Ambient Air Quality

The National Ambient Air Quality Standards (NAAQS) are standards established by the United States Environmental Protection Agency (EPA) under the authority of the Clean Air Act that apply to outdoor air throughout the country. Primary standards are designed to protect human health, with an adequate margin of safety, including sensitive populations such as children, the elderly, and individuals suffering from respiratory disease. Secondary standards are designed to protect public welfare from any known or anticipated adverse effects of a pollutant. As part of the air quality permitting process for the Project, extensive air dispersion modeling was conducted to determine the impacts the new unit will have on the local ambient air quality. These analyses show that the impact of emissions from the proposed unit will be well below the annual standards determined by the EPA.

Similarly, the dispersion modeling demonstrates that the air emissions from the Project will not cause a violation of the short-term NAAQS (graph not shown). Estimated emission rates from the Project units are identified in this table.

All air permits include restrictions on the amount of pollutants that may be emitted by a stationary source. Actual emission rates are monitored continuously and are reported to EPA on a quarterly basis.  If a unit is found to have violated these limits, a regulatory agency may issue a notice of violation. Currently, Holcomb Station operates below its permitted emissions limits and has never received a violation for excess emissions.

The emissions limits proposed for the Project are more restrictive than for the existing unit. To avoid a violation of these new limits, the control technology for the expansion unit must be operated and maintained at peak efficiency.

Occasional maintenance requirements will necessitate that some portions of the control systems will be unavailable for short periods of time. By operating at a lower “estimated emissions rate,” the overall emissions limit will not be exceeded while maintenance activities are being performed. Routine maintenance activities are required for optimum and continuous emissions reduction performance.

The new unit will be constructed with the best available control technology (BACT) to ensure that sulfur dioxide, nitrogen oxides, and particulate matter emissions will be within the standards set by federal and state regulations. Sulfur dioxide (SO2) emissions will be controlled with a dry-lime Flue Gas Desulfurization (FGD) system. Particulate emissions will be controlled with a fabric filter baghouse that removes more than 99.9 percent of the particulate matter or dust. NOx emissions are controlled with a combination of low-NOxburners, Overfire Air (OFA), and a Selective Catalytic Reactor (SCR).

The flue gas from the steam generator passes through the SCR, PAC injection system, FGD, and fabric filter emissions control systems before large, induced draft fans exhaust the flue gas through the stack. The stack will be equipped with a Continuous Emissions Monitoring System (CEMS).

With the application of these technologies, the air emissions from the Project will, like those from the existing unit, reflect technology solutions that will minimize the emission of criteria pollutants.