Desulfurization characteristic by molten alkali carbonates at high temperature.
Proceedings of the International Conference on Fluid and Thermal Energy Conversion 2006
Jakarta, Indonesia, December 10 – 14, 2006
© FTEC 2006
ISSN 0854 - 9346
Desulfurization Characteristic by Molten Alkali Carbonates at
High Temperature
S. Raharjo1 and I. Naruse2
1
Department of Ecological Engineering
2
International Cooperation Center for Engineering Education Development (ICCEED)
Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, JAPAN
Contact Person:
Slamet Raharjo
Department of Ecological Engineering
Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, JAPAN
E-mail: slamet_281983@yahoo.com
Abstract
In order to remove H2S and COS in the gasified gas, produced by coal gasifires (e.g. IGFC) at high
temperature, molten alkali carbonates (MAC) consisting of Na2CO3 and K2CO3 were applied as
solvent. Before the removal experiments, the optimum experimental conditions were estimated in
terms of the chemical equilibrium calculations. After that, the removal experiments of H2S and/or
COS by the molten alkali carbonates were conducted in alumina tube furnace, varying the furnace
temperature. As a result, H2S and COS were completely removed by the molten alkali carbonates.
The concentrations of both H2S and COS in the clean-up gas became less than the detection limit
of FPD gas chromatograph. Additionally, the regeneration tests at high temperature of used MAC
following the removal tests were conducted by introducing CO2 as a regeneration agent into the
reaction tube to form Na2CO3 and K2CO3. The regeneration tests for Na2S and K2S in solid phase
in the CO2 atmosphere, using a thermo-gravimetric analyzer, were also carried out. Under the
present experimental conditions, it was hard to regenerate the used MAC.
Keywords: Gas clean-up, Desulfurization, Gasified gas, Molten alkali carbonate
1 INTRODUCTION
Coal gasification has been recognized as one of clean and effective technologies to produce CO
and H2, which can be used for power plant, heat generation or as a synthesis precursor[1]. In the
field of power generation for electricity, particularly, coal gasification will play a role for a
future promising technology since it can improve the net thermal efficiency and decrease CO2
emission all at once.
In the coal gasification process, sulfur content in coal is converted mainly to H2S and COS.
140-1
S. Raharjo and I. Naruse
When the coals with high sulfur content are gasified, however, H2S and COS are contained in
the gasified gas. If the gasified gas is directly used as the fuel for fuel cells or synthesis gas (H2
and CO) for chemical materials, H2S and COS may affect subsequent processes of the gasifier.
Therefore, H2S and COS in the gasified gas should be removed almost completely[2]. In the
application of coal gasification integrated with fuel cell, more strictly removal of H2S and COS
is needed due to their poisonous effect to the anode of fuel cell. Currently, clean coal technology
is a main issue in the field of power generation technology in the world. Japanese government is
now developing an important project named coal Energy Application for Gas, Liquid, and
Electricity (EAGLE). This system combines fuel cell (FC) with generation turbine (GT) and
steam turbine (ST), which is called triple combined cycle. This triple combined cycle improves
net efficiency over 53% and reduces CO2 emission 30%, compared to pulverize coal-fired
boilers (PCF)[3].
This EAGLE system needs to install cold gas cleanup system in order to satisfy the tolerance
limits of fuel cell. Therefore, the synthesis gas must be heat-exchanged at the gas/gas heater
(GGH). The cooled down gas is, then, desulfurized in a Methyldiethanolamine (MDEA)
absorber. The clean synthesis gas, which exits the MDEA absorber at approximately 313 K, is
heated to approximately 473 K by a steam heater and GGH, and is supplied to the gas turbine.
Although the cold gas cleanup system is currently applied, the hot desulphurization system
favors to increase the net thermal efficiencies[4]. Because removing sulfur compounds at high
temperature can result in a gain of 6 % in the overall efficiency for a typical power generation
system[5, 6]. Therefore, it is necessary to develop the suitable desulfurization technologies at high
temperature.
The development of reliable methods to remove sulfur compounds in the coal gasified gases at
high temperatures, instead of practical wet scrubbing techniques that operate at around 323 K, is
one of the important technological advances. Therefore, this study proposes one of the removal
technologies of H2S and COS in the gasified gas at high temperature, using molten alkali
carbonates (MAC), consisting of Na2CO3 and K2CO3 as solvent.
2 EXPERIMENTAL CONDITIONS AND PROCEDURES
Before the removal and regeneration experiments, the optimum experimental conditions were
analyzed in term of chemical equilibrium calculation. After that, the removal experiments of
H2S, COS and their mixed gas (H2S+COS) by the molten alkali carbonates are conducted.
Figure 1 shows the experimental apparatus used in this study. The furnace, which is 24 mm
142-2
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
inside diameter and 500 mm length, made from alumina. Molten alkali carbonates with
composition of 43 mol%-Na2CO3 and 57 mol%-K2CO3 inserted into the furnace is electrically
heated at a designed temperature. H2S and COS gas are introduced into the furnace. The product
gas is introduced into an FPD gas chromatograph to analyze H2S, COS and SO2 concentrations.
Following the removal tests, the regeneration tests are conducted by changing introduced gas
from sulfuric gas to CO2 as the regeneration agent. In order to investigate regeneration
characteristic of Na2S and K2S in solid phase in the CO2 atmosphere, regeneration tests by using
a thermo-gravimetric analyzer are carried out.
3 RESULTS AND DISCUSSIONS
3.1. Chemical Equilibrium Calculations
The chemical equilibrium calculations were conducted by using chemical reaction and
equilibrium software with extensive thermochemical database[7]. The initial conditions for
chemical equilibrium calculation for H2S and/or COS removal process are shown in Table 1.
N2+H2S
Mass Flowmeter
Tetra Bag
N2+COS
Alumina Tube
Mass Flowmeter
CO2
FPD Gas Chromatograph
Floatmeter
MAC
Figure 1 Experimental Apparatus for Removal and Regeneration Tests
142-3
S. Raharjo and I. Naruse
Table 1 Initial Conditions for H2S and/or COS Removal Process Simulations
Mole
Components
COS
Mix
H S
N2
99.76
99.76
99.52
H2S
0.24
--
0.24
COS
--
0.24
0.24
Na2CO3
11.3
K2CO3
14.8
Figures 2, 3, and 4 show the results of chemical equilibrium calculation of removal process of
H2S, COS and their mixture, respectively. These results suggest that, even at high temperature,
the concentration of gaseous sulfur compounds would be zero. Based on this calculation, the
removal temperature was determined as 1173 K. Figures 2, 3, and 4 also suggest that the main
products of removal process are Na2S and K2S.
In order to know the simulation characteristic of Na2S and K2S regenerations, their chemical
equilibrium calculations were also carried out. Table 2 shows the initial conditions for Na2S and
K2S regeneration simulations. Figures 5 and 6 show results of chemical equilibrium calculation
of regeneration process for Na2S and K2S, respectively. In this calculation CO2 is introduced as
the regeneration agent. These results show that Na2S and K2S mainly convert to M2CO3 at 773
K (M: Na or K).
Removal Temperature
K2SO3(g)
K2SO4(g)
80
K2S(g)
60
H2S(g)
Na2S(g)
40
20
0
400
600
800
1000
1200
1400
Removal Temperature
Melting Point of MAC
Sulfur Balance in Exhaust Gas [mole%]
Sulfur Balance in Exhaust Gas [mole%]
Melting Point of MAC
100
1600
Temperature [K]
100
K2SO3(g)
K2SO4(g)
K2S(g)
80
60
Na2S(g)
COS(g)
40
20
0
400
600
800
1000
1200
1400
1600
Temperature [K]
Figure 2 Chemical Equilibrium Calculation
Figure 3 Chemical Equilibrium Calculation
of H2S Removal
of COS Removal
142-4
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
Removal Temperature
K2SO3(g)
Sulfur Balance in Exhaust Gas [mole%]
Melting Point of MAC
100
K2SO4(g)
80
COS(g)
K2S(g)
60
Na2S(g)
40
20
0
H2S(g)
400
600
800
1000
1200
1400
1600
Temperature [K]
Figure 4 Chemical Equilibrium Calculation of Mixed Gases (H2S+COS) Removal
Table 2 Initial Conditions for Na2S and K2S Regeneration Simulations
Mole
Components
Na2S Case
K2S Case
Na2S
1
--
K2S
--
1
CO2
4
3.2 Removal Tests
Table 3 shows the initial conditions for H2S and/or COS removal tests. Figures 7, 8, 9 and 10
show result of H2S, COS and SO2 concentrations in the product during the H2S and COS
removal tests at 1173 K and 1053 K, respectively. From these figures, H2S, COS and SO2
concentrations detected become smaller than the detection limit of FPD gas chromatograph.
Consequently, sulfur species are completely captured by the MAC to form Na2S and K2S mainly.
Based on the experimental and chemical equilibrium calculations, the following reactions could
100
90
80
70
60
50
40
30
20
10
0
Potassium Balance [%]
Sodium Balance [%]
be occurred in this removal process (l: liquid, g: gas, c: condensed).
Na2S
Na2CO3
Na2SO4
400
600
800
1000
Temperature [K]
1200
1400
100
90
80
70
60
50
40
30
20
10
0
K2S
K2SO4
400
K2CO3
600
800
1000
Temperature [K]
1200
1400
Figure 5 Chemical Equilibrium Calculation
Figure 6 Chemical Equilibrium Calculation
of Na2S Regeneration
of K2S Regeneration
142-5
S. Raharjo and I. Naruse
M2CO3(l) + H2S(g)
M2S(c) + H2O(g) + CO2(g)
M2CO3(l)+ COS(g)
M2S(c) + 2CO2(g)
Table 3 Experimental Conditions for Removal Tests
H2S Case
Temperature [K]
1173
COS Case
1053
502
---
COS Initial Concentration [ppmV]
---
505
MAC amount [g]
26.7
32.4
26.7
32.4
Experimental time [min]
160
180
160
180
COS, and SO2 Concentration [ppm]
H2S, COS, and SO2 Concentration [ppm]
0.7
Gas Introduced : H2S
Initial H2S = 502 ppmV
T : 1173 K
H2S
COS
SO2
Detection Limit of FPD
0
20
40
60
80
100
120
140
160
Experimental Time [min]
COS and SO2 Concentration [ppm]
H2S, COS, and SO2 Concentration [ppm]
H2S
COS
SO2
Detection Limit of FPD
40
60
80
120
140
160
Gas Introduced : COS
Initial H2S = 502 ppmV
T : 1053 K
20
100
Figure 8 Result of COS Removal at 1173 K
Gas Introduced : H2S
0
Gas Introduced : COS
5.0
Initial COS = 505 ppmV
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
Detection Limit of FPD
0.5
0.0
0
20
40
60
80
Experimental Time [min]
Figure 7 Result of H2S Removal at 1173 K
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1053
H2S Initial Concentration [ppmV]
Flow rate [l/min]
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1173
100 120 140 160 180
Experimental Time [min]
5.0
Initial COS = 505 ppmV
4.5
T : 1053 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
Detection Limit of FPD
0.5
0.0
0
20
40
60
80 100 120 140 160 180
Experimental Time [min]
Figure 9 Result of H2S Removal at 1053 K
Figure 10 Result of COS Removal at 1053 K
3.3 Regeneration tests
Figures 11 and 12 show results of the regeneration test after the H2S and COS removal tests at
1173 K, respectively. When the reactant is changed to CO2 with the flow rate of 1 l/min, only a
small amount of COS is produced. It means that the regeneration process will be hardly
performed at high temperature.
142-6
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
According to the equilibrium calculation result of Figs. 5 and 6, the optimum regeneration
temperature seems to be 773 K. Therefore, the regeneration tests for Na2S and K2S are
conducted in the CO2 atmosphere by the thermo-gravimetric analyzer. Figures 13 and 14 show
results of the regeneration tests at 773 K for Na2S and K2S, respectively. Comparing both the
figures, Na2S can be regenerated more easily by CO2 than K2S. During this regeneration test, a
few ppm of COS gas was detected in the product gas. However, the sulfur balance was not
M2S(s) + CO2(g)
M2CO3(s) + COS(g)
M2S(s) + CO2(g)
M2SO4(s) + others
Regeneration Agent : CO2
250
220
230
Experimental Time [min]
Experimental Time [min]
Figure 11 Result of Used MAC Regeneration
Figure 12 Result of Used MAC Regeneration
Test following H2S Removal
Test following COS Removal
CO2
Temperature Profile
Sample Weight [mg]
4
3
2
1
0
Weight Changing
Moisture Profile
-1
-2
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Experimental Time [min]
Figure
13
TG
Result
of
550
500
450
400
350
300
250
200
150
100
50
0
N2
CO2
1.0
O
5
Na2S
Sample Weight [mg]
N2
K2S initial weight : 16.115 mg
Holding Temperature : 773 K
Temperature [ C]
Na2S initial weight : 14.100 mg
Holding Temperature : 773 K
0.5
Temperature Profile
Weight Changing
Profile
0.0
-0.5
Moisture
-1.0
K2S Vaporization
-1.5
-2.0
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Experimental Time [min]
Figure
Regeneration in CO2
14
TG
Regeneration in CO2
142-7
Result
of
550
500
450
400
350
300
250
200
150
100
50
0
K2S
O
240
Regeneration Agent : CO2
5.0
After COS = 505 ppmV Removal
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
0.5
0.0
190
200
210
Temperature [ C]
5.0
After H2S = 502 ppmV Removal
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
0.5
0.0
190
200
210
220
230
COS, and SO2 Concentration [ppm]
COS, and SO2 Concentration [ppm]
sufficient. Therefore, the following reactions would occur during the regeneration.
S. Raharjo and I. Naruse
4 CONCLUSIONS
Desulfurization characteristics by molten alkali carbonates (MAC) were studied experimentally,
using alumina tube furnace. Chemical equilibrium calculations were carried out prior to the H2S
and/or COS removal experiments. Additionally, regeneration tests of used MAC were also
carried out. The main results obtained are summarized below.
1. Chemical equilibrium calculations for H2S and/or COS removal process show that, even at
high temperature, the concentration of gaseous sulfur compounds become zero.
2. Molten alkali carbonates (MAC) can be applied as a liquid solvent to capture gaseous sulfur
compounds in the gasified gas even at high temperature. The following reactions could be
occurred in this removal process (l: liquid, g: gas, c: condensed):
M2CO3(l) + H2S(g)
M2S(c) + H2O(g) + CO2(g)
M2CO3(l)+ COS(g)
M2S(c) + 2CO2(g)
3. The regeneration of the used MAC is difficult under the present experimental conditions.
REFERENCES
1) Saito, I.(2003), 20th Annual International Pittsburgh Coal Conference Sept. 15 – 19.
2) Mitchell, S. C.(1998), IEA Coal Research the Clean Coal Center.
3) EAGLE. (2006). Multi Purpose Coal Gasification Technology Development. NEDO and
J-Power.
4) Henderson, C.(2004). IEA Clean Coal Center, ISBN 92-9029-406-X.
5) Furimsky, E., Yumura, M. (1986). Solid Adsorbents for Removal of Hydrogen Sulfide from
Hot Gas. Erdol und Kohle-Erdgas-Petrochemie, 39(4). Pp. 163-172.
6) Barthelemy, N.M. (1991). Improved Heat Recovery and High-Temperature Clean-up for
Coal-gas Fired Combustion Turbines. Master of Science Thesis, Department of Chemical
Engineering, University of California at Barkeley.
7) HSC Chemistry. (2002). User`s Guide for HSC Chemistry for Windows. Outokumpu
Research Oy.
142-8
Jakarta, Indonesia, December 10 – 14, 2006
© FTEC 2006
ISSN 0854 - 9346
Desulfurization Characteristic by Molten Alkali Carbonates at
High Temperature
S. Raharjo1 and I. Naruse2
1
Department of Ecological Engineering
2
International Cooperation Center for Engineering Education Development (ICCEED)
Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, JAPAN
Contact Person:
Slamet Raharjo
Department of Ecological Engineering
Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, JAPAN
E-mail: slamet_281983@yahoo.com
Abstract
In order to remove H2S and COS in the gasified gas, produced by coal gasifires (e.g. IGFC) at high
temperature, molten alkali carbonates (MAC) consisting of Na2CO3 and K2CO3 were applied as
solvent. Before the removal experiments, the optimum experimental conditions were estimated in
terms of the chemical equilibrium calculations. After that, the removal experiments of H2S and/or
COS by the molten alkali carbonates were conducted in alumina tube furnace, varying the furnace
temperature. As a result, H2S and COS were completely removed by the molten alkali carbonates.
The concentrations of both H2S and COS in the clean-up gas became less than the detection limit
of FPD gas chromatograph. Additionally, the regeneration tests at high temperature of used MAC
following the removal tests were conducted by introducing CO2 as a regeneration agent into the
reaction tube to form Na2CO3 and K2CO3. The regeneration tests for Na2S and K2S in solid phase
in the CO2 atmosphere, using a thermo-gravimetric analyzer, were also carried out. Under the
present experimental conditions, it was hard to regenerate the used MAC.
Keywords: Gas clean-up, Desulfurization, Gasified gas, Molten alkali carbonate
1 INTRODUCTION
Coal gasification has been recognized as one of clean and effective technologies to produce CO
and H2, which can be used for power plant, heat generation or as a synthesis precursor[1]. In the
field of power generation for electricity, particularly, coal gasification will play a role for a
future promising technology since it can improve the net thermal efficiency and decrease CO2
emission all at once.
In the coal gasification process, sulfur content in coal is converted mainly to H2S and COS.
140-1
S. Raharjo and I. Naruse
When the coals with high sulfur content are gasified, however, H2S and COS are contained in
the gasified gas. If the gasified gas is directly used as the fuel for fuel cells or synthesis gas (H2
and CO) for chemical materials, H2S and COS may affect subsequent processes of the gasifier.
Therefore, H2S and COS in the gasified gas should be removed almost completely[2]. In the
application of coal gasification integrated with fuel cell, more strictly removal of H2S and COS
is needed due to their poisonous effect to the anode of fuel cell. Currently, clean coal technology
is a main issue in the field of power generation technology in the world. Japanese government is
now developing an important project named coal Energy Application for Gas, Liquid, and
Electricity (EAGLE). This system combines fuel cell (FC) with generation turbine (GT) and
steam turbine (ST), which is called triple combined cycle. This triple combined cycle improves
net efficiency over 53% and reduces CO2 emission 30%, compared to pulverize coal-fired
boilers (PCF)[3].
This EAGLE system needs to install cold gas cleanup system in order to satisfy the tolerance
limits of fuel cell. Therefore, the synthesis gas must be heat-exchanged at the gas/gas heater
(GGH). The cooled down gas is, then, desulfurized in a Methyldiethanolamine (MDEA)
absorber. The clean synthesis gas, which exits the MDEA absorber at approximately 313 K, is
heated to approximately 473 K by a steam heater and GGH, and is supplied to the gas turbine.
Although the cold gas cleanup system is currently applied, the hot desulphurization system
favors to increase the net thermal efficiencies[4]. Because removing sulfur compounds at high
temperature can result in a gain of 6 % in the overall efficiency for a typical power generation
system[5, 6]. Therefore, it is necessary to develop the suitable desulfurization technologies at high
temperature.
The development of reliable methods to remove sulfur compounds in the coal gasified gases at
high temperatures, instead of practical wet scrubbing techniques that operate at around 323 K, is
one of the important technological advances. Therefore, this study proposes one of the removal
technologies of H2S and COS in the gasified gas at high temperature, using molten alkali
carbonates (MAC), consisting of Na2CO3 and K2CO3 as solvent.
2 EXPERIMENTAL CONDITIONS AND PROCEDURES
Before the removal and regeneration experiments, the optimum experimental conditions were
analyzed in term of chemical equilibrium calculation. After that, the removal experiments of
H2S, COS and their mixed gas (H2S+COS) by the molten alkali carbonates are conducted.
Figure 1 shows the experimental apparatus used in this study. The furnace, which is 24 mm
142-2
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
inside diameter and 500 mm length, made from alumina. Molten alkali carbonates with
composition of 43 mol%-Na2CO3 and 57 mol%-K2CO3 inserted into the furnace is electrically
heated at a designed temperature. H2S and COS gas are introduced into the furnace. The product
gas is introduced into an FPD gas chromatograph to analyze H2S, COS and SO2 concentrations.
Following the removal tests, the regeneration tests are conducted by changing introduced gas
from sulfuric gas to CO2 as the regeneration agent. In order to investigate regeneration
characteristic of Na2S and K2S in solid phase in the CO2 atmosphere, regeneration tests by using
a thermo-gravimetric analyzer are carried out.
3 RESULTS AND DISCUSSIONS
3.1. Chemical Equilibrium Calculations
The chemical equilibrium calculations were conducted by using chemical reaction and
equilibrium software with extensive thermochemical database[7]. The initial conditions for
chemical equilibrium calculation for H2S and/or COS removal process are shown in Table 1.
N2+H2S
Mass Flowmeter
Tetra Bag
N2+COS
Alumina Tube
Mass Flowmeter
CO2
FPD Gas Chromatograph
Floatmeter
MAC
Figure 1 Experimental Apparatus for Removal and Regeneration Tests
142-3
S. Raharjo and I. Naruse
Table 1 Initial Conditions for H2S and/or COS Removal Process Simulations
Mole
Components
COS
Mix
H S
N2
99.76
99.76
99.52
H2S
0.24
--
0.24
COS
--
0.24
0.24
Na2CO3
11.3
K2CO3
14.8
Figures 2, 3, and 4 show the results of chemical equilibrium calculation of removal process of
H2S, COS and their mixture, respectively. These results suggest that, even at high temperature,
the concentration of gaseous sulfur compounds would be zero. Based on this calculation, the
removal temperature was determined as 1173 K. Figures 2, 3, and 4 also suggest that the main
products of removal process are Na2S and K2S.
In order to know the simulation characteristic of Na2S and K2S regenerations, their chemical
equilibrium calculations were also carried out. Table 2 shows the initial conditions for Na2S and
K2S regeneration simulations. Figures 5 and 6 show results of chemical equilibrium calculation
of regeneration process for Na2S and K2S, respectively. In this calculation CO2 is introduced as
the regeneration agent. These results show that Na2S and K2S mainly convert to M2CO3 at 773
K (M: Na or K).
Removal Temperature
K2SO3(g)
K2SO4(g)
80
K2S(g)
60
H2S(g)
Na2S(g)
40
20
0
400
600
800
1000
1200
1400
Removal Temperature
Melting Point of MAC
Sulfur Balance in Exhaust Gas [mole%]
Sulfur Balance in Exhaust Gas [mole%]
Melting Point of MAC
100
1600
Temperature [K]
100
K2SO3(g)
K2SO4(g)
K2S(g)
80
60
Na2S(g)
COS(g)
40
20
0
400
600
800
1000
1200
1400
1600
Temperature [K]
Figure 2 Chemical Equilibrium Calculation
Figure 3 Chemical Equilibrium Calculation
of H2S Removal
of COS Removal
142-4
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
Removal Temperature
K2SO3(g)
Sulfur Balance in Exhaust Gas [mole%]
Melting Point of MAC
100
K2SO4(g)
80
COS(g)
K2S(g)
60
Na2S(g)
40
20
0
H2S(g)
400
600
800
1000
1200
1400
1600
Temperature [K]
Figure 4 Chemical Equilibrium Calculation of Mixed Gases (H2S+COS) Removal
Table 2 Initial Conditions for Na2S and K2S Regeneration Simulations
Mole
Components
Na2S Case
K2S Case
Na2S
1
--
K2S
--
1
CO2
4
3.2 Removal Tests
Table 3 shows the initial conditions for H2S and/or COS removal tests. Figures 7, 8, 9 and 10
show result of H2S, COS and SO2 concentrations in the product during the H2S and COS
removal tests at 1173 K and 1053 K, respectively. From these figures, H2S, COS and SO2
concentrations detected become smaller than the detection limit of FPD gas chromatograph.
Consequently, sulfur species are completely captured by the MAC to form Na2S and K2S mainly.
Based on the experimental and chemical equilibrium calculations, the following reactions could
100
90
80
70
60
50
40
30
20
10
0
Potassium Balance [%]
Sodium Balance [%]
be occurred in this removal process (l: liquid, g: gas, c: condensed).
Na2S
Na2CO3
Na2SO4
400
600
800
1000
Temperature [K]
1200
1400
100
90
80
70
60
50
40
30
20
10
0
K2S
K2SO4
400
K2CO3
600
800
1000
Temperature [K]
1200
1400
Figure 5 Chemical Equilibrium Calculation
Figure 6 Chemical Equilibrium Calculation
of Na2S Regeneration
of K2S Regeneration
142-5
S. Raharjo and I. Naruse
M2CO3(l) + H2S(g)
M2S(c) + H2O(g) + CO2(g)
M2CO3(l)+ COS(g)
M2S(c) + 2CO2(g)
Table 3 Experimental Conditions for Removal Tests
H2S Case
Temperature [K]
1173
COS Case
1053
502
---
COS Initial Concentration [ppmV]
---
505
MAC amount [g]
26.7
32.4
26.7
32.4
Experimental time [min]
160
180
160
180
COS, and SO2 Concentration [ppm]
H2S, COS, and SO2 Concentration [ppm]
0.7
Gas Introduced : H2S
Initial H2S = 502 ppmV
T : 1173 K
H2S
COS
SO2
Detection Limit of FPD
0
20
40
60
80
100
120
140
160
Experimental Time [min]
COS and SO2 Concentration [ppm]
H2S, COS, and SO2 Concentration [ppm]
H2S
COS
SO2
Detection Limit of FPD
40
60
80
120
140
160
Gas Introduced : COS
Initial H2S = 502 ppmV
T : 1053 K
20
100
Figure 8 Result of COS Removal at 1173 K
Gas Introduced : H2S
0
Gas Introduced : COS
5.0
Initial COS = 505 ppmV
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
Detection Limit of FPD
0.5
0.0
0
20
40
60
80
Experimental Time [min]
Figure 7 Result of H2S Removal at 1173 K
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1053
H2S Initial Concentration [ppmV]
Flow rate [l/min]
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1173
100 120 140 160 180
Experimental Time [min]
5.0
Initial COS = 505 ppmV
4.5
T : 1053 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
Detection Limit of FPD
0.5
0.0
0
20
40
60
80 100 120 140 160 180
Experimental Time [min]
Figure 9 Result of H2S Removal at 1053 K
Figure 10 Result of COS Removal at 1053 K
3.3 Regeneration tests
Figures 11 and 12 show results of the regeneration test after the H2S and COS removal tests at
1173 K, respectively. When the reactant is changed to CO2 with the flow rate of 1 l/min, only a
small amount of COS is produced. It means that the regeneration process will be hardly
performed at high temperature.
142-6
Desulfurization Characteristic by Molten Alkali Carbonates at High Temperature
According to the equilibrium calculation result of Figs. 5 and 6, the optimum regeneration
temperature seems to be 773 K. Therefore, the regeneration tests for Na2S and K2S are
conducted in the CO2 atmosphere by the thermo-gravimetric analyzer. Figures 13 and 14 show
results of the regeneration tests at 773 K for Na2S and K2S, respectively. Comparing both the
figures, Na2S can be regenerated more easily by CO2 than K2S. During this regeneration test, a
few ppm of COS gas was detected in the product gas. However, the sulfur balance was not
M2S(s) + CO2(g)
M2CO3(s) + COS(g)
M2S(s) + CO2(g)
M2SO4(s) + others
Regeneration Agent : CO2
250
220
230
Experimental Time [min]
Experimental Time [min]
Figure 11 Result of Used MAC Regeneration
Figure 12 Result of Used MAC Regeneration
Test following H2S Removal
Test following COS Removal
CO2
Temperature Profile
Sample Weight [mg]
4
3
2
1
0
Weight Changing
Moisture Profile
-1
-2
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Experimental Time [min]
Figure
13
TG
Result
of
550
500
450
400
350
300
250
200
150
100
50
0
N2
CO2
1.0
O
5
Na2S
Sample Weight [mg]
N2
K2S initial weight : 16.115 mg
Holding Temperature : 773 K
Temperature [ C]
Na2S initial weight : 14.100 mg
Holding Temperature : 773 K
0.5
Temperature Profile
Weight Changing
Profile
0.0
-0.5
Moisture
-1.0
K2S Vaporization
-1.5
-2.0
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Experimental Time [min]
Figure
Regeneration in CO2
14
TG
Regeneration in CO2
142-7
Result
of
550
500
450
400
350
300
250
200
150
100
50
0
K2S
O
240
Regeneration Agent : CO2
5.0
After COS = 505 ppmV Removal
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
0.5
0.0
190
200
210
Temperature [ C]
5.0
After H2S = 502 ppmV Removal
4.5
T : 1173 K
4.0
3.5
COS
3.0
SO2
2.5
2.0
1.5
1.0
0.5
0.0
190
200
210
220
230
COS, and SO2 Concentration [ppm]
COS, and SO2 Concentration [ppm]
sufficient. Therefore, the following reactions would occur during the regeneration.
S. Raharjo and I. Naruse
4 CONCLUSIONS
Desulfurization characteristics by molten alkali carbonates (MAC) were studied experimentally,
using alumina tube furnace. Chemical equilibrium calculations were carried out prior to the H2S
and/or COS removal experiments. Additionally, regeneration tests of used MAC were also
carried out. The main results obtained are summarized below.
1. Chemical equilibrium calculations for H2S and/or COS removal process show that, even at
high temperature, the concentration of gaseous sulfur compounds become zero.
2. Molten alkali carbonates (MAC) can be applied as a liquid solvent to capture gaseous sulfur
compounds in the gasified gas even at high temperature. The following reactions could be
occurred in this removal process (l: liquid, g: gas, c: condensed):
M2CO3(l) + H2S(g)
M2S(c) + H2O(g) + CO2(g)
M2CO3(l)+ COS(g)
M2S(c) + 2CO2(g)
3. The regeneration of the used MAC is difficult under the present experimental conditions.
REFERENCES
1) Saito, I.(2003), 20th Annual International Pittsburgh Coal Conference Sept. 15 – 19.
2) Mitchell, S. C.(1998), IEA Coal Research the Clean Coal Center.
3) EAGLE. (2006). Multi Purpose Coal Gasification Technology Development. NEDO and
J-Power.
4) Henderson, C.(2004). IEA Clean Coal Center, ISBN 92-9029-406-X.
5) Furimsky, E., Yumura, M. (1986). Solid Adsorbents for Removal of Hydrogen Sulfide from
Hot Gas. Erdol und Kohle-Erdgas-Petrochemie, 39(4). Pp. 163-172.
6) Barthelemy, N.M. (1991). Improved Heat Recovery and High-Temperature Clean-up for
Coal-gas Fired Combustion Turbines. Master of Science Thesis, Department of Chemical
Engineering, University of California at Barkeley.
7) HSC Chemistry. (2002). User`s Guide for HSC Chemistry for Windows. Outokumpu
Research Oy.
142-8