The document discusses combustion in kiln systems. It defines combustion and its requirements, and describes the different types of firing systems and their components. The key goals of combustion control are to maintain a short, hot flame with low oxygen and carbon monoxide levels in the exhaust gases in order to optimize heat transfer and combustion quality while minimizing heat losses.

1. Combustion
Presentation & Instructor Notes
Introduction to Kiln Control
Operator Development

2. Kiln Control: Combustion 2
Combustion
Learning Objectives
To understand the mechanism of combustion and be able to:
 discern between the 3 types of firing systems
 define combustion air and components of combustion air
 list 3 main flame characteristics and how they can be
controlled
 state importance of fuel/air mixing and variables to control
mixing
 list 3 main indicators of combustion state and how they can be
controlled
 state the main goal in combustion control

3. Kiln Control: Combustion 3
Combustion
Definition of combustion
 a rapid oxidation of a combustible with a release of heat
 a reaction between fuel and oxygen (air)
Requirements for combustion
 sufficient oxygen (combustion air) to mix with fuel
 efficient mixing of fuel and air
 heat to ignite fuel
heat
(ignition)
fuel
air

4. Kiln Control: Combustion 4
The amount of air necessary to efficiently burn at a certain fuel rate.
Combustion air consists of primary air and secondary air.
Combustion Air
COMBUSTION AIR
Primary air
 primary air fan
 solid fuel
transport air
 inleakage
Secondary air
 air from
cooler

5. Kiln Control: Combustion 5
Combustion Air Needs
 Neutral combustion air
 practically impossible to achieve due to poor mixing
of fuel and air
 Excess combustion air
 complete combustion
 too much air results in heat loss
 Lack of combustion air
 incomplete combustion => CO
 loss of efficiency
 Adequate combustion air
 low CO and low O2 at kiln exit

6. Kiln Control: Combustion 6
Types of Firing Systems
Direct Firing System
Semi-direct Firing System
Indirect Firing System (newest technology)

7. Kiln Control: Combustion 7
Direct Firing System
Cooler
Kiln
 One fan to vent the mill, convey the coal, classify the ground
coal and blow it into the kiln (no control of flame shape)
 All moisture goes to kiln
 High primary air (30-35% of combustion air) resulting in high
SHC.
 Relatively safe, simple operation and low capital cost

8. Kiln Control: Combustion 8
Semi-Direct Firing System
 Two fans to classify ground coal and to blow the fuel into the
kiln
 Can add additional fans for flame shaping
 All moisture goes to kiln
 Low primary air
 Higher capital cost than direct firing system
Cooler
Kiln

9. Kiln Control: Combustion 9
Indirect Firing System
 Coal is ground in a separate system
 Moisture removed from system
 Pulverized fuel bin with high precision metering system
 Primary air is low
 Blowers (low volume, high pressure) added to control flame
shape
 Highest capital cost; safety and environmental issues
Cooler
Kiln

10. Kiln Control: Combustion 10
Combustion Air in Indirect Firing
System
COMBUSTION AIR
Primary air w. impulse
 ~4% axial air
 ~2% swirl air
 ~9% fuel transport air
 plus inleakage
Secondary air
 ~85%

11. Kiln Control: Combustion 11
Primary Air - MOMENTUM
 Required to “drive” flame
 High momentum shortens, stabilizes and
compacts the flame
momentum Turbulence at burner tip
Higher turbulence results in better mixing of
fuel and air

12. Kiln Control: Combustion 12
Primary Air - Axial and Swirl Air
 Axial Air
 minimum flow to cool down the burner pipe
 increase or decrease the flame temperature which
changes flame length
 Swirl Air
 increase or decrease the mixing of air and fuel,
allowing a higher or lower flame temperature,
which changes the shape of the flame

13. Kiln Control: Combustion 13
Primary Air - Transport Air
 Transport Air
 for solid fuel transport only
 does not vary with fuel flow
 must be at the minimum flow
 sufficient velocity at burner tip is required for flame
momentum
 for solid fuel transfer, velocity should be 24 to
30 m/s (too low => fuel deposition, too high =>
abrasion and wear)

14. Kiln Control: Combustion 14
Primary Air - In leakage
 In leakage at the kiln hood
 an expensive nuisance
 significant impact on kiln production, kiln stability,
flame length, specific heat consumption and ID fan
capacity

15. Kiln Control: Combustion 15
Secondary Air
 Heat recuperation
 higher SAT => lower SHC (kcal/kg)
 Flow controlled by ID fan
 Temperature controlled by grate speed
 clinker bed depth
 Kiln hood pressure
 low is better for heat recuperation
 air inleakage increases with more negative
pressure
 constant kiln hood pressure => stabilizes flame

16. Kiln Control: Combustion 16
Secondary Air
 How much secondary air is required
 total combustion air required minus primary air
 Where is it coming from
 from the hottest cooler chambers
 Impact of secondary air on flame
 low SAT => long, lazy flame

17. Kiln Control: Combustion 17
Mixing of Fuel and Air
 Variables to control
 Pulverized solid fuel
 fineness
 moisture
 Natural gas
 gas pressure
 Fuel oil atomization
 pressure
 temperature
 viscosity
Faster, more effective mixing => efficient combustion

18. Kiln Control: Combustion 18
Ignition
 Fuel ignition point
 temperature at which fuel ignites
spontaneously and starts to burn
 Flame ignition point
 the point just after the plume where the brilliant part
of the flame starts
 Factors affecting flame ignition point
 secondary air temperature
 type of fuel
 design of burner
 design of kiln hood
min. ignition temp.
diesel 225 C
coal 350 C
nat. gas 500 C
coke 800 C
heat
(ignition)
fuel
air

19. Kiln Control: Combustion 19
Flame
 Definition
 Temperature
 Heat transfer
 Shape

20. Kiln Control: Combustion 20
Flame - Definition
 Controlled combustion (burning) of a
determined fuel
 All flames have a short plume of air and fuel
 Fuel ignites at end of plume and forms the
flame

21. Kiln Control: Combustion 21
Flame - Definition
A large volume of very hot gases controllably generated
CO2
SO2
NOx
H2O

22. Kiln Control: Combustion 22
Flame - Temperature
 Flame temperature is affected by:
 O2 level
 secondary air temperature
 type of fuel
flame temp.
nat. gas 1700 C
oil 1900 C
coal 2200 C

23. Kiln Control: Combustion 23
Flame - Heat Transfer Rate
 Rate at which MJ (calories) are exchanged to
the material (load), coating and refractory
 Heat transfer mechanisms:
 radiation from flame to load
 convection from kiln gases to load
 conduction from refractory/coating to load

24. Kiln Control: Combustion 24
Flame - Shape
 Shapes:
 short
 long
 snappy
 lazy
 Shape controlled by:
 type and position of burner
 type of fuel
 primary air (axial, swirl air, impulse)
 ID fan flow, secondary air temp.
 O2

25. Kiln Control: Combustion 25
Flame - Shape
 Goal
 the shortest and highest temperature flame without
adversely affecting clinker quality, coating formation,
ring formation, refractory life or causing damage to
kiln discharge area
 A hot flame is always shorter than a cold flame
 Always wait for a stable kiln to make changes
to the flame shape and discuss changes with
other operators and Production management

26. Kiln Control: Combustion 26
Combustion State
 Kiln exhaust gases:
 O2
 CO
 SOx
CO2
SO2
NOx
H2O

27. Kiln Control: Combustion 27
Combustion State - O2
 Ideal O2 level determined from:
 clinker quality
 refractory protection requirements
 shell temperature
 Goals:
 keep O2 as low as possible
 maintain constant O2 (which maintains constant kiln
temperature profile)
 low CO

28. Kiln Control: Combustion 28
Combustion State - CO
 Can we accept some CO?
 Most plants operate with some CO since it is difficult
to achieve complete combustion of fuel.
 CO caused by lack of combustion air and poor
fuel preparation (fineness, viscosity, mixing,
process of pulverization)
 Incomplete combustion => longer and colder
flame

29. Kiln Control: Combustion 29
Combustion State - SOx (SO2/SO3)
 Represents sulfur oxidation from all fuel types
 SO2 formation decreases with more oxidizing
combustion
 SO3 volatilization increases with hotter burning
zone and length of flame
 SOx reacts faster than CO to changes in
combustion

30. Kiln Control: Combustion 30
fuel + air => kiln flame + exhaust gases
C + S + O2 => heat + O2 + CO2 + SOx
Summary
 Combustion quality issues
 heat quality => calcination
 flame quality => clinkerization
 Keep O2 as low as possible, but too low O2 results in:
 kiln instability
 incomplete combustion, high CO
 sulfur volatilization
 short refractory life
 poor clinker quality

31. Kiln Control: Combustion 31
Summary
 High O2
 high SHC (kcal/kg)
 long flame
 possible production limitation
 SO2 is inverse of O2
Combustion Goal:
short, hot flame (but beware of refractory life)
with low O2 and low CO