Control and Monitoring of Hydrogen Sulfide in Digester Gas

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Control and Monitoring of Hydrogen Sulfide in
Digester Gas

• Steve Walker
• Craig Barnes
• Metro Wastewater Reclamation District

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Metro Districts Central Treatment Plant
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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

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Districts Reasons to Reduce the Amount

• Potential offsite buyer for methane - gas
  contaminated with 3000 ppm H2S
• Tried ferrous chloride from suppliers
  recommendation
• Tried ferric chloride as an alternative

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Reasons for Alternatives

• Cost Comparison
• Flexibility
• Impacts on Processes

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Results

• Cost active pounds/digester loading Same
• Reaction time slower with ferrous
• Flexibility proven could use either product

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Addition Points Impacts

• Primary Influent
• Ferrous
• Ferric
• DAF Conditioning Box
• Ferrous
• Ferric

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Use of Ferric After Testing

• H2S impacts on cogeneration equipment
• Equipment reliability
• Destruction of yellow metal parts
• Acidification of engine oil
• Struvite reduction

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12 cyl. Turbo 2000 hp w/1200 kW gen - 1 of 4
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Engines required extensive maintenance
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Trigens Turbine control panel with PLC
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Trigens pride and joy - Centaur 40 Turbine
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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

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Current Dosing Points

• Digester Feed Line
• DAF Conditioning Box
• NSEC Influent Channel

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Ni/Deni Aeration Basin - 1 of 12
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Current Results

• Gas production remains at 4MSCFD
• Holding H2S levels at 900 ppm /- 100 with
  consistent feed rate
• Minimal Process Impacts
• Foaming
• Struvite

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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.

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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

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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

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Note the floating cover on Digester 6
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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)

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Digester Flares
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Turbine Exhaust
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Continuous Monitoring System (CMS) Requirements

• Continuous gas stream H2S monitoring
• Continuous data transmission and recording
• Instrument reliability
• Relative Accuracy Test Audit (RATA) compliant

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LasIR Unit
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Control Unit
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Control Unit Display
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Modifications Needed After Initial Installation

• Inability to meet accepted calibration protocol
• Extractive method required
• Water vapor in digester gas impacted reliability

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White Cell
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LasIR Reconfiguration

• Sensor upgrade - aka White Cell

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Layout of the Multi-pass Extractive Monitor Head
Alak Chanda 04January 2001
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LasIR Upgrades

• Laser modified for extractive analysis
• Nitrogen gas used for purge and zeroing
• H2S calibration gas incorporated

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Cal and Purge Gas
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Results

• Instrument has proven reliable
• Permit compliance achieved

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Future Use

• Instrument will control ferric dosing
• ANY QUESTIONS?