PRESENTATION ON PREPARATION OF COKE By:- SUKESH KUMAR B.TECH 4TH YEAR METALLURGICAL & MATERIALS ENGG.

PREAPARATION OF COKE IN COKE OVENS BATTERIES

• The main function of Coke Ovens is to convert coal into coke which is used as a fuel and reducing agent in the Blast Furnace. Its secondary function is to recover Volatile Matter and CO gas from coal and extract chemicals known as Coal Chemicals. CO gas produced is used for heating purposes in the plant.

Contents: 1.Introduction to coal. 2.Analysis of coal. 3.Sampling of coal. 4.Construction of coke ovens. 5.Analysis of coke oven gas. 6.Charging of coal in the empty coke ovens. 7.Conversion of coal into coke. 8.By-product of coal. 7.Discharging of coke from coke ovens and cooling of coke. 8.Testing &analysis of coke. 9.Coke sorting.

INTRODUCTION TO COAL Coal is a combustile black or brownish –black sedimentary rock composed mostly of carbon and hydrogen.

Coal contents: 1. Organic mass--complex mixture of organic compounds of carbon(C), hydrogen(H2), nitrogen(N2), oxygen(O2) & sulphur(S). 2.Inorganic substance--like water & mineral matter. A coal having higher volatile matter content has lower ignition tempr. Mineral matter (M) content & Ash(A) content in coal are approximately related as M=1.1A

Life of formation of Coal: Wood--->Peat--->Lignite--->Bituminious---> Anthracite--->Graphite

Rank of coal: 1.Peat:--Peat is light brown in colour containing 90% moisture, 30% O2 & having calorific value of 4500kcal/kg.

2.Lignite:--Lignite is deep brown in colour containing 60%moisture ,20% O2.

3.Bituminious:--Bituminious is black in colour containing less than 10% moisture ,5% O2 having calorific value of 7000kcal/kg,& flame is yellow.

4.Anthracite:--Anthracite is also black in colour containing2% O2 having calorific value of 8000-8500kcal/kg,&flame is

bliush yellow .

Decrease in oxygen and volatile matter content result in increase in carbon content

i.e 30% for wood to almost 100% for anthracite.

Caking index of coal is a measure of its agglutinating(binding) property Highly caking coals produces strong coke.Caking index of coal blend used for blast furnace coke making is about 22.

ANALYSIS OF COAL

1.Proximate analysis :--Proximate analysis of coal is estimation of the moisture content ,ash content ,volatile matter content & free carbon content .free carbon=100-(%v.m+%ash+moisture).

2.Ultimate analysis:-Ultimate analysis of coal is estimation of fixed carbon, sulphur,oxygen ,nitrogen ,hydrogen.

SAMPLING OF COAL Washing of coal decreases sulphur & Ash content and increases the calorific value & caking property of coal which controls its Ash fusibility.Coal comes from Bhojudih, Sudamdih, Nandan, Rajarappa, Swang and imported coking coal is recieved from Australia & New Zealand. Imported coking coal is prime coking coal, having less than 10% ash. Blend is approximate PCC 10%, MCC 15% and imported 75% for Batteries. Range of analysis of Indian Coals : • Types of coal Ash V.M.

1)Prime Coking Coal (PCC) 20-23% 22-24 % 2)Medium Coking Coal (MCC) 20-23% 25-30 %

3)Important coal 8-10% 20-23%

Reversible Hammer crusher are provided to crush the coal blend to a fineness of 80 % below 3.2mm size. A water spray is provided after the Mixing Bins and at fixed points along the conveyors to ensure correct amount of moisture in coal.

Typical Blend Composition and Coal Charge Analysis : Blend Composition Coal Blend Analysis PCC 10% Moisture 7.0 % - 8.0% MCC 15% Ash 12.0 % - 13.0% IMPORTED 75% VM 24.0 % - 25.0% Fixed Carbon 58.0 % - 60.0%

CONTRUCTION OF COKE OVENS

There are first eight coke oven batteries 4.3 M tall having 65 ovens in each battery ( in 2.5 MT stream) and twobatteries 7 M tall having 67 ovens in each (4.0 MT stream). 4 batteries are capable of producing 1.33 MT of BF Coke per annum at an average coking period of 17 to 18 hours. Battery 9 & 10 can produce 1.23 MT of BF coke per annum at average coking period of 17 to 18 hour.The temperature of control verticals flues on pusher side is 1175°C to 1225°C and 1200°C to 1250°C on coke side. These however can be increased or decreased depending upon the coking time.The ovens are maintained under positive pressure by maintaining high hydraulic main pressure of 7 mm water column in Batteries 1 to 8 and 14 mm water column in Battery 9 &10.Coke ovens are mainly constructed of silica, fireclay, high alumina and ordinary insulating bricks. The coke oven chimney is made of concrete lined with fireclay bricks. Normal life of a coke oven battery under Indian condition is considered to be 25 years. The heating of ovens is done from both the sides in side fired batteries.

• The heat consumption per Kg. of dry coal is around 650 K.Cal. Battery-9&10 uses mixed gas heating.

Important data of Batteries :

Description Battery (1-8) Battery (9&10)

Total length of oven 14.080 m 16 .000 mTotal height of oven 4.300 m 7.000 m

Useful height 4.0 m 6.7 m No. of ovens 65 67

Width on Pusher side 382 mm 385 Mm Width on Coke side 432 mm 485 Mm

Average Width 407 mm 410 Mm Useful volume 21.6 m3 41.6 m3

Heating level (Distance from the top of cross over window to the roof of oven)

800 mm 1.1 m

Coal Charge per oven 16.4 Tonne 30.0 Tonne Chimney height 100 m 120 m

Width of the coke oven towards coke side is slightly more than that on pusher side to facilitate easy discharging of coke, as it swells during coking.narrow coke ovens compared to wider coke ovens produces smaller coke ,strong coke & require less coking time.height of coke oven is limited maxm up to 7m mainly by the problem of uniform heating along its height.

The elaborate regenerator system (of refractory bricks) ensures good preheating of mixed gas as well as heat recovery from flue gases. The heating arrangement consists of 24 port holes in battery 1 to 8 and 32 port holes in battery 9 & 10 along the length in the heating wall. Heating control & sequence is quite critical for battery performance, and life of battery itself &ensuring uniform quality of coking & least mechanical (refractory) damage to the structure due to thermal stresses.The productivity of battery is improved by increasing bulk density of coal charge - this is done by means of oil water emulsion addition to coal charge, partial briquetting of coal etc. The best case of productivity improvement was achieved by stamp.

Laser Guided Alignment :This is a new development in battery where innovative methods using Toy lasers to position the pusher car and door extractor in a straight line to oven proper has been installed and found to be successful. This ensures good & accurate aligment so that doors are not damaged while opening, closing and that oven walls are also not damaged during pushing

Methane - 26.0 %

Hydrogen - 56.5 %

Hydrocarbons - 2.3 %

Carbon – monoxide - 8.5 %

Carbon dioxide - 3.0 %

Oxygen - 0.4 %

Nitrogen - 3.3 %

Density - 0.4848 kg/m3

Calorific value - 4300 Kcal/m3

ANALYSIS OF COKE OVENS GAS

Largest constituent of coke ovan gas is hydrogen removal of hydrogen from coke ovan gas increases its calorific value

CHARGING OF COAL IN THE COKE OVENSThe coal is taken in charging car from the coal tower and is charged into the empty oven. Each oven has three charging holes and each charging car has three bunkers for charging of coal into the ovens.With an increasing thrust towards improving productivity and coke quality, especially with respect to strength (M 10 index), a partial briquetting for coal charge has been installed in coke ovens.The basic process involves briquetting a part of coal charge (28% by weight) with coal tar (soft pitch) and charging the briquettes and fine coal in a pre-determined ratio (25 : 75).The rationale for using briquettes in charge is to increase the bulk density of coal charged into the oven and to improve the micum index and crushing strength of cokeImportance of Pressure in Ovens :The control of oven pressure is quite important because pressure on lower side leads to air entry into system (dangerous) while on higher side leads to excessive gassing, leakage of doors, stand pipe etc. The oven pressure is maintained at an optimum balance between ovens, in the beginning, end of coking cycle, as they are all connected to a common exhauster.Importance of Levelling :After charging coal from charging car bunkers, battery is levelled to allow enough "Free board" space above (= 300 mm) so that gas exit is free and no choking occurs. Levelling also helps in good spreading and packing of coal within oven to ensure optimum utilization of wall heat transfer area.

Preheating of coal charge for the coke ovens minimize the- coking time-yield of gases-tar-fuel comsumption in coking.Fuel comsumption in coke making can be minimize by -preheating-blending Sometimes water is sprayed during coal charging in the coke oven which helps in -prevention of clinker formation in the oven-controlling the dust nuisance ,while charging the coal-reducing the cracking of hydrocarbons in the coke oven gas

CONVERSION OF COAL INTO COKECoke oven gas burns with yellowish flame because of the presence of CH 4.

Actual flame tempr of fuel depends upon-calorific value of the fuel -volume &-specific heats of the flue gas

-extent of endothermic dissociation of CO2 & H2O present in the flue gases.

Burning of coal in absence of air is called carbonisation of coal.-Low tempr carbonisation-High tempr carbonisationLow tempr carbonisation takes place at 700 0C.Low tempr carbonisation of coal is mainly for the production of smokeless domestic coke having calorific value of about 6500kcal/Nm3 .Yield of coke ovan gas in low carbonisation of coal is about 160Nm3/ton dry coal charged.Increase in carbonisation temperature gas yield increases.High tempr carbonisation of coal takes place at 1100 0C . During the high temperature carbonisation of coal-all Tar is evolved at <600 0C

-evolution of H2

- formation of CH4 & aromatics occurs at >7000C- heat penetration rate in narrow coke ovan in high tempr carbonisation is about 2.5cms/hr.High rate of heating of coke ovans may damage its wall ,due to abrupt excessive swelling of coke.with increase in the time of carbonisation of coal calorific value of coke ovan gas decreases due to the cracking of hydrocarbon.• HPLA :This is a system used in batteries to create suction during coal charging after the batteries are isolated from hydraulic mains. HPLA stands for high pressure liquor aspiration where ammonical liquor at high pressure (28 Atm.) is sprayed to decrease the gas tempr from about 400 to 80degrC through an orifice in the gooseneck leading to hydraulic main and the gas in the oven is sucked into hydraulic main thro' standpipe due to suction created by liquor pressure. As a result of this suction arrangement, coal dust emission during charging and consequent gassing is greatly minimised.Coke is made by heating coal in the absence of air (carbonization / destructive distillation of coal) in a series of ovens called batteries.The coal to be coked is charged through the charging holes by charging cars ( which in turn receives coal from coal tower above it). After this the coal is carbonized for 17-18 hrs, during which period the volatile matter of coal distills out as "Coke Oven" gas and is sent to CCD for recovery of chemicals. When the Central temp. of the oven is 950-1000 0C., coking is said to be complete.

BY PRODUCT OF COAL *Tar -pitch -anthracene -naphthalene -phenol -Creosote *Ammonia:ammonia sulphate *Benzol -benzene -Toulene -xylene -solvent naphthalene

One tonne of dry coal yields 300-310cubic.m of gas 330kg tar & .2% ammonia or 1.0-1.04% of ammonia sulphate.**Ammonia sulphate contain–Nitrogen content-20.6%Free acidity- 0.05%Moisture- <1% By product recovery direct & indirect depending on the method of recovery ofNH3.

DISCHARGING OF COKE FROM COKE OVENS & COOLING OF COKE At this point, the oven is isolated from hydraulic mains and after proper venting of residual gases, the doors are opened for coke pushing. The coke mass at the end of coking period has a volume shrinkage which leads to detachment of mass from the walls ensuring easy pushing.The coke after being pushed into quenching car, is quenched in quenching tower where phenolic water is pumped @50 m3/hr (quench time = 2.5 Min.). The quenched mass is unloaded in wharf, from where it is conveyed to coke sorting plant, before being conveyed to Blast furnace.

Description Battery (1-8) Battery (9&10)

Moisture 3.5% - 4.5% 4.5 % - 6.0%Ash 15.5% - 17.0% 15.5 % -17.0%V.M. <1.00% <1.00 %Sulphur 0.65% max. 0.65 % max.Fixed Carbon

Strength79.0% - 81.0% 79.0 % - 81.0%

a) M 40 >80% >80 %b) M 10 7.5% - 8.5% 7.5 % - 8.5%

TESTING AND ANALYSIS OF COKESize required by blast furnace is 25-80mmIMPORTANT QUALITY PARAMETERS OF COKE : * Size distributtion * Moisture & VM * M-10, M-40. (Strength)

Metallurgical coke should have low fusion point of its Ash & low alumina and silica content because fusion point of coke Ash increases with increase in its alumina(Al2O3) & silica(SiO2) content.

Higher Ash in coke increases the specific comsumption of coke in the blast furnance . a coal with high ash content is harder & stronger.Caking index of the coal blend used for making metallurgical coke is about 21.Swelling index is the measure of its sweeling characteristic.Grayking assay index is measure of Caking & Roga index is measure of its Swelling properties . abrasion index of the coke is a measure of its hardness.Abrasion index of B/F coke should be around 80%.

Micum-40(M40) & shatter index of coke is the measure of its strength & Micum-10(M10) of coke is the measure of its hardness and M40>78 & M10<10 should be for a good quality of coke.

critical air blast(CAB) value of coke is a direct measure of its reactivity.

Mott & wheeler test is conducted on coke to find its reactivity with O2.

Coke having higher porosity has -lower bulk density-lower strength-higher reactivity

COKE SORTINGThen coke is sieved at 80 mm, grizzly and over size crushed in toothed roll crushed. The coke is then sieved on 25 mm, grizzly screen. The size 25-80 mm is conveyed to blast furnace through belt conveyors .The under size which still contains 3-4% metallurgical coke is sieved over a 25 mm vibrating screen. Over size ismixed with metallurgical coke and 0-25 mm coke is sent to 13 mm vibrating screen, where it is sorted in to 0-13 mm size called breeze is sent to sintering plant for using in sinter production And 13-25 mm size called nut coke is sent to sintering plant through conveyor belt and dump car for adding it to sinter being conveyed to blast furnace from sintering.

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