Particle-resolved numerical study of char conversion

Sebastian Schulze∗, Fred Compart∗, Andreas Richter∗, Petr A. Nikrityuk� andBernd Meyer∗∗ IEC, TU Bergakademie Freiberg, Germany� Department of Chemical and Materials Eng., UoA, Edmonton, Canada

Particle-resolved numerical study of char conversion processesconsidering conditions in the British-Gas-Lurgi (BGL) gasifier

June 13, 2016

International Freiberg Conference, 13–16 June 2016, Cologne, Germany

Content

Introduction & Motivation

CFD model

Validation: Air and 1 atm

Influence of packed-bed structure: Air and 1 atm

BGL conditions: Reducing atmosphere and 40 bar

Conclusion

TU Bergakademie Freiberg | IEC | S. Schulze et al. | Particle-resolved CFD-Study of Char Conversion | IFC 2016, Cologne, Germany | 2016-06-13 1/23

Introduction & Motivation

Introduction

The British Gas/Lurgi (BGL) gasifier with slag tapping poses advantages over the LurgiFixed-bed dry bottom gasificationa.

higher specific throughput

high carbon conversion

production of vitrified slag

much lower steam consumption

fines can be introduced via tuyeresa

Schmalfeld (Ed.), Die Veredelung und Umwandlung von Kohle, DGMK, 2008


Motivation

The understanding of the phenomena occuring within the complex structure of themoving/fixed bed gasifiers is limited, especially in the part of the oxidation andgasification zones where conversion occurs rapidly.

Additionally, due to the harsh conditions (extreme high temperatures in and abovethe slag bath, 40 bar) data is difficult to access via measurementsa.

Thus, detailed numerical simulations can give insights of the processes as they act as’numerical experiment’.

aSchmalfeld (Ed.), Die Veredelung und Umwandlung von Kohle, DGMK, 2008


GoalsApart from challenges in modeling of reacting granular media the following goals are set:

Ensure that particle-resolved simulation can represent the reacting bed

Investigation of the impact of the packed-bed structure on the fluid flow, heat and mass transfer aswell as carbon conversion

Applying boundary conditions which can be found in a BGL gasifier1 for simulations

1M. Olschar, O. Schulze, 5th Int. Freiberg Conf., Leipzig 2012


CFD model

AssumptionsApplying the pseudo-steady-state approach (PSS)2 following assumptions are introduced:

The particle shape is spherical.

The porosity of the particles is not taken into account, thus the intraparticle diffusion is neglected.

The particles consists of carbon only. The devolatilization is not included due to the steady-statecharacter .

The gas radiation is included

For 2-D simulation an axisymmetric domain is assumed.

The buoyancy effect is neglected.

2Amundson et al., Ind. Eng. Chemistry Fundamentals, 1980.


Reactions

The chemistry is modeled using semi-global homogeneous and heterogeneous reactions3:heterogeneous (surface) reactions:

C +12

O2 → CO h0R1 = −9.2 MJ kg−1C (R1)

C + CO2 → 2CO h0R2 = 14.4 MJ kg−1C (R2)

C + H2O→ CO + H2 h0R3 = 10.9 MJ kg−1C (R3)

homogeneous (gas phase) reactions:

CO +12

O2 + H2O→ CO2 + H2O h0R4 = −10.1 MJ kg−1CO (R4)

CO + H2O→ CO2 + H2 h0R5 = −1.15 MJ kg−1CO (R5)

CO2 + H2 → CO + H2O h0R6 = 1.15 MJ kg−1CO (R6)

3Turns, An Introduction to Combustion, 2006.


Validation: Air and 1 atm

Validation case set-up: Combustion of coke particles with preheated air4

Tab. 1: Parameters for simulation ofexperiments

Particle size 32 mmVoid fraction 0.4Inlet temperature 300, 478 and 700 KInlet gas composition

YO20.233

YH2O 0.007YN2

0.760Inlet mass flux 0.202 kg m−2s−1

sam

ple

ho

les

0.508 m

1.0

67

m

preheated air

product gas

steel grate

Fig. 1: Scheme of experimental fixed bedcombustion set-up

4Nicholls, Bulletin 378, Bureau of Mines, 1935.


Results: Temperature and species distribution 1

Fig. 2: Temperature and O2 mass fraction contour plots



Fig. 3: CO2 and CO mass fraction contour plots


Results: Comparison with experiments, 300 K

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

1500

1650

1800

1950

2100

2250

Tem

per

atu

re [

K]

gas temperature CFD

solid temperature CFD

temperature experiment

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

0

0.05

0.1

0.15

Xi [

-]

YCO

2

experiment

YO

2

experiment

YCO

2

CFD

YO

2

CFD

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

0

0.1

0.2

0.3

0.4

Xi [

-]

YCO

experiment

YCO

CFD

(a) (b) (c)

Fig. 4: Validation: Temperature (a), CO2 mole fraction (b) and CO mole fraction in dependence of height abovegrate. The different colors correspond to the inlet gas temperature. Results for 300 K are shown.


Results: Comparison with experiments, 700 K

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

1500

1650

1800

1950

2100

2250

Tem

per

atu

re [

K]

gas temperature CFD

solid temperature CFD

temperature experiment

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

0

0.05

0.1

0.15

Xi [

-]

YCO

2

experiment

YCO

2

CFD

0 0.05 0.1 0.15 0.2 0.25 0.3z [m]

0

0.1

0.2

0.3

0.4

Xi [

-]

YCO

experiment

YCO

CFD

(a) (b) (c)

Fig. 5: Validation: Temperature (a), CO2 mole fraction (b) and CO mole fraction in dependence of height abovegrate. The different colors correspond to the inlet gas temperature. Results for 700 K are shown.


Influence of packed-bed structure: Air and 1 atm

Grid generation for random packed bedFrom DEM simulation of particle sedimentation to representive geometry

(a)

(b)

(c)(d)

Fig. 6: Column of monodisperse particles from DEM simulation (a), Cutting-out representative volume 2.5dP × 2.5dP × 10dP (b),bridge at contact points5,6 dbridge = 0.125dP (c) and grid comprising of ca. 6 · 106 tetrahedral cells (d)

5Ookawara et al.,European Congress Chem.Eng. 2007.

6Dixon et al., Computers Chem.Eng, 2013.


Set-up of simulation: Reacting flows in the combustion zone of fixed bed

Fig. 7: Principal scheme of the computational domain

Tab. 2: Parameters for 3-D simulation

Particle size 20 mmVoid fraction 0.42Inlet temperature 1000 KInlet gas composition ’dry air’

YO20.233

YH2O 0.001YN2

0.766Inlet velocity ReP = 100

ReP =ρ∞ u∞ dpµ∞ε

(1)


Results: Velocity isosurfaces

Fig. 8: Isosurface of velocity magnitude of 3 m/s, the colour represents the gas temperature (K) for inlet gas composition ofYO2,∞= 0.233 and YH2O,∞= 0.001.


Results: Flame propagation into the fixed bedComparison of random packed bed and simple cubic bed

(a) (b)

Fig. 9: Contour plot of gas temperature (K) at the symmetry planes for random fixed bed (a) and simple cubic packing (b) forinlet gas composition of YO2,∞= 0.233 and YH2O,∞= 0.001.


Results: Combustion profiles within fixed bed 1Carbon mass flux

0 2 4 6 8 10L/d

P

10-6

10-5

10-4

10-3

10-2

m.’’ C

[k

g/(

m2 s

)]

simple cubic packingrandom packed bed

Fig. 10: Carbon mass flux m′′C for random packed bed and simple cubic packing in dependence on depth in the bed.


BGL conditions: Reducing atmosphere and 40 bar

Defining inflow conditionsBoundary conditions within oxidation and gasification zone from basic flow sheet simulation of IEC’s BGL gasifier7 with

AspenPlus.

Fig. 11: Flow diagram of BGL gasifier

Tab. 3: Inlet gas properties

Temperature 2673 KCO2 12.7 vol.-%H2O 18.8 vol.-%CO 52.2 vol.-%H2 11.1 vol.-%

7M. Olschar, O. Schulze, 5th Int. Freiberg Conf., Leipzig 2012


Applying suitable high-pressure kinetics for char gasification

Tab. 4: Intrinsic reactivity data8

Reaction A0 [g/(m2 s atmn)] EA [kJ/mol] n

C + O2 3.0e+05 136.0 0.8C + CO2 4.0e+04 211.0 0.4C + H2O 3.0e+06 231.0 0.4

Discrepancy between intinsic reactivity data and solid particle with surface reactions

Calculation of effectiveness factor:

η = 1Φ

(1

tanh(3 Φ)− 1

3 Φ

)with Thiele modul Φ =

dP6

√(n+1) k S′′′ cn−1

i2 Deff

8Hla et al., Research Rep. 80, CSIRO, 2007.



Fig. 12: Temperature and CO2 mass fraction contour plots



Fig. 13: H2O and CO mass fraction contour plots


Conclusion

Simulations of the validated particle-resolved CFD model were showingsignificantly different flame structure for the random packed bed compared tosimple cubic packing

Considering BGL-gasifer conditions, the carbon conversion in the fixed-bed ismore mass transfer limited than the combustion with air at the same particleReynolds number.


Acknowledgement

This research has been funded by the Saxon State Ministry for Economic Affairs, Labour andTransport and the European Union in the frame of the project ’Schlackebadvergaser für die

Nutzung schwieriger Brennstoffe’ (project number P100092007).


Thank you for your attention.


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Particle-resolved numerical study of char conversion