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.
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.
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.
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.
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.
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.
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.