Adriaan van Heiningen

1
Integrated Sulfur Recovery and Causticization for
Kraft Black Liquor Gasification
Adriaan van Heiningen University of
Maine IEA, August 21st, 2002 Pitea, Sweden
2
Increased Causticizing Requirement

• H2S generation creates equal moles of Na2CO3,
  which in the recovery boiler is Na2S or
  ½(NaOHNaHS), so a 30 sulfidity white liquor
  (Na2S/NaOH0.43 mole/mole) and complete
  volatilization of the sulfur leads to 43
  increased causticization requirement
• 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

3
Solution for Low T Gasification
Direct Causticization with TiO2

• Direct causticization reactions in gasifier
• 3 TiO2 Na2CO3 ? Na2O.3TiO2 CO2
  (1)
• 5 (Na2O.3TiO2) 7 Na2CO3 ? 3 (4Na2O.5TiO2) 7
  CO2 (2)
• Hydrolysis reaction in leacher
• 3 (4Na2O.5TiO2) 7 H2O ? 5 (Na2O.3TiO2) 14
  NaOH (5)

4
Direct Causticization in the MTCI Steam Reformer
5
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

and K decreases with increasing temperature
6
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
7
H2S Removal in KBL IGCC

• Various H2S scrubbing processes are possible
• 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

8
Sulfur Capture By A Regenerative Calcium Based
Process
Sulfur Capture H2S CaO ? CaS H2O
(1a) H2S
Na2CO3 ? CaS CO2 H2O
(1b) CaS Conversion CaS NaOH ? NaHS
Ca(OH)2
(2a) CaS Na2CO3 H2O ? CaCO3 NaHS NaOH
(2b) Calcination Ca(OH)2 ? CaO H2O

(3a) CaCO3 ? CaO CO2

(3b)
Note that Na2S H2O ? NaHS NaOH
9
Calcination and H2S Recapture CaCO3?CaOCO2
Na2CO3H2S?Na2SCO2H2O
10
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
11
Integrated Low T Gasification
12
Integrated High T Gasification
13
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

14
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)

15
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)