Critical operations demand continuous power, but siting, selecting, and maintaining emergency generators requires a full understanding of of the Clean Air Act. Air pollution rules for reciprocating internal combustion engines (RICE) are poorly written and confuse even the most experienced environmental engineer. Installing backup engines might seem like a simple situation, but projects can quickly stumble into severe limitations. Avoid an overly complicated air permit with a basic understanding of the applicable regulations.

Step one is to know which regulations apply to your project. Rules vary based on the size, age, fuel, location, and quantity of generators in the project. Gather the following information. 

  • Is your site in a county designated as non-attainment for air quality? If air quality is bad, more limits apply to new combustion equipment.
  • Does the site already have sources of air pollution? Major existing facilities are only allowed small increases in emissions before triggering severe limits. Regulations give small or new facilities more flexibility.
  • Will all engines be new? Rule applicability for engines manufactured before 2010 requires a complex analysis to determine the appropriate limits. 
  • What fuels will be used? Limits differ between diesel-fired (compression ignition) and natural gas-fired (spark ignition) engines.
  • Why will the RICE operate? Engines which operate only for testing, maintenance, and in actual emergencies are considered “emergency” engines. However, if operation includes peak shaving, selling power to the grid, or running to save money in times of high energy prices, then an entirely different set of rules apply, which can, in turn, require much lower emission limits and add-on control devices.

Too many combinations of answers to the above questions exist to detail here. Instead, let’s explore the most common and simplest situation: a greenfield project of less than 10 diesel-fired, RICE for emergency use only located in a county with good air quality and no other emission sources on-site. The engines are subject to New Source Performance Standard (NSPS) Subpart IIII under 40 Code of Federal Regulations (CFR) Part 60 and must be certified by the manufacturer to meet Tier 2 emission standards (which vary on engine size). Operation for testing and maintenance will be limited to 100 hours per engine for every 12-month rolling period. Operation in an actual emergency, such as when electric power is not available from the local utility, is unlimited. Diesel fuel must qualify as ultra-low sulfur (<0.0015% sulfur). Power cannot be sold to the grid and engines cannot be run to save on electricity costs or as part of a financial arrangement. Records must be kept, and a preconstruction permit from the state is likely required.


generator tracking form
FIGURE 1: A sample generator tracking form


Vary outside these assumptions, and permitting becomes more complicated and situation-specific. For example, many large utilities entice RICE operators into peak-shaving agreements. The power company pays the facility to allow remote operation of the engine during period of high electricity demand. Power is sent from the facility to the grid, allowing the power company to avoid building an entire new facility for high demand times. However, diesel-fired engines operated for this purpose usually are required to meet the more stringent Tier 4 emission standards by using add-on control devices, such as selective catalytic reduction (SCR). Additionally, the rules do not allow the purchase of a Tier 2 engine retrofitted to meet Tier 4 standards. Controls must be installed by the original equipment manufacturer. 

When 10-plus engines are required for situations, such as data centers, total annual emissions from even emergency-only operation can easily exceed major source thresholds. At 100 tons per year (tpy) of nitrogen dioxides (NOx) a facility must obtain Title V operating permit. Likewise, for 10 tpy of a single hazardous air pollution (HAP), such as formaldehyde, or 25 tpy total of all HAPs, a Title V permit is required. In a non-attainment area, 100 tpy requires a New Source Review construction permit. These permit applications might trigger an air dispersion model to demonstrate that the facility emissions will not contribute to an exceedance of the National Ambient Air Quality Standards (NAAQS). Most RICE have short stacks, which allow little dispersion before emissions hit the ground. Multiply the impacts by 20 or 30 engines, and ground level NAAQS exceedances could occur. A computer model of the emissions allows the facility design and stack heights to be adjusted to allow sufficient dispersion of pollution, avoiding negative impacts on breathable air. Universally, a stack with a rain cap will create dispersion issues; use a tractor-flap-type exhaust instead. The worst-case and most expensive scenario is finding out after startup that your plant creates excess emissions outside your property line. A good dispersion model aids in plant layout and mitigates pollution impacts.

Regardless of the situation, all RICE must follow some common requirements:

  1. Install a non-resettable hour meter.
  2. Track hours of operations. See Figure 1.
  3. Maintain original equipment manufacturer certification of emissions limits (Tier level)

Air permitting considerations affect engine selection, site layout, and ongoing compliance burden. By understanding the applicable regulations, you can reduce the chance of noncompliance fines and post-construction design changes. Your project needs an effective layout with appropriately controlled engines to avoid pollution impacts. An upfront investment in air permitting mitigates your risk.