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

Experimental and Numerical Study of NOx Formation From the Lean Premixed Combustion of CH4 Mixed With CO2 and N2

[+] Author and Article Information
K. Boyd Fackler1

Megan F. Karalus, Igor V. Novosselov, John C. Kramlich

Philip C. Malte

Department of Mechanical Engineering,  University of Washington, Seattle, WA 98105malte@u.washington.edu

1

Corresponding author.

J. Eng. Gas Turbines Power 133(12), 121502 (Sep 01, 2011) (7 pages) doi:10.1115/1.4004127 History: Received April 11, 2011; Revised April 16, 2011; Published September 01, 2011; Online September 01, 2011

This paper describes an experimental and numerical study of the emission of nitrogen oxides (NOx ) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet, stirred reactor (JSR). In this study, CH4 is mixed with varying levels CO2 and N2 . NOx measurements are taken at a nominal combustion temperature of 1800K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NOx emissions as a function of fuel dilution: (1) more NOx is produced per kg of CH4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N2 dilution increases NOx production more effectively than equivalent CO2 dilution. Chemical kinetic modeling suggests that NOx production is less effective for the mixture diluted with CO2 due to both a decrease in N2 concentration and the ability of CO2 to deplete the radicals taking part in NOx formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH4 combustion. A global CH4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH4 and CO oxidation quite well. The CFD model illustrates the flow-field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constructed using insight from spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NOx emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Diagram of experimental setup

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Figure 2

Sampling locations within the JSR

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Figure 3

Measured NOx as EI versus mass fraction of N2 or CO2 diluent in fuel stream. Temperature is maintained constant at 1800K.

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Figure 4

Measured NOx as EI versus exit gas O2 (mole %, dry). Temperature is maintained constant at 1800 K.

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Figure 5

Measured NOx as (ppm, dry) versus exit gas O2 (mole %, dry). Temperature is maintained constant at 1800 K.

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Figure 6

Profile of temperature from reactor centerline to wall, measured and predicted by CFD for CH4 combustion (w/o diluents) for exit gas O2 of 6.6% (mole %, dry)

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Figure 7

Profile of CO from reactor centerline to wall, measured and predicted by CFD for CH4 combustion (w/o diluents) for exit gas O2 of 6.6% (mole %, dry)

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Figure 8

CO and temperature contours by CFD for JSR fired on CH4 (w/o diluents) for exit gas O2 of 6.6% (mole %, dry)

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Figure 9

Chemical Reactor Network constructed from the calculated flow field within the CFD model

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Figure 10

NOx emission index predicted by CRN model: total and by four pathways. CH4 diluted with N2 .

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Figure 11

NOx emission index predicted by CRN model: total and by four pathways. CH4 diluted with CO2 .

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Figure 12

NOx production reported as emission index for each of the four mechanisms in each of the three reactor elements of the CRN model. O2 concentration is 3.6% (dry mole fraction).

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Figure 13

O atom concentration in the recirculation zone and PSB for both diluted fuels

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Figure 14

H atom concentration in the recirculation zone and turbulent flame brush for both diluted fuels

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Figure 15

CH concentration within the flame brush for CH4 diluted with both N2 and CO2

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