Anaerobic digestion
Andrea Lanzini
Biofuels Summer School
2018
20.06.2018
• Anaerobic digestion
• Biogas resource
• Applications
• Fuel clean-up
• Case study: biogas CHP plant
Topics
Anaerobic digestion
Anaerobic digestion - Fundamentals
In anaerobic processes, microorganisms (enzymes and bacteria), that work
in absence of free oxygen or linked in the form of nitrates, sulphates, etc.,
degrade via biological pathways the organic substances (e.g. biowaste).
The organic compound is converted through subsequent oxidation and
reduction reactions in its most oxidized state, CO2, and in most reduced one,
CH4. This two gaseous compounds represents the most important metabolites
and are the main constituents of biogas.
1. HYDROLYSIS: in this phase, large organic polymers such ascarbohydrates, fats and proteins are broken into smaller constituents, likesimple sugars, aminoacids, fatty acidsand water.
3. ACETOGENESIS: the organic acids are formed. They are the raw material for the eventual methanogenesis. The bacteriaresponsible for the third phase, the acetogens, are highly sensitive to temperature fluctuations. The methanogenesis itself also slowly startsduring this phase.
2. ACIDOGENESIS: the furtherbreakdown of remainingcomponents. It is done by the acidogenic bacteria which convert the organic matter in short-chained fattyacids, alcohols, CO2, H2 and ammonia.
4. METHANOGENESIS: methane (biogas) is formed from acetate (about 70%) and 30% from CO2 and H2 in this phase. AlsoCO2 is relased and, in small proportions, also water, H2S, and N2. The content of methane in biogas typically variesbetween 50 and 70% depending on the substrate charateristcs used.
AD steps
From organic waste to biogas
CH3COOH → CH4 + CO2
Source: Girard M. et al., 2013, “Biodegradation in Animal Manure Management”, Research and
Development Institute for the Agri-Environment (IRDA), Québec, Canada
Microorganism at work
https://www.youtube.com/watch?v=rSdOGjzhtcg
Temperature ranges for AD
Psychrophilic temperature in a range of 4-25°C (optimal values 15-20°C);
Mesophilic temperature in a range of 10-40 °C (optimal value 35°C);
Thermophilic temperature in a range of 45-70°C (optimal value 50°C).
Bacteria
The temperature of the digester has to be maintained within a certain temperature range in order to prevent the bacteria from being killed.
Thermophilic condition is the most critical (unstable) to maintain and leads to the higestyield (degradation of organic matter is maximized).
In most cases, methane-forming bacteria control the process. Methane formers are
very sensitive to environmental factors (high ammonia concentrations, low phosphorus
concentrations, low pH, temperature, and the presence of toxic substances), and reproduce
very slowly. Consequently, methane formers are difficult to grow and are easily
inhibited. Therefore, process design and the operation of conventional anaerobic
digestion are tailored to satisfy the needs of the methane-forming bacteria.
Factors influencing AD
Total solids
Carbon to nitrogen (C/N) ratio
Stirring
Retention time
The amount of solids (% w/w) of the organic waste feeding
the digester is a relevant operating parameter.
Total solids
Wet digestion TS < 10% w/w Sludge from wastewatertreatment plant
Semi-wet digestion 10 < TS < 20% w/w Manure, codigestion plants(organic waste + sludge)
Dry digestion TS > 20% w/w Organic fraction of municipal solid waste(OFMSW)
Mass balance of an anaerobic digestor
t.s. 100 kg
v.s.75 kg
Ash/FC25 kg
Biogas = 37.5 kg(non-degraded v.s. = 50% of total v.s.)
Feedv.s./t.s.=75%
Digester
t.s. = total solidv.s. =volatile solid
Non-degraded v.s. = 37.5 kg
Ash/FC = 25 kg
Digester effluent (digestate) = 62.5 kg
The efficiency of the process is 50% (in terms of organic matter degradation)
Final disposal of the digestate
Courtesy of Marcopolo S.p.a.
Anaerobic digestion of sludge
Biogas gasometer
DigesterDigester
From prethickner
To thickner
Courtesy of Gruppo SMAT
Sludge heating and stirring
Courtesy of Marcopolo S.p.a.
Mechanical stirring is adopted in90% of the digesters
Tank shape
Anaerobic digestion of animal waste
aa
aa
Cow manureCow manure (separated solid)Cow manure (solid)
Chicken manureChicken litter (solid)Swine manure
Source: Special Issue of «L’Informatore Agrario» (3/2013)
Biogas and biomethane exploitation pathways
ManureSewagesludge
BiowasteOFMSW
Agricultural residues
Crops
Organic feedstock
Anaerobic digestion
Engine / Fuel Cell
Boiler
UpgradeInjection in
distribution grid
Compression & storage
Biogas Biomethane
Transportation fuel
Biogas clean-up from contaminants (e.g., H2S, siloxanes, halocarbons, etc.) is required.
SludgeElectricity
GHG emission reduction cost
Biogas resource
EU current production and potential
Anaerobic digestion
Anaerobic digestion (AD) is a proved technology to reduce the putrescible
matter of organic waste while turning part of it into useful energy (i.e.,
biogas).
The scale of AD plants ranges from the relatively small and local (farm –
rural environment) to larger plants in municipalities (urban environment) or
intensive animal farming settlements.
Ancient roots
The collection of urban organic waste is a practice with ancient roots
Biogas production in EU
Source: own elaboration of Eur’ObservER data
Biogas production in EU
Source: own elaboration of Eurostat data
EU biogas production by source and country
Source: 2016, Optimal use of biogas from waste streams. An assessment of the potential of biogas from
digestion in the EU beyond 2020
World biogas production
The total primary energy supply was 573 EJ globally in 2014
Applications
WASTE HIERARCHY
Why is Anaerobic Digestion useful?
(1) prevention
(2) preparing for re-use
(3) recycling
(4) other recovery, e.g. energy recovery
(5) disposal.
Is there a large biogas potential?
The biogas potential is related to the following sources:
Human beings (sewage sluge)
Livestock (manure)
Biowaste (organic fraction of municipal solid waste, OFMSW)
Industries with organic effluents/waste
Residual Municipal Solid Waste (MSW)
Arable land (energy crops)
Biogas from manure: energy potential
Domestic
animal
Million
Heads
Total dry solid
waste (kg
solid/head/day)
Biogas production rate
(m3 biogas/day/head)
Chemical
power rate
(W/head)
TWh/yr
Buffaloes 194 2.74 0.73 54 92
Camels 27 4.11 1.37 244 58
Cattle 1,468 3.15 0.84 158 2025
Chickens 20,887 0.03 0.01 2 390
Goats 976 0.27 0.09 19 160
Horses 60 4.11 1.37 244 128
Mules 10 4.11 1.37 244 22
Pigs 977 0.68 0.35 75 646
Sheep 1,163 0.41 0.14 26 265
A farm of 1,000 pigs can produce up to 750 kW of biogas
Biomethane as transportation fuel
Q: How far can you drive a car with the daily manure
of 1 pig?
A: About 2 km…
Biomethane
Issues/opportunities for biogas use
Daily and seasonal
fluctuation in biogas
production
Contaminants(some biogases
are heavilycontaminated;
e.g., landfillbiogas)
Thermal load of the digester (can reduce sthe net power output of
the plant)
Combined heat and power (CHP)
electric production with internal thermal
use
Fuel cell technology for modular and
high-efficiencypower
production
OPPORTUNITIESTHREATS
From waste treatment to resource recovery plants
Primary + secondary sludge(C-H-N-O substrate)
ANAEROBICDIGESTER
Biogas (CH4,CO2, N2 traces)
CARBON CYCLE IN THE SOFCOM CONCEPTUAL PROCESS
Sludge
From waste treatment to resource recovery plants
Primary + secondary sludge(C-H-N-O substrate)
ANAEROBICDIGESTER
ANODEBiogas (CH4,CO2, N2 traces)
Anode off-gas(H2O, CO2 ,H2, CO, N2 traces)
Po
wer
He
at
ELECTROLYTE
CATHODE
Fresh air
Vitiated air
SOFC
Sludge
CARBON CYCLE IN THE SOFCOM CONCEPTUAL PROCESS
From waste treatment to resource recovery plants
Primary + secondary sludge(C-H-N-O substrate)
ANAEROBICDIGESTER
ANODEBiogas (CH4,CO2, N2 traces) OXY-
COMBUSTION
WATER REMOVAL
CleanwaterCO2 (high purity)
O2 (nearly stoichiometric)
Anode off-gas(H2O, CO2 ,H2, CO, N2 traces)
H2O, CO2
(N2, O2 traces)
Po
wer
He
at
ELECTROLYTE
CATHODE
Fresh air
Vitiated air
SOFC
Sludge
CARBON CYCLE IN THE SOFCOM CONCEPTUAL PROCESS
From waste treatment to resource recovery plants
Primary + secondary sludge(C-H-N-O substrate)
ANAEROBICDIGESTER
ANODEBiogas (CH4,CO2, N2 traces) OXY-
COMBUSTION
WATER REMOVAL
CleanwaterPHOTO-
BIOREACTOR
CO2 (high purity)
O2 (nearly stoichiometric)
Anode off-gas(H2O, CO2 ,H2, CO, N2 traces)
H2O, CO2
(N2, O2 traces)
Algae (C rich)P
ow
er
He
at
Was
tew
ater
Sun
ligh
t
Cle
an w
ater
Alg
ae
surp
lus
ELECTROLYTE
CATHODE
Fresh air
Vitiated air
SOFC
Nutrients (N,P,K) removal from wastewater is achieved with
fixation in micro-algae
Sludge
CARBON CYCLE IN THE SOFCOM CONCEPTUAL PROCESS
ICO2CHEM: RES+CO2 to FT products
RWGS + low T Fischer-Tropsch(small scale integrated reactor)
RES for H2 production
Industrial CO2
CO2
H2
FT-products: Value added chemicals• White oils• High molecular weight waxes
Customer-products
ICO2CHEM reactor
www.spire2030.eu/ico2chem
Fuel clean-up
The removal of biogas contaminants
• Sulfur (H2S and organic sulfur)
• Siloxanes
• Halocarbons / halogens
• Other VOCs (tars)
Biogas contaminants
Siloxanes
Biogas clean-up
Fast voltage drop due to Ni surface coverage: the performance drop is linearly
dependent with the sulfur coverage (s).
Ni-anode deactivation by sulfur
Papurello et al. "Sulfur poisoning in Ni-anode solid oxide fuel cells (SOFCs): Deactivation in single
cells and a stack." Chemical Engineering Journal 283 (2016): 1224-1233.
Ni-anode degradation by siloxanes
Si
Ni
Zr
Even ppb(v) levels of siloxanes can degrade irreversibly the fuel cell anode performance
Hossein et al. "Solid oxide fuel cell anode degradation by the effect of siloxanes." Journal of Power
Sources 279 (2015): 460-471.
Silicon mapping on the Ni-anode
1
012h EUROPEAN SOFC & SOE FORUM – Lucerne, 7 July 2016
Very little Si is visible in the
freshly reduced
sample, could be sample
preparation.
Measurement of silicon deposition: WDS mapping of anode contact layer (channelboundary) of a freshly reduced sample (before siloxane exposure).
Si Ni
Zr
Silicon mapping on the Ni-anode
1
112h EUROPEAN SOFC & SOE FORUM – Lucerne, 7 July 2016
Preferential Si deposition at the ACL, but
present in the anode support
as well.
Measurement of silicon deposition: WDS mapping of anode contact layer(channel boundary) after siloxane exposure.
Si Ni
Zr
Quantitative analysis of deposited Si
1
212h EUROPEAN SOFC & SOE FORUM – Lucerne, 7 July 2016
Si Ni
Zr
Silicon (wt. ppm)
Stack 'A' 150
Stack 'B' 80
Stack 'C' - fuel inlet 1990
Stack 'C' - fuel outlet 370
ICP-OES: Inductively Coupled Plasma Optical Emission Spectroscopy
Quantitative chemical analysis (ICP-OES)
'A' and 'B' refer to stack cells tested in biogas reformate without contamination.'C' refer the stack tested with D4-siloxane in the anode feed.
Post-mortem stack analysis results
Higher Si concentration near the anode-channel boundary (WDS mapping)
Higher Si concentration at the stack inlet (ICP-OES).
The impact of HCl on the Ni-anode
SOFC single cell operation on H2 fuel
gas at 0.25 A/cm2, HCl contamination
from 10 to 100 ppm – the recovery
phase was carried out at the end the
whole test
Short stack operation at 750 °C
and FU = 60% FU, HCl
contamination up to 500 ppm
H2S removal: adsorption on activated carbons
0
20
40
60
80
100
0,5 2,5 4,5
H2S
outl
et c
once
ntr
atio
n
(ppm
)
Elapsed time (hr)
Sulfatrap R8G
Airdep CKC
Solid sorbents (such as activated carbons) can effectively
remove sulfur down to ppm(v) levels, as required by the fuel cell
Siloxanes removal: screening of the performance of the
different sorbents
0
10
20
30
40
50
60
70
80
90
100
0,00 2,00 4,00 6,00 8,00 10,00
C/C
0 (
%)
Time (h)
Biochar(200) CKI(200) CKC(200) C64(200)
D4-siloxane inlet concentration: 20 ppm(v)
Summary of adsorption capacities
Sorbentmaterial
Adsorption Capacity (mg X / g AC)
H2S (anaerobic) H2S (+O2) D4-siloxane
C64 (airdep) n.a. n.a. 151.2
CKC (airdep) 4.5 56.0 103.1
CKI (airdep) 4.2 n.a. 73.4
R8G (sulfatrap) 39.8 49.9 n.a.
C64: mineral-based activated carbon rich in iodineCKC: activated carbon impregnated with potassium bicarbonate (KHCO3)CKI: activated carbon impregnated with potassium iodide (KI)R8G: activated carbon enriched with metal oxides
• Competitive effects (co-vapors adsorption on solid sorbent)
• Reactors’ arrangement and catalyst employed
• Reactor geometry (L/D ratio) and Gas Hourly Space Velocity
(GHSV)
• Temperature and humidity conditions of the inlet biogas stream
Parameters affecting adsorption removal in real plants
Breakthrough time of contaminants
Case study: biogas CHP plant
Optimal plant sizing
DIGESTER
CHP
Energy use in WWTPs
The Electric Power Research Institute (EPRI) estimated that 4% of the nation’s
electricity use goes towards moving and treating water and wastewater by
public and private entities1.
Waste Water Treatment Plants (WWTPs) are among the most expensive
public industries in terms of energy requirements accounting for more than 1%
of the consumption of electricity in Europe2.
[1] Electric Power Research Institute, Water & Sustainability (Volume 4): U.S. Electricity Consumption
for Water Supply & Treatment—the Next Half Century, 1006787, Topical Report, March 2002
[2] http://www.enerwater.eu/enerwater-project-waste-water-treatment-plants/
WWTP + Fuel Cell CHP integration
Efficiency of high-temperature fuel cells
Solid Oxide Fuel Cell (SOFC)
Biogas from human waste
Sewage biogas: 10-20 L/PE/day or 2.5-5.0 W/PE100,000 PE 250-500 kW of biogas power (or 125-250 kWe with the fuel cell)PE = person equivalent
Hamburg plant
0
5
10
15
20
25
0
4
8
12
16
20
24
28
32
36
1 2 3 4 5 6 7 8 9
mg t
ot.
Si /
Nm
3
mg H
2S /
Nm
3
H2S Silicon
Biogas contaminants (H2S, siloxanes)
Plant energy needs
www.demosofc.eu
% of the overallplant electric load
% of the overallplant thermal load
Biogas fluctuations in WWTP
Optimal Plant Operation
Simulated SOFC operation with buffer volume
Energy Planner Tool (EPT) with Graphical User Interface (GUI)
Energy Planner Tool (EPT) results
GAS HOLDER SOFC
BIOGAS CONSUMP.
Plant operation: SOFC load cycling
SOFC modules power modulation during a period of highly fluctuating biogas production.
Optimal plant sizing
Number of modules 1 2 3 4
Biogas share for electricity
production26.7% 53.4% 76.5% 97.8%
Equivalent capacity factor at full
load100.0% 99.8% 95.7% 87.3%
Number of forced shut-downs
(during reference period)0 0 1 4
Average electrical efficiency 53.16% 53.15% 53.05% 52.66%
Average thermal efficiency 80.00% 79.96% 79.09% 77.35%
Plant operation: impact of CH4 % vol.
Biogas clean-up: layout
Selective removal of S and Si with different sorbents
50 kW SOFC running on biogas
SOFC module by CONVION. The fuel cell runs on biogas and provideselectricity to the wastewater treatment plant (WWTP). Biogas isavailable on site via the anaerobic digestion of sludge collected in theWWTP.
From waste treatment to resource recovery plants
Photobioreactor with micro-algae for C and nutrients removal (SOFCOM project
Thank you! Any questions?
Acknowledgements
SOFCOM project (www.sofcom.eu)
DEMOSOFC project (www.demosofc.eu)
Suggested readings
Optimal use of biogas from waste streams. An assessment of the potential
of biogas from digestion in the EU beyond 2020
