11 chemical recovery cycle

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11. CHEMICAL RECOVERY CYCLE

Table of content

CHEMICAL RECOVERY CYCLE .................................................................................................................................. 1EVAPORATION OF BLACK LIQUOR............................................................................................................................1

Black liquor properties.......................................................................................................................................2

Terminology used in the evaporation of black liquors .......................................................................................4Evaporation process...........................................................................................................................................8RECOVERY BOILER................................................................................................................................................. 11

Combustion of black liquor .............................................................................................................................. 13Recovery of heat in the recovery boiler............................................................................................................16Recovery boiler operation................................................................................................................................ 19

WHITE LIQUOR PREPARATION ................................................................................................................................ 22Dregs removal from green liquor.....................................................................................................................23Lime slaking and causticizing........................................................................................................................... 25Lime handling and washing.............................................................................................................................. 26Lime drying and reburning...............................................................................................................................29

QUESTIONS ............................................................................................................................................................ 33

Kaj HenricsonProfessor Pulping Technology

Lappeenranta University of Technology

August 2005

Educational course material and only for internal and personal use during

the course: An introduction to chemical pulping technology.


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Chemical recovery cycle

One of the main advantages of the kraft process is the efficient recovery of chemicals as

compared to, for instance, sulfite pulp production. Picture 1 shows the principal unit operations

of the kraft recovery process: evaporation of weak black liquor; combustion of thick black liquorin a recovery furnace to form a smelt, which is dissolved in water forming green liquor;

causticizing sodium carbonate (Na2CO3) in green liquor to form white liquor containing sodium

hydroxide (NaOH) and sodium sulfide (Na2S); and regeneration of lime mud in a limekiln.

Black liquor is recovered with a high recovery rate after cooking and the black liquor is

combusted in the recovery boiler to produce heat and power in amounts that exceed their

consumption at the kraft pulp mill. There is no need for external heat or power in the productionof kraft pulp. The only exception is the limekiln, where usually oil or natural gas is used. In the

lime kiln, the use of fossil fuels can be replaced with for instance combustion gases produced by

gasifying bark. The water evaporated in the evaporation plan can be reused in white liquorpreparation and in the fiber line as purified condensates.

Picture 1. Recovery cycles of chemicals for a kraft mill

Evaporation of black liquorThe amount of water that has to be evaporated during black liquor evaporation depends on the

amount of water introduced into the chemical recovery system, especially into the cooking and

brown stock handling systems. The water to be evaporated originates from:

water content of wood water formed during cooking steam condensed during direct steam heating for instance in cooking and oxygen bleaching water in white liquor water used in brown stock washing. The excess water is determined by the dilution factor. various other water sources such as sealing water to leaking seals etc.

BAT


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Table 2. Typical composition of black liquor from Scandinavian wood

Softwood (pine) Hardwood (birch)

Typical Range Typical Range

Carbon, % 35.0 32-37 32.5 31-35

Hydrogen, % 3.6 3.2-3.7 3.3 3.2-3.5

Nitrogen, % 0.1 0.06-0.12 0.2 0.14-0.2

Oxygen, % 33.9 33-36 35.5 33-37

Sodium, % 19.0 18-22 19.8 18-22

Potassium, % 2.2 1.5-2.5 2.0 1.5-2.5

Sulfur, % 5.5 4-7 6.0 4-7

Chlorine, % 0.5 0.1-0.8 0.5 0.1-0.8

Inert, % 0.2 0.1-0.3 0.2 0.1-0.3

Total, % 100.0 - 100.0 -

Heating value

The heating value of black liquor expresses the amount of heat that is released when combustinga specified amount of black liquor dry solids. The types of organic compounds present in blackliquor and inorganic material in black liquor influence the heating value of black liquor. For

instance, the type of lignin and the ratio between lignin and carbohydrates affect the heating

value of black liquor as shown in Table 3. The reactions of inorganic material duringcombustion influence the heating value, and for instance the reduction of sulfate to sulfide in the

recovery boiler consumes energy. All black liquors have to be tested separately to know the

heating value and combustion properties in a recovery boiler.

Table 3. Heating values of black liquor components

Component MJ/kg Btu/lb m

Softwood lignin 26.9 11.57Hardwood lignin 25.11 10.8

Carbohydrates 13.555 5.83

Resins, fatty acids 37.71 16.22

Sodium sulfide 12.9 5.55

Sodium thiosulfate 5.79 2.49

Boiling point rise and viscosity

When dimensioning an evaporation plant, one needs to know the boiling point rise and the

viscosity of the liquor to be evaporated at the dry solids contents and the process conditions,

such as temperature, prevailing during evaporation.

The temperature where black liquor boils is higher than the boiling temperature of water at thesame pressure. The temperature difference is called the boiling point rise. As the dry solids

content of black liquor increases, the boiling point increases as shown in Picture 2 for someblack liquors.

Fapet 6B: p.B15

Fapet 6B: p.B18


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Picture 2. Black liquor boiling point rise vs. dry solids concentration

The viscosity of black liquor depends on the temperature and dry solids content. The viscosity

increases at higher dry solids contents, and a temperature increase decreases the viscosity as

shown in Picture 3 for one black liquor after evaporation to a high dry solids content. Theviscosity behavior varies from one black liquor to another. The evaporation of black liquors to

high dry solids concentrations above 75% to 80% usually requires that these black liquors are

heat treated at higher temperatures to reduce the viscosity of these liquors.

Picture 3. Effects of temperature and dry solids on the viscosity of one high dry solids black liquor

Terminology used in the evaporation of black liquors

The evaporation stage or effect is one evaporator, or possibly several parallel evaporators

operating at the same steam pressure level. The effects are numbered in the direction of the

steam flow with the first effect operating at the highest steam pressure. Multiple-stage

evaporation means evaporation in a plant consisting of a number of evaporator effects connected

Fapet 6B: p.B21

Modified by KH;

Fapet 6B: p.B23


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in series. An evaporation plant normally consists of 5-7 stages in series. Picture 4 shows atypical configuration. The condensate streams are segregated into fractions A, B and C

according to their content of methanol and COD.

BLACK LIQUOR

FROM COOK

CONCENTRATED

BLACK LIQUOR

FRESH

STEAM

CONDENSERS

BLACK LIQUOR

FROM COOK

CONCENTRATED

BLACK LIQUOR

FRESH

STEAM

CONDENSERS

Picture 4. Typical configuration of a black liquor evaporation plant

The evaporator body or the evaporator itself is a heat exchanger unit used in the evaporation

stages. One stage or effect might have several bodies in parallel. Picture 5 shows somecommonly used evaporator designs in the evaporation of black liquors.

A

B

CA

B

C

Picture 5. Evaporators; A: rising film, B: falling film, C: forced circulation (or crystallizer)

The rising film evaporators [also called long-tube-vertical (LTV) or Kestner evaporator] were

widely used for the evaporation of black liquor in the pulp industry until the mid-1980s. In a

rising film evaporator, the forming steam causes the liquor to rise in the evaporation tubes. Inlater installations, the falling film evaporator and forced circulation evaporator are more

common. In the falling film evaporator, the liquor is fed to the bottom of the evaporator from

where it is pumped to the top of the heating element by a circulating pump and flowsdownwards on the heating surface by gravity. In the forced circulation evaporator, the black

liquor is pumped through the tube heat exchanger to a separate chamber where the vapor is

KAM: p.28

Modified by KH;

Fapet 6B: p.B46, 47, 51


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released. The power needed in black liquor evaporation and the dimensioning of the heattransfer surfaces is dependent on the viscosity of the black liquor.

The term concentrator refers to the first effect where the liquor is evaporated to its final

concentration. Earlier, the final concentrators were clearly separated from the multiple effectevaporation train and had a different design. In a modern evaporation plant, this difference does

not exist but the terminology is still in use. The concentrators have to be cleaned regularly toremove scaling on the heat transfer surfaces, and evaporation plants usually have at least two

concentrators in parallel so that one concentrator can be cleaned without influencing the

operation of the whole evaporation plant.

Live steam,primary steam, or fresh steam is the clean steam from the boilers at the mill. Live

steam is mainly only used in the first effect in the evaporation plant. The steam must be atsaturation temperature or only slightly superheated. Primary condensate is the clean condensate

from the live steam. Secondary steam or vapor is the steam evaporated from the black liquor.

Vapor from the first effect is used as a heating medium in the second effect. Vapor from the

second effect is then used in the third effect etc. The surface condenser is a water-cooled heatexchanger that condenses the vapor from the last evaporation effect.

The vapor always contains some organic materials evaporated from the liquor or even liquordroplets entrained in the steam flow. Secondary condensate is condensate derived from the

vapor. It contains various levels of organic contaminants. It is therefore not as clean as primary

condensate. Foul condensate is the most contaminated secondary condensate and has to be

purified by stripping. The amount of organic material carried over into the condensates in theform of droplets is reduced by installing droplet separators in the steam phase of the evaporators.

The condensates can on the side of the condensing steam of the heat transfer surface be

segregated into a foul and clean condensate fraction.

Picture 6. Falling film evaporator A with condensate segregation and droplet separator B at the top ofthe evaporator

Vapor condensate, which has some degree of contamination mainly consisting of methanol, is

one of four fractions determined by the degree of contamination indicated in Table 4. Often,fractions 2 and 3 are combined. Vapor condensate fraction 1 and some fraction 2 can be used in

bleaching and brown stock washing. Fraction 3 is used in the causticizing plant. Fraction 4 is

treated in the stripper and is comparable to fraction 2 after stripping.

Vapor out

Vapor i n

Liquor i n

VentFlow 1%MeOH 10%

FoulcondensateFlow 10%MeOH 80%

CleancondensateFlow 89%MeOH 10%

Liquor o ut

MPS, Andritz

B

A


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Table 4. Vapor condensate fractions from a modern 7-stage multiple falling film evaporation

Vapor

condensate

fraction

Vapor condensate

drawn

Flow,

m3/adt

MeOH

content,

mg/l

MeOH

amount,

g/adt

Remarks

Fraction 1 From effects 2, 3 and

4

3.4 30 100 -

Fraction 2 From effects 5 and 6 2.5 300 700 -

Fraction 3 From effect 7 and

surface condensate

2.0 600 1,200 Slightly odorous

Fraction 4 Foul condensate

from evaporator

1.0 6,000 6,000 Malodorous

Total - 8.9 - 8,000 -

Stripped

condensate

From evaporation

and cooking foul

condensate stripping

1.6 300 500 Stripper MeOH

purification

efficiency=95%

The stripping column purifies the foul or contaminated condensate from the evaporation and

cooking plants. Stripping is a mass transfer process where volatile gas components such as

methanol and reduced organic sulfur compounds transfer from the liquid phase into the gasphase. Picture 7 shows a typical stripping column.

Picture 7. Stripping or distillation column

Non-condensable gases (NCG) are gaseous compounds that are liberated from the black liquorduring evaporation and from condensates during stripping or come from the cooking plant. They

are odorous, poisonous and inflammable. They require extraction from the evaporation plant and

incineration in suitable process equipment such as a special burner or the limekiln.

Fapet 6B: p.B71

Fapet 6B: p.B54


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Evaporation process

The evaporation of black liquor has three principal unit operations: Separation of water from

black liquor to generate concentrated black liquor and condensate, processing of condensate tosegregate clean and fouled condensate fractions, and separation of soap from black liquor. The

by far most common principle in the evaporation of kraft black liquor is the multistage

evaporation concept illustrated in Picture 8.

The sum of the boiling point rise of black liquor in an evaporator plus the temperature gradientover the heat transfer surface in the evaporator is the temperature difference between the

incoming steam and the vapor leaving the evaporator. The total temperature difference over all

the effects in an evaporation plant is the sum of the temperature differences of the individualevaporators in the plant. There is a total temperature difference available for the evaporation

plant and this difference influences the number of evaporator stages that can be used in series in

a multistage evaporation plant.

Picture 8. Principle of multiple evaporation

Picture 9 shows a perspective of a multistage falling film evaporation plant. The steam or heat

balance of the evaporation plant is dependent on the number of effects as shown in Table 5. Thenumber of effects chosen is a balance between the investment costs of additional effects, the

investment cost and amount of heat transfer surfaces in the individual effects and the savings in

steam due to additional effects.

Picture 9. Multistage falling film evaporation plant

Fapet 6B: p.B9

Modified by KH;

Fapet 6B: p.B41


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Table 5. Steam economy and specific heat consumption in multiple evaporation including anintegrated stripper column

Number of

stages

Steam economy,

ton H2O/ton steam

Specific heat consumption,

MJ/ton H2O

4 3.7-3.6 630-6505 4.3-4.1 550-570

6 5.1-4.9 460-480

7 6.2-5.9 390-400

The mixed liquor sequence in the five-stage evaporation plant shown in Picture 10 is a favorable

sequence commonly used in the evaporation of black liquors. The mixed liquor flow pattern

gives the following advantages:

Soap separation from the intermediate liquor is optimum due to the concentration of 27%-30% dry solids and the low temperature of approximately 60C.

Liquor feed to effects 3 and 4 in parallel allows balancing the feed according to the feedliquor temperature and operating temperature in the effects.

Release of non-condensable gases that enter with the feed liquor is evened between theeffects. The gases vent from effects 4 and 5.

Picture 10.Mixed flow sequence

The main factors affecting soap separation from black liquor are soap solubility and the soap

skimming tank configuration. The solubility of soap depends on the concentration of dry solidsin black liquor and black liquor temperature as shown in Picture 11.

Picture 11.Effect of temperature and solids content on soap solubility

Soap separation is often done in two stages; from the weak liquor tank from where the blackliquor is fed to the evaporation plant and from the intermediate tank after initial evaporation in

two or three evaporation effects. In a skimmer tank, a retention time of 6h-7h is used. A double

Fapet 6B: p.B44

Fapet 6B: p.B84

Fapet 6B: p.B42


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liquor tank arrangement is commonly used where weak liquor enters tank I and flows by gravityto tank II. Soap skimming is carried out batch-wise as indicated in Picture 12.

Picture 12.Soap skimming

Black liquor is also evaporated by flashing, especially in connection with continuous digesters,

where the hot extraction black liquor at the digester is allowed to flash and the flash steam is used tosteam and preheat the chips. The flashing of black liquor and condensates takes place to some extent

inside the evaporation plant as the black liquor or condensates flow to an effect at a lower pressurelevel. During flashing, the dry solids content of the black liquor increases with an amount depending

on the amount of steam separated from the black liquor during flashing. Special multi-flashevaporator systems have been developed for the evaporation of black liquor but the number of

industrial installations is small.

Vapor compression evaporation VCE is sometimes used for the pre-evaporation of black liquors

that have a dry solids concentration of less than 25% and that have a moderate boiling point rise.Picture 13 shows the principle of vapor compression evaporation. In the system, the compressor

increases the pressure and the temperature of the released vapor so that it can be reused inside

the same unit. The normal power consumption of a vapor compression pre-evaporator is 11-14kWh/ton of H2O. The feasibility of vapor compression evaporation depends on the price of

available power.

Fapet 6B: p.B86


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Picture 13.Principle of vapor compression evaporator (VCE)

Recovery boiler

The recovery boiler is a chemical reactor for the recovery of the cooking chemicals used and for

the generation of heat from dissolved inorganic and organic material in the black liquor. Therecovery of cooking chemicals in a recovery boiler includes several steps:

combustion of the organic material in black liquor to generate heat production of sodium fume to capture sulfur-containing combustion residues reduction of the inorganic sulfur compounds to Na2S production of a smelt of molten Na2CO3and Na2S recovery of inorganic dust from the flue gas recovery of the heat formed during combustion

Picture 14 is a schematic diagram of a two-drum recovery boiler and Picture 15 shows some of

the chemical reactions taking place inside the recovery boiler. The components of the recoveryboilers in Picture 14 are: furnace, where the combustion of black liquor takes place, superheatersfor heating the steam with hot combustion gases, boiler generating bank where water boils, and

economizers that recover residual heat in the flue gases and cool the flue gases. In the steam

drum, water and steam are segregated. The air enters through primary and secondary air ports andtertiary air ports. The black liquor enters through liquor guns in the lower part of the furnace. The

smelt exits through smelt spouts to the dissolving tank. The heat transfer surfaces are cleaned by

soot blowing steam. The flue gas is cleaned in an electrostatic precipitator.

Fapet 6B: p.B42


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Picture 14.Schematic diagram of a two-drum recovery boiler

The recovery boiler consists of heat transfer surfaces made of steel tubing. The tubes containsteam or water, and the heat generated inside the recovery boiler is recovered into the water and

steam inside the tubes as in normal power boilers. Recovery boilers have a lower power-to-heat

ratio compared to normal power boilers mainly due to limitations coming from the sodium and

sulfur content in the black liquor.Recovery boilers produce steam with lower heat and pressure than normal power boilers. Someof the limiting issues concerning recovery boilers are: corrosion and cracking risks, fouling ofheat transfer surfaces, air emission requirements, today almost zero SO2 is possible, very low

NOx emissions, no or low odor from the boiler, safety, and smelt-water explosion risk.

KRB: p.5


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Picture 15.Some conceptual chemical reactions in a recovery boiler

In spite of all limitations, the recovery boiler is an efficient and working solution for the

combustion of black liquors and for the recovery of heat. The sizes of recovery boilers haveincreased as pulp mills have become bigger, and the capacity of recovery boilers matches the

capacity on the fiber line. The steam values are developing towards higher steam pressures andhigher steam temperatures, which will give a higher power-to-steam ratio.

Combustion of black liquor

Black liquor is sprayed at high dry solids content into the recovery boiler as small droplets.

During the first stage, the water in the droplets evaporates and the droplet dries. Black liquordrying occurs near the liquor gun at the wall. The droplet velocity is high, about 10 m/s, when the

droplet enters the furnace.

The second stage is the devolatilization of pyrolysis gases and the release of volatiles. At the

same time, the droplets swell and a flame appears. As black liquor volatiles start to release, the

speed of the droplet decreases due to the swelling of the black liquor droplet. Small droplets are

carried upwards by the hot gas.

During the third stage, char burning starts and the droplet size decreases. Char and smelted

sodium fall into the bed where the reduction of sulfur continues under reducing conditions.

Picture 16 shows the different stages of black liquor combustion in the black liquor droplets andthe char bed in the lower part of the furnace.

BAT


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CHAR BEDPrimary Air

1st Secondary Air

2nd Secondary Air

3rd Secondary Air

Liquor Guns

smelt

char intochar bed

smalldroplets

drying

devolatilization

charcombustion

in flight

pyrolysis gases

Picture 16.Stages in the combustion of black liquor

Table 6 presents some typical times for the different stages of the combustion of black liquor

droplets. Picture 17 shows the swelling behavior of the droplets during combustion.

Table 6. Stages in black liquor combustion

Stage Characterized by Time scale in furnace

for a 2mm droplet

Drying Water evaporation

Constant diameter after initial swelling

0.1-0.2s

Devolatilization Appearance of flame, ignition

Swelling of the dropletRelease of volatiles

0.2-0.3s

Char burning Disappearance of flame

Decreasing diameter

Reduction reactions

0.5-1s

Smelt Constant or increasing diameter

Reoxidation

long

Picture 17.Characteristic swelling behavior of black liquor during combustion

Fapet 6B: p.B109

Fapet 6B: p.B109

KRB, MPS


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15

During char combustion, reduction reactions occur. Carbon has a major role in the reductionreactions. Na2SO4 reacts with carbon to form Na2S. When the carbon in the char bed burns, it

causes the reduction of sodium.

Na2S+2O2-> Na2SO4Na2SO4+2C -> Na2S+2CO2

Na2SO4+4C -> Na2S+2CO

Degree of reduction (%) =( )

100422

2

+ SONaSNa

SNa

The degree of reduction of sulfur in the white liquor is determined by the operation of the

recovery boiler. The Na2SO4 is an inert circulating compound in the liquor system, and this

reduces process efficiency. A normal degree of reduction in recovery boilers is slightly above90%.

Picture 18 shows the bottom of a recovery boiler with black liquor entering the boiler through theliquor guns shown in Picture 19. Air is added above the bottom and the char bed in such a way

that reducing conditions are maintained near the char bed. The smelt flows from the recovery

boiler into the smelt dissolver.

Picture 18.Recovery boiler lower furnace withhot char bed

Picture 19.Arrangement of a black liquor gun,port opening and liquor spray

The recovery boiler has a risk of smelt and water explosion due to the presence of the porous hotchar bed containing molten sodium salts. The water and smelt can come into direct contact due to a

recovery boiler tube failure causing a smelt and water explosion in a recovery boiler. An emergencyrapid drain system minimizes potential damage. Water remains at the lowest part of the furnace to

provide cooling for the floor tubes. If all water drains from the floor tubes, the heat from the hot char

bed will damage the boiler tubes.

Fapet 6B: p.B113

KRB: p.8


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The risk of a recovery boiler tube failure isdecreased and the use of corrosion resistant

materials in the recovery boiler has increased

the lifetime of a boiler. The materials vary in

different parts of the boiler according to theconditions prevailing in the different parts of

the boiler. For instance, the lower portion of

the recovery boiler is often made of composite

tube material as shown in Picture 20. The

outer surface of the boiler tube is covered bycorrosion-resistant stainless steel. Also,

keeping the amounts of certain corrosive

chemicals, like chlorine and potassium, belowcritical levels in the chemical recovery cycle

reduces corrosion.

Picture 20. Lower furnace arrangement withcomposite tube wall construction

Recovery of heat in the recovery boiler

Picture 21 shows the material flows in a steam boiler such as a recovery boiler. A recovery boiler is

a steam boiler that has the following components or systems:

Black liquor handling and feeding equipment for feeding the thick black liquor into the boiler.The black liquor is fed at a dry solids concentration above 65%, in some boilers built after 1990

the dry solids content is even over 80%. At high dry solids concentrations, the black liquorbecomes highly viscous and is difficult to handle unless kept at an elevated temperature.

Air fans, air ducts and air pre-heater. The combustion air is pre-heated in the recovery boilerhouse and with air pre-heaters before being fed into the recovery boiler furnace.

Flue gas cleaning. The heat is recovered from the flue gas in the economizer and the flue gasesare cleaned with an electric precipitator to recover dust-containing sodium and sulfur.

Smelt outlet ducts. Combustion chamber or furnace where the combustion of black liquor occurs. Tubing for steam and water that forms the heat exchangers that cool the hot combustion gases. Feed water system. Control systems.

The recovery boiler produces superheated steam through the combustion of black liquor with air,

and the steam is taken to a turbine to produce power as shown in Picture 21. A higher

temperature and pressure of the superheated steam going to the turbine generates more powerfrom the heat generated in the boiler.

From the turbine, the steam for the pulp production process is taken out at two pressure levels:

extraction or high pressure steam with a pressure of about 12 bars that is used for instance to heatthe digester and back pressure or low pressure steam that is used for instance at the evaporation

plant and the pulp dryer. Primary condensate from the condensing of fresh live steam can be

returned to the feed water tank feeding the boiler with clean water as shown in Picture 22. Feed

water is usually preheated with steam. Exhaust steam from the turbine is taken to a surfacecondenser.

KRB: p.12


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Picture 21.Material flows in a steam boiler Picture 22.Flow diagram of a cogeneration

steam cycle

The black liquor is combusted in the furnace of the recovery boiler. About 70%-75% of the heat

value of the black liquor will convert into superheated steam when the dry solids content of theblack liquor is 72%-85%. The smelt consumes heat due to the heat of the smelt and the chemical

reactions to form the reduced sulfur in the smelt. About 15% of the heat will go to the smelt.

The rest of the heat is lost with the flue gases. Picture 23 shows how the combustion heat is

divided between recovered heat, heat to smelt and heat losses in the flue gases as the flue gastemperature and the dry solids content of the black liquor vary.

Picture 23.Black liquor dry solids heat to smelt, as loss in flue gases, and recovered into steam

Picture 24 shows the principle of the water system in a recovery boiler and Picture 25 a

perspective of the recovery boiler construction. Feed water is pumped into the boiler and enters

the first heat transfer surface or economizer (11). The water flows from the economizer to thesteam drum (6). In the steam drum, steam separates from water. Separation occurs by gravity,

Fapet 6B: p.B206 Fapet 6B: p.B207

Fapet 6B: p.B316


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18

screens and cyclone separators in order to minimize the amount of water droplets in the steam.Downcomers (7) feed saturated water to the evaporative surfaces that are the walls of the

combustion chamber in the boiler and tubes in the boiler bank. Saturated steam (12) exits from

the drum and flows through the superheaters (13) and is heated to superheated steam. The role

of superheaters is to heat the steam well above saturation before feeding the steam to the turbine.

11

12

13

10

6

7

11

12

13

10

6

7

1111

1212

1313

1010

66

77

Picture 24.Principle of a water tube boiler

The hot flue gases carrying the heat from combustion meet the heat surfaces in the following

order: superheater boiler bank economizer. The hottest gas heats the steam in the superheaterand the coldest gas heats the feed water interring the boiler. The boiling water cools the walls of

the boiler.

The feed water system of the recovery boiler

consists of the feed water tank (1), deaerator,boiler feed water pump (2), control valve, and

feed water piping (3). The water from the

economizer generates clean attemperating

water (5) from steam in a sweet watercondenser (4). The attemperating water is

used to control the temperature of the steam in

the superheaters and thus to control thesurface temperature of the tubes in the

superheaters. Picture 26 shows some typical

superheater platens. The main steam valvescontrol steam flow (9). The steam from

superheaters flows through the main steam

line (10) to the turbine.

Picture 25.Water circulation system

Modified by KH;

Fapet 6B: p.B205

Fapet 6B: p.B122


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19

Picture 26.Arrangement of two superheater

platens Picture 27.Air system for a recovery boiler

The air is typically delivered at several horizontal elevations to ensure complete combustion and to

minimize emissions. In boilers built after the year 2000, air is often introduced over almost thewhole height of the furnace section of the recovery boiler. Picture 27 shows a boiler air system. It

has an inlet duct with silencer (1), venturi for airflow measurement (2), air blower (3), air heater (4),

distribution ducts (5), and air ducts (6 and 7). Pressure and temperature measuring devices are also

necessary at proper locations. Air intake is typically high inside the boiler house to ensure an evenvertical temperature profile and to recover heat losses through boiler walls and openings.

Recovery boiler operation

Spent kraft pulping liquors contain inorganic salts. During the combustion reactions, the

temperatures increase to levels where significant amounts of inorganic chemicals vaporize.

Small char fragments and black liquor particles also entrain into the flue gas flow. Combustion

and chemical reactions take place in the hot gas phase and cause deposits that collect on heat

transfer surfaces and foul them.

The deposits formed on the heat transfer surfaces consist mainly of Na2CO3, Na2SO4, Na2S and

small amounts of NaCl and potassium salts. The composition of the deposits varies according to

the mechanics of deposit formation, type of black liquor and combustion conditions. Deposits inthe lower part of the furnace generally contain more Na2CO3and Na2S and less Na2SO4, NaCland potassium salts that deposits in the upper furnace. Picture 28 shows typical compositions of

deposits in a recovery boiler.

KRB: p.13Fapet 6B: p.B121


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Picture 28.Composition of fireside deposits

The fouling of individual heat transfer surfaces is removed and controlled by soot blowing. Sootblowing is done with steam and is directed to the critical heat transfer surfaces. Soot blowers are

usually in pairs on the left and right side of the boiler as shown in Picture 29. Picture 30 shows a

soot blower cleaning a tube bank.

Picture 29.Recovery boiler Picture 30.Sketch of a soot blower cleaning atube bank

Potassium and chloride in the black liquor influence the fouling of heat transfer surfaces and the

corrosion of recovery boilers. Even small amounts of potassium and chloride influence the stickytemperature of dust. The sticky temperature is the temperature above which dust contains sufficient

amounts of sodium in molten state to stick to the heat transfer surfaces and form deposits as shown

in Picture 31. The deposits of chemicals on the heat transfer surfaces become an effective barrier toheat transfer from the flue gas to the tube wall. From the wall, the heat is transferred to the steam or

water inside the tube.

KRB: p.15

KRB: p.296

Fapet 6B: p.B9


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21

Tgas

Tsteam

Tube wall

Deposit

Steam(water)

Flue gas

T0 T70

Q.

Picture 31.Deposit on superheater

0

20

40

60

80

100

500 600 700 800 900 1000

Temperature [C]

Percentmelt[wt-%]

Tflow

Tsticky

Picture 32.Different compositions give different

melting behavior

The chloride and potassium levels influence the melting behavior of dust in recovery boilers and

decrease the melting point of dust as shown in Picture 32. The graphs in the picture show the

melting behavior of some typical recovery boiler dusts with varying potassium and chlorine

content. Chloride levels at Scandinavian and North American mills are typically 0.2%-0.8% of

mass. Potassium levels at Scandinavian and North American mills can vary widely: 1%-4% ofmass. Chloride and potassium enter the chemical recovery cycle mainly with the wood and

enrich into the recovery cycle. The level of chloride and potassium must be kept sufficiently low.The levels are controlled by purging dust from the recovery boiler as dust tends to enrich more in

chlorine and potassium compared to the rest of the recovery cycle.

Increasing chloride and potassium contents reduces the melting point of sodium-containing dust.Only a few percentages on molten metals make the metals sticky. Tsticky is the temperature when

enough sodium is in molten state and makes a dust particle stick to surfaces of the recovery boiler.

Tstickyis usually in recovery boiler defined to the temperature when 15% of the metals are in moltenstate. Tflowis the temperature when enough sodium is in molten state to make the metal a flowing

smelt. Metals start to flow when over 70% is in molten form but usually in connection with recovery

boilers Tflow is defined as the temperature when 85% of the metals are in molten state.

Picture 33 shows the material balance of a recovery boiler and how the generated steam is usedinside the boiler and as steam being fed to the turbine at the mill. Dust from the heating surfaces is

mixed with the black liquor and fed back into the boiler. Some of the dust may be purged from the

recovery cycle to control the content of non-process elements, in particular chloride and potassium.

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Picture 33.Material balance based on 100kg of black liquor solids

White liquor preparation

Several separate processes are needed when converting the green liquor into white liquor: dregs

removal from green liquor, slaking of lime, causticizing reaction, white liquor separation from lime

mud with lime mud washing, and drying and reburning or calcining of lime. Picture 34 shows thewhite liquor preparation system and the separate processes in the system.

Picture 34.A diagram of white liquor preparation and lime reburning

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Dregs removal from green liquor

Green liquor results when the smelt from the recovery boiler is dissolved in weak white liquor or

water in the dissolving tank. Green liquor is an aqueous solution mainly consisting of sodiumsulfides, sulfates and carbonates. It contains some unburned char and some non-process elements

that have a low solubility in the green liquor and that can be removed by sedimentation or filtration.

Most impurities in the lime cycle come from green liquor. Solid particles in green liquor mix with

the lime and accumulate in the lime cycle. Many of these impurities can be removed as dregs fromthe green liquor. Without dregs separation, the need for lime makeup may be 30% of the lime

circulation. With good purification of green liquor, the lime makeup required can decrease to 3%-

5%. The separation of the solids, called dregs, from green liquor normally uses sedimentation orfiltration methods. Green liquor purification concepts in industrial use include:

Clarification Sedimentation Sludge blanket clarification

Filtration Cake filtration with or without precoat Cross flow filtration

The closing of the process cycles has led to an increased need to control the amount of non-processelements in the recovery cycle by separating dregs in causticizing. The non-process elements enter

with wood and with chemicals used in the process as for instance magnesium used in oxygen

delignification. Table 7 shows an example of green liquor prior to and after filtration. The greenliquor treatment process in Picture 35 includes the following main process stages:

Separation of solid impurities and dregs from green liquor Cooling of green liquor for causticizing Treating the dregs for proper disposal

Picture 35.Green liquor treatment

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Table 7. Removal of various elements from green liquor

Raw green liquor Filtered liquor

Dregs, mg/l 310 7

Potassium (K), mg/l 5.3 5.3

Magnesium (Mg), mg/l 53 1.1

Aluminum (Al), mg/l 60 40

Iron (Fe), mg/l 17 8.2

Manganese (Mn), mg/l 16 2.2

Copper (Cu), mg/l 1.3 0.88

Phosphorus (P), mg/l 43.8 41.2

Green liquor can be clarified by sedimentation. In sedimentation, the density difference between the

solid material and the liquid in green liquor separates the solid material by allowing it to settle. Aclarifier is normally an open cylindrical tank as Picture 36 shows. The solid material that is heavier

than the liquid settles to the bottom of the clarifier to form sludge. The clear liquor in the upper part

of the clarifier is removed as an overflow.

Picture 36.A clarifier

In green liquor cross flow filters a thin film of green liquor fall onto a vertical filter cloth. Some

falling liquor is forced through the vertical filter cloth while most of the liquor including the dregs

continues to fall to the bottom in the film. The liquor is recirculated to the top of the filter tomaintain the film by pumping. Filtration continues and the sludge thickens in the circulation untilreaching the desired sludge density. Thickened sludge is discarded from the filter by the pressure

inside the filter.

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Picture 37.Cross-flow filter for green liquorfiltration Picture 38. Green liquor cooler

It is necessary to cool the green liquor and to control the temperature of the green liquor that enters

the slaker in recausticizing to prevent the liquor from boiling in the slaker. Boiling may occur due tothe heat generated as a result of the exothermic slaking reaction. Cooling of green liquor, and even

of weak liquor, is difficult with indirect heat exchangers because of the rapid scaling and plugging

due to reduced solubility at lower temperatures.

Lime slaking and causticizing

Green liquor is mixed with calcium oxide, CaO, and the calcium oxide slakes with water and forms

calcium hydroxide, Ca(OH)2. The calcium hydroxide reacts with the Na

2CO

3in the green liquor to

produce NaOH and CaCO3. Picture 39 shows a slaker in which lime is slaked into green liquor.

Picture 39.Slaker

The slaking of lime is a strongly exothermic reaction:

CaO + H2O - Ca(OH)2+ 65 kJ/mol

The slaking reaction takes about 10-30 minutes to complete depending on the quality of lime. Theheat released due to the slaking reaction must be distributed evenly to the liquor by agitation toavoid local overheating. The temperature of the solution has a major influence on the reaction rate.

Below 70C, the reaction rate is significantly slower than at the normal operating temperatures close

to 100C.

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The causticizing reaction is an equilibrium reaction:

Ca(OH)2(s) + Na2CO3(aq) - 2 NaOH (aq) + CaCO3(s)

The reaction does not have any significant heat effect. Calcium hydroxide and CaCO3are insoluble

and participate in the reaction as solids. The causticizing reaction begins and continues to about 70%completion during the slaking which takes 10-20 minutes.

The main purpose of the causticizer following the slaker is to give time to complete the causticizing

reaction that has already progressed to a completion rate of two-thirds in the slaker. The slaked lime

slurry (lime milk) must remain in the causticizers sufficiently long, and the liquor must have propercontact with the lime mud particles at all times. A causticizer is shown in Picture 40.

The necessary residence time in the causticizer depends on the selected method for white liquor

separation. If white liquor is separated from lime by settling, a residence time of 1.5-2 hours is

adequate in the causticizer. The reactions can continue in the clarifiers. If a filter is used, the

residence time in the causticizers is usually about 2.5 hours as there is not much time for reactions in

the filter. Too short a residence time results in excessive amounts of unreacted calcium hydroxide,which remains in the lime mud and will deteriorate the filterability properties and blind the filter

media with precipitated CaCO3during the separation of lime mud and white liquor.

Picture 40.Causticizer

The causticizing efficiency, that is the ratio between NaOH and NaOH plus Na2CO3in the white

liquor, is dependent primarily on the strength of the white liquor, expressed as total alkali pervolume of liquor. A higher strength will reduce causticizing efficiency according to Goodwin's

curve. Industrial white liquors have a causticizing efficiency or degree slightly above 80%.

Lime handling and washing

White liquor and lime mud are separated after causticizing. There are some different

technologies in use for the separation of white liquor and lime mud and for the washing of lime

mud. Picture 41 shows the water balance when a pressurized disc filter is used for white liquorseparation and lime mud washing. The separated lime mud is taken to the next stage of lime

mud washing and thickening.

Filtrate from lime mud washing and thickening is taken to smelt dissolving as weak wash liquor

or weak white liquor. After further treatment and drying, the washed lime mud is taken to thelime kiln. In lime washing, condensates, and to some extent bleach plant filtrates, are used to

replace fresh water.

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Picture 41.Water balance for causticizing with a pressurized disc filter for white liquor separation

Other commonly used machines for white liquor and lime mud separation and mud washing arethe candle filter shown in Picture 42 and the disc filter shown in Picture 43. The heat and waterbalances when using these machines are somewhat different than the ones when using

pressurized disk filters.

Picture 42.A candle filterPicture 43.Pressurized disc filtration for white

liquor separation and lime mudwashing

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The white liquor produced and separated from the lime mud is taken to the digester for cooking.Table 8 shows typical properties of white liquor.

Table 8. An example of white liquor composition

Compound Concentration,g NaOH/l

Concentration,g Na2O/l

As compound,g/l

NaOH 95 73.6 95

Na2S 40 31.8 39

Na2CO3 23 17.8 30.5

Na2SO4 4 3.1 7.1

Na2S2O3 2 1.6 4.0

Na2SO3 0.5 0.4 0.8

Other compounds - - 3.0

Effective alkali 135 105.4 -

Active alkali 115 89.5 -

Total alkali 164.5 128.3 179.4

Sulfidity, % 29.6

S/Na equivalent ratio 0.58

Causticizing degree, % 80.5

Reduction degree, %

based on total S

82.2

Reduction degree, %

based on total SO4

90.9

The target in lime mud dewatering is to complete lime mud washing and increase the dry solids so

that the lime mud can be dried thermally and fed to the lime kiln. The moisture in the lime mud hasa considerable effect on the energy consumption of the lime kiln. Dry solids contents of 80%-90%

are possible after mechanical dewatering. The water soluble alkali content in the lime mud aftermechanical filter dewatering is under 0.1% as Na2O on dry mud. A low content of sodium isnecessary for the trouble-free operation of the lime kiln.

Picture 44.Lime mud washing and dewatering

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Lime drying and reburning

The lime reburning process is shown in Picture 45 and consists of the following unit operations:

Pumping of lime mud, that is CaCO3, from lime mud storage

Mechanical dewatering of lime mud to a high dry solids content Thermal dewatering in the kiln or in a separate cyclone Heating and calcining Cooling of the lime, that is calcium oxide, product Screening and crushing of lime Conveying of lime to storage

Picture 45.Lime reburning process

The physical size of a rotary lime kiln, with a capacity of 500 to 550 tons of reburned lime/day, is

4m-4.5m in diameter and 100m-140m in length. The production of a kiln of this size matches awhite liquor production of about 7,000 m

3/day, corresponding to a pulp production of slightly above

2,000 t/day. The kiln is supported by three or four piers.

Picture 46.Limekiln

A pneumatic dryer for lime mud is an emerging drying method where the lime is dried with the

exhaust gases from the limekiln in a separate cyclone. Lime mud is fed to a flue gas stream, wherethe heat of the flue gases dries the mud. A cyclone separates dry mud and feeds it to the kiln as

shown in Picture 47.

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Picture 47.A pneumatic dryer for lime mud

The traditional method for the drying of mud is the chain section of the rotary kiln. Picture 48 showsa chain system. The length of the chain section of the limekiln is chosen so that the mud is

completely dry when leaving the chain section. The length of the chain section is usually about 20%

of the total kiln length.

Picture 48.Chain system

Lime reburning

Heating or reburning of lime mud converts lime mud to lime according to the following reaction:

CaCO3- CaO + CO2

Dissociation of CaCO3to CaO and CO2starts when the temperature is above 820C. Increasing the

temperature accelerates the reaction. Reburning is done at a temperature of about 1,100C in order

to obtain sufficiently fast reaction rates.Lime mud entering the limekiln also contains some unreacted lime CaO, water, a small amount of

alkali, other impurities and non-process elements. The amount of impurities in the lime mud istypically about 7%-10% and depends on the amount of impurities introduced into the lime cycle

with the green liquor and the makeup lime. Non-process elements normally present in the lime cycle

are magnesium (Mg), aluminum (Al), silica (Si), phosphorus (P), iron (Fe), manganese (Mn) and

sulfur (S). Silica is a major problem when pulping annual plants or tropical wood. Sulfur comes forinstance from the burning of non-condensable gases in the lime kiln. Table 9 provides an example of

different lime compositions in the case of a Scandinavian mill and a mill for mixed tropical

hardwood. The major difference between the mill limes is in the content of silica.

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Table 9. An example of different lime compositions

Component

(expressed as

oxides)

Makeup

lime

Scandinavian mill with

softwood or hardwood

as raw material

Mill with mixed

tropical hardwood

as raw material

CaO, wt. % 93 N/A N/A

Na2O, wt. % 0.06 N/A N/A

SiO2, wt. % 2.5 0.4 7.9

MgO, wt. % 1.3 1.3 1.2

Fe2O3, wt. % 0.5 0.06 0.4

MnO, wt. % 0.04 0.1 0.01

Silica is a serious impurity in the lime cycle and causes problems especially when pulping annual

plants and tropical wood. The lime mud becomes difficult to handle and dewater with an increasing

silica content as shown in Picture 49.

Picture 49.Lime mud dry substance and silica in lime mud

The cell structure of annual plants contains solid silica particles that dissolve in the cooking liquor,proceed with the black liquor into the smelt, and end up in the green liquor in dissolved form. Silica

precipitates with lime mud during recausticizing because of the low solubility of calcium silicate inwhite liquor. Lime mud from the pulping of annual plants cannot often be reburned at all. A large

portion of the lime mud from the pulping of tropical hardwood must be dumped to keep the

recausticizing plant in operation.

Depending on the design of the burner in the lime kilns, also fuels other than fuel oil and natural gas

can be used. A combination burner can have ports for non-condensable gases and arrangements forfeeding methanol. Primary air passes through the burner, cools it, and stabilizes the flame.

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Picture 50.Burner

Picture 51.Temperature profile in the limekiln

Lime kilns have a refractory lining that protects the kiln shell from overheating and reduces the heat

loss to an acceptable level. The temperature distribution of lime kilns is wide from the cold feed-end

housing to the hot firing hood. In conventional kilns, where the mud is dried inside the kiln, thelowest temperature is about 100C. In the flame zone, the temperature is about 1,250C. In large

kilns with a shell diameter of over 4.5m, the flame zone lining has to tolerate even higher

temperatures like 1,300C-1,350C. In modern kilns with a separate mud dryer, the cold-endtemperature is 400C-600C. Picture 52 shows the lime kiln brick lining zones. Each zone has a

lining of a certain material and thickness.

Picture 52.Lime kiln brick lining zones

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Questions

1. The source of water, organic material and inorganic material in black liquor. / Mustalipensisltmn veden, orgaanisen aineen ja eporgaanisen aineen alkuper.

2. Properties of black liquor influencing the evaporation plant. / Mustalipen ominaisuudethaihduttamon kannalta.

3. Multistage evaporation plant, number of stages and steam consumption, soap separation. /Monivaihehaihduttamo, vaiheiden lukumr ja hyrynkulutus, suovan erotus.

4. The most common evaporator designs used in black liquor evaporation. / Yleisimmthaihdutinkonstruktiot mustalipen haihdutuksessa.

5. Condensate handling and reuse at kraft pulp mills. / Lauhteiden ksittely ja uudelleenkyttsulfaattisellutehtailla.

6. List the main steps of the kraft recovery process. / Luettele sulfaattiprosessintalteenottopuolen trkeimmt vaiheet.

7. The smelt reactions in a recovery boiler. / Sulareaktiot soodakattilassa.8. The combustion of black liquor in a recovery boiler; the black liquor droplet during

combustion, and the different stages of combustion. / Mustalipen poltto soodakattilassa;

mustalipepisaran kyttytyminen polton aikana ja palamisen eri vaiheet.

9. Recovery of heat in a recovery boiler. / Soodakattilan energian talteenottojrjestelm.10.Scaling of heat transfer surfaces in the recovery boiler. / Soodakattilan lmmnsiirtopintojen

likaantuminen.

11.The main factors influencing the heating value of black liquors. / Polttoarvoon vaikuttavattrkeimmt tekijt.

12.Influence of chlorine and potassium on recovery boiler operation; what can be done tocontrol chlorine and potassium levels? / Kloorin ja kaliumin vaikutus soodakattilan

toimintaan; mit voidaan tehd kloori- ja kaliumtasojen hallitsemiseksi.13.Green liquor treatment, measures to reduce the need for makeup lime. / Viherlipen

ksittely, miten vhennetn korvauskalkin tarvetta.

14.Slaking of lime and causticizing reactions. / Kalkin sammutus ja kaustisointireaktiot.15.The operation principle and construction of the lime kiln. / Meesauunin toimintaperiaate ja

rakenne.

11 chemical recovery cycle

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