Combustion Air Blower Sizing From Fuel Input: Stoichiometry in Five Steps

By the NextAir Systems engineering team. Last reviewed October 2026.

The airflow a combustion air fan has to deliver is not a judgement call. It follows from the chemistry of the fuel and the fuel input. If you know the burner’s rating and the fuel, you can calculate the airflow on the back of an envelope, and it is worth doing before you ask for a quotation, because it tells you whether the number on the enquiry is sensible.

We work through natural gas in five steps, give the stoichiometric figure for propane, and tabulate the airflow for burners from 1 to 20 MW at three levels of excess air.

The short version

• Burning methane needs two molecules of oxygen per molecule of fuel. Air is 20.95% oxygen, so stoichiometric air is 2 ÷ 0.2095 = 9.55 m³ per m³ of gas.
• Per megajoule of heat, natural gas needs about 0.27 normal cubic metres of stoichiometric air and propane about 0.26.
• A 10 MW burner on natural gas with 15% excess air needs about 12,055 m³/h of actual air at 25 °C, using a gas lower heating value of 35.8 MJ/Nm³.
• The fan has to be sized for the system pressure and the worst-case air density, and AMCA 201 is a reminder that installed performance depends on the connections around the fan.^[1]

Step 1: the reaction

For methane, the main component of natural gas, the combustion reaction is CH₄ + 2 O₂ → CO₂ + 2 H₂O. Each volume of methane needs two volumes of oxygen. For propane, C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O, it is five volumes.

Step 2: from oxygen to air

Air is 20.95% oxygen by volume. The stoichiometric air is the oxygen requirement divided by that fraction.

The heating values are standard figures for the gases at normal conditions and vary slightly with composition. Use the figure from your gas supplier. The air per megajoule is nearly the same for the two fuels, which is why a burner’s airflow is often estimated from its heat input alone.

Step 3: excess air

Burners never run at exactly stoichiometric air, because mixing is imperfect and incomplete combustion wastes fuel and makes carbon monoxide. They run with excess air, typically a modest percentage for gas burners with good control, and more for burners that are less controlled. The right figure comes from the burner manufacturer. We use 10, 15 and 25% to show the effect.

Step 4: the airflow for a burner rating

Gas flow in normal m³/h is the heat input divided by the heating value, times 3,600 seconds. Air is that times 9.55 times one plus the excess air, converted from normal to actual m³/h at the air temperature.

Worked example. A 10 MW burner on natural gas at 35.8 MJ/Nm³ burns 1,006 Nm³/h of gas. Stoichiometric air is 1,006 × 9.55 = 9,607 Nm³/h. With 15% excess it is 11,048 Nm³/h, and corrected to 25 °C it is 12,055 m³/h of actual air. That is the volume flow on which the fan is selected.

Step 5: margins and pressure

• Density. Air on a hot day is less dense, so the fan delivers a lower mass flow at the same volume flow. Check the maximum site temperature and altitude, as in the density correction guide.
• Turndown. Burners run at part load much of the time. The fan must cover the range, and speed control is the usual way to hold efficiency, as in flow control for blowers.
• Pressure. The fan must overcome the burner’s air-side pressure drop, ducts, dampers and the furnace or chamber pressure. The burner manufacturer states the first and the designer calculates the rest.
• Margin. Add a modest margin on airflow and pressure for fouling and tolerance, not a large one. Oversizing costs energy, as in the life-cycle cost article.
• Other fuels. Oil, coal and biomass need their own stoichiometry, and the air requirement per megajoule differs. Ask the burner supplier for the figure.

A note on oil, coal and biomass

The method is identical for other fuels, with a different oxygen demand and heating value. Liquid and solid fuels also contain moisture, ash and, in the case of biomass, a variable composition, so the supplier’s analysis matters more than a textbook figure. As a rough sense of scale, most hydrocarbon fuels need between 0.25 and 0.28 normal cubic metres of stoichiometric air per megajoule of heat, which is why the burner rating is such a good first guide to the fan duty. Do the calculation on the actual fuel analysis before you finalise the fan.

Checking an enquiry

A quick test of any combustion air enquiry is to divide the airflow by the burner input. For natural gas with modest excess air the result is about 0.27 to 0.34 normal cubic metres per megajoule, or about 1,000 to 1,200 Nm³/h per MW. A figure far outside that range suggests an error in the enquiry, a different fuel, or a unit confusion, and is worth a call before the order.

Where this fits with NextAir

NextAir builds centrifugal blowers up to 800 HP and 500 mm WC for combustion air, in MS and high-temperature construction. Send the burner rating, fuel, maximum air temperature and system pressure to our centrifugal blower manufacturer team. Our centrifugal blower guide explains the selection, and power generation and glass, ceramics and refractories are two typical applications.

Sources

1. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.
2. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

Frequently Asked Questions