477 In cement manufacturing, formation of clinker nodules occurs at the entrance to the hottest part of the kiln with a material temperature of around 1280°C. The clinker is preferably in the form of 10-mm to 25-mm size nodules that exit from the front end of the kiln into the cooler. It is critical that cooling of the clinker is rapid to secure a phase composition that imparts adequate cementi- tious properties. It is equally important that the heat exchange between clinker and air is efficient to ensure proper cooling, and at the same time maximize the recovery of heat to secondary air, tertiary air, and the related process requirement. The modern cooler must accomplish all of these tasks efficiently and simultaneously. Like other processing equipment, clinker coolers have undergone significant development over the past years. This chapter describes the advent of clinker coolers with discussion and description of various types of coolers presently available. The chapter also focuses on the reciprocating grate *Technical Director, Western Region, Ash Grove Cement Co., 6720 SW Macadam Ave. #300, Portland, Oregon 97219, Tel: (503) 293-2333. Figure 3.8.1. Grate clinker coolers. Chapter 3.8 by Hans E. Steuch* Clinker Coolers
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477
In cement manufacturing, formation of clinker nodules occurs at the entrance to the hottest part
of the kiln with a material temperature of around 1280°C. The clinker is preferably in the form of
10-mm to 25-mm size nodules that exit from the front end of the kiln into the cooler. It is critical
that cooling of the clinker is rapid to secure a phase composition that imparts adequate cementi-
tious properties. It is equally important that the heat exchange between clinker and air is efficient
to ensure proper cooling, and at the same time maximize the recovery of heat to secondary air,
tertiary air, and the related process requirement. The modern cooler must accomplish all of these
tasks efficiently and simultaneously.
Like other processing equipment, clinker coolers have undergone significant development over the
past years. This chapter describes the advent of clinker coolers with discussion and description of
various types of coolers presently available. The chapter also focuses on the reciprocating grate
*Technical Director, Western Region, Ash Grove Cement Co., 6720 SW Macadam Ave. #300, Portland, Oregon 97219,Tel: (503) 293-2333.
Figure 3.8.1. Grate clinker coolers.
Chapter 3.8
by Hans E. Steuch*
Clinker Coolers
cooler and the latest developments in cooler designs, while tracing the historical development of
the reciprocating grate cooler in relation to increasingly fuel-efficient kiln systems. The theoretical
mass and heat balance equations that describe the steady state and heat recuperating efficiency are
presented, followed by a more practical discussion of how to automate and optimize the operation
of the cooler. Figure 3.8.1 shows the interior of most commonly operated grate coolers in cement
manufacturing.
At the discharge end of the kiln, the clinker is red hot and contains around 1.0 million Btu per
short ton thermal energy. The clinker is also to some extent still reacting chemically toward
creation of various clinker minerals. The purpose of the clinker cooling is to recoup some of the
heat in the clinker, thereby making it cool enough to handle. We also want to stop the chemical
reactions in the clinker at the point most favorable to the cement quality.
TYPES OF CLINKER COOLERS
What governs the design and selection of a clinker cooler? Surely, today, any design project would
include some of the following requirements: low capital cost; optimum cooling rate for good
clinker quality; low clinker discharge temperature; least possible impact upon the environment;
high heat recovery; low power consumption; low wear and maintenance cost, and reliable to oper-
ate, causing minimal downtime; and easy to control so it delivers a steady flow of combustion air
at an unvarying temperature to the kiln and calciner. These criteria are of immediate interest to a
manufacturer of cement who buys a cooler for clinker. The designer of the clinker cooler looks at
these criteria and tries to optimize the design, depending upon the weight of each of these individ-
ual criteria.
Over the years, the criteria that are used to select coolers have changed. The technology of clinker
cooling has developed as well, so that many different types of clinker coolers have been applied
since the infancy of the portland cement manufacturing industry in the late l9th century. The
following sections will describe the most common clinker coolers with particular emphasis on the
reciprocating grate cooler.
Planetary Coolers
The name of the planetary cooler is
derived from the fact that it circles the
kiln like planets circle the sun. A
planetary cooler consists of a number
of cooling tubes mounted around the
circumference of the kiln shell
(Figure 3.8.2). The advantage of the planetary cooler is its simplicity: it requires no excess air to
handle, no fans or motors, and no instruments. It is self-adjusting. The power consumption is only
about 0.5 to 1 kilowatt-hours per ton of clinker added to the kiln drive and exhaust fan, making it
Innovations in Portland Cement Manufacturing478
Figure 3.8.2. Planetary cooler.
the lowest for any kind of clinker cooler. The heat losses through radiation and sensible heat in
clinker are between 0.40 and 0.45 mega-joules per kilogram of clinker for an economical dry-
process kiln even and lower for wet-process kilns. Planetary coolers have been used successfully for
kilns as big as 4000 metric tons per day, though not in North America.
These coolers were popular in the 1960s and 1970s when many dry process 4-stage preheater kiln
systems were built around the world. In North America, most of the dry process kilns were
supplied with grate coolers.
The planetary cooler does not allow withdrawal of tertiary air for a calciner. As most kiln systems
built today have calciners, the planetary cooler is becoming a relic of the past. One weakness of the
planetary coolers is that they can be costly to maintain. The cooler inlets often wear out too fast
due to the thermal, mechanical, and abrasive stress to which they are subjected. To decrease the
resulting maintenance and downtime, over the years there has been continuing improvement by
trials with inlets made of high temperature metal alloys or ceramic materials.
Rotary Coolers
Some of the earlier coolers were
almost like another kiln following
the clinker burning tube or, using
another picture, take the planetary
coolers, combine them into one
tube with its own support and
drive, and you have a rotary cooler
(Figure 3.8.3).
The modern rotary cooler is equipped with ceramic lining and lifters based upon the development
of the planetary cooler. Special seals at the kiln outlet and the cooler inlet are required. To avoid
spillage from the inlet, the cooler is inclined 2.5°
and given a speed of rotation of 3 rpm. The
power consumption for the drive is about 3.5
kWh/ton. The clinker temperature is 200°C to
250°C, but is reduced to about 150°C by water
injection in the outlet. Presently, no cooler of
this type is used in North America.
Shaft Coolers
As a curiosity, we should mention the shaft
cooler (Figure 3.8.4), which has been operating
with a 3000 metric ton per day kiln in Europe
479Clinker Coolers
Figure 3.8.3. Rotary cooler.
Figure 3.8.4. Shaft cooler.
since 1976, but apparently has not gained a foothold in the cement industry. The cooler requires
fairly even clinker size distribution. The upper part is operated as a fluid bed in order to avoid
agglomeration and to ensure even distribution. The power consumption is high, 10 to 12 kWh/ton,
because the cooling air has to be compressed to about 20 kPa. With minimum air to the cooler, the
clinker temperature is 300 °C – 350°C, but it is reduced by water injection in the lower part.
It should be added that shaft coolers of somewhat different design, such as the Niems cooler, have
been used very successfully for modern lime burning kilns. Burnt lime has a rather uniform grain
size distribution and therefore is much easier to cool in a shaft cooler than cement clinker.
Traveling Grate Coolers
It should be mentioned that travelling grate coolers have been used in the past; but, generally, they
were never developed to the same high standard of operational reliability as the reciprocating grate
cooler. Travelling grate coolers have been used mostly in connection with grate preheater kilns,
which produce a very uniform clinker size. The travelling grate cooler has the disadvantage that the
clinker is conveyed as a solid bed. To obtain effective clinker and air distribution, it is often neces-
sary to use pulsating air.
Grate Coolers
The grate cooler is by far the most common clinker cooler in North America. Where the air and
clinker move in opposite directions (also called counter current) in the planetary, rotary, and shaft
coolers, the grate cooler is based on the cooling air moving cross current to the direction of the
clinker movement. This type of cooler can produce clinker discharge temperatures around 80°C;
but it needs more air for cooling than can be used in the kiln, and the excess air has to be removed
and dedusted. The amount of air needed varies according to the clinker size distribution and to the
clinker temperature required. It is costly to cool to low temperatures. The amount generally lies
between 2.3 and 3.3 kg of air per kilogram of clinker; but in order to cope with forced conditions
and fluctuations, the cooling fan capacity is normally designed to allow the introduction of 4.5 kg
of air per kilogram of clinker. The specific load on grate coolers built since the mid-1970’s is often
35 to 45 metric ton per day per square meter grate area compared to 20 for grate coolers built in
earlier times. This is the result of the tendency to improve heat recuperation by working with a
thicker clinker bed on the grate.
The cooler consists of one or several grate sections. The sections are defined by their location or
their function, or by whether they are connected to a certain drive (for instance, ‘inlet grate,’ ‘2nd
movable grate,’ etc.). Each grate consists of a certain number of rows of plates. The plates have
been the subject of much development in the 1990s, as will be described later. The air to the grates
is supplied in various ways: through air blown into compartments under the grates or blown into
ducts (often called ‘airbeams’) connected directly to a limited number of grates.
Innovations in Portland Cement Manufacturing480
A typical cooler built between 1970 and 1990 works in the following fashion. From the kiln, the
clinker drops onto a stationary air-quenching grate. This grate may be horizontal or inclined. It
consists of one or several rows of plates. In the cooler shown in Figure 3.8.5, there are three
movable grates; the first is with an inclination of a few degrees, and the other two are horizontal.
Below the grate, the cooler is divided into a number of compartments, each provided with fans
equipped with adjustable guide vanes for automatic air flow control and minimum power
consumption. Clinker spillage through the grate is collected in hoppers and removed through
airtight flap valves to the
clinker conveyor. Since the
1990s, the underside of the
plates in the quench grate
and the first grate have
been connected directly to
cooling fans. This has
allowed better individual
adjustment of air to differ-
ent parts of the grate.
The efficient sealing between the compartments permits operation at high and different pressures
in the various compartments. With a normal clinker bed thickness of 600 mm, the pressure drop at
a constant air flow per unit area will decrease from about 5.9 kPa in the hot end to about 2.0 kPa in
the cold end. The fans are sized accordingly so that the maximum pressure decreases from 7.3 kPa
to 2.9 kPa. For trouble-free operation, it is an advantage to use more air per grate or unit area in
the hot end, up to 200 kg/min/m2, and less in the cold part, say 40 kg/min/m2.
The width of the grate is reduced in the inlet in order to spread the clinker more evenly. Together
with the high air flow and the thick layer of clinker, this helps to provide a uniform clinker bed
thickness, which in turn gives a uniform air flow over the width of the grate. This is essential not
only to avoid local overheating of the grate, but also to avoid “snowmen” – the clinker is kept
moving throughout the whole grate until the individual particles have lost their stickiness and
ability to cling together.
The clinker is pushed through the cooler by the reciprocating movement of rows of plates. Usually,
every second row of plates in a grate is movable. The other rows are stationary.
A crank arm moves the movable frame on older coolers. The rows of plates are moved by a
connecting rod which is centrally fixed to the movable frame, so that twisting is avoided. The rod
goes through the wall via an airtight seal and is driven by a direct current motor or by a hydraulic
piston. In the 1980s one supplier started to offer a pendulum suspended frame, such as shown in
481Clinker Coolers
Figure 3.8.5. Reciprocating grate cooler – side view.
Figure 3.8.6. This method of moving the frame
is claimed to be particularly effective at keeping
tight tolerances of movement to minimize wear
on side castings. The activation by a single
hydraulic cylinder with an asymmetric stroke
(slow forward, fast back), helps minimize
mixing of the clinker and, thereby, bed resistance
to airflow. The speed of frames, whatever way
they are moved, can be varied between 3 and 30
strokes/min. In normal operation, 5 strokes/min
is adequate, providing ample spare capacity.
Before the 1990s, all grate plates, both the movable and stationary, were of identical design. They
were cast with circular holes – in the front part of the cooler they were made of heat-resistant steel;
in the cold part, of cast steel. The shoes of the plates were bolted to a cross beam away from the
heat. All designs allow removal from underneath where there is easy access to the grate through the
undergrate compartments.
In the late 1980s a new type
of grate plate connected to an
airbeam was introduced. This
plate contains inclined and
curved slots rather than holes
(Figure 3.8.7). The slots are
shaped by small blades that
are easily replaced from the
top of the plate. This innova-
tion was so effective that by
the 1990s all major suppliers
were offering grate plates
with slots instead of holes for
the hot part of the cooler. The suppliers’ plate designs varied, but they all contained a pocket where
cooled clinker could rest and minimize metal wear, and they were all connected directly via an
airbeam to a fan rather than being supplied with air through an undergrate compartment. These
changes resulted in better protection of the grate from thermal and abrasive stress caused by hot
moving/sliding clinker and improved cooling of the clinker by better control of air flows.
The clinker discharges from the cooler across a grizzly to a hammer mill or hydraulic roll crusher
located in the cooler outlet. The crusher may be installed in the middle of the cooler, before the
last grate, to break up lumps and large clinker, and to ensure their efficient cooling. The thermal
stress on the crusher is obviously greater in the middle than at the end of the cooler.
inlet grate plates), 4) stationary quench grates at the front of the cooler, and 5) spreader beam across
the cooler. In the 1990s another interesting method was introduced. It consists of aeration of a slop-
ing bed at the inlet end of the grate. This area is provided with a series of fixed windboxes arranged
stepwise and equipped with cast metal grate elements designed so that no particles can fall through
them (Figure 3.8.20), that is, with the airbeam and pocket grate technology mentioned earlier.
A considerable pile is built up over the grate plates, which contain pulsating air. The air expands
the pile and in particular moves and mixes the finer clinker with the coarser. At the same time
making the upper portion of the pile slide gently into the cooler while it is being spread out.
Final Words
Clinker coolers are an integral part of the kiln system. Select them carefully, keep working at opti-
mizing them, and overall plant performance is bound to improve!
497Clinker Coolers
• Static Aeration Zone• Suitable Clinker Distribution to Avoid Red River• Autogenous Wear Protection of Cooler Inlet
Figure 3.8.20. Example of fixed cooler inlet.
Innovations in Portland Cement Manufacturing498
REFERENCES
Gagnon, Denis, “Upgrading a Clinker Cooler,” Proceedings 38th IEEE/PCA Cement IndustryTechnical Conference, Los Angeles, April 1996, pages 156-170.
Herchenbach, Horst, “Cement Cooling - The Key To An Economic Kiln Operation and GoodClinker Quality,” Proceedings 21st International Cement Seminar, Rock Products, Chicago, Illinois,1985, pages 41-54.
Keefe, Brian P., and Christensen, Kim Pandrup, “The Cross-Bar Cooler: Innovative and Proven,”Proceedings 42nd IEEE-IAS/PCA Cement Industry Technical Conference, Salt lake City, Utah, May2000, pages 135-147.
Klotz, Bryan, “Design Features of the Polysius Clinker Cooler,” Proceedings 42nd IEEE-IAS/PCACement Industry Technical Conference, Salt lake City, Utah, May 2000, pages 159-170.
Lecture 55, “Cooling of Clinker,” F. L. Smidth’s Cement Production Seminar, 1981.
Nobis, Rainer, “Evaluation and Optimization of Clinker Cooler Operations,” Proceedings 25thInternational Cement Seminar, Rock Products, Chicago, Illinois, 1989 pages 119-140.
von Wedel, Justus, “The IKN Pendulum Cooler,” Proceedings 42nd IEEE-IAS/PCA Cement IndustryTechnical Conference, Salt lake City, Utah, May 2000, pages 149-157.