Developing a power project requires a delicate balancing of the needs and wants—perceived or real—of dozens of stakeholders. It only takes one dissatisfied party to slow or even stop a project. The tension between electricity production and environmental protection will only increase as more generating capacity—much of it coal-fired—is proposed to meet rapid load growth in most areas of the U.S. During the week of July 17, as this article was being written, all seven regional independent grid operators set new demand records that exceeded their 2005 peaks by as much as 9.9%. Those increases alone would require another 7,516 MW to come on-line.
Construction of any new plant entails an extensive permitting process, including obtaining a Prevention of Significant Deterioration (PSD) ruling and/or a nonattainment New Source Review, depending on the ambient air quality of the project site. Applicants must demonstrate that the air pollution control method chosen either implements a best available control technology (BACT) or would achieve a lowest achievable emission rate (LAER), compared with other emissions sources in their source category. Once a major source of pollutant emissions becomes operational, it must comply with the terms of its Title V operating permit and be recertified for compliance annually, and submit semiannual compliance monitoring reports.
Extracting data
Generally, the emissions data included in a permit application for a plant come from the manufacturer of its boiler or gas turbine. For example, a gas turbine's NOx emissions level and rate are specified at various ambient temperatures and load conditions, at typical (but not worst-case) humidity and barometric pressures (Figure 1). One conclusion we can draw from Figure 1 is that, although a unit's stated NOx emission level depends on a particular set of operating conditions, its NOx emission rate varies with ambient temperature and relative humidity, as well as fuel quality. Also not to be discounted is the role that the accuracy of instrumentation plays in quantifying the actual emissions of a unit or a plant.
2. Hooked on NOx. The typical straight-extractive continuous emissions monitoring system (CEMS) instrumentation is usually protected by an enclosure. Courtesy: Mostardi Platt Environmental
3. Pulling a sample. CEMS sample lines run up the side of the stack to the sample ports. Courtesy: Mostardi Platt Environmental
Dilution-extractive systems are a more common adjunct to solid-fueled boilers, such as utility units firing coal. The flue gases are diluted 25:1, 50:1, 100:1, or even 200:1 with instrument air prior to measurement. One advantage of dilution-extractive systems is that they do not require the gases to be conditioned prior to analysis. They are analyzed on a wet basis, with no correction for moisture content. After being delivered by the various analyzers, the gas concentrations are multiplied by the dilution ratio to retrieve the original concentration.
Another advantage of appending a dilution-based system to a coal-fired boiler is that flow monitoring is typically used to calculate mass emission rates. Because measurements of both gaseous components and flue gas flow rate are on a wet basis, calculations can be completed without regard to the moisture content of the flue gas.
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