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Investigation of Flashback Propensity in Turbines with Syngas Fuels

Bidhan Dam1, Gilberto Corona2, and Ahsan Choudhuri3,

Center for Space Exploration Technology Research The University of Texas at El Paso

El Paso, TX 79968

The paper presents experimental measurements of combustion induced vortex breakdown (CIVB) flashback propensity for hydrogen (H2)-carbon Monoxide (CO) flames. The effects of H2 concentration, diluents and swirl number on the flashback propensity of H2-CO flames are discussed. For a given Ubulk, the stoichiometric ratio (%F) at which the CIVB flashback occurs decreases with the increase in H2 concentration in fuel mixtures. However, the flashback propensity decreases with the increase in the swirl number. Combustor flashback maps for syngas compositions derived from different coal source shows the distinct behavior due to the presence of various diluents in fuel mixtures.

Nomenclature S = swirl number CIVB = combustion induced vortex breakdown SL = burning velocity %F = percentage of fuel CFD = computational fluid dynamics PIV = particle image velocimetry Ubulk = bulk velocity

I. Introduction urbine combustors of advanced power systems have goals to achieve very low pollutants emissions (NOx < 2-ppm), fuel variability, and fuel flexibility.1 Future generation gas turbine combustors should tolerate fuel

compositions ranging from natural gas to a broad range of syngas without sacrificing operational advantages and low emission characteristics.1-2 However, issues of fuel variability and NOx control through premixing also cause a number of concerns, especially combustor flashback and flame blowout. Flashback is a combustion condition at which the flame propagates upstream against the gas stream into the burner tube. Flashback is a critical issue for premixed combustor designs, because it not only causes serious hardware damages but also increases pollutant emissions. In swirl stabilized lean premixed turbine combustors, onset of flashback3-4 may occur due to: (i) boundary layer flame propagation (critical velocity gradient), (ii) turbulent flame propagation in core flow, (iii) combustion instabilities, and (iv) upstream flame propagation caused by combustion induced vortex breakdown (CIVB). Flashback due to the first two foregoing mechanisms has been studied extensively for pure fuels.3 Generally, analytical theories and experimental determinations of laminar and turbulent burning velocities model these mechanisms with sufficient precision for design usages. However, effects of composition variations on flashback propensity of fuel blends, such as syngas, are largely unknown. The presence of hydrogen (H2) in syngas significantly increases the potential for flashback. Due to high laminar burning velocity and low lean flammability limit, H2 tends to shift the combustor operating conditions towards flashback regime. Even a small amount of H2 in fuel mixtures triggers the onset of flashback by altering the kinetics and thermo physical characteristics of the mixture. Furthermore, the swirling flow complicates the flashback processes in premixed combustors, and the boundary

1 Graduate Research Assistant, Student Member 2 Undergraduate Research Assistant, Student Member 3 Director. Associate Professor, Mechanical Engineering Department, Eng. M-305, The University of Texas at El Paso, El Paso, Texas

79968, Senior Member.

T

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition4 - 7 January 2010, Orlando, Florida

AIAA 2010-1172

Copyright © 2010 by Ahsan Choudhuri. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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layer flame propagation inadequately describes the flashback propensity of most practical combustors. Recent investigations suggest that the CIVB mechanism is an important flashback process in swirl stabilized burners.4-5 Motivated by these issues the present work investigates the effects of syngas compositions on flashback resulting from combustion induced vortex breakdown.

II. Experimental Techniques The swirl flow combustor rig (Fig. 1) has three configurable modules: (i) inlet manifold with a static mixture,

(ii) swirl burner with mixing tube, and (iii) optically accessible combustion chamber. The module integrates a pilot flame ring with a mixture of methane and air. The swirl burner module is fitted with a quartz mixing tube. The fuel and air enter into the inlet manifold through five alternate injection holes. The fuel-air mixture then passes through the static mixer to eliminate injection induced flow irregularities. The quartz glass mixing tube is needed for the high speed imaging of the flashback inside the premixer. The swirl burner module can accommodate both center body and hub less swirlers. Depending on the test conditions, the burner system can accommodate a rectangular or a circular combustion chamber. Digital images of the flame are captured with a high definition camcorder to see the sequences of flashback. High resolution direct imaging and high speed PIV systems with intensified camera systems were used to capture the flashback sequences. The details of the experimental system and methodologies can be found elsewhere in the literature.17 Experimental uncertainties (bias + random errors) of present measurements are less than ± 0.5% of the mean value. All H2-CO mixture compositions reported in this article are a volumetric percent. A high speed PIV system which includes a high speed camera (2kHz), Litron LDY 300 series laser (1 kHz), and Scitek PS-10 Remote Operation Powder Seeder was used to track the flow field during the CIVB driven flashback. The transient OH chemiluminescence flame images during the CIVB flashback process were captured using a high speed intensified camera (200Hz) fitted with an OH line filter.

III. Results and Discussions

A. Visual Observation and Quantification Fig. 2 shows the typical CIVB flashback sequence. Initially, the flame was stabilized in front of the swirler. The

flame subsequently moves (slowly) upstream of the center body and then starts to oscillate. The oscillation frequency increases with the increase in the equivalence ratio. With a further increase in the equivalence ratio, the flame stabilizes upstream of the center‐body. Fig. 3 shows the CFD data of the flow-field superimposed with experimental measurements. The computed data agree fairly well with the experimental measurement.

Figure 1. Experimental setup: swirl combustor.

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Table 1 shows the typical synthesized gas compositions derived from different types of coal and Fig. 5 the

burning velocity at those compositions. The maximum burning velocity point of the synthesized gas compositions derived from brown and bituminous coal is in the lean condition due to the presence of more H2 percentage than CO in the mixtures.

Figure 5. Burning velocities of actual syngas compositions.

0

20

40

60

0.2 0.6 1 1.4 1.8

Lam

inar

bur

min

g ve

loci

ty(c

m/s

)

Ф

coke

lignite

Brown coal

bituminous

Figure 4. Burning velocities at different H2-CO mixture compositions. 

20

50

80

110

140

170

0.40 0.80 1.20 1.60 2.00 2.40

Lam

inar

Bur

ning

Vel

ocity

(cm

/s)

φ

10%H2+90% CO15%H2+85%CO20%H2+80%CO25%H2+75%CO30%H2+70%CO

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Another observation is the effect of CO2 diluents on the burning velocity of these two mixtures. Although H2

and CO percentage is almost same, the syngas compositions derived from bituminous coal have higher burning velocity than brown coal derived syngas compositions due to the presence of less CO2 concentrations. The maximum burning velocity of lignite and coke derived syngas compositions shifted to the right due to the presence of higher CO percentage than H2 concentration. Also, diluent effect is dominant here due to high CO2 concentration in lignite; the burning velocity of coke is higher than lignite coal derived syngas compositions. The addition of more diluents to the mixtures, the recombination (H+H+M →H2+M) step is faster compared to the chain branching (H+O2→O+OH) step.9 In addition, the presence of diluents (CO2, N2) with higher heat capacity in sufficient quantities in the mixtures will reduce temperature and eventually decrease the burning velocity.9 C. Effects of Fuel Compositions on CIVB Flashback Map

Fig. 6 shows the flashback limits (slow and oscillating; swirl number 0.71) of the combustor at different mixture compositions and equivalence ratios. The effect of H2 concentration on the CIVB flashback is clearly evident in Fig. 6. For a given Ubulk, the %F at which the CIVB flashback occurs decreases with the increase in H2 concentration in fuel mixtures. Similarly, Fig. 7 shows the flashback limits of the combustor for a swirl number 0.97.

Figure 6. Flashback map of the swirl combustor with 6 vane swirler (swirl number S = 0.71).

10.00%

12.00%

14.00%

16.00%

18.00%

40 120 200 280 360

%F

Ubulk (cm/s)

10%H2+90%CO15%H2+85%CO20%H2+80%CO25%H2+75%CO

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Fig 8 shows the flashback propensity of the combustor measurd for different fuel compostions. As mentioned earlier, for a given Ubulk, the %F at which the CIVB flashback occurs decreases with the increase in H2 concentration in fuel mixtures. For a given Ubulk of brown and bituminous coal derived syngas compositions, the %F at which the CIVB flashback occurs close to each other because of nearly similar compositions, but differs from lignite and coke coal derived syngas compositions due to the presence of higher percentage of H2 and diluents (N2, CO2). The %F at which the flashback occurs increases with the increase in diluents (N2, CO2) concentration in fuel mixtures.

D. Effects of Swirler Strength

Fig. 9 shows the effect of swirler strength on the CIVB flashback. The 12 vane swirler (swirl number S = 0.97 ) provides a more stabilized recirculation zone and flame as compared to the 6 vane swirler (swirl number S = 0.71).

Figure 8. Flashback map of the swirl combustor for actual syngas compositions with 12 vane swirler (swirl number S = .97).

15

20

25

30

35

40

45

50 100 150 200 250 300

%F

Ubulk (cm/s)

Brown CoalBituminous CoalLigniteCoke

Figure 7. Flashback map of the swirl combustor with 12 vane swirler (swirl no. S = .97). 

12.00%

14.00%

16.00%

18.00%

20.00%

22.00%

40 120 200 280 360

%F

Ubulk (cm/s)

10%H2+90%CO15%H2+85%CO20%H2+80%CO25%H2+75%CO

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Fig. 10 shows that the 6 vane swirler is more prone to CIVB flashback than the 12 vane swirler. For a given Ubulk, the %F at which the CIVB flashback occurs increases with the increase of the swirl number.

E. Characterization of CIVB Driven Flashback

Figs.10 through 12 show the OH emission intensity of the flame front during a flashback. The 12 vane swirler

(swirl number S = 0.97) produced more OH chemiluminiscnece emission (1 through 4 sequence images) after the flow separation as compared to the 6 vane swirler (swirl number S = 0.71). The strong swirled flow produced a more stabilized reaction zone (OH concentration) just after the swirler. Fig. 13 also shows less OH concentration because of the presence of diluents in the bituminous coal derived syngas compositions.

Fig. 13 through 15 show the PIV images of the flow field during flashback at different operating conditions.

Initially, the flame stabilizes in front of the swirler creating a stabilized recirculation zone. Next, the flame slowly starts to distort the flow field with the increase in an equivalence ratio. With a further increase in the equivalence ratio, the flame completely distorted the stabilized recirculation zone and propagated upstream.

Figure 9. Comparison between 6 vane (swirl number S=.71) and 12 vane (swirl number S=.97) swirler strength for flashback map.

14.00%

16.00%

18.00%

20.00%

22.00%

100 150 200 250 300 350

%F

Ubulk(cm/s)

6 Vane swirler(swirl no,S=.71)

12 vane swirler(swirl no,S=.97)

Figure 10. OH concentration distribution of the flame front during flashback [6 vane swirler with 10%H2+90%CO].

Figure 11. OH concentration distribution of the flame front during flashback [12 vane swirler with 10%H2+90%CO].

Figure 12. OH concentration distribution of the flame front during flashback [12 vane

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Figure 14. Vector flow field sequences (1-4) during flashback [12 vane swirler with brown coal derived syngas compositions]. 

Figure 13. Vector flow field sequences (1-4) during flashback [6 vane swirler with 10%H2+90%CO].

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IV. Conclusions i. The study concludes that,

ii. For a given Ubulk, the %F at which the CIVB flashback occurs decreases with the increase in H2

concentration in fuel mixtures. iii. For a given Ubulk, the %F at which the CIVB flashback occurs increases with the increase of swirl number. iv. For a given Ubulk, the %F at which the flashback occurs decreases with the decrease in diluents (N2, CO2)

concentration in fuel mixtures. 

V. Acknowledgments This research was done with the support of the U.S. Department of Energy, under awards

DE‐FG26‐08NT0001719 (Project Manager Robie Lewis). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the Department of Energy.

VI. References 1Turbine Technologies Website, National Energy Technology Laboratory, Department of Energy, Web address:

http://www.netl.doe.gov/technologies/coalpower/turbines/; Accessed on June 6, 2007. 2Narula, R.G. (1998), “Alternative Fuels for Gas Turbine Plants – An Engineering Procurement, and Construction

Contractor’s Perspective,” ASME Paper No. 98-GT-122 presented at the International Gas Turbine and Aeroengine Congress & Exhibition, Stockholm, Sweden.

3Lewis, B. and von Elbe, G. (1987),Combustion, Flames, and Explosion of Gases, 3rd edition, Academic Press, Orlando.

Figure 15. Vector flow field sequences (1-4) during flashback [12 vane swirler with coke coal derived syngas compositions].  

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4Kroner, M., Fritz., J., and Sattelmayer, T. (2003), “Flashback Limits for Combustion Induced Vortex Brakdown in a Swirl Burner,” Journal of Engineering Gas Turbines and Power, Vol. 125, pp. 693-700.

5Kroner, M., Fritz., J., and Sattelmayer, T. (2001), “Flashback in a Swirl Burner with Cylindrical Premixing Zone,” ASME Paper No. 2001-GT-0054.

6Gupta, A. K., Lilley, D. G., and Syred, N. (1985), Swirl Flows, Abacus Press, Cambridge, Massachusetts, USA. 7Choudhuri, A. R. (2005) Investigation of the Effects of Composition and Combustion Instabilities on the Flashback

Propensity of Syngas Premixed Flames, Final Technical Report, Department of Energy Grant DE-FG26-04NT42133. 8Turns, S.R., “An Introduction to Combustion” 2nd edition, Mc Graw Hill. 9Glassman, I., “Combustion” 3rd edition, Academic Press.