Excess air from fuel gas
Excess air, air needed, flue gas volume and composition and the water dew point — from your fuel gas analysis and the flue gas oxygen.
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Fuel to flue gas, excess air, acid dew point and fired-heater checks — with print-ready reports, any units and visible formulas.
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How this calculator works
Each fuel gas is burned completely: CₓHᵧ needs x + y/4 moles of O₂ and gives x CO₂ and y/2 H₂O; H₂ and CO need ½ O₂; H₂S needs 1½ O₂ and gives SO₂ and H₂O; CO₂ and N₂ pass through. Theoretical air is the O₂ needed ÷ 0.2095. The O₂ left over in the flue gas is set equal to your reading, which gives the excess air directly: e = O₂·V0/(A0·(20.95 − O₂)), where V0 is the flue gas at zero excess air (dry or wet, to match the analyser) and A0 the theoretical air.
The example is a natural gas (90 % methane, 5 % ethane, 1.5 % propane, 0.5 % butanes, 1 % CO₂, 2 % N₂) at 3 % dry O₂: 15.1 % excess air, 9.94 Nm³ of theoretical air per Nm³ of fuel, 12.48 Nm³ of wet flue gas, water dew point 56 °C. As a check, pure methane at 3 % dry O₂ gives 15.0 % excess air and 9.55 Nm³ of air per Nm³ — the textbook stoichiometry.
Limits: complete combustion (no CO or unburned fuel), dry combustion air; air leaking in after the burner raises the O₂ reading. References: Perry's §27 (combustion calculations: theoretical and excess air); Baukal, Industrial Combustion (method basis); IAPWS-IF97 for the water dew point.