Continuous Emission Monitoring. James A. Jahnke
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extractive systems and the components that are used in their design. Knowledge of these extractive techniques will assist in evaluating proposed system designs or systems that have already been installed.
SOURCE‐LEVEL EXTRACTIVE SYSTEMS
Source‐level extractive systems remove gas directly from the stack or duct, filter out particulate matter, and transport the gas for analysis. Three types of source‐level extractive systems are marketed commercially:
1 Hot/wet systems (Figure 3‐1)
2 Cool/dry systems with conditioning at the probe (Figure 3‐2)
3 Cool/dry systems with conditioning at the CEM system shelter (Figure 3‐3)
Hot/Wet Systems
The simplest type of extractive system uses a heated line to transport the flue gas to an analyzer that incorporates a sample cell heated above the flue gas temperature. The gas is delivered to the analyzer both hot and wet, but it minimally conditions the gas by removing particulate matter with a coarse filter located at the probe. The technique is useful when monitoring of water‐soluble gases is required, or when wet‐basis emission values are to be reported. Because water is not removed from the sample gas, problems associated with condensation systems are avoided. However, care must be exercised in maintaining the temperature of the sample above the dew point, from the probe to the analyzer exhaust. If the heating system should fail, moisture will readily condense and foul the system. This can lead to corrosion, plugging, or damage to the analyzer. Accordingly, a hot/wet system must be designed to shut down and flush with clean, dry air or nitrogen in case the system begins to cool down due to an event such as a power outage or heater failure.
Figure 3‐1 A hot/wet CEM system without sample conditioning.
Hot/wet systems are often used in association with ultraviolet analyzers designed for the measurement of SO2 and NOx. When a wet‐basis pollutant measurement is obtained in conjunction with a volumetric flow measurement, the pollutant mass concentration can be obtained directly from the product of the two measurements (pollutant mass rate = concentration(wet) × volumetric flow rate (wet)). In contrast, when a cool/dry concentration measurement is obtained, a knowledge of the flue gas moisture content is necessary to calculate the pollutant mass rate. This adds both complexity and possible errors to such a monitoring system.
Hot/wet systems are also useful for measuring water‐soluble gases such as HCl, NH3, and certain volatile organic compounds. The chiller in a cool/dry extractive system will remove these gases either in part or entirely, so either a hot/wet system or dilution extractive system would be necessary to deliver a representative sample to the analyzer. A hot extractive system and a hot analyzer can also minimize the adsorption of gases on the extractive system surfaces. Chemical reactions such as NO to NO2 conversion (Sneek 1997) and ammonium sulfate (White 1995) or ammonium chloride (Peeler et al. 1997) formation can be minimized by using higher‐temperature extractive systems.
Cool/Dry Systems with Conditioning
In a more widely applied extractive system design, the gas is conditioned before it enters the analyzer. The gas temperature is reduced to ambient temperature and moisture is removed, so that the sample is both cool and relatively dry. The conditioning may be conducted either at the probe location (Figure 3‐2) or at the analyzer shelter (Figure 3‐3). Conditioning at the probe location offers the advantage of using unheated sampling line; however, preventive maintenance of the conditioning system at the probe may be inconvenient. Conditioning at the shelter or CEM room enables the CEM system operator to check the system performance conveniently. However, the necessary heat‐traced lines must be maintained at a proper temperature over their entire length.
Figure 3‐2 A cool/dry CEM system with conditioning at the probe.
Figure 3‐3 A cool/dry CEM system with conditioning at the CEM system shelter.
Extractive systems that condition the flue gas allow greater flexibility in the choice of analyzers and are commonly used when emission calculations are performed on a dry basis or when monitoring a number of different gases is required. Although this type of system is not as sophisticated as some others, it is flexible enough to accommodate engineering changes when application problems arise. In problem applications, the system components can be readily modified or replaced so that the system can meet performance specifications.
A source‐level extractive system is made up of a set of basic components: probe, sample line, filters, moisture removal system, and pump. Because the operation of an extractive system is dependent on the design and quality of each component, as well as on their arrangement in the system, it is necessary to review these characteristics.
Sample Probes
A sample probe (sometimes called a “stinger”) can be made by merely inserting an open metal tube into the stack or duct, or multipoint probes may be used to sample from several locations when the flue gas is stratified (Shahin 2016). This may be adequate in sampling situations where no particulate matter is present. However, flue gases free of particulate matter do not often occur in those sources that are subject to CEM regulation. An open tube can be easily plugged when particulate matter is present, especially if the flue gas has high moisture content. Also, water may condense and combine with the particulate matter to produce an agglomerated material that can plug the probe more readily. To minimize such problems, a filter can be placed at the end of the probe (Figure 3‐4a–c).
Filters made of sintered stainless steel and porous ceramic materials are commonly used to prevent particles from entering the sample tube. Sintered metal is made by compressing micrometer‐sized metal granules under high pressure and elevated temperatures. The metal fuses and acquires porosity depending on the compression pressure. Sintered stainless steel filters that are capable of filtering out particles of 5 to 50 μm have been used as probe filters. Some systems use filters that exclude particles greater than 1–2 μm in size, but the finer the filter, the more difficult it will be to draw the sample gas through the filter, and pump capacity will need to be increased.
Sintered filters or ceramic filters can become plugged by the particles impacting on and penetrating into the porous material. To minimize plugging, a baffle plate can be attached to the filter to deflect particles from the filter surface (Figure 3‐4b). Particles will then follow streamlines formed around the plate, whereas pollutant gases will still diffuse into the probe. Another way to minimize plugging is to attach a cylindrical sheath around the filter (Figure 3‐4c). Gas will still