COPAC-5
ABATEMENT OF
PARTICULATE EMISSIONS
FROM
STATIONARY SOURCES
National Academy of Engineering
National Research Council
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COPAC-5
ABATEMENT OF
PARTICULATE EMISSIONS
FROM
STATIONARY SOURCES
Prepared by
Ad Hoc Panel on Abatement of Particulate
Emissions from Stationary Sources
Committee on Air Quality Management
Committees on Pollution Abatement and Control
Division of Engineering
National Research Council
National Academy of Engineering
Washington, D.C.
1972
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This is the report of a study undertaken
by the Committee on Air Quality Management
Ad Hoc Panel on Abatement of Particulate
Emissions from Stationary Sources for the
National Academy of Engineering in execution
of work under Contract No. CPA 70-48 with the
Office of Air Programs of the Environmental
Protection Agency
As a part of the Division of Engineering of
the National Research Council, National
Academy of Engineering, the Committees on
Pollution Abatement and Control perform study,
evaluation, or advisory functions through
groups composed of individuals selected from
academic, governmental, industrial, and public
sources for their competence and interest in
the subject under consideration. Members of
these groups serve as individuals contributing
their personal knowledge and judgment and
not as representatives of any organization in
which they are employed or with which they may
be associated.
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PREFACE
In this report, a program of government and
industry research for dealing with particulate emissions
from stationary sources is outlined. It is based on an
evaluation of current technology and an assessment of
what developments in control and measurement techniques
can reasonably be expected during the next five to ten
years.
The effects of particulate emissions on human
beings and the environment are complex. Detailed con-
sideration of these effects is beyond the scope of this
report but, along certain broad lines, they have been
taken into account in the Panel's discussion.
It is important to understand that each in-
crement in control of air pollution requires a shift in
our national resources and capital investment and that
such shifts are almost certain to lead to increased
costs for energy and materials. The challenge is to
minimize such increases while accomplishing our goals
by properly designing industrial processes and systems
for pollution abatement and control.
The Panel's evaluation of the current status
of and trends in particulate-emission-control technology
and its recommendations for improving and maintaining
the quality of our atmosphere have been based on pre-
sentations by manufacturers of gas-cleaning devices and
the Office of Air Programs, several comprehensive re-
views prepared by industrial organizations and research
institutes, and the review of many reports and tech-
nical documents, in addition to the general knowledge
of the field possessed by the Panel, Committee, and
Academy review groups. The Panel is particularly
grateful to Mr. Robert C. Lorentz, Division of Control
Systems, Office of Air Programs, Environmental Pro-
tection Agency, and his staff for their valuable sup-
port.
Technical problems are identified in this
report, the solution of which will require the initia-
tion of significant new engineering and scientific
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research programs. These problems are summarized in the
first chapter in which a six-point program for improving
particulate-emission control is proposed. The background
and scope of the study are discussed in the second chap-
ter, while emission sources and particle-collection tech-
nology are reviewed in the third and fourth chapters,
respectively. Since the measuring and monitoring of
particle concentrations is an important and difficult
task, a separate chapter on this subject has been in-
cluded. Finally, the characteristics of fine particles,
which have been identified as a major problem area, are
discussed in the sixth chapter.
Despite the institution of new emission-control
measures, particulate levels remain high in many of our
cities. The programs proposed here can substantially
improve and maintain the quality of our air environment.
The need for such programs is now recognized by a
concerned citizenry.
S. K. Friedlander
Chairman
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NATIONAL ACADEMY OF ENGINEERING
NATIONAL RESEARCH COUNCIL
DIVISION OF ENGINEERING
COMMITTEE ON AIR QUALITY MANAGEMENT
Jack E. McKee, California Institute of Technology,
Chairman
Reid A. Bryson, The University of Wisconsin,
Ex Officio*
Thomas H. Chilton, Retired, E. I. du Pont de Nemours
and Company, Inc.
Merrell R. Fenske,** The Pennsylvania State University
S. K. Friedlander, California Institute of Technology
Robert L. Hershey, Retired, E. I. du Pont de Nemours
and Company, Inc.
Chalmer G. Kirkbride, Retired, Sun Oil Company
Charles N. Satterfield, Massachusetts Institute of
Technology
Thomas K. Sherwood, University of California at Berkeley
Staff
R. W. Crozier, Executive Secretary,, Committees on Pollu-
tion Abatement and Control, National Research Council
J. M. Marchello, Staff Engineer^ Committees on Pollution
Abatement and Control, National Research Council
Barbara P. Sowers, Administrative Secretary, Committees
on Pollution Abatement and Control, National Research
Council
Liaison Representative - EPA
John 0. Smith, Chief, Office of Engineering Analysis,
Division of Control Systems, Stationary Sources
Pollution Control Programs, Office of Air Programs,
Environmental Protection Agency
*Liaison—NAS-NAE Environmental Studies Board
**Deceased
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NATIONAL ACADEMY OF ENGINEERING
NATIONAL RESEARCH COUNCIL
DIVISION OF ENGINEERING
COMMITTEE ON AIR QUALITY MANAGEMENT
AD HOC PANEL ON ABATEMENT OF PARTICULATE
EMISSIONS FROM STATIONARY SOURCES
Sheldon K. Friedlander, California Institute of
Technology, Chairman
Allen D. Brandt,* Bethlehem Steel Corporation
Melvin W. First, Harvard School of Public Health
T. T. Frankenberg, American Electric Power Service
Corporation
John L. Gilliland, Ideal Cement Company
A. Lieberman, Royco Instruments, Inc.
Elmer Robinson, Washington State University
Kenneth Whitby, University of Minnesota
Staff
R. W. Crozier, Executive Secretaryj Committees on Pollu-
tion Abatement and Control, National Research Council
J. M. Marchello, Staff Engineer•> Committees on Pollution
Abatement and Control, National Research Council
Barbara P. Sowers, Administrative Secretary, Committees
on Pollution Abatement and Control, National Research
Council
Liaison Representative _-_ EPA
Robert C. Lorentz, Division of Control Systems, Office
of Air Programs, Environmental Protection Agency
*Deceased
VI
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CONTENTS
I. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS . . . 1
A. Summary ]_
B. Conclusions 2
C. Recommendations 4
II. INTRODUCTION 7
A. The Clean Air Amendments of 1970 7
B. Role of the Academies 9
C. Scope of the Particulate-Pollution Problem.10
D. Importance of Particle Size and Chemical
Composition 12
III. EMISSION SOURCES 14
A. Fugitive or Open Sources 17
B. Captive or Contained Sources 17
C. Sources of Secondary Particles 18
D. Projections and Conclusions 20
IV. PARTICLE COLLECTION 21
A. Collection Equipment—Performance
and Cost 21
B. The Need for Size-Efficiency Data for
Fine Particles 23
C. The Gas-Cleaning Industry 24
D. Conclusions 25
V. SOURCE-SAMPLING AND -MEASUREMENT METHODS. . . .27
A. General 27
B. Requirements for Stack-Gas Measurement
Systems 29
1. Gas-Flow Characterization 29
2. Sample Acquisition and Transport. . . .29
3. Sample Analysis 29
4. Other Requirements 30
C. Methods Currently Used 31
D. Conclusions 33
VI. FINE PARTICLES 36
A. Properties of Fine Particles 36
B. Atmospheric Particle-Transformation
Processes 38
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C. Principal Sources of Fine Particles . . . .38
1. Primary Particles 38
2. Secondary Particles 39
D. Status of Knowledge 40
E. Conclusions 40
TABLES
1. Estimated concentrations of Los Angeles
aerosol particles by source 11
2. Sources of particulate pollution (weight
basis), United States, 1968 15
3. Major stationary industrial sources of
particulates (weight basis), United
States, 1968 16
4. Estimated gaseous-pollutant emissions,
United States, 1968 19
5. Major features of sampling trains 32
6. High-performance coal fly-ash electrostatic
precipitator measurements with different
sampling trains 34
FIGURE
Typical particulate distribution 37
BIBLIOGRAPHY AND REFERENCES 42
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I
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
A. SUME1ARY
Particulate emissions are readily visible
when in high concentration and, for this reason, were
the first form of air pollution to be controlled. In
recent years, gaseous pollutants have received the major
attention, but continuing deterioration of visibility
in our urban areas, together with the possibility of
unfavorable climatic and health effects, has led to re-
newed concern for controlling particles in the atmo-
sphere.
The Panel finds that existing technology for
particle collection is adequate for the removal of the
major fraction of the mass of participate matter cur-
rently being emitted to the atmosphere from confined
sources, but reducing the total mass of such emissions
is only a first step.
Particles in effluent gases are distributed
over a wide range of sizes from a few angstroms to
several microns in diameter. Since collection is less
efficient for small than for large particles, signif-
icant numbers of particles less than 2 microns in di-
ameter escape into the atmosphere. These small par-
ticles, equivalent in mass to a considerably smaller
number of large particles, have a much greater impact
on visibility, health, and water-droplet nucleation
than do the larger particles. Therefore, tonnage-
collection figures and weight-removal efficiencies are
inadequate to delineate the entire particle-emission
problem.
Thus, the Panel finds that small particles,
together with particulate matter formed in the atmo-
sphere from gas-phase reactions and condensation, may
continue to limit visibility and may affect health even
when presently uncontrolled sources of particulate
matter are equipped with the best collection devices
available.
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B. CONCLUSIONS
On the basis of its review of particulate-
emission-control technology, the Panel draws the following
conclusions:
1. At the 1970 level of control it is esti-
mated that partioulate emissions from in-
dustrial sources will insrease from the
present estimated level of 18 million
tons per year to over SO million tons
per year by the year 2000.
2. Existing methods for the removal of par-
ticulate matter from gases are adequate
for control of the greatest part, by
weight, of the emissions from stationary
sources. On a total mass basis, the
average efficiency that can be achieved
with many types of particle-control equip-
ment now being installed is well over 90
percent for most industrial applications
and may exceed 99 percent.
3. Four major categories of industrial par-
ticle-control equipment (electrostatic
precipitators, fabric filters, scrubbers,
and inertial or centrifugal mechanical
collectors) , each having special charac-
teristics that determine its suitability
for a particular gas-cleaning application,
have been available for many years, but
they are not universally applied to all
sources in all industries. For example,
it is estimated that 38 percent of stoker-
fired industrial boilers and 75 percent
of crushed-stone operations are not con-
trolled, thus leading to the release of
2 million tons of coal fly ash and 4.5
million tons of rock dust per year—almost
40 percent of the estimated mass of par-
ticulate emissions from all major in-
dustrial sources.
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A. The application of the best existing tech-
nology to presently uncontrolled and in-
adequately controlled stationary industrial
sources will permit major reductions in
the total weight of particulate matter
emitted to the atmosphere. Depending upon
the level of control required, such appli-
cations may be accompanied by significant
increases in production costs.
5. It may not be sufficient to evaluate the
performance of pollution-control equip-
ment on the basis of mass emissions alone.
Particle size, composition, and the
presence of co-contaminants must also be
taken into account; special emphasis needs
to be placed on the amount of material in
the fine-particle size range and on its
chemical nature.
6. Portions of certain polluting gases are
converted to aerosols in the atmosphere
and contribute significantly to the local
and national burden of suspended particles.
However, this formation of fine particles
by secondary processes is not well under-
stood. The relative importance of sec-
ondary-particle formation must be taken
into account when setting standards.
Otherwise, reduction in particulate emis-
sions may not lead to the expected im-
provement in air quality.
7. Presently available control equipment has
achieved only limited effectiveness in
that: its collection efficiency for fine
particles may not be adequate; and it is
not generally applicable to situations
where co-contaminants must be dealt with
because of the characteristics of the
materials. Significant increases in
efficiency over certain size ranges,
particularly the fine-particle, as well
as much needed improvements in reliability,
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may be achieved by intensifying engineer-
ing research and development in the par-
ticle-control field.
8. In general3 the ranges of operation of the
various collection devices with respect
to temperature,, humidity3 and corrosiveness
of the gases should be significantly ex-
tended. Other current needs include better
fabric-filter cleaning techniques, improved
filter fabrics for longer life and better
performance, more reliable electrostatic
precipitator components, better designs
to increase the charging-wire life, and
more efficient and reliable equipment.
9. Major gaps exist in instrumentation for
sampling and measuring the characteristics
of particulate pollution. These gaps have
hampered the development of rational emis-
sion and ambient-air-quality standards.
The state of technological and scientific
knowledge is sufficiently advanced, how-
ever, to support significant improvements,
especially in aerosol instrumentation.
C. RECOMMENDATIONS
There is a great need for Environmental Pro-
tection Agency leadership in planning and coordinating
a national program to ensure that the needs identified
in this report are met in a reasonable and timely manner.
To meet this need, the Panel recommends that:
1. Collaborative federal programs be develop-
ed with gas-cleaning equipment manufac-
turers and users to gather basic engineer-
ing performance data on particle-collection
equipment as a function of particle size;
special emphasis should be placed on
measuring collection efficiencies in the
fine-particle range.
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2. The Environmental Protection Agency sup-
port an expanded and coordinated experi-
mental program designed to gather informa-
tion on the size distribution3 especially
in the fine-particle range, and on the
chemical composition of particles and
carrier gas emitted from stationary
sources.
3. Engineering research and development on
particle-control technology be intensified
and special emphasis be placed on the 0.2-
to 2. 0-micron diameter range. This could
involve a cooperative effort jointly funded
by the Environmental Protection Agency,
manufacturers of industrial gas-cleaning
equipment, and major equipment users, thus
obtaining better control without creating
an intolerable individual plant or in-
dustry-wide economic burden and impairing
international competitive positions.
4. Support be given a major research program
to improve existing in-stack sampling
systems and to develop new stack monitoring
systems for measuring the physical and
chemical characteristics of particulate
emissions. A sizable research investment
must be made, and it is particularly im-
portant that efforts continue on the de-
velopment of parallel systems for ambient-
air measurement.
5. Improved methods be developed to relate
the characteristics of emission sources
to atmospheric particulate pollution.
This will require an expansion of research
on the origin and behavior of fine par-
ticles and their interaction with re-
active gases.
6. Emission standards and ambient-air-quality
standards be reviewed to more strongly
take into account hazardous particles.
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Criteria and standards for agents such as
lead, asbestos, and carcinogens, associated
with particulate matter, should be related
to particle-size and other aerosol char-
acteristics.
The level of support by the Environmental
Protection Agency for research and development on the
abatement of particulate emissions is approximately
$500,000 in fiscal year 1972. The Panel believes that
this level of support, together with industrial expen-
ditures, is grossly inadequate for the program needs
outlined above. A considered estimate of the cost of
such a program indicates that in excess of $5 million
per year over the next 10 years should be spent by the
Environmental Protection Agency on research and develop-
ment.
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II
INTRODUCTION
A. THE CLEAN AIR AMENDMENTS OF 1970
The increasing concern of Americans for the
present and future state of the nation's air resources
is reflected in the Air Quality Act of 19671 (Public
Law 90-148 as amended) and the Clean Air Amendments of
19702 (Public Law 91-604 in which P.L. 90-148 was further
amended to provide for a more effective program to im-
prove the quality of the nation's air). These Acts
provide for emissions standards for existing sources,
which are to be established and enforced by the states,
and efficiency standards for new sources, some of which
were promulgated during 1971 by the Environmental Pro-
tection Agency (EPA).3 All standards are subject to
review and revision. Responsibility at the federal
level for carrying out the provisions of these laws
remains with the EPA Administrator.
On April 30, 1971, the EPA published in the
Federal Register the National Primary and Secondary
Ambient Air Quality Standards1'' which were set by the
Administrator. These standards are based on air-
quality-criteria documents8"16 published by the EPA
and allow an adequate margin of safety to protect the
public health (primary standards) and the public welfare
(secondary standards) from all known or anticipated
adverse effects associated with the presence of pol-
lutants in the ambient air:
The national primary ambient-air-quality stan-
dards for particulate matter are:
(a) 75 micrograms per cubic meter—annual
geometric mean.
(b) 260 micrograms per cubic meter—maximum
24-hour concentration not to be exceeded more than once
per year.
'The national secondary ambient-air-quality
standards for particulate matter are:
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(a) 60 micrograms per cubic meter—annual
geometric mean3 as a guide to be used in assessing im-
plementation plans to achieve a 24-hour concentration.
(b) 150 micrograms per cubic meter—maximum
24-hour concentration not to be exceeded more than once
per year.
Under the provisions of these laws, the EPA
published, in August 1971 in the Federal Register,
guidelines to the states5 for preparation, adoption, and
submittal of implementation plans for enforcement of
national ambient-air-quality standards.
Under Section 110,2 the states were given
until February 1972 to submit a plan providing for im-
plementation, maintenance, and enforcement of the pri-
mary standards in each air-quality-control region and
an additional nine months to submit a similar plan for
secondary standards. These plans are to include a time-
table for compliance with primary standards within three
years and with secondary standards within a reasonable
time. States may adopt standards more stringent than
the federal standards.
The EPA recommendations5 to the states for
their implementation plans cover maximum emissions of
particles and gases. These, together with recently
promulgated federal standards of performance for certain
new stationary sources, are as follows for particulate
matter:
Required for Recommended for
New Sources* Existing Sources
Fuel-Burning Equipment 0.10 Ib per 0.30 Ib per
million Btu, million Btu
20% opacity
*Mass emission limitations provide control for all par-
ticles. Opacity restrictions limit emission of fine par-
ticles that have high light-scattering capability per
unit mass.
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Required for
New Sources
Recommended for
Existing Sources
Incinerators
0.08 gr/scf
corrected to
12% C0n
Portland Cement Plants Kiln; 0.30
Ib per ton
of feed, 10%
opacity
Clinker
Cooler; 0.10
Ib per ton of
feed, 10%
opacity
0.20 Ib per 100
Ib charged
Use of process
weight formula,
ranges between
1.0 and 0.1 Ib
per ton of feed
Sulfuric Acid Plants
0.15 Ib of mist
per ton of acid
produced, 10%
opacity
B. ROLE OF THE ACADEMIES
In 1967 the National Academy of Sciences and
the National Academy of Engineering established the
Environmental Studies Board to coordinate activities
of the two Academies in the environmental field. One
of the first acts of this Board was to recommend the
establishment within the Division of Engineering,
National Research Council, of four committees on air,
water, noise, and solid-waste management. These com-
mittees have an engineering orientation and are avail-
able to offer advice and assistance to the Congress and
to agencies of the executive branch of government having
responsibility for pollution abatement and control.
In June 1969, the National Air Pollution Con-
trol Administration (NAPCA) of the U.S. Department of
Health, Education, and Welfare (now the Office of Air
Programs of the Environmental Protection Agency) re-
quested the National Academy of Engineering to make a
comprehensive review of present industry and government
research, development, and demonstration programs and
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to include an examination of technical and economic
potentials, adequacy of scope, proper integration with
similar efforts, and responsiveness to national needs
directed toward control of sulfur oxides effluents from
stationary sources of combustion. The report of that
review, Abatement of Sulfur Oxide Emissions from Sta-
tionary Combustion Sources, 17 was published in May 1970.
The newly formed Office of Air Programs (OAP)
of the EPA then requested the National Academy of Engi-
neering-National Research Council to provide additional
studies of sulfur oxides emissions from industrial
sources, nitrogen oxides emissions from stationary
sources, and particulate emissions from stationary
sources. In this report on particulate emissions
from stationary sources, the Panel was asked to review
the status of particulate emissions and control tech-
nology and to recommend research, development, and
demonstration programs to meet the national need for
control of emissions, including fine particles, from
stationary sources.
C. SCOPE OF THE FARTICULATE-POLLUTION PROBLEM
Atmospheric particulate matter is produced
from natural sources such as the sea and the soil and
from man-made sources such as fuel combustion in power
plants, various industrial processes and internal-
combustion engines. Atmospheric particulate matter can
be classified as primary—introduced into the atmo-
sphere in particulate form, or secondary—formed in
the atmosphere from certain gases by chemical and
physical processes.
The particulate-pollution problem is only
partly a primary-particle-emission problem. For example,
as shown in Table 1, in Los Angeles about 35 percent
of the particulate matter is formed in the atmosphere
in a secondary fashion from gaseous pollutants; an
additional 15 to 20 percent is derived from natural
sources. The motor vehicles in Los Angeles are respon-
sible for approximately 15 percent of the primary-par-
ticulate-matter emissions. Los Angeles is especially
subject to photochemical smog formation and cannot be
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TABLE 1
ESTIMATED CONCENTRATIONS OF LOS ANGELES AEROSOL PARTICLES BY SOURCE18
(annual average)
Source
Mass Concentration
(ug/m3)
Natural Background
Primary
Dust rise by wind
Sea salt {^l"
Spores, pollen, etc.
Secondary
Vegetation (organic vapors)
Biological (soil bacterial action,
decay of organics)-NH3, NOX, S...
Man-Made
Primary
Motor vehicles
Organic solvent usage
Petroleum
Aircraft
Combustion of fuels
Other
Secondary
Reactive hydrocarbon vapors
NOI
so;
8-20
3
3
Unknown
3-6
0.7
15
6
1.3
4
5
5
11
13.5
14.A
14-26
4-7
36
39
TOTAL ACCOUNTED FOR
MEASURED TOTAL
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considered typical of the areas where coal and fuel-oil
combustion products represent the major emissions (i.e.,
the industrial centers of the East and Midwest).
Secondary conversion processes and sedimenta-
tion make it difficult to relate atmospheric particulate
pollution levels to the characteristics of pollution
sources, even when the turbulent dispersion patterns
are known. Ihis difficulty has hampered the development
of effective control strategies and the establishment
of meaningful emission standards. Currently, methods
ace being developed to relate air quality to emission
sources, both gaseous and particulate. High priority
should be given to such studies for urban and indus-
trial regions with differing source characteristics.
During fiscal year 1970, the EPA budget for
measurement and control of particulate emissions from
stationary sources was about $1 million; it decreased
to $640,000 in fiscal year 1971 and is $500,000 in fiscal
year 1972. This effort includes development of measure-
ment methods, emissions inventory, and improved control
equipment and systems for all applications. This level
of funding is grossly inadequate. Beginning a balanced
federal program to improve particulate-control capa-
bility of the type described in the Recommendations
will require expenditures at least 10 times greater.
Substantial increases in both federal and industrial
support are required to make progress toward the level
of control commensurate with the national need.
D. IMPORTANCE OF PARTICLE SIZE AND CHEMICAL COMPOSITION
A difficulty arises in assessing the impor-
tance of particulate emissions because the particles
cover a wide size range. Most of the data on emissions
and collection-equipment efficiency are reported on a
gross-weight-collected basis. It is, therefore, not
possible to distinguish between the emission of a few
large particles or of many smaller particles114'15 of
equal weight, whereas the difference between equal
weights of small and large particles released to the
atmosphere is very great with respect to atmospheric
residence time, visibility, lung deposition, cloud nucle-
ation, and weather modification.
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Another Important factor is the chemical nature
of the particles as it relates to toxicity, e.g., power-
plant fly ash produced by the combustion of pulverized
coal consists of a diverse mixture of metal oxides and
silica.19 Other particulate sources produce different
mixtures of substances. On both a regional and global
scale, atmospheric particulate matter plays a principal
role in the transport through the air of lead, zinc,
barium, vanadium, and other substances.
The chemical composition and size distribution
of fine particles are important in determining the
effects of this type of air pollution. For example,
the health hazard associated with inhaled airborne
particles depends on: 1) the site of deposition in
the respiratory tract, which is determined by particle
size; and 2) the effect on biological tissues at the
deposition site, which depends on chemical composition.
Also, the effect of particles on visibility depends on
the size and refractive index of the particles, both of
which are influenced by chemical composition.18
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Ill
EMISSION SOURCES
Midwest Research Institute (MRI), under con-
tract with EPA, completed a survey in 1971 of particu-
late emissions and control.20'21 Some of the results
are summarized in Tables 2 and 3. Nationwide, 80 per-
cent by weight of the total particles in the atmo-
sphere36 are estimated to arise from natural sources.*
In urban and industrial regions, man-made contributions
to particulate pollution are usually many times greater
than those from natural sources. The national goal of
lowering urban ambient-air concentrations to 60 yg/m3
from present levels of approximately 120 yg/m3 will
require close control over all man-made sources.
The latest detailed summary information on
industrial particle emissions and properties, based on
mass measurements of particles collected from stack
effluents, is presented in Table 3. The MRI study21
also provided information on particle size and chemical
composition. Other measurement and rating methods for
determining suspended-particle characteristics, such
as light-scattering or visibility range, toxicity, and
the potential of gaseous emissions to form aerosols in
the atmosphere, are needed.
The objective of the MRI study was to iden-
tify, characterize, and quantify particulate emissions
to the atmosphere from stationary sources in the con-
tinental United States. Emissions from each source
or industry were determined from: 1) emission factors
for an uncontrolled source based on a unit of produc-
tion; 2) the material processed per year; 3) the
average or expected efficiency of control equipment;
and 4) the percentage of production capacity equipped
with control devices. No new experimental measure-
ments were made.
*0ther authors have made different estimates but all
indicate that natural sources produce more emissions
than do man-made sources.
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TABLE 2
SOURCES OF PARTICIPATE POLLUTION (WEIGHT BASIS)21
United States - 1968
Emissions Percent by
Source (tons) Weight
Natural dusts 63,000,000 44.7
Forest fires 56,400,000 40.1
Subtotal 84.8
Major stationary
industrial sources 18,000,000 12.9
Transportation 1,200,000 0.8
Incineration 930,000 0.7
Other Sources 1,300>000 0.8
Subtotal 15.2
TOTAL 140,800,000 100.0
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TABLE 3
MAJOR STATIONARY INDUSTRIAL SOURCES OF PARTICULATES (WEIGHT BASIS)20
United States - 1968
Source
Fuel combustion
Coal
Electric utility
Pulverized
Stoker
Cyclone
Industrial boilers
Pulverized
Stoker
Cyclone
Fuel oil
Natural gas and LPG
Crushed stone, sand, and gravel
Grain elevators and other
agricultural operations
Iron and steel
Cement
Forest products
Lime
Clay
Primary nonferrous metals
Fertilizer and phosphate rock
Asphalt
Ferro-alloys
Iron foundries
Secondary nonferrous metals
Coal cleaning
Carbon black
Petroleum-catalyst regeneration
Acids
Efficiency
of Control
Application
of Control
92
80
91
85
85
82
—
—
Total
80
70
90 - 99
94
70 - 95
80 - 97
80
40 - 98
80 - 97
97
80 - 99
80
90 - 95
—
95 - 97
m Other
97
87
71
95
62
92
0
0
from fuel combustion
25
40
35 - 100
94
33 - 99
25 - 87
75
35 - 100
25 - 100
99
35 - 100
25 - 33
20 - 95
100
100
85 - 90
than Fuel Combustion
Emissions
(tons)
2,710,000
217,000
182,000
322,000
2,234,000
39,000
134,000
108,000
5,946,000
4,600,000
1,768,000
1,344,000
931,000
666,000
573,000
468,000
464,000
337,000
218,000
160,000
143,000
127,000
94,000
93,000
45,000
16,000
12,047.000
Total Major Stationary Industrial Sources 18,000,000
16
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The results of the MRI study21 differ from
those reported earlier by NAPCA.^*15 This difference
is probably due to the better information available
for the later study. However, reported emissions may
understate the true average because they are often
measured only on the best of newest units operating
under optimum conditions. Future research may also
show that the sampling methods used were less than ad-
equate for the task. A more detailed study to identify
all sources of particulates and accumulate information
on emissions is needed as a next step.
A. FUGITIVE OR OPEN SOURCES
As shown in Table 2, the largest sources of
particles are natural dusts and forest fires. These
are estimates based upon measurements made by researchers
in the field of soil conservation and by the U.S. Forest
Service.21 These sources account for an estimated 85
percent of the national atmospheric primary-particulate
loading on a mass basis and are a substantial portion
of background levels. The direct effect of these par-
ticles on people, however, is much less than that of
particles from man-made sources because of the concen-
tration of the population in urban areas.
Fugitive or unconfined sources directly related
to man's activities originate from agriculture, mining,
construction, and transportation. A number of uncon-
fined sources are included in the data in Table 3 (i.e.,
some crushing and grinding and materials-handling op-
erations are unconfined and are important sources with-
in a number of the processes listed).
B. CAPTIVE OR CONTAINED SOURCES
The mass (or weight) of emissions from
most of the industrial sources listed in Table 3 may
be highly controlled by the installation of presently
available control equipment. Estimates of the mass
efficiency of control equipment and the extent to which
it is presently being used are given in Table 3. In
some cases, such as the combustion of coal by electric
utilities, over 90-percent collection efficiency is
routinely achieved. Nevertheless, because of the
17
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large amounts of coal burned, total particulate emis-
sions from coal combustion remain a major portion of
total emissions.
Not reflected in Table 3 is the relative con-
tribution that the emissions make to the long-lived
suspended-particulate levels in the atmosphere. A
portion of the industrial emissions deposit near the
emission point in a short time. In recognition of
this, MRI also reported information on particle-size
distribution, outlet particulate loadings, and chemical
composition for the sources listed in Table 3.21 Un-
fortunately, few data relating to size are available,
and those only for a limited number of process sources.
C. SOURCES OF SECONDARY PARTICLES
Secondary particles, formed in the atmosphere
from gaseous pollutants, make a significant contribution
to urban air pollution. 2 For example, there are nearly
equal weights of primary and secondary particulate
matter from man-made sources in the Los Angeles atmo-
sphere (Table 1). The relative proportions vary from
city to city but there is little doubt that secondary-
particle levels are significant in all major cities.
Thus, efforts to meet the ambient-air standards for
particulate matter must include control of both pri-
mary- and secondary-particle sources.
Emissions of sulfur oxides, hydrocarbons, and
nitrogen oxides are given in Table 4. These gases
undergo complex reactions in the atmosphere. A major
portion of the gases ultimately form atmospheric
aerosols and are removed by the natural cleaning pro-
cesses of the atmosphere. What fraction of the gases
emitted contribute to the urban atmospheric particulate
problem is not known for most urban regions. However,
comparison of Tables 2 and 3 with Table 4 indicates
that the amount of such gaseous pollutants emitted by
man's activities exceeds the amount of primary particles
emitted.
18
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TABLE 4
ESTIMATED MAJOR GASEOUS-POLLUTANT EMISSIONS13
United States - 1968
(106 Tons/Year)
Sulfur Nitrogen Carbon
Source Oxides* Hydrocarbons Oxides** Monoxide
Transportation 0.8 16.6 8.1 63.8
Fuel combustion
in stationary
sources 24.4 0.7 10.0 1.9
Industrial
sources 7.3 4.6 0.2 9.7
Solid-waste
disposal 0.1 1.6 0.6 7.8
Miscellaneous 0.6 8.5 1.7 16.9
TOTAL 33.2 32.0 20.6 100.1
* S0x expressed as
**NOX expressed as
19
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D. PROJECTIONS AND CONCLUSIONS
Forecasts of future particulate-emission levels
have been made by several investigators. 20'21 »21+ These
are based on: 1) changes in production capacity; 2) im-
provements in control devices; and 3) increased applica-
tion of control devices based on legislative or regula-
tory enforcement.
If no improvement is made in the application
of controls for particulate-emission sources, industrial
emissions of particles may be expected to increase from
the present level of 18 million tons per year to over
50 million tons per year by the year 2000. Use of the
best presently available control equipment on all
sources would reduce emissions to about 8 million tons
per year by the year 2000. Improvements in control de-
vices and practices might reduce emissions to as low as
2.5 million tons by the year 2000,20 but simply re-
ducing total mass emissions may be insufficient since
emission and atmospheric formation of fine particles
may continue to increase and result in more severe
particulate pollution.
Control of emissions may in some instances be
achieved by significant process changes that alter the
quantity and character of emissions and reduce the
magnitude of the control problem.37
There is a pressing need for detailed physical
and Qhemioal information on the nature of the par-
ticulate emissions from various sources. This should
include information on particle-size distribution and
on chemical composition, with special emphasis on trace
metals and hazardous organic compounds. The EPA should
take the lead in gathering and publishing such emis-
sion-source data.
20
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IV
PARTICLE COLLECTION
Equipment for the control of particulate emis-
sions from confined sources includes inertial and centri-
fugal collectors, low- and high-energy scrubbers, elec-
trostatic precipitators, and cleanable fabric filters.
These devices have been available for many years and are
in widespread use, although, as pointed out in the pre-
vious chapter, many sources remain uncontrolled. Com-
prehensive reviews of the basic principles underlying
the design and application of such devices, together
with practical operating data, have been published
elsewhere.15,25-30
Construction and agricultural operations rank
high on the list of emission sources. Most of these
operations are unconfined and result in particulate
matter entering the atmosphere. Control is difficult
because of the large areas encompassed by such operations
and the transitory (construction) or intermittent (agri-
culture) nature of the activities. Equipment changes,
including use of enclosed conveyors and vehicles, and
improved methods of operation, such as wetting down sites
and use of wind barriers, are some of the ways of re-
ducing emissions from these open operations. However,
much move needs to be known about application of proper
control methods to fugitive and unconfined sources.
A. COLLECTION EQUIPMENT—PERFORMANCE AND COST
The estimated level of control, on a weight
basis, of major sources of particulate matter for new
installations is generally over 90 percent and in some
cases over 99 percent. There has been a steady improve-
ment in the collection efficiencies.
Costs of particulate-control equipment1^*21
vary widely. The costs cover a range of values because
of local conditions and the nature of the particles, the
gas stream, equipment size (gas volume), and design
collection efficiency. Published average cost figures
frequently do not reflect all cost components and fail
to illustrate the great range in costs.
21
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Some operational variables can have great
effect on collector-system performance and cost. For
example, the removal of sulfur oxides from effluents
of coal-burning power plants through use of low-sulfur
fuel can drastically lower the performance of electro-
static precipitators because this produces a more resis-
tive fly ash and requires the use of larger and more
expensive precipitators for maximum particulate-removal
efficiency. An alternative method of sulfur-emission
control—injection of dry limestone with the coal to
remove sulfur oxides from flue gases—greatly increases
the dust burden on the precipitators as well as the
dust resistivity.
In the field of electrostatic-precipitator
design, there is a need for more information on: 1) the
influence of dust properties, such as resistivity, on
its behavior in the precipitator in the presence of the
electrical field; and 2) the re-entrainment effect of
gas flow past the deposit.31 This information is im-
portant because present precipitator technology is
limited to a narrow band of operation in terms of
collected-dust resistivity and mass-collection effi-
ciency.
The greatest reliability problem associated
with precipitators is the integrity of the charging
wires. A tendency to wire failure is related to com-
peting demands for good mechanical and electrical prop-
erties. For example, the finer the wire, the better .
the electrical field; however, heavier wires are re-
quired to withstand vibrations caused by electrical
wind and arcing. When vibration is inhibited by fixing
the top and bottom, mechanical fatigue eventually pro-
duces broken electrode wires. Several new designs use
rigid electrodes in an attempt to overcome this problem.
Particle collection will, in many instances,
require dealing simultaneously with collection of one
or more co-contaminants. There is much interest in the
use of high-energy scrubbers for the removal of sulfur
oxides from flue gases. Efficient particle collection
in such systems could then be achieved simultaneously
with the removal of sulfur oxides. If this proves to
22
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be practical, scrubbers may replace precipitators in
some applications. The use of scrubbers has several
disadvantages, such as high energy requirements, stack
plumes of low buoyancy, corrosion problems, and water-
disposal problems, which must be overcome. However3
combined particle and sulfur oxides control is suffi-
ciently attractive economically to warrant support by
the EPA as part of a coordinated study. Control of
particulate and gaseous pollutants cannot be considered
entirely separate since both contribute to the particu-
late burden of the atmosphere (through gas-phase re-
actions). 36
The principal limitation of fabric-filter
systems is their inadequate performance when there are
high temperatures in the gas stream because of the low
heat tolerance of most fabrics. Although the practical
upper temperature limit of fibers has been extended as
new fibrous materials have become available, the maxi-
mum practical operating temperature at present is about
550° F using woven glass cloth. In the future, metal
and ceramic fibrous materials may facilitate operation
at temperatures up to 1000° F. Fabric-cleaning develop-
ments, such as the reverse-jet and pulsed-jet methods,
have kept pace with cloth improvements. However, there
is a need to develop filter housings and cleaning de-
vices that are suitable for operation at the higher
temperatures that can be reached with new fabrics.
B. THE NEED FOR SIZE-EFFICIENCY DATA FOR FINE PARTICLES
The high mass efficiencies often reported for
particle collection can lead to overly optimistic con-
clusions about emissions because of the large tonnages
involved. The small weight percentages of particles
that pass through high-airflow-capacity control equip-
ment still represent large numbers of particles escaping
to the atmosphere. Moreover, collection efficiencies
for the finest particles, which play a key role in air-
pollution effects, are significantly less than for
larger particles. For some types of collectors, such
as fibrous filters, theory indicates that particle-size-
removal efficiency passes through a minimum. Very
small particles are removed by diffusional processes,
23
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while large particles are captured by inertial effects.
The minimum in the efficiency curve tends to occur for
sizes between 0.1 and 1.0 micron, which is a particularly
important range with respect to visibility, health ef-
fects, and weather modification. Some data reported
for deep-bed fibrous filters indicate collection ef-
ficiencies as low as 60 percent for 0.3-micron test
aerosols.32 This is, however, not the case for fabric
filters that utilize a filter cake, or dense layer of
collected material, as the filter medium; these exhibit
much higher retention efficiency once the cake has formed.
There are too few reliable data on efficiency as a
function of particle size for large operating installa-
tions to confirm the existence of a minimum based on
theoretical or laboratory studies.
In the case of high-temperature particulate
sources such as combustion of fossil fuel, there may be
a concentration of metals such as cadmium, chromium, and
lead in the smaller size fractions—possibly created by
vaporization and condensation of metal fumes. Actual
conditions of operation are considerably more compli-
cated than can be accounted for in theory; hence, there
is an urgent need for reliable data on size distribution
and composition of particles emitted from commercially
installed and operating industrial collection equipment.
C. THE GAS-CLEANING INDUSTRY
Present membership of the Industrial Gas
Cleaning Institute consists of 29 companies in the
United States and Canada. The membership includes most
of the major gas-cleaning device manufacturers.
Total 1969 sales of equipment by the member
companies were in excess of $100 million, not including
costs of erection. In the field of particulate-emission
control, it is estimated that the member companies
account for over 95 percent of the dollar value of equip-
ment sold. It is evident that the industry is of modest
size. In one or two cases, consolidation has taken
place among the companies, but the tendency has been
for the number of manufacturers to grow. This is due
in part to the anticipated market and in part to the
24
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fact that substantial capital investment is not required
for the manufacture of much of the equipment. Indeed,
a large fraction of the production by some member com-
panies is subcontracted to fabricating shops.
The Panel suggests that the gas-cleaning -in-
dustry lacks a basic research commitment. The reasons
are: 1) the modest value of total equipment sales
divided among many competing companies; 2) the lack of
specialization among the companies, many of which pro-
duce a number of different types of devices based on
different physical principles; and 3) the varying re-
quirements as to equipment performance for different
applications.
D. CONCLUSIONS
Action should be taken on a national basis to
develop improved capability for control of particulate
emissions from both open and confined sources. This
will require developing stronger research capability
in the gas-cleaning industry. The Panel recommends
institution of cooperative programs jointly funded by
the EPA, manufacturers of industrial gas-cleaning equip-
ment, and major equipment users.
There is a pressing need for reliable data on
the size distribution and chemical composition of emis-
sions from well-maintained, efficient gas-cleaning de-
vices operated by industry. The EPA should assume pri-
mary responsibility for the collection of such data,
either by its own personnel or on a contractual basis.
Data obtained from such measurements would permit the
enlargement of research programs for the improvement
of collector efficiency. Ideally such research should
be sponsored jointly by the EPA and industry.
The equipment manufacturers should be en-
couraged to assume, perhaps with support from major
user groups and the federal governmenty primary respon-
sibility for the improvement of the durability and re-
liability of control equipment. Such improvements
should also strive to lower costs, improve efficiency
in the small-particle range, and widen the equipment-
application range. Examples of research and development
25
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needed are: better cleaning techniques for fabric
filters; improved filter-fiber characteristics and fabric
life; better electrostatic precipitator components; im-
proved precipitator reliability; better flue-gas con-
ditioning techniques for electrostatic precipitation;
improved materials of construction; improved methods
for handling both wet and dry collected material; better
treatment methods for liquid wastes from wet collectors
to prevent water pollution; improved erosion-control
techniques for inertial collectors; and improved con-
trol instrumentation.
26
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V
SOURCE-SAMPLING AND -MEASUREMENT METHODS
A. GENERAL
Emission-source measurements are made to
satisfy the following objectives: to determine com-
pliance with a legal emission limit; to define the con-
tribution of the source of atmospheric pollution; to
determine the amount and nature of emissions for purposes
of specifying control-equipment requirements; to de-
termine the effectiveness of a control device; and to
reveal the effect of a process-parameter variation.
The measurement system must be able to describe part-tote
size, shape,, concentration) composition^ and light-
scattering potential and the carrier-gas characteristics
associated with the emission source.
The Panel finds that major gaps exist in in-
strumentation for sampling and measuring the character-
istics of particulate pollution; these gaps have hamper-
ed the development of rational emission and ambient-air-
quality standards. Since no system is likely to be
universally applicable, numerous devices designed for
specific particulate compositions and conditions are
needed. The Panel believes that the state of techno-
logical and scientific knowledge is sufficiently ad-
vanced for speedy major improvements in particulate-
measurement instrumentation, but it is convinced that
setting priorities and producing the needed hardware
on planned schedules will be difficult.
Measurement of particles in process-gas
streams, in stack effluents, and in the ambient atmo-
sphere is necessary for the control of air pollution,
and each location involves special requirements. Some
progress has been made in the development of instruments
for the measurement of particulate pollution in atmo-
spheric air. Size spectra, total particle number, and
mass can be measured on a continuous, close to real-
time basis in ambient air using a variety of commercially
available devices based on different physical princi-
ples:
27
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(1) Several optical particle counters are
marketed that, when properly calibrated and maintained,
can be used to measure individual particles from 0.2-
0.3 microns to about 20 microns in diameter. Efforts
are currently under way to extend the counting range
of such sensors below the 0.1-micron size by using con-
ventional and laser optics. When existing sensors are
combined with a multichannel analyzer, detailed size
distributions can be obtained on a continuous, on-line
basis.
(2) The electrical mobility analyzer can be
used in the range below a few tenths of a micron to
measure particle-size distribution with a response time
of a few minutes.
(3) Particle concentrations in terms of total
number can be monitored continuously using condensation
nuclei counters, which function as automated Wilson
cloud chambers operated intermittently.
(4) Particle mass can be monitored continu-
ously using a vibrating quartz crystal on which the
particles are deposited by electrical precipitation or
impaction.
(5) Particle mass can also be monitored con-
tinuously using a filter and associated beta gauge with
a response time for atmospheric air of about one hour.
None of the above principles has been used in
a commercial instrument suitable for routine source
monitoring. Also, there is no proven commercial in-
strument that can measure any of the chemical con-
stituents of particulate pollution on a continuous,
on-line basis. All the commercially available instru-
ments or techniques in these categories are of the
laboratory type and are not applicable for reliable,
long-term untended field operation. Such constituents
as lead, sulfate ions, and carcinogens, and many others
are currently determined by collecting samples of par-
ticulate matter over periods of up to many hours and
then subjecting the accumulated material to chemical
analysis.
28
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B. REQUIREMENTS FOR STACK-GAS MEASUREMENT SYSTEMS
Stack-gas sampling imposes demands on instru-
ments that are much harsher than the requirements for
measuring ambient-air quality. High temperatures, con-
densation, and chemically active gases, as well as com-
plex geometric and flow configurations, complicate the
problem. Requirements for stack-gas sampling include:
1. Gas-Flow Characterj.zat.ion
Important characteristics of the gas stream
are temperature, pressure, composition, and flow uni-
formity. At present, the accepted method of flow
measurement involves determination of velocity profile
with a pitot tube and composition analysis by volume
percentage of certain gases (Orsat analysis). Steady-
state flow is assumed but atmospheric wind and process
variability may result in large flow fluctuations across
the opening of a large-diameter stack, perhaps as ex-
treme as flow reversal in some portions of the system.
Orifice, venturi, and nozzle meters are used in some in-
stances to obtain average-flow results directly.
2. Sample Acquisition and Transport
In stack sampling, it is often necessary to
remove an effluent sample from the gas stream and trans-
port it to an analytical or collection device. The pri-
mary requirement is that the sample acquisition and
transport system deliver a representative sample of
particles. Thus, the sample inlet (nozzle) must not
classify or fractionate the particles. This requires
isokinetic sampling for particles larger than a few
microns. During subsequent transport, significant
deposition will occur on the walls of the sampling lines
unless special design and construction of the sample
line (as well as control of temperature, velocity, and
partial pressure gradients in the line) are used.
3. Sample Analysis
Most accepted methods of analysis are designed
to collect the total mass of the particles emitted from
29
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a source1* and may permit the finest particles to pass
through without significant error in the mass concen-
tration.37 Further, the primary particles in the
collector may be modified by agglomeration, dispersion,
dissolution, or reaction. Usually sample analysis is
carried out on collected materials. Composition anal-
ysis is made by any of the conventional wet or dry
micro-chemical methods on all or part of the collected
sample. Particle-size analysis is made by redispersing
all or part of the collected sample and determining the
number or mass in several size fractions. In a few in-
stances, particle-size analysis is carried out by micro-
scopic techniques. Data from two analytical methods
seldom agree, not only because of non-ideal sample
treatment, but also because methods in use measure dif-
ferent size characteristics—diameter, surface, etc. At
this time, no standard procedure exists to resolve these
variations in results when comparing data from different
sampling methods.
A. Other Requirements
Some additional requirements or desirable
features for operational particle-measurement systems
to be used in source effluent streams are that they:
a. Give reproducible results;
b. Operate over the entire stack-temperature
range;
c. Operate with corrosive gases and vapors;
d. Operate in flammable and/or explosive
atmospheres without hazard;
e. Operate reliably during both short and
long sample times (complex start-up and
shutdown procedures should be avoided);
f. Operate with low power requirements and
are not overly heavy;
30
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g. Operate simply; and
h. Provide internal standardization or
calibration, particularly for in situ
instrumental analysis.
C. METHODS CURRENTLY USED
Methods of in-stack monitoring used currently
incorporate the following elements:
1. A sampling nozzle, which may be an iso-
kinetic nozzle;
2. A probe for extending the nozzle into the
stack;
3. A particle collector;
4. A cooling section or condenser to remove
excess moisture;
5. A gas-flow measuring device;
6. A gas pump;
7. A gas-temperature and gas-pressure
measuring device; and
8. A flow-regulating device.
Table 5 summarizes the major features of
several common stack-sampling trains. The differences
among the systems give rise to problems in obtaining
comparative results. For example, the line loss in
the probes before the collector in the EPA and IIA
trains will be greater than the loss in the in-stack
collector used in the ASTM train. Because of con-
densation, the material collected in the trains where
the collector is cooled may be different in composition
as well as in amount from that collected in a hot
collector. These effects are significant when the
measurement requirements are based on mass alone. As
concern with submicron particles increases, the dis-
crepancies in measurements among the trains are likely
31
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u>
Collector
TABLE 5
MAJOR FEATURES OF SAMPLING TRAINS
Environmental
American Society American Society for Incinerator
Sampling
Element
Nozzle
Probe
Protection
Agency7
Gooseneck
Glass-lined
and heated
of Mechanical
Engineers PTC-27
Gooseneck or
elbow
Stainless
steel
Testing & Materials
D2928-71
Gooseneck or elbow
Stainless
steel
Institute of
America T-6
Elbow
Stainless
steel
Cyclone, glass
fiber filter
and impingers
Cooling section Wet impingers
Flow measure- Pitot tube
ment and control and nomograph
Gas mover
Vane pump
Any at 99%
efficiency for
particles
Condenser
Totalizing gas
meter
Vane pump
Filter or thimble
in stack
Condenser
Totalizing gas
meter or flowmeter
Vane pump
Cyclone and
bag filter
Condenser
Null nozzle
Vacuum
blower
-------
to become even greater. Measurements made with the
ASME and with partial and full EPA trains on high-per-
formance fly-ash electrostatic precipitators are shown
in Table 6. The two systems give significantly different
particle-outlet concentrations.
Cascade impactors have been used for in situ
size fractionation of stack samples.33 Satisfactory
size fractionation from approximately 1 micron upwards
is reported, but some dispersion of large floes occurs.
The impactor can sample the stack atmosphere directly,
thereby minimizing the effects of transport and storage.
A sample large enough for chemical analysis may be ob-
tained if the chemical method is sufficiently sensitive.
The cascade impactor is useful for very low dust con-
centrations. If it is to be used for usual stack con-
centrations, brief sampling periods are necessary.
Radiation interference (wavelengths up to the
infrared) has been used for analysis of particulate
loadings in emission sources. Qualitative stack-effluent
monitoring is often done visually using Ringelmann
numbers or equivalent opacity and is part of many local
codes and regulations. Optical methods based on light-
scattering properties are used for continuous monitoring
of stack gases in many industries.
D. CONCLUSIONS
Much additional research and development is
required to produce measurement systems capable of
giving reliable and accurate analysis of particulate
effluents. Methods are needed to characterize par-
ticle-size distribution in relation to mass, aero-
dynamic diameter, or optical diameter. This must be
done for the particles in the stack to define control-
system performance and for a variety of atmospheric
conditions to evaluate effects on the environment.
The methods must be capable of analyzing particle sizes
over a wide range, including those from 0.01 to 1
micron in diameter, and must be usable with different
particle and carrier-gas compositions. Comparative
studies of in-stack sampling trains are valuable and
should be continued.
33
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TABLE 6
HIGH-PERFORMANCE COAL FLY-ASH ELECTROSTATIC PRECIPITATOR
MEASUREMENTS WITH DIFFERENT SAMPLING TRAINS
OJ
Item
Coal
Frecipitator
Performance*
ASME Train
1/2 EPA Train**
FULL EPA Train
Plant
Sulfur
Ash
Btu/lb
Elec. Load
Gas flow
Plate Area
A/V
Temp.
Efficiency
Outlet loading
Efficiency
Outlet loading
Efficiency
Outlet loading
Unit
%
%
(As Rec.)
MW
acfm
ft2
ft2/1000 cfm
F°
%
gr/scf
lb/106Btu
%
gr/scf
lb/106Btu
%
gr/scf
lb/106Btu
Plant A
1.5
15.7
11,800.0
108.0
497,100.0
120,000.0
248.0
676.0
98.9000
0.0652
0.120C
97.0000
0.1784
0.3290
96.5000
0.2120
0.3900
Plant B
2.49
13.13
12,566.00
519.00
1,515,000.00
270,400.00
178.00
280.00
—
98.0000
0.0828
0.1680
96.3000
0.1530
0.3100
Plant C
2.71
10.63
10,349.00
357.00
1,239,000.00
200,340.00
162.00
280.00
99.3000
0.0263
0,0597
98.4000
0.0590
0.1330
97.1000
0.1030
0.2340
Plant D
3.70
17.64
13,837.00
540.00
1,630,000.00
276,480.00
170.00
252.00
98.8000
0.0600
0.1240
98.8000
0.0615
0.1280
97.9000
0.1025
0.2130
*Efficiency calculated from estimated ash carryover at 80 percent of calculated total ash.
Precipitator inlet gas loading not sampled.
**Impinger catch excluded. Source: EPA Control Techniques Test Data
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Analytical methods must be capable of
describing the mass or number concentration of particles
and their composition in the source effluent. Data that
are integrated over a fixed gas volume or related to
time variations should be obtained. A fractionating
method is required for collecting sized fractionst es-
pecially in the submieron range, for analysis of com-
position where composition may be a function of par-
ticle size.
Simpler methods for measuring fine-particle
concentrations and the efficiency of gas cleaners for
fine particles are needed. Instruments that measure
particles in stacks (at stack conditions) or at ambient
conditions (outside the stack) should be developed.
Unfortunately, many existing sampling trains measure
the emission at some ill-defined and nonreproducible
condition.
The EPA should support the development of
instruments capable of continuous., on-line measure-
ment of important chemical constituents of particulate
matter. Such substances as lead and other heavy metals,
sulfate and nitrate ions, and pH are of interest for
public health reasons and should be accorded highest
priority. Instruments capable of measurement of com-
position averaged over particle size will be easiest
to develop. Eventually, instruments that can measure
composition with respect to particle size should be
developed. Because of the many possibilities for hard-
ware development, careful attention must be given to
the setting of program priorities. Planned programs
for development of the most essential hardware on
schedule are needed. Such plans should take cognizance
of all problems, technical and economic, involved in
making the equipment available to the prospective
users.
35
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VI
FINE PARTICLES
The environment in a stack or process usually
differs greatly from that in the atmosphere. Stack en-
vironments are often characterized by high temperatures,
high moisture content, high particle-number concentra-
tions, the presence of co-contaminants, and a short
time between particle formation and emission. Atmo-
spheric conditions are usually characterized by low
temperature, long exposure to ambient conditions,
variable humidities, and particle-number concentrations
so low that coagulation is negligible.
In a typical urban atmosphere most of the par-
ticles, by number, are smaller than 0.1 micron in di-
ameter. Most of the particle surface area is in the
light-scattering range from 0.1 to 1 micron in diameter
and, based on the most recent studies, the mass dis-
tributions for the urban regions that have been studied
show two peaks, since about half of the mass is in the
0.1- to 2-micron range while the rest is in the 2- to
30-micron range. Also, a substantial fraction of the
mass of smaller particles may be liquid. The figure
(p. 37) presents particulate-distribution data for
Los Angeles smog. Data from Colorado and foreign areas34
substantiate the twin-peak form of the mass and volume
distribution.
A. PROPERTIES OF FINE PARTICLES
Fine particles with aerodynamic diameters
smaller than 2 microns have a number of important char-
acteristics that distinguish their behavior from that
of larger particles. Fine particles compared to large
particles:
1. Scatter more light per unit mass;
2. Have greater penetration through gas
cleaners;
36
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3.0 r
<
h-
z
iii
o
z
o
o
HI
<
_l
LU
IT
2.0
1.0
0
i Number
.001
Estimated
.01 .1 1 10
PARTICLE DIAMETER, MICRONS
FIGURE: Typical particulate distribution
37
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3. Penetrate more deeply into the respiratory
system but may be largely exhaled over
certain ranges;
4. Remain airborne in the atmosphere for
longer periods;
5. Account for a greater number of particles
in urban air pollution; and
6. Are usually generated by thermal or chem-
ical processes.
B. ATMOSPHERIC PARTICLE-TRANSFORMATION PROCESSES
Fine particles typically undergo significant
transformations between the time they are created and
the time that they are collected in control equipment
or emitted into the atmosphere. Such transformations
include diffusion and coagulation, reactions with gases,
growth by vapor condensation, evaporation, and adsorption
or desorption of water.
Although generation mechanisms are not com-
pletely understood, it appears that most fine fumes and
smoke particles resulting from flames or other high-
temperature processes at the time of emission to the
atmosphere consist of hundreds of thousands (105 to 106)
of 0.005- to 0.05-micron-diameter primary-particle
aggregates per cm^ having a mean size of about 0.2
micron. Very small primary-particle aggregates may
also grow by condensation of higher vapor-pressure
liquids such as hydrocarbons and water (when they are
present) during their transport in the ducts and stacks
before emission to the atmosphere. Thus, at the point
of emission into the atmosphere, fine particles may
consist of aggregates of primary particles plus con-
densed vapors.
C. PRINCIPAL SOURCES OF FINE PARTICLES
1. Primary particles are produced by:
a. Condensation. Many chemical reactions
38
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produce vapors at elevated temperatures. Heat loss by
several mechanisms produces supersaturation, which re-
sults in nucleation of solid or liquid particles.
b. Chemical reactions. Particles that result
from combustion, as well as noncombustion aerosols such
as sulfuric acid mist, are the indirect result of chemi-
cal reactions.
c. Comminution or powder handling. This is
a prime source of large particles, but fractures of
these particles produce some fragments in the fine-
particle range. Soils and sands contain fine particles
that may become suspended in the atmosphere.
2. Secondary particles are those formed after
leaving the stack. They can be produced by:
a. Condensation of vapors in the plume. The
most common particles are water and hydrocarbon drops,
which often form at an early stage in the plume and
then evaporate as the plume mixes further with the
ambient air. Exceptions are sulfuric acid mist and
some chemical compounds that are sufficiently hygro-
scopic to hold a substantial amount of water at ambient
humidities or are of low enough vapor pressure so that
evaporation is very slow.
b. A variety of reactions, chemical and photo-
chemical, that occur outside of the stack and result
in nonvolatile products that form aerosols. Sulfate-
containing particles resulting from the oxidation of
SC>2 and combination with other compounds are an example.
Los Angeles-type smog is also the result of photochem-
ical reactions of emissions.22
c. Evaporation of droplets containing dis-
solved solids or nonvolatile liquids to form residue-
solid particles. Examples are sea salt particles from
bubbles bursting at the ocean surface and the residues
from droplets and bubbles formed in cooling towers.
39
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D. STATUS OF KNOWLEDGE
There are relatively few data on size dis-
tribution and concentration of fine particles from
various sources. Because most sampling and evaluation
in the past has been directed at measuring mass and
since the fine particles usually do not contribute the
majority of the mass, such measurements have not yielded
good data on fine particles.35
Transformations of fine particles occur in
stacks and in the atmosphere, but very little more is
known about the transformation process of gases and
particles and few studies have been made of the complete
process from source to emission point and then to the
atmosphere. These are difficult investigations but
the results are needed to relate control techniques
to effects in the atmosphere.
Because particles in stacks are exposed to
conditions that are usually quite different from con-
ditions that exist in the ambient atmosphere, measure-
ments of fine-particle concentration and size dis-
tribution in the stack and in the atmosphere after re-
lease frequently give different results.
Most sampling methods put simplicity and low
cost ahead of an ability to assess the in situ state of
the aerosol in the stack in a satisfactory manner. The
preoccupation with measurements of mass emissions and
efficiency of air cleaners has severely limited measure-
ments of the performance of gas cleaners on fine par-
ticles.
E. CONCLUSIONS
1. More fine-particle-removal efficiency data
on gas cleaners are needed. Gas cleaners with 99 per-
cent mass efficiency frequently have a significantly
lower percent efficiency on particles below 2 microns
in size. Since fine particles may have 10 or 100
times the half-life of coarser particles in the atmo-
sphere, it is apparent that the importance of fine
particles can be greater than their relative mass con-
centration might indicate.
40
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2. Techniques directed particularly toward
collecting and controlling emissions of fine particles
need improvement. Improvements in collection of fine
particles might be accomplished through particle-con-
ditioning processes using electrostatic-charging, con-
densation, and agglomeration-promoting techniques.
3. A strong research effort should be initiated
to study ways of improving removal of fine particles and
of vapors and gases that have a potential to form fine
particles in the atmosphere after emission from the stack.
4. Studies are needed to define the processes
that fine particles undergo from the instant of formation
until they are no longer a significant component in
atmospheric processes or an air-pollution hazard.
41
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