Continuous Emission Monitoring. James A. Jahnke

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      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. NO_x 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 NO_x 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, calibration gas flow, and so on.

      External Dilution Systems (Probes)

Another approach to diluting the flue gas is not to dilute it in the stack, but outside of the stack in a dilution assembly contained in a protective housing close‐coupled to the stack. External dilution systems can be more easily serviced and temperature controlled than an in‐stack dilution probe; however, in principle, their response time is somewhat slower due to the length of time necessary to transport the sample from the probe tip to the dilution orifice. External dilution probes do offer several advantages over in‐stack dilution probes. They can be heated relatively easily to maintain a more constant temperature. In wet scrubber applications, they can resolve droplet and aerosol condensation problems that may be more difficult to address with an in‐stack critical orifice. For example, by sloping the stinger of an external system, droplets can run off into the stack before they reach the critical orifice. The external systems also offer a significant maintenance advantage by not requiring the probe to be extracted to examine or replace the filter or critical orifice.

Figure 3‐21 An in‐situ dilution probe extractive system.

      Several external dilution system designs are available. Each design is associated with a particular manufacturer and different industrial sectors, such as coal‐fired power plants or pulp and paper plants, tend to favor one particular design over others. In CEM system upgrades, external dilution systems often replace in‐stack dilution probes, not so much for any inherent faults in the in‐stack probes, but more often for not having to remove the probe from the stack for maintenance or because the designs are newer and “trendy.”

       Cross‐Piece Fitting Design.

In a system designed by M&C Tech Group, an assembly, close‐coupled to the stack, is unique by cleverly incorporating both the dilution eductor and a dilution capillary within a Swagelok cross fitting (Figure 3‐22). This makes for a compact design where the temperature can be easily maintained.

      In locating the dilution system outside of the stack at the end of the probe, the characteristic low flow rate of a dilution system would decrease the response time of the system to real‐time flue gas concentration fluctuations. This is in contrast to an in‐stack dilution probe, where the flue gas is diluted essentially at the probe tip and the diluted sample is transported relatively swiftly to the analyzers. A solution СКАЧАТЬ