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)
Flame Shape
Could be long and “lazy” as heat is re leased 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
Highest flame temperature as possible
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
The following factors serve to raise the flame temperature:
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
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)
Unsafe operation (explosion in precipitator or anywhere in the system)
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
(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 (inoC oroF) 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
oC (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%
o o o
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)
HEAT TRANSFER IN A ROTARY KILN
Radiation (Flame Zone)
Flame/Gas by Material
Flame/Gas by Kiln shell
Very important because heat transfer x (Tf 4 – Tm4) Conduction
Wall to Material
Chain/crosses to Material
Convection
Gas to Material
Gas to Wall
BURNER PIPES AND NOZZLES
What is required from a burner?
A stable flame with proper geometry Versatility
Safety
Parameters Affecting the Flame
Fuel characteristics
Primary air and secondary air The burner design
Key Parameters of Burner Design Number of circuits
Primary air Quantity Ejection velocities Minimum velocity Back pressure Specific impulse Diameter
Number of Air Circuits (Figure 1 to 6)
Single Circuit
Control is minimal
High velocity requires high fan pressure Results in more wear
Two Circuits
Swirl + high velocity transport air Additional control due to swirl High fan pressure, high wear rate
Three Circuits
Swirl + high velocity axial Low velocity transport air More versatile
Primary Air Quantity
Natural gas: 0 to 7% of total combustion air Liquids: 7 to 10% of total combustion air Solid fuels: Firing system dependent
Direct firing system: 30 to 35% of total combustion air
Semi-direct firing: Mill exit air is 18 to 25% (moisture dependent) Air to burner can be controlled by diverting
“Overflow” to kiln hood
Indirect firing: 7 to 12% of total combustion air Theoretically, indirect firing is the ideal solution.
Ejection Velocities
Gaseous and liquid fuels (with atomizing fluid) Sonic range: 330 m/s (1083 ft/s) Solid fuels
Single circuit: 50 –80 m/s (164 –263 ft/s)
Multiple circuits: 80 –170 m/s (262 –558 ft/s) for axial 50 –90 m/s (164 –295 ft/s) for swirl 20 –50 m/s (65 –164 ft/s) for transport
Minimum Velocities
Required to prevent solid fuel accumulation, 20 m/s (66 ft/s) in transport line and in axial $ swirl annulus if these streams carry any fuel dust
Knowing the minimum flow rates, the pipe sizes can be determined.
Back Pressure
Typical values for a three-circuit burner;
700 – 1000 mmH2O for axial air 150 – 600 mmH2O for swirl air 600 – 1000 mmH2O for transport air
(up to 1200 mmH2O for modified three-circuit burner)
Specific Impulse
Typically, 4 to 8 N/Gcal/h
Definition
Sp. impulse = Impulse (Newton) Heat input (Gcal/h)
For Solid Fuel;
Impulse = air stream mass flow rate (kg/s) x tip velocity (m/s) For Gaseous and Liquid (pneumatically atomized) Fuels;
Impulse = (abP – c) S
Where: P = pressure (bars)
S = cross-section area (mm2)
a,b,c are atomizing fluid dependent
* 5 figures of burners to be attached at the above information.
FUELS IN THE CEMENT INDUSTRY
Solid Fuels:
Coal
Coal tailings
Petroleum coke (fluid, delayed) Wood
Tires
Municipal waste, etc.
Liquid Fuels:
Oil (bunker C) Liquid waste
Coal slurry, etc.
Gaseous Fuels:
Natural gas (95% methane) Landfill gases, etc.
Heating Value
Quantity of heat generated from 1 unit (kg, lb, ton, m3, liter) of fuel Measured in kcal/kg, Btu/lb, MJ/ton, MJ/m3, kcal/liter
Can be approximated from the fuel composition
High Heating Value (HHV) vs. Low Heating Value (LHV) High (or gross) Heating Value
Heat produced at constant volume by complete combustion of fuel, combustion product condensed to liquid state, measured in the laboratory in an “oxygen bomb calorimeter”.
Low (or net) Heating Value
Calculated from HHV by subtracting the latent heat of vaporization
The difference between HHV and LLV depends upon the hydrogen content of the fuel.
LHV = HHV - 92.7 x % H2 (Btu/lb)
Examples of HHV and LHV for Various Fuels
Fuel % H HHV
Btu/lb
LHV
Btu/lb (% of HHV)
Coal 5 12,000 11,540 (96%)
Coke 4 14,000 13,630 (97%)
Waste fuel 10 9,000 8,070 (90%)
Fuel oil 10 19,000 18,070 (95%)
Natural gas 25 23,300 20,680 (90%)
SOLID FUELS SOLID FUELS
Coal, oil, gas and in recent years, petroleum coke are the main fuels used in cement kilns.
Coal, oil, gas and in recent years, petroleum coke are the main fuels used in cement kilns.
Coals are judged on what is called an proximate analysis which tells the percentage moisture, volatile Coals are judged on what is called an proximate analysis which tells the percentage moisture, volatile matter, fixed carbon, ash, sulfur, and heat value.
matter, fixed carbon, ash, sulfur, and heat value.
Coal is very complicated in structure, containing carbon, hydrogen, oxygen, nitrogen and sulfur in Coal is very complicated in structure, containing carbon, hydrogen, oxygen, nitrogen and sulfur in various stages of
various stages of combinations.combinations.
With the application of heat, these substances from various combustible gases are classed as volatile With the application of heat, these substances from various combustible gases are classed as volatile matter. It is the first constituent of the coal to be liberated on heating.
matter. It is the first constituent of the coal to be liberated on heating.
The carbon that remains after the volatile matter is
The carbon that remains after the volatile matter is driven off is called driven off is called fixed carbon.fixed carbon.
Combustion is the chemical combination of oxygen with certain elements of the fuel to form compounds Combustion is the chemical combination of oxygen with certain elements of the fuel to form compounds with the release of heat. See figure below.
with the release of heat. See figure below.
Every combustible substance has what
Every combustible substance has what is called an is called an ignition temperaturignition temperature.e.
This is the temperature to which it must be raised before chemical combinations with oxygen or This is the temperature to which it must be raised before chemical combinations with oxygen or combustion will take place.
combustion will take place.
These combinations liberate definite amounts of heat depending on the elements entering into the These combinations liberate definite amounts of heat depending on the elements entering into the combinations.
combinations.
The principal combustible elements in coal are carbon and hydrogen. Any sulfur present is also The principal combustible elements in coal are carbon and hydrogen. Any sulfur present is also combustible, but is of minor importance from a
combustible, but is of minor importance from a heat standpoint.heat standpoint.
The chief non-combustible elements in coal are silica, alumina and
The chief non-combustible elements in coal are silica, alumina and iron which form the ash.iron which form the ash.
Bomb Calorimeter Bomb Calorimeter The heat value of coal
The heat value of coal can be determined by using the bomb calorimetercan be determined by using the bomb calorimeter. A precise amount of dry coal is. A precise amount of dry coal is fired in an oxygen atmosphere in a sealed container which is immersed in a water bath. The change in fired in an oxygen atmosphere in a sealed container which is immersed in a water bath. The change in temperature of the water is measured and knowing the specific heat of water (1.0 Btu/lb
temperature of the water is measured and knowing the specific heat of water (1.0 Btu/lbmm**ooF) the heatF) the heat content of the coal is determined.
content of the coal is determined.
Volatile Matter (VM) Volatile Matter (VM)
Portion of solid fuel liberated as gases and vapors, when it is heated in the absence of air. (result from Portion of solid fuel liberated as gases and vapors, when it is heated in the absence of air. (result from thermal decomposition)
Combustion start to be difficult when VM < 15%
Combustion start to be difficult when VM < 15%
Fixed Carbon (FC) Fixed Carbon (FC)
Residue left after volatile matter is driven off.
Residue left after volatile matter is driven off.
FC
Residue remaining after the fuel has been burnt.
Residue remaining after the fuel has been burnt.
Composed of compounds of silicon, aluminum, iron and calcium Composed of compounds of silicon, aluminum, iron and calcium Also some traces of Mg, Na, K and Ti.
Also some traces of Mg, Na, K and Ti.
Proximate Analysis Proximate Analysis
Determination of VM, FC, ash and moisture Determination of VM, FC, ash and moisture Used for quick, preliminary appraisal of solid Used for quick, preliminary appraisal of solid fuelfuel
Ultimate Analysis Ultimate Analysis
Quantitative determination of moisture, C, H, S, O and N Quantitative determination of moisture, C, H, S, O and N
DRYING, GRINDING AND FIRING SOLID FUELS DRYING, GRINDING AND FIRING SOLID FUELS
Ball Mill or Roller Mills are used.
Ball Mill or Roller Mills are used.
Air from cooler / air heater / pre-heater exit is used for drying.
Air from cooler / air heater / pre-heater exit is used for drying.
The drying systems can be; (Figure # 1, 2, 3) The drying systems can be; (Figure # 1, 2, 3)
Direct
Comparison of fuel drying system Comparison of fuel drying system
Direct Semi-direct
Direct Semi-direct IndirectIndirect
Simple
Simple operation operation Most Most difficult difficult to to operate operate Simple Simple operationoperation Relatively
Relatively safe safe Safer Safer than than indirect indirect Safety Safety is is most most importantimportant Primary air is high (30-35%)
Primary air is high (30-35%) All moisture to kiln
All moisture to kiln
Primary air is low but all Primary air is low but all moisture to kiln
moisture to kiln
Primary air can be as low as Primary air can be as low as desired (operation
Heat penalty is lower than Heat penalty is lower than direct
direct Good heat Good heat consumpticonsumptionon Lowest capital cost
Lowest capital cost Capital cost is in betweenCapital cost is in between direct and indirect
direct and indirect Highest capital costHighest capital cost
Liquid Fuels Liquid Fuels
Essentially composed of C, H, and S Essentially composed of C, H, and S Evaporation at 200-300oC or 572oF Evaporation at 200-300oC or 572oF Atomizati
Atomization on promotes evaporationpromotes evaporation Classified according to:
Classified according to:
Viscosity (measure of internal friction) Viscosity (measure of internal friction) Specific gravity
Specific gravity
Heating value
Gaseous Fuels
Natural gas most commonly used (95% methane) No preparation required for firing
Minimum ignition temperature (+/- 650oC or 1200oF) Very little primary air is required
Injected at high pressure (i.e. high tip velocity) to promote turbulence
Comparison of the Three Major Fuels
Coal Oil Natural Gas
Installation is expensive Installation is simple Installation is simple Can be stored in large storage
capacity
Storage capacity depends on
refinery No storage
Radiant flame Radiant flame Non-radiant flame
Ash can be used as a raw
material n/a n/a
Cheap Expensive Expensive
Noisy and dirty Depend on refinery Clean
Risk of explosion and fire n/a Flame control reacts
Quality can vary (% H2O &
Ash) Quality is constant Quality is constant
COMBUSTION REACTIONS
Combustibles Reactions Heat Released
Carbon C + O2 = CO2
C + ½ O2 = CO
14650 Btu/lb 4340 Btu/lb
Hydrogen 2H + ½ O2 = H2O 62100 Btu/lb
Sulfur S + O2 = SO2 4032 Btu/lb