Black Liquor Char Oxidation How Different is Black Liquor Char from Chars of Other Fuels?

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1 Black Liquor Char Oxidation How Different is Black Liquor Char from Chars of Other Fuels? Mikko Hupa, Mikael Forssén Process Chemistry Group Åbo Akademi University Colloquium on Black Liquor Combustion and Gasification, May 13-16, 23, Park City Marriott, Park City, Utah Outline Introduction: Fuel Char Oxidation Reactions Black Liquor Char Oxidation Special Features Char Nitrogen Conclusions

2 Stages of Black Liquor Gasification Oxygen gases CO 2 SO 2 NO gases Pyrolysis Char gasification Droplet Char Smelt Heat H 2 O, CO 2, etc. Char? - What remains after devolatilization (pyrolysis) - Char properties very dependent on devolatilization conditions - Char properties change easily

3 Why Study Char Oxidation? Final Burn-Out in Combustion Carbon Conversion in Gasification Carbon in the Smelt, Dregs (BL) Reduction Reactions (BL) Char-N Ash Forming Matter Release (BL: Na, S) Trace Metal Release Proximate Analysis for Eight Nordic Black Liquor Water 28% BLS 72% Organics 54% Inorganics 46% VM 46% FC 54% VM = Volatile Matter FC = Fixed Carbon (Char) Org. and Inorg. from chemical analysis - Org=C+H+N+.5*O - Inorg.= Na+K+Cl+S+.5*O Pyrolysis yield at 7C defined to be the VM

4 1% Volatile Matter (VM) and Fixed Carbon (FC) (Ash free bases) VM Bit. Coal Lignite Peat Wood Black Liquor FC Char Carbon Conversion Black Liquor Char Carbon Oxidation Reactions (All Fuels) Low temperature gasification Volatiles Char Carbon O 2 CO 2 H 2 O Na 2 SO 4 (l) CO CO CO, H 2 Na 2 CO 3 (l) CO (Na 2 S) CO (Na )

5 Gasification Reactivity for Different Fuels TGA: Mostly Under Conditions for Chemical Control Rate given: 1/mo dm/dt Coals, Lignite, Wood, Peat, Black Liquor: Rate Varies by six orders of magnitude! Moilanen et al from Liekki Combustion Research Program Technical Review , Turku 1993

6 BL Char Gasification Rate (TGA) K. Whitty, PhD thesis 1997 Moilanen et al from Liekki Combustion Research Program Technical Review , eds Hupa and Matinlinna

7 Rate Determining Steps Chemistry slow chemistry, low T Pore diffusion External mass transfer fast chemistry, high T ox. ox. ox. Outline Introduction: Fuel Char Oxidation Reactions Black Liquor Char Oxidation Special Features Char Nitrogen Conclusions

8 Char Carbon Conversion Black Liquor Char Carbon Oxidation Reactions (Black Liquor) CO Volatiles Low temperature gasification Char Carbon O 2 CO 2 H 2 O Na 2 SO 4 (l) CO CO, H 2 Na 2 CO 3 (l) CO (Na 2 S) CO (Na ) Char Oxidation Rate Equations Overall char oxidation rate equations are available: CO 2 Ea = 21 kj/mol (Li & van Heiningen 1991) H 2 O Ea = 187 kj/mol (Li & van Heiningen 199) Na 2 SO 4 (l) Ea = 213 kj/mol (Cameron&Grace 1983) Na 2 CO 3 (l) Ea = 244 kj/mol (Li & van Heiningen 199, Frederick 1996)

9 Char Carbon Conversion Rates Estimated rates 1 Time to 2% char carbon conversion, seconds 1 H2O 1 CO2 Na2SO4.1 Na2CO Temperature, C Black Liquor Carbon Conversion Total Conversion in Gasification Process (Whitty 1997) 1 8 Overall conversion Conversion, % 6 4 Pyrolysis reactions Gasification reactions 2 Time Inorganic reactions

10 Outline Introduction: Fuel Char Oxidation Reactions Black Liquor Char Oxidation Special Features Char Nitrogen Conclusions NO and CO 2 Formation During Combustion Polish coal 12mg particle, 9 C, 1% O 2 Nitrogen % O mg mgn/1 g l Carbon % O mg 74.4 gc/1 g sample NO formation (g N /s*g sample) *1e time (s) (CO2+CO) formation (g C /s*g sample) time (s)

11 NO and CO 2 Formation During Combustion Polish coal Nitrogen 12mg particle, 9 C, 1% O 2 Carbon % O mg mgn/1 g l % O mg 74.4 gc/1 g sample NO formation (g N /s*g sample) *1e time (s) logarithmic scale of y-axis (CO2+CO) formation (g C /s*g sample) time (s) Conversion of NO vs Conversion of CO 2 Polish coal 12mg particle, 9 C, 1% O 2 Conversion of total NO formation Polish coal x Conversion of total CO2 formation x = end of pyrolysis

12 NO and CO 2 Formation During Combustion Birch Nitrogen 12mg particle, 9 C, 1% O 2 Carbon % O mg 55.5 mgn/1 g l *1e-5 1 NO formation (g N /s*g sample) time (s) logarithmic scale of y-axis % O mg 45.9 gc/1 g sample (CO2+CO) formation (g C /s*g sample) time (s) Conversion of NO vs Conversion of CO 2 Birch 12mg particle, 9 C, 1% O 2 Conversion of total NO formation Finnish birch.8.6 x Conversion of total CO2 formation x = end of pyrolysis

13 NO and CO 2 +CO Formation During Combustion Black liquor Nitrogen 12mg particle, 9 C, 1% O 2 Carbon BL % O mg 15.1 mgn/1 gbls BL % O mg 26.5 gc/1 gbls NO formation (g N /s*g BLS) *1e time (s) logarithmic scale of y-axis (CO2+CO) formation (g C /s*g BLS) time (s) Conversion of NO vs Conversion of CO 2 Black liquor 12mg particle, 9 C, 1% O 2 BL312 Conversion of total NO formation x Conversion of total CO 2 formation x = end of pyrolysis

14 NO and CO2 Formation During Char Burning (9 C, 3 % O 2 ) (Forssen et al 1997) ORIGINAL CHAR WASHED CHAR NO, ppm 5 CO2, ppm Total amount of NO formed per 1 g original char: 21.3 mg N 8.8 mg N Total amount of [CO2 + CO] formed per 1 g original char: 17.9 g C 16.5 g C NITROGEN DISTRIBUTION DURING BL COMBUSTION N Pyrolysis N 2 N 2 N BL N Char NO N 2 NH 3 NO NaOCN smelt OCN - (aq) GL

15 Formation of Cyanate during Char Oxidation Cyanate, mg N/kg smelt Char Inorganic residue Extent of oxidation, % (= Smelt) Kymäläinen et al. 22 NITROGEN DISTRIBUTION DURING BL COMBUSTION N Pyrolysis N 2 N 2 N BL N Char NO N 2 NH 3 NO NaOCN smelt OCN - (aq) GL

16 % of original N in BLS Small droplets (1 mg) 9 C, 1% O 2 8 C, 1% O 2 8 C, 3% O char-no pyrolysis-no NO formation of three black liquors under different conditions. % of original N in BLS Large droplets (4 mg) 9 C, 1% O 2 8 C, 1% O 2 8 C, 3% O char-no pyrolysis-no Kymäläinen et al. 22 Outline Introduction: Fuel Char Oxidation Reactions Black Liquor Char Oxidation Special Features Char Nitrogen Conclusions

17 BL Char is Very Different! Very High Oxidation Reactivity (Alkali Catalysis) Large Internal Surface (Swelling) Additional Oxidation by Sulfate and Carbonate Heavy Alkali Vaporization Capture of Char N in Inorganic Smelt (NaOCN) Research Topics on BL Char Conversion Sodium reactions: Carbonate and Fume Formation K, Cl, trace metals Nitrogen Reactions in the Char Sulfur Behavior Char properties vs. devolatilization conditions Char porosity/internal surface vs. conversion Swelling and Fragmentation