CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Fluidized Bed Conversion –

A Challenge for Simulation

Franz Winter

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Fluidized Bed

Reactors

gas – solid contact

Grace et al. 1997

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Circulating

Fluidized Bed

Reactors

complex

two-phase flow

Horio et al. 1997

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Fluidized Bed

Boilers

Kaiser et al. 2004

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Solid Fuel Combustion complex chemistry

0

40

80

120

160

200

0 200 400 600 800 1000 1200 1400 1600 1800 2000

time [s]

0

1

2

3

4

NO N2O

HCN CO2Char Combustion

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Solid Fuel Combustion

NOhet.

hom.

het.

N2

N2O

NO

Char Surface

hom.

hom.

hom.hom.

hom.

hom.

hom.

hom.

hom.

hom.

hom.

hom.hom.

hom.

N2O

NO

NH3

NCO

HCN

CHAR - N

N2O

NO

NCO

NH3

HCN

VOLATILE -

N

TAR - N

FUEL - N

hom.

N2O

NO

hom.

complex chemistry

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Solid Fuel Combustion CO oxidation - main reactions

• CO – Oxidation:– Initiation

H2O = OH + H

O2 + M = 2 O + M

– Propagation

CO + OH = CO2 + H

– chain

H + O2 = OH + O

O + H2O = 2 OH

– Termination

CO + O = CO2H + O2 + M = HO2 + M

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Solid Fuel

Combustion

CH4 oxidation

reaction paths

CH4

CH3

CH2O

CH3O

C2H5

C2H6 CH2(S)

CO

HCO CH2

C2H4

C2H3

CH2CHO

CO2

CH

CH2CO

+O, +H, +OH

+O

+HO2

+OH

+O

+H

+H

+O, +H, +OH

+O2

+N2

+H

+O2

+O2

+O2

+O2

+H

+OH

+O2

+O

+O

+O

+O2

+O2

+H +H

+O

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

time scales

for chemical reactions

(10-10s to >1s )

Chemistry – problems with different time scales

Warnatz et al. 2001

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

are there two or more different

scales (time) of the

independent variable on which

the dependent variables are

changing:

=> stiffness occures.

example of an instability encountered in integrating a stiff equation

(schematic).

=> implicit integration methods

Chemistry – problems with different time scales

Press et al. 1992

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

Aim: Prediction of the Location of

Ash Deposition

Müller et al. 2003,

17-FBC

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

- Turbulence: k-ε model

- Radiative heat transfer: discrete ordinate model

- Turbulence chemistry interaction: Eddy-Dissipation

Combustion Model

- Software: Fluent

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

Chemistry

• 4-Step Approach:

CxHyOz + aO2 = bCO + cH2 + dH2O

CxHyOz + eH2O= fCO + gH2 + hH2O

H2 + 0.5O2 = H2O

CO + H2O = CO2 + H2

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example:

CFD Model for

Ash Deposition

Meshing: structured - unstructured

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

Mass flows: bed and freeboard, fuel split

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

CO Distribution

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

Temperature Distribution

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Model for Ash Deposition

Ash Particle Hitting Maps

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Kallio et al. 2004,

11-Fluidization

Aims:

- Prediction of NO Emissions- Effect of Particle Size

Example: Semi-Empirical Models

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example:

Semi-Empirical Models

- Flow and mixing simplified

- Zones defined

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: Semi-Empirical Models

NO Profiles comparison with

measurements

Different models

considered:

A – Attrition, SF –

Sec. Fragmentation

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Visual

observations of bubble flow

cold conditionssand bed

Peirano et al. 2002,

Powder Technology

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Eulerian model:

- gas-phase (continuous phase):

Turbulence: k-ε model

- particle-phase (discrete phase):

kinetic theory of granular flow

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Calculation time:

- 500 hours

on a CRAY 90 for 20 s of real time with 2.3x105 nodes

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Calculated particle volume

fraction

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Calculated and

measured pressure spectra

for high pressure drop distributer

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Calculated and

measured pressure spectra

for low pressure drop distributer

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Example: CFD Simulation of a Single Bubble

Time averaged

gas velocity field

for low pressure drop distributer

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Conclusions

• The current status is:

- CFD simulation of two-phase flow is very limited

- Flow is simplified with semi-empirical zone models

- Chemistry is simplified, reduced models are used

- Heat transfer is simplified based on Nusselt number

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Conclusions

• Future challenges are:

- Simulation of full chemistry

- Development of reduced mechanisms

- CFD simulation of two-phase flow

- Heat transfer models (based on local flow conditions)

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Thank you for your attention!

CD-Laboratory for Chemical Engineering at High Temperatures

Institute of Chemical Engineering

Conference on

Small-scale Combustion

18-20 November 2004, Vienna

www.semaco.co.at/dvv