What Is a Gas Flare? How Flares Work, What They Emit and How They Are Modelled

The flame is only the beginning when asking what is a gas flare

If you've ever driven past a refinery, gas plant or oilfield and wondered what is a gas flare, the short answer is that it is a controlled combustion device used to safely burn combustible gas that cannot be captured, processed or otherwise handled.

• The longer answer is much more interesting.
• Why is the gas being burned?
• What is actually coming out of the flare?
• How much does the flare release?
• Why can changing the flare stack or adding fuel gas become necessary?
• And how can engineers and air-quality professionals predict where the resulting emissions will go?

Those questions connect combustion, fluid mechanics, meteorology, air-quality modelling, engineering economics and regulation. The basic chain is:

Gas composition → combustion → emissions → plume rise → meteorology → dispersion → ground-level impact

That is where flare modelling becomes useful.

So...What Is a Gas Flare?

A gas flare is designed to burn combustible gas in a controlled manner rather than allowing it to accumulate or be released directly to the atmosphere. 

Flares are used in oil and gas operations as well as refineries, chemical and petrochemical facilities, natural-gas processing plants and other industrial settings. They can operate during:

• startup and shutdown
• maintenance
• testing
• pressure relief
• emergency conditions
• well cleanup and testing
• temporary operating conditions
• situations where capturing the gas is impractical

Safe combustion is HOT

So when you ask what is a gas flare, it is important to include its safety role. A flare can be an important safeguard against dangerous pressure accumulation or other operating problems.

At the same time, combustion produces emissions that may need to be assessed. For sour gas, sulphur-containing compounds are particularly important because combustion can produce sulphur dioxide (SO₂).

The useful question therefore becomes: What is being burned, how efficiently is it being burned and what reaches the atmosphere afterward?

That question takes us from the equipment itself into air-quality modelling.

Why Flares Are More Complicated Than Ordinary Stacks

An ordinary stack can often be described with a relatively familiar set of source parameters:

height + diameter + temperature + velocity + emission rate

A flare can require much more thought. 

• The flame itself changes the release characteristics.
• Flow affects exit velocity.
• Heat release affects plume rise.
• Gas composition affects heating value and combustion products.
• Supplemental fuel can change the heating value of the mixture.
• The flare tip affects the characteristics of the release.

The duration of the event affects which modelling  approach and averaging periods are relevant. So a flare isn't simply a tall pipe with an emission rate attached.

That's why understanding what is a gas flare is only the first step. The modeller also needs to understand how the equipment behaves.

Four Common Flare Situations

The details vary by facility and regulatory framework, but these broad categories are useful:

A single facility may use the same flare under more than one operating mode. If those modes produce substantially different source conditions, they may need to be assessed separately.

What Actually Comes Out of a Flare?

Now that we know what is a gas flare, the next question is more useful to an air quality modeller: What does the flare actually put into the atmosphere?

Depending on the fuel and combustion conditions, an assessment may require information about:

• raw-gas composition
• H₂S concentration
• gas heating value
• raw-gas flow
• supplemental fuel gas
• lift gas
• steam or air assist
• flare geometry
• exit diameter
• exit velocity
• conversion efficiency
• residence time
• heat release
• operating duration

AERflare contains calculations and checks related to many of these quantities. For example, it can calculate or estimate quantities associated with heating value, heat release, excess air, flame temperature, buoyancy flux, momentum flux, conversion efficiency and several combustion-related emission rates.

Modelling variation of heights and other parameters

The modeller doesn't necessarily need every intermediate result in the final report. But understanding what sits behind the calculated source parameters is important because a change in one input can affect several parts of the physical problem at once.

A Flare Is a Design Problem Too

Flare modelling can be useful while the equipment is still being designed.

A client or engineering firm may be looking for a practical design that meets increasingly demanding regulatory requirements without adding unnecessary capital or operating cost. That can involve iterating through:

• flare-stack height
• flare-tip diameter
• raw-gas flow
• supplemental fuel
• combustion conditions
• conversion efficiency
• expected operating duration

A taller stack may improve dispersion. A larger tip diameter can reduce exit velocity. Supplemental fuel can affect mixture heating value and combustion performance. But every change has an engineering or economic consequence.

A solution that looks attractive from an air-quality perspective may be expensive to operate. A lower-cost solution may create a problem elsewhere.

This is where modelling becomes more than a regulatory exercise. It becomes a design tool.

Exit Velocity: Small Tip, Big Problem

One practical issue is flare-tip exit velocity. If the flare tip is too small for the flow, velocity can become very high. That can create noise and other operational concerns. Under sufficiently high flow conditions, velocities can approach a significant fraction of the speed of sound.

Increasing the diameter can reduce velocity. But making the flare excessively large can introduce other design considerations. AERflare evaluates maximum, average and minimum exit velocities along with recommended diameter information for applicable conditions.

This illustrates an important engineering principle: A flare still has to function as equipment.

The best design is not necessarily the one that produces the lowest modelled concentration. It has to work physically, meet the applicable requirements and remain practical to build and operate.

Why Supplemental Fuel Can Matter

Supplemental fuel costs money. Sometimes it is nevertheless needed to achieve an acceptable heating value or combustion condition.

AERflare can evaluate whether fuel gas is required and can calculate fuel-gas-to-raw-gas ratios for applicable scenarios. That creates a genuine design trade-off.

More supplemental fuel can improve the combustion or dispersion characteristics of the flare while increasing operating cost. Less fuel can reduce cost while potentially changing combustion performance or dispersion.

There can even be circumstances in which the amount of additional fuel that would be desirable from an air-quality perspective is impractical for the facility. 

That makes this a useful question for what is a gas flare in the real world: How do we find the least-cost practical design that still meets the applicable air-quality and safety requirements?

Ground-Level Thermal Radiation Matters

Air quality is not the only concern in flare design. Thermal radiation at ground level can be an important safety consideration.

Environmental Impacts of Gas Flares

A taller flare may improve dispersion while changing the thermal-radiation footprint. A shorter stack carrying a high flow can create more concentrated ground-level radiation.

During blowdown scenarios, the highest flow occurs early in the event. Those conditions can deserve particular attention when assessing thermal radiation and other safety considerations.

So a serious flare assessment may need to examine both: Where do the emissions go? and What does the equipment physically do to the surrounding area?

This is one reason flare projects often bring together environmental, engineering and safety considerations.

The Biggest Flow Rate Does Not Automatically Produce the Biggest Ground-Level Concentration

Here is one of the best lessons in flare modelling. Suppose a flare operates at three different flow conditions.

• High flow - More gas is released. And the plume may also have greater buoyancy and plume rise.
• Medium flow - Less gas is released and the release characteristics change.
• Low flow - The instantaneous emission rate is lower, but plume rise can also be lower.

Which one produces the highest ground-level concentration? You cannot answer that from flow rate alone. The physical release conditions matter too.

AERflare's approach reflects this non-linearity by considering representative flow conditions rather than assuming the highest flow is automatically the controlling case.

So, if someone asks what is a gas flare from an air-quality perspective, the useful answer includes this: It is a changing source whose emissions and plume behaviour can interact in surprising ways.

Blowdowns Make the Problem Even More Interesting

A blowdown is a transient event. The flow changes continuously instead of remaining constant.

AERflare can represent the changing release with representative phases. By default, the blowdown can be divided into three phases containing approximately equal released mass.

The first two phases can occur relatively quickly. The third phase, often called tailout, can last much longer. For some pressure-release curves, the drop toward the final low-pressure condition takes dramatically longer than the initial pressure drop.

That creates a counterintuitive modelling result: The phase with the lowest instantaneous flow may still control an applicable averaging period because it lasts much longer.

So a blowdown should not simply be represented by its initial maximum flow rate. The modeller has to consider:

flow rate + duration + plume rise + averaging period

...together.

That is one of the clearest real-world examples of why what is a gas flare cannot be answered just by describing the flame.

What AERflare Actually Does

AERflare is much more than a spreadsheet for entering a flow rate. Depending on the project, it can work with information such as:

• facility or well information
• raw-gas volume and flow
• H₂S concentration
• flare geometry
• fuel and lift gas
• gas composition
• combustion conditions
• conversion efficiency
• heat release
• exit velocity
• residence time
• emission factors
• transient blowdown characteristics

It can also generate useful modelling statistics such as:

• maximum concentration
• distance to maximum concentration
• associated wind speed
• Pasquill-Gifford stability class
• distances to selected objectives
• high-percentile concentrations
• risk-based criteria
• 24-hour concentrations
• monthly averages
• annual averages

The exact outputs required depend on the assessment. The larger point is that the tool connects detailed source information with the dispersion analysis that follows.

AERflare Can Help With Design Iterations

AERflare can also prepare AERMOD input files for independent runs. That matters when an engineering team wants to compare several designs. For example:

• What happens if the stack is 10 m taller?
• What happens if the flare-tip diameter changes?
• What happens if supplemental fuel increases?
• Which design gives the best combination of air-quality performance, safety and cost?

The modeller can prepare multiple scenarios and process them systematically rather than rebuilding every case manually. AERflare's batch-oriented workflow can also support repeated scenario calculations such as stack-height iterations. That turns a modelling assessment into something useful during design.

What the Post-Processing Can Tell You

A maximum concentration alone doesn't explain why the model produced it. AERflare post-processing can examine information such as:

• distance to maximum concentration
• wind speed associated with the maximum
• atmospheric stability
• concentration statistics
• high-percentile values
• distances to selected objectives
• likelihood of remaining below a selected objective

For blowdown assessments, multiple representative AERMOD output files can be combined and analysed as part of the post-processing workflow. This lets the modeller investigate the circumstances surrounding the result.

Was the maximum associated with a particular wind speed? Was it associated with a stable atmosphere? Did it occur during the first phase of a blowdown or the long tailout?

Those questions help turn a number into an explanation.

Alberta: Where the Regulatory Question Begins

In Alberta, flare and incinerator requirements are established through the applicable regulatory framework, including AER requirements for upstream petroleum operations.

The current Directive 060: Upstream Petroleum Industry Flaring, Incinerating and Venting was released March 27, 2026.

For applicable flare and incinerator assessments, Directive 060 identifies AERflare-incin for SO₂ dispersion modelling. The exact requirements depend on the facility, event and regulatory pathway. A project may involve:

• a permanent facility
• temporary flaring
• non-routine flaring
• well testing
• an approval application
• an amendment or renewal
• an emergency or safety assessment

The first step is therefore understanding which requirement applies to the particular project. That is a regulatory question before it is a modelling question.

British Columbia: Similar Physics, Different Rules

Calvin Consulting also provides substantial air-quality modelling work in British Columbia. The physical behaviour of a flare does not change when the facility crosses the provincial boundary. The modelling requirements can.

Assess and reduce health impacts of air pollution

BC's dispersion-modelling guidance provides specific approaches for well-test flaring involving sour gas.

Its Level 1 approach uses AERSCREEN with a 55% default radiative heat-loss fraction for the cited application. More detailed modelling may be required depending on the predicted results and circumstances.

BC also requires a larger, denser receptor grid for flare assessments than is normally used for our Alberta flare work. That's a useful practical lesson: The same physical source can require a different modelling setup in a different jurisdiction.

The model should follow the applicable regulatory framework.

A Model Can Run Perfectly and Still Be Wrong

A computer can process every file successfully and produce an impressive contour plot. That does not establish that the assessment is representative.

1. The gas composition may be wrong.
2. The source data may be incomplete.
3. The flare geometry may be outdated.
4. An operating scenario may have been misunderstood.
5. A receptor grid may be inadequate.
6. The wrong model may have been selected.
7. A regulatory requirement may have been interpreted incorrectly.
8. Or the proposed equipment may need to change.

That's why modelling projects at Calvin involve review and discussion rather than simply producing a set of numbers.

The question is always: Does the modelling represent the physical and regulatory problem well enough to support the decision?

What We Look For at Calvin Consulting

Calvin Consulting has more than 30 years of air-quality consulting experience and extensive practical experience with industrial flare assessments in Alberta and British Columbia.

Using AERflare and ABflare with applicable dispersion models, we work with clients and design firms to develop practical solutions that can satisfy increasingly demanding requirements while keeping the overall project cost in view.

That may involve iterating through:

• Stack height - How much additional height is actually useful?
• Stack diameter - How large does the flare tip need to be to keep exit velocities reasonable?
• Supplemental fuel - How much fuel is technically useful and economically practical?
• Air quality - Which design produces acceptable predicted concentrations?
• Thermal radiation - Does the flare remain safe at ground level?
• Combustion - Does the proposed operating condition provide an appropriate conversion efficiency?
• Operating scenarios - Which flow rates and durations actually control the assessment?

The useful result isn't simply a passing number. Rather, it is a design that the facility can realistically build and operate.

From flame to ground impact

When Should You Call a Flare-Modelling Specialist?

You can enter numbers into a spreadsheet.

You can learn how to make AERMOD run.

You can produce a contour plot.

The difficult part is knowing whether you have selected the right scenario, used defensible source information, represented the flare correctly and interpreted the result appropriately. That becomes important when modelling may affect:

• flare-stack height
• flare-tip diameter
• supplemental fuel requirements
• equipment selection
• temporary-flaring approvals
• well-test planning
• safety decisions
• operating costs
• regulatory submissions

An early modelling review can reveal that a design needs to change while the engineering team still has choices. It can identify a problem in the supplied data. It can show that a less expensive configuration is adequate. It can prevent a team from spending money refining a model that was set up incorrectly.

A beautifully calculated answer is still the wrong answer if the wrong flare was modelled. That is where specialist experience earns its keep.

Calvin Consulting works with operators, engineering firms and project teams across Western Canada to evaluate flare emissions, dispersion, design options, temporary flaring and related regulatory questions.

Before you spend money optimizing the model, make sure you're optimizing the right problem. Contact Calvin Consulting Group Ltd. to discuss your flare, temporary-flaring or dispersion-modelling requirements.

Rest easy knowing you're in good hands.

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

The Short Answer

A gas flare is a controlled combustion device used to safely burn combustible gas when it cannot be captured or otherwise handled.

But a proper air-quality assessment has to go further. It has to ask:

• What gas is being burned?
• How much?
• For how long?
• How efficiently?
• Under what physical conditions?
• How does the plume rise?
• Where will the emissions go?
• What happens at ground level?
• What does the applicable regulator require?

And perhaps most importantly:

Which combination of equipment design and operating conditions gives the facility a practical and defensible solution?

That is the real subject behind what is a gas flare once the question moves beyond the definition and into engineering and air-quality modelling.

What an Experienced Flare Modeller Checks

A technically sophisticated spreadsheet cannot replace professional judgement. Before accepting the result, an experienced modeller may ask:

Is the gas composition appropriate? - Does the analysis use a representative gas analysis and an appropriate H₂S concentration?

Is the emission rate defensible? - Do flow rates and operating conditions agree with the available engineering and operating information?

Is the combustion efficiency reasonable? - Could the assumptions materially affect the emissions?

Is the flare geometry correct? - Are stack height, tip diameter, location and other physical parameters current?

Is the exit velocity acceptable? - Does the proposed tip size remain practical at the relevant flow rates?

Is supplemental fuel practical? - Can the facility actually supply the required fuel without creating an unacceptable operating burden?

Does the design provide adequate dispersion? - Could a different stack height or diameter provide a better long-term solution?

Does the thermal-radiation result make sense? - Could a design that looks attractive for air quality create an unacceptable safety problem?

Are the scenarios complete? - Have the relevant continuous, routine, non-routine, well-test or blowdown conditions been considered?

Does the controlling result make physical sense? - What meteorology and operating condition produced the maximum?

These questions are part of what makes professional flare modelling different from simply operating software.

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.

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

A Flare Modelling Checklist

Before modelling

☐ Define the purpose of the assessment

☐ Identify the applicable regulatory requirements

☐ Confirm the flare type and operating mode

☐ Establish gas composition and H₂S content

☐ Establish representative flow rates and durations

☐ Confirm stack height and flare-tip diameter

☐ Check fuel, lift gas and assist conditions

☐ Review conversion efficiency assumptions

☐ Review terrain, receptors and nearby sources

During modelling

☐ Test relevant operating scenarios

☐ Check plume-rise behaviour

☐ Check maximum, average and minimum exit velocity where appropriate

☐ Consider ground-level thermal radiation where applicable

☐ Review meteorological conditions associated with maxima

☐ Confirm the receptor grid is appropriate

☐ Examine whether the result is physically plausible

After modelling

☐ Identify the controlling scenario

☐ Identify the controlling meteorology

☐ Compare results with applicable objectives or criteria

☐ Explain important assumptions

☐ Document inputs and outputs

☐ Review conclusions independently

☐ Confirm that the proposed design remains practical