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Anaerobic Digestion Process Modeling

WOLFS

1

Keith Hodge, PE

Graduate Research Assistant

Department of Civil, Construction, and Environmental Engineering

Jim Levis, PhD

Research Assistant Professor


Morton Barlaz, PhD, PE

Professor and Head


E. Velvet Gaston

Undergraduate Assistant


http://go.ncsu.edu/SWM-LCA

Outline

• Introduction

• Reactor Configurations & System Mass Flows

• Biogas Beneficial Use

• Digestate Management

• Illustrative Results

2

INTRODUCTION

3

• Organic matter is biologically degraded in the absence

of oxygen to produce biogas

Anaerobic Digestion (AD)

4

biogas

Organic matter + microbes + H2O ---> CO2 + CH4 + cell mass

Energy

Nutrients

Soil Amendment?

Objectives of Anaerobic Digestion

• Energy recovery

• Landfill diversion

– “Avoided” methane emissions

• Secondary benefit – Beneficial use of digested solids

– Depending on feedstock

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Commingled MRF

Composting

Thermal WtE

Soil Amendment

Mixed Waste MRF

Remanufacturing

Ash Landfill

Landfill

Mixed Waste Recyclables Combustibles Organics Ash

Anaerobic Digestion

Mixed Waste/Residual

Collection

Comingled Recyclable Collection

Organics Collection

Solid

Waste

Systems

6

The Anaerobic Digestion Process Model

7

Anaerobic Digestion

Process Model

Residual

Contaminants

(Mgout/ Mgin)

Direct Emissions (kg/Mgin)

Equipment Fuel Use (L/Mgin)

Electricity Use (kWh/Mgin)

Capital Cost ($/Mg-yr-1)

Operating Cost ($/Mgin)

Incoming Waste

Materials (Mgin)

User Inputs

Recovered

Energy

(kWhout/ Mgin)

Transportation Use (kg-km/Mgin)

Recovered

Solids/Nutrients

(Mgout/ Mgin)

Organic Matter

• Yard waste (grass, leaves, branches)

– Paper bags, biodegradable plastic bags

• food waste

• soiled paper (paper towels, tissues)

• sewage sludge (biosolids)

• special wastes

– agricultural

– food processing industry • seafood, vegetable canning, brewery, etc.

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Organic Matter in SWOLF

9

Waste Component

Ultimate Methane

Yield (m3/dry Mg)

Yard Trimmings, Leaves 65.3

Yard Trimmings, Grass 194.8

Yard Trimmings, Branches 62.6

Food Waste – Vegetable 399.5

Food Waste – Non-Vegetable 399.5

Wood 13.7

Wood Other 13.7

Textiles 46.4

Waste Component

Ultimate Methane

Yield (m3/dry Mg)

Newsprint 74.3

Corr. Cardboard 195.1

Office Paper 263.6

Magazines 84.4

3rd Class Mail 263.6

Folding Containers 195.1

Paper Bags 195.1

Mixed Paper 164

Paper – Non-recyclable 155

• Also use % of ultimate yield produced (material-specific)

Food Waste Methane Yields

Source

Moisture Content

(%ww)

VS Content

(%TS)

Methane Yield

(m3/dry Mg)

Mohan and Bindu, 2008 78 95 274

Zhang et al., 2007 74 87 387

Cho and Park,1995 - 95 448

Heo et al., 2004 82 92 450

EBMUD, 2008 72 88 370

CIWMB, 2008 - - 343

Eleazar et al., 1997 - 94 300

Staley et al., 2006 - - 180

Zhang et al., 2012 76 91 321

Qiao et al., 2012 80 86 459

Browne and Murphy, 2013 71 95 475

Trzcinski and Stuckey, 2011 - - 327

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REACTOR CONFIGURATIONS

& SYSTEM MASS FLOWS

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AD Reactor Configurations

• Wet vs. Dry – Based on solids content in reactor. – Typically <20% = wet; >20% = dry

• Single-stage vs. Two-stage – Two-stage systems separate hydrolysis and

methanogenesis into distinct reactors.

• Mesophilic vs. Thermophilic – Reactor operating temperature impacts gas

production rate and process stability – Typically ~95-100°F = mesophilic;

~130-135°F = thermophilic

• Continuous vs. Batch Feed – Flow of feedstock into AD reactor

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AD Reactor Configurations in SWOLF

• Process model is black-box with respect to reactor

configuration. – i.e. microbial kinetics are not explicitly modeled.

• All configurations can be represented if relevant performance data is available.

• Default inputs based on: – Wet – Single-stage – Mesophilic – Continuous

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AD System Mass Flows in SWOLF

• Solid, liquid, and gas flows are present in all AD

systems.

• Mass flows are tracked through the system: – Water – Solids – Volatile Solids – Carbon – Biogas

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Key Mass Flow/Process Default Inputs

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Input Units Value Source

Reactor target moisture content

- 0.92 Levis and Barlaz 2010

Maximum percent of reactor water from recirculation

% 80 -

Incoming VS, water, carbon, nitrogen mass

Varies (waste component-specific)

Incoming methane potential Varies (waste component-specific)

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Anaerobic

Digestion

Material Flow

Base results using wet,

single-stage, mesophilic

default inputs

BIOGAS BENEFICIAL USE

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Biogas Beneficial Use

• Primary objective of AD is energy recovery

• Typical biogas methane content ~60%

• Combustion for electricity production

– internal combustion engine, turbine

– offset conventional electricity production

• Upgrade biogas for other uses:

– Natural gas quality for pipeline injection.

– Upgrade and compress for vehicle fuel (CNG).

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Biogas Beneficial Use in SWOLF

• Biogas production estimated using material-specific:

– Methane potential

– Percent of methane potential reached in modeled AD

system

• Combustion for electricity production

– Generation estimated using heating value of methane

and heat rate of engine/turbine system.

– System downtime, biogas leakage considered.

– Offset electricity generation for chosen grid.

• Other biogas end uses not yet modeled in SWOLF. 19

Key Biogas/Energy Default Inputs

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Biogas Engine Heat Rate MJ/kWh 8.5 Sanscartier et. al. 2012

AD Facility Electricity Use kWh/Mg 58 Sanscartier et. al. 2012

Methane Leakage % 3 Sanscartier et. al. 2012

System Downtime % 3 -

DIGESTATE MANAGEMENT

21

Digestate Beneficial Use

• Nutrient value

– Can be land applied in place of N, P, K mineral

fertilizers

• Soil conditioner/amendment

– Can improve soil quality (difficult to quantify) or

– Meet specialty soil needs (potting soil, etc.)

• Carbon storage

– Biogenic carbon can be bound in soil for years-

climate change benefit.

• Refer to Composting presentation for additional details

(digested solids are handled similar to compost).

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Typical Digestate Management Options

Direct land application, no separation

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Solids to: • Direct land application • Aerobic curing • Drying

Liquids to: • Wastewater treatment

OR

Separation

Digestate Management Options in SWOLF

Direct land application, no separation

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Solids to: • Direct land application • Aerobic curing • Drying (under development)

Liquids to: • Wastewater treatment

OR

Separation

Carbon Storage

AND

Fertilizer Production

OR

Peat Production

Emissions

counted for all

processing steps

Digested Solids Beneficial Use Inputs

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Land application diesel use (digestate) gal/ton 0.16 Berglund and Börjesson (2006)

Land application diesel use (compost) gal/ton 0.19 Berglund and Börjesson (2006)

Carbon remaining after 100 years % 10 Bruun et al. (2006)

Carbon stored from humus production in soil

kg C/kg C in compost

0.19 U.S. EPA (2006)

Percent applied N that evaporates as N2O

% 1.5 Hansen et al. (2006)

Nitrogen mineral fertilizer equivalent kg N in compost/kg N in fertilizer

0.4 Boldrin et al. (2009)

Digested Solids Beneficial Use

Offset Emission Factors

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Emission N (kg/kg N) P (kg/kg P) K (kg/kg P)

Peat (kg/Mg)

Carbon Dioxide – Fossil 5.18 0.411 0.342 18

Methane 0.04 0.002 0.02 0.27

Nitrous Oxide 2.19E-02 5.44E-05 8.16E-04 7.29E-04

GWP (kg CO2e) 12.7 0.48 1.09 25

*Emission Factors adopted from Ecoinvent v3

ILLUSTRATIVE RESULTS

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Illustrative Results –

Comparison of Digestate Management

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-400

-300

-200

-100

0

100

200

Separation and AerobicCuring

Direct Land Application

Glo

bal

War

min

g P

ote

nti

al

(kg

CO

2-e

q/M

g)

Electricity Offset

Digestate Off-Gases

Plant Operations

Soil Carbon Storage

Fertilizer Offset

Net


Influence of Electricity Offsets

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-600

-500

-400

-300

-200

-100

0

100

200

National Average High-CarbonElectrcity Offset

Low-CarbonElectricity Offset

Glo

bal

War

min

g P

ote

nti

al

(kg

CO

2-e

q/M

g)

Electricity Offset

Digestate Off-Gases

Plant Operations

Soil Carbon Storage

Fertilizer Offset

Net


Modeling of Various AD Configurations

30

0

20

40

60

80

100

120

140

Wet, Meso., Single,Semi-Cont.

Wet-Dry, Thermo.,2-Stage, Semi-Cont.

Dry, Thermo.,Single, Cont.

Dry, Thermo.,Single, Batch

Met

han

e Y

ield

(m

3/M

g fe

ed

sto

ck)

Modeled

Empirical

References

• Berglund, M. & Börjesson, P. (2006) Assessment of energy performance in the life-cycle of biogas production.

Biomass and Bioenergy, 30,254– 266.

• Boldrin, A.; Andersen, J.K.; Moller, J.; Christensen, T.H.; Favoino, E. (2009) Composting and compost utilization:

accounting of greenhouse gases and global warming contributions Waste Manage. Res., 27 (8), 800-812.

• Bruun, S., Hansen, T.L., Christensen, T.H., Magid, J. & Jensen, L.S. (2006) Application of processed organic

municipal solid waste on agricultural land: a scenario analysis. Environmental Modeling and Assessment, 11, 251-

265.

• Hansen, T.L., Bhander, G.S., Christensen, T.H., Bruun, S. & Jensen, L.S. (2006) Life cycle modelling of

environmental impacts of application of processed organic municipal solid waste on agricultural land

(EASEWASTE). Waste Management & Research, 24, 153-166.

• Levis, J.W. et al., 2010. Assessment of the state of food waste treatment in the United States and Canada. Waste

Management, 30(8-9), pp.1486–94.

• Levis, J.W. & Barlaz, M. a, 2011. What is the most environmentally beneficial way to treat commercial food waste?

Environmental Science & Technology, 45(17), pp.7438–44.

• Levis, J. W., Barlaz, M. A., (2013). Anaerobic Digestion Process Model Documentation. Raleigh,

NC. http://www4.ncsu.edu/~jwlevis/AD.pdf.

• Sanscartier, D., Maclean, H.L. & Saville, B., 2012. Electricity production from anaerobic digestion of household

organic waste in Ontario: techno-economic and GHG emission analyses. Environmental Science & Technology,

46(2), pp.1233–42.

• U.S. EPA (2006) Solid Waste Management and Greenhouse Gases: A Life-Cycle Assessment of Emissions and

Sinks http://www.epa.gov/climatechange/wycd/waste/downloads/fullreport.pdf Date accessed: 06-02-2010.

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http://www4.ncsu.edu/~jwlevis/AD.pdf

http://www4.ncsu.edu/~jwlevis/AD.pdf

References (cont’d)

• Anaerobic Digestion of Food Waste, (2008) East Bay Municipal Utility District, Oakland, CA,

http://www.epa.gov/region9/organics/ad/EBMUDFinalReport.pdf

• Browne J.D., Murphy J.D., 2013. Assessment of the resource associated with biomethane from food waste,

Applied Energy 104 (2013) 170–177.

• Cho, J. K., Park, S. C., & Chang, H. N. (1995). Biochemical methane potential and solid state anaerobic digestion

of Korean food wastes. Bioresource Technology, 52(3), 245–253. doi:10.1016/0960-8524(95)00031-9

• CIWMB (California Integrated Waste Management Board) (2008). Current Anaerobic Digestion Technologies Used

for Treatment of Municipal Organic Solid Waste. Sacramento, CA. http://www.calrecycle.ca.gov/Publications/

Documents/1275/2008011.pdf.

• Eleazer, W. E.; Odle, W. S.; Wang, Y.; Barlaz, M. A. Biodegradability of Municipal Solid Waste Components in

Laboratory‐Scale Landfills. (1997), 31, 911–917.

• Heo, N.H., Park, S.C., Kang, H., (2004). Effects of mixture ratio and hydraulic retention time on single‐stage

anaerobic co‐digestion of food waste and activated sludge. J. Environ. Sci. Health A39 (7), 1739-1756.

• Mohan S, Bindu BK. (2008). Effect of phase separation on anaerobic digestion of kitchen waste. NRC Research

Press, jees.nrc.ca; Feb. 19, 2008.

• Qiao W., Yan X., Ye J., Sun Y., Wang W., Zhang Z., (2011). Evaluation of biogas production from different biomass

wastes with/without hydrothermal pretreatment, Renewable Energy 36 (2011) 3313‐3318.

• Staley, B. F.; Xu, F.; Cowie, S. J.; Barlaz, M. A; Hater, G. R. (2006) Release of trace organic compounds during the

decomposition of municipal solid waste components. Environ Sci Technol, 40, 5984–91.

• Trzcinski A.P. and Stuckey D.C., (2012). Determination of the Hydrolysis Constant in the Biochemical Methane

Potential Test of Municipal Solid Waste, ENVIRONMENTAL ENGINEERING SCIENCE, Volume 29, Number 9.

• Zhang Y., Banks C.J., Heaven S., (2012). Anaerobic digestion of two biodegradable municipal waste streams,

Journal of Environmental Management 104 (2012) 166-174.

• Zhang, R.; El‐Mashad, H. M.; Hartman, K.; Wang, F.; Liu, G.; Choate, C.; Gamble, P. (2007) Characterization of

food waste as feedstock for anaerobic digestion. Bioresource technology, 98, 929-35.

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http://www.epa.gov/region9/organics/ad/EBMUDFinalReport.pdf

http://www.calrecycle.ca.gov/Publications/Documents/1275/2008011.pdf

http://www.calrecycle.ca.gov/Publications/Documents/1275/2008011.pdf