Hydrogen Sulfide Removal from Biogas

1. Hydrogen Sulfide Removal from Biogas Anaerobic Treatment Short Course Part 1-Supplement

2. Introduction • Biogas – Methane 50-70%, Carbon dioxide 30-50%, trace gases (nitrogen, ammonia, and hydrogen sulfide) • Hydrogen Sulfide—the most notorious • Corrosion to concrete and metal pipes • Odorous (recognition at 4.5 ppb) • Heath concerns (10 ppmV and death at 600 ppmV) • Combustion  SOx (esp. SO2) acid rain • Removing of H2S is recommended as soon as possible to protect downstream equipment • Total sulfide at 90-250mg-S/L, inhibit methanogenesis

3. Introduction • Requirement of H2S reduction in several processes (Zicari, 2003) • Microturbines: up to 70,000 ppmV • Boilers and external combustion engine: < 1000 ppmV • Internal combustion engines: <100 ppmV • Kitchen stoves and fuel cells: < 10 ppmV • Thermo-catalytic conversion: nothing

4. Introduction Where are sulfides come from ? • Wastewater containing: sulfate, sulfite, thiosulfate, protein • Sulfate in most city: up to 40 mg/L (Ames: 70-90 mg/L) • Reduction of sulfate by Sulfate reducing bacteria (Desulfovibrio, Desulfobacter, Desulfuromonas, many Archaea etc.) SO42- + organic matter  S2- + H2O + CO2

5. Sulfur cycle—various forms • http://mcb.berkeley.edu/labs/kustu/mcb112/SulfurCycle.GIF

6. Background: Method to remove sulfides from biogas 1) Raising pH H2S (aq)  HS- + H+ pKa = 7.04 at 25 C HS-  S2- + H+ pKa = 12.89 at 25 C H2S (g) = KH H2S (aq) Alkalinity addition  Chemical cost Operating pH of Anaerobic digester ?

7. Background: Method to remove sulfides from biogas 2) Precipitation using bivalent metals (Cu2+, Fe 2+, Zn 2+) Fe2+ + HS-→ FeS(S) + H+, [Fe2+][S2-] = KS, KS = 6x10-18 High chemical cost, clogging, consume reactor volume, increase sludge production

8. Background: Method to remove sulfides from biogas 3) Pass biogas through, iron sponge, Fe2O3 column or Fe2O3 impregnated wood • Formation of Fe2S3 can be regenerated by aeration High capital cost, life of media, operation problem—regeneration of the media

9. Background: Method to remove sulfides from biogas 4) Gas stripping, absorption, internal recirculation and regeneration of absorbents • Absorbents: NaOH, NaCO3, NaHCO3 • Regeneration: Aeration, Nitrate addition (Thiobacillus sp.) Chemical cost, nutrient cost, sulfate as product

10. Background: Method to remove sulfides from biogas 5) Bioscrubber, Biofilter • Use wetted organic material bed Addition of large amount of air, >30% by volume (safety issue) , moisture control 6) Addition of air (2-6%) directly to head space Still about 200 ppmV left in biogas

11. Background: Method to remove sulfides from biogas 7) Microaeration: Injection of oxygen or air to liquid content (<0.1 mg/L O2) • Biotic oxidation of sulfides 2HS- + O2 2S0 + 2OH-G° = -129.50 kJ/mol HS- 2HS- + 4O22SO42- + 2H+G° = -772.43 kJ/mol HS- • Colorless sulfur bacteria (Thiobacillus, Beggiatoa, Thiothrix, etc.), Chemoautotroph • Use reduced inorganic sulfur compounds as electron donors and CO2 as C-source

12. Background: Method to remove sulfides from biogas The explosive limits of some gases and vapors are given in percent by volume of air. Lower explosive limit (LEL) and Upper explosive limit (UEL)

13. Background: Method to remove sulfides from biogas • Anaerobic baffled reactor + aerobic sulfide oxidizing reactor treating pharmaceutical WW (Fox and Venkatasubbiah, 1996) • Janssen et al. (1998) used ORP controlled oxygen dosing for sulfide oxidizing reactor but did not integrate with anaerobic treatment • Janssen et al. (1998) used ORP controlled oxygen dosing for sulfide oxidizing reactor but did not integrate with anaerobic treatment • Direct aeration condition in an anaerobic fluidized bed reactor treating high COD high sulfate WW (Zitomer and Shrout, 2000) • ORP control pure O2 injection to anaerobic digesters to remove sulfide toxicity in high sulfate WW (Leung, 1998; Khanal and Huang, 2001) • Krishnakumar et al. (2005) used Reverse fluidized loop reactor (RFLR) to remove sulfide in liquid phase with ORP control.