EPA 340/1-76-002
FEBRUARY 1976
Stationary Source Enforcement Series
EMISSION CONTROL AND
ALTERNATIVES
COMBUSTION OF WOOD
RESIDUE IN CONICAL
WIGWAM BURNERS
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Enforcement
Office of General Enforcement
Washington, D.C. 20460
-------
COMBUSTION OF WOOD RESIDUE
IN CONICAL (WIGWAM) BURNERS,
EMISSION CONTROLS AND ALTERNATIVES
Contract No. 68-01-3150
Task No. 5
Technical Service Area No. 4
PN-3210-4-E
EPA Project Officer: James E. Casey
Prepared for
U.S. Environmental Protection Agency
Division of Stationary Source Enforcement
Washington, D.C. 20460
October, 1975
Ubran?
-------
This report was furnished to the U.S. Environmental
Protection Agency by PEDCo-Environmental Specialists, Inc.,
Cincinnati, Ohio, in fulfillment of Contract No. 68-01-3150
(Task No. 5). The contents of this report are reproduced
herein as received from the contractor. The opinions,
findings, and conclusions expressed are those of the author
and not necessarily those of the Environmental Protection
Agency.
11
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ACKNOWLEDGMENT
This report was prepared for the U.S. Environmental
Protection Agency by PEDCo-Environraental Specialists, Inc.,
Cincinnati, Ohio. Mr. George Jutze was the PEDCo Project
Manager. The principal author of the report was Dr.
Richard W. Boubel, Oregon State University, Corvallis,
Oregon. Mr. Steve Walsh assisted in preparation of the final
draft.
Mr. James E. Casey was the Project Officer for the
U.S. Environmental Protection Agency. The authors appreciate
the assistance and cooperation extended by Mr. Casey, and
staff members of EPA Regional Office VIII and Oregon State
University.
111
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TABLE OF CONTENTS
Page
1. INTRODUCTION 1-1
The Wood Residue Problem 1-1
History of Wood Waste Burners 1_2
Enforcement Actions 1_4
2. COMBUSTION IN WIGWAM BURNERS 2-1
Properties of Wood as a Fuel 2-1
Wigwam Burner Design 2-5
Wigwam Burner Construction 2-13
3. WIGWAM BURNER OPERATION 3_1
Operating Log 3_1
Startup 3-2
Fuel Charging 3-4
Shutdown 3_5
Cleaning and Maintenance 3_5
4. ATMOSPHERIC EMISSIONS FROM WIGWAM BURNERS 4-1
Regulations 4_1
Theoretical Emissions 4-3
Measured Emissions 4-4
Opacity 4_g
Operation to Minimize Emissions 4-10
IV
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TABLE OF CONTENTS (continued)
Page
5. WIGWAM BURNER MODIFICATIONS 5-1
Physical Modifications 5-1
Fuel Modifications 5-3
Air Systems Modifications 5-4
Systems Monitoring and Operating Modifications 5-5
Gas Cleaning Equipment 5-7
Other Modifications 5-7
Costs and Schedules for Modifications 5-9
6. EMISSIONS FROM MODIFIED WIGWAM BURNERS 6-1
Well Controlled Burners 6-1
7. ALTERNATIVES TO WIGWAM BURNERS 7-1
Residue Classification and Separation 7-2
Landfill 7-4
Soil Additives 7-4
Convenience Fuels 7-5
Wood Fiber Usage 7-6
Chemical Extractives 7-7
Alternative Incinerators 7-7
Direct Energy Production 7-9
Conversion to Other Fuels 7-10
8. COSTS OF ALTERNATIVES 8-1
v
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TABLE OF CONTENTS (continued)
Page
References
Appendices
Appendix A A-l
Oregon State Sanitary Authority Regulations:
Construction and Operation of Wigwam Waste
Burners
VI
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LIST OF FIGURES
Figure Page
2-1 Typical Tepee Waste Burner 2-4
2-2 Correlation Between Temperature and 2-6 ^
Excess Air
2-3 Correlation Between C0_ and Excess Air 2-7
for Douglas Fir
2-4 Relationship Between Smoke and Excess Air 2-8
2-5 Burner Sizing Graph 2-10
2-6 Installation Drawing; Thermocouple with 2-16
. Indicating Pyrometer
3-1 Format for Burner Operating Log 3-3
6-1 Particulate Loading as a Function of Exit 6-5
Temperature, for Modified Wigwam Burner
with Dirty Fuel
VII
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LIST OF TABLES
Table Page
4-1 Particulate Emissions from 19 Wigwam Waste 4-6
Burners in Oregon, 1968
4-2 Average Gaseous Emissions from Wigwam 4-8
Burners
4-3 Approximate Equivalent Values of Opacity 4-9
and Grain Loading for Wood Fired Combustion
Sources
4-4 Relationship Between Opacity and Operating 4-10
Parameters of a Wigwam Burner
5-1 Wigwam Burner Modifications, Cost Estimates, 5-10
and Delivery Schedules, 1975
6-1 Modified Wigwam Burner Test Results 6-3
7-1 Classification and Separation Unit Costs, 7-3
1975
8-1 Costs, Values Recovered, Time Schedules, and 8-1
Pollutional Potential for Alternatives to
Wigwam Burners
VI11
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1. INTRODUCTION
THE WOOD RESIDUE PROBLEM
The lumber and plywood manufacturing processes generate
large quantities of wood residue or waste material. Some
of this residue^or waste material is converted to useful by-
products such as chips for pulp, particle board, Presto
logs, and even heat which is utilized for electrical power
or steam. The remaining residue may be incinerated at the
mill site in a tepee-shaped, single-walled, steel waste
burner. The amount and type of residue fed to the waste
burner depends upon the practices of the particular mill,
species of log being processed, time of year, and mill
location with respect to by-product markets. All of these
factors are variable so the fuel fed to the burner may be
expected to also vary widely with time.
Since only approximately half the log which enters the
mill is converted to lumber or plywood, the volume of
residue produced nearly equals the volume of the primary
product produced by the mill. While many mills utilize all
of their residue for by-products, other mills incinerate all
of it. Therefore, it can be stated that utilization of
wood residue varies from 0 to 100 percent. Incineration
in wigwam waste burners is the usual method of wood waste
disposal when utilization is something less than 100
*. !'2
percent.
Wigwam waste burners at some operations have done a
creditable job. Waste products delivered to the burners
1-1
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have been consumed with only a minimum of smoke and cinders
(unburned material) issuing from the top with the exit
gases. At other operations, the great quantities of smoke
produced have caused severe air pollution problems and
hazardous visibility conditions for automobile and air
travel. Cinders ejected not only have created a nuisance
to owners of property in surrounding areas, but also have
constituted a fire hazard.
f
HISTORY OF WOOD WASTE BURNERS
During the first 20 years of this century, many types
of waste burners were designed. Some were designed for
complete combustion, some for low initial cost, and others
for low maintenance. Most accounts described design and
construction of new burners and predicted results, but
actual success of operation or economy of maintenance of
burners which had been in operation an appreciable length
of time was seldom mentioned.
The steel-jacketed, brick-lined, cylindrical burner
was perhaps the most common type. A shell made from steel
plates was lined with common brick and firebrick in the
following manner: two courses of common brick and 1 course
of firebrick from the base upward for 15 feet; 1 course of
common and 1 of firebrick from 15 feet to 40 feet; 1 course
of firebrick from 40 feet to 75 feet; and 1 course of
common brick above 75 feet. Foundations were made of brick
or concrete, and often consisted of a central core several
feet in diameter and an outer base on which the burner
rested. There were grates between core and base. Fuel
dropped from about 40 feet onto the central core. The top
of the burner was covered with a 3 by 3 mesh, 14-gauge wire
1-2
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screen. The base area was from 3.5 to 5.5 square feet per
1000 board feet of mill output. Exteriors were often
painted or tarred to prevent corrosion. Maintenance costs
were high because the brick-work had to be replaced
annually.
Some steel, water-cooled burners were used which had
no brick lining but, instead, had a watertight steel
jacket surrounding the inner shell, which was from 12 to
15 inches thick. These burners were constructed in all
sizes up to 50 feet in diameter and 115 feet in height.
Paint and asphalt were used to protect the outer surface.
Very few were built, however, because of high construction
and maintenance costs.
Open pit fires were used then, as they sometimes are
today. These consisted of a semicircular screen or wall rr
rising 20 to 30 feet on the side of the fire toward the
mill.
Brick shell burners were cylindrical and similar to
brick-lined shell burners, except that they were shorter.
Steel straps were placed around them for support.
An air-cooled burner was placed on the market in 1916.
It had a conical base and a cylindrical stack without any
brick lining. The foundation was a concrete wall 1 foot
thick and extending 2 feet above the grate level. The
framework was made of structural steel and iron pipe, with
an outside covering of medium-weight steel plates riveted
together. The conical shape placed the base of the burner
farther away from the fire, and air circulation cooled it.
These burners cost 40 to 50 percent less than brick-lined
steel burners.
This was the beginning of the tepee burner commonly in
1-3
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use today. Most builders have abandoned the cylindrical
stack and now construct burners which are conical from
base to screen.
Most tepee burners have two screens at the top - a
flat, horizontal screen and a hemispherical one. Tangen-
tial openings at the base to supply overfire air to the
fire have been used for many years. Underfire air is
usually supplied by one or two blowers to cones or burner
grates. Some burners are elevated to provide tunnels
below the grate level to admit underfire air.
The practice of building prefabricated burners has
been developed since 1946. These are built in sections,
running from the base to the screens. The structural
framework is on the outside, with the plates on the inside.
The burners are raised into place on location and bolted
together in a short time. They can be dismantled easily,
transported, and re-erected at another location.
ENFORCEMENT ACTIONS
Early enforcement actions were taken by fire insurance
underwriting agencies which specified design and operation
to minimize burning cinder discharge. These regulations
were concerned with screen size openings, burner sizing,
fuel delivery rates, and maintenance of the burner.
Air pollution regulations appeared during the early
1960"s. Most of the first regulations were simplistic
approaches relating to dustfall downwind from burners and
control of the burners to minimize visual smoke. From the
mid 1960's until the early 1970's many state and regional
air pollution control agencies adopted regulations covering
burner design and construction to minimize particulate
1-4
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emissions, particulate emission standards (grains per cubic
foot), and opacity regulations based on a modified Ringel-
mann scale (0 - 100% opacity). No Federal Standards con-
cerning emissions have been promulgated nor have any regu-
lations been adapted for gaseous emissions.
Enforcement actions based on non-compliance of emission
or design standards have been upheld in most cases to date.
Many burners have been eliminated as a result of these
enforcement actions and the mills concerned have converted
to alternative means of residue disposal.
1-5
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2. COMBUSTION IN WIGWAM BURNERS
PROPERTIES OF WOOD AS A FUEL
Wood is probably man's oldest fuel and the combustion
of wood probably man's first attempt to use a chemical
process for his betterment. Combustion is defined as the
union of a substance with oxygen accompanied by the evolu-
tion of heat and light. Even though scientists do not
completely understand all the mechanisms of combustion,
they can use the combustion reaction to their advantage.
Combustion may be used to produce energy, as in an automo-
bile or a steam generator, or as a destructive reaction to
eliminate an unusable material. The latter is the case in
the waste burner.
Ultimate Analysis*
Wood can be a widely varying fuel with different
physical and chemical properties, depending upon the
species, age, location, etc. A chemical analysis for dry
Douglas-fir would indicate the following percentages of
material:
Hydrogen 6.3%
Carbon 52.3%
Nitrogen 0.1%
Oxygen 40.5%
Ash 0.8%
Such an analysis is called an Ultimate Analysis. All non-
combustibles are lumped together as ash.
*
For a complete discussion of fuel analysis, see:
Mingle, J.G., and R.W. Boubel. Proximate Analysis of Some
Western Wood and Bark. Wood Science. !_:!• PP 29-36.
July, 1968.
2-1
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Proximate Analysis
Another type of analysis used by combustion engineers,
is the Proximate Analysis. This analysis indicates how
the fuel will be burned. The Proximate Analysis for dry
Douglas-fir would be:
Volatile Matter 82.0%
Fixed Carbon 17.2%
Ash 0.8%
The heating values of wood will vary considerably.-:
Dry Douglas-fir has a heating value of 9,050 BTU per pound.
This is only about one-half the heating value of petroleum
products. The main reason it is lower is the high oxygen
content which in this respect dilutes the heating value of
the wood.
Steps in Wood Combustion
The steps in wood combustion are rather specific and
well defined. Assume a pile of fuel, such as in a tepee
burner; fresh fuel falls on the top of the pile where it'
is dried as the moisture is driven off. This is an endo-
thermic process in that it requires heat.
The volatiles are next distilled from the wood.
These may be combustible gases (hydrocarbons) or non-
combustibles (oxygen and nitrogen). The process is endo-
thermic because it requires heat for the distillation and
also exothermic because the volatile gases are burned.
This combustion takes place above the fuel pile where
sufficient oxygen is available. The combustion reactions
of interest are: (1) C + 07 + CO and (2), 2H_ + O~ -> 2H0O.
£ £ 2, 2* 2
After all the volatile matter has been distilled,
all that remains is the fixed carbon. This is the material
of which briquets, which are used in home barbecues, are
2-2
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made. This fixed carbon is burned in the fuel pile if
sufficient oxygen is available (C + 02 + CO2). The heat
is all released within the fuel pile if the combustion ?is
complete. If not enough oxygen' from the air is available,
the reaction in the pile is: 2C + O2 •*• 2 CO, and only a por-
tion of the heat is released within the pile. The remainder
of the reaction takes place over the pile where sufficient
oxygen is available: 2 CO + 0» -> 2 C09. Additional heat is
£* £•
released in this reaction.
The remaining material that is left after combustion is
the ash. This collects at the base of the pile and must be
periodically disposed of.
Combustion in a Tepee Burner
When we analyze the combustion in a waste burner, we
have simply enclosed the open fuel pile within a shell.
Figure 2-1 illustrates such a situation.
To dry the incoming fuel, the combustion products H~0
and CO9 may be used. These were generated from the combus-
^£
tion of H2 and C. 02 and N2, which were forced through the
hot fuel pile by the forced-draft system, help dry the fuel.
Radiant heat from the shell will also help to dry the fuel.
The radiant heat is a function of the absolute temperature
squared, so a cool shell doesn't help dry much fuel.
To distill the volatiles, the hot C02 and CO are avail-
able from the combustion of the fixed carbon below. The
hot 09 and N9, which were forced through the burning bed,
£, £
are available as heat sources. Again the radiant heat from
the shell is available.
The fixed carbon is burned in the pile. If enough
forced-draft air is supplied, it burns to C02 in the pile.
2-3
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THERMOCOUPLE
PYROMETER
OVERFIRE
AIR INLET
FORCED DRAFT UNDERFIRE AIR SYSTEM
Figure 2-1. Typical tepee waste burner.
2-4
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Any CO generated burns above the pile.
Of course the ashes accumulate at the bottom and must
be periodically removed to keep the forced-draft system
operative.
It is apparent that once sufficient oxygen is supplied
to complete the combustion, additional oxygen (and its asso-
ciated nitrogen) will only tend to cool the reacting pro-
ducts and the exhaust gases. Air greater than theoretical
is termed "excess air." Test data from several waste
burners indicate that exhaust-gas temperatures may be re-
lated to excess air as shown in Figure 2-2. Because of this
relationship a good indication of excess air may be obtained
if the exhaust-gas temperature of the burner is known. The
usual procedure for determining the amount of excess air
from a combustion process is to take an exhaust-gas analysis,
and from the fuel analysis and gas analysis calculate the
excess air. For Douglas-fir such a calculation yields a
curve as shown in Figure 2-3.
It has been generally found from field observations
that if tepee burners can be operated so that the tempera-
ture of the gases leaving the top.of the burner are greater
than 600°F, emissions of smoke and other particulates will
be minimized. A maximum temperature of 900°F is recommended,
which leaves a satisfactory margin of safety before struc-
tural damage occurs. A summary of several field observations
of smoke and exit-gas temperatures is shown in Figure 2-4.
WIGWAM BURNER DESIGN
The size of a burner required to consume a given
quantity of waste is fairly critical. Too large a burner
will operate at a low temperature and smoke severely,
2-5
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1200
I I 1 T
1000
800
600
400
200
400 800 1200 1600
PERCENT EXCESS AIR
2000
Figure 2-2. Correlation between temperature and
excess air.
2-6
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CD
C\J
o
O
o
ct:
0
400 800 1200
PERCENT EXCESS AIR
1600
2000
Figure 2-3. Correlation between C09 and excess air
3
for Douglas-fir.
2-7
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g 100
o
QL
UJ
c/>
CO
o
^
o
2:
i/o
—i
<
o
o
CtL
80
60
40
NONE
••iS'&rtl-s^
"'>' r';^
2000 3000
2000
PERCENT EXCESS AIR
Figure 2-4.
Relationship between smoke and
excess air.
2-8
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while too small a burner will emit burning material. The
correct size of a burner may be determined from the equa-
tion:
D = 2.3Q1/3 (1)
where;
D = diameter of base and height, ft
Q = quantity of waste burned, Ib per hr.
Figure 2-5 is a graph of this sizing equation.
Example
An example of a typical burner design problem is the
best way to illustrate the necessary calculations. For
an example mill, assume the following as the necessary
design factors:
Species - Douglas-fir (50% moisture and 50% dry wood)
Amount of Waste - 25,000 Ib/hr (wet)
Desired Excess Air - 500% (which corresponds with
650°F exit-gas temperature)
Sizing - The sizing curve, Figure 2-5, indicates that for
12.5 tons per hour of wet fuel a 65-foot burner will
be needed. The sizing equation verifies this:
D - (2.3) ^25,000
= (2.3) (29.25)
= 67.2 ft
Air Supply - The air supply to the burner should be
calculated so that sufficient forced-draft air is
supplied to burn the fixed carbon. All other air,
including the excess, should be supplied over the
fire to burn the volatile gases and cool the exhaust
products.
2-9
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CO
S-
c
o
-M
UJ
20 40 60 80
BURNER HEIGHT AND BASE DIAMETER, ft
0
100
Figure 2-5. Burner sizing graph.
2-10
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Air for Forced Draft - Each pound of dry wood contains
0.17 pounds of fixed carbon: C + 0_ -> CO- , so each 12
pounds of carbon requires 32 pounds of oxygen for com-
plete combustion.
lbC x 32 lb °2 x 10° lb = 1.97 Ib air
Ib fuel 12 lb C 23 lb 02 lb fuel
A 50 percent overload capacity for the forced-draft
system should be provided, so:
(150%) 1.97 lb air 2.95 lb air
lb fuel lb fuel
Calculating this volume at the fan:
2.95 lb air ft3 air 12,500 lb dry fuel hr _ f
lb fuel X0.075 lb airx hr X60 min 8'2°° C±m
Air for Overfire - Carbon in volatile matter needs
air:
0.52 lb C 0.17 lb fixed C 0.35 lb volatile C
lb fuel ~ lb fuel lb fuel
The theoretical air required is :
0.35:lb C 32 lb 00 100 lb air 4.06 lb air
v - 3- v =
X £ X
lb fuel 12 lb C 23 lb 02 lb fuel
Hydrogen in volatile matter needs air. The theoretical
air required is: 2H~ + 0 ^ ->• 2 H^O, so each 4 pounds of
hydrogen requires 32 pounds of oxygen for combustion.
0.06 lb H 32 lb 0- 100 lb air _ 2.09 lb air
lb fuel 4 lb H2 23 lb 02 lb fuel
The total overfire air for theoretical combustion is
the amount for the carbon plus the amount for the hydrogen
minus the amount which the oxygen in the fuel can supply.
In other words, the air needed can be reduced by the amount
of oxygen in the fuel plus the associated amount of nitro-
gen. The reduction in air because of oxygen in the fuel is:
2-11
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0.405 Ib 00 100 Ib air _ 1.76 Ib air
£., X.
Ib fuel 23 Ib C>2 Ib fuel
Theoretical overfire air is therefore:
Air for C + Air for H2 - Air replaced by G>2 =
4.06 Ib air 2.09 Ib air 1.76 Ib air _ 4.39 Ib air
Ib fuel Ib fuel ~ Ib fuel Ib fuel
Five hundred percent excess air means that we must
supply six times the theoretical so:
... . _ .,. 4.39 Ib air 26.34 Ib air
Overfire air- (6) lb fuel - Ib fuel
The volume of overfire air is:
26.34 lb air ft3 air 12,500 lb dry fuel hr _-, ._. f
lb fuel X0.075 lb air x hr X60 min /J'1/U
To size the overfier-air openings, the draft must be
calculated. At 650°F the exit gases have a weight com-
pared to the surrounding air of:
70 + 46° or 47.7%
650 + 460
In a 65-foot burner the draft produced would be:
(1.000 - 0.477) 65 ft - 34 ft of air
(A draft gauge at the base of the burner would read
this as about 1/2 inch of water.)
The velocity through the overfire-air openings pro-
duced by this draft would be:
V = /2gh = /(64.4) ft34 ft = 46.8 fps
2 X
sec
The area of the overfire-air openings would be:
= 73,170 ft3 sec min ,fi -2
min 46.8 ftX 60 sec
2-12
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Assuming a 50 percent oversize to take care of any
overload, the overfire-air openings would have to have a
2
combined Volume of 39 ft . This could be accomplished,
for example, by using ten 2-foot by 2-foot openings.
The overfire-air openings should be of the tangential
type, with dampers for the control of the air volume
passing through them. The damper design should be such
that it does not interfere with the cyclonic action in-
duced by the tangential openings when the dampers are
partially closed.
The air supply for the burner would be 8,200 cfm
forced draft, or underfire air, and 73,170 cfm overfire.
This breaks down to a total of 81,370 cfm of which 10 per-
cent is supplied by the forced-draft system. Two 15-
horsepower centrifugal fans would adequately supply the
forced-draft requirements.
Propeller fans are not recommended for forced-draft
systems on waste burners. Propeller fans are designed
for high-volume flows at low-static pressures, and the
forced-draft system will be easily plugged if they are
used.
WIGWAM BURNER CONSTRUCTION
General
Most construction details have been standardized by
the industry. A review of some, however, seems appropriate,
All structural members should be external to the shell.
This will enable them to carry the load and be shielded
from the heat within the burner. Some builders are using
annular trusses, particularly at the top of the burner.
At high operating temperatures the truss is not weakened
2-13
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and satisfactorily supports the shell plates.
Adequate doors and other provisions must be made for
cleaning the burner. Doors should be sized so that a
loader, or similar vehicle, can enter the burner. The
forced-draft system must be designed to carry the weight
of the loader. If projections, such as cones or elbows,
are used in the forced-draft system, they should be removable
or protected during the cleaning operation.
The fuel should be admitted as low in the burner as
possible. This can be aided by using water-cooled conveyor
bearings which allow for an overhang of the conveyor system.
Discharge pipes from air-cyclone conveying systems should
deposit their planer shavings, sawdust, etc., as low as
possible to permit combustion of these small bits of fuel
rather than entrainment in the exit gases.
Draft Systems
The forced-draft system should be designed to give an even
air distribution throughout the entire fuel pile. A satis-
factory system consists of one or more blowers ducted to
the grates in the burner floor. The grates may be flat
grids or plates with perforations, conical (bee-hive) type
grates, or elbows or discharge boxes designed to impart a
tangential flow to the inlet air if they become uncovered.
All of the forementioned grate types have operated satis-
factorily when properly sized.
Dampers should be provided in the forced-draft system
to throttle the flow of air under startup and light load
conditions. These may be either at the fan inlet or out-
let, but they should be equipped with some type of position
indicator so that settings may be consistent once proper
operation is established.
2-14
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The overfire-air system should contain suitable dampers
at each port. Barometric dampers have been used, but they
require delicate adjustments which tend to change as the
bearings and shafts weather and corrode. Most satisfactory
systems are manually adjustable with the ports and dampers
arranged so that they are not damaged by falling slabs,
edgings, or other fuel.
Thermocouple
A very necessary and inexpensive part of the waste
burner is a thermocouple installed at the top to indicate
exit-gas temperature. This permits a burner to be operated
at optimum conditions for disposal of waste with a minimum
of atmospheric pollution. Figure 2-6 shows a very satis-
factory arrangement for such a thermocouple system which
can be installed for less than $500. The thermocouple is
relatively trouble free and with normal maintenance will
outlast the burner itself.
2-15
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NOTE: PYROMETER INSTALLED IN WEATHERPROOF
HOUSING FURNISHED BY OWNER. LOCATE
AT EYE LEVEL (APPROX. 5 ft) AT
LEAST 15 ft. FROM BURNER SHELL.
THERMOCOUPLE - INSTALL 90°
FROM CONVEYOR OPENING
LEAD WIRE AS
REQUIRED
PYROMETER
Figure 2-6.
Installation drawing; thermocouple with
indicating pyrometer.
2-16
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3. WIGWAM BURNER OPERATION
The correct firing of a tepee burner becomes both an
art and a science. A properly operated burner will dispose
of the residue with a minimum of smoke and other pollutants,
A poorly fired burner will smoke and deposit particulate
over a wide area even though it may be properly sized and
designed. One of the most common mistakes is to fire the
burner with the access door open. This severely upsets
the draft balance and air distribution in a properly de-
signed burner. The fire may appear to burn more rapidly
for a short period of time because the fuel pile glows
more. This is due to the increased "forge effect" on the
fixed carbon. Fixed carbon is only about one-seventh of
the fuel by weight. Firing with the door open, in order to
get apparently better combustion for a relatively small per-
centage of the fuel, actually chills the volatile gases and
hinders their combustion. If the forced-draft system was
adequate and properly operated in the first place, opening
the door would not aid the combustion of the fixed carbon.
An open access door on an operating tepee burner is a
glaring indication that something is improper with the de-
sign, loading, or operation of the burner.
OPERATING LOG
A written log is necessary for proper burner operation,
Only if entries concerning draft settings, gas temperature,
smoke, fuel, etc., are faithfully made will the operator
3-1
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know the optimum firing situation. If it is noted in the
log that a certain series of settings gave smokeless opera-
tion for a particular fuel load and type, then the settings*
can be used for future firings. If no written record is
available, the proper settings are quickly forgotten. A
suggested form for the written log is given on the following
page. It can be modified for any particular burner or
mill. '•
STARTUP
Probably the most difficult period of burner operation
is during the startup. The residue starts coming from the
mill and the burner is expected to handle it. Unless the
burner is previously heated and contains a good fire, this
fresh residue will tend to extinguish the fire rather than
increase the rate of combustion.
An adequate fire must be started in the burner well
before the first residue is sent to it. An hour before the
start of production is a good time to get the burner
operating. Dry planer ends, slabs, edgings, etc., should
be accumulated from the previous day's operation and avail-
able to build a satisfactory fire in the burner. The fire
should be built and started with the overfire-draft doors
closed and the underfire air set very low. A good-sized
preliminary fire should be the goal by startup time of the
mill. Once the residue starts entering the burner from the
mill, the fire will sustain itself only if proper control
of the air supply is exercised. The general tendency is
toward too much overfire air, which only tends to excessively
chill the fresh fuel. Cold air does a poor job of drying
wood fuel. The overfire air should be kept from the fire
3-2
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BURNER LOG
Date.
Operator.
Name of Co._
Address
Burner size.
Est. hourly production
Est. hourly waste to burner_
Species and type of waste
Time
Overf ire
draft
setting
Forced
draft
setting
Exit
gas
temp.
Smoke
density
and
color
Fallout
emission
Weather
Wind
direction
and
velocity
-"
-
Auxiliary
fuel
used
Remarks
CO
I
00
Figure 3-1. Format for burner operating log.
-------
until the fire is self-sustaining, and then the draft
settings may be opened to their normal operating positions.
Many state and regional air pollution control agencies
have recognized that wigwam burners will smoke so much
during the startup period that they cannot meet opacity
regulations. To permit the burners to operate most regu-
lations are written with a clause allowing a deviation
from opacity standards for the first 1/2 hour or hour of
burner operation. If only 1/2 hour is allowed the operator
must build an extremely hot fire before the normal fuel
flow starts. This is usually done using dry wood plus an
auxiliary fuel such as gas or diesel oil. If one hour is
allowed for startup it is usually possible to get an adequate
fire established using only dry wood at startup.*
FUEL CHARGING
Combustion is properly established when the fuel is
consumed at the same rate it enters the burner. The opera-
tor should adjust the burner so that the exit-gas tempera-
ture is between 600°F and 900°F and smoke and particulate
are at a minimum. A good clue to overall burner operation
is smoke. Since smoke is particulate, it indicates how
well the fire is consuming the residue material. With "no
smoke" you can be sure that the burner is doing its best
job and that a minimum of pollutants and particulate are
leaving the top of the burner.
* Actual regulations and startup times should be obtained
from the appropriate State Implementation Plan (SIP)
covering the specific burner.
3-4
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Slight adjustments in burner operation may have to be
made even during periods of apparently continuous operation.
Waste quantities will vary or even stop completely during
breaks and lunch hour. Weather or wind changes will affect
the combustion. ,The fireman should be continually aware .
of the situation and make small corrections to the draft
settings as required.
SHUTDOWN
Another critical period in burner operation is when
production stops for the day. If the burner has been pro-
perly operated, only a normal fuel pile will exist inside.
About one half hour after the mill stops this will be re-
duced to practically 100 percent fixed carbon, and the
overfire draft doors should be closed. Only after the fixed
carbon has been consumed may the forced-draft blowers be
shut off.
If the burner was not operating properly during the
period of mill production, an extremely large fuel pile may
have accumulated by the end of the working day. A fireman
must remain in attendance at the burner until this large
pile is consumed. Many burners have been badly damaged
because a large fuel pile fell against the side of the
burner and the excessive heat and lack of cooling buckled
the structure and shell.
CLEANING AND MAINTENANCE
Care of the burner is another important factor for
proper firing with a minimum of pollution. The burner
should be completely cleaned of ashes at least once a week.
A thorough inspection should accompany this cleaning and
3-5
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any faults or defects reported to the millwright or main-
tenance superintendent. Leaks in the shell and warped doors
are more easily spotted from inside the burner than from
outside. If repairs are needed, they should be made imme-
diately to put the burner back into proper operating condi-
tion. Remember that the burner costs the mill owners
somewhere around $10,000 per year. It deserves to be
treated with care.
3-6
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4. ATMOSPHERIC EMISSIONS FROM WIGWAM BURNERS
REGULATIONS
Regulations covering emissions from wigwam burners are
concerned with only particulate matter and/or visual
emissions. No regulations are known which apply to gaseous
pollutant emissions from wigwam burners. Some states and
regions have design and construction standards which apply -
Regulations for Particulate Emissions
Particulate regulations can be expressed in several
ways:
1. Mass emission per cubic volume of exhaust gas,
usually normalized at 12% CO- to account for dilution
by excess air. Many agencies have adopted a regula-
tion of 0.2 grains per standard cubic foot of gas
corrected to 12% CO- for existing burners and 0.1
grains per standard cubic foot of gas corrected to
12% CO2 for new construction of burners after a cer-
tain date. Some of the regulations state that the
standard cubic foot is "dry", meaning the water volume
present in the gas phase must be subtracted. Other
regulations may not state whether the cubic foot is
wet or dry. The "standard" cubic foot may also be
ambiguous. The "standard" temperature for a cubic
foot depends upon regulation and may be 32°F, 60°F,
68°F, 70°F, or 20°C, which is equivalent to 68°F.
The "standard" pressure for the same cubic foot may
4-1
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be expressed as 29.92 inches of mercury which is the
same as 14.7 pounds per square inch or one atmosphere.
Some agencies, however, use 30.00 inches of mercury
for the standard cubic foot.
2. Mass emission per unit of energy input to overcome
the problems of defining the standard cubic foot. The
usual emission standard is 0.1 pounds of particulate
per million BTU of input energy- Some agencies may
allow a higher value (0.2 pounds per million BTU) for
burners existing before a certain date.
3. Mass emission per unit of process weight, usually
included with the allowable mass emission for the
entire mill. If a process weight chart (such as the
Bay Area APCD chart) shows an allowable atmospheric
discharge the wigwam emission is included along with
emissions from cyclones, driers, etc. , to determine
the total. If the wigwam burner is the only source
at the mill it can emit particulate up to the maximum
allowed by the process weight chart and still be legal,
Regulation of wigwam burners by means of visual
emission standards usually refers to the opacity of the
effluent from the burner. A certified observer must read
the opacity periodically and determine if the burner is in
compliance or not. Typical regulations may state, "No
visual emissions exceeding 10% opacity will be permitted
except for 3 minutes in any one hour •" Some agencies will
use 20% opacity for existing burners and 10% opacity for
new burners. The time exemption may be differently stated
in the regulations to clarify it or further define it.
The observer making the opacity readings must be
trained and certified by the control agency. He must be
4-2
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aware that opacity readings can be affected by such things
as stack exit diameter, moisture content of the plume,
particulate size, particulate shape and color, background
lighting and textures, sun position and angle, sky color,
etc.
The Oregon State Sanitary Authority* was one control
agency which chose to adopt Construction and Operation
Standards rather than emission standards (Appendix A).
These standards, adopted in 1965, are typical of similar
standards adopted by other agencies.
THEORETICAL EMISSIONS
Particulate Emissions
The particulate emissions from a wigwam waste burner
may be theoretically approximated. The example problem
used earlier will be used for calculation purposes:
Fuel - Douglas Fir; 50% moisture, 0.8% ash
12,500 pounds of dry fuel per hour,
Heating Value 9,050 BTU per dry pound.
Excess Air - 500% for 650°F exit temp.
The air supply was calculated at 81,370 standard cfm
total which yields exhaust gas of 27.34 pounds per pound of
fuel. If we assume that one pound of exhaust gas occupies
the same volume as one pound of standard air, we can calcu-
late the following value:
scfm of _ 27.34 Ib gas/lb fuel (81,370 cfm)_
exhaust gas 26.34 Ib air/lb fuel «4,4bu cm
* Presently the Oregon State Department of Environmental
Quality.
4-3
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Assuming that one half the ash in the fuel goes; out the
top of the burner as fly-ash and that the fly-ash is usually
50% ash and 50% unburned carbon:
particulate emission = (12,500 Ib fuel/hr) - — lb S
\
iff
ash
On a basis of mass emission per energy input:
100 Ib. partic.\ [ hour _ \ /Ib fuel \= 0.88 pounds
hour Ml2,500 Ib fuel! (9050 BTUJ per million
/ \ / \ / BTU
Or on a basis of grain loading:
[100 Ib partic.\ /7000 grain\ / min J / hour
1 hour J I pound ) 184,460 ft I 1,60 min.
0.138 grains per scf (not corrected)
Since 500% excess air corresponds with 3 1/2% CO the
corrected grain loading is:
0.138 grains\ /1 2. 0\ „.-,., . i. j j
3 - — o — F- = 0.473 grains per standard cubic
/ \ / foot corrected to 12% C0
From the previous theoretical calculations it becomes
quite obvious that in order to meet an emission standard
of either 0.1 pounds per million BTU of input or 0.1 grains
per standard cubic foot corrected to 12% C0«, some control
must be considered.
MEASURED EMISSIONS
Particulate Emissions
Actual emissions from wigwam burners have been found
4-4
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to be of the same order of magnitude as the previously cal-
culated theoretical emissions. On a well designed and
operated burner, firing a clean fuel at a rate high enough
'to operate in the 600°F to 900°F range, the actual emissions
may be as low as 0.1 grains per standard cubic foot cor-
rected to 12% C02. On a poorly maintained burner, firing
a finely divided and dirty fuel (such as bark coated with
soil), at a low or intermittent fuel feeding rate, the
actual emissions may be greater than 1.0 grains per standard
cubic foot corrected to 12% CO_.
£-•
Tests of 100 samples from 19 different wigwam burners
in Oregon, taken in 1968,vare summarized in Table 4-1.
The average emission temperature was 485°F which is
considerably below the 600°F - 900°F temperature range
recommended for smoke-free operation.
The average loading to the atmosphere was 0.168 grains
per cubic foot of gas corrected to 12% CO- and standard
temperature (60°F) and pressure (30.00 inches of mercury).
This value is considerably below the value used by many con-
trol agencies of 0.2 grains per cubic foot for allowable
incinerator emissions. Converted to metric units, the
average particulate emission is 384 mg/m (corrected to
12% CO- and STP). If the air/fuel ratio for a typical wood
is assumed (12% CO,, is approximately equivalent to 9.5
pounds of air per pound of fuel) the average emission can
be calculated as 10.7 pounds of particulate per ton of fuel
consumed. Possibly it would be easier to remember, as well
as being simpler, if it were rounded to 11 pounds per ton.
4-5
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Table 4-1. PARTICULATE EMISSIONS FROM 19 WIGWAM
WASTE BURNERS IN OREGON, 1968
Burner
number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Overall
average
Average
gas temp.
o F
389
539
400
455
291
544
525
598
866
435
405
379
338
208
166
519
791
230
308
485
Particulate
emissions, 3
grain/ft
0.171
0.105
0.080
0.120
0.312
0.155
0.129
0.224
0.130
0.284
0.191
0.163
0.252
0.194
0.132
0.021
0.128
0.160
0.252
0.168
Two distinct size distributions were noted upon micro-
scopic examination of the collected material. One size
distribution was noted for a larger particulate which was
capable of settling to the ground as dustfall. Another
distribution was noted for the smaller sized particles which
are seen as "smoke" and are referred to as suspended parti-
culate. An average value of about one quarter of the mass
of the particulate would be considered as "smoke" or sus-
pended particulate.
4-6
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A correlation matrix of variables was run to see if
there was a significant relationship between variables
measured during this study. Only three significant corre-
lations were found but they were very interesting as they
indicate how a burner might be operated to reduce air
pollution:
1. The particulate emission correlates inversely
with the emission temperature, i.e., the higher the
temperature, the lower the emissions.
2. The draft ratio (actual/theoretical) correlates
directly with temperature. High temperatures, and
hence lower emissions, are achieved with a tighter
burner (better maintenance and the doors closed).
3. The percent of ash in emissions correlates directly
with temperature. Higher emission temperature indi-
cates more complete combustion with less material to
be emitted as an air pollutant.
The size of the particulate emitted did not correlate
significantly with temperature which would indicate that it
was more a function of the material being fed to the burner
than how the burner was operated.
Gaseous Emissions
Although generally not mentioned in the emission stan-
dards, the gaseous pollutants from a wigwam burner are
significant, greatly exceeding particulate emissions.
One study from California during the mid 1960"s reported
extensive gaseous pollutant analyses which are summarized
in Table 4-2.
The gaseous emissions show the same inverse correlation
with,temperature as noted for particulate emissions. The
4-7
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higher the operating temperature, the lower the gaseous
emissions.
Table 4-2. AVERAGE GASEOUS EMISSIONS FROM
WIGWAM BURNERS
Shell temp,
range, a °F
90 - 150
160 - 200
210 - 250
260 - 300
310 - 350
410 - 450
Average pounds of gaseous pollutant per ton
of wood residue burned
CO
189
176
144
125
78
62
Total hydrocarbon
17.5
15.0
10.6
13.8
4.5
1.4
C~ + hydrocarbon
7.2
4.5
4.7
5.5
1.7
0.8
Shell temperatures are approximately 1/2 of exit gas
temperatures, °F.
Oxides of nitrogen from wigwam burners are not con-
sidered a serious problem because of the low combustion
temperatures involved. It is generally assumed that oxide
of nitrogen emissions are "negligible", even when exit gas
temperatures reach values as high as 900°F.
OPACITY
One would expect opacity to be directly correlated with
particulate emissions. This is true, so whatever affects
particulate emissions will also affect opacity. Opacity may
also be related to other variables of firing conditions or
ambient weather conditions. When firing wet fuel, for
example, as long as the exit gas temperature is below the
dew point, a visible plume will persist due to the condensing
water vapor. Above the exit gas dew point temperature, the
opacity may be expected to vary as particulate emissions.
4-8
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(This may not always be true as some studies show poor
correlation over about 70% relative humidity).
The opacity of the plume of course relates to the size,
shape, and color as well as the mass of the particulate. A
burner emitting 0.1 grains per standard cubic foot with a
size distribution of 0.1 - 0.5 micrometers would have a
high opacity plume while one emitting 1.0 grains per stan-
dard cubic foot with a size distribution of 2.0 - 20.0
micrometers would show a nearly clear plume.
In general, the information in Table 4-3 is a good
rule of thumb for wood burning combustion sources, including
wigwam burners. It should be noted that Table 4-3 relates
to about 50 - 150% excess air so the values of opacity read
from a wigwam burner at higher excess air values should be
lowered accordingly.
Table 4-3. APPROXIMATE EQUIVALENT VALUES OF OPACITY
AND GRAIN LOADING FOR WOOD FIRED COMBUS-
TION SOURCES
Grains
0
0
0
0
per SCF
.00
.10
.20
.50
Opacity, %
0
10
20
50
Figure 2-4, shown earlier in this report, is developed
1 2
from data obtained from over 200 waste burner tests. '
The equivalent opacity values corresponding to the descrip-
tive terms used in this figure are shown in Table 4-4 which
summarizes Figure 2-4.
4-9
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Table 4-4. RELATIONSHIP BETWEEN OPACITY AND OPERATING
PARAMETERS OF A WIGWAM BURNER '
Smoke
description
as per Fig.
2-4.
None
Slight
Moderate
Heavy
Opacity
range ,
%
0-25
25-50
50-75
75-100
Percent of total smoke observations
in specific smoke & opacity ranges
250%
x ' s air
850°F
100
0
0
0
500%
x ' s air
675°F
81
18
1
0
750%
x ' s air
500°F
65
30
4
1
1000%
x ' s air
430°F
52
40
6
2
1500%
x ' s air
325°F
36
46
14
4
2000%
x's air
270°F
25
43
24
8
OPERATION TO MINIMIZE EMISSIONS
Combustion Control
The previous data, tables, and figures indicate that
all emissions, visual, gaseous, and particulate, decrease
as combustion improves within the burner. The exit gas
temperature is probably the best single indicator of com-
bustion conditions and may therefore be relied on as the
variable of interest to the operator. If an operator is
familiar with the combustion characteristics of a given
burner, he can use this information to minimize the atmos-
pheric emissions. Suppose, for example, a certain wood
waste burner log shows that an exit temperature of 600°F
and above results in a clear plume when burning hemlock
trim and bark. If the exit gas temperature is only 400°F
with this fuel, and the burner is smoking, the operator
should cut back on his air or increase the fuel to get to
the higher temperature. Since the fuel feeding rate is
usually fixed, the operator will most likely adjust the air
4-10
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supply.
If on the same fuel the exit gas temperature is 900°F
and the operator is concerned about the structural members
should the temperature increase, he can increase the air-
flow to the burner. He knows he can drop the exit gas
temperature to as low as 600°F without increasing particu-
late or opacity appreciably.
The State of Oregon, Department of Environmental
Quality, has determined that minimum emissions from a
wigwam burner will be obtained if the following conditions
4
are satisfied:
a. That the waste burner size be compatible with the
fuel load.
b. That fuel be correctly introduced at a reasonably
uniform rate and be of such physical characteristics
as not to obstruct passage of underfire air or com-
bustion gases from the heat release within the fuel
pile.
c. That an adequately designed underfire air system
be provided. Such a system must be adjustable, or
sufficient capacity for the maximum rate of fuel
supply, and must introduce air with sufficient dis-
persion to preclude "channeling" through the fuel pile.
d. That adjustable, tangential overfire ports be
provided of ample capacity to supply at least 10 times
the underfire air volume at a differential pressure
corresponding to the burner stack effect at 300°F exit
and 90°F ambient temperatures.
e. That the burner shell be reasonably airtight to
preclude parasitic leakage and thus cooling effect and
lack of control of overfire air.
4-11
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f. That adequate maintenance practices be observed
to assure optimum performance of the underfire air
system at all times.
g. That operational practices include frequent adjust-
ment of underfire air volume (firing rate) and over-
fire air volume as required to maintain optimum exit
temperature at all times.
It may be generally stated that if a wigwam burner is
smoking it needs less excess air to eliminate the smoke.
Opening an access door, for example, lets in more cold air
which may momentarily cause an apparent increase in the com-
bustion rate but immediately starts to chill the entire
fire. Only in the rare case where exit gas temperature is
in the range above 900°F and the burner starts emitting
dense black smoke (indicating unburned carbon) is it neces-
sary to increase the airflow to get rid of the smoke.
Ash Removal
The burner must be kept free of excessive ash buildup
and in good repair. The fans and grate system must be
inspected periodically to assure they are operating at peak
efficiency. If a handful of sawdust is thrown into a fan
inlet, and it is not sucked into and through the fan, it
indicates that the system is plugged and needs immediate
attention.
4-12
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5. WIGWAM BURNER MODIFICATIONS
When one looks at the simple wigwam burner with its
natural draft overfire air supply, uncontrolled forced
draft air, and variable fuel feed it becomes apparent that
modifications could be made to improve combustion. All of
the following suggestions for modification have been tried
at one time or another, either singly or in combination.
PHYSICAL MODIFICATIONS
Sizing
Many mills are faced with the situation of having a
wigwam burner too large for disposal of their available
residue. The wigwam may have been originally sized to
handle all of the waste and now part of the material is
being utilized and only a portion is being incinerated. In
this case it may be desirable for the mill to construct a
smaller, properly sized burner for optimum disposal of the
residue with a minimum of pollution.
In the case where too much fuel is available for the
burner the obvious solution of going to a larger, properly
sized burner should be considered.
In either case, where a new burner of the proper size
is indicated, other modifications listed in this report
should be considered. It is much easier, and less expen-
sive, to include the necessary modifications when the new
burner is designed and constructed rather than trying to
retrofit an existing burner.
5-1
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Dampers
Dampers allow the flow of gas or air to be restricted.
Dampers in wigwam burners may be installed in three
separate places to adjust the air or gas flow:
1. In the top of the burner just below the horizontal
screen. This decreases the gas volume leaving the
burner and allows the combustible material to remain
in the burner longer. The overall effect of this
damper is the same as going to a smaller burner, when
it is partially closed. For light fuel loading the
damper may be nearly closed and yet it can be opened
for heavy fuel loadings.
2. Individual dampers at each overfire air port.
These may be closed to restrict the overfire air flow
during times of startup or light fuel feeding. They
may be opened as necessary as the fuel loading in-
creases. Control with these dampers will be indicated
as a change in the exit gas temperature indicated by
the thermocouple. If the exit temperature is too low
the dampers should be closed to cut back on the cold
air entering the burner. If too hot, the dampers
should be opened.
3. Dampers in the forced draft system may be either
ahead of or after the forced draft fan. They should
be adjusted to give less air flow during startup or
with light fuel loading. When the burner is operating
near design capacity they should be fully opened to
allow proper air through the grates. Caution should
be exercised in design of these dampers that they be
not completely closed. Some air flow must be kept
through the grates to cool them.
5-2
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FUEL MODIFICATIONS
Fuel Introduction
The ideal way to introduce the fuel would be to mix
all the residue in an external fuel house and then feed
the burner at a constant rate. This would result in a
uniform fuel as far as both quality and quantity. Constant
fuel feed would simplify the burner operation as it could
be set for the constant fuel flow and then left unattended
unless an upset occurred.
Another alternative is to fire the burner so that some
fuel is entering at all times and there are no periods with-
out fuel. For example, if a burner is being operated on a
mixture of bark and sawdust have the barker crew take their
lunch break at a different time than the sawmill crew.
This would assure at least partial fuel feed without the
normal, complete interruption during lunch.
The fuel mixture is important to the combustion within
the burner. If, for example, sawdust falls on one side of
the burning pile and trim ends and edgings fall on the
other, the sawdust may smolder due to packing too tightly
while the other half of the pile burns properly. By mixing
the fuel external to the burner, on conveyors or other
i
handling systems, the fuels can complement each others
characteristics. Dry Sander dust mixed with wet bark will
make an excellent fuel. If they were fired separately in
the same burner we might expect problems.
If light fuels such as sander dust, planer shavings, or
sawdust are to be introduced directly to the burner from the
discharge of a cyclone, they should be introduced as low
as possible. This will enable them to remain in the combus-
tion zone longer before being carried to the top of the
5-3
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burner.
Auxiliary Fuel Systems
If a burner cannot reach proper operating temperature
an auxiliary fuel system may be considered. Natural gas or
propane is the most commonly used auxiliary fuel although
diesel oil is sometimes used. The auxiliary fuel is fired
in long-flame burners pointing at the top of the fuel pile.
For startup, or low temperature conditions, the auxiliary
burners are ignited and kept in operation until the waste
fuel can maintain the desired exit gas temperature. The
auxiliary burners may be mounted on wheels or tracks to
allow their removal from the burner once the fire is
established. Some wigwam burners firing very wet fuel use
as many as 3 auxiliary burners to get the fire started in
the morning and then leave one burner operating contin-
uously throughout the shift.
AIR SYSTEMS MODIFICATIONS
Overfire Air Systems
The overfire air system on many wigwam burners consists
simply of openings in the shell, a foot or two above the
base. Construction of ducts at the openings to cause a tan-
gential flow of the overfire air inside the burner is an
improvement. This allows the lighter fuel particles to
remain in the combustion zone longer. Dampers can be
fitted to the overfire air openings so that the airflow to
the burner may be regulated.
On some burners the overfire air openings are replaced
with an overfire air fan and manifold around the burner.
The airflow to the burner may then be easily controlled by
5-4
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a single damper at either the fan inlet or outlet. The
ultimate modification to the overfire air system is an auto-
matic fan control using a modulating damper on the fan
inlet. As the exit gas temperature increases above a cer-
tain set point,the damper opens to admit more overfire air.
Should the exit gas temperature decrease below a certain
value the damper closes and the excess air decreases there-
by raising the temperature.
Underfire Air Systems
Modifications to underfire air systems are generally
directed toward rebuilding the grate system to give more
uniform air flow with fewer blow holes and adding dampers
to control the total air flow. Many of the existing under-
fire air systems are inadequate for their operating con-
ditions. Fans may be of the propeller type which do not
develop enough static pressure to keep the air flowing
through the fuel pile.
Changes in fuel, since the burner was originally de-
signed, may be plugging the grates. Sawdust is a material
which will plug many grate types and is best burned in a
burner with elbow or conical grates.
SYSTEMS MONITORING AND OPERATING MODIFICATIONS
Instrumentation
Instrumentation that may be added to a burner to aid
in proper combustion with a minimum of air pollution is
similar to that used on other combustion systems. The pre-
viously mentioned thermocouple and pyrometer system is the
minimum that should be considered. At the other extreme
would be a fully instrumented burner with oxygen or C0
5-5
-------
recorder, draft guages for overfire and underfire systems,
damper position indicators, and possibly even an opacity
monitoring instrument at the burner exit.
A further modification would be to take one or more of
the instrument output signals and utilize these directly
or through a computer to control an appropriate system on
the burner. The ultimate system would be a completely
computer controlled system using exit temperature, draft
gauge readings, gas analysis data, and outlet gas opacity
readings to adjust the burner for optimum combustion with
minimum opacity or particulate emissions.
Any instrumentation should be installed in weather-
proof cabinets and shielded from the extreme temperature
near the burner shell. Recording instruments are preferable
to indicating only instruments because they yield a per-
manent record which may be used for optimizing the burning
process.
Maintenance and Operation
The burner may have to be modified to permit satis-
factory maintenance. The door must be large enough to get
a vehicle inside to clean out ashes. The grate system
must be strong enough to support the vehicle or have remov-
able elbows, boxes, or cones to allow complete ash removal.
A complete maintenance schedule for the burner should
be adhered to. Of course, a fully modified burner with
auxiliary burners, installation, and controls will re-
quire more maintenance than one that is only the simple
wigwam burner.
Operation of the burner should be assigned to a person
knowledgable about combustion. To properly fire a wigwam
5-6
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burner requires as much skill as to properly fire a boiler.
Unfortunately, the job of firing the burner is usually de-
tailed to someone who does not really understand combustion
reactions in wigwam burners.
GAS CLEANING EQUIPMENT
Wigwam burners do not really lend themselves to install-
ation of gas cleaning equipment. Scrubber installations have
been used with moderate success. Usually, the visible, con-
densing water plume from the scrubber is more objectionable
to the population than the clear to slightly opaque plume
from a properly operated burner. Since the cost of the
scrubber approximately equals the cost of a fully modified,
new wigwam burner it appears to be a rather poor investment.
The tremendous volumes of hot gases leaving the wigwam,
at near atmospheric pressure, result in large and expensive
gas cleaning devices with large, high horsepower fan re-
quirements. Also, the particulate emitted from the burner
may cover a size range from sub-micron (smoke) to millimeter-
plus material (cinders or partially burned fuel). For such
a wide variation of particle sizes it is probable that no
one single piece of control equipment could do a satisfac-
tory job. Double or higher costs are necessary for multiple
control devices in series.
OTHER MODIFICATIONS
The modifications mentioned previously are those con-
sidered as being practical with some real benefit. Several
other modifications have been suggested over the years and
some of them tried on various burners. The following modi-
fications fall into this category and are listed here to
indicate that they have been considered and then rejected
5-7
-------
as being unpractical .
Fuel Drying Systems
Fuel drying systems enable combustion of excessively
wet fuels. This would be extremely expensive and would only
accomplish the drying of the fuel external to the burner
rather than within the burner. Condensation problems in
the ductwork might be severe.
Preheating Combustion Air
Preheating of combustion air is achieved by recircu-
lating hot gas from the top of the burner. To do any kind
of proper heating would require effective insulation of
ducts to prevent condensation. The costs would far exceed
the small benefits gained.
Sprinklers for Cinder Control
Lawn sprinklers on top of the burner do nothing to
control emissions. They may wet down the area downwind and
prevent fires from a poorly operating burner that is dis-
charging burning cinders.
Refractory Linings
Refractory linings are installed either in the lower
portion only or all the way to the top. These refractory
linings are very expensive and need continual maintenance.
They are subject to damage by falling material and during
clean-out. A properly designed and operated burner will
burn just as clean without refractory as it will with
refractory -
5-8
-------
Natural gas or propane mixed with the underfire air
This mixture does not burn until it has passed through
the fuel pile. It then burns with less effectiveness than
if it had been fired in auxiliary burners. Explosion
hazards also should be considered.
COSTS AND SCHEDULES FOR MODIFICATIONS
The capital costs, yearly operating costs (including
maintenance) and time schedules for the wigwam burner
modifications discussed are shown in Table 5-1. The costs
are approximate for today (1975) and should of course be
verified. The same applies to stated delivery schedules.
Several burner manufacturers are presently constructing
and modifying burners as mentioned in this report. A few
of these are:
1) Rees Burner Company, Memphis, Tennessee
2) Medford Steel and Blowpipe Co., Medford, Oregon
3) Steelcraft Corporation, Memphis, Tennessee
4) Industrial Construction Co., Eugene, Oregon
5-9
-------
Table 5-1. WIGWAM BURNER MODIFICATIONS, COST ESTIMATES AND DELIVERY SCHEDULES, 1975*
Modification
New, properly sized burner
Top damper
Overfire air dampers
Forced draft fan dampers
Fuel house & fuel regulating system
Fuel mixing system using cyclones
& conveyors
Change of conveyors & cyclones for
better fueling
Gas fired auxiliary burners-^
Convert overfire air system to
tangential flow
Convert overfire air system to
manifold with blower & dampers
Add to the above for automatic
control from exit temperature
Capital
cost ,
$
40
foot
burner
40,000
2,000
400
100
35,000
7,000
1,500
3,000
600
4,000
3,000
60
foot
burner
70,000
4,000
600
150
40,000
9,000
2,000
4,500
900
5,000
4,000
Yearly
cost ,
$
40
foot
burner
8,000
100
40
10
7,000
1,500
150
3,000
60
800
600
60
foot
burner
10,000
200
60
15
8,000
2,000
200
4,500
90
1,000
800
Time,
months
concept
to
operation
6
2
1
1
6
6
3
6
1
6
8
Ul
* Costs and time schedules will vary with location
-------
Table 5-1. WIGWAM BURNER MODIFICATIONS, COST ESTIMATES AND DELIVERY SCHEDULES, 1975*
Modification
New grate system & forced draft fan
Thermocouple & recording pyrometer
Gas analysis recording (02 or CO2)
Draft gauges & controls
Complete computer control system
Proper maintenance & operation
including labor
Multiple cyclone particulate collector
Wet scrubber
Opacity monitoring system
Capital
cost,
$
40
foot
burner
3,000
500
2,000
500
15,000
1,000
25,000
40,000
10,000
60
foot
burner
4,000
500
2,000
500
20,000
1,000
70,000
120,000
12,000
Yearly
cost,
$
40
foot
burner
600
100
500
50
3,000
2,000
5,000
12,000
1,000
60
foot
burner
800
100
500
50
4,000
2,000
14,000
36,000
1,200
Time,
months
concept
to
operation
6
2
3
1
12
1
12
12
6
* Costs and time schedules will vary with location
-------
6. EMISSIONS FROM MODIFIED WIGWAM BURNERS
Not nearly as much data are available on emissions from
modified wigwam burners as are available from simple wigwam
burners. The data that are available may be more reliable,
however, because it was pbtained recently with modern equip-
ment and methods. In some cases the actual emission data
may not be measured but can be estimated from correlation
with other known data, such.as exit gas temperature.
WELL CONTROLLED BURNERS
Specific Tests
1. Tests on a fully modified.wigwam burner with a top
damper, manifolded and electronically controlled (as a
function of exit temperature) overfire air system, balanced
forced draft system and three propane fired auxiliary
burners indicated a loading of 0-23 grains per standard
cubic foot, corrected to 12% CO- at startup and an average
value of 0.11 grains per standard cubic foot, corrected to
12% CO- under steady firing conditions and 800°F gas temp-
erature. The opacity readings on this burner were io% or
less after initial startup. All three propane burners
were used at startup and one was left on continuously
throughout the day to aid in drying the wet fuel. The fuel
was Douglas Fir bark and sawdust. At steady state firing,
this burner was indicating 5.2% C02 and 15.0% 02 for an
exit gas analysis. (Publishers Pape^r Co., Liberal, Oregon)
6-1
-------
2. A burner which had been modified by installing a
top damper and a controlled, modulated overfire air system
was tested over a wide range of varying air flows. The
fuel was fed at a constant rate and was Douglas Fir bark
and sawdust. With the air systems completely open the
emissions were 0.14 grains per scf at 12% C02, about 10%
- 15% opacity, 3.5% CO- and 16.0% 02. With the dampers
partially closed the emissions dropped to 0.07 grains per
scf at 12% C02, 10% opacity, 5.7% C02 and 13.5% 02. This
burner was a relatively new, properly sized burner, free of
air leaks, and with good maintenance. The exit gas temper-
ature varied from a low value of 820°F with the dampers open
to a high of 965°F with partial damper closure. Only
limited damper adjustment was possible because the system
had previously been set within narrow control limits based
upon visual observations. This burner had been operated
for several months in the 850°F - 950°F temperature range
with no evidence of damage. (U.S. Plywood, Roseburg, Oregon)
3. An older burner was modified by changing the over-
fire air system from natural draft openings to 3 tangentially
directed blowers. The blowers were controlled so that they
operated individually. The burner was in excellent repair
and was fired with a mixture of Douglas Fir and Hemlock,
bark and sawdust. The test results are summarized in
Table 6-1. (U.S. Plywood, Idanha, Oregon)
6-2
-------
Table 6-1. MODIFIED WIGWAM BURNER TEST RESULTS
Test Conditions
Normal - all 3 blowers
Only 2 blowers
Normal - conveyor
partially closed
Exit
gas
temp. ,
OF
750
940
860
co?,
%**
3.0
•5.6
3.6
?2'
17-0
14.4
16.5
Particulate
emissions ,
Grain
scf @ 1"° C°2
0.22
0.09
0.15
Opacity
10%
0%
10%
4. A highly modified wigwam burner was tested shortly
after it was completed at the mill site. It had auxiliary
burners for startup and a series of 4 separately controlled
overfire air blowers which could be modulated to take in
ambient air or any proportion of exhaust gas recirculated
from just below the top damper. The damper system on these
blowers was controlled by a thermostat at the top of the
burner. The theory was that at high exit gas temperatures
they were to take in ambient air to cool the fire and at
low exit gas temperatures they were to recirculate the exit
gases which would give a hotter fire. The recirculation
ducts were not insulated and by the time the recirculated
gases reached the overfire air blowers they were within
50°F of ambient air temperature. The fuel to this burner
was very dirty, containing a large amount of clay and other
inorganic matter from the dry handling of the logs. The
fuel was a mixture of Englemann Spruce and White Fir, bark
and sawdust. Because of the dirty fuel, the particulate
emissions were primarily inorganic material. Both particu-
late loading and opacity were excessive, because of the
high ash fuel being fed to the burner. The particulate
6-3
-------
matter averaged 0.70 grains per standard cubic foot cor-
rected to 12% CO,, and the opacity was in the 20% - 40%
range. The exit gas recirculation system did not seem to
be effective in changing the emission significantly. The
operator of the burner stated that, ".... it really smokes
during the winter when the bark (fuel) is frozen." This
burner again exhibited the fact that lower emissions are
a function of higher temperature as shown in the results
plotted as Figure 6-1. (Boise-Cascade, Council, Idaho)
6-4
-------
1.2
1.0
O
o
CM
< 0.8
o
K—1
oo
0.6
0.4
o:
0.2
_J | I
400 500 600
EXIT TEMPERATURE, °F
700
800
Figure 6-1. Particulate loading as a function
of exit gas temperature for a modified Wigwam
burner with dirty fuel (Boise-Cascade, Council, Idaho)
6-5
-------
7. ALTERNATIVES TO WIGWAM BURNERS
If a mill must eliminate their wigwam burner they must
go to alternative means of residue disposal. Some mills are
disposing of all their residue at a profit and have not
used a wigwam burner for years. Other mills in the same
general area are convinced that the most economical method
for them is to dispose of all of their residue in a wigwam
burner. Since mill practices vary, and residues also vary,
it becomes obvious that a thorough study must be done by
each individual mill. This study consists of taking an
accurate inventory of present and future residues, analyzing
all alternatives for their costs and benefits, determining
which alternatives are feasible and finally selecting the
alternative, or set of alternatives, most favorable to the
mill. When studying alternatives, a mill must be sure to
establish costs for residue disposal by their presently
used system. Many times mill operators have discovered
that disposal by wigwam burner is one of the more expensive
alternatives.
Conversion from a system burning the residue in a
wigwam burner to an alternative system may not solve the
pollution problem entirely. The alternative system selected
also has some pollution potential which must be thoroughly
analyzed and evaluated. The regulations governing the area
where the alternatives may be applied must also be studied.
Landfill regulations, for example, may be more restrictive
than the regulations covering wigwam burners.
7-1
-------
The following alternatives are some which might be
considered for a medium sized mill (50,000 FBM/8 hr shift).
Not all of the alternatives would apply to every mill and
there may be other, more desirable alternatives for a
specific mill, which are not even mentioned. Those men-
tioned have been tried and have been found satisfactory in
specific situations.
RESIDUE CLASSIFICATION AND SEPARATION
Many of the alternative methods of residue disposal
require rather exact material separation. If a mill is
currently not concerned with residue separation this can
be a large expense. If the bark must be removed to sell
the white wood as bark free chips, a barker plus conveying
system is required. Sawdust, chips, and planer shavings
will need classification and then storage facilities at the
mill site. New cyclones, conveyors, bins, and residue con-
version systems such as chippers or hogs will be required.
Since many alternatives involve initial classification and
separation of residues, the costs of the system components
are developed in Table 7-1 for inclusion with the alterna-
tives mentioned later. Most of these systems will require
6 months to a year from conception to completion. The
schedule and costs, of course, should be verified in
advance.
7-2
-------
Table 7-1. CLASSIFICATION AND SEPARATION UNIT COSTS, 1975a/b
System component
Barker for 20" dia. logs w/motor & drive
Barker hog & conveyor system, in & out
Bark conveyor
Sawdust screening system
Sawdust shaving or chip storage bin
Chipping head w/motor & drive
Chipping knife grinder
Chipping conveyor system, in & out
Chipping building
Chip screen
Hog for fuel w/motor & drive
Hog conveyor system, in & out
Hogged fuel storage bin
Planer shavings screening system
Blower, blowpipe, and cyclone (each)
Capital
outlay,
$
50,000
45,000
10,000
8,000
15,000
40,000
10,000
15,000
10,000
8,000
35,000
20,000
10,000
8,000
6,000
Operating
cost,
$/Year
4,000
3,000
500
500
500
8,000
1,000
1,000
500
500
3,500
1,500
300
500
500
Assume mill capacity 50 M FBM/8 hr shift.
From report prepared for William Nelson, P.E., Anchorage,
Alaska by Richard W. Boubel, P.E., and David C. Junge,
P.E., Corvallis, Oregon, 1974.
Does not include operators labor costs.
7-3
-------
LANDFILL
Wood residue is similar to municipal refuse in that
it contains a high percentage of organic material. There-
fore, landfill practices and costs are very similar for
the two residues. There are several factors that require
investigation if wood and bark wastes are to be disposed of
in a landfill. These factors include:
1. Haul distance, mill to landfill.
2. Landfill life at present and future mill production,
3. Fire hazards at landfill site.
4. Pollution of ground or" surface water.
5. Gas evolution.
6. Settling of finished landfill.
The cost of disposal at a typical landfill site is
approximately $1.50 per ton or $3.00 per unit. Trucking
costs, for a 10 mile haul, are estimated to be about $1.00
per ton or $2.00 per unit. Landfill disposal of wood
residue offers the mill no return for residue in terms of
either money or energy.
Table 8-1 summarizes landfill costs as well as those
for all other alternatives to burning in a wigwam burner.
One note of caution concerning Table 8-1; the annual net
cost or profit to the mill is calculated at the mill site.
The distance of hauls and availability of markets will of
course determine the actual net cost or profit.
SOIL ADDITIVES
Sawdust and bark have been used for years as mulches
and soil amendments. These residues have been used by
nurseries and garden stores around shrubs and flowers to
retain moisture and beautify displays. Sawdust is used on
7-4
-------
highway cutbanks and along roadsides. Bark and sawdust
can be used to lighten heavy soils, mulch orchards, provide
drainage for wet lands, and help stabilize drifting sand
dunes. Bark chunks are being used extensively for land-
scaping.
If the sawdust and bark could all be marketed, the
mill would still be faced with disposal of other residues
such as white wood and planer shavings. To amortize the
large capital investment in the systems necessary for
barking the logs it would probably be necessary to find
markets for the shavings and chipped white wood.
CONVENIENCE FUELS
The two primary convenience fuels used in the U.S. are
Presto-Logs and charcoal briquets. In some areas, bundled
white wood or slab may also be sold for home use.
Presto-Logs, and some similar products, are made from
primarily white wood residue which is compressed with or
without a binder into a shape and size which is easily
handled. These manufactured fuels retail for approximately
1C per pound which certainly looks attractive to a mill
owner faced with 40 tons of white wood residue per day (40
tons per day X 2,000 pounds per ton X $0.01 per pound =
$800 per day). The problem is the large installed equipment
cost plus the limited market for the finished product,
as well as transportation. Usually the manufacturer does
not retail the product and a wholesale price of 1/2C per
pound is more realistic.
Charcoal briquets are similar in that they retail for
approximately IOC per pound. They are generally easier to
handle because they are bagged after manufacture. A single
7-5
-------
mill could not consider a charcoal plant due to the cost
(several million dollars). It might be a possibility in an
area where several mills could construct and operate the
plant cooperatively. Again, haul distances to the plant
and for the finished product, size of plant, demand for the
product, etc., should be carefully considered for an
accurate estimate of costs and returns made before con-
sidering any expenditure on the plant.
WOOD FIBER USAGE
By far the largest usage of wood residue in the U.S.
is for the fiber material it contains. Most paper pulp is
made from wood fiber. White wood chips are preferred but
varying small quantities of bark or sawdust can be incor-
porated. Chips, delivered to the pulp mill, are currently
selling for approximately $20 per unit, although the price
has been as high as $40 per unit at times past. The tre-
mendous fluctuation in the price the pulp mills are willing
to pay can turn a profit item into a loss for a mill. A
thorough analysis of market conditions, transportation
costs, contract arrangements, and the pulp mills' long
range plans must be considered along with the usual con-
cerns of equipment and labor costs at the forest products
mill.
Shavings are in demand for particle board manufacture
and small quantities are used for animal bedding. One must
be cautious of relying heavily on particle board manufac-
turing to consume the mill residue. Lately, several
particle board plants have closed because of the construc-
tion industry slump and mills have found themselves with
7-6
-------
planer shavings to dispose of. This has meant reactivating
wigwam burners which had previously been retired. The
same may be said for pulp chips. Today, a large amount of
white wood which was previously chipped is being consumed
in wigwam.burners.
CHEMICAL EXTRACTIVES
Wood residue is a vast potential source of chemical
raw materials. As the price of petrochemicals increases
the economics of extraction of chemicals from wood becomes
more favorable. Currently wax and cork are being profitably
manufactured from Douglas Fir bark and as oil prices con-
tinue to increase,the economics look even more favorable.
The bark is also a potential source for a wide variety of
pharmaceutical chemicals and much research is directed
toward this recovery and use.
The profitable extraction of chemicals from wood
residue probably requires a parallel use for the wood
fiber, either as fuel or raw material for pulp mills. There
still needs to be considerable research on the size of
extractive plants to achieve optimum utilization from the
wood residue.
ALTERNATIVE INCINERATORS
The reason the wigwam incinerator is used so exten-
sively by the forest products industry is that it does the
job most economically. Any other type of incinerator that
destroys the residue is, therefore, going to cost the mill
more money- If a mill were to replace a wigwam burner with
a more expensive type of incinerator they might want to go
to a .fuel bin system to obtain continuous feul feeding.
7-7
-------
For example, if a mill is incinerating all the residue from
50 thousand board feet cut in an 8 hour shift, they would
be feeding the wigwam approximately 60 tons of residue per
8 hours or 15,000 pounds per hour. If this was sent to
a fuel storage bin and then taken from the bin at 5,000
pounds per hour for the entire 24 hour day, and a new
incinerator operated continuously, a much smaller incinera-
tor could be used. The savings in incinerator -cost might
pay for the fuel storage system plus the additional opera-
tion of 16 hours per day.
The multiple chamber type of incinerator is the most
efficient destructor of combustible residue and also the
most costly- The fuel would have to be hogged or somehow
be reduced in size before going to the multiple chamber
incinerator. Costs for a system using a multiple chamber
incinerator approach those for a boiler plant (without the
turbine) and can easily be 10 times the amount which would
be spent for a new, properly sized wigwam burner.
The open pit incinerator or air curtain destructor
has been advanced as the replacement for the wigwam burner
but it has not been proven as the universal solution.
These open pit incinerators, with ducted overfire air sys-
tems, are not capable of consuming small sized fuel particles
such as sawdust and planer shavings. The particulate air
pollution from these burners using light fuels is inaccep-
table in both grain loading and opacity. They do work
fairly well on large material such as slabs and edgings
and possibly could be used in a mill where a market for the
light fuels was nearby.
-------
DIRECT ENERGY PRODUCTION
Wood fired boilers have been in existence nearly as
long as the forest products industry. The original boiler
type, which is still widely used today, is the dutch oven,
which relies on large masses of refractory material to dry
and then gasify the fuel. The combustible gases then
travel to a second chamber where additional air is added
and combustion completed. The hot gases then flow to the
boiler.
Spreader stokers rely on hot air from a stack air
heater to dry the fuel and start gasification. The com-
bustion reaction occurs in a water walled boiler and much
of the energy is transferred as radiant heat.
Both the dutch oven and spreader stokers are used with
boilers to generate process steam or electricity for mill
or public uses. As energy prices increase rapidly, wood
residue is re-entering the energy picture. Many large
mills are installing new boiler capacity to make themselves
"energy independent." Smaller mills that are dependent
upon electricity, oil, or natural gas may find that suddenly
their wood residue, which had previously been incinerated,
is now a relatively high value item. Hogged fuel prices at
the mill site have gone from about 50£ to over $4.00 per
unit in just a few years. At todays energy prices, the $
energy equivalent of a unit of hogged fuel is approximately
$25.00.
Suspension burners are being used with both incinera-
tors and boilers. The fuel must be relatively dry and
finely divided before firing with a suspension burner (dry
plywood sander dust makes an excellent fuel). Suspension
burners require surge prevention in their fuel system to
7-9
-------
maintain a steady flame with fluctuating fuel supplies.
Heavy surges of fine dry fuel can lead to dense black smoke
issuing from the boiler stack. To use suspension burners
on the total wood residue from a mill would require exten-
sive fuel drying facilities (capital cost of approximately
$100,000 for a 50 M FBM mill) and grinding or hammermilling
facilities. These facilities would be in addition to the
other, usual boiler costs.
Fuel cells have been suggested as an alternative to
spreader stokers, dutch ovens, and suspension burning. A
fuel cell is best described as a combination of all three
of these systems. The fuel must be in finely divided state
(but not as fine as for suspension burning) and introduced
with a primary air supply. Hot air is added tangentially
iii a cylindrical, refractory lined chamber which serves to
dry arid gasify the fuel. The hot gases then pass from the
fuel cell into the boiler section. Some fuel cells attain
such a high temperature that the ash may be drawn off as,
a liquid slag. Fuel cells show some future promise but
currently the problems of control, non-uniformity of fuel,,
and refractory problems appear to limit their use.
Other combustion systems for wood fuel can be mentioned
as future possibilities, not practical at this time. In-
cluded in this category would be fluidized bed combustion,
direct fired gas turbines, and hot air engines of various
types.
CONVERSION TO OTHER FUELS
Wood as a fuel raw material is generally thought of
in its native, solid form. Conversion to other forms is
possible and in many cases practical to extend liquid .and
7-10
-------
gaseous fuels. Wood burning automobiles, trucks, and buses
are occasionally pictured in the news. Usually these have
generators that convert the carbon in the wood into CO and
other combustible gases which are directed to the engine
where they burn to give energy.
A more practical, and acceptable alternative appears
to be the conversion of wood residue to methanol (methyl
alcohol) or ethanol (ethyl alcohol) for use as a gasoline
substitute. Current automobiles can be operated on mix-
tures up to 30% methanol in gasoline with only minor tune-
up changes. Vehicles operating on 100% methanol could be
designed if necessary. Many research projects are now
looking into methanol (and ethanol) as gasoline substitutes.
Wood residue, along with agricultural products, is a renew-
able energy resource which could ease the gasoline shortages
predicted for the U. S.
Pyrolysis is a process of distilling a material with
heat to break it into its gaseous, liquid, and solid com-
ponents. Wood residue is an excellent material to use as
the raw material for pyrolysis. Turpentine, wood tars,
and gaseous, liquid, and solid hydrocarbons are just some
of the materials that are currently being produced by
pyrolysis. Through pyrolysis it is possible to produce
substitutes for current gaseous and liquid fuels, using
solid fuels, such as wood, as both the raw material and
source of heat energy.
Pyrolytic decomposition is still in the investigation
stage for wood residue distillation and disposal and
no firm cost data is available at this time.
7-11
-------
Table 8-1.
COSTS, VALUES RECOVERED, TIME SCHEDULES, AND POLLUTIONAL POTENTIAL
FOR ALTERNATIVES TO WIGWAM BURNERS3
Alternative
Landfill, 10
nile haul
Barx. logs.
sold; white
vood shipped l
sold, planer
sr.avin-js sold
Hog fc mix
all residue,
hoi fuel
Hog fc mix
al 1 res idue,
fire in own
boilar for
energy
Multiple
char.be r
incinerator
for all
residue
Air curtain
destructor
for all
r-2-iiduea
Air curtain
destructor
for bark t
course residue
sell sawdust
^havin'js3
sell bark, all
other to
Presto-Log
plant
System needed at mill with
units per day processed
Storage bin 30 units
Barker 8 units, stoxage bin 30
units. Chipping system 11
units, chip bin 30 units,
planer shavings system l storag<
bin 2 at 30 units, 2 new
Hog system 34 units, storage
bin 2 at 30 units 2 new
conveyor systems , 2 new
cyclone systems , wood- fired
boiler with wet scrubber fc
turbine generator
Hog system 34 units, storage
bin 2 at 30 units, 2 new
conveyor systems , 2 new
cyclone systems , multiple
units / hour (24 hours)
units per hour (8 hours daily
operation)
for 3 units per hour
(8 hours daily operation)
sawdust 5 units, bin
30 units, planer shavings
for bark 30 units, chipping
system 11 units, 3 new
plant (3 employees to operate)
Capital
cost, $
10,000
262,000
97,000
950,000
500,000
80,000
106,000
498,000
Annual operating
COBtb, $
39,000
21,300
7,600
80,000
20,000
8,000
6,000
49.500
Recovered
products,
value i unit
None
Bark SB.OO/unlt
Sawdust $6.00/unit
Chips $25.QO/unit
Shavings $8.00/unit
Total
Mixed hogged
fuel $4.00/unit
Mixed hogged
fuel 9 $20.00
per unit
energy
equivalent
None
None
Sawdust $6.00/unit
Shavings SB.OO/unlt
Total
Bark 58.00/unit
Presto-Loga
» l«/lb
Total
Annual value
products or
energy, $
None
16,000
7,500
68,750
10,000
lQJ,i50
30,000
150,000
None
None
7,500
10,000
17,500
16,000
100,000
116,000
Annual net
or (excess
return)0, $
37,000
(60,950)
(22,400)
(70,000)
20,000
8,000
(9,500)
(66,500)
Time to
install
total system,
months
2
12 - 16
9-15
16 - 36
18 - 36
6-8
6-8
12 - 18
Potential
pollution
from new
system
Solid waste,
possible water
pollution
Some fugitive
dust at
loading systems
Some fugitive
dust
Boiler
emissions
range froa
controlled £
0.05 grains/
SCF to uncon-
trolled 0.5
grains per 5CP
Incinerator
einiss ions
about same
ranae as
boiler
emissions
Emissions
approximately
0.1-0.2
grains per SCF
Sane as for
air curtain
destructor
above
Some
fugitive
dust
05
I
a Data from Drlall, Corp., Attica, Indiana (foe mill producing
Approximately 50,000 board feet per 8 hour shift.)
b Mot Including labor coats.
c Not including any capital coeta. At mill lita except landfill.
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References
1. Boubel, R. W., M. N. Northcraft, A. VanVliet, and
M. Popovich. Wood Waste Disposal and Utilization.
Bulletin No. 39. Engineering Experiment Station,
Oregon State University. Corvallis, Oregon.
August, 1958.
2. Popovich, M., R. W. Boubel, M. Northcraft, and G. E.
Thornburgh. Wood Waste Incineration. Technical
Report A 61-3. Robert A. Taft Sanitary Engineering
Center, U.S. Public Health Service. Cincinnati,
Ohio. 1961.
3. Boubel, R. W. Wood Residue Incineration in Tepee
Burners. Circular No. 34~. Engineering Experiment
Station, Oregon State University. Corvallis, Oregon.
July, 1965.
4. McKenzie, H. W. Wigwam Waste Burner Guide and Data
Book. Oregon State Sanitary Authority. Portland,
Oregon. March, 1968.
5. Boubel, R. W. Particulate Emissions from Sawmill
Waste Burners. Bulletin No. 42. Engineering
Experiment Station, Oregon State University- Corvallis,
Oregon. August, 1968.
6. Atherton, G. H. and S. E. Corder. A Study of Wood
and Bark Disposal in the Forest Product Industries.
Forest Products Laboratory, Oregon State University.
Corvallis, Oregon. February, 1969.
7. Droege, H. and G. Lee. The Use of Gas Sampling and
Analysis for the Evaluation of Tepee Burners. Pro-
ceedings of the Seventh Conference on Methods in Air
Pollution Studies. Los Angeles, California.
January, 1965.
8. Hammond, V. L. , L. K. Mudge, C. H. Allen, and G. F.
Schiefelbein. Energy From Solid Waste by Pyrolysis-
Incineration. BNWL-SA-4471. Battelle Pacific
Northwest Laboratories. Richland, Washington.
November, 1972.
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APPENDIX A
Oregon State Sanitary Authority Regulations:
Construction and Operation of Wigwam
Waste Burners
A-l
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STATE SANITARY AUTHORITY
CH. 334
Subdivision 4
CONSTRUCTION AND OPERATION
OF WIGWAM WASTE BURNERS
[ED. NOTE: Unless otherwise speci-
fied, sections 24-005 through 24-025 of
this Chapter of the Oregon Administra-
tive Rules Compilation were adopted by
the State Sanitary Authority, June 24,
1965 and filed with the Secretary of
State, July 6, 1965 as Administrative
Order SA 22.]
24-005 DEFINITIONS. (1) "Approved"
means approved in writing by the Sani-
tary Authority staff.
(2) "Auxiliary Fuel" means any car-
bonaceous material which is readily com-
bustible (includes planer ends, slabs and
sidings).
(3) "Overfire Air" means air intro-
duced directly into the waste burner in the
upper burning area around the refuse or
fuel pile.
(4) "Underfire Air" means air intro-
duced into the waste burner under the
fuel pile,
(5) "Wigwam Waste Burner" means a
burner which consists of a single com-
bustion chamber, has the general features
of a truncated cone, and is used for in-
cineration of wood wastes.
24-010 WIGWAM WASTE BURNERS -
PURPOSE. Section 24-010 through Sec-
tion 24-025 are adopted for the purpose
of preventing or eliminating air pollution
or public nuisance caused by smoke, gases
and particulate matter discharged into
the air from wigwam waste burners.
24-015 WIGWAM WASTE BURNER
CONSTRUCTION PROHIBITED. Con-
struction of wigwam, waste burners is
hereby prohibited after July Ij 1965, un-
less plans and specifications have been
submitted to and approved by the Sanitary
Authority prior to construction.
24-OZu ••.•-,} ;.PLLANCE, All existing
Wigwam waste burners shall comply by
January 1, 1966, with the following:
(1) Adjustment of forced draft under-
fire air shall be by variable speed blower
or fans, dampers or by-passes or by
other approved means.
(2) The introduction of overfire air shall
be principally by adjustable tangential
air inlets located near the base of the
wigwam waste burner or by other approved
means.
(3) A thermocouple and pyrometer or
other approved temperature measurement
device shall be installed and maintained.
The thermocouple shall be installed on
the burner at a location six inches above
and near the center of the horizontal
screen or at another approved location.
(4) During,,burner operation the burner
exit temperatures shall be maintained
as high as possible so as to maintain
efficient combustion.
(5) A daily written log of the waste
burner operation shall.be maintained to
determine optimum patterns of operation
for various fuel and atmospheric con-
ditions. The. lag shall include, but not be
limited to, the time of. day, draft settings,
exit gas temperature, type of fuel and
atmospheric conditions. The log or a
copy shall be submitted to the Sanitary
Authority within ten days upon request.
(6) Auxiliary fuel shall be used as
necessary during start up and during pe-
riods of poor combustion to maintain exit
temperatures required under subsection
(4). Rubber products, asphaltic materials
or materials which cause smoke- dis-
charge in violation of Section 21-011
or emissions of air contaminants in vi-
olation of Section 21-016 or Section
21-021 shall not be used as auxiliary
fuels.
(7) Light fuels or wastes shall be in-
troduced into the burning area in such a
manner as to minimize their escape from
the burner.
24-025 VARIANCE. (1) Waste burners
operating within the modifications and
criteria of Section 24-020 are granted
a variance for" one year from the effec-
tive date of these rules from compliance
with Section 21-011 Smoke DUcharee,
Section 21-016 Particle Fallout Kate and
Section 21-021 Suspended Particu.latc
8- 15-65
A-2
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CH. 334 OREGON ADMINISTRATIVE RULES
Matter. » lution problem in the immediate or sur-
(2) Wigwam waste burners located in rounding area is slight, may be granted
sparsely populated areas of the state where variances from the provisions of Sec-
their potential for causing an air pol- tion 24-020 pursuant to ORS 449.810.
8-15-65
A-3
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA 340/1-76-002
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Combustion of Wood Residue in Conical
(Wigwam) Burners, Emission Controls and
Alternatives
5. REPORT DATE
October 1975
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Dr. Richard W. Boubel, Oregon State University
N. Stephen Walsh, PEDCo-Environmental
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
PEDCo-Environmental Specialists,
Suite 13, Atkinson Square
Cincinnati, Ohio 45246
Inc.
11. CONTRACT/GRANT NO.
Contract No. 68-01-3150
Task Order No. 5
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Division of Stationary Source Enforcement
Washington, D. C. 20460
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The lumber and plywood manufacturing process generate large quantities of wood
residue and waste material, much of which is incinerated in conical or "wigwam"
burners. This report provides technical information on air pollution control
techniques and alternatives for conical burners. Background information is
given regarding the design, operation and combustion activities, including fuel
composition. Air pollution emissions, both gaseous and particulate, are cal-
culated and current regulations are reviewed. Modifications to existing burners
are suggested as well as alternative methods of residue disposal. Capital and
operating costs and approximate time schedules are given for both modifications
and alternatives to wigwam burners.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Burners
Wood Wastes
Combustion
Air Pollution
Industrial Wastes
Waste Disposal
Wigwam Burners
13B
B. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
70
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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