In Virginia, the air permit for a data center generator plant is now judged one engine at a time. On 4 November the State Air Pollution Control Board's new Article 7 of 9VAC5-50, "Emission Standards for Data Center Engine Generator-Sets", takes effect. The board adopted it on 18 August and it appeared in the Virginia Register on 5 October, according to the environmental consultancies Encino and Alliance Technical Group.
What the Rule Says
The rule carries out House Bill 507, now Virginia Code § 10.1-1322.6, which has applied to data center air permit applications filed since 1 July 2026. Virginia DEQ may not issue a permit, general permit coverage included, unless the emission limit for each engine-generator set is at or below that of a Tier 4 equivalent unit. A cap for the whole site does not satisfy it. A data center, for this purpose, is a facility mainly storing, managing or processing digital data that houses at least 1 MW of power and cooling equipment.
"Tier 4 equivalent" is defined by the control technology rather than a certificate: selective catalytic reduction or an equivalent for NOx, an oxidation catalyst for CO and a particulate filter for PM. Anyone proposing something else has to agree with DEQ how equivalence will be shown. DEQ can still ask for more under other state and federal requirements.
The Guidance Behind It
DEQ's companion guidance, APG-576, sets a presumptive NOx limit of 0.60 g/hp-hr for SCR-controlled units and calls for monitoring of catalyst temperature and reagent injection. As Data Center Knowledge reported in May, DEQ also dropped the old argument that low operating hours justify lighter controls, and expects most backup sets at data centers to be permitted as non-emergency units.
- Initial compliance tests for CO and NOx: three one-hour runs per pollutant on each unit tested, at above 90 % load unless DEQ approves testing across several load bands.
- A visible emissions reading to EPA Method 9 at the same load.
- Typically 20 to 25 % of the units of each generator type are tested; that does not waive the limit for the rest.
Not Everyone Is Happy
Compass Data Centers' chief executive Chris Crosby told Data Center Knowledge that Compass stayed with Tier 2 generators because SCR-equipped engines can slow down or shut down when the emission controls fail during an outage. On the other side, a Southern Environmental Law Center attorney called the rule an improvement but noted that Tier 4 units are allowed to run for non-emergency purposes, which may mean more running time. The Piedmont Environmental Council counts more than 11 GW of backup capacity at Virginia data centers. The Data Center Coalition cites a state legislative study (JLARC) finding that the generators usually run 10 to 30 minutes a month and account for less than 4 % of regional NOx.
What Lands on the Project Team
Two things get decided earlier than before. One is the exhaust aftertreatment on every unit, including its reagent storage and the way a failed catalyst or dosing fault is handled while the site is on generator power. The other is the test programme: a run above 90 % load on selected units means someone has to supply that load on site, and a load bank booked late is a classic reason for a commissioning slip. Crosby's point about shutdown on control failure is one to put to the engine supplier in writing before the permit application is filed.
Sources: Encino, Alliance Technical Group, Data Center Knowledge