Predictive vs. Continuous Emissions Monitoring Systems

Predictive Emissions Monitoring Systems


  • Lower system capital cost.
  • A PEMS can be configured, delivered, and installed in 30-45 days.
    Installing a predictive emissions monitoring system requires the installation of one computer with the DAS/PEMS software installed & interfaced to the boiler control system. Typically, a one-day installation and start up process.
  • PEMS have lower startup costs, normally one day including DAS.
  • PEMS require less on-site training.
    PEMS training is usually 1 to 1 1⁄2 days plus DAS training.
  • PEMS should require no on-site emergency service.
    A direct modem to the system takes care of 99% of all service requirements.
  • PEMS do not require dual range analyzers.
    CMC PEMS measure the full dynamic range of emissions from a source, including start up to shut down.
  • PEMS do not require EPA protocol calibration gases, piping, wiring, gas regulators or software to operate the auto-calibration sequence.
    CMC PEMS analyzers do have auto calibration just like a CEMS.
  • PEMS require no air source.
  • PEMS have no ongoing preventative maintenance requirements as do CEMS.
    The only related maintenance for a PEMS is a periodic cleaning of the PEMS computer. Normal maintenance and calibration of process instruments that provide data to the PEMS is required. This maintenance should already be ongoing within a plant to maintain maximum efficiency and output; therefore, it is not considered an additional associated cost.
  • PEMS have higher data availability which should normally approach 100%.
    The PEMS does not rely on any one process input to maintain system uptime or accuracy of emissions quality. The PEMS uses numerous inputs for a robust data base. Even with a PEMS failure the data from the DCS inputs for the predictions still reside within the DCS. This data can be retrieved, calculated, and back filled by the PEMS when brought back on line. Therefore, very little if any down time or missing data should ever be reported.
  • PEMS can display emission process and combustion efficiency outputs.
  • PEMS can be used to determine the source of excess emissions.
    Combustion input parameter(s) that are out of normal range can be identified and alarmed for immediate action and provide a trail to the diagnosis of excess emissions.
  • PEMS require no field hardware or additional equipment beyond the plant or process control system.
    In fact, the PEMS in some cases can run on a control system computer and completely eliminate the need for additional hardware.
  • PEMS maintenance and service is minimal.
    A PEMS annual service contracts after warranty period typically runs $300.00 per month, which includes maintenance, regulatory and software upgrades. PEMS maintenance and quarterly audits with on-site support for report generation run $750.00 to $1000.00 per month including emergency support with the annual service contract.
  • A CMC PEMS provides accuracy that is equal to that of a CEMS.
    PEMS inherently do not drift. PEMS rely on process inputs and instruments that typically drift no more than 0.5 to 1% per year. Typical models utilize 12 or more input parameters that are in some cases redundant. The resulting emissions prediction is resilient to input failure and drift, such that no single input parameter is critical to the accuracy of the stated emission.
  • The PEMS analyzer is not vulnerable to obsolescence. CMC PEMS are continually upgraded for any new regulatory changes or software upgrades through inexpensive ongoing service contracts.
    Typically $300.00 per month.
  • PEMS require no on site spare parts or consumable spare parts.

Continuous Emissions Monitoring Systems


  • PEMS typically cost much less than CEMS. The purchase price of a PEMS is even lower if a CEMS is on site to provide QA’d emissions data.
  • Depending on complexity and location, CEMS are generally delivered in 12-14 weeks.
    Installation can take 7 to 14 days using various skilled trades (electrical, mechanical, computer) to install ports, probes, umbilical/trays, etc. CEMS installation may cost more than the CEMS equipment.
  • CEMS normal start up is 2-4 days, depending on complexity.
  • CEMS vendors typically offer 3-day introductory system training.
    (Probe, sample conditioner, analyzers, controller.) In-depth training is an additional 5 days. DAS training can be another 2 days.
  • On site emergency service for a CEMS is inevitable.
    Rates run $1200.00 to $1600.00 per day plus portal to portal expenses.
  • CEMS may be required to have dual ranges to cover the entire range of operation,
    including additional calibration gases, hardware and software.
  • CEMS require calibration systems and the ongoing purchase of EPA Protocol Gases.
    Cylinders of calibration gases also require safe storage and supplier documentation is necessary. No more demurrage expenses with a PEMS.
  • CEMS require air sources generally of instrument grade.
    The air is required for probe blow back and or probe dilution air. CEMS can require expansion and additional maintenance to the plant air supply in many cases.
  • CEMS require daily, monthly, quarterly, semi and annual maintenance.
    Consumable and spare parts inventories are required to maintain uptime, and generally are equal to 7-10% of the system’s cost. Specifically, trained personnel are required to perform maintenance on CEMS, including on-call duties.
  • CEMS typically are considered doing well if they maintain 95% uptime which is a minimum requirement.
    If an analyzer fails (NOx, CO or O2 etc.) or a critical component fails the system is consider down and down time is logged. When a CEMS analyzer or critical component fails the system has no capability to retrieve any missing data and data substitution may be required.
  • CEMS do not.
    CEMS provide outputs of stack emissions and do not typically provide process data or source efficiency beyond unit load, fuel feed, or fuel consumption data.
  • CEMS do not provide any insight as to the cause of an excess emission or the ability to facilitate process control or correction of the problem.>
    CEMS cannot point the operator to the cause or solution of excess emissions.
  • CEMS can, and do, require external equipment.
    Cabinets with HVAC, environmentally controlled shelters or clean rooms must be provided which can take up a considerable footprint and eliminate valuable space for other process equipment.
  • CEMS annual service contracts typically cost $1000.00 to $2500.00 per month plus portal to portal expenses. (depending on complexity)
    They normally do not include emergency service, daily, weekly or monthly maintenance, such as changing gas bottles, unless specifically requested at an additional cost. Contracts that include a quarterly site visit, quarterly cylinder gas audits, and emergency support for CEMS are also priced at an additional cost.
  • CEMS analyzers are designed to drift no more than 1-2% daily. Long-term CEMS drift is generally experienced through contamination of sample transport and the sampling train.
    Total system drift is associated with the sampling probe, transport / conditioning system, sample line, temperature control, ambient conditions and specified analyzer drift. Each of these components contribute to a total system drift or inaccuracy.
  • CEMS are always vulnerable to analyzer obsolescence and replacement. The replacement of 2 analyzers can be equal to the purchase of a PEMS system.
    Two of the main suppliers of analyzers have both replaced existing analyzers with new models and cancelled support over the last ten years.
  • CEMS require spares and consumable spare parts to maintain performance and up time requirements and are a sizable ongoing expense.


CMC Solutions certified 12 PEMS in the months of March 2006 through 2007. The Relative Accuracy for all PEMS was below 5.0%. This includes the PEMS on gas turbines, natural gas, and BFG fired boilers, natural gas and fuel oil fired boilers, and an ethanol production source. Many CEMS typically find it difficult to achieve a low RA on a regular basis especially following several years of continuous operation and minimal maintenance.

Regulatory acceptance is not an issue in today’s environment. CMC assists in all regulatory communication and documentation required for site installation. The EPA demonstration ending in early 2003, was conducted and endorsed by the EPA so that sources using CMES or installing CMES have a much lower cost alternative that satisfies all the monitoring requirements / regulations. With the advent of the PEMS Performance Specification 16 (PS-16, March 2009) PEMS now have an objective criteria for certifications across teh United States.

CMC Solutions guarantees PEMS certification for State, Local, and Federal EPA Part 60/75 regulations. The PEMS purchase has a return on the investment (ROI) of normally less that one year. References provided.