Biogas production

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Biogas production
Gas out
Biomas in
Biomas out (Digestate)
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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?
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Biogas knowledge piramide
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Biogas plant concept
Pig slurry
Fe
Biomas out (Digestate)

• Additional income for the farmers
• Clean energy (kitchen)
• Digestate is an excellent fertilizer
• Less odour
• Sanitaion

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The microbiology process
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Methane produktion

• 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

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Physical process
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Biological and chemical process
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Biogas knowledge pyramideInhibition
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H2inhibition
Acetic acid
VFA component
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Ammonia inhibition

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

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Ammonia chemistry

• 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

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Ammonia-ammonium equilibrium
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Ammonia inhibition

• In literature ammonia inhibition has been
  assessed relating biogas production to
• Reactive ammonium (NH3)
• Total Nitrogen
• Ammonium
• How is NH3 related to NH4
• How would you recommend that the inhibition is
  expressed (reactive ammonium, total nitrogen or
  ammonium

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Inhibition at high and low pH
SH2
NH3
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VFA inhibition

• Inhibition at a ratio of propionic acid to acetic
  acid at 1.41
• Inhibition at 2 g VFA Ltr-1

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TemperatureBacteria adaptation
Batstone et al. 2002
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What happens if the temperature suddenly drops?
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Metane production as affected by NH4
koncentration interacting with temperature
20 days retention time in CSTR digester
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Biogas knowledge piramideDigestibilty
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Definitioner

• VS (Volatile solids)
• The fraction of dry matter (DM) in slurry that is
  transformed to gas at high temperature/incineratio
  n (550oC) for one hour
• How would you measure VS?
• Methane productivity
• CH4 production pr. unit VS
• CH4 production pr. unit COD

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Source of energy in animal slurry
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Energy production
Biogas, CH4 CO2
CH4 - source
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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

Biomas
Inoculum
BMP is estimated in batch fermentation at
35oC Fermentation time 70-100 days
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Anaerobic Digestibility

• The theoretical biogas production can be
  calculated from knowing the chemical composition
  of the biomassTBMP
• In the biological process the maximum biogas
  production BMP
• Anaerobic digestibility BMP/TBMP
• Question
• - BMP/TBMP ? digestibility??
• - BMP/TBMP ? digestibility ??

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Biodegradability (BMP/TBMP) examples

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Lignocellulose

• Low digestibility
• lignin Non degradable in anaerobic environments
  
• hydrolysis of cellulose blocked by lignin.
• Lignin
• glue to hold lignocellulosic matrix
• protective coat
• used to assess digestibility of feed in animal
  science

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Lignocellulose in VS (volatile solid)
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Fermentation result - animal manure
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Digestability of the biomass
CH4 L kg(VS)-1
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Energy potential of biomass
Dry 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
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Methane produkcion crops and organic waste
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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

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Hashimoto equation
HRT or ? Hydraulic retention time
SRT Solid retention time
G 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)
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Hashimoto model predictions
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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