Coal
Preparation
Asec
Academy
Fuels in cement industry
In physical state :
fuels classified as solid ,liquid ,or gaseous.
Solid Fuels Coal , lignite
used in cement rotary kilns
Coal (Anthracite)
Lignite Coke
used in cement shaft kilns
Peat
Wood Also heavy oil and natural gas
used as a fuels in cement industry.
Coke
Asec Academy Coal Preparation
The consumption of fuel in a
cement plant
Dry process Wet process
83 % kiln operation 96% kiln operation
14 % raw material drying 4 % coal drying
3% coal drying
Asec Academy Coal Preparation
In 1890s pulverized coal firing achieved in cement industry in USA
Fuel oil and natural gas have replaced coal for economic reasons
In Germany coal in total fuel consumption was 2% in 1973 increased
to 6% in 1975.
Investments cost of coal mines roughly 20 times higher than oil or
gas.
The exploitation cost of natural gas is 3-5 times lower than oil or
Coal.
Gas does not required elaborate transportation and construction cost
of pipe lines are only fraction of costs of railroads for transportation
of coal.
Asec Academy Coal Preparation
The costs of coal preparation department of a cement plant is about
15-20% of cost of total plant machinery .
When switching from coal to natural gas in existing plant production
cost lower by 8-10%
Practically fuel oil and gas is ash free so chemical composition of
raw mix is free of variation due to different ash content of coal.
Asec Academy Coal Preparation
Coal Formation
Introduction
The majority of fossils recovered in Cape Breton can be traced to
coal areas or coal fields.
Fossils have been found ranging from whole or partial tree trunks
and branches to shrubs and vine growth.
Evolution dates back to pre-historic times to approximately 325
million years ago when the region was covered in lush, dense
vegetation.
Coal beds consist of altered plant remains.
When forested swamps died, they sank below the water and began
the process of coal formation.
Asec Academy Coal Preparation
However, more than a heavy growth of vegetation is needed for the
formation of coal.
The debris must be buried, compressed and protected from erosion.
Even though all the biological, geographic and climatic factors may
be favourable, coal could not be formed unless the plant debris was
submerged and buried by sediments.
There are 4 stages in coal formation:
peat, lignite, bituminous and anthracite.
The stage depends upon the conditions to which the plant remains
are subjected after they were buried - the greater the pressure and
heat, the higher the rank of coal.
Higher-ranking coal is denser and contains less moisture and gases
and has a higher heat value than lower-ranking coal.
Asec Academy Coal Preparation
Peat Stage 1
Peat is the first stage in the formation of coal.
Normally, vegetable matter is oxidized to water and carbon dioxide.
However, if plant material accumulates underwater, oxygen is not
present and so only partial decomposition occurs.
This incomplete destruction leads to the accumulation of an organic
substance called peat.
Peat is a fibrous, soft, spongy substance in which plant remains are
easily recognizable.
It contains a large amount of water and must be dried before use.
Therefore, it is seldom used as a source of heat. Peat burns with a
long flame and considerable smoke.
Asec Academy Coal Preparation
Lignite Stage 2
Lignite, the second stage, is formed when peat is subjected to
increased vertical pressure from accumulating sediments.
Lignite is dark brown in colour and, like peat, contains traces of plants.
It is found in many places but is used only when more efficient fuel is
not available.
It crumbles easily and should not be shipped or handled before use.
Asec Academy Coal Preparation
Bituminous Coal Stage 3
Bituminous Coal is the third stage.
Added pressure has made it compact and virtually all traces of plant
life have disappeared.
Also known as "soft coal, bituminous coal is the type found in Cape
Breton and is our most abundant fuel.
It is greatly used in industry as a source of heat energy.
Asec Academy Coal Preparation
Anthracite Stage 4
Anthracite, the fourth stage in coal formation, is also known as
"hard coal" because it is hard and has a high lustre.
It appears to have been formed as a result of combined
pressure and high temperature.
Anthracite burns with a short flame and little smoke
Asec Academy Coal Preparation
Mill Types
Ball mill - roller mill
The operating principles of these two types of mill differ in the
following ways:
A A roller mill requires more air to convey the material than does a ball
mill.
This means that only part of the air from a roller mill can be used as
primary air for a rotary kiln of the modem, high-economy type, as the
amount of primary air would be too great in relation to kiln combustion
requirements, thus increasing heat consumption
All or some of the air from a roller mill must therefore be dedusted in a
separate filter.
If the raw coal has a low or moderate moisture content, the amount of
air from a ball mill is generally no greater than can be used as primary
air in the kiln.
Asec Academy Coal Preparation
B Grinding plants with roller mills take up less space and have a
lower specific energy consumption than ball mill plants, the power
saving being 15-25% for the whole grinding plant.
C The feed to ball mills must be precrushed to approx. 25 mm lump
size, and the drying capacity is limited to a reduction in moisture
content of 12-15%, whereas a roller mill will handle material of up
to 85 mm feed size, depending on mill dimensions, and remove
up to 20% moisture.
D Roller mills are more flexible than ball mills, since the roller mill
can easily be adjusted to varying throughputs within a range of
approx. 40-100% of nominal capacity, as required.
D Ball mills are usually preferred for grinding highly abrasive types
of coal.
Asec Academy Coal Preparation
Grinding
Coal contains varying quantities of volatiles, both combustible and
non-combustible
All types of coal also contain hygroscopic (inherent) moisture as well
as varying quantities of impurities which are released in the form of
ash in the combustion process.
The degree of grinding required depends on the type of coal used.
Coal with a low volatile content requires a high ignition temperature
and must be finely ground Coal with a high volatile content, however,
must not be ground too fine, otherwise the volatiles will be expelled too
quickly to be able to mix properly with the combustion air.
Asec Academy Coal Preparation
Drying - Air circulation
There are three factors which are decisive in determining
the amount of air to be drawn through a coal mill:
The amount of air at a given temperature must be sufficient to
ensure effective drying of the moist coal
The amount of air must be adequate in relation to the
evaporated moisture so that the dew point of the air leaving the
mill is maintained at a suitably low level. In practice, the dew
point must be 15-20C lower than the mill exit air temperature to
prevent condensation in the ducts and the dedusting installation
after the mill.
Asec Academy Coal Preparation
Drying - Air circulation
There are three factors which are decisive in determining
the amount of air to be drawn through a coal mill:
In air-swept mills, of both the ball and roller types, the air
velocity must be high enough to extract the ground material
from the mil When specifying the moisture of coal, a distinction
is made between surface moisture, which evaporates at
ambient temperature, and hygroscopic moisture which is more
closely bound to the coal.
Asec Academy Coal Preparation
The inherent moisture content
depends on the geological age of the
coal, the moisture ranging from 1-2% in
anthracite to 10-20% in lignite
The amount of air to be extracted from the grinding
plant, including false air and water vapour will in this
case be less than 1.5 kg air per kg coal, corresponding
to approx 17% of the combustion air, based on coal
with a calorific value of 6500 kcal/kg (27,200 kJ/kg).
Asec Academy Coal Preparation
Safe handling of coal
The coal must be sufficiently dry for grinding, storing, and feeding
purposes.
Excessive drying should be avoided, due to the accelerating process
of oxidation which occurs as the moisture content is reduced, with the
consequent risk of spontaneous ignition.
In practice, drying off the small part of the hygroscopic moisture will
ensure a high degree of safety combined with good processing and
handling properties.
Asec Academy Coal Preparation
Direct firing
Implies a simple and cheap installation.
Regulation of firing in the kiln is effected by varying the feed to the
coal mill.
The coal meal produced is precipitated in a cyclone, and after
passing an air sluice, it is blown into the kiln.
Kiln and mill operation are interdependent with direct firing, which
means that mill throughput must correspond to kiln requirements.
Asec Academy Coal Preparation
Indirect firing
is the most commonly applied method of firing a rotary kiln.
After precipitation in a cyclone, the coal meal is stored in a bin from
which the burner is fed.
With indirect firing the mill throughput is kept constant irrespective of
kiln output fluctuations, and short interruptions of mill operation do
not affect kiln operation so long as there is an adequate supply of
coal meal in the bin.
Asec Academy Coal Preparation
In both systems, depending on mill and
kiln type, all or part of the drying air
may be used as primary air in the kiln
provided it is passed through a cyclone
to remove most of the coal meal.
Asec Academy Coal Preparation
Drying with inert kiln gases
Plants using inert kiln gases with an oxygen content of about 4% for
drying and transport of coal are becoming more commonplace.
They have the advantage that the risk of fires and explosions is
reduced considerably when the oxygen level is kept below 14%.
Operating conditions and plant maintenance must be such that this
limit is not exceeded during operation.
This is effectively controlled by an 02-meter placed at dedusting
installation outlet.
Asec Academy Coal Preparation
In an inert coal grinding plant the temperature of the drying gas can
to some extent be reduced either by adding cold, atmospheric air or
by recirculating the mill exit air.
The dew point of the exit gas when using cold atmospheric air is
lower than when using recirculation air.
In order to ensure efficient material transport, the air flow rate
through the system must be maintained within a certain range.
It may therefore prove necessary to inject water into the mill to keep
the system inert while operating with coal with a low moisture
content.
Asec Academy Coal Preparation
Dedusting Installation
When all the air passed through the mill cannot be utilised as
primary air for kiln firing, an electrostatic precipitator or a bag filter
must be installed to dedust the air.
An electrostatic precipitator must be preceded by a cyclone for
collecting most of the coal meal so as to reduce the average dust
concentration in the precipitator to a level at which explosions do not
occur.
Asec Academy Coal Preparation
Safe Operation
1. Electric calorifier for preheating the precipitator to ensure even heat
distribution during start-up.
2. Electric heating elements with thermostatic control to prevent
condensation during operation.
3. Vibrators for bottom hoppers
4. CO monitoring device
5. Thermal monitoring
6. C02 or N2 injection equipment for precipitator inertisation
Asec Academy Coal Preparation
Rotary Drum Coal Drying
Technology & Plant Design
Coal (Wet Basis) from Coal storage yard or coal mine is loaded into
belt conveyor and conveyed to sealing feeder for a direct fed into the
saving drum dryer
After undergoing a process of medium heat exchange drying, coal
is loaded into discharging conveyor belt from the discharging box
then the coal is conveyed to the storage yard
Hot medium (hot fuel gas) for coal drying is
produced and prepared in coal-fired hot blast
heater.
Asec Academy Coal Preparation
After undergoing , the coal is loaded into discharging conveyor
belt from the discharging box , then the coal is conveyed to the
storage yard.
Hot medium(hot fuel gas) for coal drying is produced and
prepared in coal-fired hot blast heater.
The environment air, which is sucked into the flue pipe with air
blower and hot fuel gas from the hot blast furnace, is sent to drum
dryer after temperature adjustment where it heats the wet coal
and absorbs the water vapor.
The mixture of hot gas and water vapor go through discharging
box and flue pipe and is sent to dust collect cyclone for powdered
coal recovery.
Asec Academy Coal Preparation
The preliminary purified gas is sent to bag filter for further
purification.
Then the waste gas goes through the exhaust funnel and is
discharged to the atmosphere by the induced draft fan.
The powdered coal collected by First Stage dust collector is
loaded onto discharging conveyor by discharging spiral and
sealed coal discharger, then mix with the dried coal.
The powder coal collected from the second stage bag filter is
loading into discharging conveyor by discharging spiral.
Asec Academy Coal Preparation
Drying System Process Flow
System principle of drying
process
The
dried
Coal
Asec Academy Coal Preparation
Coal
after
Drying
Asec Academy Coal Preparation
Drum Coal Drying System
The drying machine is a rotary metal drum, which is set at an angle
about 3-5 degree and rotates at a speed of 2-8 R/Min.
The coal remains in the drum for 8-12min.
The coal drum dryer are uniflow which mean the hot fuel gas and
coal are fed in one end and discharged at the other.
The heat exchange takes place during the process of gas-solids
concurrent flow in the drum
Asec Academy Coal Preparation
1 The process is simple and operation reliable.
The factory is reasonably and compactly laid out as a whole
with a combination of feeding, crushing, drying and
discharging.
The total investment is low and service life is long.
2 The temperature of the heat transfer medium can be up to
600, and down to 100 when discharging.
The drying thermal efficiency is high compared with other
drying system.
Asec Academy Coal Preparation
3 The feeding quantity is adjustable with sealed coal feeder.
4 Measures are adopted to collect the dust and prevent the
smoke at the drying drum, hot blast furnace and system
transfer point
5 Automatic monitoring system can monitor temperature and
pressure at four points:
furnace, dryer inlet, and dryer outlet, inlet of induced
draft fans which guarantee the normal and safe
production
Asec Academy Coal Preparation
6 Big volume drum dryer is adopted to shorten the residence
time of coal in the dryer to prevent coal burning and
pulverization.
7 In designing the drum dryer, we consider reliable
performance, high capacity, high thermal efficiency,
avoidance of caking, etc
In designing furnace, we consider easy ignition, quick heating, etc.
In designing the dust collector, we chose deducting cyclone, which is
high temperature resistant and without moving components, and high
efficient pulse bag filter.
Asec Academy Coal Preparation
Coal firing
Direct firing Indirect firing
Asec Academy Coal Preparation
Coal
Great care has to be taken handling and burning coal owing to the
risk of spontaneous ignition, fire and explosion.
As a result, the design and operation of coal firing systems requires
greater specialist knowledge than gas and fuel oil systems.
Coals are usually classified according to their content of volatile
substances, spanning from the younger, gas rich lignites over
ordinary coals to older anthracites poor in volatiles.
Following the same pattern is also the ability of the various coal
types to retain moisture due to their structure, which gets more and
more compact as the coal gets older.
Irrespective of the age of the coals it is found that the content of inert
ashes are also subject to considerable variation
Asec Academy Coal Preparation
The calorific value of a coal sample depends quite naturally on
the content of inert substances like moisture and ashes. But the
content of volatile material will also affect the specific calorific
value yet in a more complex way.
Since the calorific values of hydrocarbons are considerably
higher than that of carbon, the calorific value will increase with
the volatiles.
To achieve good combustion and a satisfactory flame formation
in the kiln, the proper fineness of the coal meal is very important.
Immediately one would think that the finer the coal the quicker
the ignition, but this is only true for low volatile coals, coals of
higher contents of volatiles must be ground coarser in order to
control the rate of expulsion of the gases from the solid particles
Asec Academy Coal Preparation
Properties of typical coals
Type of coal Lignite Bituminous coal Anthracite
%C 56 70 78
%H 4 3 2
Composition
%S 1 1 1
% (N+O) 19 3 2
Total moisture
40 -50 5 - 10 0-3
%
Volatiles % 40 - 50 10 - 40 5
Ranges
Hygroscopic
10 - 25 1-3 1
water %
Ash 5 - 25 10 - 20 5 - 10
Calorific value Gross 5120 6625 7100
(kcal/kg) Net 4820 6310 6900
Kg/kg 7.1 9.2 9.9
Combustion air
Nm3/kg 5.5 7.1 7.6
Flame
Adjustment Primary air
quantity
Gaseous fuels Liquid fuels Pulverized fuels
8 9% 89% 10-13%
of total of total of total
combustion air combustion air combustion air
Total combustion air = min combustion air for max (designed)
burner capacity + 10% air excess
Asec Academy Coal Preparation
Influence of Coal
properties on flame shape
1 Volatile Content
The combustion time of pulverized Coal increases as the volatiles
decreases (longer ignition distance) so low volatiles Coal must
grind more fine.
2 Fineness
Burning time and distance of Coal dust grain increases with its
square diameter
3 Ash Content
High ash content has retarding effect on burning time
(reduce coal dust concentration & absorbed heat )
Asec Academy Coal Preparation
