Emissions monitoring from combustion processes requires careful design to provide all the required data continuously and economically. Here’s how to approach your application.
CEMS design insights
- Environmental regulations around industrial combustion processes demand continuous emissions monitoring systems (CEMS).
- How gas samples are extracted, conditioned and delivered to analyzers can significantly impact measurement accuracy.
- Designing a CEMS can be complex, involving multiple technologies and compliance demands (e.g., EPA Part 60/75).
Anywhere there is an industrial combustion process beyond a basic size threshold, there will be some specific type of environmental regulations imposed by relevant agencies. In the U.S. it is the Environmental Protection Agency (EPA), as well as various state agencies in some cases; in the EU it is Industrial Emissions Directive 2010/75/EU of the European Parliament; and virtually every country has something similar, or it relies on U.S. or EU regulations. Such regulations invariably become complex because the range of possible applications is so broad. A basic steam boiler fired by natural gas is at the simpler end of the scale, while a municipal solid waste incinerator or a cement plant utilizing varying fuels is far more complicated due to the potential for different pollutants.
The common element is that applications must be monitored continuously whenever operating, and a continuous emissions monitoring system (CEMS) must quantify whatever individual pollutants and emission rates the regulatory agency specifies. Each agency generates a list of pollutants connected with these fuels, which may include:
- Nitrogen oxides (NOX)
- Sulfur dioxide (SO2)
- Hydrogen sulfide (H2S)
- Carbon monoxide (CO)
- Carbon dioxide (CO2)
- Hydrogen chloride (HCl)
- Mercury and other heavy metals
- Dioxins and furans
- Unburned hydrocarbons
- Other acid gases
- Ammonia (NH3)
- Particulates
Most monitoring points are installed after all process control stages and just before flue gas is released to atmosphere. For example, if a process has a selective catalytic reduction system to control NOX, the monitoring point will be downstream from this system.
The two main questions to answer at this point are:
- What analyzer technology, or multiple technologies, are necessary to detect and measure the full range of pollutants and report emission rates?
- How are flue gas, or gas samples, delivered to the analyzer in representative fashion?
Analyzer technology selection
Once the list of specific pollutants to be measured is finalized, it is possible to select which technologies may be necessary to cover the required measurements. Most situations are unique, but there is a large degree of commonality within various industries. Traditional analyzer selections and the analytes they can measure include:
- Non-dispersive infrared spectroscopy (CO, CO2, CH4, SO2, NO, NH3)
- Non-dispersive ultraviolet spectroscopy (NO2, SO2)
- Ultraviolet and fluorescence technology for SO2
- Gas chromatography (total hydrocarbons, H2S, sulfur species)
- Paramagnetic technology (oxygen)
- Chemiluminescent technology (NOx)
When a combination of technologies is necessary to cover multiple pollutants, operators often encounter headaches as each may have different consumables, range limitations, and calibration requirements. Fortunately, traditional technologies improve, and new ones emerge built upon better data processing and simplified operation.
For example, quantum cascade laser (QCL) and tunable diode laser (TDL) analyzers (Figure 1) have grown in popularity thanks to their ease of operation and ability to measure a wide variety of analytes. Generally, QCL covers the mid-infrared spectra, while TDL covers near-infrared spectra. These two technologies, working together in a single unit, can replace all the technologies just mentioned in many applications, and provide measurements of all the pollutants common to CEMS applications.
Figure 1: A hybrid QCL/TDL analyzer, such as Emerson’s Rosemount™ CT5100 Continuous Gas Analyzer, can be configured with up to six laser modules, some of which can measure two analytes. Each laser type covers its respective pollutants.
Analyzer placement
Once an analyzer technology is selected, the challenge is delivering the source flue gas to the sensor mechanism for measurement (Figure 2). Some analyzer technologies are relatively simple, and a point sensing probe can be inserted in-situ directly into the stack, or a beam source and receiver can be mounted on opposite sides, with the beam passing through the flue gas. But this is only practical for a few specific analyzer technologies, such as oxygen levels and particulates, but not usually on a compliance basis.
Figure 2: A CEMS may use more than one sampling system. For example, a basic oxygen sensor may be installed in-situ, while other pollutants may require an extractive approach using a sampling system.
The more common method is an extractive sampling process where a flue gas sample is drawn from the stack and sent via a sample handling system to the analyzer. Designing a sample handling system is complex due to the chemical complexity of flue gas. Depending on the combustion process involved, it comprises primarily nitrogen, residual oxygen, various pollutants, and varying levels of water vapor. Since the gas is hot, typically over 100°C, (212°F) its ability to carry water vapor is greater than air at typical ambient temperatures. Therefore, water condenses as it reaches the atmosphere and cools.
Complexity of sample handling
The capabilities of a given extractive sample handling system must match the requirements of whatever analyzer technologies it serves and the pollutants it must handle. All analyzers have some limitations due to temperature, moisture, pollutant profiles, and other factors. Similarly, some pollutants can change during handling, affecting the measurement. For example, if a sample is allowed to cool, water vapor will condense. If liquid water is present, any sulfur dioxide in a sample will tend to dissolve into the water, creating a corrosive acidic liquid and reducing the gaseous component. If this is an analyte of interest, the reading will be inaccurate.
Consequently, sample handling systems (Figure 3) must determine how they will handle temperature and therefore water vapor.
- Cold/dry systems either actively chill the gas to condense moisture, or they allow it to drop to ambient temperature so moisture falls out and can be separated by a coalescing filter. As mentioned, this can cause some pollutants to condense as well, so it is best suited to a minimally complex process, such as a boiler fired by natural gas.
- Dilution systems inject clean, instrument air into the gas stream through an orifice to reduce moisture and particulate matter, thereby minimizing condensation and additional sample handling requirements. However, dilution systems potentially lose accuracy and linearity in hydrocarbon applications.
- Hot/wet systems keep the sample gas at its original high temperature, 120 to 190 C (248 to 375 F). This avoids condensation without dilution, so it is easier to measure pollutant levels, and no corrosive liquids form. However, the analyzer must be able to handle high temperatures, so this may limit the options.
Figure 3: These diagrams from an EPA field audit manual illustrate the two most common extractive approaches. A cold/dry system (A) cools the sample gas, but this type of system is not suitable for all pollutants. A hot/wet system (B) eliminates the need for cooling but must maintain the sample gas temperature. (Courtesy of U.S. EPA)
Sample handling systems must also provide mechanisms for purging the lines through blowback, and injecting calibrating gases for routine sequences.
Avoiding crippling complexity
The process of designing and implementing a complete CEMS installation can be a major undertaking, and often a thankless task within a facility. It does not generate any product or income, neither core nor critical, but if it does not work well, it can create major maintenance headaches. Worse, if a CEMS does not work reliably and continuously, it can result in permit violations, fines from regulators, and even forced shutdowns until problems are solved. Any design approach must therefore consider the comprehensive operational picture.
First, analyzer technology selection. Naturally a CEMS must be able to handle the full list of pollutants required for the industry and combustion process, including any potential alternative fuels that could be used. For example, if a refinery wants the option of burning some process off-gas in a fired heater as well as pipeline natural gas, the analyzer must handle both options.
This is not as easy as it sounds because there will be options available, and more than one analyzer technology may be required to cover the full list. Most pollutants do not depend exclusively on one measurement technology, but this does not mean that all are equally practical. Reviewing a list of options must consider initial cost, but also the costs of consumables, the frequency and cost of maintenance and calibrations, and overall reliability.
Second, the sample handling system must be suited to the analyzer, and in some cases, it may drive the selection. For example, the analyzer technology suggested may call for a higher degree of cost and complexity for the sample handling system, such that it overwhelms any analyzer cost advantage. If a hot/wet system is more economical but the analyzer selected can’t handle it, which element should be changed?
The consequence of these points is that a CEMS must be optimized as a system with the cost and complexity of every element weighed in the larger picture. Making this work depends on having designers with access to the widest range of technologies, rather than having perhaps one analyzer option which forces the use of sub-optimal approaches.
Part 60 and Part 75 Data Acquisition System
The final component of a compliant CEMS is the data acquisition and handling system (DAHS). Emerson offers its own DAHS for all world areas, including those regulated by EPA CFR Part 60 and Part 75. The selected DAHS should provide the following functionality:
- Monitor multiple analyzers in real-time.
- Provide flexible communications with support for the Modbus TCP, Modbus RTU, OPC-UA, MQTT protocols, along with hardwired signals.
- Include a web server to provide secure remote access to the system from any device capable of hosting a web browser, such as a laptop PC, desktop PC, smartphone, or tablet.
- Initiate and schedule calibrations and blowback procedures.
- Provide full cylinder inventory management.
- Provide alert and event handling in real-time, including summary reports for audit purposes.
- Provide historical reports that show compliance with emissions regulations.
- Include a powerful trending engine to graphically show compliance over custom time periods.
- Include operator and supervisor logbooks.
One-stop shopping
The thought of having to create a CEMS with a rack of multiple analyzer technologies, combined with one and maybe even two sample handling systems and a reliable DAHS can be daunting, but this does not have to be the case. Emerson’s Engineered Products Group has designed hundreds of CEMS based on its Rosemount CT5100 Continuous Gas Analyzer (Figure 1) and Rosemount™ X-STREAM Enhanced XEGP Continuous Gas Analyzer (Figure 4).
Figure 4: This continuous gas analyzer can measure multiple components, including CO2.
The CT5100 uses hybrid QCL/TDL technology, with up to six laser modules in a single enclosure. It allows monitoring of up to eight analytes, including the most frequently specified as pollutants. This approach has exceptional stability with virtually no consumables, plus it works well with a hot/wet sample handling system, minimizing operational costs, but it is not the only choice. There are others, including the Rosemount CT4400 Continuous Gas Analyzer, which excels for NOX applications.
The XEGP measures up to five components simultaneously in various combinations. This analyzer is designed for applications where it is not necessary to measure analytes on a hot/wet basis. It incorporates a gas conditioner to remove all moisture, delivering a dry and ambient sample (cold/wet) to the analyzer. Its versatile design allows physical benches to be installed in their own compartment separate from the electronics, and it is available in a 19-inch rackmount or tabletop enclosure.
With this approach, a CEMS can be contained in the smallest possible enclosure or shelter with all the required support software and any hardware, including programable logic controllers, human machine interfaces, computers, dashboards, and interfaces to larger automation systems. The ability to get a complete system, fully integrated and optimized, from one source, can save significant amounts of design time, while ensuring reliable long-term performance.
All figures courtesy of Emerson except as noted.
Keith N. Linsley is the senior global product manager for CEMS and Process Analyzer Systems at Emerson, and he has nearly 30 years of expertise in the analytical systems integration industry. With a focus on business management, engineering design, fabrication, and the start-up of complete continuous emissions monitoring systems, Linsley has successfully implemented projects across the globe for the electric power generation, petrochemical, and other industries. He holds a bachelor of science degree in mechanical engineering from Purdue University.
