Combustion Theory
![triangle](img/general/xCombustionTriangle.jpg.pagespeed.ic.bQkDyMML7J.jpg)
What is Combustion?
- High speed fuel oxidization that produces light, heat and flame
- Three necessary components: oxygen, ignition, fuel
- If one of the components is not present, combustion does not occur
- Removal of either fuel or air will stop combustion
![propane and methane](img/general/xhydrocarbon.jpg.pagespeed.ic.qaxXIOVP9-.jpg)
Gaseous Fuel Composition
- Hydrocarbons determine calorific value
- More carbon content means more heating value
A hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. A molecule of propane and methane is pictured on the left.
![fuelairmix](img/general/xFuelAirMix.jpg.pagespeed.ic.XQSPxB_u_D.jpg)
Fuel to Air Mixture
- Hydrocarbons determine calorific value
- More carbon content means more heating value
![fuel to air ratio](img/general/xFuelAirRatio.jpg.pagespeed.ic.rrtOi3nOR0.jpg)
Fuel to Air Ratio
- There are two ways of creating CO with the wrong fuel/air ratio
- Insufficient air (sub-stoichiometric)
- Too much air (over-airing)
- Sub-stoichiometric conditions create CO very quickly
- CO production with over-airing is less rapid
![air](img/general/xAir.jpg.pagespeed.ic.oNlJUc_1EI.jpg)
Air is Composed of:
- 21% Oxygen
- 78% Nitrogen
- 1% Carbon Dioxide and other gasses (argon, krypton, etc.)
![products of combustion](img/general/xProdsComb.jpg.pagespeed.ic.Mlik__Kc3M.jpg)
Combustion of Natural Gas
In simple terms, the chemical equation of natural gas is:
AIR + NATURAL GAS -> HEAT + WATER VAPOR + CO2
However, since the oxygen in real-world combustion comes from the air around us, undesirable byproducts in the form of nitrogen will be produced.
Perfect (Stoichiometric) Combustion
To mimimize the amount of 'undesirable' emissions, a perfect air to fuel ratio has to be achieved.
When theoretical perfect air/fuel mixtures are applied, stoichiometric combustion is achieved.
Eg: 10 parts air to 1 part natural gas
Each fuel has a maximum CO2 percentage when stoichiometric combustion is achieved
- Natural gas: 11.8 - 12.5% CO2
- Propane gas: 13.6 - 13.8% CO2
- Fuel oil: 15.4 - 15.6% CO2
Real World Combustion
In the real world, it is very difficult to maintain the perfect 10:1 air/gas ratio due to a number of factors.
- Air Temperature Changes
- Air Pressure Changes
- Varying Furnace Pressures
- Varying Draft Pressure
- Fan Contamination
![excess air](img/general/xExcessAir.jpg.pagespeed.ic.HooK9NqsID.jpg)
Excess Air
The common way of expressing the usage of more than the stoichiometric combustion air is percent excess combustion air.
-Too little excess air creates CO and/or soot
-Too much excess air is a waste of energy
-For safety, the burner is usually adjusted for an additional 17-30% excess air
![measuring flue gas](img/general/xmeasuring.jpg.pagespeed.ic.MZJtfMU6er.jpg)
Measuring Excess Air
Excess air is measured by analyzing flue gas (exhaust gas)
Excess air is expressed as Lambda
![incomplete combustion](img/general/xIncCombustion.jpg.pagespeed.ic.txzXRSgCUp.jpg)
Carbon Monoxide
Incomplete combustion causes single carbon atoms to combine with single oxygen atoms, productin carbon monoxide.
Reasons why CO is formed:
- Insufficient air
- Insufficient heat (chilled flame)
- Too much air
CO is a an odourless, colourless gas
CO is undetectable without instruments such as a flue-gas analyzer
CO is a flammable gas that will ignite in the range of 12.5 to 74% CO to air
CO is created when the fuel/air ratio is not set correctly
Effects of Exposure to Carbon Monoxide
- 12,800 ppm - Death within 1 to 3 minutes.
- 1,600 ppm - Nausea within 20 minutes. Death within one hour.
- 800 ppm - Nausea and convulsions. Death within two hours.
- 400 ppm - Frontal headaches 1-2 hours. Life threatening after 3 hours.
- 50 ppm - Maximum concentrations for continuous exposure in any eight-hour period.
- 9 ppm - Maximum indoor air quality level.
- 0 ppm - Desirable level.
Oil Combustion
- Oil contains considerably more nitrogen than gaseous fuels
- NOX levels are generally much higher than on natural gas
- Oil flue gasses are tested with a Bacharach smoke spot test and combustion analyzer
- Test gives a number from 0 (no smoke) to 10