This document discusses methods for measuring the efficiency of biogas production. It presents data on the energy balance of a biogas plant, showing inputs, outputs, and losses at each stage of the process. Mass balance is identified as the basis for calculating conversion efficiency. Challenges include a lack of standardized methods for characterizing substrates and determining biogas potential. The document concludes that developing standard methods and validating approaches for full-scale applications would advance the evaluation of biogas process efficiency.

1. Efficiency of biogas production
Jan Liebetrau, Sören Weinrich, Jürgen Pröter
Conference of the European Biogas Association 2014

2. Efficiency – why and where to measure?
Purpose determines the boundaries
For any process comparison:
assumptions, methods and
evaluation need to be the same
2

3. Energy
crops
(60
%
mass) 79,21
Manure
(40
Ma
%
mass) 20,79
Substrate
provision Energy
input 100,00
Preservation/Ensilaging
process
(12
%
energy
crops) 9,51
Digestion available
energy 90,49 100,00
Flare
(4
%
methane
production) 2,57 2,84
Leaks
(0,1
%
methane
production)
0,064 0,071
Heat
losses
(4
%
overall
energy) 3,62 4,00
fuel
value
digestate
including: 22,67 25,05
gas
potential
digestate
(7
%
methane
potential) 4,49 4,97
CHP Methane
61,57 100,00
gross
electricity
including: 21,35 34,68
plant
requirements
(8
%
of
gross
electricity),
including: 1,71 2,77
Feed
in
systems
(7,3
%
of
plant
req.
) 0,12 0,20
Mixer
(40,1
%
of
plant
req.) 0,69 1,11
CHP
(3,5
%
of
plant
req.) 0,75 1,21
Misc.
(8,9
%
of
plant
req.) 0,15 0,25
net
electricity,
including: 19,65 31,91
Transformer
losses
(1
%
of
gross
electricity) 0,21 0,35
feed
in
electricity 18,65 30,28
gross
heat
including:
24,64 40,02
plant
requirements
(20
%
gross
heat) 4,93 8,00
net
heat 19,71 32,02
Conversion
losses
including 15,57 25,29
CHP
methane
slip
(1,2
%
methane
production) 0,77 1,25
Plant:
500
kWel,
ηel=39
%
577
kWth,
ηth=45
%
Full
load
hours
8000h/a;
Portion
gross
energy
quantity
%
Energy
fraction
for
components
%
not
included:
energy
for
energy
crop
production:
4,93
%
;
Transport
(energy
crops
and
digestate):
0,87
%
gross
energy
3

4. Hours of operation
Betriebsstunden 2011, geordnet nach Inbetriebnahmejahr (Daten aus Betreiberbefragung DBFZ 2011/12)
● Operational hours give impression of „downtime“
● Increasing quality
● Downtime – consequences of overproduction ?
● No data what happens during downtime
4
Year of start up Average operational
hours
Number of
questionnaires
Average full load
hours
Number of
questionnaires
(h/a) (number) (h/a) (number)
vor 2000 6911 47 5161 49
2000 - 2003 7801 90 6570 94
2004 - 2008 8248 297 7323 287
2009 - 2010 8273 146 7242 132

5. Overpressure valve opening
CH4-Emission Temperatur ÜUDS
Lufttemperatur Luftdruck
Stromausfall
120
100
Plant operators does not recognize the losses
80
Because there is no method in place to
predict precisely 60
biogas production
Temperatur in °C
15,9m3
10,7m3
y = -10927x2 + 8940,8x - 1681,2
R² = 0,976
y = -49,363x + 166,16
R² = 0,99
160
140
120
100
80
60
40
20
0
40
20
CH4-Emissionsvolumenstrom in m3 h-1
12,3m3
4,3m3
5,7m3
14:00 14:45 15:30 16:15 17:00
0 0,75 1,5 2,25 3
CH4-Emissionsvolumenstrom in m3 h-1
Uhrzeit
1.000
995
990
985
980
975
0
Luftdruck in hPa
18,2m3
0,9m3
8,8m3
02.07.2014 07:00 02.07.2014 08:30 02.07.2014 10:00 02.07.2014 11:30 02.07.2014 13:00
Uhrzeit
Bild 1: REMDE, C.: Methanemissionen aus Über- und Unterdrucksicherungen bei Biogasanlagen in Deutschland. Leipzig, Deutsches
Biomasseforschungszentrum gGmbH, Universität Stuttgart, Institut für Feuerungs- und Kraftwerkstechnik, Diplomarbeit, 2013
Bild 2: noch unveröffentlichte Messungen des DBFZ 5

6. Conversion efficiency of biological process
• Benchmarking
• Comparison of substrates
• Comparison of plant concepts
• Monitoring and optimization of processes
(e.g. evaluation of disintegration processes)
6
Conversion efficiency is
Input vs. Energy output (methane output)
Mass balance is basis for energy balance
Precondition: Knowledge of material flows within the process

7. Fractions of substrate
7
• Correction of volatile organic materials
• Not degradable VS
• Integrated water
Substrate component in the biogas process (changed according to WEIßBACH)
wet mass
ü specific to substrates, difficult analysis
ü necessary to obtain comparable results
Recalcitrant VS
Total solids (corr.)
VS (corr.)
Comparison is only possible if absolute reference value is existent –
degradable substrate with specific gasproduction
Not
degraded VS
Ash1
Integrated
water
conversion
Biogas
Degradable VS
Converted VS
Microorg.
Water
Not integrated
water
Ash Org. Digestate Biogas Water
1 Asche = anorganische TS, enthält mitunter Substanzen und Nährstoffe (z.B. N, P, S) welche von Mikroorganismen für das Wachstum und die Biogasbildung benötigt werden

8. Determination of gas potential
8
Simple?
Degree of degradation?
Standard (resp. literature) values?
Batch Test? Continuous experiments?
Feed value analysis?
Challenge:
• few precise biochemical measurement values
• lack of standards and individual methods for evaluation (interlaboratory tests)
9 Development of standardized and robust methods for field tests

9. Calculation of Biogas potential
Calculation of degradable VS according to WEIßBACH
9
Example corn silage (Average of samples at DBFZ)
Ash [g/kgTS] NfE [g/kgTS] Fiber [g/kgTS] Protein [g/kgTS] Lipids [g/kgTS]
44 621 226 79 30
Standardized digestion tests considering metabolic excreta
FoTS = 1000 − Ash − 16 − 0 − ( 0,47 · Fiber + 0,00104 · Fiber² )
Non degradable Carbohydrates (Function of fiber)
Non degradable Lipids (constant)
Non degradable proteiin(constant)
FoTS = 984 − Ash − ( 0,47 · Fiber + 0,00104 · Fiber² ) = 780 [g/kgTS]
Calculation of Biogas potential according to WEIßBACH
Stoichiometric calculation considering 5 % MO Biomass
Biogas = 780 ∙ 800/1000 = 624 [l/kgTS] Methan = 780 ∙ 420/1000 = 327,6 [l/
Gasproduction coefficient fkorg cTorSn] s ilage [l/kg FoTS]

10. Biogas potential and yield
10
BIOGASPOTENTIAL
Theoretical maximum biogas production
• Calculation based on chemical composition
• Potential can be modified by means of disintegration processes " up to complete VS
• Can be obtained with the ideal batch test (infinite retention time)
Real full scale application
(THEORETICAL) BIOGASYIELD
Biogas production under conditions of continuous processes (retention time)
• Calculation based on process kinetics
• Can be obtained with continuously operated processes
9 Standard methods necessary!

11. Biogas potential and yield within a CSTR
Yield and Gas Production Rate
in a CSTR
140
120
100
80
60
40
20
0
0 5 10 15 20 25 30 35 40 45 50 55 60
Retention Time (d)
Yield (%)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Gas Production Rate
(m3/m3 daily)
Yield
gasproduction rate
theory
gasproduction rate
praxis Organic Load (kgVS/m3)
19.2 9.6 6.4 4.8 3.8 3.2 2.7 2.4 2.1 1.9 1.8 1.6
11

12. Evaluation disintegration processes
12
§ Degradation of stillage proteins
§ Two desintegration effects: a) rate increase;
b) increase of biogas potential TS (Enzym B = Effekt a+10%b)
Degree of degradation (%)
[Mauky, 2011]
Increase of gas production(%)
Retention time

13. Biogasmessprogramm II (Plant evaluation)
13
Gas potential digestate (37°C, > 60 Tage) and VS at 50 Biogas plants
Portion of gas potential and recalcitrant substrate is different and process specific

14. Basics of mass balance
dm/dt ≈ Δm/Δt = Input − Output ± reaction
14
Substrate
Additives
Water
Biogas
Process model
Kinetics
Stoichiometry
System boundary
Digestate
Mass balance of biogas process
Input Output
Rezirculation
rate Transport over boundaries Biochemical process
9 Efficiency on basis of conversion rate

15. Mass balance
15
Biogas plant (500 kWel) without recirculation
Fermenter BHKW
k 0.132 [1/d] ηel 38 [%]
Maissilage HRT 75.4 [d] Biogas
ṁS 31.5 [t/d] Uf oTS 90.9 [%] ṁB 7.99 [t/d] PFeu 1315 [kW]
TS 33.47 [% FM]̇VB 6085 [m³ i.N./d] Pel 500 [kWel]
oTS 95.6 [% TS] cCH4 52 [%]
foTS 78 [% TS] Gärrest cCO2 48 [%]
ṁW 20.96 [t/d] ṁG 23.51 [t/d] ρB 1.314 [g/l]
ṁTS 10.54 [t/d] TSG 13.07 [% GR] H 9.97 [kWh/m³]
ṁoTS 10.08 [t/d] foTSG 24.49 [% TSG]
ṁf oTS 8.22 [t/d] ṁW,G 20.43 [t/d]
YB/f oTS 740 [m³ i.N./t foTS] ṁTS,G 3.07 [t/d]
YB/oTS 604 [m³ i.N./t oTS] ṁf oTS,G 0.75 [t/d]
●
Allgemeine
Vorgaben
Inputcharakteris ierung
und
Gas zusammensetzung ●
Vorgabe
Variante
B
Elektris che
BHKW-­‐Leistung
Berechnung unter Berücksichtigung des stöchiometrischen Wassereinbaus und unter Vernachlässigung des Biomasseaufbaus
●
Vorgabe
Variante
A
Spezifis cher
Biogasertrag
des
Subs trats
bzw.
der
Anlage ●
Vorgabe
Variante
C
Reaktionskinetik
1.
Ordnung
und
HRT
9 Variation of methods for validation dependent on available data

16. How to measure and calculate?
1. Substrate gas potential (degradable VS)
2. Masses at plant (corr. TS/VS)
• Input
• Output
◦ Digestate
(Ash scaling, (subtraction of biogas,
gas potential of digestate))
◦ Biogas
(measurement or estimation from electricity production,
CHP efficiency and losses??)
(Weißbach 2009)
16
Challenge is representative sampling
• Dynamic process
• Heterogeneous substrates
• Lack of standard methods and individual methods

17. Example Weissbach
17
Mass balance,
no kinetic model
included

18. Conclusion
18
Options for process evaluation:
• 1. Biogas potential: Calculation for common substrates according to WEIßBACH
(based on feed value analysis)
• 2. Biogas yield: Calculation based on simple first order kinetic
9 Aim: Application of available methods and higher precision
9 Validation for full scale applications need to be done
Challenges
• Standard methods for substrate characterization necessary (degradable VS, kinetic parameters)
• Transfer of results from different methods (e.g. batch to continuous)
• Models for transfer of Labscale experiments to full scale results
• Portion and influence of MO mass on results
9 Development of a standard for mass balances for biogas plants

19. DBFZ Deutsches
Biomasseforschungszentrum
gemeinnützige GmbH
Torgauer Straße 116
D-04347 Leipzig
Tel.: +49 (0)341 2434 – 112
www.dbfz.de
Contact
Dr.-Ing. Jan Liebetrau
Jan.Liebetrau@dbfz.de
+49 341 2434 716