Transcript CONTROL AND MONITORING OF HYDROGEN SULFIDE IN DIGESTER …

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 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
12 cyl. Turbo 2000 hp w/1200 kW gen - 1 of 4
Engines required extensive maintenance
Trigen’s Turbine control panel with PLC
Trigen’s pride and joy - Centaur 40 Turbine
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
Current Dosing Points
• Digester Feed Line
• DAF Conditioning Box
• NSEC Influent Channel
Ni/Deni Aeration Basin - 1 of 12
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
Note the floating cover on Digester 6
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)
Digester Flares
Turbine Exhaust
Continuous Monitoring
System (CMS) Requirements
• Continuous gas stream H2S monitoring
• Continuous data transmission and
recording
• Instrument reliability
• Relative Accuracy Test Audit (RATA)
LasIR Unit
Control Unit
Control Unit Display
Modifications Needed After
Initial Installation
• Inability to meet accepted calibration
protocol
• Extractive method required
• Water vapor in digester gas impacted
reliability
White Cell
LasIR Reconfiguration
• Sensor upgrade - aka “White Cell”
Layout of the Multi-pass Extractive Monitor Head
Gas Out
(1/2” or ¼” Teflon tubing)
Fiber-optic cable
From LasIR Analyzer
Diaphragm
Pump
Infrared detector
Pressure/Flow
Controller
12-m multiple reflection (White) cell
Manual Valve
Cal. Gas IN
~ 15”
Coaxial cable
Three-way valve
To LasIR Analyzer
Stack Gas IN
(¼” or 1/8” Teflon tubing)
Alak Chanda – 04January 2001
LasIR Upgrades
• Laser modified for extractive analysis
• Nitrogen gas used for purge and
zeroing
• H2S calibration gas incorporated
Cal and Purge Gas
Results
• Instrument has proven reliable
• Permit compliance achieved
Future Use
• Instrument will control ferric dosing
• ANY QUESTIONS?