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Effects of CH4 content on NO formation in one dimensional adiabatic flames investigated by saturated laser-induced fluorescence and CHEMKIN modeling Yajun Zhou, Zhi-hua Wang, Yong He, Ronald Whiddon, Dongxiang Xu, Zhongshan Li, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02434 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017

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Energy & Fuels

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Effects of CH4 content on NO formation in one dimensional adiabatic flames investigated by saturated laser-induced fluorescence and CHEMKIN modeling

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Yajun Zhou†‡, Zhihua Wang†*, Yong He†, Ronald Whiddon†, Dongxiang Xu†, Zhongshan Li‡, Kefa Cen†

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†

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

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‡

Division of Combustion Physics, Lund University, P.O. Box 118, S-221 00 Lund, Sweden

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ABSTRACT: Experiments applying Saturated Laser-induced Fluorescence (LSF) technology were

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performed focusing on the influence of equivalence ratio, syngas mixture contents on NO formation in

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H2/CO/CH4/CO2/N2/O2 premixed flat flames supplied by a Heat Flux burner. Experimental data was

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extracted to validate calculation by CHEMKIN software. Both experiments and CHEMKIN calculation

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draw one conclusion that, NO mole fraction in CH4-air flame peaks at stoichiometric and φ1.3, due to

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thermal NO and prompt NO routes. All mechanisms applied can well predict NO mole fraction at fuel

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lean side, but failed at fuel rich side. NO mole fraction in syngas flame is propotional to CH4 ratio as

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shown in the experimental data, but all the mechanisms failed to predicted it with some even have a

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wrong tendency. A rate of NO production and sensitivity analysis suggests, that thermal and prompt NO

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routes play different roles in each mechanism, and modifications to mechanisms are required to improve

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NO concnetration predictions at CH4 containing syngas flame.

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KEYWORDS:

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Saturated Laser-induced Fluorescence

22

Syngas combustion

23

NO formation

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Effects of CH4 content

25

Mechanism validation

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1. Introduction

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NOx pollution triggers numerous destructive processes in the earth’s environment. NOx in the upper

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troposphere interrupts the natural cycle of ozone regeneration1 and can influence the natural greenhouse

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due to solar trapping and reduction of CH4 through complex chemical processes2. The conversion of NOx

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in the lower atmosphere to nitrate and HNO also creates acid rain which has been known to cause

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acidification of lakes and streams, deforestation and corrosion of natural and manmade structures3-5. At

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ground level, NOx affects cardiopulmonary function in humans and animals.6 When NOx, sometimes

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together with other acid gas like SOx, forms in combustion applications such as engines and burners, the

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combustion chamber itself is prone to corrosion7, 8.

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NOx is produced during combustion via three distinct pathways: thermal NOx, prompt NOx and fuel

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NOx.9 Both thermal and prompt NOx pathways arise from the presence of N2 in the combustion mixture,

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typically in the oxidizer stream, while fuel NOx results from nitrogen that is bound in the fuel. The

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thermal NOx pathway, first published by Zel’dovich10 in 1946, is composed of reactions between Oxygen

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and Hydrogen: O2=2O，N2+O=NO+N, N+O2=NO+N. This NOx pathway becomes dominant when

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flame temperature exceeds 1773 K; production of NOx by the thermal NOx pathways exhibits

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exponential growth as temperature increases above this temperature11. In 1971, Fenimore12 described the

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prompt NOx pathway which contains a cluster of reactions between N2 and CHn which eventually

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oxidize to NOx. The prompt NOx pathway is important in low temperature and fuel rich cases. NO and

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NO2 are the most important species contributing to NOx, with NO accounting for more than 90% of NOx

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in normal combustion implementations9.

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Owing to the impact of combustion related NOx on the environment, much effort has been made to

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accurately measure and model NOx production in reacting systems13-15. Numerous mechanisms, for

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example, GRI-mech 3.0, CRECK, the Mendiara and Glarborg mechanism, GDF-kin 3.0, Konnov 6.0, and

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more, have been developed to model combustion systems; most practical mechanisms include a

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customized NOx formation sub-mechanism that includes thermal NOx, prompt NOx and fuel NOx

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reaction subsets16. Despite the amount of time dedicated to improving the performance of these

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mechanisms, there is still a great potential for improvement. Christian et al. used LIF to measure NO

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mole fraction in premixed ammonia/air flames on a water-cooled stainless steel porous-plug burner at

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atmospheric pressure. They found the Mendiara and Glarborg mechanism to produce the most accurate

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prediction among various models, but noted that prediction of NO formation above the stoichiometric

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ratio needed improvement17. Giménez-López et al. experimentally and numerically studied the HCN

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radical which is an important intermediate in NO formation and removal for O2-CO2 atmosphere18. Carla 2


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et al. found a strong connection between CH radical chemiluminescence and NO formation in premixed

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ethanol flames19, which led to the identification of prompt-NO as the governing mechanism for NO

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production in their reaction system. Wang et al. published results20 of NO formation in an adiabatic H2-

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CO syngas flame, finding that several widely used mechanisms over-predict NO mole fraction in

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premixed combustion.

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Syngas is an energy carrier of interest which is created by converting solid fuels to cleaner burning

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gaseous fuels via gasification and pyrolysis9 of biomass or fossil fuel. Syngas is a mixture of different

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combustible and non-combustible components, with principle flammable components of H2, CH4 and CO.

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The exact composition of the syngas varies greatly with the energy source. The range of H2 ratio varies

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from 20% to 40%, that of CO is 10-57%21, and CH4, N2, CO2 ratios are 0.1-15%, 0.6-43%, and 2-35%

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respectively. The gas ratios investigated in this paper were chosen to represent typical coal gasification

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products22. NOx is a notable problem for syngas combustion due to the high flame temperatures and

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presence of hydrocarbon in the fuel. However, conversion of fossil fuels or biomass to syngas removes

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fuel nitrogen, NOx is only produced through the thermal and prompt NOx pathways23.

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Building on the previous research on H2-CO syngas, the present work studied the influence of CH4 on

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NO

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CH4/H2/CO/CO2/N2/O2 syngas using Saturated Laser-induced Fluorescence (LSF). The experiments

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provide a comprehensive data set for the effect syngas composition at several equivalence ratios on NO

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formation. The test conditions were modeled in CHEMKIN using several well known chemical

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mechanisms; the reactions that contribute to thermal peak and prompt peak were discussed.

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2. Methodology

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2.1 Experiment Setup

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An illustration of the experiment setup is shown in Fig. 1. The experimental system is separated into

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formation

experimentally

and

numerically.

Experiments

measured

NO

formation

in

three sections: the combustion system, laser system and signal collection system.

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2.1.1 Combustion system

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Test flames were stabilized by Heat Flux method; this burner provides a one-dimensional flat flame for

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fuels with flame speeds range from 10 cm/s to 60 cm/s. The original Heat Flux burner was announced by

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Eindhoven University of Technology24 and detailed design operational principles of this method can be

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found in references25, 26. The Heat Flux burner used in this study is modified from the initial design as

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described in our previous publication20. The modified experimental method with Teflon coating was

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validated in previous works20, 27, 28. Different water circulations were used to balance heat transferation in 3


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the burner: low temperature circulation was kept at 298 K to preheat the fresh gas prior to combustion

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procedure; high temperature circulation was kept at 358 K to replenish heat loss from flame to the metal

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plate, ensuring an adiabatic flame front. Seven T-type thermocouples are distributed across the underside

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burner plate to provide a radial temperature profile of the burner as in Ref.25, e.g., 𝑇（𝑟） = 𝑇𝑐 + a𝑟 2 . Tc is

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temperature measurement dots away from the center29. Before flame fixed in measurement, fresh gas

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supply was adjusted according to temperature information, until the radial gradient of it was zero,

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indicating a flat, approximately one-dimensional, stretch-less, adiabatic flame.

the central temperature of the burner plate, a means the parabolic coefficient, r denotes the radius of

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Each gas bottle used in this investigation was supplied by Jingong Gas Co., Ltd, China. Here, H2 is in

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99.999% purity, CO in 99.95% purity, CH4 in 99.99% purity, CO2 in 99.995% purity, N2 in 99.999%

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purity, O2 in 99.999% purity. Mass flow controllers (MFCs) from Alicat Scientific, Inc., were applied

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for different gas mass flow controlling; Agilent 34970A, from Agilent Technologies Inc., a data logger,

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was for thermocouple recording read from burner plate. The gas mixtures investigated in experiments

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were chosen to reflect realistic syngas compositions22, and the laminar burning speed for all the chosen

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fuel mixtures are within the range of the Heat Flux method. Constitution of fuel components

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(Volumetrically) investigated in all experimental cases are shown in Table 1, and air was mixed from

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bottled sources at a mixture of O2(21%) and N2(79%).

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2.1.2 Laser system

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The third harmonic output at 355nm, from a Spectra-Physics Quanta-Ray Nd:YAG laser was used to

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pump a dye laser (Coumarin 460 dye, Continuum, ND6000) producing a fundamental laser. The dye

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beam was frequency doubled, reaching the final UV wavelength around 225.4 nm, which is a suitable

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choice for the NO radical Q2(26.5)13 transition excitation in the flame. The UV laser beam had a power of

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1.5±0.05 mJ per pulse, which is sufficient for NO LSF measurement20. The laser line passed horizontally

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through flame at 10 mm above the burner (HAB) plate, and was focused by a 1000mm spherical lens at

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the plate center with a beam weight of ~300 μm. Laser scatter was avoided by shielding the laser path

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with black craft paper and a beam dump placed after the measurement zone. The NO mole fraction was

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measured with Laser-saturated fluorescence (LSF) measurement as it is considered to be insensitive to

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both laser irradiance20 and the electronic quenching rate coefficients30.

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2.1.3 Signal collection system

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The LSF signal from the measurement region was imaged at 1:1 through a spherical lens (f=+60mm)

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transmitting into Acton SP2300, a monochromator with a 1200 grooves/mm grating and blaze wavelength

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at 300 nm. It has a 100 μm wide entrance slit, which was aligned horizontally to cater laser path. The slit 4


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was fine aligned to overlap with the center of pump beam and LSF signal; positioning the slit to be

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parallel to the beam path is preferred in saturated fluorescence measurement31. Photomultiplier tube (PMT,

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-800V, R928), from Hamamatsu was attached to the exit slit of monochromator, which was set to

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1000 μm covering a spectral bandwidth ~3 nm centered at the γ(0,1) band of the NO emission spectrum.

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The PMT signal was recorded with an oscilloscope from Agilent, (Infinium DSO80604B, 6GHz). Then

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signal was integrated with a gate width of 500 ps at the peak (as shown in fig 1), and the average value of

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300 single shots was reported in each case.

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Absolute NO mole fraction, was calculated according to the following formula13: 𝑇

𝑓 (𝑇 )

𝑐 𝑁𝑎𝑏𝑠 = �𝑇 � · � 𝑓𝐵 (𝑇) � · 𝑁re 𝑐

(1)

𝐵

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where Nabs is the absolute NO mole fraction, Nre is the measured LSF signal, T represents the local flame

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temperature at 10 mm downstream, the subscript C indicates parameters in calibration flame, and fB is the

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Boltzmann factor according to difference temperature. The Heat Flux burner produces a flames with an

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adiabatic flame front. The temperature at the measurement position was predicted from modeling the

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adiabatic combustion with heat loss compensation by 100 K/cm, as Coppens et al.32 suggested.

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2.2 Kinetic models

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All numerical calculations of NO mole fraction were performed by CHEMKIN 3.7 software, applying

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the burner stabilized module, with 300 to 500 grid points to provide grid-independent results. The NO

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mole fraction at the point 10 mm downstream from the burner surface used for comparison with the

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experimental data. The initial temperature profile was provided from a freely propagating module in

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CHEMKIN. There were four mechanisms, i.e., CRECK 140733, GRI-mech 3.034, GDF-kin 3.035 and the

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Mendiara and Glarborg mechanism36, chosen specifically for their ability to predict flame characteristics

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in simple hydrocarbon fuels, for results validation.

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CRECK (1407 version) was published by the Chemical Reaction Engineering and Chemical Kinetics

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group (CRECK)

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reactions, to describe the oxidation of a wide range of hydrocarbons. The sub-mechanism used in this

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work is “C1-C3 High temperature kinetic mechanism with NOx”, with 115 species and 2142 reactions.

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. The complete model (version 1407) has 425 species and 13,532 nonreversible

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GRI-mech 3.0 is composed of 325 elementary reactions involving 53 chemical species34. It is widely

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used in premixed combustion modeling within temperature ranges between 1000 and 2500 K, pressures

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from 10 Torr to 7600 Torr, and equivalence ratios from 0.1 to 5.

5


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The GDF-Kin mechanism for natural gas combustion, was available through reference37 in 1998. GDF-

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Kin 3.0 35 includes an NOx sub-model, focusing on NCN chemistry35, 38 for prompt NO formation which

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contains 120 species and 883 reactions. It was designed to predict combustion characteristics for simple

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hydrocarbon fuel, and was validated for temperatures between 400 and 2200 K and pressures between

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0.04 and 10 Bar.39

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The Mendiara and Glarborg mechanism, first published by P. Glarborg and J.A. Miller in 1998,

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contains 97 species and 779 reactions, including HCN reactions which are important in NOx chemistry

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both in formation and removal processes18. The Mendiara and Glarborg mechanism performed well,

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showing agreement with experimental results in terms of temperature and flame front position for

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nitrogen diluted flames, as well as for radical concentration profiles in lean flame17.

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3. Calibration and validation

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Validation of the Heat-Flux burner and calibration of the LSF technique was performed prior to

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measurements of syngas cases. In the present research, the equivalence ratio 𝜙 is calculate from the

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following equation:

𝜙 = (𝐹)

(𝐹) �(𝑂)

�(𝑂) 𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐

(2)

where F/O indicates the volumetric ratio of fuel to oxidizer.

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3.1 Validation

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In order to achieve a flat, approximately one-dimensional, stretch-less, adiabatic flame, the fresh gas

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flow rate was adjusted for each case according to temperature profile, until its gradient across the Heat

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Flux burner plate reaching flat condition. In this operation, gas flow rate is equal to the laminar flame

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speed. Flame speeds for different equivalence ratios used in the CH4-air measurement sets are shown in

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fig 2(a) together with results from CHEMKIN GRI 3.0 calculation and previously reported values27, 40. All

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of the data sets were created using the Heat Flux method. In general, the present laminar flame speed

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measurements agree well with previously reported values, with peak flame speed occurring slightly rich

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of stoichiometry. The speed measured for a stoichiometric CH4-air flame is 36.1 cm/s, which agrees with

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Bosschaart’s conclusion that the stoichiometric laminar flame speed is approximately 36 cm/s. The

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differences between the multiple of reported stoichiometric flame speeds do not vary by more than ~1

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cm/s41, indicated that the Heat Flux burner is indeed producing a near adiabatic flame.

6


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NO is generated by reactions in flame front and adjacent downstream9 region of the flame, with little

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production or loss occurring in the burnt gas region, NO mole fraction in flue gas is largely constant13. To

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ensure that the measurement of NO mole fraction is performed in a stable region of the post combustion

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flow, the NO mole fraction as a function of distance from the flame front was analyzed numerically and

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experimentally. Figure 2(b) presents the NO mole fraction along the vertical axis (height above the burner,

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HAB) of CASE Eqrt.1 by CHEMKIN calculation for the four different mechanisms, together with the

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temperature profile. It was found that, NO mole fraction dramatically increases as temperature increases,

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at a compact region from 0.7 mm-3 mm HAB, above this, the increasing slope gradually decreases until

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stabilizing at a constant value after 5 mm HAB. There is a difference of calculated NO mole fraction at

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downstream for the different mechanisms. Experiments were also conducted at different heights (10mm,

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15mm, 20mm HAB) for fuel lean, stoichiometric and fuel rich flame. As shown in figure 2(c), there is

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little difference among the data obtained at different height (with scattering of no more than 0.5ppm).

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There is no trend that can be abstracted from the measured NO mole fractions versus measurement height,

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which shows essentially random discrepancy due to system uncertainty. Both numerical and experimental

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results indicate that measuring the NO mole fraction at 10mm HAB is reflective of the global NO product.

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3.2 Calibration

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Calibration was performed by seeding a known mole fraction NO gas into the reference flame (CASE

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Eqrt.2) to create a global NO mole fraction ranging from no seeding to 90 ppm. The seeding gas is

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basically Helium, with 0.29% (±0.001%) NO gas in volumetric fraction. It was customized from ZHGM

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R&D Institute of Chemical Industry Corporation. A CH4-air flame, φ = 0.8, was chosen as the reference

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flame for the calibration condition, as shown in Table 1. The intrinsic NO, generated in the CH4 flame

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was subtracted from the measurements. Each case was repeated 3 times, and the average results with

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standard deviation are shown in fig 2(d). The standard error for 3 repetitions did not exceed 6% for all

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measurements points. According to existing research19, 42, 43, there is consumption of NO by the flame,

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which leads to a linear signal response at low NO mole fraction cases while saturated response at high

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NO mole fraction cases creates a plateau initiating around 100 ppm13. In this work, saturation limits were

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done evaluated by increasing the NO mole fraction in 15 ppm per step from non-seeded to 90 ppm. The

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calibration showed the onset of saturation at around 60ppm. Still, every measurement was performed at

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NO mole fraction lower than linear threshold, so NO fluorescence signal intensity versus NO seeding in

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flame was regarded as a linear relationship here.

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4. Results and Discussion

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4.1 NO formation routes in CH4-containing syngas flame 7


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Measurements of NO in a CH4-air premixed flame at various equivalence ratios were conducted and

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are shown, with uncertainty, together with simulation predictions from 4 different kinetic in fig 3. These

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data are also partially included for calibration in previously published work20. In this work, a new

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mechanism is included, CRECK 1407 C1-C3 mechanism, which performed well in subsequent test

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conditions. As discussed in the previous paper20, experimental uncertainty includes that from uncertainty

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in the calibration (~2.2%) and variance in the measurement techniques. Uncertainty is presented with data

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as error bars; generally, there was no more than 9% standard error except fuel lean cases with a very low

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absolute value. As shown in figure 3, an NO peak that is attributable to the thermal NO pathway is found

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to occur at the stoichiometric ratio and a second NO peak attributable to the prompt NO peak at around

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φ = 1.3. All four chemical mechanisms produced reasonable predictions of NO mole fraction at fuel lean

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conditions, especially GRI-mech 3.0 and GDF Kin 3.0. As the equivalence ratio is increased, the GRI

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prediction starts to deviate from the experiment value; similarly, CRECK 1407 gives reasonable

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prediction of the NO mole fraction below φ = 1.1, and then rapidly separates from the measured NO

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values. GDF Kin 3.0 was the most accurate mechanism regarding NO prediction; however, it also over-

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predicts NO mole fraction above φ 1.2. In the φ = 1.3 case, the best prediction was from the M&G

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mechanism which was 8.4ppm (23%) away from experimental data. So under fuel rich conditions, no

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mechanism in current set can provide a reasonable prediction.

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The 10 most influential reactions for NO Rate of Production in each mechanism were picked out for

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further discussion, as shown in table 2. Due to the development history and choices of the respective

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research groups in developing their chemical kinetic mechanisms, the top 10 reactions vary from

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mechanism to mechanism, case to case.

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7 reactions, R1, R3, R4, R5, R7, R9, R13, (reactions in italic had negative influence on NO production)

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were found to be significant for NO production in all four mechanisms. Reactions R5 and R9 show the

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thermal NO inter-conversion of NO-NO2 by consuming HO2 and H and producing OH radicals, which

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facilitates NO generation by reactions R2 and R3. Consumption of O radicals promotes the product side

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of reaction R4 which generates more NO. Reaction R7 is the conversion of HNO to NO, while R13

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consume NO for HCN formation. In CRECK and GDF mechanism, there is an NO production cycle

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HCNO-HCO-HNO-NO (R14, R11, R6, R7, R8)18, which consume O and OH radicals. Reactions R2, R12

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and R15 become important in fuel rich cases, while R10 and R16 are most significant in fuel lean

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conditions in the CRECK mechanism.

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In the GDF mechanism, two reactions, R18 (NO-HNO) and R20 (NCN-NO) are enhanced. Reaction

240

R18 converts NO to HNO; however, this is offset by R7, R8 and R17 whereby HNO is converted to NO. 8


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In all 8 cases, the biggest loss of NO is through reaction R6 (NO-HNO), while the biggest contribution to

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NO is through the prompt NO reaction, R7. Reaction R3 was enhanced as the equivalence ration was

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varied from fuel lean to fuel rich, and contributes at almost an equal value as R7 in case 1.3 and 1.4.

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Compared with former work20, which considered low NO mole fractions produced in an H2/CO flame,

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NNH and N2O pathways were never important in NO generation in cases of these work.

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In the GRI mechanism, the top 10 reactions show less of a cascade or cycle of NO

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production/consumption. At fuel lean side, the thermal pathways R4, R5 and R9 dominate the total NO

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production. As the equivalence ratio increases, contribution by both R5 and R9 are weakened (relative to

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total amount) while the prompt NOx pathway R7 and R24 are strengthened and R3 is enhanced at

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stoichiometry and decreased at rich conditions. Instead of R8, a NNH reaction, R22, moves into the top

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10 most significant reactions at fuel lean conditions. The over-prediction of NO mole fraction at

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stoichiometric and fuel rich conditions may be due to the weak contribution of R14 and absence of

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reaction R6 which are the key consumption reactions in both the CRECK and GDF mechanism.

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Reactions R3, R4, R5, R7, R9 play the same role in the M&G mechanism as in the GRI mechanism,

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while more reactions involving hydrocarbons (R29-31) are promoted into top 10. Reaction R30 is the

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biggest consumption reaction across the entire φ set, which suggests that this reaction may be the reason

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for the under-estimate of NO mole fraction in all 8 cases. One interesting occurrence is that reaction R31

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begins as a loss pathway for NO initially during combustion, and then abruptly switches to an NO

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production pathway at around 0.068cm HAB. This phenomenon corresponds to the temperature change

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in the combustion system, which sharply increased downstream. Thus, reaction R31 initially proceeds to

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the right, generating CH3CH2ONO and consuming NO, and then proceeds to left above a threshold

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temperature, decomposing the NO fixed hydrocarbon and producing NO.

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Considering the large prediction deviation at the second peak around φ = 1.3, a first-order sensitivity

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analysis was done for case 7. The top 10 reactions with the largest sensitivity coefficients for each of the

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four mechanisms are listed in Table 3. One notable reaction is R5, which is of significance in NO

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consumption in all mechanisms, with a coefficient of -0.96, -0.83, -1.22 and -0.34 for CRECK, GDF, GRI

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and the M&G mechanism respectively. Reaction R40 is listed in the first 3 mechanisms, but fall out of

268

top 10 of the M&G mechanism owing to the inclusion of a large number of reactions with negative

269

sensitivity coefficient (R39, R43, R53). As noted above, the impact of these inhibitors may be the cause of

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the NO underestimate seen in the M&G mechanism. The GDF mechanism produced a reasonable result

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in which NO generation is promoted through an NCN pathway(R41, R42), CH2 pathway (R37, R38) and

272

reaction R35. Consumption of NO via reactions R5 and R40 are a somewhat weakened in comparison

273

with CRECK and GRI mechanism. In evaluation of the two over-predicting mechanisms, CRECK and 9


Energy & Fuels

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274

GRI, large positive values were found for specific NO producing reactions. For CRECK, reaction R47 is

275

found to have a value of 0.92 and for the GRI mechanism, reaction R48 is found to have a value of 0.93.

276

As a general rule, each of the four mechanisms predict the NO mole fraction in fuel lean conditions with

277

reasonable accuracy and locate the thermal NO peak near stoichiometry. Additionally, the presence of a

278

prompt NO peak is predicted in all cases, but most failed in predicting NO value under fuel rich

279

conditions.

280

The first-order sensitivity analysis which was performed on the CRECK and GDF mechanisms

281

indicates that these two mechanisms share 8 reactions in common (fig.4). The coefficient of thermal NOx

282

reaction, R58, decreases with equivalence ratio, while prompt NO, reaction R40, increases. The

283

sensitivity coefficient of reactions R34, R56, R5 and R54 peak near stoichiometry; these reactions

284

correspond to temperature variation. Reaction R38 is of interest, since its coefficient is proportional to

285

equivalence ratio in the GDF mechanism and inversely proportional to equivalence ratio in the CRECK

286

mechanism. Among reactions of the GDF mechanism, R35, R36, R39, R42 vary in concert with the

287

temperature profile, R37, R55, R62 and R41 increase together with the equivalence ratio increase. In

288

CRECK set, R43, R44, R61 are temperature sensitive, while some reactions sensitive to equivalence ratio

289

were strengthened and promoted into the top 10 reactions, especially the proportional reactions R46, R47,

290

R59 and R60. This may lead to over-prediction of NO at fuel rich conditions.

291

4.2 Effect of CH4 ratio variation on NO formation

292

Following calibration and validation of the four chemical kinetic models, experiments were performed

293

to measure the NO mole fraction in syngas flames containing various CH4 ratios at 3 representative

294

equivalence ratios: 0.8 (fuel lean, CASE Fuel.1-5), 1.0 (stoichiometric, CASE Fuel.6-10) and 1.3 (fuel

295

rich, CASE Fuel.11-15). The CH4 ratio in the fuel mixture varied from 0 to 21.3% while the CO ratio

296

varied from 21.3% to 0 so that a constant ratio of flammable gases was maintained. Other components

297

were kept at a constant ratio in the fuel blend for all cases, as shown in table 1, i.e., ratio of Hydrogen,

298

CO2, and N2 is 24.8%, 11% and 42.9% respectively.

299

The NO mole fraction was measured by LSF, and experimental data were used to compare against the

300

four previously discussed chemical mechanisms, as shown in figure 5(a)-(c). The experimental data

301

points are shown together with error values, with isolated measurement points connected by a black line

302

to describe the changing tendency. Unlike the predictions introduced above for CH4-air flames, each of

303

the four mechanisms poorly predict the NO mole fraction for most cases. In addition to predicting

304

unreasonable values for NO at all 3 representative equivalence sets, the mechanisms also may predict a

305

completely different trend in NO mole fraction as a function of the CH4 mixture fraction, even in lean 10


Page 11 of 41

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Energy & Fuels

306

conditions. Relative to the predictions, GDF and CRECK mechanisms more correctly predicted the trend

307

in NO production with CH4 mixture fraction with lower deviation between the measurement and

308

predictions. The GDF mechanism over-predicted NO mole fraction in high CH4 cases at lean conditions

309

and under-estimated it in low CH4 cases. It showed increasingly greater deviation as the equivalence ratio

310

and CH4 ratio increase. The CRECK mechanism produced the most accurate curve for NO mole fraction

311

in all cases, though the predicted values were slightly shifted below the experimental data in all three

312

equivalence ratio series. The deviation between measurement and models is worse as the CH4 ratio

313

increases, just as in the GDF mechanism, reaching a maximum deviation of 28%. Unlike the former

314

work20, in which all mechanisms over-predicted NO mole fraction in CO/H2 syngas, the mechanisms

315

showed under-estimation in most cases in this work (syngas with CH4).

316

To determine the influence that CH4 exerts on NO formation, a sensitivity analysis was done for the

317

GDF and CRECK mechanism stoichiometric series, shown in figure 5(d)-(f). First order sensitivity

318

coefficients from the GDF mechanism show a good continuity in all experiments. A total of 15 reactions

319

were listed, among which 11 reactions showed only a slight change in sensitivity coefficient across the

320

test conditions, 3 reactions become important when CH4 is absent and 1 reaction becomes prominent at

321

the highest CH4 ratio. Comparing these reactions with former work20, R5, R18, R34, R35, R55, R58, R62,

322

R63 and R68 are again notable in NO production. The prompt NO reactions involving hydrocarbons: R36,

323

R38, R39, R42, and R57, are promoted to top 10 for these CH4-syngas cases, replacing the OH reactions

324

in CO/H2 syngas. The CRECK mechanism shares 3 reactions in common with the GDF mechanism: R34,

325

R38 and R57. The reactions populating the top 10 reactions vary from case to case, especially for the zero

326

CH4 case. For clarity, only 2 reactions from CASE Fuel.6 were included in the comparison in Figure 5(e),

327

with the value adjusted to fit the graph (true values are labeled above the respective bar). Reaction R34

328

showed excessive positive contribution in the non-CH4 case by CRECK mechanism, while it was

329

negative in the other cases for the CRECK mechanism and in all cases for the GDF mechanism. The

330

whole set of top 10 reactions of non-CH4 case for the CRECK 1407 “C1-C3 High temperature kinetic

331

mechanism with NOx” is listed in figure 5(f). Unsurprisingly, NO formation is vastly different with and

332

without hydrocarbons9, having only R43 and R34 carrying over from the reactions in figure 5(e). Unlike

333

the sensitivity coefficients mentioned before, which are in the range of -1 to 1, values in non-CH4 case

334

change over a much greater span, from -23.2 to 4.3. As discussed before, a cluster of reactions involving

335

OH radicals become important in the absence of CH4. Additionally, a calculation with the CRECK 1407

336

“H2/CO kinetic mechanism” was done for the same case, CASE Fuel.6. Again, there is little continuity of

337

reaction list with that of the CH4-syngas cases, though R34, R43, R44, R56 do appear in the list. The

338

same case calculated by “C1-C3 High temperature kinetic mechanism with NOx” and “H2/CO kinetic 11


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Page 12 of 41

339

mechanism” yield similar values of 5.97 ppm and 6.52 ppm respectively, but share only R34 and R43 in

340

common from the sensitivity analysis.

341

The calculated temperature profiles of case Fuel.3, 8 and 11-15 are plotted in figure 6, with the region

342

of rapid temperature variation occurring in the 0-3mm HAB zoomed in. Case 3, 8, 13 have the same fuel

343

composition but different equivalence ratios, case 11-15 have the same equivalence ratio and different

344

CH4 ratio. All the temperature profiles exhibit a sharp increase at 0-1.5 mm HAB, and then plateau and

345

slightly decrease at downstream. At 10 mm HAB (which corresponds to the measurement height),

346

temperature ranks as case8>case15>case14>case13~case3>case12>case11. In case 3, with low

347

temperature and low fuel ratio, both thermal and prompt NO routes are weak, resulting in low NO mole

348

fraction about 4.5 ppm. At stoichiometric case (CASE 8), flame temperature increases so that the thermal

349

NO route governs total NO formation (11 ppm). As equivalence ratio increases, flame temperature

350

decreases due to uncompleted combustion, and thermal pathway weakened while the prompt pathway

351

strengthened, leading to the prompt peak (12.5 ppm) around φ = 1.3 (CASE13).

352

For the different CH4 ratio sets, the starting location of temperature increase was delayed and the slope

353

in the 0-1mm region became smaller as CH4 ratio increased from 0 to 21.3%. However, cases with higher

354

CH4 ratio display a wider region of increase, Case 15 didn’t plateau until 1.5 mm HAB while Case 11

355

plateaued around 1.0 mm HAB. This phenomenon may due to the higher specific heat capacity of CH4 (2-

356

3 times that of CO at the combustion temperature)44,

357

oxidization is the last and the key heat release step of hydrocarbon combustion). Consequently, high CH4

358

cases gradually reach peak temperature, producing a high NO mole fraction in the plateau region.

45

and different heat release characteristics(CO

359

A comparison among the top 10 reactions of highest rate of product NO from the four mechanisms for

360

case Fuel. 13 are presented in table 4. These four mechanisms share 4 positive reactions in the set,

361

NH+O=NO+H, N+OH=NO+H, N+O2=NO+O, and HNO+H=NO+H2. Both thermal NOx reaction

362

N+O2=NO+O and prompt NOx reactions contributed to the total NO production. Unlike the other three

363

mechanisms, reaction HNO+H=NO+H2 in the GDF mechanism was the most positive, even stronger than

364

N+OH=NO+H. In the CRECK mechanism, N+O2=NO+O was ranked second in positive contribution,

365

which shows that thermal NOx route was predicted to be more significant in this mechanism than in other

366

three mechanisms. In addition to the reactions that were shared between the four mechanims,

367

NH+OH=NO+H2 and NCO+O=NO+CO were important NO contributors in the CRECK mechanism,

368

HNO+OH=NO+H2O, NO2+H=NO+OH and NCN+O=CN+NO were important in the GDF mechanism.

369

The M&G and GRI mechanisms had more similarity in the top 6 positive reactions, sharing

370

NO2+H=NO+OH and NNH+O=NH+NO in common together with the four previously noted shared

371

reactions. 12


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Energy & Fuels

372

In stark contrast to positively contributing reactions, the most significant negative reactions varied from

373

mechanism to mechanism. NO was evenly consumed by four reactions: CH+NO=HCN+O,

374

HCCO+NO=HCNO+CO, HCO+NO=CO+HNO and C+NO=N+CO in the CRECK collection. Reactions

375

HCO+NO=CO+HNO, CH+NO=HCN+O, NO+HO2=NO2+OH, and NO+H+M=HNO+M were showed

376

up in two of the model predictions. Unique reactions, CH2+NO=H+HNCO in the GRI mechanism and

377

CH3CH2O+NO=CH3CHO+HNO in M&G mechanism also contributed to NO consumption. The NO

378

consumption was predicted to be much stronger by the GDF mechanism than the others, where two

379

negative reactions yield a consumption at a rate order of 10-7mol/cm3-s. This may lead to the under-

380

estimation in total NO mole fraction observed for the GDF mechanism.

381

5. Conclusion

382 383

Both experiments and CHEMKIN calculation indicate that NO mole fraction in CH4-air flame peaks at stoichiometric and φ1.3, due to thermal NO and prompt NO routes, respectively.

384

All four mechanisms can adequately predicted the thermal peak and NO mole fraction at fuel lean

385

conditions, but show a bad performance at fuel rich conditions. An NO-NO2 cycle was included in the top

386

10 reactions of rate of NO productions for all mechanisms. The dominant reactions are NO-NO2 and

387

HNO-NO pathways for the CRECK and GDF mechanisms, while reactions of NO-NO2 and reactions

388

with hydrocarbons contribute the most in the GRI and M&G mechanisms respectively. The strenthening

389

of reactions that are sensitive to equivalence ratio in the GDF mechanism may lead to the over-prediction

390

at fuel rich conditions.

391

The NO mole fraction in the syngas flame increases as the CH4 ratio increases, but all the mechanisms

392

failed to predicted the value. The GRI and M&G mechanisms over-predicted NO at fuel lean conditions

393

and under-estimated it at fuel rich conditions, while the CRECK and GDF mechanisms under-estimate it

394

in most cases. The CRECK mechanism gives the most accurate NO profile, though the deviation reached

395

28% from the measurement. However, an analysis of the relatively better performing CRECK and GDF

396

mechanisms shows that GDF has greater continuity of reactinos across the measurement conditions, while

397

the CRECK mechanism was more random. A research on Rate of Prodcut NO showed that the strong

398

consumption of NO predictied by the GDF mechanism may result in its under-estimation.

399

Acknowledgments

400

The authors acknowledge the support of the National Natural Science Foundation of China (51422605),

401

Program of Introducing Talents of Discipline to University (B08026). Yajun Zhou also acknowledges the 13


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402

Chinese Scholarship Council (CSC) for sponsoring abroad, and Zhongshan Li acknowledges the Swedish

403

Energy Agency.

404

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(1)Rabl, A.; Eyre, N., Environment International 1998, 24, (8), 835-850. (2)Jain, A. K.; Tao, Z. N.; Yang, X. J.; Gillespie, C., Journal Of Geophysical Research-Atmospheres 2006, 111, (D6). (3)Mason, B. J., Acid rain; its causes and its effects on inland waters. Clarendon Press: Oxford, 1993. (4)Mason, B. J., The surface waters acidification programme. Cambridge University Press: London, 1990. (5)Chameides, W. L.; Li, X. S.; Tang, X. Y.; Zhou, X. J.; Luo, C.; Kiang, C. S.; St John, J.; Saylor, R. D.; Liu, S. C.; Lam, K. S.; Wang, T.; Giorgi, F., Geophysical Research Letters 1999, 26, (7), 867-870. (6)Mauzerall, D. L.; Sultan, B.; Kim, N.; Bradford, D. F., Atmospheric Environment 2005, 39, (16), 28512866. (7)Marchand, D. Method and apparatus for reducing acid pollutants in smoke US4704972 A, 1987. (8)Huijbregts, W. M. M.; Leferink, R. G. I., Anti-Corrosion Methods and Materials 2004, 51, (3), 173-188. (9)Cen, K. F.; Yao, Q.; Luo, Z.; Li, X., Advanced combustion theory. Zhejiang University Press: Hangzhou, 2002. (10)Zeldovich, J., Acta Physiochimica U.R.S.S., XXI, Academy of Science of the USSR 1946, 21, (4), 577. (11)Liu, Y. W., zhongguo., Oil-gasfield surface engineering 2007, 26, (4), 32-33. (12)Fenimore, C. P., Symposium (International) on Combustion 1971, 13, (1), 373-380. (13)Li, B.; He, Y.; Li, Z. S.; Konnov, A. A., Combust. Flame 2013, 160, (1), 40-46. (14)Watson, G. M. G.; Munzar, J. D.; Bergthorson, J. M., Energy & Fuels 2013, 27, (11), 7031-7043. (15)Watson, G. M. G.; Munzar, J. D.; Bergthorson, J. M., Fuel 2014, 124, 113-124. (16)Whitty, K. J.; Zhang, H. R.; Eddings, E. G., Combustion Science and Technology 2008, 180, (6), 11171136. (17)Brackmann, C.; Alekseev, V. A.; Zhou, B.; Nordström, E.; Bengtsson, P.-E.; Li, Z.; Aldén, M.; Konnov, A. A., Combust. Flame 2016, 163, 370-381. (18)Gimenez-Lopez, J.; Millera, A.; Bilbao, R.; Alzueta, M. U., Combust. Flame 2010, 157, (2), 267-276. (19)Marques, C. S. T.; Barreta, L. G.; Sbampato, M. E.; dos Santos, A. M., Experimental Thermal and Fluid Science 2010, 34, (8), 1142-1150. (20)Wang, Z.; Zhou, Y.; Whiddon, R.; He, Y.; Cen, K.; Li, Z., Combust. Flame 2016, 164, 283-293. (21)Clarke Energy. https://www.clarke-energy.com/synthesis-gas-syngas/ (22)YangTianming. Research on Low NOx Burning Technologies for Hydrogen-rich Syngas. North China Electric Power University, 2012. (23)Chaos, M.; Dryer, F. L., Combustion Science and Technology 2008, 180, (6), 1053-1096. (24)Degoey, L. P. H.; Vanmaaren, A.; Quax, R. M., Combustion Science And Technology 1993, 92, (1-3), 201-207. (25)Bosschaart, K. J. Analysis of the Heat Flux Method for Measurinng Burning Velocities. Technische Universiteit Eindhoven, Eindhoven University Press, 2002. (26)Bosschaart, K. J.; de Goey, L. P. H., Combust. Flame 2003, 132, (1-2), 170-180. (27)Weng, W.; Wang, Z.; Liang, X.; Huang, Z.; Zhou, J.; Cen, K., Proceedings of the Chinese Society of Electrical Engineering 2013, 33, (8), 74-80. (28)Wang, Z. H.; Yang, L.; Li, B.; Li, Z. S.; Sun, Z. W.; Aldén, M.; Cen, K. F.; Konnov, A. A., Combust. Flame 2012, 159, (1), 120-129. (29)W.B. Weng, Z. H. W., Y. He, Y.J. Zhou, J.H. Zhou, K.F. Cen In H2/CO Syngas Laminar Burning Velocity Measurement Using Teflon Coated Heat Flux Burner, European Combustion Meeting, 2013; 2013. 14


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(30)Cooper, C. S.; Ravikrishna, R. V.; Laurendeau, N. M., Applied Optics 1998, 37, (21), 4823-4833. (31)Daily, J. W., Applied Optics 1977, 16, (3), 568-571. (32)Coppens, F. H. V.; De Ruyck, J.; Konnov, A. A., Experimental Thermal and Fluid Science 2007, 31, (5), 437-444. (33)Cuoci, A.; Frassoldati, A.; Stagni, A.; Faravelli, T.; Ranzi, E.; Buzzi-Ferraris, G., Energy & Fuels 2013, 27, (2), 1104-1122. (34)Institute, G. R. GRI mech home page. http://combustion.berkeley.edu/gri-mech/releases.html (2015.03.30), (35)Lamoureux, N.; Desgroux, P.; El Bakali, A.; Pauwels, J. F., Combust. Flame 2010, 157, (10), 1929-1941. (36)Mendiara, T.; Glarborg, P., Combust. Flame 2009, 156, (10), 1937-1949. (37)Sick, V.; Hildenbrand, F.; Lindstedt, P., Quantitative laser-based measurements and detailed chemical kinetic modeling of nitric oxide concentrations in methane-air counterflow diffusion flames. 1998; p 1401-1409. (38)El bakali, A.; Pillier, L.; Desgroux, P.; Lefort, B.; Gasnot, L.; Pauwels, J. F.; da Costa, I., Fuel 2006, 85, (7-8), 896-909. (39)Venot, O., 2012. (40)Dyakov, I. V.; Konnov, A. A.; Ruyck, J. D.; Bosschaart, K. J.; Brock, E. C. M.; De Goey, L. P. H., Combustion Science and Technology 2001, 172, (1), 81-96. (41)Bosschaart, K. J.; de Goey, L. P. H., Combust. Flame 2004, 136, (3), 261-269. (42)Ravikrishna, R. V.; Cooper, C. S.; Laurendeau, N. M., Combust. Flame 1999, 117, (4), 810-820. (43)Thomsen, D. D.; Laurendeau, N. M., Combust. Flame 2001, 124, (3), 350-369. (44)Methan. http://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Mask=1 (45)Carbon monoxide. http://webbook.nist.gov/cgi/cbook.cgi?ID=C630080&Type=JANAFG&Table=on

470

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471

Page 16 of 41

Tab.1.Volumetric fraction of fuel components used in experimental cases.

Series

Case

H2

CO

CH4

CO2

N2

Note

Eqrt.1-8

0

0

1

0

0

Φ=0.7-1.4

Fuel.1/6/11

24.8

21.3

0

11

42.9

CH4 Fuel.2/7/12

24.8

16.3

5

11

42.9

Fuel.3/8/13

24.8

11.3

10

11

42.9

Fuel.4/9/14

24.8

6.3

15

11

42.9

Fuel.5/10/15

24.8

0

21.3

11

42.9

Validation/ CH4-Air φ Syngas varies

16


Φ= 0.8,1.0,1.3

Page 17 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels Tab.2. List of top 10 reactions of Rate of Production NO in each case (Case1-8). Numbers 0.7-1.4 at first line: equivalence ratio (φ) for each premixed gas blend; Numbers in colored cells: Reaction No. from each corresponding mechanism which was labeled at the top of the column (CRECK, GDF, GRI and P&M); Red colored cell: reactions prompt in NO product; Green colored cell: reactions consumption NO; Grey colored cells: reactions appears in all 4 mechanisms.

Reactions in CRECK

Reactions in GDF

CH4-air Equivalence Ratio variation Reactions

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30

472

R31 R32 R33

NH+O=NO+H NH+OH=NO+H2 N+OH=NO+H N+O2=NO+O HO2+NO=NO2+OH HCO+NO=CO+HNO HNO+H=H2+NO HNO+OH=NO+H2O NO2+H=NO+OH NCO+O=NO+CO HCNO+O=HCO+NO C+NO=CO+N CH+NO=HCN+O HCCO+NO=HCNO+CO HCCO+NO=HCN+CO2 CH3+NO2=CH3O+NO HNO+O=NO+OH H+NO+M=HNO+M C+NO=CN+O C+NO=CN+O NO+O+M=NO2+M NNH+O=NH+NO CH+NO=N+HCO CH2+NO=H+HNCO CH3+NO=HCN+H2O N+CO2=NO+CO HNO+O2=HO2+NO CH3+HNO=NO+CH4 CH3OO+NO=NO2+CH3O CH3CH2O+NO=CH3CHO+HNO CH3CH2O+NO(+M)=CH3CH2ON O(+M) NCN+OH=HCN+NO H2CN+O2=CH2O+NO

17


Reactions in GRI

Reactions in P&M

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

Energy & Fuels

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473 474 475

Tab.3. Top 10 reactions of the highest sensitivity coefficients in CASE Eqrt.7. Figures in black is positive, figures in red color mean negative.

CH4-air φ 1.3 Reactions H+O2+M=HO2+M

CRECK -0.17598

GDF 0.27813

CH3+O=CH2O+H

-0.40293

CH2+H=CH+H2

P&M

-0.29421

H+HO2=OH+OH CH2+OH=CH+H2O

GRI

0.40278 -0.22734

0.28524 0.2157

CH+H=C+H2

0.33532

0.33907

-0.18412

-0.38276

CH+H2O=CH2O+H

-0.29133

-0.17508

-0.31562

NO+HO2=NO2+OH

-0.96107

-0.82678

-1.22401

-0.34769

NCN+H=HCN+N

0.28509

-0.33928

CH+N2=NCN+H

0.51124

0.8423

H+O2=OH+O

-0.40986

HO2+HO2=H2O2+O2

0.37985

H+CH3(+M)=CH4(+M)

-0.31853

NO+HO2=NO2+OH

9.29E-05

HCN+O=NCO+H

0.18804

CH+N2=HCN+N

0.91963

HCCO+NO=HCNO+CO

-0.63168 0.32449 0.48931

-0.22171

H+NO+MHNO+M

-0.3817

HNO+HH2+NO

0.28681

CH+N2(+M)HCNN(+M)

0.92662

O+CH3H+CH2O H+HO22OH

-0.10094 0.256

OH+CH4CH3+H2O

0.35364

CH+H2H+CH2

0.24712

H+O2(+N2)=HO2(+N2)

0.72297

H2CN+O2=CH2O+NO

0.49768

476 477

18


Page 18 of 41

Page 19 of 41

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Energy & Fuels

478 479 480

Tab.4. Top 10 reactions of the highest rate of product NO in CASE Fuel. 13. Figures in black is positive, figures in red color mean negative.

Rate of product NO （mol/cm3-s） NH+O=NO+H NH+OH=NO+H2 N+OH=NO+H N+O2=NO+O HCO+NO=CO+HNO HNO+H=NO+H2 NCO+O=NO+CO C+NO=N+CO CH+NO=HCN+O HCCO+NO=HCNO+CO HNO+OH=NO+H2O NO+HO2=NO2+OH NO+H+M=HNO+M NO2+M=NO+O+M NCN+O=CN+NO NO2+HNO+OH NNH+ONH+NO CH2+NOH+HNCO N+CO2NO+CO CH3CH2O+NO=CH3CHO+HNO

CRECK E-8 E-8 E-7 E-8 -E-8 E-8 E-8 -E-8 -E-8 -E-8

GDF GRI E-8 E-8

P&M E-8

E-7 E-8 -E-7 E-7

E-7 E-8 -E-8 E-8

E-7 E-7 E-7

-E-8 E-8 -E-8 -E-7 -E-7 E-8

-E-8 -E-8

E-8 E-8 -E-8 E-8

-E-8

E-8 E-8

-E-8

481 482

19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

483 484

Figure Caption

485 486 487

Fig.1. Schematic diagram of the experiment setup. UV laser passes through the flame 10mm above the burner plate. The spectrometer entrance is parallel to LSF signal horizontal center line. Inset pictures show the flame image and LSF signal (the upper pink trace) on oscilloscope screen.

488 489 490 491

Fig.2. Pre-experiments results. (a) Flame speed validation with CH4-air flame; (b)NO accumulated along the vertical axial by four mechanisms and temperature profile for CASE Eqrt.1; (c)Height independence validation for fuel lean, stoichiometric, and fuel rich cases; (d)LSF signal versus seeded NO mole fraction in calibration.

492 493 494

Fig.3. NO mole fraction versus equivalence ratio in premixed CH4-air flames at 10mm HAB. Measured values are shown together with error bars as scatters, and calculation by 4 mechanisms are plot in fitted lines.

495 496

Fig.4. First-order sensitivity coefficients analysis from fuel lean, stoichiometric, and fuel rich case (CASE Eqrt. 1,4,7). (a)Results by GDF-Kin 3.0 mechanism; (b)Results by CRECK 1407 C1-C3 mechanism.

497 498 499 500 501 502 503

Fig.5. Results for CH4 ratio various cases in syngas flame at 3 typical equivalence ratio 0.8 (fuel lean, CASE Fuel.1-5), 1.0 (stoichiometric, CASE Fuel.6-10) and 1.3 (fuel rich, CASE Fuel.11-15): (a) Experimental and Numerical NO mole fraction for CASE Fuel.1-5; (b) Experimental and Numerical NO mole fraction for CASE Fuel.6-10; (c) Experimental and Numerical NO mole fraction for CASE Fuel.1115;(d) First-order sensitivity coefficients analysis for GDF-Kin 3.0 mechanism; (e) First-order sensitivity coefficients analysis for CRECK 1407 C1-C3 mechanism; (f) First-order sensitivity coefficients analysis for CASE Eqrt.6 by CRECK 1407 C1-C3 mechanism.

504

Fig.6. Given temperature profile in CASE Fuel. 3, 8, 11-15.

505

20


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Energy & Fuels

506 507 508 509

Nd：YAG Laser

Gas Supply N2

CO2

CH

CO

H2

Oscilloscope Dye Laser (450.923nm→ 225.462nm)

510 511 512

N2

O2

513

PMT

514

Spectrograph Spherical Lens Beam Dump

Fig. 1. Schematic diagram of the experiment setup. UV laser passes through the flame 10mm above the burner plate. The spectrometer entrance is parallel to LSF signal horizontal center line. Inset pictures show the flame image and LSF signal (the upper pink trace) on oscilloscope screen.

21


Heat Flux Burner

Rectangular Prism

Energy & Fuels

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Page 22 of 41

515 516 517 518

(b)

(a)

(c)

(d)

Fig. 2. Pre-experiments results. (a) Flame speed validation with CH4-air flame; (b)NO accumulated along the vertical axial by four mechanisms and temperature profile for CASE Eqrt.1; (c)Height independence validation for fuel lean, stoichiometric, and fuel rich cases; (d)LSF signal versus seeded NO mole fraction in calibration. 22


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Energy & Fuels

519 520 521 522 523 524 525

Fig. 3. NO mole fraction versus equivalence ratio in premixed CH4-air flames at 10mm HAB. Measured values are shown together with error bars as scatters, and calculation by 4 mechanisms are plot in fitted lines.

23


Energy & Fuels

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526 527

(a)

(b)

Fig.4. First-order sensitivity coefficients analysis from fuel lean, stoichiometric, and fuel rich case (CASE Eqrt. 1,4,7). (a)Results by GDF-Kin 3.0 mechanism; (b)Results by CRECK 1407 C1-C3 mechanism.

24


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528

Energy & Fuels

(a)

(b)

(d)

(e)

(c)

(f)

Fig.5. Results for CH4 ratio various cases in syngas flame at 3 typical equivalence ratio 0.8 (fuel lean, CASE Fuel.1-5), 1.0 (stoichiometric, CASE Fuel.6-10) and 1.3 (fuel rich, CASE Fuel.11-15): (a) Experimental and Numerical NO mole fraction for CASE Fuel.1-5; (b) Experimental and Numerical NO mole fraction for CASE Fuel.6-10; (c) Experimental and Numerical NO mole fraction for CASE Fuel.11-15;(d) First-order sensitivity coefficients analysis for GDFKin 3.0 mechanism; (e) First-order sensitivity coefficients analysis for CRECK 1407 C1-C3 mechanism; (f) First-order sensitivity coefficients analysis for CASE Eqrt 6 by CRECK 1407 C1-C3 mechanism 25


Energy & Fuels

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529 530 531

Fig.6. Given temperature profile in CASE Fuel. 3, 8, 11-15.

26


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Energy & Fuels

Nd：YAG Laser (355nm)

Gas Supply N2

CO2

CH4

CO

H2

Oscilloscope Dye Laser (450.923nm→ 225.462nm)

N2

O2

PMT Spectrograph Rectangular Prism Spherical Lens Beam Dump


Heat Flux Burner

Energy & Fuels

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(a) Flame speed validation with CH4-air flame; shown in fig 2(a) 177x143mm (300 x 300 DPI)


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Energy & Fuels

(b)NO accumulated along the vertical axial by four mechanisms and temperature profile for CASE Eqrt.1; Figure 2(b) 177x132mm (300 x 300 DPI)


Energy & Fuels

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(c)Height independence validation for fuel lean, stoichiometric, and fuel rich cases; figure 2(c) 177x139mm (300 x 300 DPI)


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Energy & Fuels

(d)LSF signal versus seeded NO concentration in calibration. shown in fig 2(d) 177x149mm (300 x 300 DPI)


Energy & Fuels

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Fig.3. NO concentration versus equivalence ratio in premixed CH4-air flames at 10mm HAB. Measured values are shown together with error bars as scatters, and calculation by 4 mechanisms are plot in fitted lines. shown in figure 3 177x133mm (300 x 300 DPI)


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Energy & Fuels

(a)Results by GDF-Kin 3.0 mechanism; fig.4 177x147mm (300 x 300 DPI)


Energy & Fuels

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(b)Results by CRECK 1407 C1-C3 mechanism. fig.4 177x144mm (300 x 300 DPI)


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Energy & Fuels

(a) Experimental and Numerical NO concentration for CASE Fuel.1-5; figure 5(a) 177x151mm (300 x 300 DPI)


Energy & Fuels

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(b) Experimental and Numerical NO concentration for CASE Fuel.6-10; figure 5(b) 177x146mm (300 x 300 DPI)


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Energy & Fuels

(c) Experimental and Numerical NO concentration for CASE Fuel.11-15; figure 5(c) 177x149mm (300 x 300 DPI)


Energy & Fuels

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(d) First-order sensitivity coefficients analysis for GDF-Kin 3.0 mechanism; figure 5(d) 177x144mm (300 x 300 DPI)


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Energy & Fuels

(e) First-order sensitivity coefficients analysis for CRECK 1407 C1-C3 mechanism; figure 5(e) 177x147mm (300 x 300 DPI)


Energy & Fuels

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(f) First-order sensitivity coefficients analysis for CASE Eqrt.6 by CRECK 1407 C1-C3 mechanism. figure 5(f) 177x145mm (300 x 300 DPI)


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Energy & Fuels

Given temperature profile in CASE Fuel. 3, 8, 11-15. 221x167mm (300 x 300 DPI)


Effects of CH4 Content on NO ... - ACS Publications

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