The influence of the structure of perfectly premixed flames on NOx formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630 K, an adiabatic flame temperature of 1840 K, and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: • molecular diffusion of temperature and species; • flame strain; • local quench in highly strained flames and subsequent reignition; • turbulent diffusion (no preferential diffusion); • small scale mixing (stirring) in the flame front. Either no relevant influence or an increase in NOx production over that of the one-dimensional laminar flame is found. As a consequence, besides the improvement of mixing quality, a future target for the development of low-NOx burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (“flamelets”) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NOx-abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the primary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO penalty. However, the complexity of such a system with a larger flame tube area to be cooled will increase the reaction zone temperatures, so that the full advantage cannot be realized in an engine. Of greater technical relevance is the potential of a lean-mixlean combustion system within an improved thermodynamic cycle. A reheat process with sequential combustion is perfectly suited for this purpose, since, first, the required low inlet temperature of the second stage is automatically generated after partial expansion in the high pressure turbine, second, the efficiency of the thermodynamic cycle has its maximum and, third, high exhaust temperatures are generated, which can drive a powerful Rankine cycle. The higher thermodynamic efficiency of this technique leads to an additional drop in NOx emissions per power produced.

1.
Nicol
D. G.
,
Steele
R. C.
,
Marinov
N. M.
, and
Malte
P. C.
, “
The Importance of the Nitrous Oxide Pathway to NOx in Lean-Premixed Combustion
,”
ASME JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER
, Vol.
117
,
1995
, pp.
100
111
.
2.
Miller
J. A.
, and
Bowman
C. T.
, “
Mechanisms and Modeling of Nitrogen Chemistry in Combustion
,”
Prog. Energy Combustion Sci.
, Vol.
15
,
1989
, pp.
287
338
.
3.
Tennekes, H., and Lumley, J. L., A First Course in Turbulence, MIT press, 1985.
4.
Koochesfaharni
M. M.
, and
Dimotakis
P. E.
, “
Mixing and Chemical Reactions in a Turbulent Mixing Layer
,”
LFM
, Vol.
170
,
1986
, pp.
83
112
.
5.
Borghi
R.
, “
Turbulent Combustion Modeling
,”
Prog. Energy Combustion Sci.
, Vol.
14
,
1988
, pp.
245
292
.
6.
Zimont, V. L., “Theory of Turbulent Combustion of a Homogeneous Fuel Mixture at High Reynolds Numbers,” Combust. Expl. and Shock Waves, 1979, pp. 305–311.
7.
Meneveau
C.
,
Poinsot
T.
, “
Stretching and Quenching of Flamelets in Premixed Turbulent Combustion
,”
Combustion and Flame
, Vol.
86
,
1991
, pp.
311
332
.
8.
Poinsot, T., Veynante, D., and Candel, S., “Diagrams of Premixed Turbulent Combustion Based on Direct Simulation,” 23rd Symp.(Int.) on Combustion, 1993, pp. 613–619.
9.
Roberts
W. L.
,
Driscoll
J. F.
,
Drake
M. C.
, and
Goss
L. P.
, “
Images of the Quenching of a Flame by a Vortex to Quantify Regimes of Turbulent Combustion
,”
Combustion and Flame
, Vol.
94
,
1993
, pp.
58
69
.
10.
Abdel Gayed
R. G.
, and
Bradley
D.
, “
Criteria for Turbulent Propagation Limits of Premixed Flames
,”
Combustion and Flame
, Vol.
62
,
1985
, pp.
61
68
.
11.
Abdel Gayed
R. G.
, and
Bradley
D.
, “
Combustion Regimes and the Straining of Turbulent Premixed Flames
,”
Combustion and Flame
, Vol.
76
,
1989
, pp.
213
218
.
12.
Kee, R. J., Grear, J. F., Smooke, M. D., and Miller, J. A., “A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames (Chemkin Manual),” Sandia Report SAND85-8240, 1985.
13.
Correa
S. M.
, “
Turbulence-Chemistry Interactions in the Intermediate Regime of Premixed Combustion
,”
Combustion and Flame
, Vol.
93
,
1993
, pp.
41
60
.
14.
Curl
R. L.
, “
Dispersed Phase Mixing: Theory and Effects in Simple Reactors
,”
AIChE J.
, Vol.
9
, pp.
175
181
.
15.
Tonouchi, J. H., Pratt, D. T., “A Finite-Rate Macromixing, Finite-Rate Micromixing Model for Premixed Combustion,” The Combustion Institute Fall Meeting, 1995, Stanford University, Paper 95F-167.
16.
Rogg
B.
, “
Response and Flamelet Structure of Stretched Premixed Methane-Air Flames
,”
Combustion and Flame
, Vol.
73
,
1988
, pp.
45
65
.
17.
Rogg, B., “RUN-1DL; The Cambridge Universal Laminar Flamelet Computer Code,” Reduced Kinetic Mechanisms for Applications in Combustion Systems, Springer, 1993.
18.
Kampmann
S.
,
Leipertz
A.
,
Do¨bbeling
K.
,
Haumann
J.
, and
Sattelmayer
T.
, “
Two Dimensional Temperature Measurements in a Technical Combustor With Laser Rayleigh Scattering
,”
Applied Optics
, Vol.
32
, No.
30
, Oct.
1993
, pp.
6167
6172
.
This content is only available via PDF.
You do not currently have access to this content.