their toxic effects. Monitoring requirements for these materials, implemented incrementally over the past 20 years, are discussed in Chapter 13.
Data Acquisition and Handling Systems
CEM system analyzers do not stand alone, but are part of a larger system as seen in Figure 1‐1. The assembly of analyzers is coordinated and controlled to provide emissions data that are subsequently recorded and reported. These roles were all originally considered part of the data acquisition and handling system (DAHS or DAS). Today, control functions are commonly separated from the DAHS by using data loggers, programmable logic controllers (PLCs), or separate microprocessor systems. This simplifies the CEM system by providing more flexibility for both system control and data acquisition and reporting.
Hardware used in CEM recording and reporting systems has evolved from the now archaic strip chart recorders to computer systems integrated into the plant distributive control system and the plant local area network, corporate wide area network, or intranet. In special cases, remote terminal units are used to provide emissions data to environmental control agencies either continuously on a real‐time basis or on demand.
CEM software has evolved significantly, principally due to the demands of the U.S. EPA Acid Rain Program. Requirements to report all emissions data plus plant operational data, on a quarterly basis, have led to sophisticated multitasking programs, having the capability of editing and back‐filling data according to prescribed algorithms. Today’s programs offer flexibility to both CEM system operators and environmental engineers in evaluating data quality and in preparing internal and external reports.
Programming tends to be customized to meet the demand of each plant installation with flexible and user‐configurable programs, making this task somewhat easier. The integration of CEM system data into plant distributive control systems and information networks has become a larger task. This requires considerable coordination between plant information technology personnel, plant engineers, the CEM system integrator, and the data acquisition system provider. This is often the most difficult job associated with the installation of a new CEM system.
THE ROLE OF QUALITY ASSURANCE
In the 1970s, industry frequently presented quite valid arguments that the performance of continuous monitoring systems was questionable. Two basic principles of CEM technology were soon learned:
1 There is no one “best” type of system for all applications.
2 A CEM system must be maintained if it is to operate.
During this period, aggressive CEM system vendors frequently sold their systems to anyone who could be convinced to buy their product. This resulted in misapplications of both in‐situ and extractive systems. The resulting poor performance led to unfortunate perceptions about the reliability of the technology and bankruptcy and absorption of several companies. This is a process that continues still today. From this experience, formal procedures for specifying and evaluating CEM systems have been developed and should be used by companies planning major CEM system purchases.
Errors in application have not been the only reason for poor CEM system performance. It is often assumed that after a CEM system is installed, it can generate data as routinely as a thermocouple or pressure gauge. It must be realized that routine maintenance programs are necessary for the continuing operation of extractive system plumbing and electro‐optical systems. Although this necessity is now well understood, awareness of this need did not develop in the United States until the early 1980s. A CEM specialty conference of the Air Pollution Control Association held in Denver in 1981 pointed out the need for established and effective CEM system quality assurance (QA) programs. By the time of a subsequent conference held in Baltimore in 1985, the U.S. Environmental Protection Agency proposed CEM system quality assurance requirements and many companies were reporting the success of their own QA programs in improving CEM system performance. In 1989, the U.S. EPA promulgated quality assurance procedures for CEM systems used for compliance determinations, specifying requirements for calibration and periodic audits. With this lesson learned, when new performance specifications are promulgated, quality assurance procedures specific to the pollutant are published concurrently. This can be seen in the almost concurrent publication of Appendix F quality assurance procedures for particulate monitoring systems, mercury monitoring systems, and hydrochloric acid monitoring systems with the publication of their respective performance specifications in Appendix B of Part 60.
Like an automobile, where the oil must be changed and the tires rotated, a CEM system requires routine checks and replacements. QA programs incorporating daily and weekly checks, periodic audits, and preventive maintenance procedures have been found to be the key to continuing CEM system operation. Systems with such programs today show better than 98% data availability.
APPLICATION
CEM systems were originally required by regulatory agencies in the United States for monitoring the effectiveness of air pollution control equipment in removing pollutants from flue gases. As indicators of control equipment performance, the data could be used to track plant performance and target sources that were not meeting their emission limitations. Manual source tests would then be conducted on targeted sources to determine if, indeed, they were failing to be in compliance with emission limitations.
However, the extension of CEM data to direct enforcement applications has grown in both federal and state programs. By stating specifically in a rule or permit that a CEM system provides enforceable data that determines if an emission limit is being met or exceeded, the earlier link to control equipment performance is not as important. In addition, the promulgation of the “credible evidence” rule now allows the use of CEM system data in litigation.
CEM systems also provide the basis for the U.S. EPA acid rain control program mandated in the 1990 Clean Air Act Amendments. Here, CEM systems determine the “allowances,” the number of tons per year of SO2 emissions that are traded between the electrical utilities. This successful regulatory program has led to significant SO2 reductions in the United States within two years of its implementation. The over 2000 utility CEM systems installed to track allowances have contributed much to this success, providing an accurate database necessary to instill confidence in the trading market.
Another trading program is found in the Cross‐State Air Pollution Rule, where NOx emissions are traded between electrical utilities and other sources in the northeastern states. These regulatory programs as well as economic forces led to an 89% decrease in SO2 emissions and an 82% decrease in NOx emissions from 1995 to 2017.
Although emission monitoring systems have been applied principally to satisfy such regulatory requirements, CEM system data can also be used proactively by plant and corporate management by providing a base of information on compliance status or for consideration in legal issues. As a result of both agency environmental programs and proactive source monitoring, the CEM database provides assurance to the public that emissions are being monitored to address environmental concerns.
However, the essential purpose of CEM systems should not be forgotten when providing for the timely submission of emissions reports. That purpose is to use CEM system data to control plant operations to meet emissions limitations (Figure 1‐3). The continuous record of emissions data enables plant operators and engineers to optimize plant performance and control equipment operation. On a continuous basis, emissions can then be maintained within regulated limits. In some cases, operating costs can be reduced and data can be gathered for plant design and maintenance information. In sum, the focus should not be on the quarterly emissions report, but rather, on how to use the data to improve operational efficiencies to minimize emissions.