on the analyzer – the instrument that measures. However, it was soon found that the process of transporting the gas to the analyzer was a source of many problems. Such problems were addressed in a number of ways by CEM “systems” integrators. Those who understood the effects of corrosive stack gases on materials and the effects of pressure and temperature on gas transport were the first to design and successfully market systems that worked under severe sampling conditions.
The difficulties associated with extractive systems led to the idea of measuring the flue gases as they exist in the stack or duct, without conditioning. This idea was realized in the development of “in‐situ” analyzers, second‐generation source monitoring systems especially designed to avoid problems inherent in extracting gases. In‐situ systems have been designed to measure gas concentrations, flue gas opacity, flue gas flow, and particulate matter concentrations. Many present‐day CEM systems are a combination of extractive and in‐situ systems (Figure 1‐1).
Third‐generation systems emerged with the promulgation of the Clean Air Act Amendments of 1990 and the implementation of the acid rain allowance trading program. Requirements to report emissions in tons per year led to the demand for systems that could measure pollutant mass rate. As a result of this program, flow monitors and dilution‐extractive systems were added to the inventory of monitoring methods. It was in the acid rain program that CEM systems found their maturity. To assure the success of plant monitoring programs, both equipment and personnel resources were made available by corporate management. And here, it was found that for the success of monitoring programs, trained and experienced personnel can be equally important as the equipment.
A fourth wave of continuous emission monitoring applications came after 2000 with the delayed and piecemeal regulatory development of rules for hazardous air pollutants. The program to control hazardous air pollutant emissions was promulgated in the 1990 Clean Air Act Amendments along with the acid rain program; however, due to the enormity of the task of regulating 187 hazardous air pollutants from 174 source categories, the program was slow to start. It wasn’t until after 2000 that continuous monitoring requirements for air toxics began to come into effect, but the need to develop new and more sophisticated monitoring systems for measuring particulate matter, mercury, and hydrochloric acid was apparent before that time. A new generation of monitoring systems was developed, measuring a wider range of compounds and materials, at ever lower concentrations, by incorporating advanced measurement and miniaturization techniques developed in response to national military and security concerns.
The business of continuous emission monitoring is largely dependent upon environmental regulations and is almost cyclical with the ebb and flow of environmental rule‐making. As new regulatory programs are developed, it is found necessary that a means be provided to keep track of progress or lack of progress. When emission limits are mandated, enforcement programs require a measure of whether emission limits are met or not. Intermittent manual stack testing is clearly not adequate for this purpose, and it has been demonstrated that continuous emission monitoring systems can provide data of sufficient precision and accuracy to support enforcement programs and allowance trading programs. With emission limits becoming ever more stringent and the proper operation of pollution control equipment ever more critical, continuous emission monitoring systems have evolved where they can today, meet the most demanding applications.
Figure 1‐1 A continuous emission monitoring (CEM) system.
TYPES OF MONITORING SYSTEMS
A CEM system is actually composed of several subsystems: the sampling interface, the gas analyzer(s), and the data acquisition/controller system. The sampling interface is a subsystem that either transports or separates the flue gas from the analyzer. CEM systems are characterized in terms of the design of this interface. In extractive systems, the interface consists of a system that extracts and conditions the gas before entering the analyzer. In in‐situ systems, the interface is simpler, composed of flanges designed to align or support the monitor and blower systems used to minimize interference from particulate matter. The data acquisition/controller subsystem is integral to the proper operation of the total system. The control system controls automatic functions of the system, such as calibration, probe purging, and alarming. The data acquisition system receives the analyzer data, converts it into appropriate units, records it, and provides reports for both internal and external use. Today, CEM data acquisition systems are frequently networked to engineering, corporate, and even agency offices, where the data are used for a variety of operational and management purposes.
Both extractive and in‐situ systems operate in the source environment and must operate continually under changing stack and ambient conditions. Although this is not necessarily a problem for properly designed and maintained systems, alternative approaches have been sought. One of these approaches, remote sensing, has been applied with limited success, but has not been used widely. Another alternative, the correlation of stack emissions to process parameter data, has led to computerized “predictive emission monitoring systems” or PEMS. These systems have been employed successfully in a number of applications and have considerable potential if used in tandem with extractive or in‐situ monitoring hardware. The monitoring methods discussed earlier are classified in Figure 1‐2 and discussed further in the following sections.
Extractive Systems
Extractive gas monitoring systems were the first to be developed for source measurements. In these systems, gas is extracted from a duct or stack and transported to analyzers to measure the pollutant concentrations. Many of the early extractive systems first diluted the gas using rotameters, and then applied ambient air analyzers for measurements. However, frequent problems occurred in maintaining stable dilution ratios, so analyzers were subsequently developed to directly measure the flue gas at source‐level concentrations in the range of 100–1000 ppm or higher. These source‐level extractive systems were quite successful and received their widest application in the 1970s and early 1980s.
Many of the problems associated with the earlier dilution systems have since been eliminated by new techniques developed in the 1980s. The advent of the “dilution probe” made dilution systems viable for source measurements. Dilution systems are now relatively easy to construct and exhibit good performance. They are particularly useful for monitoring water‐soluble gases and provide a platform for the application of a new generation of analyzers that are able to measure part per billion concentration levels.
In order for an instrument to measure gas concentrations, the gas sample must be free of particulate matter. Often, water vapor is removed and the sample is cooled to instrument temperature. This requires the use of valves, pumps, chillers, sample tubing, and other components necessary for gas transport and conditioning. “Hot‐wet” systems, which measure hot gases without water removal, operate continuously at elevated temperatures, eliminating the need for water removal systems. Extractive systems use an umbilical line to transport the flue gas sample from the stack or duct to an analyzer cabinet or temperature‐controlled shelter. This line is heated in source‐level systems where the conditioning system is located in the shelter.
Figure 1‐2 Types of monitoring systems.
Dilution‐extractive systems became popular in the 1990s for determining pollutant mass emission rates at U.S. coal‐fired power plants subject to acid rain cap‐and‐trade regulations. Dilution‐extractive CEM systems measure on a wet basis,