1. Energy Technology & Innovation Initiative
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Optimisation of a gas-mixed anaerobic
digester using a combined CFD and
biochemical kinetic modelling approach
International Conference on Advances in Energy
Research
10-12th December 2013
Indian Institute of Technology Bombay
Dr Mark Walker, Dr Lin Ma and Prof Mohamed Pourkashanian
m.walker@leeds.ac.uk

2. Introduction

Anaerobic Digestion (AD) is an attractive energy and waste management
technology

At industrial scale AD plants consist of a mixed tank e.g. mechanical, gas, jet
mixed

Small unmixed systems can digest a limited range of feedstocks

Mixing is parasitic to the energetic and economic feasibility of the biogas plant
but allows a greater variety of substrates to be digester

Presented is a novel coupled CFD and AD model → optimisation of the mixing of
based on biogas production

Mixing with biogas has low capital and operation cost relative to other mixing
methods → potential to increase the efficiency of many thousands of small-scale
digesters operated worldwide
Mark Walker, ICAER, 10-12th December 2013, IITB

3. Energy Technology & Innovation Initiative
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Model Description

4. Modelling Scenario
14.7 m3 cylindrical tank
Biogas recirculation
Output
digestate
Biomass
Biogas
mixing
Feedstock
flow rate
Biogas
flow rate
Working fluid
(Digestate)
Sparger
Mark Walker, ICAER, 10-12th December 2013, IITB

5. Geometry
Free shear wall
Geometry 1
Geometry 2
Draught
tube
Opening
Gravity
Geometry 1
Geometry 2
Mass inlet
Gas sparger
Ø 2.65
Ø 2.65
0.2
Ø 0.36
2.65
φ 0.2
1.325
2.65
Ø 0.2
Ø 0.2
1.325
1.325
0.2
Ø 0.3
Mark Walker, ICAER, 10-12th December 2013, IITB

6. Model Structure




CFD
 Laminar
 Non-Newtonian
 Multiphase (gas-liquid)
 Steady
Biochemical reactions
 Contois kinetics
 Microorganism (X) and 1
Substrate/Feedstock (S)
Rheology
 TS based formulation of nonNewtonian model parameters
Non-Newtonian fluid (digestate)
 Power law
 Shear thinning/pseudoplastic
Mark Walker, ICAER, 10-12th December 2013, IITB

7. CSTR Model – Fully Mixed
Q (m3 day-1)
Biomass
Inlet Conditions
Xin = 0
Sin =
Tsin =
Q=
Anaerobic Digester
S, X
Qbiogas
Biogas
Parameter Values
V = 14.7 m3
β = 25 kg m-3
µm = 0.125 day-1
Ks = 12
Y = 0.1
α = 1 m3 kg-1
Initial Conditions
S0 = 20 kg m-3
X0 = 5 kg m-3
Mark Walker, ICAER, 10-12th December 2013, IITB

8. Model Comparison
Phenomenon
CFD + Contois
Model
Contois CSTR
Model
✓✓
✓
Hydraulic overload
✓✓
✓
Microorganism growth kinetics
✓✓
✓
Degree of Digestion
✓✓
✓
Biogas Yield
✓✓
✓
Effect of Mixing on other phenomena modelled
✓
××
Short circuiting of biomass
✓
××
Contact between microorganism and biomass
✓
××
Other mixing related phenomena - Sedimentation,
crust formation, foaming, gas entrainment, shear…
×
××
Other biological phenomena - Organic overload,
inhibition…
×
×
CSTR Model
Dilution/Washout
- modelled (better)
- modelled
- not modelled
- cannot be modelled
Mark Walker, ICAER, 10-12th December 2013, IITB

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Results and Discussion

10. Results and Discussion
TS Distributions
G1
Increasing mixing flow rate
0
0.01
0.1
1
Approx. mixing energy (W m-3)
10
G2
Mark Walker, ICAER, 10-12th December 2013, IITB

11. Results and Discussion
Biogas Production

Compared with CSTR model and no mixing

Large process gain by a small mixing flow rate (x10+)

Above a threshold mixing rate system begins to act as a CSTR

G1 threshold lower than G2

Optimal mixing rate in G1 ~ 0.02 W m-3 (G1)

Slight predicted process gain by using threshold mixing (+1.8%)

12. Discussion

Despite potential applications that this type of model requires further
development and is not fully validated. Relevant input data would be required
before the model could be used reliably in a predictive capacity

Validation;
 Model converges to a CSTR model at higher mixing rates
 The sub-models used have all been previously validated

Some issues regarding applicability of input data;
 The rheological data did not span the expected strain rates found in anaerobic digesters
 Contois parameters based on household solid waste, rheological data from cattle slurry

Model optimisation only addresses a subset of the phenomena relating to mixing
in anaerobic digestion and does not account for;
 Temperature and pH distribution
 physical stratification through sedimentation
 Foaming and gas entrainment
 effect of shear on the microorganisms
Mark Walker, ICAER, 10-12th December 2013, IITB

13. Conclusion

3D coupled CFD and AD model developed

Optimisation of the biogas mixing of two idealised digester designs for biogas
production and biomass degradation

An increase in biogas production from 2.6 to 33 m3 day-1 was predicted by the
introduction of mixing (0.02 W m-3)

Potential applications include enhancing the design and operational
characteristics of biogas plants

The model was partly validated using comparison with a CSTR case

For reliable predicative modelling more comprehensive biochemical and
rheological data specific would be needed

The model does not include some of the phenomena relating to the mixing of
anaerobic digesters
Mark Walker, ICAER, 10-12th December 2013, IITB

14. Energy Technology & Innovation Initiative
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Thank you!
Any questions?