Emission Standards for Stationary & Mobile Generators

Since the mid 20th century, the U.S. government has taken steps to reduce air pollutants. The Clean Air Act of 1963 was the first federal legislation aimed at monitoring and controlling air pollution, followed by the Clean Air Act of 1970 which limited emissions from both stationary sources and mobile sources. It also started four major programs for regulating stationary emission sources: 

• The National Ambient Air Quality Standards (NAAQS)
• State Implementation Plans (SIPs)
• New Source Performance Standards (NSPS)
• National Emission Standards for Hazardous Air Pollutants (NESHAPs)

Today, information about these programs is found in the Environmental Protection Agency federal regulations. These rules guide the answer to a common question: How do you comply with EPA standards for generator emissions?

Understanding EPA Generator Emission Standards

Emission levels for generators vary based on how a generator engine is used: Engines can be either nonroad or stationary engines. 

Nonroad engines are any portable engines, other than in cars and motor vehicles. Stationary or standby engines are engines that will remain at one location for 12 months or more. Generators that fall into this category are then broken into subcategories for new or existing and emergency or non-emergency engines.

Manufacturers classify different stationary engines with ratings based on how they will be used, their total power load, and their operation time. These three ratings are:

• Standby rating: primarily used in stationary emergency applications and applies to variable loads with an average load as a factor of the standby rating, with 100% of the rating available for the duration of a power outage
• Prime rating: primarily used in non-emergency applications and applies to varying loads and unlimited operating hours
• Continuous rating: used for non-emergency applications where the generator set is used to supply constant or non-varying load for unlimited hours, and the continuous rating is equal to the generator’s average power

Lastly, engines are further segmented based on their method of ignition. Engine ignition types are compression ignition, which use diesel fuel, and spark ignition, which can use gas including natural gas, landfill gas, gasoline, propane and others. Both compression and spark ignition engines are required to meet the U.S. EPA New Source Performance Standards (NSPS), which regulate emissions based on the manufacturing date of the engine and its maximum horsepower rating. 

The EPA has developed a series of summary tables to help determine if generator emissions are compliant with the regulations for stationary engines, with additional requirements for maintenance and reporting.

Determining Diesel Generator Emission Standards

In the case of compression ignition engines, the EPA implemented “progressive” regulations (called Tier levels) for diesel generator emission standards in the 1990s. These levels became more stringent over the intervening years, and they substantially lowered levels of air pollutants for both nonroad and stationary engines. 

Today, new non-emergency and nonroad diesel engines are designed to meet Tier 4 emission rates and often use the most advanced emission control technologies. Stationary emergency engines have varying EPA emission requirements based on their maximum horsepower, and they are less stringent because their operation time is typically only during power outages, generating less pollution than non-emergency generators.

Emergency Generators Can Require Permits as Sources of Pollutants

Permitting is required under the federal Clean Air Act Permit Program (CAAPP) for new sources of air pollutants, for any stationary-non emergency unit with a prime or continuous rating, and for major modifications to existing sources of air pollutants. Permits are often also issued by state or local air pollution control agencies. 

Buckeye Power Sales distributes, installs and services stationary and mobile generator sets in three states: Illinois, Indiana and Ohio. These states each have different permitting requirements for new or existing generators.

Ohio State Air Permits

The Ohio EPA has an optional provision in the state code that only applies to low-emitting air pollution sources, like emergency electrical generators, called a permit-by-rule (PBR). A PBR exempts the source from the formal permit process, and instead effectively functions as both an installation and operating permit for the source. However, the state PBR rule does not exempt any source from the U.S. EPA requirements, and under PBR, emergency generators must maintain records logging operation time and the type of fuel used.

Illinois State Air Permits

The Illinois Environmental Protection Agency (IEPA) requires equipment that emit any air pollutants be evaluated for Air Permits. The state issues both construction permits and operating permits. Construction permits are required before beginning construction of an emission source or air pollution control equipment, and operating permits are required to run that source. Though there are exceptions to the permitting rules, like Ohio, sources are still required to comply with the U.S. EPA regulations.

Indiana State Air Permits

Air permits are also required in Indiana by the Indiana Department of Environmental Management (IDEM), but their requirements are different. They are issued holistically based on the total, combined potential to emit (PTE) air pollution of all stationary air polluting equipment at a single property or source — not for each individual piece of equipment. 

The PTE is based on a worst-case calculation, assuming all equipment will operate at maximum capacity for 24 hours a day, 365 days a year or for the 500 hours per year emergency use engines are allowed to operate, per the U.S. EPA. If the PTE of a single pollutant for all equipment meets or exceeds the threshold, then a permit is required. And regardless of whether a permit is required, U.S. EPA regulations still apply for all engines.