See the latest EHS federal and state regulatory updates due to COVID-19

Many industrial facilities with pollutant emitting units subject to regulatory emissions limits must implement sophisticated Continuous Emissions Monitoring Systems (CEMS) to demonstrate compliance. Regulations that contain CEMS requirements include a number of NSPS and MACT standards, some construction and operating permits programs, EPA’s new greenhouse gas mandatory reporting rule, the Acid Rain program, the Cross State Air Pollution Rule, and local initiatives such as California AB 32 and RECLAIM programs.

Prior to CEMS equipment specification, vendor section, and installation, industrial facilities should have a solid understanding of the federal regulatory requirements under 40 of the Code of Federal Regulations (CFR) Parts 60 and 75 (as well as state and local requirements). Furthermore, the ongoing operation of an effective CEMS program that consistently meets compliance requirements entails significant effort related to quality control, maintenance, training, auditing, and reporting. The level of effort associated with these tasks requires specialized expertise and an organized, well defined approach. Following a brief overview of CEMS equipment, this article focuses on the operational elements of a CEMS program and provides recommendations for best practices.

CEMS Equipment

CEMS continuously analyze source emissions for specific pollutant(s) generated from industrial processes and combustion sources. The monitoring equipment is used to demonstrate compliance with applicable emissions limits and standards – either concentration-based limits/standards (ppm, ppb, %, etc.) or mass-based limits/standards (pounds per hour, pounds per million BTU of heat input, etc.). Essentially, a sampling system constantly extracts a slip stream of stack gas from a process unit and routes it to the monitoring equipment which consists of various analyzers. Once the gas sample has been analyzed in the monitoring equipment, the associated results are fed into a data acquisition and handling system (DAHS) which processes the raw data and outputs results in the required format to the facility’s reporting system. 

EQ Summer 2012_CEMS diagram

There are four major types of CEMS systems: 1) dry extractive systems, 2) dilution extractive systems, 3) wet extractive systems, and 4) in-situ systems. Dry extractive systems are used throughout the U.S. in a petrochemical plant night power, pulp and paper, steel, cement, landfill gas combustion, and glass bottle manufacturing. Approximately 55% of CEMS which measure nitrogen oxides (NOX) are dry extractive CEMS. This equipment performs well at both high and low concentrations, is versatile, and fairly straightforward to operate. However, it requires frequent maintenance and has a relatively high purchase price. Dilution extractive equipment has historically been used to comply with 40 CFR Part 75 sulfur dioxide (SO2) monitoring requirements for coal-fired combustion units. It is also commonly used for dusty emissions sources with high particulate matter (PM) concentrations in the sample, such as cement plant kilns. These systems are easily maintained and often cheaper than dry extractive systems. However dilution extractive systems may not be appropriate for measuring pollutants in lower concentrations. Furthermore, the systems require more maintenance than other CEMS, can be difficult to troubleshoot, and cannot measure oxygen levels.

Wet extractive CEMS, in which the sample is delivered to the analyzer above dew point, are useful for incinerators where acid gases are present (HCl and HF). These systems are relatively easy to calibrate and require minimal sample conditioning. However performance of these units at low concentrations is sub-par. In situ CEMS are located at the source of the pollutant (e.g., on the stack). These units analyze the exhaust in or across the stack or duct without extracting and treating a sample for analysis. Examples of in-situ systems include Fourier Transform Infrared (FTIR) systems, flow measurement systems using ultrasonic technology, and thermal sensing. Fast response time, lower cost, and lower maintenance requirements for these units are offset by shorter life spans due to environmental exposure, maintenance difficulty, poor accuracy, and calibration challenges.A qualified CEMS contractor can assist with the selection of the most suitable equipment as well as the installation and certification of the system. However, the development of adequate Quality Assurance and Quality Control (QA/QC) programs and maintenance plans, ongoing training, reporting and auditing remain largely the responsibility of the facility.

Ensuring Quality Control

Every federal, state, or local regulation that requires CEMS to demonstrate compliance with an emission limit includes provisions for ensuring data quality. In fact, Quality Assurance and Quality Control (QA/QC) programs are the backbone of any continuous emissions monitoring system. Setting up appropriate QA/QC procedures, establishing missing data substitution provisions, and specifying DAHS calculation procedures are just a few key elements of an effective QA/QC program.

Quality Assurance pertains to the accuracy and reliability of the emissions data, whereas Quality Control includes the activities involved in maintaining or improving the accuracy and reliability of the emissions data. The Quality Assurance Plan, as well as routine audits comprise the quality assurance section of the QA/QC program, whereas the maintenance plan and calibrations act to ensure quality control for the CEMS.

Regulatory requirements for CEMS QA/QC vary, but all include tasks such as daily calibration, linearity tests, calibration gas audits (CGAs), and relative accuracy test audits (RATAs). Furthermore, data must be accurate, representative, reliable, complete, and precise, as defined by applicable regulations. A QA/QC manual should be developed by either internal staff or by external resources such as an environmental consultant or CEMS vendor. To develop an effective QA/QC manual, the author(s) should first consult and request the input of every person who will have interaction with the CEMS. Once everyone’s input has been assembled, the author should review the entire system (analyzers, pump, conditioning system, temperature controllers, peristaltic pump, heated sample line, probe, probe filter, etc.), photograph the equipment, and document normal operating parameters and reporting procedures. The QA/QC Manual should be reviewed frequently to document any procedure changes, equipment changes, personnel changes, and improvements. The manual should include written procedural tasks and responsible parties as well as training requirements and records. Annual or bi-annual auditing is needed to identify deficiencies in compliance or provide assurance of continuing compliance. Audits should be performed by an outside resource that is experienced with CEMS program audits. The audit should include a review of the DAHS emissions calculations and data replacement procedures. The audit should also include an evaluation of the entire CEMS, including equipment inspection.

Maintenance Programming

Elements of a maintenance programs can be categorized as either preventative (with systematic scheduling to avoid breakdowns), predictive (where breakdowns are anticipated and maintenance is performed prior to the expected failure), or reactive (where equipment is fixed after it fails). An effective CEMS maintenance program must include all three elements to ensure that the CEMS provides accurate and reliable data for compliance with applicable air quality regulations. The preventive maintenance (PM) program should be organized, scheduled, and efficient. Tasks are scheduled by frequency (daily, weekly, etc.) and should include logging of the status of tasks (completed/ not completed) as well as the responsible person. 

EQ Summer 2012_CEMS Activity MatrixIn the case of daily calibrations, the operator should ensure a quick response to failed calibrations, monitor and record calibration trends, and for facilities with multiple CEMS, stagger the scheduled daily calibration times. Other maintenance tasks should be scheduled to avoid conflict with auto-calibrations, maximize data collection, and minimize downtime. Prior to initiating any preventative or maintenance activity, it is important to place the system into maintenance mode. This will mark the associated monitoring data with a maintenance flag, thus preventing the data from being used in hourly averages. Likewise, the system must be taken out of maintenance mode immediately following any maintenance activity.

Daily Maintenance Activities
As part of the preventative maintenance activities the CEMS operator must visit each CEMS shelter daily and record in the log book the following parameters:

  • Outlet pump pressure
  • Sample system vacuum
  • Sample flow to analyzers
  • Temperature
  • Presence of alarms or faults
  • Cooler temperature
  • Peristaltic pump status (on or off) 

Frequently, the facility’s air permit requires log books to be kept with each CEMS. The log book provides a performance history of the system including information on previous issues and how those issues were resolved. The log book also provides a record of trends in flow, vacuum, pressure, and temperature. Another way some facilities maintain the performance history of CEMS is the use of detailed maintenance forms which can be enabled on mobile devices and stored electronically.

Weekly Maintenance Activities
In addition to daily activities, the CEMS operator is responsible for weekly activities including recording analyzer test function readings, such as slope and offset, recording calibration gas bottle pressures, replacing calibration gas bottles if the pressure is below an acceptable level (e.g., 150-200 psi), and monitoring for trends in daily calibrations, adjusting zero and span if needed. With respect to the analyzer test functions, a downward trending slope could indicate a problem with the calibration response, such as a dirty lens or UV filter. This can be easily addressed, the system recalibrated, and availability preserved.

Less Frequent Maintenance Activities
Monthly maintenance includes all of the daily and weekly tasks as well as preparing for any upcoming audits such as RATAs, CGAs, and linearity tests. It is also a good interval to check the status of air conditioning filters and plant air filters and to make sure the air conditioning unit is in good working order, especially in warmer climates or during hotter times of the year.

Quarterly requirements for CEMS include the completion of linearity tests, CGAs, and NO2 converter tests. The results of these tests can be used to improve accuracy and reliability, thus improving CEMS data availability. Annually, many CEMS consumables have reached the end of their “service life” and must be replaced, as dictated by manufacturers’ recommendations. Annual maintenance activities include probe maintenance (replacing o-rings and bushings) and replacing peristaltic pump tubing as well as the pump diaphragm and gaskets. CEMs maintenance programs should be reviewed annually to ensure that they comprise the appropriate tasks. An audit of the program can assist in improving effectiveness and efficiency, as well as, highlighting areas of positive achievement. Ultimately, the effectiveness of the maintenance program can be measured by CEMS data availability.

DAHS and Quarterly Reporting

CEMS Landfill Gas

The data acquisition and handling system (DAHS) is the brains of the operation. The DAHS receives emissions data from the programmable logic controller (PLC) or data logger and arranges it according to programs demands. It also performs data substitution, calculates emissions, and creates quarterly reports. The DAHS portion of the CEMS is where most non-compliance issues are discovered. Data acquisition and handling is not a “do it yourself” task; rather, the facility should purchase a system with proven reliability. Annual DAHS audits are critical for ensuring reported emissions are accurate, assessing data availability calculations, and affirming data substitution procedures. Recently, most of EPA’s CEMS enforcement activities have focused on DAHS review and problems associated with calculations.

Under 40 CFR Part 60, a quarterly Data Assessment Report (DAR) is required which characterizes the data accuracy and corrective actions over the previous three months. Quarterly emissions monitoring reports are required under 40 CFR Part 75 Subpart G. Compliance audits are required under both 40 CFR Part 60 and Part 75 with specific frequency and criteria for RATAs, CGAs, and linearity tests.

Essential Training

A final element necessary for proper operation of CEMS is training. Both environmental managers and facility operators should understand the applicable underlying air quality regulations that require CEMS including how to effectively and appropriately operate CEMS units. Nearly all analyzer manufacturers offer training for their equipment. Although this training can be somewhat costly, it should be considered for any technician who will be tasked with maintaining a CEMS. Analyzer manufacturer training provides insight and knowledge and will assist in increasing CEMS data availability. If a facility is installing new CEMS, it is important to ensure the CEMS manufacturer provides training on proper operation of the CEMS to all personnel who will have a role in the CEMS operation, maintenance and reporting. Furthermore, facilities should consider sending any new technicians with CEMS responsibility to appropriate training.


Worldwide, regulations are increasingly requiring the use of CEMS to demonstrate compliance with emission limits. Creating and maintaining a successful CEMS program can be a daunting task for any environmental professional. Selecting the most appropriate type of CEMS, from among the various technologies that are available, is essential to demonstrating compliance with applicable regulations. Creating a structured and well defined CEMS QA/QC program is equally as important for complying with applicable regulations. A well organized and executed preventative maintenance program will assist in maximizing the performance and reliability of the CEMS and increase data availability. Finally, having a well trained and knowledgeable CEMS team will contribute to the accuracy and reliability of the CEMS data.

CEMS Petrochem Plant