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| CEMENT
INDUSTRY |
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Thermal Energy Consumption
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The
thermal energy contributes nearly half of the
energy cost in a cement plant. The major consumption
is the fuel used in the kiln. The other thermal
energy consumers are
- Coal furnace - Used for drying of coal in some
plants
- Raw mill furnace - Used for drying of raw meal
during starting of the plant
- Cement mill furnace - Used in some plants for
drying of slag
The major aspects which affect the thermal energy
consumption in a cement plant are listed below.
Pre-heater - stages & sizing
The number of stages in the pre-heater system
has a major bearing on the thermal energy consumption
of the kiln.
The more the number of stages in the preheater,
the higher the thermal efficiency of the system.
Presently, it is recommended to install a 6-stage
/ 5-stage pre-heater system. One additional stage,
will on one hand reduce the thermal energy consumption
by 10 - 15 kcal / kg of clinker. On the other
hand it increases the pre-heater fan power consumption
by 1 unit / ton of clinker. The net benefit is
to be quantified before taking the decision. It
is also important to note that, the pre-heater
outlet temperature reduces by about 25oC, when
an additional stage is added. This reduces the
heat available for drying the limestone and coal.
This aspect also has to be borne in mind, while
deciding the number of stages in the pre-heater.
Target specific thermal energy
consumption - 715 kCal / kg of clinker
Pre-heater - monitoring
The continuous monitoring of the pre-heater is
very essential, to maintain the pre-heater system
leak proof. Hence, it is recommended to install
oxygen and CO analysers at the kiln inlet, pre-heater
outlet to enable continuous monitoring of the
system.
Kiln - inlet & outlet seals
The inlet & outlet seals of the kiln are important,
as it helps reduce the air infiltration into the
system. The inlet seal is more important, as the
infiltration chances are higher. Based on the
experiences of the Indian cement plants, we recommend
installing
- v Pneumatic seal for kiln inlet
- Spring loaded mechanical seal for kiln outlet
The pre-heater flanges, poking holes & inspection
holes are all potential sources of leakage during
operation of the plant. Hence the number of flanges
should be reduced to a bare minimum. The inspection
& poking holes in the pre-heaters should be
provided with air tight covers, so that, the air
infiltration into the system is minimum.
Coolers
The cooler is a critical equipment used for cooling
and also recuperate the heat back into the kiln
system. In majority of the existing plants, conventional
grate coolers are being used. These coolers have
lower recuperation efficiency, occupy more space
& need more cooling air. The latest high efficiency
coolers, have higher recuperation efficiency,
need lesser cooling air, are compact in size and
has lower radiation loss. The typical comparison
of a conventional cooler and a high efficiency
cooler are as below.
|
| Parameters |
Units |
Conventional
cooler |
High
efficiency
cooler |
| Loading |
ton/m2/day |
35
|
50
|
| Cooling air |
Nm3/kg of Clinker |
2.2
|
1.8 |
| Cooler vent air
|
Nm3/kg of Clinker |
1.0 |
0.6- 0.8 |
| TA temperature
|
deg C |
800
- 900 |
950
- 1050 |
| Power cons. |
units / ton of
clinker |
5 |
5 |
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Typically, the high efficiency
cooler can reduce the thermal energy consumption
by 30 - 40 kCal/kg of clinker.
Hence, it is recommended to
install a high efficiency grate cooler at the
initial stage itself.
Wherever possible, the cooler exhaust air should
be used for raw material drying/coal drying/slag
drying.
The cooler should also have an ESP/bag house for
collection of clinker dust.
Burners
The burner system also plays a vital role, in
determining the thermal efficiency of the system.
The present operating burners operate with, a
primary air ranging from 12% - 18%. The latest
trend is to install multi-channel low primary
air burners. These burners have the following
advantages
Lower primary air requirement (7 - 8%)
Sharper and shorter flame
Better flame control
In an operating plant, the replacement of a conventional
burner with a new burner (7 % primary air, including
conveying air) resulted in reduction of thermal
energy consumption by nearly 15 kCal / kg of clinker.
Hence, it is recommended to
install a high efficiency low primary air burner.
Refractory lining - kiln, pre-heaters
& coolers
The refractory lining in the kiln, pre-heaters
& coolers are very vital for conserving thermal
energy within the system and protect the outside
supporting metallic part. The older systems used
to utilise all along only high alumina bricks.
The present trend is to install
basic bricks in the kiln, which have comparatively
higher life (more than twice ) and lower consumption.
Since this reduces the stoppages of the plant,
the runnability of the plant improves, resulting
in lower thermal and electrical energy consumption.
In all other areas of the pre-heaters and the
cooler, it is recommended to install alumina bricks
(30 - 40%), preceded by insulation bricks.
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