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

NOx Reduction by Air-Side Versus Fuel-Side Dilution in Hydrogen Diffusion Flame Combustors

[+] Author and Article Information
Nathan T. Weiland

 National Energy Technology Laboratory, Pittsburgh, PA 15236-0940; Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106nathan.weiland@mail.wvu.edu

Peter A. Strakey

 National Energy Technology Laboratory, Morgantown, WV 26507-0880

J. Eng. Gas Turbines Power 132(7), 071504 (Apr 14, 2010) (9 pages) doi:10.1115/1.4000268 History: Received May 01, 2009; Revised May 21, 2009; Published April 14, 2010; Online April 14, 2010

Lean-direct-injection (LDI) combustion is being considered at the National Energy Technology Laboratory as a means to attain low NOx emissions in a high-hydrogen gas turbine combustor. Integrated gasification combined cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent CO2 separation. The IGCC’s air separation unit produces a volume of N2 roughly equivalent to the volume of H2 in the gasifier product stream, which can be used to help reduce peak flame temperatures and NOx in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and it has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on NOx emissions. From a peak flame temperature perspective, greater NOx reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower NOx production potential for fuel-side dilution due to its smaller flame size, whereas air dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal NOx generation. For more complex staged-air LDI combustor designs, the dilution of the primary combustion air at fuel-rich conditions can result in the full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for NOx reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with the dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and NOx emissions. Strategies for accurate computational modeling of LDI combustors’ stability characteristics are also discussed.

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Figures

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Figure 13

Computed velocity vectors (max 6 m/s shown) and stoichiometric location in the wake region of the fuel tube. Same conditions as in Fig. 1.

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Figure 12

Computed near-field flame temperatures. Same conditions as in Fig. 1.

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Figure 11

Comparison of CFD H2O formation rate with flame luminosity for fuel and air dilutions using dA=6.35 mm, 5 slm H2, N2/H2=0.5, and Φcoax=1, up to x/dF=10

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Figure 10

Flame base photos with UF=75 m/s and dA=5.00 mm: (a) UA=0 m/s, no dilution; (b) UA=15 m/s, no dilution; (c) UA=15 m/s, 17.4% coaxial air O2; (d) UA=15 m/s, 14.8% coaxial air O2; (e) UA=0 m/s, fuel N2/H2=1; (f) UA=15 m/s, fuel N2/H2=0.5; (g) UA=15 m/s, fuel N2/H2=1; and (h) UA=45 m/s, fuel N2/H2=1

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Figure 9

Corrected NOx measurements for N2/H2=1, as a function of diluent placement, dA, and Φcoax (=1 unless otherwise noted)

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Figure 8

Stability regimes for coaxial air-side dilution versus fuel-side dilution using dA=5.00 mm. Air dilution is expressed in terms of the resulting percent oxygen in the air. NOx test conditions of Fig. 9 for N2/H2=1 are shown as lines of constant UF/UA emanating from the origin.

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Figure 7

Split flame formation for dA=5.00 mm, N2/H2=0.25, and Φcoax=0.5. (a) Stable flame at UF=100 m/s, (b) near split flame at 120 m/s, and (c) split flame at 140 m/s

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Figure 6

Effect of air-side dilution versus fuel-side dilution on NOx with dA=6.35 mm and Φcoax=1

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Figure 5

NOx reduction with increased mixing from coaxial air flow without dilution

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Figure 4

Simple jet flame temperature and residence time scaling of NOx emissions with air- and fuel-side dilutions

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Figure 3

Flame length versus the nitrogen dilution of hydrogen in a simple turbulent jet flame

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Figure 2

Adiabatic flame temperatures for premixed and diffusion flames with varying N2/H2 dilution ratios

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Figure 1

Dilute diffusion flame combustor

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