Dear Mr. Faggert,

Virginia has drafted amended guidance proposed by DEQ (March 9, 2026) including provisions related to the testing of source emissions. Revisions to DEQ Guidance Memo APG-576 address presumptive Best Available Control Technology (BACT) requirements for emergency and non-emergency diesel engine-generator sets located at data center stationary sources.

Additional revisions will ensure Virginia maintains adherence to the emissions compliance goals while managing more efficient and accurate emissions assessments.  EMA encourages compliance with the most updated standards as technically appropriate for the given application and mission. 

The following is provided in summary with more detailed proposals included below:

1. Section B – Definitions and Section C – Applicability
   • Update proposed language for BACT applicability to the appropriate section
2. Section D – Emission controls and BACT under Article 6

1. • Avoid suggestion of technology-forcing requirements with specific components (e.g., DOC, DPF); it is critical to achieve specific emissions reductions
     • EPA requirements do NOT specify technology for achieving emissions limits
     • BACT should specifically clarify emissions limits (PM, CO, etc.)

2. • Align NOx limits with federal standards (engines >30 L/cyl)

3. Section F – Emission Limits / Calculations
   • Update language to align with current EPA referenced sections (e.g., 40 CFR Part 1039)
4. Section I – Testing Requirements (data centers) and Section J – Testing Requirements (non-data center source categories)
   • Retire visual emissions evaluations (VEE)
     • Tier 4-certified or compliant systems already certify to ensure compliance with PM limits
     • PM testing follows EPA 40 CFR 1065; accurate emissions sampling procedures specific for measurement of low emissions engines
     • EMA is working with EPA on CI NSPS regulation amendments to alternatively accept 40 CFR 1065 methodologies for stationary source testing
   • Exempt initial stack testing requirements for certified units
     • Performance of initial stack testing for certified engines is redundant as the system was already approved under strict test procedures by the EPA; testing would create emissions without benefit
       • Include language to exempt Tier 4 certified products from any site testing like some AQMDs in California (e.g., SCAQMD)
     • Initial stack testing for uncertified engines is appropriate but should consider potential updates to in-use testing process
       • Update procedures with alternative stationary source requirements under review by EPA
       • Defined modal test points – 90 to 100 percent
       • Shortened minimum test intervals – 15 minutes

EMA is reviewing proposed regulatory amendments to the CI NSPS with the EPA to more effectively support operation of engine-generator sets that meet specific testing requirements while demonstrating emissions performance via on-site testing. Proposals submitted by EMA to Virginia DEQ are consistent with those submitted to EPA and we encourage alignment between federal and state agencies.

• Support proposed alternative of accurate and efficient field testing requirements and methods; essential for Tier 4 certified engines
• Align requirements that avoid redundant stack testing for Tier 4 certified engines; provides no value while increasing site emissions

Respectfully,

The Truck and Engine Manufacturers Association (EMA)

Specific Draft Comments

Section B. Definitions

• Propose to remove the applicability note from the definition of “Data Center”
• Move applicability to section “C. Applicability”

Section C. Applicability

• Propose addressing applicability in section “C. Applicability” with additional clarification of applicability dates

“The data center specific Best Available Control Technology (BACT) provisions of this guidance document will be applied to all air permit applications submitted to DEQ on or after July 1, 2026. The data center specific BACT provisions of this guidance document do not apply to engine-generator sets for which air permit applications have been submitted prior to July 1, 2026.”

Section D. Emission Controls and BACT under Article 6

1. Particulate Emissions
   1. Emergency and non-emergency engines (located in data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Particulate emissions shall be controlled by a diesel particulate filter (DPF), or equivalent (as required to meet the applicable Tier 4 standard. If a diesel particulate filter (DPF) is required, it shall be equipped with a backpressure monitor that notifies the permittee when the high backpressure limit of the engine is approached.”

• Propose alternative language to ‘DPF-level control’ that is a non-specific limit; language should focus on meeting Tier 4 emission standards

“DEQ’s experience with the data center source category does support limiting the application of DPF-level control to T4 emissions standards to gen-sets that do not serve as secondary or tertiary backup power sources (i.e., backup other emergency gen-sets) or do not provide power to non-server resources at the site. Therefore, presumptive BACT...”

• Propose alternative language that focuses BACT to comply with Tier 4 emission standards rather than a specific technology

“However, any new or replacement gen-sets included in a project would be subject to the DPF (or equivalent) applicable Tier 4 PM emission standard presumptive BACT described in this section.”

“As defined in 9VAC5 Chapter 50, a BACT limitation is always a case-by-case determination… The evaluation would need to demonstrate that the site specific factors are so significantly different from the rest of the data center industry that the cost of DPF (or equivalent) control equipment required to meet T4 PM standards for the gen-sets covered by the application is substantially greater than the cost of DPF (or equivalent) control the equipment required to meet T4 PM standards for gen-sets included in the data center source category as a whole. A showing that simply indicates a “high” cost or average cost-effectiveness for DPF (or equivalent) systems emissions reduction systems should not be expected to succeed.”

3. For non-emergency engines (source categories other than data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Particulate emissions shall be controlled by a diesel particulate filter (DPF), or equivalent (as required to meet the applicable Tier 4 standard. If a diesel particulate filter (DPF) is required, it shall be equipped with a backpressure monitor that notifies the permittee when the high backpressure limit of the engine is approached.”

• Propose language that removes reference to ‘cost prohibitive’; language should focus on meeting Tier 4 emission standards

“If a DPF is too cost prohibitive or is not needed to meet the applicable Tier standard, then PM emissions shall be controlled by the use of good operating practices and performing appropriate maintenance in accordance with the manufacturer recommendations.”

2. Carbon Monoxide Emissions
   2. Emergency and non-emergency engines (located in data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Carbon monoxide (CO) emissions shall be controlled by a diesel oxidation catalyst (DOC), or equivalent (as required to meet the applicable Tier 4 standards.)”

• Propose alternative language to ‘oxidation catalyst-level control’ that is a non-specific limit; language should focus on meeting Tier 4 emission standards

“DEQ’s experience with the data center source category does support limiting the application of oxidation catalyst-level control Tier 4 emissions standards to gen-sets that do not serve as secondary or tertiary backup power sources (i.e., backup other emergency gen-sets) or do not provide power to non-server resources at the site. Therefore, presumptive BACT...”

• Propose alternative language that focuses BACT to comply with Tier 4 emission standards rather than a specific technology

“In general… gen-sets that were previously permitted as emergency units and that will remain permitted as emergency units continue to have a presumptive BACT equivalent to that specified for emergency gen-sets for non-data center source categories. However, any new or replacement gen-sets included in a project would be subject to the DOC (or equivalent) Tier 4 CO emission standards presumptive BACT described in this section.”

“As defined in 9VAC5 Chapter 50, a BACT limitation is always a case-by-case determination… The evaluation would need to demonstrate that the site specific factors are so significantly different from the rest of the data center industry that the cost of DOC (or equivalent) control equipment required to meet T4 CO standards for the gen-sets covered by the application is substantially greater than the cost of DOC (or equivalent) control equipment required to meet T4 CO standards for gen-sets included in the data center source category as a whole. A showing that simply indicates a “high” cost or average cost-effectiveness for DOC emissions reduction systems should not be expected to succeed.”

3. 3. For non-emergency engines (source categories other than data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

" Carbon monoxide (CO) emissions shall be controlled by a diesel oxidation catalyst (DOC). A DOC may be required for this type of engine in order to meet Tier 4 standards."

• Propose language that removes reference to being ‘cost prohibitive’; language should focus on meeting Tier 4 emission standards

“If a DOC is too cost prohibitive or is not needed to meet the applicable Tier standard, then CO emissions shall be controlled by the use of good operating practices and performing appropriate maintenance in accordance with the manufacturer recommendations.”

4. Nitrogen Oxide Emissions
   4. Emergency and non-emergency engines (located in data centers), Emission limit – 0.60 g/hp-hr

• Propose language that clearly specifies the load point(s) at which emissions limits apply; 90% engine-generator set load is proposed based on Section I: Testing Requirements
• Accurate measurement of this emissions limit is not achievable in the field with current reference methods; acceptance of 40 CRF Part 1065 compliant equipment – including PEMS – would support these detection limits

“a. Emergency and non-emergency engines (located at data centers): Emissions limit = 0.60g/hp-hr at 90% engine-generator set load.”

• Provide flexibility for stable catalyst bed temperature
• Propose clarifying language for load point to reflect the generator load; engine loads will be difficult to assess in the field
  • Propose load point to be changed from “…when the engine(s) are operating at or above 20% load…” to 25% of generator load

“(b)Engine exhaust gas shall be treated with urea or an ammonia solution when the engine(s) engine-generator set(s) are operating at or above 20% 25% load and the catalyst bed exhaust temperature of 570°F is achieved, except for periods of start-up or shutdown. The temperature is the preferred metric, but minimum load is available applicable if for some reason (e.g., certain Tier 4-certified units), the catalyst bed temperature is unavailable. Note that both the minimum temperature and load values are general estimates that should be revised if data specific to the SCR and/or gen-manufacturer, which may be higher or lower, is available.”

• Remove explicit catalyst bed exhaust temperature not otherwise specified by the manufacturer; retain limit (930 deg F) if not specified
• Define limit based on ‘exhaust temperature’

“(c)The permittee shall operate the gen-sets and SCR systems such that the catalyst bed exhaust temperature specified by the manufacturer is not exceeded. does not exceed 930°. 930°F may be used in the absence of more specific data from the SCR and/or gen-set manufacturer.”

5. 5. Emergency engines (source categories other than data centers), Emission limit – 6.00 g/hp-hr

• Propose language that clearly specifies the load point(s) at which emissions limits apply; 90% engine-generator set load is proposed based on Section I: Testing Requirements

“b. Emergency engines (source categories other than data centers): Emission limit = 6.0 g/hp-hr at 90% engine-generator set load.”

6. 6. Non-emergency engines (source categories other than data centers), Emission limit – 0.60 g/hp-hr

• Provide flexibility for stable catalyst bed temperature
• Propose clarifying language for load point to reflect the generator load; engine loads will be difficult to assess in the field
  • Propose load point to be changed from “…when the engine(s) are operating at or above 20% load…” to 25% of generator load

“(b)Engine exhaust gas shall be treated with urea or an ammonia solution when the engine(s) engine-generator set(s) are operating at or above 20% 25% load and the catalyst bed exhaust temperature of 570°F is achieved, except for periods of start-up or shutdown. The temperature is the preferred metric, but minimum load is available applicable if for some reason (e.g., certain Tier 4-certified units), the catalyst bed temperature is unavailable. Note that both the minimum temperature and load values are general estimates that should be revised if data specific to the SCR and/or gen-manufacturer, which may be higher or lower, is available.”

• Remove explicit catalyst bed exhaust temperature not otherwise specified by the manufacturer; retain limit (930 deg F) if not specified
• Define limit based on ‘exhaust temperature’

“(c)The permittee shall operate the gen-sets and SCR systems such that the catalyst bed exhaust temperature specified by the manufacturer is not exceeded. does not exceed 930°. 930°F may be used in the absence of more specific data from the SCR and/or gen-set manufacturer.”

7. Visible Emissions (Opacity)

• EPA Method 9 is a subjective test, more prone to variation and error. Suggest a more quantitative test or removal of opacity test requirements.

• The opacity limit of 5% is within the error of an observer; ref., Appendix A-4 to Part 60, Title 40, Method 9  

• • Alternatively, replace with applicable PM measurements methods defined in 40 CFR 1065

• Site Testing requirements for EPA Tier 4 certified units are redundant; remove requirement if maintained according to manufacturer instructions
  • Additional Stack Testing will only increase site emissions
  • EPA is reviewing language to support planned amendments to the CI NSPS
  • Other regional districts have aligned (e.g., SCAQMD)

Section F. Emission Limits / Calculations

• Propose regulatory alignment; EPA migrated regulatory requirements for engines under 40 CFR Part 89 to 40 CFR part 1039 in 2021

“NSPS standards should not be used or compared to the manufacturer specifications… The NSPS standards specified in 40 CFR Part 1039 Table 2 to Appendix I Table 1 (40 CFR 89.112) are measured using the procedures in 40 CFR 1039 Subpart C Subpart E of 40 CFR Part 89. Looking closely into Subpart F (Part 1039)E (89.404), the test cycles consist of various steady state operating modes)…”

1. Uncontrolled Emissions for Permit Applicability

• The definition references “manufacturer Not to Exceed”
  • Request clarification of the NTE reference definition; 40 CFR 1039 or 40 CFR 60.4212(c)
  • It is a common error to use the terms “Not to Exceed” and “Maximum Potential to Exceed” interchangeably

• Calculation should establish emissions limits from the exhaust of the system including the aftertreatment
• The definition for “Uncontrolled Emissions” requires an update to clarify that emissions from Tier 4 engine-generator sets be taken at the exhaust of the system
• Air pollution control equipment is considered as part of the engine system; separating the two is not representative

“Uncontrolled emissions for permit applicability should be calculated using the manufacturer Not To Exceed (NTE) (or equivalent) emission factors at the worst case load for that specific engine and pollutant. Load is pollutant specific. For example, NOX will have the worst case emission factor at 100% load while PM may have a worse case emission factor at 25% load. Uncontrolled emissions measurements for engine-generator sets which include either a 3^rd party emissions control unit, or include an emission control system as part of an EPA T4 certification, must be measured at the system exhaust.”

Section I. Testing Requirements (data centers)

AND

Section J. Testing Requirements (non-data center source categories)

• Accept alternative PM measurements methods including those defined in 40 CFR 1065
  • Include additional language that each of the three test runs must be at least 15 minutes; aligns with current proposal to EPA for planned amendments to the CI NSPS

• Site Testing requirements for EPA Tier 4 certified units are redundant; remove requirements if maintained according to manufacturer instructions
  • Additional Stack Testing will increase site emissions
  • EPA is reviewing language to support planned amendments to the CI NSPS
  • Other regional districts have aligned (SCAQMD)

“Initial VEE PM  Stack Testing: This boilerplate does contain a requirement for PM emissions an initial visible emissions evaluation for emergency (and non-emergency) gen-sets located at data center stationary sources. PM measurement An initial visible emissions evaluation shall be conducted according to the requirements listed in 40 CFR 60.4212. to show that the unit is in compliance with the appropriate opacity standard. Each test shall consist of 30 sets of 24 consecutive observations (at 15 second intervals) to yield a six minute average. Testing shall be conducted with the engine(s) operating at greater than 90% capacity, unless multiple load band testing is approved by DEQ. Engine generator-sets containing engines certified to EPA Tier 4 are exempt from these stack testing requirements.”

“Initial Stack Testing:

Emissions testing of each pollutant for each selected engine-generator set shall consist of three one-hour test runs under load. Each test run must last at least 15 minutes. The average of the three runs shall be reported as the short-term emission rate for that engine-generator set. Testing shall be conducted with the engine(s) operating at greater than 90% capacity, unless multiple load band testing is approved by DEQ. Engine generator-sets containing engines certified to EPA Tier 4 are exempt from these stack testing requirements.”

• Propose a reduction of the Initial VEE and Stack Testing application
  • Reduce from 20-25% to 10% for each type of generator-set
  • May alternatively align with the same provision in section J. Testing Requirements
  • Excessive Stack Testing increases site emissions without additional benefit

“Initial VEE PM Stack Testing and Stack Testing: The initial compliance determination testing described above will typically be applied to 10% 20-25% of the units for each type of gen-set permitted at a data center.”

• Add language to clarify field measurement procedures aligned with EPA for engines with a displacement ≥ 30 liters per cylinder; reference 40 CFR Subpart