1. Coal gasification
Coal gasification is the process of producing coal
gas, a type of syngas (mixture of carbon
monoxide (CO) and hydrogen (H2) gas) from coal.
3C (i.e., coal) + O2 + H2O → H2 + 3CO
If, however, hydrogen is the desired end-product,
the coal gas (primarily the CO product) undergoes
the water gas shift reaction:
CO + H2O → CO2 + H2
2. Coal gasification
The chemistry of gasification is quite complex and is really accomplished through a
number of chemical reactions within the gasifier. Some of the major chemical
reactions are shown in the diagram.
3. Coal Gasification
Coal gasification offers one of the most versatile and
clean ways to convert coal into electricity, hydrogen, and
other valuable energy products.
Coal gasification electric power plants are now operating
commercially in the World
Experts predict that coal gasification will be at the heart
of future generations of clean coal technology plants.
4. Coal Gasification
Rather than burning coal directly, gasification (a thermo-
chemical process) breaks down coal - or virtually any
carbon-based feedstock - into combustible gaseous fuel.
In a modern gasifier, coal is typically exposed to steam
and carefully controlled amounts of air or oxygen under
high temperatures and pressures.
Under these conditions, molecules in coal break apart,
initiating chemical reactions that typically produce a
mixture of carbon monoxide, hydrogen and other
gaseous compounds.
5. Coal Gasification
The composition of the gas obtained from a gasifier
depends on a number of parameters such as:
1. Fuel composition
2. Gasifying medium
3. Operating pressure
4. Temperature
5. Moisture content of the fuels
6. Mode of bringing the reactants into contact inside
the gasifier, etc.
6.
7. Gasifier Types
Gasification processes are classified on the basis
of the method used to bring the coal into contact
with the gasifying medium (air or oxygen).
The three principal commercial modes are
1. fixed-bed,
2. fluidized-bed, and
3. entrained-flow
9. Temperature Profile of Gasifiers
FLUIDIZED BED GASIFIER
(800 – 10500C, 10 to 25 bar)
MOVING BED GASIFIER
(400-1100 0 C, 10 to 100 bar)
ENTRAINED FLOW GASIFIER
(1200-16000C, 25 to 80 bar)
11. Fixed-Bed Gasifier
In a fixed-bed gasifier, 1/4- to 2-in. coal is
supplied countercurrent to the gasifying medium.
Coal moves slowly down (sometimes this type of
gasifier is called a moving-bed gasifier).
Reaction zones typically consist of drying and
devolatilization, reduction, combustion, and ash
zones.
In the drying and devolatilization zone, located
at the top of the gasifier, the entering coal is
heated and dried and devolatilization occurs.
In the reduction / gasification zone, the
devolatilized coal is gasified by reactions with
steam and carbon dioxide.
12. Fixed-Bed Gasifier
Heat exchanged with the entering
gasifying medium and fuel
As a result both the ash and the
product gas leave at modest
temperature
Fixed-bed gasifiers operating on
low-rank coals have exit
temperatures lower than 800 ◦F.
Low oxidant requirements;
Design modifications required for
handling caking coal;
Limited ability to handle fines.
13. Fluidized-Bed Gasifier
In this gasifier, coal with 1/8 to ¼ in. in
size enters the side of the reactor and
is kept suspended by the gasifying
medium.
Similar to a fluidized-bed combustor,
mixing and heat transfer are rapid,
resulting in uniform composition and
temperature throughout the bed.
The temperature is sustained below
the ash fusion temperature to avoid
clinker formation
14. Fluidized-Bed Gasifier
Char particles entrained in the product gas are recovered
and recycled back into the gasifier via a cyclone.
Acceptance of a wide range of solid feedstock
(including solid waste, wood, and high ash content coals);
Uniform temperature;
Moderate oxygen and steam requirements;
• Extensive char recycling
15. Entrained-Flow Gasifier
Entrained-flow systems gasify pulverized fuel particles suspended in a
stream of oxygen (or air) and steam.
Residence time in this type of gasifier is very short.
Entrained-flow gasifiers generally use oxygen as the oxidant and
operate at high temperatures.
Temperature is well above ash-slagging conditions, to ensure high
carbon conversion.
Ash in the coal melts at the high operating temperature of the gasifier
and is removed as liquid slag
The product gas and slag exit close to the reaction temperature
16. Entrained-Flow Gasifier
. Entrained-flow gasifiers have the following characteristics :
Ability to gasify all coals regardless of coal rank, caking
characteristics, or amount of coal fines,
Feedstocks with lower ash contents are favored
Uniform temperatures
Very short fuel residence times in the gasifier;
Very finely sized and homogenous solid fuel required;
Relatively large oxidant requirements;
High-temperature slagging operation
Entrainment of some molten slag in the raw gas.
17. Entrained-Flow Gasifier
Their use for biomass gasification is rather limited, as it
requires the fuel particles to be very fine (in the order of
80 to 100 µm).
A number of manufacturers offer commercial entrained-
bed gasifiers for large-scale applications, such as
Texaco, Shell, and Koppers–Totzek.
19. Pipeline gas from coal via Lurgi pressure gasification.
COAL
PREPARATION
OXYGEN-BLOWN
COAL
GASIFIERS
SCRUBBING
AND COOLING
CRUDE GAS
SHIFT CONVERSION
H2O+CO=H2+CO2
RECTISOL
GAS
PURIFICATION
METHANATION
3H2+CO=CH4+H2O
COMPRESSION
AND
DEHYDRATION
ELEMENTAL
SULFUR * 128 LTPD
CO2
H2S+CO2
GAS LIQUOR
25,303 TPD @ 8,920 Btu/lb
FEED
COAL
GAS LIQUOR SEPARATION
AND EFFLUENT
WATER TREATMENT
PIPELINE GAS
290 MMSCFD
@ 950 Btu/Scf
RECLAIMED WATER
TO PLANT USE
NAPHTHA
COAL TAR
COAL OIL
PHENOLS
AMMONIA
TOTAL LIQUIDS
8940 BBLS/day
155 tons/day
OILY
GAS
LIQUOR
SULFUR
PLANT
TARRY GAS LIQUOR
N2
OXYGEN
PLANT
AIR
COAL FINES
STEAM
OXYGEN
STEAM AND
POWER PLANT
ELECTRIC POWER
TO PLANT USE
20. System for In-Situ Gasification of
Coal
From Wiser, 2000
Inject air
and steam
Produce
low-Btu
gas
Chemical reactions
21.
22.
23. UCG – ADVANTAGES
Energy obtainment from vast coal reserves which are not
commercially viable to mine
Increase of worker safety as no mining operations involved
Low environmental impact
No atmospheric pollution
No appreciable change in landscape
Less subsidence than conventional mining
Less Resettlement and Rehabilitation (R&R) issues
No surface disposal of ash and coal tailings
Potential GHG (greenhouse gas) reduction activity
(CO2 storage in cavities)
24. Selection of a Gasifier
Type of bed
Operating pressure
Slagging, non-slagging
Lurgi (fixed bed, high P, non-slagging)
Koppers-Totzek (entrained bed, low P,
slagging)
Wellman-Galusha (fixed, low P, non-slagging)
Winkler (fluidized, low P, non-slagging)
