The Kraft Recovery Process Kraft Pulping Process
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
Lignin Fibre
(NaOH + Na2S) Wood 155oC 900 kPa 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 Chemicals Chemicals 2000 Black t/d Dissolved 1500 Liquor Organics 1000 Wood
500 Fibre Pulp
0
2 Kraft Recovery Process
A Closed Cycle Process with 3 Main Functions:
Eliminate the waste material (black liquor) Pulping Recover pulping chemicals (NaOH and NaOH, Na, S, Organics Na2S Na2S) Recovery Generate steam and power Steam & Power
Kraft Recovery Process
Wood PulpingDigester
Pulp WashingWashing
Weak Black Liquor 15 – 18% ds
3 Kraft Recovery Process
Wood PulpingDigester
Pulp WashingWashing
Weak Black Liquor
Evaporators Ash Heavy Black Liquor Recovery 65 – 85% d.s. Boiler Makeup
Na2SO4
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
Composition K 2.0 1 – 3 Cl 0.5 0.1 – 4
4 Smelt Formation 0.4 kg smelt /kg BL d.s. C
H Na2CO3 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 PulpingDigester
Pulp NaOH WashingWashing Weak Black Liquor
Evaporators
Na2CO3 Green Water Liquor Heavy Black Liquor Na2S Recovery Smelt Boiler
Kraft Recovery Process
Wood Lime Kiln PulpingDigester
White Pulp Liquor WashingWashing Lime Lime Mud Weak Black Liquor CausticizingCausticizing Plant Plant Evaporators
Green Water Liquor Heavy Black Liquor Recovery Smelt Boiler
6 Na and Ca Cycles
Makeup Wood Lime Kiln PulpingDigester
NaOH Pulp CaO WashingWashing Na2S CaCO3 Calcium Na, S CausticizingCausticizing Sodium Plant Evaporators
Na2CO3 Water Na, S Na2S Recovery Makeup Boiler
Sulfur Cycles
SO2 Fuel Wood Lime Kiln PulpingDigester S Pulp NCG/ CaSO Na S 4 2 SOG WashingWashing Na2SO4
S Loss/ Scrubbing/ S Incineration CausticizingCausticizing Recovery Plant NCG/SOG Evaporators Na S SO 2 Water 2 Na SO 2 4 S Recovery S Makeup 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
Wood Loss Lime Kiln PulpingDigester Pulp White WashingWashing Liquor Lime Lime Mud Power LP Steam Weak CausticizingCausticizing Black Liquor Plant PulpingTurbine Plant Evaporators
Green HP Steam Liquor Heavy Black Liquor Recovery 22 GJ/ADt Smelt Boiler 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 5000 Maximum 4000
3000
2000 Capacity (t ds/d) (t Capacity 1000
0 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year Vakkilainen (2006)
Maximum Recovery Boiler Steam Temperature and Pressure 600 180 Temperature 960 F 500 150 1813 psi Pressure 400 120
300 90
200 60
100 30 Steam Pressure (bar) Steam temperature (C) 0 0 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year Vakkilainen (2006)
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
12 Common Operating Problems
Evaporators Lime kiln and Recausticizing Scaling High kiln fuel consumption Corrosion Ring formation High steam consumption Refractory damage Low solids in product liquor Chain damage Recovery Boilers NCG/SOG burning Fouling and plugging Gaseous/particulate emissions Tube corrosion and cracking Poor lime quality/availability Spout corrosion and cracking Overliming/underliming Low steam production Poor causticizing efficiency Poor sootblowing efficiency Poor mud settling and low solids Poor water circulation Clarifier corrosion Smelt-water emergencies Process control Gaseous/particulate emissions Liquor Cycle Tube damage by falling deposits NPE Accumulation (Cl and K) Unstable combustion/blackouts High deadload “Jelly roll” smelt/smelt run-off Chemical makeup Low reduction efficiency Na and S imbalance High dregs in smelt 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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