Melting properties of the smelt, carry-over particles and superheater deposits

in the recovery boiler

Daniel Lindberga, Rainer Backmana,b, Patrice Chartrandc, Mikko Hupaa,

a) Åbo Akademi Process Chemistry Centre b) Energy Technology and Thermal Process Chemistry

Umeå University, Swedenc) Centre de Recherche en Calcul Thermochimique, Montréal, Canada

Outline

– Thermodynamic modeling of molten alkali saltsin the pulp and paper industry• Purpose and goal• Historical developments• Thermodynamic model

– Recovery boiler smelt• Polysulfides

– Carry-over particles and superheater deposits• Pyrosulfates

– Summary

Thermochemistry and melting properties of inorganic compounds in black liquor conversion processes

The formation and the existence of molten alkali salts isof great importance in the recovery boiler

• Behavior of the smelt

• Formation of sticky deposits on superheaters

• Corrosion of alloys in contact with a melt

• Important reactions involving a molten phase

Na2SO4(l)+(2+2X) C(s) ⇋ Na2S(l)+4X CO(g)+(2-2X) CO2(g)

Na2CO3(l) + NaBO2(l)⇋ Na3BO3(l) + CO2(g)

GOAL

To predict the melting behaviour for the Na+,K+/CO32-,S2-,SO4

2-

,Cl-,S2O72-, Sn

2- system in the recovery boiler at varying Temperature, Pressure and Composition

– Experiments

€€ + → set of discrete data

– Empirical equations (curve fits of experiments)

– Thermodynamic models

( )

ioii

nnPTii

nP

nP

NaClCONaSONaSNa

aRTnG

TGS

T

TG

H

nnnnPTfG

kji

i

ln1

...,,,,,

,,,,

,

32422

=−⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

=⎟⎠⎞

⎜⎝⎛∂∂

−=⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

∂

∂=

=

μμμ

(For every phase)

Thermodynamic modellingDevelopments & Highlights (1)

• 1960-1964: Erik Rosén• Computerized calculations of multicomponent/

multiphase equilibrium of pressurized black liquor gasification. Activity coefficients for Na2CO3-Na2S

• 1984: Pejryd & Hupa • Equilibrium modeling of furnace gas and smelt bed• Non-ideal interactions for liquid Na2CO3-Na2S

• 1988-1990: Sangster & Pelton• Thermodynamic model of non-ideal liquid and solid

solutions of alkali salts without sulfide• Na+,K+//CO3

2-, SO42-, Cl-, OH-

Thermodynamic modellingDevelopments & Highlights (2)

• 1984→present: Backman et al. • Thermodynamic model for the recovery boiler smelt • Na+,K+//S2-, S2O7

2-, CO32-, SO4

2-, Cl-, OH-

• Equilibrium modeling of recovery boiler chemistry, melting properties of superheater deposits, pressurized black liquor gasification etc.

• 2000: Pelton, Chartrand and Eriksson• Modified Quasichemical Model in Quadruplet

Approximation• Thermodynamic model developed for complex

molten salts• Large amount of salt systems have been modelled

and multicomponent phase equilibrium is accurately predicted

Thermodynamic model for the molten phase

• New thermodynamic model - Modified Quasichemical Formalism in the quadruplet approximation

• Based on molten salt theory and takes into account short-range ordering (on molecular level) in the liquid

• Previous models for molten salts can be incorporated →previous optimized binary systems can be directly incorporated

• New components can easily be incorporated in the model– Ca2+, Mg2+ , Heavy metals etc.

Procedure for thermodynamic modelling

• Evaluation of experimental thermodynamic data for pure phases and experimental phase equilibrium data

• Choice of thermodynamic model (mathematical description) for the solution properties of solutions (liquid, solid solutions, gas)

• Optimization of solution interaction parameters and unknown or uncertain thermodynamic data of pure phases using experimental data as input

• Calculation of phase equilibrium and comparison with experimental equilibrium data (FactSageTM)

Thermodynamic modelling

• A good thermodynamic model/database for solids and liquid should:– predict the phase equilibrium (=melting properties) of binary

and higher order systems within the uncertainties of experimental investigations of these systems

– give good predictions of the phase equilibrium for conditions where no experimental data exist

• NOTE! All experimentally determined melting data are NOT equilibrium data

• Examples of non-equilibrium phenomena:– Supercooling of liquids– Equilibration of solid solutions

Examples of modelled subsystems relevant for the recovery boiler

Recovery Boiler Smelt

• The liquid phase in the char bed consists mainly of Na2CO3, Na2S and Na2SO4, with minor amounts of other K, Cl and S species

• Existence of polysulfides has been shown to reduce the first-melting temperature of the smelt

Liquid

Na 2

(SO

4,S)(s

s)

Na2(SO4,S)(ss)+Na2S

Liquid+Na2S

Na2S - Na2SO4

x(Na2S)

T (°

C)

0.0 0.2 0.4 0.6 0.8 1.0500

700

900

1100

Liquid

Na2(CO3,SO4) (ss)

Na2CO3 - Na2SO4

x(Na2CO3)

T (°

C)

0.0 0.2 0.4 0.6 0.8 1.0800

820

840

860

880

900

Liquid

Na 2

(CO

3,S) (

ss)

Na2(CO3,S)(ss) + Na2S

Liquid + Na2S

Na2S - Na2CO3

x(Na2S)

T (°

C)

0.0 0.2 0.4 0.6 0.8 1.0400

600

800

1000

1200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.10.20.30.40.50.60.70.80.9

0.10.2

0.30.4

0.50.6

0.70.8

0.9

S

Na2S K2Smole fraction

1100

1000

900

800

700

400

300

500

600

200

Twoliquids

400

300

700

800

900

K2S2Na2S2

Na2S-K2S-S

1175 °C 948 °C

475 °C 491 °C

254 °C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.10.20.30.40.50.60.70.80.9

0.10.2

0.30.4

0.50.6

0.70.8

0.9

S

Na2S K2Smole fraction

1100

1000

900

800

700

400

300

500

600

200

Twoliquids

400

300

700

800

900

K2S2Na2S2

Na2S-K2S-S

Lowest melting point 73 °C

Melting properties of carry-over particles and superheater deposits

• The liquid phase is of great importance for deposit formation on superheaters and for the corrosion of superheaters

• Deposits are generally consist of Na+,K+//SO4

2-,CO32-,Cl-,S2-

• Acidic sulfates (S2O72-, HSO4

-)may form in boiler bank and economizers

Mixtures of NaCl, Na2SO4 and Na2CO3

Na2SO4 - (0.35 Na2Cl2 + 0.65 Na2CO3)

x(Na2SO4)

T(°C

)

0.0 0.2 0.4 0.6 0.8 1.0500

600

700

800

900

Bergman et al. (1958)

Åbo Akademi, unpubl.

Liquid

Liquid+Na2(SO4,CO3)

NaCl+Na2(SO4,CO3)

Na2CO3 - (0.85 Na2SO4 + 0.15 Na2Cl2)

x(Na2CO3)

T(°C

)

0.0 0.2 0.4 0.6 0.8 1.0600

700

800

900

Bergman et al. (1958)

Åbo Akademi, unpubl.

Liquid

Liquid+Na2(SO4,CO3)

Mixtures of Na+, K+/SO42-, Cl-

(NaCl, KCl, Na2SO4, K2SO4)

x(Na2SO4)

T(°C

)

0.0 0.2 0.4 0.6 0.8 1.0500

600

700

800

900Na2SO4-(KCl)2 Solidus, T0

KCl Na2SO4

Na2SO4-(KCl)2 Liquidus, T100

KCl Na2SO4x(Na2SO4)

T(°C

)

0.0 0.2 0.4 0.6 0.8 1.0500

600

700

800

900

Stability of alkali pyrosulfates(Na2S2O7, K2S2O7)

Na2S2O7(s) Na2SO4(s)

Liquid

Na2SO4-Na2S2O7p(O2) = 0.05 bar

T(°C)

log 1

0(p(

SO

3)/b

ar)

200 300 400 500 600 700 800-6

-5

-4

-3

-2

-1

ppm

SO

3

10

10000

1000

1002200 ppm SO3

ppm

SO

3

10

10000

1000

100

Liquid

K2S2O7(s)

K2SO4(s)

K2SO4-K2S2O7p(O2) = 0.05 bar

T(°C)

log 1

0(p(

SO

3)/b

ar)

200 300 400 500 600 700 800-6

-5

-4

-3

-2

-1

150 ppm SO3

ppm

SO

3

10

10000

1000

100

Liquid

(Na,K)2SO4 - (Na,K)2S2O7p(O2) = 0.05 bar, K/(Na+K)=0.1

T(°C)

log 1

0(p(

SO

3)/b

ar)

200 300 400 500 600 700 800-6

-5

-4

-3

-2

-1

50 ppm SO3

Summary

The melting behaviour of multicomponent alkali salts in the recovery boiler has been modelled using a new thermodynamic model for the liquid phase

The thermodynamic model can reproduce most experimental data of melting of the complex salts within error limits