The Kraft Recovery Process
Honghi Tran
Pulp & Paper Centre
University of Toronto
Toronto, Canada
Tappi Kraft Recovery Short Course
St. Petersburg, Florida, January 7-10, 2008
Kraft Pulping Process
Most important chemical pulping process
Global kraft pulp production: 130 million
metric tons/year (67% of total)
Advantages
High pulp strength
Versatility - ability to handle a wide range of wood
species
Favorable economics - High chemical recovery
efficiency (96 - 98%)
1
Kraft Pulping Process
Fibre
Fibre
Lignin
+
White
Liquor
(NaOH + Na2S)
155oC
900 kPa
Wood
Black Liquor
A 1000 t/d Kraft Pulp Mill
produces 1500 t/d BL d.s.
8000 ~ 10,000 t/d weak black liquor
3000
2500
t/d
Chemicals
Chemicals
2000
Dissolved
Organics
1500
1000
500
Black
Liquor
Wood
Fibre
Pulp
0
2
Kraft Recovery Process
A Closed Cycle Process with 3 Main Functions:
Eliminate the waste
material (black liquor)
Recover pulping
chemicals (NaOH and
Na2S)
Pulping
NaOH,
Na2S
Na, S, Organics
Recovery
Generate steam and
power
Steam &
Power
Kraft Recovery Process
Wood
Pulping
Digester
Pulp
Washing
Washing
Weak
Black Liquor
15 – 18% ds
3
Kraft Recovery Process
Wood
Pulping
Digester
Pulp
Washing
Washing
Weak
Black Liquor
Evaporators
Ash
Recovery
Boiler
Heavy
Black Liquor
65 – 85% d.s.
Makeup
Na2SO4
As-Fired Black Liquor Composition
(750 liquor samples; All Wood Species)
Typical
Range
72
65 – 85
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
Solids content, %
Composition
HHV, MJ/kg
4
Smelt Formation
0.4 kg smelt /kg BL d.s.
C
Na2CO3
H
O
2/3
Na
1/3
Na2S (Reducing Conditions)
S
K
Others:
Cl
Na2SO4, NaCl
K2CO3, K2S, K2SO4, KCl
Smelt Spout
Smelt
Na2CO3 + Na2S
5
Kraft Recovery Process
Wood
Pulping
Digester
Pulp
Washing
Washing
NaOH
Weak
Black Liquor
Evaporators
Na2CO3
Na2S
Green Water
Liquor
Smelt
Recovery
Boiler
Heavy
Black Liquor
Kraft Recovery Process
Wood
Lime Kiln
Lime
Mud
Pulping
Digester
Lime
Pulp
White
Liquor
Washing
Washing
Weak
Black Liquor
Causticizing
Causticizing
Plant
Plant
Evaporators
Green Water
Liquor
Smelt
Recovery
Boiler
Heavy
Black Liquor
6
Na and Ca Cycles
Makeup
Wood
Lime Kiln
CaCO3
Pulping
Digester
Calcium
Pulp
NaOH
CaO Na2S
Washing
Washing
Na, S
Sodium
Causticizing
Causticizing
Plant
Evaporators
Na2CO3
Na2S
Water
Na, S
Recovery
Boiler
Makeup
Sulfur Cycles
SO2
Fuel
Wood
Lime Kiln
Pulping
Digester
NCG/
SOG
CaSO4 Na2S
Na2SO4
Causticizing
Causticizing
Plant
S Loss/
Recovery
Na2S
Na2SO4
S Makeup
S
Pulp
Washing
Washing
S
Scrubbing/
Incineration
NCG/SOG
Water
SO2
Evaporators
S
Recovery
Boiler
7
Makeup Chemicals (Na, S and Ca)
Sodium
8 to 16 kg/ADt as Na2O
NaOH, Na2CO3, Na2SO4 (salt cake)
Organic Na compounds (acetate, etc.)
Sulfur
1.4 to 4 kg/ADt as S
Na2SO4, NaSH, H2SO4, Na3H(SO4)2, Elemental S
S in lime kiln fuels
Calcium
4 to 8 kg/ADt as CaO
Purchased lime
Lime rock
Main Sources of NPEs
(Non Process Elements)
Wood: Si, Al, Cl, K, Mg, Mn, P, Fe, Ni, Cr, etc.
Makeup caustic: Cl
Additive: Si, Mg
Makeup lime: Si, Al, Mg, P, Fe
Refractory bricks: Si, Al
Corrosion products: Fe, Ni, Cr
8
Types of NPEs
Form soluble compounds: Cl, K
Be with the liquor
Accumulate
Form partially soluble compounds: Si, Al
Precipitate under appropriate conditions
Form insoluble compounds: Most other
elements
Do not accumulate
Be removed from the recovery system with grits,
dregs and lime mud/dust
Energy Recovery
1.9 GJ/ADt
Loss
Wood
Lime Kiln
Pulping
Digester
Pulp
Lime
Mud
Lime
Washing
Washing
White
Liquor
Power
LP Steam
Weak
Black Liquor
Causticizing
Causticizing
Plant
Plant
Pulping
Turbine
Green
Liquor
Smelt
Evaporators
HP Steam
Heavy
Recovery
Boiler
Black
Liquor
22
GJ/ADt
Loss
1 GJ/t = 0.860 MM Btu/ton
9
Recovery Boiler Energy
Production
Steam
Typically 3.5 kg/kg BLds
May vary from 2.5 to 3.8 kg/kg BLds
Power
A 1000 t/d kraft pulp mill may generate 25 to
35 MW of electricity from black liquor
combustion
Technological Advancements
Evaporators/concentrators
Falling film, plate-type
High solids (75 to 85%)
Recovery boilers
High solids firing
High steam temperature/pressure
High efficiency
Recaust and lime kilns
Pressurized filters
Lime mud dryer
10
Recovery Boiler Firing Capacity
6000
11 million lbs/d
Capacity (t ds/d)
5000
Maximum
4000
3000
2000
1000
0
1930 1940 1950 1960 1970 1980 1990 2000 2010
Year
Vakkilainen (2006)
Maximum Recovery Boiler Steam
Temperature and Pressure
180
Temperature
960 F
500
400
1813 psi
Pressure
150
120
300
90
200
60
100
30
Steam Pressure (bar)
Steam temperature (C)
600
0
0
1930 1940 1950 1960 1970 1980 1990 2000 2010
Vakkilainen (2006)
Year
11
Benefits of High Solids Firing
Significantly increases steam generation
Improves combustion stability
Lower TRS and SO2 emissions
Less boiler fouling and plugging
Increase capacity in existing units.
Operating Problems
Many problems can occur
They can be:
Equipment related
Process related
Liquor chemistry related
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Common Operating Problems
Evaporators
Scaling
Corrosion
High steam consumption
Low solids in product liquor
Recovery Boilers
Fouling and plugging
Tube corrosion and cracking
Spout corrosion and cracking
Low steam production
Poor sootblowing efficiency
Poor water circulation
Smelt-water emergencies
Gaseous/particulate emissions
Tube damage by falling deposits
Unstable combustion/blackouts
“Jelly roll” smelt/smelt run-off
Low reduction efficiency
High dregs in smelt
Lime kiln and Recausticizing
High kiln fuel consumption
Ring formation
Refractory damage
Chain damage
NCG/SOG burning
Gaseous/particulate emissions
Poor lime quality/availability
Overliming/underliming
Poor causticizing efficiency
Poor mud settling and low solids
Clarifier corrosion
Process control
Liquor Cycle
NPE Accumulation (Cl and K)
High deadload
Chemical makeup
Na and S imbalance
High sulphidity operation
Problems Can Be Costly
Recovery Boiler Plugging
0.8 - 1.5 million USD
Recovery Boiler Tube Corrosion/Cracking
5 - 20 million USD
Lime kiln ringing
10,000 - 500,000 USD
Environmental impacts
??? USD
13
This Course is Designed to
Help You To:
Understand process principles
Understand possible causes of problems
and find solutions to them
Recognize potential problems that may
occur and devise means to prevent them
from occurring
Challenges
Increased economy
Increase production capacity
Lower operating costs
Increased environmental protection
Increase recovery efficiency
Reduce emissions and discharge
New technologies, sensors and control
strategies
Process simplification
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