Introduction to Kraft process

-

Black liquor

and

The black liquor recovery boiler

Black liquor

+

Black liquor

Wood

Lignin etc. Fibers

Pulping chemicals

(Na2S, NaOH)

Fibers

• Kraft chemical pulping

(Honghi Tran)

155°C 900 kPa

0

500

1000

1500

2000

2500

3000

Chemicals

Wood

Dissolved

Organics

Fibre

Chemicals

t/d

Black

Liquor

Pulp

produces 1500 t/d BL d.s.

A 1000 t/d Kraft pulp mill

(Honghi Tran)

As-Fired Black Liquor Composition

(750 liquor samples; All Wood Species)

Typical Range

Solids content, % 72 65 – 85

HHV, MJ/kg 13.9 12.5 – 15.5

C, wt% d.s. 33.9 30 – 40

H 3.4 3.2 – 4.0

O 35.8 34 – 38

Na 19.6 17 – 22

S 4.6 3.6 – 5.6

K 2.0 1 – 3

Cl 0.5 0.1 – 4

Co

mp

os

itio

n

(Honghi Tran)

Black liquor recovery boiler

Chemical

pulping

• Wood

• Chemicals

• Water Fibers

Black liquor

• Organic matter

• Spent chemicals

• Water

Recovery processes

• Recovery Boiler

• Black liquor is burned

Recovered

chemicals

Heat

• Recovery of chemicals and heat

800 °C, 19% O2

Black liquor droplet burning

Stages of combustion

Drying Devolatilization Char

combustion Smelt bead

Removal of water and

combustion of organic matter

Inorganics

→ pulping

chemicals

Black liquor combustion

Digester

Wood chips

Pulp

White

liquor

Black liquor 15% DS

NaOH

Na2S

Na2CO3

Na2SO4

Recovery boiler

Evaporators

Smelt

Black

liquor 75% DS

Flue gas

Na2CO3

Na2SO4 Na2CO3

Na2S

Causticizing

Green

liquor

Dissolver

Water Na2CO3

Na2S

Steam turbine

Steam

(Power)

Process steam

CaCO3 CaO Lime

kiln

(K Whitty)

Chemicals recovery in the Kraft process

CaO + H2O → Ca(OH)2

Ca(OH)2 + Na2CO3 → 2NaOH + CaCO3

Wood Yard

Chip Handling

Pulping area

White Liquor Preparation

BLStorage

Evaporators

Recovery Boiler

Lime Kiln

85 m

Black liquor recovery boiler

15 m

First Tomlinson recovery boiler (1937)

First Tomlinson

Recovery Boiler Modern Recovery Boiler

(1937)

15 m

85 m

Black liquor combustion in the recovery boiler

Recovery of

1) Heat

2) Chemicals

By

1) Oxidizing organic matter

2) Reducing Na2SO4 to Na2S

Flue gas

Superheaters

Black liquor

Air

Steam Power & process steam

Smelt

Black liquor combustion in the recovery boiler

Kankkunen et al

• Black liquor spray

• In-flight combustion

Black liquor combustion in the recovery boiler

• Char bed

• Material input by droplets

• Char bed burning

Black liquor spraying

Black liquor combustion in the recovery boiler

Black liquor spraying

(Miik

kula

inen a

nd K

ankkunen)

Splash Plate

Nozzle

V-Type

Nozzle

Typical Nozzle

Gun Assembly

Swirlcone

Nozzle

Beer Can

Nozzle

Nozzle Types

(Rick Wessel)

Splash Plate Nozzle Geometry

Looking down

on a splash

plate nozzle 35

plate angle

45 plate angle

Barrel Tilt

Coupling Angle

Splash Plate

Angle Black Liquor

Coupling Barrel

(Rick Wessel)

Three Temperature Regimes

Non-Flashing Flashing Transitional

Solids = 69%, EBP = 115C

Excess Temperature, Te = T - TEBP

(Rick Wessel)

(Miik

kula

inen a

nd K

ankkunen)

Black Liquor Flashing

Flashing

Higher nozzle velocity

due to expansion of

steam bubbles

Non-flashing

(Rick Wessel)

Effect of Temperature on Drop Size

Dmedian 1/T0.14

Note: liquor mass flow rate, solids, and nozzle diameter are fixed

93 115

(°F)

(°C) 121

(Rick Wessel)

Black liquor spraying – droplet size distribution

Texcess

18°C

17°C

14°C

Black liquor spraying – droplet size distribution

Texcess

Mass flow 4.3 kg/s 5.3 kg/s 6.1 kg/s

18°C

17°C

14°C

Black liquor spraying – droplet size distribution

Texcess

Mass flow 4.3 kg/s 5.3 kg/s 6.1 kg/s

18°C

17°C

14°C

Droplets should deliver char carbon to bed

goal: Na2SO4(l) + 4C(s) → Na2S(l) + 4CO(g)

avoid: Na2S(l) + 2O2(g) → Na2SO4(l)

Avoid too wet droplets to bed

- Bed temperature

Combustion air

Liquor spray

Smelt

(Honghi Tran)

primary

secondary

tertiary

Liquor spraying & Char bed burning

Droplet burning

Drying

Droplet burning

Pyrolysis

Char

combustion

Smelt

bead

Drying

Effect of droplet size

10 mm

Drying Pyrolysis

Char

combustion

6 mm

Effect of droplet size

2 mm

Effect of droplet size

0.5 mm

Effect of droplet size

3.6 x 6 mm droplet

No swelling

Effect of swelling

3.6 x 2 mm droplet

No swelling

Effect of swelling

Fate of Droplets in a Recovery Boiler

1

2

3 4

Entrained - too small and/or too high swelling properties

- Na2S Na2SO4 -

- “carry over” deposits -

1

Reach bed as smelt - still too small and/or too high swelling properties

- Na2SO4 to bed -

- no carbon to bed -

2

Reach bed while burning - optimium size of droplets and/or optimum swelling prop.

- Na2S to char bed +

- Carbon to char bed +

3

Reach bed wet - too large droplets and/or too small swelling properties

- carbon to bed +

- Na2S produced in bed +

- H2O to char bed char bed temperture drops -

4

(Hupa & Frederick)

Char bed

”Härkä”

Matti Reitti, 1975

Char bed

Char bed V

ideo a

t 64 x

norm

al speed

Char bed burning

Char-carbon oxidation

C(s) + ½O2 → CO

C(s) + CO2 → 2CO

C(s) + H2O → CO + H2

C(s) + ¼Na2SO4(l) → CO + ¼Na2S(l)

Combustion air

Smelt

Liquor spray

(Honghi Tran)

primary

secondary

tertiary

Char bed burning

(g/s-m2)

O2 CO2 H2O SO4

35% 17% 13% 35%

Share of total carbon conversion

Bergroth et al. 2004

Char bed size and shape

- Goal: sufficient amount of char for sulfate reduction

Avoid: too high bed/uncontrolled growth

- Material balance: Input vs. Conversion

- Controlled via changes in spraying and air delivery

Char bed size and shape

Stable char bed burning important for

stable operation of RB

- Example: impact on steam production

Stable bed Growing bed

15

16

17

18

19

20

09.02 10.02 11.02 12.02 13.02

Date

liq

uo

r fe

ed

pe

r w

all

( li

ters

/se

co

nd

)

Left

Right

Liquor feed per wall (liters/second)

• Process data on liquor spraying

Stable Growing

Impact of bed operation on steam production

15

16

17

18

19

20

09.02 10.02 11.02 12.02 13.02

Date

liq

uo

r fe

ed

pe

r w

all

( li

ters

/se

co

nd

)

138

139

140

141

142

Liq

uo

r te

mp

era

ture

(°C

)

Left

Right

Liquor temperature (°C)

• Lower T → bigger droplets → more carbon to bed

Impact of bed operation on steam production

Stable Growing

Impact of bed operation on steam production

115

120

125

130

135

8:00 10:00 12:00 14:00 16:00 18:00

Time

Ste

am

(kg

/s)

115

120

125

130

135

8:00 10:00 12:00 14:00 16:00 18:00

Time

Ste

am

(kg

/s)

Stable Bed Growing Bed

Data

- 1 Minute

- 1 Hour

• Process data on steam production

Impact of bed operation on steam production

Impact of bed operation on steam production

300

320

340

360

380

400

420

15:00 16:00 17:00 18:00

Time of day during 12.2.2009

Po

wer

(MW

)

Heat to steam

Time of day

Fouling

• Carryover

– Smelt from droplets carried up with flue gases

– Size mm range

• Fume (dust)

– Condensed vapors

– Size μm range

• Cleaning – Soot blowing

Carryover

• Smelt from burning droplets carried up with combustion

gases

• Overloaded boilers most suseptible because of large gas

flow

• Composition slightly reduced in K and Cl

• If between ~15 and 70% molten (T15 and T70) it will stick

Fume

• Comprised of alkali salt vapors

• Condense as the gas cools

• Carried to heat transfer surfaces by thermal gradients

• Enriced in K and Cl when compared to BL

• Typically deposit in back end of boiler bank and

beginning of economizers

• Problem if sinter significantly before removed by soot

blowers

• T0 key variable

Entrained BL char

Fume Particles

1 mm

1 mm

Particles From

Black Liquor

Burning

100 mm

Intermediate Particles

1 μm

Fume particles attached on a carry over particle

Composition of Recovery Boiler Fly Ash

Carry over

• Burning BL droplets

• Size mm range

Fume

• Released Na, K, S and Cl from

burning BL droplets

• Evaporated Na, K, S and Cl

from smelt bed

• Formed from gas phase

• Size μm range

Na2CO3

Na2SO4

Na2S

Cl, K, etc. Cl, K, etc.

Na2CO3

Na2SO4

Acidic sulfates - NaHSO4

• Requires surplus of SO2 in flue gases

• Partly molten already at 400 - 450oC

• Not stable above 450oC

• Indications by pH measurements

Cl , K, etc.

Na 2 SO 4

NaHSO 4 *

°C

200

400

600

800

1000

LOWER

FURNACE

TERT.

AIR

SUPER

HEATER

BOILER

BANK

ECONO-

MIZER

H2S(g) SO2(g) Na2SO4(s) Na2CO3(s)

Na2CO3 (l,s)

O2 (g)

O2 (g)

CO2 (g)

H2S (g)

SO2 (g)

2NaOH(g)

Na2SO4 (l,s)

REACTIONS OF S AND Na IN FLUE GASES

(S/Na2 = 0.8, hot lower furnace, low sulphidity)

°C

200

400

600

800

1000

LOWER

FURNACE

TERT.

AIR

SUPER

HEATER

BOILER

BANK

ECONO-

MIZER

H2S(g) SO2(g) Na2SO4(s) Na2CO3(s)

O2 (g)

O2 (g)

CO2 (g)

NaHSO4(s)

O2 (g)

H2O (g)

SO3(g)

H2S (g)

SO2 (g)

2NaOH(g)

Na2SO4 (l,s)

Na2CO3 (l,s)

2NaHSO4 (l,s)

SO3 (g)

REACTIONS OF S AND Na IN FLUE GASES (S/Na2 = 1.5, cool lower furnace, high sulphidity)

Types and locations of fouling

Super Heaters -Carry over

Boiler Bank & Economizer - Sintering of fume

Economizer & ESP filter Acidic sulfates

Summary