Owners or operators planning to install or modify diesel-fueled emergency engines within the jurisdiction of the Bay Area Air Quality Management District (BAAQMD) must either ensure the engine meets EPA Tier 4 Final (Tier 4F) emission standards or submit the Authority to Construct (ATC) application on or before December 1, 2024.

Regulatory Basis

After December 1, 2024, all diesel-fueled emergency engines will be subject to Tier 4F emission standards to comply with the BAAQMD’s Best Available Control Technology (BACT) requirements. The BAAQMD previously determined in December 2020 that the Tier 4F BACT requirement only applied to diesel-fueled engines rated greater than 1,000 brake horsepower (BHP), citing multiple “achieved in practice” (AIP) examples within the BAAQMD and outside the state of California. During the latest informational webinar held on October 21, 2024, the BAAQMD announced that engines rated less than 1,000 BHP have also been demonstrated as achieved in practice. The AIP examples cited include thirty-five Permits to Operate (PTOs) issued within BAAQMD. BAAQMD also cited BACT requirements from other California air districts (e.g., Sacramento Metropolitan Air Quality Management District and San Joaquin Valley Air Pollution Control District), and the EPA RACT/BACT/LAER Clearinghouse. Based on how BAAQMD implements and enforces BACT, applications that trigger BACT requirements must at a minimum meet the technology determined to be AIP, regardless of the cost. Applications that do not meet AIP BACT requirements are at risk of being denied an ATC permit.

When Does BACT Apply?

BACT is trigged on a pollutant-specific basis for “District BACT Pollutants” defined as: precursor organic compounds (POC), non-precursor organic compounds (NPOC), oxides of nitrogen (NO_X), sulfur dioxide (SO₂), particulate matter with an aerodynamic diameter of 10 micrometers or less (PM₁₀), particulate matter with an aerodynamic diameter of 2.5 micrometers or less (PM_2.5), and carbon monoxide (CO). If the proposed new or modified source has the potential to emit 10.0 pounds per day or more of a District BACT Pollutant, the source is subject to the BACT requirements for that pollutant. It is therefore possible to trigger BACT for one District BACT Pollutant and not the others. BAAQMD is proposing three compliance options to meet the new Tier 4F BACT requirements and as shown in the webinar slides (slides 13 and 14), which include certified engines, compliant engines, and retrofit engines.

Compliance Options by BACT Pollutant and Considerations

Table 1 summarizes the BHP at which engines will exceed a daily potential to emit of 10.0 pounds per day using the BAAQMD’s BACT2 emission standard (Document #96.1.3, dated 12/22/2020) and assuming 24 hours per day of runtime per BAAQMD’s current policies. Table 1 also summarizes typical control technology used to meet the Tier 4F BACT.

Table 1. Minimum Diesel-Fueled Engine BHP Rating to Trigger BACT

<sup>1. Based on BAAQMD’s BACT Guidelines for Diesel-Fueled Emergency Engines <1,000 BHP (Document #96.1.3, dated 12/22/2020).
2. POCs are represented as non-methane hydrocarbon (NMHC), which is parsed out from the NMHC+NO_X standard assuming a 95% NMHC/5% NO_x ratio using BAAQMD’s engineering policy
3. The upcoming BACT requirement for CO aligns with lower EPA tiers (i.e., Tier 2, Tier 3, Tier 4 interim) and does not change technology needs. Certified diesel engines achieve nearly complete combustion, producing minimal CO.</sup>

For both compliant and retrofit Tier 4F engines (i.e., not manufacturer-certified), BAAQMD will likely require periodic monitoring via source testing or portable analyzers to demonstrate compliance with the Tier 4F emissions standards. Trinity’s considerations for meeting the upcoming BACT standards include the following:

• Procure an engine capable of meeting at least Tier 4F for NO_X standards;
• If the engine is rated at ≥787 BHP, meet the Tier 4F NMHC standard;
  • The typical technology used to meet NO_X and NMHC emissions standards is a selective catalytic reduction (SCR) system, which requires the system to utilize a diesel exhaust fluid (DEF) supply for abatement.
  • Trinity anticipates that the source test obligations will also require testing for the ammonia slip in the exhaust from unreacted DEF to prevent applicants from over-injecting DEF to get favorable NO_X results.
• PM BACT is not triggered for engines <1,000 BHP; however, PM from diesel combustion is considered a potent toxic air contaminant (TAC) in BAAQMD. As such, TAC emissions from these engines may require Best Available Control Technology for Toxics, or TBACT.
• TBACT for diesel-fueled emergency engines effectively requires meeting the Tier 4F PM standard.
• Applicants can avoid needing to source test for PM by procuring a CARB-verified Diesel Particulate Filter (DPF) which has been verified for the engine’s EPA Family Name.

As part of BAAQMD’s informational webinar, BAAQMD has introduced a Health Risk Assessment (HRA) Streamlining process which will allow single-engine applications that meet certain criteria (such as sufficient distance to nearby residences and business and/or annual hour restrictions) to bypass the full HRA process. The full HRA process is a time-consuming and resource-intensive exercise which requires air dispersion modeling and is often the bottleneck for permit issuance. The HRA Streamlining criteria is summarized on slide 17 of the webinar slides.

Frequently Asked Questions

At the end of the webinar, BAAQMD committed to publishing a Frequently Asked Questions (FAQ) document addressing the most common questions from participants. In the meantime, Trinity has summarized the responses that were provided during the webinar:

• Will applications that are currently undergoing District review be subject to the new Tier 4F BACT requirements?
  • No, all applications that are submitted before December 2, 2024, are not required to meet Tier 4F for BACT.
• Are existing engines subject to the new Tier 4F BACT requirements?
  • Existing permitted engines are not subject to the new Tier 4F BACT requirements. However, any modified engines or engine replacements will be subject to the new Tier 4F BACT requirements.
• How was the HRA streamlining table developed?
  • The HRA streamlining table is applicable for single engine applications. The table lists engine capacity ranges, engine distance from the nearest receptor (i.e., nearest residence or business), and annual operating hours that trigger an HRA.
    Whether the engine is located within an Overburdened Community (OBC) will also play a factor in qualifying for streamlining.
• In the HRA streamlining table, are the maximum hours allowed to avoid triggering an HRA emergency hours or only reliability related maintenance and testing hours?
  • The maximum hours allowed to avoid triggering an HRA in the HRA streamlining table are only reliability related maintenance and testing hours. BAAQMD excludes emergency hours from the HRA.

Trinity is Available to Help

If the proposed Tier 4F BACT requirement impacts your operations, Trinity is available to support, which may include the following:

• Provide subject matter expertise on recommended procedures for source testing and portable analyzer monitoring;
• Prepare expedited emergency engine applications, including evaluating HRA applicability and compliance;
• Prepare due diligence HRA demonstrations to evaluate the most favorable design for HRA compliance; and
• Address questions on implications of the upcoming BACT requirements for your facility.

If you are interested in any of the aforementioned support, please contact Emily Wen or Trinity’s Oakland office at 510.285.6351.