02 Biogas Chemistry 11 08

BIOGAS ‐ PRINCIPLES


SHORT INTRODUCTION

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Overviewdegradation pathwaysspecific gas yieldeffect of temperaturebuffer systemkineticsdissociationinhibitiondigester control

BIOGAS

Three substances will always produce

less suitable:

unsuitable:

degradation pathways

carbohydrates

proteins

fats

lignin

cellulose

polymers carbon hydrates proteinfat anorganic substance

monomercarbohydrates amino acids long chain

fatty acids

Hydrolysis

CO2 + H2 acetate ethanolbutyrate valeriatepropionate

Acidogenesis

Acetogenesis

Methanogenesisseconds to minutes

minutes to hours

minutes to days

hours to days

Degradation pathways with time axis

Pind et al: Monitoring and Control aof Anaerobic Reactors 2003

CH4 + CO2 + H2O

CO2 + H2 acetate


starch

amylose~ 27 %

amylopectin~ 73 %

maltose

glucose

acetic acid

methane carbon dioxide

biogas

decomposition by

amylase

decomposition by

maltase

degradation of starch

( )C H On

H On

C H OnAmylase

6 10 5 2 12 22 112 2+ ⎯ →⎯ ⎯

decomposition by amylase

C H O H O C H OGlu idase12 22 11 2 6 12 62+ ⎯ →⎯ ⎯ ⎯⎯−α cos

decomposition by maltase (α‐glucosidase)


exoenzyme

H2

maltoseglucose

bacteria

glucose

exoenzyme

glucose

acetic acid

degradation of carbohydrates

C6H12O6 + 4 H2O 2 CH3COO¯ + 2 H2CO3 + 2 H+ + 4 H2

Bacteria which are involvedin the anaerobic degradation

of carbohydrates:

Saccharolytical Clostridia

‐ Clostridium butyricum‐ Clostridium acetobutylicum‐ Clostridium cellulosae‐dissolvens

pH optimum5.3 – 6.7


lipids

fatty acids

acetic acid

methanecarbon dioxide

biogas

hydrolysis by

lipase

β‐oxidation

hydrogen

glycerin

fermentation

degradation of lipids – stoichiometryLipase producing bacteria:Pseudomonas cepacia

Pseudomonas fluorescensPseudomonas species 242610455 CO16CH39OH26OHC +→+


proteins

oligopeptide

dipeptide, amino acids

acetic acid

methane carbon dioxide

biogas

hydrolysis by

proteases

decomposition by

peptidase

deamination

ammonia

C13H25O7N3S1 + 9 H2O → 6.5 CH4 + 3.5 CO2 + H2S + 3 NH4+ + 3 HCO3

‐

degradation of proteins – stoichiometry

∆ G0 = - 77,8 kJ / reaction

part of the reaction: deamination

Bacteria which are involved in the anaerobic fermentation of amino acids:

amino acids bacteriaAlanin Clostridium propioniciumArganin Clostridium spp.

Streptococcus spp.Glutamat Clostridium tetanomorphiumGlycin Peptostreptococcus microsLysin Clostridium sticklandii

Gerardi 2003

pH optimum7.0


methanogenesis

CH3COOHCH3COO־ H+

CH4 CO2

CH3COOH → CH4 + CO2

reception of acetic acid by methane bacteria only in undissociated form

CO2 + 4 H2 → CH4 + 2 H2O

CO2

H2

CH4 H2O

pH optimum6.8 – 7.5

decarboxylation of acetic acid∆ G0 = ‐ 31 kJ

reduction of carbon dioxide∆ G0 = ‐ 135.6 kJCH3COOH → CH3COOˉ + H

acetic acidundissociated

acetic aciddissociated= acetate

substance gas yield

m³/kg

methane content

% by volume

carbohydrates 0.830 50

proteins 0.610 65

lipids 1.430 71

specific gas yield

sewage sludge gas yield m³/kg TOS

primary sludge 0.500

secondary sludge 0.400

TOS = total organic solids

substrates

Component

Volatile solids

Lipids

Cellulose

Hemicellulose

Crude Protein

Ash

Primary sludge

75.0

10.3

32.2

2.5

19.0

25.0

Secondary sludge

70.0

5.8

9.7

‐

53.7

21.0

Municipal Refuse

82.1

6.4

35.0

16.5

5.8

17.9

Meat packing waste

92.0

54.6

‐

‐

28.7

8.0

Cattle manure

72.0

3.5

17.0

19.0

19.0

28.0

Chicken manure

76.0

1.5

28.3

11.9

28.8

24.0

Composition of Various Substrates (% of Dry Matter)

Pavlostathis/Giraldo‐Gomez 1991

COD balance

DIGESTERCOD influent COD effluent

COD biogas

calculation COD of methane

CH4 + 2 O2 → CO2 + 2 H2O

1 mol CH4 = 2 mol O216.04 g CH4 = 2 * 31.99 g O222.4 l CH4 = 2 * 31.99 g O2COD CH4 = 16.04/2*31.99

= 0.25 g CH4= 0.350 Nl CH4

10 % of the COD bounded as bacteria0.350 Nl – 0.0525 Nl = 0.315 Nl CH4

m³ CH4 / kg CODreduced

0.315 m³/kg CSB

ATV‐FA 7.5: Technologische Beurteilung zur anaeroben Abwasserbehandlung, KA 40(1993)S.217‐223

effect of temperature

Optimal growth temperature and optimal pH of some methane‐producing bacteria

Genus Temperature range°C pH

Methanobacterium 37 – 45

Methanobrivibacter 37 – 40

Methanosphaera 35 – 40 6.8

Methanothermus 83 – 88 6.5

Methanococcus 35 – 40

65 – 91

Methanocorpusculum 30 – 40

Methanoculleus 35 – 40

Methanogenium 20 – 40 7.0

Methanoplanus 30 – 40

Methanospirillum 35 – 40 7.0 – 7.5

Methanococcoides 30 – 35 7.0 – 7.5

Methanohalobium 50 – 55 6.5 – 7.5

Methanolobus 35 – 40 6.5 – 6.8

Methanosarcina 30 – 40

50 – 55

Methanothrix 35 – 50 7.1 – 7.8

Michael H. Gerardi: The Microbiology of Anaerobic Digesters, 2003

effect of temperature

Comparison of Mesophilic and Thermophilic Digesters

Feature Mesophilic Digester

Thermophilic Digester

Loading rates Lower Higher

Destruction of pathogens Lower Higher

Sensitivity of toxicants Lower Higher

Operational costs Lower Higher

Temperature control Less difficult More difficult


Temperature Range for Methane Production for Municipial Anaerobic Digester

Temperature [°C] Methane Production

35 Optimum

32 ‐ 34 Minimum

21 – 31 Little, digester going „sour“

< 21 Nil, digester is „sour“


CO2Gas

↑↓

buffer system

CO2 + H2O H2CO3 H+ + HCO3ˉ

carbonic acid buffer

NH3 + H2O NH4+ + OHˉ

NH3 + CO2 + H2O NH4+ + HCO3ˉ

ammonia buffer

CH4 H2S

C13H25O7N3S1 + 9 H2O → 6.5 CH4 + 3.5 CO2 + H2S+ 3 NH4

+ + 3 HCO3 ˉ

protein degradation

The concentration of ammonia is limited by the carbonate concentration

pH > 6.7[NH4

+] = [HCO3‐] + [CH3COO‐]

[CH3COO‐] = ~ 0

[NH4+] = [HCO3

‐]

carbonic acid buffer HCO3‐/CO2

influencepH 6.0 – 6.6

ammonia buffer NH3/NH4+

influencepH ≥ 7.7

kinetics

concentration

limitation inhibition

rate of growthrelative

μ

definition

limitiationthe optimum concentrations are not reached

inhibitionexceeding of the optimum concentrations

nutrient exhaustionlimitation by exhaustion of nutrients

substrate surplus blockingsubstrates are present in too high concentrations and damagethe cell or ligate an enzym at the wrong position

The blocking of the anaerobic generation of methane is caused by exceeding of the optimum concentrations

undissociated acetic acid depends on pH-value

02

46

810

1214

1618

2022

2426

2830

6,5 6,6 6,7 6,8 6,9 7,0 7,1 7,2 7,3 7,4 7,5 7,6 7,7 7,8 7,9 8,0

pH-value

mg

CH

3CO

OH

/ l

500 1.000 1.500 2.500 5.000 mg CH3COOH / l

dissociation acetic acid

CH3COOH → CH3COO‐ + H+

pK =4,76

dissociation acetic acid

0

10

20

30

40

50

60

70

80

90

100

pH

Mol.%

acetate acetic acid

1 2 3 4 5 6 7 8 9 10 11 12 13 14

pK =4.76

I SK

I

I

=+

1

1

I = inhibition function [‐]SI = concentration of the inhibition component [mg/l]KI = inhibition constant (50%‐inhibition) [mg/l]

inhibition = 100 – (I *100) [%]

inhibition

pH NH4 NH3

kg/m³ kg/m³

7.0 1.020 0.005

7.1 1.284 0.008

7.2 1.616 0.013

7.3 2.035 0.020

7.4 2.562 0.032

7.5 3.225 0.051

7.6 4.060 0.081

7.7 5.112 0.129

7.8 6.435 0.205

7.9 8.101 0.324

8.0 10.199 0.514

Inhibition of the anaerobic digestion by ammonia NH3

0

10

20

30

40

50

60

0 50 100 150 200 250

ammonia [mg/l]

inhibition [%]

ammonia

KI = 210 mg/l

[ ][ ]

[ ][ ]pH pK

HCOCO

pKNHNHs C s N= + = +

−

+, ,log log3

2

3

4

[ ][ ]pH pK

cc

A

HA

= + log

With the Henderson‐Hasselbalchen equatation a relation between the dissociation constant, the concentrations and the pH‐value could be built:

Because the ammonium content in the digested sludge is proportional to the hydrogen‐carbonate content [NH4

+] = [HCO3ˉ ], the ammonium content could be defined from the CO2‐content of the gas

Henderson‐Hasselbalch‐equation

pAT

B C TKs N, *= + −

temperature dependency of pKs,N

[ ]CO K p K p p cH CO CO H CO H O CO2 2 2 2 2 2= = −, ,* *( ) *

pHNHCO

NHNH

= +⎡

⎣⎢

⎤

⎦⎥ = +

⎡

⎣⎢

⎤

⎦⎥

+

+6 31 8 954

2

3

4, log , log

[ ] [ ]log , logNH pH CO4 26 31+ = − +

[ ] [ ]log , logNH pH NH3 48 95= − + +

The degree of CO2‐saturation can be calculated with the Henry‐coefficient and the CO2‐partial pressure as:

If the ph‐value is known, the NH4+ and the NH3‐content

could be defined by changing the term:

Henderson‐Hasselbalch‐equation

substrate additionweighing mass

gas production

gas quality

control spezific gas production no need for actionOK

no need for actionOK

moresubstrate addition

OK

calculation undissociated acidcalculation undissociated ammonia

OK

stop substrate addition

temperature no need for actionOK

no heat

to heat

< reference

> reference

measure : acetic acid‐concentration

measure : CO2‐concentration

measure :propionic acid‐concentration

measure : pH‐value

digester control

< reference

< reference

< reference

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02 Biogas Chemistry 11 08