Continuous Emission Monitoring Systems (CEMS) generally refers to a packaged system of gas analyzers, gas sampling system, temperature, flow and opacity monitors that are integrated with a data acquisition system to demonstrate environmental regulatory compliance of various industrial sources of air pollutants.
Technical requirements and approved analytical techniques for continuous emission monitoring systems are found in 40 CFR (Protection of the Environment) part 60 and 40 CFR part 75. State implementation plans are based on the USEPA’s regulatory authority in 40 CFR, though some technical variation exists from state to state. Here are some details on CEMS: https://www.epa.gov/emc/emc-continuous-emission-monitoring-systems
The most widely used type of continuous emission monitor is an extractive CEMS in which a sample of gas is continuously drawn from the process point, filtered, transported, conditioned and presented to a gas analysis system. Gas concentrations are measured, recorded and stored as data. The data is used to generate reports, alarms or control some aspect of the client’s process. Extractive systems offer the advantages of choosing the most appropriate analysis technique for the desired components and concentration ranges and a properly designed system provides the flexibility of easily upgrading, expanding or contracting the scope of emissions analysis.
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CEMS are useful tools in gathering process emissions data for environmental compliance demonstration and process control and optimization. Other types of CEMS include in-situ and electrochemical cell type monitoring and may be appropriate in particular applications.
A CEMS consists of the system hardware, data acquisition and system integration.
An Extractive CEMS hardware generally consists of the following major sub-systems:
Sample transport and conditioning
Sample gas analysis
Data acquisition, reporting and system control
Sample Transport and Conditioning
To accurately monitor a source, a gas sample must be presented to the gas analyzers in a timely fashion. The gas analyzers however, must be presented with clean gas in order to function reliably. In addition, gas analyzers are intolerant of contamination by condensed liquids in the source gas. Condensable water vapor and particulate can plug passages and flow components, obscure optical sensors and lenses and cause failure of pumps, valves and flow-meters.
Prior to presentation the gas analyzers, the sample gas is often filtered and conditioned to remove particulate matter and moisture. This process however, cannot alter the composition of the gas species of interest in the conditioned gas sample.
Various techniques may be employed to filter and condition sample gas depending on the levels of particulate and/or moisture present, required system response time, and solubility of the gas sample component species of interest.
Primary filtration and dilution using a dilution probe with clean, dry air may be appropriate when it is required to measure on a wet-basis and when the use of unheated or ambient-level gas analyzers is desirable. If properly designed and maintained, such systems are sufficiently accurate for measuring high levels of pollutants and may allow the use of un-heated sample transport tubing. Such systems are commonly used in situations with long sample lines and soluble constituents as in fossil fuel-powered utilities.
A heated sample transport system (heated filtered probe, heated sample lines, heated pumping and distribution, as appropriate) and the use of heated gas analyzers allow wet-basis gas analysis without dilution and is the most accurate means of analyzing a wet-basis gas sample. Such systems can be used to measure very wide variances in concentrations total hydrocarbon, oxides of nitrogen, oxygen and moisture. For the measurement of very low levels of nitrogen oxides (without loss of NO2, this is the only appropriate technique currently available.
Dry-basis systems are appropriate for the measurement of a wide variety of constituents and the use of un-heated gas analyzers. This type of system allows the greatest flexibility in analytical technology but requires the most thoughtful design. Most continuous emission monitoring systems incorporate elements of dry-extractive systems in their design.
Dry-basis CEMS generally consist of primary filtration and a heated sample system from the sampling point down to the gas conditioner. The heated sample system minimizes the exposure of soluble sample constituents to liquid condensate and their subsequent potential removal from the sample gas. In the sample conditioner, condensable vapor is quickly removed and separated from the gas sample.
To remove the water content from the gas sample, the gas may be cooled by compressor or Peltier (thermo-electric) refrigeration. Alternatively, water vapor may be removed using a nafion dryer system where appropriate.
Gas Sample Analysis
To measure the concentration of sample gas constituents, gas analyzers measure some physical characteristic of the species of interest. Ideally, the characteristic is unique to the species. Such techniques commonly include infrared and ultraviolet adsorption, photo or flame ionization, catalytic or chemiluminescence photon emission from chemical reaction or excitation.
In cases where gas sample constituents create cross-interference, techniques are used to reduce or measure and cancel these interferences. Selection of appropriate technology for gas conditioning and analysis is important in the production of reliable and accurate emissions data.
Data Acquisition, Reporting and Control
The data acquisition system acts as a system controller as well as a means to collect and record data. It should be capable of initiating daily calibrations, monitor important system parameters, generate alarms and reports and communicate over the client’s network. A means of accounting for maintenance, validating data and future system upgrade should be included in the package.
Often a DAS includes some level of redundancy. All or part of the DAS may be redundant, depending on the application requirements. An uninterruptable power supply or battery-powered data logger is almost a requirement. Often local storage is supplemented with a multi-pen recorder or regular back up over the client’s network.
System Integration: Putting it Together
Integration of a CEMS is the packaging, arrangement and connection of all of the various components so that they operate as one coherent and highly reliable system. The selected components and technologies should compliment each other and be appropriate for the application.
Successful integration of a CEMS starts with good basic design practice. The designer(s) must seriously consider not only source characteristics and how the equipment will be used, but also the way the client (including outside testing firms and regulatory officials) will be working with it. Reliable long-term function and complete data capture are the result of careful planning, taking into account source constituents, materials compatibility, system siting, minimal maintenance, redundancy and ease of use.
In particular, source characteristics and constituents in the sample gas play a very important role in determining the best analytical technologies as well as selection of sample transport and conditioning components. A “pre-engineered CEMS” is more a product of a vendor’s marketing priorities than consideration of the client’s monitoring requirements. While allowing a vendor to streamline inventory, documentation and the production of sales literature, the resulting monitoring system inevitably incorporates less than optimal design features. A surprising number of large, well-known companies that manufacture analytical products, fail to provide well-designed custom CEMS. Mistakes with continuous emission monitoring systems design or vendor selection are often costly and contentious to resolve.
Often overlooked by CEMS vendors, is the importance of periodic RATA testing. While the selection of a reputable testing firm is most important, the CEMS must use appropriate gas sampling rates and analytical technique(s) to produce source data that tracks well with a reference CEMS. In addition, the data acquisition system must be flexible and accessible to the testing firm; capable of archiving and exporting appropriate data sets for periodic testing. A DAS that produces graphical and scalable trending data can assist tremendously in the set-up and monitoring of periodic testing progress. Often, problems that would have resulted in re-testing (at considerable expense) have been avoided by a cursory visual comparison of the two systems’ data trends.
System redundancy and maintainability are very important in any CEMS. They are absolutely essential during periodic testing. The basic CEMS design must be as simple and robust as possible. Sampling system parameters that are important to measure performance and condition of probes, filters, sample lines and pumps should be accessible, but not overwhelm the operator with irrelevant data.
Corrective action for probe-blinding or component failure should take minutes, at the maximum, to correct. Probes should be self-cleaning and sub-assemblies within the equipment enclosure should be either redundant or easily removed and replaced.
Finally, when selecting a vendor to supply a CEMS, get references and take a look at the work they have done previously. Be sure that the displays and controls are placed optimally for operator convenience. Check that the equipment, wiring, tubing, valves and services within the enclosures are intelligently and professionally arranged. If you are favorably impressed, check to make sure that the designer whose work you admired is still on staff. Ask questions about maintenance, technical support, consumables usage and anything you can think of relating to how the equipment will be used in your application. Keep in mind that this equipment will be used for at least several years hence so operational quirks should be excluded from its design.