COAL DESCRIPTION:-

The input for the coke oven is coal. In technical terms, there are two types of coal available; one is cooking coal (i.e. which gives useful metallurgical coke on carbonization) and the other is non-cooking coal. There are several types of coking coal,depending on their ash content. In India the ash content is around 24 to 32%, whereas the imported coal is divided into four major types:

a)        Primary coking coal(PCC)

b)       Medium coking coal(MCC)

c)        Blendable coking coal(BCC)

d)       Primary coking coal(IC)

So, in an integrated steel plant, we have to use the above four types in definite proportions. In DSP the percentages of the various types are as follows:-

·        PCC- 30%

·        MCC-33%

·        IC-30%

·        BCC-7%

This   proportion gives the maximum productivity and losses in washing,filtering, etc, are minimized. Also the ash content is such that the overall effect is very less in the carbonization process. There is another difference in grades of coal i.e. lump coal and washed coal. Lumped coal  is one that can be directly charged into coke ovens after giving the exact size where as washed coal has to be washed  before being charged into the coke oven.

COAL HANDLING PLANT:-

          Raw material from different mines, mostly from Jharia, Barakar, and Bishergarh is received in open box wagon and mechanically unloaded by machines called tipplers. After being unloaded all types of coal are sent to Bradford Breaker for primary crushing. Jharia coal is diverted to coal washery (which is exclusive for DSP) and the other two varieties are sent to the stock files by a system of conveyers.

          There are 14 coal beds made of concrete floor in all. Stacking is done by neat of wing tippler. In each pile different types of coal having different ash content are stacked in different layer so that while taking a coal out from the pile for use in the coke oven, these type gets mixed up and a composite mixture called Reclaimer. This blended coal is then further pulverized in hammer mills to (-1/8) size and stored in bunkers.

To cater the need(both qualitative and quantitative)for the existing coke oven batteries through the adoptive strategies    like grade wise stacking, selective coal crushing and bunker blending. Reclaimed coal from different beds is stored in blending bunkers marked as prime, medium and blendable.  Coal is drawn from blending bunkers through automatic proportioning device at predetermined rates and is sent to  selective crushing unit through first stage crusher.

                     In selective ‘crushing unit’, and fine fractions of coal is separated by pneumatic classifier. The finer fraction is sent to coal tower, the coarser fraction is crushed again in secondary mills. The output of this mill is fed again fed to pneumatic classifier in order to arrest infiltration of course fraction going into the coal tower. The final coal from the pneumatic classifier (i.e. -3mm) only is fed to the coal tower. This coal is sent through conveyers to the coal tower, the capacity of each tower being 4000T.During this crushing  a little amount of moisture (6-7%) is ensured to minimize the loss and it also helps in pollution control. This coal is now ready for carbonization.

COAL WASHERY:-

DSP is the only steel plant, which has a coal washery inside the plant. The coal from Jharia has to be washed to reduce the ash content, after primary crushing to (-3”) size. The coal is separated into two sizes, namely (+1/2”) to     (-3”) and (-1/2”); each variety is treated separately in Heavy media section and Jig section respectively.

       Heavy media section

        Coal of size (+1/2”) to (-3”) is fed to primary  DREWBOY unit. The specific  gravity of the bath is maintained at 1.41 to 1.42 with the help of magnetic slurry(300 mesh). The float of the primary unit  is the clean coal and is sent to the stock piles. The sink is further treated in secondary DREWBOY unit, specific  gravity being 1.51.The float of the secondary unit is called middlings and the sink is called rejects. Middlings is being used by the captive power plant while rejects are sold to outside parties.

Jig section :

Coal of size (-1/2”) is charged in jig box along with water at a rate of 1000GPM. By the action of air pulsation in each of the fines compartments of the jig box, a batch of coal is separated depending in the heaviness. Coal from first two and a half compartments is comparatively lighter and is collected as middlings. These are sent to their respective bunkers. Clean coal which is lighter compared  to the other two is carried over by water and subsequently separated in three stages to the stock piles by a common washed coal belt conveyer.

NEW FACILITIES AFTER MORDERNISATION

·        A new wagon tippler of 720 T/hr capacity.

·        Four new coal beds 17000T each.

·        Two new stackers 720 T/hr capacity each.

·        Three bridge type bucket wheel, which reclaims 600 T/hr each.

·        Two primary hammer mills, 400 T/hr capacity each.

·        Two secondary mills, 300 T/hr capacities each.

·        Four pneumatic classifiers, 400 T/hr each.

·        Additional Reclaiming conveyer series.

·        14 blending bunkers, 1250 T capacities each.

·        Additional conveyer for feeding coal to coal tower, 600 T/hr each.

·        24 existing conveyers are to be modified to improve their capacity & performance.

COKE OVEN BATTERY:-

There are four batteries, each battery consisting of two independent and separate blocks of 39 ovens. The ovens are of the under jet, compound, twin flue type, using BF gas as normal fuel and with facilities for burning CO gas when available. Each chamber is of rectangular shape and lined inside with refractories. Coal is charged inside the ovens and is heated in absence of air to produce coke. All the volatile matter is removed from coal as raw or flue gas and the mass that is left behind is coke. When coke formation is complete, it is pushed out and quenched with water.

There are four types of special cars, which assists the coke making are as followed:

CHARGING CAR:

          This car collect the coal from the coal tower and charges it into the individual oven through charge holes, which are open by the charging car itself.

PUSHER CAR:

          This car does the job of pushing the coke out of the oven. It also opens and closes the ovens’ door, which provide for the escape of volatile gases.

GUIDE CAR:

          This car guide the coke during pushing operation and it also helps in opening and closing of the oven door.

QUENCHING CAR:

          This car receives the hot cake for quenching.

          Other than the coke oven chamber, there are heating chamber that provide the necessary heat for carbonization of coal. Each coke chamber is flanked on either side by heating chamber. Thus for a battery of 79 ovens there are 80 heating chamber. In these heating chamber, BF gas or CO gas is sent under pressure and air is supplied under natural suction pressure (through regenerator). The gas is sent in one direction in the flues numbered one, three, five, seven etc. to two, four, six, eight etc. for 30 minutes, the cycle reverses after the lapse of 30 minutes to the opposite direction i.e. from 2,4,6,8 etc. to 1,3,5,7 etc. Thus the heat supplied to the coal inside the coke chamber via the wall through conduction. The heating temperature is around 1220-1250⁰C. The coke temperature is 1050⁰C.

SPECIFICATION:

          Input coal to oven = 17.5 T/Day

          Heat = 600 Mcal/T of dry coal

          Output crushed coke = 77-78% of dry coal

          Output BF coke = 57-58% of dry coal

          CO gas = 290 Nm³/T of dry coal

          With calorific value = 4200 Kcal/m³

OVEN DIMENSIONS:

Height = 4.45 m

Length = 13.95 m

Width = 420-480 mm

Effective Volume = 23.8 m³

COKE OVENS AFTER MODERNIZATION:

Battery Features

No. of coke oven = 4 battery of 78 ovens each

Coal tower capacity = 4000 T

Total oven available = 273

Total pushing capability = 307 ovens/day

Coal through put = 0.6688 MT/Year

Coke Output = 0.4250 MT/Year

Gas Make = 22,700 m³/Year

MAIN EMPHASIS:

1.     Mechanization of operation in the following areas:

^·         Hot and hard job like oven charging coke wharf operation, cleaning operation in even top and battery platform.

^·         Lubrication of machines and equipments motorized with “auto” mode operation.

^·         Operation of large sized gas valve (>26” Dia) motorized.

2.     Pollution control measures introduced as follow:

^·         To achieve smokeless serving of oven a system of “ON-MAIN CHARGING OF OVEN” with the help of high pressure Ammonia Liquor at 30kg/cm² introduced.

^·         Water filled A.P. lids provided on oven top to minimize gas leakages.

^·         Chimney height increased to 110 m to reduce air borne pollutant (mainly CO).

^·         Arrangement for gas transfer during charging to adjacent oven to reduce pressure inside the oven being charged.

^·         Fly-ash arrester in coke quenching tower provided.

^·         Dust extractor and dust suppression system provided in coke plant.

3.     Thermal efficiency of battery increased by providing:

·        Insulation layer at different point to reduce lot by radiation.

·        Low circuit resistance in regenerator.

·        Electro-hydraulic reversing hinged machine with “PAUSE HEATING”.

4.     Equipment safety and electrical interlocks provided in all critical areas.

5.     Better control of operation by VLC control.

COKE HANDLING PLANT:

          The quenched coke is supplied to the wharfs. These are collected into a belt conveyer system and are sent for screening. The size of the coke must be supplied to the Blast Furnace is the major aspect of the coke handling plant. Coke plays a major role in the BF, as

·        It provide the necessary fuel for the preparation of the hot metal

·        It act reducing agent, i.e. help in reduction of iron oxide to iron.

·        It help in paving way for the blast gases to rise upword due to it porosity.

The size specified for the BF is +20mm to -80mm. The under sized coke is under different name, like peace coke, nut coke and coke breeze.

PROCESS:

          The coke is first made to pass through primary screen i.e. positive type 20mm, the output is that whose size is greater than 20mm, are sent to the next screen called 80mm screen, while the under sized coke is used is Nut coke. From the 2^nd i.e. 80mm screen the coke whose size is between 20mm & 80mm is sent to secondary screen (10mm), while the coke whose size is greater than 80mm is sent to coke cutter where the cutter cut the coke, this coke is again sent to the secondary screen where the coke size (between 20mm & 80mm) is collected as Pearl Coke and those less than 10mm are collected as Breeze Coke.

COAL CHEMICAL:

INTRODUCTION:

          The gas evolved during the carbonization of coal content various chemical. For the economy of the carbonization process and also for using this gas as a fuel, the recovery of these chemicals from the gas is essential. The coal chemical plant recover these chemicals from the coke oven gas and allows the clean gas to be used in coke oven and other plant as fuel.

          The hot gas coming out of the oven through astension pipe is cooled in the goose neck by direct contact with the spray of dilute Ammonia liquor called a flushing liquor to a temperature about 80⁰C. Most of the tar is condensed from the gas and is collected in the hydraulic mains. The hydraulic mains conduct the gas to primary condenser where the gas is cooled to 30-35⁰C and through a downcomer at first and the tar and liquor discharges into a tar catch tank and the other separated liquor is again used as flushing liquor.

          The exhauster cool the gas from the oven through the primary condenser and push it through electrostatic detarrers into the other unit of the plant. In detarrers the gas is subjected to the action of a high voltage electric field, the gas is ionized and the charged particle collect on the surface of the grounded positive electrode. Detarrers remove 95-99% of tar mist from the gas passed through it. From detarrers the gas is delivered to saturators, where ammonia present in the gas is absorbed by dilute sulfuric acid. Ammonium sulfate crystals produced are ejected into centrifuge to remove adhering liquids and then dried in a rotary drier.

          The gas leaving the saturator is cooled in the secondary cooler cum primary naphthalene washer. Here a portion of the naphthalene is removed from the gas by wash oil in secondary naphthalene washer. Then the gas is cooled in final cooler to 20⁰C and then passed through Benzol washers and final naphthalene washers for removal of Benzol and remaining naphthalene from the gas. The clean coke oven gas is used as a fuel.

AMMONIUM SULPHATE PLANT

The plant is subdivided into three sections:

a)     Gas absorption

b)    Intermediate storage liquor tank

c)     Salt production section

          Gas absorption section handles 33,000N /hr of CO gas each from 1M stage & 1.6 M stage. Product liquor from both the stages is stored in buffer tank and in large evaporator feed tank. Production capacity of salt section is 98 tons/day.

BASIC REACTION:

 +      →  (  + ∆H;                  ∆H=-65440 Kcal/kmol

   RAW MATERIALS:

a)     Coke oven gas to absorber ( flow rate=33000 /hr)

b)     concentration = 6-8 g per N/  of CO gas

c)     Inlet pressure 900 to 1500 mm water gauge.

d)    Inlet temperature=40-45 degree Celsius.

e)     Composition of CO gas:

     =58.2%              =20%              = 4.2%

     =0.4%            =4.2%              Unsatd. HC= 2%

UTILITES:

a)     Sulphuric acid =98% concentration (w/w)

b)    IP steam (superheated): P=5 kg/ ,T=25  C

c)     LP steam (superheated): P=1.5 to 2.75 kg/ ,T= C

d)     Cooling  water: P=2 to 2.5 kg/ , T= C 

e)    Compressed air : P=2 to 3 kg/ , T = ambient

EQUIPMENT OF SALT HANDLING

·        Belt conveyer

·        Bucket elevator

·        Centrifuge

·        Rotary drum drier (dia=1600mm,length=10000mm)

·        Drier exhaust fan:

Flow rate =11000 N /hr, 15 Kw motor, 1450 rpm, fluid is air with entrained dust at 80 degree Celsius with suction pressure  of -225mm water(g)

·        Spray nozzle

a)     Fluid: (  solution with free sulfuric acid.

b)    Type : non-clogging full cone without atomization

c)     Temperature=55 to 60  degree Celsius; pressure=1.5 bar

d)    Specific gravity= 1.24; spray angle=60 degree

e)     Average droplet size=500-600µ

f)      Gas velocity = 2 to 2.3 m/s

The ammonium sulphate crystals obtained are white , free flowing with acidity content of 0.04 and a concentration of 20.6% of nitrogen.

          BENZOL PLANT:

After the recovery of ammonia from the CO gas, naptha and crude benzol are removed from the gas in scrubbers by direct contact with wash oil. The ratio of oil : gas is kept low in naphthalene scrubber as compared to benzol scrubbers. Napthalene enriched oil from naphthalene scrubbers is sent to naphthalene stripping unit and some crude benzol, which was absorbed along with naphthalene.

          In crude form benzol is a mobile liquid, which distills almost completely below 200⁰C. The wash oil, which has absorbed benzol from the gas, is treated in benzol plant. Benzol is separated from the oil by steam stripping and the benzol-free wash oil is sent back for circulation. The separated benzol is called crude benzol that is then fractioned (refined) in the rectification unit to get pure products like benzene, toluene, xylene and heavy and light solvent naptha. These are stored in respective storage tanks.

          TAR FRACTIONATOR AND MNCP:

          The crude tar that is separated earlier is further distilled to get the following primary products viz. light oil, naphthalene oil, creosote oil, residue oil and pitch. Napthalene oil is thus separated from tar, and is treated in Mechanized Napthalene Crystallization Plant (MNCP); it is allowed to cool and settle down in big traps in naphthalene plant, where naphthalene crystallizes out. The remaining oil is drained and stored as Drained naphthalene oil. The naphthalene flakes are pressed into cakes in a hydraulic press. The pressed naphthalene is then crushed and bagged except the initial filling of pumps.