EPA 910/9-82-089f
United States
Environmental Protection
Agency
Region 10
1200 Sixth Avenue
Seattle WA 98101
Air & Waste Management Division February 1984
£EPA Residential Wood
Combustion Study
Task 4
Technical Analysis of Wood Stoves
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TECHNICAL ANALYSIS OF WOOD STOVES
Combustion Principles
Design Considerations
Operating Techniques
-.'.3. Env::on<~en'a! Pn/ectien Agency
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RESIDENTIAL WOOD COMBUSTION STUDY TASK 4
Technical Analysis of Wood Stoves
Combustion Principles
Design Considerations
Operating Techniques
FINAL REPORT
PREPARED BY:
DEL GREEN ASSOCIATES, INC.
Environmental Technology Division
1535 N. Pacific Highway
Woodburn, Oregon 97071
(503) 982-8304
PREPARED FOR:
U.S. ENVIRONMENTAL PROTECTION AGENCY
Region X
1200 Sixth Avenue
Seattle, Washington 98101
TASK MANAGER
Wayne Grotheer
March 1983
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THIS REPORT CONSISTS OF SEVERAL DIFFERENT PARTS.
THEY ARE LISTED BELOW FOR YOUR CONVENIENCE.
EPA 910/9-82-089a Residential Wood Combustion Study
Task 1 - Ambient Air Quality Impact
Analysis
EPA 910/9-82-089b Task 1 - Appendices
EPA 910/9-82-089c Task 2A - Current & Projected Air Quality
Impacts
EPA 910/9-82-089d Task 2B - Household Information Survey
EPA 910/9-82-089e Task 3 - Wood Fuel Use Projection
EPA 910/9-82-089f Task 4 - Technical Analysis of Wood Stoves
EPA 910/9-82-089g Task 5 - Emissions Testing of Wood Stoves
Volumes 1 & 2
EPA 910/9-82-089h Task 5 - Emissions Testing of Wood Stoves
Volumes 3 & 4 (Appendices)
EPA 910/9-82-089i Task 6 - Control Strategy Analysis
EPA 910/9-82-089J Task 7 - Indoor Air Quality
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DISCLAIMER
This report has been reviewed by Region 10, U. S. Environmental
Protection Agency, and approved for publication. 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 or commercial products constitute
endorsement or recommendation for use.
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ACKNOWLEDGEMENT
The assistance of the Oregon Department of Environmental Quality
in researching this topic is greatfully acknowledged. The O.D.E.Q.
furnished numerous reports and studies on this topic, as well as
furnished names of p ertinent contacts within the field. Barbara
Tombleson's assistance, knowledge, and guidance in preparing this
report is particularly appreciated.
iii
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TABLE OF CONTENTS
page
Disclaimer
Acknowledgement •LlL
Executive Summary vl
I. Introduction x
II. Combustion Principles
III. Residential Wood Combustion Systems
*J /
IV. Modifications and Retrofit
V. Fuel Selection and Preparation
VI. Stove Selection 44
VII. Stove and Fireplace Operation
VIII. Bibliography and Reference
IV
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LIST OF FIGURES
page
Figure 1 Basic Stoves *®
Figure 2 Air Flow Patterns
Figure 3 Catalyst Installation
Figure 4 Heat Loss vs Moisture Content '
Figure 5 Energy Efficiency vs Moisture Content
Figure 6 Heat Loss vs Design Temperature ""
Figure 7 Adjusted Heat Requirement vs Estimated Heat Requirement 49
Figure 8 Wood Combustion Rate vs Adjusted Heating Requirement 51
Figure 9 Firebox Volume vs Charging Rate "
LIST OF TABLES
Table 1 Four Stages of Wood Combustion 6
Table 2 Typical Overall Efficiencies of Wood Burning Appliances 23
Table 3 Relative Heating Value Per Cord of Wood 35
Table 4 Outside Design Temperatures ^
•Table 5 Effects of Design Differences on Total Heat Loss 48
Table 6 Stove Sizing Process Summary and Illustration -*b
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EXECUTIVE SUMMARY
Design and operation of residential wood combustion devices influence
both performance and emissions. Important design considerations include
mechanisms to increase thermal efficiency and improve combustion efficiency.
Both these efficiencies must be relatively high to have an overall efficient
residential wood combustion (RWC) device. Until the last five years or so,
levels of residential wood combustion were low enough that there was no real
demand for improved stove designs which increase efficiencies and decrease
emissions. There remains considerable room for improvement in the design of
stoves. Some of these improved units are beginning to appear, but emission
test results from these units are limited. It is expected that in the next
few years the emerging stove technology will result in substantial emission
reductions, possbily by as much as 75%. However, design alone is not sufficient
to assure an efficient operation and reduce emissions from RWC devices. The
overall efficiency of all these devices is ultimately determined by the
operator. Variables such as fuel, charging rate, and combustion air regulation
greatly impact performance and emissions.
In order to obtain the highest overall efficiency while still minimizing
air contaminant emissions, the following practices must be observed.
1. Stoves should be sized to encourage operation at a moderate to high
burning rate (greater than 32 kg/hr-m , or 2 Ib/hr-ft , dry fuel basis)*.
Automatic regulation of combustion air to facilitate an even and
The recommended burn rate is expressed as mass of wood consumed per hour.per
volume of combustion chamber; e.g., for a stove with a firebox of 1.5 ft
this is a burn rate of 3 Ib/hour.
VI
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moderate burning further improves overall efficiency.
2. Charging s. stove with an excessive amount of wood decreases
efficiency. When this occurs, combustion air must be restricted
to maintain the desired output or the heat generated must be wasted
which reduces the overall efficiency. Overnight banking of fires
should be discouraged since the resulting combustion is poor and
excessive amounts of pollutants are generated.
3. Seasoning the fuel properly increases the usable heat of the fuel.
Therefore, less seasoned fuel is required to operate the appliance
to provide the same amount of heat.
4. Operate fireplaces only during mild weather (temperature) conditions.
Use doors to close off heated room air losses during low fires or
during periods when the fireplace is not in use. Burn fireplace
with full hot fires to maximize efficiency and reduce emissions.
Existing information is contradictory on whether or not fireplace
doors in conjunction with outside combustion air significantly
improves efficiency.
*5. Add-on (retrofit) devices such as catalysts and automatic thermo-
stats can improve efficiencies and reduce emissions in some instances.
However, improvements in operator firing techniques appear to have
a far more significant impact on efficiencies and emissions for
existing units.
Continued effort needs to be undertaken to investigate new stove designs.
There appears to be considerable room to improve both combustion and thermal
VII
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efficiency while reducing air contaminant emissions. It is expected that
some of the currently emerging units with advanced engineering designs will
enable significant emission reductions to be achieved. Nonetheless, it
appears that a considerable reduction in contaminants from existing units
could be realized by providing more public information and education on proper
firing techniques.
viii
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I. INTRODUCTION
Fire has been used as a source of heat for thousands of years. During
that time, wood heating has progressed from the open pit to a semi open pit
(a fireplace) to an enclosed pit (a wood stove). When fuel was plentiful and
air pollution of no concern, there was little need for an efficient fireplace
or stove with reduced air pollution.
It was recognized that wood heating provided only a local source of
heat and warmth. They also required great care and abundant labor to minimize
fire danger and supply sufficient fuel. As new fuels and heating systems were
developed, the use of the fireplace and wood stove as a primary heat source
diminished.
It was not until this past decade, when conventional fuel prices escalated,
that wood heating started to be more popular. People who had switched away
from wood heat as a fuel are now starting to switch back. The popular trend to
use wood, either as a primary or auxiliary fuel, has been gaining momentum.
This is placing a demand on the availability of fuel and a burden on our air
23
quality. In an effort to reduce fuel demand and improve air quality, numerous
studies have been instituted on wood heating systems, particularly on wood
stoves. Although wood heating systems include fireplaces, stoves, fireplace
inserts, central furnaces and boilers, this report will emphasize data regarding
fireplaces and stoves. An overview of general information in provided in this
report. Basic combustion principles, efficiencies and combustion variables are
discussed in Section II. The influence of design configurations on combustion
with the resultant formation of pollutants is investigated in Section III.
1
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Modification to the combustion system and the heat transfer mechanism by
design or retrofit is explored in Section IV.
Many of the technical reports analyzed in reference to this task stressed
the need for good operating practices and the use of quality fuel to improve
operation and reduce emissions. The impact of fuel species, fuel moisture
content and operational variables are, therefore, discussed in detail in
Section V.
It also became apparent that proper stove selection is critical to reduce
emissions and improve efficiencies. Section VI is provided to assist in making
this selection. Section VII serves to summarize the design parameters and
firing techniques recommended to operate a stove or fireplace with minimum
emissions.
A bibliography of technical reports reviewed in conjunction with this
task also is provided (Section VIII) to assist in additional detailed investiga-
tion where desired.
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II. COMBUSTION PRINCIPLES
Combustion of wood is a process in which the hydrogen and carbon in the
fuel are chemically combined with oxygen to form combustion products and
I9
release heat energy. Complete combustion is dependent on the "Three T's"
of combustion: time for the combustion reaction to occur, a high enough tempera-
ture to maintain combustion, and enough turbulence to allow sufficient oxygen
to mix with the fuel. If combustion is complete, carbon dioxide and water
vapor are formed. When complete combustion does not occur, which is common in
wood burning appliances, particulate matter, carbon monoxide, hydrocarbons
and other gases also are formed. The latter are emitted as air contaminants
and represent an energy loss to the user. As combustion becomes more complete,
less contaminants are formed. With proper heat transfer, this means more
usable heat energy is available.
STAGES OF COMBUSTION
Combustion of wood involves four basic processes: moisture evaporation,
pyrolysis, gas vapor burning, and surface char burning. The rate of heat release
and the formation of pollutants is dependent on these processes and the rates
at which they occur. In the wood stove, these processes are all occurring
simultaneously within the combustion chamber.
MOISTURE CONTENT
As wood is heated, moisture in the wood is evaporated to form a vapor
(steam). This evaporation of water uses energy rather than releasing it, un-
like the combustion processes of gas vapor and surface char burning. Since the
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vaporation uses energy released from the combustion processes, it lowers the
temperature in the combustion zone which retards the combustion process. In
wood fired boilers, for example, it has been found that the combustion process
cannot be maintained if the wood moisture content exceeds 68% - this is, the
wet wood requires so much energy to evaporate the water that temperatures are
12
reduced below the minimum temperature required to sustain combustion. Con-
sequently, fuel moisture content (seasoned vs "green" wet wood) is an important
variable.
PYROLYSIS
Pyrolysis involves a chemical decomposition of the original molecules
into other molecular species because of high temperature. Combustible gases
evolve from the wood as the temperature rises. Wood will not burn until this
26
chemical change occurs.
GAS VAPOR BURNING
Initially these gases near the surface of the wood are not ignited due
to the high concentration of carbon monoxide and water vapor. However, as the
rate of pyrolysis and the temperature increases combustion can occur in the
presence of oxygen. Thus with an increase in temperature and turbulence to
mix with oxygen, combustion becomes more rapid and heat is generated.
CHAR BURNING
In a wood stove the charred surface of wood does not usually burn until
well into the combustion process. Charcoal does not vaporize at the temperature
achievable in a wood stove. Consequently, combustion can occur only when
oxygen is available and can come in direct contact with the charcoal on the wood
surface. Oxygen can get to the surface only when the flow of gases coming out
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49
of the wood has subsided. This occurs after moisture evaporation and pyrolysis.
COMBUSTION CHEMISTRY
The chemical reactions involved in the combustion process are complex
due to the complex chemical nature of wood. However, for the purpose of this
section, a simplistic approach to this subject will be utilized.
Wood consists basically of cellulose fibers and lignin with water
trapped within its structure. The weight of the trapped water in green wood
can equal the weight of the dry wood (502 water).
When dry wood burns completely, the following basic reaction occurs:
C^H Oc + 6 CL -* 6 CO +5 H?0 + Heat
6 10 5 2 i L
(cellulose) + (oxygen) ->• (carbon dixoide) + (water) + Heat
Simply stated, when wood vapors mix with oxygen present in the air at a tempera-
ture sufficient to promote combustion, carbon dioxide and water are formed and
heat is generated. Table 1 summarizes the energy involved in the various
stages of combustion involving one pound of wood.
In theory, the amount of moisture in the wood does not affect the available
energy, but it drastically affects the ease of burning and the usable energy.
With wet wood, more of the heat released during combustion must be used to
vaporize water within the wood thus reducing the heat output of the appliance.
The relationship of moisture content to efficiency is discussed in more detail
in Section V.
EFFICIENCY
The design of a wood heating appliance has considerable effect on combustion
efficiency, the resultant emissions, and heat output. Many reports and sales
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TABLE 1
The Four Stages of Wood Combustion
35
1. Water vaporizes
2. Wood pyro lysis to make
charcoal, wood gas, and
wood oil vapors
3. Wood-gas and vapors burn
4. Charcoal burns in air
Temp
Range
•F
200-250
500-750
Above 1100
1200-1800
Ib Air/
Ib Wood
0
0
5
1
Energy
BTU/lb
-ioob
- 43
+1600
+3200
Negative value represents a required energy input; positive value
represents an energy output.
For wood containing 10% moisture. These numbers are for oak, but are
quite similar for all woods. See Table 3, page 34 for heat values of
other species. '
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brochures are quick to report the efficiency of an appliance without defining
what is meant by efficiency. Efficiency can be defined as a ratio of output
to input.
However, heating a home with any combustion device involves two
basic processes, each with its own efficiency. These processes are combustion
and heat transfer. The combustion of these two yield the overall efficiency.
COMBUSTION EFFICIENCY
Combustion efficiency is defined as the heat energy generated during
combustion divided by the wood energy input. Combustion efficiency is
affected by parameters that affect the basic combustion process. These
include fuel, air supply, and temperature in the combustion zone. Combustion
efficiency is a measure of how well combustion is occurring.
HEAT TRANSFER EFFICIENCY
Heat transfer efficiency is used to describe how well heat is transferred
from the combustion zone to the area being heated. This is dependent on the
design of the heating appliance. Factors that affect heat transfer efficiency
include such items as mass of the stove, its ability to retain or transmit heat,
and loss of heat out the appliance exhaust stack.
OVERALL EFFICIENCY
The product of the combustion efficiency and heat transfer efficiency
determines the overall efficiency. These efficiencies in turn affect air
contaminant emissions from the stove. Overall efficiency can be defined as
the useful heat energy output divided by the wood energy input.
Both combustion and heat transfer losses can be measured by the movement
of combustion exhaust gases out the stack. These stack gases contain energy
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in the form of unburned gases and particulace matter which results through
incomplete combustion. Heated excess combustion air and hot combustion
products represent a heat transfer energy loss.
In any stove design, both the combustion and the heat transfer efficiency
are extremely important considerations. Ideally, both efficiencies should be
very high to provide an overall efficient stove. However, there are certain
practical considerations which must be taken into account. As heat transfer
increases, stack gas temperature decreases. Certain minimum stack temperatures
must be maintained to prevent condensation and provide adequate draft. If
there were no condensible gases in the exhaust stream other than the water
formed by combustion and through evaporation from the fuel, stack temperature
could,, as a practical matter, be reduced until condensation of the water vapor
in the stack occurred. However, the condensible hydrocarbon gases produced
from incomplete combustion cannot be ignored in normal operation. Unless these
are removed from the gas stream they condense on the walls of the exhaust stack.
causing a creosote buildup and a safety hazard. Current operating practices
stress the need to maintain stack temperatures high enough to reduce this
formation and buildup of creosote. Furthermore, as the temperature in the
stack is reduced thermal buoyance is reduced, decreasing draft and air flow
through the combustion zone.
If heat transfer efficiency is going to be increased significantly with
a. resulting decrease in stack gas temperatures, the condensible organic hydro-
carbons must be removed from the stack gases to eliminate the creosote problem;
this is achieved through more complete combustion.
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COMBUSTION VARIABLES
Combustion is dependent upon the characteristics of the fuel and the
adequate supply of oxygen. To enhance the combustion process, the combustion
air must mix with the fuel (turbulence) at an adequate temperature to ignite
and must remain in the combustion zone for a long enough time to complete the
chemical reaction. This is often referred to as the 3T's of combustion (time,
temperature, and turbulence).
Combustion efficiency increases with an increasing fuel burn rate.
However, this is not to be confused with the size of che charge or with the
overall efficiency. The fuel burn rate represents how rapidly the charge is
being burned, and the combustion efficiency represents the completeness of the
combustion reaction. A rapidly burning fire increases both the temperature and
the turbulence and, therefore, combustion efficiency normally increases.
Turbulence is necessary to provide mixing of the fuel with the combustion
air (oxygen) permitting the oxidation (burning) process to proceed. The tem-
perature of the gases must also be sufficient to allow the reaction to continue.
Therefore, the location of the draft air inlet on an appliance is very important
and should be located to provide combustion air preheating. The quantity of
air supplied to the combustion process also is important. Too little air supply
limits the reaction resulting in incomplete combustion. Air must be supplied
in proper proportion to the fuel to provide for proper combustion. Theoretically
' 35
it requires 5.7 pounds of air to burn 1 pound of dry wood.
Air which is supplied to promote the primary combustion process is referred
to as primary combustion air. Although it requires 5.7 pounds of air to burn 1
pound of wood, additional air must be supplied to make up for the incomplete
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mixing of the fuel and oxygen. This "extra" air is referred to as excess air.
One problem that exists, however, is that as excess air increases, overall
26
efficiency decreases due to a decrease in thermal efficiency.
In an attempt to reduce the amount of primary air required for complete
combustion, some combustion appliances introduce secondary combustion air.
Secondary combustion refers to the ignition of the volatile gases which are
released by the burning fuel and not ignited directly in the fire. This igni-
tion is accomplished by adding the secondary air to the primary combustion pro-
ducts, thus providing the oxygen needed for combustion of these hot gases.
To maintain secondary combustion there must be sufficient fuel to mix with the
49
incoming oxygen at a high enough temperature to support combustion. This
process provides the dual benefit of reduced emissions, as well as a reduction
of heat that would otherwise escape out the chimney with excess primary air.
Unfortunately, secondary combustion is difficult to attain and maintain. To
49
support secondary combustion, very high gas temperatures are needed (at least
1100*F), which generally require the stove to be operated at a very high tem-
perature. In concept secondary combustion should be very effective, yet
secondary combustion is very difficult to obtain at the lower burning rates
due to the reduced operational temperature that is typical of consumer operation.
Unless the stove is properly sized (i.e., small enough to maintain a hot fire
without generating excessive heat output), the heat output becomes uncomfortable
to the homeowner and operator and the firing rate is reduced which results in
lower combustion temperatures causing the secondary combustion to cease.
Other important combustion variables include the fuel and the firing
techniques used by the operator. Variables in these areas include the species,
10
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moisture content, size, and frequency of the fuel charge. These are discussed
in detail in Sections V and VI of this report.
The effects of reduced heat transfer and combustion efficiencies on air
26
contaminant emissions is still being investigated. According to Harper
emissions of CO, THC, and N0x decrease as overall efficiency increases. He
further states that creosote is a result of incomplete combustion. Findings by
19 31
Shelton ' indicate that combustion is more complete in hotter fires and
that more creosote is formed in low temperature fires. Another study by
20
Stockton indicates that at higher temperatures particulate emissions are
42
reduced. Hubble's results indicate that CO, particulate, and creosote emis-
sions increase with a decreasing fuel combustion rate (i.e., slow burning fires
with low combustion efficiencies produce more emissions); however, for Hubble's
study the highest thermal efficiencies were calculated to occur at the low burn
41
conditions. Emission tests conducted under Task 5 of this study also indicated
that emissions are inversely proportional to the burn rate.
It must be remembered that overall efficiency includes both combustion and
thermal efficiencies. A proper combination of efficiencies, therefore, is
desirable to obtain the minimum air contaminant emissions. Theoretically, by
maximizing the combustion efficiency, to maintain the highest overall efficiency,
emission rates (g/BTU) can be reduced. Conversely, reducing the combustion
efficiency and improving the thermal efficiency may result in higher emissions
with no resulting increase in overall efficiency.
However, this limited data precludes any definitive answers regarding emis-
sions as a function of overall efficiency. It appears that emissions decrease
as combustion efficiency increases and that emissions increase as thermal efficiency
11
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increases (mass per mass of fuel consumed; g/kg fuel). Note, however, that
as the thermal efficiency of an appliance increases, the amount of fuel which
must be consumed is decreased; therefore, the net emissions to the atmosphere
may decrease. Section V discusses creosote formation and operating efficiencies
in more detail.
12
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III. RESIDENTIAL WOOD COMBUSTION SYSTEMS
RWC has been in use for centuries. However, until recently its use
had been rapidly fading. Consequently, research into improved RWC appliance
was virtually non-existent. Now that additional importance is being placed
on RWC systems to provide heat, research is beginning. This research
includes investigating systems with respect to heating characteristics, effects
of fuel variables, and identification of air contaminant emissions as to type
and amount.
Three basic RWC systems are in use. These include central heating systems,
fireplaces, and wood stoves.
The central heating system involves combustion of wood with a simple
combustion appliance and then distribution of the heat generated to other areas
of the home. Typically this involves a central furnace or boiler where sawdust
or logs are used as the fuel. The hot combustion gases are then used to heat
either air, which is routed to other areas of the home, or water that is pumped
to other areas of the home for heat. A significant feature of central heating
systems is that they utilize a sophisticated (relative to stoves) heat transfer
system, and often include heat storage systems.
A second system used to generate local heat is the fireplace. The fire-
place is an open combustion appliance without means to effectively regulate the
combustion air. Consequently, these generally operate with 500-600% excess air.
Although originally constructed of masonry, many are now manufactured of metal
and may be of free standing design.
The third system is the s tove where combustion occurs in a closed combustion
13
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chamber. Consequently, combustion air can be regulated by draft controls or
by inlet air restrictions. These airtight or semi-airtight appliances operate
in the range of -25 to 100% excess air."" These units are customarily manufac-
tured from iron, steel, or ceramics.
Although these are the three basic systems utilized for RWC, there are a
multitude of hybrids. Fireplace inserts are commonly installed in existing
fireplaces. These units are stoves that use the chimney rather than a stove
exhaust pipe, and are operated in a similar manner to regular wood stoves.
Some fireplaces are being modified to incorporate some design characteristics
of the wood stove (i.e., glass doors to partially control excess air) in an
effort to increase efficiency, while some wood stoves are being modified to
operate like fireplaces to retain the "romance of a fireplace" (e.g., wide
open doors). Both systems are being modified to extract additional thermal
energy by including air to air heat exchangers and in some cases, hot water
heat exchanger coils. Only the basic systems are discussed in any detail in
this task with emphasis placed on fireplaces and wood stoves.
CENTRAL HEATING SYSTEMS
Central heating systems are designed to include a heat transfer system
within the overall, system. They also are designed to be operated on a nearly
continuous basis since they are often the primary source of heat for the home.
According to the RWC survey conducted as part of this project, it is estimated
that these systems account for only 1-2% of the current RWC.
Central furnace systems operate with an overall efficiency of 40 to 75%.
This is a slightly higher efficiency than typical wood stoves and much higher
14
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efficiencies than associated with fireplaces.
LOCALIZED HEATING SYSTEMS
A fireplace or wood stove is a localized heat source. Although some
units may employ fans to move heat away from the appliance or even a hot water
coil to assist in heat transfer, they are really designed to add heat to their
immediate surroundings only. If the appliance is to be used as a central heat-
ing system, some other heat transfer system must be added to accomplish this
task.
FIREPLACES
The efficiency of a fireplace is quite low. Typical masonry fireplaces
have an overall efficiency of 20%. Other sources have reported efficiencies
to range from -10 to +10% and from 20 to 42%. The apparent wide range in
efficiencies may be caused in part by the lack of standardized operating and
testing conditions. However, due to the high volume of excess air associated
with a fireplace, the outside temperature has a significant effect on overall
efficiency. When in operation, the draft created from the fire draws in large
volumes of cold air through leaks under doors and around windows. The cooling
from the outdoor air drawn into the heated space may actually result in a net
heat loss from an open fire. A decrease in efficiency of 3% exists for each
2
10°F difference between outside and inside air temperature. Thus, assuming
most fireplaces are only 20% efficient, most fireplaces will consume more
energy than they produce when the outdoor temperature is below 0*F (a tempera-
ture difference of 70 degrees between the outside and inside air results in an
efficiency reduction of 21%).
15
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However, some energy efficient fireplaces have been designed which may
have efficiencies as high as 30 to 35%.~ At an efficiency of 35% this means
the best factory built energy efficient metal fireplaces have efficiencies as
good as the worst wood stoves.
Emissions from fireplaces have become of more concern since their apparent
o
increase in usage. A study by J.L. Muhlbaier indicates that the most important
parameter controlling emissions is the average burning rate. As the burning rate
increases, the emissions of particulate matter and hydrocarbons decrease. The
study further indicates that there is no obvious correlation between fuel moisture
content and emissions, although particulate emissions increased greatly with
large log sizes. Based on this study, to minimize emissions from fireplaces
(pounds emissions vs burning rate) hot full fires are necessary. In this study,
there was no attempt made to correlate emissions as a function of heat output
delivered to the surrounding room. However, these results are consistent with
other findings and recommendations; in short, to maximize energy efficiency,
full, hot fires should be built and also should be maintained for long periods
of time. Obviously, when the fireplace is not in use, the dampers should be
closed to prevent losS of warm air up the chimney. It was further recommended
that on cold days, the fireplace should not be used since an actual energy loss
may result.
WOOD STOVES
Stoves may be classified by the heat transfer technique utilized. Useful
heat energy can be transmitted by radiation, convection, or by a combination of
these two. Although all stoves utilize both forms of heat transfer they are
generally classified as radiators or circulating (convection) stoves depending
16
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on the principle mode of heat transfer. These are illustrated in Figure 1.
Some literature states that circulators do not perform as well (do not have a
high overall efficiency) as radiant stoves, with circulators having a 40 to
50% efficiency and radiant stoves having a 45 to 70% efficiency. Other
studies'" indicate that over a range of conditions, a difference cannot be
statistically determined. In any event, the overall efficiency of the wood
stove exceeds the fireplace but is still relatively low in relation to conven-
tional methods of home heating.
Radiating Stoves
Radiating stoves supply most of their useful heat by radiation from the
stove's surface. Therefore, heat transfer from the combustion chamber to the
surface of the stove is essential. If this is done improperly, the temperature
of the combustion chamber may be significantly reduced adversely affecting com-
bustion efficiency.
Circulating Stoves
Convection units, or circulating stoves typically use hot air circulation
as their principle form of heat release. Air passes or is forced over the
stove's surface between an outer shell and the shell containing the combustion
chamber, resulting in the warming of the air before it is blown into the room.
Stoves also are classified on the basis of air flow paths through the
combustion chamber. Five basic classifications commonly are sold, with numerous
combinations or modifications of these classifications available. The airflow
of the primary combustion air determines whether the stove is an updraft, down
draft, cross draft, diagonal, or "S" draft stove. These designs are illustrated
in Figure 2. The path the air follows in relation to the combustion zone would
17
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FIGURE 2
Air Flow Patterns
UPDRAFT
\
DOWNDRAFT
DIAGONAL
CROSS DRAFT
"S" FLOW
19
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appear to be extremely important. It would appear to affect pyrolysis rates,
oxygen availability and mixing, combustion temperatures, and consequently,
combustion efficiencies. In addition the air path would appear to influence
the heat transfer within the stove and therefore, its overall efficiency.
However, data indicate that the air flow path has negligible effect on the
wood stove overall efficiency.
•Down Draft
Combustion air is drawn down through the charge of wood to the grate area
where combustion is occurring. The gases generated in the pyrolysis process
are carried with the primary air to the area of combustion facilitating more
thorough combustion and improving combustion efficiency. Heating of the
primary air, which occurs with the air being drawn through the fire, also reduces
emissions by increasing the combustion efficiency.
•Updraft
In this design, the primary air flow passes upward through the burning wood.
Heating of the air occurs under the grates providing the fuel with preheated
air. This results in the heating and pyrolysis of some of the wood in an oxygen
deficient environment since these gases have already passed through the combustion
zone and the oxygen consumed. This effectively limits the rate of combustion.
Combustion which occurs in an oxygen deprived atmosphere is incomplete and,
therefore, higher in air contaminants.
•Cross Draft
In the cross draft or side draft stove, the primary air in introduced at
one side of the charge and leaves at the base of the charge. The pyrolysis
products are not released directly into the primary air stream but into the fuel
20
-------
magazine. The air is heated by radiation and convection before being drawn
into the primary combustion area.
•Diagonal Flow
This flow pattern is typical with many box stove designs. Air enters
near the base of the charge and travels directly to the stack. It travels
across the face of the burning area as overfire air. There is little mixing
of the incoming air with the fuel to promote complete combustion.
."S" Flow
This design permits the slower "end burning" of the charge. Air is intro-
duced at the base of the charge with the combustion products leaving the com-
bustion chamber on the same side. This design is conducive to a very slow burn-
ing rate since the effective surface area of the fuel log is relatively small.
•Ceramic Stoves
Stoves which utilize a ceramic combustion chamber in lieu of metal or fire-
brick lined also are on the market. There was little technical data available
9 33
in reference to this type of design. Sales literature ' indicates improved
overall efficiencies resulting from the massive structure designed to accumulate
the heat and release it slowly. This heat "accumulation" theoretically
increases the combustion temperature resulting in increased combustion efficiencies
and reduced emissions. The sales literature claims very little heat loss up the
41
stack. Limited test data conducted in RWC Task 5 of this project indicates
low mass emissions but very high stack temperatures (i.e., high heat loss).
•Fireplace Inserts
Stoves which are manufactured to be installed in existing fireplaces are
defined as fireplace inserts. Basically the inserts are designed and operated
21
-------
like the stoves previously discussed. The major difference is that they use an
existing fireplace chimney rather than a "stove pipe". Theoretically combustion
principles and the combustion efficiency would be the same as wood stoves. How-
ever, the heat transfer efficiency should be lower than that of wood stoves
since no stove pipe exists (considerable heat is transmitted through a stove
pipe). When comparing the insert to the fireplace much higher overall eff-
iciencies are expected from the insert, since the combustion air can. now be
regulated.
50
Table 2 is a summary of overall efficiencies as reported by T. Burch.
In summary it should be noted that fireplace and non-airtight stove efficiencies
ranged from negative to 40%. Airtight stoves had higher efficiencies, ranging
from 35 to 70% which was little diffe^^nt than the central heating systems. The
overall efficiency appears to be directly related to the ability of the appli-
ance to regulate combustion air. RWC appliances that regulate combustion have
higher overall efficiencies.
22
-------
TABLE 2
Typical Overall Efficiencies
of
Wood Burning Appliances*
Appliance Efficiency Range
Masonry Fireplace -10% to 10%
Manufactured Fireplace with
heat circulation and -10% to 10%
outside combustion air 15% to 35%
Free-Standing Fireplace 20% to 40%
Fireplace Stove 20% to 40%
Non-Airtight Stoves 15% to 40%
Radiant Stoves 45% to 70%
Circulator Stoves 49% to 55%
Fireplace Inserts 35% to 55%
Supplement Furnaces 40% to 60%
Central Furnaces 40% to 75%
* From Wood Burning Safety & Efficiency by Burch et al.~ The sources of
these efficiency ranges are not cited in this reference, and may conflict
with other data presented in this report. Nonetheless, this table is
useful for comparing efficiency ranges for the various devices.
23
-------
IV. MODIFICATIONS AND RETROFIT
A variety of stove and fireplace features can affect operation, heat
output, and emissions. Several possible combustion modification techniques
appear feasible to reduce emissions and improve combustion efficiency. Many
of these techniques are operator and fuel dependent. Physical modification to
existing fireplace and stove installations is possible by retrofit.
The retrofit devices currently on the market are used to regulate air
flow, improve combustion, treat the exhaust gases or improve heat transfer
efficiency. Many of the retrofit units are designed with the intent of influencing
more than one of the above traits in a positive manner. However, little data
exists that either supports or refutes the claims made on these units.
WOOD STOVE MODIFICATIONS
DRAFT CONTROLS
Controlling combustion air to the fire is one of the design features
that can most readily be modified. Draft control has a significant effect on
wood stove efficiencies and wood usage. Draft control can be accomplished at
the stove by regulating the area which allows the combustion air into the firebox
or can be accomplished at the stack through the use of a barometric damper. The
barometric damper allows room air to be bled into the stack which simultaneously
reduces the air flow through the combustion zone. For non-airtight stoves, only
the barometric damper type control will work. Draft control is important because
the amount of primary air introduced into the wood stove acts as a throttle and
determines burning rate.
-------
The combustion air can be regulated through the use of a damper controlled
manually or automatically by a thermostat. Typically a thermostat is installed
on the stove. A bi-metallic spring attached to a butterfly damper closes off
or opens up the primary air intake passage thus regulating the air flow. The
thermostat acts automatically to regulate the primary air damper after the stove
16
reaches a certain temperature. According to the manufacturer , overall stove
efficiency increases when controlled by an automatic thermostat. As the amount
of primary air reaching the fire is reduced the rate of combustion decreases due
to oxygen starvation. After the stove starts to cool, the damper opens which
allows additional oxygen to reach the fuel increasing the rate of combustion.
Through the use of a damper, the temperature of the stove is kept relatively
constant. Since the stove temperature can be automatically maintained at the
peak of the stove's efficiency curve , the net thermal efficiency of the stove
is increased. The increased thermal efficiency requires less wood to be burned
and, therefore, results in less emissions to the atmosphere. In Barnett's
study , the actual stove emission factor (mass emissions/mass fuel) was not
reduced by the automatic damper. These data should not be construed to indicate
that restricted air flow conditions improve efficiency. Instead, the data
indicate that regulated air flow improves thermal efficiency over non-regulated
air flow.
Utilization of outside combustion air ducted directly to the combustion
chamber has been suggested as a means to improve overall efficiency (i.e., the
unit is not consuming warmed room air and, therefore, not using energy already
transmitted to the room). However, the use of outside combustion air for air-
56
tight wood stoves apparently does very little toward saving energy.
-------
AFTERBURNERS AND CATALYSTS
Afterburners and catalysts are used to promote additional combustion of
the exhaust gases. Theoretically the improved combustion will provide more
heat and at the same time reduce emissions. The afterburning process can be
accomplished through the addition of a secondary fuel into the gas stream to
promote and maintain combustion or by the placement of an active catalyst into
the exhaust.
There were no data available regarding the introduction of a secondary
fuel to promote additional combustion and there is only limited data in
reference to use of catalysts on wood stoves. A typical catalyst consists of
a ceramic support in a configuration which permits the combustion gases to pass
over or through the support (e.g., a honeycomb structure); the support is
coated with platinum or other metal catalyst. Figure 3 shows one installation.
The catalyst acts by substantially reducing the temperature at which the unburned
combustion gases will ignite and burn. For example, combustion gases from a
wood stove typically will combust at 1100*F but in the presence of a catalyst
would combust at 500*F (given adequate oxygen, mixing, and residence time).
Proper operation of the catalyst requires that it be operated in a hot
exhaust stream. At temperatures below 400*F a noble metal catalyst becomes
32
inefficient. Some catalysts require a minimum operating temperature of 500-
550°F. Temperaturs below 400°F do occur at low burn settings or at the end of
the burn cycle for typical stoves.
In order to effectively use a catalyst system, the operator must be more
attentive to stove operation since catalysts are designed to work in temperatures
above 400BF. In addition to contacting the catalyst, the flue gases also must
26
-------
FIGURE 3
Catalyst Installation
Catalvst
\J
Secondary
Combustion
Primary Combustion
Chamber
Regulated Bypass Damper
Flue Outlet
27
-------
contain sufficient oxygen in order to undergo additional combustion. Since the
combustion gases must contact the catalyst in the presence of oxygen, catalyst
surface area, residence time of the gases with the catalyst, oxygen content,
and mixing of the combustion gases all become important considerations.
According to one study conducted by a catalyst manufacturer, catalysts can
reduce total emissions.
5 18
It has been reported by the manufacturers ' that as the loading
(uncombusted emissions) to the operating catalyst increased, catalyst combus-
tion efficiencies increased since more "fuel" was being provided to complete
the combustion reaction. Emissions that continued through the catalyst were
reported to be more soot-like than tar-like, indicating volatile organics had
41
been oxidized. Other tests conducted on catalyst equipped stoves indicated
little or no reduction in emission rate compared to emission rates typically
expected from non-catalytic stoves.
The effectiveness of a catalyst appears to be very dependent on the proper
operation of the stove, as well as stove design. This includes proper location
of the catalystic combustor within the appliance and properly designed second-
ary air inlets for adequate combustion air distribution. The catalyst must be
operated in a hot environment with fuel and oxygen present in proper amounts in
order to promote additional combustion and reduce emissions. It is important
that the stove be properly sized so that it can be operated at high enough tem-
peratures for catalytic action without overheating the house. Using catalysts
on wood stoves is relatively new technology. Many potential problems and
unanswered questions remain. One significant question involves the amount and
type of secondary pollutants which may be generated during the catalytic combustion
28
-------
32
process, particularly hazardous or toxic emissions. Among secondary pollutants
that theoretically could be formed in the presence of a catalyst are ammonia
and hydrogen cyanide, although tests have not confirmed the formation of such
32
pollutants. Operating the catalyst in an excessively rich exhaust stream
32
below the ignition temperature may cause fouling of the catalyst and possibly
plugging of the pathway for combustion gases resulting in a safety hazard if a
bypass around the catalyst is not available. In addition, combustion of certain
products such as magazines and some pressed wood products containing metals in
the ink and glue resins may poison the catalyst and eliminate its effectiveness.
The expected life of a catalyst will be an important consideration in the success
of the new technology.
DESIGN MODIFICATIONS
A properly designed wood burning stove would include bricklining to pro-
mote high combustion temperature, preheated primary and secondary air to promote
combustion, baffling to increase retention time of the combustion gases, com-
bustion air regulation, and an efficient means of extracting the useful heat.
o /•
Design considerations such as these are present in the more efficient stoves.
Fire Brickliners
Theoretically fire brick liners can decrease air pollution emissions by
helping to maintain higher temperatures in the firebox, thus promoting more
complete combustion. The stove takes longer to attain its heating temperature
due to its increased mass but once attained, the temperature of the stove is
more uniformly maintained.
Baffles
Baffles are used to keep the hot gases in the stove longer, rather than
29
-------
allow their immediate escape at the exhaust. Theoretically this causes the
heat to be released to the room through the stove instead of lost out the
chimney. The baffles also theoretically act to decrease emissions by allowing
extra time for more complete combustion to occur in the firebox. However,
there is insufficient data available to provide any indication as to the effec-
tiveness of baffles.
Pollution Control Equipment
Only one system to date has been identified as being designed specifically
to reduce air contaminant emissions. This system utilizes a stainless steel mesh
which is inserted in the exhaust stack. When operated at low temperatures,
contaminants theoretically condense and agglomerate on the mesh. At elevated
temperatures, this accumulation would provide a fuel source to promote combus-
tion. Based on one test , this system was 50% effective in particulate con-
trol. However, it must noted that one other study indicated no decrease in
emissions nor significant increase in overall stove efficiency associated with
the use of this unit.
FIREPLACE MODIFICATION
As previously discussed, the overall efficiency from a fireplace is very
low (-10% to 20%) because of the large amounts of cold air drawn into the house
by the fire's draft. Modifying an existing masonry fireplace can be very
difficult. However, many retrofit devices are becoming available which claim
to improve overall efficiency on these units. This improvement in overall
efficiency typically is accomplished by improving the thermal efficiency (heat
transfer) of the fireplace. Improvements in thermal efficiency should result
in a corresponding reduction in air pollution since less fuel would need to be
30
-------
consumed to generate the same amount of heat. Some of the more common retrofit
devices include glass doors or tube grates.
COMBUSTION AIR REGULATION
Fireplaces typically operate with 500 to 600% excess air. This means
that for each pound of wood burned, approximately 37 pounds (500 ft ) of air
is used. To control the loss of this heated room air, outside combustion air
preheated by the fireplace, should be utilized. One report states this is
the most effective means of increasing fireplace efficiency. However, another
report is quick to point out that no hard evidence exists that use of outside
air is beneficial. The latter report also identifies many potential negatives,
when using outside air, such as heat loss through these ports when the fireplace
is not in use.
A second means to reduce the loss of preheated room air is by using glass
fireplace doors. However, these doors, when closed, reduce the gross heat
2 31
transfer to the room of the fireplace by 50 to 55%. ' To eliminate this
high heat loss, the fireplace should be operated with the doors open. The doors
should be closed when the fire burns down or when the fireplace is not in use.
The doors are more effective at eliminating heat losses after the fire then
improving efficiency of operating during the fire.
HEAT TRANSFER SYSTEMS
Fireplaces can be built with air to air heat exchangers incorporated
into their design. Typically these are an envelope placed behind the fire
pit that allows air to come into contact with the metal back of the fireplace.
These are similar to the convection stove illustrated in Figure 1. Heat transfer
in this form is not high. When not in use, it may in fact provide a source of
31
-------
heat less. When used with a fan to promote movement of heated air an improve-
0 1J 1 "" '
ment of S.6% may occur.^ Other studies *' report that fans help but only by
2
about 5%. Without fans, the increase in efficiency is only about 2.5%.'" Fire-
places that do not have these heat exchangers built in may receive some of the
same benefits by installing tube grate systems or forced air heat exchange
systems. Tube grates (hollow tubes which support the burning logs and are
shaped to draw in room air, heat it, and return it directly to the room) may
increase efficiencies 5 to 8 percentage points when these units are equipped
with a fan. Without fans, this increase is more in the neighborhood of 1
2
percentage point."
Improving fireplace efficiency appears extremely difficult to accomplish
whereas reducing recurring heat losses during non-burning periods through the
installation of glass doors or other fireplace sealers may be quite feasible.
HEAT STORAGE SYSTEMS
Wood stoves are designed to produce usable heat during the combustion
process. The combustion process must be regulated to meet the required heating
demand. This requires considerable operator attention to match the output to
the demand. When the operator is not available to regularly tend the fire it
must be "banked". Frequently this involves placing large charges of wood on
the fire and reducing the combustion air in an effort to sustain a long burning
period. In such a case, little- attention is given to the actual heat output
or the efficiency of operation; the primary concern is simply to sustain a fire
that gives off some heat until the operator can return to properly tend the fire.
Theoretically overall stove efficiencies could be significantly increased
if heat storage principles could be utilized to first efficiently accumulate
32
-------
the heat generated during ideal combustion and then later dissipate the heat as
needed. This would allow the operator to operate the appliance at its highest
overall combustion efficiency. The concept of massive rock, or water heat sinks
to provide this accumulation capacity appears to be the idea most frequently
voiced. No technical data were available that provided comparison of efficiencies
or emissions when using these systems for wood stoves. However, wood burning
furnaces designed according to these principles of operation and intended for
use with central heating systems have been tested and shown capable of attaining
low emission rates; heat transfer efficiencies were not measured during the
study.
33
-------
V. FUEL SELECTION AND PREPARATION
The chemical composition of dry wood as measured in percent carbon,
hydrocarbon and oxygen is very similar for hardwoods and softwoods. All wood
49
has approximately the same energy content on a per pound basis. The elemental
content typically is about 40-52% carbon, 6% hydrogen, and 40-44% oxygen.
The cellulose content does vary however, with the hardwoods containing more
volatile hemicellulose and less char-forming lignin than the softwoods. Due
to hardwood's higher density it has a higher heating value on a volume basis.
Wood is usually purchased by the cord which is a volume measurement. Thus,
there are more BTU's per cord which makes them a more desirable fuel from an
operator's standpoint. Furthermore, softwoods burn more rapidly, therefore,
requiring more frequent charging.
The utilization of a high quality fuel that has been properly prepared
for use in the wood stove is essential. Species, moisture content, and log
size all must be considered in maximizing heat output while minimizing air
pollution. These aspects also play an important role in creosote buildup and
potential fire problems.
FUEL SPECIES
Species selection is limited by the geographic area and the availability
of the desired species. If several species are available, price often is used
as the sole consideration when purchasing firewood. This can be a serious error
since the heat output among species varies significantly. The relative heating
value can vary by up to 50%, as illustrated in Table 3. Based on heating value
34
-------
TABLE 3
^
Relative Heating Value Per Cord of Wood
(Millions of Bill's per Cord)
HIGH (24-31
Live oak
Shagbark hickory
Black locust
Dogwood
Slash pine
Hop hornbean
Persimmon
Shadbush
Apple
White oak
Honey locust
Black birch
Yew
Blue beech
Red oak
Rock elm
Sugar maple
American beech
Yellow birch
Longleaf pine
White ash
Oregon ash
Black walnut
MEDIUM (20-24)
Holly
Pond pine
Nut pine
Loblolly pine
Tamarack
Shortleaf pine
Western larch
Juniper
Paper birch
Red maple
Cherry
American elm
Black gum
Sycamore
Gray birch
Douglas fir
Pitch pine
Sassafras
Magnolia
Red cedar
Norway pine
Bald cypress
Chestnut
LOW (16-20)
Black spruce
Hemlock
Catalpa
Red alder
Tulip popular
Red fir
Sitka spruce
Black willow
Large-tooth aspen
Butternut
Ponderosa pine
Noble fir
Redwood
Quaking aspen
Sugar pine
White pine
Balsam fir
Cottonwood
Basswood
Western red cedar
Balsam popular
White spruce
35
-------
along, a significant price differential could be offset rapidly. Other consider-
ations, such as residual ash and creosote generation, make the use of high
quality fuel very desirable.
MOISTURE CONTENT
The moisture content of wood affects both the heat value of the wood and
the combustion process. As the wood burns, the moisture in the wood is evaporated
to form steam. This change from the liquid to gaseous state requires energy,
which reduces the overall heating value of the wood. The evaporation process
also lowers the temperature in the firebox, which further inhibits combustion.
Proper air drying or seasoning of firewood for three to four months can
29
increase the heating value 10 to 12%. The actual amount of moisture that can
be removed through air drying depends on the relative humidity of the air around
the wood and proper storage practices. Wood that is stored on the ground
during wet periods with plastic completely covering the pile will rot rather
than dry, for example. Depending on ambient conditions, properly air dried
wood will have a moisture content of 10 to 20% (moist wood basis), which corre-
sponds to the maximum overall efficiency range. Figure A illustrates the effect
of moisture content on the heating value of wood.
Combustion of very dry wood also increases emissions and decreases the
overall efficiency. ' Kiln dried wood (less than 10% moisture content,
wet basis) tends to pyrolyze and burn very rapidly producing a very hot fire.
The gases which evolve during the rapid pyrolysis are not adequately mixed with
combustion air so that complete combustion does not occur. These unburned gases
20
represent increased emissions and a significant energy loss.
36
-------
TO
OJ
tt>
a.
% Decrease In Heat Value of Wood
C"
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3-
O
o
00
1X3
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r+
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r+
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o
Q.
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-------
The effect of wood moisture content on energy efficiency is shown in
Figure 5. For the wood stove tested to produce this data, an overall 60%
or greater energy efficiency corresponds to wood moisture content between
about 10% and 25%, wet basis. Using Figure 4, this corresponds to a seasoning
time of four months or more.
OPERATION
19
Operation is more important than either fuel species or moisture content.
The size, number of pieces, and loading techniques significantly influence the
rate of combustion, hence, the stove efficiency.
BURN RATE
As wood burns, it releases energy'in the form of heat. As heating demand
increases, more fuel must be consumed to satisfy this demand. The burning rate
is, therefore, normally adjusted to meet the demand. Emissions for a stove
42
also are related to the burn rate , although the specific relationship is not
well defined with different reports indicating different relationships.
In general, however, the data indicates that total particulate emissions
/ O
(g/kg fuel) decrease as burn rate increases. According to Hubble , both the
highest thermal efficiency and highest emissions are associated with lowest
burn rates. Therefore, as the burn rate increases, emissions would decrease.
At least two other studies ' support the concept that emissions decrease as
the burning rate increases. These findings are further supported by a study
conducted by Harper that indicates that optimal efficiency lies between the
low and medium burn rates. Condensible organics decrease as the burn rate de-
creases, but condensible organics increase as the burn rate decreases.
38
-------
FIGURE 5
Efficiency vs Moisture Content
90 -
Combustion Efficiency
Heat Transfer
A \Efficiency
Overall Efficiency
0 10 20 30
Moisture Content (%-Moist Wood Basis)
The dependence of efficiencies on fuel moisture content in an
airtight stove. The air inlet setting was varied to maintain an
average power output of about 17,000 BTU's per hour for all moisture
contents. The fuel load volume was approximately constant.
39
-------
The reasons for che differences in the burn rate — emissions relationship
are not known but many variables are likely to be involved. Differences may
be caused by the way the burn rate was established or by other variables such
as appliance design or fuel moisture. For example, emissions associated with a
large charge of wood that has its burning rate restricted by oxygen starvation
would be expected to be different than emissions generated in a series of small
charges burning in an excess air condition even if the overall burn rate were
the same (Ib wood/hr). Both these operating conditions are common for wood
stoves and is dependent upon the operator.
PIECE SIZE
42
Emissions from the stove are related to log size. The shape and size
of the log determines the surface area and the distance that water trapped in
the fuel must travel before reaching the surface. Thus the rate of evaporation
and the rate of pyrolysis are affected, as well as the char burning that occurs
on the surface. Excessive fuel surface area in the stove requires larger
26
quantities of oxygen to facilitate combustion. Without a proper air to fuel
ratio in the combustion zone, these gases are exhausted out the stack without
undergoing combustion. Basically, combustion of large wood pieces (small sur-
face area per volume) generates less emissions than combsution of small wood
7 42
pieces (large surface area per volume). ' Ideal wood piece size is 3% to
13
5 inches in diameter providing it can maintain the desired burning rate.
Wood larger than this provides insufficient surface area to promote proper com-
bustion, while pieces smaller have too much surface area. However, it must be
25
noted that according to one study the organic emission rate is not a function
of log size, although the distribution between the different constituants
40
-------
(creosote, particulate, and condensible organics) is affected. At comparable
burn rates, the particulate and creosote emissions are higher while the com-
densible organic emissions are lower for the small logs when compared to the
25
large logs.
The pile must be properly stacked to allow hot combustion gases and oxygen
to come in contact to promote combustion. Stacking the fuel pile loosely pro-
motes combustion. On the other hand, tightly packing the pile, thus eliminating
air movement and combustion, can extinguish a fire. Therefore, the operator
needs to load the firebox with care to maintain proper air flow around the fuel.
CHARGING RATE
The size of the firebox establishes the maximum charge size that can be
loaded into the stove. Typically, this charging capacity is restricted to 40%
10
of firebox volume. The charge size has an effect on emissions.
Overcharging (too large a load) causes the premature volatilization of
combustion gases in zones where temperatures are below the ignition temperature,
causing excessive emissions and reduced efficiencies. ' ' To heat the
entire volume of the combustion chamber up to temperature may result in a much
higher rate of combustion than desired and result in overheating the room, again
representing a loss of efficiency. Many manufacturers claim this problem can be
overcome by reducing the amount of combustion air; however, this produces a
46
slow smoldering fire, low on energy and high in air pollution. "Banking" a
stove with a large charge of wood for overnight or sustained burns without
frequent charging creates the same effect. The slow smoldering fire typically
generates a lot of combustion gases that are never ignited, decreasing com-
bustion efficiency and increasing air pollution. However, one study indicates
41
-------
that overall efficiency was not significantly different at half capacity or
39
full capacity , since at full capacity thermal efficiency increased while
combustion efficiency decreased. However, this is a less desirable situation
since emissions are expected to increase with decreasing combustion efficiency
39
(emissions were not measured in the just referenced study ). On the other
hand, undercharging (too small a load) allows for excessive combustion air.
Since overall efficiency decreases as excessive combustion air increases this
again generates an energy loss. Maximum efficiency occurs when approximately
1/3 of a load is added (i.e., 30 to 35% of the firebox volume) at each charge.
Therefore, careful attention should be given to selecting a stove that will
give the desired heat output when properly charged.
CREOSOTE FORMATION
In addition to heat output and energy efficiency a stove operator needs
to be aware of creosote buildup to prevent a potential fire hazard. As
condensihle hydrocarbons (tars) leave the combustion zone a condensation process
begins. The rate of condensation depends on initial gas temperature and the
amount of cooling that occurs in the stack. The quantity of condensible hydro-
carbons that accumulate on the stack wall also is dependent on the amount of
condensible hydrocarbons generated during the combustion process, which is dependent
upon several variables already discussed, primarily combustion efficiency. A study
26
by Harper concludes that air dried (approximately 25% moisture content) hard-
woods are the most desirable fuel regarding the reduced formation of creosote.
19
In another study by Shelton reference can be found regarding creosote buildup
as a function of temperature with low temperature fires forming more creosote.
-------
This appears logical in chat creosote is a product of incomplete combustion and
diminishing stack gas temperatures. This study is further supported by findings
47
contained in Hubbies' ~ report. These findings indicate that as the burning rate
increases, the formation of creosote decreases.
Contrary to popular belief, high wood moisture content does not automatically
mean more creosote formation. According to one study neither moisture
content nor species had a significant effect on creosote formation when operating
under a restricted air smoldering condition. However, when operating under a
medium to high fire condition in a closed combustion chamber there was a sub-
stantial decrease in creosote formation as moisture content of the fuel
19
increased. This may have been the result of a decreased pyrolysis rate
resulting from more moist fuel. Under the same conditions (i.e., medium to high
19
fires) more creosote was reported from pinon pine (softwood) than from oak
(hardwood). This same study investigated creosote formation when combustion
occurred in an open door mode rather than a restricted air mode. Under these
conditions, creosote formation increased as the moisture content of the fuel
increased. This type of combustion would more likely occur in a fireplace rather
than a wood stove.
43
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VI. STOVE SELECTION
Selecting a wood stove that suits one's particular need can be very
frustrating. There are numerous designs made by a multitude of manufacturers
with an equal number of claims of superiority. Basic areas of differences
include construction materials, size, internal configuration, heat output,
charge size, and overnight burning capabilities. Not all of these parameters
are of equal importance in the selection of a stove, however. Selecting the
proper size stove to fit the use and heating needs is probably the most impor-
tant aspect to be considered and often the most misunderstood.
In selecting a wood stove, the concept "the bigger the better" is incorrect.
Actually, the reverse is true, and this basic premise should be kept in mind
throughout the selection process.
SIZE DETERMINATION
Size determination should be based upon anticipated heating demand. If
the wood stove is to be the sole source of heat in the house, the unit must be
able to equal the house's total heat loss. The amount of heat loss from a house
is a function of many variables including temperature, wind speed, relative
humidity, and siting variables such as exposure to wind and available sunlight.
In addition, the insulating properties of individual houses vary considerably
with the size and shape of the dwelling and amount of insulation used in con-
struction.
Table 4 has been developed to aid in the selection of a wood stove and to
a
illustrate theoretical heat losses as a function of outside design temperatures.
Outside design temperature is the lowest temperature that is expected to occur
once in 13 years.
-------
TABLE 4 a
Outside Design Temperatures
of
Pacific Northwest Cities
CITY DEGREES FARENHEIT (Ff)
Idaho
Boi se
Lewi ston
Pocatel lo
Twin Falls
Oregon
Eugene
Pendleton
Portland
-10
5
-5
-10
15
-15
10
Washi ngton
Seattle 15
Spokane -15
Tacoma 15
Walla Walla -10
Yakima 5
Alaska
Juneau -5
3 ASH REA 1980
Normal design conditions winter - occurs once in 13 years.
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Calculating heat losses utilizing the outside design temperature is an
accepted practice that minimizes the number of variables that must be considered
in determining the theoretical heat loss. Table A lists numerous Pacific North-
west cities and the corresponding outside design temperature. The value obtained
from this table can then be utilized in conjunction with Figure 6 to estimate
the hourly heat loss. Since the outside design temperature is expected to be
reached only once in L3 years, it is advisable to use a slightly warmer tempera-
ture to represent the more typical situation that is anticipated unless the
stove is to be used as the sole source of heat.
After the design temperature and square footage of the home has been
determined, the estimated heat loss can be obtained using Figure 6. This value
is for a "typical home" and needs to be adjusted for the specific design situation.
The "typical home" used here is a single-story frame house with wood siding and
single pane windows. Ceilings were insulated to R-30, walls to R-ll, and floors
to R-19, consistent with today's construction standards. Adjustments to the
"typical home" heat loss value must then be made. This can readily be accomplished
using Table 5. Differences in design characteristics between the "typical case"
and the "specific case" should be noted on Table 5 and the total percent heat
loss or heat savings computed. Figure 7 can then be used to obtain the adjusted
heat requirement based on the estimated heat requirement and the computed
percent heat difference.
After the "adjusted" hourly heat loss has been determined, the intended
specific application must be considered. Most homes are not constructed to
readily accommodate a complete conversion from conventional heat to wood heat.
Homes that were constructed with individual room heat or forced air heat systems
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Estimated Heat Loss (1000's BTU/Hr)
c:
OJ
—^
c
1
n:
o
3
ro
Design Temperature *F
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TABLE 5
Effect of Differences
on
Total Heat Loss
Design Differences
Single story wood construction*
Two story home (wood or masonry)
Walls
Wood R-5 (no insulation)
Wood* R-ll (2" insulation)
Wood R-15 (3" insulation)
Masonry R-5 (Brick or cinderblock, no
insulation)
Masonry* R-ll (2" insulation)
Masonry R-15 (3" insulation)
Windows
Single pane*
Insulated
Floor
Insulated R-5
Insulated R-ll (2" insulation)
Insulated R-19* (3%" insulation)
Insulated R-30 (5" insulation)
Ceil ing
R-15 (3" insulation)
R-20 (4" insulation)
R-30* (5" insulation)
R-40 (6" insulation)
Infiltration**
1/4 (tightly sealed home)
1/2
3/4*
1
Percent
Heat
Loss
--
--
4
--
--
9
—
—
7
--
—
10
. 1
—
--
—
10
Percent
Heat
Savings
--
3
--
--
6
—
—
13
—
--
13
—
—
3
20
10
__
R Values = Thermal resistance. The higher R value the better the insulating performance,
* Used in "typical home" example.
** Air changes per hour. 48
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FIGURE 7
CO
l/l
o
o
o
c
dj
CT
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may prove very difficult to heat with a wood stove. For example, trying to
heat a large ranch style home formerly equipped with electric ceiling heat
to a uniform temperature would prove extremely difficult. To heat the distant
rooms to the desired temperature may cause areas nearer the stove to be uncom-
fortably warm due to the high output which would be required of the stove.
Without an adequate air movement system the exclusive use of a wood stove would
create significant cold and hot spots. This type of heating leads to user
discomfort and to additional fuel expenses resulting from overheating part
of the house to accommodate other areas. Rather than trying to heat the
entire home with a wood stove designed for localized heating, careful considera-
tion should be given to heating just a portion of the home with wood.
Using the wood stove as an auxiliary heat source may prove to be more
cost effective and provide better user comfort. By reducing the size of the
stove and burning less wood, the cost of the stove and the firewood would be
reduced. Also, labor associated with wood heat can be reduced, which will
enhance user comfort.
Once the percent of overall heat load to be carried by the stove and the
adjusted heat requirement have been determined, Figure 8.can be used to deter-
mine the approximate required wood combustion rate to supply the necessary heat.
This will be used in selecting the proper size of wood stove. Although the wood
combustion rate required to provide proper heat to the home has been determined,
the stove selection process is not complete yet. The charge size and volume
(firebox size) of the stove must still be selected. These parameters will, in
effect, establish the required frequency of charging. As previously discussed
in this report, a well controlled hot fire minimizes emissions and improves
50
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FIGURE 8
14
10
s-
:c
(O
ac.
c
o
E
O
o
o
o
6
2 _
0 —I
60
50 —
40
CO
in
O
o
o
c
n> n>
Q- CX
^ O
~O
rn
o
ni
O
~n
o
a
51
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overall efficiency. Consequently, using a charge size of about 1/3 the firebox
volume and allowing it to burn rapidly minimizes emissions and improves overall
efficiency. Therefore, one of the prime considerations in selecting a stove
should be its ability to operate under these above conditions, i.e., operate
with a charge of 1/3 to 1/2 firebox capacity, at a moderate to high burn rate
without producing more heat than is needed. Trying to maintain a high tempera-
ture fire in a large stove may produce far more heat than is needed. This
results in either reducing the size of the charge or dampering down the stove
(and sacrificing efficiency of operation) or letting the excess heat out the
windows, requiring excessive fuel usage and again promoting inefficiency. As
already mentioned, there is greater combustion efficiency and reduced air
pollution from a hot fire than from a slow smoldering fire. Adding wood to the
stove every two or three hours generally has proved to be the most effective
charging rate regarding efficiency and pollution considerations. More frequent
charging requires excessive operator attention. Less frequent charging of
large loads causes operator inattention, resulting in reduced efficiency and
increased emissions; this process is similar to fire banking.
Consequently, the choice of burn rate, charge size, and charge frequency
is the key to the stove sizing process. Using the required combustion rate
previously established and a theoretical time between charges, allows the use of
Figure 9 to establish the desired firebox volume and thus completes the selec-
tion process. Table 6 summarizes the sizing process and provides an example
illustration to assist in the size selection process.
52
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FIGURE 9
9.0 —1
60
7.5 —
50 —
6.0 —
40 —
Ol
X
O
-Q
o
3.0 —
20 —
1.5 —
10 —
Firebox Volume
vs
Charging Rate
O
O
n
c
-s
cr fD m
CU r+ X
3 £ o
A- fD (D
-•*• fD •
fD 3
^ C
3
o
rr ri-
cu _i.
-S 3
U3 fD
fD
O XI
T3 fD
fD J3
-s e
fD
O
-s
cu ro
rf X
r+ O
53
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TABLE 6
Stove Sizing Process Summary
and
II lustration
1. Determine outside design temperature - Table 4.
2. Estimate square footage of home.
3. Determine estimated hourly heat loss - Figure 6.
4. Estimate effect of design difference from "typical home"
Table 5.
5. Determine adjusted heat requirement - Figure 7.
6. Establish percent of home to be heated by wood heat or amount
stove is going to be used as auxiliary heat.
7. Determine actual BTU requirement and required wood combustion
rate - Figure 8.
8. Establish desired charging frequency.
9. Select firebox volume - Figure 9.
Illustration:
Home located in Boise, Idaho. From Table 4 Outside Design Temper-
ature determined to be -10° F.
Home size estimated at 1600 square feet. Using Figure 6, the
estimated heat loss is 47,000 BTUs/Hr. Using Table 5 an additional
heat savings of 13% is expected since the home is equipped with insu-
lated windows. Therefore, from Figure 7, the adjusted heat requirement
is estimated to be 41,000 BTUs/Hr.
Stove is only going to be used to heat 40% of the living area of
the home. Using Figure 8, it can be determined that the actual heating
requirement is only 15,500 BTUs/Hr and the wood combustion rate should
be approximately 3.8 pounds per hour. Since the stove will be charged
every 3 hours, it is evident from Figure 9 that a stove with a firebox
volume just overl.Sfr will be adequate in size. Therefore, a small
stove should be used.
54
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VII. STOVE & FIREPLACE OPERATION
Air contaminant emissions and overall efficiency of operation of a stove
and fireplace are influenced by many variables. These include design, fuel
characteristics, and operator technique. The following is a list of comments
and suggestions provided to minimize air contaminant emissions and improve the
heating characteristics of the stove or fireplace.
FUEL:
1. Select a good quality fuel. This provides more BTU's per dollar
and less creosote, maintenance, and air pollution. Do not burn
trash or plastics — these can produce hazardous pollutants and
affect the life of your stove and chimney. Air dried hardwoods
reduce creosote formation. Hardwoods provide more BTU's per cord
than softwoods, although about the same BTU's per pound as softwoods.
2. Properly season the fuel. By providing air circulation for your
fuel pile and tine to let it season, you will get more BTU's of heat
with less air pollution. Provide a full year for optimum seasoning.
Proper preparation of the fuel is essential for clean, efficient burn-
ing. The wood should be stacked properly to allow air to circulate
within the pile to dry the wood. Sheltering the wood pile against
rain and snow is extremely desirable. However,.if covering the wood
pile with plastic, one must arrange the plastic so as to prevent
trapping the ground moisture within the pile.
55
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WOOD STOVES:
3. Select a properly sized stove. Remember, bigger is not better.
Determine the heating demand for your particular installation
and select a stove that meets that specific heating requirement.
4. Combustion air dampers properly operated or regulated in reference
to the optimum air to fuel ratio can improve efficiency and reduce
emissions. Stove efficiency increases when controlled by an automatic
thermostat. There is a tendency to purchase a large stove with an
automatic damper to allow overnight and all day operation without
additional charging. These stoves are generally grossly oversized.
Used in this manner this feature significantly reduces efficiency
and increases air pollution, since the ideal burning rate is not
maintained.
5. Burn hot fires. Stoves should be batch fed every two to three hours
to produce the required heat. Overnight banking of the fire at a
low combustion rate should be eliminated to improve emissions and
overall efficiency. This also reduces creosote formation. Leave air
dampers open to promote combustion. This improves efficiency and
heat output while reducing emissions.
6. Use a stack or surface thermometer to monitor the operation of the
stove to assure it is operating at maximum efficiency. Typically a
«
stack temperature in the range of 300*F - 350*F where it connects to
the stove assures a hot fire to promote combustion without excessive ,
heat loss out the stack.
56
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7. Don't overcharge or undercharge the firebox. Maximum efficiency
and minimum air pollution depends on a proper charge. Typically this
is approximately 1/3 the firebox volume. Stack the charge in the
firebox loosely to allow air movement throughout the pile.
3. Use properly sized wood — 34 to 5 inches in diameter. Wood too
large tends to smolder, while wood too small undergoes pyrolysis too
fast — both of these promote incomplete combustion.
9. Heat exchangers could be used more effectively. A run of non-
insulated stove pipe is an example of a heat exchanger. These units
increase thermal efficiency. However, precautions need to be taken
to prevent excessive creosote accumulation and the creation of a
possible fire hazard.
10. Heat sinks such as water reservoirs, could be utilized to allow the
more effective operation of the stove while providing a storage area
for heat for a later use. This would allow the stove to be operated
continuously at its maximum efficiency with the heat being stored for
later use.
FIREPLACES-:
11. Fireplaces should be used for recreational/aesthetic reasons rather
than for space heating, because of their very low energy efficiency.
12. Use of a fireplace in cold weather should be avoided. The low net
overall energy efficiency may even be negative (more heat loss than
gained) when the outside temperature approaches 10"F.
8, 31
13. Hot full fires should be burned in the fireplace to reduce emissions.
57
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1A. Fireplaces equipped with glass doors should be operated with the
doors open for maximum overall efficiency. When closed, glass doors
severely retard the flow of heat from the fireplace into the room.
15. When not in operation and during the burn down period, glass doors
should be closed to prevent the loss of heated room air up the
fireplace stack.
16. Heat exchangers with blowers and tube grates help to improve overall
efficiency of the fireplace. However, this increase in efficiency is
typically very minor.
58
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VIII. BIBLIOGRAPHY AND REFERENCES
1. American Forests, John Zerbe, October 1978, pg 33.
2. Analysis of Heat-Saving Retrofit Devices for Fireplaces, Robert D.
Busch, PhD., New Mexico Energy Institute, March 1979, NMEI Report No.
77-1102.
3. Applied Ceramics, Dennis A. Carlson (Sales Brochure).
4. Blair & Ketchum County Journal, "Woodburning Furnaces", Larry Gay,
October 1979.
5. Catalytic Combustion in Residential Wood Stoves, Robert V. Van Dewoestine,
Frank Zimar, and Robert A. Allaire, Corning Glass Works.
6. Catalytically Assisted Combustion in Residential Wood-Fired Heating
Appliances, J.W. Shelton, February 1981.
7. Characterization of Emissions from Residential Wood Combustion Sources,
W. Marcus and John M. Allen, Battelle-Columbus Laboratories, presented
at 1981 International Conference on RESIDENTIAL SOLID FUELS, Environmental
Impacts and Solutions, Portland, Oregon, June 1981.
8. A Characterization of Emissions from Wood Burning Fireplaces, Jean L.
Muhlbaier, Environmental Science Department, General Motors Research
Laboratories, presented at 1981 International Conference on RESIDENTIAL
SOLID FUELS, Environmental Impacts and Solutions, Portland, Oregon, June 1981.
*FM
9. Concord Catalytic (Advertising Brochure).
10. Consumer Reports, "The Return of the Wood Stove", October 1981, pg 566-573.
11. Control of Emissions from Residential Wood Burning by Combustion Modification,
J.M. Allen, W.M. Cooke, Battelle-Columbus Laboratories, May 1981, EPA-
600/7-81-091.
12. Control of Particulate Emissions from Wood-Fired Boilers, EPA-340/1-77-026.
13. "Converting to a Wood Stove", K.P. Maize, Rodales New Shelter, September 1981.
14. "The Creosote Problem and How to Reduce It", R.K, Jorstand, Wisconsin Energy
Extension Service, June 1979.
15. A Design of a Domestic Wood-Burning Stove, G.R. Katzer and A.F. Ward,
February 1979.
59
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16. Determination of Wood Stove Efficiency In-Home Conditions, Stockton G.
Barnett, Prof., Dept. of Earth Sciences, State University of New York,
presented at 1981 International Conference on RESIDENTIAL SOLID FUELS,
Environmental Impacts and Solutions, Portland, Oregon, June 1981.
17. The Domes trie Fireplace and The Energy Crisis, L. Cranberg, PhD.
18. The Effects of Catalytic Combustion on Creosote Reduction, Combustion
Efficiency and Pollution Abatement for Residential Wood Heaters, Frank Zimer,
Robert V. Van Dewoestine, and Roger A. Allaire, Research and Development
Division, Corning Glass Works presented at 1981 International Conference on
RESIDENTIAL SOLID FUELS, Environmental Impacts and Solutions, Portland,
Oregon, June 1981.
19. The Effects of Fuel Moisture Content, Species, and Power Output on Creosote
Formation, Jay W. Shelton and James McGrath, 1981.
20. Effects of Woodburning Stove Design on Particulate Pollution, Stockton G.
Barnett and Damian Shea.
21. Effects of Wood Stove Design and Operation on Condensible Particulate
Emissions,Stockton G. Barnett and Damian Shea, Dept. of Earth Sciences
and Dept. of Chemistry, State University of New York.
22. Efficient Wood Stove Design and Performance. A.C.S. Hayden and R.W. Braaten,
Canadian Combustion Research Laboratory.
23. Environmental Impact of Residential Wood Combustion Emissions and Its
Implications, J.A. Cooper, APCA, August 1980, pg 853-863.
24. EPA's Research Program for Controlling Residential Wood Combustion Emissions.
R.E. Hall and D.G. DeAngelis, APCA, August 1980, pg 862-867.
25. Experimental Measurements of Emissions from Residential Wood Burning Stoves,
B.R. Hubble, J.R. Stetter, E. Gebert, J.B.L. Harkness and R.D. Flotard,
Energy and Environmental Systems Division, Argonne National Laboratory
presented at 1981 International Conference on RESIDENTIAL SOLID FUELS,
Environmental Impacts and Solutions, Portland, Oregon, June 1981.
26. Factors Affecting Wood Heater Emissions & Thermal Performance, J.B. Harper
and C.V. Knight, TVA.
27. Forbes, "Look Who's Setting the World on Fire", November 12, 1978.
28. Heat Recovery for Efficient Fireplace Operation, P.M. Sturges.
29. Heating With Wood, U.S. DOE, May 1980, DOE/CS-0158.
60
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30. Letter - S.G. Barnett to Janet Gillaspie, April 7, 1981.
31. Measured Performance of Fireplace and Fireplace Accessories, Dr. J.W. Shelton.
32. Measurements of Chemical Changes Due to Catalysis of Wood Stove Effluent,
Dr. Dennis R. Jaasma, Virginia Polytechnic Institute.
33. The Meridian (Advertising Brochure).
34. Method for Measuring Heat Output and Efficiency on Wood Heating Appliances
and Results from Tests on Ten Wood Stoves and Fireplaces, Lars Sundstrom,
National Testing Institute, Boras, Sweden, presented at 1981 International
Conference on RESIDENTIAL SOLD FUELS, Environmental Impacts and Solutions,
Portland, Oregon, June 1981.
35. Net Energy Available from Wood. Thomas B. Reed.
36. Particulate Emissions from New, Low Emission Wood St-oves Designs Measured
by EPA Method 5. John F. Kowalczyk, Peter B. Bosserman, and Barbara J.
Tombleson, Dept. of Environmental Quality, Oregon, presented at 1981
International Conference on RESIDENTIAL SOLID FUELS, Environmental Impacts
and Solutions, Portland, Oregon, June 1981.
37. Pollution and Fireplaces in California, Peter H. Kosel, California Air
Resources Board.
38. Popular Science, "The Secrets of a Good Wood Stove1!, Jason Schneider,
November 1977.
39. Preliminary Results on the Effects of Some Fuel Operator Variables on
Stove Efficiencies. Jay W. Shelton.
40. Reduction of Losses from Heat Emitters Sited Against External Walls -
A New Approach. U.S. Dept. of Commerce, May 1977, PB-277117.
41. Residential Wood Combustion Study, Task. 5, Del Green Associates, Inc.,
December 1981, EPA Contract No. 68-02-3566.
42. Results of Laboratory Tests on Wood Stove Emissions and Efficiency,
B.R. Hubble and J.B.L. Harkness.
^3. Source Assessment: Residential Combustion of Wood, Monsanto Research
Corporation, Contract No. 68-02-1874.
44. Standard Handbook of Engineering Calculations, T.G. Hicks, 1972.
45. Standard Handbook for Mechanical Engineers, Baumeister and Marks, 7th Edition.
61
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46. A Study of Wood Stove Parciculate Emissions, Samual S. Butcher and Edmund
M. Soreson, APCA, 1979.
47. Thermal Performance Testing of Residential Solid Fuel Heaters, Jay W. Shelton,
Shelton Energy Research, Santa Fe, New Mexico.
48. Waterbury, Vermont: A Case Study of Residential Woodburning, Vermont Agency
of Environmental Conservation, C.R. Sanborn, et al, August 1981.
49. The Woodburners Encyclopedia, Jay Shelton, Vermont Crossroads Press, Waits-
field, Vermont, 1976.
50. Woodburning Safety & Efficiency-Woodburning Innovations, Burch, et al,
Auburn, Alabama, 1980.
51. Wood-Fired Boilers and Multi-Fuel Control Heating Systems, J.M. Rummlel,
August 1977.
52. "Wood 'N Energy", Solid Fuel Journal, Vol. 1 No. 7, June 1981.
53. Woodstove Directory, Volume V, Energy Communication Press, Albert J. Myer,
Editor, 1982.
54. Wood Stove Selection, Walter E. Matson, OSU Extension Service
55. Wood Stove Testing Methods and Some Preliminary Experimental Results,
Dr. J. W. Shelton, T. Black, M. Chaffee, and M. Schwartz, ASHRAE Transac-
tions, Vol. 48, Part 1, 1978.
56. Wood Stoves - How to Make and Use Than, OleWik, Alaska Northwest Publishing
Company, Anchorage, Alaska, 1979.
62
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