Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 15

In‐situ systems consist primarily of an analyzer that employs some type of sensor to measure the gas directly in the stack, or projects light through the stack to make measurements. The opacity monitor and flow monitor illustrated in Figure 1‐1 are typical examples of in‐situ analyzers. There are two classifications of in‐situ analyzers: point and path. Point analyzers consist of an electro‐optical or electroanalytical sensor mounted on the end of a probe that is inserted into the stack. The point in‐stack measurement is usually made by a sensor over a distance of only a few centimeters. Path analyzers, on the other hand, measure along a path across the width of the duct or diameter of the stack. In these “cross‐stack” gas analyzers, light is transmitted through the gas, and the interaction of the light with the flue gas is used to obtain a quantitative value of the pollutant concentrations. In single‐pass instruments, light is transmitted from a unit on one side of the stack to a detector on the other side, making only one pass through the stack. In a double‐pass system, light is reflected from a mirror on the opposite side, doubles back on itself, and is detected back at the “transceiver.”

In‐situ analyzers are used to measure the concentrations of pollutant and combustion gases and particulate matter, flue gas opacity, and flue gas velocity (flow). Both point and path techniques are used to monitor gas and particulate concentrations. Opacity monitors (transmissometers) are path monitors and can be either single‐pass or double‐pass systems, measuring the transmittance of visible light through the stack. Flow monitors are designed in either point or path configurations, depending upon the analytical technique.

Using wavelength‐tunable lasers, in‐situ gas monitoring systems are experiencing renewed popularity, particularly for the measurement of reactive gases such as HCl and NH₃. More attention is being paid to verifying system calibration using NIST traceable calibration gases, an important issue in the United States. In‐situ point monitors using laser‐light scattering techniques have become popular for monitoring flue gas particulate matter concentrations. Light‐scattering particle sizing techniques are developing, but this technology has lagged behind in source monitoring applications for many years.