United States
                           Environmental Protection
                         Air and Radiation
            December 1991
Continuous  Emission
Proposed Acid  Rain  Rule
                            The U.S. Environmental Protection Agency (EPA) has proposed four rules
                            containing the core acid rain requirements: the Permits Rule (40 CFR Part 72),
                            the Allowances Rule (40 CFR Part 73), the Continuous Emission Monitoring
                            Rule (40 CFR Part 75), and the Excess Emissions Rule (40 CFR Part 77). EPA
                            will also propose additional rules at a future date. These rules will include
                            requirements for facilities that elect to opt in to the Acid Rain Program (40 CFR
                            Part 74) and for the nitrogen oxide (NOX) control program (40 CFR Part 76).
                            This fact sheet summarizes the key components  of the proposed Continuous
                            Emission Monitoring Rule (40 CFR Part 75).
T Tnder Title IV of the Clean Air Act
l^J Amendments of 1990, Congress
authorized the  U.S. Environmental
Protection Agency (EPA) to establish
the Acid Rain Program.  The overall
goal of this program is to significant-
ly reduce sulfur dioxide (§02) and
nitrogen oxide (NOX) emissions, the
precursors of acid rain.  To achieve
this goal at the lowest cost, the
 Sgram will employ both traditional
^2 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 ouy, sell, or hold
allowances as part of its compliance
strategy.) Complete and accurate
emissions data are key to implement-
ing this market-based approach.
                                           Printed on Recycled Paper.

Table 1 . CEM Monitor Components Required for Proposed Acid Ra
(units required)
SOz (Ibs/hr)
NO, (Ibs/mmBtu)1
Opacity (%)
CO2 (Ibs/hr)2
n Monitoring Regulations
Required CEM Monitoring Component






Diluent Gas


Data Handling
1Heat input in mm/Btu/hr is also required.  Alternative methods may be used to monitor CO?.
  An essential feature of smoothly
operating markets is a method  for
certifying the existence of the com-
modity being traded. The CEM data
will, in effect, supply the "gold
standard" to back up the paper cur-
rency of emissions allowances. The
CEM requirements, therefore, will
instill confidence  in the market-
based approach by certifying the ex-
istence  and value of the trading
commodity (the allowance).
  CEM  will also be instrumental in
ensuring that the mandated reduc-
tions of  SO2 and NOx are achieved.
While traditional emissions limita-
tion programs  have  required
facilities to meet specific emissions
rate limitations, the Acid  Rain
Program requires an accounting of
the total emissions from regulated
units during each year. Compliance
for each unit is then determined
through a direct comparison of
SO2 emissions totals reported  by
CEM and  allowances held for  the
What Are the Proposed
T  Tnder the proposed rule, the
LJ owner or operator of a unit regu-
lated under Phase I or Phase II (or a
unit that opts in to the program) and
any new unit must install a CEM sys-
tem on  the unit unless otherwise
specified in the regulation.  A CEM
system includes:
   • An SO2 pollutant concentration
   • A NOx pollutant concentration
   • A volumetric flow monitor.
   • An opacity monitor.
   • A diluent gas monitor.
  • A data acquisition and handling
    system (computer-based) for
    recording and performing cal-
    culations with the data.
  Table 1 summarizes  the CEM
monitors required by the proposed
rule. In all cases, a data acquisition
and handling system must be used to
collect and report the data.
  To monitor SO2 emissions using
a CEM system, a facility must use
both an SQz pollutant concentration
monitor and a  volumetric  flow
monitor to measure the emissions in
pounds per hour (the units needed to
determine compliance). If the SQz
monitor measures on a dry basis and
the flow monitor measures on a wet
basis,  the owner  or  operator  must
determine the hourly moisture content
of the flue gases and correct the
measured flow rate for moisture using
procedures specified in the rule.
  To measure NOX emissions, both
an NOx pollutant concentration
monitor and a diluent gas monitor
are required to calculate an emissions
rate in pounds per  million British
thermal units (Ibs/mmBtu).
  Opacity  monitoring,  which
involves a percentage calculation,
requires only an opacity monitor.
(Under  the proposed rule, units
firing 90 percent or more natural gas
with oil as the backup fuel are ex-
empted from the opacity monitoring
  The proposed  rule does not re-
quire a utility to use a CEM system to
measure CQz. If a utility chooses to
use a CEM system, however, a COa
diluent monitor plus a flow monitor
may 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 system, a
separate CEM system is required on
the bypass flue gas stream.
  All CEM systems must be in con-
tinuous operation and must be able
to sample, analyze, and record data
at least every 15 minutes. All emis-
sions and flow data will be  reduced
to 1-hour averages.  The proposed
rule specifies procedures for  convert-
ing  the hourly emissions data into
the  appropriate units (pounds per
hour for SO? and pounds per million
Btus for NOx).

How Will Emissions Be
Calculated for Periods
of Missing Data?
  Four or more data points are
  needed to  compile valid 1-houd
averages for emissions flow  data, ex"
cept during calibration, main-
tenance, repair, or other required
quality assurance activity  periods,
where two or more data points may
comprise a valid hour. Failure of the
system to acquire the data points
would result in the loss of data for the
entire hour. The proposed rule con-
tains procedures for "filling  in" data
when no valid hour or hours of data
have been recorded by the SO2
monitor, the flow monitor,  and the
NOX CEM system consisting  of the
NOx monitor and the diluent gas
monitor. The proposed rule uses a
conservative approach to substitut-
ing  for missing data. The proposed
procedures are summarized in Table 2.

What Are the
Requirements for Units
Using a Phase  I
Qualifying Technology?
   The proposed rule specifies addi-
   tional monitoring requirement!
for units that  are implementing an
optional  compliance   method
specified under the Permits  Rule
that allows a  unit to use a Phase I

qualifying technology (a system that
achieves  a 90-percent reduction in
SC>2 emissions). Each such unit must
be equipped with pollutant  and
diluent gas monitors to measure SOa
^missions at the inlet to the control
device, in addition to the monitors
required  for measuring  SC>2 emis-
sions discharged to the atmosphere.

What Are the
Requirements for
Multiple Units with a
Common Stack?

  If  two  or  more units share a
  common stack, the  proposed  rule
allows the owner or operator to com-
bine SO2  allowances according to
the procedures outlined  in  the
Allowance System Rule and install
one monitoring system. If a Phase I
and Phase II unit share a common
stack, the owner or operator must
either (1)  install a separate CEM sys-
tem in each duct leading to the stack;
(2) declare the Phase II unit as a sub-
stitute unit in accordance with the
requirements of the Permits Rule; or
(3) obtain the  Administrator's ap-
proval to differentiate between the
ynits parametricly.
F If  a   regulated  unit  and  a
nonregulated unit share a common
stack, the owner or operator must
either (1)  install a separate CEM sys-
tem in each duct leading to the stack;
(2) declare the nonregulated unit as
an opt-in unit under me Opt-in Rule;
or (3) obtain the Administrator's ap-
proval to differentiate the units

What Tests Are Required
for Performance
HPhe proposed rule requires the fol-
 JL lowing performance certification
tests for CEM systems:
  • Calibration error tests  for
    each pollutant concentration
    monitor, diluent gas monitor,
    and flow monitor capable of
    daily calibration tests.
  • An electronic stability test for
    each flow monitor not capable of
    daily calibration tests.
  • A relative accuracy test for each
    SC»2 pollutant concentration
    monitor,  flow monitor, and the
    NOx CEM  system. (Units with
    SC>2 concentrations of 250 parts
    per million or less, gas velocities
    of 10 feet per second or less, or
    NOx emissions rates of 0.5
    Ibs/mmBtu or less are not re-
    quired to meet the relative ac-
    curacy requirement for the SO2
    monitor, the flow monitor, or the
    NOx monitoring system, respec-
    tively, but must meet an alterna-
    tive requirement specified in the
  • A bias test for each SO2 pol-
    lutant concentration monitor,
    flow monitor, and the NOx
    CEM systems.

  • A cycle response test for SQz pol-
    lutant concentration monitors
    and NOx CEM sytems.
  • For differential pressure  flow
    monitors, an orientation sen-
    sitivity test, and for ultrasonic
    and differential pressure  flow
    monitors, an  interference test,
    and  calibration error tests
    whenever possible.

  • For   continuous   opacity
    monitoring systems, perfor-
    mance verification tests for
    calibration error, response time,
    zero drift, and calibration drift.
    These tests must be conducted
    according to the requirements of
    40 CFR Part  60,  Appendix B
    ("Performance Specification 1—
    Standards of  Performance  for
    New Stationary Sources").

What Are the  Certification
Requirements for CEMS?

T Tnder the proposed rule, the EPA
IteJ Administrator must certify the
CEM system (CEMS) before it can be
used in the Acid Rain Program. To
Table 2. Summary of CEM Substitution Criteria for Estimating Values for Missing Data Periods
Annual availability (%) of monitor
or system1
Greater than or equal to 95%
Less than 95% but greater than or equal to
Less than 90%
Number of hours missing (N)
N 5 24 hours
N > 24 hours
N S 6 hours
N > 6 and £ 24 hours
N > 24 hours
N > 0 hours
Value substituted for each missing hour
Average of the hour before missing period
and the hour recorded after missing period
90th percentJIe value recorded in previous
30 days of service or the before/after
value, whichever is greater
Average of the hour recorded before
missing period and the hci«r recorded after
missing period
90th percentile value recorded in previous
30 days of service or the before/after
value, whichever is greater
90th percentile value recorded in previous
365 days of service or the before/after
value, whichever is greater.
90th percentile value recorded in previous
365 days of service or the before/after
value, whichever is greater.
1SO2 and flow monitors are Individually evaluated for missing data. For NOx monitoring, the monitor system (NOx polllutant concentration
monitor and diluent gas monitor) are considered In combination. NOx and flow monitoring data Is correlated to unit gross operating load
before selecting the percentile values. SOz data are correlated to fuel sulfur content before selecting the pVcentlle value.

obtain certification, the owner or
operator  of a unit must submit a
request to the EPA Administrator.
The request must include the follow-
ing information:

   • Unit identification.

   • Stack inside diameter at monitor
    location and stack height.

   • Description of CEMS and con-
    tinuous opacity monitoring sys-
    tem (COMS) installed.

   • Results and date of each perfor-
    mance verification test for cer-

   • Supporting documentation to
    substantiate the test results.

   • Calculations to verify that the
    data  acquisition and handling
    system properly calculates and
    converts the recorded emissions
    data into units of the standard.
   EPA will issue a notice approving
or disapproving the request for cer-
tification within 120 days. If the
proposed system is disapproved, the
owner or operator must revise the
equipment, procedures, or methods
as necessary and resubmit a request
for certification.
What Quality Assurance/
Quality Control
Procedures Are Required?
   The proposed rule requires the
   owner or operator to develop and
implement a written quality as-
surance/quality control plan for
each system. This plan must be sub-
mitted as part of the utility's acid rain
permit application. The quality con-
trol plan must  include complete,
step-by-step procedures and opera-
tions   for  calibration  checks,
calibration adjustments,  preventive
maintenance, audits, and  record-
keeping  and  reporting.  The
proposed rule specifies procedures
for assessment of calibration error,
relative accuracy,  and  bias. The
quality assurance plan must in-
clude relative accuracy test audits,
calibration error tests, and bias tests.

What Alternative
Monitoring Systems Are
   he owner or operator of an af-
   fected unit may apply to the
EPA Administrator for approval of
an alternative monitoring system to
determine hourly emissions data for
SO2, NOx, and/or volumetric flow.
An alternative   system  must
provide the   same  or  better
precision, reliability, accessibility,
and timeliness as  a certified  CEM
system.  The owner or operator
must  submit certain information
and data to demonstrate that the
alternative system meets these
  The proposed rule  includes an
exception for units fired 90 percent
or more by natural  gas because
emissions from the 525 units in this
category emitted only about  7,600
tons of SO2 in 1985.   These  units
may use in-line oil flow meters and
oil sampling and  analysis to es-
timate tons of  SQz emitted. Units
burning greater than 10 percent oil
may also use  this approved, ex-
cepted method, although they must
sample the  oil  hourly using flow-
proportional or continuous  drip
methods. This exception is based on
a combination of factors: (1) the
relative ease of  obtaining oil
samples; (2) the fact that oil is much
more  homogeneous in sulfur con-
tent than coal; and (3) the approved
oil sampling and analysis methods
produce SO2 emission estimates
consistently higher than actual
CEM data.

What Are the
Recordkeeping, Reporting,
and Notification
   The   CEM   rule   proposes
   requirements for notification,
recordkeeping, and reporting for the
Acid Rain Program. The require-
ments include:
  • Submission of monitoring plans
    as part  of the  compliance plan
    and permit required under the
    Permits Rule.
  • Written   notifications of
    monitor certification tests.
  • Daily recording of hourly emis-
    sions data and other informa-
  • Maintaining records of emis-
    sions, flow data, and other in-
  • Initial and  quarterly reports
    of quality assurance  and
    quality control tests for the
    CEM system.

   • Reports of recorded emissions,
    flow, unit operating status, and
    monitoring performance data.
   The proposed rule also requires
the owner or operator to electronicaa
ly report the required information on
a quarterly basis as an ASCII flat file
via either an IBM-compatible per-
sonal computer floppy diskette or a
modem. EPA will use the information
to determine compliance  with the
emissions reductions mandated by
the Clean Air Act.

What Are the Deadlines
for Compliance?
   The proposed rule applies to ex-
   isting units  regulated under
Phase I or Phase II of the Acid Rain
Program, and to each new utility
unit  when  it begins its operation.
(A "new unit" is a defined as a unit
that  begins commercial operation
on or  after November 15, 1990.)
Equipment  required   by  the
proposed rule must  be installed,
certified,  and  operational by
November  15, 1993, for Phase I af-
fected units, and by January 1,1995,
for any Phase II units. A unit that
formally commits to retiremei^
before December 31, 1994, will flQ
exempt from the requirements of
the proposed rule.

For More Information

     more information, write to:

   U.S. EPA Office of Air and
   Acid Rain Division
   Washington, DC 20460

   If you would like to receive other
fact sheets in this series, call the Acid
Rain Hotline at (617) 641-5377 or the
EPA Public Information Center (PIC)
at 202-260-2080.

   Fact sheets are available on the fol-
lowing subjects:

   • Allowance System

   • Environmental Benefits

   • Excess Emissions

   • Permits
   • Proposed Acid Rain Rules