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

# The Influence of Diluent Gases on Combustion Properties of Natural Gas: A Combined Experimental and Modeling Study

[+] Author and Article Information
Sandra Richter, Jörn Ermel, Thomas Kick, Clemens Naumann, Uwe Riedel

German Aerospace Center (DLR),
Institute of Combustion Technology,
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany

Marina Braun-Unkhoff

German Aerospace Center (DLR),
Institute of Combustion Technology,
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany
e-mail: Marina.Braun-Unkhoff@dlr.de

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received December 21, 2015; final manuscript received March 4, 2016; published online April 26, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(10), 101503 (Apr 26, 2016) (9 pages) Paper No: GTP-15-1572; doi: 10.1115/1.4033160 History: Received December 21, 2015; Revised March 04, 2016

## Abstract

Currently, new concepts for power generation are discussed, as a response to combat global warming due to CO2 emissions stemming from the combustion of fossil fuels. These concepts include new, low-carbon fuels as well as centralized and decentralized solutions. Thus, a more diverse range of fuel supplies will be used, with (biogenic) low-caloric gases such as syngas and coke oven gas (COG) among them. Typical for theses low-caloric gases is the amount of hydrogen, with a share of 50% and even higher. However, hydrogen mixtures have a higher reactivity than natural gas (NG) mixtures, burned mostly in today's gas turbine combustors. Therefore, in the present work, a combined experimental and modeling study of nitrogen-enriched hydrogen–air mixtures, some of them with a share of methane, to be representative for COG, will be discussed focusing on laminar flame speed data as one of the major combustion properties. Measurements were performed in a burner test rig at ambient pressure and at a preheat temperature T0 of 373 K. Flames were stabilized at fuel–air ratios between about φ = 0.5–2.0 depending on the specific fuel–air mixture. This database was used for the validation of four chemical kinetic reaction models, including an in-house one, and by referring to hydrogen-enriched NG mixtures. The measured laminar flame speed data of nitrogen-enriched methane–hydrogen–air mixtures are much smaller than the ones of nitrogen-enriched hydrogen–air mixtures. The grade of agreement between measured and predicted data depends on the type of flames and the type of reaction model as well as of the fuel–air ratio: a good agreement was found in the fuel lean and slightly fuel-rich regime; a large underprediction of the measured data exists at very fuel-rich ratios (φ > 1.4). From the results of the present work, it is obvious that further investigations should focus on highly nitrogen-enriched methane–air mixtures, in particular for very high fuel–air ratio (φ > 1.4). This knowledge will contribute to a more efficient and a more reliable use of low-caloric gases for power generation.

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## Figures

Fig. 1

Characteristic combustion properties. Left: laminar flame speeds of fuel–air mixtures at p = 1 bar, T0 = 373 K. Calculations (curves) for methane, NG, syngas, biogenic gas: GRI 3.0 [19]; for ethanol (10% ethanol + 90% RG): DLR [20]. Right: ignition delay times measured (symbols) and predicted (curves, DLR reaction model [20]) of fuel–air mixtures diluted in argon 1:5 at φ = 1.0, p = 4 bar, oxidizer: 79% Ar–11% O2. Fuel: H2 [21]—circles, 50% H2–50% CO [22]—squares, biogenic gas [16]—stars, ethanol [20]—dark diamond, RG + 10% ethanol [20]—triangles down; NG [21]—rhombus, and CH4—triangles.

Fig. 2

Experimental setup of the burner system

Fig. 3

Typical flames burned in air at p = 1 atm and T0 = 373 K, for two fuel–air mixtures: 20% CH4–20% H2–60% N2 (a) and (c); 80% H2–20% N2 bar (b) and (d). Fuel–air ratio: φ = 1.12 (a), φ = 1.01 (b), φ = 1.39 (c), and φ = 2.03 (d).

Fig. 4

Comparison between measured burning velocities (symbols) and calculated laminar flame speeds (curves) of fuel–air mixtures of system I, at p = 1 bar and T0 = 373 K. Fuel: 100% methane (a), 50% CH4–20% H2–30% N2 (b), 20% CH4–20% H2–60% N2 (c), and 50% H2–50% CH4 and all three mixtures given before (d). Calculations with reaction models of DLR [20]—full, GRI 3.0 [19]—dashed, Li et al. [30]—dashed–dotted, and PeWi [31]—dotted.

Fig. 5

Comparison between measured burning velocities (symbols) and calculated laminar flame speeds (curves) of fuel–air mixtures of system II, at p = 1 bar and T0 = 373 K. Fuel: 100% H2 (a), 80% H2–20% N2 (b), 60% H2–40% N2 (c), and 40% H2–60% N2 (d). Calculations with reaction models of DLR [20]—full, GRI 3.0 [19]—dashed, Li et al. [30]—dashed-dotted, and Petrova–Williams [31]—dotted.

Fig. 6

Comparison between measured burning velocities (symbols) and calculated laminar flame speeds (curves; DLR reaction model [20]) of two fuel–air mixtures (systems I and II), at p = 1 bar and T0 = 373 K

Fig. 7

Comparison between measured burning velocities (symbols) and calculated (curves) laminar flame speeds of two fuel–air mixtures (system II, and H2-RG [16]), at p = 1 bar and T0 = 373 K. Calculations with reaction models of DLR [20]—full; GRI 3.0 [19]—dashed. Fuel–air ratio φ = 1.0 (left); φ = 0.8, 1.0, and 1.5 (right).

Fig. 8

Sensitivity analysis with respect to laminar flame speed of a (50% CH4–20% H2–30% N2) air mixture (system I), for three fuel–air ratios. Calculations with reaction models: DLR [20] left; GRI 3.0 [19] right.

Fig. 9

Sensitivity analysis with respect to laminar flame speed of a (80% H2–20%N2) air mixture (system II), for three fuel–air ratios. Calculations with reaction models: DLR [20] left; GRI 3.0 [19] right.

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