Intern

Indian Institute Of Technology, Madras

HEG Limited, Mandideep

Internship Report

Optimization Of Energy Used In ATunnel Kiln

Author:

Sanket Wani

CH12B059

Supervisor:

Lalit Chandra

.

December 2014 - January 2015

List of Figures

1.1 Graphite Electrodes ready for transport . . . . . . . . . . . . . . . . . . . . . . 9

2.1 Production Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 Production Flow Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Tunnel Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2 Positioning of the kiln cars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3 Overview of Tunnel Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.4 Zoomed overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.5 Zoomed overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.6 Zoomed overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1 Specific Energy Consumption with Time . . . . . . . . . . . . . . . . . . . . . 38

5.2 Specific Energy Consumption with mass flow rate . . . . . . . . . . . . . . . . 38

5.3 Specific Energy Consumption with temperature at the inlet of heating zone . . 39

5.4 Specific Energy Consumption with average temperature over heating zone . . . 39

5.5 Specific Energy Consumption with average temperature over heating zone . . . 40

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List of Tables

3.1 Kiln Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.2 Re baked properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.1 Observed and calculated data . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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Contents

Acknowledgement 5

Abstract 6

1 Company Overview 7

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 Graphite Electrode Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Graphite Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Graphite Manufacturing Process 10

2.1 Product Development Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 Green Electrode Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.1 Base Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.2 Binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.3 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.4 Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Baking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.1 Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.2 Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4 Impregnation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.4.1 Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.5 Rebaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.6 Graphitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.7 Machining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 Analysis of the process in Tunnel Kiln 21

3.1 Rebaking with packing media . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Rebaking without packing media . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3 Overview of Tunnel Kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3.1 System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3.2 System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.3.3 Brief description of pitch fumes burning system of tunnel kiln . . . . . 24

3.3.4 Importance of oxygen during combustion . . . . . . . . . . . . . . . . . 26

3.3.5 Need of excess air for liquid fuel . . . . . . . . . . . . . . . . . . . . . . 26

3.3.6 Process Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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3.4 Advantages of tunnel kiln over conventional baking . . . . . . . . . . . . . . . 31

3.5 Limitations of tunnel kiln . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Energy balance in tunnel kiln 32

4.1 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2 Mathematical Derivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5 Calculations 35

5.1 Key observations from graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.3 Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.4 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

References 43

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Acknowledgement

The internship opportunity I had with HEG Limited, Mandideep was a great chance for

learning and professional development. Therefore, I consider myself a very lucky individual as

I was provided with an opportunity to be a part of it. I am also grateful for having a chance

to meet so many wonderful people and professionals who led me though this internship period

(December 2014- January 2015).

I express my deepest thanks to my project guide, Mr. Lalit Chandra, Deputy Manager for

taking part in useful decision and giving necessary advices and guidance and arranging all

facilities to make life easier. I choose this moment to acknowledge his contribution gratefully.

It is my radiant sentiment to place on record my best regards, deepest sense of gratitude to

Mr. Shubro Sen, Graduate Trainee for his careful and precious guidance which was extremely

valuable for my study both theoretically and practically.

I perceive this opportunity as a big milestone in my career development. I will strive to use

the gained skills and knowledge in the best possible way, and I will continue to work on their

improvement, in order to attain my desired career objectives.

Abstract

Graphite manufacturing is a lengthy step by step process. The steps involved are green

electrode production, baking, impregnation, rebaking, graphitization and machining before

transportation of the final product. All the processes are briefly studied so as to gain an

overall understanding of the working of the industrial plant.

In this report, the rebaking process which is done in tunnel kiln is critically analyzed. The

system (tunnel kiln) is studied in detail. All the system components are identified, the system

is described in a simple manner. The process in the kiln is described in a simple manner using

the available process flow diagrams and equipment.

A simple mathematical model is also used to predict the specific energy consumption and

heat losses in a tunnel kiln. The heat transfer mechanism is complex inside the tunnel kiln,

due to the interactions of kiln car, kiln furniture, product types and product arrangements in

the kiln. Therefore, it is important to establish a model to analyze the heat transfer in the

kiln. The specific energy consumption, the heat losses and the efficiency of the tunnel kiln

are calculated. Several sources of heat losses are identified and inputs have been given. Also,

recommendations are made to make the process safe (less emissions).

1 Company Overview

1.1 Introduction

HEG Limited was established in technical and financial collaboration with Societe Des Elec-

trodes Et Refractaires Savoie (SERS), a subsidiary of Pachiney of France in the year 1977.

HEG is the leading manufacturer and exporter of Graphite Electrodes in India. HEG is the

world’s largest single site plant of Graphite Electrodes with a production capacity of 80,000

MT per annum. It is an ISO 9001 and ISO 14001 Certified Company. HEG also operates

three power generation facilities with a total rated capacity of about 77 MW.

HEG Ltd, a premier company of the LNJ Bhilwara group, is today India’s leading graphite

electrode manufacturer. It has one of the largest integrated Graphite Electrode plants in

the world, processing sophisticated UHP (Ultra High Power) Electrodes.The company ex-

ports over 80 percent of its production to more than 25 countries of the world.The position

the company enjoys today in India and abroad is largely due to its commitment to constant

upgradation of its product quality to match international standards and to meet new chal-

lenges to win and excel in all situations.

In the 1990’s, the company set its Vision to be : A vibrant globally acknowledged top league

player in Graphite Electrodes and allied businesses with commitment to growth, innovation,

quality and customer focus.

In Graphite, its focus is on UHP grade electrodes, and it has expanded its product range

and established the same on some of the toughest furnaces of customers. Today, it has years

of experience supplying quality UHP grade electrodes all over the world.The encouragement

from their customers has led them to increase production capacity and become a significant

global producer of quality UHP grade electrodes for EAF application. The ability to source

the best raw materials from sources worldwide and the skills of human resources has been the

key to their growth.

As a responsible graphite electrode manufacturer, the company continues to invest in tech-

nology, development of new products and in human resources.

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1.2 Graphite Electrode Division

The main business of HEG is graphite which accounts for 80 percent of the revenue. Set up

in 1977, in technical and financial collaboration with Societe Des Electrodes Et Refractaires

Savoie (SERS), a subsidiary of Pechiney of France, HEG is now the largest integrated graphite

plant in the world. Spread over an area of about 170 acres, HEG (graphite division) has

facilities for production of Graphite Electrodes and Graphite Specialities. Its plant is located

at Mandideep near Bhopal (MP).The plant has a annual capacity to make 80,000 MT of UHP

grade electrodes. It has three captive power generation facilities which can together produce

around 77 MW which fulfils almost the entire requirement of the graphite plant. HEG also

has a dedicated R&D Set-up for Carbon and Graphite.

1.3 Graphite Electrodes

Chemically highly corrosion resistant and an excellent conductor of electricity, graphite is an

essential component of welding rods, coatings, most high duty refractory bricks and electric

arc electrodes.

Graphite electrodes have the following properties

• Graphite electrodes have good electrical conductivity in order to withstand the high

current density required by metallurgical processes.

• High thermal conductivity to minimize the temperature difference inside the electrodes

when in use and to reduce internal stresses.

• Low thermal expansion resulting in high thermal stress resistance.

• Strength at high temperature to withstand the stresses when in use.

• Chemical inertness and non-wetting to glass and most metals.

Since graphite electrodes have strength bearing capacities, it can withstand temperature rang-

ing above 30000C and sublimes above the maximum temperature of 35000C. Hence are used

frequently as consumable electrodes.

The essential use of graphite electrodes are can be seen by its absolute necessity in electric

arc furnace. They are used to manufacture steel in arc furnaces. They are also used to manu-

facture various Ferro-alloys like stainless steel, fero-carrides etc. where high strength material

is required to be manufactured maintaining its quality, quantity, and its economic aspects.

8

Graphite electrodes are used in metallurgical applications as a source of energy or melting

scrap iron in an electric furnace, refining certain types of ceramic materials, manufacturing

chemicals (i.e. calcium carbide), and other applications requiring a high temperature, clean

energy source. These electrodes must be highly conductive and free of contamination. High

conductivity and high purity are attributes of synthetic graphite.

Figure 1.1: Graphite Electrodes ready for transport

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2 Graphite Manufacturing Process

The production flow chart is shown in Figure 2.1.

Figure 2.1: Production Flow Chart

The process is carried out in step by step process as shown in Figure 2.2

2.1 Product Development Stages

• GEP- Green Electrode Production

– Raw materials

– Transformation

– Green Stock

• Baking

• Impregnation

• Re-baking

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Figure 2.2: Production Flow Chart

• Graphitization

• Machining

2.2 Green Electrode Production

The raw materials required in the manufacturing may be classified as

• Product Raw Material

• Process Raw Material

Product Raw Material can be further subdivided in

- Base Material

- Binder/Impregnants

- Additives

2.2.1 Base Material

The base material used is Calcined Petroleum Coke (CPC) whose most common feature are-

• It is the purest form of carbon available in large quantity.

• It has higher degree of crystallinity upon heat treatment to temperature of 25000C or

higher. Also considered as most grpahitizable (requires temperture upto 28000C) of

common filter raw materials.

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• Good electrical and thermal conductivity, good mechanical strength

• Low coefficient of thermal expansion and high thermal shock resistance

• No wettability by molten metal and glass, high sublimation temperature (no melting

point)

2.2.2 Binder

Historically, every viscous and adhesive material which gives carbon residue upon pyrolysis

can be used as binder. However, for economic as well as technical reasons, material derived

from coal tar has proved to be the most suitable binder for graphite manufacture.

• High carbon yield, usually 40 to 60 % weight of pitch

• Show good wetting and adhesive properties to bind the coke filter together

• Exhibit acceptable softening behaviour at forming

• Contain less ash and extraneous matter that could reduce strength and other physical

properties

• Produce binder coke that can be graphitized to improve electrical and thermal properties

2.2.3 Additives

Stearic Acid

It is a fatty acid, an organic material, recovered in oil industries as a by product. It has

lubricating properties, which is used in preparation of green products to overcome the friction

between container, die and green paste to facilitate extrusion.

Iron Oxide

Reduces volume expansion (puffing) of the artifact which occurs with filler materials in the

entry stage of graphitization process ( temp range 1600-21000C) . Sulphur present in CPC

causes puffing or swelling of the coke. This can cause uncontrolled expansion resulting into

cracking of product. Usually Iron Oxide is required as 30 kgs for every 1000 kgs of CPC.

2.2.4 Process

The process of green stock manufacturing starts with receiving of raw material i.e. CPC,

binder pitch and additives. The CPC is crushed and milled and stored in different bins.

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The particle size of the calcined coke is selected according to the end use of the electrode.

Generally, particles up to 25 mm in average diameter are employed in the blend. The particu-

late fraction includes a small particle size filter comprising cake powder. Other additives that

may be incorporated into the small particle size filter include iron oxides to inhibit puffing

(caused by release of sulphur from its bond with carbon inside the coke particles), coke powder

and oils or other lubricants to facilitate extrusion of the blend.

Following this, predetermined quantities of different fractions are weighed and transferred to

a mixer which is heated to a certain predetermined temperature. The required quantity of

binder and additives is also weighed separately and added to the mixer. The mixing is con-

tinued for a predetermined period known as the mixing cycle.

After the blend of particulate fraction, pitch binder etc is prepared, the body is formed(or

shaped) by extrusion through a die r molded in conventional forming molds to form what is

referred to as green stock. The forming, whether through extrusion or molding, is conducted

at a temperature close to the softening point if the pitch.

After cutting the electrode in desired length it is dropped inside a cooling bath. The electrode

keeps rolling slowly and is cooled,. The cooling and rolling is necessary, so that the green

product can retain its shape. After cooling, the electrode is taken out form the bath and is

put on inspection.

Good electrode must be of proper density, have smooth outside surface be free from cross

checks or crack and splits, contain no foreign materials, be cut to proper length.

2.3 Baking

The general idea of baking is that the green electrodes from the GEP shop are heated causing

the volatile matters of electrode to evaporate and also help in the conductivity and reduction

in resistivity of electrodes.The green electrodes from GEP shop contains volatile matter like

benzene, toluene and moisture which reduces its mechanical strength.

In baking process the electrodes are fed to riedhammer furnace which is lined by refractories

where the electrodes are lined up vertically in layers of two or three. The spaces between the

electrodes are filled with grains of met coke and then the furnace is sealed. The furnace used

is Riedhammer furnace.

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2.3.1 Firing

The firing process takes place which is the setting up of the furnace for heating the electrodes.

The fuel used is furnace oil which on burning gives heat. The heat is not directly fed to the

electrodes instead are fed indirectly through fluid transfer pipe (FTP). The oil from the main

tank comes to the respective tank on each geared pump which is then filtered and sent for

preheating up to 600C. This oil from the pumps is then delivered through pipes to a nozzle

and from the nozzle, oil is sprayed which burns in presence of air generating heat. After firing

of the furnace it starts generating heat which bakes the electrodes. The electrodes are baked

upto 9000C. Thermocouples are placed inside the furnace to record the temperatures of the

gas and the fuel products at three different places continuously until the temperature reaches

a value between 800 to 10000C. This heating process takes about 36 to 48 hours.

2.3.2 Cooling

Next the cooling of the product takes place. After the desired temperature is achieved the

furnace is shut down and the electrodes are allowed to cool but in closed atmosphere first as

directly exposing them in open atmosphere may result in cracking of electrodes. This process

takes place for 3 to 4 days after which the chambers are opened and the grains are removed

using hardware and bin which creates a suction pressure of about 150 bar. Then the electrodes

are removed using EOT cranes for natural cooling.

The complete baking process (including cooling) takes a lot of time before the electrodes

are sent to next shop.The met coke grains which are fed with the electrodes are sticking to

the electrodes when they are takes out causing their surface to become uneven and thus are

further processed (scrapping and facing) before sending it to the next shop.

2.4 Impregnation

During the baking process the electrodes becomes porous due to outgassing and thus their

density is reduced. The expansion and compression process disturbs the arrangement of par-

ticles and weakens the internal bonds thereby making it vulnerable to damage and defects.

To overcome this, electrodes are impregnated with coal tar pitch which would be carbonized

in further processes. These pitches have lower melting points. These are in form of a fluid at

impregnation temperature.

Impregnates are generally coal tar products but differ from coal tar binders in characteristics

Fluidity (reverse of viscosity) of impregnates is high at the temperature of use. For good

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impregnation, softening points of pitch should not be high. Also alpha resin need to be as low

as possible so as to avoid clogging of porosity.

2.4.1 Process

Electrodes are placed in the baskets which carry 6 to 8 electrodes depending on the diameter

which are first preheated in pre-heaters so that the pores are somewhat widened and then

transferred to the autoclaves creating vacuum inside it. Preheating process is done using air

which is heated from the oil supplied from HOB and the hot air is blown using a blower to

the preheating tanks. This vacuum helps in the easy penetration of pitch in the electrodes

with specified pressure and temperature. Preheating process takes much more time than the

vacuuming process. After vacuuming pitch is supplied to autoclave from PVC and allowed to

impregnate.Then after the process the pitch is sent back to the PVC and the electrodes are

allowed to cool in cooling tank. Electrodes from cooling tank are then moved out for natural

cooling from where they are transferred to next shop.

2.5 Rebaking

The electrodes after being impregnated have increased densities but contains volatile matters

and excess of pitch which needs to be removed so that to reduce to carbon in form of coke. This

happen through re-baking of electrodes in a tunnel kiln. This tunnel kiln process also helps

in improving the conductivity of the electrodes. Here indirect heating of electrodes takes place.

Inside the tunnel there are primarily 3 zones namely

• Heating zone

• Precooling zone

• Cooling zone

The main baking process takes place inside the heating zone. Here the trolley is pushed inside

the tunnel by pushers (which are hydraulic cylinders). Preheating of electrodes is done for

about 3-5 hours to remove any trapped moisture inside the electrodes. Generally the pushing

rate of trolleys inside the tunnel is slow for bigger diameter electrodes. At a time only a single

diameter electrodes are pushed inside the tunnel. Even if there are two different diameter

electrodes the time duration for the larger diameter electrode is followed until all of larger dia

electrodes are taken out from the tunnel. Heating of electrodes removes the volatile matter

present.

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After heating, the electrodes are pushed to the precooling zone. Here the electrodes are not

directly heated by atmospheric air instead the heat supply is cut-off and the temperature is

lowered. Further there is a heat exchanger which takes the heat from the furnace oil exiting

the heating zone and loses it to the atmosphere. Then this lower temperature oil is used to

cool the electrodes in the precooling zone. Precooling is done to avoid any kind of cracks or

defects on electrodes.

Figure 2.3: Tunnel Kiln

After precooling of electrodes, cooling of electrodes takes place in atmospheric air. Atmo-

spheric air is pumped inside and then drained off. The electrodes are then taken out of the

tunnel by another set of hydraulic cylinders known as pullers, unloaded and kept in atmo-

spheric air for natural cooling. The complete cooling process takes place.

All doors in the tunnel which separates different zones are all laser aligned and they have

proximity sensors at the ends which is used for opening and closing of the doors. Doors help

in maintaining the vacuum inside the tunnel thus are of utmost importance. It is this vacuum

which helps in removing the volatile matter. During this process of rebaking the impregnation

pitch is reduced to coke by destructive distillation and finally the electrodes are converted to

carbon which is to be converted to graphite in the next shop.

One cycle of tunnel kiln takes about 3 days to complete after which the electrodes are taken

to graphitization shop.

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Unlike in Figure 2.3 the graphite electordes are covered by a sagger.

2.6 Graphitization

Till now all the processes were dealing with carbon. Graphitization is the actual process in

which the amorphous carbon in converted to the crystalline carbon (graphite).This conversion

requires very high temperature of around 35000C. The heating takes place in the furnace using

the electric resistance method as the high temperature of 3000 can be reached with the help

of electricity. The graphite so obtained is artificial graphite and has properties different from

the natural graphite.

The electrodes are setup length wise in layers. The space between the layers is filled with

grains of met coke and the whole setup is completely covered with the grains. The packing

layers serve as hot blankets in which relatively impure material might be used in manufacture

of very pure graphite electrodes. Recti former is used to convert the high magnitude AC

current into DC current. The heat is generated using electric resistance method in and is of

highest magnitude of approx. 30000C. For one cycle of firing it takes about 56 hours and after

which cooling of electrodes takes place.

Depending on the raw materials and the processing parameters various degrees of convergence

to the ideal structure of a graphite single crystal are achieved. There is no significant change in

dimensions of electrodes. Hydrogen and sulphur are evolved when the temperature increases

beyond the previous baking temperature.

HEG has two kinds of furnace setups namely

1. HSN/Acheson (Height Wise Setup)

2. LWG (Length Wise Graphitisation)

HSN furnace setup is old setup as compared to LWG. In this type of setup the firing is done

on the grains which are earlier used for covering the electrodes. The electrodes are placed

vertically with spaces covered with grains. Most of the energy is used up in heating the grain

and a little is transferred to the electrodes.

LWG furnace setup is more effective than HSN setup. The electrodes are placed end to end

in a series which increases the resistance and so greater amount of heat is supplied in less

current. Firing is done directly on the electrodes which increases its effectiveness. During

graphitization the crystals are developed. The material gets softer and machinable and the

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impurities gets vaporized.

An Acheson type furnace consists of a central chamber surrounded by external walls made of

some refractory material such as firebrick. The chamber is roughly rectangular in outline. The

top is open. The Acheson furnace is nothing more than a room without a ceiling, designed to

keep in the heat generated by electrical resistance heating of the carbon charge.

The end walls of the Acheson furnace are fitted with graphite buss bars. These buss bars

extent to the outside wall of the furnace where they are coupled to large caliper copper buss

bars that may or may not be water cooled. Copper or other metallic materials are not per-

mitted within the inter confines of the furnace since they would vaporize at the high process

temperatures.

Since graphitization process temperatures are expected to reach 28000C or more, it is of

paramount importance that oxygen be excluded from the furnace. This is accomplished by

covering the electrodes with some oxygen scavenging material which is the grains of met coke.

This packing as it is called must also be somewhat conductive to electricity since it will form

part of the electrical pathway through the furnace.

In addition to restricting oxygen from the electrodes the packing material used to protect the

electrode blanks also serves as a thermal insulator. It is important to keep the heat in the

furnace so that the extreme process temperature required for graphitization to occur can be

reached. Once the electrodes are placed in the Acheson furnace and covered with the grains

a DC current of low voltage and very high amperage is applied to the furnace charge. The

furnace load heats up due to its own electrical resistance. As the heating progresses the fur-

nace resistance goes down due to the increase in conductivity that results from the formation

of graphitic carbon at the expense of amorphous carbon.

Furnace conditions are constantly monitored, this includes the power consumed. At the point

where the operator determines that the proper furnace temperature has been achieved, or

that the charge is fully graphitized, power to the furnace is cut. The furnace is allowed to cool

and the electrodes are removed. Graphitized carbon is cleaner than amorphous when taken

from furnace and they have a fairly smooth surface. These electrodes have only a quarter of

resistivity of amorphous carbon.

The electrodes are then sent to Product Finish Shop (PFS) for finishing purpose.

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2.7 Machining

After the amorphous carbon is converted to graphite it is sent to PFS, which is the last shop

before the product is delivered to the customer. Here the graphitized electrodes and nipples

are tested and then given the final shape using different machines (most of which are CNC).

Nipples are used to connect two electrodes end to end and are mechanically stronger than the

electrodes as they have to hold two electrodes together.

The machining of electrodes is basically done in four basic steps

1. Boring

2. Turning

3. Threading

4. Resistivity Check

Boring

In this operation, internal holes are made in the electrodes at the faces or we can say that

internal diameter is increased. Facing is also done in this step simultaneously so that the faces

becomes flat.

Turning

The bored electrode are fastened and rotated along its length and the tool moves from one

end to another. This makes the electrode into desired diameter as required by the customer.

Threading

In this operation, threads are made in the hole made earlier so as to fasten the nipple in the

electrode.

Resistivity Check

The machined electrode is then weighted and their resistivity is checked in this final machine.

Also the electrodes are checked for any damages and cracks.

Packing of the electrodes is also done in PFS. In the final assembly station, nipples are fas-

tened on one end of the electrode and other end is left as it is, after this polystyrene capping is

placed on both nipple and thread so as to protect them from abrasion and damage during its

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transport. Electrodes are delivered on wooden rafters strapped with steel bands, the threads

and faces being protected by end caps in order to minimize the risk of damage to the threads

and end faces.

All processes are carried out on FIFO basis i.e. first in first out basis. The requirement of

a customer are first taken at customer engineering section which then sends it to Production

Planning & Control section which plans the production of the desired product to carry out

the above listed processes.

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3 Analysis of the process in Tunnel Kiln

When high mechanical strength, density and current carrying capacity are required in finished

products, the baked stocks are impregnated with special pitch to fill the porosity. Impreg-

nated stocks are again baked to ensure coking of impregnated pitch. The process is called

second baking. This can be done with or without the packing media. The transfer of heat

can be done directly to the impregnated pitch or through media similar to first baking.

3.1 Rebaking with packing media

Rebaking with packing media is similar to that of first baking

• Loading and unloading is done as per similar practice

• The difference between first and second baking is in the heating rate

• Heating rate in rebaking can be much higher as compared to the first baking

• Since heating rate is higher and heating cycle is shorter, cooling of the cell becomes very

important to maintain the turn around cycle

3.2 Rebaking without packing media

This process is significantly different from the first baking or rebaking with packing media.

Here, heating is done directly to the product through convection.

3.3 Overview of Tunnel Kiln

In Figure 3.1 an overview picture of tunnel kiln is shown and in Figure 3.2 the positioning of

the kiln cars is shown.

The second baking is divided into following zones

• Zone 1 : Entrance lock D10

• Zone 2 to 25 : Heating zone D07

• Zone 26 to 34 : Precooler D08

• Zone 35-39 : Main cooler D09

21

Figure 3.1: Overview

Figure 3.2: Positioning of the kiln cars

The entrance lock D10 will be locked by gates A10 and A11. The heating zone will be locked

by gates A11 and A22. The precooling zone D08 will be locked by gates A22 and A23. The

main cooling zone will be locked by gates A23 and A24.

Length 90.6m

Width 4.95m

Height 5.17 m

Table 3.1: Kiln Dimensions

22

The kiln dimensions are shown in Table 3.1.

With second baking kiln systems, it is possible to carry out continuous second baked process

of impregnated products of different length and diameters at required temperatures.

3.3.1 System Components

The main components of the tunnel kiln are:

• Heating System

• Fuel pumping, heating and filtration unit

• Heat Recovery Unit

• Hot air circulation system

• Cooling air system : Precooler

• Cooling air system : Main cooler

• Transport system

• Haulage system

• Water sealing system

• Hydraulic system

• Pneumatic system

• Control and measuring system

3.3.2 System Description

The heating system consists of incinerator A40, burner A50, hot gas duct and hot gas boxes

including all necessary valves.

Furnace oil system consists of storage tank with heater, oil filter, pumps and pipe lines in-

cluding all necessary valves.

The Heat Recovery Unit (HRU) consists of heat exchanger W51, flaps and ducts.

23

Hot air circulation system consists of recirculation fan V52, axial fans V20.1-V20.11 and re-

circulation gas duct including all necessary flaps and compensators.

Cooling air system for precooling consists of cooling air fan V40, recirculation fan V50, heat

exchanger, cooling air pipe including all necessary flaps and compensators.

Cooling air system for main cooler consists of cooling air fan V41, cooling air pipe lines in-

cluding all necessary flaps and compensators.

Combustion air system consists of combustion air fan V71, heat exchanger W51 and combus-

tion air pipe lines including all necessary flaps and compensators.

The transport system of kiln consists of kiln track H18, return track H16, track 23.1 and 23.2

for transfer car H06 and H08, pushing unit H48, pulling unit H34 from precooler to main

cooler and stopper H36.

Haulage system is wire rope driven car shifting mechanism consists the gear box, motor, pul-

ley wire rope dock etc.

Water sealing system is to avoid heat to go in the bottom of the kiln car to avoid bearing

seizing and it consists of water circulation pump P1 and P2, water tanks B3.1, B3.2 and B3.3,

and water trough.

3.3.3 Brief description of pitch fumes burning system of tunnel kiln

The Hot Air Generator(HAG) will generate hot air for circulation in the system for rebaking

of impregnated products placed inside the kiln to attain required coke temperature, by direct

heating.

The impregnated materials are loaded on the kiln car in a defined manner. Inside the kiln,

during heating, pitch is likely to give out a number of VOCs and other combustibles, which

may mix with the circulating hot air. Hence it is essential to completely combust the VOCs

and other combustible gases as soon as they enter the HAG. Hence, the HAG is designed in

line with VOC incinerators, with a predetermined residence time of the gases in the combus-

tion chamber for ensuring complete combustion of the fuel as well as the other combustibles.

The combustion chamber is refractory lined with and outer shell. The process air will enter

the annular gap between this outer shell and the combustion chamber shell and will then get

24

Fig

ure

3.3:

Ove

rvie

wof

Tunnel

Kiln

25

injected into the combustion chamber through strategically laced air nozzles. This arrange-

ment will ensure safe and complete combustion.

Complete combustion of pitch fumes in the combustion chamber gives good amount of energy,

which is finally reducing the requirement of primary fuel.

3.3.4 Importance of oxygen during combustion

It is established process need that while re baking of impregnated stock, oxygen% has to be

maintained at lower side to avoid product oxidation, but at the same time as we are burning

liquid fuel it is required to have excess air for complete combustion. Therefore, a balanced

approach has to be adopted to fulfill opposite requirements.

3.3.5 Need of excess air for liquid fuel

Theoretically, if air and fuel mixed in chemically equivalent proportion would be stoichiomet-

ric, but in a dynamic process, like a burner, the mixing and reaction time is short. Pockets of

combustion may not have enough time for these pockets to mix with remaining oxygen. The

result is incomplete combustion and formation of carbon monoxide.

Carbon monoxide is an intermediate product in the combustion process and it contains signif-

icant thermal energy, depending on the burner, 2 to 4 % excess oxygen significantly reduces

carbon monoxide concentration in the flue gases which finally make combustion process safe

and efficient.

3.3.6 Process Description

First of all kiln cars are loaded at loading station. Pitch pans are put and adjusted on the

bricks of the kiln car. Subsequently electrodes are put on the electrode furniture. After load-

ing of electrodes, the sagger is set on the pitch pan.

Loaded kiln car is transported with transfer car H06 in front of A10 door.

With the pushing unit on transfer car, the loaded kiln car is pushed into entrance lock. The

transport in heating zone D07 takes place with main pushing unit H48. In heating zone,

product is heated with hot gases. The necessary volume of hot gases into different zones is

controlled with MCV 4.1 to 4.8 depending on temperature in these zones.

26

Fig

ure

3.4:

Zoom

edov

ervie

w

27

Fig

ure

3.5:

Zoom

edov

ervie

w

28

Fig

ure

3.6:

Zoom

edov

ervie

w

29

The suction of recirculation air from heating zone takes place through suction pipes with fan

V52 to incinerator A40.

In incinerator, hydrocarbons are oxidized and recirculation gas is heated up to necessary pro-

cess temperature. The heat up of hot gases inside the incinerator takes place with generator

gas and furnace oil. The necessary combustion air for the burner is supplied by combustion

fan V71. The combustion air is preheated in the combustion air/exhaust gas heat exchanger

W51 with the exhaust gases.

The exhaust gases is sucked off the system with exhaust gas fan V61 and is blown out through

stack X10 into atmosphere.

From heating zone, product is transported to pre cooling zone D08. The cooling air for pre

cooler circulates via cooling air heat exchanger W70 and the recirculation fan cooling air V50

in a closed circuit. In heat exchanger the recirculation cooling air is cooled with fresh air

sucked off directly from atmosphere by cooling fan V40. After heating up of fresh air, it is

blown directly to the atmosphere.

From pre cooling zone, the product is transported to main cooler D09. In this zone, the prod-

uct is cooled to necessary outlet temperature. The cooling air fan V41 sucks air directly from

atmosphere. The outlet of heat up cooling air takes place in zone 35 to 39 through cooling

air pipe line directly to atmosphere.

With pulling unit H38 mounted on transfer car, the product is pulled out from main cooler on

transfer car H08. After the transfer car reaches its position in front of return track, the kiln

car is pushed on return track and transported with the return draw mechanism in direction

to the transport car H06.

In the unloading area, kiln car is unloaded and prepared for next loading.

Figure 3.3 shows a different figure of the overview of tunnel kiln. Since the picture is not clear

enough, some more figures have been added.

Re baked properties are shown in table 3.2.

30

Weight Loss % 6 to 7

Apparent Density gm/cc 1.74 to 1.8

Table 3.2: Re baked properties

3.4 Advantages of tunnel kiln over conventional baking

• In-out duration is very less

• Very fuel efficient system

• Less pollution

• Cold in cold out process

• Continuous process

3.5 Limitations of tunnel kiln

• Not suitable for nipple processing

• Loading of smaller length is difficult and less output per car

• Percentage weight loss is more in smaller size electrodes

• Continuous burner operation is required to avoid product oxidation

• Precise air fuel ratio is required to maintain safe kiln operation

31

4 Energy balance in tunnel kiln

The heat transfer mechanism is complex inside the tunnel kiln, because of the interaction

of kiln car, kiln furniture, product types, and product arrangements in the kiln. Therefore,

it is important to establish a simplified model to understand the principal behavior of the

kiln process. This model considers the heating zone in which the solid materials (product,

furniture, and car) are heated up to the required temperature. The cooling air is sucked away

from the kiln and then used in the dryer. It is assumed that the enthalpy of the cooling covers

the energy for the drying process. As a consequence, the energy of the fossil fuel of the kiln is

the required energy for the total process. This is the case for the most processes of graphite

manufacturing.

4.1 Assumptions

• The process is assumed to be a steady state process.

• The temperature of product and of gas is assumed to be constant at any cross section.

As a consequence, the temperature depends only on the length of the tunnel kiln.

• Due to unavailability of data, the temperature of product and transportation materials

are same.

• The heat transfer coefficient is constant.

• The material properties (specific heat capacity, density) are assumed to be constant.

• The material does not store latent energy.

4.2 Mathematical Derivation

The basis of all kiln processes is an energy balance. The enthalpies in heat balances are always

referred to the reference temperature (0C). In energy balance analysis, the energy inserted by

fuel is equal to the heat gain by solid, heat removed from gas, and heat loss through walls.

By applying the energy balance, the following equation is obtained

Mscs(Tout − Tin) + Mgcpg(Tg,out − Ta) + Qw = MFhu (4.1)

Where

Ms represents solid mass flow rate,

cs specific heat of solid,

Tin temperature at the inlet of heating zone,

32

Tout average temperature over the heating zone,

Qw heat losses,

MF fuel mass flow rate,

Mg gas flow rate,

cpg gas specific heat at constant pressure,

Ta the inlet gas temperature (ambient air temperature), and

Tg,out outlet gas temperature.

The solid mass flow rate represents the mass flow rate of both the products and transporta-

tion (due to unavailability of data). Hence, the specific heat capacity of the product and the

transportation material are also assumed to be the same.

The outlet gas mass flow rate from the combustion process is the summation of the fuel mass

flow rate MF , and the air mass flow rate Ma.

Mg = MF + Ma (4.2)

The mass flow rate of air can be expressed in terms of air demand (L), excess air number (λ)

and fuel mass flow rate as

Ma = λLMF (4.3)

Then the gas mass flow rate can be written as

Mg = MF (1 + λL) (4.4)

Now, the energy equation requires the mean specific heat capacity. Therefore the following

equation is used to calculate the mean specific heat capacity with gas properties referred to a

reference temperature, To = 273 K

cpg(T )

cpo(T0)=

1

nc + 1

((T/To)

nc+1 − 1

(T/To) − 1

)(4.5)

For this specified case the specific heat of coal tar pitch with (cpo(To)= 1.0) and index (nc=

0.1) are used.

Moreover, equation 4.5 is used to simplify the energy consumption equation

cpg(1 + λL) ∼= cpgoλL (4.6)

The specific energy consumption referred to the solid flow is

Es =MFhu

Ms

(4.7)

33

Solving the above equations gives the following form for specific energy consumption

Es =cs(Tout − Tin) + Qw/Ms

1 − cpg(1 + λL)(Tg,out − Ta)

hu

(4.8)

Air demand (L) is defined as the ratio of mass flow rate of air to the mass flow rate of fuel.

Excess air number (λ) is defined as

λ =(moles of O2)fed

(moles of O2)theoretical

It can be easily proved that the excess air is the same as excess oxygen in a system. Since the

available data in our system is percentage of oxygen, the air number can be expressed as

λ = 1 +% excess air (or oxygen)

100

The heat losses considered above do not include the heat loss due to exhaust gases which has

been considered as a separate term. However, while defining efficiency, this term will also be

considered as lost heat.

Efficiency(η) is defined as

η =Mscs(Tout − Tin)

MFhu× 100 (4.9)

34

5 Calculations

There are three tunnel kilns at HEG, Mandideep operating in coordination with each other.

The following observations have been made for data obtained from TK3 (Tunnel Kiln 3). All

the tunnel kilns are equipped with a SCADA unit from which run time data was obtained.

• As mentioned earlier, due to unavailability of data, the temperature and the specific

heat capacity of the product and the transportation material were assumed to be the

same. The product that is fed to the tunnel kiln is essentially, 88% coke and 12% im-

pregnation pitch, which converts to graphite only at the end of the graphitization pro-

cess. The specific heat capacity of coke and impregnation pitch (coal tar pitch), were

obtained and a mean specific heat capacity for the solid material was found as following.

Specific heat capacity of coke = 0.85 kJ/kg K

Specific heat capacity of coal tar pitch = 1.47 kJ/kg K

Therefore, the specific heat capacity (cs) of the solid material

cs = (0.88 × 0.85) + (0.12 × 1.47)

= 0.9244 kJ/kg K

• The SCADA unit was monitored for the mentioned dates and an average mass flow rate

of air and fuel was obtained.

< MF > = 180 kg/hr = 0.05 kg/s

< Ma > = 3300 kg/hr = 0.9167 kg/s

Using equation 4.2 , the average mass flow rate of gases is

< Mg > = (180 + 3300) kg/hr

= (0.05 + 0.9167) kg/s

= 0.9667 kg/s

• The average air demand is the ratio of mass flow rate of air to mass flow rate of fuel.

L =0.9167

0.05

= 18.334(kg)air(kg)fuel

35

• The average excess oxygen percentage inside the heating zone of the tunnel kiln was

found to be 5.42 %. Therefore, the excess air number (λ) is 1.05.

• The exhaust gases leaving the tunnel kiln consist mainly of coal tar pitch fumes which

have more than 400 components. Again, due to unavailability of values, the specific

heat capacity of gases was assumed to be that of the tar pitch which is 1.47 kJ/kg K.

• The average temperature of exhaust gases over the given period was found to be 2500C

or 523.15 K. The average ambient temperature is assumed to be 250C or 298.15 K.

• The fuel that is used furnace oil whose heating value is around 10,000 cal/g or 4.186 x

104 kJ/kg.

The other data required for our calculation is the temperature at the inlet of heating zone

(Tin), the average temperature over the heating zone (Tout) and the mass flow rate of solid

(Ms). The data that was available gave the tonnage per day. Assuming that the process was

carried out for 22 hours in a day giving 2 hour rest time, the mass flow rate of solid was

calculated.

After this data was obtained, the calculations were done in Microsoft Excel. The heat losses

(QW ) and the specific energy consumption (Es) were calculated.

The calculated values are shown in Table 5.1

Day 1 in Table 5.1 refers to 23 November 2014 and day 2 refers to 24 November 2014 and so on.

The specific energy consumption (Es) was plotted with various quantities such as

• time(day number)

• mass flow rate(Ms)

• temperature at the inlet of the heating zone(Tin)

• the average temperature over the heating zone(Tout)

• heat losses (QW )

36

Day Tonnage

Mass flow

rate in

kg/s(Ms)

Temperature

at inlet of

heating

zone in0C(Tin)

Average

tempera-

ture over

heating

zone in0C(Tout)

Heat losses

in kW(QW )

Specific

Energy

Consumption

in kJ/kg(Es)

1 74.8 0.944 159.89 663.28 1333.778 2.216

2 99.61 1.258 201.89 664.65 1235.246 1.664

3 108.82 1.374 225.71 652.83 1230.768 1.523

4 139.4 1.76 227.81 678.54 1039.905 1.189

5 120.47 1.521 194.61 654.85 1126.12 1.376

6 129.88 1.64 216.16 659.53 1101.145 1.276

7 138.09 1.744 210.5 665.51 1039.899 1.2

8 129.32 1.633 201.41 654.17 1089.87 1.281

9 132.56 1.674 232.05 658.16 1113.981 1.25

10 146.63 1.851 229.05 673.19 1013.148 1.13

11 146.03 1.844 227.53 676.15 1008.623 1.135

12 156.59 1.977 249.95 672.14 1001.634 1.058

13 159.25 2.011 249.9 666.7 998.545 1.041

14 150.25 1.897 240.64 674.85 1011.797 1.103

15 152.84 1.93 232.64 661.99 1007.34 1.084

16 155.94 1.969 237.04 658.68 1005.838 1.063

17 151.35 1.911 199.51 683.31 918.62 1.095

18 129.8 1.639 191.33 682.23 1029.552 1.277

19 141.11 1.782 175.13 684.91 933.654 1.174

20 157.6 1.99 165.32 663.37 857.116 1.052

21 129.92 1.64 160.44 672.91 996.156 1.276

22 132.53 1.673 152.86 672.51 969.438 1.25

23 122.66 1.549 133.33 676.93 995.014 1.351

24 137.28 1.733 133.65 678.46 900.315 1.207

25 142.83 1.803 145.87 680.4 882.16 1.16

26 116.35 1.469 169.22 678.91 1081.099 1.424

27 112.23 1.417 178.81 670.36 1129.37 1.477

28 114.78 1.449 172.57 663.32 1115.812 1.444

29 101 1.275 172.75 674.38 1181.917 1.641

30 120.31 1.519 181.71 660.56 1100.847 1.377

Table 5.1: Observed and calculated data

37

Figure 5.1: Specific Energy Consumption with Time

Figure 5.2: Specific Energy Consumption with mass flow rate

Due to our definition of specific energy consumption in equation 4.7, the total energy con-

sumption in a day(Eday) is nothing but the energy supplied by the fuel in 22 working hours.

38

Figure 5.3: Specific Energy Consumption with temperature at the inlet of heating zone

Figure 5.4: Specific Energy Consumption with average temperature over heating zone

Eday = MF × hu× time

= 0.05 kg/s × 4.186 × 104 kJ/kg × 22 × 3600 seconds

= 165.72 GW

39

Figure 5.5: Specific Energy Consumption with average temperature over heating zone

Therefore, the total energy consumption in 30 days is

Emonth = 30 × 165.72 GW

= 4971.6 GW

= 4.97 TW

The efficiency is calculated using equation ?? for the observed period and the average is found

to be 34.64%.

5.1 Key observations from graphs

• The average specific energy consumption over the period of 30 days is 1.29 MJ/kg.

• The minimum specific energy consumption is 1.041 MJ/kg on 05 December (day 13).

The maximum being 2.216 MJ/kg on 23 November (day 1).

• From Figure 5.2, it can be clearly seen that the specific energy consumption reduces

with increased mass flow rate. This is verified by the fact that on 05 December the

mass flow rate was the highest and specific energy consumption was the least observed.

Same for 23 November, where the mass flow rate was the lowest and specific energy

consumption the highest.

40

• As the temperature at inlet of the heating zone is increased, the specific energy con-

sumption decreases by a significant amount.

• Although there is, but nothing can be said about the dependence of Tout and heat losses

QW on specific energy consumption from Figure 5.4 and 5.5.

5.2 Results

The energy consumed in Tunnel Kiln 3 (TK3) of HEG Limited, Mandideep in a period of 30

days (from 23 Nov 2014 to 22 Dec 2014) was found to be 4.97 TW. The average specific

consumption was found to be 1.29 MJ/kg. The average efficiency of the kiln over 30 days

was 34.64%.

The above results are derived assuming a lot of things such as a steady state process which

was not observed in the real industrial plant due to manual operational challenges and care-

lessness. However, if the process is steady and the kiln is operated for 24 hours, the energy

consumption can be reduced and the productivity can be increased.

To avoid further complications in measuring the specific energy consumption the temperature

of the product and gas is assumed to vary only with the length of the kiln. This is not possible

because of the geometry of the loaded product.

Obviously, the temperature of the product and the temperature of the transportation material

is going to be different in a tunnel kiln because of different specific heat capacities but due to

unavailability of the material as well as the temperature of the transportation material, the

assumption had to be made, If it was available, the result would have been more accurate.

Other assumptions included constant heat transfer coefficient and constant specific heat ca-

pacity and density over a wide range of temperatures. Again, if sufficient literature was

available, the results would have been more accurate.

5.3 Findings

• The solid mass flow rate (or the tonnage per day) is inversely proportional to the specific

energy consumption. If the mass flow rate is increased, the specific energy consumption

decreases and vice versa.

41

• The specific energy consumption decreases when the temperature at the inlet of the

heating zone is high. A decent amount of energy is saved when the temperature is high

enough.

5.4 Recommendations

• The tonnage per day (solid mass flow rate) could be increased by a significant amount

because an inverse relation was observed between the specific energy consumption and

the mass flow rate. This could save a lot of energy.

• If the temperature at inlet of heating zone is increased, the energy consumption can be

reduced. So effective use of energy between gates A10 and A11 is recommended.

• As observed at the actual site, one of the reasons for heat loss was leakage of gases

through the gates due to increased pressure in the kiln. If this is improved, significant

energy would be saved.

• In a normal tunnel kiln operation, hot exhaust gases are released into the atmosphere.

The gases take away some part of the energy. A couple of heat exchangers in addition

to the one already present could be installed strategically to utilize this energy.

• There is a proper mechanism in place to open and close the doors of the tunnel kiln.

On consulting with the officials at HEG, it was found that a sufficient amount of energy

is convected out when the doors of the tunnel kiln are opened.

• Better fuel combustion would result in lower emissions from the kiln.

• The ducts and valves should distribute the heat uniformly so that the percentage of

good quality product is higher.

42

References

1. Hassanein Refaey, Mathematical Model to Anaylze the Heat Transfer in Tunnel Kilns for

Burning of Ceramics, PhD Dissertation 2013, Otto von Guericke University Magdeburg

2. Michael E. Hanyak, Jr., Excess Air = Excess Oxygen, 2010

3. Solids-Specific Heat Capcities, A comprehensive list of some common solids as brick,

cement, glass and many more - and their specific heat capacities - imperial and SI units

4. Greentech Knowledge Solutions Pvt Ltd, Factsheets about brick kilns in south and

south-east Asia, March 2014

5. V. Srivastava, Tunnel Kinl-for Rebaking (On HEG internal servers)

6. Reabaking Process Flow, Training Center, HEG Ltd, Mandideep

7. Trainin material available on HEG internal servers

8. http://www.hegltd.com

9. http://www.hegltd.com/brochure.pdf

43