A numerical and experimental study of in-situ NO formation in laminar NH3-seeded syngas diffusion flames

by Miao Li

Institution: Iowa State University
Year: 2011
Keywords: CFD; Chemical Mechanism; Combustion; NO Formation; PLIF; Syngas; Mechanical Engineering
Record ID: 1906972
Full text PDF: http://lib.dr.iastate.edu/etd/10434



Oxides of nitrogen formed during combustion are significant threats to our environment. They result in the formation of "acid rain", smog, and depletion of the ozone layer. These combustion systems often include diffusion flames of hydrocarbon and air, where the NO can be formed at high levels (100's to 1000's of parts per million). In fuels produced from biomass or waste streams, small amounts of ammonia (NH3) can significantly enhance the production of NO. However, the chemical kinetic mechanisms for NO formation in the presence of NH3 are not well understood or validated. In the current work, a series of laminar diffusion flames of CH4/air and syngas/air are investigated using in situ measurements and detailed numerical simulations with varying levels of NH3 to understand the dominant mechanisms of NO formation. For these flames, the 2-D flame structure, as well as the 2-D NO formation and distribution within the flame are of major interest. This includes investigation of (1) the basic flame structure in the meridian plane of flames, (2) the NO distribution in the meridian plane of flames, (3) Relative contributions of each NO-formation sub-mechanism (e.g. thermal NO, prompt NO, N2O intermediate, NNH intermediate, fuel NO), (4) effects of syngas composition on NO formation, and (5) effects of fuel-bound nitrogen (such as NH3) on NO formation. Numerically, a well-validated research code - UNICORN (UNsteady Ignition and COmbustion with ReactioNs) is used to solve 2-D axisymmetric equations of continuity, momentum, enthalpy, and species. Two detailed chemical mechanisms - GRI-Mech 3.0 and Tian are incorporated into UNICORN to describe the chemical reactions in flames. Experimentally, an in-situ laser-diagnostics technique  – Planar Laser Induced Fluorescence (PLIF) is implemented to diagnose the 2-D OH concentration profiles qualitatively, and then NO concentration profiles quantitatively. Qualitative measurement of the OH radical assures agreement between the CFD simulation and experiment, in terms of flame structure. Quantitative measurement of NO concentration is compared with the CFD simulation to validate predictions with respect to NH3 concentrations in CH4/air and syngas/air flames. The amounts of NH3 in the fuel stream are varied to investigate the effects of fuel-bound nitrogen on NO formation. Two syngas mixtures (F1: 10 vol% CH4, 45 vol% CO, 45 vol% H2 vs. F2: 50 vol% CO, 50 vol% H2) are used to study composition effects. Results of the current work can be summarized as follows: 1. Both CFD and PLIF agreed well on the diffusion flame structure on the meridian plane of studied flames. 2. Both CFD and PLIF agreed well on the "one-peak" and "two-peak" structures of NO radical concentration on the centerlines of CH4/air and syngas/air diffusion flames. 3. Peak(s) of the NO radical concentration along the centerline of flames are due to different NO-formation sub-mechanisms. This depends on the amount of seeded NH3 and syngas composition (F1 vs. F2). 4. For each test/simulation condition, NO-formation…