Operational Challenges in Energy and Chemical Recovery in Kraft Pulp Mills Honghi Tran University of Toronto Toronto, ON, CANADA Roberto Villarroel Eldorado Brazil Sao Paulo, BRAZIL The 7 th International Colloquium on Eucalyptus Pulp Vitoria, Brazil, May 28, 2015 Presentation Outline Brief Overview Energy & Chemical Recovery from Black Liquor Operational Issues Operational Challenges in Modern Eucalyptus Pulp Mills 2 1
Kraft Pulping Process Uses sodium hydroxide (NaOH) and sodium sulfide (Na 2 S) as cooking chemicals Predominant pulping process ~ 130 million metric tons/year globally 77% of all wood pulp (>90% in Brazil) Advantages High versatility High pulp strength Favorable economics (high chemical and energy recovery efficiency) 3 A 1000 t/d Kraft Pulp Mill produces 1500 t/d BL d.s. 8000 ‐ 10000 t/d weak black liquor 3000 2500 Chemicals Chemicals 2000 Black t/d Liquor Dissolved 1500 (dry) Organics Wood 1000 Fibre 500 Pulp 0 4 2
Black Liquor Recovery (Courtesy of Wood Valmet) Digester Chips Recovery Boiler Power Boiler Waste Water Treatment Green Liquor Na 2 CO 3 , Na 2 S 5 Slaker Common Operating Problems Evaporator Recausticizing and Lime Kiln Fouling/scaling Overliming Corrosion Poor causticizing efficiency Poor efficiency Poor green liquor filterability Low solids in product liquor Poor mud settling/low solids Recovery Boiler Corrosion High kiln fuel consumption Fouling/plugging Ring/ball formation Corrosion/cracking/tube leakage Refractory damage Low steam production Chain damage Poor sootblowing efficiency Gaseous/particulate emissions Poor water circulation High residual carbonate Gaseous/particulate emissions Poor lime quality/availability Tube damage by falling deposits Unstable combustion/blackouts Liquor Cycle Jelly ‐ roll smelt/smelt run ‐ offs Non ‐ process elements Low reduction efficiency High deadload Smelt ‐ water explosions Na and S imbalance Dissolving tank explosions High sulphidity operation High dregs in smelt Corrosion 6 3
RB Fouling RB Superheater Corrosion 7 Lime Kiln Ring Formation Ball Formation 8 4
Recovery Boiler Capacity Year 1933 1976 1996 2004 2010 2016 Capacity 120 1500 3800 6000 7000 11600 t/d d.s. (25.6 million lbs/d d.s.) 9 (Courtesy of Valmet Power) Technological Advancements Recovery Boilers High firing capacity High solids (80+%) firing High steam temperature/pressure (515 o C/130 bar, 960 o F/1885 psig) High heat recovery efficiency Sootblowing technology NCG burning Cl and K removal systems 10 5
Technological Advancements Evaporators/Concentrators High solids (80 + %) Falling film (tube ‐ type & plate ‐ type) Reynolds enhanced crystallizers Recaust and Lime Kilns Pressurized filters Lime mud dryers and product coolers Multi ‐ fuel burners Alternative fuel burning Improved Process Control and Metallurgy 11 Technological advancements have alleviated many problems, but have also made some problems worse and have led to new problems 12 6
Problems in Modern Pulp Mills Evaporator Recausticizing and Lime Kiln Fouling/scaling Overliming Corrosion Poor causticizing efficiency Poor efficiency Poor green liquor filterability Low solids in product liquor Poor mud settling/low solids Corrosion Recovery Boiler High kiln fuel consumption Fouling/plugging Ring/ball formation Corrosion/cracking/tube leakage Refractory damage Low steam production Chain damage Poor sootblowing efficiency Gaseous/particulate emissions Poor water circulation High residual carbonate Gaseous/particulate emissions Poor lime quality/availability Tube damage by falling deposits Unstable combustion/blackouts Liquor Cycle Jelly ‐ roll smelt/smelt run ‐ offs Non ‐ process elements Low reduction efficiency High deadload Smelt ‐ water explosions Na and S imbalance Dissolving tank explosions High sulphidity operation High dregs in smelt Corrosion 13 Challenges in Modern Eucalyptus Mills Non Process Elements (NPEs) Low lime mud solids and poor green liquor filterability Cl and K accumulation and control Na and S balance (or imbalance) 14 7
Non ‐ Process Elements (NPEs) Elements that do not participate in the pulping process Commonly referred NPEs: Cl and K (most K actually exists as a process element, not an NPE) Mg, Si, Al, P, Mn, Fe V, Cr, Ni, Zn, Pb, Cu, Ti, Ba, etc. (amounts typically too small to be significant) Can be problematic if present in large amounts 15 Sources of NPEs Sources Elements Wood K, Cl, Mg, P, Si, Al, Mn, Fe, etc. Makeup lime Mg, Si, Al, P, Fe Makeup chemicals Cl Makeup water Cl, P Additives Mg, Si Refractory bricks Si, Al Corrosion products Fe, Ni, Cr Biosludge Si, Cl, Mg, K, P, Mn, Fe, Al Petcoke V, Ni 16 8
Whether or not an NPE accumulates in the recovery cycle depends on the solubility of its compounds in the liquor 17 Solubility of NPE Compounds Solubility Elements Consequences in Liquor Cl, K, P, B, V High • Accumulate in the liquor cycle Si, Al Medium • Accumulate in liquor & lime cycles • Accumulate in the lime cycle Mg, Mn, P, Fe, Low • Removed from the system with Ni, Cr, Si, Al grits, dregs, mud and lime dust 18 9
Accumulation Factors LIQUOR CYCLE LIME CYCLE 10 50 Mill 1 Mill 1 Accumulation Factor Accumulation Factor 8 Mill 2 40 Mill 2 Mill 3 Mill 3 Mill 4 6 Mill 4 30 4 20 2 10 0 0 Mg Mn Al Si S K Cl Mg Mn Al Si K Cl 19 Richardson et al, 1998 ICRC Problems and Contributing NPEs Operational Problems Contribut ing NPEs Recovery boiler fouling Cl, K Recovery boiler SH corrosion K, Cl Evaporator scaling Si, Al, Ca, Fe Poor green liquor filterability Si, Mg Poor mud settling/dewatering Si, Mg, Al, Fe Low lime availability/reactivity S, Mg, Si, P, Mn, Fe, Al 20 10
Effect of SiO 2 on Mud Solids Content 100 Kiln A1 90 Kiln A2 Mud Solids (wt%) 80 Kiln B Kiln C 70 Arpalahti et al. (1999) 60 50 40 30 0 4 8 12 16 SiO 2 Content in Lime (wt%) 21 Problems with of SiO 2 Reacts with Na 2 CO 3 to form CSH Gel Na 2 SiO 3 in the recovery boiler Na 2 SiO 3 reacts with lime in the causticizing plant to form Calcium Silicate Hydrate gel ( x CaO y SiO 2 n H 2 O) A small amount of Si or SiO 2 can result in a large amount of CSH gel Filter clogging 22 11
Poor Green Liquor Filterability Experienced in a number of Brazilian mills equipped with pressurized filters Possible Causes High suspended solids in green liquor (char/dregs from RB and suspended solids from weak wash) Large amount of Si and/or Mg gel ‐ like particles Overliming Tight filter cloth 23 Green Liquor Filter Cloth Clean Clogged Deposit 100 µm 100 µm 24 12
Cl and K Accumulation in Ash (No Purging) 6 K 5 Conc. in Ash (wt%) 4 3 Cl 2 1 0 0 60 120 180 240 300 360 420 Days 25 Factors Affecting Cl and K Accumulation Input Output (Degree of mill closure) Chemical losses Ash purging/ash treatment Recovery boiler operation Sulphidity (Cl only) 26 13
Control Targets for Cl and K Contents in Precipitator Ash Chloride Potassium (wt% Cl) (wt% K) Very low < 0.4 < 1.8 Low 0.4 – 1.2 1.8 – 2.5 Typical 1.2 – 2.9 2.5 – 5 High 2.9 – 8.5 5 – 9 Very high > 8.5 > 9 27 Precipitator Ash Amount produced: Typically about 75 kg/ADt pulp Enriched in Cl and K Cl enrichment factor = 2.5 K enrichment factor = 1.5 Ash purging is the easiest/most effective way to control Cl and K accumulation Amount purged: 10 to 30% of the total ash Disadvantage: High Na and S losses 28 14
Cl and K Accumulation (with Ash Purging) 6 Start Purging K 0% 5 Conc. in Ash (wt%) 10% 4 20% 3 40% Cl 10% 2 20% 1 40% 0 0 60 120 180 240 300 360 420 Days Takes 1 to 2 months to reach steady state Precipitator Ash Treatment Various technologies available Ash treatment helps remove Cl and K but also recover some Na and S Most technologies rely on effective separation of solids from ash ‐ water slurries Separation is difficult when Slurry concentration: > 1 kg/L H 2 O Ash carbonate content: > 6 wt% CO 3 30 15
Effect of CO 3 on Liquid ‐ Solids Separation 0.3 wt%CO 3 12.3 wt%CO 3 Ash with High CO 3 Contents Poor solids ‐ liquid separation Low Cl and K removal efficiency Low Na and S recovery efficiency CO 3 may be neutralized with sulfuric acid Add more S into the system, resulting in Na and S imbalance (high sulphidity) 32 16
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