United States Office of Air Quality EPA-340/1 -83-007
Environmental Protection Planning and Standards January 1983
Agency Research Triangle Park NC 27711
Stationary Source Compliance Series
An Introduction
to Continuous
Emission
Monitoring
Programs
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EPA-340/1 -83-007
An Introduction to
Continuous Emission Monitoring Programs
Prepared by:
James W. Peeler
Entropy Environmentalists, Inc.
Research Triangle Park, North Carolina
Prepared for:
Louis R. Paley'
Stationary Source Compliance Division
and
Anthony Wayne
Region VII
United States Environmental Protection Agency
SSCD Contract No. 68-01-6317
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Stationary Source Compliance Division
Washington, D.C. 20460
January 1983
U.S.^Environmental Protection Agency
• -'-< o Library (5PL-16)
167°
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The Stationary Source Compliance series of reports is issued by the
Office of Air Quality Planning and Standards, U. S Environmental
Protection Agency, to assist Regional Offices in activities related to
compliance with implementation plans, new source emission standards,
and hazardous emission standards to be developed under the Clean Air
Act. Copies of Stationary Source Compliance Reports are available -
as supplies permit - from Library Services, U.S. Environmental
Protection Agency, MD-35, Research Triangle Park, North Carolina
27711, or may be obtained, for a nominal cost, from the National
Technical Information Service, 5285 Port Royal Road, Springfield,
Virginia 22151.
This report has been reviewed by the Office of Air Quality Planning
and Standards, U.S. Environmental Protection Agency, and approved for
publication as received from Entropy Environmentalists, Inc. Approval
does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement or
recommendation for use.
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ABSTRACT
This docunent provides a general introduction to continuous emission
monitoring for those persons not previously involved in this field. Information
is presented on continuous opacity monitoring, as well as instrumental and
alternative monitoring techniques for S0? and NO (i.e., continuous
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wet-chemical measurement methods and fuel sampling and analysis methods). This
document presents an outline and review of the fundamental concepts,
terminology, and procedures used in a continuous emission monitoring program.
Also presented are selected technical details necessary to understand the
operation of emission monitors, the use of continuous emission monitoring data
by air pollution control agencies, and references to other available documents
which provide additional detailed information.
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TABLE OF CONTENTS
Page
I. Introduction 1
II. CEM Programs 7
Key Technical Elements of a CEM Program 7
CEM Data Quality Definitions and CEM Reliability 8
Installation and Location of CEMs 11
Instrument Design and Performance Specifications 13
Operation and Maintenance 15
Quality Assurance for CEMs 19
Reporting and Record Keeping Requirements 20
CEM Inspections and Performance Audits 22
Use and Interpretation of CEM Data 23
Alternative S0» and NO Continuous Monitoring Methods 25
III. Opacity Monitoring Systems 27
Basic Design and Operation Features of Opacity Monitors 27
Analyzer System 27
Sample Interface 32
Data Recorder 34
Calibration Mechanism 34
Design and Performance Specifications for Opacity Monitors. ... 35
Transmissometer Installation Criteria 39
IV. Gas Continuous Emission Monitors 41
Gas Monitoring Systems and Monitoring Measurements 41
Basic Features of Gas Emission Monitors 44
Extractive Gas Monitors 45
In-situ Gas Emission Monitors 47
Performance Specifications for Gas Emission Monitors 51
Gas Monitor Installation Considerations 56
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I.
INTRODUCTION
Continuous emission monitoring uses automatic instruments to provide semi-
continuous measurement and recording of air pollutant emission levels (i.e.,
opacity, S0_, and NO ) at stationary sources. Tne term "continuous" applies to
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the on-going process of monitoring emission levels, rather than to the
frequency of measurements. Depending on the type, design, and application of
the continuous emission monitor (CEM) , the sampling frequency may vary. Some
instruments may provide an almost instantaneous or truly continuous record of
emissions, while others may provide measurements taken at 10- to 15-minute
intervals. In either case, the sampling frequency is generally sufficient to
characterize variations in emission levels over time.
Alternative emission monitoring techniques are currently being developed.
These techniques include the use of continuous vet-chemical S02 and NOX
measurement methods similar to those employed in Reference Methods and the use
of various fuel sampling and analysis techniques for predicting S02 emission
levels. Although the approval of these methods differs from that of the
traditional CEM, it provides an essentially equivalent characterization of
emission levels.
CEMs and alternative monitoring methods provide direct estimates of
emission levels, control equipment collection efficiencies, and/or evaluation
of process and control equipment operation and maintenance procedures. Tne
obvious advantage over the more traditional compliance tools (e.g., source
tests, source inspections, and visible emission observations) is that the CEM
provides continual surveillance of source emissions.
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Most CEMs in the United States are installed and operated to comply with
Environmental Protection Agency (EPA), State, or local monitoring regulations.
However, in some cases, CEMs are utilized by industry for process and/or
control equipment operation. Currently, the EPA requires CEMs to be installed
and operated at specified sources primarily through the New Source Performance
Standards (NSPS). To date, some types of CEMs have been promulgated for 13
NSPS categories. Also, the EPA requires the use of CEMs through Prevention of
Significant Deterioration (PSD) permits, Section 113 orders, Section 114
authority, and State Implementation Plans (SIPs) . As a result, many states have
now adopted CEM requirements for existing sources and have revised SIPs to
include CEM regulations.
EPA and State monitoring regulations most often require the source owners
and operators to monitor opacity, S02, and N0x emissions. In addition, total
reduced sulfur (TRS) or CO monitoring is required at some sources.
Occasionally, at other sources where emissions cannot be measured directly,
monitoring of velocity, pressure drop, temperature, and/or other process and
control system parameters is required. (Figure 1-1 tabulates the various NSPS
emission monitoring requirements.)
The use of CEMs and alternative monitoring techniques can provide
significant benefits to the control agency and to the affected source
owner/operator only when a comprehensive monitoring program is established. An
effective CEM program requires that: (1) suitable and reliable instruments are
used, (2) measurements representative of the entire effluent stream are
obtained, (3) proper operation and maintenance of the monitors are performed,
(4) an adequate quality control program is followed, (5) appropriate record
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Figure 1-1. CONTINUOUS EMISSION MONITORING REQUIREMENTS FOR FACILITIES SUBJECT
TO NEW SOURCE PERFORMANCE STANDARDS (NSPS)
Regulation Source AffecFecT
40CFR60 Category Facilities
(Subpart)
J 1 ' — _,..-,,, — — — -f — —
0 FFFSG >250 x 106 Btu/hr
Da FFFSG >250 x 106 Btu/hr
(Electric Utility)
G Nitric acid Process equipment
Sulfuric acid Process equipment
J Petroleum refineries FCCU
FCC
Claus plants
' Primary copper smelters Dryer
Roaster, smelting furnace ,
or copper converter
Q Primary zinc smelters Sintering machine
Roaster
R Primary lead smelters Blast or reverberatory
furnace, sintering machine
Sintering machine,
electric furnace, or
converter
Z Ferroalloy production Electric arc furnace
AA Iron and steel Electric arc furnace
BB Kraft pulp mil Is Hecovery furnaces
Lime kilns, digester,
washer, evaporator,
condensate stripper, or
black liquor oxidation
system, smelt tanks
DO Grain elevators Loading, unloading
handl ing or dryers
fill Lime plants Rotary lime kiln
Emissions RequfrT
Opacity
X
X
X
X
X
X
X
X
X
X
X
so?
X
X
X
X
X
X
X
X
NOY
X
X
X
07JCQ?
X
X
X
X
\
MoniTdrfng
nzsi
X
X
TRS1
X
X
X
rni
X
Not effective until monitor performance specifications are proposed and promulgated.
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keeping and reporting practices are utilized, and (6) appropriate procedures
are used to interpret continuous monitoring results.
The degree to which each of the above activities must be performed and the
corresponding complexity and detail of the CEM regulations depend directly on
the intended use of the data. For instance, greater precautions and effort must
be expended to acMeve accurate results when the CEM data are used to determine
compliance. However, when CEM data are employed as a relative indicator of
source process/control system operation and maintenance practices, less effort
need be expended .
The design of the CEM program must consider realistically the limitations
of monitoring technology, methodologies, expertise, and manpower available to
industry for complying with the regulations. Allowances must be made for
unavoidable CEM malfunctions and inherent errors in CEM data.
CEM instruments vary widely in design and construction. In general, CEMs
are inherently complex devices composed of a number of subsystems. They
typically have complex physical-chemical analytical mechanisms, sophisticated
electronic circuitry, and data recording systems ranging from simple strip
charts to digital computer automatic data processors. The actual source
conditions and situations often present additional problems which must be
resolved on a case-by-case basis. In many situations, unforeseen specific-
applications problems are encountered, and the CEM user is required to expend
significant time and considerable effort in their resolution.
Historically, the inherent complexity of many CEMs, the difficulties of
applying relatively new technology to new situations, and the general lack of
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successful long-term demonstrations of GEM performance have affected
significantly the implementation of CEM programs and have impeded the effective
use of CEM data. However, in spite of the technical and administrative
problems, the field of continuous emission monitoring has progressed very
rapidly in recent years. Alternative monitoring methods are being developed,
and CEMs are being applied to increasing numbers of source categories and new
situations. Many CEM applications problems have been identified and resolved,
and CEM instrumentation continues to evolve and improve. Much additional
operating experience has been obtained, and effective quality assurance
programs are being developed. In general, much more information is now
available on achievable, long-term CEM performance.
Recent technical and methodological progress clearly aids the CEM user in
obtaining high quality emission monitoring data. Existing regulations and
procedures are being revised while new ones are being developed to establish
more effective CEM programs which will facilitate the utilization of monitoring
data in documenting pollutant emission levels from stationary sources. Current
efforts by CEM manufacturers, industrial CEM users, and control agencies will
further improve the technological feasibility and cost-effectiveness of
continuous emission monitoring, thereby culminating in more effective
measurement, regulation, and control of air pollution from stationary sources.
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II.
CEM PROGRAMS
This section presents an overview of CEM program implementation. The key
elements of a CEM program are delineated, followed by brief discussions of
basic CEM data quality definitions, reliability, and the key elements of
conventional CEM instrumentation. Finally, a brief summary is provided of the
status of alternative SC- and NOY continuous monitoring techniques (i.e.,
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continuous wet-chemical measurement methods and fuel sampling and analysis
methods for estimating S02 emission levels). Throughout this section,
references to other documents are included that provide additional information
and/or in-depth discussion of particular subject areas.
Key Technical Elements of a CEM Program
Successful implementation of any CEM program or alternative monitoring
methodology depends upon a number of key program elements encompassing a range
of activities and regulatory provisions, from the selection of CEM measurement
locations to the utilization and interpretation of monitoring results. Tnese
key elements include appropriate procedures to ensure:
1. Representative measurements of the entire effluent stream.
2. Proper performance testing of monitoring instrumentation and
adequate criteria to ascertain the acceptability of monitoring
instrumentation.
3. Proper operation and maintenance of monitoring equipment.
4. Adequate quality assurance that data quality levels are
consistent with the intended use of the data.
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5. Acceptable reporting and recordkeeping practices.
6. An effective control agency inspection-audit program to provide
independent validation of the accuracy of reported
measurements.
7. Correct interpretation of GEM (and alternative method) data to
facilitate the initiation of follow-up activities.
All of the above program elements are interrelated and interdependent, and
none can be neglected or eliminated without seriously diminishing the
effectiveness of the entire CEM program. Conversely, excessive emphasis
directed at any one (or all) of the elements may surpass the needs of the
source owner/operator and the control agency, thereby resulting in excessive
CEM program implementation costs.
CEM Data Quality Definitions and CEM Reliability
CEM data, lite any other scientific measurements, are estimates of the
actual or "true" values. The accuracy and/or errors associated with the data
must be considered to arrive consistently at valid and supportable conclusions.
Thus, to be useful, the quality of the data must be maintained within
reasonable limits. The confidence level associated with CEM data is directly
proportional to the degree of data quality.
CEM data reliability is indicative of the overall data quality and is
generally defined in terms of accuracy, precision, representativeness, and
availability. Because confusion often results from the practical application
of these terms, they are defined for this document as follows:
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accuracy - the closeness of the measured value
to the true value (usually the degree
of closeness of the mean of a data
set to the mean of the corresponding
true emission values) .
precision - the repeatability of the data ob-
tained by the measurement system
(consistency of the relationship
between measured values and true
values) .
representativeness - the degree to which the effluent
samples obtained represent the entire
effluent stream, and the degree to
which the measured values are indica-
tive of the parameters of interest.
availability - the portion of source operating
time for which CEM data is obtained
(% of time monitor is actually opera-
ting and providing data, with respect
to the total time the monitor is re-
quired to operate) .
CEM "reliability" represents the degree to which CEM data yield consistent
and valid opacity, SO-, and NO measurements.
For any particular emission measurement to be meaningful, three
fundamental criteria must be met: (1) samples must be representative of the
entire effluent stream, (2) sampling must be conducted with the maximum
accuracy obtainable under the existing test conditions, and (3) sufficient
sampling and analysis must be conducted to minimize the effects of test site
parametric variations and the imprecision of the measurement method. While
these criteria provide a basis for evaluating the validity of a given set of
emission measurement data, the time-dependent characteristics of the data must
be considered.
Historically, CEM reliability and long-term level of performance have been
the center of much controversy, because of the lack of available information.
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Recently, however, several studies characterizing long-term CEM performance
have been completed, and additional studies are on-going. Information relevant
to the performance of SO and NO monitors is included in "A Compilation of S02
and NO Continuous Emission Monitor Reliability Data Information," SSCD CEM
Report Series No. 340/1-83-012 (J. W. Peeler, Entropy Environmentalists, Inc.,
Contract No. 68-01-6317, Task No. 29). Information regarding the performance
of continuous opacity monitoring systems is included in "A Compilation of
Opacity Monitor Psrformance Audit Results," SSCD CEM Report Series No.
340/1-83-011, (Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task
No. 29).
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Installation and Location of CEMs
Installation and location criteria specify how and where CEMs are to be
installed. The purpose of these requirements is to reduce the possibility that
a poor monitor location will adversely affect the representativeness of the
monitoring data. Two distinct issues must be addressed.
First, because a CEM samples only a very small portion of an effluent
stream, the samples must be consistently representative of the entire effluent
stream at the measurement site. Stratification (i.e., variations in the
pollutant concentration across the duct or stack cross section) at the selected
monitoring location must be considered to ensure that CEM samples have the same
pollutant concentration as the average of the total effluent stream.
Stratification tests are sometimes required to determine vfcether particular
monitor locations will provide representative measurements.
Second, the CEM data must represent the effluent exit stream. For example,
consider a coal-fired steam generator with twin electrostatic precipitators
(ESPs) and a common exhaust stack. An opacity monitor can be located in the
ductwork following each precipitator, or a single opacity monitor can be
located in the stack. The final decision depends on whether the opacity
monitors are intended to monitor control equipment operation and maintenance
practices (in which case a monitor should be installed in each duct), or
whether the opacity monitor is intended to provide data on the opacity of the
effluent discharged to the atmosphere (in which case a single opacity monitor
should be installed in the stack). Thus, the choice of the monitor location is
dependent on the monitoring program goals.
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Other factors which should be addressed in locating the CEM include: (1)
accessibility for monitor maintenance, (2) environmental conditions (i.e.,
ambient temperature, exposure to weather, presence of vibrations, etc.), and
(3) effluent conditions (i.e., temperature, pressure, moisture content, etc.).
All of these factors will affect the degree of maintenance required and CEM
data availability.
The monitoring requirements of 40 CFR 60.13 include a general requirement
for obtaining representative measurements; specific installation and location
criteria are included in the Performance Specifications of Appendix B.
Additional location criteria are provided in the applicable sub parts of Bart 60
for some source categories.
The proposed revisions to Performance Specification 1 for opacity monitors
(published in the October 10, 1979 Federal Register) will provide improved
guidance in selecting and evaluating opacity monitoring installation locations.
In addition, revisions to Performance Specifications 2 and 3 for S02> N0x, C02,
and 02 monitoring systems (first proposed in the October 10, 1979, Federal
Register and subsequently reproposed in the January 26, 1981, Federal Register)
will affect the choice and evaluation of gas CEM installation locations. Until
the final revisions are promulgated, it is not possible to determine the impact
of these new requirements on CEM test parameters and methodology. However, it
is expected that revisions to Performance Specifications 2 and 3 will clarify
the source operator's responsibility for the selection of representative
monitoring locations.
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Existing NSPS monitoring requirements do not provide procedures for
conducting stratification tests to determine the representativeness of gas CEM
monitoring locations. Draft procedures have been developed, however, and may
be found in "Transportable Continuous Emission Monitoring System Operational
Protocol: Instrumental Monitoring of SC>2, NOX, C02, and 02 Effluent
Concentrations," SSCD CEM Report Series ffo. 3W1-83-016, (G. D. Deaton and J.
W. Peeler, Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task No.
3D.
Instrument Design and Performance Specifications
Instrument specifications are necessary to ensure that CEMs are capable of
providing data of sufficient quality to fulfill the requirements of the
monitoring program. Instrument specifications are classified in two
categories: performance specifications and design specifications.
Performance specifications prescribe operational criteria, such as
response time, accuracy, drift, etc. The performance of the instrument in
terms of these parameters is verified according to prescribed evaluation
procedures. Performance specifications do not dictate specific instrument
design criteria, but instead provide latitude in the instrument design,
requiring only that the instrument be capable of being evaluated.
Design specifications, in contrast, prescribe physical design and
construction details. Tne assumption is that if an instrument complies with
specific design criteria, then it will perform satisfactorily.
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Performance specifications are generally preferred to design
specifications because the desired instrument operating characteristics are
verified directly through testing of the monitor. Design specifications are
generally utilized where testing of instrument performance is not practical or
feasible. CEM regulations usually contain both design and performance
specifications. EPA instrument specifications (both design and performance
specifications) for opacity, SO NO , 02, and C02 monitors are contained in
Performance Specifications 1, 2, and 3 of Appendix B, 40 CFR 60. These
regulations specify performance test procedures and design criteria for
evaluating the acceptability of CEM instrumentation.
Instrument specifications ensure only that CEMs are capable of accurately
analyzing effluent samples. They do not ensure the validity of monitoring
data, except when the monitors are demonstrated to comply with the performance
specifications during the actual testing periods. (Instrument design and
performance specifications are discussed in greater detail in Sections III and
IV of this document for opacity monitors and gas emission monitors,
respectively.)
NSPS monitoring regulations and most state CEM regulations require source
owners/operators to conduct field tests in accordance with the procedures
specified in Performance Specifications 1, 2, and 3, which require that the
control agency be notified in advance of such tests. The control agency should
then designate a representative to observe the monitor performance tests. A
manual for use by control agency observers has been prepared, entitled,
"Guidelines for the Observation of Performance Specification Tests of
Continuous Emission Monitors," SSCD CEM Report Series No. 3^0/1-83-009,
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(Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task No. 28). An
additional manual, which addresses the review and evaluation of CEM Performance
Specification test reports submitted to the agency, has also been prepared,
entitled "Performance Specification Tests for Pollutant and Diluent Gas
Emission Monitors: Reporting Requirements, Report Format, and Review
Procedures," SSCD CEM Report Series No. 340/1-83-013, (G. B. ddaker III,
Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task No. 28).
Operation and Maintenance
Proper operation and maintenance procedures are vitally important for the
successful CEM application. Improper operation and/or lack of maintenance is
often the cause of invalid monitoring data and excessive monitor downtime. The
appropriate procedures for operating and maintaining CEMs are very monitor- and
source-specific. Thus, it is difficult to prescribe general guidelines.
For NSPS, minimum operating requirements for CEMs are included in 40 CFR
60. 13; these include specification of the sampling frequency and minimum
procedures for checking CEM calibration on a daily basis. In addition, the
span value (upper limit of the CEM measurement range) is specified for each
source category in the applicable subpart of 40 CFR 60. Also, Subpart Efe for
electric utility steam generators specifies a minimum data capture rate
(minimum acceptable monitor availability).
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CEM regulations generally specify only that proper maintenance practices
be followed and that the CEM user follow the manufacturer's written
instructions. Thus, the adequacy and completeness of the manufacturer's
instructions become an integral part of the CEM program. It must be kept in
mind that monitor vendors are somewhat hesitant to specify more than minimum
maintenance procedures, because an apparently extensive operation and
maintenance program would affect a potential user's decision to purchase a
particular continuous emission monitor.
The routine calibration of CEMs is probably the most important aspect of
operation and maintenance procedures. Calibration involves a check of monitor
system operation by introducing known input conditions to the monitor and
observing the resultant instrument responses. Routine calibration checks are
generally performed at the zero value and at one upscale value. The known
conditions are simulated by the use of devices or materials (i.e., calibration
standards) for which there is some assurance of the equivalent value in units
of the monitoring measurement. Filters that attenuate a known quantity of
light are used to calibrate opacity monitors. Calibration gas mixtures
containing known quantities of the gas of interest are often used to calibrate
gas emission monitors.
Calibration of a monitoring system allows the operator to adjust the
monitor to obtain the correct monitor response to the calibration standards.
Thus, the validity of the monitoring data is directly dependent on the
calibration procedure and on the accuracy of the calibration standard values.
For example, if the values of the calibration standards are in error, then the
monitoring system will be misadjusted and errors will be introduced into the
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monitoring data. Similarly, where a particular calibration procedure fails to
check the entire monitoring system, errors arising from the unchecked portion
of the system may affect the validity of the monitoring data, even though the
monitor is apparently calibrated correctly. The latter situation has occurred
far too frequently, particularly for in-situ gas CEMs. It is anticipated that
more attention will be directed at the validity of calibration procedures as
effective quality assurance procedures are developed and as additional CEM
operational experience is obtained.
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Quality Assurance for CEMs
Quality assurance (QA) consists of procedures and practices to ensure an
adequate level of monitor data accuracy, precision, representativeness, and
availability. Generally, monitor location criteria, instrument design and
performance specifications, monitor operation procedures, and maintenance
procedures can all be considered as QA procedures. However, in common usage, QA
is usually considered to mean the procedures and practices employed in addition
to the above criteria to ensure valid and reliable CEM data. To date, QA
procedures for CEMs have not been included in the EPA monitoring regulations.
Efforts are currently underway, however, to develop Appendix F of 40 CFR 60 to
fulfill the need for QA procedures for CEMs at NSPS sources.
The need for CEM QA procedures is apparent from the past experience of CEM
operation at industrial sources. Although the performance specification test
shows that a particular monitoring system can produce valid data and although
the rather general requirements for operating and maintaining CEMs should ensure
that the CEM data will fall within some error range, it has been very difficult
to address the reliablity or accuracy of CEM data over any extended period of
time.
QA procedures may be divided into two distinct areas: quality assessment
and quality control. Quality assessment procedures provide methods for
estimating the accuracy and precision of monitoring data. Quality control
consists of specific procedures and corrective actions taken to improve data
quality. These procedures are implemented when quality assessment procedures
indicate that data quality is inadequate.
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Because CEMs vary widely in design and application, general QA procedures
are difficult to devise. Efforts are currently underway to develop
monitor-specific and source-specific QA procedures. Specifically, Appendix F,
Procedure 1, will apply to SC>2 and NOX emission monitors used to determine
compliance with emission limitations. It is anticipated that this procedure
will contain relatively general quality assessment procedures, including daily
precision estimates based on calibration data and periodic relative accuracy
tests (comparisions of monitoring data with independent measurements of the
pollutant emission levels) . Appendix F will require that each CEM user develop
a specific set of quality control procedures.
Additional information regarding QA procedures for gas CEMs is contained in
"A Compilation of Quality Assurance Procedures for SO and NO Continuous
t— A
Emission Monitoring Systems," SSCD CEM Report Series No. 3^0/1-83-014,
(J. W. Peeler, Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task
No. 27). Furthermore, the results and conclusions presented in "Iransmissometer
Field Audit Results" (see previous citation) provide information relevant to
appropriate QA practices for opacity monitoring systems.
Reporting and Record Keeping Requirements
Reporting and record keeping requirements are of fundamental importance to
any CEM program. Obviously, if CEMs are to provide any benefit for either the
control agency or the source, then adequate data records must be maintained and
specific information must be reported to the control agency.
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Basically, records of all emission measurements and information documenting
monitor performance and operation should be maintained. The second category
should include records of: (1) monitoring system performance evaluations, (2)
calibration data, (3) adjustments and maintenance performed on the monitoring
system, and (4) all periods of monitor malfunction or downtime.
The type of information that should be reported to the control agency by
the CEM user depends directly on the intended utilization of the data. For
example, under "never to be exceeded" emission standards, reporting only periods
of excess emissions (periods when the standards are exceeded) is appropriate.
In contrast, for 30-day rolling average standards, reporting daily averages of
pollutant emission levels is probably more appropriate. In either case, the
agency should require only the information necessary to decide whether
additional action is necessary within the overall context of the particular
monitoring program to be reported. For additional information, the agency
should rely on the records maintained by the source.
Reporting and record keeping requirements for CEMs installed to comply with
NSPS are contained within Part 60.7 of 40 CFR 60. Reporting requirements
include: (1) the magnitude and duration of all periods of excess emissions, (2)
identification of each excess emission period that occurs during startup,
shutdown, or malfunction of the affected facility, (3) the nature and cause of
each malfunction and the corrective action taken, and (4) all periods when the
monitoring system was inoperative. These reporting requirements provide a basis
for determining whether proper process/control system operation and maintenance
practices are followed by the affected source, and for initiating appropriate
follow-up activities by the control agency.
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GEM Inspections and Performance Audits
Control agency inspections and performance audits comprise a critical
element of any CEM program. They provide an independent means (not subject
to the control of the source operator) for determining the validity of the
data reported to the agency, the adequacy of monitor operation and
maintenance procedures, and compliance with various monitoring regulations.
Performance audit procedures for opacity monitors are presented in
"Performance Audit Procedures for Opacity Monitors," SSCD CEM Report Series
No. 3MO/1-83-010, (Entropy Environmentalists, Inc., Contract No. 68-02-3431,
Tasks No. 40 and 166, and Contract No. 68-01-6317, Task No. 28). These
procedures afford a quantitative measure of monitor performance and indicate
whether a source is utilizing proper monitor operation and maintenance
procedures. Over 100 audits have been conducted to date, providing an
extensive data base for evaluating opacity monitor performance. The results
of the opacity monitor performance audit program are presented in
Transmissometer Field Audit Results" (see previous citation).
Performance audits of SO and NO CEMs quantitatively determine
ci X
compliance with both monitoring regulations and emission limitations. Audit
procedures that include traditional reference method testing and
transportable extractive monitors are delineated in two manuals:
"Performance Audit Procedures for SOp, NO , CO-, and 02 Continuous Emission
Monitors," SSCD CEM Report Series No. 340/1-83-015, (J. W. Peeler and
G. D. Deaton, Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task
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No. «3L), and "Transportable Continuous Emission Monitoring System
Operational Protocol: Instrumental Monitoring of S02, NOX, C02, and 02
Effluent Concentrations" (see previous citation). The results of gas CEM
performance audits are included in "A Compilation of S02 and NOX Continuous
Emission Monitor Reliability Data" (see previous citation).
Use and Interpretation of CEM Data
Throughout the foregoing discussions, there have been numerous
references qualifying other requirements and activities in terms of the
intended use of CEM data. Although, too often, the use and interpretation
of the monitoring data is the least discussed and least well-defined aspect
of continuous emission monitoring, the applicability and appropriate level
of effort for other aspects of a CEM program hinge on the intended use of
the data.
TWo major categories of CEM data utilization are included in the
existing NSPS: (1) the use of CEM data as an indicator of process and
control systems operation and maintenance practices (40 CFR 60.11d), and (2)
the use of CEM data to determine compliance with emission standards (Subpart
Da) . The original promulgation of NSPS monitoring requirements (October 5,
1975, Federal Register) employed CEMs to assess a source's process/control
system operation and maintenance practices. As such, CEMs are required to
provide only a relative indication of emission values; the absolute accuracy
of the data is not of fundamental concern. For example, if the opacity
monitor indicated levels significantly above those measured during the last
23
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particulate performance test and no malfunction of the process or control
system was apparent, then it may be appropriate to require a new particulate
emission test to determine whether the source is still in compliance with
the particulate emission standards. In this situation, the CEM is used to
indicate a relative change in emission levels, rather than to provide an
absolute value.
The second use of CEM data within NSPS is contained in the recently
promulgated NSPS for electric utility steam generators, Subpart Da. These
regulations require the use of SO,, and NOX CEM data to determine compliance
with S02 and NOX emission standards, and the use of S02 CEM data to
determine compliance with SCL percent removal requirements. Subpart Da
requires that these compliance determinations be made on a 30-day rolling
average basis. Although the Subpart Da promulgation does not specify
procedures to be used by the control agency to interpret and to evaluate the
CEM data, it does require that affected sources report the appropriate
30-day rolling average values. Subpart Da also specifies alternative
calculation procedures for use in reporting CEM results where the required
minimal data capture rates are not achieved.
Some control agencies are reluctant to discuss specific procedures used
to evaluate CEM data because such procedures are expected to vary between
Regions and States to reflect local policies and control strategies.
Efforts currently underway should enhance the basis for establishing the
error band associated with CEM data, and thus, should enhance appropriate
procedures for inter pretating CEM results. Also, the promulgation of
improved monitor performance specifications and quality assurance procedures
should reduce the potential error band associated with CEM data. In
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addition, procedures using quality assessment to interpret GEM data are
being developed. Draft procedures for interpretating continuous opacity
monitoring results for NSPS sources have been developed but are still being
evaluated by the EPA. These procedures focus on the relationship of opacity
monitoring results to: (1) proper control system operation and maintenance
practices, (2) visible emission observations, and (3) particulate emission
levels.
Alternative SO,, and NO Continuous Monitoring Methods
An alternative S0_ monitoring method (i.e., proposed Method 6B) is
currently under development by the EPA's Emissions Measurement Branch,
Quality Assurance Division, and Stationary Source Compliance Division. A
limited quantity of field testing has been conducted to demonstrate and
evaluate the feasibility of this monitoring technique when emission
standards are expressed in terms of 24-hour and longer averaging periods.
Promulgation of Method 6B is expected fairly soon; this method should prove
to be a relatively low cost, highly reliable S02 emission monitoring
technique. A current assessment of the status of Method 6B is provided in
"An Update and Discussion of the Critical Aspects of Proposed EPA Reference
Method 6B," SSCD CEM Report Series No. 5-411-11/82, (G. B. Oldaker III,
Entropy Environmentalists, Inc., Contract No. 68-01-6317, Task No. 28).
A method similar to proposed Method 6B, referred to as the
"permanganate method," is also being developed. This method will provide
for concurrent measurement of SO NO , and CO- effluent concentrations, and
£. A ^
25
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together with proposed Method 6B, will provide industry with increased
flexibility in meeting S0_ and/or NOX monitoring requirements, thereby
reducing the cost of conducting a CEM program.
Coal sampling and analysis (CSA) procedures for determining flue gas
desulfurization (FGD) inlet S0_ levels have been promulgated in Method 19,
Appendix A, 40 CFR 60. CSA procedures for non-FGD equipped steam generators
are currently under development. A number of alternative CSA approaches are
being considered, spanning the range of "as received" to "as fired"
sampling. A preliminary protocol has been developed to allow source
operators to demonstrate the adequacy of existing CSA procedures in lieu of
utilizing SOp CEM. A limited amount of field testing has been conducted,
and further development of CSA methods is expected to provide industry with
increased flexibility in meeting SO monitoring requirements while reducing
the costs of conducting a CEM program.
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III.
OPACITY MONITORING SYSTEMS
Basic Design and Operation Features of Opacity Monitors
Continuous opacity monitoring systems use transmissometers to determine
the in-stack opacity of an effluent stream. The transmissometer operates on
the principle of light attenuation by the particulate matter in the stack
effluent. The transmissometer generates a light beam, projects it across the
stack effluent, and detects the amount of light transmitted across the stack
effluent relative to the amount of light generated by the light source. (Figure
3-1 shows typical transmissometer configurations.) The basic components of the
opacity monitoring system are the analyzer, sample interface, data recorder,
and calibration mechanism. Each of these system components is discussed
separately in the following paragraphs.
Analyzer System
The analyzer system contains the light source, detector, and signal
generator, and measures the amount of light attenuated (i.e., absorbed and
scattered) by the stack effluent. The percentage of visible light attenuated
is defined as the opacity of the emission. Transparent stack emissions will
have a transmittance of 100?, or an opacity of zero percent. Opaque stack
emissions that attenuate all of the visible light will have a transmittance of
zero percent, or an opacity of 100?.
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LIGHT SOURCE
COLLIMATING LENS
DETECTOR
COLLIMATING
LENS
ROTARY
BLOWER
SINGLE PASS TRANSMISSOMETER
(Many single pass opacity monitoring systems do not conform
with EPA continuous monitoring requirements)
LIGHT
BEAM ^"/" "A'
SPLITTER DETECTOR
RETRO-
REFLECTOR
STACK
ROTARY
BLOWER
DUAL PASS TRANSMISSOMETER
Figure 3-1. Typical Transmissometer Configuration
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The opacity of an effluent stream is a function of the light beam path
length: the longer the measurement path length, the greater the resulting
opacity for a given particulate concentration. The measurement path length at
the transmissometer installation may not be the same as the stack exit
diameter. However, existing opacity monitoring regulations usually require the
correction of opacity measurements to the stack exit diameter. The following
equation is used for this calculation.
L1
log (1 - Op ) = log (1 - Opp)
L2
where: Op = opacity at the stack exit
L^ = stack exit diameter
Lp = monitor pathlength
Opp r opacity based on L~
The light attenuation characteristics of a particulate laden stream are
dependent on the wavelength of the light passing through the effluent. In
traditional visual opacity measurement, the in-stack opacity represents the
attenuation of visible light. This convention restricts the optical
characteristics of the transmissometer. Visible light encompasses the region
of the electromagnetic spectrum between 0.3 and 0.7 microns (see Figure 3-2).
Consequently, the transmissometer system must be designed for peak response
within this range. Most transmissometers use a tungsten filament lamp as a
light source. Figure 3-2 shows that the tungsten lamp's output encompasses a
broader range than the visible spectrum. Part of the tungsten lamp's emission
is also in the region where water vapor absorbs light strongly. Therefore,
transmissometers must optically filter the lamp's output before it crosses the
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SPECTRAL CHARACTERISTICS
PHOTOPIC TUNGSTEN FILAMENT
SPECTRAL RESPONSE
100
INCANDESCENT LIGHT 2500° K
LU
ULTRAVIOLET
VISIBLE
1000 1500 2000 2500
- INFRARED
LIGHT
WAVELENGTH IN NANOMETERS
Figure 3-2. Electromagnetic Spectrum
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stack effluent, both to eliminate water vapor interference and to provide a
light beam of the proper spectral characteristics. The optical system for both
the light source and the detector must be designed such that the peak and mean
spectral responses are within the visible light range, as previously described,
to minimize the adverse effects of water vapor and CCL.
Most transmissometers use either a single- or dual-pass beam to determine
the amount of light transmitted across a stack effluent relative to the amount
of light emitted by the light source. Some dual-pass instruments use a
multilobed, perforated, rotating disc, which alternately gates the light
between measurement and reference signals. The reference beam is projected
internally to the detector, with measurement and reference beams being compared
on the same detector using time-shared optics.
Single-pass transmissometers cannot use the same techniques for generating
the reference beam. Fiber optic cables may be employed to transmit a reference
signal to the detector. Fiber optics are flexible "light pipes" that transmit
light with minimal spectral distortion and reduction in intensity. With these
cables it is possible to transmit a reference beam generated by a beam splitter
around the outside of the stack and couple the light beam to the detector.
As with any line of sight optical measurements, optical alignment is
important. The light source and detector must be aligned so that the light
beam falls squarely on the detector. The transmissometer alignment must be
carried out under actual stack conditions because of thermal expansion effects
occurring when the stack is heated. Long slotted tube transmissometers are not
practical if sagging occurs because of excessive tube length. Some dual-pass
transmissometers employ special reflectors to reflect the light beam parallel
to the incident light path independent of small variations in reflector
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alignment.
The transmissometer1 s optical system must be sensitive only to light
actually transmitted through the stack effluent. Slotted tube transmissometers
must be designed so that no light is reflected off the walls of the pipe and
into the detector. The optical system of all transmissometers must be
insensitive both to ambient light and to scattered light. Modulation of the
light source may be used to eliminate the detection of ambient light. In this
approach, the detector system is designed to respond only to light at the
modulation frequency, thereby eliminating responses to ambient light. In order
to avoid detection of scattered light, the light beam must be properly
collimated. Simply put, collimation is the focusing of the light beam using
lenses and apertures to prevent scattered light from reaching the detector.
Figure 3-3 shows a typical collimation method. Collimation of transmissometers
is characterized in terms of the angle of projection and angle of view of the
instrument. The angle of projection is the total included angle which contains
95% of the light radiated from the lamp. The angle of view is the total
included angle for which the detector has greater than a 5 percent response.
Sample Interface
The transmissometer's optical surfaces must be protected from the stack
effluent. Farticulate matter deposited on the optical surfaces can cause
erroneously high opacity readings. The sample interface generally provides a
constant flow of highly filtered air (purge air) across the optical surfaces to
prevent participate accumulation on the exposed surfaces. In addition, a
method for isolating the optical surfaces in the event of a loss of filtered
air should be provided. Some transmissometer models provide an automatic
protection device that is actuated when a loss of filtered air is detected.
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ANGLE OF VIEW
DETECTOR /'APERTURE
COMPACT
(LAMENT
LAMP
COLLIIYIATING
LENS
LENS
CLEANir,
AIR
LENS
CLEANING
AIR
Figure 3-3. Transmissometer with Col 1imating Features
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Data Recorder
The data recorder provides a hard copy record of the analyzer output.
Data recorders may range in complexity from strip chart recorders to
mini-computers. The opacity may be recorded as the instantaneous value, the
integrated value, or some combination of the two. Some systems provide a
summary of excess emissions for each 1-hour or 24-hour period. It is important
that the data recorder have sufficient resolution to permit proper calibration
of the instrument. The recorder must be sensitive enough to enable the
performance tests to be carried out, and should have a resolution of
approximately 0. 5% opacity.
Calibration Mechanism
EPA monitoring regulations require that the calibration of opacity
monitors be checked daily (and adjusted if necessary) at the zero opacity level
and at a prescribed upscale opacity level. These checks are referred to as
zero and span checks. Most commercially available transmissometers provide an
automated method of performing the zero and span checks. The most commonly
encountered approach to performing a zero check for dual-pass instruments uses
a mirror (located on the effluent side of the window separating the analyzer
from the effluent) which can be rotated in and out of the light path. During
the zero check, the mirror is automatically positioned in the light path, and
it returns the same level of light to the analyzer as would be returned by the
stack mounted reflector under clear stack conditions. The span check is
accomplished by inserting both a calibrated filter and the zero mirror into the
light path simultaneously to produce a simulated upscale opacity condition.
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Design and Performance Specifications for Opacity Monitors
Instrunent design and performance specifications for opacity monitors are
contained in Performance Specification 1 of Appendix B, 40 CFR 60. The
existing specifications were promulgated on October 5, 1975. Revisions to
Performance Specification 1 were proposed October 10, 1979; however, final
revisions to the specifications have not yet been promulgated. For the
purposes of this discussion, the existing specifications will be used.
Performance Specification 1 includes design specifications for peak
spectral response, mean spectral response, angle of view, and angle of
projection. The peak and mean spectral response criteria require that
transmissometers measure the attenuation of visible light. These
specifications are important in ensuring the accuracy of transmissometer
measurements, because the attenuation of light by a particulate laden stream is
wavelength dependent. The angle of view and angle of projection specifications
(i.e., collimation specifications) ensure that the accuracy of the measurements
obtained by an instrument meeting these specifications will be relatively
unaffected by scattered light. Performance Specification 1 includes general
procedures for demonstrating that a particular instrument complies with the
design specifications. However, because compliance with these specifications
is essentially a design feature of the monitors, instrument manufacturers are
only required to test one instrument from each month's production. Performance
Specification 1 also includes performance specifications and compliance test
procedures for calibration error, response time, zero drift, calibration drift,
and an operational test period.
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The calibration error test provides an evaluation of the accuracy,
precision, and linearity of the analyzer portion of the transmissometer. This
test is performed before the instrument is installed at the source, and is most
often performed at the instrument manufacturer's facility. This test involves
inserting calibrated neutral density filters into the light path of the
transmissometer and comparing the instrument response to the known filter
values. Three different filter values, spaced over the operating range of the
instrument, are used, and five measurements are obtained with each filter. For
each set of 5 measurements, the mean difference (a measure of accuracy) and the
95% confidence interval of the data set (a measure of the precision) are
calculated. The calibration error for each filter is the sum of the mean
difference and confidence interval, and must be less than 3% opacity.
The response time is defined as the time required for an instrument to
reach 95% of the final value in response to a step change in the monitored
value. The response time specification is 10 seconds for transmissometers;
this ensures that opacity monitors will be able to track the relatively rapid
changes in effluent opacity which are typical of many participate emission
so ur ce s .
Transmissometers are required to operate in the "normal operating manner"
without malfunction or repair, first for a 168-hour conditioning period, and
then for a 168-hour operational test period. Both of these requirements ensure
that the transmissometer is capable of operating for sufficient periods of time
to provide a useful amount of data.
During the 168-hour operational test period, the tests for zero drift and
calibration drift are conducted. These tests involve an initial calibration of
the transmissometer at the zero value and at an upscale value, followed by
36
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subsequent calibrations at 24-hour intervals during the 168-hour operational
test period. The difference in zero readings at 24-hour intervals and the
difference in span readings at 24-hour intervals are used to calculate the zero
and span drift, respectively. Thus, the zero and calibration drift tests
evaluate the stability of the instrument calibration over time.
Performance Specification 1 as initially promulgated does not contain an
accuracy specification. This provision was omitted because of the absence of an
independent method to measure in-stack opacity other than the use of
transmissometers. Together, the prescribed design and performance
specifications for transmissometers attempt to ensure the accuracy and validity
of opacity monitoring data by limiting critical instrument design criteria and
by requiring those performance tests that are feasible for transmissometers.
Nonetheless, there is no means available for checking the absolute performance
of the entire transmissometer system after it is installed on the stack except
when the source is not operating. When clear stack conditions do exist,
performance audit techniques (described in "Performance Audit Procedures for
Opacity Monitors," cited previously) can be used to evaluate the performance of
the opacity monitoring system.
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Transmissometer Installation Criteria
An ideal transmissometer installation location would provide
representative measurements of the effluent stream and easy access to the
instrument for routine servicing and maintenance. Although both of these
criteria are important, compromises must often be made.
Installation criteria for transmissometers are provided in Performance
Specification 1. Generally, the transmissometer must be installed such that
the flow of particulate material through the optical volume of the
transmissometer is representative of the flow of the particulate matter through
the entire duct or stack. Additional location criteria specified by
Performance Specification 1 require that the transmissometer be installed: (1)
downstream of all particulate control devices, (2) as far from bends and flow
obstructions as possible, (3) in the plane of the bend when it is necessary to
be located after a bend or turn, and (4) in accessible locations.
The above criteria provide only the most general framework for selecting a
transmissometer installation location. In practice, proposed locations must be
evaluated on a case-by-case basis. It must be kept in mind that almost all
effluent streams are stratified with respect to particulate matter
concentration. However, because relatively small effluent stream particles are
responsible for the opacity of the effluent as measured by a transmissometer
and because small particles tend to remain fairly well mixed and evenly
distributed throughout the effluent stream, the effects of particulate
stratification on opacity measurements are generally minimal .
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When a particular transmissometer location is suspected of being
non-representative, Performance Specification 1 allows the agency to require
the source to conduct an examination of the opacity profile at the monitoring
location. This type of test, usually performed with a portable
transmissometer, facilitates a determination of whether a particular monitoring
location is acceptable. The feasibility of conducting these opacity profile
examinations has not been demonstrated , and specific procedures for conducting
this type of test are not included in Performance Specif ication 1.
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IV.
GAS CONTINUOUS EMISSION MONITORS
This section introduces the terminology and outlines some of the important
measurement concepts associated with continuous emission monitoring of gaseous
pollutants. A general discussion of monitoring systems and monitoring
measurements is followed by discussions of basic monitor design and operation
features, performance specifications, and installation considerations for gas
CEMs. The variety and complexity of gas emission monitoring analytical
techniques, combined with the wide variety of adaptations of these techniques
to emission monitoring, prohibit an extensive discussion of technical details
contained in other literature and in specific source testing regulations.
Gas Monitoring Systems and Monitoring Measurements
Monitoring of emission levels of gaseous pollutants is required at many
sources. Almost all gas emission monitoring regulations require measurements
in the units of the applicable standard, which are generally specified in units
of concentration, mass emission rate, or production rate (i.e., mass of
pollutant emitted per unit of product or mass of pollutant emitted per unit of
heat input).
Individual emission monitors provide measurements of a particular gas
constituent in units of concentration, usually expressed in ppm (parts per
million). Thus, monitoring of other parameters in addition to pollutant
concentration is required to determine emissions in units of the standards.
41
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For exanple, to monitor mass emission rates of SO,,, both the concentration of
SO and the effluent volunetric flow rate must be measured. In some cases,
monitoring of process or production rate parameters is required in addition to
monitoring of pollutant concentrations.
Monitoring emissions of S0p and NO in units of mass of pollutant per unit
^ A
of heat input (lbs/10^Btu) at fossil fuel-fired steam generators presents a
special case which deserves attention because of the frequency with which it is
encountered. At steam generators, a pollutant monitor (measuring S02 or N0x
concentrations) and a diluent monitor (measuring 0 or C02 concentrations) are
used in conjunction with the F-Factor to calculate emissions in units of
lbs/10^Btu. In this situation, the accuracy of the S02 or N0x monitor and that
of the 0 or C02 monitor directly affect the accuracy of the measured emission
levels.
The F-Factor method of calculating emissions in units of lbs/10 Btu is
included in the NSPS for steam generators, Subpart Da , and in the more recently
promulgated Method 19, "Determination of Sulfur Dioxide Removal Efficiency and
Particulate Sulfur Dioxide and Nitrogen Oxides Emission Rates from Electric
Utility Steam Generators." There are a number of formulations of the F-Factor
approach. The appropriate equation to be used depends on: (1) whether 02 or
CO measurements are obtained, and (2) whether pollutant and diluent
concentrations are obtained on a wet or a dry basis. The two equations which
are applicable when all concentrations measurements are on a dry basis are:
20.9
E = CF - ~
20.9 -
100
E = CF
C
42
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where: E = emissions in lbs/10 Btu
C = pollutant concentration
F, F = constants for various types of fuels
%0_ = oxygen concentration
5&CCL = carbon dioxide concentration
The above equations show that errors in either the pollutant or diluent
concentration measurements will affect the calculated emission values.
Therefore, in assessing the accuracy of the emission monitoring data, the error
contribution of both measurements must be considered.
CEMs may provide concentration measurements on either a wet or a dry
basis. Wet concentrations measurements are equivalent to the ratio of the
volume of pollutant to the total volume of effluent gases including water
vapor. In contast, dry concentration measurements exclude the volume occupied
by the water vapor and are, therefore, equivalent to the ratio of the volume of
pollutant to the volume of dry effluent gases. For an effluent stream
containing water vapor, wet basis measurements yield lower concentration values
than dry basis measurements. At most sources, a significant fraction of the
effluent gases is attributable to water vapor, and therefore, the distinction
between wet and dry basis measurements is important. Care must be exercised
when gas emission monitoring data are converted to units of the standard, or
when CEM data are compared to Reference Method sampling values, to ensure that
all measurements are expressed on the appropriate moisture basis.
The term "system" as it applies to gas emission monitoring often causes
confusion. For example, an SO^ monitor is composed of a number of components
which function together to sample, analyze, and record effluent S0_
43
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measurements. The aggregate of the various components is typically referred to
as a "monitoring system." It should be remembered that the proposed revisions
to the performance specifications and corresponding test procedures for gas
emission monitors evaluate the performance of the system, rather than the
components within the system. At steam generators, where both a pollutant
monitor and a diluent monitor are required, the term "system" may be used to
refer to the combined monitoring system composed of the two monitors, or it may
be used to refer to either monitor separately. Both usages are quite common,
and it is often important to distinguish between the two usages in discussing
monitoring at steam generators. The proposed revisions to the CEM performance
specifications (October 10, 1979, Federal Register) redefine "system" to
include both the pollutant and diluent monitors at steam generators. However,
some of the performance specifications of the proposed revisions apply to each
monitor separately, while others apply to the combined monitoring system.
Basic Features of Gas Emission Monitors
Gas emission monitors may be categorized into two general groups:
extractive monitors and in-situ monitors. Extractive monitors withdraw a
sample of the effluent stream and transport the sample to an analyzer at
another location. 3h-situ monitors measure the gas concentration at the
effluent stream sampling location: a sample is not removed. Both extractive
and in-situ monitors are composed of subsystems performing separate functions.
The major monitoring system components are the sample interface, the analyzer,
and the data recorder. The nature of these components varies greatly between
extractive and in-situ monitors.
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Extractive Gas Monitors
The analyzer is the portion of the monitoring system that senses the gas
component of interest and generates an output signal proportional to the
concentration of that component. A wide variety of analyzers for S02, N0x,
CO and CU are available for use in extractive monitoring systems. Commonly
encountered analytical methods include such diverse techniques as:
nondispersive infrared spectroscopy, differential absorption spectroscopy,
chemiluminescence , pulsed fluorescence, electrocatal ysis, and paramagnetism.
Fortunately, NSPS performance specifications typically require evaluation of
only the overall system performance, which allows monitor performance
evaluations to be conducted without requiring familiarity with or knowledge of
the above analytical techniques. Selection of the most appropriate analyzer is
usually dependent on the source-specific conditions encountered and the gas
components to be monitored.
Frequently, a single analyzer is used to determine concentrations of more
than one gas component. For example, many analyzers employing ultraviolet
differential absorption are used to monitor both SO and NO concentrations.
In some cases, a single analyzer processes samples obtained at several
monitoring locations. Thus, the analyzer is time-shared between several
sampling locations and costs are greatly reduced v*iere monitoring of several
emission points or effluent streams is required at a single facility. Although
some manufacturers of CEM gas analyzers provide only the analyzer, others
provide complete systems, including the sample interface, analyzer, and data
recording components.
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The sample interface for extractive monitoring systems performs three
basic functions: (1) sample acquisition; (2) sample transport; and (3) sample
conditioning. Samples are extracted from the effluent stream using either a
single point or multi-point sampling probe. Rarticulates are usually removed
from the sample stream by filtration. In all cases, the condensation of water
vapor in either the sample transport lines or in the analyzer must be
prevented. Therefore, where water is not removed at the sampling probe outlet
(i.e., as a function of the conditioning system) , heated sample transport lines
are used to prevent condensation.
Most gas analyzers require that specific sample conditions at the analyzer
inlet be maintained. Thus, sample conditioning systems are usually employed to
remove particulates and water vapor from the sample stream and to ensure that
the samples are within the temperature and pressure operating limits of the
analyzer. The degree of water vapor removal is dependent on the analytical
technique employed by the analyzer. For some instruments, removal of enough
water vapor to prevent condensation within the analyzer is sufficient. For
other instruments, water vapor severely impedes the measurement process, and
essentially all of the water vapor must be removed. Water is usually removed by
refrigeration of the sample and separation of the resulting condensate, or by
permeation tube dryers.
Regardless of the design or configuration of the sample interface system,
the sample interface must not affect the concentration of the gas constituent
of interest. The two most common problems are absorption-adsorption of
pollutant gases and dilution of the sample stream by air in-leakage into the
system. In the case of absorption-adsorption, the sample stream gas
concentrations are changed when constituent gases are trapped in the sample
46
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interface system prior to entering the analyzer. In contrast, sample stream
dilution by air infiltration results in an erroneously low concentration of
pollutant gases reaching the analyzer.
The proper calibration of an extractive monitoring system is verified by
introducing calibration gas into the system. Calibration gases are
quantitatively known mixtures of the gas of interest in an appropriate diluent
gas. A zero gas (usually nitrogen or "clean" air) and a span gas (gas mixture
with a concentration of approximately 90? of the maximum concentration which
can be measured by the monitor) are used to verify proper instrument
performance. For extractive monitors, the calibration gases must be introduced
as near to the sampling probe as possible to provide a check of both the sample
transport/sample conditioning system and the analyzer. If gases are introduced
at the analyzer, as happens too frequently, then dilution or absorption effects
in the sample interface may go undetected, resulting in errors in the
monitoring data.
In-Situ Gas Emission Monitors
Ih-situ monitors analyze the gas concentration within the effluent stream.
Most in-situ analytical techniques utilize optical analytical methods, in which
the interaction of light with the gas component of interest is employed to
generate an output signal proportional to the particular gas component
concentration. Analytical techniques employed for in-situ monitoring include:
ultraviolet differential absorption, second derivative ultraviolet
spectroscopy, nondispersive infrared correlation spectroscopy, and
electrocatalysis. Again, because the applicable monitoring regulations include
only system performance specifications, monitoring systems can be adequately
evaluated in most cases with little knowledge of these techniques.
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With the exception of electro-catalytic monitors (used for 0^ measurements
only) , in-situ monitors project a beam of light across the duct (referred to as
a path monitor) or project the light beam through a shorter segment of the
effluent (limited path or point monitor). If the light source and detector are
located on opposite sides of the effluent stream, the monitor is a single-pass
instrument. If the light source and the detector are located on the same side
of the effluent stream and a reflector is used to return the light source
radiation to the detector, then the instrument is referred to as a dual-pass
instrument (i.e., the light traverses the effluent twice). The distinction
between single-pass and dual-pass in-situ monitors is important in determining
the applicability of some instrument specifications. Dual-pass instruments are
somewhat easier to deal with, because both of the critical components (light
source and detector) are located at the same place and because the calibration
procedures are generally simplified.
In-situ monitors are typically calibrated using calibration gas cells that
contain known quantities of the gas constituent s) of interest. These cells are
placed in the light beam of the instrument during calibration. Difficulty has
been encountered in calibrating some in-situ monitors, because the calibration
procedure devised by the manufacturer does not always check the entire
monitoring system. For single-pass instruments, the interference of the other
stack effluents cannot be eliminated to provide a check of the instrument zero
value .
The chief advantage offered by in-situ monitors, as compared with
extractive monitors, is the virtual elimination of the sample interface system
and of the corresponding sample handling problems. Disadvantages include the
restriction that an in-situ monitor cannot be time-shared between several
48
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locations, calibration is more difficult, and effluent stream conditions are
not always suitable for their use.
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Performance Specifications for Gas Emission Monitors
The existing regulations for gas emission monitors (Performance
Specification 2 for SO and NO instruments, and Performance Specification 3
for 0 and COp instruments) were promulgated October 5, 1975. Proposed
revisions to the Performance Specifications were included in the October 10,
1979, Federal Register. Since then, additional and extensive revisions have
been considered. For the purposes of this discussion, the existing
specifications are generally cited as examples, and where significant revisions
are expected, they are pointed out.
The Performance Specifications for SO NO , COp, and Op monitors are
indeed performance specifications. The regulations do not mandate the use of
any particular analytical technique or design criteria. Thus, the regulations
allow a great deal of freedom in the analytical technique employed and in the
electro-mechanical configuration of gas monitoring systems. Essentially, the
only design specifications contained in Performance Specifications 2 and 3 are
the implicit requirements that the monitors can be tested according to the
prescribed methods.
The performance specifications applicable to S0? and NO monitoring
^ A
systems are :
Relative Accuracy £ 20%
Calibration Error <_ 5%
Response Time <^ 15 minutes
24-Hour Zero Drift < 2%
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24-Hour Calibration Drift £2.5$
2-Hour Zero Drift £ 2%
2-Hour Calibration Drift < 2%
Conditioning Period 168 hours
Operational Test Period 168 hours
Ihe requirements for diluent monitors are similar to those listed
above. Performance Specifications 2 and 3 also prescribe test procedures for
determining compliance with the performance specifications. Each individual
monitor must be tested to determine compliance with the specifications.
Approval of a particular monitor design cannot be granted in place of the
testing requirement, because of the source-specific problems and conditions
that may affect monitor performance.
After a CEM is installed at a source, the monitor must first complete a
168-hour conditioning period. The purpose of the conditioning period is to
ensure that the monitor can operate continuously in the "normal operating
manner" for at least a week without requiring non-routine maintenance.
After the conditioning period is successfully completed , a 168-hour
operational test is conducted. During this period, conformance with the
other performance specifications is determined. The existing specifications
allow the calibration error test to be performed either in the field or in
the laboratory, and therefore, the calibration error test is not necessarily
conducted during the operational test period.
During the operational test period, the monitor must again operate
without failure or malfunction. Cnly routine maintenance can be performed
during this period. Both the conditioning period and the operational test
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period serve to ensure that monitors that comply with the Performance
Specifications can operate reliably and can achieve sufficient data
availability to fulfill the purposes of the monitoring program.
The relative accuracy of gas CEMs is determined by conducting Reference
Method sampling of the effluent stream and comparing the sampling results to
concurrent CEM data. Under the existing specifications, the relative
accuracy of SO and NO monitors is determined in units of concentration by
conducting a series of nine measurements using Reference Method 6 for S02
and Method 7 for NO . Concurrent moisture sampling is also conducted where
the CEM provides wet basis measurements. The moisture sampling results may
be used to adjust either the wet basis CEM data or the dry basis Reference
Method data, so that the two sets of concentration data are expressed on the
same moisture basis. The relative accuracy is computed from the differences
between the 9 pairs of concurrent monitor/manual sampling results. Tne
relative accuracy is calculated as the sum of (1) the absolute value of the
mean difference and (2) the two-sided 95% confidence interval, divided by
the mean Reference Method value (to express the relative accuracy as a
percentage) . The relative accuracy calculated using this procedure is
actually expressed in terms of error; smaller calculated relative accuracy
values indicate better monitor performance.
The results of the relative accuracy test will be affected by errors in
the CEM data or by errors in the Reference Method sampling results. Tne
Reference Methods are neither totally accurate nor totally precise;
therefore, a portion of the allowed relative accuracy is attributable to the
inherent variability of Reference Method sampling results. Although the
relative accuracy test provides a direct measure of the accuracy of the
monitoring data, the results of the test may be representative only at the
53
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effluent conditions encountered during the test.
The existing Performance Specifications do not require a relative
accuracy test to be conducted for diluent monitors, even when a pollutant
and a diluent monitor are employed to provide emissions data in units of
lbs/10 Btu at steam generators. Also, the existing specifications do not
require a system relative accuracy test in which Reference Method sampling
results expressed are compared to CEM data. However, the proposed revisions
to the Performance Specifications do require a system accuracy. Thus, a
measure of the accuracy of the CEM data in units of the standards will be
available.
The calibration error test is a check of an instrument's accuracy,
precision, and linearity in response to a range of calibration standards.
The calibration error test for extractive gas monitors is performed by
introducing into the monitoring system calibration gases equal to 0, 50%,
and 90% of the span value. For in-situ monitors, calibration gas cells are
utilized instead of calibration gases. The calibration error, a measure of
the difference between the monitor response and the value of the calibration
standard, is computed from the 5 measurements obtained using each gas.
Zero and calibration drift tests must be conducted on both a 2-hour and
24-hour basis. For this discussion, zero drift is defined as the change in
the measurement system output over a stated period of time when the
pollutant concentration at the time of the measurements is zero. Similarly,
span drift is the change in the measurement system output over a stated
period of time when the pollutant concentration at the time of the
measurements is the same upscale value. Calibration drift is equivalent to
span drift with the effects of zero drift removed from the upscale value
54
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measurements. For the 2-hour drift test, 15 sets of zero and span drift
measurements are obtained over 2-hour intervals. For the 24-hour drift
tests, 7 sets of drift measurements are obtained over 24-hour intervals. For
all drift measurements, the parameter of interest is the change in the zero
or span values over time. Thus, the 2-hour and 24-hour drift tests provide
a basis for evaluating the stability of the instrument calibration over the
short and long term.
Response time is defined as the time interval from a step change in
pollutant concentration at the input of the measurement system to the time
at which 95% of the final monitor output value is reached. The response
time specification for pollutant gas monitors is 15 minutes; the response
time specification for diluent monitors is 10 minutes. According to the
existing specifications, the response time is determined by alternately
injecting zero gas and 90% span gas into the monitoring system and measuring
the time required for the monitor to reach 95% of the final response. For
in-situ monitors, the alternation between the simulated zero conditions and
the upscale calibration value determines the response time. According to
the proposed revisions to the specifications, the response time would be
determined by alternately switching from the zero value to monitoring the
effluent for upscale response time determinations, and switching from the
upscale calibration values to monitoring the effluent for downscale response
time determinations. The change in the test procedure reflects the fact
that the response time observed during the calibration procedures is not
always representative of the response time associated with changes in the
effluent concentration. The proposed method would provide a more realistic
determination of response time.
55
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Gas Monitor Installation Considerations
Obviously, from a data quality point of view, the location of gas CEMs
is of fundamental importance. The accessibility of the location is also
very important, however, because service and maintenance of the monitoring
system is vital to achieving acceptable monitoring data availabilty. Often a
trade-off between the most representative location and the most practical
location is required.
Performance Specifications 2 and 3 prescribe monitor installation
location criteria for pollutant and diluent monitors. Location criteria are
particularly important where stratification exists. Stratification usually
exists when the mean concentration and the concentration at any point more
than 1 meter from the duct wall differs by more than 10%. Stratification of
gaseous constituents may occur following any point in the effluent handling
system where the mean concentration of the effluent stream is expected to
change. Examples of situations where gaseous stratification may occur are:
(1) following a point where two effluent streams having different
concentrations are combined; (2) after a flue gas desulfurization (FGD)
device; or (3) after points where air infiltration exists.
Where only a pollutant monitor is employed, the monitor must be located
to sample a portion of the effluent stream where the concentration of the
samples are equivalent to the mean concentration of the entire effluent
stream. When both a pollutant monitor and a diluent monitor are required,
the two monitors should be located so as to sample essentially the same
portion of the effluent stream. Stratification due to air infiltration will
56
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not affect emission monitoring results expressed in terms of lbs/10 Btu
where both the pollutant and diluent monitors sense the same quantity of air
infiltration, because the F-Factor method of computing emissions will cancel
out the biases associated with dilution of the effluent stream by ambient
air.
Where monitoring location stratification cannot be determined according
to the prescribed criteria, tests may be required to determine whether
stratification exists and/or whether particular sampling points will provide
representative measurements. Specific procedures for conducting these tests
have not been prescribed; the only practical method of performing the tests
utilizes portable extractive monitoring equipment. An important aspect of
this type of test is to ensure that the monitoring location is
non-stratified and is representative at all processs operating conditions,
because the concentration profile of the gas constituent s) of interest may
vary with process operating conditions. The need for consistent GEM
measurement representativeness must be balanced against the cost and
feasiblity of performing numerous stratification tests. Realistically,
stratification tests at two process conditions (80-100? and 40-60/t of the
maximum production rate) should suffice.
57
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
. REPORT NO.
EPA-340/1-83/007
3. RECIPIENT'S ACCESSION NO. •
. TITLE AND SUBTITLE
An Introduction to Continuous Emission Monitoring
Programs
5. REPORT DATE
January 1983
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
James W." Peeler
8. PERFORMING ORGANIZATION REPORT N(
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Entropy Environmentalists, Inc.
P.O. Box 12291
Research Triangle Park, NC 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-01-6317
12. SPONSORING AGENCY NAME AND ADDRESS
OAQPS
Stationary Source Compliance Division
Waterside Mall, 401 M Street, SW
Washington, DC 20460
13. TYPE OF REPORT AND PERIOD COVEREI
FINAL - IN-HOUSE
14. SPONSORING AGENCY CODE
EPA/200/04
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This document provides a general introduction to continuous emission monitoring
for those persons not previously involved in this field. Information is presented
on continuous opacity monitoring, as well as instrumental and alternative
monitoring techniques for SO and NOX (i.e., continuous wet-chemical measurement
methods and fuel sampling an3 analysis methods). This document presents an out-
line and review of the fundamental concepts, terminology, and procedures used
in a continuous emission monitoring program. Also presented are selected tech-
nical details necessary to understand the operation of emission monitors,^the
use of continuous emission monitoring data by air pollution control agencies,
and references to other available documents which provide additional information.
17 KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
Air Pollution
Continuous Emission Monitoring
if.
18. DISTRIBUTION STATEMENT
Release to Public
b.lDENTIFIERS/OPEN ENDED TERMS
Monitoring Techniques
Monitoring procedures
Operation of emission
monitors
unclassified
20. SECURITY CLASS (This page)
unclassified
c. COS AT I Field/Group
21. NO. OF PAGES
66
22. PRICE
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
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- c. tnvlrormien'iTiJ Protection Ji/?en
?&}-:; 5. Lilirury (5?L-lfi
,.'JC f- Dearcorn ;?t,rer+,, F.iok ;-"'"'
••-, • ,' ;• rev v:
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United States Office of Air Quality Planning and Standards
Environmental Protection Stationary Source Compliance Division
Agency Washington, D.C. 20460
Official Business Publication No EPA-340/1-83-007 Postage and
Penalty for Private Use pees pai(j
5300 Environmental
Protection
Agency
EPA 335
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