3. What Is Coal Preparation?
• Coal preparation is the removal of undesirable material (ash,
Sulphur, moisture)from the Run-of-Mine (ROM) coal by
employing separation processes which are able to differentiate
between the physical and surface properties of the coal and the
• Coal preparation is also called as Washing, Cleaning, Processing
• Coal processing technologies play an important role in the
electrical power supply chain by providing high-quality fuel for
coal-fired utilities and industrial boilers.
• The upgrading and cleaning of the coal is achieved by
using following separation process :
i) Low cost separation.
ii) Solid-solid separation.
iii) Solid-liquid separation
• The separation process removes waste rocks and water
from the mined coal.
5. Why is Coal Preparation Needed?
• Coal preparation is required because freshly mined coals contains:
i) Heterogeneous mixture of organic (carbonaceous) matter.
ii) Inorganic (mineral) matter.
• These impurities reduce coal heating value, leave behind an
undesirable ash residue.
• Increase the cost of transporting of coal to market.
• Reduces heating value and can lead to handling and freezing
issues for consumers.
7. Coal Processing Operations
• Run-of-mine coal produced by mechanized mining
operations can contain particles as small as fine powder
and as large as several hundred millimeters.
• There are different type of sizing equipments used to
produce desired size of coal which are as:
8. Vibrating Screens
• Screening is used to separate different
sizes of crushed coal. In this process
coarse and fine coal is separated so to
accommodate for specific markets and
• Screens are mechanical sizing devices
that use a mesh to sort particles into
fine (particles that pass through the
screen openings) and coarse (particles
that are retained on the screen surface).
• Vibrating screens, use a shaking rotating
mechanism to segregate particles and to
move material along the screen surface.
• High-frequency screens vibrate very
rapidly to enhance the passage of fine
particles and are normally used for
dewatering fine coal or rock.
• A cyclone is a conical vessel in which coal along with finely
ground magnetite is pumped tangentially to a tapered inlet and
short cylindrical section at a predetermined flow rate and pressure
followed by a conical section where the separation takes place.
• The higher specific gravity fractions being subject to greater
centrifugal forces pull away from the central core and descend
downwards towards the apex along the wall of cyclone body and
pass out as rejects.
• The lighter particles are caught in an upward stream and pass out
as clean coal through the cyclone overflow outlet.
• Cyclones are used where conventional screening or sieving
• Classifying cyclones are commonly applied to size (cut) at 0.10 to
• And represent the only practical option for sizing ultrafine particles
(at a cut of 0.045 mm).
• This sizing device exploits differences in the settling rates of
particles of different size (i.e., smaller particles settle slower than
12. Solid - Solid Separation
The separation of valuable carbonaceous material from waste rock is
typically accomplished using low-cost processes that exploit
differences in physical properties that vary with mineral content.
The common properties that are used to separate coal and rock are
13. What is Dense Medium Separation?
• Dense medium separation or heavy medium separation is used
in coal preparation to produce a commercially graded end
product ,clean coal being separated from heavier shale or high
• A popular process for cleaning coarse coal which is having a
size greater than 12.5 mm.
It is the simplest of all gravity processes and has long
been a standard laboratory method for separating
minerals of different specific gravity.
Heavy coal particles of suitable density are used ,so
that those minerals lighter than the heavy particles will
float, while those denser will sink to bottom .
15. Water-Based Density Separators
• Water-Based Density Separators is a variety of density-based separators
available for separating coal and rock in the particle size range between 0.2 and
• The most common methods include water-only cyclones and spirals.
• A water-only cyclone (WOC) is similar to a classifying cyclone, but typically
has a broad wide angled conical bottom .
• Separation of coal and rock occurs because of the formation of dense
suspension created by the natural fines already in the feed slurry.
16. • A spiral consists of a corkscrew-shaped device that sorts coal from rock by
selective segregation that occurs as particles move in the flowing film
along the helical trough.
• Because of the low unit capacity (two to four tons per hour), spirals are
usually arranged in groups that are fed by an overhead distributor. WOCs
and spirals are often employed in two stages or in combination with other
water-based separators to improve performance.
18. Solid-Liquid Separation
• Solid-liquid separators are used downstream of coal cleaning processes to
remove unwanted surface moisture that lowers heating value, leads to
handling/freezing problems, and increases transportation costs.
• Several different types of mechanical dewatering methods are required,
depending on the size of particles to be treated.
• The removal of water from the surfaces of coarser (> 5 mm) coal is
predominantly carried out using simple screens.
• Fine particles, which have a higher surface area and tend to have
correspondingly higher moisture content, are typically dewatered using
centrifugal methods or filtration system.
19. Centrifugal Dewatering
• Centrifugal dewatering systems, which use centrifugal force to pull water
away from the surfaces of coal particles, operate in much the same fashion
as the spin cycle in a home washing machine.
• For coarse particles, centrifugal dryers that use either a rotating scroll or
vibratory action to transport solids are commonly used.
• For fine particles (< 1 mm), another popular design, known as a screen-
bowl centrifuge, may be used.
• These units are capable of providing low moisture products, although some
ultrafine solids can be lost as waste effluent with the bulk of the water.
20. Filtration Dewatering
• Filtration processes may be used to dewater fine coal in cases where high
coal recovery is desirable.
• Filtration involves the entrapment of fine solids as a cake against a porous
• Traditionally, flotation concentrates have been dewatered using some form
of vacuum filtration.
• These units are capable of maintaining high coal recoveries (greater than 97
percent) while generating product moisture contents of approximately 20 to
• The most popular type of vacuum filter used in the United States is the disc
• The process of converting coal into coke is called as
carbonization of coal.
• When a coking coal is heated in the absence of air, the
porous , hard and strong residue left is called coke.
• Coke making process is multistep complex process and
variety of solid liquids and gaseous products are produced
which contain many valuable products.
• Coke is white , lustrous, dense porous mass.
Depending on the behavior of coal , when heated in
the absence of air, the coal is classified into:
1. Non-coking coal which undergoes practically no
fusing effect and also called as free burning coal.
2. Coking coal which gives porous , hard and strong
residue after heating in the absence of air. The
residue is used for metallurgical purposes and is
known as coke.
25. Types of coal
• Depending upon the extent of carbonization , coal can
be classified into four types as follows:
Classification of coal
38% (soft coal/ brown coal)
• Bituminous 65% (household coal)
• Anthracite 90% (hard coal)
27. Carbonization at various
• Low temperature carbonization is used to produce liquid fuels
while high temperature carbonization is used to produce
• Low temperature carbonization (450-7500 C): In low
temperature carbonization quantity of gaseous product is less
while liquid products are large.
• High temperature carbonization (above 9000 C): In high
temperature carbonization, the yield of gaseous product is more
than liquid products with production of tar relatively low.
29. Conversion of Coal to Liquid
• Several very old processes… since WWII
• Same general theme: increase H/C ratio
• Can make a wide variety of hydrocarbon products (e.g.
synthetic crude or synfuel)
• Two basic methods: direct and indirect
• CTL Improves national and economic security
• Lessens dependence on foreign oil
• Uses domestic resources and produces more jobs for
• Provides environmental benefits
- Cleaner fuels that reduce NOx and particulate emissions
- Enables use of higher efficiency engines
• Is capable of capturing CO2 emissions
• Provides geographic diversity as energy source
31. Coal Liquefaction
• Very expensive
• Liquefaction attractive for transportation fuel
• Indirect liquefaction commercially proven (>50 years)
• Acid gas removal by amines (CO2, H2S)
• S removal by Claus Process
H2S + O2 = H2O + SO2 H2S
+ SO2 = S + H2O
32. Direct Liquefaction Features
• Low transportation cost
• Less chemical transformation required
• Higher efficiency than high Btu gas production
• Easy to store
• Less water required for manufacture
However, processing slurries at high temperature and pressure
presents difficulties with equipment life and solid/liquid separation
(still not commercial)
33. SASOL in South Africa
Indirect Coal liquefaction is proven technology
• South Africa’s SASOL Co. developed a commercial coal
liquids industry (fuel plus chemicals)
•The plant produces about 150,000 barrels daily at its second
34. Conversion Approaches
1. Direct Liquefaction:
• Dissolves coal in a solvent at elevated temperature and
• Combined with hydrogen gas and a catalyst
2. Indirect Liquefaction:
• Involves first gasifying coal, followed by reacting carbon
monoxide and hydrogen together
nCO + (2n+1)H2 = CnH2n+2 + nH2O
Adds hydrogen to break down the
Dissolves in a solvent followed by
Operates at 450 C and 170 bars
Light products are distilled
Medium and heavy distillates
obtained from vacuum distillation
Liquid yields of 70% of the dry
weight of coal feed
Further upgrade is needed for use
as transportation fuels
Complete breakdown of coal with
steam and oxygen
Sulfur is removed from the syngas
Syngas reacted over catalyst at
300 C and 20 bars
Produces a lighter suite of
products; high quality gasoline and
Comparison of Processes
37. • Coal-based liquid fuel becomes viable when the per-barrel price of oil is
expected to exceed the $70-100 range for 20+ years.
• CTL has high front-end capital cost
- A50,000 barrel-a-day plant would cost over $3 billion to construct.
• The product refinement process is three to four times more expensive than
refining an equivalent amount of oil.
• The cost of sequestering the captured CO2 would increase the price of the end
product by $10-20 a barrel.
• The imposition of a carbon cap and trade policy would also raise the cost of
fuel produced with CTL technology
38. Liquids Fuels Summary….
• Proven technologies
• All processes require adding hydrogen
• All processes remove sulfur and ash
• Product include liquid, gas, and combustible solid(char)
• Several long-standing commercial processes (e.g., S.Africa -
• So far no large-scale use without government support
39. Coal to Liquids Summary….
• Good promise because of very large coal deposits in the world
• Numerous processes that can generate a wide variety of
• Environmental issues include some hazardous wastes and
• Does not yet effectively compete with natural gas or oil
production … requires some form of subsidy
41. Introduction :--
Gasification – conversion of carbonaceous fuel to gaseous product.
Dominant chemical process – partial oxidation.
Coal gasification – process for producing syngas.
3 steps of coal gasification process :--
GASIFICATION GAS CLEANING
42. 4 ranks of coal – based on the carbon content
Lignite, bituminous, sub-bituminous, anthracite – different
Factors Recommended specification
Coal rank Sub-bituminous, bituminous
Ash content <(8-12)% & max. 25%
Volatile content No limit
Coal reactivity Moderate - high
Ash viscosity < 250 poise at operating temp.
43. Gasification process
Gasification - coal - properly preheated
High moisture coal - drying required
Caking coals - partial oxidation required
Fluid or entrained bed gasifier - crushing &
Fixed bed gasifier - briqueting of fines coal
44. Gasification process
Pretreated coal – charged in gasification reactor
Temp. :- 800-1900 °C & Pressure :- 10 MPa
Combustion reaction :-
C + 0.5O2
C + O2
Gasification reaction :-
C + CO2
C + H2O
CO + H2
Water – gas shift reaction :-
CO + H2O CO2 + H2
Methanol production :-
CO2 + 3H2 CH3OH + H2O
45. Gasification process
Low heat content product gas – coal
burning - < stoichiometric quantity of air
with or without steam
Medium heat content product gas –
oxygen replaced by air
Water-gas shift conversion – additional
Ash – removed in dry condition
Main product – synthesis gas – 1:2 =
CO : H2
46. Gasification process
Gas products – CO, CO2, H2, CH4, H2S, other organic
vapour, etc. - Impurities – particulate matters - Gas
product purification – prior to combustion –removal
Tar & oil removal – quenching method
Ammonia removal – scrubbing process
aqueous or organic liquid
Acid gases removal – solvent treatment technology
Water-gas shift conversion process – CO2 removal -
passing through absorber – carbon capture
47. Different gasification technique
Updraft moving bed gasifier – coal from the top & oxidants
from the bottom
Downdraft moving bed gasifier – coal from the top & air from
Entrained flow gasifier – reaction between fined coal particles &
oxygen & steam
Fluidized bed gasifier - for low rank coals like lignite
48. Underground coal gasification
Process – under ground – within the coal
seam - 2 vertical walls drilled to the coal
seam – horizontally linked – combustion
process & syngas formation – same as
above ground gasification
Economic benefits - no need to mine the
coal - no need for coal handling - no
need to prepare the coal to be fed into a
Environmental benefits - minimum land
use - coal seams below the fresh
drinking water supplies - no landfill
disposal required for ash or slag
Synthesis gas production – electricity generation, transportation
fuels (e.g. gasoline & diesel)
Hydrogen formation – ammonia, powering hydrogen economy,
upgrading fossil fuels
Ammonia - fertilizer
Hydrocarbon – methane, natural gas from coal gasification –
liquefied natural gas – can replaced gasoline & diesel
50. Current Scenario
Rising population – increasing
energy demand – CO2 emitted from
gasification – released in atmosphere
– green house effect - research on
carbon capture technique – capture &
compression – transportation –
China - Recent development in
converting coal to clean fuels –
advance domestic gasification
technology – energy efficient –
Fischer – tropsch synthesis combined
– moving bed gasification – high
USA - Transport Integrated
Gasification (TRIG) technology – low
rank coals can also used – power