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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 above 15 wt-% molten - 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
°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)
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 *
Types and locations of fouling
Super Heaters -Carry over
Boiler Bank & Economizer - Sintering of fume
Economizer & ESP filter Acidic sulfates
Summary