1

CFD Model for Optimisationof an

Entrained Flow Gasifier for Black Liquor

Magnus Marklund1, Rikard Gebart1, David Fletcher2

1 Energy Technology Centre in Piteå, Sweden2 Department of Chemical engineering, University of Sydney, Australia

Introduction

• Development plant for

pressurised black liquor

gasification under construction

• Underpinning research

program with 4 subprojects at

ETC, LTU, UmU and Chalmers

• Aim of current project:

development of a CFD model

for optimisation of the reactor.

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Introduction

• Three main parts, a hot

reactor, a quench cooler and a

counter current condenser.

• Atomisation to fine droplets

(~200-300 µm)

• Partial combustion of fuel

spray with oxygen

The CFD paradigm

The conceptual model is an approximation of reality

The numerical model is an approximation of the conceptual model

Verification of the numerical model must be done before validation.

Reality

Numerical model Conceptual modelwith sub models

(AIAA, ERCOFTAC)

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Current model

• Based on CFX4 CFD code

• Distribution of non interacting discrete droplets forming a spray

• Droplet conversion by customised subroutines in the solver

• k-ε and Reynolds stress turbulence models

• Gas combustion modelled by Eddy Break Up/Kinetic model

• Discrete transfer model for thermal radiation

• Coupled solver for heat conduction in the walls

Model status

• Included gas species: O2, H2O, CO, H2, CO2, CH4

• Sulphur and sodium reactions neglected

• Combustion is controlled by mixing and kinetics

• Gas phase combustion reactions:

222

22

222

2224

2224

5.0 5.0

2 242

HCOOHCOCOOCOOHOH

OHCOOCHHCOOHCH

+⇔+⇒+⇒+

+⇒++⇒+

Temporarily neglected

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Model verification

• Unexpectedly high flame temperature predicted by the model.

• Input parameters, model, numerical or programming errors?

• Simplified model of a long rectangular duct to verify the results.

• Mass flux of O2 (λ=1.1) and droplets at inlet

• Droplets consist only of CO (volatile matter) and smelt

• Constant specific heat

• Endothermic reactions from pyrolysis neglected

• Resulting outlet temperature can be computed by hand

Model verification

• Reactions run to completion well before the outlet from the plug

flow reactor

• CFD simulation predicts an outlet temperature of 2163 K.

• Hand calculation yields 2171 K.

• Overall error less than 1% likely to come from numerical errors

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Uncertainty from input data

Pyrolysis rate

0102030405060708090

100

300 500 700 900 1100 1300 1500

T (K)

Rate

(1/s

)

Case ICase II

Pyrolysis rate parameters (λ (O2) = 0.4 in test case) :

• Case I: Standard values used for coal (Ubhayakar et. al.)

• Case II: 90 % of the activation energies in Case I

(Case II results in a faster pyrolysis compared to Case I, see graph)

Uncertainty from input dataCase I Case II

Axial velocity and

streamlines

Note:• Recirculating flow

outside of flame

• Peak axial velocity

slightly lower for

case II

• Overall solution

very similar

U (m/s)U (m/s)

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Uncertainty from input dataCase I Case II

Temperatures

Note:• High flame

temperatures

predicted

• Shorter flame and

higher flame

temperature for

case II

• Temperature at

outlet close to

adiabatic flame

temperature

T (K)T (K)

Predicted flame temperature(Case I)

CO O2 T (K)

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Predicted flame temperature

• Recirculation brings hot reactive gases (e.g. CO) in contact with oxygen in the flame region

• The preheating of the fuel gas explains the high temperatures

• Conclusion: Flame temperature reasonable after all

• Addition of the water/gas shift reaction will lower the flame temperature

• Additional reactions (e.g. CO2 dissociation) may have to be included in the model if local temperatures becomes too high

Summary

• Partial verification of gasifier model for a plug flow reactor with an error less than 1%

• Relatively low sensitivity in the model from different devolatilization rates• Higher flame temperature and shorter flame length was detected for the

faster devolatilization rate• Locally high flame temperatures can be explained with recirculation of hot,

partially burnt fuel gas that gets in contact with oxygen• Water/gas shift reaction creates numerical instabilities but needs to be

included for accurate predictions of the gas composition and temperature

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Future work

• Continue work on verification and sensitivity to input parameters• Perform optimisation studies on flame shapes in the DP1 reactor• Construct a pressurised test rig for qualitative (high speed photography

and flow visualisation) and quantitative (PDA) measurements of atomisation

• Candidate nozzles for the Chemrec reactor will be tested in the rig • Refine conjugate heat transfer model for prediction of temperature in

refractory lining and pressure vessel• Add sodium sulphate reduction chemistry to CFD model

PDA equipment at ETC

• Simultaneous measurement of velocity (up to 3 components) and size of spherical particles as well as mass flux, concentration

• High accuracy and high spatial resolution (small measurement volume)

• Particle sizes between ~1 µm and several millimetres