Flue gas composition and excess air are evaluated by combining gas analysis with material and elemental balances around the combustion unit. In a pilot plant, operators typically measure dry flue gas concentrations of CO₂, O₂, CO, SO₂, and N₂ using an Orsat apparatus or continuous gas analyzers. They then calculate the theoretical oxygen requirement from the fuel composition and compare it with the actual oxygen supplied in the combustion air.

Core takeaway: Flue gas analysis indicates how much oxygen remains after combustion, while fuel and air-flow data establish the theoretical requirement. Comparing actual air supplied with stoichiometric air, and checking the result against measured flue gas composition, gives the percentage excess air and verifies the combustion material balance.

Establishing the Combustion Basis

Define the Fuel Composition

The analysis begins with the dry or as-fired fuel composition. The balance should identify carbon, hydrogen, sulfur, oxygen already bound in the fuel, moisture, ash, and any nitrogen or other relevant constituents.

For gaseous fuels, the composition may be expressed as mole fractions of methane, ethane, hydrogen, carbon monoxide, carbon dioxide, and other components. For solid or liquid fuels, ultimate analysis is commonly expressed as mass percentages of carbon, hydrogen, oxygen, sulfur, nitrogen, ash, and moisture.

Select a Consistent Basis

A calculation is performed on a defined basis, such as 100 kmol of fuel, 100 kg of coal, or a measured pilot-plant operating period. All fuel, air, flue gas, and residue quantities must use the same basis.

The analysis also distinguishes between wet flue gas, which includes water vapor, and dry flue gas, from which water has been removed. Most Orsat and stack-gas results are reported on a dry basis.

Calculating the Theoretical Oxygen Requirement

Apply Complete-Combustion Stoichiometry

The theoretical oxygen requirement is calculated for complete oxidation to the highest expected oxidation states:

Carbon is converted to CO₂.
Hydrogen is converted to H₂O.
Sulfur is converted to SO₂.

For a fuel containing elemental carbon, hydrogen, sulfur, and oxygen, the net oxygen required is commonly written on a mass basis as:

[ n_{O_2,\text{net}}

n_C+\frac{n_H}{4}+n_S-\frac{n_O}{2} ]

Here, (n_C), (n_H), (n_S), and (n_O) are the amounts of carbon, hydrogen, sulfur, and fuel-bound oxygen expressed in consistent atomic units.

Fuel-bound oxygen is subtracted because it contributes to oxidation. The theoretical requirement is based on complete combustion, so measured carbon monoxide or unburned fuel does not redefine the theoretical oxygen requirement.

Convert Oxygen Requirement to Stoichiometric Air

For ordinary dry air, oxygen is approximately 21% by volume, or mole fraction, with nitrogen making up most of the balance. Therefore:

[ n_{\text{air,stoich}}

\frac{n_{O_2,\text{stoich}}}{y_{O_2,\text{air}}} ]

Using the appropriate actual air composition is important when the pilot plant uses oxygen-enriched air, recycled flue gas, humidified air, or another oxidant.

Measuring Flue Gas Composition

Use Orsat or Instrumental Analysis

A traditional Orsat analyzer measures dry flue gas constituents by selectively absorbing carbon dioxide, oxygen, and sometimes carbon monoxide. Nitrogen is commonly obtained by difference:

[ y_{N_2}

1-y_{CO_2}-y_{O_2}-y_{CO}-y_{\text{other}} ]

Modern pilot plants often use continuous analyzers based on techniques such as paramagnetic oxygen measurement, infrared carbon dioxide and carbon monoxide measurement, or ultraviolet sulfur dioxide measurement.

Report the Basis and Sampling Conditions

A flue gas result should state whether concentrations are on a dry or wet basis, whether they are volumetric or molar fractions, and the sampling temperature and pressure. Sampling lines may require filtration, cooling, condensation control, and moisture correction.

Poor sampling can produce misleading results. Condensed water, leaks, particulate loading, analyzer cross-sensitivity, or an unrepresentative sampling location can all distort the apparent gas composition.

Interpret the Major Components

CO₂ indicates carbon oxidation and is useful for checking the carbon balance. A low CO₂ concentration may result from high excess air, incomplete combustion, fuel dilution, or carbon not represented in the sampled gas.

O₂ represents oxygen that remains after the combustion reactions. It is the most commonly used flue-gas indicator for controlling excess air, but it must be interpreted together with CO and unburned fuel measurements.

CO and residual hydrocarbons indicate incomplete combustion. Their presence may result from insufficient air, poor fuel-air mixing, inadequate residence time, low temperature, or unstable burner operation.

SO₂ is related primarily to sulfur conversion and helps evaluate sulfur balances and downstream acid-gas treatment. Nitrogen is mostly carried through from the combustion air, although fuel-bound nitrogen and nitrogen oxide formation may require additional analysis.

Determining Percentage Excess Air

Calculate It from Air-Flow Data

If the actual air feed is measured, percentage excess air is:

[ %\text{ excess air}

\frac{n_{\text{air,actual}}-n_{\text{air,stoich}}} {n_{\text{air,stoich}}} \times 100 ]

Equivalently, using oxygen supplied from the air:

[ %\text{ excess air}

\frac{n_{O_2,\text{actual}}-n_{O_2,\text{stoich}}} {n_{O_2,\text{stoich}}} \times 100 ]

The excess-air coefficient is:

[ \lambda

\frac{n_{\text{air,actual}}}{n_{\text{air,stoich}}}

1+\frac{%\text{ excess air}}{100} ]

Thus, 15% excess air corresponds to (\lambda=1.15).

Estimate It from Dry Flue Gas Oxygen

For complete combustion with ordinary air and negligible oxygen in the fuel, a common approximation based on dry flue gas oxygen is:

[ %\text{ excess air} \approx \frac{y_{O_2,\text{dry}}} {0.21-y_{O_2,\text{dry}}} \times 100 ]

This relation assumes that the measured oxygen is the principal indicator of excess oxidant and that the air composition is known.

The approximation becomes less reliable when carbon monoxide, unburned fuel, oxygen enrichment, air leakage, flue-gas recirculation, or significant fuel-bound oxygen is present.

Cross-Check with a Complete Material Balance

The strongest pilot-plant procedure compares two independent estimates:

Calculate theoretical air from the fuel elemental analysis.
Calculate actual air from the measured air flow.
Analyze dry flue gas.
Reconstruct the outlet stream using carbon, hydrogen, sulfur, oxygen, and nitrogen balances.
Compare calculated and measured compositions.

Agreement supports the validity of the operating data. A substantial discrepancy may indicate analyzer error, air infiltration, inaccurate fuel composition, unmeasured solid residue, or incomplete combustion.

Understanding the Trade-offs

Too Little Air Causes Incomplete Combustion

Operating below the stoichiometric air requirement can produce CO, soot, smoke, hydrogen, methane, or other unburned hydrocarbons. It also lowers recoverable heat because part of the fuel's chemical energy remains in the exhaust or residue.

Low oxygen readings alone do not prove efficient operation. A low-O₂ flue gas accompanied by elevated CO can indicate oxygen deficiency or poor mixing.

Too Much Air Reduces Thermal Efficiency

Excess air helps promote complete combustion, but it also increases the volume of nitrogen and unused oxygen heated by the furnace. These gases leave through the stack and carry sensible heat away from the process.

Excessive air can therefore lower thermal efficiency, increase fan and ducting requirements, and dilute pollutant concentrations. The appropriate operating point is the lowest excess-air level that maintains stable, complete combustion and satisfies safety and emissions requirements.

Air Leakage Can Corrupt the Calculation

Air entering through furnace seals, inspection ports, duct joints, or sampling points increases measured flue-gas oxygen without necessarily representing combustion air supplied to the burner. This can make the calculated excess air appear higher than the burner setting suggests.

Pilot-plant balances should therefore distinguish between combustion air and uncontrolled dilution air wherever possible.

Moisture and Dew Point Must Be Considered

Fuel moisture and water formed from hydrogen combustion appear in wet flue gas but are absent from dry Orsat results. This distinction matters when converting measured concentrations into total flow rates or performing an oxygen balance.

The flue gas must also remain above its relevant water or acid dew point in heat-recovery equipment unless condensation is intentionally managed. Sulfur-containing fuels can create acidic condensate and corrosion risks when sulfur oxides and water condense together.

How to Apply This to Your Pilot Plant

The appropriate method depends on the purpose of the experiment and the available instrumentation.

If your primary focus is combustion efficiency: Measure dry O₂, CO₂, and CO continuously, calculate theoretical air from fuel composition, and operate at the minimum excess air that keeps CO and unburned fuel acceptably low.
If your primary focus is material-balance verification: Use a fixed fuel basis, measure air and flue-gas flow rates, analyze the dry gas, and close carbon, hydrogen, sulfur, oxygen, and nitrogen balances.
If your primary focus is burner or furnace control: Use flue-gas O₂ as a control indicator, but validate it against CO, air leakage, fuel flow, and periodic analyzer calibration.
If your primary focus is emissions or exhaust treatment: Track CO₂, CO, SO₂, oxygen, moisture, and relevant pollutants on a clearly stated dry or wet basis before sizing or evaluating scrubbers and other treatment equipment.

Reliable excess-air evaluation requires both accurate flue-gas measurement and a stoichiometrically consistent combustion balance.

Summary Table:

Evaluation Area	Key Method	Main Considerations
Fuel basis	Analyze carbon, hydrogen, sulfur, oxygen, moisture, and ash	Use a consistent mass or molar basis
Theoretical oxygen	Apply complete-combustion stoichiometry	Subtract fuel-bound oxygen
Stoichiometric air	Divide oxygen demand by air oxygen fraction	Adjust for enriched, humidified, or recycled air
Flue gas analysis	Measure dry O₂, CO₂, CO, SO₂, and N₂	State dry or wet basis and sampling conditions
Excess air from flow data	Compare actual air with stoichiometric air	% excess air = (actual air - stoichiometric air) / stoichiometric air x 100
Excess air from O₂	Use dry flue-gas oxygen concentration	Approximation: O₂ / (0.21 - O₂) x 100
Balance verification	Reconcile carbon, hydrogen, sulfur, oxygen, and nitrogen	Check for leaks, analyzer error, moisture, and unburned fuel
Operating trade-off	Minimize excess air while maintaining complete combustion	Monitor O₂ together with CO and unburned hydrocarbons

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