United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-85/043 Dec. 1985
SEPA Project Summary
Feasibility Study of Enhanced
Combustion Via Improved
Wood Stove Firebox Design
Karen T. Fuentes and Leonard J. Hodas
Emissions from incomplete combus-
tion in wood burning stoves are becom-
ing an increasing environmental prob-
lem. In this study, materials were
examined which might be used to line
the firebox of a wood burning stove to
produce more uniform and complete
combustion. Although many materials
were initially considered, refractory
materials appear to possess the quali-
ties desired relative to heat transfer, re-
sistance to the firebox environment,
availability, and cost. Specific refrac-
tory materials have been further inves-
tigated, resulting in a list of material
properties of potentially useful refrac-
tories and a determination of relative
installed costs. The approach used in
this study was to establish the condi-
tions for a "basic" wood stove, then to
apply various candidate lining materi-
als to the basic stove and analytically
estimate the effect of the lining addi-
tion. Basic heat transfer calculations
were used. The use of refractory materi-
als permitted an increase in stove inner
wall temperatures and an increased
cooldown time for a stove. The study
showed that refractory materials could
aid in maintaining internal firebox tem-
peratures above the ignition tempera-
tures of common emissions. This
would not be a practical operational
mode for an uninsulated stove.
The study concludes that there is a
need for actual tests to confirm the re-
sults cited in the study.
This Project Summary was devel-
oped by EPA's Air and Energy Engineer-
ing Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report or-
dering information at back).
Introduction
The use of residential wood burning
stoves has grown considerably in re-
cent years. As the use of these stoves
has increased, so has the amount of pol-
luting emissions. Since these stoves
come in a very wide range of designs,
combustion and resulting emissions
can vary widely. Additionally, opera-
tional parameters can result in signifi-
cantly different emission levels from the
same stove design. One design ap-
proach for reducing emissions is to
place material inside the firebox that
can lead to more complete combustion
of the wood charge and any emission
constituents present. Ideally any design
modification should be essentially pas-
sive; i.e., requiring little or no attention
by the user. Also, due to the large num-
bers of existing wood burning stoves, it
is desirable that any modification lend
itself to the retrofit of existing stoves.
Existing Stove Designs
From a review of the design and oper-
ational characteristics of existing resi-
dential wood stoves, it was determined
that various design factors could have a
significant effect on wood stove emis-
sions. These design factors include
combustion chamber usage, combus-
tion controls and drafting design, and
certain operational factors, particularly
draft and flue adjustments.
-------
Resistance to the Combustion
Process and Increasing Com-
bustion Efficiency
Any material placed in the firebox (for
emission reduction purposes) must be
resistant to the combustion process, in-
cluding: firebox temperature extremes
and heating and cooling rates, the fuels
utilized, and the products evolved dur-
ing combustion. Additionally, the mate-
rial should enhance the combustion
process while not appreciably degrad-
ing stove operation or requiring signifi-
cant user attention. If a material in the
firebox can be maintained above the ig-
nition temperature of emission con-
stituents (see Table 1), combustion can
take place which can lead to emissions
reduction in the firebox.
Calculation Method: Basic
Stove
The initial approach used in this study
was to employ standard heat transfer
calculation techniques to determine a
temperature profile for a generic or
basic stove. The basic stove was de-
fined as a cast iron firebox with 1/4-in.
(0.64 cm) wall thickness and no lining.
The steady state temperature of the fire-
box gases was assumed to be 1500°F
(816°C). A room temperature of 60°F
(16°C) was assumed. When these tem-
peratures plus appropriate heat transfer
coefficients were inserted into the
standard heat transfer equations (con-
sidering radiation, convective conduc-
tance, and conservation of energy), the
basic stove wall temperature was found
to be 720°F (382°C) and the heat trans-
ferred (per square foot of stove wall)
was about 4000 Btu/hrft2 (12.6 kW/m2).
These values define the steady state
condition of the basic stove.
Calculation Method: Lined
Stove
Several assumptions were required
to determine the steady state tempera-
ture profile for a lined stove. The heat
transfer rate and the outer wall temper-
ature of the lined stove were assumed
to be the same as for the basic stove.
(Thus the heat output of the stove will
not be affected by the installation of a
lining.) Additionally, an inner (liner) wall
temperature of 1200°F (649°C) was cho-
sen. This temperature was selected as a
conservative adjustment to the carbon
monoxide ignition temperature of
1128°F (609°C) (see Table 1). That is, the
purpose of the liner is to raise the tem-
perature of the inner stove wall such
that the combustion of emission con-
stituents can take place. The use of
these temperatures, appropriate heat
transfer coefficients, and certain physi-
cal properties of a lining material permit
Table 1. Some Combustion Products Re-
sulting from Burning of Wood
Carbon monoxide3
Naphthalene*
Biphenyl0
Benzened
Example of ROMs
(polycyclic or-
ganic matter)
"Ignition temperature 1128°F (609°C)
blgnition temperature 898-1017°F (481-
547°C)
clgnition temperature 1071°F I577°C)
dlgnition temperature 1097°F (592°C)
the calculation of the required lining
thickness and a firebox gas tempera-
ture. The required lining thickness will
vary for different materials in direct pro-
portion to the conductivity of the mate-
rial.
Calculation Method: Cooldown
After the steady state temperature
profiles of the basic and various lined
stoves were determined, the tempera-
ture loss or cooldown of the stoves was
calculated. The cooldown rate of the
stoves was assumed to be governed by
an exponential time function. The abil-
ity of a lining material to store heat in-
creased the cooldown period of lined
stoves compared to the basic stove.
(See Figure 1.)
Candidate Materials for Inclu-
sion in the Firebox
A wide range of materials were con-
sidered as candidate materials for inclu-
sion in a wood stove firebox. Factors
such as resistance to the process envi-
ronment, cost, and ease of fabrication
were considered during the initial eval-
uation of candidate materials. Addition-
ally, a material must enhance the com-
bustion process by the heat transfer
methodology previously described.
Based on these considerations and cal-
culations, refractory type materials
were determined to be the most poten-
tially viable firebox lining materials.
The effect of various thicknesses of
typical refractory materials on inside
wall (liner) temperatures is shown in
Figure 2. An inside wall temperature of
1200°F (649°C) was used as an effective
temperature relative to the ignition of
emissions. An outside wall temperature
of 720°F (382°C) was used to maintain
an equivalent heating rate compared to
the basic stove. The required wall thick-
nesses for various refractory materials
were calculated based on the indicated
temperatures. Some of these wall thick-
nesses are shown in Table 2.
Ranking of Candidate Materials
There are various ways of rating or
ranking the numerous candidate mate-
rials that exhibit acceptable process re-
sistance and potential enhancement of
combustion efficiency. A rating criteria
of Total Installed Cost (TIC) per unit of
surface area would be of interest to both
wood stove manufacturers and owners.
The ratios of TICs for various refractory
materials were calculated based on the
previously described assumed temper-
ature profile, with medium duty fire
brick assigned a ratio of 1.0 for new fab-
rication. Table 3 gives examples of the
TICs for various refractory materials.
Conclusions
Various types of refractory materials
can be installed in a wood stove firebox.
These materials can lead to tempera-
tures (occurring at and near the refrac-
tory surface) that are above the ignition
temperatures of many wood stove
emission constituents.
A refractory lined stove has a longer
cooldown period than a metal stove.
A quantification of the effectiveness
of emission reductions attributable to
the types of firebox designs considered
here should be determined by various
tests.
The refractory materials that are rec-
ommended for testing include medium
duty fire brick, HW Super Castable, and
silica brick. These materials were se-
lected because their relative total in-
stalled cost ratios (TICs) are less than
1.5. The calculated wall thickness re-
quirements for each of these materials
are similar, which would allow for a
more consistent comparison during
testing. They also represent typical ex-
amples of the broad spectrum of poten-
tial materials that could be used.
-------
1400
(760)
1200
(649)
G 7000
£ <53a>
o
«> 500
I <427>
^
§. 600
| (315)
400 •
(204)
200
(93)
Tiw = Temperature of inner stove wall
Tow = Temperature of outer stove wall
Tiw Medium Duty
Firebrick (MDFB)
f
Basic Stove
Tiw - Tow
5 6
Time, hr
10
11
Figure 1.
Cooldown of a basic stove and a lined stove (1 in.—2.54 cm—of medium duty fire
brick.
1
1500 -\
(816)
1400 •
(760)
1300
(704)
1200
(649)
1100
(593)
Figure 2.
01234567
(2.54) (5.08) (7.62) (10.16) (12.70) (15.24) (17.78)
Insulation Thickness, in. (cm)
Inner wall temperature versus insulation thickness.
Table 2. Calculated Wall Thickness for
Assumed Design Conditions of
1200°F (649°C) Inside Wall
Temperature, and 720°f
(382°C) Outside Wall Tempera-
ture for Various Refractory Ma-
terials
Material
Bricks
MDFB
K23 IFB
Silica
Mullite
Alumina (99/->
Silicon Carbide
Castables
Kast-seta
HW Super Castable"
Wall Thickness
in. (cm)
1.0
0.2
1.3
1.6
3.2
12.5
0.5
0.8
(2.54)
(0.51)
(3.301
(4.06)
(8.13)
(31.75)
(1.27)
(2.03)
8A. P. Green Company
bHarbison Walker Company
Table 3. Total Installed Cost (TIC) Ratios
Per Unit of Surface Area for Var-
ious Refractory Materials
TIC
Material Ratio
Bricks
MDFB
K23 IFB
Silica
Mullite
Alumina (99/-)
Silicon Carbide
Castables
Kast-set'
HW Super Castableb
1.0
0.5
1.4
8.8
69.8
438.5
0.6
1.0
M.P. Green Company
^Harbison Walker Company
-------
K. Fuentes and L. Hodas are with Radian Corp., Austin, TX 78766.
Michael C. Osborne is the EPA Project Officer (see below).
The complete report, entitled "Feasibility Study of Enhanced Combustion Via
Improved Wood Stove Firebox Design," (Order No. PB 86-121 373/AS; Cost:
$11.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-85/043
0000129 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
------- |