United States	Air and Radiation	EPA430/F-92/Q21
Environmental Protection	(6204J)	October 1992
Agency
*>EPA Acid Rain Program
Continuous Emission
Monitoring (CEM)
In order to reduce acid rain in the United States and Canada, Title IV of
the Clean Air Act Amendments of 1990 established the Acid Rain
Program. The program will cut sulfur dioxide emissions in half and
substantially reduce nitrogen oxides emissions from electric aitility
plants. This fact sheet discusses the continuous emission monitoring
requirements of the Acid Rain Program and is one of a series containing
information about the program.
The overall goal of the Acid
Rain Program is to achieve
significant environmental bene-
fits through reductions in emis-
sions of sulfur dioxide (SO2) and
nitrogen oxides (NOx), the pri-
mary causes of acid rain. To
achieve this goal at the lowest
cost to society, the program
employs both traditional and
innovative, market-based ap-
proaches for controlling air pollu-
tion. In addition,, the program
encourages energy efficiency and
pollution prevention.
Title IV of the Qean Air Act sets
as its primary goal the reduction
of annual SO2 emissions bv 10
million tons below 1980 levels. To
achieve these reductions, the law
requires a two-phase tightening
of the restrictions placed on fossil
fuel-fired power plants.
Phase I begins in 1995 and
affects 110 mostly coal-burning
electric utility plants located in 21
eastern and midwestern states.
Phase II, which begins in the year
2000, tightens the annual emis-
sions limits imposed on these
large, higher emitting plants and
also sets restrictions on smaller,
cleaner plants fired by coal, oil,
and gas. The program affects ex-
isting utility units with an output
capacity of 25 megawatts or
greater and new utility units
under 25 megawatts that use fuel
with a sulfur content greater than
0.05 percent.
The Act also calls for a 2 mil-
lion-ton reduction in NOx emis-
sions by the year 2000. A
significant portion of this reduc-
tion will be achieved by coal-fired
utility boilers that will be re-
quired to install low NOx burner
technologies and to meet new
emissions standards.
The innovative, market-based
SO2 allowance trading compo-
nent of the Acid Rain Program
allows utilities to adopt the most
cost-effective strategy to reduce
SO2 emissions at units in their
system. The Acid Rain Program
operating permit outlines the
specific program requirements
and compliance options chosen
by each source. Affected utilities
also will be required to install
systems that continuously moni-
tor emissions of SO2, NOx, and
other related pollutants in order
to track progress, ensure compli-
ance, and provide credibility to
the trading component of the
program. In any year that com-
pliance is not achieved, excess
emissions penalties will apply,
and sources will be required to
submit a plan to EPA that speci-
fies how the excess SO2 emis-
sions will be offset.
Introduction
Continuous emission monitor-
ing (CEM) will be instrumental in
ensuring that the mandated re-
ductions of SO2 and NOx are
achieved. While traditional emis-
sions limitation programs have
required facilities to meet specific
emissions rates, the SO2 portions
of the Acid Rain Program require
an accounting of each ton of emis-
sions from each regulated unit.
Compliance is then determined
through a direct comparison of
total annual SO2 emissions re-
ported by CEM and allowances
neld for the unit.
CEM is the measurement on a
continuous basis of pollutants
emitted into the atmosphere in
exhaust gases from combustion
processes or as the by-product of
industrial processes. EPA has es-
tablished requirements for the
continuous monitoring of SO2,
NOx, volumetric flow, opacity,
and diluent for units regulated
under Phase I and Phase II of the
Acid Rain Program. In addition,
procedures for monitoring or es-
timating carbon dioxide (CO2)
are specified. The rule also con-
tains requirements for equipment
performance specifications, certi-
fication procedures, and record-
keeping and reporting.
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Why Is CEM Necessary?
The Acid Rain Program estab-
lishes an allowance trading sys-
tem as a market-based approach
to reduce SO2 emissions in a cost-
effective manner. (One allowance
is an authorization to emit 1 ton
of SO2 during or after a specified
calendar year; a utility may buy,
sell, or hold allowances as part of
its compliance strategy.) Com-
plete ana accurate emissions data
are key to implementing this
market-based approach.
An essential feature of
smoothly operating markets is a
method for certifying the exist-
ence of the commodity being
traded. The CEM data will, in ef-
fect, supply the "gold standard"
to back up the paper current of
emissions allowances. The CEM
requirements, therefore, will in-
still confidence in the market-
based approach by certifying the
existence and value of the traded
commodity (the allowance).
What Are the Monitoring
Requirements?
The owner or operator of a unit
regulated under Phase I or Phase
II and any new unit must install a
CEM system on the unit unless
otherwise specified in the regula-
tion. A CEM system includes:
•	An SO2 pollutant concentra-
tion monitor
•	A NOx pollutant concentra-
tion monitor
•	A volumetric flow monitor
•	An opacity monitor
•	A diluent gas (oxygen or
CO2) monitor
•	A data acquisition and
handling system (computer-
based) for recording and per-
forming calculations with the
data
Table 1 summarizes the CEM
requirements of the Acid Rain
Program. In all cases, a data ac-
quisition and handling system
must be used to collect and report
the data.
TABLE 1. CEM MONITOR COMPONENTS REQUIRED FOR ACID RAIN
MONITORING REGULATIONS
Monitoring
Requirement (units
required)
Required CEM Monitoring Component
SO2
NOx
Flow
Opacity
Diluent
Gas
Data
Handling
SO2 (Ibs/hr)
Yes

Yes


Yes
NO* (lbs/mmBtu)1

Yes


Yes
Yes
Opacity (%)



Yes

Yes
CO2 (ibs/hr)2


Yes

Yes
Yes
1Heat input in mmBtu/hr is also required.
2Altemative methods may be used to monitor CO2.
To monitor SO2 emissions us-
ing a CEM system, a facility must
use both an SO2 pollutant concen-
tration monitor and a volumetric
flow monitor to measure the
emissions in pounds per hour.
To measure NOx emissions,
both a NOx pollutant concentra-
tion monitor and a diluent gas
monitor are required to calculate
an emissions rate in pounds per
million British thermal units
(lbs/mmBtu).
Opacity monitoring, which
measures the percentage of light
that can be seen through flue gas,
requires only an opacity monitor.
Under the CEM Rule gas-fired
units that combust natural gas for
at least 90 percent of their total
heat input auring the 3 previous
calendar years, and diesel and
dual-fuel reciprocating internal
combustion engine units are
exempt from opacity monitoring
requirements.
The rule does not require a util-
ity to use a CEM system to meas-
ure CO2. If a utility chooses to use
a CEM system, however, a CO2
diluent monitor plus a flow moni-
tor would be used to compute
emissions in pounds per hour.
If a CEM system is installed in
such a way that any portion of the
flue gases from an affected unit
can bypass the monitoring sys-
tem, a separate CEM system is
required on the bypass flue gas
stream.
All CEM systems must be in
continuous operation and must
be able to sample, analyze, and
record data at least every 15 min-
utes. All emissions and flow data
will be reduced to 1-hour aver-
ages. The rule specifies proce-
dures for converting the hourly
emissions data into tne appropri-
ate units of measure.
Who Do These
Requirements Apply To?
All units over 25 megawatts
and new units under 25 mega-
watts that use fuel with a sulmr
content ereater than 0.05 percent
by weight are required to meas-
ure and report emissions under
the Acid Rain Program. The new
units under 25 megawatts using
clean fuels are required to certify
their eligibility for an exemption
every 5 years. A unit that formally
commits to retirement before
December 31, 1994, will be ex-
empt from the requirements of
the rule. The following is a sum-
mary of monitoring method re-
quirements and options.
•	All existing coal-fired units
greater than 25 megawatts
and all new coal units must
use CEM for SO2, NOx, flow,
and opacity.
•	Units burning natural gas
exclusively are exempt from
SO2 monitoring require-
ments.
•	Units burning oil may moni-
tor SO2 mass emissions by
one of the following three
methods: (1) daily manual oil
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It
s
sampling and analysis plus
oil now meter (to continu-
ously monitor the amount of
oil consumed), (2) automatic
continuous oil sampling plus
oil flow meter, or (3) SO2 and
flow CEM.
•	Gas-fired and oil-fired base-
loaded units must use NOx
CEM.
•	Gas-fired peaking units and
oil-fired peaking units may
either estimate NOx emis-
sions by using site-specific
emission correlations and
periodic stack testing to
verify continued repre-
sentativeness of the correla-
tions, or use NOx CEM.
•	All gas-fired units using
natural gas for at least 90 per-
cent of their annual neat
input, and diesel and duel-
fuel reciprocating internal
combustion engine units
are exempt from opacity
monitoring.
•	For CO2, all units can use
either (1) a mass balance esti-
mation, (2) CO2 CEM, or (3)
oxygen CEM in order to esti-
mate CO2 emissions.
If a unif s utilization or fuel use
changes so that excepted moni-
toring methods no longer apply,
the unit would become subject to
CEM requirements in the follow-
ing calendar year.
How Will Emissions Be
Calculated for Periods of
Missing Data?
Four or more data points are
needed to compile a valid 1-hour
average for emissions flow data,
except during calibration, main-
tenance, repair, or other required
quality-assurance activity peri-
ods, where two or more data
?oints may compose a valid hour,
ailure ot the system to acquire
the data points would result in
the loss of data for the hour. The
rule contains procedures for
filling in data when no valid hour
or hours of data have been re-
corded by the SO2 monitor, the
flow monitor, and the NOx CEM
system consisting of the NOx
monitor and the diluent gas
monitor. The rule uses a conserva-
tive approach to substituting for
missing data. This offers an incen-
tive to keep monitor down-time
to a minimum, giving the most
accurate and reliable results. The
procedures are summarized in
Table 2.
What Are the
Requirements for Units
Using a Phase I Qualifying
Technology?
The CEM Rule specifies addi-
tional monitoring requirements
for units that are implementing
an optional compliance method
specified under tne Permits Rule
that allows a unit to use a Phase I
qualifying technology (a system
that achieves a 90-percent reduc-
tion in SO2 emissions). Each such
unit must be equipped with pol-
lutant and diluent gas monitors to
measure SO2 emissions at the in-
let to the control device, in addi-
tion to the monitors required for
measuring SO2 emissions dis-
charged to the atmosphere.
What Are the
Requirements for Multiple
Units with a Common
Stack?
If two or more units share a
common stack, the CEM Rule
allows the owner or operator to
combine SO2 allowances accord-
ing to the procedures outlined in
the Allowance System Rule and
to install one monitoring system.
If a Phase I and Phase II unit share
a common stack, the owner or
TABLE 2. SUMMARY OF CEM SUBSTITUTION CRITERIA FOR
ESTIMATING VALUES FOR MISSING DATA PERIODS
Annual availability (%) of
monitor or system1
Number of hours missing
(N)
Value substituted for each
missing hour
Greater than or equal to
95%
N s 24 hours
Average of the hour
recorded before missing
period and the hour
recorded after missing
period
N £ 24 hours
90th percentile value
recorded in previous 30
days of service or the
before/after value,
whichever is greater
Less than 95% but greater
than or equal to 90%
Ni 8 hours
Average of the hour
recorded before missing
period and the hour
recorded after missing
period
N > 8 hours
95th percentile value
recorded in previous 302
days of service or the
before/after value,
whichever is greater
Less than 90%
N > 0 hours
Maximum value recorded
in previous 30 days of
service or the before/after
value, whichever is greater
SO2 and flow monitors are individually evaluated for missing data. For NOx, the NOx
pollutant concentration monitor and diluent gas monitor are considered in combination. NOx
and flow monitoring data are correlated to unit gross operating load before selecting the
percentile values.
2NOx CEM systems review the previous 90 days of service.
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operator must either (1) install a
separate CEM system in each
duct leading to the stack, (2) de-
clare the Phase II unit as a substi-
tution unit in accordance with the
requirements of the Permits Rule,
or (3) obtain the Administrator's
approval to differentiate between
the units parametrically.
If a regulated unit and a
nonregulated unit share a com-
mon stack, the owner or opera-
tor must either (1) install a
separate CEM system in the duct
leading to the stack of each regu-
lated unit, (2) declare the non-
regulated unit as an opt-in unit
under the Opt-in Rule, or (3) ob-
tain the EPA Administrator's
approval to differentiate the
units parametrically.
What Are the Certification
Requirements?
The Acid Rain Program re-
quires the following performance
certification tests for CEM sys-
tems:
•	A 7-day calibration error test
for each monitor
•	Alinearity check for each pol-
lutant concentration monitor
•	A relative accuracy test audit
(RATA) for each monitor
•	A bias test for each pollutant
concentration monitor and
flow monitor
•	A cycle time/response test for
eacn pollutant concentration
monitor
The EPA Administrator must
approve the CEM system before it
can be used in the Acid Rain Pro-
gram. The owner or operator of a
unit must conduct a certification
test and submit the results to the
EPA Administrator.
EPA will issue a notice approv-
ing or disapproving the reouest
for certification within 120 aays.
If the proposed system is disap-
proved, tne owner or operator
must revise the equipment, pro-
cedures, or methods as necessary
and resubmit a request for
certification.
The operator also must per-
form periodic performance
evaluations of the equipment,
including daily calibration error
tests, daily interference tests for
flow monitors, and semi-annual
(or annual) RATA and bias tests.
What Quality Assurance/
Quality Control Procedures
Are Required?
The owner or operator must
develop and implement a written
quality assurance/quality control
plan for each system. The quality
control plan must include com-
plete, step-by-step procedures
and operations for calibration
checks, calibration adjustments,
preventive maintenance, audits,
and recordkeeping and reporting.
The rule specifies procedures for
assessment of calibration error,
relative accuracy, and bias. The
quality assurance plan must
include relative accuracy test
audits, calibration error tests, and
bias tests.
Are Alternative
Monitoring Systems
Allowed?
The owner or operator of an
affected unit may apply to the
EPA Administrator for approval
of an alternative monitoring sys-
tem to determine hourly emis-
sions data for SO2, NOx, and/or
volumetric flow. An alternative
system must provide the same or
better precision, reliability, acces-
sibiUtyand timeliness as a certi-
fied CEM system. The owner or
operator must submit substantial
information and data to demon-
strate that the alternative system
meets these criteria.
What Are the
Recordkeeping, Reporting,
and Notification
Requirements?
The CEM Rule indicates a num-
¦ ber of requirements for notifica-
tion, recordkeeping, and reporting
for the Acid Rain Program. The
requirements include:
•	Submission of monitoring
plans as part of the precer-
tification testing form
submittal
•	Written notifications of moni-
tor certification tests
•	Report of certification test
results in a "certification
application"
•	Daily recording and main-
taining of hourly emissions
data, Tlow data, and other
information
•	Quarterly reports of emis-
sions, flow, unit operating
status, and monitoring per-
formance data
The rule also requires the
owner or operator to report the
CEM data electronically. EPA will
use this information to determine
compliance with the emissions
reductions mandated by the
Clean Air Act.
What Are the Deadlines
for Compliance?
All required equipment must
b* installed, certified, and
operational by November 15, ¦-
1993, for Phase I affected units,
and by January 1,1995, for any
Phase II units. A new unit (a
unit that begins commercial
operation on or after November
15,1990) must meet all require-
ments no later than 90 days
after commencing commercial
operations.
For More Information
Write to:
U.S. EPA
Acid Rain Division (6204J)
401 M Street, SW.
Washington, DC 20460
If you would like to receive
other fact sheets on the Acid Rain
Program, call the Acid Rain
Hotline at 617-674-7377 or the
EPA Public Information Center
(PIC) at 202-260-2080.
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