Thermodynamics

Gas Combustion Calculator

Determine theoretical oxygen, theoretical air, excess air ratio (λ), and combustion products from a pure gaseous fuel's chemical formula and its measured dry flue gas analysis. Calculation basis: 1 kmol of fuel.

Gas Combustion Molar Basis CxHyOz Excess Air (λ)
Fuel CxHyOz

Enter the formula: CH4, C2H6, C3H8, C2H4O — only C, H and O atoms.

C (x) — atoms/mol
H (y) — atoms/mol
O (z) — atoms/mol
Molar Mass — kg/kmol
Dry Flue Gas Analysis kmol / 100 kmol dry gas
kmol %
kmol %
kmol %
auto
S CO2 + CO + O2 —
Calculation
Basis: 1 kmol of fuel

About This Simulator

Combustion is a high-temperature exothermic redox reaction between a fuel (the reductant) and an oxidizer — usually atmospheric oxygen. For gaseous fuels this reaction occurs rapidly in the gas phase. Complete combustion happens when the fuel reacts entirely with oxygen, producing only fully oxidized products such as carbon dioxide (CO2) and water vapor (H2O). Incomplete combustion occurs when there is insufficient oxygen or poor mixing, leading to carbon monoxide (CO), unburnt hydrocarbons, or soot.

This calculator solves the stoichiometry of pure gaseous fuels (CxHyOz) on a molar basis, referenced to 1 kmol of fuel. It determines the theoretical oxygen and air requirement, then back-calculates the actual air supplied from a measured dry flue gas analysis via a carbon balance — a standard technique for evaluating real combustion equipment such as boilers and furnaces.

How to Use

  1. Step 1. Enter the fuel's chemical formula (CxHyOz), e.g. CH4, C3H8, or C2H4O. The parsed atom counts and molar mass appear automatically.
  2. Step 2. Enter the measured dry flue gas analysis: CO2, CO and O2, all in kmol per 100 kmol of dry gas. N2 is calculated automatically by difference.
  3. Step 3. Click Calculate. The gas analysis must contain at least CO2 or CO so the carbon balance can be solved.
  4. Step 4. Read the diagnostic badge (stoichiometric, excess air, or air deficit) and the results below.

Understanding the Results

Theoretical O2 and Theoretical Air (L0)

Theoretical O2 is the exact molar quantity of oxygen required for complete stoichiometric combustion of the fuel. Since air is approximately 21% O2 and 79% N2 by volume, theoretical air (L0) is the amount of air needed to supply that oxygen. It is the baseline reference for designing and evaluating combustion equipment.

Excess Air Ratio (λ)

λ = Actual air (L) / Theoretical air (L0). If λ = 1 the combustion is stoichiometric; λ > 1 means the system runs lean (excess air); λ < 1 means it runs rich (air deficit). Real equipment operates with excess air to ensure complete combustion, but too much excess air wastes energy heating unnecessary nitrogen and lowers thermal efficiency. Typical industrial burners operate with λ between 1.05 and 1.20.

Actual Air (L) and Air-Fuel Ratio

Actual air is back-calculated from the dry gas analysis via a carbon balance, then reported on a molar basis (kmol air/kmol fuel) as well as a mass basis (kg air/kg fuel).

Combustion Products

The carbon in the fuel is split between CO2 and CO in the same proportion measured in your dry gas analysis — so if you entered CO > 0, it shows up as a real product here instead of being silently converted to CO2. Hydrogen is assumed to fully oxidize to H2O, N2 is inert (from the combustion air), and if λ > 1 there is leftover O2. The table reports each in kmol per kmol of fuel and as % volume on a wet basis.

Frequently Asked Questions

What's the difference between theoretical and actual air?
Theoretical air (L0) is the minimum air needed for complete combustion, calculated purely from the fuel's formula. Actual air (L) is what a real system actually supplies, determined here from the measured dry flue gas composition. Real systems always need L = L0 to avoid incomplete combustion.
Why is N2 calculated automatically instead of entered?
A dry gas analysis of CO2, CO, O2 and N2 must sum to 100 kmol %. Since N2 is the remainder once CO2, CO and O2 are known, the simulator computes it by difference to keep the analysis internally consistent.
Why do I need at least CO2 or CO in the gas analysis?
The actual air supplied is back-calculated using a carbon balance: the carbon atoms in the fuel must match the carbon found as CO2 and CO in the flue gas. Without measured carbon in the gas, that balance cannot be solved.
What are the limitations of this simulator?
It only supports pure gaseous fuels made of C, H and O (no sulfur or fuel-bound nitrogen). CO shown in the products table comes directly from your measured gas analysis — the simulator does not predict how much CO would form under a given air deficit, nor does it model other intermediate species (soot, unburnt hydrocarbons, H2). Air is assumed to be exactly 21% O2 / 79% N2, and the flue gas analysis is assumed to be on a dry basis.