Stationary internal combustion engines (ICE) remain one of the most frequently permitted emission source types in Maricopa County. This article reviews engine-related air permitting considerations and reviews common pitfalls when permitting engines with the Maricopa County Air Quality Department (MCAQD). This includes the General Permit renewal and BACT requirements, with particular attention to requirements for emergency generators.
General Permit Expiration and Revision
MCAQD issues engine-specific general permits, as allowed by Rule 230, under an Emergency Internal Combustion Engines General Permit. This general permit authorizes facilities through an Authority to Operate (ATO) framework where standardized, general permit conditions are combined with a facility-specific ATO sheet in any issued permit.
The Stationary Emergency ICE General Permit (GP) was just revised on April 10, 2026 (https://www.maricopa.gov/DocumentCenter/View/16988/Emergency-ICE-General-Permit-PDF ). Facilities authorized under this general permitting program prior to the revision date were required to apply for a permit renewal, submitted through the AQD Online Portal, before April 10, 2026. Paying the annual administrative fee associated with this general permit does NOT constitute a renewal, as a separate renewal application is required. Renewals are free and require no attachments under standard circumstances. Prior to submitting the renewal application, it is important to verify that facility contact information in the portal is current as MCAQD distributes renewed ATOs by email only.
The new GP includes several changes throughout the 30-page document. If you were previously authorized under the program, review the new GP carefully to identify the changes that will matter for your operations. These include recordkeeping and reporting requirements, affirmative defense provisions, and timing requirements for renewal when the current GP expires in five years. There is also a new section detailing requirements associated with engine operation to support utility grid stability. Additionally, the applicability of the GP has changed and may broaden the range of facilities that could apply for coverage.
The 2026 Emergency ICE GP governs engines used solely for emergency or standby operation. Such engines qualify for the new GP if they are at facilities with total potential NOx emissions of no more than 24.9 tons per year from the combination of external combustion units (including heaters and boilers) plus emergency ICE, under one of the following options:
- Option 1: The combined rating of all stationary emergency ICE does not exceed 2,735 brake horsepower (bhp) AND the combined rating of all external combustion units at the same facility does not exceed 6.0 MMBtu/hr.
- Option 2: The total combined NOx emissions of emergency engines and external combustion units are less than 24.9 tons per year based on a demonstration assuming 500 hours of operation per year for the emergency engines, and 8760 hours of operation per year for the external combustion units. ICE equipped with selective catalytic reduction shall estimate NOx assuming an emission factor of 0.0011 lb NOx/hp-hr.
Engines at certain facilities, including those that have been issued Emission Reduction Credits and facilities with rotary engines/turbines that are not insignificant activities, are not eligible for the GP.
Key conditions in the general permit include a 500-hour total operating hour limit per engine per 12-consecutive-month period, a 100-hour annual limit for reliability testing and maintenance, a strict prohibition on peak shaving, and a non-resettable hour meter requirement to verify compliance.
BACT Requirements Under Rule 241
While BACT requirements have not undergone any recent updates, this article reflects on considerations for engines specifically, as numerous industries set their sights on expanding into Maricopa County. BACT is required under Rule 241 when a new source or modification exceeds minor NSR emission thresholds: 40 tpy for NOx, VOC, or SO2; 15 tpy for PM10; 10 tpy for PM2.5; and 100 tpy for CO. For engine operators, BACT is commonly triggered upon installation of new engines such that facility-wide potential emissions push above one or more of these thresholds.
MCAQD provides three routes for satisfying a BACT obligation:
- Option 1: Applicants may consult the MCAQD BACT Clearinghouse, which lists approved control technologies for commonly permitted source categories.
- Option 2: Applicants may adopt BACT already determined for the same or similar source category by one of three California air quality management districts: the South Coast AQMD (SCAQMD), San Joaquin Valley Air Pollution Control District (SJVAPCD), or Bay Area AQMD (BAAQMD). Selecting an established California District determination allows the applicant to forgo a full top-down analysis.
- Option 3: Where no Clearinghouse or California District determination applies, a full top-down BACT analysis is required.
SCR Requirement for Emergency Engines ≥ 1,000 BHP
For large emergency engines subject to BACT, compression ignition (CI) engines are required to meet EPA Tier 4 Emission Standards. California District BACT workbooks, which MCAQD expressly relies upon as the shortcut alternative to top-down analysis, treat Tier 4 certification as achieved-in-practice BACT for NOx and PM in this size class. For engines at or above 1,000 bhp, Tier 4 standards can either be met through purchase of Tier 4 certified engines, or installation of aftermarket selective catalytic reduction (SCR) to control NOx to be in compliance with the Tier 4 standard.
BACT Requirements for Emergency Engines < 1,000 BHP
For emergency CI engines below 1,000 bhp, installing an engine certified to the emission standards in New Source Performance Standard (NSPS) Subpart IIII for CI engines or NSPS Subpart JJJJ for spark ignition (SI) engines satisfies the BACT requirements. For CI engines, Tier 2 certification typically satisfies BACT without requiring additional controls such as SCR. MCAQD retains the authority to reconsider and update what is considered BACT, however, so if you are concerned about what will be required of you we recommend discussions with MCAQD to ensure your proposed BACT will be acceptable.
Takeaways
Understanding engine permitting requirements remains a common pitfall for many facilities. On top of the complex federal regulations, it is important to understand local regulations such as the ones discussed in this article. Knowledge of local regulations can prevent unexpected delays or costs.
The most recent updates to the federal rules are discussed here at https://trinityconsultants.com/resources/epa-issues-stationary-reciprocating-internal-combustion-engine-rice-rule-revisions, and Trinity’s highly regarded classroom training on federal engine requirements will be offered in New Mexico in May: https://shop.trinityconsultants.com/training/158147/Understanding-Engines:-Their-Emissions-and-Your-Compliance-Requirements .
If you have any questions on Arizona engine permitting requirements, please contact Trinity’s Phoenix office at 602.274.2900.
