Home / News / Regulatory Updates / Bay Area Air District Requires Tier 4-Level Engines as BACT 

Bay Area Air District Requires Tier 4-Level Engines as BACT for Small- to Medium-Sized (<1000 hp) Emergency Engines

February 28, 2025

As of December 2, 2024, any permit application for a diesel emergency standby engine rated 50 brake horse-power (BHP) or greater now triggers Best Available Control Technology (BACT) requirements to meet Tier 4 emissions levels for combustion con-taminant emissions of nitrogen oxides (NOx), carbon monoxide (CO), precur-sor organic compounds (POCs) (non-methane hydrocarbons, or NMHCs), and particulate matter, as set by the U.S. EPA. The BACT and BACT for Toxics (TBACT) requirement is now Tier 4 and is consistent for all standby diesel generators 50 BHP and greater. BACT is required when a source of emissions has the potential to emit 10 or more pounds per day of a pollutant. TBACT is required for sources with an associated cancer risk greater than 1.0 in one million and/or a chronic hazard index greater than 0.20.

There are three ways that an engine may meet Tier 4 standards:

1. Be certified by the U.S. EPA to meet Tier 4 standards;
2. Be packaged by the equipment manufacturer with abatement equipment that brings the emis-sions levels to Tier 4 standards; or
3. Be a lower-tier engine with after-market abatement equipment from a third-party vendor.

All three methods are acceptable; how-ever, additional requirements, such as source testing, may apply if the engine is not certified to Tier 4 standards. Source testing may be required annu-ally, which is an additional cost that would be incurred by the operator. A retrofitted engine might also require usage restrictions on the hours of oper-ation for maintenance.

This change in BACT/TBACT levels applies to emergency engines, such as those that power backup generators or fire pumps. In cases where the equip-ment would need to meet National Fire Prevention Act (NFPA) Section 110, Level 1 (direct risk to human life) re-quirements, such as an engine driving a fire pump or used for backup gener-ation of medical life-sustaining sys-tems, an auxiliary emissions control device (AECD) may be installed on the Tier 4 engine to allow for the tempo-rary disabling of any emissions con-trols during a qualified emergency sit-uation. This is permitted per Code of Federal Regulations (CFR) Title 40, Chapter I, covering U.S. EPA and air pollution controls.

When choosing an engine for a project, these new limits should be taken into consideration. Limitations on the avail-ability of smaller (under 75 horse-power) Tier 4 engines for emergency use may require upsizing the engine to meet BACT requirements.

The Bay Area Air District has also pro-vided guidance on how to shorten the permitting time for these types of en-gines. Based on the engine’s proximity to a nearby receptor, location in an Overburdened Community, size, and hours of use, a health risk assessment (HRA) analysis may be avoided or “streamlined.” A streamlined HRA would still require public notice if the engine will be located in an Overbur-dened Community; however, it will shorten the overall permitting time from the typical full HRA.

___________________

Yorke Engineering, LLC has assisted over 2,000 industrial and government facilities with EH&S permitting and compliance throughout California. 

If you would like a proposal, click the button below.

View our informative archived Newsletters ...