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Excess Air


Combustion air and flue gas for some common fuels:

Fuel Theoretical air for combustion
Volume at standard pressure
Theoretical flue gas
Volume at standard pressure
Solids (m3/kg) (ft3/lb) (m3/kg) (ft3/lb)
Anthracite 9,4 150 9,5 152
Bituminous coal 6,9 110 7,0 112
Charcoal 8,4 134 8,4 135
Coke 8,4 134 8,0 135
Lignite 5,7 92 8,4 93
Peat 5,7 92 5,9 94
Semi anthracite 8,4 135 8,6 137
Wood (dry) 4,4 70 5,0 80
Gas (m3air/m3fuel) (ft3air/ft3fuel) (m3air/m3fuel) (ft3air/ft3fuel)
Butane C4H10 31 31 27 27
Natural gas 9,5 9,5 8.5 8,5
Propane C3H8 24 24 22 22
Town gas 4,0 4,0 3,8 3,8
Fluids (m3air/literfuel) (ft3air/galfuel) (m3air/literfuel) (ft3air/galfuel)
Gas oil 9,8 1.570 10,4 1.670
Heavy fuel oil 10,8 1.730 11,6 1.860



Fuel Excess of Air (%) Coefficient
Anthracite 40 1,40
Coke oven gas 5 - 10 1,05 – 1,10
Natural Gas 5 - 10 1,05 – 1,10
Coal, pulverized 15 - 20 1,15 – 1,20
Coal, stoker 20 - 30 1,20 – 1,30
Oil (No. 2 and No. 6) 10 to 20 1,10 – 1,20
Semi anthracite, hand firing 70 to 100 1,70 – 2,00
Semi anthracite, with stoker 40 to 70 1,40 – 1,70
Semi anthracite, with traveling grate 30 to 60 1,30 – 1,60

Excess Air and O2 and CO2 in Flue Gas

Excess Air
%
Carbon Dioxide - CO2 - in Flue Gas (% volume) Oxygen in Flue Gas for
all fuels (% volume)
Natural Gas Propane Butane Fuel Oil Bituminous Coal Anthracite Coal
0 12,0 14,0 15,5 18,0 20,0 0,0
20 10,5 12,0 13,5 15,5 16,5 3,0
40 9,0 10,0 12,0 13,5 14,0 5,0
60 8,0 9,0 10,0 12,0 12,5 7,5
80 7,0 8,0 9,0 11,0 11,5 9,0
100 6,0 6,0 8,0 9.5 10,0 10,0


An optimal content of carbon dioxide - CO2 - after combustion is approximately 10% for natural gas, and approximately 13% for light oils.
Normal combustion efficiencies for natural gas at different amounts of excess air and flue gas temperatures are indicated below:

Combustion Efficiency (%)
Excess % Flue Gas Temperature °F (°C)
Air Oxygen 200 (93) 300 (150) 400 (204) 500 (260) 600 (315)
9.5 2.0 85.4 83.1 80.8 78.4 76.0
15 3.0 85.2 82.8 80.4 77.9 75.4
28.1 5.0 84.7 82.1 79.5 76.7 74.0
44.9 7.0 84.1 81.2 78.2 75.2 72.1
81.6 10.0 82.8 79.3 75.6 71.9 68.2


Flue Gas Loss Combustion Oil

The relationship between temperature difference flue gas and supply air, CO2 concentration in the flue gas, and the efficiency loss in the flue gas combustion oil, is expressed in the diagram below.


The recommended boiler exhaust velocity should be within certain limits indicated below:


Exhaust Gas Exhaust Velocity
m/s ft/s
Ducts at minimum load < 4.0 < 13
Stack at minimum load < 5.0 < 16
Boiler with one-step burner (on - off) 5.0 - 8.0 16 - 26
Boiler with two-step burner (high - low) 10 - 15 31 - 49
Boiler with modulating burner 15 - 25 49 - 82
To keep the surface free from soot the velocity should exceed 3.0 - 4.0 9.8 - 13