1. Regulatory Overview
Air quality permitting for natural gas facilities is governed by the Clean Air Act (CAA) and implemented through federal EPA regulations and state air quality programs. Understanding these requirements is essential when planning new facilities or modifications to existing stations.
Key Air Quality Regulations
- Clean Air Act (CAA): Federal law establishing air quality standards
- 40 CFR Part 60: New Source Performance Standards (NSPS)
- 40 CFR Part 63: National Emission Standards for Hazardous Air Pollutants (NESHAP)
- 40 CFR Part 70/71: Title V Operating Permits
- State Implementation Plans (SIPs): State-specific requirements
Regulatory Framework
| Program | Authority | Applicability |
|---|---|---|
| Title V | EPA / State | Major sources (≥100 TPY criteria pollutant) |
| PSD | EPA / State | Major sources in attainment areas |
| NSR | EPA / State | Major sources in nonattainment areas |
| Minor Source | State | Sources below major thresholds |
| NSPS | EPA | New or modified equipment categories |
| NESHAP | EPA | Sources of hazardous air pollutants |
Impact on Driver Selection
Air emissions significantly impact compressor driver selection. When evaluating prime mover options, consider:
- Existing station emissions and cumulative impact
- Attainment status of proposed location
- Cost to meet required emission levels (controls, offsets)
- Time required to obtain permits
- Electric motor options to avoid combustion emissions
Permitting Timeline
Air permit applications can take 6-18 months or longer to process depending on permit type and state workload. Major source permits (Title V, PSD) require public notice and comment periods. Plan permit applications early in project development to avoid schedule delays.
2. Emission Sources at Natural Gas Facilities
Natural gas compressor stations and processing facilities have multiple emission sources that must be inventoried and permitted.
Combustion Sources
| Source | Primary Pollutants | Typical Controls |
|---|---|---|
| Natural gas engines | NOx, CO, VOC, HAPs | Lean-burn, SCR, oxidation catalyst |
| Gas turbines | NOx, CO, VOC | Dry low-NOx, water injection, SCR |
| Heaters/boilers | NOx, CO | Low-NOx burners |
| Flares | NOx, CO, VOC | Proper design, assist gas |
| Thermal oxidizers | NOx, CO | Combustion optimization |
Fugitive and Process Sources
| Source | Primary Pollutants | Typical Controls |
|---|---|---|
| Equipment leaks (LDAR) | VOC, HAPs, methane | LDAR program, low-emission components |
| Compressor blowdowns | VOC, methane | Vapor recovery, flare |
| Pneumatic devices | VOC, methane | Low-bleed or zero-emission devices |
| Dehydration units | VOC, HAPs (BTEX) | Flash tank separator, condenser |
| Storage tanks | VOC | Vapor recovery, floating roof |
| Pig launchers/receivers | VOC, methane | Vapor recovery, flare |
Methane Emissions
EPA has increasingly focused on methane emissions from oil and gas facilities. The Methane Emissions Reduction Program (MERP) under the Inflation Reduction Act imposes fees on methane emissions above certain thresholds. Operators should inventory and minimize methane sources including compressor blowdowns, pneumatic devices, and fugitive leaks.
3. Regulated Pollutants
Air permits regulate multiple categories of pollutants. Understanding which pollutants apply to your facility determines permit requirements.
Criteria Pollutants
The six criteria pollutants have National Ambient Air Quality Standards (NAAQS):
| Pollutant | Primary Standard | Averaging Time | Gas Facility Source |
|---|---|---|---|
| NOx | 100 ppb | 1-hour | Engines, turbines, heaters |
| CO | 35 ppm | 1-hour | Engines, turbines, flares |
| VOC | (Ozone precursor) | - | Engines, dehy, tanks, leaks |
| SO₂ | 75 ppb | 1-hour | Usually negligible (sweet gas) |
| PM₁₀ | 150 μg/m³ | 24-hour | Combustion particulates |
| PM₂.₅ | 35 μg/m³ | 24-hour | Combustion particulates |
Hazardous Air Pollutants (HAPs)
HAPs are toxic pollutants regulated under Section 112 of the Clean Air Act. Common HAPs at natural gas facilities:
| HAP | Source | Health Effect |
|---|---|---|
| Benzene | Dehydrators, tanks, leaks | Carcinogen |
| Toluene | Dehydrators, tanks | Neurological |
| Ethylbenzene | Dehydrators, tanks | Possible carcinogen |
| Xylene | Dehydrators, tanks | Neurological |
| n-Hexane | Process vents, leaks | Neurological |
| Formaldehyde | Engine/turbine exhaust | Carcinogen |
Major Source HAP Thresholds
- Single HAP: ≥10 tons per year
- Combined HAPs: ≥25 tons per year
BTEX from Glycol Dehydrators
Glycol dehydration units are often the largest source of HAPs (benzene, toluene, ethylbenzene, xylene - collectively BTEX) at natural gas facilities. Flash tank separators and condensers can reduce BTEX emissions by 90% or more. Subpart HH of 40 CFR 63 establishes MACT standards for dehydration units at major sources.
4. Permit Types
The type of air permit required depends on facility emissions and location. Understanding permit categories helps determine project requirements and timeline.
Major Source Permits
Title V Operating Permits
Title V permits are comprehensive operating permits required for major sources. They consolidate all applicable air quality requirements into a single document with monitoring, recordkeeping, and reporting requirements.
Major source thresholds:
- ≥100 TPY of any criteria pollutant (attainment areas)
- Lower thresholds in nonattainment areas (varies by pollutant)
- ≥10 TPY single HAP or ≥25 TPY combined HAPs
Prevention of Significant Deterioration (PSD)
PSD applies to major new sources or major modifications in attainment areas:
| Requirement | Description |
|---|---|
| BACT Analysis | Best Available Control Technology for each pollutant |
| Air Quality Analysis | Modeling to demonstrate NAAQS protection |
| Additional Impacts | Visibility, soils, vegetation analysis |
| Class I Area Review | If within 100 km of national parks/wilderness |
Nonattainment New Source Review (NSR)
NSR applies to major new sources or major modifications in nonattainment areas:
| Requirement | Description |
|---|---|
| LAER | Lowest Achievable Emission Rate |
| Emission Offsets | Purchase offsets at ratio >1:1 |
| Compliance Certification | All sources in compliance |
| Alternative Analysis | Benefits outweigh environmental costs |
Minor Source Permits
Sources below major thresholds may still require state minor source permits:
- Synthetic minor: Accept enforceable limits to stay below thresholds
- True minor: Actual emissions below thresholds
- Permit by rule: Registration for small standardized sources
- General permits: Pre-approved permits for common source types
Synthetic Minor Strategy
Many compressor stations accept federally enforceable emission limits (hours of operation, fuel usage, emission rates) to qualify as minor sources. This avoids Title V and PSD requirements but requires careful tracking to ensure compliance with limits.
5. Potential to Emit (PTE)
Potential to emit (PTE) is the maximum capacity of a source to emit a pollutant, considering physical and operational design but excluding air pollution control equipment unless it is federally enforceable.
PTE Calculation Basis
| Factor | Uncontrolled PTE | Controlled PTE |
|---|---|---|
| Operating hours | 8,760 hours/year | Permit-limited hours |
| Emission rate | Maximum rated capacity | Permit-limited rate |
| Controls | Not credited | Credited if federally enforceable |
| Fuel | Maximum consumption | Permit-limited consumption |
Common Emission Factors
Emission factors from AP-42 and manufacturer data for typical gas facility equipment:
| Source | NOx (lb/MMBtu) | CO (lb/MMBtu) | VOC (lb/MMBtu) |
|---|---|---|---|
| 4-stroke lean burn engine | 1.0 - 2.0 | 0.5 - 1.5 | 0.1 - 0.3 |
| 4-stroke rich burn engine | 8.0 - 14.0 | 1.0 - 3.0 | 0.3 - 0.7 |
| 2-stroke lean burn engine | 2.0 - 4.0 | 1.5 - 3.5 | 0.5 - 1.0 |
| Gas turbine (uncontrolled) | 0.2 - 0.5 | 0.02 - 0.08 | 0.002 - 0.01 |
| Gas turbine (DLN) | 0.03 - 0.10 | 0.02 - 0.08 | 0.002 - 0.01 |
| Natural gas heater | 0.05 - 0.10 | 0.02 - 0.05 | 0.005 |
PTE Calculation Example:
1,000 hp lean-burn engine, 9,000 Btu/hp-hr heat rate, NOx = 1.5 lb/MMBtu
Annual PTE = 1,000 hp × 9,000 Btu/hp-hr × 8,760 hr/yr × 1.5 lb/MMBtu ÷ 10⁶
= 118 tons NOx/year
6. Emission Control Technologies
When emissions exceed permit thresholds, control technologies can reduce emissions to acceptable levels. Control selection depends on pollutant, source type, and cost-effectiveness.
NOx Control Technologies
| Technology | Reduction | Application | Cost ($/ton removed) |
|---|---|---|---|
| Lean-burn conversion | 80-90% | Rich-burn engines | $500-2,000 |
| Non-selective catalytic reduction (NSCR) | 90-98% | Rich-burn engines | $1,000-3,000 |
| Selective catalytic reduction (SCR) | 70-90% | Lean-burn engines, turbines | $2,000-10,000 |
| Dry low-NOx (DLN) combustors | 85-95% | Gas turbines | Included in new equipment |
| Water/steam injection | 60-80% | Gas turbines | $1,000-3,000 |
| Low-NOx burners | 40-70% | Heaters, boilers | $500-2,000 |
CO and VOC Control Technologies
| Technology | Reduction | Application |
|---|---|---|
| Oxidation catalyst | 90-98% CO, 50-90% VOC | All combustion sources |
| Thermal oxidizer | 98-99% | Process vents, tanks |
| Vapor recovery unit (VRU) | 95-98% | Tanks, blowdowns |
| Condenser | 70-95% | Dehydrator vents |
| Enclosed combustor (flare) | 95-99% | Various process vents |
BACT Determination
Best Available Control Technology (BACT) is determined through a top-down analysis:
- Identify all available control technologies
- Eliminate technically infeasible options
- Rank remaining by control effectiveness
- Evaluate cost-effectiveness ($/ton removed)
- Select most effective unless cost or other impacts are unreasonable
7. Attainment Status
The attainment status of a facility's location determines which permit programs apply and the stringency of requirements.
Attainment Designations
| Status | Definition | Permit Program |
|---|---|---|
| Attainment | Meets NAAQS for pollutant | PSD for major sources |
| Nonattainment | Exceeds NAAQS | NSR with offsets |
| Unclassifiable | Insufficient data | Treated as attainment |
| Maintenance | Former nonattainment, now meets NAAQS | PSD with monitoring |
Nonattainment Area Classifications (Ozone)
| Classification | Major Source Threshold (VOC/NOx) | Offset Ratio |
|---|---|---|
| Marginal | 100 TPY | 1.1:1 |
| Moderate | 100 TPY | 1.15:1 |
| Serious | 50 TPY | 1.2:1 |
| Severe | 25 TPY | 1.3:1 |
| Extreme | 10 TPY | 1.5:1 |
Emission Offsets
In nonattainment areas, new major sources must obtain emission offsets - emission reductions from existing sources that more than compensate for new emissions. Offsets can be extremely expensive (tens of thousands of dollars per ton) and may not be available in some areas. This can make nonattainment area locations economically infeasible for combustion-based facilities.
8. Permitting Process
Understanding the permit application process helps ensure timely project execution. Requirements vary by permit type and state.
Typical Application Contents
| Section | Contents |
|---|---|
| Facility description | Location, process description, site maps |
| Emission inventory | All sources, emission factors, calculations |
| Control equipment | Description, efficiency, monitoring |
| BACT analysis | Top-down analysis (if PSD) |
| Air quality modeling | Dispersion modeling results (if required) |
| Compliance plan | Monitoring, recordkeeping, reporting |
Typical Timeline
| Permit Type | Review Time | Public Notice |
|---|---|---|
| Minor source (state) | 2-6 months | Usually not required |
| Synthetic minor | 3-6 months | May be required |
| Title V (initial) | 12-18 months | 30-day comment, EPA review |
| PSD | 12-24 months | 30-day comment, hearing possible |
| NSR (nonattainment) | 12-24+ months | 30-day comment, offset procurement |
Common Permit Conditions
- Emission limits: lb/hr, lb/day, tons/year
- Operating limits: Hours, throughput, fuel usage
- Monitoring: CEMS, parametric monitoring, testing
- Recordkeeping: Operating logs, maintenance records
- Reporting: Semi-annual, annual, deviation reports
- Testing: Initial and periodic stack testing
Pre-Application Meeting
Request a pre-application meeting with the permitting agency before submitting. This meeting can identify potential issues early, clarify requirements, and establish expectations for the review process. Many agencies require or strongly encourage pre-application meetings for major source permits.