In August 2025, we discussed how air quality due diligence and permitting impact the design process of new data center developments in Illinois. This month, we consider these same factors as they relate to expanding capacity at existing data center properties in the state. We will also address the relevance of Illinois Senate Bill 25, otherwise known as the Clean and Reliable Grid Affordability (CRGA) Act, to future data center expansions.
Air Permitting Considerations for Illinois Data Center Expansions
Companies seeking to expand operations at existing Illinois data centers may already be familiar with the basics concerning the Illinois air permitting process from permitting initial operations at the property. However, some of these procedures may change when seeking to increase capacity at existing properties (considering that the additional servers and networking equipment necessary to enable these increases are likely to require their own backup gensets). The following sections discuss these changes, and the CRGA Act, as they relate to permit strategy.
Existing Source Status
Chief among the factors shaping permit strategy for expansions at existing data centers is existing source status. That is, do emissions for the already-permitted operations at the property classify the source as an existing minor source with respect to Prevention of Significant Deterioration (PSD) and Nonattainment New Source Review (NA NSR) (if applicable), or as an existing major source? The first step in this exercise is to determine what the major source thresholds for PSD and NA NSR (if applicable) are based on the property’s location.
The attainment status of an area refers to how the ambient pollutant concentrations in that area compare to the National Ambient Air Quality Standards (NAAQS) for that pollutant. The primary concern for data centers in Illinois is whether they are located in a “nonattainment area” (NAA) for ozone. In Illinois, the Chicago area counties of Cook, DuPage, Kane, Lake, McHenry, and Will, the Townships of Aux Sable and Goose Lake in Grundy County, and the Township of Oswego in Kendall County, in addition to the Metro East area counties of Madison, Monroe, and St. Clair, are all classified as “serious” nonattainment for the 2015 ozone NAAQS.
For data centers located in these NAAs, the major source thresholds for volatile organic material (VOM) and nitrogen oxide (NO) emissions (pollutants which are considered precursors to ozone formation) are currently only fifty (50) tons/year, each. This was changed (reduced) as of January 2025, so may be different than the thresholds in place when the current data center was originally permitted. The major source thresholds for the remaining criteria pollutants [i.e., carbon monoxide (CO), sulfur dioxide (SO2), and particulate matter (PM)] remain at 250 tons/year, each, for data center sources considering PSD applicability. Therefore, a source with emissions greater than or equal to 50 tons of VOM or NOx per year are considered an “existing major source” with respect to NA NSR, and a source with emissions greater than or equal to 250 tons of CO, SO2, and/or PM are considered an “existing major source” with respect to PSD. Data centers located outside these nonattainment areas are not subject to NA NSR. In all other areas of the state, a data center with potential emissions greater than or equal to 250 tons of VOM, NOx, CO, SO2, and/or PM is considered an “existing major source” with respect to PSD.
Major Modification
Understanding existing source status is key to determining the magnitude of emissions increases that would trigger complex requirements under PSD and/or NA NSR otherwise referred to as the significant emission rates (SERs). A net emissions increase exceeding a pollutant’s SER would cause the project to be considered a “major modification” for that pollutant.
If the data center is an existing minor source with respect to these regulations, then the SERs are much higher. An existing minor source in a NAA may increase emissions of VOM and NOx by up to 50 tons/year (each) and emissions of CO, SO2, and PM by up to 250 tons/year (each). An existing minor source data center outside of these areas may increase emissions of any criteria pollutant by up to 250 tons/year.
If classified as an existing major source, however, the SERs are far lower. For existing major sources of VOM and/or NOx with respect to NA NSR, the SER for whichever pollutant(s) the data center is considered major is only twenty-five (25) tons/year. Further, this 25-ton/yr threshold does not only include the net emissions increases attributed to the current project. Existing major sources in Illinois’ NAAs are presently required (due to the serious ozone nonattainment status) to aggregate net emissions increases from the current project with any other net emissions increases in the calendar year in which the project will occur and the previous (4) calendar years. (E.g., for a project that would begin operation in 2026, this aggregation would include projects occurring in 2026 and 2022-2025.) This can be challenging for data centers in the NAAs pursuing rapid expansion. To avoid triggering NA NSR, NOx emissions controls may be required—potentially both on the new engine generator sets and also on generator sets that were permitted as part of the initial construction of the data center.
The SERs for existing major sources with respect to PSD, summarized in the table below for criteria pollutants, are also lower.
| Pollutant | SER (tons/yr) |
| VOM | 40 |
| NOx | 40 |
| SO2 | 40 |
| CO | 100 |
| PM | 25 |
| PM10 | 15 |
| PM2.5 | 10 |
Unfortunately, it is not always possible to avoid exceeding an SER. Should that be the case, PSD or NA NSR would be triggered for the proposed data center expansion project. We welcome you to reach out to Trinity for additional details and assistance.
Clean and Reliable Grid Affordability (CRGA) Act
On October 30, 2025, the Illinois General Assembly passed the CRGA Act, sending the bill to Gov. Pritzker for final approval. The CRGA Act, which Gov. Pritzker has pledged to sign, spans several issues from incentives for energy storage projects to lifting a ban on the construction of large-scale nuclear plants in Illinois. The CRGA Act also includes new provisions for any data center that is/would be required to obtain a federally enforceable state operating permit (FESOP) or a Clean Air Act Permit Program (CAAPP) permit.
Per the CRGA Act, all new or modified backup (i.e., emergency) gensets at such a data center will be required to adhere to stricter emissions standards than previously necessary for compliance with state and federal regulations. Specifically, diesel-fired backup gensets will be required to meet emissions standards at least as protective as the Tier 4 standards for non-road diesel engines codified at Title 40 of the Code of Federal Regulations (40 CFR), Part 1039. Natural gas-fired backup gensets will similarly be required to meet emissions standards at least as protective as the Tier 2 standards for non-road large spark-ignition engines codified at 40 CFR Part 1048. In addition, the CRGA Act will limit subject gensets to emergency or standby use only (with exceptions for maintenance and readiness testing), thereby excluding such gensets from participating in any Demand Response programs.
The above requirements will be reflected in any relevant construction permit for which an affected data center source applies beginning six (6) months after the effective date of the CRGA Act. Thus, it is important that companies understand how the CRGA Act will impact the design of future projects. Companies should also consider the potential impact to project timeline, as gensets designed to meet these new standards often have longer lead times for delivery.
Other Considerations
Existing source status, SERs, and the CRGA Act are just a few pieces of the permitting puzzle. Data centers should also consider the following items:
Is the data center located in an Environmental Justice (EJ) area? Those seeking to expand operations at a data center located in an EJ area will likely face air dispersion modeling requirements to demonstrate that the proposed project does not inhibit compliance with the NAAQS.
- Projects in EJ areas are also subject to an EJ-related public notice process (and potentially a public comment period, if the EJ notice generates enough interest).
If the data center is being developed as part of a phased approach, how are these phases technically and/or economically related? If phases are found to be technically and/or economically dependent on each other (e.g., Phase 1 would not be viable without ultimately implementing Phase 2 shortly thereafter), this may require emissions from the phases to be aggregated—even if not an existing major source for NA NSR.
Would the emissions increase from the project warrant a public comment period? The Illinois EPA currently requires permits that would authorize an increase of ≥80% of an SER or cause sitewide emissions to exceed ≥80% of a major source threshold to undergo a public comment period of at least thirty (30) days prior to permit issuance.
Recommendations
Given the complex issues addressed above, the following best practices are recommended to inform strategic planning in the expansion of existing data center properties in Illinois:
- Begin the planning process early, making sure to consider the following key items and their impact on timeline:
- Existing major source status
- Potential public comment period
For phased data center developments, complete an analysis of whether each phase may be considered economically and technically independent of each other.
If an existing major source for NA NSR, consider whether it may be necessary to pursue projects on a delayed timeline to mitigate concerns regarding the required aggregation window.
If an existing minor source, consider leveraging additional restrictions (e.g., reduced runtime) on new and existing generators to avoid becoming a major source.
- Maintaining minor source status may also help to avoid additional compliance requirements, such as more frequent emissions testing or supplemental reporting (e.g., Annual Compliance Certifications).
- If becoming a major source is unavoidable, expect to apply for a Title V permit within one (1) year of starting operation of the project.
For more information, please reach out to Connor Rutledge or by phone at 630.495.1470.
