This guide will provide prospective owners and operators with an in-depth look at the process for acquiring and commissioning a new Continuous Emissions Monitoring System (CEMS) and explain some of the potential pitfalls that could exist in such a process. If you are an owner or operator of a regulated source and you operate based on a limit set forth in a Title V permit, chances are that at some point a CEMS will become a necessary part of your emissions monitoring program.
What is a Continuous Emissions Monitoring System and what are the benefits of a CEMS?
A Continuous Emissions Monitoring System or CEMS is a system comprised of multiple components that draw a sample from the monitored source, condition that sample to make it ready for analysis, analyze the conditioned sample via a gas analyzer, and then collects, processes, and stores the emissions data. These systems are purpose built for the applications in which they are to be used, considering factors such as stack temperature, particulate matter volumes, sample humidity, and permit required measurement parameters.
CEMS can operate alone for the purpose of measuring pollutant or diluent content of flue gases, or they can be paired with a flow rate monitor to become a CERMS or Continuous Emissions Rate Monitoring System. These systems can be applied effectively to various types of equipment. Most notably, CEMS are used to measure the flue gases from Power Boilers, Recovery Boilers, Lime Kilns, and other sources of monitored pollutants and diluents in industry.
CEMS can provide operators with the information in real time to adjust operating parameters to compensate for fluctuations in the flue gas composition. This allows facilities to be better stewards of the environment and of the air quality in their area. CEMS can often also be used to determine the relative efficiency of combustion systems or be an indicator that related systems may need to be evaluated for potential maintenance issues.
What to know before installing a CEMS
A CEMS is typically comprised of 5 main groups of components: the probe, the umbilical, the sample conditioning system, the analyzer, and the data acquisition system. Individually, these groups of components each have their own purpose relative to measuring the stack sample.
The probe is where the sample is first removed from the stack. The probe can serve to filter the sample, dry the sample, measure the sample for wet O2 content, take flue gas temperature readings, meter the flow of sample down to the conditioning panel, or any combination of these tasks. A properly designed probe is vital to the longevity of a CEMS and thus comes with critical considerations when designing a new system. The probe needs to be rated to handle the temperature extremes of the stack for which it is designed, it must be able to provide an adequate sample consistently and reliably to the system, it must have proper filters or cleaning options to keep it operating over long periods without assistance, and it must be impregnable to the pollutants/diluents that it handles. Some probes do not utilize filters, whereas others have various types of filters to remove larger particles very early in the process. Most probes have some form of “blow back” system that uses clean air to blow through the probe and into the stack to remove any contaminants that could be building up on the probe sampling inlet. Many probes are made of high-quality stainless steel that provides the necessary temperature stability and strength needed for the application. Some incorporate orifice tubes that serve to limit the sample from the probe. Instead of pulling down a much larger sample than is needed, these probes are designed to only provide a sufficient sample flow and this in turn reduces the volume of sample that must be filtered, conditioned, and analyzed by the system. There are numerous different probe designs available on the market currently and each is built based on experience and various design preferences of their manufacturers. The good news is that there is a probe for every application and there should be no shortage of options for each type. When trying to decide which probe to purchase for an upcoming project, it may be prudent to consult with someone who has experience with various probes in use for applications like the one in question. These individuals can provide very useful information as to what worked well and what was problematic with these components.
The umbilical is our next step on the path to sample analysis. The umbilical is much like what its name implies, it is generally a tube that connects the probe to the sample conditioning panel and is responsible for maintaining the sample temperature to avoid condensation of water vapor in the sample. It does this by having a heat trace near the sample line and the whole bundle is insulated and protected by various layers of insulating materials and abrasion resistant jackets. The heat trace is accompanied and controlled by thermocouples that are part of the feedback loop that regulates the temperature of the heat trace to maintain a constant temperature above the dewpoint of the sample. Additionally, the umbilical typically houses the tubing for the blow back function of the probe and the communications wiring, and it could also incorporate the sample lines for a flow monitor used for rate monitoring. The umbilical is made specifically for the intended application and the lead times can be in excess of a few months for this part of the system. It is important to add in additional spare sample lines, communications wires, and thermocouples to allow for quick fixes during maintenance issues involving the umbilical components. The method of supporting the umbilical between the probe and the CEMS shelter is vital to the success of the umbilical over time. A final consideration, and one that may not be obvious to most in this decision, is to make sure that the sample tubing is made of material that is impregnable to the gases that it will encounter. This is important because the sample gases can contaminate the sample tubing, and this could cause the readings to be incorrect due to a lack of stack gases arriving at the analyzer due to absorbed gases interacting with gases in the sample line or the sample line “scrubbing” the gas from the sample. There are plenty of reputable and professional sources for properly designed and well manufactured umbilical bundles for use in CEMS. The umbilical may have a more simplistic list of functions, but it is also the most exposed and therefore vulnerable component of the system and only a quality umbilical is going to stand the test of time, so look for the most robust and well supported umbilical available when deciding what to purchase for your project.
The umbilical delivers the flue gas sample to the sample conditioning system. This system is responsible for drying, filtering, pumping, and delivering the sample to the analyzer. Typically, the sample conditioning system utilizes a chiller to condense and remove the moisture from the sample to protect the analyzer from excess moisture. This chiller needs to be sized appropriately to accommodate the volume of sample as well as the volume of water that might be encountered. Some sample conditioning panels utilize an O2 cell to measure wet O2 values at the panel. This feature can be problematic because the wet O2 sample must be taken before the chiller can remove the moisture. This can lead to the O2 sensor becoming inundated with water or the sample flow through the wet O2 becoming highly variable which can affect the system flows downstream. Careful consideration and ample research should be conducted before utilizing the sample conditioning panel to collect data on wet O2. A critical function built into most sample conditioning systems is its role in the system’s automated “calibration check”. The sample panel will be equipped with solenoid valves that are controlled by a Programmable Logic Controller to allow the introduction of certified calibration gas into the analyzer each day as is required by the applicable regulation. This same system would be in use during maintenance calibrations and regulatory testing of the system. The sample conditioning panel is also usually responsible for pumping the sample and controlling sample flows. An analyzer must be provided with the proper volume of sample flow to accurately produce proper readings. If there is too much sample flow, the analyzer will typically read extremely high values and if there is too little flow, small or near zero values can be observed. It is critical that the sample flow be maintained at a very steady rate to provide the most accurate readings from the system. The sample conditioning panel must be built to the desired range of flow rates and must be user adjustable to allow for the system to function properly and be adaptable for changing conditions that may develop over a long service life. High quality pumps and filters as well as redundancy in components are great to include when designing the sample conditioning panel. When trying to procure a proper sample conditioning system, it can be difficult to compare the various designs and approaches that are available due to variability in components that can be used. A few considerations that can make this decision easier are the cost of filters and other components that must be replaced periodically, and the ease of maintaining and tuning the system as the owner/operator. Simplicity can often yield the most desirable results, so there is no need to overcomplicate this process by adding too many extra components or processes to the system.
The analyzer is the heart of the system and may require the most consideration when trying to determine the specifications for a new system. The rest of the system is typically built according to the demands of the analyzer and therefore changing the analyzer could completely change the function of the system. Many great analyzers are available that are more than suitable for most applications. Many also come with great support and proper warranties from the manufacturer. It is best to settle on an analyzer that has the most desirable range of measurement, and one that requires the least amount of maintenance. Another great consideration has to do with the interface between the analyzer and its user. Some analyzers incorporate a touch screen and graphical user interface that can be very intuitive whereas others may utilize physical buttons and may require the navigation of a list tree to operate. These features become even more relevant when it is considered that someone will need to periodically recalibrate or perform maintenance checks and functions on the analyzer, and an intuitive system or very simple process for interfacing can lessen the learning curve for all involved. Most analyzers are capable of multiple communication configurations allowing them to be very nearly “plug and play” with popular process control software already in use in industry. It is important that the analyzer be installed so that all necessary alarms can be communicated to the operator in real time so that these alarms can be addressed as soon as possible.
The final portion of the CEMS is the data acquisition and handling system or DAHS. This system is tasked with capturing and storing the data from the emissions values, alarms, and system parameters of the entire system. The DAHS will collect the data, calculate averages according to the applicable regulations, act as an interface for users with the system, determine data validity, store alarms, and control automatic functions of the system. The DAHS usually consists of a PLC and the associated program used to interface with the system. A robust PLC with appropriate redundancy features is a must for a proper DAHS and may be more expensive but is a very critical consideration for any new CEMS. It is possible to utilize a desktop or laptop computer to satisfy the requirements of a DAHS, although this is not recommended for multiple reasons. A computer typically does not incorporate an uninterruptable power supply to keep the DAHS operating when power outages occur. A proper DAHS should always be designed with this possibility in mind. Computers also must be updated periodically, and this can cause the loss of monitoring data if the computer restarts automatically. PLCs are also less likely to receive any unwanted attention whereas it is possible that someone may unknowingly use a DAHS PC for reasons outside of its intended duties and potentially cause the loss of data or improper operation of the system. PLCs also provide many more interface and control options to the system administrator than does a computer. If you must cut costs somewhere to fund a project, the DAHS may not be the best place to cut corners. Remember that it is most desirable to have a system that is free from obvious pitfalls and a robust PLC-based DAHS does a lot to avoid the shortcomings of a standard computer operating as a DAHS.
In conclusion, CEMS can be as simple or as complex as our perception of them. They are no more than a collection of many parts with various tasks that hopefully work well together to provide the most accurate monitoring data and the least amount of user input. Once the function of each part is understood, the whole system is quite simple. When looking into a new CEMS, experience is better than luck. Find a knowledgeable source and let them help you find the right solution for your application. Start the planning process well in advance of any deadlines and assume that manufacturing and shipping may take longer than expected. We should all strive to have the best monitoring equipment available so that we may continually protect our environment and maintain it for years to come.
Environmental 360 can help with CEMS installation, complying with new requirements, and maintenance. Contact us today for more information.
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Cover Photo: Dmytro_Mykhailov – stock.adobe.com