Continuous Emission Monitoring. James A. JahnkeЧитать онлайн книгу.
by inexperienced personnel, and ad hoc supporting and fastening methods have led to numerous umbilical failures in the past (Robinson 2010). It is highly recommended that the umbilical manufacturer either supervise or be on‐site during installation. Lastly, the umbilical should be routed to avoid the possibility of damages by facility vehicles, cranes, forklifts, or other plant equipment.
Calibration Gas System and Cylinder Gas Pressure Regulators
A calibration gas delivery system is a component of any extractive sampling systems designed for meeting regulatory requirements in the United States and Canada. In North America, CEM systems are required to be calibrated against NIST‐certified calibration gases meeting established traceability protocols (Schakenbach et al. 2002; U.S. EPA 2012). Monitoring system calibration is required to be verified daily, and monitoring system linearity must also be audited quarterly using “protocol” gases injected as close to the probe as possible, passing through all components of the extractive system to the analyzers.
The calibration gas system typically extends from the calibration gas cylinders, through a gas pressure regulator, to an automated gas injection control system, through a dedicated line in the umbilical, and on to the injection port (Figures 3‐15 and 3‐21). The gas returns through the sample line to the analyzer(s). The calibration gas line may be either in the umbilical (which may be heated or unheated) or separated from the main umbilical in an unheated line. Components typically in contact with the calibration gas will be the calibration gas cylinder pressure regulator, calibration gas delivery line, the sample gas probe, the sample line, and the internal surfaces of the analyzer sample cell.
The use of cylinder gas pressure regulators is often taken for granted and can be a factor in the delivery of reactive calibration gases. Reactive gases such as HCl and NH3 can stick to surfaces in the regulator as well as to the calibration gas delivery line, where the gas adsorption is dependent upon the materials of construction, flow rate, temperature, and the prior status of the delivery system. Moisture or other contamination in the regulator can have effects that may be difficult to uncover if proper precautions are not taken. Water in the air can condense in a regulator and regulators used for other purposes may have an unknown history that could lead to losses of the gas. Similar to calibration gas lines, regulators treated with SilcoNert® reduce surface absorption.
For reactive gases, calibration system problems are often blamed for the quality of the calibration gases. However, problems are more often due to the lack of consistent procedures used to deliver the calibration gas to the monitoring system. It is recommended that for reactive gases the regulator first be flushed with nitrogen for a half hour, installed on the cylinder, and then purged with the calibration gas. The regulator should remain on the cylinder and should be back‐purged when it is removed to exchange cylinders. Recommendations provided by Marshak (2015) are as follows:
Use new regulators – leave on the cylinder all the time.
Purge for 30 minutes prior to installing on cylinder.
Pressure purge with the cylinder gas 10 times.
Gas vendors are not always helpful on these issues, assuming that their purpose is to ensure the quality of calibration gases and not their usage.
Moisture Removal Methods
Moisture is usually removed before the hot flue gas enters a sample pump because water vapor and acid gases can easily condense in an unheated pump and corrode the interior. Cooling beyond the dew point (the temperature at which air is saturated with moisture) will cause moisture to condense, and many of moisture removal systems are designed to reduce the sample temperature below the dew point. Thermoelectric and compressor‐operated gas coolers are most commonly used for this purpose in extractive systems. Coolers should be correctly sized for the sample gas flow rates and stack‐gas moisture content; also, coolers must remove condensed water rapidly from the gas stream to minimize contact with dried gas.
Thermoelectric Coolers.
Hot flue gases can be cooled in thermoelectric Peltier coolers, which take advantage of the Peltier effect to chill the sample gas. The Peltier effect occurs when two dissimilar metals or semiconductors are joined in a loop and a voltage source generates a current through the loop. Because of the differing electron distributions in the dissimilar materials, a small voltage difference exits across their junction. The junction will heat up where the voltage difference opposes the voltage difference of the battery. At the other junction, thermal energy is absorbed from the surroundings and converted into electrical energy to balance the electron flow. This absorption, of course, reduces the temperature of the surroundings.
Semiconductor thermoelectric Peltier coolers are used in many commercial sample conditioning systems. Typically, impingers (laminar flow heat exchangers (Figure 3‐10)) are enclosed by a heat transfer block, which is contacted and cooled by a Peltier plate (Baldwin 1995). Radiator fins and a cooling fan dissipate heat from the hot side of the Peltier element.
In the impinger, the flue gas flows through the central tube, which is surrounded by a vacuum jacket. The flue gas remains above the dew point until it reaches the bottom where it is cooled rapidly. Water vapor condenses and is removed at the bottom of the impinger by typically using a peristaltic pump. The gas is cooled further by the cool impinger wall as it moves up to the impinger outlet. In a unique feature of the design, the gas is reheated by the unjacketed part of the central tube before exiting to the analyzer.
The effectiveness of this type of cooler is dependent upon the surface area and length of the impinger, gas flow rate, materials of construction, temperature of the ambient air, and the cold side temperature. The sizing of a Peltier cooler/impinger condensation system is dependent upon the demands of the total sampling system, so care must be taken in their proper application. The chillers normally operate to output a gas having a dew point of 4 °C (0.5% moisture concentration), but subzero coolers are available down to −25 °C to reduce the residual moisture concentration and to prevent the formation of acid gases such as SO3.
Figure 3‐10 Impinger (laminar heat exchanger) used with a Peltier cooling system.
Compressor Gas Coolers.
Another approach to moisture removal is to use a compressor‐run refrigerator to cool the gas. Either laminar‐flow heat exchangers such as that shown in Figure 3‐10 or coils of Teflon, glass, or Kynar such as that shown in Figure 3‐11 are cooled by the refrigeration system. The condenser can be immersed in a liquid such as an antifreeze solution or water to facilitate better heat exchange. To avoid freezing condensate in the coil or impinger, temperature is typically not allowed to decrease to below 3 or 4 °C. Condensed water vapor is again continuously removed using a peristaltic pump. If condensate is removed only periodically or manually, failure to perform this operation may cause the trap to fill and overflow into the sample line. Most systems will incorporate a thermal conductivity detector or other means to detect or prevent moisture breakthrough into the sample line and analyzer.
Another condenser can be added to further reduce the water content, but a more efficient technique is to place the sample pump after the first coil and transport the gas from the chiller, under pressure, to a second chiller as shown in Figure