Download Diapositiva 1

Università Degli Studi di Milano
http://www.diprove.unimi.it/
reference:
Fabrizio Adani
[email protected]
Giuliana D’Imporzano
[email protected]
Biogas production by
anaerobic digestion:
process basis and
monitoring
• Anaerobic digestion biochemical basis
• Process phases
• Process parameter
• Full scale application
• Problems identification
Biochemical
By a biochemical point of view anaerobic
digestion is the breakdown of organic
matter by bacteria in the absence of
oxygen, where the final electron
acceptor is not oxygen but carbon (carbon
dioxide and acetic acid ) and the product is
methane.
Industrial process
By an industrial point of view anaerobic
digestion is a monitored process run in
anaerobic digestors
in lack of oxygen
to transform an organic substrate into
biogas (methane and carbon dioxide) and a
stabilized digestate (residual material).
Materials
Almost any organic material can undergo
anaerobic digestion





Animal waste
Vegetal by product
Sewage
Organic Fraction of Municipal Solid Waste
Energy crops
Biogas yield
material
Biogas yield
(m3/ton organic
matter)
Cow slurry
400
Pig slurry
450
silage
500-600
Oil seed by
product
600
distillation byproduct
450-500
whey
800
OFMSW
500
Glycerin
600
Source:
Baserga 2000,
Adani data not pubblished
AD promote a
Closed Cycle of Carbon
AD promote a
Closed Cycle of Carbon
Vegetal and
animal by
products
OFMSW
Renewable energy
CO2 saving
Fertilizer
atmosphere
soil
Phases and biochemical steps
Carbohydrates
Sugars
Carbonic Acids
and Alcohols
Fats
Hydrogen
Acetic Acid
CO2
Fatsacids
Fatty
Hydrogen
CO2
Ammonia
Proteins
Methane
CO2
Proteins
Aminoacids
Hydrolysis
Acidogenesis
Source: T Al Seadi (2001)
Acetogenesis
Methanogenesis
Phases and biochemical steps
Selonomonas
Clostridium
Desulfovibrio
Syntrophomonas
Eubacterium
Clostridium
Metanotrix
Metanosarcina
Metanobacterium
Metanococcus
Source: Lechner et al, 2000
What actually happens when organic
matter undergo anaerobic digestion:
Biochemical reactions coexist but at macroscopic
level we detect:
 Prevalence of acidification reactions (hydrolisys,
acidogenesis, acetogenesis ) in the first time
 Increasing and stabilization of methanogenic
reactions in second time
Acidification phase
During acidification phase:
 increase of organic acids and volatile fatty
acids (VFA) and pH fall down
 pH fall support acid-generating bacteria, but
inhibit methane-generating bacteria
 Biogas produced is rich in CO2 but not in
methane
Methane phase
During methane phase:




VFA concentration falls down
pH value increases (7-8)
pH increase support methane-generating bacteria
growth
biogas produced is rich in methane
VFA concentration and pH trend during
anaerobic digestion
Acidification phase: low pH and high concentration of
VFA
Methane phase: pH value raises and steady around 7-8
VFA concentration drops.
Biogas production and methane
concentration during AD phases
250
100
90
200
80
70
60
50
40
30
20
10
0
150
100
50
0
time
biogas quantity
methane percentage
Methane phase: biogas production increases and methane percentage raises
(over 60%)
methane percentage (%)
biogas (ml*gTS-1)
Acidification phase: biogas production is low and methane
concentration below 50%.
Different reactions coexist during AD
Methane phase is the most sensitive,
slow and limiting step
In order to optimize an industrial AD
process we have to maintain conditions
favouring methane-forming bacteria
(methane steady state) in the digester
To favour the growth of methane-forming
bacteria we balance the rate between
digestate material (e.g. material that
previously underwent AD) and fresh
material still to be degraded.
Digestate material works like seeding for
methane-forming bacteria and helps to
buffer the acids deriving from the first
step of organic matter degradation.
Each industrial process has its own
strategy in order to balance the rate
between digestate and fresh material and
to keep the methane phase steady in the
digester.
Process parameter to monitor in a
digester
Volatile Fatty Acids (VFA)
VFA concentration
conecentration.
is
expressed
as
acetic
acid
It depends on the quantity and quality of the material
loaded in the digester and on the ballance between acidforming bacteria and methan-forming bacteria.
VFA concentration in a digester can range between 500
and 3000 mg Ac/l, but higer values are reported.
More than the VFA concentration value is important the
variation during time. Sharp increase of VFA points out
that the AD process is turning towards acid phase.
Process parameter to monitor in a
digester
Ammonia concentration
Ammonia is formed during the degradation of proteins.
High concentration may inhibit
acid and methane-forming bacteria.
concentration ranges (van Velsen 1979):
200 e 1500 mg/l :
1500 -3000 mg/l :
3000 mg/l :
never toxic,
inhibiting if pH is under 7.4
always inhibiting
Neverless ammonia concentration is importan to buffer the system and
ballance high accumulation of VFA
CO2 + H2O
HCO3- + NH4+
HCO3- + H+
NH4HCO3
Process parameter to monitor in a
digester
Total alkalinity
Is the system capacity to
neutralize protons
and buffer the system.
It is expressed as CaCO3mg/l
Total alkalinity depends on ammonia, carbonate
systems (CO2 HCO3-) and VFA.
Total alcalinity values around 3000-5000 mg
CaCO3/l (and more…) are reported in steady state
digesters (Stafford et al., 1980).
Process parameter to monitor in a
digester
pH
In a steady state digester the pH value should range
around 7-8.
It indicates equilibrium in acid forming and methane
forming methabolism.
pH value depends, and to some extent resume, the
parameters previously proposed (VFA, ammonia,
alkalinity).
pH fall below 7 indicates VFA accumulation (sometimes
due to digester overloading)
pH raise over 8, usually indicates ammonia accumulation
Process parameter to monitor in a
digester
Biogas quantity and methane concentration
Variation in the production of biogas and in the
concentration of methane indicates instability in the
system (Stafford et al., 1980).
If biogas quantity decrease and methane percentage
falls down 50%, it indicates inhibition probably due to
VFA accumulation.
Methane concentration is the only parameter that
shows digester instability faster than pH. Always to
detect on line
Criteria to separate AD process
Total solids (TS) content in the digester:



Wet process (TS = 5–10%),
Half- dry process (TS = 10–20%),
Dry process(TS > 20%).
Biological phases:


One- stage system: all the process phases (acid and methane phase) are run in one digester
Multi-stage : hydrolitic and acid forming phase are run in different digesters than methane phase
Feeding system:


continuous feeding system
Batch system
Operating temperature:


35–37°C
55°C
Industrial process
technologies widely applied in industrial scale:
 Wet one stage continuous system
 Dry one stage continuous system
 Dry one stage batch system
 Dry sequence batch system
Each tecnology run at any operating temperature (37-55 °C)
Continuous system
Steady methane phase: pH around 7-8
Methane concentration over 50%
250
100
90
200
80
70
60
50
40
30
20
10
0
150
100
50
0
time
biogas quantity
methane percentage
Methane phase: biogas production increases and methane percentage raises
(over 60%)
methane percentage (%)
biogas (ml*gTS-1)
Continuous system
Batch system
Acidification phase: pH around 5
methane concentratione below 50%
Methane phase: pH around 7-8, methane concentration over 50%
250
100
90
200
80
70
60
50
40
30
20
10
0
150
100
50
0
time
biogas quantity
methane percentage
methane percentage (%)
biogas (ml*gTS-1)
Batch system
Wet one stage continuos
system
Digester design for this system
is very simple and is called
CSTR (Completely Stirred Tank
Reactor).
Fresh material is
continuously fed in the
digester and
completely mixed
Digestate is
continuously drawn
from the digester
Dry continuous system
Total solids content is higher than
20%, so dry process are not run in
completely stirred reactors but in
plug-flow type.
Dranco system
Inoculum
recycling mixture 20-50%.
Totalal solid
of feeding
Feeding: from the top of the digester, extraction
from the bottom
Balance between digestate and fresh material:
liquid digestate is recycled from the bottom and
mixed with fresh material
Feed
Feed
Digestate
Digestate
Kompogas system
Inoculum recycling
Total al solid of feeding mixture 2540%.
balance between digestate and fresh
material : inoculum recycling
Mixing: slow rotating mixer
Feed
Feed
Digestate
Digestate
Valorga system
Biogas Off-take
Digestate
Recirculated Biogas
Feed
•
Total al solid of feeding mixture :30%.
•
balance between digestate and fresh
materia:inoculum recycling
•
Mixing: biogas bubbling
Batch system
The material is put into the digester in one single
feeding
In sequential bach
system liquid digestate
is recycled from
digester with old
material to the one
with new material
and from the one with
new material to the one
with old material
after a stated period is drawn out
Criteria
Wet Single Stage
Technical
Advantages
Disadvantages
Inspired from known process
Environmental
Dilution of inhibitors with fresh
water
Short-circuiting
Sink and float phases
Abrasion with sand / grit
Complicated pre-treatment if MSW is used
Particularly sensitive to shock loads as
inhibitors spread immediately in reactor
Economic
Equipment to handle slurries is
cheaper
High consumption of water
Larger reactor volume (because of dilution)
High energy requirement for heating large
volume
No moving parts inside reactor
Robust (inerts and plastics need not
be removed)
No-short-circuiting
Larger OLR (high biomass)
Limited dispersion of inhibitors
smaller reactors
Complete hygienisation
Low water usage
Low heat requirement (no water to
heat up)
Wet wastes (<20% TS) cannot be treated
alone, co-digestion is needed
Simple
Low-tech
Robust (inerts, plastics need not be
removed)
Reliable process due to niches and
use of several reactors
Clogging
Need for bulking agent
Risk of explosion during emptying of
reactor
Poor biogas yield due to channelling of
percolate
Small OLR
Large land requirement (similar to aerobic
processes)
Dry Single Stage
Technical
Environmental
Economic
Batch Systems
Technical
Environmental
Economic
Cheap
Low water consumption
Little possibility to dilute inhibitors
Though more robust, equipment is more
expensive
Eunomia 2005
Parameters value
Parameter
Methane percentage in biogas
pH
VFA concentration
Ammonia concentration
Rate VFA/Total alkalinity
C/N ratio
NaCl concentration
Sulphur concentration
Ranges
>50
7-8
<6000
<3000
<0.4
10-30
<500
<22
Measure unity
%
mg acetic acid/l
mg/l
mM
mg/l
Possible problems
Biogas quantity
pH
1. System overloading
Methane percentage
pH
Ammonia> 3000
2.ammonia accumulation
Ammonia<3000
3. salinity
pH
VFA/total
alkalinity> 0.3
pH
VFA/total
alkalinity< 0.3
4. Presence of
inhibitors (antibiotics,
pesticides, heavvy
metals…)
Possible solution
1. System overloading
Stop feeding
Dilute VFA concentration
digestate recirculation
Water addition
Buffer addition (NaCO3)
2.ammonia accumulation
Stop digestate recirculation
3. salinity
Dilute concentration
enhance the feeding of fresh matherial (if condition allow)
Add water
4. Presence of
inhibitors (antibiotics,
pesticides, heavvy
metals…)
Check inlet matherial
AD plant managing parameter
Retention time (RT)
it indicates the time (days) organic material resides in the reactor.
In continuous feeding system RT is equal to:
V
Fv
RT =V/Fv
reactor volume [m3];
feeding volume for day [m3/day].
Organic Loading Rate (ORL)
it indicates the loading of organic matter in the reactor
OLR
F
C
V
OLR = F* C/V
[kg VS/m3 reactor/day];
feeding volume for day [m3/day].
substrate concentration in feeding , [kgVS/m3];
reactor volume [m3];
Mass ballance
Organic matter degradation during AD can reach 60-80%
of VS content
The digestate material present a relative enrichment in
lignin and recalcitrant molecules
VS
(%)
Ash
(%)
Lignin like
material
(%)
Feeding
mixture
90.6
10.5
7.3
Digestate
71.3
28.7
24.3
Adani, data not pubblished
References
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Biocycle, April 2004. Kranert M, Hillebrecht K. (2000) – Anaerobic digestion of organic waste, process parameters and balances
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Glossary
Anaerobic Digestion: breakdown of organic matter by bacteria in the absence of oxygen, where the final
electron acceptor is not oxygen but carbon (carbon dioxide and acetic acid ) and the product is methane It
occurs in enclosed vessels (in this case, reactors, or digestion tanks).
Biogas: The gas produced by anaerobic bacteria in the anaerobic digestion process. Typically, it is composed
primarily of methane and carbon dioxide, with low levels of other gases such as ammonia, hydrogen sulphide,
hydrogen and water vapour.
Biogas Production Rate: The biogas production rate is the volume of biogas, at standard temperature and
pressure, that can be produced per unit mass of digestion input. It is expressed in this report as m3 /
tonne of waste to digestion.
Digestate: The material resulting from anaerobic digestion of organic materials.
Mesophilic: A specific temperature range in which AD reactions may take place, nominally 25°-40°C, but usually
around 35° C
Methanogenesis: The generation of methane in landfills and in AD by anaerobic processes.
Retention Time: The average amount of time that material remains in the digester. It is a parameter used to
determine adequate digestion.
Organic Fraction of Municipal Waste (OFMSW): organic wastes (kitchen scraps, nonrecyclable paper, animal
wastes and sanitary wastes) which are separated from mixed waste collected from householders.
Thermophilic: A specific temperature range in which AD reactions may take place, nominally 45°-65°C, but
usually around 55°C.
Total Solids (TS): The amount of dry solids in a material. This includes both the organic and non-organic
fractions of the solids.
Volatile Fatty Acids (VFA): short chain organic acids such as acetic acid, formic acid, propionic acid. VFA are
produced as intermediate during first stages of anaerobic digestion and can cause the pH fall in the
medium. High accumulation of VFA in a digester can result in process failure, due to inhibition of methane
forming bacteria.
Volatile Solids (VS): The organic, or carbon-containing, fraction of TS. VS concentration is expressed as a
percentage of the TS. It is determined by incineration of the sample at 550C; volatile solids burn off while
the fixed solids (non-organic fraction) will remain in the sample.
Thank you for your attention
For any further question please mail to
[email protected]