Biogas production

Biomas in Biomas out (Digestate)Gas out

Where are we?Why do we want?

How do we manage?What do we need?

From where should we start?What do we want to know?

Biogas knowledge piramide

Biogas plant concept

Pig slurryFeBiomas out (Digestate)

Additional income for the farmersClean energy (kitchen)Digestate is an excellent fertilizerLess odourSanitaion

The microbiology process

Biomasse:Animal manureOrganic waste

Hydrolysis

Dissolved substrate

Acidogenesis

H2+CO2

Acetognesis

VFA>C2

CH4+H2O+ CO2

CH3-COOH

Methanogenesis

Methane produktion

Biomasse:Animal manureOrganic waste

Hydrolysis

Dissolved substrate

Acidogenesis

H2+CO2

Acetognesis

VFA>C2

CH4+H2O+ CO2

CH3-COOH

Methanogenesis

Hydrolysis is process rate controlling

VFA transformation reduced due to:

•High NH3

•Sudden changes in environment

•High H2 concentration

Feedback:

High VFA conc. reduces hydrolysis

Physical process

COMPOSITE MATERIAL

MACROMOLECULES

Physical process: Disintegration

• Lysis• Non enzymatic decay• Phase separation• Physical breakdown (shearing)

Biological and chemical process

MACROMOLECULES

SIMPLE SUBSTRATES

Hydrolysis (chemical)A→B1+B2 (H2O is used)

Hydrolytic enzymes (biological/chemical)Made by micro-organisms – same outcome

SIMPLE SUBSTRATES

VOLATILE FATTY ACIDS

ACETATE & HYDROGEN

Acidogenesis: (biological)Volatile fatty acids are generated from monosaccarides, fat and aminoacids.(sugar-degraders & aminoacid-degraders)

Acetogenesis: (biological)Acetate is generated from LCFAs. (lcfa-degraders) and from sugar (sugar-degraders)

ACETATE & HYDROGEN

BIOGAS

Methanogenesis (biogas production)

Acetoclastic methanogenesisCH3COOH → CH4 + CO2

Hydrogenotrophic methanogenesisCO2 + 4H2 → CH4 + 2H2O

Biogas knowledge pyramideInhibition

H2inhibitionReaction

)()(3)(3)( 3233223 aqHCOaqHOHaqCOOCHOHaqCOOCHCH

)()()( 423 gCHaqCOaqCOOHCH )()(3)()(3)( 3224223 aqHCOaqHaqCOaqCHOHaqCOOCHCH

O2H(aq)CH(aq)4H(aq)CO 2422 -130.4

1

VFA component Acetic acid

Ammonia inhibition

• Ammonia inhibition: 1,5 – 2,5 g N/L, after adaptation inhibition at 4 g N/L (Angelidaki og Ahring 1998)

Ammonia chemistry OHNHOHNH NK

3324

)(

)()(

4

33

NH

OHNHKN

)(

)(

3

43 OH

NHKNH N

pH= -log(H+)•Thus if the concentration of [H+] is •Neutral: 10-7 mol then pH = –log(10-7)=7•Acid: 10-2 mol then pH = –log(10-2)=2•Basic: 10-10 mol then pH = –log(10-10)=10

Ammonia-ammonium equilibrium

pH

7 8 9 10 11 12

NH

4+, N

H3,

TA

N (

mol

l-1)

0.00

0.02

0.04

0.06

0.08

0.10 TAN

NH4+ NH3

Ammonia inhibition

• In literature ammonia inhibition has been assessed relating biogas production to

• Reactive ammonium (NH3)

• Total Nitrogen• AmmoniumHow is NH3 related to NH4

+

How would you recommend that the inhibition is expressed (reactive ammonium, total nitrogen or ammonium

Inhibition at high and low pH

SH2 NH3

VFA inhibition

• Inhibition at a ratio of propionic acid to acetic acid at 1.4:1

• Inhibition at 2 g VFA Ltr-1

TemperatureBacteria adaptation

Batstone et al. 2002

SIMPLE SUBSTRATES

VOLATILE FATTY ACIDS

ACETATE & HYDROGEN

Acidogenesis: (biological)Volatile fatty acids are generated from monosaccarides and aminoacids.(sugar-degraders & aminoacid-degraders)

Acetogenesis: (biological)Acetate is generated from LCFAs. (lcfa-degraders) and from sugar (sugar-degraders)

What happens if the temperature suddenly drops?

Metane production as affected by NH4+

koncentration interacting with temperature

20 days retention time in CSTR digester

0

100

200

300

400

0 10 20 30 40 50 60 70

Temperature oC

Lite

r C

H4/

kg V

S

NH4-N<3 g/l

NH4-N>3 g/l

Biogas knowledge piramideDigestibilty

Definitioner

• VS (Volatile solids):– The fraction of dry matter (DM) in slurry that is

transformed to gas at high temperature/incineration (550oC) for one hour

– How would you measure VS?

• Methane productivity:– CH4 production pr. unit VS

– CH4 production pr. unit COD

Source of energy in animal slurry

Fermented slurry

Digestible

Non-digestible

Transformed to CH4

Energy production

Concentration, %

0

20

40

60

80

100

Concentration, %

0

1

2

3

4

5

6

Water

Volatile solids

Ash

Volatile solids

Ash

Untreated slurry

Digestible

Non-digestible

Biogas plant

CH4 - source

Biogas, CH4 +CO2

Characterisation of biogas potential• In the biological process the maximum biogas production BMP• (liter CH4 kg(VS)-1)

– Volume of methane produced when residence time is in principle very long

Inoculum

Biomas

BMP is estimated in batch fermentation at 35oCFermentation time 70-100 daysDays

0 10 20 30 40 50 60 70CH

4 pr

oduc

tion

, lit

er C

H4

kg(V

S)-1

0

100

200

300

400

Pig slurry

Cattle slurry

Anaerobic Digestibility The theoretical biogas production can be calculated from knowing the chemical

composition of the biomass:TBMP In the biological process the maximum biogas production: BMP Anaerobic digestibility =BMP/TBMP• • Question • - BMP/TBMP ↑ digestibility??• - BMP/TBMP ↓ digestibility ??

•

Biodegradability (BMP/TBMP) examples

Low digestibility Low digestibility

lignin : Non degradable in anaerobic environments

hydrolysis of cellulose blocked by lignin.

Lignin Lignin glue to hold lignocellulosic

matrix protective coat used to assess digestibility of

feed in animal science

LignocelluloseLignocellulose

Lignocellulose in VS (volatile solid) Lignocellulose in VS (volatile solid)

Fermentation result - animal manure Fermentation result - animal manure

Digestability of the biomass

CH4 L kg(VS)-1

Energy potential of biomassDry matter Volatile

solids in pct of dry matter (DM)

Total energy content

Energy production in biogas plant

% % MJ/ kg DM MJ/kg DM

Pig slurry 6 80 16,3 9,8

Cattle slurry

10 80 15,3 7,6

Clover grass

20 90 18,3 14,6

Straw 90 90 19,1 9,6

Why is biogas energy production of straw so low

Methane produkcion crops and organic waste

0100200300400

Gra

ssM

aize

Whea

t stra

w

Slaug

hter

was

tes

Flotta

tions

slud

ge

Bleac

hing

clay

Whey

Mun

icipa

l sol

id was

teNm

3 C

H4/

ton

ne

s

Biogas production estimates

With the Hashimoto equation one can assess production of biogas as affected by:

•temperature, •hydraulic retention time, •micro-organism activity•biomass composition

Hashimoto equation

HRT or θ Hydraulic retention timeSRT Solid retention timeΓ Is the specific gas yield

B0 The ultimate or specific methane yield, measured with batch fermentation at more than 60 days and at 35oC.

µ maximal specific growth rate of the micro organisms, µm a

function of temperature and residues feed to the reactor K is a kinetic parameter depending of the rate of feed, feed

composition and bacterial consortium,

S0 Concentration of organic components in feed to the reactor

Nm3 The volume CH4 produced, calculated at 0oC (273oK)

Hashimoto model predictions’

Summarising

• Biogas is efficient in producing energy from biomasses with a high water content

• Biogas transform the biomas reducing VS and thus reduced GHG emission potential of the slurry

• Biogas transform biomas organic N into ammonium that is an efficient fertilizer