- Combustion Definition: Combustion is a rapid reaction where fuel and oxygen combine to release heat energy.
- Stoichiometric Air Requirement: The exact amount of air needed to fully oxidize fuel’s combustible elements.
- Coal Combustion Equation: For sufficient air, 2.67 gm of oxygen is needed for 1 gm of carbon, producing 3.67 gm of CO2.
- Excess Air Importance: Providing 20% more air than theoretically needed ensures efficient combustion.
- Combustion Losses: Unburnt gas, dry flue gas, and ash losses occur during combustion, with optimal air minimizing these losses.
Coal combustion theory treats combustion as a rapid reaction in which the fuel’s carbon, sulfur and hydrogen combine with oxygen and release heat. Dry air supplies that oxygen: about 21% by volume, with the rest mostly nitrogen. Nitrogen does not burn.
Nitrogen still carries heat from the furnace to the steam boiler stack. As per combustion theory the air quantity required is the amount that supplies enough O2 to oxidise every combustible element in the fuel. That quantity is the stoichiometric air requirement.
That air mass per unit mass of fuel depends on the fuel analysis. Typical stoichiometric air figures for several fuels are listed below.
| Fuel | STOICHIOMETRIC AIR mass / unit mass of fuel |
| Bituminous Coal | 11.18 |
| Anttiasite Coal | 10.7 |
| Coke | 9.8 |
| Liquite | 7.5 |
| Peat | 5.7 |
| Residual Fuel Oil | 13.85 |
| Distillate Fuel Oil(Gas Oil) | 14.48 |
| Natural Gas(Methane Base) | 17.3 |
Combustion of Coal
For complete combustion of carbon in sufficient air,
By mass, dry air contains about 23.2% oxygen. The air mass needed to supply 2.67 g of oxygen is therefore 2.67 / 0.232 g of air per gram of carbon.
After 1 g of carbon burns completely, the products are 3.67 g of CO2 and 
of N2.
Coal Combustion for Insufficient Air
The air mass that supplies that O2 is
After 1 g of carbon burns to carbon monoxide, the products are 2.33 g of CO and 
of N2.
Comparing the two carbon reactions, the heat left unreleased when 1 g of carbon stops at CO instead of carbon dioxide in coal combustion is 
.
Combustion of Sulfur
Air required for 1 g of sulfur is
The products from 1 g of sulfur are then 2 g of SO2 and 
of N2.
Combustion of Hydrogen
From the C, S and H2 balances, 1 g of carbon needs 2.67 g of oxygen, 1 g of sulfur needs 1 g of oxygen and 1 g of hydrogen needs 8 g of oxygen. For C g of carbon, S g of sulfur and H g of hydrogen those demands become 2.67 C, S and 8 H grams of oxygen.
So 1 g of coal that contains those masses needs (2.67 C + S + 8 H) g of oxygen if none is already in the fuel.
Fuel often already holds some oxygen. If O is that oxygen mass in 1 g of fuel, the net oxygen for complete coal combustion is (2.67 C + S + 8 H – O) g. The matching air mass is
That calculation is the usual ultimate-analysis air demand.
Complete combustion still needs two plant conditions:
- Enough oxygen must reach the fuel.
- The furnace must mix fuel and air well enough that oxygen meets the combustibles.
Coal Content in Proximate Analysis
One example proximate analysis is moisture 8%, volatile matter 20 to 25%, fixed carbon 40% and ash 30%. Fixed carbon typically ignites near 900oC. Ash is mostly silica, alumina and other oxides, not elemental silicon. Pure silicon is not what melts at 1200oC.
If local gas temperature stays above about 1100oC, ash can fuse and slag the tubes, which cuts heat transfer.
Operators then add some excess air to complete burnout and to limit peak temperature.
Volatile matter also shapes the flame. Low-volatile coal can throw a short, hot flame that may impinge on a superheater coil.
In a real furnace, fuel and air do not mix completely at the burner, so the plant supplies more air than the stoichiometric figure. That extra air cuts unburnt carbon and CO, at the cost of fan power and stack heat.
Pulverized-coal boilers often run about 15 to 20% excess air. 20% is a common allowance, not a fixed law.
| % of Excess Air | Unburnt Carbon in Ash | C.V. Liberated in Furnace | Unburnt Gas Loss |
| 0 % | 10 % | 75 % | CO2, O2, N2, H2O, CO, CH4(15 %) |
| 15 % | 2 % | 97 % | CO2, O2, N2, H2, CO(1 %) |
| 100 % | 0.5 % | 99.5 % | CO2, O2, N2 |
The 100% excess-air row is a poor operating point: the fans use more power and the stack carries more unused heat.
A 200-mesh sieve passes particles of about 74 microns. Pulverisers are used for
- More complete burnout of the coal
- Faster reaction of the finer particles.
Three heat losses dominate coal combustion:
- Unburnt gas loss
- Dry flue gas loss
- Combustible in ash loss.
Unburnt Gas Loss
Unburnt-gas loss is mainly carbon that stops at carbon monoxide instead of carbon dioxide. Heat released in the CO reaction is about one third of the heat released when the same carbon goes to CO2. Extra oxygen cuts that loss, but it does not fall all the way to zero if mixing stays poor.
Dry Flue Gas Loss
Dry flue-gas loss, also called stack loss, is the sensible heat leaving in nitrogen, carbon dioxide and unused oxygen. More excess air raises that loss.
Combustible in Ash Loss
This loss is high when there is little or no excess air, because fuel and oxygen mix poorly. As air flow rises, the loss falls. It does not reach zero, because it still depends on mixing and on how fine the pulverized coal is. Finer coal burns more completely, but extra grinding power can cost more than the recovered heat. A common grind is about 75% of a high-volatile coal through 200 mesh, and about 80% of a low-volatile coal through the same mesh.
Unburnt-carbon loss falls, then stack loss rises, so the combined loss has a minimum at one excess-air setting for a given fuel and mill grind. For many bituminous pulverized-coal furnaces that minimum sits near 15 to 20% excess air. 15.5% is one handbook figure, not a universal set point.
