OPTIMISATION OF THE ROTARY KILN
OPTIMISATION OF THE ROTARY KILN
((Work done atWork done at SESHASAYEE PAPER AND BOARD LIMITED,SESHASAYEE PAPER AND BOARD LIMITED,ERODE.ERODE.))
PROJECT REPORT PROJECT REPORT SUBMITTED BY SUBMITTED BY BALAJI.A BALAJI.A BALAMURASU.R BALAMURASU.R SAMPATHKUMAR.G SAMPATHKUMAR.G AYYAPAN.S AYYAPAN.S SUBMITT
SUBMITTEDED ININ PAR PAR TITIAL FAL FUULFLFIILLMMEENTNT FOR FOR THTHE AWARD OFE AWARD OF THTHEE
DEGREE OF DEGREE OF BACHELOR OF ENGINEERING BACHELOR OF ENGINEERING IN IN THTHEE
CHEMICAL ENGINEERING BRANCH CHEMICAL ENGINEERING BRANCH
SCHOOL OF CHEMICAL AND FOOD SCIENCE SCHOOL OF CHEMICAL AND FOOD SCIENCE
KONGU ENGINEERING COLLEGE KONGU ENGINEERING COLLEGE
(AUTONOMOUS) (AUTONOMOUS) PERUNDURAI-638052 PERUNDURAI-638052 APRIL 2011 APRIL 2011
KONGU ENGINEERING COLLEGE
KONGU ENGINEERING COLLEGE
PERUNDURAI, ERODE-638052
PERUNDURAI, ERODE-638052
BONAFIDE CERTIFICATE BONAFIDE CERTIFICATE
C
Certified that this project report ³ertified that this project report ³OPTIMISATION OF ROTARY KILNOPTIMISATION OF ROTARY KILN³is³is the bonafide work of
the bonafide work of
BALAJI.A (07CHR009) BALAJI.A (07CHR009) BALAMURASU.R (07CHR011) BALAMURASU.R (07CHR011) SAMPATHKUMAR.G (07CHR040) SAMPATHKUMAR.G (07CHR040) AYYAPPAN.S (07CHR008) AYYAPPAN.S (07CHR008)
Who carried out the project work under my guidance. Who carried out the project work under my guidance.
SIGNATURE SIGNATURE
SIGNATURE SIGNATURE
Dr.K.SARAVANAN Dr.K.CHANDRAMOHAN
Dr.K.SARAVANAN Dr.K.CHANDRAMOHAN
HEAD
HEAD OF OF THE THE DEPARTMEDEPARTMENT NT GUIDEGUIDE
Department
Department of of chemical chemical engineering engineering Professor, Professor, Dep Dep of of CChemical Enghemical Eng
S
School of chemical and food sciencechool of chemical and food science SSchool of chemical and food sciencechool of chemical and food science
Kongu
Kongu engineering engineering college college Kongu Kongu engineering engineering collegecollege
Perundurai Perundurai
Perundurai Perundurai
Erode
KONGU ENGINEERING COLLEGE
KONGU ENGINEERING COLLEGE
PERUNDURAI, ERODE-638052
PERUNDURAI, ERODE-638052
BONAFIDE CERTIFICATE BONAFIDE CERTIFICATE
C
Certified that this project report ³ertified that this project report ³OPTIMISATION OF ROTARY KILNOPTIMISATION OF ROTARY KILN³is³is the bonafide work of
the bonafide work of
BALAJI.A (07CHR009) BALAJI.A (07CHR009) BALAMURASU.R (07CHR011) BALAMURASU.R (07CHR011) SAMPATHKUMAR.G (07CHR040) SAMPATHKUMAR.G (07CHR040) AYYAPPAN.S (07CHR008) AYYAPPAN.S (07CHR008)
Who carried out the project work under my guidance. Who carried out the project work under my guidance.
SIGNATURE SIGNATURE
SIGNATURE SIGNATURE
Dr.K.SARAVANAN Dr.K.CHANDRAMOHAN
Dr.K.SARAVANAN Dr.K.CHANDRAMOHAN
HEAD
HEAD OF OF THE THE DEPARTMEDEPARTMENT NT GUIDEGUIDE
Department
Department of of chemical chemical engineering engineering Professor, Professor, Dep Dep of of CChemical Enghemical Eng
S
School of chemical and food sciencechool of chemical and food science SSchool of chemical and food sciencechool of chemical and food science
Kongu
Kongu engineering engineering college college Kongu Kongu engineering engineering collegecollege
Perundurai Perundurai
Perundurai Perundurai
Erode
CERTIFICATION OF EVALUATION
CERTIFICATION OF EVALUATION
C
Collegeollege NName ame : : KongKongu u EngineeringEngineering CCollege.ollege. B
Branch ranch :: CChemical Engineering.hemical Engineering. S
Semester emester : : VVIIIIII..
S
Sl.l.NNoo NNames of the studentsames of the students TTitle of theitle of the
project project
N
Name of the guideame of the guide
with designation with designation 1. 1. 2. 2. 3. 3. 4. 4. B Balaji. alaji. A A (07chr009)(07chr009) B Balamurasu.R alamurasu.R (07chr011)(07chr011) S Sampathkumar.G (07chr040)ampathkumar.G (07chr040) Ayyappan. Ayyappan.SS (07chr008)(07chr008) Optimisation of Optimisation of rotary kiln in rotary kiln in paper mill paper mill Dr.K.
Dr.K.CChandrahandraMMohanohan
Professor Professor Department of Department of chemical chemical T
The reports of the project work submitted by the above students in partialhe reports of the project work submitted by the above students in partial
fulfilment for the award of
fulfilment for the award of BBachelor of Enachelor of Engineergineering ining in Chemical EngineeringChemical Engineering
was evaluated and confirmed to be reports of the work done by the above was evaluated and confirmed to be reports of the work done by the above
students. students.
S
Submitted foe theubmitted foe the UUniversity Eniversity Examinatioxamination held onn held on...... ......
(EXA
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
1.7MANUFACTURE 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 ILNFLAMES 11
2.4 ROTAR YK ILN CHAIN SYSTEMS 11
2.5. ROTAR YK ILN REFRACTOR Y SYSTEMS 12
2.6. ROTAR Y K ILNPRODUCT 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. MAJOR HEAT RATE IMPROVEMENTS 17
2.13. LIME K ILN FUELS 19
CHAPTER. 3
DESIGN CALCULATION OF LIME KILN
3.1. MASS FLOW RELATIONSHIPS
3.2FUEL HEATING VALUE LOSSES
3.3 K ILNENERGY BALANCE
3.4. HEAT RATE CALCULATION PARAMTER S
3.5. SHELL HEAT LOSS CALCULATION
3.6. RESIDENCETIME CALCULATION
LIST OF FIGURES
GF = Fuel flow rate
GCaO = CaO production rate
Gco2 = 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
Gcp = Combustion product flow rate
HR = Kiln Heat Rate
Hr =Heat of reaction of Caco3=> CaO + Co2, atTref HHV = Higher heating value
LHV = Lower heating value
Tp = Kiln product temperature Tref = 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 Tgc 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
CpCaco3 = 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
CHAPTER-1
INTRODUCTION
1.1 ABOUT THE COMPANY
Seshasayee Paper andBoards Limited (SPB), the flagship company
belonging to 'ESVIN GROUP', operates an integrated pulp, paper and
paper boardMill at Pallipalayam, Erode -638 007, DistrictNamakkal, and Tamilnadu, India.
SPB, incorporated in June 1960, was promoted by Seshasayee Brothers (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 Financial Institutions (Rs 31 Millions).
In 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
1.2 EXPANSION/MODER NISATIONPROJECT
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 Millions.
The Said Expansion / Modernization Project Was Completed In December 2010
After SuccessfulTrials, the Commercial Production Out Of the New Paper Machine 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
BagasseUnbleached Pulp And Balanced Is Purchased. SPB Has Four Stationary Digesters, Brown Stack Washing (BWS), Screening & Centri ±
Cleaning And Bleaching System (CEPHH) For Production of Bleached Wood
Pulp
1.3 ENVIRONMENTAL PROTECTION
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 Boilers To Control Dust Emissions 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
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. In 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 SPB
Seshasayee Paper & Board Li ited manufactures paper of var ious qualities
using Bagasse as raw mater ial. The pr oduction pr ocess is as f ollows:
1.7 MANUFACTUR E OF PAPER
R AW MATEIALS:
Hard wood and Bagasse are the two basic raw mater ials mostly
used in SPB f or manufactur ing paper. Following 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 Bagasse, is either fed to the continuous
Step-3: BROWN STACK WASING:
The 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 processes involved: thickening / dilution, displacement and diffusion. The 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 ANDBLEACHING
The 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.
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
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! The machine itself
consists of 7 distinct sections. The 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
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 weight of the kiln is supported on the riding rings that encircle kiln. These 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
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 ILNFLAMES
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 ILNREFRACTOR 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
2.6. ROTAR Y K ILN PRODUCT COOLER S
All modern kilns are being offered with product coolers. Satellite
coolers are tubes attached to the kiln shell and rotating with kiln. 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 wet lime mud is introduced into the duct leading to a cyclone. The 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 ILNFANS
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.
The 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. Heat
is the reciprocal of energy efficiency, and is usually expressed as MM Btu/ton
of CaO. or as GJ/tone of CaO. Lower values of Heat Rate indicate more
efficient operation.
Fans, draft and O2
y PA fan only for flame shaping y ID fan is main air moving fan
- Use ID fan to control O2
- Often limited production capacity - Wet scrubber is main for restriction
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 asMM Btu/ton ³product´
y Typical range
-5to 9 MM Btu/ton Cao
- 5.8 to 10.5 GJ/tonnes Cao - Lower is better
The main chemical reaction in a lime kiln is calcining, the conversion of the
calcium carbonate (Caco3) 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.
2.10. EXAMPLE FOR K ILN HEAT RATE
Kiln Parameters
Fust Net Gas
Production 25TPD
Mud dry solids 78%
Kiln exit O2 3%
Lime availability 85%
Dust loss 18%
Product temp 6000F
Cold and gas temp 6000F
Shell heat loss 11.4MM Btu/hr
Lime Kiln Energy Balance Components
y Drying
y Calcining :CaCO3 + heat-> CaO +CO2 y Losses
Heat loss through shell
Heat loss in hot lime product
Heat loss with gas and dust exiting at cold end
y A portion of fuel higher heating value is unavailable y IIIIV is measured and reported
Lime Kiln Heat Rate
Overall Heat Rate = 8.4 MM Btu/CaO
contribution Heat Rate fraction Remedy Calcining reaction 2.82 34% No change possible Heat to evap water
1.75 21% Increase mud dry
solids
Heat in exit gas 1.50 16% Decrease exit gas 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
2.11. MAJOR HEAT RATE IMPROVEMENTS
Kiln Base Refractory Fuel Type Chains Fuel Nal ZGas Nal Gas Fuel Gas Net Gas
Production, Rate TPD
250 250 260 250
Mud dry solids 75% 78% 78% 78%
Kiln exit o2 3% 3% 3?% 3% Lime availability 85% 85% 85% 85% Dust loss 19% 1`2% 18% 18% Product temp,F 600 600 600 600
Cold and gas temp,F 600 475 500 525 Shell Heat Loss,MM Btu/tu 11.4 6.3 11.4 11.4 Heat Rate,MM Btu/tu 8.4 7.0 7.4 8.0 change -17% -11% -4%
Improving the Kiln refractory to reduce the shell heat loss obviously has a very
impact on Kiln Heat Rate, but the fuel used in firing the kiln is almost as
2.12. MINOR HEAT RATE IMPROVEMENTS
Kiln Base Refractory Fuel Type Chains Fuel Nal ZGas Nal Gas Fuwel Gas Net Gas
Production, Rate TPD 250 250 260 250 Muddry solids 75% 78% 78% 78% Kiln exit o2 3% 2% 3% 3% Lime availability 85% 85% 85% 85% Dust loss 18% 18% 12% 18% Product temp,F 600 600 600 600
Cold and gas temp,F 600 575 600 600 Shell Heat Loss,MM Btu/tu 11.4 11.4 11.4 11.4 Heat Rate,MM Btu/tu 6.4 8.1 8.2 8.2 change -3% -3% -2% 2.13. LIME K ILNFUELS
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.
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.
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 can be high y Improves best rates
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
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 andHeat Rate similar to that or natural.
The 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 Heat
Rate of the kiln.
Gasification Fuels
y 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
Fuels from the pulp mill
y Turpentine or methanol liquid y Stripper off-gas (SOG)
y Non ± condensable gas (NCG)
- Can be wet and sulfur level can be high
y Tall oil and tall oil pitch
- Can fire 100% tall oil, similar to fuel oil - 16,000Btu/lb and low sulfur
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 3.2FUEL 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 ILNENERGY 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 Gf = 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
PROPER TIES
Combined product specific heat Cp Cp 1.267 J/kg/0C
Reburn specific heat Cp CaO 989 J/kg/0C
Co2 specific heat Cpco2 919 J/kg/0C
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 3.5. SHELL HEAT LOSS CALCULATION y Convection, W/m2 = 1.175 = 1101.3 W/m2 y Radiation, W/m2 = 5.668 * *0.75* = 1025.8 W/m2