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Continuous Emissions Monitoring Fact Sheet


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Continuous emissions monitoring (CEM) is instrumental in ensuring that the mandated reductions of SO2 and NOx under the Acid Rain Program are achieved. While traditional emissions limitation programs have required facilities to meet specific emissions rates, the Acid Rain Program requires an accounting of each ton of emissions from each regulated unit. Compliance is then determined through a direct comparison of total annual SO2 emissions reported by CEM and allowances held for the unit.

CEM is the continuous measurement of pollutants emitted into the atmosphere in exhaust gases from combustion or industrial processes. EPA has established requirements for the continuous monitoring of SO2, volumetric flow, NOx, diluent gas, and opacity for units regulated under the Acid Rain Program. In addition, procedures for monitoring or estimating carbon dioxide (CO2) are specified. The CEM rule also contains requirements for equipment performance specifications, certification procedures, and recordkeeping and reporting.

Frequently Asked Questions About CEM

  1. Why is CEM Necessary?
  2. What are the Monitoring Requirements?
  3. Who Do these Requirements Apply To?
  4. How Will Emissions Data be Calculated for Periods of Missing Data?
  5. What Are the Certification Requirements?
  6. What Quality Assurance/Quality Control Procedures are Required?
  7. What Are the Deadlines for Compliance?
  8. What Are the Recordkeeping and Reporting Requirements?

Why is CEM Necessary?

The Acid Rain Program uses 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.) Complete and accurate emissions data are key to implementing this market-based approach.

An essential feature of smoothly operating markets is a method for measuring the commodity being traded. The CEM data will supply the gold standard to back up the paper currency of emissions allowances. The CEM requirements, therefore, will instill confidence in the market-based approach by verifying the existence and value of the traded allowance.

What Are the Monitoring Requirements?

The owner or operator of a unit regulated under the Acid Rain Program must install CEM systems on the unit unless otherwise specified in the regulation. CEM systems include:

Table 1 summarizes the CEM requirements. In all cases, a data acquisition and handling system must be used to collect and report the data.

To monitor SO2 emissions in pounds per hour using a CEM system, a facility must use both an SO2 pollutant concentration monitor and a volumetric flow monitor.

For NOx, both a NOx pollutant concentration 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.

The rule does not require a utility to use a CEM system to measure CO2. If a utility chooses to use a CEM system, however, a CO2 or oxygen monitor plus a flow monitor would be used to compute emissions in tons per hour.

All CEM systems must be in continuous operation and must be able to sample, analyze, and record data at least every 15 minutes. All emissions and flow data will be reduced to 1-hour averages. The rule specifies procedures for converting the hourly emissions data into the appropriate units of measure.

Table 1: CEM Monitor Components


Monitoring
Requirement
(units required)
Required CEM Monitoring Component
SO2 NOx Flow Opacity Diluent
Gas
Data
Handling
SO2(lbs/hr) Yes   Yes     Yes
NOx(lbs/mmBtu)*   Yes     Yes Yes
Opacity(%)       Yes   Yes
CO2(lbs/hr)**     Yes   Yes Yes

*Heat input in mm/Btu/hr is also required
**Alternative methods may be used to monitor CO2

To Whom Do These Requirements Apply?

All units over 25 megawatts and new units under 25 megawatts that use fuel with a sulfur content greater than .05 percent by weight are required to measure 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 five years. A unit that formally committed to retirement before December 31, 1994 is exempt from the requirements of the rule. The following is a summary of monitoring method requirements and options:

If a unit's operation or fuel use changes so that excepted monitoring methods no longer apply, the unit would become subject to CEMS monitoring requirements in the following calendar year.

How Will Emissions Be Calculated for Periods of Missing Data?

The CEM rule contains procedures for filling in data when no valid hour or hours of data have been recorded by a monitor or monitoring system. The rule uses a conservative approach to substitute for missing data. This methodology offers an incentive to keep monitor down-time to a minimum, giving the most accurate and reliable results. The procedures for SO2, NOx, and flow are summarized in Table 2. The recently revised regulation also clarifies missing data procedures for CO2, heat input, and fuel flow.

Table 2: Substitution Criteria for CEM Missing Data Periods


Annual Availability (%)
of Monitor or System*
Number of Hours
Missing (N)
Value Substituted for Each
Missing Hour
Greater than or equal
to 95%







N is less than or
equal to 24 hours


N is greater than 24 hours




Average of the hours
recorded before and after
missing period

90th percentile value
recorded in the previous 30**
days of service or the
before/after value,
whichever is greater
Less than 95% but
greater than or equal
to 90%







N is less than or
equal to 8 hours


N is greater than 8 hours




Average of the hours
recorded before and after
missing period

95th percentile value
recorded in the previous 30**
days of service or the
before/after value,
whichever is greater
Less than 90% N is greater than 0 hours
Maximum value recorded in
previous 30** days of service

*SO2 and flow monitors are individually evaluated for missing data. For NOx monitoring, the monitor system NOx pollutant concentration monitor and diluent gas monitor are considered in combination. NOx and flow monitoring data is correlated to unit electrical output (load) before selecting the percentile values.

**NOx CEM systems and flow monitors review the previous 90 days of service.

What Are the Certification Requirements?

The Acid Rain Program requires the following performance certification tests for CEM systems:

An accuracy test is required for fuel flowmeters. Stack tests are required for a NOx emission correlation for gas- and oil-fired peaking units. For all monitoring methods, the DAHS must be tested.

EPA must certify the CEM system (CEMS) before it can be used in the Acid Rain Program. To obtain certification, the owner or operator of a unit must conduct certification tests and submit the results to the EPA and the appropriate State agency.

EPA will issue a notice approving or disapproving the request for certification within 120 days after receiving a complete certification application. 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 Are the Deadlines for Compliance?

All required equipment must be installed, certified, and operational by the dates in Table 3.

Table 3: CEM Rule Certification Deadlines

Type of Unit Certification Testing Deadline
Phase I 11/15/93
Phase II (except for NOx/CO2
at oil and gas-fired units
1/1/95
Phase II oil and gas-fired in
ozone non-attainment areas
7/1/95 for NOx and CO2 only
Other Phase II oil and gas-fired
(not in ozone nonattainment area)
1/1/96 for NOx and CO2 only
New units 90 days after commercial operation
Units with new stack 90 days after sending emissions through
new stack
Units shutdown on original
deadline
earlier of: 45 unit operating days or 180 days
after recommencing commercial operation
Emergency fuel for gas and oil-fired
units
30 unit operating days after using emergency fuel
Backup fuel for gas & oil-fired
units (App. E only)
later of: 30 unit operating days after using
backup fuel after testing primary fuel or NOx
monitoring deadline for gas and oil units

What Quality Assurance/Quality Control Procedures Are Required?

The operator must perform 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.

The owner or operator must develop and implement a written quality assurance/quality control plan for each system. The quality control plan must include complete, step-by-step procedures and operations for calibration checks, calibration adjustments, preventive maintenance, audits, and record-keeping and reporting. The quality assurance plan must include procedures for conducting periodic performance tests.

What Are the Recordkeeping and Reporting Requirements?

The CEM rule includes requirements for notification, recordkeeping, and reporting for the Acid Rain Program, such as:

The owner or operator also must report the data in a standard electronic format available through the Acid Rain Hotline. EPA will use this information to determine compliance with the emissions reductions mandated by the Clean Air Act. Most new recordkeeping and reporting requirements under the revised rule are optional through December 31, 1995 and are mandatory beginning January 1, 1996.

 


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