Methusalem (M2dcR2) Advisory Board Meeting, Ghent, 24/06/2013

CFD Investigation on Fire-Side of Steam Cracking Furnace

Yu Zhang, Kevin M. Van Geem and Guy B. Marin

http://www.lct.UGent.be E-mail: yuzhan.Zhang@UGent.be

Laboratory for Chemical Technology

Technologiepark 914, 9052 Ghent, Belgium

European Research Institute of Catalysis

Furnace Geometry

• The furnace is a double radiant box with

common convection section

• Equipped with only long-flame (floor)

burners, which have separated fuel and

air inlet, situated at the bottom of the

furnace

• 44 reactor tubes are suspended in one

row, each has two passes which are

connected by a S-band and U-band

one fourth of a radiant box

Aim To investigate the flow, combustion and

radiation in the fire-side of an industrial

steam cracking furnace and to obtain the

precise heat transfer into tubular reactor

and the NOx emission

Significance • The precise heat flux profile is required

by the cracking reactor simulation

software to better predict the yields,

temperature, and coking rate inside the

tubular reactor, which are essential for

reactor design and optimization

• The radical combustion kinetics which

take NOx formation into account will

give burner designer clues to reduce

NOx emission in the furnace

Fuel Composition

• The fuel gas can be considered

as CH4/H2 mixture

• Excess air inlet flow rate to

ensure the fuel gas is fully

combusted

• Fuel gas flow rate : 2.22 kg/s

• Fuel gas inlet temperature :

288.75 K

• Oxygen excess (vol%) : 2

Models

• Flow model : Re-Normalisation Group

(RNG) k-ε turbulence model

• Species transport model : Finite-

Rate/Eddy-Dissipation model by Magnussen

and Hjertager (1977), the reaction rate is

governed by the minimum of Arrhenius and

eddy-dissipation reaction rates

• Combustion kinetics : two-step reaction

mechanism based on Westbrook and Dryer

(1981)

• Radiation model : discrete ordinates (DO)

radiation model is used to solve the radiative

transfer equation (RTE) for a finite number of

discrete solid angles

• Flue gas absorption model : weighted-sum-

of-gray-gases model (WSGGM)

Magnussen and Hjertager (1977), Symp.

(Int'l.) on Combustion, 16: 719-729

Westbrook and Dryer (1981), Combustion

Science and Technology, 27: 31-43

Conclusions

• The effects of burner geometry on the

simulated flow, combustion and radiation

heat transfer inside steam cracking furnace is

not negligible

• When using detailed combustion kinetics to

predict NOx formation in the furnace ,the

geometry effect must be taken into account

Future Work • Perform furnace simulations in which detailed

combustion kinetics is taken into account to

predict the NOx emission

• Evaluate the applicability of Eddy Dissipation

Concept (EDC) model coupled with detailed

combustion kinetics in non-premixed

combustion

Effect of burner structure on furnace simulation

In most of previous study on fire-side of steam

cracking furnace, the long-flame burners are always

simplified as a plane at the bottom of the furnace

In order to evaluate the geometry effect, two

simulations are preformed with simplified and detailed

burner structure respectively, as shown

Due to the symmetry consideration, only a quarter of

one radiant box is simulated, coupled with cracking

reactor simulation software

simplified burners detailed burners

• The initial velocity of flue gas above the

burners with detailed structures are

significantly higher than that of the simplified

burners, resulting in a stronger recirculation in

the furnace

• Flame shifts towards the furnace wall in the

detailed burner simulation whereas remains

the same place in the simplified one

Flue gas velocity and temperature

Heat transfer into process

• The shifted flame temperature maximum towards the higher elevation and furnace wall leads to shorter

distance between the two heat flux maximum of the tubular reactors

• Non-uniform heat loads between 22 tubular reactors are observed

No. 18

reactor tube No. 1-22

Heat flux of the tubular reactors

No. 9

Heat transfer rate to all tubes

Acknowledgement

This work was carried out using the STEVIN Supercomputer

Infrastructure at Ghent University, funded by Ghent University,

the Flemish Supercomputer Center (VSC), the Hercules

Foundation and the Flemish Government – department EWI