Continuous Emissions Monitoring Fact Sheet

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Continuous emissions monitoring (CEM) is instrumental in ensuring that the mandated reductions of SO₂ and NO_x 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, NO_x, 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

Why is CEM Necessary?
What are the Monitoring Requirements?
Who Do these Requirements Apply To?
How Will Emissions Data be Calculated for Periods of Missing Data?
What Are the Certification Requirements?
What Quality Assurance/Quality Control Procedures are Required?
What Are the Deadlines for Compliance?
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:

An SO₂ pollutant concentration monitor.
A NO_x pollutant concentration monitor.
A volumetric flow monitor.
An opacity monitor.
A diluent gas (O₂ or CO₂) monitor.
A computer-based data acquisition and handling system (DAHS) for recording and performing calculations with the data.

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 NO_x, both a NO_x 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
Monitoring Requirement (units required)	SO₂	NO_x	Flow	Opacity	Diluent Gas	Data Handling
SO₂(lbs/hr)	Yes		Yes			Yes
NO_x(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:

All existing coal-fired units serving a generator greater than 25 megawatts and all new coal units must use CEMs for SO₂, NO_x, flow, and opacity.
Units burning natural gas may determine SO₂ mass emissions by: (1) measuring heat input with a gas flowmeter and using a default emission rate; or (2) sampling and analyzing gas daily for sulfur and using the volume of gas combusted; or (3) using CEMs.
Units burning oil may monitor SO₂ mass emissions by one of the following methods:
1. daily manual oil sampling and analysis plus oil flow meter (to continuously monitor oil usage)
2. sampling and analysis of diesel fuel oil as-delivered plus oil flow meter
3. automatic continuous oil sampling plus oil flow meter
4. SO₂ and flow CEMs.
Gas-fired and oil-fired base-loaded units must use NO_x CEMs.
Gas-fired peaking units and oil-fired peaking units may either estimate NO_x emissions by using site-specific emission correlations and periodic stack testing to verify continued representativeness of the correlations, or use NO_x CEMS. The emission correlation method has been significantly streamlined in the revised rule.
All gas-fired units using natural gas for at least 90 percent of their annual heat input and units burning diesel fuel oil are exempt from opacity monitoring.
For CO₂ all units can use either (1) a mass balance estimation, or (2) CO₂ CEMs, or (3) O₂ CEMs in order to estimate CO₂ emissions.

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, NO_x, 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 NO_x monitoring, the monitor system NO_x pollutant concentration monitor and diluent gas monitor are considered in combination. NO_x and flow monitoring data is correlated to unit electrical output (load) before selecting the percentile values.

**NO_x 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:

A 7-day calibration error test for each monitor.
A linearity check for each pollutant concentration monitor.
A relative accuracy test audit (RATA) for each monitor.
A bias test for each SO₂ pollutant concentration monitor, flow monitor, and the NO_x CEM system.
A cycle time test for each pollutant concentration monitor.
A daily interference test for flow monitors.

An accuracy test is required for fuel flowmeters. Stack tests are required for a NO_x 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 NO_x/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 NO_x and CO₂ only
Other Phase II oil and gas-fired (not in ozone nonattainment area)	1/1/96 for NO_x 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 NO_x 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:

Submission of monitoring plans.
Written notifications of monitor certification tests.
Report of certification test results in a "certification application".
Recording and maintaining of hourly emissions data, flow data, and other information.
Quarterly reports of emissions, flow, unit operation, and monitoring performance data.

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.