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Control and Monitoring of Hydrogen Sulfide in Digester Gas . Steve Walker Craig Barnes Metro Wastewater Reclamation District. Metro District’s Central Treatment Plant. What to Use to Control H 2 S. Equipment Iron vs. Aluminum Salts Dewatering Issues
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Control and Monitoring of Hydrogen Sulfide in Digester Gas Steve Walker Craig Barnes Metro Wastewater Reclamation District
What to Use to Control H2S • Equipment • Iron vs. Aluminum Salts • Dewatering Issues • 503 Regulations – Aluminum under scrutiny in Round 2 • Iron beneficial for crops and other plants
District’s Reasons to Reduce the Amount • Potential offsite buyer for methane - gas contaminated with 3000 + ppm H2S • Tried ferrous chloride from supplier’s recommendation • Tried ferric chloride as an alternative
Reasons for Alternatives • Cost Comparison • Flexibility • Impacts on Processes
Results • Cost – active pounds/digester loading – Same • Reaction time slower with ferrous • Flexibility proven – could use either product
Addition Points Impacts • Primary Influent • Ferrous • Ferric • DAF Conditioning Box • Ferrous • Ferric
Use of Ferric After Testing • H2S impacts on cogeneration equipment • Equipment reliability • Destruction of yellow metal parts • Acidification of engine oil • Struvite reduction
Current Usage and Why • Goal – Title V Air Permit Compliance • Limit of 169 tons per year SO2 to atmosphere • H2S ceiling of 2000 ppm with running 3 hour average of 1800 ppm • Ferric onsite – used for struvite control in dewatering process • Handling one chemical vs. two - KISS
Current Dosing Points • Digester Feed Line • DAF Conditioning Box • NSEC Influent Channel
Current Results • Gas production remains at 4MSCFD • Holding H2S levels at 900 ppm +/- 100 with consistent feed rate • Minimal Process Impacts • Foaming • Struvite
NOTE • High flow in spring 2001 reduced sulfide generators in the collection system by flushing the piping. This made results seem better. Dosages are back to traditional levels now.
Dosage Rate • Roughly 0.3 gpm ferric solution to 650 gpm digester feed or…… • Roughly 2000 pounds/day to 340,000 pounds TS = 12 active lb/ton • Digesters are fed sequentially so all get equivalent dose
Calculation • 0.3 gal/min x 8.34 lb/gal x 1.4 (specific gravity) x % iron in solution (40%) x 1440 min/day = 2000 lb/day of iron • Lb/day x $0.10/lb x 365 days/yr = $73,000
Monitoring • Effectiveness checked with GC Mass Spec – twice per week from grab sample • This would not meet the requirements of the current air permit • APCD permit required installation of Continuous Monitoring System (CMS)
Continuous Monitoring System (CMS) Requirements • Continuous gas stream H2S monitoring • Continuous data transmission and recording • Instrument reliability • Relative Accuracy Test Audit (RATA) compliant
Modifications Needed After Initial Installation • Inability to meet accepted calibration protocol • Extractive method required • Water vapor in digester gas impacted reliability
LasIR Reconfiguration • Sensor upgrade - aka “White Cell”
Gas Out (1/2” or ¼” Teflon tubing) Fiber-optic cable From LasIR Analyzer Diaphragm Pump Infrared detector Pressure/Flow Controller Manual Valve 12-m multiple reflection (White) cell Cal. Gas IN ~ 15” Coaxial cable To LasIR Analyzer Three-way valve Stack Gas IN (¼” or 1/8” Teflon tubing) Layout of the Multi-pass Extractive Monitor Head Alak Chanda – 04January 2001
LasIR Upgrades • Laser modified for extractive analysis • Nitrogen gas used for purge and zeroing • H2S calibration gas incorporated
Results • Instrument has proven reliable • Permit compliance achieved
Future Use • Instrument will control ferric dosing • ANY QUESTIONS?