Combustion Presentation & Instructor Notes

Introduction to Kiln Control Operator Development

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

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

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

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

Kiln Control: Combustion 6

Types of Firing Systems

Direct Firing System

Semi-direct Firing System

Indirect Firing System (newest technology)

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

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

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

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%

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

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

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)

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

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

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

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

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

Kiln Control: Combustion 19

Flame

Definition

Temperature

Heat transfer

Shape

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

Kiln Control: Combustion 21

Flame - Definition

A large volume of very hot gases controllably generated CO2

SO2

NOx

H2O

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

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

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

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

Kiln Control: Combustion 26

Combustion State

Kiln exhaust gases:

O2

CO

SOx

CO2

SO2

NOx

H2O

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

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

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

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

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