We were discussing “Cements
and its types” and “Material preparation technology in cement
manufacturing” in our previous posts. We have also see the concept of cement manufacturing process and its
selection in our recent post.
Long dry kiln is easy to operate as it is not very sensitive to upset conditions as a result of high chloride or alkali content in the raw mix. However, it is not very economical from the point of view of fuel economy.
Today we are going to start here one very important
topic i.e. burning technologies in cement manufacturing process. We will
understand the various terminologies and engineering concepts used in cement
technology with the help of this category.
Before reading this post, please find link “Raw meal homogenization and storage in cement industries” and “Grinding and drying of coal in cement industry”.
Further we will go for discussion of firing
technology, clinker cooling and grinding, cement packing and dispatch,
pollution control and much more facts about cement technology in our next post
in this category of cement technology.
Cement burning technology
The kiln of the pyro - processing plant is the heart
of the cement plant. Cement manufacturing processes, termed according to the
condition of the feed entering the kiln, are Wet, Dry, Semi - wet and Semi - dry.
The rotary kiln used in the cement burning may be
classified into various types as shown below
Type
of kiln
|
Dimension
(m)
|
Heat
consumption
|
kiln
load
|
|
length
|
dia.
|
kcal
/ kg
|
t
/ m3
|
|
Long wet process
|
80 - 230
|
3 - 7.6
|
1200 - 1450
|
0.4 - 0.8
|
Long dry process
|
70 - 230
|
3 - 7.0
|
850 - 950
|
0.6 - 1.0
|
Short dry process
(with suspension- preheater + precalciner)
|
40 - 110
|
3 - 6.0
|
720 - 900
|
1.4 - 2.2
|
Wet process
Wet processes require long rotary kiln (L / D ~ 40)
and degree of kiln filling not more than 17 %. Practically, it should be ~ 13 %
as a higher value may impair heat transfer.
Slurry
dewatering may be done mechanically by using proper filters (drum disc filter
or filter press) or chemically using thinners. Thinners are ions and molecules
which are adsorbed on the surface of raw mix particles preventing their
agglomeration, thus, reducing internal friction and increasing flowability of
slurry. Chain systems installed inside the kiln bring about dewatering of
slurry. There are two types of systems - Garland chain and Chain curtain.
Two other installations in the kiln are slurry
preheater located before the chain section and slurry dryer located behind the
chain section to remove the remaining water and preheat the dried raw mix.
Dry Process
The dry process kiln can be of the following types:
1. Long dry kiln with internal /external heat exchanger
2. Kiln with suspension preheater
3. Kiln with suspension preheater and precalciner
Long dry kiln is easy to operate as it is not very sensitive to upset conditions as a result of high chloride or alkali content in the raw mix. However, it is not very economical from the point of view of fuel economy.
The heat liberated in the rotary kiln can, besides
being used for the actual processing of the feed material, advantageously be
utilized for preheating the material. to promote this, the inlet zone of the
kiln may be equipped with suspended chains made of heat resistant steel or
ceramics which assist heat exchange. All such devices aim to provide a large
contact surface area between the hot gases and the kiln feed material in order
to promote heat exchange.
Devices external to the kiln and installed at the
feed end, where hot exit gases flow through them, preheat the feed material
and, if necessary, dry it. With the dry process, the drying operation is unimportant;
what is important is the preheating attainable in suitable devices.
Cyclone Preheater
In cyclone preheater system, an efficient heat
transfer takes place between the finely dispersed raw material powder falling
downwards and the hot gases flowing upwards. With this preheater it has been
able to obtain exit gas temperature of about 320 - 350 C.
Due to efficient heat transfer between raw mix and
hot gases, it is possible to achieve about 25 - 30 % calcination of raw mix in
the preheater before entry into the kiln. Thermal efficiency of 800 - 900 kcal
/kg of clinker are possible with this system.
Now days, the
kiln plays use low pressure drop cyclones in the preheater system. The gain in
pressure drop enables to design the system with increased number of cyclone
stages. Cyclone preheaters with four or five stages are used in the cement
plants.
Preheater Bypass
The volatilized alkalis in the kiln (at 1470 F)
condense on the colder kiln feed (stages 3 and 4). The condensed alkali arrives
with the preheater raw mix and volatilize again in the high temperature zone of
the kiln. This causes the internal alkali circuit.
Hence a part
of the gases is diverted through the bypass valve to lower the alkali content
in the clinker. This, however, has the following disadvantages:
Increased investment cost.
Negative influence on kiln’s heat economy due to
increased heat consumption
Shaft Preheater
In this system, heat exchange is accomplished by the
raw mix descending down a vertical shaft against the counter current hot gases.
Different designs of shaft preheaters are available with thermal efficiency
ranging from 850 - 950 kcal / kg. The raw material is calcined to about 25 ~ 30
% the shaft preheater has found its operation abroad with smaller cement units producing 1600 tpd or lower, because of its self -
supporting construction and hence, lower civil costs.
We will see in our next post “Pre calciner in
cement plant “with continuation of firing technology
Cooling
of the shell
Water cooling of the burning zone of the shell has a
protective influence on the lifetime of the refractory; it extends the life by
1.5 - 2.0 times and facilitates the formation of a protective coating on the
refractory.
If proper cooling of the shell is not provided for,
a thin layer of low viscosity liquid clinker phase is formed which fails to
provide the protective coating.
Do you have any suggestions? Please write in comment
box.
We will see other topic i.e. “Basic
properties of cement compounds ” in our next post in the category of
Steel and cement technology.
I am very thankful to Mr. Subrata Bhaumik,
Independent cement consultant, for providing such beautiful information and
contents about cement technology.
Mr. Subrata Bhaumik has more than 50 (Fifty) Years
(1965 - 2016) of Experience in Cement and other related Industry covering more
than 100 assignments in cement plants with capacities ranging from 100 tpd to
10,000 tpd in India and abroad involving visit to 25 countries overseas in
connection with work.
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Author of this content, please click the Author profile link below.
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