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Clean Air: International Journal on Energy for a Clean Environment

ISSN for PRINT: 1561-4417

Institutional price:	$451.00
Issues per year:	4

Best Paper Award Selection - Editorial Board Site

2006, Volume7

123 pages

DOI: 10.1615/InterJEnerCleanEnv.v7.i3

Issue price - $128.00

NO_x PRODUCTION IN VITIATED AND ADIABATIC COMBUSTION SYSTEMS

Y. Levy
Faculty of Aerospace Engineering, Technion— Israel Institute of Technology, Haifa, Israel

P. Arfi
Faculty of Aerospace Engineering, Technion— Israel Institute of Technology, Haifa, Israel

ABSTRACT

The accuracy of NO_x predictions in combustion systems is highly dependent on the temperature and concentration fields inside the combustor. Turbulence-chemistry interaction affects local temperature, local extinction, and hence flame stability. Some of the advanced dry low NO_x combustion systems are based on exhaust gas recirculation or staged combustion. Combustion with air diluted by combustion products is a known NO_x reduction technique, however, it may significantly deteriorate combustion stability. Therefore modeling effects of air vitiation on flame stability as well as on fuel and nitrogen chemistry are a key point for improving NO_x predictions. In the present study, a local extinction phenomenon is considered, based on a perfectly stirred reactor model, representative of the reactive fine structure. It is shown that current CFD models based on global chemistry mechanisms are unable to reproduce local extinction phenomena. However, comparisons to experimental data show that combination of a combustion model accounting for local extinction and a NO_x model allowing for nonequilibrium O concentrations could improve trend predictions. Results show that even though air vitiation by combustion products impairs flame stability, it can be counterbalanced by an increase in the gas mixture inlet temperature. In addition, the combined effect of high inlet air temperature and air vitiation enables reduction of local temperature peaks and thereby allows significant NO_x reduction. It appears that the NO_x species formation, even when based on a detailed chemistry of the hydrocarbon chemistry oxidation, is poorly represented by simplified CFD emission models. In addition, it was observed that the classic approach based on the assumption of fast fuel chemistry, combined with the O−O₂ equilibrium assumption in the emission model, may be inaccurate when finite rate kinetic effects become preponderant.