LITMIR.BIZ

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. 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.