EPA-450/2-78-044
Wood Residue-Fired Steam
Generator Particulate Matter
Control Technology Assessment
Emission Standards and Engineering Division
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
October 1978
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This report is issued by the Environmental Protection Agency to report technics I data of interest to a
limited number of readers. Copies are available - in limited quantities - from the Library Services
Office (MD-35), U.S. Environmental Protection Agency, Research Triangle Park, North Carolina
27711; or, for a fee, from the National Technical Information Service, 5285 Port Royal Road,
Springfield, Virginia 22161.
Publication No. EPA-450/2-78-044
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Table of Contents
Page
I. Introduction 1
II. Description of Industry 3
III. Characteristics of Wood Residue . 5
IV. Boiler Design and Operation 6
V. Characteristics of Particulate Emissions 9
VI. Emission Control Technology . .10
VII. References 14
List of Tables
Page
Table 1. Summary of Tests on Bark Boilers 16
Table 2. Comparative Chemical Analysis of Wood and Bark,
Coal, and Oil 17
Table 3. Chemical Analyses of Hogged Fuel 18
List of Illustrations
Figure 1. Particle Size Distribution of Bark Boiler Fly Ash
Page
19
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I. INTRODUCTION
\
In the United States the paper and pulp industry produces approximately
80 million tons of wood waste each year, which in heating value corresponds
to 110 million barrels of oil or 32 million tons of anthracite coal. This
is approximately 75 percent of our annual consumption of residual fuel oil
and 5 percent of our annual consumption of coal.
Prior to the 1960's the method of disposal for wood waste consisted
mostly of incineration in Dutch ovens or open air tepees. Since then the
advent of the spreader stroker boiler and the increasing costs of fossil-
fuels has made wood residue an economical fuel to burn in large boilers for
the generation of process steam.
There are several hundred steam generating boilers in the pulp and
paper and allied forest product industry that use fuel which is partly or
totally derived from wood residue. These boilers range in size from 6
megawatts (20 million Btu per hour) heat input to 146 megawatts (500
million Btu per hour) heat input and are estimated to be emitting 225
tons of particulate matter per day after application of existing air
2
pollution control devices.
On November 22, 1976, EPA amended the standards of performance for
new fossil-fuel-fired steam generators to allow the heat content of wood
residue to be used for determining compliance with the standard. Prior
to this amendment, performance tests on such boilers had to be conducted
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with the boiler consuming 100 percent fossil fuel. After the wood residue
amendment, a combination boiler was required to burn its normal fuel mix
to determine compliance with the Federal particulate matter standard of
43 nanograms per joule. In most cases the use of mechanical collectors
has provided State compliance for wood residue boilers., but in those
States where a stringent regulation exists 108 to 215 nanograms per joule
more efficient control systems are used.
The control system that has the widest application is a mechanical
collector in series with a wet-impingement scrubber. This system has
demonstrated the ability to provide compliance with the Federal
particulate matter standard for boilers firing nonsalt-laden wood
residue.
For boilers burning salt laden wood residue, the high temperature
baghouse has been proved to provide compliance with the Federal
particulate matter standard. Also the baghouse provides compliance on
boiler burning nonsalt-laden wood residue, but concern over fire hazards
from burning cinders has inhibited the use of this system.
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II. DESCRIPTION OF INDUSTRY
There are several hundred steam generating boilers in use in the
pulp and paper and allied forest product industry which burn wood
residue waste, both to alleviate a potential solid waste disposal
problem and to provide an economical fuel substitute for fossil fuels.
The number of bark boilers is growing at the rate of 15 percent each
year. The trend is toward fewer but larger sized boilers.
The major breakthrough came in the mid-40's with the development
of the spreader stoker, which offered a much more efficient method of
burning than the earlier "Dutch oven" type furnaces. A modern bark
boiler, in conjunction with other heat and chemical recovery units,
can supply all the steam requirements of most mills by utilizing wood
residue as the dominant fuel.
The first processing step to provide this fuel involves debarking
of the logs to be used by the Kraft process or for lumber. The
debarking is accomplished in a variety of methods which utilize natural
or mechanical friction. Succeeding this, the various sizes of bark are
fed through a disintegrator or "hog" to produce a uniform size bark chip.
After hogging, the bark is conveyed to a surge bin or storage facility,
sized to allow for one to two hours of boiler firing at maximum capacity.
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Final distribution of the bark is accomplished from the individual boiler
chutes with either a pneumatic or mechanical distributor.
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III. CHARACTERISTICS OF WOOD RESIDUE
In wood processing approximately 50 percent of the weight of the
raw wood (logs) is removed to produce sound lumber. Though the total
waste usually averages approximately 50 percent, distribution of the
different types of waste such as slab edgings, trimmings, bark, sawdust,
and shavings may vary depending on mill conditions and product desired.
Usually, this residue is divided into 18 percent slab edgings and
5
trimmings, 10 percent bark, and 20 percent sawdust and shavings.
The mills frequently use sawdust or shavings and sawdust mixtures
for steam production, because they can be burned without further
processing. The remainder of the waste products requires further size
reduction in a "hog" machine to facilitate storage, feeding, and
combustion. These newly sized products, in addition to a varying
percentage of sawdust and shavings, present constitute "hog fuel."
This so-called hog fuel may contain on the average, 50 percent
moisture, but the moisture content may vary considerably, depending
on the particular type of wood processing involved. Other than the
moisture content, wood shows a remarkably uniform composition. It
usually contains less than 1 percent ash and little or no sulfur.
Table 2 shows a comparative chemical analysis of wood and bark, coal,
and oil. Table 3 analyzes hogged fuel, while covering moisture content
on an "as received" as well as an "air dried" basis.
5
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IV. BOILER DESIGN AND OPERATION
In general, bark boilers can be divided into two ranges of sizes,
less than 1133.7 kilograms steam per minute (200 million Btu per hour)
and 113.7 kilograms steam per minute to 6048.0 kilograms steam per
c
minute (1066 million Btu per hour). Three basic classes of furnace
design are commonly used for wood firing: Dutch ovens, spreader stokers,
and suspension burners.
In a Dutch oven the burning is done in two stages: (1) drying and
gasification, and (2) combustion of gaseous products. The first stage
is accomplished in a cell separated from the boiler section by a bridge
wall; the combustion stage takes place in the main boiler section.
This type of unit is not responsive to steam load changes and has poor
temperature stability. It has poor combustion control and is not
commonly used to fire multiple fuels.
The spreader stoker type boilers operate with three stage burning
in a single chamber: (1) drying (in suspension), (2) distillation and
burning of volatile matter, and (3) burning of fixed carbon. This
type of operation has a fast response to load changes, has improved
Q
combustion control, and can be operated with multiple fuels.
In order to increase steam production in a spreader stoker boiler
or to keep the boiler temperature up while burning wood fuel of poor or
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variable quality, gas or oil is often burned as an auxiliary fuel. Gas or
oil is the auxiliary fuel normally used, but coal can also be used when
9
provided for in design.
Another type of boiler that, as yet, has had limited application, is
the suspension-fired boiler which is similar to a pulverized coal-fired
boiler. This type of boiler requires finely hogged bark to guarantee that
the bark will burn in suspension.
Most bark boilers have spreader stoker firing equipment and burn the
bark on the grate in a thin layer. The most popular type of stoker is
the traveling grate stoker. It is ideally suited for areas with high ash
content bark, since it provides for continuous ash discharge. It also can
better compensate for bad bark distribution than can a dump grate stoker.
Because of this, the traveling grate stoker requires less grate area and
results in a physically smaller sized boiler.
At present, bark boiler emissions are more affected by the various
process operations than they are by the application of air pollution
control equipment. Boiler design, auxiliary equipment design, bark
handling techniques, excess air used, wood moisture content, and equipment
operation all have a significant effect. The most predominant effect by
far, however, is the type and amount of fly ash reinjection.
Fly ash reinjection systems, which return collected particles to the
combustion zone to achieve more complete combustion of the carbon, represent
a compromise between two conflicting objectives. While reinjection increases
boiler efficiency from 1 to 4 percent and minimizes the emissions of
uncombusted carbon, it also increases boiler maintenance requirements,
decreases the average fly ash particle size, increases the dust load to the
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collector, and makes collection more difficult. Properly designed reinjection
systems should separate the sand and the char from the exhaust gases, reinject
the larger carbon fraction to the boiler, and reinject the fine sand particles
to the ash disposal system.
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V. CHARACTERISTICS OF PARTICULATE EMISSIONS
The particulate carried in the wood-fired boiler exhaust gas consists
of two separate and distinguishable materials - sand and bark char or fly
ash. The sand is usually entrained with the char, the quantity depending
on the method by which the original wood was logged and delivered. The
fly ash particulates have a low specific gravity, 0.15 to 0.5, and a large
surface area to particle mass ratio. A typical size distribution curve
is given in Table 4.
The bark fly ash, unlike most fly ash, is primarily unburned carbon.
With collection and reinjection to the boiler, greater carbon burnout can
increase boiler efficiency from one to four percent. The reinjection of
collected ash also significantly increases the dust loading since the sand
is also recirculated with the fly ash.
Bark boiler emissions are more affected by the various process opera-
tions than they are by the application of air pollution control equipment.
Boiler design, auxiliary equipment design, bark handling techniques, and
equipment operation all have a significant effect. The process operation
which has the greatest adverse effect is the type and amount of fly ash
reinjection. A tenfold dust loading increase has been reported with 100
percent reinjection.
12
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VI. EMISSION CONTROL TECHNOLOGY
A survey of currently operated facilities which fire wood residue alone
or in combination with fossil fuel shows that most operate with mechanical
collectors; some operate with low energy wet scrubbers, and a few facilities
currently use higher energy venturi scrubbers (HEVS) or electrostatic
precipitators (ESP). One facility reviewed is using a high temperature
baghouse control system.
Practically all of the facilities currently meeting the new source
particulate matter standard (43 nanograms per joule) are using wet scrubbers
of the venturi or wet impinger type. Table I presents a summary of the
best systems in operation today.
Currently, the use of multitube cyclone mechanical collectors on
hogged fuel boilers provides the sole source of particulate removal for a
majority of existing plants. The most commonly used system employs two
multicyclones in series, allowing for the first collector to remove the
bulk of the dust and a second collector with special high efficiency vanes
for the removal of the finer particles. Collection efficiency for this
arrangement ranges from 65 to 95 percent. This efficiency range is not
sufficient to provide compliance with the Federal particulate matter
standard, but does provide a widely used first stage collection to which
other control systems are added.
10
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Of special note is one facility using a Swedish designed mechanical
collector in series with conventional multiclone collectors. The Swedish
collector is a small diameter multitube cyclone with a movable vane ring
that imparts a spinning motion to the gases while at the same time main-
taining a low pressure differential. This system is reducing emissions
13
from the largest boiler found in the review to 107 nanograms per joule.
Electrostatic precipitators have been demonstrated to allow compliance
with the particulate matter standard when coal is used as an auxiliary fuel.
Available information indicates that this type of control provides high
collection efficiencies on combination wood residue coal-fired boilers.
One ESP collects particulate matter from a 50 percent bark, 50 percent
coal combination-fired boiler. An emission level of 13 nanograms per
joule (.03 pounds per million Btu) was obtained using test procedures
recommended by the American Society of Mechanical Engineers.
The fabric filter (baghouse) particulate control system provides the
highest collection efficiency available, 99.9 percent. On one facility
currently using a baghouse on a wood residue-fired boiler, the sodium
chloride content of the ash being filtered is high enough (70 percent)
that the possibility of fire is practically eliminated. Source test data
collected with EPA Method 5 showed this system reduces the particulate
emissions to 5 nanograms per joule (.01 pounds per million Btu) while
this boiler is firing 100 percent bark.
The application of fabric filters to control emissions from hogged
fuel boilers has recently gained acceptance from several facilities of
the paper and pulp industry, mainly due to the development of improved
designs and operation procedures that reduce fire hazards. Several
11
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large sized boilers, firing salt and non-salt laden wood residue, are
being equipped with fabric filter control systems this year and the
performance of these installations will verify the effectiveness of
fabric filtration.
Practically all of the facilities currently meeting the new source
particulate matter standard are using wet scrubbers of the venturi or
wet-impinger type. These units are usually connected in series with a
mechanical collector. Three facilities reviewed which are using the
wet-impingement type wet scrubber on large boilers burning 100 percent bark
are producing particulate emissions well below the 43 nanograms per joule
standard at operating pressure drops of 1.5 to 2 kPa (6 to 8 inches, hLO).
Five facilities using venturi type wet scrubbers on large boilers, two
burning half oil and half bark, and the other three burning 100 percent bark,
are producing particulate emissions consistently below the standard at
pressure drops of 2.5 to 5 kPa (10 to 20 inches, H20).
One facility has a large boiler burning 100 percent bark emitting a
maximum of 5023 nanograms per joule of particulate matter into a multicyclone
dust collector rated at an efficiency of 87 percent. The outlet loading
from this mechanical collector is directed through two wet-impingement type
scrubbers in parallel. With this arrangement of scrubbers, a collection
efficiency of 97.7 percent is obtained at pressure drops of 2 kPa (8 inches,
H20). Source test data collected with EPA Method 5 showed particulate
matter emissions to be 15 nanograms per joule, well below the 43 nanograms
per joule standard.
Another facility with a boiler of similar size and fuel was emitting a
maximum of 4650 nanograms per joule into a multicyclone dust collector
12
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operating at a collection efficiency of 66 percent. The outlet loading from
this collector is drawn into two wet-impingement scrubbers arranged in
parallel. The operating pressure drop on these scrubbers was varied within"
the range of 1.6 to 2.0 kPa (6 to 8 inches, H^O), resulting in a proportional
decrease in discharged loadings of 25.8 to 18.5 nanograms per joule.17
Source test data collected on this source was done with the Montana
Sampling Train.
Facilities using a venturi type wet scrubber were found to be able to
meet the 43 nanograms per joule standard at higher pressure drops than the
impingement type scrubber. One facility with a large boiler burning 100
percent bark had a multi-cyclone dust collector in series with a venturi wet
scrubber operating at a pressure drop of 5 kPa (20 inches, HpO). Source
test data using EPA Method 5 showed this system reduced emissions to an
average outlet loading of 17.2 nanograms per joule of particulate matter.
Another facility with a boiler burning 40 percent bark and 60 percent
oil has a-multicyclone and venturi scrubber system obtaining 25.8 nanograms
per joule at a pressure drop of 2.5 kPa (10 inches, H20).19 The Florida
Wet Train was used to obtain emission data at this source. A facility
of similar design but burning 100 percent bark is obtaining the same
emission control, 25.8 nanograms per joule, at a pressure drop of 3 kPa
on
(12 inches, h^O). Source test data collected on this source was obtained
with EPA Method 5.
13
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VII. REFERENCES
1. "Economics of Environmentally Acceptable Wood Waste Burning" by
Jorgen 6. Nedenhaz. February 1978.
2. "Air Pollution Control Technology and Costs in Nine Selected Areas."
Industrial Gas Cleaning Institute, Inc. September 1972.
3. "Handling Ash from Bark-Fired Boilers." Robert L. Bump. Power, p. 94,
February 1977.
4. Ibid.
5. "Comparison of Fossil and Wood Fuels," E. H. Hall, EPA Report Number
EPA-600/2-76-056, March 1976.
6. "Kraft Mill Bark Boilers," L. C. Hardison, EPA Contract Number 68-02-0301.
September 1972.
7. "An Investigation of Source Particulate Measurement Procedures, Particle
Sizes, and Practical Control Technology for Wood Fuel-Fired Boilers,"
National Council of the Paper Industry for Air and Stream Improvement.
Technical Bulletin No. 72. June 1974.
8. Ibid.
9. "Boilers Fired With Wood and Bark Residues," David C. Junge, Oregon State
University. Research Bulletin 17. November 1975.
14
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10. Reference 2.
11. "Studies on the Collection of Bark Char Throughout the Industry,"
Alvah Barren, Jr. Tappi, August 1970. Volume 53, Number 8.
12. Ibid
13. "Combination Fuel Boiler Particulate Emission Control Pilot Studies,"
National Council of the Paper Industry for Air and Stream Improvement.
Technical Bulletin Number 73, June 1974.
14. Source Test Data on Westvaco Co., Covington, Virginia, from Westvaco
Co. (September 1977)
15. Source Test Data on Simpson Timber Co., Shelton, Washington, from
Olympic Air Pollution Control Authority. (September 1977)
16. "Wet Scrubber Application to Hogged Fuel Boiler." John W. Robinson.
Kirby Timber Corp. A paper for presentation at the 68th Annual
Meeting of the Air Pollution Control Association, June 1975.
17. "Hogged Fuel Boiler Emissions Control - A Case History." Herman K.
Effenberger. A paper for presentation at Environmental Division
Conference of Tappi, May 1972.
18. "An Examination of the Performance of Wet Scrubbers on Combination
Fuel-Fired Boilers." Andrew 6. Kutyna. November 1976.
19. Ibid.
20. Ibid.
15
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TABLE 3
CHEMICAL ANALYSES OF HOGGED FUELS
(%, except as noted)
Item
Moisture, as received
Moisture, air-dried
Proximate analysis, dry:
Fuel
Volatile matter
Fixed carbon
Ash
Ultimate analysis, dry:
Fuel
Hydrogen
Carbon
Nitrogen
Oxygen
Sulfur
Ash
Heating value, dry: _
(joule/kg)
Western
Hemlock
57.9
7.3
74.2
23.6
2.2
5.8
50.4
0.1
41.4
0.1
2.2
20.05 x 106
Type of Fuel
Douglas
Fir
35.9
6.5
82.0
17.2
0.8
6.3
52.3
0.1
40.5
0
0.8
21.05 x 106
Pine
Sawdust
—
6.3
79.4
20.1
0.5
6.3
51.8
0.1
41.3
0
0.5
21.25 x 106
18
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40
80
CO
CD
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jxi 200
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325
WITHOUT
REIWJECTION
100/Li
10 30 50
% UNDER BY WEIGHT
70
90
Figure 1. Particle size distribution of bark boiler flyash.
19
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-450/2-78-044
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Wood Residue-Fired Steam Generator
Particulate Matter Control Technology Assessment
5. REPORT DATE
Ort.nhpr 1Q7R
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U. S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
DAA for Air Quality Planning and Standards
Office of Air, Noise, and Radiation
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/200/04
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This document presents the results of a performance review of particulate matter
control systems on wood-fired steam generators. This review describes the industry,
presents the characteristics of wood residue and particulate emissions, and discusses
the control equipment and emission limits which represent Best Available Control
Technology (BACT).
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Wood Residue-Fired Steam Generator
Hogged-Fuel Boilers
Particulate Matter
Air Pollution Control
8. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
23
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220—1 (Rev. 4—77) PREVIOUS EDITION is OBSOLETE
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