Continuous Emission Monitoring. James A. Jahnke
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will condense more readily than a gas under vacuum. Since it is more difficult for water molecules to escape the liquid surface to vaporize when the gas is under pressure, this pressurization will, in effect, reduce the moisture content to a level lower than the content under atmospheric pressure.
Figure 3‐11 Refrigerated condenser moisture removal system with a secondary chiller under positive pressure.
In refrigerator systems, Freon sometimes escapes, pumps wear out, or algae start growing in the liquid. Another cooling option of using vortex coolers avoids these problems by using a counterflow of high‐pressure plant air to cool the sample gas. However, the power requirements necessary to generate the high flow rates of air needed may be excessive in some applications.
NafionTM Dryers.
Another method of removing water vapor from a flue gas sample is to use a material called “Nafion™.” Nafion™ is a copolymer of tetrafluoroethylene‐perfluoro 3, 6‐dioxa‐4‐methyl‐7‐octene sulfonic acid (Permapure 2019). The sulfonic acid group of the copolymer has high water‐of‐hydration, absorbing up to 13 molecules of water. Water vapor will transfer across the material by absorption as water‐of‐hydration if the partial pressure of the water vapor is different on each side of the membrane. Water molecules are initially bound on the material surface. However, unhydrated sulfonic acid groups deeper in the Nafion™ membrane will have higher affinity for water molecules and will absorb them and in turn pass them onto other unhydrated sulfonic acid groups, causing them to migrate into and through the membrane.
A typical dryer uses a bundle of Nafion™ tubes as shown in Figure 3‐12. In this dryer assembly, the wet sample gas enters the tubes and the dry purge gas flows in the opposite direction on the outside of the tubes to sweep the transferred water molecules away from the membrane. The driving force in this exchange can also be provided by either evacuating the shell side of the dryer or by back‐purging with dry gas taken from the tube exit. The dryer requires that the flue gas sampled be held above the dew point upon entering the dryer. The drying efficiency increases with the length of the tubes and is also dependent upon the sample inlet and purge gas pressures. Capacity can be increased by increasing the number of tubes in the bundle. In contrast to gas coolers, having an exit moisture content of 0.5%, gas exiting a Nafion™ drier is significantly lower, at 0.05% H2O, corresponding to a dew point of −22 °C (Geary and Sinada 2018). When using a gas analyzer where water vapor interferes in the measurement, a Nafion™ dryer, or a Nafion™ dryer following a gas cooler, may help to minimize the interference.
The Nafion™ dryer offers a number of advantages over refrigerated chillers because no mechanical parts are incorporated into the system, no condensate trap is required, and the question of pollution absorption in condensed water is avoided. However, dryers using small‐diameter Nafion™ tube bundles can be prone to plugging, either from droplets of condensed material or from particulate matter introduced by improperly filtered samples or by the precipitation of salts. Problems of condensing liquids may be minimized by heating the entrance side of the dryer, whereas particulate plugging and precipitation can be resolved by using larger diameter tubing. When using large‐diameter tubing (1/4 in.), greater lengths of tubing must be used to achieve performance equivalent to that obtained from employing bundles of smaller diameter tubing.
Pollutant Losses in Condensation Systems.
One limitation of condensation‐type moisture removal systems is that pollutant gases can be absorbed in the condensate. This occurs readily for gases such as HCl and NH3, which have high solubility in water and where, clearly, condensation methods are not applicable. NO has little solubility in water, but gases such as SO2 and NO2 can also be lost in the condensate. This issue has not always been recognized because SO2 and NO2 losses are less pronounced at higher concentrations (>100 ppm) and the losses can go undetected during a certification test if the source tester uses an apparatus that also incorporates a condensation moisture removal method. However, with the advent of more stringent emission standards requiring gas monitoring in the range of 50 ppm or less, more care must be taken to avoid biases due to analyte condensate losses.
Figure 3‐12 A Nafion™ dryer assembly.
Unfortunately, research on this problem has often failed to be sufficiently comprehensive in addressing a number of factors that affect analyte losses. McNulty et al. (1974) studied SO2 losses at 1200 ppm, Freitag (1993) at 100–1000 ppm, and Baldwin (1995) at 300–400 ppm. Complicating the issue, the design and construction of the condenser can contribute to solubility losses, where poorly designed systems may allow too great a contact time between the dried gas stream and the collected liquid. Also, losses are dependent upon the inlet moisture concentration, sample gas flow rate, condenser temperature, and condenser material (glass, Kynar, PTFE, stainless steel). The most comprehensive, independent evaluation conducted to date is by Swaans et al. (2018), who studied SO2 and NO2 losses in four different coolers using glass, stainless steel, and PTFE condensers, at nominal concentrations of 25, 90, and 100 ppm for SO2 and 10, 50, and 100 ppm for NO2, at flow rates of 2, 3, and 5 l/min, and flue gas and moisture contents of 4, 11, and 20% H2O.
Freitag (1993) found that for SO2 at concentrations on the order of 100–1000 ppm, under a variety of conditions, 3–15% of the SO2 could be lost in the chiller. Freitag also projected that at SO2 levels of 20 ppm at 20% moisture, losses can be on the order of 30%. He observed that SO2 losses increase with the increasing moisture content, decreasing SO2 content, and decreasing trap temperature. In thermoelectric coolers at 380 ppm SO2, Baldwin (1995) found losses of 1.7% of the SO2 concentration at an inlet moisture concentration of 20% using glass impingers. Using stainless steel impingers and an SO2 concentration of 245 ppm, he found losses of 4.1% SO2 at an inlet moisture concentration of 20%. He also found greater losses at lower flow rates. In a comparison between thermoelectric coolers and permeation dryers, Dunder and Leighty (1997) sampled gases having SO2 concentrations of 20, 50, and 100 ppm at moisture levels of 15 and 30%, at flow rates of 5 l/min. In the study, higher SO2 concentrations were found using a permeation drier compared to a thermoelectric cooler, under all conditions. In agreement with these earlier studies and extending the work to lower concentrations while examining additional variables, Swaans and associates (2018) found, for SO2 at a concentration of 25 ppm in a flue gas at 20% moisture content and a flow rate of 3 l/min, relative losses of 7–17% SO2 using either Peltier or compressor coolers, depending upon the manufacturer of the cooler. The higher losses were seen in PFA‐coated heat exchangers relative to glass heat exchangers. At a lower SO2 concentration of 17 ppm, Geary and Sinada (2018) found, at moisture levels of 6–9%, losses of up to 50% of the original sample concentration in a condensing cooler system compared to measurements using a permeation drier. Pellikka et al. (2019) have also demonstrated losses using a chiller in comparison to hot–wet and dilution extractive systems.
The primary conclusion from the work of Swaans et al. is that when monitoring SO2 at low concentrations (<25 ppm) SO2 losses are highly dependent upon the manufacture of the cooler, flue gas variables, and cooler operational variables. It is also apparent by comparing the work of others that the lower the SO2 concentrations