Anaerobic Digestion - EIHP · Anaerobic digestion peat bog permafrost ruminants • anaerobic =...

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Dr. agr. Melanie HechtNQ-Anlagentechnik GmbH

Anaerobic Digestion

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NQ-Anlagentechnik GmbH

• since 1993 a full-line supplier of biogas plants and components

• approval planning - construction - start-up & services/maintenance

• by now constructed ~ 250 biogas plants in Germany, Switzerland, Austriaand Croatia

• biological service: lab analysesconsulting (feeding/stirring, crop rotation and more)on-site visitsdimensioning

NQ-concrete digester NQ-bp 620kWel.NQ-150 agitator

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Veterinarska stanica Dvor

• start-up 04/2009

• 2x16/6 digesters (2x1205 m3)

• 135 kWel.

(MAN, gas engine, 6 cyl.)

• chicken/bull manure, cattle slurry

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Anaerobic digestion

NQ-Anlage 380kW

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Anaerobic digestion

peat bog

permafrost

ruminants

• anaerobic = without oxygen (O2)

• digestion = mechanical and chemical breakdown of organic

material into smaller compounds (macro to micro)

• renewable energy sourcereplaces fossil fuels (oil, gas)

� methane (CH4) = combustible

23 times the climate warming potential of CO2

� digestate = fertilizer

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Substrates for Anaerobic Digestion

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• organic materials:

• agricultural: manure, slurry, good availability (€)methanogenic bacteriasolvent (4-8 % DM)trace elements/nutrients

energy crops (corn, grass, sunflowers)by-products

• organic wastes: MSW, catering-/biowaste

agro/food industryoften requires separation techology

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• non-degradable: lignin (wood)• hard to degrade: cellulose, hemicellulose

• continuity: keep a calm hand

Types of digestion:• dry digestion: > 35 % dry matter (DM)• wet digestion: < 20% DM

referring to DM in main digester

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• Contaminants: heavy metalsdisinfectants antibioticspesticides, fungicides

sand, stonesbones, feathersplastic, plastic bands, glasshairwire, metal parts

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Fundamentalsof Anaerobic Digestion

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micro:CH4 (methane)

CO2 (carbon dioxide)

macro:starchproteinsfat ?

The anaerobic digestion process

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4 steps of microbial degradation

parallel in space and time

acidogenesis

hydrolysishydrolysis

acetogenesis

methanogenesis


macro:starchproteinsfat

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hydrolysis

acetogenesis

methanogenesis

acidogenesis

complex polymers

biogas (CH4 + CO2) + residue


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complex polymers

monomers

hydrolysis

carbohydrates, fat and proteins are broken down into mono/di-saccharides, fatty acids and amino acids by exo-enzymes



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complex polymers

monomers

H2 + CO2propionic / butyric acid

acetic acid

acidogenesis

monomers are taken up by bacteria and converted to H2, CO2, volatile fatty acids (VFA) and alcohols by fermentation



hydrolysis


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complex polymers

monomers

H2 + CO2

H2 + CO2

propionic / butyric acid

acetic acid



acetogenesis VFA, H2 and CO2 are partly metabolized into acetic acid

acidogenesis


hydrolysis


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H2 + CO2

H2 + CO2


acetic acid

complex polymers

monomers



acetogenesis VFA, H2 and CO2 are partly metabolized into acetic acid

acidogenesis


hydrolysis


methanogenesisacetic acid, H2 and CO2 are converted into CH4 and CO2

30%70%

very sensitive to environmental changes, e.g. T, pH, VFArate-limiting reaction in anaerobic digestion!

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Archaea

peat bogblack smoker

archaea

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Environmental Parameters

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Louis Pasteur (1822 □ 1895)founder of the germ theory of disease

„The germ itself is nothing, the terrain is everything.“

Bacteria need...

...certain environmental conditions.

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Environmental parameters

alcalinity /buffer

capacity

VFA

pH nutrients

ammoniumNH4

electricconductivity

temperature

bacterial growth /process control

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alcalinity /buffer

capacity

VFA

pH nutrients

ammoniumNH4

electricconductivity

temperature



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volatile fatty acids (VFA):

C5 valeric acid CH3 - CH2 - CH2 - CH2 - COOH

C4 butyric acid CH3 - CH2 - CH2 - COOH

C3 propionic acid CH3 - CH2 - COOH

C2 acetic acid CH3 - COOH

Complex polymers


Monomers

H2 + CO2

H2 + CO2


acetic acid

acid = H+ (protons) = pH


pH (alcalinity/acidity)

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Effect on pH by acids:


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C2 acetic acid CH3 – COO- + H+

H+

pH

14

1

7

pH

alkaline

acidic



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H+

pH

14

1

7

pH

alkaline

acidic



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H+

pH

14

1

7

pH

alkaline

acidic



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H+

pH

14

1

7

pH

alkaline

acidic



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Effect of alkalinity on pH:

hydrogen carbonate HCO3-

H+

pH

14

1

7

pH

alkaline

acidic


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H+

pH

14

1

7

pH

hydrogen carbonate HCO3-

alkaline

acidic



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CH3–COOH CH3–COO- + H+ HCO3

- H2CO3

acetic acid acetate hydrogen hydrogen water carboncarbonate bicarbonate dioxide



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• alkalinity (= hydrogen carbonate / HCO3-)

neutralizes (= buffers) acids (H+)

stabilizes pH

keeps environmental conditions in digesters stable


- H2CO3

acetic acid acetate hydrogen hydrogencarbonate bicarbonate



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• alkalinity (= hydrogen carbonate / HCO3-)

neutralizes (= buffers) acids (H+)

stabilizes pH

keeps environmental conditions in digesters stable


- H2CO3

• by buffering alkalinity is lost via CO2 emissions!!

acetic acid acetate hydrogen hydrogen water carboncarbonate bicarbonate dioxide



H2O + CO2

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source: M. Hecht, 2008

• alkalinity (= buffer capacity) stabilizes pH

• pH as sole parameter unsuitable

time (d)

0 1 2 3 4 5 6 7 8 9 10 11 12

pH

-va

lue

6,0

6,5

7,0

7,5

8,0

bu

ffe

r ca

pa

city /

VF

A (

g/l)

0,00

0,05

0,10

0,15

0,20

0,25

0,30pH-value

buffer capacity



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source: M. Hecht, 2008

• alkalinity (= buffer capacity) stabilizes pH

• pH as sole parameter unsuitable

• buffers VFA-fluctuations / rises

time (d)

0 1 2 3 4 5 6 7 8 9 10 11 12

pH

-valu

e

6,0

6,5

7,0

7,5

8,0

buff

er

capacity / V

FA

(g/l)

0

1

2

3

4

5

6pH-value

buffer capacity

VFA



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Measuring pH / VFA / alkalinity

pH-meter + pH-electrode

calibrate regularly (pH 7 + pH 4.01)

store correctly (3 M KCl), cool, dry place

pH-meterpH-electrode

titration

quantifies VFA / alkalinity

via acid consumption

pH-meter required


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Temperature

condition temperature (T) minimum retention time

psychrophilic < 20 °C 80 - 90 days

mesophilic 39 – 43 °C 30 – 50 days

thermophilic 50 – 52 °C 10 – 20 days

• keep temperature constant

• increasing T accelerates digestion

processes

• thermophilic conditions are more

sensitive to environmental changes,

e.g. T, pH, NH4-N


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Temperature

(Bavarian Biogashandbook, 2004)days

rela

tive b

iogas y

ield

[%

]


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poultry farm

inhibition of CH

4-production (%)

inhibitory effect increases with rising pH value and temperature


Temperature: ammonia inhibition

ammonium (NH4) ammonia (NH3 + H+)ion, fertilizer gas, toxin

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Equilibrium of NH3 / NH4

ammonium (NH4) ammonia (NH3 + H+)ion, fertilizer gas, toxin


Temperature: ammonia inhibition

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Electric conductivity

ion content, „salt content“

20 - 25 mS/cm2

min: 15 mS/cm2

max: 30 mS/cm2

increase of osmotic pressure

critical substrates: food wastes, bread

salt crystalssalt crystal

electricconductivitymeter

electricconductivity electrode


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Nutrients

• macronutrients: nitrogen (N), phosphorus (P), potassium (K)

• micronutrients: iron (Fe), sulphur (S), copper (Cu), manganese (Mn),

cobalt (Co), selenium (Se) and others

• build-up of microbial biomass (enzymes)

• animal manure/slurry is a good medium to supply them

dairy cows liquid manure transport


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Engineering Parameters

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Engineering parameters

Optimal bacterial growth / process control

biogasquantity &

quality

hydraulicretention

time

organicloading

rate

substrateavailability

organicdry

mattercontent

drymattercontent


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Dry matter / Organic dry matter

substrate = (1) water + (2) organic compounds + (3) mineral compounds

dry matter (DM): water eliminated (at 60 - 100 °C)

organic dry matter (oDM): DM + organic compounds eliminated (550 °C), e.g. cellulose, lignin, starch

detects contamination by stones, soil etc.

DM content after degradation: 8 - 12 %

oDM content after degradation: 5.5 – 8 %

straw grass silage maize


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Substrate availability

Biogas production strongly depends

1. on the substrates used (proteins, fat, starch)

2. environmental conditions

Difficult to degrade: cellulose, hemicellulose

(crude fiber, cf. ruminant nutrition, C/N-ratio)

Non-degradable: lignin (wood)

straw grass silage maize


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Organic loading rate (OLR)

daily addition of substrates (kg organic dry matter) per m3 active digester volume:

OLR (kg oDM/d*m3) = substrates (kg oDM) active digester volume (m3)

organic loading rate: 3-4 kg oDM m3*day-1

external feeding system


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Hydraulic retention time (HRT)

= mean, theoretical presence of input material in digesters (+ sec. digester)

HRT = active digester volume (m3) volume fresh matter (m3/d)

HRT can differ substantially due to recirculation of digested material!


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Hydraulic retention time

Biogas yield [m³/kg oDM]

Gas production rate [m³/m³ d]

Mean hydraulic retention time (HRT) [d]


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The biological limit

Hydraulic retention time [d]

Organic loading rate [kg oDM / m³ d]

Gas y

ield

[m

³ gas

/ kg o

DM

]

Gas p

roductivity [

m³ g

as

/ m

³ ferd]


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Biogas quality

CH4 methane > 50 %

CO2 carbon dioxide < 50 %

H2S hydrogen sulfide < 1 %

NH3 ammonia < 1 %

H2Ov water vapour traces

N2 nitrogen traces

O2 oxygen < 0.5 %

Biogas is a mixture of different gases:


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Biogas quality: corrosion by hydrogen sulfide (H2S)

0 % required

usually measured in biogas (ppm): smell of rotten eggs

critical substrates: abattoir wastes, blood, manure

technical problems:

corrosion: damages CHP-unit

manure heap abattoir CHP unit


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Biogas quality: desulfurization

Internal: by air (= oxygen/O2) addition to digester (biological)

requirements: surface area for sulfuric MO growth

gas storage

advantages: inexpensive

low maintenance

disadvantages: no optimized H2S-degradation

corrosion possible

activated carbon filters

sulphur bacteriaon digester inside/headspace

air pump


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Biogas quality

biogas analysis systems (online / permanent measurement)measure biogas components:CH4

(CO2)

O2

H2S


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Any Questions?

Thanks very much for your attention!

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Contaminants

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Contaminants

Optimal bacterial growth / process control

others

disinfec-tants

antibiotics

heavy metals

ammonia

VFA

hydrogensulfide

oxygen


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Ammonia (NH3)

NH3 > 3 kg/m3

high adaptative ability of bacteria

poultry farm

inhibition of CH4-production (%)

inhibitory effect increases with rising pH and temperature

Contaminants

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Heavy metals

copper (Cu) > 50 mg/l

zinc (Zn) > 150 mg/l

chromium (Cr) > 100mg/l

also lead (Pb), iron (Fe), cadmium (Cd) a.o.

only dissolved metals have

inhibitory effect pig farm

Contaminants

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Disinfectants, antibiotics (vaccinations)

e.g. chloroform, aldehydes, phenoles, alcohols, acids a.o.

e.g. Flavomycin, Salinomycin, Tetracyclin a.o.

inhibitory effect is product- and dosage-specific

most disinfectants and antibiotics are degraded within 2 -3 weeks

storage of (liquid) manure before feeding

way of administration of antibiotics plays important role

Contaminants

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Others

Temperature

when de- / increased rapidly

Spoilt substrates

substrates that are mouldy or otherwise contaminated

fresh silage

Abrupt changes in:

feeding intervals

choice of substrates

agitation intervals

Bacteria need adaptation time!

mouldy silage

Contaminants

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Oxygen (O2)

> 0.1 mg/l O2

inhibition of strict anaerobic methane bacteria

entry of O2 via feeding, overflow-systems and biological desulfurisation (air-inflow)

inflow system

feed mixermono-feeder

Contaminants

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Hydrogen sulfide (H2S)

> 50 mg/l H2S

usually measured in biogas (ppm): smell of rotten eggs

critical substrates: abattoir wastes, blood, manure

technical problems:

corrosion

damages CHP-unit

manure heap abattoir CHP unit

Contaminants

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Hydrogen sulfide (H2S)

inhibitory effect increases with sinking pH value

Contaminants

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Hydrogen sulfide (H2S): desulfurization

Internal: by air (= O2) addition to digester (biological)Fe-salts (Fe-II/III-chloride/oxide)

requirements: surface area for sulfuric MO growth

gas storage

advantages: inexpensive

low maintenance

disadvantages: no optimized H2S-degradation

corrosion possible

activated carbon filterssulphur bacteriaair pump columns

Contaminants

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Hydrogen sulfide (H2S): desulfurization

External: desulfurization-columns (chemical ZnO2/biological O

2)

activated carbon filters

requirements: explosion precautions necessary

advantages: optimized management (nutrients, air, T, H2S-content)

no corrosion

variations in gas yield do not affect gas quality

disadvantages: additional costs

additional maintenance

activated carbon filterssulphur bacteriaair pump columns

Contaminants

Anaerobic Digestion - EIHP · Anaerobic digestion peat bog permafrost ruminants • anaerobic =...

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