Emissions & Environmental Compliance
Fired heaters are major regulated emission sources on most refinery sites. This page covers the pollutants generated, how they form, what the operator can influence, and the compliance obligations that flow from environmental permits. All indicative limit values require verification against your site-specific permit document before use in operations.
Regulated pollutants
Four pollutant categories are regulated at most fired heater installations under federal and state environmental programmes. Each has a distinct formation pathway and a different set of operational levers available to the operator.
Formed during combustion primarily from atmospheric nitrogen reacting with oxygen at high flame temperatures. The dominant regulated pollutant in gas-fired heater emissions and the focus of most permit limit-setting.
Ozone precursor regulated under the NAAQS. Both NO and NO₂ are counted as NOₓ for compliance purposes.
Product of incomplete combustion. Elevated CO indicates oxygen deficiency, poor fuel/air mixing, or flame chilling. Directly toxic at elevated concentrations and a regulated air pollutant.
CO and NOₓ have opposing responses to excess air — this is the primary operational tradeoff in combustion management.
Generated stoichiometrically from sulphur in the fuel. Essentially all fuel sulphur converts to SO₂ during combustion — operational controls have minimal effect unless fuel composition changes.
Control is via fuel sulphur specification, not combustion management. Fuel certification records are the primary compliance document.
Primarily unburned carbon (soot) from incomplete combustion, particularly during fuel oil operation. Visible black smoke from the stack is an exceedance indicator and often a reportable event in its own right.
Measured as opacity (% light extinction) in continuous monitoring, or as PM₁₀/PM₂.₅ mass rate during periodic stack testing.
NOₓ formation mechanisms
NOₓ is not a single phenomenon. Three distinct formation pathways contribute to total stack NOₓ, and understanding which mechanism dominates determines which operational levers are effective. In gas-fired process heaters, thermal NOₓ is dominant — typically 80–95% of total NOₓ.
Operator levers: reduce peak flame temperature via excess air increase or O₂ trim; reduce residence time in the hot zone. Low-NOₓ burners reduce thermal NOₓ through staged combustion and internal flue gas recirculation, which dilute the reaction zone.
Operator levers: limited. Staged combustion (sub-stoichiometric primary zone) can suppress conversion to NOₓ. Fuel specification and sourcing is the primary control mechanism.
Operator levers: minimal. Prompt NOₓ is inherent to hydrocarbon combustion chemistry and is not practically reducible through field operating adjustments.
CO as a combustion quality indicator
Carbon monoxide is the most operationally immediate emissions parameter. Unlike NOₓ, which responds gradually to setpoint changes, CO can spike rapidly with any deterioration in combustion quality. Stack CO monitors or CEMS provide near-real-time feedback — elevated readings demand prompt investigation, not just logging.
| Condition | Mechanism | Operator action |
|---|---|---|
| Low O₂ / insufficient excess air | Oxygen starved — incomplete oxidation of CO to CO₂ | Increase air register opening; open arch damper; monitor flue gas O₂ to target range |
| Burner tip fouling or misalignment | Poor fuel/air mixing; localised fuel-rich zones | Inspect tips and registers; adjust register positions; schedule maintenance if persistent |
| Flame impingement on tubes or refractory | Flame chilling — CO oxidation quenched before completion | Adjust burner pattern; check for tip damage, erosion, or misalignment |
| Air infiltration through casing | Cold dilution air stratifies with flue gas; local O₂-deficient pockets | Inspect casing seals, peepholes, and observation doors; verify draft control |
| Fuel gas composition change | Higher hydrocarbons or H₂S shifts stoichiometric air requirement | Recalculate excess air target for new composition; notify engineering if Wobbe Index shifts significantly |
| Partial burner outage | Remaining burners fire richer to maintain COT; mixing degraded | Investigate cause of outage; do not over-fire remaining burners; consult shift supervisor |
Stack monitoring equipment
Most environmental permits require either continuous emissions monitoring (CEMS) or periodic stack testing to demonstrate compliance. The equipment type required, validation frequency, and data quality objectives are all specified in the facility operating permit.
| Equipment | Measures | Output | Notes |
|---|---|---|---|
| CEMS Continuous Emissions Monitoring System |
NOₓ, CO, SO₂, O₂, volumetric flow; some include CO₂ and moisture | Real-time ppm and mass rate (lb/hr or kg/hr); integrated to DAS for compliance record | Required for major sources under Title V. Subject to EPA PS-2 through PS-7 performance specifications. RATA (Relative Accuracy Test Audit) required periodically. |
| Opacity monitor (COMS) Continuous Opacity Monitoring System |
% opacity of stack plume via light transmittance across the stack | % opacity averaged over 6-minute blocks; equivalent to EPA Method 9 | Primary continuous indicator for particulate compliance. Opacity >20% is typically a permit exceedance requiring immediate investigation and reporting. |
| Portable analyser | O₂, CO, NOₓ, CO₂, SO₂, combustibles (electrochemical and NDIR sensors) | Field ppm and % readings — not a compliance instrument unless permit-specified | Used for combustion tuning, troubleshooting, and cross-checking CEMS. Calibration records required. Results do not substitute for CEMS data in compliance reports. |
| Stack test (EPA Reference Method) | Pollutant-specific: PM (Method 5), NOₓ (Method 7E), SO₂ (Method 6C), and others per permit | Mass emission rate (lb/mmBTU or g/GJ) integrated over the test run | Required periodically per permit schedule, or after major modification. Three test runs at representative operating conditions. Test contractor must be pre-approved in some jurisdictions. |
Environmental permit limits
Permit limits for fired heaters are site-specific and vary with heater size, technology vintage, BACT/LAER determination, and applicable state regulation. The values below are indicative industry ranges only — they are not a substitute for your operating permit. All values require verification before operational use.
| Pollutant | Reference basis | New / ULNB unit (indicative) | Existing unit (indicative) | Averaging period |
|---|---|---|---|---|
| NOₓ | ppm @ 3% O₂ dry; or lb/mmBTU | 0.02–0.04 lb/mmBTU ≈ 10–20 ppm |
0.06–0.10 lb/mmBTU ≈ 30–50 ppm |
Rolling 30-day average or 3-hour block |
| CO | ppm @ 3% O₂ dry; or lb/mmBTU | 50–100 ppm | 50–200 ppm | Rolling 30-day average or 3-hour block |
| SO₂ | Derived from fuel sulphur spec | Fuel spec dependent | Fuel spec dependent | Often 30-day rolling; varies |
| Opacity | % light extinction (COMS) | <10% (6-min avg) | <20% (6-min avg) | 6-minute block average |
| PM₁₀ | lb/mmBTU (stack test) | 0.01–0.03 lb/mmBTU | 0.03–0.06 lb/mmBTU | Stack test average (3 runs) |
Title V compliance framework
US refineries typically operate under Title V operating permits issued under Clean Air Act §502 / 40 CFR Part 70. These are facility-wide consolidated permits that incorporate all applicable federal and state requirements for each emission unit on site. The fired heater appears as a specific emission unit (EU) within the permit.
| Permit element | What it means for operations |
|---|---|
| Emission unit (EU) identifier | Each fired heater is designated as a specific EU in the permit. Emission limits, monitoring requirements, and operational restrictions are tied to that EU ID. Operators must know the EU designation for every heater on their unit. |
| Applicable requirements | The permit consolidates all federal NSPS (New Source Performance Standards), NESHAP, and MACT standards alongside state requirements. NSPS Subpart J or Subpart Ja typically applies to refinery process heaters depending on construction date. |
| Operational restrictions and caps | May include maximum heat input rate (mmBTU/hr), maximum annual fuel throughput, fuel type restrictions, or operating hour caps. Operating outside these restrictions — even briefly — can constitute a permit violation independent of emission levels. |
| Monitoring, recordkeeping and reporting (MRR) | Specifies what must be measured, at what frequency, in what format, and when reports must be submitted to the regulatory agency. The permit's MRR conditions are legally binding. Operators must understand which MRR requirements attach to their specific heater EU. |
| Permit modifications | Planned changes to heater operation, fuel type, heat input rate, or control equipment may require a minor or significant permit modification before the change is made. Confirm with the environmental team before making any operational change outside established permit conditions. |
| Permit renewal cycle | Title V permits are renewed every 5 years. During renewal, new BACT/LAER determinations may be required for any modifications made during the permit period. Late renewal applications must be submitted before permit expiry to maintain operating authority. |
Exceedance — recognition and reporting
A permit exceedance occurs when a monitored parameter exceeds its permit limit for the applicable averaging period. The operator's obligations are to recognise it promptly, take corrective action, and ensure it is reported to the regulatory authority within the timeframe specified in the permit. Failure to report is treated as a separate and often more serious violation.
2 hours of identification. Record time of notification in the shift log.30 days of the end of the reporting period, or within a shorter timeframe for acute events. The environmental team owns this obligation — operators must ensure they have been reached and have confirmed the deadline.Operational levers — emissions management
The following summarises the primary field adjustments available to the operator and their effect on each regulated pollutant. Note the opposing NOₓ/CO response to excess air changes — the dominant constraint in day-to-day combustion management.
| Adjustment | NOₓ | CO | SO₂ | Opacity / PM |
|---|---|---|---|---|
| ↑ Increase excess air | ↓ Decreases — lower flame temperature reduces thermal NOₓ formation | ↓ Decreases — more O₂ available for complete oxidation | No effect | ↓ Decreases — better combustion completeness |
| ↓ Decrease excess air toward stoichiometric | ↑ Increases — higher flame temperature | ↑ Risk increases — insufficient O₂ | No effect | ↑ Risk increases — incomplete combustion |
| ↓ Reduce firing rate | ↓ Decreases — lower heat release, lower flame temperature | Neutral if air adjusted proportionally | Proportional reduction with fuel rate | Proportional reduction |
| Low-NOₓ burner at correct register position | ↓↓ Significant — staged combustion reduces peak flame temperature and NOₓ formation zone | Neutral if correctly commissioned and tuned | No effect | Neutral |
| Fuel switch: gas → fuel oil | ↑ Fuel NOₓ increases with nitrogen content of oil | ↑ Risk increases if atomisation or mixing is poor | ↑↑ Sulphur content typically much higher than gas | ↑↑ Soot and unburned carbon significantly higher |
| Improved O₂ trim control (tighter setpoint) | ↓ More consistent NOₓ; avoids inadvertent over-air spikes | ↓ Avoids under-air episodes | No effect | ↓ More consistent combustion completeness |
Recordkeeping and reporting — operator responsibilities
The specific MRR (Monitoring, Recordkeeping, and Reporting) obligations are defined in the operating permit. The table below represents a minimum framework; confirm exact requirements with the environmental team for each specific heater EU.
| Record type | Frequency / trigger | Retention | Notes |
|---|---|---|---|
| CEMS hourly averages (NOₓ, CO, SO₂, O₂, stack flow) | Continuous — DAS generated | 5 years | Operator verifies DAS operational status each shift; documents any gaps |
| CEMS daily calibration drift checks | Each calendar day of operation | 5 years | Zero/span drift must remain within EPA Performance Specification limits; out-of-control periods require data flagging |
| Opacity monitor 6-minute block averages | Continuous — COMS generated | 5 years | Visible emissions observation records (EPA Method 22) required when COMS is offline |
| Fuel gas usage and heat input (mmBTU/hr or daily) | Per shift or daily | 5 years | Required to calculate lb/mmBTU emission rates and annual mass emissions totals for permit compliance demonstration |
| Fuel sulphur content (if SO₂ not CEMS-monitored) | Per delivery or monthly composite | 5 years | Supplier certificates of analysis acceptable under most permit conditions; must match the fuel stream actually fired |
| Exceedance and deviation event records | Event-triggered | 5 years | Must include: date, start/end time, duration, parameter exceeded, magnitude, probable cause, and corrective action taken |
| Quarterly deviation reports (semi-annual or annual in some states) | Per permit schedule | 5 years | Submitted by environmental team; operators provide operating data, CEMS summaries, and event records as input |
| Annual compliance certification | Annual (date per permit) | 5 years | Responsible official sign-off confirming permit compliance. Operators must ensure their records support certification accuracy. |