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