Optimal Combustion Processes - Fuel vs. Excess Air
To understand efficient boiler operation the combustion process must be understood.
Stable combustion conditions requires right amounts of fuels and oxygen. The combustion products are heat energy, carbon dioxide, water vapor, nitrogen, and other gases (excluding oxygen). In theory there is a specific amount of oxygen needed to completely burn a given amount of fuel. In practice, burning conditions are never ideal.
Therefore, in practice more air than ideal must be supplied to burn all fuel completely. The amount of air more than the theoretical requirement is referred to as excess air.
Power plant boilers normally run about 10 to 20 percent excess air. Natural gas-fired boilers may run as low as 5 percent excess air. Pulverized coal-fired boilers may run with 20 percent excess air. Gas turbines runs very lean with up to 300 percent excess air.
Typical values of excess air for some commonly used fuels are shown in the table below:
| Fuel | Excess of Air (%) |
|---|---|
| Anthracite | 40 |
| Coke oven gas | 5 - 10 |
| Natural Gas | 5 - 10 |
| Coal, pulverized | 15 - 20 |
| Coal, stoker | 20 - 30 |
| Oil (No. 2 and No. 6) | 10 to 20 |
| Semi anthracite, hand firing | 70 to 100 |
| Semi anthracite, with stoker | 40 to 70 |
| Semi anthracite, with traveling grate | 30 to 60 |
To determine the excess air at which the combustion system will operate we have to start with the stoichiometric air-fuel ratio known as the perfect or ideal fuel ratio - or stoichiometric combustion. With stoichiometric combustion there is
- chemically correct mixing proportion between air and fuel
- no fuel or air left over
In practice process heating equipment almost never runs stoichiometric. Even so-called "on-ratio" combustion, used in boilers and high temperature process furnaces incorporates a modest amount of excess air - 10 to 20% more than needed to burn the fuel completely.
If insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide are exhausted from the boiler. The results is heat transfer surface fouling, pollution, lower combustion efficiency, flame instability and a potential for explosion. To avoid inefficient and unsafe conditions, boilers normally operate at an excess air level. This excess air level also provides protection from insufficient oxygen conditions caused by variations in fuel composition and "operating slops" in the fuel-air control system.
Related Topics
• Combustion
Boiler house topics, fuels like oil, gas, coal, wood - chimneys, safety valves, tanks - combustion efficiency.
• HVAC Systems
Heating, ventilation and air conditioning systems - design and dimensions.
Related Documents
Air Supply to a Boiler House
Incomplete boiler combustion may generate carbon monoxide - CO - and re-ignition may cause disastrous effects on personnel and property.
Alternative Fuels - Properties
Properties of alternative fuels like biodiesel, E85, CNG and more.
ASTM Standard - Volume 05.06 Gaseous Fuels, Coal and Coke
An overview of ASTM Section 5 - Petroleum Products, Lubricants, and Fossil Fuels - Volume 05.06 Gaseous Fuels, Coal and Coke.
Boiler - Efficiency
Combustion gross and net calorific value.
Boiler Rooms - Sizing
Minimum area in a boiler room.
Coal - Classification
Classification of coal based on volatile matter and cooking power of clean material.
Combustion Appliances - Exhaust Temperatures
Exhaust and outlet temperatures fuels like natural gas, liquefied petroleum, diesel and more.
Combustion of Fuels - Carbon Dioxide Emission
Environmental emission of carbon dioxide CO2 when combustion fuels like coal, oil, natural gas, LPG and bio energy.
Combustion Processes and Combustion Efficiency
Typical furnace combustion efficiencies in fireplaces, space heaters, boilers and more.
Flue Gases - Dew Point Temperatures
Flue gas dew point temperatures and condensation of water vapor.
Fossil vs. Alternative Fuels - Energy Content
Net (low) and gross (high) energy content in fossil and alternative fuels.
Fuels - Boiling Points
Fuels and their boiling points.
Fuels - Combustion Air and Flue Gases
Combustion air and flue gas for common fuels - coke, oil, wood, natural gas and more.
Fuels - Higher and Lower Calorific Values
Higher and lower calorific values (heating values) for fuels like coke, oil, wood, hydrogen and others.
Fuels Flue Gases - Average Dew Points
Flue gas dew point temperatures for some fuels.
Gaseous Fuels - Chemical Composition
Chemical composition of gaseous fuels like coal gas, natural gas, propane and more.
Heating Fuels - Cost Comparing
Cost comparison formulas for heating fuels like Natural Gas, Propane LP Gas, Fuel Oil and Electricity.
Intermittent Combustion and Boiler Efficiency
Efficiency reduction due to intermittent boiler operation.
Nitrogen Oxides (NOx) Emission from Fuels
Emission of Nitrogen Oxides - NOx - with combustion of fuels like oil, coal, propane and more.
Paraffins and Alkanes - Combustion Properties
Properties like heat values, air/fuel ratios, flame speed, flame temperatures, ignition temperatures, flash points and flammability limits.
Stoichiometric Combustion
Stoichiometric combustion and excess air.