The Engine Rules: More Key Elements

In the Fall 2014 issue of Environmental Quarterly, the article "The Engine Rules: Ten Key Elements" noted the voluminous rules and guidance issued by EPA regarding permitting and compliance with the engine rules (e.g., 40 CFR Part 63, Subpart ZZZZ) and provided clarification surrounding several confounding issues (see page 9 for summary). This article addresses several additional elements related to these complex rules and EPA's interpretation thereof.

Understanding Rich Burn and Lean Burn…EPA's Way

Technically (but not regulatorily), the difference between rich burn and lean burn combustion depends on which side of the ideal air-to-fuel ratio (AFR) line the engine operates. The ideal, or "stoichiometric", AFR for any fuel can be calculated using a simplified combustion chemistry equation and the molecular weights, in pounds per mole (lb/mol), of carbon, C (12); hydrogen, H (1); oxygen, O (16); and nitrogen, N (14). An example for methane, CH4, is shown below.

CH4 + 2O2 + 7.5N→ 2H2O + CO2 + 7.5N2 

In this equation, the methane weighs 16 lb and the air (oxygen plus nitrogen) weighs 274 lb, resulting in an ideal AFR of 17.1.

Each fuel has a different AFR. For example, a typical average diesel formulation has an AFR of 14.6. In order to discuss combustion generically, lambda, λ = 1 is used to represent the ideal AFR for any fuel. Any λ value less than 1 (i.e., for methane, an AFR of less than 17.1) represents rich burn conditions, and any λ value greater than 1 (meaning that more air is present than the ideal AFR calls for) represents lean burn conditions. This is shown graphically below.

 EQ Summer 2015 - Engine rules- Fig. 1 alt

The reason for the divorce between technical and regulatory definitions is that non-selective catalytic reduction (NSCR), a.k.a., three-way catalyst, which is applied to rich burn engines only, can operate with high efficiency even with some excess air (approximately 2 to 4 percent excess oxygen) present.

Non-certified Parts for Certified Engines

Some engine manufacturers assert that, for certified engines, all maintenance/replacement parts (e.g., spark plugs) must be purchased from the certifying manufacturer or else the certification is voided. While this may be ideal, EPA seems to indicate that it is not required. A 2007 regulatory preamble states that EPA "disallows tampering (including the installation of clearly inadequate replacement components), but EPA does NOT require the use of original OEM parts or certified components when replacing emission-related parts."

Fire Pump (NFPA) Certifications v. EPA-manufacturer Certifications

Caution should be used when reviewing whether an engine is properly certified for compliance with the engine rules, as more than one certification could be required. The engine rules establish different requirements depending on whether an engine is certified as a National Fire Protection Association (NFPA) fire pump engine. While the requirements for a fire pump engine may be different, purchasing a fire pump does not necessarily preclude the need to purchase an engine that has been certified to meet the emission standards established in the engine rules. An owner/operator of a fire pump engine should ensure that proof of certification to the relevant emission standards as well as the NFPA fire pump certification are readily available (e.g., included on engine nameplate or included in owner's manual).

EQ Summer 2015 - Engine rules- Fig. 3

Controlling to Tier Standards Not Allowed / Flex Engines

Be aware that overuse of a stationary emergency engine could trigger non-emergency engine status and non-emergency emission limits. Typically, when more stringent requirements are triggered under Clean Air Act programs, the owner could add controls to meet the more stringent requirements. However, EPA has clearly disallowed this strategy for engines, dictating that compliance must be demonstrated by purchasing an engine certified to the appropriate Tier emission standards.

Also, it is important to have an individual designated as the expert on stationary engines purchased/operated at a facility. Some engine dealers and manufacturers may claim that facilities can demonstrate compliance with NSPS IIII by purchasing what are known as "flex" engines (described in 40 CFR 1039.625(e)(2)). These standards are less stringent, and they apply only to non-road/mobile engines. EPA has been explicit that "flex engines are not allowed for stationary applications."

Addressing Related Modeling Issues

While not directly related to NSPS and NESHAP regulations, modeling is a critical element to many air permitting programs. Modeling of engines is often challenging because short, horizontal exhausts result in large fence line concentrations, especially for sources near a fence line, which is also often the case for engines.

Many regulatory authorities allow "quick and easy"EQ Summer 2015 - Engine rules- Fig. 4 screening modeling, but sometimes those models are so conservative that passing is nearly impossible. For the same scenario of engines, a screening model can predict impacts at least 50 percent greater than a refined model, primarily because a screening model adds together the peak impacts for each source (red and gray lines in the figure on the right) to estimate an aggregate maximum impact (blue line). This methodology ignores the likelihood that each source's peak impact is occurring at different locations and times.

The problem has been exacerbated by the continual ratcheting down of ambient air quality standards. NO2 is the most problematic pollutant when modeling engines. The current National Ambient Air Quality Standard (NAAQS) for NO2 is 188 micrograms per cubic meter (µg/m3) on a 1-hour basis, which is many times more stringent than the annual-average standard that was in place for many years prior to the 2010 update.

When modeling for NO2, it is important to remember that refined modeling can account for the fact that, for most engines, the amount of NOX emissions in the form of NO2 is quite small (NOX from combustion is often dominated by NO, not NO2). Various techniques are available as model refinements, e.g., Ozone Limited Method (OLM), Ambient Ratio Method (ARM), and Plume Volume Molar Ratio Method (PVMRM), all of which require additional model inputs such as in-stack NO/NO2 ratio and therefore are normally used only after careful review of the myriad of guidance available and/or negotiations with the regulatory agency.

Unfortunately, in some situations model refinement is simply not enough, and changes to emissions/controls, adjustments to exhaust parameters, relocation/rearrangements of sources and buildings, or even replacement of engines may be necessary.

Although initial compliance dates for the engine rules have passed, ongoing compliance is essential. Recent enforcement activity, particularly regarding state permitting provisions, indicates an attention to engines that was largely absent previously. For assistance related to the engine rules, contact your local Trinity office at (800) 229-6655.