Thermochemical Studies of
Relevance for Black Liquor
Combustion and Gasification - The
System Na2CO3-Na2S
Mathias Råberg
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
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
I. Research program
Inorganic
Inorganicreactions
reactions
(UmU,
(UmU,Åbo)
Åbo)
Construction
Construction
materials
materials
(Åbo)
(Åbo)
•Gas phase reactions
•Smelt formation
•Green liquor quality
•Corrosion
•Smelt layer thickness
Design
Design
verification
verificationprogram
program
(Chemrec)
(Chemrec)
•Technical design
Center
Centerof
of
Black
BlackLiquor
Liquor
Gasification
Gasification
CFD
CFDmodeling
modeling
(LTU,
(LTU,ETC)
ETC)
•Gasification reactor
•Quench
•Counter current condensor
Gasification
Gasification
reactions
reactions
(CTH,
(CTH,Åbo)
Åbo)
•Kinetic models
•Evaporation, pyrolysis,
char conversion
II. Uncertainties in thermochemical data
Earlier published data of the binary
phase diagram Na2CO3-Na2S
The phase diagram Na2CO3 – Na2S according The phase diagram Na2CO3 – Na2S according
to Ovechkin (Zh. Neorg. Khim. 16, 1971)
to Tegman and Warnqvist (Acta Chem. Scand
26, 1972)
III. Phase diagram studies on the
system Na2CO3-Na2S
• 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 the
Na2CO3-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)
Experimental set-up, HTM
TS 1500 Hot Stage and the heater assembly
Experimental equipment, HT-XRD
• BRUKER AXS (Analytical
X-ray System), D8 Advance
• HTK 16 High
Temperature Camera
• Pt sample stage
HTM results
Sample with XNa2S = 0.15
a. Sample at 25 ºC
c. Close to melting point,
822 ºC
b. Partly melted, 815 ºC
d. Crystallized during cooling,
700 ºC
Results from the melting point study
XNa2S
0.1
First melt appears,
mean (ºC)
795
Melting point,
mean (ºC)
842
0.15
780
830
0.2
757
817
0
-
858
1
-
1190
HT-XRD results
Na2CO3 (25-800 °C)
T (ºC)
800
600
550
520
510
500
490
480
470
460
450
440
430
420
400
375
365
355
345
335
200
25
23
24 25
26
27
28
29
30
31
32
33
34
35 36 37 38
2θ-Scale
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
Na2S (25 °C)
Lin (Cps)
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
23
30
40
50
2θ-Scale 60
70
Diffraction pattern of pure Na2S at 25 ºC
80
90
XNa2S == 0.15
0.1
0.2
X
Na2S
Pure Na2CO3
Pure
Pure Na2CO3
Na2CO3
Na2CO3
in mixture
Na2CO3
in
Pure
Na2CO3
Na2CO3
in mixture
mixture
Na2S in mixture
mixture
Na2S
Na2S in
in mixture
800
800
700
700
600
600
TTT(C)
(C)
(C)
500
500
400
400
300
300
200
100
0
23
28
33
33
38
38
2θ-Scale
2θ-Scale
2θ-Scale
43
43
43
48
48
48
Phase Diagram Na2CO3-Na2S
1200
1100
1000
T ( oC)
900
800
700
600
500
400
0
Na2CO3
0.2
0.4
0.6
XNa2S
0.8
1
Na2S
Phase Diagram Na2CO3-Na2S
HTM data
Previous estimated data
1200
1100
1000
T ( oC)
900
800
700
600
500
400
0
Na2CO3
0.2
0.4
0.6
XNa2S
0.8
1
Na2S
Phase Diagram Na2CO3-Na2S
HTM data
HT-XRD data
Previous estimated data
1200
1100
1000
T ( oC)
900
800
700
600
500
400
0
Na2CO3
0.2
0.4
0.6
XNa2S
0.8
1
Na2S
Phase Diagram Na2CO3-Na2S
1200
1100
Liquid
1000
900
T ( oC)
Na2CO3(s) + L
Na2S(s) + L
800
700
Na2CO3(ss)
600
Na2CO3(s) + Na2S(s)
500
400
0
Na2CO3
0.2
0.4
0.6
XNa2S
0.8
1
Na2S
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
VI. Uncertainties in thermochemical data
Sensitivity analysis
• 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 in
thermochemical data has been developed. The program
uses ChemApp for equilibrium calculations.
Stability of Na2S relative to Na2CO3
Na2S(s)+CO2+H2O=Na2CO3(s,l)+H2S
with different data, pCO2 = 0.15 bar, pH2O = 0.20 bar
log[H2S/bar]
0
SGPS-s,l
SGTE96-s,l
Fact51-s,l
HSC5-s,l
Rosén-s
Backman-s
Na2S
-1
-2
Na2CO3
-3
400
500
600
700
800
Temperature [°C]
900
1000
VII. Thermochemical equilibrium studies
• 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
Preliminary results
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)
Acknowledgements
• Swedish Energy Agency, STEM