FLAME
A flame is a stream of gases at extremely high temperature (around 3500oF or 1930oC) where the reactions of combustion of the fuel with secondary and primary air are taking place. Anything exposed to such a flame is bound to receive heat from it.
Flame Evaluation
Should always be evaluated during stable kiln condition
Flame Length
Could refer to the distance between the burner tip and the end of the flame which is a total flame length
It could also refer to a distance between the point where ignition of the fuel start and where the reaction of fuel combustion ends
It is desirable to operate a kiln with the flame as short as possible, as long as it will not create problem in front of the kiln, hood, nose ring and refractory (Figure #1)
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Flame Shape
Could be long and “lazy” as heat is released over a relatively long distance (example A) Could be “snappy” as heat is released over a shorter distance (example C)
Flame Direction
The flame path is not a straight line
The flame has a tendency of lift upward toward the top resulting in uneven entrance of secondary air, or mechanical condition of the primary air pipe nozzle
A good direction target for the flame could be 2A or 2B in Figure # 2, or one inch down center line and one inch towards the material load
The flame temperature is related to: 1. Quality and type of fuel used
Gas: 1830oC or 3325oF
Fuel oil: 1956oC or 3553oF
Coal: 1927oC or 3500oF
2. Total combustion air temperature Secondary air temperature Primary air temperature Air in-leakage temperature
3. Oxygen level at kiln outlet
4. Brick and coating temperature in the burning zone
Flame Target
Ignition as quick as possible
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Length as short as possible As constant as possible
Primary air flow minimum to carry fuel in kiln
All above combined in such a way for not making erosion and direct contact of the flame on the refractory
Shell temperature scanner is a good indication of flame profile.
Flame Adjustment
Increase in primary air
The speed will increase Temperature will increase The volume will become wider
Increase in primary air temperature The plume will get shorter The flame will become shorter The flame will become wider
Increase in secondary air temperature The flame temperature will increase The flame length will decrease The plume will decrease
Increase on the oxygen level
The flame length will increase The flame temperature will decrease
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Increasing the secondary air temperature
Using less primary air, thus making it possible to utilize more secondary air which is preheated to higher temperature
Promoting rapid mixing of the air and fuel upon leaving the burner by improving the design of primary air pipe and burner
Better atomization of the fuel oil by increasing the fuel oil temperature or employing a mechanical device in the burner nozzle to bring a better atomization
By keeping hood pressure as close as possible from “0” in order to avoid air in-leaking in front of kiln
Operating the kiln with neither a deficiency or excess of air by maintaining the oxygen content of not less than 0.7% and not more than 3.0%
Rules on Flames
a) When the primary air pipe nozzle has accidentally been warped, resulting in an erratic flame shape and direction, immediate steps should be taken to repair this condition
b) A flame should never be allowed to impinge upon the coating or bare refractory for a prolong length of time
c) A flame should never be allowed to strike too hard upon the feed bed
d) Oil burners or gas burners should be centered well in the primary air pipe in order that an even envelopment of air around the fuel jet takes place
e) Flame direction should be adjusted only when the kiln is in stable operating conditions and the temperatures, fuel pressures, and air flow rates are at normal level. Flame direction changes can be caused by unusual operating conditions. If any attempt were made to adjust the flame at such a time, there will most likely be an undesirable flame once the kiln returns to normal operating conditions again
f) It is better to make the desired adjustments in flame direction in several small steps instead of a large one in order that the operating stability of the kiln is not affected adversely
g) Once the ideal flame direction has been obtained, the primary air pipe position should not be changed unless a definite reason (such as to combat a ring formation or hot shell conditions) makes it desirable
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h) To protect the primary air pipe from possible damage during a shutdown, a certain amount of primary air flow must be maintained until the temperature inside the kiln is low enough (approximately 600oF or 315oC) that the pipe cannot be damaged. Upon power failure when primary air fan stops, the primary air pipe must be immediately removed from the burner hood.
COMBUSTION
What is combustion?
Rapid combination of oxygen with fuel resulting into heat
Fuels contains, Carbon Hydrogen Sulfur
Oxygen Comes from Combustion Air
Carbon + Oxygen = Carbon dioxide + heat Hydrogen + Oxygen = Water vapor + heat Sulfur + Oxygen = Sulfur dioxide + heat
Proper Proportioning of Fuel, Oxygen and Heat Perfect combustion
Combustion air = Neutral (stoichiometric) combustion air
Deficiency of Air (Reducing Conditions) Incomplete combustion
Heat released is low (4500 Btu vs. 14500 Btu per lb carbon)
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Excess of Air (Oxidizing Conditions) Complete combustion
Flame temperature decreases with increasing air, lower fuel economy Recommended back-end oxygen is 1.0 to 1.5%
Combustion Air + Fuel = Combustion Gases
Combustion Air = Primary Air + Secondary Air + Leakage
Combustion Gases:
Carbon monoxide (CO) with incomplete combustion Carbon dioxide (CO2) with complete combustion Water vapor (H2O)
Sulfur dioxide (SO2) Nitrogen (N2) from air Excess oxygen (O2)
Good Combustion Requirements:
Proper proportioning of fuel and air Thorough mixing of fuel and air
Initial and sustained ignition of the mixture
Mixing of Fuel and Air
Good mixing is important for mixture to be uniform throughout Every particle of fuel must be in contact with an air particle
Solids must be pulverized to increase surface area for mass transfer
Liquids must be atomized (breaking up into tiny particles) to speed up evaporation (resulting to vapors burn as gases)
Process of starting combustion
Can start at low temperatures, but may not be sustained Minimum ignition temperature required for sustained ignition
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(Ignition continues without any external source of heat)
At this point:
Heat from Reaction > Heat Lost to Surroundings
Fixed carbon:* 400 – 450oC or, 752 – 842oF Volatile Matter: 500 – 600oC or, 932 – 1112oF
C & H (methane): 632oC or, 1170oF
Coke: +/- 800oC or, 1472oF
Fuel oil: 200 – 300oC or, 392 – 572oF
* can be considered ignition temperature of coal
Theoretical Flame Temperature: Tf = LHV / (NCA + 1) S
Where Tf = Maximum (theoretical) flame (in oC or oF) LHV = Fuel low heating value (in kg/kgf or Btu/lbf) NCA = Neutral combustion air (in kg/kgf or Btu/lbf) S = Specific heat of combustion gases (=/- 0.29)
Typical Fuel Data:
Fuel LHV
Kg/kgf (Btu/lbf) NCA
Theo. Max. Flame T o
C (oF)
Coal 6500 (11,700) 9.1 2460 (4460)
Oil 9870 (17,770) 13.7 2480 (4500)
Gas 11,500 (20,700) 16.6 2400 (4350)
Influences and Impact on Flame Temperature
Impact of Oxygen content of Kiln Gases on Flame Temperature (Figure #3)
Oxygen 1% 5%
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Impact of Secondary Air Temperature on Flame Temperature
Sec. air To 420oC (770oF) 845oC (1553oF) 1093oC (2000oF) Flame To 2180oC (39560oF) 2445oC (4433oF) 2610oC (4730oF)