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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Turbulent Flame Speed as an Indicator for Flashback Propensity of Hydrogen-Rich Fuel Gases

[+] Author and Article Information
Yu-Chun Lin

e-mail: yu-chun.lin@psi.ch

Peter Jansohn

Combustion Research Laboratory,
Paul Scherrer Institut (PSI),
Villigen PSI 5232, Switzerland

Konstantinos Boulouchos

Aerothermochemistry and Combustion
Systems Laboratory,
ETH Zurich,
Zurich 8092, Switzerland

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 1, 2013; final manuscript received July 4, 2013; published online September 17, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(11), 111503 (Sep 17, 2013) (8 pages) Paper No: GTP-13-1215; doi: 10.1115/1.4025068 History: Received July 01, 2013; Revised July 04, 2013

The turbulent flame speed (ST) is proposed to be an indicator of the flashback propensity for hydrogen-rich fuel gases at gas turbine relevant conditions. Flashback is an inevitable issue to be concerned about when introducing fuel gases containing high hydrogen content to gas turbine engines, which are conventionally fueled with natural gas. These hydrogen-containing fuel gases are present in the process of the integrated gasification combined cycle (IGCC), with and without precombustion carbon capture, and both syngas (H2 + CO) and hydrogen with various degrees of inert dilution fall in this category. Thus, a greater understanding of the flashback phenomenon for these mixtures is necessary in order to evolve the IGCC concept (either with or without carbon capture) into a promising candidate for clean power generation. Compared to syngas, the hydrogen-rich fuel mixtures exhibit an even narrower operational envelope between the occurrence of lean blow out and flashback. When flashback occurs, the flame propagation is found to occur exclusively in the boundary layer of the pipe supplying the premixed fuel/air mixture to the combustor. This finding is based on the experimental investigation of turbulent lean-premixed nonswirled confined jet flames for three fuel mixtures with H2 > 70 vol. %. Measurements were performed up to 10 bar at a fixed bulk velocity at the combustor inlet (u0 = 40 m/s) and preheat temperature (T0 = 623 K). Flame front characteristics were retrieved via planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) diagnostics and the turbulent flame speed (ST) was derived, accordingly, from the perspective of a global consumption rate. Concerning the flashback limit, the operational range of the hydrogen-rich mixtures is found to be well represented by the velocity gradients prescribed by the flame (gc) and the flow (gf), respectively. The former (gc) is determined as ST/(Le × δL0), where Le is the Lewis number and δL0 is the calculated thermal thickness of the one-dimensional laminar flame. The latter (gf) is predicted by the Blasius correlation for fully developed turbulent pipe flow and it indicates the capability with which the flow can counteract the opposed flame propagation. Our results show that the equivalence ratios at which the two velocity gradients reach similar levels correspond well to the flashback limits observed at various pressures. The methodology is also found to be capable of predicting the aforementioned difference in the operational range between syngas and hydrogen-rich mixtures.

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References

Figures

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Fig. 2

Schematic of the high-pressure combustor

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Fig. 3

ST versus the normalized flame length for the fuel mixture H2-N2 85-15

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Fig. 4

The velocity gradients gf and gc (symbols) plotted against Φ for the fuel mixture H2-N2 85-15 (T0 = 623 K). The nearly horizontal curves indicate the gf at various pressure levels. The black regression curves of gc are added to illustrate the “critical” conditions when gc reaches gf.

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Fig. 6

The predicted and measured ΦFB plotted against the pressure for the syngas mixture (H2-CO 50-50; T0 = 573 K, 623 K, and 673 K)

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Fig. 5

The velocity gradients gf and gc (symbols) plotted against Φ for the syngas mixture (H2-CO 50-50). The color coding corresponds to the respective preheat temperature. Dark blue: T0 = 673 K; brown: T0 = 623 K; and light blue: T0 = 573 K. The symbol of gc indicates the respective pressure level. Filled triangle: 10 bar; empty square: 5 bar.

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Fig. 7

The predicted ΦFB plotted against the pressure for the syngas and H2-rich mixtures (T0 = 623 K)

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Fig. 8

The velocity gradients gf and gc (symbols) plotted against Tad for the syngas and H2-rich mixtures at 10 bar

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Fig. 1

(a) Stable, and (b) “anchored” flames. The red bars indicate the boundaries and inlet of the combustor.

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Fig. 9

The velocity gradient gc plotted against SL0 for the syngas and H2-rich mixtures. For the syngas, the color coding indicates the respective preheat temperature. Dark blue: T0 = 673 K; brown: T0 = 623 K; and light blue: T0 = 573 K. For the H2-rich mixtures, the color coding indicates the compositions (refer to Fig. 8).

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