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Thermochemical Studies of Relevance for Black Liquor

Combustion and Gasification - The System Na2CO3-Na2S

Anders Larsson, Gustav Lindberg, Anders Nordin, Dan Boström,

Björn Warnqvist, Erik Rosén, Rainer Backman*

Energy and Process Technology, University of Umeå,SE-901 87 Umeå

*Åbo Akademi, Process Chemistry Group

Mathias Råberg

Contents

I. Research program (Black Liquor Gasification)

II. Uncertainties in thermochemical data

III. Phase diagram studies

- Na2CO3-Na2S

IV. Conclusions

V. Future work

-----------On-going projects------------

VI. Uncertainties in thermochemical data

VII. Thermochemical equilibrium studies

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Center ofBlack LiquorGasification

Inorganic reactions(UmU, Åbo)

•Gas phase reactions•Smelt formation•Green liquor quality

CFD modeling(LTU, ETC)

Gasificationreactions

(CTH, Åbo)Design

verification program(Chemrec)

Constructionmaterials

(Åbo)

•Corrosion•Smelt layer thickness

•Technical design•Kinetic models•Evaporation, pyrolysis, char conversion

•Gasification reactor•Quench•Counter current condensor

I. Research program

The phase diagram Na2CO3 – Na2S according to Ovechkin (Zh. Neorg. Khim. 16, 1971)

The phase diagram Na2CO3 – Na2S according to Tegman and Warnqvist (Acta Chem. Scand 26, 1972)

II. Uncertainties in thermochemical data

Earlier published data of the binary phase diagram Na2CO3-Na2S

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• Objectives:

- Re-determination of liquidus lines, in the Na2CO3 rich area, and melting points of the pure components

- Determination of the extension of the Na2CO3(ss) solid solution in theNa2CO3-Na2S system

•Methods: - High Temperature Microscopy, HTM

- High Temperature X-Ray Powder Diffraction, HT-XRD

• Chemicals- Na2CO3 and Na2S prepared according to Tegman and Warnqvist

(Acta Chem. Scand. 26, 1972)

III. Phase diagram studies on the system Na2CO3-Na2S

Experimental set-up, HTM

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TS 1500 Hot Stage and the heater assembly

Experimental equipment, HT-XRD

• BRUKER AXS (Analytical X-ray System), D8 Advance

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• HTK 16 High Temperature Camera

• Pt sample stage

HTM results

a. Sample at 25 ºC b. Partly melted, 815 ºC

c. Close to melting point,822 ºC

d. Crystallized during cooling,700 ºC

Sample with XNa2S = 0.15

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1190-1

858-0

8177570.2

8307800.15

8427950.1

Melting point,mean (ºC)

First melt appears, mean (ºC)

XNa2S

Results from the melting point study

Na2CO3 (25-800 °C)

2θ-Scale

800

600

550

520

510

500

490

480

470

460

450

440

430

420 400

375

365

355

345

335

200 25

T (ºC)

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

A series of HT-XRD diffraction patterns of pure Na2CO3 in the temperature interval 25-800 ºC

HT-XRD results

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Na2S (25 °C) Lin (Cps)

0

10

20

30

40

50 60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

2θ-Scale 23 30 40 50 60 70 80 90

Diffraction pattern of pure Na2S at 25 ºC

0

100

200

300

400

500

600

700

800

23 28 33 38 43 482θ-Scale

T (C

)

Pure Na2CO3

XNa2S = 0.1

0

100

200

300

400

500

600

700

800

23 28 33 38 43 482θ-Scale

T (C

)

Pure Na2CO3 Na2CO3 in mixture Na2S in mixture

XNa2S = 0.1

0

100

200

300

400

500

600

700

800

23 28 33 38 43 482θ-Scale

T (C

)

Pure Na2CO3 Na2CO3 in mixture Na2S in mixture

XNa2S = 0.15

0

100

200

300

400

500

600

700

800

23 28 33 38 43 48

2θ-Scale

T (C

)

Pure Na2CO3 Na2CO3 in mixture Na2S in mixture

XNa2S = 0.2

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Phase Diagram Na2CO3-Na2S

400

500

600

700

800

900

1000

1100

1200

0 0.2 0.4 0.6 0.8 1

XNa2S

T (o C

)

Na2CO3 Na2S

Phase Diagram Na2CO3-Na2S

400

500

600

700

800

900

1000

1100

1200

0 0.2 0.4 0.6 0.8 1

XNa2S

T (o C

)

HTM data Previous estimated data

Na2CO3 Na2S

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Phase Diagram Na2CO3-Na2S

400

500

600

700

800

900

1000

1100

1200

0 0.2 0.4 0.6 0.8 1

XNa2S

T (o C

)

HTM data HT-XRD dataPrevious estimated data

Na2CO3 Na2S

Phase Diagram Na2CO3-Na2S

400

500

600

700

800

900

1000

1100

1200

0 0.2 0.4 0.6 0.8 1

XNa2S

T (o C

)

Na2CO3 Na2S

Na2CO3(s) + Na2S(s)

Liquid

Na2S(s) + LNa2CO3(s) + L

Na2CO3(ss)

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IV. Conclusions

• Phase diagram studies:- HTM: Re-determination of liquidus lines and melting

points of the components were made

- HT-XRD: The extension of the Na2CO3(ss) solid solution in the Na2CO3-Na2S system was determined

V. Future work

• Other systems:- K2CO3-K2S

- Na2S-K2S

- Na2S-NaCl, K2S-KCl

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• Objective:

- Perform sensitivity analysis for the black liquor combustion/gasification chemistry in order to identify species with the most uncertain data and to compare the effect of these uncertainties with the variation in fuel and process variables

• Method:- Systematically performing chemical equilibrium

calculations (with H2S, COS, Na-gases, melting temperatures etc. as target/indicator variables) with uncertainties in thermochemical data included according to an extensive factorial design.

- A program that handles input/output files and changes inthermochemical data has been developed. The program uses ChemApp for equilibrium calculations.

Sensitivity analysisVI. Uncertainties in thermochemical data

Na2S(s)+CO2+H2O=Na2CO3(s,l)+H2Swith different data, pCO2 = 0.15 bar, pH2O = 0.20 bar

-3

-2

-1

0

400 500 600 700 800 900 1000

Temperature [°C]

log[

H2S

/bar

]

SGPS-s,lSGTE96-s,lFact51-s,lHSC5-s,lRosén-sBackman-s

Na2CO3

Na2S

Stability of Na2S relative to Na2CO3

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• Objective:

- Investigation of the equilibrium between Na2S and Na2CO3 to get more reliable data

• Method:

- Equilibrium studies in a vertical tube-furnace

• Improvements from previous studies (Köszegi, Rosén. Trans Roy Inst. Technol. 1964):

- Lower and better controlled gas flow

- In-situ production of Na2S

- Reducing atmosphere to prevent formation of Na2S2

- Aqueous titration method for the determination of H2S formed

VII. Thermochemical equilibrium studies

Temperature dependence for the equilibrium constant of the reaction Na2S(s) + H2O(g) + CO2(g) Na2CO3(s) + H2S(g)

(Previous data: Köszegi, Rosén. Trans Roy Inst. Technol. 1964)

Preliminary results

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Acknowledgements

• Swedish Energy Agency, STEM