Continuous Emission Monitoring. James A. Jahnke
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for monitoring CO.
The example shows a simple sample probe (stinger) attached to a heated, coarse filter external to the stack. Instrument air is provided for particulate blowback from the filter. Two sets of impingers, cooled with a Peltier cooler, condense moisture from the flue gas sample. The condensed water vapor is removed from the system using peristaltic pumps. A water break‐through detector is incorporated into the line after the first set of impingers and before the sampling pump. The sampling pump transports the dry sample gas to the analyzer after passing through a fine filter. The sample system control panel monitors vacuum, pressures, and flow rates at various points in the system. The control panel may provide for either manual or automatic control of the gas distribution although today most systems are automatically controlled. The system illustrated shows only one gas analyzer, but can be expanded to monitor multiple gases.
Although assembling such components to construct a CEM system may appear straightforward, it is important that the system design be appropriate for the application. If emissions need to be reported on a mass rate basis (kg/hr), if the pollutant is partially soluble in water and the emission limit is 10 ppm instead of 200 ppm, a hot/wet or dilution extractive system might be more appropriate. Other factors such as flue gas temperature, moisture content, and particulate concentration must also be considered in the system design. High temperatures and/or high levels of particulate matter or sticky particulate matter may limit the choice of probes and filters. Similarly, high moisture levels or the presence of acid gases may limit the choice of sample coolers. For challenging applications, experience in system design is important. It is this experience that distinguishes CEM system integrators from each other. It also may make a difference in the success of the system in passing its initial certification test and its success in operating continually without excessive maintenance demands.
Not shown in the diagram are the electrical and communication systems necessary for system control and data utilization. These aspects of the CEM system are the most difficult to accommodate into a CEM system installation because they must be integrated into existing plant networks. Most CEM systems do not stand alone, but are often used for control and optimization of the monitored operating unit. Alarms, emission values, calibration results, and real‐time as well as summary data are typically routed to the plant distributed control system, or separately to the environmental manager or corporate office. All of this takes coordination and collaboration with plant personnel and contractors to assure that CEM system electrical connections and communications are properly integrated into the existing systems of the plant. Compared to these challenges, assembly of monitoring system hardware is the simplest part of the installation program.
CLOSE‐COUPLED SYSTEMS
Many of the problems associated with source‐level extractive systems can be resolved by coupling the extractive and analytical system directly to the stack or duct. This technique minimizes problems associated with reactive, condensable, or adsorbing gases by essentially eliminating the sampling line. The method is especially appropriate for the extraction and measurement of volatile organic compounds. Close‐coupled systems have also been designed for monitoring criteria pollutants (Mandel and Gottlieb 1995), the most recent incorporating modular gas sensors with internal microprocessor control to regulate temperature, flows, and calibration sequencing (Jahnke 1997). A typical close‐coupled system is illustrated in Figure 3‐16.
Close‐coupled systems allow the use of analytical techniques that may need a more controlled sampling volume than is afforded in in‐situ measurements. By measuring the sample directly outside of the stack, sampling conditions can be controlled, while minimizing reactivity and sample line problems. Close‐coupled systems can be designed as either cool/dry or hot/wet systems.
Figure 3‐16 A close‐coupled extractive systems.
Costs of such systems are lower due to the elimination of the sampling line and the use of temperature‐controlled cabinets, which removes the need for a CEM shelter. However, challenges of the ambient environment, such as lightning, rain, ice, or extremes in temperature, may compromise its operation unless it is designed to accommodate such varying conditions.
DILUTION EXTRACTIVE SYSTEMS
The main problem associated with source‐level extractive systems is the need to filter and condition relatively large volumes of stack gas. This problem can be largely avoided by using dilution systems where gas is drawn into the probe at low flow rates, sometimes two orders of magnitude less than in a source‐level system (e.g. 0.05 vs 5 l/min.). This means that there will be less particulate matter to filter and less moisture to remove. Because the flow is relatively low, particles are more likely to follow the flue gas streamlines around the probe than to enter the probe.
Dilution systems are used in conjunction with ambient air level analyzers – a feature that can provide significant advantages to a source that has had previous experience with ambient air level analyzers or that is operating an ambient air network. In the case of analyzer problems, CEM system analyzers could be swapped for ambient air level analyzers or spares maintained for both purposes. Plant technicians already may be familiar with the operation and maintenance needs of the analyzers and may not require additional training.
There are two approaches to designing dilution systems. One approach is to dilute the stack gas in an in‐situ, “in‐stack” sample probe (Figure 3‐17). Here, the eductors and dilution orifice are contained in the probe itself. Another approach is to dilute the stack gas outside of the stack (Figure 3‐18) in a module containing the eductor and the dilution capillary or orifice. As with fully extractive‐system conditioning systems, using a dilution module outside of the stack, dilution can be performed at either the stack or the CEM shelter. If diluted at the stack, unheated sample lines can be used to transport the gas to the analyzers, but if diluted at the shelter, heat‐traced lines must be used.
Figure 3‐17 An in‐situ (in‐stack) dilution probe CEM system.
Figure 3‐18 An external dilution CEM system.
The dilution systems have seen widespread application in monitoring emissions from electrical utilities affected by EPA's acid rain program. In this program (U.S. EPA 2020a), emissions are reported in units of mass/time (i.e. lbs/hr or tons/year), calculated by multiplying a wet basis flow rate measurement and a wet basis pollutant gas measurement. Since dilution systems measure gases on a wet basis, the calculation is straightforward, not requiring a determination of flue gas moisture content. This feature, as well as the advantages inherent in sampling at low flow rates, has made it popular.
Dilution Probes
A dilution probe dilutes the stack gas in the probe to such a degree that the dew point of the diluted gas will be less than the lowest ambient temperature at the sampling location. This enables the CEM system to avoid the use of heat‐traced line and simplifies the gas transport system.
One of the first and most successful dilution systems uses a critical orifice coupled with an ejector pump designed into