Causticizing Aspects and Sulfur Recovery in Black Liquor Gasification. Low Temperature Gasification

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1 Causticizing Aspects and Sulfur Recovery in Black Liquor Gasification Adriaan van Heiningen University of Maine Colloquium on Black Liquor Combustion and Gasification May 13-16, 2003, Park City, Utah Low Temperature Gasification (Sulfur-free) Atmospheric (600 C) (Na 2 CO 3 ) 1

2 High Temperature Gasification 950 C Pressurized Entrained Flow Reactor Consequence of Sulfur Release during KBL Gasification Sulfur must be removed from gasification gas for liquor preparation and turbine protection Formation of sulfur-rich and sulfur-lean white liquor streams Possibility to produce elemental sulfur, and thus polysulfide liquor 2

3 Continuous System-Hi/Lo Sulfidity High Sulfidity White Liquor 12% Chips Total Alkali Charge 19.5%AA To Evaps Low Sulfidity White Liquor 4%AA Low Sulfidity White Liquor 3.5%AA Pulp 20 Kappa Preparation of Polysulfide Dissolve elemental sulfur in white liquor (WL) NaHS + NaOH + n S o Na 2 S n S + H 2 O (n=2-3) Oxidation of white liquor 3 NaHS + O 2 Na 2 S 2 S + NaOH + H 2 O 2 Na 2 S NaOH 4 Na 2 S + Na 2 S 2 O 3 + H 2 O 1. Elemental sulfur decreases alkali content of WL 2. WL oxidation does not significantly change alkali content of cooking liquor 3

4 H 2 S Removal in KBL IGCC Various H 2 S scrubbing processes are possible: 1. Absorption in weak wash. Na 2 CO 3 + H 2 S NaHS + NaHCO 3 (1) Na 2 CO 3 + H 2 O + CO 2 2 NaHCO 3 (2) This increases lime requirements as can be inferred from: 2NaHCO 3 + 2Ca(OH) 2 2CaCO 3 + 2NaOH + H 2 O (3) Na 2 CO 3 + Ca(OH) 2 CaCO NaOH (4) 2 Physical/chemical H 2 S absorption-stripping systems. Adds complexity of Claus plant for H 2 S to S conversion. Also recovered H 2 S still contains CO 2 (about equimolar) Increased Causticizing Need (Includes Physical Absorption of H 2 S) Gasification Temperature ( C) 600 Increase in Causticization (%) Increase in lime kiln fuel (% of gasifier output)

5 Direct Causticization with TiO2 Direct causticization reactions in gasifier: 3 TiO 2 + Na 2 CO 3 Na 2 O.3TiO 2 + CO 2 (1) 5 (Na 2 O.3TiO 2 ) + 7 Na 2 CO 3 3 (4Na 2 O.5TiO 2 ) + 7 CO 2 (2) Hydrolysis reaction in leacher: 3 (4Na 2 O.5TiO 2 ) + 7 H 2 O 5 (Na 2 O.3TiO 2 ) + 14 NaOH (5) Direct Causticization in the MTCI Steam Reformer 5

6 Fluid Bed Operating Conditions with Direct Causticization Complete direct causticization in 50 hours at 675 C in steam reformer At 700 C or higher the reformer must be pressurized to convert all Na 2 S in H 2 S (Na 2 S + CO 2 + H 2 O Na 2 CO 3 + H 2 S) At 800 C complete direct causticization in 0.3 hrs and pressure of 3.5 bar in reformer KBR oxygen blown recirculating FB at 827 C and 25.6 bar appears consistent complete H 2 S release Minimum Charge of TiO 2 or Na 2 O.3TiO 2 For kraft black liquor with a sodium content of 19 weight % based on dry solids: TiO 2 charge of 41 w/w % (kbl dry solids basis) Na 2 O.3TiO 2 charge of 89 % (on kbl dry solids) In practice a minimum Na 2 O.3TiO 2 charge equal to the dry black liquor solids weight is needed, because of incomplete conversion to 4Na 2 O.5TiO 2 6

7 Sulfate Reduction Kinetics Na 2 SO 4 + 3C Na 2 S + CO + CO 2 Temperature Sulfate Conversion in 1 hour % C F Heats of Reaction and Temperatures of Titanate and Lime Causticizing Process/Reaction Causticizing Leaching Calcination Slaking/Causticizing Temperature ( C) Titanate Lime Heat of Reaction (kj/mol NaOH)

8 Advantages of Direct Causticization with TiO 2 for Low T Gasification 4.5% increase in overall energy efficiency compared to conventional recovery. Elimination of lime cycle (Annual saving of 100,000 barrels of oil for 1000 TPD mill. Increased carbon conversion Increased tar conversion Conversion of sulfate to H 2 S Potential Problems or Risks Large Amount of titanate solids handling Disintegration of titanate particles during leaching Agglomeration of bed particles Too large TiO 2 losses in solids handling and NPE removal 8

9 Sulfur Capture By A Regenerative Calcium Based Process Sulfur Capture: H 2 S + CaO CaS + H 2 O H 2 S + CaCO 3 CaS + CO 2 + H 2 O CaS Conversion: CaS + NaOH NaHS + Ca(OH) 2 CaS + Na 2 CO 3 + H 2 O CaCO 3 ++ NaHS + NaOH Calcination: Ca(OH) 2 CaO + H 2 O CaCO 3 CaO + CO 2 (1a) (1b) (2a) (2b) (3a) (3b) Note that: Na 2 S + H 2 O NaHS + NaOH 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 CaCO 3 and avoiding H 2 S recapture by Na 2 CO 3 are incompatible Desulphurization with CaO in separate reactor at 100 C above steam reformer with TiO2 as bed; however no temperature restrictions with dolomite Desulphurization with CaCO3 or CaO may be combined with high T gasification 9

10 Integrated Low T Gasification CaO Desulfurization Reactor CaS High Pressure CaS Slaker Weak Wash Ca(OH) 2 NaHS White Liquor Clarifier Ca(OH) 2 Slaked Lime Washer Cool Combustion Gas Slaked Lime Calciner Raw Gasification Gas HP Steam KBL + Steam TiO 2 Make-up H 2 O Steam and/ or O 2 /air Pressurized air KBL Gasifier Recirculating or Fluidized Bed 4 Na 2 O. 5 TiO 2 Solids Heat Exchanger Solids Heat Exchanger Titanate Causticizer Na 2 O. 3 TiO 2 O 2 / Steam Weak Wash NaOH Desulfurized Medium BTU Gas H 2 O Venturi Scrubber Caustic NaOH Clarifier Na 2 O. Na 3 TiO 2 O. 3 TiO 2 2 Sodium and Sulfur-free Gas H 2 O Titanate Washer H 2 O Combustor Titanate Dryer Sulfur-rich White Liquor (NaHS, NaOH) Combustion Gas to Waste Heat Boiler Gas Electricity Turbine Sulfur-free White Liquor (NaOH) Cool Combustion Gas Acid TiO 2 NPE Removal NPE s Process Integration Advantages for Low T Process Version Production of NaOH and sulfur rich liquors Conversion of sulfate into H 2 S Increased carbon conversion Increased tar conversion (also in CaS reactor!) Elimination of lime cycle 10

11 Conclusions Black Liquor Gasification Technology may make the P and P Industry independent of fossil fuel, a net exporter of electricity, and dramatically reduce greenhouse gas emissions needs further development in areas of causticizing requirement and sulfur recovery 11