Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 80

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 NO_x. 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.