Understanding emission requirements for standby gensets

Engineers should know the current emission regulatory requirements to ensure their designs comply.


Figure 1: The yellow haze shown in this photo is an indication of NOX emissions. Courtesy: Safety Power Inc.To obtain an air emissions permit for facilities that have stationary diesel emergency standby generators, it is necessary to comply with U.S. Environmental Protection Agency (EPA) and local regulatory requirements. Ensuring that the system design takes into account these regulatory requirements can have a significant impact on overall facility layout and cost. 

The EPA’s regulations are relatively complex and have been in a state of flux. This article provides an overview of the EPA regulatory framework with a concentration only on those requirements for stationary diesel emergency standby generators that are greater than 500 hp. This size range is commonly encountered in larger data centers, hospitals, and municipal infrastructure. This article also focuses on new installations only; it does not offer insight into the rules that govern existing retrofit installations.

This article also attempts to identify some of the key EPA terminology, such as National Ambient Air Quality Standards (NAAQS), New Source Performance Standards (NSPS), Reciprocating Internal Combustion Engine (RICE) National Emission Standards for Hazardous Air Pollutants (NESHAP), and Tiers 2, 3, and 4, and put them into a framework that allows critical power engineers to understand the current regulations and how those regulations influence the design of new facilities. 

Targeted emissions

A diesel engine generates certain emissions that the EPA considers to be criteria pollutants. Criteria pollutants are deemed to be serious health risks and are measured by the EPA throughout the U.S. in geographic entities called areas. The key criteria pollutants associated with diesel engines are: nitrogen dioxide (NO2), particulate matter (PM), and carbon monoxide (CO) (see Figure 1). 

Explaining the regulatory environment

Figure 2: The overall EPA regulatory framework can impact stationary diesel gensets. Courtesy: Safety Power Inc.The Clean Air Act forms the regulatory basis for all air compliance activity (see Figure 2). It was originally established in the early 1970s. The most important recent major amendments to the Act occurred in 1990. These amendments recognized the need to consider the available technology as a component in determining achievable standards. The EPA terminology for this is maximum achievable control technology (MACT). Cost-effective technology advances in MACT have created the platform for the EPA to look at new emission requirements for diesel engines. 

As part of its risk assessment, the EPA allows emergency engines to meet somewhat lower standards than nonemergency units because of the lower annual operating hours. The definition of “emergency” can be relatively complex. Clearly, a utility outage is an emergency condition. There are specific definitions for voltage and frequency variations for electrical reliability that can also constitute an emergency situation. In general, a total of 100 hr/yr is allocated to emergency generators for maintenance and testing. Of these 100 hr, the EPA currently allows up to 50 hr to be used for demand response programs in some jurisdictions. However, this aspect is currently under review and may be removed. There are no restrictions on the number of run hours for the engine when it is being used under emergency conditions. 

EPA regulatory framework

The EPA regulatory framework has several components based on the details of implementation. These components include NAAQS; RICE NESHAP; NSPS; and Tiers 2, 3, and 4. They are covered in detail in the following paragraphs.

NAAQS: Maintaining a NAAQS is a fundamental concept of the Clean Air Act. A NAAQS is based on limits that are designed to ensure healthy air quality for all citizens regardless of where in the U.S. they live. As part of a NAAQS, the EPA defines six criteria pollutants. The modern lean-burn diesel engine has improved dramatically in recent years, but can still contribute significantly to three of the criteria pollutants: NO2, PM, and CO.

NO2 is one of the constituents of NOX, the formation of which is largely a function of combustion temperature. Typically, a higher combustion temperature results in a higher level of NOX formation. PM is also a function of combustion temperature. Typically, a higher combustion temperature results in less PM formation. As a result, undesirable NOX and PM formation act in opposing directions when engine designers are investigating combustion temperature. Engine efficiency also typically improves at higher combustion temperatures, which is another important consideration for engine designers. CO is often a reflection of incomplete oxidation of fuel in the combustion chamber. Most major diesel engine manufacturers have optimized their combustion processes to such an extent that often CO regulatory requirements are not an issue. 

The required targets and the timetable for NAAQS implementation are always changing and apply to each of the criteria pollutants. The EPA goes through a public consultation process to establish the required NAAQS levels for each criteria pollutant. The U.S. is divided into a set of areas, and the EPA performs measurements of the criteria pollutants in each area. Areas that do not meet the NAAQS targets for criteria pollutants are deemed nonattainment areas. For each nonattainment area, the affected state is required to prepare a state implementation plan (SIP) to resolve the issue and achieve attainment. The U.S. Northeast (Maine to Northern Virginia) is a special case. Because this air shed is highly populated, it has more stringent air quality standards. The EPA calls this area the Ozone Transport Region. 

When seeking an air permit for a new diesel emergency generator, if there is a NAAQS issue, it will most likely relate to NO2. In 2010, the EPA proposed limits based on an hourly worst-case scenario of 100 parts per billion. It is not uncommon, during certain times, for background concentrations in nonattainment areas to be high enough that very little NO2 needs to be added to make an installation exceed the limit. Prior to 2010, the NO2 limit was based on a yearly average. 

By mid-2013, each state was to have submitted a SIP for its nonattainment areas with respect to NO2. When a major data center, hospital, or other installation installs significant capacity of new diesel standby generators, the typical hourly worst-case scenario occurs during the full load test of the units. Modeling is done of the site, typically using the EPA’s atmospheric dispersion modeling (AERMOD) system. AERMOD is a mathematical simulation of how pollutants will disperse into the atmosphere. The modeling takes into account the topography of the site, its major emissions sources, prevailing wind conditions, and other factors that could lead to worst-case conditions.

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