It should be noted that DEQ acknowledges that there will be additional costs and complexities to apply the new presumptive BACT to modified gen-sets that were previously permitted as emergency units and will remain as emergency units. As such, DEQ will not require application of the new presumptive BACT to the modified emergency gen-set that will remain emergency gen-sets, however, if unit is replaced or a new unit is installed, the new presumptive BACT will apply.
Additionally, the application of the new presumptive BACT will be limited to engines that do not 1) serve as secondary or tertiary backup power sources or 2) provide power to non-server resources at datacenters.
The proposed revision to APG-576 requires gen-sets equipped with SCR at datacenters to meet specific operating parameters as follows:
- Temperature probe to continuously monitor the Catalyst bed exhaust temperature
- Urea or ammonia solution injection when engine operating at or above 20% load and the catalyst bed temperature is at least 570°
- Catalyst bed temperature not to exceed 930°
- For open loop systems, urea or ammonia injection rate.
- For closed loop systems, NOX concentration (ppm) measured after the catalyst.
The proposed revision to APG-576 also formalize DEQ’s expectation that typically 20-25% of the units in each type of gen-site permitted at a datacenter is expected to be stack tested.
The guidance also makes clear that applicant who want to propose controls that are less stringent than the proposed presumptive BACT will need to submit a full top-down BACT analysis and demonstrate that their site-specific circumstances are so unique that costs are substantially greater than those typically incurred across the data center industry.
The proposed BACT requirements significantly changes air permitting strategy for data centers. Since the same control technologies are required regardless of emergency classification, permitting generators as non-emergency units under Virginia regulation (still qualifying as emergency under federal regulations) no longer results in additional BACT obligations. As a result, facilities may include limited non-emergency operation, such as testing, commissioning, or up to 50 hours per year of non-emergency use, without triggering new control requirements. Operational limits are instead addressed through enforceable throughput limits, monitoring, and recordkeeping. DEQ has indicated that application submitted on or after July 1, 2026 are expected to reflect this approach by permitting most backup generators as non-emergency units.
The proposed APG-576 Diesel Engine-Generator Set Procedure for Writing New and Modified Permits guidance was published in the Virginia Register on December 29, 2025 and can be found here. Please contact Susan Barnes at Trinity’s Washington, D.C. office (240.379.7490) or Farshid Kiani at Trinity’s Virginia office (540.400.8041) if you would like to understand the impact of this rule on your operations or would like assistance with submitting any comments on the proposed guidance document.
