174107168-Combustion-Kiln-Control.pdf

Introduction to Kiln Control Operator Development

Combustion Presentation & Instructor Notes

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

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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

fuel

heat (ignition)

air Kiln Control: Combustion

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Combustion Air The amount of air necessary to efficiently burn at a certain fuel rate.

Combustion air consists of primary air and secondary air.

Primary air

Secondary air

 primary air fan  solid fuel transport air  inleakage

 air from cooler

COMBUSTION AIR

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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

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Types of Firing Systems

Direct Firing System Semi-direct Firing System Indirect Firing System (newest technology)

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Direct Firing System    

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

Kiln Cooler

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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

Kiln Cooler

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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

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Combustion Air in Indirect Firing System Primary air w. impulse  ~4% axial air  ~2% swirl air  ~9% fuel transport air

Secondary air  ~85%

 plus inleakage

COMBUSTION AIR

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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

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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

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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)

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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

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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

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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

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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

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Ignition 

fuel

Fuel ignition point  temperature at which fuel ignites spontaneously and starts to burn



heat (ignition)

Flame ignition point

air

 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

diesel coal nat. gas coke

min. ignition temp. 225 C 350 C 500 C 800 C

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Flame    

Definition Temperature Heat transfer Shape

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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

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Flame - Definition

CO2 SO2 NOx H2O

A large volume of very hot gases controllably generated

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Flame - Temperature 

Flame temperature is affected by:  O2 level  secondary air temperature  type of fuel nat. gas oil coal

flame temp. 1700 C 1900 C 2200 C

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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

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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

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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

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Combustion State 

Kiln exhaust gases:  O2  CO  SOx

CO2 SO2 NOx H2O

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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

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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

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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

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Summary fuel + air => kiln flame + exhaust gases C + S + O2 => heat + O2 + CO2 + SOx 

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 Kiln Control: Combustion

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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

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