REHABILITATION AND OPTIMIZATION OF ANAEROBIC DIGESTION MIXING SYSTEMS by Rebecca Schaefer, P.E Joe Gorgan, P.E. WATERCON 2012 March 19, 2012
Presentation Outline Existing Digester Mixing Systems New Digester Mixing Systems Mixer Advantages and Disadvantages Digester Mixing System Evaluation Design Considerations Beneficial Uses of Sludge Gas Energy Cost Considerations Questions and Answers
Existing Digester Mixing Systems
Gas Lance Mixing System
Pump Recirculation Pump Multi-Duty 7.5-HP Circulating Sludge Pumps (2 Pumps per Tank, 8 Pumps Total) Multi-Duty 7.5-HP Circulating Sludge Pumps (2 Pumps per Tank, 8 Pumps Total) Multi-Duty 7.5-HP Circulating Sludge Pumps Recirculation Mixing Systems
New Digester Mixing Systems
Jet Mixing System Advantages Low capital cost Liquid Dynamics Corp. Jet Mix TM Disadvantages Nozzle clogging No back-up Requires grinder High energy demand Liquid Dynamics Corp. 7
External Draft Tube Mixers Multiple external draft tube mixers are placed around perimeter of digester Eimco Advantages Proven technology Heat exchange jackets for low cost digester heating Maintenance access exterior to digester System can reverse direction Back-up Disadvantages Require a crane for maintenance Capital costs Upper discharge pipe limits travel distance for floating covers
External Draft Tube Mixers Operation
External Draft Tube Mixer Installation
Vertical Linear Motion Mixers Eimco Works on displacement Literature shows comparable to draft tubes Advantages Low energy demand Low capital cost Low maintenance, easy access Disadvantages Newer technology for digester mixing No back up 11
Gas Cannons Large bubbles every 3-4 seconds 14 24-inch diameter, 18 long mixers for 110 diameter Advantages Less energy demand Redundancy Disadvantages Gas compressors Bubble generator inside digester Degremont Technologies 12
Digester Mixing System Evaluation
Cost Cost Relative Mixing Technology Cost Comparison* Mixer Capital Cost Mixer Energy Cost Gas Cannon External Draft Tube Mixer Type Vertical Linear Motion Jet Mixer Relationships may vary depending on manufacturer and digester size. Gas Cannon External Draft Tube Vertical Linear Motion Mixer Type *Based on 110 Diameter Digester Jet Mixer
Digester Performance Evaluation Volatile Solids Reduction (%) 60 50 40 30 1998 2011 20 10 0
Digester Performance Evaluation Specific Biogas Production Rate 20 18 16 14 12 10 8 1998 2011 6 4 2 0 per LB VS Fed per LB VS Degraded
Digester Performance Evaluation 16% Increase in VSS Destruction Over 5.0 Million CF of Additional Digester Gas Produced on an Annual Basis Improved Mixing Targeting Hydrophobic Solids Resulting in Higher Gas Production More Consistent SRT s through Elimination of Short Circuiting More Consistent Temperature Control
Design Considerations
Design Considerations Basis of Design (110 dia) Mixer Type External Draft Tube Unit Horsepower HP Per Digester System Power Consumed (KW) Turnover Time (minutes) Unit Flow Rate (gpm) Digester Flowrate (gpm) 10.0 40.0 30.0 33.2 15,000 60,000 Jet Mixers Gas Canon Mixers Linear Motion Mixers 100.0 100.0 75.0 343.1 5,800 5,800 45.0 45.0 67.1 28.0 5,070 70,980 12.5 12.5 9.4 N/A N/A N/A
Design Considerations Cover Types Digester Covers Fixed Cover Gas Holder Covers Membrane Covers Floating Covers Floating Covers
Greater Reliance on Gas Conditioning Moisture Removal H 2 S Removal Siloxane Removal Gas Pressure Boosting
Digester Gas Conditioning 1600 1400 1200 1000 800 600 400 200 0 Engine H 2 S Limits uturbine & Fuel Cell H 2 S Limits Hydrogen Sulfide, ppmv 30 25 20 15 10 5 0 Engine Siloxane Limits uturbine & Fuel Cell Siloxane Limits Siloxanes, mg/m^3 Siloxane Damage
Gas Conditioning Unit
TWAS Facility Location
Digester Gas Heat Consider All Heating Loads
Design Considerations Electrical Improvements
Beneficial Uses of Sludge Gas
System Alternatives Thermal Only Boilers Combined Heat and Power Fuel Cells IC Engine-Generators Microturbines Absorption Chillers Prime Movers
Energy Cost Considerations
Value of Equivalent Natural Gas +5,884,385 Additional CF @ 600 BTU/CF= 35,306 Therms @ $1.00/Therm= $35,306/Yr Natural Gas (NG) Equivalent As much as 80% can be recovered in boilers, for an annual NG savings of $28,245 If this gas is used to fire a combined heat and power system with 30% mechanical and 30% electrical efficiency, at an $.08/kWh electric rate, the annual NG and electricity costs replaced could be as much as $38,060
PLANT HEATING LOAD: BTU/Hour x 1,000 Statistical Thermal & CHP Energy Modeling Heating Loads: MBH in Each Outside Air Temperature Bin 8000 7000 6000 5000 4000 Digester Building Total DG 3000 2000 1000 0-10 0 10 20 30 40 50 60 70 80 90-1000 OUTSIDE AIR TEMPERATURE: ºF DG Boiler uturbine
Hourly Thermal & CHP Energy Modeling Heating Loads Calculated for each Hour of Occurrence Temperature (Degrees F) 100 80 60 40 20 0-20 South Bend, Indiana TMY3 (Typical Meteorological Year) Data National Solar Radiation Database U.S. National Renewable Energy Laboratory 1 1 2 3 4 5 6 7 8 9 10 11 12 Month Number
Million BTU Under the Curve 3500 3000 2500 2000 1500 Additional Energy from increased DG production. Burned In DG boilers reduces annual Natural NG Gas cost Cost 12 % or more DG Natural Without Boiler Gas DG Heat Savings Usage Output = With having DG With $254,450 Boilers uts= no beneficial $84,032 = $156,720 use ~ (Green 20% + Blue) Σ MMBTU Blr MMBTU ut MMBTU 1000 500 0-10 0 10 20 30 40 50 60 70 80 90 Outside Air Temperature: ºF
Benefits: Why External Draft Tube Mixers? Very accurate temperature control Minimized solids settling Reduced digester cleaning maintenance Elimination of digester heat exchanger cleaning Reduced chance of process upsets Superior heating and mixing reliability and redundancy Savings percent potentials extend, to a greater extent, to FOG & other digester feed supplements Increased digester gas production Reduced electrical mixing energy usage compared to most alternatives
Questions and Answers