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Bay Area AQMD Emergency Engine BACT Ruling

Posted: June 15th, 2021

Authors: Graham B. 

On December 22, 2020, the California Bay Area Air Quality Management District (BAAQMD) adopted a new Guideline Document that revised the Best Available Control Technology (BACT) emissions standards found under BAAQMD Regulation 2, Rule 2 for emergency standby-power diesel-fired engines (emergency engines).  The guideline requires emergency engines larger than 1,000 brake horsepower (bhp) to meet U.S. Environmental Protection Agency (U.S. EPA) Tier 4 Final (Tier 4) emissions standards as BACT.  The Tier 4 emissions standards are defined in 40 CFR Part 89 (Control of Emissions from New and in-Use Nonroad Compression-Ignition Engines).  With this revision to Regulation 2, Rule 2, any new or modified source that results in an increase in emissions of ten or more pounds per day (lb/day) of any district BACT pollutant (as defined in BAAQMD Regulation 2-2-210) is subject to BACT review.  For emergency engines, the district pollutants that most commonly trigger BACT review are nitrogen oxides (NOX) and particulate matter (PM).  The most common methods for meeting Tier 4 emissions standards for NOX and PM are the use of selective catalytic reduction (SCR) and diesel particulate filter (DPF) control equipment, respectively.

BACT Guideline

Previously, emergency engines in BAAQMD were only required to meet Tier 2 emissions standards, consistent with U.S. EPA requirements.  With this change, facilities that want to install emergency engines larger than 1,000 bhp will need to purchase U.S. EPA Tier 4 emissions certified engines or U.S. EPA Tier 4 emissions compliant engines, which can be done by purchasing Tier 2 engines with aftermarket air pollution control equipment installed.  These changes will apply to any permit application for an emergency engine deemed complete after January 1, 2020.  This means that even if you submitted an application for an air permit (referred to as an Authority to Construct) before this guideline was adopted on December 22, 2020, Tier 4 emissions certified or equivalent engines are required if your application had not been deemed complete prior to the January 1, 2020 adoption date.  Furthermore, BAAQMD will not refund the cost of the initial fee and filing fee if an application is withdrawn.

Impacts to Data Centers (and Other Facilities)


Data centers play a crucial role in the day-to-day operations for thousands of businesses across the United States.  Most businesses rely on the internet, and therefore a data center, for their day to day operations.  Because of the services that data centers provide, they require electricity at all times.  Network and server processes, equipment cooling, and facility operations all require an uninterruptible power supply (UPS).  Normally, this electricity is supplied by electric utilities, but during emergencies (i.e., when electrical service from the servicing utility is interrupted by an unforeseeable event) other sources must provide the electricity.  This is often achieved by the use of emergency engines – and a lot of them.  ALL4 frequently permits as many as 50 emergency engines at individual data centers in one initial application, and although this requirement will affect any facility with an emergency engine larger than 1,000 bhp, the number of engines typically seen at data centers complicates the process of making the necessary changes to comply.  Other types of facilities ALL4 frequently works with when permitting emergency engines include hospitals, universities, distribution centers, and retail stores.  Remember, any emergency engine larger than 1,000 bhp in BAAQMD will now need to meet Tier 4 emissions standards, not just engines located at data centers.

Project Costs and Other Considerations

Purchasing, installing, and maintaining 50 or more emergency engines is expensive, and for someone managing these projects the first question that comes to mind is likely: will the requirement to purchase Tier 4 engines increase project costs?  The short answer is yes.  With the requirement to purchase Tier 4 engines, there are additional costs one must consider when installing emergency engines.  As mentioned previously, the most common method of meeting Tier 4 NOX and PM emissions standards is through the use of SCR and DPF control equipment, respectively.  The cost of turnkey installations (i.e., packaged, ready to operate upon installation) of Tier 4 emergency engines will vary wildly but is more expensive than installing Tier 2 engines that lack SCR and DPF control equipment.  For aftermarket SCR and DPF control equipment installations on 2-Megawatt (MW) Tier 2 emergency engines one can expect a capital cost of approximately $220,000, but this too can vary.  Additionally, proper operation of SCR control equipment requires diesel exhaust fluid (DEF), a mixture of 32.5% urea and 67.5% water that is injected into the engine exhaust stream to ultimately convert NOX to gaseous nitrogen (N2) and oxygen.  DEF needs to be in a storage tank that is cool, dry, well-ventilated, out of direct sunlight, and kept between a temperature of 10 and 77°F.  DEF has a shelf life of approximately two years when stored under these conditions, meaning facilities will need to determine the appropriate storage tank size and the amount of DEF stored on-site at any one time.  Facilities must have enough DEF on-site to ensure proper operation of the control equipment in the case of an emergency, but if the engines are only being run for maintenance checks and readiness testing (MC/RT) then the owner runs the risk of DEF spoiling from being stored too long.

Are other Counties/States likely to follow suit?

As of June 15, 2021, two other agencies have adopted the same rule requiring Tier 4 emissions standards be met as BACT for emergency engines: Sacramento Metro Air Quality Management District (SMAQMD) in California and Maricopa County Air Quality Department in Arizona.  However, it is possible that other agencies will adopt the same or similar requirements in the future, so it is important to consider this when planning projects.

If you have questions about air permitting or compliance for emergency engines now subject to Tier 4 emissions standards, please reach out to Graham Brittain at gbrittain@all4inc.com.


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