Course 3 – Clinker Production

3.2 Firing
Imprint

German Cement Works Association

Research Institute of the Cement Industry
P.O. box 30 10 63, 40410 Duesseldorf, Germany
Tannenstrasse 2, 40476 Duesseldorf, Germany
Phone: +49 211 45 78–1
Fax: +49 211 45 78–296
info@vdz-online.de
www.vdz-online.de

info@elearning-vdz.de
www.elearning-vdz.de

Issued: 13th February 2013

Contents
1 Introduction....................................................................................................... 1
2 Construction and Operation............................................................................ 1
2.1 Rotary Kiln Firing Systems ................................................................................ 2
2.1.1 Firing Systems for Solid Fuels ........................................................................... 3
2.1.2 Firing Systems for Liquid Fuels ......................................................................... 5
2.1.3 Firing Systems for Gaseous Fuels ...................................................................... 7
2.1.4 Firing Systems for Mixed Fuels/Multifuel Firing Systems ................................ 9
2.2 Kiln Inlet Firing Systems.................................................................................... 11
2.3 Calciner Firing Systems...................................................................................... 12
2.4 Other Types of Firing System............................................................................. 12

3 Operation and Quality ..................................................................................... 12

4 Wear, Maintenance and Inspection ................................................................. 13

5 Health and Safety Practices ............................................................................. 14 i

6 Environmental Protection and Energy Consumption ................................... 15
7 Questions on Course LB 3.2 – Firing Systems ............................................... 16

Solutions............................................................................................................................... 18
Glossary ............................................................................................................................... 21

Index..................................................................................................................................... 22

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1 Introduction

In cement production most of the thermal energy is used to burn the cement clinker. The
firing systems used in the kiln plants for cement production are greatly influenced by the
type of kiln plant used and how it is operated as well as by the type and nature of the raw
materials and fuels used.
In this course you will learn about the basic principles of rotary kiln firing systems, kiln
inlet firing systems and calciner firing systems. You will learn more about flame formation
and the effects of the respective fuels. You will also learn how firing systems influence the
Course Summary
operation of rotary kiln plants and how they can affect clinker quality.

2 Construction and Operation

In traditional rotary kiln plants the thermal energy is input solely via the rotary kiln firing Firing Systems in
system. Nowadays many rotary kiln plants are operated with cyclone preheaters and a RotaryKiln Plants 1
kiln inlet firing system, which supplies part of the thermal energy for the thermal clinker
production process to the inlet region of the rotary kiln and the preheater for calcining the
kiln meal and feed material . Because the rotary kiln firing system is so important for the
sintering zone and the clinker burning process it cannot be dispensed with. In modern kiln
plants a precalcining system, in which the majority of the calcination of the kiln meal
takes place, is also positioned between the kiln inlet region and the bottom cyclone stage.
Fig. 2.0-1 shows a modern rotary kiln plant with cyclone preheater and precalciner. The
locations of the various firing systems are also indicated.

raw meal

preheater

calciner

cooler
fuel
exhaust air

tertiary air duct

raw gas Fuel

rotary kiln
cooling air
cooler
clinker

Figure 2.0-1: Diagram of a Rotary Kiln Plant with a Preheater, Calciner and Reciprocating Grate Cooler
.

A large part of the thermal energy is required for calcining the kiln feed. Nowadays many Calcination, Secondary
kiln plants are also fitted with a kiln inlet firing system, which may account for up to 30 % Firing System

of the thermal rating of the firing system for the entire kiln plant, in order to introduce

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 2 Construction and Operation

the energy at the site of calcination in the preheater and kiln inlet region. The kiln inlet
firing system is also known as a secondary firing system.
Precalciner In new plants or in kiln plants that have been completely modernized the preheater is
supplemented with a precalciner where most of the calcination of the kiln meal takes place.
The degree of calcination achieved may be up to 90 %. The output of the calciner region
may account for up to 60 % of the thermal rating of the firing system for the entire kiln
plant. The calciner is operated with one or more calciner firing systems.

2.1 Rotary Kiln Firing Systems

Components Rotary kiln firing systems consist of
 the burner, the measuring equipment and control devices required for its operation,
 the burner mount and
 the fuel supply and fuel processing systems.
The rotary kiln firing systems are supplied with fuels by a single burner installed at the
2
outlet from the rotary kiln.
Types A distinction is made between several types of firing system depending on the fuel used:
 firing systems for solid fuels
 firing systems for liquid fuels
 firing systems for gaseous fuels
 multifuel firing systems/mixed firing systems
At this point in the online course you would watch a short video showing the inside of a
rotary kiln.
 The combustion air consists of all the air fed to the flame (primary air + secondary
air).
Terms
 The primary air comprises all airflows fed through the burner.
Primary air may be separated into two airflows:
 flame-shaping airflows
 conveying and atomizing airflows for the fuels
 The airflow from the clinker cooler is known as secondary air.

Operating Principle The fuel is injected via the burner at high speed. In this process, a swirl is imparted to part
of the unheated primary air by the burner tip to give the flame the necessary momentum
and swirl for mixing the fuel and combustion air. The proportion of cold primary air is
kept as low as possible, i.e. to the minimum amount of air required for the combustion
process (8 to 15 %), in order to obtain high flame temperatures. Most of the combustion
air is drawn from the clinker cooler as preheated secondary air (700-1,100 ◦ C) and mixed
into the flame outside the burner. The rotary kiln flame therefore consists of a fuel-rich
flame core.
Axial Air, Various burner manufacturers offer different systems for imparting a swirl to the flame.
Swirl Air Burners with a traditional design split a large part of the primary air into axial air and
swirl air. The flame swirl and combustion can be controlled by varying the division
between axial and swirl air. Modern burners of the latest generation forego the traditional
splitting process and have a mechanically adjustable blast pipe integrated into the burner
tip. Varying degrees of swirl are imparted to the flame by adjusting the exit flow direction
of the primary air.
The fuel-rich flame core is fanned out by adjusting the axial and swirl air or the adjustable
blast pipe to obtain a favourable flame shape for the sintering process. The flame length

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 2.1 Rotary Kiln Firing Systems

may be three or four times the kiln diameter. Combustion temperatures of over 1,800
◦
C must be achieved in the rotary kiln with low excess air to ensure the quality of the
clinker. The level of the combustion temperature and its behaviour pattern in time and
space are determined by the properties of the kiln feed and are influenced by adjustment
of the burner, the properties of the fuel and the mode of operation of the clinker cooler.

It is very important for the clinker quality that a short, hot flame and a specific temper-
ature profile are formed in the kiln.

The flame can be shaped to suit the sintering process by configuring and adjusting the exit
flow system and by setting the volume of the primary air flow. This also applies to the
formation of pollutants and to the co-combustion of secondary fuels.
Flame Shaping

At this point in the online course you would watch a video provided by Polysius showing
how the flame can be shaped using the burner.
Nowadays a mix of different types of natural fuels and secondary fuels is used to fire Fuels
cement rotary kiln plants. 3
 Natural fuels are solid, liquid and gaseous fuels from fossil energy sources. They
are also known as primary or standard fuels, since they are used to control kiln
operation.
 Fossil fuels are being replaced by secondary fuels to protect the environment and
conserve natural resources. The secondary fuels used in the German cement industry
are divided into liquid fuels, such as waste oil and solvents, solid air-entrainable fuels,
such as wood residues and plastic waste, processed fractions from industrial, business
and municipal waste, animal meal or sewage sludge and solid lumpy fuels, such as
used tyres.

Further Information
Further information can be found in the corresponding course on fuels.

2.1.1 Firing Systems for Solid Fuels

The main primary solid fuels used are pulverized lignite and coal. However, other com-
bustible products, such as petroleum coke, processed production waste and other by-
products may also be introduced in the correct proportions on the primary side. Origi-
nally it was sufficient to inject the pulverized coal into the rotary kiln with air through a
simple burner pipe. A development from the simple burner pipe, via the installation of
flame control systems to the current multi-channel burners took place during the 80s and
90s with the main aim of saving energy and reducing environmental pollution by cutting
combustion-dependent nitrogen oxide emissions. Fig. 2.1-1 shows the exit flow system of
a three-channel burner for solid fuels. The pipe for the start-up fuel (gas or oil) is located
in the central jacket tube.
The volume of the axial and swirl air flows (burner air) is changed by internal regulating
devices and by regulating the output of the fans.

Adjustable Flame Shapes

The flame shape of a rotary kiln burner must be established so there is optimal provision
of heat in every kiln zone in accordance with the requirements of the process, without the
kiln shell or the refractory lining being subjected to excessive stressing at any point.

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 2 Construction and Operation

2 4 6 5 3 1

1 swirl air channel 4 exit flow system, pulverized coal

2 exit flow system, swirl air 5 axial air channel

3 pulverized coal channel 6 exit flow system, axial air

Figure 2.1-1: Three-Channel Burner for Pulverized Coal.

If the burner is configured optimally by the design engineer and supplier for the specific
rotary kiln plant then the normal flame will constitute the basic setting. In the absence
of any further information and operational experience with the burner, the rotary kiln is
started up and monitored until its nominal output has been reached.
If this flame shape does not satisfy the requirements of the burning process, the following
measures should be taken in small steps:

Swirl Air Axial Air

narrower, longer flame reduce reduce

wider, shorter flame increase increase

soft, wide flame increase reduce

Table 2.1-1: Adjustment of Axial and Swirl Air.

Note on Tab. 2.1-1 The procedure described in Tab. 2.1-1 only applies to rotary kiln firing systems in a hot
combustion chamber (> 600 – 800◦ C). A greater swirl with smaller amounts of air
must normally be achieved in a cold kiln and during the start-up process (< 600 – 800◦
C). In this instance the regulating device for axial air should be closed completely and
that for the swirl air should be opened carefully.

A possible regulating device is shown in Fig. 2.1-2.

The airflows are adjusted either by controlling the fan or by adjusting a throttle valve. The
throttle device opens the flow cross-section in the air feed to the burner and regulates the
volume flow by the flow resistance at the throttle.

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 2.1 Rotary Kiln Firing Systems

18 19 20

10 11 9 1 10
11
9
12
16
2 21 8

4 13 5 3 14 6 17 7 15 12

1.flame stabilizer 11.swirl air channel 5

2.pulverized coal 12.central air channel
3.swirl air 13.axial throttle valve
4.axial air 14.swirl throttle valve
.5 axial adjustment 15.central throttle valve
6. swirl adjustment 16.jacket tube
7. coal channel adjustment 17.instrument panel
8.central air adjustment 18.wear ring
9.pulverized coal channel 19.wear cone
10.axial air channel 20.wear half-shell
21.pulverized coal cooling

Figure 2.1-2: Rotary Kiln Burner with Adjustable Flame Shape for Solid and Liquid Fuels.

2.1.2 Firing Systems for Liquid Fuels

Fuel oils of classes G (heavy oils) and E (light oils) are used as liquid fuels and are fed into
the combustion chamber via an atomizing system. The viscosity of the fuel oils should be
less than 15 cSt to achieve optimal atomization. This means that fuel oils in class G must
be heated appropriately. The fuel oil flows toward the burner from a processing station.
The burner consists of the burner pipe with fuel feed and atomization systems as well as
the air supply system comprising one or more primary air channels.
The fuel oil is fed to the burner with dual circuit atomizer by two routes (Fig. 2.1-3). The
primary oil (tangential oil) flows from the primary oil pressure regulator via the oil feed
system to the burner tip and enters the chamber before the nozzle opening from outside
through tangential slots in the nozzle swirler. The secondary oil (axial oil) flows from
the secondary oil pressure regulator, via the oil feed line, toward the burner tip and enters
the chamber before the nozzle opening in the axial direction through small holes without
any swirl. Oil flowing in radially and axially is combined in this chamber and enters the
combustion chamber through the nozzle opening with a swirl. The total swirl of the oil
flow can be varied by changing the ratio of primary oil to secondary oil. This controls the
width of the flame and the progress of the combustion process over time.
With return-flow atomizers (Fig. 2.1-4) any oil that is not required is fed back into the
system. The return-flow principle achieves a considerably wider range of control than the
dual circuit atomizer.

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 2 Construction and Operation

cross-section A-A
axial oil A
tangential oil

Figure 2.1-3: Dual Circuit Atomizer in Rotary Kiln Firing Systems for Liquid Fuels .
6
A

cross-section along A-A

tangential oil A
return oil flow

Figure 2.1-4: Return-Flow Atomizer in Rotry Kiln Firing Systems for Liquid Fuels.

The primary air is supplied in the same way as in burners for solid fuels. The outer
primary air flows out axially via the axial air channel. The inner primary air flows
through the swirl air channel, via the swirler, toward the nozzle and exits with a strong
swirl. The amount and speed of the two primary air flows are adjusted using throttle
valves.

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 2.1 Rotary Kiln Firing Systems

Adjustable Flame Shapes

Primary and Sec- Swirl Air Axial Air

ondary Oil

long, narrow flame  match the throttle valve com- throttle valve moder-
secondary oil pletely closed ately open
pressure with the
primary oil
pressure
 secondary oil valve
fully open

short, wide flame  select a nozzle that throttle valve fully open throttle valve fully open
allows high
primary oil
pressures to be set
 increase the
amount of
secondary oil to 7
match the primary
oil

Warning! This adjust-

ment may lead to local
overheating of the re-
fractory lining and to
damage of the kiln
shell. The temperature
of the kiln shell must
therefore be monitored
continuously.

average flame ensure atomization by throttle valve moder- open step-wise until op-
average adjustment ately open timal flame shape is
of primary oil and obtained
secondary oil

Table 2.1-2: Adjustment of the Flame Shape.

The procedure described in Tab. 2.1-2 only applies to rotary kiln firing systems in a hot Note on Tab. 2.1-2
combustion chamber (> 600 – 800◦ C). A greater swirl air with smaller amounts of air
must normally be achieved in a cold kiln and during the start-up process (< 600 – 800◦
C). In this instance the regulating device for axial air should be closed completely and
that for the swirl air should be opened carefully.

In Germany, liquid fuels from fossil energy sources are only used for the start-up and Use
heating of rotary kiln plants owing to high purchase costs. Waste oil and solvents may
be used as liquid secondary fuels and often serve to provide an auxiliary flame for solid
fuels that are difficult to ignite.

2.1.3 Firing Systems for Gaseous Fuels

As a gaseous fuel for rotary kiln firing systems, natural gas is generally used as an ignition
aid for liquid and solid fuels that are difficult to ignite. Natural gas is normally provided
by gas suppliers at such a pressure that a gas off-take and pressure reduction station is
required in the works. The natural gas is then available at the burner at pressures of 4 bar
to 6 bar. The burner consists of the gas connecting and operating section, the concentric
gas channels for axial gas and swirl gas and an external air channel. This generally

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 2 Construction and Operation

acts merely as a cooling air supply so the burner does not have to be removed from the
kiln every time the kiln is stopped. In practice, natural gas burners may also be operated
without any primary air (Fig. 2.1-5).

22

33

11 A

33

22

view along A
1. pilot burner
2. axial gas
3. gas flow, directed outwards

axial gas tangential gas

Figure 2.1-5: Gas Burner .

The gas flows are divided into the axial flow and the swirl flow by throttle valves. Natural
gas burners permit a high turndown ratio of 1:30 so the rotary kiln can be started up from
cold without any major adjustment.

Adjustable Flame Shapes

Note on Tab. 2.1-3 The procedure described in Tab. 2.1-3 only applies to rtaory kiln firing systems in a hot
combustion chamber (> 600 – 800◦ C). A greater swirl air with smaller amounts of
air must normally be achieved in a cold kiln and during the start-up process (< 600 –
800◦ C). In this instance the regulating device for axial air should be closed completely
and that for the swirl air should be opened carefully.

Owing to the high purchase costs natural gas is only used to heat up the kiln plant and
as an ignition aid for liquid and solid fuels that are difficult to ignite.

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 2.1 Rotary Kiln Firing Systems

Swirl gas Axial gas

long, narrow flame close the swirl gas open the axial gas reg- When using small
regulating device ulating device and amounts of gas the ax-
admit gas into the axial ial gas outlet cross-
channel at low pressure. section should also be
reduced by adjusting
the channel.

short, wide flame open the swirl gas regu- open the axial gas
lating device and admit regulating device
gas to the swirl channel Warning! This adjust-
at high pressure. ment may lead to local
overheating of the re-
fractory lining and to
damage of the kiln
shell. The temperature
of the kiln shell must
therefore be monitored
continuously. 9
average flame starting from the setting starting from the setting
for long flames, slowly for long flames, slowly
open the regulating de- open the regulating de-
vices for axial and swirl vices for axial and swirl
gas to obtain an average gas to obtain an average
flame. flame.

Table 2.1-3: Adjustment of the Flame.

2.1.4 Firing Systems for Mixed Fuels/Multifuel Firing Systems

The burners in use today are characterized by a large number of lances, tubes and annu-
lar channels. These permit versatile combustion of solid, pulverized fuels, fuels in lump
form, liquid and viscous fuels as well as gaseous fuels. These multi-channel and mul-
tifuel burners are, in terms of their operating principle, solid fuel burners that have been
developed and further modified. Fig. 2.1-6 shows the cross-section of a typical burner with
axial air and swirl air channels for pulverized fuels, such as coal, as well as for secondary
fuels in lump form and liquid secondary fuels.

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10

Figure 2.1-6: Multi-Channel Burner .

1 central air

2 swirl air

3 pulverized fuels

4 axial air

5 fuels in lump form

6 swirl air for fuels in lump form

7 empty tube for receiving a lance for liquid and viscous fuels

8 baffle plate/swirler

9 perforated distributor plate

Table 2.1-4: Key to Fig. 2.1-6.

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 2.2 Kiln Inlet Firing Systems

Advantage Disadvantages

high variability heavy weight owing to large size

highly complex assembly
high procurement costs

Table 2.1-5: Advantages and Disadvantages.

A burner nozzle of complex design makes it possible to use different fuels with different
combustion properties and in varying mixtures. The amount of primary air and the way it
is introduced must be adapted to the fuel mixture. Fig. 2.1-7 shows the burner mouth of a
modern burner for solid, pulverized fuels, air-entrainable fuels and liquid fuels.

11

Figure 2.1-7: Burner Mouth of a Modern Rotary Kiln Burner for a Multifuel Firing System.

2.2 Kiln Inlet Firing Systems

In kiln inlet firing systems the fuels are introduced into the kiln inlet chamber or the riser
pipe between the kiln inlet chamber and the bottom cyclone stage. It is possible to feed
fuels that are in lump form directly without the aid of burner equipment. Simple tube
burners can generally be used for fine-grained fuels, light solid fuels and for gaseous fuels,
but liquid fuels always require atomization.
Low-calorie fuels are generally used to produce energy in the kiln inlet firing system. In
most cases unprocessed or only slightly processed secondary fuels are used. In this case
used tyres, both in shredded form and as whole tyres, are most commonly used . However,
scrap rubber, textile waste, old wood and sewage sludge may also be burned. The fuels are
introduced by rotary-vane feeders and flap valve feeders, screw feeders or chutes.

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 3 Operation and Quality

2.3 Calciner Firing Systems

The type of calciner firing system varies depending on the nature of the fuels and the
design of the calciner. The long residence times in the calciner burner chamber make
it possible to burn fuels that are difficult to burn. Low-calorie fuels can also be used
as there are only a few requirements regarding the heat release of the fuels that must be
satisfied.
The calciners may also be supplied with fuels in different ways, directly or in the form of
a lean gas or a gas with a residual calorific value (for example via an upstream preburning
chamber or fluidized bed). The type of calciner firing system depends on the nature of
the fuels and the design of the calciner. The fuels are generally injected into the lower
region of the calciner. Simple pipes and sometimes also burners are generally used for
this purpose. The fuels should be air-entrainable so that they can be carried along in their
entirety by the gas flow. Disruptive materials that are not entrained drop into the kiln inlet
where they can cause operational problems.
SecondaryFuels A large number of secondary fuels (used tyres and whole tyres, scrap wood, etc.) of
12
varying nature may be used in calciner firing systems because there are fewer applica-
ble combustion-related requirements (calorific value of the fuel) than for primary firing
systems.
Fuels in lump form are fed directly by flap valve feeders or rotary-vane feeders. Viscous
and shredded fuels are fed by screw feeders or chutes. Fine and gaseous fuels are generally
injected via a simple tube. Liquid fuels must be atomized.
A large number of fuels of varying nature can be used in calciner firing systems owing to
the few combustion-related requirements. An auxiliary flame fired with pulverized coal
is generally required when using fuels that are difficult to burn.

2.4 Other Types of Firing System

Special firing system designs are sometimes encountered in cement works. These special
systems are not generally the actual firing system for the kiln plant but instead tend to be
systems for thermally processing the fuels. The fuels are gasified in a separate preburn-
ing chamber or fluidized bed. The combustion chamber or fluidized bed chamber is
supplied with the fuels, and gasification air is introduced substoichiometrically.

Sub- and Superstoichiometric Combustion

The air excess factor represents the ratio of the actual amount of combustion air to
the stoichiometric amount of combustion air. With substoichiometric combustion less
combustion air is available than the amount required for complete combustion of all fuel
components (λ < 1). With superstoichiometric combustion unutilized combustion air
remains in the flue gas (λ > 1).

Tertiary air is used as the gasification air. The fuels are gasified in the chamber so that a
low-calorie lean gas with high carbon monoxide concentrations and coke are produced as
residue. The lean gas is supplied to the calciner firing system. The coke can be fed both to
the calciner firing system and to the kiln inlet firing system.

3 Operation and Quality

Spheres ofInfluence The firing systems, and the burners in particular, are major components of a kiln plant and

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affect plant production and product quality. The availability and output of the burner es-
sentially determine plant production and clinker output. The flame shape and temperature
profile can be established selectively in the kiln by adjusting the burner, thus affecting
clinker burning and therefore product quality. In addition, the thermal energy requirement
can be lowered by adapting the burner setting to the fuels used.
 The required burner output is calculated from the maximum clinker throughput, the Operation
theoretical heat consumption for clinker burning based on the desired clinker quality,
the burning properties of the raw meal produced and the plant-specific heat losses from
exhaust gas, dust, clinker waste heat and radiation.
 The burner is also responsible for the combustion operation and flame formation
functions so any variation in the burnability of the raw material can be offset by chang-
ing the amount of fuel or the flame shape and flame temperature.
The burner can therefore influence
 clinker output,
 specific heat consumption,
 clinker quality, 13
 recirculation and emission of pollutants and
 service life of the refractory lining.
The main transformations from raw material to clinker take place during the burning pro- Quality
cess as a function of temperature. To a large extent this establishes the subsequent prop-
erties of the end product. In addition to the calorific value and the degree of processing
of the fuel, the burner itself also influences clinker quality through its structural features
and range of adjustments.
 With sintering zone burners it is important that the flame shape and flame length are
set in such a way that the amount of heat necessary for the transformation is available in
each kiln zone. This avoids under-burning, i.e. incomplete clinker phase formation,
on the one hand, and overburning (shell burning with encased, incompletely burnt
core) on the other.
 An important criterion for clinker quality is the cooling phase from the sintering
temperature to less than 1,200 ◦ C. On the one hand, the clinker should be cooled
quickly enough that the alite does not decompose and sets in a finely crystalline form
and, on the other hand, it must be cooled slowly enough that the melt does not set as a
glass. If cooling is too slow then this will affect the early strength of the cement.

4 Wear, Maintenance and Inspection

During plant production the firing systems and burners are exposed to mechanical wear, Types of Wear
thermal stressing and chemical attack. The fuels supplied act abrasively on all internal
components of the firing systems, while the clinker dust in the secondary air subjects the
refractory casing to mechanical and thermal stresses. Both the fuels and the kiln atmo-
sphere subject the materials to chemical attack. Firing systems generally require little
maintenance during operation. However, the factors affecting wear and corrosion must be
monitored and regular maintenance and inspection must be carried out to ensure reliable
operation.
 When solid fuels are used mechanical wear occurs mainly at the transition from the
circular cross-section of the feedpipe to the annular cross-section of the channel for
pulverized fuels. Wear protection is already provided in this area by the manufacturer
in the form of a welded half-shell or a split wear cone. Further wear protection may
also be provided by applying ceramic tiles with a long service life in the transition

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 5 Health and Safety Practices

zone. Mechanical wear also affects the exterior of the refractory casing, where the
abrasive, sharp-edged clinker dust in the secondary air comes into contact with the
burner lance. This generally affects the underside of the burner lance. If the refractory
casing becomes worn away as a result of abrasion the supporting pipe will then be
subjected to attack. In this case the mechanical and thermal attack will jeopardize the
stability and function of the burner lance.
 Thermal wear occurs predominantly at the burner outlet when the burner is cooled
insufficiently or when it is not cooled at all if the primary air fails while the system is
hot. In this situation the burner should, if possible, be removed immediately from the
kiln. An emergency air fan with an independent power supply is generally available to
ensure cooling of the burner lance if the burner air fails.
 Corrosion occurs as a result of attack by the alkalis released from the clinker and pos-
sibly by chlorine or sulfur contained in the fuel. These substances generally intensify
the wear on the refractory casing of the burner lance, including the retaining anchors.
The burner may be subjected to sulfuric acid attack due to condensation when using
heavy fuel oil owing to its high sulfur content.
14
Inspection The following system components should be inspected regularly:
 refractory casing
 flame stabilizer (if present)
 exit flow system or burner tip
 internal wear fittings (wear shell, wear cone or wear ring, ceramic wear protection)
 measuring points must be cleaned and examined
In addition the position of the burner in the kiln should be checked during operation (it
may have been displaced slightly due to coating formation) and the condition of all line
connections and control joints should also be examined.
All adjustment devices and throttle devices should also undergo regular maintenance to
ensure that they always function correctly. The measuring accuracy of any manometers
and volume flow meters used should also be checked regularly.

5 Health and Safety Practices

Firing systems and burners are closed systems so do not pose any risk to individuals or the
equipment during routine checks. The actual risk lies in handling the fuels. Particular
risks arise during the lighting of the kiln, if fuel escapes via leaks and broken seals as well
as on open stockpiles and during transport of the fuels in an open system.

Industrial Safety
When operating rotary kiln firing systems it should be ensured that
 the operational staff have been trained,
 the burner tunnel is well ventilated,
 the operating instructions have been circulated, read and understood,
 all fixing components on the burner and burner carriage or any suspension device are
firmly screwed and secured,
 all connections for fuel supply are sealed,
 no fuel residues can build up in the burner,
 the exit flow systems are free from caked-on material (for example clinker dust, fuel
ash, carbon, etc.),

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 the pressure and speed of the fuel and primary air are set correctly (for example selec-
tion of the correct nozzles for oil atomizers),
 uniform metering of the fuel and uniform air supply mean that the flame does not
pulsate,
 operation is carried out with an excess of air,
 the burner itself is flexibly connected to rigid feed pipelines via joints or hoses,
 measuring and adjusting equipment is in good working order,
 the position of the burner in the rotary kiln is precisely fixed.
The burner itself has no rotating parts. Any risks therefore stem mainly from the fuel used
and the improper or negligent handling thereof.
A specific risk is posed by igniting the kiln when there is little or no heat in the combus-
tion chamber. The ignition process must therefore be described in specific instructions.
Special interlocking conditions and CO limits must be established for igniting the kiln and
operating the firing system in conjunction with electrostatic precipitators. Fuel deposits on
hot surfaces are to be avoided outside combustion chambers.
15
Health and Safety Practices
 avoid endangering health as a result of gases issuing into closed spaces. Ventilate
spaces well if necessary.
 implement noise protection measures in the region of noisy primary fans.
 avoid skin contact with fuels that contain corrosive substances or are biologically
contaminated.
 wear a helmet visor when inspecting the inside of the kiln.

6 Environmental Protection and Energy Consumption

Firing plants and burners are closed systems so, under normal operating conditions, no
emissions are released directly into the environment. However, some environmentally rel-
evant substances may escape through broken seals and leaks. The emissions are primarily
produced when the fuels are burned and leave the kiln plant with the exhaust gases.

Factors Affecting Emissions

Emissions can be affected by the manner in which the fuel is injected and by the manner in
which primary air is supplied. The burner should be set so that even the most varied fuels
remain within the flame until they have burnt out completely and therefore hardly any CO
is produced. The use of modern burners in conjunction with staged combustion makes it
possible to control NOx emissions.
Measures and Adjustment Options include: NOx Emissions
 avoiding unnecessary temperature peaks
 using a flame that is not too hot (avoiding thermal NOx )
 operating the kiln system with low CO at the kiln inlet (possibly assisted by a sec-
ondary fuel feed)
 secondary combustion of the CO produced in the kiln inlet in a further combustion
stage (for example in the calciner)
Burn-out that is as complete as possible is to be ensured in the calciner or preheater in
order to keep CO emissions low and to save on fuel costs.

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 7 Questions on Course LB 3.2 – Firing Systems

Environmental Protection Measures

Firing systems for rotary kilns pose no risk to the environment so no measures are nec-
essary. However, a precondition for this is that kiln operation and the firing system must
be set in such a way that no explosive mixtures can be formed in the kiln system or in
the exhaust gas ducts. Furthermore, any leaks in the fittings and supply hoses in oil firing
systems must be repaired immediately so that any hot oil that escapes does not seep into
the ground. Specific construction regulations apply to oil tank plants.

Firing systems must be electrically interlocked with the rotary kiln system so the firing
system is switched off immediately in the event of kiln failure or a lack of air.

Factors Affecting Energy Consumption

Options for reducing energy consumption:
 an exit flow system that is optimized with regard to flow mechanics,
16  selective fuel enrichment in the flame core,
 minimization of the amount of primary air,
 direct feed of the combustion air to the fuel flow and
 the position of the burner in the kiln hood (in conjunction with the secondary air flow).
It goes without saying that the fuel is processed in such a way that it is possible to obtain
optimal burn-out conditions. With solid fuels it is particularly important to avoid coarse
fractions. Fluctuations in the calorific value of the individual fuels should be minimized
by, for example, mixing. Where possible, moist fuels can be pre-dried by waste heat from
the kiln.

7 Questions on Course LB 3.2 – Firing Systems

You can test your knowledge by answering the following questions.

Question 7.0 A:
1. Name the different types of firing system.
2. How might you influence flame shape in a burner for solid fuels?
3. What is the difference between dual circuit atomizers and return-flow atomizers in
a firing system for liquid fuels? Briefly describe the oil flow paths.
4. What is another name for the rotary kiln firing system?
5. Why is it not possible to dispense with the rotary kiln firing system in a kiln plant?
6. Briefly name the most important channels in a three-channel burner for solid fuels.
7. How might you adjust the burner to obtain a wide, short flame?
8. What are the options for adjusting the volume of the axial air and swirl air flows?
9. What is the advantage of return-flow atomizers compared to dual circuit atomizers
for liquid fuels?
10. Why can a firing system for gaseous fuels be operated without primary air?
11. How must the fuels be processed for the kiln inlet firing system?
12. What is the purpose of special types of firing system and what is their advantage?
13. What measures may be taken to reduce the energy consumption of a rotary kiln
firing system (at least 3 measures)?

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14. What safety measures need to be taken when operating rotary kiln firing systems
(at least 5 measures)?
15. How does the rotary kiln firing system affect plant production (at least 3 points)?
16. What are the main types of wear in a rotary kiln firing system?
17. What must be examined regularly on a burner during an inspection?
18. What environmental factors may be affected by the rotary kiln firing system?
19. When does thermal wear of the rotary kiln burner occur?
20. What measures may be taken to avoid NOX by adjusting the firing system?
Solutions see p. 18

17

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 7 Questions on Course LB 3.2 – Firing Systems

Solutions
Solution for 7.0 A:
1.  Firing systems for solid fuels
 Firing systems for liquid fuels
 Firing systems for gaseous fuels
 Firing systems for mixed fuels and multifuel firing systems
2. By increasing or reducing the axial air and swirl air. The volume of the axial air
and swirl air flows is controlled either by the fan or by throttle devices.
3. With dual circuit atomizers the fuel oil is fed to the burner by two routes. The pri-
mary oil flows from the primary oil pressure regulator via the oil feed system to the
burner tip and enters the chamber before the nozzle opening from outside through
tangential slots in the nozzle swirler. The secondary oil flows from the secondary
oil pressure regulator, via the oil feed line, toward the burner tip and enters the
chamber before the nozzle opening in the axial direction through small holes with-
18 out any swirl. Oil flowing in radially and axially is combined in this chamber and
enters the combustion chamber through the nozzle opening with a swirl. The total
swirl of the oil flow can be varied by changing the ratio of primary oil to secondary
oil. This controls the width of the flame and the progress of the combustion pro-
cess over time. With return-flow atomizers any oil that is not required is fed back
into the system. The return-flow principle achieves a considerably wider range of
control than the dual circuit atomizer.
4.  Rotary kiln firing systems are also known as primary firing systems.
5.  The rotary kiln firing system heats the kiln feed to the sintering temperature and
burns the clinker. The energy required for this is generated by the rotary kiln
firing system. The other firing systems merely assist with calcination.
6.  axial air channel
 pulverized coal channel or fuel channel
 swirl air channel
7.  A wider and shorter flame is obtained by reducing the axial air and increasing
the swirl air.
8.  The volume of the flows is controlled either by the output of the fans or by
adjusting the throttle devices.
9.  The return-flow principle of return-flow atomizers achieves a considerably greater
range of control than dual circuit atomizers.
10.  The gaseous fuels is divided into axial gas and swirl gas. The desired flame
shape can be established by controlling the axial gas and swirl gas in such a
way that it is possible to dispense with flame-shaping using primary air.
11.  For combustion in the kiln inlet firing system the fuels do not need to be pro-
cessed or are only slightly processed. It is possible to burn a wide range of
fuels, from those in lump form to whole tyres.
12.  Special types of firing system are preburning chambers or fluidized bed cham-
bers. Fuels that are difficult to burn and highly inhomogeneous fuels are ther-
mally processed in these types of firing system. The processing leads to ho-
mogenization of the energy input into the kiln plant.
13.  an exit flow system that is optimized with regard to flow mechanics,
 selective fuel enrichment in the flame core,
 minimization of the amount of primary air,
 direct feed of the combustion air to the fuel flow and

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  the position of the burner in the kiln hood (in conjunction with the secondary
air flow).
14. It should be ensured, for example, that:
 all fixing components on the burner and burner carriage or any suspension de-
vice are firmly screwed and secured,
 all connections for fuel supply are sealed,
 no fuel residues can build up in the burner,
 the exit flow systems are free from caked-on material,
 the pressure and speed of the fuel and primary air are set correctly,
 uniform metering of the fuel and uniform air supply mean that the flame does
not pulsate,
 operation is carried out with an excess of air,
 the burner itself is flexibly connected to rigid feed pipelines via joints or hoses,
 measuring and adjusting equipment is in good working order,
 the position of the burner in the rotary kiln is precisely fixed.
15. The rotary kiln firing system affects:
19
 clinker output,
 specific heat consumption,
 clinker quality,
 recirculation and emission of pollutants and
 service life of the refractory lining
16. The main types of wear are:
 mechanical wear
 thermal stressing and
 chemical or thermochemical decomposition
17. The following components should be inspected regularly:
 refractory casing
 flame stabilizer (if present)
 exit flow system or burner tip
 internal wear fittings (wear shell, wear cone or wear ring)
In addition, the position of the burner in the kiln should be checked during op-
eration and the condition of all line connections and control joints should also be
examined. All adjustment devices, throttle devices, pressure measuring devices and
volume flow measuring devices should undergo regular maintenance and inspection
to ensure that they always function correctly.
18.  avoidance of the formation of explosive mixtures in the kiln system or in the
exhaust gas ducts
 control of the formation of emissions, such as NOX and CO
19. Thermal wear on the rotary kiln burner occurs at the burner outlet when there is
insufficient cooling. The burner lance is also subject to thermal stress if the primary
air fails or if the burner is insufficiently cooled by primary air.
20.  avoidance of unnecessary temperature peaks
 using a flame that is not too hot (avoiding thermal NOX)
Questions see p. 16

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Glossary
abrasive
When hard particles of a material penetrate the outer layer, this leads to scratching and material removal on a
micro scale. This wear is called abrasive wear or erosion wear.
air excess factor
the air excess factor is the ratio of the actual amount of combustion air to the stoichiometric amount of combustion
air. With substoichiometric combustion less combustion air is available than the amount required for complete
combustion of all fuel components (λ < 1). With superstoichiometric combustion unutilized combustion air
remains in the flue gas (λ > 1)
calcination
Calcination is a process in which limestone is decomposed into calcium oxide and carbon dioxide. Calcination
is one of the most important processes of clinker burning. Calcium carbonate (limestone) is broken down into
calcium oxide (CaO, also known as free lime) and carbon dioxide (CO2). This reaction starts at about 650 ◦ C,
but most of the limestone is only completely calcined at temperatures between 800 ◦ C and 900 ◦ C.
carbon monoxide
CO; carbon monoxide; colourless, odourless and tasteless poisonous gas
cSt 21
Centistrokes (cSt) – a unit for kinematic viscosity
viscosity
measure of the flow resistance of a fluid

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 Index

A
abrasive 13
air excess factor 12

C
calcination 1
carbon monoxide 15
cSt 5

V
viscosity 5

22

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