Flare Stack oil and gas - Air Quality Modelling for Alberta Energy Operations

A flare may look simple: gas goes up, it burns and the products disperse into the atmosphere. From an air quality modelling perspective, it is rarely that simple. Except for a typical flare stack oil and gas operations uses.

Flare performance can change with gas composition, flow rate, heating value, stack geometry, meteorological conditions and the duration of the event. For sour gas facilities, the questions can become more serious: Where will SO₂ and H₂S concentrations be highest? Will Alberta Ambient Air Quality Objectives be met? Does the flare design satisfy Directive 060?

That is where air dispersion modelling becomes important.

At Calvin Consulting Group Ltd., we use AERMOD and the Alberta Energy Regulator's AERFlare methodology to assess continuous incineration, routine and emergency flaring, plant blowdowns, acid gas flaring and other combustion scenarios.

The objective is straightforward: Produce a modelling assessment that realistically represents the facility and provides a defensible answer to the regulatory question.

The modelling required to get there, however, can be considerably more involved.

Flares, flares and more flares

What question does flare dispersion modelling answer?

The fundamental question is usually: Will emissions from this flare or incinerator result in unacceptable ground-level concentrations or otherwise fail to meet applicable regulatory requirements?

Depending on the project, that may involve evaluating:

• Sulphur dioxide (SO₂)
• Hydrogen sulphide (H₂S)
• Nitrogen dioxide (NO₂)
• Continuous combustion sources
• Emergency flaring
• Blowdowns
• Acid gas flaring
• Different flare flow rates
• Short-duration events
• Background air quality concentrations
• Elevated terrain
• Nearby facilities or sensitive locations

In Alberta, the modelling may support requirements under AER Directive 060, the Alberta Air Quality Model Guideline, an EPEA approval application or another regulatory process.

The important point is that there is no single flare model that automatically answers every question. The modeller first needs to understand what is actually happening at the facility.

Why modelling for a flare stack oil and gas companies use is different from modelling an ordinary source

For a conventional stack, the modeller may be given relatively stable parameters:

• Stack height
• Stack diameter
• Exit temperature
• Exit velocity
• Emission rate

A flare can be more dynamic.

A hot flare in a cold climate

The gas flow rate may change. The flare stack oil and gas composition may change. The heating value may change. The flame characteristics and resulting effective source parameters can also vary with atmospheric conditions.

Simply choosing one set of assumed parameters and calling it worst case is not necessarily the most representative approach.

For applicable AERFlare scenarios, Calvin Consulting regularly uses the algorithms provided by the Alberta Energy Regulator to calculate flare emission and source parameters that vary with meteorological conditions. These changing parameters are incorporated into the modelling rather than assuming that one fixed flare condition represents every hour of the meteorological record.

That distinction can matter. The highest emission rate does not always produce the highest ground-level concentration.

Likewise, the lowest exit velocity or largest flow rate may not independently represent the worst dispersion conditions.

The atmosphere and the flare interact. That is one reason flare modelling requires more professional judgment than simply entering a flow rate into a dispersion model.

AERFlare: More Than a Spreadsheet

AERFlare is an important tool used in Alberta to support flare and incinerator assessments. It helps calculate flare characteristics and prepare modelling inputs using the methodology associated with Alberta regulatory requirements.

Behind the scenes, the process can involve:

• Gas composition
• Heating value
• Flare or stack dimensions
• Gas flow rate
• Combustion parameters
• SO₂ and H₂S emissions
• Thermal radiation
• Meteorological conditions
• Terrain
• Receptors
• AERMOD dispersion modelling

The spreadsheet is therefore only one part of the assessment. A good flare study also requires the modeller to decide:

• Which operating cases need to be assessed
• Which assumptions are representative
• Whether a scenario is continuous, steady-state or transient
• Which meteorological data are appropriate
• How terrain should be treated
• How background concentrations should be incorporated
• Which results are relevant to the regulatory decision

Those decisions often have a much greater influence on the usefulness of the assessment than simply knowing which button to press in the spreadsheet.

Example: Continuous Incineration and Emergency Flaring at the Same Facility

AERFlare for flare stack oil and gas clients

One recent Calvin Consulting project involved a proposed facility with both continuous incineration and potential emergency sour gas flaring.

These were not treated as the same source operating under one set of assumptions.

The assessment evaluated:

Continuous incineration - The continuous incinerator was assessed using its normal stack and emission parameters to determine whether predicted SO₂ and H₂S concentrations would comply with applicable Alberta Ambient Air Quality Objectives.

Emergency flaring - Separate emergency flaring scenarios were then evaluated for different flare configurations and flow rates. The allowable flow rates were not selected arbitrarily. They were constrained by the applicable ground-level thermal radiation requirements.

The flare modelling parameters were calculated using the AERFlare methodology, with parameters varying by hour according to the meteorological algorithms provided by the AER.

This is an important example of why an assessment for a flare stack oil and gas firms have is not always just a matter of modelling the largest possible gas flow. A scenario may be constrained simultaneously by:

1. Air quality objectives
2. Thermal radiation requirements
3. Gas composition
4. Flare geometry
5. The expected operating event

The technically appropriate solution has to consider all of them together. 

Emergency Blowdowns Can Create a Different Modelling Problem

Another recent project involved a sour gas plant undergoing an emergency blowdown. The question was not simply, What happens if the flare operates continuously?

Instead, the assessment considered a short-duration event associated with rapidly depressurizing the plant (a blowdown). Separate scenarios included:

• Emergency high-pressure sour gas flaring
• Emergency acid gas flaring
• Different flare flow rates
• Fuel gas added to acid gas to achieve an appropriate heating value
• Short-duration flaring events
• SO₂ and H₂S dispersion
• Background concentrations
• Ground-level thermal radiation

In one case, three different acid gas flow rates were assessed. Interestingly, the highest flow rate did not automatically produce the highest predicted ground-level SO₂ concentration.

That is exactly the type of result that illustrates why dispersion modelling cannot always be reduced to a simple assumption that:

More gas = higher concentration everywhere.

A larger flow can also produce a hotter or more buoyant plume with different dispersion behaviour. The interaction between emissions, plume rise and meteorology needs to be evaluated rather than guessed.

Meteorology Matters More Than Many People Expect

A flare assessment can involve years of hourly meteorological data. For Alberta regulatory modelling, Calvin Consulting commonly processes five years of meteorological data for use with AERMOD. The meteorological data influence:

• Wind direction
• Wind speed
• Atmospheric stability
• Turbulence
• Mixing conditions
• Plume rise
• The location of maximum concentrations

For many facilities, terrain can also be important. A facility located in relatively flat terrain may behave very differently from one located near significant elevation changes. Receptors may need to be added around facility boundaries or concentrated near predicted maximum concentrations to properly define the results.

The objective is not simply to create the largest possible receptor grid. It is to use a receptor system that can adequately identify the maximum concentrations relevant to the assessment.

What Does a Good Flare Modelling Assessment Actually Look Like?

Components of assessment using AERflare

The final report should not simply say that the model was run and everything passed. A useful assessment should clearly explain: 

1. The facility and the scenario - What is being modelled? Continuous operation? A pressure relief event? A blowdown? Acid gas flaring? A temporary maintenance flare?
2. The assumptions - What gas composition, flow rates, flare dimensions and operating durations were used?
3. The modelling method - Which dispersion model and regulatory methodology were applied?
4. The meteorological and terrain data - Where did the data come from and why are they appropriate?
5. The receptors - How was the modelling domain designed and where are maximum concentrations occurring?
6. The results - What concentrations were predicted, how do they compare with applicable objectives and where do the maximum impacts occur?
7. The engineering and regulatory implications - If there is a potential problem, what can realistically be changed?

For example:

• Flare height
• Gas flow rate
• Operating duration
• Gas composition
• Fuel gas requirements
• Stack or flare configuration
• Operating limits

The best modelling report does more than just produce a number; it helps the client understand what the number means and what can be done about it.

When Air Quality Modelling Can Help Before a Design Is Final

Flare modelling is often associated with regulatory submissions, but it can be more useful when completed before every design decision has been locked in.

Modelling can be used to compare alternatives. For example:

• Would a taller flare reduce ground-level concentrations? - Usually, but the answer depends on the entire scenario.
• Would reducing the flow rate improve air quality? - Not necessarily. Lower flow rates may change plume rise and dispersion.
• Would adding fuel gas improve combustion characteristics? - Normally yes, but the resulting gas composition, heating value and emissions must be evaluated.
• Can a facility operate at a lower incinerator temperature? - Potentially, if the resulting concentrations still comply with applicable requirements.

One recent Calvin Consulting assessment supported an application to reduce the minimum operating temperature of a sulphur recovery unit incinerator. The analysis evaluated the resulting air quality implications rather than assuming that a lower temperature was automatically acceptable or unacceptable.

This is where modelling becomes a decision-support tool rather than simply a regulatory checkbox.

Flare Modelling for Pipeline Maintenance Operations

Not every flare assessment involves a permanent sour gas facility. Calvin Consulting has also reviewed the regulatory requirements affecting mobile flares used during natural gas transmission pipeline maintenance.

In that type of work, the questions may extend well beyond dispersion modelling. They can include:

• When flaring, incineration or venting is permitted
• Notification requirements
• Record keeping
• Operating procedures
• Flare performance requirements
• Heating value requirements
• Thermal radiation
• Equipment design
• Public and stakeholder notification
• Requirements under AUC Rule 007 and AER Directive 060

For a pipeline operator, the practical question may be: What do our crews actually need to do before, during and after a flaring event?

A technically useful regulatory review translates the requirements into something that operations personnel can apply.

Why Experience Matters

AERFlare and AERMOD are tools. They do not replace professional judgment.

Two assessments using the same software can produce very different, and potentially very different quality, results depending on:

• The scenarios selected
• The input data
• The treatment of meteorology
• The receptor design
• The handling of background concentrations
• The interpretation of regulatory requirements
• The understanding of flare physics

The most important part of an assessment is often the thinking that occurs before the model is run. At Calvin Consulting, our work is based on the principle that the model should represent the real facility as clearly as reasonably possible.

That means avoiding unnecessary complexity while also avoiding oversimplifications that could distort the result. Sometimes a simple model is the right model.

Under other circumstances, the project requires multiple scenarios, hourly varying flare parameters, short-duration event analysis, refined receptors or detailed regulatory interpretation.

The objective is the same: Use a defensible approach that answers the actual question the client and regulator need answered.

Flare Stack and Air Quality Modelling by Calvin Consulting

Calvin Consulting Group Ltd. provides air quality dispersion modelling and regulatory support for industrial facilities across Western Canada. Our experience includes:

• AERMOD modelling
• AERFlare methodology
• Continuous incineration
• Emergency flaring
• Sour gas facilities
• Acid gas flaring
• Plant blowdowns
• SO₂, H₂S and NO₂ assessments
• Five-year meteorological data processing
• WRF meteorological data
• Complex terrain
• Background concentration analysis
• Regulatory support for AER Directive 060
• Flare Performance Assessments and operating considerations

Instead of every project needing the most complicated possible model, we believe the modelling needs to be appropriate for the facility, defensible to the regulator and useful to the people making decisions.

If you have a flare, incinerator, blowdown or industrial emission source that needs to be assessed, Calvin Consulting can help determine what level of modelling is actually required and then complete the assessment without adding unnecessary complexity.

Send a message with your situation today.

With us, you’re not just meeting requirements - you’re leading with trust, expertise and certainty.

​Clean air is our Passion...Regulatory Compliance is our Business.

When Should You Call an Air Quality Modeller?

Consider getting a flare assessment started when you are:

• Designing a new flare or incinerator
• Preparing an EPEA approval or approval renewal application
• Evaluating a sour gas facility
• Planning emergency blowdown scenarios
• Assessing acid gas flaring
• Changing flare height, capacity or operating conditions
• Considering a lower incinerator operating temperature
• Preparing a Directive 060 assessment
• Developing a Flare Performance Assessment
• Using mobile flares for pipeline maintenance
• Trying to understand whether a proposed operating scenario will meet Alberta air quality requirements

The earlier the modelling is considered, the more opportunity there may be to evaluate alternatives before they become expensive design changes.

Do you have concerns about air pollution in your area??

Perhaps modelling air pollution will provide the answers to your question.

That is what I do on a full-time basis.  Find out if it is necessary for your project.

Have your Say...

on the StuffintheAir         facebook page

Other topics listed in these guides:

The Stuff-in-the-Air Site Map

And, 

See the newsletter chronicle. 

Thank you to my research and writing assistants, and the author remains responsible for the content.

Background Concentrations: A Small Number That Can Matter

Air dispersion models generally predict the contribution from the facility being assessed.

They do not automatically include everything already present in the atmosphere.

Where appropriate and required, background concentrations must be considered. This can require professional judgment.

For example, the closest monitoring station may not necessarily have suitable data for every contaminant or averaging period. The modeller may need to examine:

• Available monitoring stations
• Data completeness
• Distance from the facility
• The period of available data
• The method used to estimate representative background concentrations

In one recent project, Calvin Consulting evaluated multiple years of ambient monitoring data to establish background concentrations for SO₂ and H₂S before adding them to the predicted concentrations.

The calculation itself may be straightforward. Choosing and documenting a defensible approach is where experience becomes valuable.