Biomass & BiofuelsAnareobic Digestion

San Jose State University

FX RongèreMarch 2009

Biofuels

Biofuels cover a broad range of technologies and applications:

Thermochemical Conversion

Direct Combustion

Direct Combustion

Gasification

Gasification

PyrolysisLiquefaction

PyrolysisLiquefaction

Biochemical Conversion

Anaerobic Digestion

Anaerobic Digestion

Fermentation

Fermentation

Extraction

Extraction

Heat Electricity Transportation

Steam Gas Oil Charcoal Bio-dieselBiogas Ethanol

Source: From Boyle, Renewable Energy, 2nd edition, 2004

Anaerobic Digestion Breakdown of biodegradable material by micro-

organisms (bacteria) in absence of gaseous oxygen It applies to 3 different waste treatments:

Landfill Animal waste – Manure Waste water treatment

It generates a biogas composed of:Matter  %

Methane, CH4 50-75

Carbon dioxide, CO2 25-50

Nitrogen, N2 0-10

Hydrogen, H2 0-1

Hydrogen sulphide, H2S 0-3

Oxygen, O2 0-2

Anaerobic Digestion

The process includes 4 major biological or chemical stages:

Carbohydrates

Polymer base on Glucose molecule C6H12O6

α-D-glucopyranoseMono-mere

CellulosePoly-mere

Carbohydrates

Proteins:

C — CO — H

O

—

NH2

H

R — N — C — C

O

H H

R’

peptide bond

Carboxyl GroupAmino Group

Example of a protein structure:Myoglobin

Carbohydrates

Fats:Polymers of fatty acids and alcohols

O

C

H — CH — CH — CH — H

O

R

O

C

O

R

O

C

O

RFatty acids

Alcohols

Hydrolysis Converts Complex organic matters (Carbohydrates,

Proteins and fats) in Soluble organic molecules (Sugar, Amino-acids, Fatty acids)

Sugar - Glucose

R — C

O — H

O

Fatty Acids

N — C — C

O — H

O

R

H

H

Amino Acids

Fermentation - Acidogenesis

Decomposition in volatile Fatty Acids (C3 and C4), acetic acid and H2

CH3 — C

O — H

O

ethanoic acid(acetic acid / vinegar)

propionic acid

CH3 — CH2 — C

O — H

O

O — H

butanonic acid(butyric acid)

CH3 — CH2 — CH2 — C

O

Acetogenesis

Conversion of the volatile fatty acids in acetic acid and H2

Bacteria syntrophic (mutually beneficial) with the methanogens

Acetic Acid

Methanogenesis

Acetotrophic methanogens2 CH3COOH 2 CO2 + H2

Methylotrophic methanogens4 CH3OH + 6 H2 3 CH4 + 2 H2O

Hydrogenotrophic methanogensCO2 + 4 H2 CH4 + 2 H2O

CH3COOH CO + CH3OH

Matter  %Methane, CH4 50-75

Carbon dioxide, CO2 25-50

Nitrogen, N2 0-10

Hydrogen, H2 0-1Hydrogen

sulphide, H2S 0-3

Oxygen, O2 0-2Resulting Biogas

Anaerobic digestion drawbacks

Bacterias are sensitive to: Oxygen Temperature (35o optimal - Mesophilic) pH (stability and slightly acidic) Toxic component (H2S, NH3, metals)

Optimization and control of an anaerobic bio-digestor are complex

Takes time (several days to several years)

Municipal Solid Waste

MSW production in the USA

Source: EPA Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2005

EuropeJapan

Domestic waste sources in the USA

Municipal Solid Waste

Source: EPA Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2005

Landfill

MSW treatment in the USA

Source: EPA Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2005

Landfill/Recovery/Combustion in the USA

Recovery-Combustion-Landfill ratio evolution from 1960 to 2005

0%

20%

40%

60%

80%

100%

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

Landfill

Recovery

Combustion

Source: EPA Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2005

Landfill

MSW treatment in the world

Source: Kai Sipilä, VTT Processes, Finland MUNICIPAL AND COMMERCIAL SOLID WASTE FOR PYROLYSIS (OILS ) AND GASIFICATION MARKETS 2002

1995

Landfill

Structure of a modern landfill

Landfill

Biogas is used to generate electricity or is sold to utilities

Major issues: Pollutants: Siloxanes, SO2, Solanes, Aliphatics

components, explosive cycloalcnes CH4 content variation

Internal Combustion Engine

Diesel cycle

inW

P1

P2

V1 V2

Pressure

Volume

outW

combQ

inout hhm

.

Methane

CH4

Energy Content: 802 kJ/mol=50,125 kj/kg

Methane Combustion:CH4+2 O2 -> CO2 + 2 H2O

Stoichiometry: 17.4 Auto-ignition temperature: 537°C Adiabatic Flame Temperature: 1,950°C Explosive limits: 5–15%

Diesel Cycle

Adiabatic Compression

AA

AA

Closed

WQdt

dE

A A

A

Closed T

Q

dt

dS

P

dPR

T

dTCpds

dTCpdh

dTRCpdTCvdu

..

.

).(.

Air is close to an ideal gas:

),(

),(

).(.

TPss

TPuu

kuMeME

Perfect compressor

Compression ratio Compression ratio

Is determined by the methane auto-ignition temperature: 537oC

1

2

P

Pr

1

1

1

2

112 ..

rT

P

PTT

40r

Constant pressure Combustion

Max temperature is limited by the materials typically 1,200oC

AA

AA

Closed

WQdt

dE

A A

A

Closed T

Q

dt

dS

Perfect transfer

12. VVPW F VF is the volume at

the end of the combustion

A part of the power is generated during the combustion phase

4.0 5.0 6.0 7.00

500

1000

1500

2000

s [kJ/kg-K]

h [

kJ

/kg

]

1 bar

40 bar

10 bar Air

Diesel Cycle

Adiabatic Expansion (similar to the compression)

AA

AA

Closed

WQdt

dE

A A

A

Closed T

Q

dt

dS

Venting pressure is not equal to P1. It is defined by the expansion to V2

Conversion rate of a Diesel cycle is about 40%. Venting temperature of 400oC allows cogeneration

Advantages/Disadvantages

Engine Advantages Disadvantages

Internal Combustion Engines

EfficiencyCostMaturity

Sensitive to CH4 concentration and pollutants (Siloxanes and silanes)Maintenance costPollutions (NOx, SOx)

Stirling Engines

Low MaintenanceNo compression requiredAccept CH4 variationLess pollution

New technologyComplexity

Gas Turbines

Low MaintenanceAccept CH4 variationLess pollution

CostEfficiency without recuperation

Other Engines

Stirling Engines

Other Engines Gas turbines

50 30 kW Capstone microturbines convert flare gas at Lopez Canyon Landfill in Sylmar, CA for 1.5 MW

20 30 kW Capstone microturbines convert flare gas at in La Ciotat, France

Liquefied Natural Gas Generation

Prometheus generates Liquefied Natural Gas from landfill biogas.

California MSW 70 MM tons per year of

MSW generation leading to 11 lb/day/capita (more than 2 times the USA average)

43 MM tons are sent to landfills (61% of total MSW), thermal conversion is about 20% of total MSW

There are three MSW mass-burn for 67 MWe and 46 landfill gas to energy facilities (LFGTE) for 280 MW

Spittelau incineration plant in Vienna.

Potential: .15 to .45 m3 of biogas/kg Biogas = 60% methane 1,800 MM Therm/y 21 TWh/y Electricity 2.8 GW

MSW Component and properties

* Ash: proportion of non burning residues† HHV: Higher Heating Value, energy content for combustion in absence of water

Direct Combustion potential: 4,000 MMTherm/y, 30 TWh/y, 4.1 GWSource: CEC-500-2006-095

Manure bio gas

Potential in California

California is the first state of the USA for dairies

California

Dairies 1.7 Millions cows 2,153 dairies (2002)

5 leading counties Tulare Merced Stanislaus San Bernadino King

Potential is: 2.4kWh/day/cow (assuming 20% used for digester heating, conversion rate:30%) 170 MM Therms/y, 1,500 GWh/y, 200 MW

Process

Digestion duration: 15-20 days

Temperature: 36oC

Energy in Dairies

Electric Energy Use on a Representative California Dairy Farm

Water Systems8%

Air Circulation10% Lighting

13%Compressed Air

4%

Milk Cooling27%

Miscellaneous2%

Waste Handling24%

Milk Harvest12%

Wide variation: 300 – 1,500

kWh/cow/y1

Narrower interval: 700 – 900

kWh/cow/y2,5

Total California: 1,190 GWh/y3

SCE guide - 2004

1: SCE guide – 20042: Audit PG&E3: A Consumer’s Look at California’s Dairy Industry - 20024: PG&E electricity bills – 2004 (Livestock)5: Scott Sanford University of Wisconsin (Wisconsin data)

Using digesters, California dairies may be energy neutral

Examples Biogas by-products can be used by microturbines for on-site

energy use, such as at this Wisconsin wastewater treatment plant

Digesting tanks at Microgy, Inc.'s biogas plant process manure from about 10,000 cows into methane and compost. Credit: Microgy, Inc.

Types of Anaerobic Digesters

There are many different types of digesters: Covered lagoons Complete mix digesters

Complete Stirred Tank Reactors (CSTR)Completely Mixed Flow Reactors (CMF)Continuous Flow Stirred Tank (CFST)

Plug flow digesters Anaerobic Sequencing Batch Reactor Fixed film digesters

Covered Lagoons

Advantages Low cost (relative) Low tech / easy to

construct

Disadvantages Cover maintenance / life Large footprint Solids / nutrient

accumulation

Complete Mix Digesters

Advantages High level of experience Works over wide range of influent Total Solids (TS) Can be used with scrape or flush systems and

swine or dairy systems

Disadvantages Poor biomass immobilization Mechanical mixing requirement

Plug Flow Digesters

Advantages Good track record with Dairy manure Works well with scrape systems

Disadvantages Requires high solids manure (11 - 14 %) Not compatible with sand bedding

Haubenschild Farm

750 cow dairy in Minnesota 59,500 pounds of milk per day Plug-flow manure digester:

130'LX30'WX14'D 1/2 million gallons 20,000 gallons each day constant 100F degrees 72,500 cf of biogas per day Biogas is 60% methane & 35% CO2

Electric generator: 150kW diesel cycle generator Waste heat recovery for digester operation and

building heat

Haubenschild Farm

Cost Analysis

Source: C. Nelson, J. Lamb Final Report: Haubenschild Farms Anaerobic Digester The Minnesota Project Aug. 2002

Revenues

yearsk

kSP 2.4

85$

355$

Simple Payback:

Internal Rate of return:Horizon: 20 years

0

1

20

1

t

tt

IRR

RCostNPG

IRR=23%

Other animals

California farm animal population

Source: CEC-500-2006-095

Companies to follow

Capstone www.capstone.com STM-bio Prometheus-Energy

www.prometheus-energy.com RealEnergy www.realenergy.com Cummins www.cummins.com Solar www.mysolar.cat.com Microgy

www.environmentalpower.com/companies/microgy/