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Achieving Economic and Environmental Sustainability Objectives through On-Site Energy Production from Digester Gas

C. Michael Bullard, Bryan R. Lisk, and Scott A. Hardy

22 June 2011Sandusky, OH

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Where are we going…

Anaerobic Digestion and Site Specific Considerations

Biogas-to-Energy Technologies

Case Studies on Biogas Utilization

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Anaerobic Digestion and

Site Specific Considerations

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Energy production from digester gas represents an untapped energy resource. ~16,000 Centralized Wastewater Treatment

Plants in the USA

~544 (3.4%) of plants with flow rate of > 5.0 MGD use anaerobic digestion for stabilization

~106 (20.6%) of those with anaerobic digesters generate electrical and/or thermal energy from digester gas in a CHP system

Reference: USEPA, Opportunities for and Benefits of Combined Heat and Power at Wastewater Treatment Facilities, April 2007

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Anaerobic digestion converts complex organic material into gaseous byproducts

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Not all “complex organic matter” is created equal in the anaerobic digester

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Practical implications…what does it mean if you don’t have a primary sludge feedstock…

Primary + WAS Feedstock 50/50 PS/WAS Blend DG = 9,360 CFT/MG Heat = 6.08 MMBTU/MG

Sufficient excess DG heat is generated, even in winter, to cover anticipated heating demands and provide heat for other uses.

WAS Feedstock• 100% WAS• DG = 3,390 CFT/MG• Heat = 2.20 MMBTU/MG

Heat generation, particularly under winter conditions, is marginal and excess heat availability for other uses may only be seasonal

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Digester gas quality also must be considered in a beneficial use system

ConstituentWILM

NSWILM

SSNDDG

IC #1DG

RWSADG

ROANDG

CH4 (%) 66.2 61.7 71.5 60.4 58.7 61.2

CO2 (%) 31.5 29.6 25.5 32.9 38.3 30.8

Nitrogen (%) 2.0 5.1 2.3 3.8 1.6 7.4

Oxygen (%) 0.3 1.7 0.7 0.9 0.4 0.6

Heat Value(BTU/CFT)

670 620 720 610 595 620

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Micro-constituents in digester gas can not be ignored in the decision making process

ConstituentWILM

NSWILM

SSNDDG

IC #1DG

RWSADG

ROANDG

Specific Gravity(relative to air)

0.81 0.88 0.85 0.88 0.94 0.89

H2S (ppmV) 550 250 690 53 118 ND

Total Siloxane (ppbV) 1,045 2,246 3,103 360 2,569 3,477

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Biogas to Energy Technologies

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Conventional digester gas utilization equipment as a bare minimum…

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Gas utilization in a combined heat and power cycle system increases efficiency

Raw Biosolids

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Typical Combined Heat and Power (CHP) Technologies Gas Driven

Internal Combustion Engines

Micro-turbines

Stirling Engines

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Internal Combustion Gas Driven Engine

Image Courtesy GE/Jenbacher Engines

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Microturbine

Images: Courtesy Ingersoll-Rand

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Comparative performance and gas quality requirements between options

Internal CombustionGas Engine

Microturbine

Electrical Power Conversion Efficiency 31.1% to 32.9% 26.1% to 27.0%

Hydrogen SulfideConcentration (ppmV)

< 1,000 < 25

Siloxane Content< 1,800 ppbV

< 10 ug/L (as Si)< 60 ppbV

< 0.33 ug/L (as Si)

Inlet Gas Pressure(psig)

> 1.0 > 75

Electrical Power Output Capacity 300kW to 3,000kW 30kW to 250kW

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Several Case Studies for

Digester Gas Utilization

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Cape Fear Public Utility AuthorityWilmington, North Carolina - Northside WWTP

Design Basis 16-MGD Permitted Flow Primary Clarification Nitrification Activated

Sludge

Anaerobic Digestion 3 @ 0.380 MG each 2 @ 0.695 MG each Total = 2.53 MG

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Digester gas energy production is a function of influent loading.

Unit Digester Gas Heat Available vs. Influent Flow Rate

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0Influent Flow (MGD)

DG

Hea

t Ene

rgy

(MM

BTU

/DAY

)

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Seasonal heating demands and energy capture effectiveness should be evaluated.

Seasonal Digester Gas Heat Balance - Average Loading Conditions(Full Combined Heat and Power System with NG Seasonal Supplement)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Dig

este

r Gas

Hea

t (M

MB

TU/D

AY)

CHP to Power CHP to HW Boiler to HW Loss - Boiler Loss - CHP HW Loss - CHP Exhaust

~ 33% of Total DG Energy is Not Recovered with this System

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Multiple digester gas energy recovery configurations have been evaluated…

PlantOperating

Flow(mgd)

Hot WaterBoilerOnly

Hot Water Boiler

+ ICGE

Generatorwithout CHP

Hot WaterBoiler

+ICGE

Generatorwith CHP

10-mgd(Current)

16-mgd(Future)

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Beneficial energy recovery varied depending on flow and process configuration.

PlantOperating

Flow(mgd)

Hot WaterBoilerOnly

Hot Water Boiler

+ ICGE

Generatorwithout CHP

Hot WaterBoiler

+ICGE

Generatorwith CHP

10-mgd(Current) 38% 54% 66%

16-mgd(Future) 33% 50% 63%

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Power production potential varied based on flow and process configuration.

Process Configuration

PlantAverage

Flow10-MGD

PlantAverage

Flow16-MGD

Conventional Hot WaterBoiler + ICGE without

CHP1,013 MWH/YR 1,729 MWH/YR

Conventional Hot WaterBoiler + ICGE with CHP 1,944 MWH/YR 2,921 MWH/YR

% Change with CHPProcess Configuration +91.9% +68.9%

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Both energy recovery configurations were economically attractive.

ICGE Generator

without CHP

ICGE Generatorwith CHP

Net Present Operating Benefit, $MM +$1.923 +$3.402

Capital Cost, $MM -$1.246 -$1.405

Net Value Created, $MM +$0.677 +$1.997

Benefit-to-Cost Ratio 1.543 2.421

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Greenhouse gas reductions are achieved with digester gas-to-energy systems

Process Configuration

PlantAverage

Flow10-MGD

PlantAverage

Flow16-MGD

Conventional Hot WaterBoiler + ICGE without CHP -890 tCO2e/year -1,520 tCO2e/year

Conventional Hot WaterBoiler + ICGE with CHP -1,700 tCO2e/year -2,665 tCO2e/year

% Change with CHPProcess Configuration -91.0% -75.3%

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Rivanna Water and Sewer AuthorityMoores Creek WRF Multiple Objectives

Decouple process air and digester heating

Provide beneficial use of digester gas across all seasons

Increase reliability in digester heating capacity

Reduce overall greenhouse gas production

Project Components Replace engine driven

blowers with high speed turbo blowers.

Install new CHP system

Reduction in GHG quantities from approximately 765 tCO2e/year to 175

tCO2e/year

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New CHP system includes both an engine-generator and hot water boiler.

Engine-Generator 335kW Output Electrical

Energy 1.17 MMBTUH Output

Thermal Energy Engine Jacket & Exhaust

Gas Heat Recovery

Hot Water Boiler Back-up Heat Capacity Seasonal Heating

Supplement for Winter Operating Conditions

Hot Water Boiler Output Energy – 4.0 MMBTUH

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Western Virginia Water AuthorityRoanoke WRF Historical Usage

Digester gas driven influent pumping system with a combined heat system

Gas driven influent pumps replaced with screw pumps in mid 2000’s with a plant upgrade

Project Components Install new combined heat

and power (CHP) system to produce electricity from digester gas and use excess heat for digester heat demand

Estimated capacity 1,000 kW based on current plant influent flow.

Benefit-to-Cost Ratio approximately $3.00 per $1.00 invested with an estimated

payback on capital of 7-8 years

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Gwinnett County Water ResourcesF. Wayne Hill WRC – G2E Project Engine-Generator/CHP

GE/Jenbacher J616 Dual Fuel DG/NG Capable “On-the-Fly” Fuel Blending 2.147MW Output Electrical

Energy 7.50 MMBTUH Output

Thermal Energy Engine Jacket & Exhaust

Gas Heat Recovery Gas Treatment

Moisture Removal via Glycol Chilling

Hydrogen Sulfide Removal via Iron Sponge

Siloxane Removal via Non-Regenerative Activated Carbon Media Filter

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Heat recovery from the engine generator can meet anticipated winter demands.

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The day will come where “normal” should be to see a waste gas flare….without a flame.

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Questions

MBULLARD@HAZENANDSAWYER.COM