1. Major Source Overview
A major source is any stationary source or group of stationary sources that emits or has the potential to emit (PTE) pollutants at or above specific threshold levels established under the Clean Air Act (CAA). The designation determines which federal and state air quality permits and regulatory programs apply to a facility.
Why Major Source Status Matters
Crossing a major source threshold triggers significantly more stringent regulatory requirements, including Title V operating permits, MACT/NESHAP compliance, New Source Review (NSR), and potentially Best Available Control Technology (BACT) or Lowest Achievable Emission Rate (LAER) requirements. For midstream facilities, maintaining minor source status can save millions in compliance costs and project timelines.
Key Regulatory Programs
| Program | Regulatory Basis | Applicability |
|---|---|---|
| Title V | CAA Section 502, 40 CFR Part 70 | Operating permit for major sources of criteria pollutants or HAPs |
| PSD | CAA Section 165, 40 CFR 52.21 | Pre-construction review for major new sources or modifications in attainment areas |
| NNSR | CAA Section 173, 40 CFR Part 51 | Pre-construction review in nonattainment areas |
| NESHAP / MACT | CAA Section 112, 40 CFR Part 63 | Maximum Achievable Control Technology for HAP sources |
| NSPS | CAA Section 111, 40 CFR Part 60 | New Source Performance Standards (applies regardless of major/minor) |
Pollutant Categories
| Category | Pollutants | Notes |
|---|---|---|
| Criteria Pollutants | NOx, SO2, CO, PM10, PM2.5, VOC, Pb | NAAQS-regulated; VOC and NOx are ozone precursors |
| HAPs | 187 listed substances (benzene, toluene, formaldehyde, etc.) | CAA Section 112(b) list; separate major source threshold |
| GHGs | CO2, CH4, N2O, HFCs, PFCs, SF6 | Subject to PSD if facility is already major for another pollutant |
2. Title V Permitting
Title V of the Clean Air Act requires major stationary sources and certain other sources to obtain an operating permit that consolidates all applicable air quality requirements into a single document. The permit is renewed every five years and includes monitoring, recordkeeping, and reporting requirements.
Title V Major Source Thresholds
A source is considered major under Title V if its potential to emit (PTE) exceeds the following thresholds for any single regulated pollutant:
| Pollutant | Threshold (tons/year) | Area Classification |
|---|---|---|
| Any criteria pollutant | 100 | Attainment / unclassifiable areas |
| VOC or NOx | 100 | Marginal or moderate ozone nonattainment |
| VOC or NOx | 50 | Serious ozone nonattainment |
| VOC or NOx | 25 | Severe ozone nonattainment |
| VOC or NOx | 10 | Extreme ozone nonattainment |
| CO | 50 | Serious CO nonattainment |
| PM10 | 70 | Serious PM10 nonattainment |
Potential to Emit (PTE)
PTE is the maximum capacity of a source to emit a pollutant under its physical and operational design, considering any enforceable emission limitations (permit conditions, consent decrees, state regulations). PTE assumes continuous operation at maximum rated capacity, 8,760 hours per year, unless a federally enforceable limitation restricts operating hours. This distinction between actual emissions and PTE is critical for major source determinations.
Title V Permit Components
| Component | Description |
|---|---|
| Emission limits | Enforceable limits for each emission unit and pollutant |
| Monitoring | CEMS, periodic testing, parametric monitoring requirements |
| Recordkeeping | Equipment logs, fuel usage, operating hours, emission calculations |
| Reporting | Semi-annual monitoring reports, annual compliance certifications |
| Compliance schedule | Timeline for achieving compliance with applicable requirements |
3. PSD Applicability
Prevention of Significant Deterioration (PSD) is a pre-construction permitting program that applies to new major sources or major modifications at existing major sources located in areas that meet the National Ambient Air Quality Standards (NAAQS). PSD review requires ambient air quality analysis, BACT determination, and public notice.
PSD Major Source Thresholds
| Source Category | Threshold (TPY) | Midstream Relevance |
|---|---|---|
| 26 Named categories (power plants, refineries, etc.) | 100 | Rarely applies to midstream |
| All other source categories | 250 | Standard threshold for compressor stations and gas plants |
PSD Significant Emission Rates
Once a source is classified as major under PSD, any modification that increases emissions above the significant emission rate (SER) for any pollutant triggers PSD review for that pollutant:
| Pollutant | Significant Emission Rate (TPY) |
|---|---|
| CO | 100 |
| NOx | 40 |
| SO2 | 40 |
| PM10 | 15 |
| PM2.5 | 10 |
| VOC | 40 |
| Lead | 0.6 |
| GHGs (CO2e) | 75,000 |
PSD Netting Analysis
When evaluating whether a modification at an existing major source triggers PSD, contemporaneous emission increases and decreases at the facility over the preceding 5-year period may be "netted" to determine whether the net emission increase exceeds the significant emission rate. This netting analysis can be complex and typically requires careful documentation of baseline actual emissions and creditable emission decreases.
BACT Requirements
Sources subject to PSD must apply Best Available Control Technology (BACT) for each pollutant that triggers review. BACT is determined on a case-by-case basis through a top-down analysis that evaluates technically feasible control options in descending order of effectiveness.
| Step | Top-Down BACT Analysis |
|---|---|
| 1 | Identify all available control technologies |
| 2 | Eliminate technically infeasible options |
| 3 | Rank remaining options by control effectiveness |
| 4 | Evaluate economic, energy, and environmental impacts |
| 5 | Select BACT (most effective unless adverse impacts justify lower tier) |
4. HAP Major Source
Hazardous Air Pollutants (HAPs) have a separate major source definition under CAA Section 112. The HAP major source thresholds are significantly lower than criteria pollutant thresholds, making HAP emissions a frequent driver of major source status for midstream facilities.
HAP Major Source Thresholds
Single HAP: PTE ≥ 10 tons per year of any individual HAP
Combined HAPs: PTE ≥ 25 tons per year of any combination of HAPs
Common HAPs in Midstream Operations
| HAP | Primary Sources | Typical Concern |
|---|---|---|
| Benzene | Glycol dehydrators, storage tanks, fugitive emissions | Often the single HAP that drives major source status |
| Toluene | Storage tanks, loading operations, process vents | Present in condensate and NGL streams |
| n-Hexane | Gas processing, condensate storage, fugitive emissions | Significant in rich gas processing |
| Xylenes | Storage tanks, process vents | Present in condensate streams |
| Formaldehyde | Combustion sources (engines, turbines, flares) | Combustion byproduct; can be significant for large engine fleets |
| Ethylbenzene | Condensate handling, storage tanks | Typically smaller contributor than BTEX components |
MACT/NESHAP Implications
HAP major sources are subject to Maximum Achievable Control Technology (MACT) standards under 40 CFR Part 63. For oil and gas operations, key subparts include Subpart HH (glycol dehydrators), Subpart ZZZZ (reciprocating engines), and Subpart YYYY (combustion turbines). Area sources (below major source thresholds) may be subject to less stringent Generally Available Control Technology (GACT) standards.
HAP Speciation Methods
Determining the HAP content in process streams is essential for accurate emission calculations. Common approaches include:
| Method | Application | Accuracy |
|---|---|---|
| Extended gas analysis | Full chromatographic analysis of gas composition including C6+ fractions | Highest accuracy for gas streams |
| AP-42 speciation profiles | EPA default HAP fractions based on source type | Conservative; used when site-specific data unavailable |
| Process simulation | HYSYS or ProMax modeling of HAP partitioning | Good for process vents and flash emissions |
| Stack testing | Direct measurement of HAP emissions from combustion devices | Most accurate for combustion HAPs (formaldehyde, acetaldehyde) |
5. Emission Calculations
Accurate emission calculations are the foundation of major source determinations. Emissions must be quantified for each emission unit at the facility and summed to determine the facility-wide PTE.
Combustion Source Emissions
Internal combustion engines and turbines are typically the largest emission sources at compressor stations and gas processing plants. Emissions are calculated using manufacturer emission factors or AP-42 factors.
E = EF × A × H / 2000
Where E = annual emissions (tons/year), EF = emission factor (lb/MMBtu or g/hp-hr), A = activity level (MMBtu/hr or hp), H = annual operating hours (hr/yr), 2000 converts lb to tons
Typical Emission Factors
| Source Type | NOx (g/hp-hr) | CO (g/hp-hr) | VOC (g/hp-hr) | Formaldehyde (g/hp-hr) |
|---|---|---|---|---|
| 4SLB engine (rich burn, no catalyst) | 11.0 | 2.4 | 0.68 | 0.21 |
| 4SLB with NSCR | 1.1 | 2.4 | 0.27 | 0.08 |
| 4SLB (lean burn) | 2.2 | 2.4 | 0.68 | 0.21 |
| 2SLB engine | 14.0 | 4.0 | 1.2 | 0.35 |
| Gas turbine (uncontrolled) | 3.5 | 0.67 | 0.08 | 0.07 |
Manufacturer Data vs. AP-42
For new equipment, manufacturer-guaranteed emission factors should be used when available as they are typically more accurate than AP-42 generic factors. However, AP-42 factors may be required by some state agencies for PTE calculations. When using manufacturer data, ensure the emission factors are guaranteed at site conditions (altitude, ambient temperature, fuel composition) and at the maximum rated capacity of the equipment.
Fugitive Emission Estimation
Equipment leaks from valves, connectors, flanges, pumps, and compressor seals contribute to both VOC and HAP emissions. Fugitive emissions are estimated using one of three EPA-approved methods:
| Method | Description | Typical Use |
|---|---|---|
| Average emission factors | EPA Protocol component-specific average factors | PTE calculations for new facilities without leak data |
| Screening value correlation | Emission rate correlated to OGI or Method 21 screening values | Existing facilities with LDAR program data |
| Unit-specific correlation | Site-specific emission factors from bagging studies | Refinement of estimates at large facilities |
6. Common Emission Sources at Midstream Facilities
A comprehensive emission inventory must account for all emission points at the facility. Missing sources in the inventory can lead to permit violations and enforcement actions if actual emissions exceed permitted levels.
Emission Source Inventory
| Emission Source | Primary Pollutants | Calculation Method |
|---|---|---|
| Compressor engines | NOx, CO, VOC, HAPs, PM | Manufacturer EF or AP-42 Section 3.2 |
| Glycol dehydrators | VOC, HAPs (BTEX), CO, NOx | GRI-GLYCalc or AP-42 Section 5.3 |
| Storage tanks | VOC, HAPs | TANKS 4.09d or AP-42 Section 7.1 |
| Flares / combustors | NOx, CO, VOC, SO2 | AP-42 Section 13.5 with destruction efficiency |
| Fugitive components | VOC, HAPs | EPA Protocol emission factors |
| Pneumatic devices | VOC, HAPs (methane) | Device count and bleed rates |
| Truck loading | VOC, HAPs | AP-42 Section 5.2 |
| Produced water tanks | VOC, HAPs, H2S | E&P TANKS or flash analysis |
Contiguous or Adjacent Definition
Under EPA's major source determination, all pollutant-emitting activities that belong to the same industrial grouping (same 2-digit SIC code), are located on contiguous or adjacent properties, and are under common control are aggregated as a single stationary source. For midstream operations, this means compressor stations, treating facilities, and associated equipment on the same or adjacent properties under the same operator may be combined for threshold purposes.
7. Synthetic Minor Limits
A facility that would otherwise be a major source can accept federally enforceable emission limitations to maintain its PTE below major source thresholds. This is called a synthetic minor or permit-by-rule approach, and it avoids the cost and complexity of Title V permitting.
Common Synthetic Minor Strategies
| Strategy | Mechanism | Requirements |
|---|---|---|
| Operating hour limits | Restrict annual operating hours per engine or unit | Hour meters, monthly logging, annual reporting |
| Fuel usage limits | Cap annual fuel consumption | Fuel meters, monthly records |
| Throughput limits | Restrict gas processing or liquid handling volumes | Flow measurement, monthly records |
| Emission controls | Install catalysts, VRUs, or combustors to reduce PTE | Compliance testing, monitoring, maintenance records |
| Equipment limitations | Limit number of engines, tanks, or emission units | Equipment inventory, notification of changes |
Enforceability Requirements
Synthetic minor limitations must be practically enforceable and federally enforceable to be creditable in PTE calculations. The permit must contain: (1) specific emission limits or operational restrictions, (2) monitoring and recordkeeping sufficient to demonstrate compliance, (3) reporting requirements, and (4) a legally binding commitment. Voluntary or internal company policies are not enforceable and cannot reduce PTE.
8. State Permitting Programs
Each state implements the CAA through its own State Implementation Plan (SIP) and permitting programs, which may include requirements more stringent than federal minimums. Understanding state-specific thresholds and permit types is essential for compliance.
State Variations for Key Oil and Gas States
| State | Regulatory Agency | Minor Source Permit | Notable Requirements |
|---|---|---|---|
| Texas | TCEQ | Standard Permit, PBR | Permit by Rule for small sources; standard permits for compressor stations |
| Oklahoma | ODEQ | Minor Source Permit | Permit required above de minimis thresholds |
| Colorado | CDPHE / AQCC | APEN, Minor Source | Air Pollutant Emission Notice required; AQCC Regulation 7 for O&G |
| Pennsylvania | PA DEP | GP-5, GP-5A | General permits for natural gas operations |
| New Mexico | NMED | NSR Permit | Methane regulations; ozone precursor requirements in Permian Basin |
| Louisiana | LDEQ | Minor Source Permit | Toxic air pollutant screening via SCREEN3 or AERMOD |
Permit Application Timeline
Understanding typical permitting timelines is important for project planning:
| Permit Type | Typical Timeline | Notes |
|---|---|---|
| Permit by Rule / General Permit | Immediate to 30 days | Pre-authorized; registration or notification only |
| Minor source construction permit | 2 – 6 months | State review and approval required |
| Synthetic minor permit | 3 – 9 months | Requires enforceable conditions; may need public notice |
| Title V initial permit | 12 – 24 months | Public notice, EPA review, 30-day comment period |
| PSD permit | 12 – 36 months | BACT analysis, air modeling, public hearing |