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CFD for Flameless Burners Development

Valerio Battaglia1, Francesco Crippa1, Enrico Malfa1,

Massimiliano Fantuzzi2

1. Centro Sviluppo Materiali - Italy 2. Tenova LOI Italimpianti - Italy

Abstract

As a result of US market requirements for a new Ultra Low NOx burner, Tenova started in 2002 the FlexyTech® research program in order to definitely lower NOx emission of steel reheating furnace equipped with central recuperator and traditional flame burners through application of flameless technology. In the development process extensive CFD simulations performed by CSM allowed reducing field tests and time to market for the new burners. The paper describes the CFD approach adopted by CSM and TENOVA in the designing process of new burners.

1. CFD contribution to the design process of Tenova burner

The FlexyTech® burners were developed to reach a target level of NOx emission for standard operative condition of steel reheating furnace equipped with central recuperator and at the same time to guarantee flexibility in furnace operability (start up and power modulation) [1]. One of the main tool adopted during the design of the new burner has been Computational Fluid dynamic and three aspects in particular were objects of numerical analysis:

• Conceptual design: simulation of different configurations before prototype stage. • Experimental data interpolation: simulation of the burner tests and comparison if the

results with experimental data, for deep understanding of combustion phenomena. • Burner scaling: to provide a reliable method of scaling to larger or smaller

applications. CSM experience has been applied to define the numerical approach adapted to answer the request of the burner development.

2. Numerical study

To support the design process CSM has set-up a CFD activity divided in steps: • Model validation. • Grid optimization. • Analysis of combustion control parameters.

As a first step CFD has been applied to simulate the traditional high impulse Tenova TS12 burner (2MW nominal power based on natural gas) with the goal to set-up a model to apply as engineering tool for the design of new burners. The CFD simulations have been performed by commercial fluid dynamic code FLUENT version 6.3. The selection of the physical models (table 1) has been based on a previous extensive validation work dedicated to evaluate the performance of the different turbulence representations for simulating high velocity round jets and combustion scheme for natural gas. The Wilcox κ−ω model with shear flow correction resulted the most appropriate to

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simulate the turbulent generation produced by strong velocity gradient in near jet zone [2,3] The Finite Rate/Eddy Dissipation model [4] with two steps reaction mechanism was utilized to model turbulence-chemistry interaction. Recent studies [5,6] have confirmed that Eddy Dissipation represents the temperature field with accuracy comparable to the more complex and time consuming PDF and EDC models.

Physical model and reactive scheme

Fluid Ideal gas

Turbulence k-ω with shear stress correction and wall functions Chemistry

2 steps reduced mechanism: CH4+1.5O2 = CO+2H2O CO+O2 = CO2

Rate of reaction: Eddy Dissipation/ Arrhenius Radiation DOM (Discrete Ordinate Model)

Absorption coefficient: WSGGM Numerical approach Mesh type Hybrid

Solver Segregated

Discretization Implicit/finite volume

Non linear term Second order scheme

Table 1 Physical model, reaction scheme and numerical approach The computational domain is composed of the burner installed in the CSM pilot furnace (square section 2mx2m, variable length 7.5m) where characterisation tests have been performed. The grids were built by Gambit, Fluent pre-processor, with a high resolution non structured zone in the burner region and a structured zone to describe the furnace volume, as shown in Fig.2. The computational domain has been reduced exploiting periodic configurations of the burner outline.

Fig. 2 Mesh near the burner tip

The baseline grid has been generated with 880k cells for the complete CSM furnace. The starting grid was modified to test grid independency of the numerical results. The mesh was progressively reduced to evaluate the minimum grid resolution that guarantees difference of wall heat flux prediction inside an tolerance interval of ±4 kW/m2, corresponding to ±5 oC of wall temperature (fixed at 1250 oC), Fig3.

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Fig.3 Heat flux on furnace walls (left) and on test line for different grid sizes (right)

The results are very sensitive to grid resolution in the unstructured zone where it is necessary to keep cell skewness under 0.85. The minimum grid that guarantees results in the validation interval has 640k cells. The CFD model has been utilized to evaluate the TS12 burner performance in different operative conditions. This results has been used to evaluate the recirculation ratio of flue gas generated by an high impulse burner, that is one of the main parameters that is necessary to take under control in the flameless condition regime [7]:

AirFuelV MFRMFR

latedMFRrecircuK+

=

Having fixed the burner tip geometry, Kv is not affected by air mass flow (jet velocity) but depends on combusting air temperature (air density), as shown in Fig.4 where Kv is plotted in sections along furnace axis. Further analysis show that Kv depends on furnace gas density (process temperature).

Fig.4 Recirculation ratio function of combusting air mass flow (left) and temperature (right)

The analysis of the real burner confirms the well known formula for the jet entrainment [8]:

air

furnacev d

xKρ

ρ32.0=

where d is the jet diameter and x is the axis coordinate.

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The same CFD analysis has been applied in the development of the FlexyTech® TSX burner series, implementing flameless technology [1]. The CFD has been used to design burners that allow to maximize the recirculation factor close to the tip and to define the proper gas injections in order to assure the condition for flameless regime.

Fig. 5 TSX burner simulation (Tfurnace=1250°C / T Air=450°C)

Moreover the modelling helps to found the right burner tip geometry for coupling both flame (start up) and flameless (operative) mode switching fuel injection from central to lateral position without any mechanical switching tool operating on heated air. The temperature fields in the two operative mode are shown in Fig.6 for TSX10 and TSX12 burner. Cross correlating the CFD simulation and the experimental data allowed to define scaling criteria of FlexyTech® burner, based on the distribution of recirculation factor and of temperature peak inside the burner chamber.

Fig. 6 Temperature fields for flameless (lateral gas) and flame (central gas) mode

Using this criteria the family of FlexyTech® burners has been enlarged with small size flameless burners (TLX [9]) and flameless roof burner (TRX [1] – Figure 7).

a) Path line colorate conil modulo della velocità 70

0mm

b) Dettaglio path line coloratein prossimità del bruciatore

Path line coloured by velocity

X=0.7m X=0.35m

Iso-superfacev=25m/s

X=0.7m X=0.35m

Iso-superfacev=25m/s

6%

30%

CH4 molar fraction

O2 molar fraction

6%

30%

CH4 molar fraction

O2 molar fraction

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Fig. 7 Simulation of different TRX roof burner configuration during development stage

3. NOx calculation

Calculation of thermal and prompt NOx have been attempted using Fluent Post-processor, however comparison between calculated and measured NOx value, for both TS and TSX burner, confirms the limitation of Fluent NOx model approach. In particular the model over estimate NOx emission in flame regime due to over prediction of the flame temperature and under predict NOx concentration in case of diluted combustion regime (flame temperature lower then 1600°C due to dilution). To overcome this limitation two different approaches has been followed:

• Cross correlation between the temperature peak obtained by CFD simulation and NOx measured during experimental test to obtain the interpolation curve able to estimate emissions of new simulated burner [10].

• CFD simulation with EDC model coupled with complex reaction mechanisms that directly simulate NOx concentration. This activity is on going and results obtained by EDC with ARM9-5steps mechanism [11] are presented in Figure 8.

Fig. 8 NOX prediction with different combustion model vs. experimental data

Tem

pera

ture

[°C

]

T m

ax =

155

1°C

T m

ax =

152

5°C

2 steps2 steps ARM9ARM9

Tfurnace = 1250°C

0

10

20

30

40

50

60

70

0 1 2 3 4Oxygen Concentration in Dry Flue Gas [%]

NO

x [p

pm@

3%O

2 D

FG]

T air =450°C

T air =520°C

TSX - EXP

TSX-ARM9

TSX-2step

TRX (A), Tmax=2081 °C TRX (B), Tmax=1851 °C TRX (C), Tmax=1702 °CTRX (A), Tmax=2081 °C TRX (B), Tmax=1851 °C TRX (C), Tmax=1702 °C

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In spite of a very close value of maximum temperature in the furnaces predicted by 2steps and ARM9, the value of NOx calculated differs of one order of magnitude (2ppm vs.15ppm respectively). However the value of ARM9 is still half of the measured NOx emission.

4. Conclusion

CFD allows to abandon the traditional “trial and error” approach in the design process of new flameless burners and results synergic with experimental test:

• Certain observation or data can only be obtained using mathematical modelling technique due to practical limitation of taking measurements on the plant → more information

• Modelling can enable production plant modification to be evaluated without having to carry out on-site experiments which may cause loss of production → lower risk

• Modelling allows a wider range of design options to be evaluated before testing → lower development cost

This allow to obtain design goal more quickly reducing significantly the time to market of new burners The positive results obtained using CFD in the FlexyTech® burner development and the increasing computational power installed in CSM has allowed starting the extension of the CFD analysis to a furnace equipped with several burners. Considering that the main goal is to have a tool able to reproduce the velocity and temperature field in a complex geometry with engineering accuracy the 2step reduced mechanism with Westbrook&Drives kinetic seems to be the best compromise between accuracy and CPU time. In parallel more advanced reduced mechanism, such as ARM19, will be evaluated with the goal to verify if some improvement can be obtained in the prediction of NOx and to give guideline for the selection of proper reaction scheme when the goal is evaluate also the NOx and CO concentration in the flue gases.

5. References

1. M.Fantuzzi et others: 7th International Symposium on High Temperature Air Combustion and Gassification, January 13-16, 2008, Phuket, Thailand

2. Wicox D.C.: Turbulence Modelling for CFD, DCW Industries Inc.. 2000

3. E.Malfa, E.Colombo, F.Inzoli: 2005 Summer Heat Transfer Conference, 17-22/07/2005, San Francisco, USA

4. B. F. Magnussen and B. H. Hjertager: 16th Symp. (Int'l.) on Combustion. The Combustion Institute, 1976.

5. Malfa E., Venturino M., Macchi E., Tota V.: Fluent User Group Meeting, Maranello, Oct. 2002

6. Mancini M.: Analysis of mild combustion of Natural gas with preheated air Ph.D. Thesis. Clausthal University of Technology

7. Wunning J.: Prog. Energy Combust. Sci. 23 (1997) 1243-1245

8. Beer J.M., Chigier N.A.: Combustion Aerodynamics, Applied Science Publishers LTD, 1972

9. Malfa E et al.: paper #002 in this conference

10. Malfa E. et al.: 15° IFRF Members’ Conference, Pisa, Italy – June 13-15, 2007

11. C.J. Sung et others, Combustion and Flame 125:906-919 , 2001