OPTIMISATION OF THE ROTARY KILN

OPTIMISATION OF THE ROTARY KILN

(Work done at SESHASAYEE PAPER AND BOARD LIMITED, ERODE. ) PROJECT REPORT

SUBMITTED BY BALAJI.A BALAMURASU.R  SAMPATHKUMAR.G AYYAPAN.S SUBMITTED IN

TIAL FULFILMENT FOR THE AWARD OF THE PAR TI

DEGREE OF BACHELOR OF ENGINEERING IN THE

CHEMICAL ENGINEERING BRANCH SCHOOL OF CHEMICAL AND FOOD SCIENCE KONGU ENGINEERING COLLEGE (AUTONOMOUS) PERUNDURAI-638052 APRIL 2011

KONGU ENGINEERING COLLEGE PERUNDURAI, ERODE-638052

BONAFIDE CERTIFICATE

Certified that this project report ³ OPTIMISATION OF ROTARY KILN ³is the bonafide work of  BALAJI.A BALAMURASU.R

(07CHR009) (07CHR011)

SAMPATHKUMAR.G (07CHR040) AYYAPPAN.S

(07CHR008)

Who carried out the project work under my guidance.

SIGNATURE Dr.K.SARAVANAN

SIGNATURE Dr.K.CHANDRAMOHAN

HEAD OF THE DEPARTMENT DEPARTME NT

Department of chemical engineering School

of chemical and food science

Kongu engineering college Perundurai Erode -638 052

GUIDE

Professor, Dep of C of Chemical Eng School

of chemical and food science

Kongu engineering college Perundurai Erode-638052

KONGU ENGINEERING COLLEGE PERUNDURAI, ERODE-638052

BONAFIDE CERTIFICATE

Certified that this project report ³ OPTIMISATION OF ROTARY KILN ³is the bonafide work of  BALAJI.A BALAMURASU.R

(07CHR009) (07CHR011)

SAMPATHKUMAR.G (07CHR040) AYYAPPAN.S

(07CHR008)

Who carried out the project work under my guidance.

SIGNATURE Dr.K.SARAVANAN

SIGNATURE Dr.K.CHANDRAMOHAN

HEAD OF THE DEPARTMENT DEPARTME NT

Department of chemical engineering School

of chemical and food science

Kongu engineering college Perundurai Erode -638 052

GUIDE

Professor, Dep of C of Chemical Eng School

of chemical and food science

Kongu engineering college Perundurai Erode-638052

CERTIFICATION OF EVALUATION

College Name

: Kongu Kongu Engineering College.

Branch

: Chemical Engineering.

Semester

Sl.No

Names

: VIII.

of the students

Title

of the

project A

Name

of the guide

with designation

1.

Balaji.

(07chr009)

2.

Balamurasu.R

(07chr011) Optimisation of 

3.

Sampathkumar.G

(07chr040)

rotary kiln in

Professor 

4.

Ayyappan. S

(07chr008)

paper mill

Department of 

Dr.K.C Dr.K. ChandraMohan

chemical

The

reports of the project work submitted by the above students in partial

fulfilment for the award of Bachelor of Engineer En gineering ing in Chemical Engineering was evaluated and confirmed to be reports of the work done by the above students.

Submitted

(EXAMINER I)

foe the University Examinatio Examination n held on...... on...... ..............

(EXAMINER II)

ACKNOWLEDGEMENT:

We

wish

to

express

our

sincere

thanks

to

our

correspondent

Mr.R.K.VISHWANATHAN , and our generous contributors of  KVIT Trust

for providing us the necessary infrastructure to compl ete this project. We owe our deepest gratitude and sincere thanks to our beloved principal, professor  .S. KUPPUSWAMI B.E., M.sc (Engg). Dr.Ing (France) for his encouragement during the courses of study. We whole heartedly thank  Dr.K.SARAVANAN M.E., PhD head of the chemical engineering department for providing us the necessary facilities to do our project successfully. We extend our sincere thanks to our project review committee members Dr.K.SARAVANAN M.E., PhD and N.SIVARAJA SEKAR M.E., for their 

constant support to complete this project successfully. We take extreme delight in expressing our warm and sincere gratitude to our  guide Dr.K.CHANDRAMOHAN M.E., PhD for his valuable suggestion and guidance throughout our project duration, which have all been cardinal to finish this project successfully. We thank all teaching and non-teaching staff of the chemical department for  their support, encouragement and cooperation in letting us make use of various amenities in the department. Keeping in mind that ³little drops make the mighty ocean´, we also take this opportunity to thank all our friends who have helped in so many ways to finish this project successfully.

CONTENTS PAGE NO. SYNOPSIS

I

LIST OF FIGURES

II

CHAPTER 1 INTRODUCTION

1.1 ABOUT THE COMPANY

1

1.2 EXPANSION/MODER NISATION PROJECT

2

1.3 ENVIRONMENTAL PROTECTION

2

1.4 CHEMICAL RECOVER Y PROCESS

3

1.5 OR IGINOF PAPER 

4

1.6 OVER ALL PAPER  MANUFACTUR ING PROCESS IN SPB

5

1.7 MANUFACTURE

OF PAPER 

5

CHAPTER 2 LITERATURE SURVEY

2.1 ROTAR Y LIME K ILNS

10

2.2 NECESSITY OF ROTATION IN ROTAR Y LIME K ILN

11

2.3 ROTAR Y K ILN FLAMES

11

2.4 ROTAR Y K ILN CHAIN SYSTEMS

11

2.5. ROTAR Y K ILN REFRACTOR Y SYSTEMS

12

2.6. ROTAR Y K ILN PRODUCT COOLER S

13

2.7. EXTER NAL LIME MUD DR IES

13

2.8. LIME K ILN FANS

14

2.9. LIME K ILN HEAT RATE

15

2.10. EXAMPLE FOR K ILN HEAT RATE

16

2.11.

17

2.12.

MAJOR  HEAT

MINOR HEAT

RATE IMPROVEMENTS

RATE IMPROVEMENTS

18

2.13. LIME K ILN FUELS

CHAPTER. 3 DESIGN CALCULATION OF LIME KILN

3.1. MASS FLOW RELATIONSHIPS 3.2FUEL HEATING VALUE LOSSES 3.3 K ILN ENERGY BALANCE 3.4. HEAT RATE CALCULATION PARAMTER S 3.5. SHELL HEAT LOSS CALCULATION 3.6. RESIDENCE TIME CALCULATION

LIST OF FIGURES

GF

= Fuel flow rate

G CaO

= CaO production rate

G co2

= Co2 production rate

G Caco3 = Caco3 production rate Gd

= Dust flow rate

GW

= Flow rate of water in mud

Gi

= Inerts flow rate

Ga

= Air flow rate

19

Gcp

= Combustion product flow rate

HR

= Kiln Heat Rate

Hr 

= Heat of reaction of Caco 3 => CaO + Co2, at Tref 

HHV

= Higher heating value

LHV

= Lower heating value

Tp

= Kiln product temperature

T ref 

= Reference temperature

Tge

= Kiln exit gas temperature

Tsh

= shell temperature, 0C

FLHV

= Fraction of  HHV in LHV

f cp

= Fraction of  HHV in combustion products at

hgc

= Enthalpy of vaporization of water 

Qsh

= Shell heat loss, W

hi

= Heat of component i above T ref 

AFS = Stoichiometric air-to-fuel ratio e

= Excess air 

s

= Mud solids

a

= Lime availability

d

= Dust loss

Gm

= Mass flow rate

T gc

Cp Caco3 = Specific heat of Caco3 Cp CaO = Specific heat of CaO Cp co2

= Specific heat of Co2

Cp w

= Specific heat of water 

Cp Cp = Specific heat of combined product Cp i

= Specific heat of Inerts

Cps

= Specific heat of Steam

 = Shell emissivity X

= Residence time, hr 

D =Kiln outside diameter, m V = Wind velocity, m/s L = Length, m R

= Angle of inclination, degrees

q c = Convection heat transfer , W/m2 q r = Radiation heat transfer , W/m2

CHAPTER-1 INTRODUCTION 1.1

Seshasayee

ABOUT THE COMPANY

Paper and Boards Limited (SPB), the flagship company

belonging to 'ESVIN GROUP', operates an integrated pulp, paper and paper board Mill at Pallipalayam, Erode -638 007, District Namakkal, and Tamilnadu, India.

SPB, Brothers

incorporated in June 1960, was promoted by Seshasayee

(Pvt) Limited in association with a foreign collaborator  M/s

Parsons and White horse, South East Asia Inc., USA. After  commencement of commercial production, having fulfilled their  performance guarantee obligations , the foreign collaborators withdrew in 1969. SPB

commenced commercial production in December 1962, on

commissioning a 20000 tpa integrated facility, comprising a Pulp

Mill

and two Paper Machines (PM -1 and PM-2), capable of producing, writing, printing, Kraft and poster varieties of paper. The

Plant capacity was expanded to 35000

TPA

in 1967-68, by

modification of PM -2 and addition of a third Paper  Machine (PM -3). The cost of the expansion scheme, at Rs 34 Millions, was part financed by All India In

Financial Institutions (Rs 31 Millions). the second stage of expansion, undertaken in 1976, capacity was

enhanced to 55000

TPA,

through addition of a 60 tpd new Paper  Machine .Cost

of the project, including cost of a Chemical Recovery Boiler and other facilities for enhanced requirement of utilities, was estimated at Rs. 176

Millions.

1.2

EXPANSION/MODER NISATION PROJECT The

Company Embarked On An Expansion /

Modernizati on

Project To

Enhance Its Production Capacity From 60000 Tons Per Annum, To 1 , 15,000 Tons

Per Annum And To Upgrade Some Of The Existing Facilities, At An

Estimated Cost Of Rs 1890 The Said

Millions.

Expansion / Modernization Project Was Completed In December 2010

After Successful Trials, the Commercial Production Out Of the Machine

New

Paper 

Commenced On July 1, 2010.

The

Current Installed Capacity of the Company Stands at 1, 15,000 Tons

per Annum. At Present Five Machines Produce An Average Of 330 Tpd of Paper. To Meet

its Pulp Requirement, SPB Produce About 235 Tpd Of Wood And

Bagasse Unbleached Stationary

Pulp And Balanced Is Purchased. SPB Has Four 

Digesters, Brown Stack Washing (BWS), Screening & Centri ± 

Cleaning And Bleaching System (CEPHH) For Production of  Bleached Wood Pulp 1.3 ENVIRONMENTAL PROTEC TION The

Company attaches paramount importance to the conservation and

improvement of the environment. In its efforts to improve the environmental protection measures, the Company has installed: y

Two

Electro Static Precipitators For Its

y

An Anaerobic Lagoon For  High BOD Liquid Effluents

y

A Secondary Treatment System For Liquid Effluents And

y

An Electro Static Precipitator and Cascade Evaporator to the Recovery Boiler.

Boilers To

Control Dust Emissions

1.4. CHEMICAL RECOVER Y PROCESS The

excess black liquor is at about 15 % solids and is concentrated in

a multiple effect evaporator. After the first step the black liquor is about 20 30 % solids. At this concentration the rosin soap rises to the surface and is skimmed off. The collected soap is further processed to tall oil. Removal of  the soap improves the evaporation operation of the later effects. The

weak black liquor is further evaporated to 65% or even 80% solids and

burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process. Higher solids in the concentrated black liquor increases the energy and chemical efficiency of the recovery cycle, but also gives higher  viscosity and precipitation of solids . The

molten salts from the recovery boiler are dissolved in process water 

known as "weak wash". This process water, also known as "weak white liquor" is composed of all liquors used to wash lime mud and green liquor precipitates. The

resulting solution of sodium carbonate and sodi um sulphide is known as

"green liquor". This liquid is mixed with calcium oxide, which becomes calcium hydroxide in solution, to regenerate the white liquor used in the pulping process through an equilibrium reaction. Calcium carbonate precipitates from the white liquor and is recovered and heated in a limekiln where it is converted to calcium oxide (lime). Calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used.

The

recovery boiler also generates high pressure steam which is

fed to turbo generators, reducing the steam pressure for the mill use and generating electricity. A modern Kraft pulp mill is more than self-sufficient in its electrical generation and normally will provide a net flow of energy which can be used by an associated paper mill or sold to neighbouring industries.

1.5 OR IGIN OF PAPER  Paper derives from the word "papyrus". Today, paper includes a wide range of products with very different applications: communication, cultural, educational, artistic, hygienic, and sanitary as well as storage and transport of  all kinds of goods. It's almost impossible to imagine a life without paper.

Paper has a long history, beginning with the ancient Egyptians and continuing to the present day. After hand-made methods dominated for thousands of years, paper production became industrialized during the 19th century. Originally intended purely for writing and printing purposes, a wide variety of paper  grades and uses is now available to the consumer. Paper is a natural product, manufactured from a natural and renewable raw material, wood. The advantage of paper is that it is biodegradable and recyclable. I n this way, the paper industry is sustainable, from the forest through the production of paper, to the use and final recovery of the product.

Paper is an essential part of our lives and satisfies many human needs. We use it to store and communicate infor mation (newspapers, books, documents and writing paper), for cultural and artistic purposes, to transport and protect food .

.6. OVER ALL PAPER MANUFACTURING PR OCESS IN SP B Seshasayee Paper & using

Bagasse

Board

Li ited manufactures paper  of var ious qualities

as raw mater ial. The pr oduction pr ocess is as f ollows:

1.7

R AW

MANUFACTUR E

OF PAPER 

MATEIALS:

Hard wood and

Bagasse

are the two basic raw mater ials mostly

used in SPB f or manufactur ing paper. Following paper mak ing.

steps are

involved in the

Step

-1: CHIPING OF WOODEN LOGS

Wooden logs with a width of more than 6 cm are saw in a band saw Stripped

logs are chipped into small pieces by knives mounted in massive steel

wheels The

chips pass through vibrating screens, whereby undersized chips, dust etc

and oversized chips are rejected. Accepted chips are then stored in huge bins ready for the next process. Step-

2: COOK ING OF CHIPS

The

wood chips are cooked in huge pressurized vessels called digesters.

Some

digesters operate in batch manner and some in continuous processes.

There

are several variations of the cooking processes both for the batch and the

continuous digesters. Digesters producing 1,000 tons of pulp per day and more are common with the largest producing more than 3,500 tones of pulp per day. In

a continuous digester the materials are fed at a rate which allows the pulping

reaction to be complete by the time the materials exit the reactor. Typically delignification requires several hours at 130 to 180 °C (266 to 356 °F).

Under 

these conditions lignin and hemicelluloses degrade to give fragments that are soluble in the strongly basic liquid.

The

solid pulp (about 50% by weight based

on the dry wood chips) is collected and washed. At this point the pulp is quite brown and is known as brown stock. The

combined liquids, known as black liquor (so called because of its color ),

contain lignin fragments, carbohydrates from the breakdown of  hemicellulose,sodium carbonate, sodium sulphate and other inorganic salts. The Bagasse

is depithed in a wet depither, in which pith is removed. The depithed,

in which pith is removed .the depithed digester for further process.

Bagasse,

is either fed to the continuous

Step-3: BROWN STACK The

WASING:

brown stock from the blowing goes to the washing stages where the

used cooking liquors are separated from the cellulose fibres. Normally a pulp mill has 3-5 washing stages in series. Washing stages are also placed after  oxygen delignification and between the bleaching stages as well. Pulp washers use counter current flow between the stages such that the pulp moves in the opposite direction to the flow of washing waters. Several The

processes involved: thickening / dilution, displacement and diffusion.

dilution factor is the measure of the amount of water used in washing

compared with the theoretical amount required to displace the liquor from the thickened pulp. Lower dilution factor redu ces energy consumption, while higher  dilution factor normally gives cleaner pulp. Thorough washing of the pulp reduces the chemical oxygen demand ( COD). Step-4: SCREENING The

AND BLEACHING

pulp from the washers is screened in screens inn screens and cleaners to

remove the sand particles The

primary objective of bleaching is to achieve a whiter or brighter pulp. If a

mill produces brown paper such as linerb oard, a bleaching sequence is not required.

However,

if white paper such as writing or magazine paper is

produced, bleaching is required.

Bleaching

removes the lignin which remains

following digester cooking. Lignin is the source of colour and odour for pulp. It

is extensive reuse of washer filtrate to reduce fresh water usage. This

reduces the amount of effluent to be treated prior to discharge from the mill. Some

modern plants use totally enclosed pressure diffusion washers following

O2 delignification to further reduce toxic effluent. It

involves increased substitution of chlorine dioxide for chlorine gas.

Chlorine dioxide does not release the chlorine ions responsible for forming

dioxin. Although chlorine dioxide is more expensive to prod uce, it requires 2.5~3 times less to bleach the same amount of pulp. Some processes which use O2 delignification prior to bleaching have achieved 100% substitution of  chlorine dioxide. Although many changes have evolved which have decreased dioxin emissions, the future continues to hold change. Federal and state regulatory agencies continue to disagree on allowable emission limits. Future technology will continue to move toward zero discharge limits for dioxins and other by products of the bleaching process. Step

-5: STOCK PREPARATION The

stock is prepared is a series of steps that converts logs to a suitable

form for use in the pulp mill. Logs from the forest are usually received from a truck, rail car, or barge. Large overhead cranes are used to unload and sort the logs into piles for long or  short logs. Logs may pass through a slashed if a certain length is required. The

next step involves debarking which removes both dirt and bark from the

logs. The most common method employed is mechanical debarking via a barking drum. Logs are fed into the rotating cylinder and the rotating/tumbling action rubs the bark from the logs. The bark falls out of the cylinder via slots and debarked logs exit the opposite end of the cylinder.

Bark

is used as fuel for the

power boiler. Following debarking, the logs are fed to the chipper. The chipper  uses high speed rotating blades to reduce the logs to chips of a suitable size for  pulping. Chips are then screened for acceptable sizes by passing them over a set of vibratory screens. The rejects are returned for further chipping and acceptable chips stored in large outdoor piles or silos for pulp mill use.

Step-6:

PAPER MACHINING

The

Paper Machine is a very large piece of machinery. A typical machine is

about the length of two football pitches and around 4 meters wide. It can run up to speeds of 2000 m per minute - or 60 miles per hour! consists of 7 distinct sections.

The

The

machine itself 

flow box, wire, press section, drier section,

size press, calendar and reeling up. The

first section of the machine is called the 'Wet End'.

This

is where the

diluted stock first comes into contact with the paper machine. It is poured onto the machine by the flow box which is a collecting box for the dilute paper stock. A narrow aperture running across the width of the box allows the stock to flow onto the wire with the fibers distributed evenly over the whole width of the paper machine. Step

7: CHEMICAL MEASUR ING METHOD (KAPPA NUMBER) The

Kappa number is an indication of the residual lignin content

or bleach ability of wood pulp by a standardized analysis method. Measuring

method

The

Kappa

number

is

determined

and

applicable

to

all

kinds

of chemical and semi-chemical pulps and gives a Kappa number in the range of  1-100.

The

Kappa

number

is

a

measurement

of

how

much

a

standard permanganate solution that is consumed by the pulp. These compounds are formed during the chemical pulping process, from the hemicelluloses. Application The

Kappa number estimates the amount of chemicals r equired during

bleaching of wood Pulp to obtain a pulp with a given degree of whiteness.

CHAPTER ±2 LITERATURE SURVEY

LIME K ILN PR INCIPLES AND OPERATIONS 2.1 ROTAR Y LIME K ILNS Rotary lime kiln are large steel tubes that are lined on the inside with refractory bricks.

They

are slightly inclined from the horizontal and are slowly

rotated on a set of riding rings. Lime mud is introduced at the uphill, feed end and slowly makes it away to the discharge end due to the inclination and rotation. A burner is installed at the downhill or discharge end of the kiln fuel is burned to from an approximately cylindrical flame.

Heat

transfer from this

flame and the hot combustion gases tha t flow up the kiln dries, heats, and calcines the counter-flowing lime solids. Rotary kiln in the pulp and paper  industry range in size from 7 ft (2.1m) in diameter by 175 ft (53m) long to 13.5 ft (4m) in diameter by 400ft (122m) long .

The

refractory lining is from 6 in

(15.2cm) to 10 in (25.4cm) thick. Production capacities for these units range from 50 tons/day of capacity for these units range from 50 tons /day of  Cao (45 metric tons/day) to 450 tons/day of Cao (400 metric tons/day). The These

weight of the kiln is supported on the riding rings that encircle kiln.

riding rings contact carrying rolls supported by concrete piers. A large

electric motor operating through a reducing gear box and pinion drives a main gear attached to the kiln. Typical ly the kiln is driven at speeds of 0.5 to 2 RPM, often with variable speed arrangements.

Typically

transit times for the lime

through the kiln are from 1.5 hours to 4 hours under normal operating conditions.

This

is set by the speed and by the slope of the kiln, which is

between 1.5 0 and 3 0 (5/16 to 5/8 inches/foot).

2.2. NECESSITY OF ROTATION IN ROTAR Y LIME K ILN The

rotation of the kiln is necessary for the use of hoods and seals at each

end for connection to stationary ancillary equipment. At the hot end, the firing hood provides support for the burner and the flame management equipment, as well as openings and passages for the discharge of the reburned lime product. At the cold end, the hood provides openings for a lime mud feed screw or belt, a connection to the induced draft fan and an important seal to limit the flow of  tramp air. In order installations this often an enlarged chamber in which dust and mud can be sluiced out of this area.

Newer

installations incorporate smaller 

hoods to improve the seal and shorten the length of the mud screw or belt.

2.3. ROTAR Y K ILN FLAMES The

burner and flame play an important role in product quality and

refractory service life. As with all combustion fired heat exchange equipment, higher flame temperature means higher production capacity and efficiency. However,

excessive temperatures cause refractory damage, and over -burned,

slow-reacting lime product.

This

tradeoff in performance results in a

compromise in flame length. Slide 5 shows ske tches of three types of rotary kiln flames. Shorter flames are too hot and cause refractory damage and overburden lime, while longer flames cause some loss in production capacity and efficiency, and loss of control of the product quality. A compact, medium length flame approximately three times the kiln diameter in length is a good tradeoff between efficiency and refractory service life.

However,

irrespective of 

the shape, the flame must not touch the refractory, or serious refractory washing will occur.

2.4. ROTAR Y K ILN CHAIN SYSTEMS At the cold end of the kiln, the relatively low gas temperature hampers heat transfer. To improve this, a section of chain is hung from the shell in this part of the kiln. This chain is made up of links tha t are typically ¼ in. by 3 in(1.9cm x 7.6cm) .Hangers attach lengths of this chain directly to the kiln shell either from one end of this chain directly to the kiln shell either from one end or both ends. When chain is hung from one end it is referred to a s curtain chain. When hung from both ends it is most often called a garland system. Slide 6 shows sketches of these two types of chain systems, and shows the difference between high-density and low-density chain hanging arrangements. The

method of hanging the chain makes little difference in this

effectiveness as a regenerative best exchange surface. As long as the chain alternatively contacts the combustion gases and the lime mud as the kiln rotates, it is effective. Like any low-temperature heat exchanger, it is the available surface area that is most important to effectiveness. The chain surface area in a lime reburning kiln can represent two -thirds of the entire heat transfer surface.

2.5. ROTAR Y K ILN REFRACTOR Y SYSTEMS There

are several different types of refractory materials available for 

application in lime reburning, and usually two or three of these are used at different locations along the length of the kiln. A very common refractory system consists of bricks that are either shaped to fit the curvature of the shell or  are in thin wedges that can be laid in an arch pattern in order to produce a complete shell lining.

The

refractory bricks are composed of special heat -resistant and

chemical attack resistant materials that are most often alumina and silica compounds. Traditionally, the bricks in the hot sooner of the kiln near the flame

are composed of 70% alumina in order to resist the high temperatures and chemical attack in this region. About one-third of the way up the length of the kiln, this is changed to 40% alumina bricks, which have better insulating characteristics. Finally, a cast able low -temperature refractory is used in the chain section at the cold end of the kiln.

Many

modifi cations of this pattern are

now available including cast or packed refractories in place of bricks, or two brick systems that use insulating bricks against the steel shell and chemical ±  attack resistant bricks in contact with the lime solids and combustion gases.

The

ability of the refractory lining to withstand chemical attack by the lime

and its constituents is crucial to the service of life of this part of the kiln. Although sudden changes in temperature can damage the lining, it is p rimarily due to chemical attack that refractory is washed from the kiln and requires periodic replacement. Quite aside from the increased heat loss associated with thin, worn refractory lining, it is important for structural reasons to maintain the lining to avoid exposure of the steel shell to combustion temperatures.

Refractory wastage y

Most

refractory damage due to wastage

- Smooth,´birdbath´refractory thinning y

Due to high temperature chemical attack 

y

Product refractory with a coating of lime

y

Operate kiln for lower refractory face temp - Low primary air flow, avoid flame impingement - Cold ±end excess O2 > 1.5%

2.6. ROTAR Y K ILN PRODUCT COOLER S All modern kilns are being offered with product coolers. coolers are tubes attached to the kiln shell and rotating with kiln.

Satellite The

hot

reburned lime product drops through holes in the shell just uphill from the lip of  the kiln into the tube coolers. Internal structures move the lime back uphill in these tubes as they orbit with the kiln rotation. They also bring the hot lime into contact with air, which preheats this combustion air and results in a substantial improvement in energy efficiency for the kiln.

There

are now two types of 

product coolers for lime reburning kilns that can be installed on new kilns or  retrofit to older kilns.

2.7. EXTER NAL LIME MUD DR IES The The

wet lime mud is introduced into the duct leading to a cyclone.

mud dries in flight, separates from the gases in the cyclone, and flows into

the kiln as a dry powder. The

lime dust that escapes the cyclone is usually captured in an electrostatic

precipitator and also enters the kiln dry. With this system, chains are not needed to dry the lime mud; the entire kiln length is available for heating and calcining. 2.8. LIME K ILN FANS The

fans at the hot end and cold end of the kiln. The primary Air 

(PA) fan is at the hot end and supplies a small amount of air to the burner for  flame shaping and stability.

Typically

the PA fan supplies only 5% to 25% of 

the total air required for complete combustion. The induced Draft ( ID) fan at the cold end of the kiln is the main gas moving fan.

It

pulls the combustion

products, carbon dioxide from calcining, and the water vapor from the wet mud out of the cold end of the kiln.

The ID

fan is used to control the total air flow

into the kiln for combustion so controls the excess air or excess oxygen in the flue gas from the kiln.

Fans, draft and O2 y

PA fan only for flame shaping y

The

ID

fan is main air moving fan -

Use ID

fan to control O 2

-

Often limited production capacity

-

Wet scrubber is main for restriction

capacity of the ID fan often limits the production capacity of the kiln.

When the ID fan reaches its maximum capacity, no more combustion air can be brought into the kiln.

This

limits the fuel firing rater and the lime production

rate. For many installations the wet scrubber that follows the ID fan in the flue gas system is the biggest resistance to flue gas flow, so scan limits the ID fan capacity. Changes in wet scrubber pressure-drop for emission control or  changes in fuel type can decrease the ID fan capacity and kiln production capacity. 2.9. LIME K ILN HEAT RATE The

energy efficiency of lime kilns is expressed as the heat Rate.

is the reciprocal of energy efficiency, and is usually expressed as of  CaO. or as GJ/tone of  CaO. Lower values of 

Heat

MM Btu/ton

Rate indicate more

efficient operation. Lime Kiln Heat Rate y

Heat

rate is a measure of energy effiency

-Units are MM Btu/ton Cao or GJ/tone Cao -Often stated as MM Btu/ton ³product´ y

Typical

range

-5to 9 MM Btu/ton Cao - 5.8 to 10.5 GJ/tonnes Cao - Lower is better 

Heat

The

main chemical reaction in a lime kiln is calcining, the conversion of the

calcium carbonate (Caco 3) in the lime mud into calcium oxide (Cao) in the kiln product. Energy is required to cause this endothermic reaction to occur, but there are other energy components to the overall energy demand of the kiln. Lime Kiln Energy Balance Components y

Drying

y

Calcining :CaCO3 + heat-> CaO +CO2

y

Losses Heat Heat Heat

loss through shell loss in hot lime product loss with gas and dust exiting at cold end

y

A portion of fuel higher heating value is unavailable

y

IIIIV

y

Only Lower  Heating Values (LHV) is actually available

is measured and reported

2.10. EXAMPLE FOR K ILN HEAT RATE Kiln Parameters Fust

Net

Gas

Production

25TPD

Mud dry solids

78%

Kiln exit O 2

3%

Lime availability

85%

Dust loss

18%

Product temp

6000F

Cold and gas temp

6000F

Shell heat loss

11.4 MM Btu/hr 

Lime Kiln Heat Rate Overall Heat Rate = 8.4 MM Btu/CaO

contribution

Heat

fraction

Remedy

2.82

34%

No

evap 1.75

21%

Increase

Rate Calcining

change possible

reaction Heat

to

mud

dry

exit

gas

solids

water

1.50

Heat in exit gas

16%

Decrease temp

Shell heat loss

1.10

13%

Insulating

refractory

LHV/HHV loss

0.82

10%

Change fuel

Heat in product

0.29

3%

Product coolers

Heat in dust

0.11

1%

Improve

chain system

CAUTION WITH HEAT RATE CALCULATION y

Gives instantaneous value

y

-Does include down time, upsets

y

Good to asses where improvements needed

y

Some

changes a\affect more than one parameter 

-refractory changes loss & exit temp[ Mud

solids affects evaporates loss and exit temp

Fuel changes LHV/HHV, exit gas temp, and diet loss.

2.11. MAJOR HEAT RATE IMPROVE MENTS Kiln

Base

Refractory

Fuel Type

Chains

Fuel

Nal

Nal

Fuel Gas

Net

Production,

250

250

260

250

Mud dry solids

75%

78%

78%

78%

Kiln exit o2

3%

3%

3?%

3%

Lime

85%

85%

85%

85%

Dust loss

19%

1`2%

18%

18%

Product

600

600

600

600

Cold and gas 600

475

500

525

6.3

11.4

11.4

7.0

7.4

8.0

-17%

-11%

-4%

ZGas

Gas

Gas

Rate TPD

availability

temp,F

temp,F Shell

Heat 11.4

Loss,MM Btu/tu Heat Rate,MM 8.4 Btu/tu change

Improving

the Kiln refractory to reduce the shell heat loss obviously has a very

impact on Kiln important.

Heat

Rate, but the fuel used in firing the kiln is almost as

2.12. MINOR HEAT RATE IMPROVE MENTS Kiln

Base

Refractory

Fuel Type

Chains

Fuel

Nal

Nal

Fuwel Gas

Net

Production,

250

250

260

250

75%

78%

78%

78%

Kiln exit o 2

3%

2%

3%

3%

Lime

85%

85%

85%

85%

Dust loss

18%

18%

12%

18%

Product

600

600

600

600

Cold and gas 600

575

600

600

11.4

11.4

11.4

8.1

8.2

8.2

-3%

-3%

-2%

ZGas

Gas

Gas

Rate TPD Muddry solids

availability

temp,F

temp,F Shell

Heat 11.4

Loss,MM Btu/tu

6.4

Heat Rate,MM Btu/tu change

2.13. LIME K ILN FUELS The

common lime kiln fuels used in the pulp and paper industry.

Natural

gas and fuel oil are widely used, but a growing number of Kilns are at least partially fired with petroleum coke. Pet coke is an efficient, though messy,

Kiln fuel as long as the sulfur and metals contamination are not too high.

The

sulfur content of petroleum coke slightly derates the Kiln due to the formation of  CaSO4 and the metals require somewhat higher use of purchased lime, but these two are offset by the lower cost and better efficiency. Common Lime Reburning Kiln Fuels y

Nat

gas and fuel oil or most common

y

Fuel Oil in more efficient, gives higher capacity

y

Petroleum`, Coke

y

Many

applications, low cost

y

Safer

and metals can be high

y

Thermal Nox

y

Improves

can be high

best rates

Wood and bark powder have been fired directly in kilns as the main kiln fuel. The NPEs

in these fuels are usually low enough so that modest increase in like

makeup can control build up of NPEs in the recovery loop. There

are several schemes to separate lignin from the black liquor and use it

as product or as fuel for the like kiln.

Tests

of lignin as a fuel both in test

facility and in the field have shown this is feasible, though the sulfur content is relatively high. Pyrolysis oils have also been proposed for lime kilns but fuel handling problems need to be overcome to make this attractive. Other Solid / Liquid Kiln Fuels y

Wood not bark powder  -

NPEs

can be high

y

Lime purge & makeup sanded to central NPEs

y

Lignin

y

sulfur can be high

y

Pyrolysis Oils -

Not

currently and, pH & possible issues

On the gasification of wood, coal and other materials, which have been used for  many years to provide clean fuel -gas for firing lime kilns.

The

lower cost of the

gasification fuel offsets the high capital¶s cost of the equipment needed to gasify these fuels. Good, stable operations possible with gasification with production capacity and Heat Rate similar to that or natural.

The

y

Gasification Fuels Coal, wood and other fuels can be gasified

y

Fuel-gas can be used to fire kiln -

Similar

to natural gas

-

Lower inert level, well established technology

-

On-line availability ± 85%

-

Wet gasifier feedstock¶s derate kiln

-

Burner

and chains must be designed for fuel -gas

³fuels´ that ate generated in the pulp mill. Turpentine, methanol,

stripper off-gas ( SOG), and non condensable gases (NCG) have all been burned in lime reburning kilns. The energy content of these ³fuels´ varies considerably, but each makes a contribution to over all heat input. These materials contain some sulphur that can derate the kiln capacity, and all of them lower the Rate of the kiln. Fuels from the pulp mill y

Turpentine

y

Stripper

y

Non

-

off-gas ( SOG)

± condensable gas (NCG) Can be wet and sulfur level can be high

Tall

y

or methanol liquid

oil and tall oil pitch

-

Can fire 100% tall oil, similar to fuel oil

-

16,000Btu/lb and low sulfur 

Heat

CHAPTER. 3. DESIGN CALCULATION OF LIME KILN

CALCUATION 3.1. MASS FLOW RELATIONSHIPS 







    

=250+ (











)*(

)

= 598.31 tons /day 







  





 

= 598.31 (









)*(

)

= 107.69 tons /day 

  

  







 



= 598.31*(









)*(

= 168.75 tons/day 





= 598.31* (



  

 

= 44.12 tons/day

)



)*(



)







= 598.31*



 

 

= 196.428 tons/day 



 

     

= 36.83        = 432.8 tons/day 3.2FUEL HEATING VALUE LOSSES y

Losses to use of higher heating value  

 

=         = 0.953 y

Losses due to flow of combustion products  

           

=                = 0.0762.

3.3 K ILN ENERGY BALANCE  

  



 



   



  





                 

*4.27*10^7*

(0.983-0.0762) = 250      

    ) +44.12(80-25)*1046 +168.75     + 107.69*795.5(175-25) +4704.5 G f  = 36.74 tons/day. Heat

rate calculation

HR= 

 

= (      = 6275192 J/kg 3.4.

HEAT

RATE CALCULATION PARAMTER S

FUEL CONSTANTS Nat.gas

Fuel oil

Units in SI

HHV

5.37E+07

4.27E+07

J/kg

LHV

4.86E+07

4.07E+07

J/kg

AFS

16.80

13.44

PROPER TIES

Combined product specific heat

Cp Cp

1.267

J/kg/0C

Reburn specific heat

Cp CaO

989

J/kg/ C

Co2 specific heat

Cp co2

919

J/kg/ C

Inerts specific heat

Cp i

1,046

J/kg/0C

Steam specific heat

Cp s

1,991

J/kg/0C

Heat of calculation

Hr 

3,270,045

J/kg

Enthalpy of vaporization

hfg

2,439,465

J/kg

0

0

3.5. SHELL HEAT LOSS CALCULATION y

Convection, W/m

2

              

= 1.175            =

y

2

W/m

1101.3

Radiation, W/m2            

= 5.668 *  *0.75*      2

= 1025.8 W/m