What techniques are typically used for Extractive CEMS?

Question: What techniques are typically used for Extractive CEMS, which withdraws flue gas samples from a heater or boiler stack to CEMS analyzer in a shelter at grade?

Answer: The most widely used technique is Hot Extraction Method by which a wet gas sample is extracted from the stack through a sample probe and transported through a heat traced sample line to a sample conditioning system (SCS) in the CEMS shelter. This method requires the samples to be maintained above the dew point (no cold spots) until they reach the SCS sample cooler, where the water vapor is removed without removing the water soluble components to be measured. The SCS sample pump continuously pulls the samples from the stack to the sample cooler. Once properly set up, Hot Extraction Method will allow the analyzer technician to perform most of the maintenance tasks at the CEMS shelter.

Alternatively, Cold Dry Method is also used if Hot Extraction Method is not preferred or to avoid the use of a long heat traced sample line if the CEMS shelter must be placed far away from the stack. In this method, the water vapor is removed at the stack using a gas dryer (Nafion dryer). A sample probe with a gas dryer is mounted on the stack nozzle or a short heat traced sample line is installed if a gas dryer unit needs to be mounted away from the sample probe. Since the water vapor is removed through the Nafion dryer, the use of a heated sample line can be avoided. To make the dryer function properly, the instrument air supply to the dryer must be moisture free and continuous. In addition, the analyzer technician will need to climb the stack to the platform where the sample probe and gas dryer are located for their required maintenance work more frequently than Hot Extraction Method.

Another extraction technique is Dilution Method. The flue gas sample is diluted with dry, clean air in the sample probe to avoid scrubbing of water soluble components by lowering the sample dew point below the ambient temperature. Like Cold Dry Method, a heated sample transport line is not needed and a water solubility concern can be avoided. If the sample is toxic or highly corrosive, the dilution system will allow safer monitoring. Because of dilution with air, the required sample flow rate will far less than the other extraction methods above, making the probe filter last longer from less particulates loading. For the accurate measurements of the sample constituents, the dilution ratio of the sample and diluent air must be maintained through a critical orifice in the probe, i.e., the critical flow through the orifice. Note that since the ratio will be affected by the flue gas properties ( pressure, temperature and molecular weight) and diluent air condition (pressure, temperature and flow), the analyzer technician must continuously monitor the proper functions of the instruments involved.