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Coal Combustion Theory
|Fuel||STOICHIOMETRIC AIR mass / unit mass of fuel|
|Residual Fuel Oil||13.85|
|Distillate Fuel Oil(Gas Oil)||14.48|
|Natural Gas(Methane Base)||17.3|
Combustion of CoalFor sufficient air, We have already said that mass wise there is 23.2 % O2 presents in air. Hence the amount of air required to provide 2.67 gm of O2 is As per ideal combustion theory, after combustion of one gm carbon (C), product of combustion contains only 3.67 gm of CO2 and (11.5 - 2.67 =) 8.83 gm of N2
Coal Combustion for Insufficient AirBy weight, the requirement of air for providing this much O2 is After combustion of one gm carbon(C), product of combustion contains only 2.33 gm of CO and (5.75 - 1.33 =) 4.42 gm of N2. From equation (1) and (2) it is clear that due to insufficient air combustion, the heat lose during 1 gm of coal combustion is (33.94 - 10.12) = 23.82 kj
Combustion of SulfurSo, air required for 1 gm sulfur combustion, is So, combustion product, after completing 1 gm of sulfur combustion, contains 2 gm of SO2 and (4.31 - 1 = ) 3.31 gm of N2
Combustion of HydrogenFrom combustion theory of C, S and H2 it is found that 2.67 gm oxygen is required for 1 gm carbon combustion, which implies 2.67 C gm oxygen is required for C gm carbon, 1 gm oxygen is required for 1 gm sulfur combustion, which implies S gm oxygen is required for S gm sulfur and 8 gm oxygen is required for 1 gm hydrogen combustion, which implies 8H gm oxygen is required for H gm hydrogen.
Hence 1 gm of coal (fuel) which contains C gm carbon, S gm sulfur and H gm hydrogen, requires (2.67 C + S + 8 H) gm of oxygen for efficient combustion. Some amount of oxygen may be contained in the fuel itself in form of different compounds and it takes part in combustion also. If O is the original weight of the oxygen presents in 1 gm of fuel, net requirement of oxygen for sufficient coal combustion is (2.67 C + S + 8 H - O) gm.For that the amount of air required is This above mentioned analysis is called coal analysis for combustion. Before efficient combustion can take place, several basic requirements must be fulfilled, most important of them are,
- The combustion must be done with sufficient oxygen.
- There must be sufficient turbulence to promote throughout mixing of combustible and oxygen.
Coal Content in Proximate AnalysisMoisture = 8 %, volatile material = 20 to 25 %, fixed carbon = 40 %, ash = 30 %. Fixed carbon's combustion temperature = 900°C. Basic component of ash is Si, Al and others. Now fusion temperature of Si is 1200°C. If the furnace temperature raises above 1100°C then Si will be fused and deposited on the tubes, as slag, causing improper heat transfer. Now to dilute the temperature excess air and complete combustion are required. Now, the volatile material plays important role in combustion. Less the volatile material flame will be high which may be chance for flame impingement of S/H coil. For fulfilling the point some practical steps to taken. In practice it is always necessary to supply more air to the combustion system than it is theoretically required. Reason for that air and fuel mixing process in any combustion system, as it is not possible to ensure complete and intimate mixing of the fuel with the necessary oxygen at the point of injection. So some excess air is required for proper combustion to a reasonable minimum power, stack loss and unburnt carbon in ash. Generally 20% excess air is allowed.
|% 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|
- Better utility of coal
- Saving of time.
- Unburnt gas loss
- Dry flue gas loss
- Combustible in ash loss.