PROJECT

REPORT ON RECOVERY PROCESS AT NPM ANALJYOTI BARUAH S. NAVEEN KUMAR

PROJECT REPORT ON BLACK LIQUOR RECOVERY

PROCESS AT NAGAON PAPER MILL, HPCL

Under the guidance of

MR. A.K. GARG

Senior Manager (Recovery)

Nagaon Paper Mill

Submitted by

ANAL JYOTI BARUAH S.NAVEEN KUMAR

ROLL- UG116203 ROLL- UG116135

CLASS- II/IV B.Tech- Chemical CLASS- II/IV B.Tech- Chemical

NIT Warangal NIT Warangal


CONTENTS

INTRODUCTION 1

RECOVERY PLANT 4

1. EVAPORATORS 8

1.1 MELONE FILTER 11

1.2 LONG TUBE EVAPORATOR 12

1.3 FALLING FILM EVAPORATOR 15

1.4 PREHEATER 18

1.5 CONDENSER 19

2. BOILER 20

2.1 REACTIONS INVOLVED 23

2.2 DIFFERENT PARTS OF THE BOILER 24

2.3 ZONES IN THE BOILER 25

2.4 BLACK LIQUOR FIRING 26

2.5 COMBUSTION 27

2.6 ECONOMIZER 29

2.7 SUPERHEATER 30

2.8 ELECTROSTATI PRECIPITATOR 31

2.9 SOOT BLOWER 32

3. CAUSTICIZING PLANT 33

3.1 LIME SLAKER 38

3.2 CAUSTICIZER 40

3.3 WHITE LIQUOR CLARIFIER 42

3.4 LIME MUD WASHER 43

4. LIME MUD REBURNING PLANT 44

4.1 LIME MUD STORAGE TANK 48

4.2 LIME MUD PRECOAT FILTER 49

4.3 LIME KILN 51


INTRODUCTION

Hindustan Paper Corporation (HPC) was launched on May 29, 1970 and is now

synonymous with the quest for quality paper, especially for mass consumption.

HPC today owns four paper mills: -

Two are directly managed units-

1. Nagaon Paper Mill.

2. Cachar Paper Mill.

Two are managed through subsidiary companies-

3. Hindustan Newsprint Limited (HNL).

4. Nagaland Pulp & Paper Company Limited (NPPL).

The Nagaon Paper Mill (NPM) is a unit of the Hindustan Paper Corporation Ltd set

up in the northeastern part of the country. The mill is located at Jagiroad in the

Morigaon district of Assam at a distance of about 70 kms from Guwahati. The mill site

was basically selected by a group of experts on techno-economical grounds to boost

the economy of the region and to use the abundant bamboo crop of the northeastern

forests.

The N.P.M occupies a total land area of 428.10 acres for area mill and township. It is a

public sector undertaking under Ministry of Heavy Industry, Govt. of India. The mill

was commissioned in Oct 1985. At the time of commissioning, it was India’s largest

paper mill. It has a capacity of producing 300 Metric Tonnes (MT) of finished paper

per day and 100,000 MT from the year 2003-2004 (writing and printing including

20,000 newsprint)

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The Kraft Process is the dominant pulping process in the pulp and paper industry,

which uses sodium hydroxide (NaOH) and sodium sulphide (Na2O) to convert

wood into pulp. In this process about half of the wood is dissolved, and together

with the spent pulping chemicals, forms a liquid stream called as weak black liquor.

The weak black liquor is separated from the pulp by washing and is sent to the

Kraft recovery system, where the inorganic pulping chemicals are recovered for

reuse, while the dissolved organics are used as a fuel to make steam and power.

Pulp Mill performs the preparation of the raw material (in this case bamboo) for

making paper which means that the raw material is converted to slurry form after

cooking and then bleaching with chemicals it to give brightness to the pulp.

Pulp mill can be divided into four sections as follows:

Chipper House

Digester House

Washing and Screening

Bleaching

The Paper Machine is a device for continuously forming, dewatering,

pressing, and drying a web of paper fibers. Paper machine at NPM consist of

Fourdrinier type wire part, where a dilute suspension of fibers (typically 0.3-

0.6% consistency) is applied and water is removed by gravity, or the

developed by hydrofoils or suction equipment, and the drilled couch. The web

at this point is 18-23% consistency. More water is squeezed out in the press

section to a consistency of 40-45 %. Finally the sheet is dried with steam

heating in the dryer section. There are two paper machines at NPM.

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3


RECOVERY PLANT

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INTRODUCTION: RECOVERY

Sodium compounds used in the cooking of sulfate pulp are a significant cost

item. Even in the early days of pulping, the sodium compounds were

recovered from the black liquor and recycled back into the process.

The main objective of Recovery section at Nagaon Paper Mill is to recover

chemicals used in cooking of fibrous raw materials and to recover and

beneficially use the thermal energy from combustion of organics present in the

black liquor.

This process eliminates pollution hazards of spent liquor.

Black liquor

formed when white liquor used in sulfate pulping reacts with lignin and

other ingredients in the wood.

a black aqueous solution of lignin residues, hemicelluloses, and

the inorganic chemicals(mainly sulfur based) used in the process. The

black liquor comprises 15% solids by weight of which 10% are inorganic

and 5% are organic.

approximately 7 tonnes is produced in the manufacture of one tonne of

pulp.

obtained from brown stock washing section of the Paper Mill. It is known

as Weak Black Liquor(WBL) as its concentration is low

After the passage through the evaporators it is known as Strong Black

Liqour (SBL).

5

http://en.wikipedia.org/wiki/Inorganic_compound


Major sections

Recovery plant at NPM can be summarized in the following sections

Evaporation plant

Recovery boiler

Causticizing plant

Lime mud reburning plant

Brief description of the recovery process

Weak black liquor from brown stock washers is concentrated in multi-effect

evaporators. Concentrated black liquor is sprayed into the lower part of the

recovery boiler where it is burned in an oxygen deficient environment so that

Na2S is formed. The extent of sulphide formation is measured by the reduction

efficiency, typically over 90%. The inorganic sodium and sulphur are recovered

as a molten smelt which consists mostly of Na2S and sodium carbonate

(Na2CO3). The molten smelt enters a dissolving tank where it is dissolved in

water to form green liquor. The green liquor is then sent to the causticizing

plant, where it is reacted with lime, CaO, to convert the Na2CO3 to NaOH.

Conversion is measured by causticizing efficiency, typically 80 to 83%. The

Na2S passes through the causticizing step unchanged.

The causticized green liquor is known as “white liquor” which contains mostly

NaOH and Na2S. It is returned to the digester for reuse in pulping. The

precipitated CaCO3 (lime mud) from the causticizing reaction is washed, and

sent to a lime kiln where it is heated to a high temperature to regenerate CaO for

reuse.

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Recovery Flow Diagram

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1. EVAPORATION PLANT

Weak Black Liquor (WBL) from the brown stock washers is typically at 13-

18%TS. Most of this water content must be evaporated to produce a material

with high enough solids to support effective combustion in the recovery boiler,

typically between 65% and 80% TS.

Major Components

Multiple effect Long Tube Vertical(LVT) or Rising Film Evaporators(6

Nos)

Falling Film Evaporators(3 Nos)

Preheaters(4 Nos)

Condensers( 2 Nos)

Brief Description of the Evaporation Plant at NPM

WBL from Pulp Mill at 09-13oTw is received in WBL Tank1 and Tank2. From

the tanks, it is taken to three Malone filters for removing fine pulp and filtrate

WBL is sent to WBL Tank3 and Tank4.

From these tanks, WBL is taken to Multiple Effect Evaporator including FF

concentrator to concentrate the liquor to 62-78oTw by utilizing standard steam(

140-155oC) in one effect and consequent vapour of each effect is utilized as

heating medium for next effects. Thus, produced SBL is stored in two SBL

tanks for further use in Recovery Boiler. Pure condensate produced is sent to

DM Plant and combined condensate is sent to Causticizing Plant for further use.

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Limitations

During the evaporation to this level of solids, various volatile components

(sulphur compounds, methanol, etc) are released from the liquor and must

be separated from the condensate to allow reuse in the fiberline and

recausticizing.

Black liquor also contains a substantial fraction of inorganic compounds

which, during the evaporation process, reach their solubility limit and can

deposit as scale on the evaporator heat transfer surfaces greatly limiting

the operating capacity of the evaporation plant and of the entire recovery

island.

Parameters of the evaporator plant

WBL feed flow-110 to 135 MT/hr

WBL density-10 to 15oTw

SBL density-62 to 78oTw

LP steam to 1st effect calendria-3.5 to 4.5 kg/cm

2

Steam consumption- 16 to 25 MT/hr

Surface condenser water flow- 1488 to 1740 m3/hr

Vacuum at surface condenser- 68 to 720 mm of Hg

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EVAPORATION PLANT FLOW DIAGRAM

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1.1 MALONE FILTER

The weak black liquor (WBL) from the pulp mill carries large amount of fine

pulp fibres and particles, which have to be removed before the recovery process.

For this purpose WBL is passed through a series of 3 Malone filters at NPM.

Construction

Malone filters are basically drum washers.

These filters are situated at a height above the ground level.

There is a dumping space exactly below the filters at ground level

where the separated pulp falls.

Working

The WBL which is stored in WBL tanks is pumped to the Malone

filter section.

The filter employs low pressure steam to extract the liquid and

separate the solid.

The pulp after being separated falls onto the ground from where it is

manually taken back the pulp mill for reuse.

The WBL which is now purely in liquid form goes back to the storage

tanks.

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1.2 LONG TUBE EVAPORATOR

Long Tube Vertical (LTV) evaporator is also known as Rising Film evaporator.

The heating element is a shell and tube heat exchanger using 2” OD tubes of

30ft height.

Working

Liquor is fed into the

bottom liquor chamber

and then into the tubes. It

is heated with condensing

steam on the outside of

the tubes. The lower

portion of the tubes is

used to preheat the liquor

to its boiling point.

Evaporation then begins

at that height, where the

vapour pressure of the

feed liquor equals the

system pressure.

As the liquor climbs up

the inside of the tubes,

additional vapours are generated and the velocity of the liquid-vapour mixture

increases to a maximum at the tube exit. The outlet mixture impinges upon a

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deflector, mounted above the top tubesheet of the heat exchanger, where gross,

initial separation of the liquid from the vapours occurs.

Additional liquor is separated from the vapour by gravity as the vapours rise in

the vapour body. An entrainment separator is installed near the top of the vapour

body to remove most of the remaining traces of liquid from the vapours prior to

their exiting the vapour body. The concentrated liquor is discharged from a

connection near the bottom of the vapour body.

Heat Transfer Rate

Heat-transfer rates in the preheating section are quite poor due to the slow

moving liquor pool inside the tubes, but are several times greater in the boiling

section due to the turbulence enhancement provided by nucleate boiling. It is

therefore critically important to reduce the non-boiling zone to a minimum.

Multiple Effect

Multiple effect evaporators (MEEs) are always used in black liquor service. The

term multiple effect comes from the multiple effective use of energy to perform

the evaporation task.

At NPM 6 effect LVT evaporators are being used. Steam generation is a

significant operating expense and every effort must be made to conserve its use.

So, the vapours formed in Lamella falling film evaporators are used in LVT

instead of steam.

Due to radiation losses and changes in latent heat of evaporation the full

theoretical efficiency value cannot be attained, but it is around 0.7 lbs for each

1.0 vapours condensed in the first effect of the MEE.

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Rising film evaporators are sensitive to the amount of ∆T available for heat

transfer operation. A ∆T of less than 13-150F will cause the unit to stall and

perform poorly. For this reason MEE train limits the number of effects to 6.

Parameters of LTV Evaporators at NPM

SL NO. PARAMETER VALUE

1 Type of evaporators Long tube vertical

2 No. of effects 6

3 Weak black liquor conc. 12.5%

4 Temperature of WBL 800C

5 Liquor flow for RB 30.6 t/h

6 Concentration- SBL 70%

7 Temperature – SBL 1080C

8 Vacuum system Steam ejectors

9 Year of installation 1980

10 Temperature at various effects(1,2,3,4..)

11 Evaporation capacity 141 t/h

12 Evaporation being achieved 90-95 t/h

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1.3 FALLING FILM EVAPORATOR

This evaporator design relies on plates as heat transfer surfaces. Liquor is

processed on the outside of the heat transfer surface in plate designs. At NPM

three lamella falling film evaporators are installed.

Construction

FF evaporators consist of

Liquor sump from which a defined

volume of liquor is continuously

recirculated to the heating element.

Distribution device, typically a tray

or a spray nozzle that distributes the

flow of liquor over the entire heating

surface.

Slots for plate units are positioned to

allow the liquor to fall onto the

tubesheet or the plates.

Bottom liquor chamber which

stores the concentrated liquor giving it

retention time for higher efficiency

Mist eliminator installed near the

bottom of the vapour body unlike that in long tube evaporator, which

removes the mist from the concentrated liquor.

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Working

The feed inlet is present at the top of the evaporator. A thin film of

liquor is established on the heating surface and flows downward back

to the liquor sump while being partially evaporated.

Steam enters the vapour body and flows in the plates. The plates get

heated which in turn heat the liquor.

The concentrated liquor exits from the bottom

Parameters of Falling Film evaporator, NPM

S.No. PARAMETER VALUE

1 Evaporator units IA, IB, IC

2 Unit 3

3 Shell diameter 3100 mm

4 Cylindrical height 9000 mm

5 Total height 11060 mm

6 Shell thickness 10 mm

7 Shell material SS304

8 No. of lamellas per body 54

9 Width of lamella 1219 mm

10 Height of lamella 7315 mm

11 Lamella material SS2333

12 Lamella plate thickness 1.5 mm

13 Lamella design pressure 4.0 bar/ full vacuum

14 Lamella design temperature 1520C

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Heat transfer rate

Heat-transfer rates are considerably better, especially at higher concentrations,

when using falling film design over rising film design since the liquor falls

turbulently over the heating surface. Any liquor preheating requirement is also

efficiently accomplished in the falling film design.

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1.4 PREHEATER

A preheater is a device which heats the liquor without causing evaporation. It

just increases the temperature of the liquor. Thus the temperature required at the

evaporator is achieved by pre heater.

At NPM four pre heaters are being used. Preheating the liquor reduces the

irreversibility involved in evaporation and therefore improves the

thermodynamic efficiency of the system. Thus it helps in reducing the plant

operating cost.

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1.5 CONDENSER

The vapours arising from the evaporation of the black liquor are to be

condensed or converted into liquid form.

The reasons for condensation are

The vapours contain methane gas which may be fatal if the vapours leak

into the atmosphere. The methane gas is condensed and discarded.

The condensed vapours can be reused for heating

The non condensable gases can be separated out of the vapours.

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2. Recovery Boiler

Recovery boiler is the part of Kraft process of pulping where chemicals for

white liquor are recovered and reformed from black liquor, which contains

lignin from previously processed wood. The black liquor is burned, generating

heat, which is usually used in the process or in making electricity, much as in a

conventional steam power plant. The invention of the recovery boiler by G.H.

Tomlinson in the early 1930s was a milestone in the advancement of the Kraft

process.

Recovery boilers are also used in the (less common) sulfite process of wood

pulping; this article deals only with recovery boiler use in the Kraft process.

Function of recovery boilers

Concentrated black liquor contains organic dissolved wood residue in

addition to sodium sulfate from the cooking chemicals added at the

digester. Combustion of the organic portion of chemicals produces heat.

In the recovery boiler heat is used to produce high pressure steam, which

is used to generate electricity in a turbine.

The turbine exhaust, low pressure steam is used for process heating.

Combustion of black liquor in the recovery boiler furnace needs to be

controlled carefully. High concentration of sulfur requires optimum

process conditions to avoid production of sulfur dioxide and reduced

sulfur gas emissions. In addition to environmentally clean combustion,

reduction of inorganic sulfur must be achieved in the char bed.

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SCHEMATIC OF RECOVERY BOILER

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The recovery boiler process has several unit processes:

Combustion of organic material in black liquor to generate steam.

Reduction of inorganic sulfur compounds to sodium sulfide, which exits

at the bottom as smelt.

Production of molten inorganic flow of mainly sodium carbonate and

sodium sulfide, which is later, recycled to the digester after being re-

dissolved.

Recovery of inorganic dust from flue gas to save chemicals.

Production of sodium fume to capture combustion residue of released

sulfur compounds.

At NPM, capacity of Recovery Boiler for handling of Black liquor is 675

MT/Day with generation of 92 Tons of Steam /Hr. at 60 Kg/cm2. Steam so

generated is utilized in process requirement. Steam generation rate varies

depending upon the Black Liquor firing rate.

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2.1 REACTIONS INVOLVED

In the smelting furnace

2NaR + air Na2CO3 + CO2

(Lignin salt)

Na2SO4 + 2C Na2S + CO2 ; H = -660kCal

In the boiler

C + 1/2O2 CO

C + O2 CO2

C + CO2 2CO

C + H2O H2 + CO

C + ½ Na2SO4 CO2 + 1/2Na2S

C + 1/4Na2SO4 CO + 1/4Na2S

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2.2 DIFFERENT PARTS OF THE

BOILER

1. Furnace 9.Smelt spouts

2. Superheaters 10.Dissolving tank

3. Boiler generating bank

4. Economizers

5. Steam drum

6. Primary and Secondary air ports

7. Liquor guns

8. Tertiary air ports

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2.3 ZONES IN THE BOILER

Drying zone where the liquor is fired.

Reduction zone where salt cake is reduced to sodium sulfite.

Oxidation zone where the various chemicals are oxidized.

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2.4 BLACK

LIQUOR FIRING

In the conventional process, black

liquor is fired into recovery

furnaces at solids contents ranging

from 60-70% and at temperatures

of 105-1200C. With high solids

firing, the solids content may

exceed 80% and the firing

temperature 1750C. Spray nozzles

form the liquor into flat, conical, or

elliptical sheets which quickly

disintegrate into droplets. Black

liquor sprays typically have a mean

droplet diameter of 2-3 mm and a

range from 0.5-5 mm.

The liquor is fired with a pressure

of 0.8-1.2 kg/cm2 at a firing temperature of 115-123

0C.

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2.5 COMBUSTION

Furnace is lit up with auxiliary fuel (F. Oil) and after taking the boiler in range

and normalizing the furnace condition, auxiliary fuel is cut out an HBL firing is

established. Incineration of HBL (Heavy Black Liquor) involves drying,

pyrolysis and gaseous combustion. Salt cake added in the mixing tank is

reduced to Na2S on reaction with char during char burning. Inorganic content of

HBL is converted to sodium salts in the form of molten smelt which comes out

through the spout and gets dissolved in the main dissolving tank with the

addition of Weak White Liquor to form Green Liquor, which is sent o the

causticizing Plant at required concentration. The generated steam is sent to the

Utility Department.

The burning chemistry can be summarized into the following groups

Pyrolysis

Volatiles Burning

Char burning

Inorganic Oxidation

Pyrolysis

Pyrolysis is a gradual series of irreversible degradation reactions that black

liquor solids undergo as their temperature is increased. The reactions in

pyrolysis result in a combustible gas and a solid carbonaceous char.

Black liquor solids pyrolysis gases + char - heat (from flue gases)

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The pyrolysis gases include H2, CO, CH4, TRS, CO2, H2O, and other high

molecular weight hydrocarbons and organic compounds.

Char – Na2CO3 + Na2SO4 + fixed carbon

Volatiles Burning

It is the combustion of volatiles produced by pyrolysis. It requires adequate air

supply in order to be accomplished properly; ensuring proper mixing between

combustibles and air. Mixing is a critical factor here as temperatures of 760-

8150C need to be reached. A combination of high injection velocities and a

relatively large amount of air is used to achieve this.

Char burning

Char consists of finely divided carbonaceous material and inorganic salts. The

average oxidation state of the sulfur compounds may be different. Also, at the

time of pyrolysis, the char is 75% inorganic and 25% carbon.

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2.6 ECONOMIZER

Economizers are mechanical devices intended to reduce energy consumption,

or to perform another useful function such as preheating a fluid. Economizers

are so named because they can make

use of the enthalpy in fluid streams

that are hot, but not hot enough to be

used in a boiler, thereby recovering

more useful enthalpy and improving

the equipment’s efficiency.

In the Recovery Boiler, the preheated

water from the economizer is

supplied to the steam drum. The

waterside of the steam drum is

connected with the furnace bottom ring header through boiler bank tubes, lower

water drums and down corners. The furnace front wall tubes slope forward to

form the furnace roof and routed in such a way that the super heater can be

easily penetrated. The rear wall tubes slope forward to form the rear furnace

arch and both furnace walls are crowned with side outlet headers. The side wall

outlet headers are connected with steam drum by a system of riser tubes. The

water in the furnace wall absorbs heat and the resulting steam water mixture is

discharged into the steam drum directly from rear and front walls whereas the

steam water mixture from the side walls are collected by the outlet headers and

discharge into the drum by means of a system of steam riser. The temperature of

the flue gas at the exit of the economizer is 4000C.

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2.7 SUPERHEATER

A superheater is a device used to convert saturated steam or wet steam into dry

steam used in steam engines or in processes, such as steam reforming. This

device is a boiler mounting which increases the efficiency of the boiler by

generating super heated steam. If superheated steam is required, the saturated

steam must pass through a superheater. This is simply a heat exchanger where

additional heat is added to the saturated steam.

It contains a series of tubes through which the wet steam and heat carried by the

flue gases.

SCHEMATIC OF A SUPERHEATER

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2.8 ELECTROSTATIC PRECIPITATOR

An electrostatic precipitator (ESP) or electrostatic air cleaner is

a particulate collection device that removes particles from a flowing gas (such

as air) using the force of an

induced electrostatic charge.

Electrostatic precipitators are

highly efficient filtration devices

that minimally impede the flow of

gases through the device, and can

easily remove fine particulate

matter such as dust and smoke

from the air stream.

In contrast to wet scrubbers which

apply energy directly to the flowing

fluid medium, an ESP applies

energy only to the particulate matter

being collected and therefore is very

efficient in its consumption of

energy (in the form of electricity).

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2.9 SOOT BLOWER

Soot blowers are most widely used equipment to control the fireside deposit

accumulation in recovery boilers.

The entrainment of fly ash particles from the recovery boiler lower furnace to

the convection sections of the boiler is an inevitable process. The accumulation

of these particles in the fireside heat exchanger surfaces may reduce the boiler

thermal efficiency, create a potentially corrosive environment at the boiler tube

surfaces and, if the accumulation is not properly controlled, it may also lead to

costly unscheduled boiler shutdowns due to plugging of the gas passages.

Construction and Working

A soot blower consists of a lance tube with two opposing nozzles mounted near

the tip of the lane. During the removal process, the soot blower lance rotates and

extends, through a small opening in the boiler wall, while blowing high pressure

steam directed into the tube banks. After the lance is fully extended, it rotates in

the opposite direction as it is inserted and retracts to its original inactive state.

The following diagram illustrates the cleaning process of a tube bank by a soot

blower.

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3. CAUSTICIZING PLANT

The function of causticizing operation is to convert sodium carbonate into active

sodium hydroxide and remove various impurities introduced from the furnace.

Reactions Involved

The causticizing reaction occurs in two steps, the lime first reacts with water to

form calcium hydroxide.

CaO + H2O Ca(OH)2

The slaking operation is the calcium hydroxide is reacts with sodium carbonate

to form sodium hydroxide.

Ca(OH)2 + Na2CO3 2NaOH + CaCO3

Major Components

The major components of this plant are

Lime Slaker

Causticizers-4 Nos

White liquor clarifier

Lime Mud Washer (LMW) - 3 Nos.

Description of the process in Causticizing plant

Green Liquor received from Recovery Boiler is stored in GL Storage

Tank from where it is taken to GL constant head Tank to increase its

temperature utilizing LP steam.

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Slacking is important to the subsequent operations of causticizing and

lime mud settling.

A significant portion of the causticizing reaction also takes place in

slaker.

If both lime and green liquor are fed at relatively high temperature to the

slaker, considerable steam generation can be expected due to exothermic

reactions.

A high temperature is helpful in accelerating the reaction rate and

ensuring a good causticizing efficiency.

Hot GL is fed to Rotary Slacker and Lime is added from Table feeder at

required rate to maintain a difference between GL to Lime liquid which is

passed to Bowl and Slant make classifier to remove out unreacted lime in

the form of grits after proper washing.

This limed liquid goes to three causticizers in series to complete the

reaction and after that finally goes settled and clear WL overflows out to

WL Storage tanks.

The mud of WLC is removed out and washed in three subsequent

washing stage systems with counter current wash water to extract out

maximum alkali in the form of Sodium salts.

Mud from the final stage wash system again sent to filter system to

extract out rest possible alkali.

Finally, the mud (Filter cake) is sent to C&C plant to dispose it out of the

mill.

White Liquor produced is sent to Pulp Machine as per requirement.

Weak White Liquor produced is sent to Rec. Boiler to dissolve smelt.

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35


Parameters of Causticizing Plant

SL.No. PARAMETER VALUE

01 Green Liquor temperature 900C

02 Green Liquor concentration 25-280Tw

03 Causticizer-I temperature 98-1020C

04 Causticizer-II temperature 95-1050C

05 Causticizer-III temperature 95-1050C

06 Green Liquor to limited liquor difference 05-090Tw

07 C.E % 78-84%

08 Sulfidity % 18-20%

09 Active alkali in White Liquor 104-110 gpl as NaOH

10 CaO % in T.F. 58-60%

11 Green Liquor TTA 125-135 gpl as NaOH

12 CaO loss in lime mud 0.5-1.0%

13 Na2O loss in Lime mud 0.5-1.0%

14 Cao loss in grits 5-9%

15 Na2O loss in grits 1.0%

16 WWL TTA 25-40 gpl

17 LMW3 TTA 05-07 gpl

18 Underflow of WLC, LMW1,LMW2,LMW3 70-1040Tw

19 Filter vat consistency 70-800Tw

20 Mud discharged consistency to C&C plant 10-150Tw

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Definitions

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3.1 LIME SLAKER

The lime slaker is the heart of the causticizing plant. The chemical reaction that

takes place in the slaker determines the chemical composition of the white

liquor used in the digester. The size of the lime mud particles is also determined

by the way the slaker is operated. Proper control of the lime and green liquor

entering the lime slaker is important for correct operation of the causticizing

plant.

The lime slaker consists of two separate components.

The mixing compartment where the lime and green liquor are

introduced is equipped with an agitator to keep the lime particles in

suspension while they are in the slaking process.

the slurry flows from the mixing compartment into the classifier

compartment. Here approximately 65 mesh or larger solids separate out

from the causticized slurry. The oversized material settles to the bottom

of the classifier section and is removed using a screw conveyor or rake

type mechanism.

The material that settles down is called grit. This material emanates from

unslaked lime, reject material that comes in purchased lime and any other

small non-slakable lime components entering the lime slaker.

The temperature of the lime slaker is maintained by controlling the green

liquor feed temperature. When the green liquor to lime ratio is set to produce

the correct strength white liquor, the only changes required are monitoring

the green liquor density and adjusting the green liquor temperature to

maintain the correct slaker operating temperature.

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Lime slakers are equipped with a gas scrubbing device since a lot of steam is

generated inside the slaker which can carry with it lime dust particles from

the lime feed.

LIME SLAKER

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3.2 CAUSTICIZER

Causticizers are mixing compartments installed for better mixing and retention

efficiency in the causticizers. Generally three or four single compartment-type

tanks are installed for causticizer systems with a residence time of around 90

minutes.

At NPM four causticizers are being used. There is very little improvement in

going over six compartments or tanks, as long as the design retention time

requirements are satisfied.

Causticizers are available in two types:

Single compartment type which are typically 4.5m diameter x 4.5m

deep. NPM employs this type of causticizers.

Multiple compartment type or stacked causticizers which are in the

order of 6m diameter x 10m high.

Causticizer tanks are connected together with large diameter pipes with ample

provision for clean-out.

The pipes or launders between the tanks are kept as short as possible to reduce

the amount of cleaning required on the main flow line. Each tank is equipped

with a bypass so that the tank can be taken out of service for maintenance.

Liquor after passing from the last tank overflows into a standpipe, from where it

is pumped to the white liquor clarifier.

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SINGLE STAGE CAUSTICIZER

MULTI-COMPARTMENT CAUSTIIZER

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3.3 WHITE LIQUOR CLARIFIER

Vertical Pressure filter, Pressure Disc filter or sedimentary clarifiers can be used

for white liquor clarification. At NPM sedimentary clarifiers are being used for

this purpose.

The clarifier is equipped with an automatic lifting device that allows the rakes to

lift if the torque level on the drive becomes too high, causing the rake to stop on

high load. They can also be used to store the white liquor.

Clarifiers are easier to control than pressure filters, however over-liming of the

lime slaker will produce a clarifier upset resulting in a cloudy overflow. The

cloudy white liquor can cause scaling problems at the digester.

SEDIMENTATION CLARIFIER WITH LIQUOR STORAGE

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3.4 LIME MUD WASHER

The lime mud slurry discharge from sedimentation clarifiers contains white

liquor at the same concentration as the feed of the clarifier. Before this slurry is

dewatered for feeding to the lime kiln, the white liquor is washed out of the mud

as much as possible. This requires dilution with water followed by second stage

sedimentation/filtration.

The dilution water is not all fresh water. Most of the water comes from mill hot

water systems, recycled filtrate from the lime mud precoat filter at the lime kiln,

and scrubber water from a wet-type scrubber used on the lime kilns. These

flows are thoroughly mixed with the underflow or lime mud slurry from the

white liquor clarifier.

Since the lime mud washing process is by dilution, it is important to have the

correct amount of water entering the lime mud washer, so that the weak wash

generated has the lowest TTA possible prior to being used for dissolving smelt

at the recovery boiler. The retention time should be 5 minutes at least.

In construction LMWs are very similar to clarifiers with the difference being

addition of water in LMW to wash the liquor. In clarifiers white liquor is

produced where as in LMW weak wash is obtained. This weak wash is sent to

recovery boiler to dissolve the smelt.

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4. LIME MUD REBURNING PLANT

The lime mud containing calcium carbonate (CaCO3) formed after white

liquor is extracted from the white liquor clarifiers and lime mud washers, is

reutilized to form lime (CaO).

At NPM this equipment for this whole process is installed but presently not in

operation. Fresh lime is brought and is fed into the slaker. So this portion of

the report is based on the theoretical aspects and installation specifications

rather than working parameters.

Phases involved

Drying the lime mud

Raising the temperature of the lime mud to the level (about 8000C )

required for calcination reaction.

Maintaining a high temperature for sufficient time to complete the

endothermic reaction.

Reactions involved

The lime mud is burnt in the lime kiln with the help of a fuel to produce lime.

The reaction is

CaCO3 + Heat CaO + CO2

A well controlled lime mud reburning plant will yield a product which is 80-

85% CaO and reacts rapidly with green liquor. Excessive temperature along

with chemical impurities can promote the formation of grits in slaker.

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Major sections of Lime mud reburning plant

Lime mud storage tank

Lime mud precoat filter

Lime kiln

Operation

The lime kiln is equipped with double burner type (i) Furnace oil (ii) Producer

gas.

Lime mud is fed into the high end of the kiln and the solid phase moves counter

current to the flow of hot air as the kiln rotates. The transfer of heat into the mud

at the cold end is optimized by providing extended surface area, usually by

means of steel chains attached to the kiln shell and hanging in the hot gases. In

the hotter zones of the lime kiln, the metal shell is lined with refractory brick.

As its temperature is raised, the lime mud material becomes plasticized and

forms into pellets, aided by the rolling and lifting action of the kiln. Normally,

the size of the aggregates ranges upto about 3 cm in diameter. Occasionally the

pellets keep on growing to form large ball rings. The soda content of the lime

mud has a significant role on its aggregating properties during the operation, and

is typically controlled less than 1%.

The hot end of the lime kiln is typically maintained at 1150-1250 0C by firing

furnace oil or producer gas, without reclaiming heat from the kiln product, the

reburned lime would be discharged at a temperature of about 950 0C.

At NPM integral tube coolers are installed to recover the heat in direct contact

with part of entering air. These coolers are attached to the discharge end in such

a way that the calcined lime falls into one of the cooler; it is then reverses the

direction and flows uphill to the opposite end of the cooler where it is

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discharged at a temperature of about 350 0C air is supplied by a forced draft fan,

but the major work of ID fan that pulls the combustion gases through the kiln.

The gases leaving the kiln are laden with lime mud dust and must be cleaned up

before discharge. The dust is removed in a suitably designed electrostatic

precipitator.

SCHEMATIC OF LIME MUD REBURNING PLANT

LIME MUD STORAGE TANK

LIME MUD PRECOAT

FILTER

LIME KILN

LIME MUD

LIME

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Specifications of lime mud reburning plant, NPM

S.No. PARAMETER VALUE

1 Plant capacity 155 MTPD @ 70% purity

2 Feeding system 70% lime mud+30% lime(s)

3 Speed of lime kiln 1.1 rpm

4 Lime mud moisture 50%

5 Make up limestone size 6-18 mm

6 Make up limestone 98 TPD

7 Kiln exhaust temperature 1750C

8 Product discharge temp( kiln) 965 0C

9 Product discharge temp( cooler) 180 0C

10 Furnace oil flow 1127 kg/hr

11 Temperature of furnace oil 100-110

12 Estimated power requirement 675 kwh

13 Limestone consumption 0.632 T/T of Product lime

14 Furnace oil consumption 28.47 KL/d @ 0.95 kg/m3

15 Steam consumption 35 TPH@ 10 kg/cm2

16 Product burnt lime purity 65%

17 Residual CaCO3

3%

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4.1 LIME MUD STORAGE TANK

The lime mud storage tank serves as a buffer between the lime kiln and the

causticizing plant. It allows continued operation of the causticizing plant when

the lime kiln or lime mud filter is not operating.

Operation

The washed, thickened lime mud is stored in a large tank with a slow

speed stirrer or agitator. It is normally stored at 40 to 45 wt% solids and,

at this consistency, is fairly homogeneous and very slow to separate. If

the consistency is allowed to drop as low as 25 wt%, settling will occur in

this tank resulting in stalling of the agitator mechanism

To alleviate problems with power outrages the agitator drive is usually

equipped with a standby engine or an auxiliary electric motor powered by

an emergency power generator.

LIME MUD STORAGE TANK

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4.2 LIME MUD PRECOAT FILTER

The 40 wt% to 45 wt% slurry from the lime mud storage tank is further diluted

to approximately 25 wt% solids and filtered on a vacuum precoat filter drum.

This filter is also equipped with cake wash pipes, allowing for washing of the

filtered solids

prior to discharge

to the lime kiln.

At NPM, lime

mud precoat

filter is being

used and then the

mud is

discharged rather

being sent to

lime kiln for

reburning.

Operation

The lime mud precoat filter operates at quite low submergence and is

equipped with a scraper blade set approximately 12 mm from the face of

the drum. When the vacuum pump is started, the filter forms a cake until

it reaches the scraper blade. At this point, the top layer of filter cake is

scrapped off and discharged into the lime kiln.

By operating the filter at high speeds of 3-6 rpm results in a thinner cake

formation on the top of the precoat, which is easier to wash and also

easier to dry.

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After washing, the cycle of the filter allows drying before discharge into

the lime kiln. Sometime after the precoat has been formed, the lime mud

precoat filter will start to blind with fines, resulting in a decrease in the

percent solids discharged to the kiln.

The filtrate from the lime mud precoat filter is generally pumped to the

lime mud mixer, or it would be directed to weak wash storage.

Lime mud precoat filters require a large amount of air, approximately

3m3/min/m

2 of filter area at approximately 560 mm of Hg vacuum.

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4.3 LIME KILN

At NPM rotary lime kiln is installed, where the calcination reaction takes place

to produce lime.

Rotary kilns started to be used for lime manufacture at the start of the 20th

century and now account for a large proportion of new installations if energy

costs are less important. The early use of simple rotary kilns had the advantages

that a much wider range of limestone size could be used, from fines upwards,

and undesirable elements such as sulphur can be removed.

On the other hand, fuel consumption was relatively high because of poor heat

exchange compared with shaft kilns, leading to excessive heat loss in exhaust

gases. Now a days, lime kilns partially overcome this disadvantage by adding a

preheater, which has the same good solids/gas contact as a shaft kiln, but fuel

consumption is still somewhat higher, typically in range of 4.5 to 6 MJ/kg. In

the design shown, a circle of shafts (typically 8-15) is arranged around the kiln

riser duct. Hot limestone is discharged from the shafts in sequence, by the action

of a hydraulic "pusher plate". Kilns of 1000 tonnes per day output are typical.

The rotary kiln is the most flexible of any lime kilns able to produce soft,

medium, or hard burned as well as dead-burned lime or dolime.

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