COKE OVEN AND BY-PRODUCT PLANT (Pakistan Steel).
The
coke used in the Blast Furnace must have a high carbon content ( Fixed carbon )
and an appropriate porosity as well as
good strength to ensure that it gives good reactivity and does not pulverize to
clog the gas flow in the Blast Furnace even at high temperatures environment
that exists throughout the lower two-thirds of the Blast furnace.
The other components such as moisture, ash,
sulfur, phosphorus, and alkalies are undesirable as they have adverse effects
on energy requirements, blast furnace operation, hot metal quality, and
refractory lining.
Cokes that meet the above requirements are derived from
bituminous coals. Coal
available in Pakistan are not suitable to convert into metallurgical coke due
to low carbon, high sulphur & moisture content. Therefore prime coking coals with fairly low ash and sulphur
content are imported.
Pakistan Steel’s coke oven plant is comprised
of two Coke Oven Batteries, each having 49 ovens. First battery was commissioned on 18
August 1981 & second battery went into production on 6 May 1985.
A view of Coke oven battery during Capital Repair at Pakistan Steel.
As per reports, Pakistan
steel’s coke oven battery # 1 has undergone capital repair in 2008 and the
battery # 2 is under capital repair currently.
Refractory bricks Laying work for a new Coke Oven Battery.
Coke ovens are normally operated for long
periods like 25 - 30 years continuously.
Therefore
refractory bricks for lining a Coke Oven Battery should have accurate shapes and precise dimensions,
an excellent mechanical strength at high temperatures, hot modulus of rupture
and excellent volumetric stability to work at temperatures 1350 0C.
Silica
bricks have sufficiently high refractoriness under load and reliable volumetric
stability at high temperatures so an enormous volume of silica bricks of standard shapes are
used to construct the coking and combustion chambers of Coke oven battery.
The
roof, regenerator Checkers and Chimney flue of Coke Oven battery are
constructed with high quality fireclay bricks.
In developed countries like Japan a campaign
life of >40 years is achieved by employing high quality refractories and through control
of standard operating parameters for Coke oven batteries.
Coke Oven Plant is mainly comprised of
following production units.
1)
Coal
Handling & Processing Plant .
2)
Coke
Oven Batteries.
3)
Coke
Dry Quenching Plant.
4)
Coke
Wet Quenching Plant.
5)
Coke
Screening.
6)
By-Product
Plant.
Process of Carbonization.
A
blend of bituminous coals is charged
into ports (holes) on the top of the ovens and is converted
into metallurgical coke by heating it in the absence of air (oxygen deficient atmosphere) to prevent combustion
at high temperatures for a period of 16-18 hours (reaction time).
Refractory
bricked regenerative chambers recover the heat from flue gases and retain high temperatures
1100
– 1350℃. This
heat is utilized in coking chambers.
In
carbonization process, the lumpy coal decomposes to form plastic layers between
temperatures 370°C to 470°C. As the temperature rises further 470°C to 620°C, a marked evolution of
tar and aromatic hydro carbons takes place. Upon further progress of
temperature from 620°C to 1100°C, a structural
development of coals take place with evolution of hydrogen where they start softening and form
plastic mass that swells and resolidifies into a porous solid coke. At
temperatures 1050
- 1100°C, the entire mass is carbonized.
The
red-hot coke is pushed out of the oven by pusher car through a guide into coke
bucket which is transported with
an Electric Locomotive to the Coke Wet or Dry
Quenching Plant.
In
wet
quenching heat energy of hot coke cannot be utilized because water is directly
showered on hot coke & the steam is evaporated in the air. So Wet quenching
process serves only as a reserve quenching facility.
Mostly
dry
quenching process of coke is executed. The process uses
the counter flow of nitrogen gas in which energy of red hot coke is
utilized to produce steam for onward supply to Turbo Blower Station. In
addition dry quenching emerges as a highly reliable system to reduce air
pollution. The dry quenched coke is harder, stronger and much low moisture
content as compared to wet quenched. The coke is cooled and screened into
pieces ranging from one inch to four inches.
The
metallurgical coke
contains 91- 93 % carbon, some ash and sulfur but compared to raw coal is very
strong. The strong pieces of coke (with a high
energy value) are required to reduce and melt the iron ore and sinter in
Blast Furnace.
The
cooled coke from CDCP (Coke Dry Cooling Plant) is separated into 3 fractions,
BF Coke i.e. +25-70 mm which is sent to Blast Furnaces, Coke Breeze i.e. +0-15
mm which is sent to Sintering plant. The nut coke i.e., +15-25 mm is also used
in the Blast Furnace.
Related
video:
Coke Oven’s By-Products
Recovery Plant.
Throughout
the coke making process, organic compounds are recovered as gas, tar, oil, and
other liquid products for reuse or conversion into by-products for sale or
internal use.
The
volatile matter of coal liberated during carbonization is collected in gas
collecting mains
at the top of the batteries
in the form of raw coke oven gas passing through stand pipes and direct contact
cooling with ammonia liquor spray.
SCHEMATIC FLOW DIAGRAM OF BY- PRODUCT PLANT.
Crude Coke Oven Gas.
The main by product in the process of coke
making is crude coke oven gas which also has lot of valuable chemicals. The
first step in the treatment of raw coke oven gas is to cool it for removal of water
vapors which greatly reduces its volume. This is done in primary gas cooler.
Tar and ammonia liquor are produced as by
products in the process of coal carbonization from materials contained in the
coal feedstock and water added to the process.
Ammonium
sulfate.
The
removal of ammonia from coke oven gas is carried out by bubbling and contacting
the coke oven gas with a solution of 4% sulfuric
acid solution to form ammonium sulfate in a saturator.
NH3 +
H2SO4 ---------è NH4HSO4
NH4HSO4 +
NH3 ---------è (NH4)2SO4
The
ammonium sulphate produced by the reaction of ammonia with sulphuric acid is
recovered by crystallization. The crystals are then centrifuged, washed, dried
and sent to packing unit for sale.
Dehydrated tar.
The
compounds present in the coal tar are formed ultimately from complex organic
materials in the coal charge. The tars are an extremely complex mixture of
organic compounds which are condensed as medium to dark liquids or solids at
room temperature, mostly aromatic compounds.
The initial separation of tar and ammonia
liquor is performed in a flushing liquor decanter, a unit based on gravity separation where
three layers are formed, the upper layer is of ammonia water, second layer of
coal tar & last layer of tar sludge. The ammonia water is pumped back into
the ammonia tank where it is condensed and again send to batteries for cooling
of coke oven gas.
Coke
oven gas after cooling in the primary gas coolers, is sent for final
purification from the remaining tar traces into electrostatic precipitators. The exracted coal tar traces are send for
water separation and finally to the storage tank for sale.
Naphthalene.
To
obtain clean and high purity coke oven gas, the gas cleaning processes such as removal
of naphthalene & benzyl to be operated effectively which needs that the gas
must be cooled in “final Gas Coolers”.
Final coke
oven gas cooler plant.
Coke
oven gas is send to the scrubber for removal of naphthalene where solar oil is
showered from the top & gas is entered from the bottom. Content of naphthalene in the spent solar oil is 7-8%.
Solar oil is stored & marketed.
After
final treatment, purified Coke Oven Gas whose major constituents are H2 (55-60%), CH4 (23-27%), CO (5-8%), with
calorific value around 16720-18810 KJ / m³ (4.6 - 5 kWh/Nm³), becomes suitable for use at coke oven batteries,
thermal power plant, sintering plant, refractory & lime production unit
etc.
The
annual production capacity of Coke Oven & By-Product Plant is as under.
Coke:
960000 metric
tons/year.
Coke
oven gas: 300- 350 M3/ ton of coal.
Ammonium
sulfate: 172000 metric tons.
Dehydrated
tar: 46500 metric tons.
Energy productivity.
In
an integrated iron & steel plants like Pakistan Steel, Coal used in coke
ovens provides not only energy in the form of metallurgical coke for iron production in Blast furnace but also internally
generates high value fuel as coke oven gas (with a
calorific value of 4.6 - 5 kWh/m³N) and steam. Both are used for power generation, making such
integrated iron & steel plant self sufficient of electrical power. Only when the coke
oven battery of the mills goes off for capital repair, the thermal power plant
of steel mill is operated on natural gas.
Further
the molten pig iron
produced in Blast furnace (at the expense of metallurgical Coke) have physical
temperature 1400
-1450 0C which is dropped down to an average 1320 0C
due to transportation, handling and
pouring of B.F hot metal ladles into/ out of storage vessel (Mixer) at Steel Making
Department. This physical temperature of molten pig iron together with the chemical heat
generated in LD-Converter Steel making, formulate the converter steelmaking process
self-sufficient in energy.
Operational prudence needed at Coke Oven.
The
exhauster
is a large blower that provides the motive force to induce the coke oven
gas to flow from the coke oven battery and through the by-product plant. Timely
maintenance of exhauster is of prime importance to the operation of the coke
oven battery. It allows the close control of the gas pressure in the collecting
main, which in turn affects the degree of emissions from doors, lids and off-takes.
Exposure to emissions is dangerous to workers, may cause lungs and intestine
cancer and malignant tumors.
The
degree of escape or release of benzole, tar compounds and acidic gases (H2S,
HCN, CO2) to the surroundings are regularly monitored to prevent exposure of plant operators
to the toxic materials and prompt preventive measures be taken timely.
Need to avoid using coals of
deviated chemical compositions from required standards for production of
metallurgical coke or operating a coke oven at too high end temperatures than targeted range, cause
the quality of metallurgical coke production below standard, excessive emission
and also the refractories of Batteries are depleted faster from thermal shocks thereby shortening the Campaign life of Coke ovens batteries.
When
the plant gets older after years of service, the number of cracks and openings
increase in refractory bricks of chamber walls and doors owing to thermal
shocks and temperature fluctuation during operation. So periodic preemptive
wall repairs and sealing of door openings become indispensible with high quality
silica bricks. The flame gunning methods are also used in advanced countries to
patch up worn out portions of coking chambers to achieve optimum service life
of campaign.
In addition to above checks, industrially developed countries have
invested in process control automation system to optimize the emission problems
and energy conservation all around coke oven battery operation ( e.g coke
discharging system ) which may be deemed as part of revamping package of
Pakistan Steel.
