Transcript No Slide Title

Integrated Sulfur Recovery and
Causticization for Kraft Black
Liquor Gasification
Adriaan van Heiningen
University of Maine
IEA, August 21st, 2002
Pitea, Sweden
Increased Causticizing
Requirement
1. H2S generation creates equal moles of Na2CO3,
which in the recovery boiler is Na2S or
½(NaOH+NaHS), so a 30% sulfidity white
liquor (Na2S/NaOH=0.43 mole/mole) and
complete volatilization of the sulfur leads to
43% increased causticization requirement
2. Absorption in weak wash leads to co-absorption
of CO2. At a selectivity factor of 10 moles
H2S/mole CO2 this increases the causticization
by another 60%
Solution for Low T Gasification
Direct Causticization with TiO2
Direct causticization reactions in gasifier:
3 TiO2 + Na2CO3  Na2O.3TiO2 + CO2
5 (Na2O.3TiO2) + 7 Na2CO3  3 (4Na2O.5TiO2) + 7 CO2
(1)
(2)
Hydrolysis reaction in leacher:
3 (4Na2O.5TiO2) + 7 H2O  5 (Na2O.3TiO2) + 14 NaOH
(5)
Direct Causticization in the
MTCI Steam Reformer
Selection of Operating Conditions
Criterium: Avoid formation of low melting point
eutectic salt mixture (Na2CO3, Na2S, NaCl, K2CO3)
• Na2CO3 (and K2CO3) to be converted to titanates
(Eutectic of Na2O.3TiO2 + 4Na2O.5TiO2 is 985 °C)
• 14 NaCl + 5(Na2O.3TiO2) + 7H2O  3(4Na2O.5TiO2) + 14 HCl
Presence of steam and titanate reduces NaCl content
• Na2S + CO2 + H2O  Na2CO3 + H2S
Volatilization of H2S is favored by high total P and low T
[H 2S ]
0.02
P

K [ H 2O][CO2 ] 0.1K
and K decreases with increasing temperature
Direct Causticization Process Conditions
•K = equilibrium constant of H2S release from Na2S
• td = time for 100% conversion of NT3 into N4T5
• tc = time for 100% conversion of organic carbon
• P = total pressure
Conclusions:
1. Minimum operating temperature of 675 °C
2. Pressurized gasification above 700 °C
3. At maximum temperature of 800 °C the solids residence time
is reduced to 0.3 hours
H2S Removal in KBL IGCC
Various H2S scrubbing processes are possible:
1. Absorption in weak wash.
Na2CO3+ H2O + CO2  2 NaHCO3
(1)
This increases lime requirements as can be inferred from:
2NaHCO3 + 2Ca(OH)2  2CaCO3 + 2NaOH + H2O (2)
Na2CO3+ Ca(OH)2  CaCO3 + 2 NaOH
(3)
2. Amine-based H2S absorption-stripping systems.
Adds complexity of Claus plant for H2S to S conversion.
Also recovered H2S still contains CO2
Sulfur Capture By A Regenerative
Calcium Based Process
Sulfur Capture:
H2S + CaO  CaS + H2O
H2S + Na2CO3  CaS + CO2 + H2O
(1a)
(1b)
CaS Conversion:
CaS + NaOH  NaHS + Ca(OH)2
CaS + Na2CO3 + H2O  CaCO3 + NaHS + NaOH
(2a)
(2b)
Calcination:
Ca(OH)2  CaO + H2O
CaCO3  CaO + CO2
(3a)
(3b)
Note that: Na2S + H2O  NaHS + NaOH
Calcination and H2S Recapture
CaCO3CaO+CO2; Na2CO3+H2SNa2S+CO2+H2O
Tempe
CO2
rature Equilibrium
Pressure
(°C)
(Atm)
Maximum
Minimum Total Pressure
Total Pressure to Avoid H2S Recapture in
at CO2 = 15%
O2 Blown System
(Atm)
(Atm)
700
750
800
0.034
0.093
0.23
0.23
0.62
1.5
2.0
4.3
8.3
850
900
950
0.52
1.08
2.12
3.5
7.2
14.1
15.8
27
45
1000
3.91
26.1
73
Combining Calcium Based Sulfur
Recovery with Gasification
• Calcination in gasification gas of 1 or 20 atm.
requires temperature above resp. 775 and 975 °C
• Calcination of CaCO3 and avoiding H2S
recapture by Na2CO3 are incompatible
Conclusions: 1. Desulphurization with CaO must be
performed in separate reactor operating 100 °C above
T of gasification reactor with TiO2 as bed material.
2. Desulphurization with CaCO3 or CaO may be
combined with high T gasification
Integrated Low T Gasification
Weak Wash
CaO
Desulfurization CaS
Reactor
High Pressure
CaS Slaker
Ca(OH)2
NaHS White Liquor Ca(OH)2 Slaked Lime
Clarifier
Washer
Slaked Lime
Calciner
HP Steam
Raw Gasification Gas
KBL + Steam
TiO2 Make-up
H2O
Steam and/
or O2/air
Pressurized
air
Na2O.3
Sulfur-rich White
Liquor (NaHS,
NaOH)
H2O
NaOH
KBL Gasifier
Recirculating
or Fluidized
Bed
TiO2
O2 / Steam
4 Na2O.5 TiO2
Desulfurized
Medium BTU
Gas
H2O
Venturi
Scrubber
Sodium and
Sulfur-free
Gas
Solids Heat
Exchanger
Combustion Gas
to Waste Heat
Boiler
Combustor
Gas
Turbine
Electricity
H2O
Sulfur-free White
Liquor (NaOH)
Solids Heat
Exchanger
Weak Wash
Titanate
Causticizer
Cool
Combustion
Gas
NaOH
Na2O.3 TiO2
Caustic
Clarifier
Na2O.3
TiO2
Titanate
Washer
Titanate Dryer
Cool
Combustion
Gas
TiO2
Acid
NPE Removal
NPE’s
Integrated High T Gasification
H2O
Steam +
KBL
O2
CaCO3
KBL
Gasifier
Chemrec
Type
Water
Quench
Vessel
Na2CO3
CaS
Flash
Tanks
H2O
Desulfurized
Medium BTU
Gas
Venturi
Scrubber
Sodium
and
Sulfurfree Gas
Electricity
Gas
Turbine
Combustor
Contaminated H2O
HP + LP
Steam
CaCO3
Causticizers
NaOH
NaHS
CaCO3
White Liquor
Clarifier
CaCO3
White Liquor
(NaOH, NaHS)
Lime Mud
Washer
Lime Kiln
Weak Wash
Pressurized
Air
Lime
Make-up
Oil
Combustion
Gas to Waste
Heat Boiler
Ca(OH)2
Lime Slaker
CaO
Cool
Combustion
Gas
Lime Mud Purge
(Grits)
Process Integration Advantages
Low T Process Version
•
•
•
•
•
Production of NaOH and sulfur rich liquors
Increased black liquor throughput
Increased carbon conversion
Increased tar conversion (also in CaS reactor!)
Elimination of lime cycle
High T Process Version
• Same causticizing requirement as conventional
• Reduced capital cost
• Simplest process
Process Integration Technology Gaps
Low T Process Version
• Pulping benefits resulting from split sulfidity and
polysulfide (with/without AQ) liquors (NCSU)
• Direct causticization kinetics; Effect of low T, CO2
pressure and TiO2 particle size and source)
• Pilot and PDU verification tests (MTCI and U of Utah)
• Removal of NPEs from leached titanate product
• Optimization of 4Na2O.5TiO2 leaching process
• Desulphurization and tar cracking kinetics of CaO/CaS;
Effect of Ca source and particle size, type of sulfur gas
• System analysis (NCSU and VTT)
Process Integration Technology Gaps
High T Process Version
• Desulphurization kinetics of CaO/CaCO3; Effect of T, Ca
quality and particle size, P (IPST), type of sulfur gas, and
mixtures with black liquor.
• White liquor generation kinetics from CaS by Ca(OH)2
suspension and/or Na2CO3 solution; Effect of T, Ca quality
and [NaOH]
• Industrial validation (Weyerhaeuser)
• System analysis (VTT)