A REPORT

ON

ANALYSIS OF FACTORS

AFFECTING THE CALORIFIC VALUE OF

COKE OVEN GAS

BY

Arpit Kishore 2009B1A1799P

Sneha Choudhury 2009A1PS495G

Yachit Mahajan 2009A1PS195G

AT

TATA STEEL, JAMSHEDPUR

A Practice School-I Station of

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

JULY, 2011

2

A REPORT

ON

ANALYSIS OF FACTORS

AFFECTING THE CALORIFIC VALUE OF

COKE OVEN GAS

BY

Arpit Kishore 2009B1A1799P B.E.(Hons) Chemical

Sneha Choudhury 2009A1PS495G B.E.(Hons) Chemical

Yachit Mahajan 2009A1PS195G B.E.(Hons) Chemical

Prepared in partial fulfilment of the

Practice School-I Course No.

BITS C221

AT

TATA STEEL, JAMSHEDPUR

A Practice School-I Station of

BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI

JULY, 2011

3

BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE PILANI (RAJASTHAN)

Practice School Division

Station: Tata Steel Centre: Jamshedpur

Duration: 8 weeks Date of Start: 23rd May, 2011

Date of Submission: 7th July, 2011

Title of the Project: Analysis of factors affecting the calorific value of coke oven gas

STUDENTS

2009B1A1799P Arpit Kishore B.E.(Hons) Chemical

2009A1PS495G Sneha Choudhury B.E.(Hons) Chemical

2009A1PS195G Yachit Mahajan B.E.(Hons) Chemical

EXPERTS

A.N. Rai

Head By Product Plant

Coke Plant

PS FACULTY

Dr. Tarun Kumar Jha

Key Words: Calorific Value (CV), Coke Oven gas, Coal Blend, ORSAT Analysis, Coking Time

Project Areas: Chemical engineering, Utilization of Coal carbonisation by products

Abstract: This report aims to analyse the various factors that affect the calorific value (CV) of coke oven gas in TATA Steel. It is TATA Steel’s vision to be the world’s steel industry benchmark. Hence, this report is of great value as it includes a detailed study on all factors which affect the calorific value of the gas. The calorific value will help determine the capacity of the gas to be used as a fuel in the Steel Plant and an analysis of the factors affecting it will be helpful in maintaining a high and stable CV value for the efficient functioning of the plant. This report contains detailed discussions on production and cleaning of coke oven gas and also includes calculation of calorific value of the gas along with theoretical correlations of the CV value with various factors like coking time, coal blends and gas cleaning process.

Signatures of students Signature of Project Guide Signature of PS Faculty

Date: Date: Date:

4

ACKNOWLEDGEMENT

In preparing the present project we have received help from a large number of people who we are thankful to. We extend our sincere gratitude to Mr. D. Kumar, the Chief of Coke Ovens, for giving us the opportunity to work in the Coke Plant at Tata Steel and Mr. Rajesh Kumar, the Chief of the Coke Plant for referring us to Mr. D. Kar in the By Product Plant who helped us understand the work of the By Product Plant and took us on a tour to the same. We would also like to acknowledge the help extended by Mr. Anupam Kumar, Training Coordinator at Coke Plant.

We are also grateful to Mr. Arvind Nath Rai for suggesting us this really illuminating project title which has helped us learn a lot in the due course of our work; and Mr. Avijit Roy and Mr. Amit Kumar for providing us valuable information that we were able to incorporate in the report.

We are thankful to Mr. S. B. Choudhury for providing us the space to work on our project.

Finally, we would like to thank our PS-I instructor Dr. Tarun Kumar Jha for his constant support which has helped us complete the project.

5

TABLE OF CONTENTS

I) Abstract 3 II) Acknowledgement 4

1) Introduction 6 2) Properties of Coke Oven Gas 7 3) Production of Coke Oven Gas

3.1) Blending of Coal 8 3.2) Blending Facility at TATA STEEL 8 3.3) Coal Carbonisation 9

4) The Coke Oven By Product Plant 4.1) Coke Oven Gas Cleaning 11 4.2) Gas Scrubbing for Naphthalene and Ammonia Removal 12

5) Coke Oven Gas Analysis by ORSAT Apparatus 5.1) ORSAT Analysis 15 5.1.1) Carbon Dioxide Test 5.1.2) Hydrocarbon Test 5.1.3) Oxygen Test 5.1.4) Carbon Monoxide Test 5.1.5) Hydrogen Test 5.1.6) Methane Test 5.2) Flow Sequence for Gas Analysis 16

6) Calculation of Calorific Value of Coke Oven Gas 17 7) Correlation with Coal Factors 19 8) Correlation with Coking Factors 22 9) Impact of Gas Cleaning on CV of Coke Oven Gas 23 10) Conclusion 24 11) Recommendation 25

III) Appendix 26 IV) List of References 31

6

LIST OF ILLUSTRATIONS

(i) Layout – Coke Plant 8 (ii) Outline – Coke Oven Gas Production 14 (iii) Schematic Layout – By Product Plant 15 (iv) Contribution of various constituents to CV of coke oven gas 19

7

1. INTRODUCTION

Tata Steel is among the top ten global steel companies with an annual crude steel capacity of over 28 million tonnes per annum (mtpa). It is now one of the world's most geographically-diversified steel producers, with operations in 26 countries and a commercial presence in over 50 countries. Tata Steel’s vision is to be the world’s steel industry benchmark. Therefore, it is undertaking several activities to expand and increase its production capacity of which setting up of new coke batteries is of primary concern since coke is used as a fuel in the blast furnace and also since the coke oven gas so evolved has a very high calorific value and is used after cleaning for various purposes in the Steel Works at Jamshedpur.

Thus, we as interns at TATA Steel have been asked to analyse the factors that affect the calorific value of the coke oven gas and thus determine the various ways of maintaining a high calorific value for the same. This project has been completed by us under the constant guidance and supervision of our mentors Mr. Arvind Nath Rai, Mr. D. Kar, Mr. Rajesh Kumar and Mr. Anupam Kumar.

This report contains a theoretical analysis of the factors that affect the calorific value of coke oven gas which includes coal factors like coal blend and its constituents etc. and coke making factors like battery operations and carbonization time. Also, a calculation of the calorific value of coke oven gas has been done from its constituent elements on a dry basis. The analysis also describes the process of evolution of coke oven gas and the cleaning process of the gas to discuss the impact of cleaning of the gas on its calorific value. Although the analysis provided in this report is entirely theoretical, it will be highly helpful for the company as the concerned people will be able to monitor the calorific value and take measures to maintain a high value because it will help in efficient functioning of the plant where coke oven gas after cleaning is used as a very important fuel.

However, although we had the numerical data we have not been able to estimate the changes in terms of figures that would occur in the calorific value of the coke oven gas. We could only theoretically correlate the parameters to the change in the value without being able to give exact or approximate numerical values to support our analysis.

Most of the information collected in this report has been from the internet and through personal interaction with the officials at the Coke Plant Office at the Steel Works in Jamshedpur, mainly our mentors Mr. Arvind Nath Rai, Mr. D. Kar, Mr. Anupam Kumar, and Mr. Avijit Roy.

The report is chiefly divided into three parts. The first part involves a detailed description of the carbonization process of coal or the coke making process in the coke ovens during which coke oven gas is evolved. It also discusses the process involved in cleaning the coke oven gas in the by-product plant in detail. The second part mentions the composition of the coke oven gas on a wet and dry basis and includes calculations to determine the calorific value of the gas. The third part discusses the various methods of measuring the calorific value and finally, the last part includes a discussion on the various factors affecting the calorific value of the coke oven gas. In the end some recommendations have also been provided in order to be able to maintain a high and stable calorific value of the coke oven gas for efficient functioning of the Steel plant.

8

LAYOUT – COKE PLANT

9

2. PROPERTIES OF COKE OVEN GAS

CONSTITUENT GASES PERCENTAGE BY VOLUME

CARBON DIOXIDE (CO2) 2.8-3.2

OXYGEN (O2) 0.8-1.0

UNSATURATED HYDROCARBONS(CmHn) 2.5-3.4

CARBON MONOXIDE (CO) 8.8-9.0

HYDROGEN (H2) 53.0-55.0

METHANE (CH4) 21-23

NITROGEN (N2) BALANCE

IMPURITIES mg/Nm3

TAR FOG 8-40

NAPHTHALENE 120-500

AMMONIA 40-150

CALORIFIC VALUE (NET) 4000±200 Kcal/Nm3

MOISTURE CONTENT SATURATED

SPECIFIC GRAVITY 0.43

FLAME CHARACTERISTICS SELF-SUSTAINING

UPPER EXPLOSIVE LIMIT 31%

LOWER EXPLOSIVE LIMIT 9%

HYDROGEN SULPHIDE (H2S) 2500 Mg/Nm3

SULPHUR DIOXIDE (SO2) NIL

10

3. PRODUCTION OF COKE OVEN GAS

As soon as the coal is charged into the ovens, the volatile matter (VM) starts evolving. The volatile matter containing the by-product and moisture escapes through the ascension pipes into the hydraulic mains and then to suction main through the cross-over main. Raw coke oven gas as produced at coke ovens is laden with impurities which make it difficult to use as a fuel. As coke oven gas is generated during the production of coke from coal, the steps in the production of coke oven gas are the same as that of coke. These are listed below:

3.1 Blending of Coal

A wide variety of coking coals with different coking properties broadly classified as prime medium and blend able are blended in different proportions for the production of blast furnace coke. Since the prime coking coals are extremely limited, special attention is given for blending to optimize the use of coking coals, consistent with their availability and the national policy of conservation

3.2 Blending Facility at TATA STEEL

For coke making at TATA STEEL stamp charging technique is used in Battery. # 5,6,7,8 & 9 and Battery # 3 have top charging process. Coal blend is achieved by controlling the quantity of coal withdrawn through the bunkers or silos having different coal. The desired quantity of coal is withdrawn from the silos and is mixed on the gathering conveyor. The mixed coal is crushed in primary and secondary crushing machine consisting of hammer mills to achieve a desired crushing fineness and sent to coal bunker. Water is added to coal mix after crushing to maintain a moisture level 8-10% in coal mix. In Top Charge Battery Coal is stored in Slot bunkers and clearing machines are used to draw coal from slot bunkers, which is then sent to hammer mill by conveyors and after crushing coal is sent to Coal Bunker. In Top Charge Battery, Briquettes are added in crushed coal to increase the bulk density of coal charged inside the ovens.

3.3 Coal Carbonisation

When coking coals are heated in absence of air they become soft and plastic over a temperature range 310°C to 500°C. The coal particles agglomerate in to a coherent mass which swells and re solidify to form a porous structure called coke.

When coal is charged in the hot oven, the temperature of oven refractories wall are at about 1100 – 1150°C. The portion of coal in immediate contact with hot wall is heated very rapidly to a high temperature, a thin layer soften, becomes plastic and melts. This layer of plastic material travels towards the centre of the oven and some of the gaseous products force their way out of the plastic material, as the temperature of the charge is raised. On the wall side, the plastic layer hardens in to a residue, and the volatile matter left in the coke is driven off gradually as the temperature rises during reminder of the coking period. Thus in an oven, during initial stage of coking the coal exist side by side in several phases e.g. coke, semi-coke, a plastic mass and granular coal.

It may be stated as follows.

• Evaporation of moisture in the temperature range up to 100°C.

11

• Release of V.M in the temperature range of 200 – 600°C. • Softening and solidifying zone remains in the temperature range of 300 – 600°C. • Plastic zone up to 450°C. • Semi coke up to range of 450 – 650°C. • Release of hydrocarbons up to 800°C. • Above 800°C only hydrogen is released and coke becomes hard.

The evolved gas is then sent for cleaning to the by-product plant.

12

4. THE COKE OVEN BY PRODUCT PLANT

The coke oven by-product plant is an integral part of the by-product coke making process. In the process of converting coal into coke using the coke oven, the volatile matter in the coal is vaporized and driven off. This volatile matter leaves the coke oven chambers as hot, raw coke oven gas. After leaving the Coke oven chambers, the raw coke oven gas is cooled which results in a liquid condensate stream and a gas stream. The functions of the by-product plant are to take these two streams from the coke ovens, to process them to recover by-product coal chemicals and to condition the gas so that it can be used as fuel gas.

In the coke ovens, the evolved coke oven gas leaves the coke oven chambers at high temperatures approaching 850°C. This hot gas is immediately quenched by direct contact with a spray of aqueous liquor (flushing liquor). The resulting cool gas is water saturated and has a temperature of 82°C. This gas is collected in the coke oven battery gas collecting main. From the gas collecting main the raw coke oven gas flows into the suction main. The amount of flushing liquor sprayed into the hot gas leaving the oven chambers is far more than is required for cooling, and the remaining un-evaporated flushing liquor provides a liquid stream in the gas collecting main that serves to flush away condensed tar and other compounds. The stream of flushing liquor flows under gravity into the suction main along with the raw coke oven gas. The raw coke oven gas and the flushing liquor are separated using a drain pot (the down comer) in the suction main. The flushing liquor and the raw coke oven gas then flow separately to the by-product plant for treatment.

4.1 COKE OVEN GAS COOLING

The coke oven gas cooling takes place in 6 Primary cum deep coolers (PCDC). The gas enters the top at a temperature of approximately 80 – 82°C and leaves the bottom at approx. 25°C. The gas includes two small recycle gas streams. One stream consists of acid gases and water vapour from the concentration of ammonia and the other is recycled naphthalene and benzoyl vapour from the naphthalene still overhead.

The coolers are horizontal tube type. The inlet gas is cooled from 80 – 82°C, over upper tube bundle carrying cooling water. Tar, naphthalene and water vapour condense from the gas. The descending fluid is further cooled to approx. 25°C by chilled water flowing through a lower bundle of tubes. Tar and liquor emulsion at 70°C is sprayed on to the tube bundle of the cooler to prevent naphthalene deposition on the outside of the tube. The emulsion can be sprayed either at the top or mid-way into the cooling water bundle. Naphthalene dissolves in the condensed coal tar. The tarry condensate flows out in the condensate collection tank.

The cooling water circulation is on a recirculating system. The return water is cooled in ID cooling tower. The chilled water system is a closed recirculating system connected with the refrigeration plant. While the bulk of the tar and naphthalene condenses in primary coolers not all of it actually separates in primary coolers. Particles of tar and naphthalene condensing towards the end of primary coolers flow out along with the gas in the form of mist or fog. There are also entrained water particles. All these entrained solids or liquids are precipitated from the gas in electrostatic precipitators. There are five operating precipitators.

13

Primary cooler and electrostatic precipitators are put into the suction side of a gas exhauster which provides the motive force for the flow of gas from batteries to gas cleaning plant. In addition to the three exhausters, one more exhauster has been installed to take care of additional gas loads.

4.2 GAS SCRUBBING FOR NAPHTHALENE AND AMMONIA REMOVAL

The method adopted for naphthalene removal is scrubbing of coke oven gas with absorbing oil. By bringing about an adequate contact between gas and oil, almost all of the naphthalene from coke oven gas can be transferred to oil phase. The process is essentially repeatedly spraying the oil over coke oven gas in large quantities. This is carried out in a set of three scrubbers. The three scrubbers are operated in series with counter flow of wash oil. Naphthalene rich oil is steam stripped and returned back for spraying in the gas scrubbers. The stripped naphthalene is returned in vapour form to foul gas main so that it condenses in primary coolers and is solved there in coal tar.

In the same way as naphthalene, ammonia is removed from coke oven gas by spraying it with absorbing water. Water is a relatively cheaper absorption media as compared to sulphuric acid. But its capacity to absorb ammonia is not as much as that of sulphuric acid. Therefore, large size scrubbers with repeated spray water are necessary. In the B. P. Plant, ammonia will be scrubbed by water in a set of three scrubbers, each having four spray stages. The water used for spray, comes from ammonia stills. As in the case of naphthalene scrubbers, these scrubbers operate in series with a counter current flow of sprayed water.

In order to ensure that the ammonia scrubbing is carried out without problems of tar fog or dust precipitation in these scrubbers, an additional scrubber has been put up in the B. P. Plant. This is called pre-scrubber. The function of pre-scrubber is to make the gas flow through a shower of sprat liquor so that dust and tar particles get separated in the scrubber itself. Flushing liquor is used as scrubbing media and the accumulated tar or dust is returned to tar decanters.

The scrubbers are not arranged in a sequence of all naphthalene scrubbers preceding all ammonia scrubbers. Instead, combinations of naphthalene and ammonia scrubbers are kept in series. This ensures that before the gas is cooled down in ammonia scrubbers it is made to lose its naphthalene in naphthalene scrubbers. There are three such pairs of scrubbers and each pair can be taken off the gas stream separately, so even if the first pair is taken off, the second scrubber combination also has naphthalene scrubber as opening scrubber. This arrangement ensures no interface of naphthalene in ammonia scrubbers.

14

OUTLINE – COKE OVEN GAS PRODUCTION

15

SCHEMATIC LAYOUT – BY PRODUCT PLANT

16

5. COKE OVEN GAS ANALYSIS BY ORSAT APPARATUS

5.1 ORSAT ANALYSIS

The amount of the various constituents of the coke oven gas is done by ORSAT analysis.

5.1.1 CARBON DIOXIDE TEST:

Take 100 cc of the sampling gas in the measuring burette. Allow the gas to pass through the KOH solutions of the 1st pipette and note the measuring burette reading. Reduction in the volume of the test gas is the percentage of the carbon dioxide gas.

KOH + CO2 → K2CO3 + H2O

5.1.2 HYDROCARBON TEST: (a)Pass the gas through pipette no.2 (bromine water) till the acidified colour water in the measuring burette becomes slightly colourless.

(b)Then pass the sample through pipette no. 1 to regain the colour of water in the measuring burette. Observe the measuring burette reading. Repeat the above procedure till constant reading is obtained. Difference in volume is hydrocarbon percentage present in the sample gas.

CmHn + Br2 → CmHnBr2

5.1.3 OXYGEN TEST:

Repeat the above procedure using the pipette no. 3, which contains pyrogallic acid solution in KOH. Note the measuring burette reading. Difference in volume is oxygen percentage.

C6H3 (OH) 2 + 4O2 → CH3COOH + 4CO2 + H2O

5.1.4 CARBON MONOXIDE TEST: (a)Pass the gas through pipette no.4, which contains Ammoniac Cuprous Chloride Solution, till constant reading in the measuring burette is obtained.

Cu2Cl2 + 2CO → Cu2Cl2 (CO2)

(b)Pass the gas through pipette no. 5 which contains acidified Cuprous Chloride Solution, till constant reading in the measuring burette is obtained.

(c)Difference in volume gives the CO% in test gas.

Cu2Cl2 + 2H2O + 2CO → Cu2 Cl2 2CO2 H2O

5.1.5 HYDROGEN TEST: (a) Heat the combustion tube (filled with cupric oxide) at temp. Of 200°C approx. Pass the gas over the heated cupric oxide and allow the gas to come into pipette no. 8 which contains acidified water to absorb H2O.

b) Take the reading of the measuring burette. Repeat the above procedure till constant reading is obtained in the measuring burette. The difference in reading gives the H2 percentage present in test gas.

17

5.1.6 METHANE TEST: a) Repeat the above procedure keeping the combustion temperature at 900-950°C and allowing the gas to come into pipette no. 7(which contains KOH solution). Repeat the above procedure till constant measuring burette reading is obtained. Difference in reading gives the percentage of methane in the test gas.

5.2 FLOW SEQUENCE FOR GAS ANALYSIS

After the percentage of the various gases in the coke oven gas has been determined, the calorific value of the gas is determined by multiplying the calorific value of the individual gases with their compositions in the coke oven gas. The method is illustrated by the example below:

THE BLUE BOXES REPRESENT THE PIPETTES

#1 #2 #3 #4 #5

COMBUSTION TUBE

HEATER

#6 #7 #8

ACIDIFIED WATER KOH

TEST GAS 100 CC

PYROGALLIC ACID SOLN.

ACIDIC CUPROUS CHLORIDE SOLN.

18

6. CALCULATION OF CALORIFIC VALUE OF COKE OVEN GAS

After the percentage of the various gases in the coke oven gas has been determined, the calorific value of the gas is determined by multiplying the calorific value of the individual gases with their compositions in the coke oven gas. The method is illustrated by the example below:

CONSTITUENT CV OF CONSTITUENT

(Kcal/Nm3)

% OF CONSTITUENT

IN COKE OVEN GAS

CONTRIBUTION TO CV (Kcal/Nm3)

HYDROGEN (H2) 2570 54 1387.8

METHANE (CH4) 8570 23 1971.1

CARBON MONOXIDE (CO)

3014 9 271.26

ETHYLENE (C2H4) 14200 2 284

PROPYLENE (C3H6) 20900 1.4 292.6

GRAND TOTAL 4206.76

The remaining of the coke gas contains carbon dioxide and nitrogen which are incombustible and does not contribute in any capacity to the calorific value of coke oven gas.

19

CONTRIBUTION OF VARIOUS CONSTITUENTS TO CV OF COKE OVEN GAS

CONSTITUENT CV OF CONSTITUENT

(Kcal/Nm3)

% OF CONSTITUENT

IN COKE OVEN GAS

CONTRIBUTION TO CV (Kcal/Nm3)

MIN MAX MIN MAX

HYDROGEN (H2) 2570 53 55 1362.1 1413.5

METHANE (CH4) 8570 21 23 1799.7 1971.1

CARBON MONOXIDE (CO)

3014 8.8 9 265.232 271.26

ETHYLENE (C2H4) 14200 1.25 3.4 177.5 482.8

PROPYLENE (C3H6) 20900 1.25 3.4 261.25 710.6

20

7. CORRELATION WITH COAL FACTORS

Naturally occurring coal contains fixed ash, volatile matter, ash and moisture. As discussed earlier, the volatile matter is given off during the process of carbonization which contributes to the coke oven gas primarily. Thus, clearly, the volatile matter content of different blends of coal used in the coke plant for coke making will affect the final calorific value of coke oven gas. Below are provided two tables to depict the volatile matter content of coal blends used in the Coke Plant and the variation of calorific value of Coke Oven Gas on a daily basis.

Table I. Volatile matter content in different blends of coal used in the Coke Plant

BLUE HIGHLIGHTED COALS REPRESENT THE COALS CURRENTLY USED IN THE PLANT

COAL TYPE Volatile Matter

WB 23.5 - 28.5

Curragh SS 19.5

IPC SS 24.5 – 26

Metropolitan SS 20

Burton SS 22

Black water weak 26.5

New Zealand 33

Illawara 21.5

Curragh Prime 20.64

Hail Creek 20

Jamadoba 19

Bhelatand 19

Goonyella 23-24

North Goonyella 24

Oaky Creek 26-26.5

Oaky North 23.5

Riverside 22-24

Saraji 18.25-18.5

CD HCC 22

Woolumbi 21

21

From the table A.1 (refer Appendix), we get a clear picture of how the calorific value of coke oven gas has varied over the months of April and May 2011. On the basis of the volatile matter content of the coal blends used in the Coke Plant we will now analyse these variations.

We can see from Table A.1, that there are some days when the calorific value of coke oven gas way beyond 4000 kcal/Nm3. The ideal value should be close to this value. So, on referring to company data on the blends of coal used each day we see that on the 3rd, 4th and 19th of April, 27% was WB incoming coal with an ash content of 13.5%, 20% was WB stock coal, 9% was Jamadoba coal, 22% of the coal was of semi-soft type obtained from Curragh B called the SS Curragh-B blend, 15% was hard coking coal obtained from Curragh Prime called HCC Curragh Prime and 7% was coal obtained from New Zealand. Now based on the volatile matter content given in Table I, we see that coal obtained from Curragh B has 19.5% volatile matter that from Curragh Prime has 20.64% volatile matter and the volatile matter content of New Zealand coal is 33%. The coal obtained from West Bokaro has around 23.5% volatile matter however due to a considerable amount of ash it doesn’t contribute significantly in the evolved coke oven gas. The Jamadoba Coal has about 19% volatile matter which is the least of all the blends used. Now, the lesser the volatile matter content in coal the higher will be the calorific value of the coke oven gas and the closer it will be to the ideal value of 4000 kcal/Nm3, as shown in the calculations earlier. Since for these two days a significant amount of SS Curragh B variety was used along with some amount of Jamadoba coal, both of which have relatively less VM content as compared to the others, the CV value was high and close to the ideal value as predicted.

Similarly, now if we take a look at the other days when CV value was more than or near about 4000 kcal/Nm3, we see that from 20th April onwards the composition of coal continued to remain the same for the rest of the month. It contained 52% coal from West Bokaro having a significant ash content of 13.5%, 9% Jamadoba coal, 16% SS Curragh B, 10% SS IPC, 7% New Zealand coal and 6% Oaky North variety. Although this blend contains a high percentage of WB incoming coal, due to the high ash content and ash being the only non-combustible matter in coal, this blend of coal does not carbonize to a large extent to produce large amount of coke oven gas. Thus, the coke oven gas produced from this blend of coal for the rest of the month has a high and relatively stable CV value of near and around 4000 kcal/Nm3 because of a considerable amount of SS Curragh B variety containing 19.5% volatile matter and a little amount of the Jamadoba variety containing 19% volatile matter which again is the least compared to all the other varieties present.

A similar analysis can be given for the month of May 2011 when the composition of the coal remained almost the same except Curragh B variety which was now increased to 23%.

Now, in the month of April, 16% of the SS Curragh B variety was used along with 9% Jamadoba coal for most of the days of the month while in May 23% of SS Curragh B variety was used. Also, from Table A.1 we can see that in the month of April the CV value was close to 4200 kcal/Nm3 while it decreased to values close to 4100 kcal/Nm3 in the month of May. This is primarily due to the increase in the SS Curragh B variety which has relatively larger volatile matter content than Jamadoba coal.

Finally, let us take a look at what appears to be an anomaly in the CV value for these two months. The only day when the CV value was below 4000 kcal/Nm3 was on 2nd April. A look at the coal blends used that day gives us the following composition:

• WB incoming (13.5% ash) – 35%

22

• WB stock coal – 11% • Jamadoba – 9% • SS Curragh B – 23% • HCC Curragh Prime – 14% • New Zealand – 8%

Although the percentage contents of SS Curragh B and Jamadoba are similar to those in the month of May, however, due to the presence of 14% HCC Curragh Prime and 8% NZ variety which have a considerably high volatile matter content of 20.64% and 33% respectively, along with the 23% SS type, the CV value is significantly reduced to 3989 kcal/Nm3. In other words, due to the relatively less amount of Jamadoba coal, which has a VM content of 19% that is less compared to the VM content of other varieties present, the CV value is affected significantly for the day.

Thus, a variation in the VM content leads to an inverse variation in the CV value of coke oven gas. An increase in the VM content leads to a decrease in the CV value while if there is a decrease in VM content it leads to an increase in the CV value. Therefore, we can say that the calorific value of coke oven gas depends largely on the coal blends used for coking purpose in the Coke Plant because each of the varieties used has a different VM content which brings about a variation in the CV value of the gas.

Moisture present in coal prevents cracking of higher hydrocarbons, so it leads to a decrease in the calorific value of coal. Although, this contribution is not as significant as that of the volatile matter content of coal.

The figure given below is a graphical representation of the variation in CV value of the coke oven gas over a period of 60 days for the months of April and May 2011.

A plot of the variations in calorific value of coke oven gas over a 60 day period for the months of April and May 2011. On the y-axis we have the calorific values while on the x-axis we have the day number with 1st April starting as day 1 and 30th May ending as day 60 for the analysis.

3800

3850

3900

3950

4000

4050

4100

4150

4200

4250

4300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59

Calo

rific

Val

ue o

f cok

e ov

en g

as

23

8. CORRELATION WITH COKING FACTORS

The main coking factor which is of primary importance in our analysis of factors affecting the calorific value of Coke Oven Gas is the Coking Time.

In TATA Steel the Coking Time is kept to 19 hours and 45 minutes which remains constant for most of the time. Hence, currently it does not contribute much to the variation in calorific value of the gas in the plant. However, in general, change in coking time will bring about a change in the calorific value of the coke oven gas.

Coking time is the time required for carbonization of coal to coke. During this period the volatile matter present in coal is given off as coke oven gas which has the capacity to be used as fuel gas. Thus, the more the volatile matter more is the amount of gas generated. However, as discussed earlier the amount of volatile matter has an impact on the calorific value of the gas as less volatile matter is desirable for a high calorific value.

If coking time is increased it means that the amount of gas generated will increase as more and more volatile matter will be given out. However, this can happen only till the point where all the volatile matter in coal is given out as coke oven gas. Thereafter, further increase in coking time will have no impact on the coke oven gas. Now, increase in coking time may lead to the cracking of higher hydrocarbons which are otherwise very difficult to burn especially if they are long chain hydrocarbons. When these hydrocarbons crack into smaller hydrocarbons like methane it increases the calorific value of the gas. In case cracking of the C-H bond in hydrocarbons take place it releases hydrogen which again contributes to an increase in the calorific value of the coke oven gas. However, sometimes due to intense cracking the hydrocarbons break into carbon and hydrogen. In such cases, the carbon forms a part of the coal tar which is removed in the cleaning process in the by-product plant, thereby, reducing its impact on the calorific value of the gas, and the hydrogen released contributes to increase the CV value. Decreasing the coking time will decrease the calorific value of the gas since less volatile matter will be given out as a result of which the hydrocarbon content will decrease reducing its contribution to the CV of the gas.

24

9. IMPACT OF GAS CLEANING ON CV OF COKE OVEN GAS

After the generation of coke oven gas, it is sent to the by product plant for cleansing. Coal tar, naphthalene and ammonia are three important impurities which are removed in the By Product Plant. Besides these chemicals, moisture, hydrogen cyanide and hydrogen sulphide are some other impurities which are also removed. Studies on the impact of gas cleansing on calorific values show that gas cleansing does not affect the calorific value of coke oven gas. It benefits the plant by removal of coal tar, naphthalene and moisture which make transportation of coke oven gas in the supply grid difficult. Thus we can safely conclude that operations carried out in the by-product plant do not affect the calorific value of coke oven gas.

25

10. CONCLUSION

Coke oven gas, industrially referred to as CO gas, is a very useful fuel because of its high calorific value. It is produced during the carbonisation of coal carried out in absence of air. It is then cleaned of tar, ammonia, naphthalene and other impurities. This cleaned gas has calorific value of 3800 – 4200 Kcal/Nm3 approximately. It mostly comprises hydrogen (H2), methane (CH4), carbon monoxide (CO), and hydrocarbons (CmHn).

The calorific value, or CV, of the gas is calculated by computing the individual contribution of the constituent gases. Their composition is found out by ORSAT analysis.

TATA STEEL uses a blend of coal in its coke plant. The volatile matter content of different blends of coal used in the coke plant for coke making affects the final calorific value of coke oven gas. As we saw from the coal blend data, an increase in the VM content leads to a decrease in the CV value while if there is a decrease in VM content it leads to an increase in the CV value. Moisture present in coal prevents cracking of higher hydrocarbons, so it leads to a decrease in the calorific value of coal. Although, this contribution is not as significant as that of the volatile matter content of coal.

During coking time the volatile matter present in coal is given off as coke oven gas which has the capacity to be used as fuel gas. Thus, the more the volatile matter more is the amount of gas generated. If coking time is increased it means that the amount of gas generated will increase as more and more volatile matter will be given out. However, this can happen only till the point where all the volatile matter in coal is given out as coke oven gas. Cracking (breaking of C-H bond) releases hydrogen and causes increase in CV, on the other side intense cracking lowers the CV.

Operations carried out in the by-product plant do not affect the calorific value of coke oven gas much.

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11. RECOMMENDATION

After having analysed the various factors which affect the calorific value of coke oven gas, our prime concern is now how to maintain a high and stable CV value for efficient utilization of the gas in the functioning of the plant. Therefore, we have presented the following recommendations.

• Since the coal blend used for coking brings about a significant change in the calorific value of the coke oven gas, we recommend, from our analysis, that the coal blend that should be used should have a very high volatile matter content overall so that a large amount of coke oven gas is produced, however, it should contain larger percentage of that variety of the coal that has relatively lesser volatile matter content which will help increase the calorific value of the gas.

• Since the coking time is constant and carbonization of the coal takes place almost completely during this period of 19 hours and 45 minutes, we recommend that this is the appropriate time that should be maintained for batteries 8 and 9. However, for the new batteries that are still under construction and will come up soon in the near future, depending upon the coal blend used, we suggest that the coking time can be increased to a little more than the existing time to determine the optimum time duration for which the coking process can be carried out in order to generate a large quantity of coke oven gas having a high calorific value for its use as a fuel.

• From the contribution of different gases to CV of coke oven gas, we see that the variation is largest for unsaturated hydrocarbons (C2H4 and C3H6), then methane (CH4) and hydrogen (H2) and the least variation is from carbon monoxide (CO). Therefore, variations can be reduced if the composition of hydrocarbons is held stable.

• We see that the highest contribution to the CV is from CH4 followed by H2. Therefore, it can be deduced that having higher composition of the above mentioned gases will result in a better CV which will be useful to the organisation.

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APPENDIX

Table A.1. Variation in calorific value of coke oven gas on a daily basis

Date Minimum Maximum Average Standard deviation

1-Apr-11

Down 2-Apr-11 3834 4081 3989 87

3-Apr-11 4039 4091 4072 16

4-Apr-11 4058 4125 4083 30

5-Apr-11

Down 6-Apr-11

Down 7-Apr-11

Down 8-Apr-11

Down 9-Apr-11

Down 10-Apr-11

Down 11-Apr-11

Down 12-Apr-11

Down 13-Apr-11

Down 14-Apr-11

Down 15-Apr-11

Down 16-Apr-11

Down 17-Apr-11

Down 18-Apr-11

Down 19-Apr-11 4138 4213 4173 28

20-Apr-11 4086 4240 4189 48

21-Apr-11 4095 4261 4198 61

22-Apr-11

Down 23-Apr-11 4194 4301 4263 31

24-Apr-11 4196 4321 4269 41

25-Apr-11 4161 4268 4216 32

26-Apr-11 4195 4260 4218 20

27-Apr-11 4192 4279 4236 27

28-Apr-11 4125 4239 4205 34

29-Apr-11

Down 30-Apr-11 3991 4230 4133 93

1-May-11 4012 4025 4019 7

2-May-11 4095 4115 4105 10

3-May-11 4034 4170 4121 38

4-May-11 4078 4150 4125 23

5-May-11

Down

28

6-May-11

Down

7-May-11

Down

8-May-11

Down

9-May-11

Down

10-May-11 3974 4044 4003 24

11-May-11

Down

12-May-11

Down

13-May-11

Down

14-May-11

Down

15-May-11

Down

16-May-11 4047 4165 4126 40

17-May-11 4077 4155 4120 22

18-May-11

Down

19-May-11

Down

20-May-11

Down

21-May-11 4083 4205 4160 37

22-May-11 4134 4207 4160 19

23-May-11 4100 4174 4146 23

24-May-11 4116 4190 4147 21

25-May-11 3999 4154 4093 41

26-May-11 4048 412 4097 22

27-May-11 4084 4155 4117 21

28-May-11 4087 4168 4138 23

29-May-11 4112 4191 4148 26

30-May-11 4010 4158 4090 41

29

Table A.2. Variation in coal blend on a daily basis

DATE

WB INCOMING (13.5%

Ash)

WB STOCK COAL

JAM SS CURR-B

SS IPC

HCC CURR.PRI

. NZ OAKY

NORTH

MIX (Moorvale+

Collinsvelle) TOTAL

1-Apr-11 35 11 9 23 14 8 100

2-Apr-11 35 11 9 23 14 8 100

3-Apr-11 27 20 9 22 15 7 100

4-Apr-11 27 20 9 22 15 7 100

5-Apr-11 27 20 9 22 15 7 100

6-Apr-11 27 20 9 22 15 7 100

7-Apr-11 27 20 9 22 15 7 100

8-Apr-11 27 20 9 22 15 7 100

9-Apr-11 27 20 9 22 15 7 100

10-Apr-11 27 20 9 22 15 7 100

11-Apr-11 27 20 9 22 15 7 100

12-Apr-11 27 20 9 22 15 7 100

13-Apr-11 27 20 9 22 15 7 100

14-Apr-11 27 20 9 22 15 7 100

15-Apr-11 27 20 9 22 15 7 100

16-Apr-11 27 20 9 22 15 7 100

17-Apr-11 27 20 9 22 15 7 100

18-Apr-11 27 20 9 22 15 7 100

19-Apr-11 27 20 9 22 15 7 100

20-Apr-11 52 9 16 10 7 6 100

21-Apr-11 52 9 16 10 7 6 100

22-Apr-11 52 9 16 10 7 6 100

23-Apr-11 52 9 16 10 7 6 100

24-Apr-11 52 9 16 10 7 6 100

25-Apr-11 52 9 16 10 7 6 100

26-Apr-11 52 9 16 10 7 6 100

27-Apr-11 52 9 16 10 7 6 100

28-Apr-11 52 9 16 10 7 6 100

29-Apr-11 52 9 16 10 7 6 100

30-Apr-11 52 9 16 10 7 6 100

1-May-11 52 9 16 10 7 6 100

2-May-11 52 9 16 10 7 6 100

3-May-11 52 9 23 7 6 3 100

4-May-11 52 9 23 7 6 3 100

5-May-11 52 9 23 7 6 3 100

6-May-11 52 9 23 7 6 3 100

30

7-May-11 52 9 23 7 6 3 100

8-May-11 52 9 23 7 6 3 100

9-May-11 52 9 23 7 6 3 100

10-May-11 52 9 23 7 6 3 100

11-May-11 52 9 23 7 6 3 100

12-May-11 52 9 23 7 6 3 100

13-May-11 52 9 23 7 6 3 100

14-May-11 52 9 23 7 6 3 100

15-May-11 52 9 23 7 6 3 100

16-May-11 52 9 23 7 6 3 100

17-May-11 52 9 23 7 6 3 100

18-May-11 52 9 23 7 6 3 100

19-May-11 52 9 23 7 6 3 100

20-May-11 52 9 23 7 6 3 100

21-May-11 52 9 23 7 6 3 100

22-May-11 52 9 23 7 6 3 100

23-May-11 52 9 23 7 6 3 100

24-May-11 52 9 23 7 6 3 100

25-May-11 52 9 23 7 6 3 100

26-May-11 52 9 23 7 6 3 100

27-May-11 52 9 23 7 6 3 100

28-May-11 52 9 23 7 6 3 100

29-May-11 52 9 23 7 6 3 100

30-May-11 52 9 23 7 6 3 100

31-May-11 52 9 23 7 6 3 100

31

LIST OF REFERENCES

Tata Steel Works Study Guide Reference Sheet provided by Fuel Management Department, Tata Steel Data and study material provided by Mr. Anupam Kumar, Training Coordinator, Coke Plant, Tata

Steel Coal Blend and Coal VM (Volatile Matter) provided by Mr. Avijit Roy, Coke Plant, Tata Steel CV (Calorific Value) charts provided by Mr. D. Kar, By Product Plant, Tata Steel