Continuous Emission Monitoring. James A. Jahnke
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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 avoid problems resulting from water droplets, adequate temperature control is required.
The dilution probe is sensitive to changes in stack pressures and temperatures (Jahnke and Marshall 1994; Myers 1986). In installations where the stack static pressure is highly negative (<−10 in. H2O), the venturi vacuum may not be sufficient to overcome the pull of the stack negative pressure.
It should also be noted that air is used for dilution. This prevents the use of an oxygen analyzer in the CEM system because the contribution of stack gas oxygen to the sample would be swamped by the background 21% oxygen level of the dilution air. Most commercial oxygen analyzers are not designed to measure differences in oxygen levels at diluted values of 0.1–1%. If it is required to correct pollutant emission data for stack dilution air, a CO2 analyzer is used in the system. When a regulation requires that pollutant concentrations be corrected to a specified oxygen concentration (such as the 6% O2 correction specified in 40 CFR 60 Subpart BB for Kraft pulp mills (U.S. EPA 2020c)), the critical orifice can be incorporated into an assembly, which first splits the flue gas into two gas streams – one that passes through a critical orifice and is diluted and one that remains undiluted. This cannot be done when using an in‐stack dilution probe, but can be done when using external dilution systems.
Dilution Air Cleanup Systems
The air used for dilution must be clean and free of any of the gases being measured or else significant errors can occur. For example, 1 ppm of NO in the dilution air will give a response of 100 ppm in a dilution system having a dilution ratio of 100 : 1. Activated charcoal, sorbents, heatless dryers, and other gas scrubbing techniques are commonly employed to provide clean, dry air to the dilution system.
Because the dilution air cleanup system is central to the proper operation of the dilution extractive system, it is usually made redundant, with two equivalent systems. If maintenance is required, the system being serviced can be isolated, while the other system continues to provide purified air. A schematic of a redundant dilution air cleanup system is shown in Figure 3‐20.
In Figure 3‐20, manual valves are used to isolate air cleanup system for filters, scrubbers, or dryer maintenance, while the other system remains in operation. As instrument air enters, a coalescing filter is used to remove oil, water, and dirt from plant instrument air, in combination with a regulator to regulate the gas pressure. The air passes to a second‐stage coalescing filter backing up the filter/regulator and then to a soda lime scrubber, containing a mixture of Ca(OH)2, NaOH, and KOH for the removal of CO2. NOx and SO2 are removed in a scrubber containing a material such as Purafil, a chemisorbent media composed of activated alumina impregnated with potassium permanganate. Water and CO2 are removed in a heatless regenerative dryer, and SO2 and other gases are removed in an activated carbon scrubber. The regenerative dryer is filled with chemisorption media to remove CO2 and H2O from the gas stream by adsorption, absorption, and chemical oxidation processes. The purifiers contain two columns packed with a desiccant material (such as activated alumina), where one column is drying the air and the other one is simultaneously being regenerated. If CO is being measured, another regenerative dryer and a heated catalytic CO scrubber can be included in the system, as shown in the figure.
Figure 3‐20 Example of a redundant dilution air cleanup system.
The dilution air pressure must remain constant. Variations in this pressure due to inadequate pressure regulation can significantly affect dilution ratios. Some systems integrators have installed mass‐flow controllers to maintain this pressure at a constant level. The air supplied to a dilution system may also serve as the air supply for the zero calibration cycles of the system gas analyzers, as well as an air supply for the ozone generator of an NOx analyzer.
Figure 3‐21 illustrates the plumbing associated with a typical dilution extractive system using an EPM probe. Note that the design differs from that of a source‐level extractive system shown in Figure 3‐15, by eliminating the sample conditioner and sample pump. In the dilution system, the motive, dilution air is used in the ejector pump to both dilute the flue gas sample and send the diluted sample under positive pressure to the analyzer. However, in exchange for a flue‐gas conditioning system in a source‐level extractive system, a dilution air cleanup system is required for dilution systems. Of course, either requires periodic maintenance. Dilution systems typically include a control unit that includes gauges and meters for monitoring the eductor vacuum, dilution air pressure, temperatures,