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.
Figure 1‐3 Industry uses of CEM system data.
SUMMARY
The technology of continuous emissions monitoring has not been static. The use of CEM systems for allowance trading programs and emissions enforcement programs, the demands for increased system availability, and advances in data handling and reporting have led to more sophisticated systems with better reliability. CEM systems have advanced considerably over 50 years of development, with improved sampling techniques, analyzers, and data processing systems being integrated into today’s systems to meet the challenges posed by new requirements. Also, by implementing CEM system quality assurance programs and by properly managing the monitoring programs, high system availability can be achieved. This high availability is a necessity today, where inaccurate data or missing data can incur both regulatory and economic penalties.
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