Continuous Emission Monitoring. James A. Jahnke
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Название: Continuous Emission Monitoring
Автор: James A. Jahnke
Издательство: John Wiley & Sons Limited
Жанр: Биология
isbn: 9781119434023
isbn:
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 the probe body (Figure 3‐19). This probe, originally developed in the Netherlands (Bergshoeff and van Ijssel 1978), is unique in its design and construction.
This probe has also been known as the “EPM” probe named after the company that marketed it in the 1980s and 1990s.
Inside the probe, an ejector pump operates at flow rates of 1–10 l/min. A glass critical orifice (consisting of a glass tube drawn to a point, as shown in the figure) is chosen to limit the flow of sample gas to flow rates from 20 to 500 ml/min. The condition for obtaining a critical flow for the glass orifice is that the ratio of the absolute pressure at the venturi throat to the stack static pressure must be less than or equal to 0.53 (Brouwers and Verdoorn 1990). The dilution ratio, D, is calculated as follows:
(3‐2)
where
Q1 = dilution air flow rate (l/min)
Q2 = sample gas flow rate (l/min)
Data obtained from the gas analyzers measuring the diluted flue gas are converted into the source‐level concentrations by multiplying the analyzer response and the dilution ratio determined or adjusted at the time of initial calibration.
Dilution ratios of 50‐to‐1 to 300‐to‐1 are typical. The higher ratios are used for hot, saturated gas streams. In coupling a dilution system to an analyzer, attention must be paid to the measurement range of the analyzer. If the lowest instrument range should be 0–5 ppm, and it is required to measure a pollutant stack gas at a nominal concentration of 50 ppm of the pollutant, a 100‐to‐1 dilution ratio would provide a sample to the analyzer of 0.5 ppm. This would be at the low end of the range where the analyzer sensitivity is lowest. If the instrument noise or drift is high at this part of the scale, it could be difficult for the system to pass a relative accuracy test.
Figure 3‐19 The in‐stack EPM dilution probe.
Although the EPM dilution probe has been successfully applied, it is not the solution to all extractive sampling problems. In the cases of wet, caking, or sticky particulate matter, the probe can still become plugged even though it is pulling at a low flow rate. Dilution probes of this type have experienced difficulties when installed after wet scrubbers, where water droplets are entrained by the flue gas. If the droplets enter the probe, or water condenses in the probe from a highly saturated gas stream, the glass wool filter can become wet and the orifice can become plugged. Under normal conditions, when the probe is heated, water droplets should be vaporized and plugging should not be a problem. To СКАЧАТЬ