showed average or higher emissions (Stove EE). It does not appear
that there is any inherent advantage to round or spherical fire-
boxes, other than that they can minimize firebox surface area for
PAGE 19
-------
60
Third generation stoves
50
40
a
JC
a
| 30
a
(0
O
55
«2 20
UJ
10
FIREBOX
(ft3)
A 1.3
« 1.3
1.3
1.4
©1.6
D2.0
A 2.1
O2.3
+ 2.6
H 1.4
STOVE
CODE
M
S
Q
F
D
00
K
B
0
C
w/o 2nd air
C.
w/2nd air
A1
A2
T
Second
generation
stoves
20
40
60
80
100
120
140
160
<*> FUEL LOAD BURNED PER HOUR
Figure 10
Emission factors as a function of the percent of fuel
load burned per hour for recent design RWC stoves.
-------
WOODSTOVE DESIGN FACTORS
a given volume and therefore reduce heat transfer away from the
firebox. This would theoretically help maintain higher temper-
atures and promote better combustion conditions in the firebox.
Firebox height shows the most variation among stove designs,
due to the. variety of flue gas baffling and venting configura-
tions. A baffling plate, extending from the rear of the stove
upward at a slight angle, is common in many stove designs and is
often the effective firebox ceiling (Figure 11). The addition of
a baffle was considered a significant improvement at the time it
was introduced, with this design feature being considered "second
generation" technology. Sloping sides or walls on a firebox,
either vertically or horizontally, are also sometimes used.
Stoves which have demonstrated low levels of particulate
emissions were reviewed for firebox shape to determine if one
shape or configuration was common to the best models. While no
universal shape or height-width-depth ratio is found on the
stoves with the best emissions performance, several models with
low emissions had relatively short heights in relation to width
and depth (Figure C-6 in Appendix C).
A "squat" shape, with a basic aspect ratio of about 1:0.5
(Figure 12), representing the ratio of width or depth to height,
is theorized to promote complete combustion by minimizing the
firebox size and fuel load while maximizing air/fuel mixing. The
low height (10 to 12 inches) may be the most significant factor.
A "door wash" air delivery system was common to all three
"squat" stove models. This design brings air in through a slot
at the top of the fuel loading door and is believed to allow good
PAGE 20
-------
Figure 11
Single Rear Baffle Design
-------
Figure 12
LOW FIREBOX HEIGHT DESIGNS
-------
WOODSTOVE DESIGN FACTORS
mixing of fresh air with fuel gases. A wide inlet opening maxi-
mizes fresh air supplies in the combustion zone. The cooler and
more dense inlet air is thought to drop into the primary combus-
tion zone. It appears that this feature, intended to keep stove
door glass clean, contributes to efficient combustion. Figure C-
7 in Appendix C illustrates low emission rates are effected by
air inlet location for several stoves at all heat output rates.
3. Pathway of Combustion Products - The third category of fire-
box geometry is the flow pattern, or the pathway of combustion
gases. Flow patterns are a function of the firebox shape and
combustion air system and can vary significantly with burn rates,
gas temperatures, or the draft generated by stove operation
and/or installation. Draft is a weak force; however, since it is
the driving force for all gas flow through the system, it is a
domninant factor in forming flow patterns. Observations indi-
cate the magnitude and direction of eddies and swirls within the
combustion zones can be changed dramatically by small changes in
burn rate, temperature distribution in the firebox, and/or fuel
load configuration. It should be noted that these factors, as
well as fuel load geometry, change throughout the cycle. Actions
such as opening and closing the stove loading door or even minor
adjustments to inlet air control settings have been observed to
affect flow patterns as evidence by flame patterns and changed
fuel load burning patterns.
Combustion efficiency and heat transfer efficiency are both
strongly affected by flow patterns, which influence gas residence
times, temperatures, and air/fuel mixing. From a theoretical
PAGE 21
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WOODSTOVE DESIGN FACTORS
standpoint, combustion efficiency will be optimized by main-
taining elevated temperatures with an adequate oxygen supply for
as long as possible. Flow patterns in the firebox are signi-
ficant in that they control gas residence times and mixing rates
in the hottest regions of the stove. Flow patterns can be con-
trolled or directed in several ways:
* Gas recirculation (backmixing) - Some stove models have
designs which enhance the recirculation of combustion gases
in the firebox. In this approach, combustion air enters the
firebox, mixes with hot fuel gases and makes one or more
passes through a swirl or eddy which supplies air to the
primary combustion zone. These gases complete their travel
through the swirl pattern within the firebox before exiting
the stove. This "recirculation" promotes turbulent mixing
of the hot aged combustion gases with fresh combustion air
and fuel gas. In some stoves (stoves B, R, and others),
the shape of flames formed in the firebox indicate that
recirculation to some extent does appear to occur, although
this is subject to burn conditions and draft.
While most stoves do achieve limited recirculation, measure-
ment and verification of this phenomenon is difficult, even
when visible flames are present. Monitoring the direction
of combustion gases is difficult, especially under the low
velocity and variable conditions of low-fire burn rates.
* Long flame path - This design attains long residence times
at high temperatures by an extended single path through the
stove. High temperatures must be maintained along the en-
PAGE 22
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Figure 13
LOW FIREBOX EXIT DESIGN
-------
WOODSTOVE DESIGN FACTORS
tire path for this concept to be effective at increasing
combustion efficiency. Stoves using this design have been
observed to be ineffective due to the inability to maintain
adequate gas temperatures far from the primary combustion
zone.
All of the non-catalytic stoves which have demonstrated
relatively low average emissions (less than 15 grams/hour) at
relatively low burn rates have primary air introduced high in the
front of the firebox and a firebox breach in the front section of
the top of the firebox (e.g. Stoves B, C, M, O, S, T, and Q).
While these designs have other differences (shape, size, with or
without secondary air supplies, etc.) the common trait of the
combustion gas flow pattern appears to be a significant factor.
It does promote good mixing of the air, fuel, and burning gases
in the combustion zone.
Most stove designs have a gas flow configuration in which
combustion products exit at the top of the stove. Some stoves
however, use a low exit, typically at the bottom rear of the
firebox, to provide a longer residence time in the stove (inten-
ded to enhance either combustion or heat exchange efficiencies
(Figure 13). These designs use the rear of the stove for primary
heat transfer rather than the top. A limited number of stoves
have been tested which use low rear exit designs.
The low rear exit design poses some potential problems on a
theoretical basis. Relatively poor test results from stove models
with this feature seem to bear this out, although other design
features may have contributed. Stoves E and Y are examples of
PAGE 23
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WOODSTOVE DESIGN FACTORS
stoves with low exhaust gas exits from the firebox. Theoretical
problems include:
* Hot combustion gases are buoyant and rise within the
firebox. In order for these gases to exit the stove, they
must either cool and drop to the exit or be pulled down to
the exit by static pressure in the flue (draft) which is
great enough to overcome the buoyant pressure of the firebox
gases.
* A low exit from the firebox can make the stove difficult to
start, as a strong draft is required to draw hot gases down
to the exit. In these cases/ a bypass damper is usually
required for startup and for refueling. Without a bypass
damper, flame and smoke spillage into the room can occur
when the stove door is open. An open door, providing a
higher opening than a low exit, is a pathway of less
resistance to the flow of hot combustion gases, unless an
extremely low flue pressure (high draft) condition in the
flue exists.
a. Baffling
Baffling is a feature which is significant in defining the
size and height of the firebox as well as the flow of combustion
gases through the stove. A baffle which is low and covers a
large portion of the firebox can help improve combustion
efficiencies by lowering the height of the firebox and restrict-
ing the size of the fuel load. This can also be an important
feature in enhancing the heat exchange capabilities of a stove.
PAGE 24
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WOODSTOVE DESIGN FACTORS
However, the term "baffle" in the woodstove industry incorporates
a wide range of designs with an equally wide range of effective-
ness.
The most common baffling arrangement is a sloping plate
extending from the rear of the firebox forward and upward at a
slight angle as shown in Figure 11. Combustion gases are forced
to travel around the baffle before exiting the stove. The
length of the baffle will affect the "route" the gases must
make; a longer baffle will increase the combustion gas pathway
and increase gas residence time. As mentioned previously,
another significant feature of a baffle is that is acts to
impose on or limit firebox ceiling height.
Multiple baffling is used by some manufacturers, usually to
increase heat exchange efficency (Figure 14). In this configura-
tion, combustion gases may be brought forward by the first
baffle, routed again to the rear of the stove by a second
baffle, forward by a third and to the rear again by the top
surface of the stove. This lengthy routing of flue gases can help
heat transfer to the room, although OMNI does not have test data
on this design. A higher resistance to flow is usually generated
by multiple baffling, which may necessitate a bypass route for
startup and fuel loading. Higher heat transfer efficiency and
closer wall clearances (determined by a stove safety testing
laboratory) are often a benefit of the resulting lower flue
temperatures. However, lower flue temperatures can contribute to
increased creosote deposition in the flue.
The baffle plate is, in effect, the roof of the primary
firebox. The height and shape of the baffle can therefore be an
PAGE 25
-------
Figure 14
MULTIPLE BAFFLING DESIGN
-------
WOODSTOVE DESIGN FACTORS
important feature in stove design.
b. Combustion Chambers - Primary and Secondary
The fueling and firing of a woodstove occurs in the primary
combustion chamber. The primary combustion chamber is the fire-
box, where fuel is loaded and burned. Virtually all combustion
processes and the release of energy from the fuel takes place
here, at high or low burning conditions. A number of stoves
currently on the market advertise secondary combustion features.
These include designs with secondary air supplies and/or secon-
dary combustion chambers. Secondary combustion, in theory, pro-
»
duces more complete burning of incomplete combustion products
("smoke") before these products leave the stove. This can theor-
etically be accomplished by introducing fresh air into the hot
gases at the exit of the primary combustion chamber. Combustion
gases leaving the primary combustion chamber often have reduced
oxygen concentrations so that with the introduction of additional
"secondary air", additional combustion can occur. However, in
order to prevent quenching of the combustion process by cold air,
secondary air must be preheated before entering the gas stream.
In a technical sense, a combustion chamber is not a second-
ary combustion chamber unless additional oxidation of incomplete
combustion products is induced and subsequent release of energy
occurs. Under this definition, very few non-catalytic stoves
have true secondary combustion systems. Most stove designs
are not capable of maintaining the temperature of gases exiting
the primary chamber at low and medium burn rates at levels high
enough to support secondary combustion. Test data indicate that
PAGE 26
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. WOODSTOVE DESIGN FACTORS
true secondary combustion on most stoves only occurs at high burn
rates when gas temperatures are very high. Most secondary combus-
tion chambers seem to act simply as a baffling chamber or heat
exchanger surface when the stove is operated at low to medium-
high burn rates. This can be seen from the fact that stoves with
secondary combustion chambers still exhibit significantly in-
creased emission rates at burn rates of less than 40% of their
fuel load per hour (Stoves B, Cf D, and T). Stove R has demon-
strated an effective secondary combustion chamber system (Figure
C-8 in Appendix C).
The key difficulty appears to be introducing secondary air
at a point where combustion gases are at temperatures high enough
to promote additional combustion (typically well above 1100 de-
grees F) . Most models have secondary air inlets at the exit of
the primary firebox where gases begin to cool below the temper-
tures necessary for the combustion of hydrocarbons and carbon
monoxide.
c. Downdraft Combustion
The principle of a downdraft stove is to draw burning gases
down through a bed of coals to promote more complete combustion
of the volatile compounds generated in the main fuel mass (Figure
15). Theoretically, the combustion zone of the downdraft stove
is confined to the lower coal bed areas of the fuel mass. By
drawing the hot combustion gases down through the coals, the fuel
mass, other than that in close proximity to the coal bed, remains
relatively cool. This limits, or more closely regulates, the
release of fuel gases from the fuel mass and promotes more
PAGE 27
-------
Air Inlet
Goal Bed
Figure 15
Downdraft Woodstove
-------
WOODSTOVE DESIGN FACTORS
complete and even combustion conditions in the primary combustion
zone. In practice, however, downdraft stoves have not exhibited
significant performance or emission improvements over the base-
line updraft type of stove. Figures C-9 and C-10 in Appendix C
show the relative emission rates of downdraft and updraft type
stoves.
PAGE 28
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WOODSTOVE DESIGN FACTORS
C. COMBUSTION AIR - DISTRIBUTION, CONDITIONING AND CONTROL
1. Primary Air - Air supply systems include all devices
affecting the volume and distribution of inlet combustion air.
Typically combustion air enters the stove through one or more
openings, often located symmetrically, through which flow can be
regulated and controlled. It also includes any intrinsic lea-
kage, intended or not, through door seals, around air controls,
and through cracks in the stove.
Primary air refers to the main flow of air into the primary
combustion zone or firebox. This air controls the overall rate
of fuel consumption and the heat output of the stove. Many
stoves use a single air inlet, although a number of other confi-
gurations are also used. Common air inlet configurations include
dual (symmetrical) inlets on the door(s) or sides of the stove,
at the rear of the stove/ at the top or bottom of the door and
from underneath the floor of the firebox. The upper firebox
inlet, or door glass air wash system, has seen increasing popu-
larity on stoves, as it helps prevent the accumulation of creo-
sote on glass windows. Most important, it promotes mixing of
fresh air with the burning and aged combustion gases.
The amount of air entering the firebox is a function of the
area of "free flow" through air ports and the draft (negative
pressure) drawing air into the stove. The flow of air is gen-
erally controlled by a device which can decrease or increase the
free flow area available for air to pass through. Common designs
include a sliding plate placed over the primary air port, "spin
PAGE 29
-------
WOODSTOVE DESIGN FACTORS
drafts," butterfly valves and flapper plates, with or without a
thermostatic control. The pathway of inlet air is controlled by
the location of entry and temperature.
The location of the air entrance into the firebox can be
important to the burning characteristics of a stove for several
reasons. If air is brought directly into the firebox too low, it
can descend in the firebox and "tunnel" into and under the coals
due to its cooler temperature and higher density relative to the
hotter aged gases in the firebox. These conditions can lead to
pockets of oxygen depletion in the coal bed and fuel mass. When
this occurs, carbon monoxide concentrations increase in the flue
gases. For example, measurements of carbon monoxide in flue gases
during non-tunnel conditions in a baseline stove were 1.25%.
During a period when tunneling was observed in the fuel mass,
concentrations increased to 3.50 - 4.60 %. In addition, as raw
wood is heated, fuel gases are volatilized and driven off from
the fuel mass. Incomplete combustions of these gases contribute
to particulate emissions loading.
Air should be brought into the firebox so that sufficient
oxygen is mixed into aging combustion gases, and that an adequate
amount of air reaches the coal bed. Most stoves showing good
performance (averaging less than 15 gram/hour) at low to medium
burn rates have air inlets elevated in the firebox (Stoves A, B,
C, D, M, S, T).
2. Underfire Air - While most stove designs introduce com-
bustion air through the walls or door of the firebox, underfire
air systems bring air into the firebox through a grid or grate
PAGE 30
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WOODSTOVE DESIGN FACTORS
under the fuel mass. This design is typical of combination
coal/wood burners. When wood is fired from below, heat released
by combustion rises through the fuel load, releasing excessive
amounts of the volatile components of the wood. However, obser-
vations in the OMNI lab show that low flue oxygen levels are
typical of underfire designs. This results in poor combustion,
high particulate and carbon monoxide emissions and is char-
acterized by very low oxygen in the flue. For this reason,
combination coal/wood burners, or any woodstove using underfire
air, have performed poorly in emission tests. Figure C-ll in
Appendix C illustrates the effect of reducing underfire air
conditions in a woodstove air supply system. Onderfire air
designs show performance similar to or worse than those which
introduce air very low in the firebox, for similar reasons
(Stoves El, E2, and FF).
One of the most effective primary air supply systems appears
to be a wide air slot at the top of the firebox (Stoves A, B, C,
D, M, O, Q, S, and T ). This approach was devised to keep glass
windows clean by preventing the deposit of creosote or carbona-
ceous material. Air is typically drawn into the firebox through
a slot extending across and directly above the stove window. The
air "washes" down the glass, providing an air curtain to keep
combustion gases away from the window. When sized appropriately,
these systems are quite effective at keeping glass windows clean,
even at relatively low burn rates.
There are several reasons for the effectiveness of door wash
designs in reducing pollutant emissions. The introduction of
primary air high in the firebox (at the top of the door) will
PAGE 31
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WOODSTOVE DESIGN FACTORS
generally allow good heating and mixing/ preventing pockets of
oxygen depletion in the coal bed and fuel mass. Also,
introducing air through a slot extending across the stove/
combustion air can be mixed more readily with hot aging fuel
gases. Most door wash stove designs also have the firebox breach
toward the front of the stove, so that the aging gases mix with
some of the high-oxygen inlet air before exiting the stove/
promoting additional combustion if temperatures are sufficiently
high. The door wash designs also appear to promote a circular,
"swirl" type gas flow pattern, which promotes efficient combus-
tion through greater turbulence and longer residence times.
3. Secondary Air - Secondary air is a term describing air
added to hot combustion gases which have left the primary combus-
tion zone. Although this term is used to describe a variety of
designs, few stoves have what can be technically considered
effective secondary air systems, especially at medium to low burn
rates. What is called secondary air is often simply an addi-
tional air supply.
A well designed secondary air system allows preheated air to
mix well with hot aging combustion gases. To be effective, this
air should be brought into an area where rapid and thorough
mixing is possible. The point of injection needs to be as close
as possible to areas where the highest temperatures occur at any
burn rate. Secondary air ports on several low emission stoves
introduce preheated air at the top of the firebox, where mixing
and combustion can occur before gas temperatures drop (Stoves C2-
2, C2-3, M, R). At low burn rates, most secondary combustion
PAGE 32
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WOODSTOVE DESIGN FACTORS
systems lose effectiveness due to low gas temperatures and
turbulence.
Effective secondary air systems are possible on woodstoves
(Stoves B, C2-2, C2-3, M, and R). These stoves introduce pre-
heated air, directly into the upper flame zone of the primary
firebox. The air is diffused across a wide area to allow com-
plete mixing. Data comparisons of various secondary air systems
are presented in Figures C-12 through C-14 in Appendix C.
Secondary air volumes are low in comparison to primary air
volumes and may be regulated together or separately from the
primary air. A feature which blocks secondary air completely
below a certain primary air setting may be desirable to reduce
high excess air conditions when the secondary air is not effec-
tively improving combustion efficiences. This is a feature on
Stove M.
4. High Minimum Burn Rates - One approach to the problem of
low combustion temperatures and efficiency at low burn rates is
to always maintain a moderate to high burn rate by not allowing
the air controls to be closed below a certain point. As presented
previously, non-catalytic woodstoves have lower combustion effi-
ciencies and higher particulate emission rates at firing rates
below about 40% of the fuel level burned per hour. This is due
to lower combustion temperatures and poorer mixing of gases. In
addition, a larger firebox (and hence a larger fuel load), will
result in higher emissions at any low burn rate, as described
previously.
Current emission testing requirements prescribed by the
Oregon DEQ and in the proposed ASTH test procedure (8) specify
PAGE 33
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WOODSTOVE DESIGN FACTORS
that an appliance be tested across the range of stove heat output
capacity. If a stove is not capable of passing an emission
standard at the prescribed lower burn rates/ a minimum burn rate
which is high enough to provide good combustion conditions and
reduced emissions can be established. This is accomplished by an
air control stop which prevents closure beyond an established
amount. Elevated minimum burn rates are used to reduce emission
rates, especially with larger firebox stove models (Stoves D, F,
and G). It is important to note that while a tested minimum heat
output may be relatively high, actual "in-use" heat output can
be reduced by firing the stove with smaller fuel charges. This
approach does reduce emissions based on units of heat delivered.
5. Controls - Adusting the air control allows a greater or
lesser opening for air entering the firebox. This adjustment
controls the rate at which fuel is consumed and the heat output
of the stove. Air inlet controls are typically either a manual
control which remains in a fixed position or a thermostatic
control which responds to changes in stove or room ambient
temperatures.
A variety of designs are used for manual controls. Sliding
plates, hinged flaps, butterfly valves and spindraft controls are
most common. An adjustment lever is usually connected directly
to the air control and is therefore located near the air inlet.
Some designs use connecting linkage between the adjustment lever
and the air control, permitting the control lever and air inlet
to be separated. This may be done for convenience and ease of
operation, or may help keep control levers cool.
PAGE 34
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WOODSTOVE DESIGN FACTORS
Thermostatic devices are often used on woodstoves, although
they are more common on catalytic models. Thermostatic air
controls typically use a bimetallic spring or coil in conjunction
with a manually adjusted control. The thermostat reduces the air
opening when desired temperatures are achieved and opens to
provide increased air supplies as stove temperatures drop.
While the bimetallic coils are most common, oil-filled sensors
and linearly expanding rods are also used.
When fuel is added to a non-thermostatically controlled
stove, temperatures peak approximately halfway through the burn-
ing cycle if the air inlet control remains constant. With a
thermostat, however, the thermostat begins to reduce the amount
of air entering the stove as stove temperatures increase. By
this time, most of the fuel charge has been heated and is in-
volved in combustion or is volatilizing fuel gases. As tempera-
tures increase and the thermostat closes, air to the fire is
reduced, and the heated fuel charge continues to emit organic
compounds. In most cases the decreased air supplied by the ther-
mostat is insufficient for complete combustion of the fuel gases
evolving from the fuel mass. Low excess air conditions and high
emissions result. The apparent effect of this is illustrated in
Figure C-15 in Appendix C. As a general rule, it has been OMNI's
experience that a thermostatic device on a non-catalytic stove
will cause increased emissions if compared to the same stove
without a thermostat. Data from Stoves E and FF exhibit this
charateristic.
PAGE 35
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WOODSTOVE DESIGN FACTORS
D. HEAT EXCHANGE SYSTEMS
The overall heating performance of a woodstove is based on
two parameters: combustion efficiency and heat transfer effi-
ciency. While the efficient combustion of wood in a naturally
drafted appliance has shown to be a complex and difficult task,
efficient transfer of heat from hot flue gases to a heat transfer
surface is relatively well understood. Many woodstove designs
under-utilize existing technology for maximizing heat transfer.
The heating capabilities of a woodstove are based on the
ability of the stove to transfer the heat released from the fuel
by combustion to the space to be heated. To accomplish this,
heat energy must be removed from the burned gases, passed through
the surfaces of the stove and transferred into the space sur-
rounding the stove. Heat transfer from a fluid (gas) to solid
surface and vice versa is a function of the velocity of the gas,
the geometry and area of the surface, the emissivity of the gas,
and the temperature differential between the gas and the surface.
Most basic stove designs use the combustion chamber as a heat
exchanger. A simple box stove design without baffling transfers
heat from hot combustion gases to the stove surfaces, which then
radiates heat to the surrounding air. On all stoves, the chim-
ney connector between the flue collar and chimney is also an
effective radiating surface. A significant drawback of this
general technology is that by removing heat from the entire
stove, firebox included, temperatures in the combustion chamber
are lowered, which can reduce combustion efficiency.
PAGE 36
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WOODSTOVE DESIGN FACTORS
For any stove design, heat transfer efficiency will usually
decrease as the burn rate increases. This is due to the de-
creased residence time of the hot combustion gases in the stove
due to higher flow rate/ even though higher temperature differen-
tials exist. There does not appear to be any way to prevent this
loss in efficiency at higher burn rates without increasing
residence time or radiating surface area. The lower heat transfer
efficiences at high burns may be compensated for by higher com-
bustion efficiencies, so that overall efficiency may remain rela-
tively constant.
The design considerations mentioned above help improve
transfer of heat from combustion gases to the stove body, and
high surface area on the stove will increase the heat radiated
into the room. Some stoves also rely on forced convective heat
transfer to the space being heated by using a fan to force air up
the back across the top, and/or across the sides of the stove.
Fans are often used on fireplace inserts, due to the reduced
stove surface area available for radiating heat into the room.
However, it has not been shown that a fan will increase the net
performance of a stove. This is especially true if a fan draws
enough additional heat away from the stove to cool the firebox
temperature and thereby decrease combustion efficiences.
A number of stoves use a natural drafted convection air
system to draw air up past the sides of the stove. In this
approach, an outer shell or cabinet (typically light gauge metal
or ceramic tiles) is placed around the firebox. Heated air
between the stove and the shell rises and produces a thermal
PAGE 37
-------
WOODSTOVE DESIGN FACTORS
siphon effect, drawing cooler air in at the bottom while venting
heated air at the top. This method also usually allows closer
clearance from the stove to combustible surfaces when tested to
the safety standards used in the industry.
While it is desirable to obtain as much usable heat as
possible from a stove, some heat must be sacrificed to ensure
proper operating conditions. If too much heat is removed from
exhaust gases, flue temperatures may be inadequate to maintain
adequate draft pressure. The lower the draft, the less air will
be drawn into the stove and the slower the wood will burn at any
given setting.
Maintaining a minimum stack temperature also reduces the
chance of condensing water in the flue. With an average flue
moisture content of 12%, condensation of moisture will occur at
about 120 degrees F. At a moisture content of 25%, which is
common in woodstove flues for short periods, condensation will
occur at 150 degrees F. This potential problem is of greater
concern on catalytic stoves, where stack gas temperatures and
velocities are typically lower. Moisture condensation can be a
nuisance problem by draining down the wall of the chimney into or
onto the stove. If moisture condensation in the flue is a chro-
nic problem, corrosion of the metal chimney connector, metal
chimney and even the stove itself may be greatly accelerated.
This problem can also be exacerbated by freezing conditions in
flue segments located in outdoor environments.
PAGE 38
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WOODSTOVE DESIGN FACTORS
E. CONSTRUCTION MATERIALS
The materials used in the construction of a stove can have
a significant impact on the performance of the device. There are
two primary functions in a woodstove: releasing heat from the
fuel and recovering heat from combustion gases before they exit
the stove. In efforts to maximize the efficiency of these two
functions, opposite tasks must be achieved. High temperatures are
essential for complete combustion, requiring that heat be re-
tained in the firebox as much as possible. Efficient transfer of
heat out of hot flue gases/ however, requires good conductivity
of heat away from the flue gases.
A traditional woodstove, constructed entirely with single
walls of plate steel or cast iron, accomplishes much of its heat
transfer by radiating heat from the firebox, which serves as both
combustion chamber and heat exchanger. The resulting lower fire-
box temperatures make efficient combustion more difficult. The
use of a full baffle in the roof of the firebox does help in-
crease firebox temperatures by reducing the temperature differ-
ential of the top of the firebox (the baffle plate) as shown in
Figure 16.
Most stoves currently use a plate steel or cast iron fire-
box. Plate steel is most common, primarily because the equipment
required to form, shape and join the metal is relatively inexpen-
sive compared to the cost of casting. Steel stoves typically use
1/4" plate for the firebox and 3/16" plate for other parts.
Cast stoves offer greater potential for unusual shapes and forms,
and parts can be formed rapidly and economically once a design
PAGE 39
-------
Figure 16
HEAT RETENTION EFFECTS OF BAFFLE
300" F
80
rp A
500 F
900*F
-------
WOODSTOVE DESIGN FACTORS
mold has been established. Parts are usually joined with screws
and clips and seams are caulked.
Maintaining high firebox temperature for any given burn rate
can be accomplished by utilizing materials which have a low heat
transfer coefficient. Recent work by Fuentes and Hodas (9) iden-
tifies the insulating effectiveness and relative costs of various
ceramic materials/ including firebrick. As a practical matter,
any material or design which reduces heat transfer away from the
firebox is desirable. A more efficient refractory material will
achieve lower heat transfer (conductivity) coefficent for a given
thickness. Less efficient insulators will require greater thick-
ness to attain the same resistence to heat loss.
It should be noted that although it is desirable to maintain
high temperatures in the primary combustion zone, there can be
undesirable effects if temperatures get too high. Excessive
temperatures cause the fuel mass to heat up too fast,- accelera-
ting the release of the volatile fuel gases. without the addi-
tion of large amounts of air, these volatiles burn inefficiently,
producing high pollutant emission rates. For example, an experi-
mental stove tested at OMNI had a highly insulated primary
combustion chamber which maintained temperatures at 1820-1980
degrees F. At the end of each burn cycle flue gas carbon mono-
xide concentrations were in the range of 0.1% to 0.2%, with 10%
oxygen. However, each time additional test fuel was added to the
coal bed, carbon monoxide levels exceeded 10% with oxygen less
than 2%. Dense smoke was also observed in flue gases during
these periods.
PAGE 40
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WOODSTOVE DESIGN FACTORS
While the use of refractory material to insulate the firebox
is recommended from a theoretical standpoint, verification of
how much is necessary has not been attempted on a controlled
basis. Tests on the addition of refractory material to an
existing stove design have been performed (Figure C-16 in
Appendix C). However, no tests have been performed to separate
the effects of reducing the volume of the firebox and the
effects of the insulating value of the refractory. Most stoves
with a refractory lining incorporate a standard firebrick used by
masons in building fireplaces. These bricks are nominally 4.5 by
9 inches and typically 1.25 inches thick, although 2.5 inch thick
bricks are available.
Heat exchange materials usually consist of the material used
in the construction of the stove. Most stove designs use the
walls and top of the stove as radiating surfaces, and virtually
all radiating metal surfaces are painted black to increase emis-
sivity. Some stove designs utilize fins to increase the radiat-
ing and conductive surface area.
Some stoves advertise "high mass" heat storage or efficiency
characteristics. These stoves work on the principle of heating a
large mass of material during the firing period and then re-
leasing the heat slowly over a longer period of time. This
concept holds potential as an emission reduction method only if
combined with both an air control system which will maintain a
high minimum fire setting and an appropriate heat transfer system
to capture the produced heat. The principle of using a large
mass to store heat is that heat absorbed by the material at a
high firing rate is radiated to the living space at a slower rate
PAGE 41
-------
WOODSTOVE DESIGN FACTORS
over a longer period of time. This allows a stove to be fired at
a higher rate than would be comfortable with a similar steel or
cast stove. The high firing rate will usually result in a lower
pollutant emission rate. However, if the air control can be set
to maintain a low burn rate, resulting in higher particulate and
carbon monoxide emission rates, no emission reduction benefit is
realized. While a high mass stove will radiate stored heat for a
longer period than a lighter stove, it will also take a longer
time for the stove mass to come up to temperature.
A high mass/high heat capacity stove allows a higher firing
rate over a longer period of time than would be acceptable in a
lower mass stove. Heat is released from the high mass stove at a
slower rate than it is accumulated during the firing period and
then releases it longer after the firing period is finished. As
mentioned previously, no emission reduction benefit is seen un-
less a high firing rate is used at all times. Heat output from
the stove could be regulated by the frequency of firings, and
continuous firing may not be necessary.
A variety of materials are used for heat storage and/or
decorative purposes. Ceramic, stone and refractory-type mater-
ials are frequently used for their low cost and high specific
heat (approximately twice that of steel).
PAGE 42
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WOODSTOVE DESIGN FACTORS
IV. OTHER NON-CATALYTIC TECHNOLOGIES
A. Pellet Fuel Stoves
Efficient combustion is difficult to accomplish in tradi-
tional wood stoves at low heat outputs due to low combustion
temperatures in the firebox, poor air/fuel mixing, and changing
combustion conditions. If air can be moved into the firebox in a
more concentrated and turbulent manner than relying on the draft
generated by hot flue .gases, higher temperatures and more effi-
cient combustion can be achieved. Mechanical draft systems, in
which air is forced or drawn into the firebox, are used on many
wood furnaces and all types of small, mid- and large scale
boilers. This technology has seen virtually no application to
residential wood stoves, however.
A forced draft system moves air into the firebox under posi-
tive pressure; higher pressures exist in the stove than the sur-
rounding air. Induced draft systems draw exhaust gases out of
the firebox, creating negative firebox pressures. Both systems
use fans to move the air and gases, hence the term mechanical
draft. (Natural draft stoves are more properly "thermally
drafted"). The use of a mechanical draft allows much greater
control of combustion air flows and patterns, resulting in im-
proved combustion conditions.
Both the induced and forced mechanical draft systems have
advantages and disadvantages. Forced draft systems require
PAGE 43
-------
WOODSTOVE DESIGN FACTORS
higher quality construction methods to prevent leaks through
doors and other openings but move clean air through the blower.
Induced draft systems do not require a perfectly sealed
system, as any leaks will simply draw air into the stove or flue.
They must move hot and sometimes dirty gases, requiring more care
in selection and maintenance of fans and motors. In addition,
mechanical draft stoves will not operate without electric power
to drive the fans, so these units can not be utilized during
power outages.
Among wood burning appliances currently available, virtually
all mechanical draft stoves are designed to burn pelletized wood
fuel. Pellets are typically 1/4 inch to 3/8 inch in diameter and
about 1/2 inch in length. The composition varies among manu-
facturers, but is primarily sawdust and chips from forest pro-
ducts operations. Some pellets are composed of wood only, while
others contain more bark and debris. Most are formed under heat
and high pressure, and most use no binder. Heat content of the
pellets are typically 8750 to 9200 Btu/lb, with a moisture con-
tent of 6-10 percent. Pelletized fuel has been successfully
used as a substitute for coal in many small boiler applications
which have traditionally used coal.
The primary advantage of using pellets in residential com-
bustors is the ability to control the amount of fuel involved in
combustion at any time. Air and fuel feed rates can both be
controlled, allowing optimized combustion conditions. Pellets
can be fed at a constant rate into a combustion zone maintained
at high temperatures and high turbulence by a forced or induced
draft.
PAGE 44
-------
WOODSTOVE DESIGN FACTORS
The mass of fuel involved in combustion at any time is very
small, while oxygen supplies and turbulence are high. Pellet
stoves can operate under steady state conditions as a continuous
process, rather than the semi-batch process of burning cordwood.
The small mass of fuel burning at any given time promotes stable
conditions, which allows more fficient combustion. Pellet
stoves have shown to have efficiences and emissions comparable or
better than catalytic cordwood stoves (Stoves U, and X). Results
are illustrated in Figures C-17 and C-18 in Appendix C.
On all existing pellet stove designs, fuel is stored in a
hopper and moved into the combustion chamber/firebox with a moto-
rized screw auger. The feed rate is controlled by a variable
speed or time-on switch. Pellets are pushed or dropped into a
small cup-shaped tray which is surrounded by combustion air inlet
jets, creating an intense burn region. Blower speed can also be
varied; some stoves combine air and fuel feed rates while others
offer combinations of independent fan and fuel feed rates, and
continuous or intermitant operation. Most pellet stoves use a
refractory lined firebox in which the pellet "cup" and air ring
are located. Gases are then vented through heat exchange baffles
or chambers similar to conventional woodstoves. Several models
of add-on pellet hopper/burner devices are also available, which
mount on most woodstoves. The performance of a pellet hopper
device on two stoves is shown in Figure C-19 in Appendix C.
Exhaust gas temperatures are generally quite low on pellet
stoves, indicating high heat transfer efficiency. Temperatures
PAGE 45
-------
WOODSTOVE DESIGN FACTORS
as low as 150 degrees have been recorded on several devices at
the OMNI laboratory.
B. Densified Fuel
While pelletized fuel requires a special combustor and fans
to move combustion air and exhaust, other processed and densified
wood waste logs have also shown lower emissions in traditional
stoves. Limited testing on densified wood residue "logs" showed
approximately one half the particulate emissions seen from burn-
ing a comparable mass of cordwood in the same appliance at simi-
lar heat outputs. Stove runs Z2 are densified fuel, compared to
Zl runs using Douglas Fir, fuel. Figure C-20 in Appendix C
illustrates these data. It appears that this is due to the
slower evolution of fuel gases because of the lower surface area
per unit mass. Fuel moisture is generally quite low (about 8%)
in the densified fuel.
C. Mechanically Drafted Cordwood Stoves
Very few mechanically drafted residential wood stoves are
currently available. Most mechanically drafted units designed to
burn cordwood are furnaces, hot water heaters or central heating
systems. Some testing has been done on an induced draft cordwood
furnace/water heaters. Results were varied, but low emissions
were obtained under proper conditions (estimated from carbon
monoxide and carbon dioxide concentrations in flue gases). High
temperatures were achieved by a refractory lined furnace and a
refractory secondary combustion chamber.
When a fuel charge is added to a firebox at very high
temperatures, the wood volatilizes very rapidly. While this can
PAGE 46
-------
WOODSTOVE DESIGN FACTORS
provide very efficient combustion, the critical requirement is
supplying adequate air to match the evolving volatile compounds.
Very high temperatures are possible with this type of system.
The high rate of heat output from such systems require some type
of heat storage or moderation.
D. Microprocessor Control
The batch process of combustion in cordwood stoves presents
a great challenge. Changing temperatures, fuel gas concentra-
tions and combustion air requirements make designing an air
supply system which will provide the proper amount of air under
all conditions difficult. Even mechanically drafted stoves can
create poor combustion conditions, primarily due to the "batch"
process of fueling.
Some design work is underway to apply microprocessor con-
trols to wood stoves. System monitoring of stack oxygen levels
or firebox temperatures could provide feedback to reduce or
increase the combustion air supply, compensating for rich or lean
conditions. For cordwood stoves not utilizing catalytic combus-
tors, this approach may be necessary to achieve very low particu-
late emission standards. The high cost of developing and instru-
menting such a system appear to be the factors preventing the
application of existing boiler technology to residential wood
combustors. Few if any small woodburning appliances with micro-
processor controls are currently available.
PAGE 47
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WOODSTOVE DESIGN FACTORS
References
1. Marshall, N. , Patten/ B., and Skog, K., "The Dynamics of
Residential Wood-Energy Use in the United States: 1970-2030 r"
Resource Policy Center, Dartmouth College, 1983.
2. Skog, K. and Watterson, I., "Residential Fuelwood Use in the
United States: 1980-81," U.S. Department of Agriculture, Forest
Service, July 1983.
"
3. Thompson, C., "Residential Energy Consumption Survey:
Consumption and Expenditures April 1982 through March 1983
DOE/EIA - 0321/2 (82).
4. Wood 'N1 Energy, "Marketing in the '80's", Special Seminar
Edition, 1984.
5. Federal Register, Vol. 50. No. 149, Friday, August 2, 1985,
"Standards of Performance for New Stationary Sources; Residential
Wood Combustion", p. 31504.
6. Burnet, P., Edmisten, N., Tiegs, P., and Yoder, R., "Acid-
Potential Emission Factors for Residential Wood Buring Stoves,"
Air Pollution Control Association, Annual Meeting, June 1985.
7. Burnet, P. and Tiegs, P., "Woodstove Emissions as a Function
of Firebox Size", Paper 84-27, 21st Annual Meeting, PNWIS/APCA,
November 12-14, 1984.
8. "Standard Test Methods for Heating Performance and Emissions
of Residential Wood-Fired Closed Combustion-Chamber Heating
Appliances", American Society of Testing and Materials,
September, 1985.
9. Fuentes, K. and Hodas, L., "Feasibility Study of Enhanced
Combustion Via Improved Wood Stove Firebox Design," Radian
Corporation, Wood Heating Alliance, Baltimore, March 16-19, 1985.
ACKNOWLEDGEMENTS
The contributions and assistance of Professor Philip Malte,
Department of Mechanical Engineering, University of Washington,
are greatly appreciated.
-------
APPENDICES
A. STOVE DESCRIPTIONS
B. STOVE TEST DATA
C. GRAPHICAL DATA PRESENTATION
-------
WOODSTOVE DESIGN FACTORS
.Notes on Stove Codes
Stoves are identified by an alpha-numeric code:
* The first alpha character(s) (A through EE) designates the
stove manufacturer and model;
* The first numeric character, immediately following the alpha
code but proceeding the hyphen, designates the test series
by a manufacturer at OMNI labs. Unless otherwise noted in
Appendix A, Stove Descriptions (see Comments column), no
modification was made to the stove between test series.
* The number following the hyphen represents the test run
number in the manufacturer's testing series.
Example:
A2-7
ttl
test run number in second test series on stove A
second test series on stove A
Stove manufacturer/model code
»
Appendix A, Stove Descriptions, shows modifications to the
stoves.
Appendix B presents stove test data identified by stove codes.
Appendix C shows stove test data from Appendix B plotted in
graphical form. Data points are identified by stove code.
Additional stove test data is compiled in a separate companion
volume entitled "Data Supplement - The Effects of Design Factors
on the Emissions from Non-Catalytic Residential Wood Combustion
(RWC) Appliances."
-------
WOODSTOVE DESIGN FACTORS
The following information represents compiled data from
woodstove emissions and performance testing conducted in
accordance with the Oregon Department of Environmental Quality
"Standard Method for Measuring the Emissions and Efficiencies of
Residential Woodstoves," June 8f 1984.
Descriptions of the stove technologies include pertinent
information on the size and configuration of stoves referenced in
the data. This is followed by test data grouped by stove model.
-------
APPENDIX A
STOVE DESCRIPTIONS
-------
Stove
Code
Al
A2
Firebox
(FT3)
1.6
1.2
Bl
B2
Cl
C2
Dl
D2
2.1
2.3
2.3
1.4
1.8
1.6
2.6
Dimensions (in.) Primary
WHO Air
19.5 13.9 10.4 a,b,c
19.5 10.6 10.4 a,b,c
16.8 12.5 17.0 a,e,f
16.9 13.0 18.5 a,e,f
17.0 12.3 19.1 a,b,c
31.1 11.1 17.0 a,b,c
16.8 10.0 19.0 a,b,c
14.7 10.3 17.9 a,b,c
22.9 20.0 9.8 e,f,h,i
Secondary
Air
no
no
no
no
c,f
Refractory
Lining
yes
yes
Firebox
Breach
g
g
no
no
yes
no
no
see comments
g
g
g
g
yes
see
comments
no
Comments
Actual depth 17"
- log irons
prevent use of
forward 6.5". A2
had grate to
reduce fuel load
with no change
in actual firebox
dimensions. Con-
vection heater.
Firebrick removed
for B2.
Convective
heater.
Cast iron fire-
box liner used
to reduce fire-
box size on C2.
Dl - brick on
floor only.
D2 - Fully
lined w/fire-
brick & reflec-
tive plates.
Stove and fire-
box made of cast
iron. Primary
air controlled
by a thermostatJ
-------
Stove
Code
F.l
F2
G2
HI
H2
Firebox Dimensions (in.) Primary
(FT3) WHO Air
1.3 18.8 12.0 10.0 c,d,o
1.44 19.5 12.1 10.5 c,d,o
2.04 20.6 15.4 11.5 c,d,o
1.0 17.0 10.5 9.5 b,e,p
1.0 17.0 10.5 9.5 b
2.2 24.0 13.0 12.0 e,p
1.7 11.5 12.0 21.0 c,f,i
2.1 13.9 17.7 14.0 a,e,f
Secondary
Air
no
no
no
a
a
no
no
f,r
Refractory
Lining
no
no
no
no
no
no
see
comments
Firebox
Breach
m
m
m
g
g
m
Comments
Back baffle and
front grate
were in place.
Stove & fire-
box are made of
cast iron. Fl
and F2 are the
same stoves.
The firebox was
remeasured for
F2 to reflect
reevaluation of
complex geometry.
Top baffle &
grate were in
place. Stove is
made entirely of
cast iron.
Cast iron fire-
box. Air on H2
entered along
sides of firebox
floor. Steeply
sloping baffle.
Underfire air
used in combus-
tion area. Cast
iron firebox.
Stove is of
down draft
design.
Refractory
lining on the
bottom, sides
and in chamber
below firebox.
Downdraft desju
-------
Stove Firebox Dimensions (in.) Primary Secondary Refractory Firebox Comments
Code (FT3) WHO Air Air Lining Breach
! 0.8 11.0 8.5 15.0 b,c,o f no see Stove is made
comments of cast iron.
Secondary air
directed into
narrow breach
at rear of
firebox.
M 1.3 16.5 10.0 13.8 a,c,f,i c,f,t yes g M2 runs were
conducted in
efforts to
improve
"Oregon
weighted"
emission
values.
N 1.7 20.0 15.9 9.3 b,e,i no v q Cast iron fire-
box surrounded
by a stainless
steel cabinet.
0 2.5 18.5 14.0 16.8 a,e,f no yes u Stove uses con-
vection air
blower.
P 1.6 .16.0 17.5 10.0 c,p f no g Convection
heater.
Ql 1.1 22.0 9.5 9.0 a,c,f no no g Ql & Q2 are
same stove with
Q2 1.3 22.0 11.5 9.0 a,c,f no no g different
interpretation
of firebox
volume changing
fuel load size.
-------
Stove
Code
R
T
U
V
Firebox Dimensions (in.) Primary Secondary
(FT3) W H D Air Air
1.4
17.4 9.8 14.5 a,b,c,i
1.3
1.3
1.2*
16.6 12.0 12.0
18.3 9.5 12.8
11.5 13.0 14.0
a,c,f
a,b,c
aa,bb,cc
*fuel placed within combustion zone
approx. 3" x 3"
5.1
23.5 21.6 17.3
bb,cc
c,f ,x
Refractory
Lining
yes
Firebox
Breach
no
no
f ,cc
yes
yes
side &
rear walls
9
1
no
yes
Comments
A second second-
ary air tube
runs perpend-
icular to the
front tube down
the top center
of the firebox.
Fireplace insert,
Firebrick on
floor and sides
of firebox.
Pellet stove
induced draft
Blower fan &
fuel feed rate
are under sepa-
rate controls.
Fuel fed by an
auger & is
introduced from
underneath the
combustion zone.
Pellet fuel
add-on device on
large stove
using induced
draft system.
Blower fart and
fuel feed rates
are controlled by
separately. Fuel
is fed by an
augor and is
dropped into
the combustion
zone from above:
-------
Stove
Code
W
Firebox Dimensions (in.) Primary Secondary Refractory
(PT3) W H D Air Air Lining
2.4
16.0 15.0 17.0 bb,cc
no
yes
Firebox
Breach
u
see comments
b,bb,cc
yes
2.4
17.0 13.5 18.0
aa,o,b
no
yes
Comments
Same pellet
add-on device as
V on smaller
stove. Primary
air closed for
use with add-on
device. Induced
draft system.
Blower fan and
fuel feed rates
controlled sepa-
rately.
Pellet stove w/
a forced draft
exhaust system.
Blower fan and
fuel feed rates
are controlled
separately.
Fuel is fed by an
auger and is
dropped into the
combustion zone
from above. New
test data; des-
criptions & data
being compiled.
Baseline stove/
very small
baffle. Z2 runs
were conducted
with densified
fuel.
-------
Stove
Code
AA
Firebox Dimensions (in.) Primary
(FT3) WHO Air
3.5
BB
0.8
CC
1.4
DD
2.0
Secondary
Air
19.0 19.0 17.0 aa,b
no
9.0 17.0 9.0 b,o,aa
no
18.0 8.0 16.75 a,c,f
no
18.0 14.0 14.0 a,c,f
no
Refractory
Lining
yes
Firebox
Breach
no
yes
bottom,
rear/ &
3" on sides,
sildes
yes
bottom,
rear, &
sides
Comments
Baseline stove.
AA1 and AA2 runs
reflect no change
in stove design.
AA2 runs with
asterisk denote
ASTM fuel loads.
Top baffle
extending 2/3
the depth of
firebox from
the backwall
toward the
loading door.
Cast iron fire-
box. BB1 runs
with asterisk
were conducted
with ASTM fuel
loads. Emission
testing was not
conducted on runs
BB1-2 and BBl-4.
Uses brick
baffle plate.
By-pass for
"secondary
combustion
chamber".
-------
Stove
Code
EE
Firebox Dimensions (in.) Primary
(FT3) WHO Air
2.2
FF
2.4
15.0 diameter x b,c,o
Secondary
Air
no
Refractory
Lining
Firebox
Breach
no
23.0 15.0 12.0 e,h
yes
yes
Comments
Cylindrical
firebox.
Double paned
window in front,
0.5" x 1.5"
deflectors at
air inlets.
Primary air
enters through
both sides.
Right side
primary air
goes through
loading door.
Top baffle.
Air enters
firebox at
rear bottom
center.
-------
WOODSTOVE DESIGN FACTORS
STOVE TECHNICAL DESCRIPTION
Notes:
a - Air enters firebox through slot located at top front of
firebox, directly over door.
b - Little or no preheating of air.
c - Air slide control.
d - Air stop maintains high minimum burn rate.
e - Thermostatic control.
f - Preheated.
g - Front top 1/3 of firebox.
h - Air enters firebox through openings located on the rear of
the firebox.
i - Air enters firebox through openings located at the bottom
front of the fireplace.
j - Exits firebox at the lower right back corner.
k - Located in the retro fit device.
1 - Exits at the top rear of the firebox.
m - Exits at the upper center of the back wall.
n - No independent secondary air introduced; primary air used.
o - Air enters firebox through an opening in the loading door.
p - Under fire air.
g - Exits the firebox at the bottom of the rear wall.
r - Secondary air flow unregulated.
s - Down through the bottom center, below the firebox floor.
t - Enters firebox horizontally across the top of the firebox.
u - Exits firebox at the top center of the firebox.
v - Refractory secondary combustion chamber.
X - Enters horizontally across the top front 1/4 to 1/3 of the
firebox.
-------
WOODSTOVE DESIGN FACTORS
aa - Air controlled by spin draft.
bb - Air enters firebox through holes in burner rings surrounding
a combustion zone.
cc - Air controlled by a blower fan.
dd - "Door wash" air inlet (slot across top part of firebox, air
directed torward glass).
ee - Air enters firebox at both bottom front corners.
-------
APPENDIX B
STOVE TEST DATA
-------
STOVE CODE
Al-2
Al-3
Al-4
A2-5
A2-6
A2-7
Bl-1
B2-1
B2-2
B2-3
Cl-1
Cl-2
Cl-3
Cl-4
C2-1
C2-4
C2-5
C2-6
C2-2
C2-3
01-1
01-2
02-2
02-3
02-4
D2-6
El-1
El-2
El-4
El-34
Fl-1
Fl-2
F2-1
F2-2
F2-3
F2-4
F2-6
F2-7
G2-B
G2-9
G2-20
C2-:i
Hl-3
Hl-4
FIREBOX BURN RATE BEAT OUTPUT P ARTICULATE PARTICULATE PARTICULATE CO CO CO CONDUSTION HEAT OVERALL
SHE (LBS/UR) (BTU/HR) EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EFFICIENCY TRANSFER EFFICIENCY
("3)
1.6
1.6
1.6
1.2
1.2
1.2
2.1
2.3
2.3
2.3
2.3
2.3
2.3
2.3
1.4
1.4
1.4
!..«
1.4
1.4
1.8
1.8
1.6
1.6
1.6
1.6
2.5
2.5
2.S
2.5
1.3
1.3
1.4
1.4
1.4
1.4
1.4
1.4
2.0
2.0
2..0
2.0
1.0
1.0
5.2
9.5
7.7
5.3
8.8
12.0
4.6
3.5
4.3
8.5
2.1
5.4
6.1
3.5
2.4
1.3
7.9
3.6
2.6
3.5
3.1
2.7
2.0
2.8
3.9
6.4
7.6
5.6
3.7
1.6
4.9
3.3
5.0
10.3 .
7.3
8.4
6.4
7.3
4.9
7.6
12.6
6.6
5.9
4.7
(G/KG) (G/UR) (G/106 JOULE)
24411.0
35126.0
30621.0
2S054.0
37257.0
45450.0
21267.0
16874.0
21074.0
42743.0
8780.0
23933.0
26780.0
15438.0
11760.0
5958.0
35473.0
16198.0
12766.0
17297.0
13383.0
11219.0
8966.0
13499.0
18851.0
30062.0
31658.0
22211.0
14181.0
6545.0
22947.0
15529.0
25214.0
44336.0
26281.0
38251.0
29707.0
33880.0
24203.0
36547.0
54498.0
30582.0
25999.0
21362.0
5.4
1.6
3.8
2.2
2.3
1.2
6.2
11.2
7.9
1.6
44.6
6.5
5.7
14.2
9.3
36.3
4.3
14.0
10.3
2.6
15.8
21.5
20.9
18.4
4.2
2.2
15.9
33.3
31.3
31.7
7.3
15.9
4.2
2.2
2.5
1.3
11.1
7.0
6.9
8.9
5.3
3.1
12.3
16.4
10.3
5.8
10.9
4.4
7.5
5.3
10.7
14.6
12.8
4.9
34.6
13.2
13.0
18.3
8.3
17.6
12.8
18.3
9.9
3.3
18.2
21.0
15.5
19.3
6.1
5.3
44.9
69.3
43.0
18.9
13.2
19.3
7.6
8.6
6.8
4.0
26.6
18.7
12.6
25.1
24.6
7.6
26.6
28.3
0.4
0.2
0.3
0.2
0.2
0.1
0.5
0.8
0.6
0.1
3.7
0.5
0.5
1.1
0.7
2.8
0.3
1.1
0.7
0.2
1.3
1.8
1.6
1.4
0.3
0.2
1.4
3.0
2.9
2.7
0.5
1.2
0.3
0.2
0.2
0.1
0.8
0.5
0.5
0.7
0.4
0.2
1.0
1.3
(G/KG)
128.0
38.5
82.8
104.6
51.8
2?. 5
117.5
166.6
122.6
64.2
327.7
120.2
118.3
143.2
183.1
328.0
73.7
163.1
167.8
124.3
180.4
175.8
278.8
190.5
117.2
90.4
144.6
187.3
248.5
426.2
116.7
.165.2
73.6
27.0
50.6
38.7
68.6
57.3
167.4
145.1
104.0
120.5
149.5
151.1
(G/HR) (G/106 JOULE)
245.0
135.1
235.0
209.3
168.9
104.9
202.1
217.4
199.2
202.8
254.4
242.5
267.4
184. 7
163.6
159.2
216.6
212.9
161.1
159.8
207.6
171.6
206.2
200.8
169.0
215.7
407.0
389.0
342.6
253.5
210.6
201.5
134.8
105. 5
135.7
120.2
165.2
154.4
304.9
409.0
484.2
293.4
323.7
261.4
9.5
3.6
7.2
7.9
4.3
2.2
9.0
12.2
9.0
4.5
27.5
9.6
9.5
11.3
13.2
25.3
5.8
12.5
12.0
8.8
14.7
14.5
21.8
14.1
8.5
6.8
12.6
16.6
22.9
36.7
8.7
12.3
5.1
2.3
4.9
3.0
5.3
4.3
12.0
11.0
B.6
3.7
11.8
11.6
EFFICIENCY
84.1
87.0
85.3
89.1
86.4
88.4
84.4
82.7
84.1
87.3
74.1
83.6
84.4
81.7
80.6
74.1
85.3
81.1
82.3
84.1
79.0
79.8
75.1
80.2
84.4
84.0
83.9
77.5
74.7
77.7
87.0
82.0
87.8
89.1
87.1
89.1
83.5
83.1
81.7
82.1
83.6
84.0
B1.9
81.7
78.7
59.6
66.0
76.5
68.7
60.3
75.9
81.2
80.0
80.2
77.8
72.9
72.4
75.8
84.7
85.2
73.0
79.3
83.8
83.0
79.6
77.6
82.2
81.7
79.5
77.1
69.9
72.4
73.4
75.7
75.6
80.6
80.4
66.1
58.3
70.6
76.6
78.3
84.0
80.1
71.5
77.5
76.3
78.5
66.1
51.9
56.3
65.7
59.4
53.3
64.1
67.1
67.3
70.1
57.6
61.0
61.1
61.9
68.2
63.1
62.2
64.3
68.9
69.8
62.9
62.0
61.7
65.6
67.1
64.8
58.6
56.1
54.8
58.8
65.6
66.1
70.6
58.9
50.8
62.9
64.0
65.0
68.6
65.7
59.8
65.1
62.5
64.1
-------
Hl-1
H2-6
H2-8
H2-9
H2-10
H2-11
11-1
Jl-1
Kl-1
Kl-2
Kl-3
Kl-4
Ll-1
Ll-2
Ll-J
Ll-4
Ll-5
Ll-19
Ml-1
. Ml-2
Ml-5
Hl-6
Ml-7
M2-1
M2-2
N2-1
N2-2
N2-3
N2-4
N2-5
N2-6
N2-7
01-1B
01-28
01-3B
01-4B
Pl-1
Pl-2
Pl-3
Ql-2
01-3
SI SB
(PT3)
1.0
1.0
1.0
1.0
1.0
1.0
2.2
1.7
2.1
2.1
2.1
2.1
0.8
0.8
0.8
0.8
0.8
0.8
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.7
1.7
1.7
1.7
1.7
1.7
1.7
2.5
2.S
2.S
2.5
1.1
1.1
ORN RATE HEAT OUTPUT PARTICIPATE PARTICOLATE PART1CULATE
(LBS/HR) (BTU/HR) EMISSIONS EMISSIONS EMISSIONS
(G/KG) (G/HR) (G/106 JOULE)
4.1
5.1
4.9
5.1
6.0
6.9
3.0
3.6
2.7
5.1
3.2
5.5
3.0
2.8
1.9
6.9
4.1
2.0
6.2
2.2
9.5
3.0
4.8
7.8
2.9
13.7
9.0
7.0
6.7
11.1
B.I
8.5
9.6
5.5
4.6
10.4
10.9
5.1
4.7
4.1
14.4
16303.0
23749.0
21721.0
23582.0
20660.0
24934.0
12611.0
18557.0
12546.0
24280.0
15778.0
27163.0
12806.0
11800.0
6887.0
32043.0
18279.0
8209.0
22118.0
8961.0
29181.0
14510.0
20116.0
35198.0
12945.0
57260.0
39260.0
' 31203.0
30311.0
48973.0
36049.0
36784.0
43775.0
27609.0
22234.0
44960.0
34S09.0
20726.0
21641.0
20376.0
60100.0
30.7
16.3
6.4
8.4
5.7
6.9
44.9
6.3
19.7
13.1
16.7
3.6
17.1
24.4
22.0
1.4
4.5
3.9
12.7
24.2
4.8
7.0
6.9
6.4
14.5
2.7
3.2
4.3
5.4
1.8
5.5
4.2
3.5
2.7
22.6
3.5
8.2
6.7
10.9
6.1
4.2
46.1
30.4
11.4
16.0
12.7
17.9
50.4
8.5
20.0
25.1
19.9
7.4
20.3
24.9
36.3
3.5
7.7
2.6
28.8
19.7
16.9
7.6
12.2
18.8
15.9
13.4
10.7
11.3
13.4
7.4
16.3
13.2
12.7
5.6
38.4
13.5
33.5
12.6
19.2
9.5
22.2
2.7
1.2
0.5
0.6
0.6
0.7
3.8
0.4
1.5
1.0
1.2
0.3
1.5
2.0
2.1
0.1
0.4
0.3
1.2
2.1
0.6
0.5
0.6
0.5
1.2
0.2
0.3
0.3
0.4
0.1
0.4
0.3
0.3
0.2
1.6
0.4
0.9
0.6
O.B
0.5
0.
-------
01-5
Q2-6
02-7
Rl-1
. Rl-2
Rl-4
Rl-5
Rl-6
Rl-7
Sl-1
Sl-2
Sl-3
Sl-4
Sl-5
Tl-1
Tl-2
Tl-3
Tl-4
Tl-5
Tl-6
Tl-7
Tl-10
Tl-11
Tl-12
Tl-13
Tl-14
Ul-1
Ul-2
Ul-3
Ul-4
Vl-1
Vl-2
Vl-3
Vl-4
H2-5
W2-6
H2-7
W2-8
Xl-1
Xl-2
Xl-3
Xl-4
11-20
Zl-21
Zl-22
Zl-23
EBOX BURN RATE BEAT OUTPUT PARTICOLATE PARTICULATE PARTICULATE CO CO CO COMBUSTION
HE (LBS/HR) . (BTU/HR) EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EFFICIENCY
T3)
1.1
1.3
1.3
.4
.4
.4
.4
.4
.4
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.1
1.1
1.1
1.1
S.I
S.I
5.1
5.1
2.4
2.4
2.4
2.4
0.3
0.3
0.3
0.3
2.4
2.4
2.4
2.4
S.2
5.2
4.7
6.1
3.0
2.5
2.3
3.4
7.4
2.5
5.2
9.9
1.4
4.1
4.2
8.2
3.7
2.7
2.0
3.4
5.1
11.1
2.6
1.3
4.3
3.0
4.4
1.6
2.6
3.3
1.4
1.8
3.0
9.3
1.2
2.0
9.2
3.0
1.5
2.3
3.0
4.0
4.1
2.3
2.8
11.7
25012.0
24346.0
20840.0
26905.0
14721.0
12065.0
10543.0
15864.0
30258.0
12447.0
23096.0
40993.0
6259.0
18500.0
17890.0
28045.0
17234.0
10455.0
8256.0
13311.0
23045.0
43350.0
13626.0
4887.0
18154.0
14717.0
29192.0
8744.0
17945.0
22745.0
7903.0
10866.0
18345.0
55349.0
6932.0
12308.0
56196.0
17961.0
9499.0
15041.0
20126.0
26638.0
18926.0
10544.0
10243.0
47569.0
(G/KG)
2.8
2.3
2.9
9.3
9.3
11.4
11.4
6.2
4.3
12.9
8.1
5.0
19.3
11.2
9.5
9.8
19.2
28.4
30.1
20.4
9.9
4.9
11.8
34.9
9.2
7.4
0.7
1.0
0.6
0.6
4.0
1.8
2.2
0.8
6.0
3.1
1.9
1.8
0.9
1.0
1.0
1.4
19.8
28.7
24.3
9.9
(G/HR) (G/106 JOULE)
5.3
4.5
5.0
21.0
10.5
10.7
9.6
7.8
11.8
12.1
15.9
18.4
10.0
16.9
14.9
30.4
26.3
28.6
21.9
26.0
19.2
20.0
12.4
16.7
14.6
8.4
1.2
0.7
0.7
0.9
2.3
1.4
2.8
3.2
2.9
2.6
7.3
2.2
0.5
0.9
1.2
2.4
29.7
24.1
25.3
42.7
0.2
0.2
0.2
0.7
0.7
0.8
0.9
0.5
0.4
0.9
0.7
0.4
1.5
0.9
0.8
1.0
1.4
2.6
2.5
1.9
' 0.8
0.4
0.9
3.2
0.8
O.S
0.0
0.1
0.0
0.0
0.3
0.1
0.1
0.1
0.4
0.2
0.1
0.1
0.1
0.1
0.1
0.1
1.5
2.2
2.3
0.9
(G/KG)
97.9
111.7
127.9
128.1
150.7
179.7
212.2
150.7
78.5
194.1
145.6
94.8
237.2
150.1
103.8
134.5
133.8
245.1
310.1
157.1
95.8
56.4
201.7
377.4
179.0
175.0
23.6
29.1
5.2
1.9
34.7
17.7
17.5
4.3
32.6
18.3
44.3
8.0
29.1
37.5
17.8
22.1
172.5
297.1
238.3
114.6
(G/HR) (G/106 JOULE)
187.3
218.3
224.2
289.3
170.6
167.9
178.0
189.0
214.3
181.7
286.5
347.7
122.4
226.0
162.3
417.2
183.6
247.1
225.6
199.4
184.8
228.7
212.8
181.0
285.4
200.3
43.1
19.3
5.7
2.4
19.8
13.5
22.7
17.0
15.9
15.6
171.8
10.0
17J4
34.8
22.0
36.5
259.0
249.2
248. 5
491.8
7.1
8.5
10.2
10.2
11.0
13.2
15.9
11.3
6.7
13.8
11.8
8.0
18.5
11.6
8.6
14.1
10.1
22.4
25.9
14.2
7.6
5.0
14.8
35.1
14.9
12.9
1.4
2.1
0.3
0.1
2.4
1.2
1.2
0.3
2.2
1.2
2.9
0.5
1.7
2.2
1.0
1.3
13.0
22.4
23.0
9.8
BEAT OVERALL
TRANSFER EFFICIENCY
EFFICIENCY
88.8
64.7
82.9
80.8
82.8
80.4
79.9
82.0
82.7
80.6
78.1
84.4
76.1
80.8
87.6
92.5
82.1
72.8
74.1
82.2
84.5
90.2
84.2
69.2
62.9
63.3
92.6
91.9
99.0
94.3
87.7
88.9
90.5
92.3
91.8
93.8
90.1
91.7
89.0
69.1
91.0
92.3
81.7
77.8
77.5
84.6
76.2
75.2
74.5
76.5
80.7
82.9
61.8
80.4
69.2
84.2
77.6
69.3
65.6
78.8
70.2
51.9
81.6
75.7
81.3
67.5
75.1
63.1
81.6
80.8
73.4
78.5
87.2
74.7
84.9
87.6
81.6
83.0
81.3
78.9
80.3
60.2
63.4
61.2
87.9
89.6
87.7
86.2
81.9
85.3
85.3
72.5
67.7
63.7
61.8
61.8
66.8
66.6
65.4
65.9
57.2
67.9
60.5
58.5
65.3
63.7
61.6
48.0
67.0
55.1
60.2
55.5
63.5
56.9
68.7
55.9
60.8
65.3
80.8
68.7
84.0
62.7
71.6
73.8
73.6
72.8
73.7
75.3
75.1
74.5
78.2
79.8
80.1
79.5
66.9
66.4
65.8
59.2
-------
12-1
12-2
AA1-6
AA1-7
AA1-8
AA1-9.
AA2-8*
AA2-9*
AA2-10
AA2-11
AA2-12*
AA2-13*
BB1-1*
BB1-2*
BB1-4*
BB1-5* ,
BB1-6*
BB1-7*
CC1-15
CC1-14
CC1-16
DD1-9
DD1-36
DD1-35
DD1-6
EE1-1**
PF1-18
FF1-21
PP1-19
FF1-17
EBOX BORN RATB BEAT OUTPUT PARTICULATE PARTICOLATB PARTICULATE CO CO CO COMBUSTION HEAT OVERALL
lit (LBS/BR) (BTO/HR) EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EMISSIONS EFFICIENCY TRANSFER EFFICIENCY
1*3) (G/KG) (G/HR) (G/106 JOOLE) (G/KG) (G/HR) (G/106 JOOLE) EFFICIENCY
2.4
2.4
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
o.a
o.a
0.8
0.8
0.8
o.a
1.4
1.4
1.4
2.0
2.0
2.0
2.0
2.2
2.4
2.4
2.4
2.4
4.0
2.2
4.6
2.2
3.0
7.6
6.4
5.9
2.1
2.6
B. 3
8.6
6.2
6.1.
2.1
1.7
11.0
10.5
3.7
5.8
10.2
2.0
3.0
4.3
14. »
4.6
2.0
2.8
3.9
6.6
24728.0
13432.0
20662.0
9159.0
11980.0
31615.0
27050.0
25781.0
8596.0
10934.0
36266.0
35213.0
29670.0
26301.0
9834.0
7919.0
44600.0
46090.0
18590.0
25342.0
36058.0
7919.0
13331.0
19606.0
62447.0
20484.0
9272.0
11115.0
17868.0
30123.0
9.4
. 12.8
22.4
28.2
29.3
7.2
13.7
22.3
24.7
32.0
10.6
12.2
9.2
-
-
33.2
3.4
2.3
12.5
11.7
2.5
52.5
27.7
17.5
8.5
31.6
28.8
37.8
24.3
9.2
15.9
11.8
39.2
23.2
32.9
20.0
32.1
48.4
19.0
30.9
32.3
38.9
21.6
-
-
21.1
13.5
6.9
17.3
24.7
9.3
38.8
31.1
27.6
47.7
53.4
22.1
38.2
35.6
22.6
0.6
0.8
1.8
2.4
2.6
0.6
1.1
1.8
2.1
2.7
0.8
1.0
0.7
-
-
2.5
0.3
0.2
0.9
0.9
0.2
4.6
2.2
1.3
0.7
2.5
2.3
3.3
1.9
0.7
73.4
97.7
215.7
298.2
281.4
118.4
169.2
206.2
362.2
328.2
139.6
162.2
154.9
168.4
186.6
250.4
16.0
29.6
119.8
100.7
44.4
352.2
211.4
127.4
104.1
226.5
321.9
305.9
199.4
120.4
124.9
90.0
374.9
247.4
309.9
336.9
398.2
446.9
279.2
316.1
423.2
518.5
364.3
377.9
144.7
159.3
72.9
117.3
164.7
211.1
167.3
259.8
237.6
200.6
579.6
382.5
246.4
308.3
292.1
295.5
4.8
6.4
17.2
25.6
24.5
10.1
14.0
16.4
30.8
27.4
11.1
14.0
11.6
13.6
13.9
19.1
1.5
2.4
8.4
7.9
4.4
31.1
16.9
9.7
8.8
17.7
25.2
26.3
15.5
9.3
89.5
89.1
80.6
73.1
75.7
85.5
80.5
76.9
70.7
71.7
82.3
79.0
80.1
78.5
79.0
76.0
91.6
88.2
83.9
87.1
89.8
70.0
77.0
83.9
64.7
77.8
75.9
75.4
81.5
88.1
81.0
81.7
78.1
85.3
79.8
72.7
73.7
79.8
82.0
81.3
75.2
75.8
80.4
76.6
83.6
84.5
62.5
68.6
81.7
70.3
57.0
79.3
83.7
75.6
69.2
80.8
82.1
77.4
77.4
74.3
72.5
72.8
63.1
59.8
56.9
60.0
59.4
61.4
58.0
58.3
61.9
59.9
64.4
60.1
66.0
64.2
57.2
60.5
68.6
61.3
51.1
55.5
64.5
63.4
58.6
62.8
62.3
58.4
63.0
65.4
Fuel load based on proposed ASTM procedures (Reference 8). Fuel
loading density was approximately 12 to 13 pounds fuel per usable
cubic foot of firebox volume.
**Partlculate samples obtained with Condar sampler.
-------
APPENDIX C
GRAPHICAL DATA PRESENTATION
-------
FIREBOX SIZE EFFECTS
1?
o
-------
BASELINE STOVES
70 -i
oc
__/
o
X
if
ir
o
i/>
f/)
^£
C
LJ
§
->
U
CL
60 -
40 -
.30 -
20 -
10 -
0
1 -20
0
Baseline stove data is used as a refer-
ence to approximate emissions from
"typical" stoves found In many Install-
ations. This stove had a 2.4 ft3 fire-
box with a refractory brick lining, a
splndraft control bringing air in
through the lower 1/3 of the door. It
had a very small baffle directly below
the flue.
21 -2,3
20 40
f Thou sands')
H EAT O Cl TP LI T (BTlf/ HO U Rj
Figure C-2
Firebox Modifications to Stove Z
60
-------
40 -,
tt?' 35 -
r>
I
to" w
3 25 -
Q 20-
f.O
1.0
^ 15 -
LJ
b
B 10-
F;
£ 5 -
0 -i
c
'FIREBOX S
ZE EFFECTS
The firebox volume of 2.3 ft3 in Cl tests was
reduced to. 1.4 ft3 in C2 tests with the
addition of refractory bricks and a cast iron
C1 1 freestanding firebox liner. This reduced the
fuel load from about 16 Ibs to just over 9 Ibs.
Significant emissions reductions are seen at the
lower heat output levels, although some overlap
of performance is seen.
/* o .1 \-'-i f- VD
Oz 4-
01-62-3 C2-5
02-1
i i i
D 20
i i
40 60
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-3
Firebox Modifications to Stove C
-------
Rl i - »-* f - »"". '. X j**»» I "" -y f ' -» »» «*MB r ..
REBOX SIZE EFFECTS
o
ft:
40
35
30
25
01 tests were conducted on a 1.8 ft^ steel
firebox with about 12 Ibs of fuel. D2 testa
were conducted on 10.3 Ibs of fuel in a
1.6 ft3 firebox with refractory brick and
reflective metal in place. This stove
appears to be relatively sensitive to burn
rate changes, as emissions drop rapidly
at medium high burns.
O
Co
LJ
LJ
.5
1
20 -
15 -
10 -
ft, _
01 -2
D2-2
02-6
0
0
20
n r~
40
(Thousands)
'ITU/
60
HEAT OUTPUT (BTU/HOUR)
Figure c-4
Firebox Modifications to Stove D
-------
FLREBO
O ri
L
-4-0 -r
..
tt:
_/
o
o:
CO
LJ
U
r.;
I
.30 -
25 -J
1 0 -
5 -J
F2-6
D2-5
02--4
Both D2 and F2 runs were conducted on
1.6 ft-* usable firebox stoves. The
actual height of the F2 stove was
about 16 Inches, although It was
measured at about 10" due to the fuel
loading door configuration preventing
fuel being placed In the upper fire-
box area. The D2 stove had an air
Intake slot over the door while the
F2 stove used an air slide control In
the lower section of the fuel loading
door. Fuel loads were about 1 Ib
lower on the F2 stove.
F2-4
0
20
60
('Thou sane? a;)
MEAT -OUTPUT (6TU/HOUR)
Figure C-5
Firebox/Fuel Load Effects on Emissions
-------
FIREBOX HEIGHT
70
C.2
O
I
2
O
0
I
60 -
50 -
40 -
30 -
20 -
10 -
0
A range of firebox heights is shown below.
Low firebox heights are usually associated
with smaller firebox sizes. Firebox heights,
in inches, are as followsi E 20, G- IS,
X 18, L - 9, M - 10, 0 14.
E2-4
E2-1
01-3B
M1-3
M1-1
L1-2 K1-2 G2-9
M2-2 Ml-5
M1-92- 8
M1 -161 - 5
LI -19
20 40
(Thousands;
HEAT OUTPUT (BTU/HOUR)
Figure C-6
Firebox Height Effects on Emissions
r
60
-------
AIR INLETS
70
o:
t/
f/}
g
t/)
£'
2
LJ
c.;
i
60 H
X
50 -
4-0 -
30 -
20>
10 -
0
tz
Four stoves with similar sizes but different air inlet
designs are ooipared below. Firebox sizes for stoves,
E and 0 were 2.5 ftj, while stoves B and C had 2.3 ftj
fireboxes. Stove E had an air inlet very low in the
firebox, while stoves B, C, and 0 had air inlets through
slots above the fuel loading doors (door window wash
design).
EI-4-
E2-1
01-30
C1-1
032H6
02-1
H-1
O1-2B
B2-3
0
1 1 1 1 T~
20 40 60
(ThoLisonds)
HEAT OUTPUT (BTLT/HOUR)
Figure C-7
Emission Reductions by Air Inlet Location
-------
ND
-
MB.
AMBER
\
t
tiL
->
O
^
i/;'
!5
(fr
&
1/3
o
LJ
I.|J
<?
13
U
F
o:
=!.
CL
t-u -
35 -
30 -
J W
25 -
20 -
15 -
10 -
5 -
0 -
All data points shown were conducted under
the same stove configuration, with the
exception of Rl-1 (leveling bolt plugs
were not installed, allowing air leakage
through the firebox floor from a convec-
tion fan blower). This design utilizes
extensive secondary air Inlets, parallel
and perpendicular to the loading door.
Consistent performance at varying,
especially low, heat outputs, is unique
in this natural draft stove.
R1-1
R1-7
KR1&,V"2
w
R 1 - 6
I i i ii i
0 20 40 60
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-8
Effective Secondary Air
-------
DOWN DRAFT
n
x
=r
i'c
CJ
o
TO
0
LJ
LJ
hp
1]
O
F
JD -
25 -
20 -
15 -
10 -
5
0
_ r>
K1-KH-3
J1-1
K 1 - 4
0
These two stove designs bring combustion
air down past fuel logs, into a burning
zone and DOWN away from the fuel before
being allowed to draft upwards. Some
individual runs appear promising, others
show higher emissions. Results from the
Kl stove may be misleading, however, as
an unusually large coal bed was necessary
to comply with test requirements. Normal
operation uses a smaller coal bed,
which does not block the firebox
breach to as great an extent.
T
20 40
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-9
Emissions from Downdraft Stoves
60
-------
DOWN DRAFT
70
'o
I
o
(7;
LJ
UJ
.0
60 -
50 -
30 -
20 -
10 -
0
K1-2
K1-K1-3
B2-;
12-2
M-1
A ,"well designed" downdraft stove, K, is
compared to a similarly sized conventional
updraft stove. The downdraft Stove K, which
introduces secondary air to combustion gases
exiting the firebox, shows,similar;performance
to the updraft design. Without secondary air,
emissions from Stove K would be expected to be
much higher.
K1-4-
B2-3
0
20 , 4-0
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-10
Comparison of Downdraft and Conventional Stoves
60
-------
UNDERFIRE AIR
70
IT
6
r.9
',0
~-r
o
l/>
10
2
u
LJ
0
I
60 -
40 -
30 -
20 -
10 -
Stoves H and I were underfire coal/wood designs.
Runs I 1-1 and H 1-1, H 1-3 and H 1-4 used combustion
air brought into the firebox from directly beneath
the fuel. All H2 runs were made with the underfire
air slots blocked, with air entering the firebox around
the bottom edges of the hearth. Although this air
inlet was still low, emissions dropped significantly.
11 -1
H1-1
H2-6
-3
0
i i r
20 4-0
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure c-11
Underfire Air Effects
i
60
-------
2ND AIR
o
r.0
\
8
u
u
5
3'
U
i
s-u -
35 -
30 -
25 -
20 -
15 H
10 -
5 -
0 -
The effectiveness of secondary air systems is shown
below. All the stoves presented here had secondary
air systems, stoves R and C2 directed secondary air
into the main firebox from above, while stoves B, BB,
and D directed secondary air into combustion gases
at the firebox breach. A wide range of effective-
ness can be seen.
BB1&S-2 BB1-1
°2"2 02-1
J V2B1-1 R1-7
R1-6
D2~"4" D2-6
C2-3
i i i
3 20
001-6
BB1-7
B2-3
i i i
40 60
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-12
Secondary Air Effects on Emissions
-------
SECONDARY AIR
'£
*
o
to"
5
:?i>
s
6
CO
1J»,
u
i-
D
F
DL
10--
.35 -
30 -
25 -
20 -
15 -
10 -
5 -
0 -
Tests were conducted on a single
stove both with (C2) and without (Cl)
a secondary air supply which intro-
duced preheated air at the top center
area of the firebox. This stove was
especially sensitive to starting con-
ditions, so it is not clear from the
limited data whether Cl-1 and C2-3
were particularly good starts or
whether the C2 test results are as
good as they appear.
C1-4 C1~6
01-5
C2-2
C1-1
C2-3
i i i i i i
0 20 40 60
(Thousands:)
HEAT OUTPUT (BTU/HOUR)
Figure C-13
Emissions With and Without Secondary Air
-------
SECONDARY AIR
IT'
D
O
X
^ji;
if
Lt:
P.
o
to
£
LJ
LJ
§
O
i£
ii'
-4-u -1
35 -
30 -
.25 -
20 -
15 -
10 -
5 -
0 -
Ml and M2 tests all reflect the same
stove configuration, which brings
M 13 ' Prftheated air into the firebox through
a tube on the top of the firebox. The
stove had a 1.3 ft^ firebox and a 9 Ib
fuel load. M2 runs were conducted
with additional secondary air inlets
1 1 -I __ -I on the sides of the firebox and
several upper secondary air holes
blocked .
Ivl 12
M2-1
M1 -5
M2-2
M1-7
Ivl 1-6
iii ill
0 20 40 60
[Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure c-14
Secondary Air Modification
-------
THERM
A
o
x
"._
10'
o
ki
LJJ
LJ
C.J
F
11'
60 -
50 -
.30 -
20 -
~ET~-2~
E1-1
El and N2 stoves had primary air inlets
which were thermostatically controlled.
The thermostats on both stoves were
bimetalic springs which operated flaps
covering the primary air inlets. El
had a 2.5 ft3 firebox and an 18 Ib fuel
load, while N2 had a 1.7 ft3 firebox
and a 12 Ib fuel load. The high mini-
mum burn rates with N2 makes comparison
of the stoves somewhat difficult. El
had a low air inlet port and a low
firebox exit, which probably contributed
to the high emission rates.
1
N2-6
t
\l 9 9
n
._T . .,
20 40
l'Tr,ou5'-:ind£')
HEAT OUTPUT (BTLf/HOUR)
Figure C-15
Effect of Thermostatic Controls
T'
'-, f!
-1 1
-------
REFRACTORY
/s
LL
O
z
ct:
tf
v -'
!£
.'
O
"'0
0
<:
LJ
LJ
C
I
35 -
30 -
25 -
20 -
15 -
10 -
5 -
0
D1 -2
D2-
D2-4
D2-6
Dl runs were conducted on a steel 1.8
ft3 firebox. D2 runs had a brick and
stainless steel firebox liner added,
which reduced the firebox volume to 1.6
ft3 and the fuel load from 12 Ibs to
about 10 Ibs. While the Insulation of
the firebox is theoretically an impor-
tant feature, no data is available
which does not also Include changes in
firebox dimensions (size) and fuel load.
0
I I I
20 4-0
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-16
Effect of Refractory Materials
I
GO
-------
MECH. DRAFT
I?
~
I
to"
-^
IL
* n
'>
6
to
U
LJ
K
=r
_J
r*
F
₯
CL
-t-u
35 -
30 -
2K
G -
20 -
15 -
10 -
5 -
0 -
Stoves U and X are very similar in
design and performance. Both use
. pelletized fuel which is burned in a
cup surrounded by air supply holes
and are driven by induced draft.
Stove U feeds fuel from under the
burn cup while Stove X drops fuel in
from above. This technology appears
to be especially promising.
U 1 _ 2 U 1 - 3J 1 - 4 IJ 1 ~ 1
I i i i i i
0 20 40 60
(Thousands')
HEAT OUTPUT (BTlf/HOUR)
Figure C-17
Emissions from Pellet Stove U
-------
MECH. DRAFT
JP
:--.
to"
^
til
to
6
to
to
iti
UJ
hp
r~:
F
tr
d
tL
f U
35 -
30 -
25 -
20-
15 -
10 -
5 -
0 -
Stoves U and X are very similar in
design and performance. Both use
pelletized fuel which is burned in a
cup surrounded by air supply holes
and are driven by induced draft.
Stove U feeds fuel from under the
burn cup while Stove X drops fuel in
from above. This technology appears
to be especially promising.
XI -4
'x 1 _ 1 X 1 2^ ' ~~ -"
ii i i i i
0 20 4-0 . 60
(Thousands')
HEAT OUTPUT (BTU/HOUR)
Figure C-18
Emissions from Pellet Stove X
-------
IviEChT. DRAFT
4-0
'£
n
±
Co"
Q
CO
CO
UJ
LJ
I
LL
x
.35 -
25 -
20 -
15 -
10
5
VI and W2 tests were conducted as a
pellet fuel hopper mounted on a 5.1
and 2.4 ft3 stove, repectively.
Other than firebox size, the tests
were conducted under identical condi-
tions, except that VI tests had a
flue damper and W2 did not. The
hopper unit dropped pellets into a
burn cup, which used a forced air fan.
While test results show higher
emissions than integrated pellet
units, particulate emissions are
below the 1988 Oregon catalytic
standard of 4 grams/hour.
*'2-7
I I I I
20 40
(Thousands)
HEAT OUTPUT (BTU/HOUR)
Figure C-19
Emissions from a Pellet Hopper Device
60
-------
DENSIFiED FUEL
70
Lt
-=!'
til
c/j
(0
u
g
it
60 -
40 -
.30 -
20 -
1 0 -
Z 1 - 2 0
'.\ -'
Zl runs were conducted on a "tradi-
tional" 2.4 ft^ firebox stove with
a fuel load of about 16 Ibs. 2.1
runs were conducted using commer-
cially available denslfied fuel logs
with the same fuel loading density.
Emissions from 2.2 runs were signifi-
cantly (by a factor of about two)
lower. Although only two runs were
conducted with the densifled fuel,
the emissions appear to follow the
same shape of curve as seen with the
dimensional fuel. It is not known
if the densified fuel changes the
range of heat outputs on the stove.
0
20
40
I ThOU Stf fids')
MEAT OUTPUT (BTIf/hOJR)
Figure C-20
Emissions from Densified Fuel
1
60
------- |