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Air Quality Permitting of Emergency Generators Located at Northern Virginia Data Centers

Posted: December 1st, 2020

Authors: All4 Staff 

Because of the infrastructure put into place decades ago, the Northern Virginia region has developed into a hub for data centers.  Due to the critical nature of data centers remaining operational at all times, their presence naturally brings along with it a sizable influx of emergency generators (EGs).  In fact, it’s not uncommon to see a new data center campus install 50-100 EGs.  Existing facilities may seek to add additional EGs in order to increase the capacity of their facility and serve additional customers.  As these facilities continue to be built and expanded, the Northern Regional Office (NRO) of the Virginia Department of Environmental Quality (VADEQ) has added some requirements to the air permit application process and created new boilerplate language for permits.  Data centers need to be aware of these changes, as some of these changes can delay obtaining a permit and the installation and operation of EGs if they are not accounted for at the beginning of a project.  The air permitting changes are primarily applicable to EGs at data centers in the Northern Virginia region; however, some of the requirements may be included in new air permits for EGs at non-data center facilities in the Northern Virginia region.  EGs located at facilities outside the Northern Virginia region may see similar changes included in their air permits at some point in the future, but this is not the case at the time of publication.  A more detailed look at what’s included in these changes is provided below.

Guess Who’s BACT

When a project involving the installation of EGs requires air permitting due to the increase in potential to emit (i.e., the maximum amount of emissions from the operation of a facility’s equipment), facilities must use the Best Available Control Technology (BACT) to control emissions from new equipment.  In the past, when it came to demonstrating that an EG would be using BACT for the control of NOX emissions, the “standard procedure” was to do a top-down BACT analysis. This involved identifying available control technologies, evaluating technical and economic feasibility of each technology, and then using that information to determine the highest level of control available that would still be cost-effective to install and operate.  The exception to this was if the maximum NOX emissions rate for an EG was no greater than 6.0 grams per brake horsepower-hour (g/bhp-hr), as this was considered to be presumptive BACT, and a top-down BACT analysis for NOX would not be required.  Often the top-down BACT analysis would result in add-on control technologies being found technically feasible but economically infeasible.  This would then result in Good Operating Practices (e.g., following manufacturer recommendations and using ultra-low sulfur diesel fuel) being selected as BACT and memorialized as air permit conditions, even if the maximum NOX emissions rate was greater than 6.0 g/bhp-hr.  However in the last couple of years, VADEQ NRO has determined that, given the number of EGs that can meet the 6.0 g/bhp-hr presumptive BACT limit, approval for a higher hourly limit would be rare and all new data center EGs must not exceed the 6.0 g/bhp-hr NOX emissions level.  EGs installed at data centers can be quite large, and it may be difficult to find larger EGs where the not-to-exceed data from the manufacturer does not exceed 6.0 g/bhp-hr for NOX.  This doesn’t necessarily mean there is no way to obtain an air permit for these EGs.  There are ways to meet this BACT limit, but coordination with the engine manufacturer is needed.  These options include:

  • Setting the presumptive BACT limit of 6.0 g/bhp-hr as the engine NOX limit (with assurances that the engine will pass a stack test to minimize risk of non-compliance),
  • Programming the engine with a U.S. EPA-approved NOX optimized fuel curve to reduce emissions to meet the 6.0 g/bhp-hr limit, or
  • Accepting an output capacity limit on the engine.

Data centers should be aware of the BACT limitation prior to applying for a permit with VADEQ so that changes do not have to be made to the engine during the permitting process.  It is beneficial to complete an emissions analysis prior to permitting to see if the engine chosen would require a change to meet the presumptive BACT limit.

Don’t Test Your Limits, Limit Your Tests!

Recent permits have included more stringent limits on scheduled maintenance checks and readiness testing activities (MC/RT), including specific emissions limits for these types of activities as a subset of the overall emissions limits for the EGs.  In order to set limits, VADEQ may request an approximate schedule of planned facility maintenance, such as monthly load testing for EGs and annual blackout tests.  This will be requested during the permit application process to ensure the limits will be acceptable to both the facility and to VADEQ.  Because the manufacturer recommendations for MC/RT activities should be used to determine the schedule of planned facility maintenance, VADEQ may also ask for a copy of the recommendations as well.

It’s the Reason for the (Ozone) Season

Because of the impact strong sunlight has on the formation of ground-level ozone, the concentration of ground-level ozone typically peaks during the warmer months of the year.  The emissions produced by EGs also contribute to this process.  Therefore, between May 1 and September 30 each year, the newer EG permits restrict facilities from performing non-emergency activities (such as scheduled maintenance, readiness testing, stack testing, or operator training that involves fuel combustion) between the hours of 7 AM to 5 PM unless a variance is received from the VADEQ Air Compliance Manager.  This includes running the EGs for initial testing and commissioning activities (referred to as the Integrational Operational Period).  However, recent permits have language specific to the Integrational Operational period that allows for daytime operation without petitioning VADEQ, as long as the Air Quality Index (AQI) for that day is less than or equal to 100.  If a facility chooses to perform these activities during the hours of 7 AM to 5 PM between May 1 and September 30, the permit will require them to keep additional records of these activities and measured and forecasted AQI conditions.

If you have questions about air permitting or compliance for EGs at data centers in Northern Virginia, please reach out to our team at info@all4inc.com or 610-933-5246



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