[PDF] - Operational Challenges in Energy and Chemical Recovery in Kraft Pulp PDF Document

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Operational Challenges in Energy and Chemical Recovery in Kraft Pulp Mills

The 7th International Colloquium on Eucalyptus Pulp Vitoria, Brazil, May 28, 2015

Honghi Tran

University of Toronto Toronto, ON, CANADA

Roberto Villarroel

Eldorado Brazil Sao Paulo, BRAZIL

Presentation Outline

Brief Overview

Energy & Chemical Recovery from Black Liquor Operational Issues

Operational Challenges in Modern Eucalyptus Pulp Mills

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Kraft Pulping Process

Uses sodium hydroxide (NaOH) and sodium sulfide

(Na2S) 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)

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A 1000 t/d Kraft Pulp Mill

500 1000 1500 2000 2500 3000 Chemicals Wood Dissolved Organics Fibre Chemicals

t/d

Black Liquor (dry) Pulp

8000 ‐ 10000 t/d weak black liquor produces 1500 t/d BL d.s.

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(Courtesy of Valmet) Recovery Boiler Digester Wood Chips Power Boiler Slaker Waste Water Treatment Green Liquor Na2CO3, Na2S

Black Liquor Recovery

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Common Operating Problems

 Evaporator

 Fouling/scaling  Corrosion  Poor efficiency  Low solids in product liquor

 Recovery Boiler

 Fouling/plugging  Corrosion/cracking/tube leakage  Low steam production  Poor sootblowing efficiency  Poor water circulation  Gaseous/particulate emissions  Tube damage by falling deposits  Unstable combustion/blackouts  Jelly‐roll smelt/smelt run‐offs  Low reduction efficiency  Smelt‐water explosions  Dissolving tank explosions  High dregs in smelt

 Recausticizing and Lime Kiln

 Overliming  Poor causticizing efficiency  Poor green liquor filterability  Poor mud settling/low solids  Corrosion  High kiln fuel consumption  Ring/ball formation  Refractory damage  Chain damage  Gaseous/particulate emissions  High residual carbonate  Poor lime quality/availability

 Liquor Cycle

 Non‐process elements  High deadload  Na and S imbalance  High sulphidity operation  Corrosion

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4 RB Fouling RB Superheater Corrosion

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Lime Kiln Ring Formation Ball Formation

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Recovery Boiler Capacity

120 1500 Year 11600 t/d d.s. (25.6 million lbs/d d.s.) 7000 1933 1976 1996 2004 2010 2016 Capacity 3800 6000

(Courtesy of Valmet Power)

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Technological Advancements

Recovery Boilers

High firing capacity High solids (80+%) firing High steam temperature/pressure (515oC/130 bar,

960oF/1885 psig)

High heat recovery efficiency Sootblowing technology NCG burning Cl and K removal systems

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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

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Technological advancements have alleviated many problems, but have also made some problems worse and have led to new problems

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Problems in Modern Pulp Mills

 Evaporator

 Fouling/scaling  Corrosion  Poor efficiency  Low solids in product liquor

 Recovery Boiler

 Fouling/plugging  Corrosion/cracking/tube leakage  Low steam production  Poor sootblowing efficiency  Poor water circulation  Gaseous/particulate emissions  Tube damage by falling deposits  Unstable combustion/blackouts  Jelly‐roll smelt/smelt run‐offs  Low reduction efficiency  Smelt‐water explosions  Dissolving tank explosions  High dregs in smelt

 Recausticizing and Lime Kiln

 Overliming  Poor causticizing efficiency  Poor green liquor filterability  Poor mud settling/low solids  Corrosion  High kiln fuel consumption  Ring/ball formation  Refractory damage  Chain damage  Gaseous/particulate emissions  High residual carbonate  Poor lime quality/availability

 Liquor Cycle

 Non‐process elements  High deadload  Na and S imbalance  High sulphidity operation  Corrosion

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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)

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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

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Sources of NPEs

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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

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Whether or not an NPE accumulates in the recovery cycle depends on the solubility of its compounds in the liquor

Solubility of NPE Compounds

Elements Solubility in Liquor Consequences

Cl, K, P, B, V High

Accumulate in the liquor cycle

Si, Al Medium

Accumulate in liquor & lime cycles

Mg, Mn, P, Fe, Ni, Cr, Si, Al Low

Accumulate in the lime cycle
Removed from the system with

grits, dregs, mud and lime dust

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Accumulation Factors

10 20 30 40 50 Mg Mn Al Si K Cl

Mill 1 Mill 2 Mill 3 Mill 4 LIQUOR CYCLE

Accumulation Factor 2 4 6 8 10 Mg Mn Al Si S K Cl

Mill 1 Mill 2 Mill 3 Mill 4 LIME CYCLE

Accumulation Factor

Richardson et al, 1998 ICRC

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Problems and Contributing NPEs

Operational Problems Contributing 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

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Effect of SiO2 on Mud Solids Content

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30 40 50 60 70 80 90 100 4 8 12 16

SiO2 Content in Lime (wt%) Mud Solids (wt%)

Kiln A1 Kiln A2 Arpalahti et al. (1999) Kiln B Kiln C

Problems with of SiO2

Reacts with Na2CO3 to form

Na2SiO3 in the recovery boiler

Na2SiO3 reacts with lime in

the causticizing plant to form Calcium Silicate Hydrate gel (xCaO ySiO2 nH2O)

A small amount of Si or SiO2

can result in a large amount

f CSH gel

 Filter clogging

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CSH Gel

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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

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Green Liquor Filter Cloth

100 µm 100 µm

Clean Clogged

Deposit

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Cl and K Accumulation in Ash (No Purging)

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1 2 3 4 5 6 60 120 180 240 300 360 420

Conc. in Ash (wt%)

Days

Cl K

Factors Affecting Cl and K Accumulation

Input Output (Degree of mill closure)

Chemical losses Ash purging/ash treatment

Recovery boiler operation Sulphidity (Cl only)

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Control Targets for Cl and K Contents in Precipitator Ash

Chloride (wt% Cl) Potassium (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

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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

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Cl and K Accumulation (with Ash Purging)

1 2 3 4 5 6 60 120 180 240 300 360 420

Conc. in Ash (wt%)

Days

Cl K

10% 20% 40% 0% 10% 20% 40% Start Purging  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 H2O Ash carbonate content: > 6 wt% CO3

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Effect of CO3 on Liquid‐Solids Separation

0.3 wt%CO3 12.3 wt%CO3

Ash with High CO3 Contents

Poor solids‐liquid separation

Low Cl and K removal efficiency Low Na and S recovery efficiency

CO3 may be neutralized with sulfuric acid

Add more S into the system, resulting in Na and S imbalance (high sulphidity)

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Effects of High Sulphidity

Bad

Increases propensity for corrosion in RB and liquor

cycle and for RB superheater fouling and plugging

Increases TRS/SO2 emissions Lowers steam production

Good

Lowers lime requirement Increases smelt fluidity Lowers Cl content in ash and liquor Lowers carbonate content in ESP ash

Bad effects outweigh good effects

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Causes of Na/S Imbalance

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Increased NaOH consumption to control sulfidity High Bed Temperature High CO3 Content in Ash Low Cl/K removal effi. Increased use of sulfuric acid to neutralize CO3

High sulfidity

Keep S in the system

Low TRS Emissions

Low Cl content in liquor

Improve in NCG collection; Burning NCG in Rec. boilers

High BL Solids, >75%

High RB Capacity

Add more S to liquor system High Cl/K removal effi.

High TTA