|
|
|
| CEMENT
INDUSTRY |
 |
Electrical Energy Consumption
|
| Section
/ Equipment |
Electrical
energy consumption
(in units / ton of cement)
|
| Mines, crusher
& stacking |
1.5
|
Reclaimer,grinding & transport |
18.0 |
Kiln feed, kiln & cooler |
22.0 |
Coal mill |
5.0 |
Cement grinding & transport |
23.0 |
Packing |
1.5 |
Lighting, pumps & services |
4.0 |
Total |
75.0 |
|
Electrical
energy consumers
Mines & Crusher
The first step in the cement manufacturing process
is the mining of limestone. Mining has conventionally
been with compressed air drilling and subsequently
blasting with explosives. The surface miner has
recently been developed primarily for mining in
locations, which are located close to human inhabited
areas. This equipment has proved to be so useful
that, they are fast replacing the conventional
mining systems.
In one of the new plants, a surface miner was
installed and the mining energy consumption (compressors)
was avoided. The size of the limestone mined is
also lower, resulting in reduction of crushing
energy consumption to 0.5 units / ton of cement,
as against 1.5 - 2.0 units / ton in conventional
plants. The surface miners are presently available
for medium limestone. Hence, wherever possible,
the surface miner can be installed for achieving
energy saving. The operation of surface miner
is also environment friendly, with less dust &
noise generation.
Hence, consider application
of surface miner for soft and medium hard Limestone.
The next step in the cement manufacturing process
is the crusher. The choice of crusher type depends
on the grindability and the type of down-the-line
grinding system.
Typically, for a VRM system/ball mill with roll
press system, it is recommended to have a single
stage hammer crusher, with a high efficiency screen
before the crusher.
Install single stage hammer
crusher with high efficiency screen.
The other points to be taken care during the design
stage are, the additional auxiliary power consumption.
The auxiliary capacity and controls should be
designed, such that, the idle power consumption
is avoided during operation of the crusher. |
| |
Per
ton of Limestone |
Per
ton of
Cement |
Target
power consumption for the crusher
(for medium hard limestone)
|
1.0 unit |
1.5 unit |
|
Transportation
of Limestone
The transportation of the mined limestone to the
crusher, is normally done through dumpers or trucks.
In case of mountainous terrain, the ropeways are
installed. The detailed life cycle cost analysis
of transportation with dumpers & ropeway indicates
that, the ropeway is more beneficial in some cases,
where the distances are more than 5 kms. The ropeway
calls for higher investment, but the operating
costs are much lower. In one of the operating
cement plants, the ropeway was installed replacing
the dumpers for transportation of limestone from
the mines to the crusher in the plant, resulting
in substantial benefits. The option of installing
a rope-way can also be considered at the design
stage.
Hence, consider installation of rope way for limestone
transportation, where the distances are more than
5 kms.
Pre-blending system
The pre-blending system plays a vital role in
providing a uniform feed to the raw mill. The
statistics of the performance of the pre-blending
stockpiles in various operating cement plants
in the country indicate that, the longitudinal
stockpile has a better pre-blending efficiency.
Hence, it is recommended to install a longitudinal
pre-blending system.
In locations where the
- Rain-fall is more
- Raw material is sticky in nature
it is recommended to install closed stockpiles.
This will help in avoiding feed failure to the
raw mill during the monsoon / rainy seasons.
The target operating pre-blending efficiency should
be 10.
Raw mill
The raw mill is a major energy consumer and should
be designed based on the grindability and moisture
of the limestone available. The Vertical Roller
Mill (VRM) has at least 30% lesser power consumption,
than the comparable ball mill system. Hence, for
upto medium hard limestone, the VRM should be
the first choice. Wherever the moisture percentage
is also higher, say >15%, VRM should be the
first choice. Only in case of very abrasive raw
materials or raw materials with higher free silica
content, the plant can consider installing a closed
circuit ball mill, with a pre-grinding system.
The VRM (wherever the design allows), should be
installed with a reject handling / mechanical
re-circulation system, to ensure lower pressure
drop across the mill.
The energy efficient alternative therefore is
to install a VRM with mechanical re-circulation
system.
The other aspects to be taken care are
Conveying system - only mechanical conveying systems
such as, bucket elevators/belt conveyor should
be installed
Classifier - dynamic high efficiency classifier
( cage type classifier ) should be installed
Mill inlet sealing - This is very essential, since
majority of air infiltration in an operating plant
is through the inlet. The present day plants use
either rotary / triple gate seals for the mill
inlet. These seals though effective in the initial
days, start developing leaks over a period, demanding
regular maintenance.
In one of the new plants, a material seal at the
VRM has been attempted with satisfactory results.
Hence, it is better to strongly consider installation
of material seal or reliable triple gate/rotary
seals at the design stage itself
High efficiency mill internals (for ball mills)
In the case of ball mills, the present trend is
to replace existing mill internals with high efficiency
internals (grinding media, liners and diaphragms).
This has the following advantages
Higher mill output
Less wear
Lower power consumption (up to 5% reduction is
expected)
Lesser maintenance
Hence, all the new plants installing ball mills,
should prefer high efficiency mill
internals.
Target power consumption of (VRM) 18 units/ton
of cement
for medium hard limestone (or) 11-12 units/ton
of limestone
Fans in raw mill
The fans are the major power consumers, next only
to the main drive in the raw mill section and
hence need attention during the design stage.
The major power consuming fans are the raw mill
fan and the vent ESP/BH fan (with kiln).
The fans should be high efficiency (> 80%)
backward curved fans with variable speed controls.
The typical variable speed mechanisms proposed
are
Variable frequency drive (VFD) - up to 250 kW
LT drives.
RRC, fluid coupling, SPRS (HT only) - in increasing
order of energy efficiency.
The best variable speed systems are VFD for all
LT fans and SPRS for all HT fans. The other major
decision, whether to adopt a 3-fan system or 2-fan
system, should be carefully weighed and taken
at the design stage itself. The various conditions
and the corresponding decision making table is
given below |
| Operational
Characteristics |
Three
Fan System
|
Two
Fan Systems |
Entire PH air
passes through
the VRM and no flow of air
through by-pass
|
Not
Advantageous |
Will
serve
the purpose |
Part of PH air is passed through by-pass
always |
Advantageous |
Higher
power
consumption |
P/H fan/ESP fan operates with damper throttling
when R/M is stopped and expected R/M stoppage
say >25% of time |
Advantageous
and lower power |
Higher power
consumption |
|
The other major decision is
to decide the excess margin for the fans. The
closer the operating point to the design point,
the higher will be the efficiency of the fan.
However, a safety margin is required to meet the
contingency requirements.
The fans are to be designed
typically with 15 % margin over the operating
condition.
Raw mill and kiln main dust
collection system
The Electro Static Separator (ESP) has been conventionally
used for collecting the dust in the kiln and raw
mill circuits. The operating efficiency of the
ESPs are normally around 99.5 %. The operation
of the ESP also depends on the CO content of the
flue gas. Whenever the CO content goes beyond
0.6 %, the ESP trips. The amount of time the tripping
occurs is less; it nevertheless, leads to particulate
emissions. With the increased emphasis on the
environmental regulations, it is necessary to
have a dust collection system, that is say 99.9
% efficient and operate continuously.
The present trend has been to install bag houses
for the kiln and raw mill exhaust. The comparisons
of the ESP and bag house are as below
- The bag house has a pressure drop of about 150
mm Wg as against 20 - 40 mm Wg in the ESP. Hence,
the power consumption of the exhaust fan is higher.
- The ESP trips, whenever the CO content goes
beyond 0.6 %, while the operation of the bag house
is not dependant upon the CO value.
- The collection efficiency of the bag house is
much higher at 99.9%, while that of the ESP is
marginally lower. However, presently ESP's with
efficiencies of 99.9% require higher investments.
- The ESP requires conditioning with power consumption
for the GCT pump and screw conveyors.
- The GCT in the ESP system needs water for conditioning,
while the bag house does not need water.
Hence, at the design stage
itself, the decision to install the Bag house
or ESP has to be taken, after carefully weighing
the above aspects.
Blending and homogenisation
This is a very important operation having a major
bearing on the operation of the kiln and the thermal
energy consumption of the kiln. The comparisons
of the effectiveness & energy efficiency of
various systems indicate that, the continuous
blending & storage system is more efficient.
Hence, it is recommended to
install a continuous blending & storage system.
The target blending efficiency should be 10.
Kiln and pre-heater
Kiln feed conveying
Kiln feed conveying should be carried out by mechanical
bucket elevator, as pneumatic conveying is more
energy consuming and also adds false air to the
system. The pneumatic conveying system consumes
about 1 unit / ton of kiln feed, as against 0.25
units / ton of kiln feed for a mechanical conveying
system.
Hence, it is recommended to
install a mechanical conveying system.
Pre-heater - stages & sizing
The more the number of stages in a preheater,
the higher the thermal efficiency of the system.
In the older plants, the conventional cyclones
offer a pressure drop of 550 to 600 mm wg resulting
in higher power consumption of the pre-heater
fan. Presently low-pressure drop cyclones, having
a pressure drop of 380 - 420 mm wg are available.
With the installation of these low pressure drop
cyclones, it is possible to achieve a pre-heater
fan power consumption of about 6 units / ton of
clinker, as against the present consumption of
more than 8 units / ton of clinker. The target
pre-heater fan air flow capacity should be below
1.8 Nm3/kg of clinker.
The margins in all the sections of the plant,
also influence the energy efficiency of the plant.
It is recommended that all the equipment in the
pre-heater system, be designed for a kiln loading
of 5 ton/m3/day.
Coal conveying system
The coal after grinding in the mill, is conveyed
pneumatically to the kiln & calciner for firing.
Generally, identical conveying equipments are
used for both calciner and kiln. The coal fired
in the calciner is about 50 % more than that fired
in the kiln. Correspondingly, the capacity of
the conveying system and the blower are lower
for the kiln firing. The installation of a lower
capacity conveying system, reduces the quantity
of the ambient air entering the kiln. This increases
the secondary air by an equivalent quantity, resulting
in increased thermal efficiency. Hence, by design
separate right capacity conveying systems should
be installed for kiln & calciner.
Install separate right capacity
coal conveying system for kiln & calciner
.
Water sprays
Water spray systems can be installed, particularly
in cooler vent, pre-heater outlet or other areas,
where further utilisation of heat is not required.
The volume of the gas can be reduced by spraying
of water, which ultimately results in reduction
of fan power consumption. Presently, water spray
systems, which can reduce temperature by more
than 100oC in a small area, are available. These
systems can aid in reducing the power consumption
of the exhaust fans.
GCT water pumps
Water is sprayed in GCT (located before R/M or
in the by-pass line) through high pressure pumps.
The capacity of pumps required depends on the
operation of raw mill and coal mill. The water
requirement will be maximum, when none of the
hot air users are in operation. The water requirement
will be least, when all hot air users are in operation.
Hence, it is better to install pumps of different
capacities to suit both the different requirements.
Install two GCT pumps of different
capacities to meet the raw mill operation and
stoppage condition.
Coolers
The cooler is a critical equipment used for cooling
and also to recuperate the heat back into the
kiln system. This has a major bearing on the thermal
efficiency of the system. This is being dealt
with in Section-5.4. The cooler is also a major
consumer of electrical energy. The major energy
consumers are the cooler fans.
It is recommended to install
high efficiency backward curved fans for cooler,
with Variable Frequency Drives (VFD) for all the
fans.
Target energy consumption - 5 units / ton of clinker
Coal section
The coal is generally procured from the coalfields
through rail or road, crushed, ground to fine
powder and then used in the mill. The coal receipts
vary in quality continuously. Hence, it will be
better to install a longitudinal pre-blending
stockpile for coal also. The installation of pre-blending
stockpile for coal will ensure uniform quality
of coal feed to the kiln.
Install pre-blending stockpile
for coal.
The coal mill is also a major energy consumer.
Presently, majority of the coal mills are air-swept
ball mills. The VRM has been installed in some
of the modern plants, with at least 40% lower
energy consumption, in comparison to the conventional
air-swept ball mills, used in older cement plants.
Hence, it is recommended to install vertical roller
mills for coal grinding.
Install VRM for coal grinding.
Fine coal bins
The calciner can accept coarser coal, in comparison
to the kiln. Hence, it is recommended to have
separate fine coal hoppers, for kiln coal and
calciner coal. This will enable differential grinding
of coal for calciner & kiln.
Install separate bins for calciner
and kiln.
Fans in kiln & coal mill
The fans are major power consumers in the kiln
and pre-heater system. Hence, by design the fans
should be of higher efficiency (>80%) and equipped
with a variable speed mechanism for speed control.
The typical variable speed arrangements for the
fans in the kiln and coal mill sections are as
below
|
|
Fan |
Type
of veriable speed drive (VSD) |
| P H fan |
SRPS/GRR/LRS
|
RM fan/ESP fan |
SPRS/GRR/LRS |
Cooler vent fan |
SPRS/DC/Fluid coupling/ GRR/LRS |
Cooler fans |
VFD |
Coal
mill fans
|
VFD/GRR (or) fluid coupling for HT |
|
The
VSDs are given in the order of reducing efficiency.
Cement mill
The cement mill is another major energy consumer
in a cement plant. In the present operating plants,
majority of the cement mills are open circuit
ball mills. In some of the plants, closed circuiting
has been done, to increase the capacity &
reduce the energy consumption. The installation
of the pre-grinding system, has also been taken
up in some plants, with substantial benefits.
Very recently, the VRM has also been installed
for grinding of cement in three cement plants
in India. The results reported by one of the plants
(23-24 units/ton of OPC-43) is very encouraging.
Hence, every new plant should
go for either a VRM or a ball mill with pre-grinding
system & high efficiency separator.
The VRM should have a high efficiency separator
and mechanical re-circulation system. The ball
mill if installed, should have high efficiency
internals (grinding media, liner plates, diaphragm).
The conveying of cement to the silo should be
through a mechanical conveying system.
Utilise mechanical conveying
for cement.
Target power consumption for
OPC - 43 grade cement are as follows
VRM - 23 units / ton of cement
Ball mill with roll press - 27 units / ton of
cement
Ball mill ( closed circuit ) - 29 units / ton
of cement
Cement silo and packing plant
The cement silo should have a blower extraction
system, to reduce the power consumption. The packing
should be through an electronic packer, for accurate
weight and reduced energy consumption.
Target power consumption -
1.5 units / ton of cement
|
|
|
|
 |
|
| |
|
|