Continuous Emission Monitoring. James A. Jahnke
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the greater will be the losses of SO2 in gas coolers. To certify a CEM system in a RATA, comparing data with a source tester’s reference system operating under a different set of variables could lead to a failed certification, or where bias correction factors are required, inaccurate bias adjustments. At low SO2 concentrations, it is recommended that instead of installing a source‐level extractive system with a gas cooler, a hot/wet extractive system, a dilution‐extractive system, or a source‐level extractive system, using a Nafion™ dryer, be considered for the application.
As a last resort in source‐level extractive systems using chillers, SO2 absorption can be decreased by increasing the acidity of the condensate. One technique has been to acidify the gas stream with an unmonitored acid (such as HCl) to reduce SO2 solubility during condensation (DeFriez 1992; Williams 1992).
Miscellaneous Drying Techniques.
Other drying techniques have been used or attempted in extractive monitoring systems. Cyclone‐type devices installed in or near the probe, coalescing filters, “knock‐out jars,” and other engineering afterthoughts are sometimes encountered.
The use of chemical desiccants (such as calcium chloride, CaCl2; concentrated sulfuric acid, H2SO4; calcium sulfate, CaSO4) to remove moisture is not common in CEM systems. Because desiccants have to be periodically regenerated or replaced, they are considered to present an unnecessary maintenance task. Also, to justify their use, it must be shown that the gases being measured do not react with, adsorb onto, or absorb into the material.
In cases where acid gas formation, such as SO3, might occur, a “freezer” chiller designed to achieve a dew point of −25 °C can be used to reduce moisture levels to less than 650 ppm and minimize the loss of SO2 by acid formation.
Sample Pumps
The sample pump is an important element of the extractive system and is used to transport the sample from the stack to the analyzer. A pump should be sized appropriately to meet the demands of gas analyzers and be designed so that no air in‐leakage occurs (i.e. around a rotary shaft seal) and that no contamination is introduced from pump lubricating oils. Two types of pumps meeting these criteria are (i) diaphragm and (ii) ejector pumps. These pumps are commonly used in source monitoring applications.
Diaphragm pumps operate by mechanically stroking a piston or a connecting rod to move a flexible diaphragm (Figure 3‐13). The diaphragm is circular and can be made of a flexible metal plate, Teflon, polyurethane, or other types of elastomer. The reciprocating action of the diaphragm moves the gas in short bursts. As the diaphragm is raised, the gas is drawn through the suction valve into the pump cavity. When the diaphragm is pushed down, the suction valve closes and the discharge valve opens. The gas is then displaced out into the sample line. Because only the chamber, diaphragm, and valves come into contact with the gas, contamination of the gas is minimized.
Figure 3‐13 A diaphragm pump.
Diaphragm pumps can be used before the sample gas conditioning system and can even be operated hot. However, particulate matter and condensed acid can weaken the pump head by particle abrasion or chemical attack. Unless the gas is properly filtered and the pump head adequately heated, it is better to locate the pump after the conditioning system. Eventually, the continual, rapid flexing action of the pump will cause the diaphragm to split or tear.
The flow rate of a diaphragm pump can be controlled by placing a throttle valve in the line on the discharge side of the pump or by installing a by‐pass line and valve from the suction side to the discharge side. If the pump is operating at less than its full capacity, controlling flow with the throttle valve will cause the pump to work against a high discharge pressure and pump life will be reduced. For this reason, it is better to control the flow using a bypass valve.
The ejector pump (also called an eductor or air aspirator) takes advantage of the Bernoulli effect to draw a vacuum through a sampling system (Figure 3‐14). In the Bernoulli effect, the moving jet of air reduces the air pressure normal to its flow. This is a common effect that is familiar in venturi flowmeters and jet carburetors. This reduced pressure pulls the sample gas through the sample line; if the jet velocity increases, the vacuum increases. Typically, the filtered plant air or compressed cylinder gas is used for the high‐velocity gas stream. The ejector pump is simple in its design and can be incorporated as part of an inertial filter probe system as shown in Figure 3‐7. Here, the pump draws in the sample through an inertial filter that would be installed in a heated cabinet located outside the stack. Another pump, usually a diaphragm pump, is used to pull the filtered sample to the analyzer. In the ejector pumps applied where the sample gas is not filtered, particulate matter can build up and dry out in the annular space of the pump. In such cases, it may be necessary to use steam instead of compressed air to provide the motive force for the pump action.
Figure 3‐14 The ejector pump or eductor.
Fine Filters
The coarse filter is used to remove larger particles from the sample gas. Because the majority of gas analyzers require almost complete removal of particles larger than 0.5 μm, additional filtration is necessary. This is accomplished by incorporating a fine filter before the analyzer inlet. The location of the fine filter is an important consideration in the CEM system design and is dependent upon the susceptibility of the other system components to the effects of fine particles. There are two types of fine filters: (i) surface filters and (ii) depth filters.
A surface filter can be simply a filter paper that excludes particles of a certain size. The filter material is porous to the moving gas, but the pores are of such a size that they prevent penetration of fine particulate matter. A filter cake can also build up on the filter, further reducing the size of particles passing into the gas stream. Because of the filter cake and developed electrostatic charges, surface filters can remove particles smaller than the actual filter pore size.
Depth filters collect particles within the bulk of a filter material. The filter may consist of loosely packed fibers of quartz wool or of filter material wrapped to a depth sufficient to remove fine particles. Such filters work particularly well for dry solids and moist gas streams containing aerosols. A depth filter can also be used as a probe filter, a technique that is used in some systems.
Assembling a Cool/Dry Extractive System
Designing and assembling a cool/dry extractive system is a relatively straightforward process. System components, such as probes, chillers, filters, and pumps are available from many suppliers and can be purchased individually to incorporate into a system design. Alternatively, several suppliers provide modular conditioning systems and modular calibration gas distribution systems that simplify the assembly. An example of a plumbing system for a cool‐dry extractive system is shown in Figure 3‐15.