Experimental and Numerical Study of the Effects of Steam Addition on

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Experimental and numerical study of the effects of steam addition on NO formation during methane and ammonia oxy-fuel combustion Yizhuo He, Xiaochuan Zheng, Jianghui Luo, Hangfei Zheng, Chun Zou, Guangqian Luo, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01550 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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Energy & Fuels

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Experimental and numerical study of the effects

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of steam addition on NO formation during

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methane and ammonia oxy-fuel combustion

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Yizhuo He, Xiaochuan Zheng, Jianghui Luo, Hangfei Zheng, Chun Zou*, Guangqian

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Luo*, Chuguang Zheng

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State Key Laboratory of Coal Combustion, Huazhong University of Science and

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Technology, Wuhan, 430074, P. R. China

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*Corresponding author. Tel: +86 2787542417-8314; fax: +86 2787545526.

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E-mail address: [email protected] (C. Zou), [email protected](G.Luo)

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Abstract: The effect of H2O addition on the oxidation of methane and ammonia during

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oxy-fuel combustion was investigated both experimentally and numerically. Comparison

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experiments between O2/CO2 and O2/CO2/H2O atmospheres were conducted in a flow

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reactor at atmospheric pressure with equivalence ratios ranging from fuel-rich to

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fuel-lean and temperature from 973 K to 1773 K. The experimental results indicate that

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the effects of H2O addition shift the onset temperature of oxidation to the lower values,

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inhibit CO formation significantly and enhance NO formation remarkably. The chemical

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kinetic mechanism, which was hierarchically structured and updated in our previous work,

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captured the main characteristics of CO and NO formation satisfactorily. The presence of

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H2O leads to far higher OH radical concentrations in the CO2/H2O atmospheres. The

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ultrahigh OH radical concentrations dramatically enhance the reactions between OH and 1

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amine radicals, resulting in the significant enhancement of pathway NH2 → NH → HNO

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→ NO and NH2 → NH → N → NO in CO2/H2O atmospheres. Meanwhile, NH2 →

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CHxNHy/HNCO → NCO → NO is vastly demoted in CO2/H2O atmospheres. The

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increase in pathways NH2 → NH → HNO → NO and NH2 → NH → N → NO is always

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much more than the decline in pathway NH2 → CHxNHy/HNCO → NCO → NO. Hence,

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H2O addition in oxy-fuel combustion enhances NO formation during the oxidation of

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methane and ammonia. In addition, the effects of H2O addition become stronger on

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enhancing NO formation with the increasing H2O concentration in CO2/H2O atmospheres

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by further amplifying the amount of OH radicals.

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Keywords: Oxy-fuel combustion; H2O addition; Reaction mechanism; Plug-flow reactor;

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NO

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1．．Introduction

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Environmental crisis caused by greenhouse gases has drawn extensive attentions of

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the international community in recent decades. Carbon dioxide (CO2) emitted from the

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combustion of fossil fuels is the primary greenhouse gas at the present stage.1,2 In

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response to the emission challenges, oxy-fuel combustion has been comprehensively

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considered as a promising alternative technology for carbon capture and storage.3-8

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Oxy-fuel combustion implies that the recycled flue gases are used to moderate the high

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temperature generated by fuel combustion with pure oxygen, instead of air, ensuring that

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the CO2 volume concentration exceeds 90% in the exhaust gas, which is almost

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sequestration-ready. 2


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It also has been found that NO emissions can be reduced during oxy-fuel

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combustion compared with conventional air combustion for coal,9-14 hence, the

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mechanisms and characteristics of NO formation impacted by CO2 has attracted

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considerable attentions in recent years.15-18 Due to the nitrogen chemistry mechanisms for

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coal combustion are extremely complicated, homogenous gas-phase chemistry was

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chosen to provide theoretical foundation for the underlying mechanisms in many

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investigations. Kim et al.19 numerically studied the effect of CO2 addition on NO

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emission in H2/N2 laminar diffusion flame and they pointed out that the C-related

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reactions affect the production of prompt NO in the case of CO2 addition. Park et al.20

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numerically investigated the chemical effect of CO2 dilution on NO emission

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characteristic in methane–air counterflow diffusion flame and results showed that the

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mole production rates of nitrogenous species are prevented considerably. Park et al.21 also

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computationally examined NO emission behavior in methane oxy-fuel combustion

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recirculated with CO2 and they pointed out that the chemical effects of recirculated CO2

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not only reduce the formation and destructions of NO through the Fenimore mechanism

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but also suppress the NO formation through the thermal mechanism. The chemical effects

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of CO2 addition in ethylene diffusion flame were demonstrated by Liu et al.22 and

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numerical results showed that CO2 reduces NO emissions and reaction CO2 + H = CO +

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OH is primarily responsible for the chemical effects of CO2 addition. Meanwhile, due to

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the absence of N2 in oxy-fuel combustion, NO emissions almost completely derive from

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fuel-NO. Hence, the conversions of hydrogen cyanide and ammonia, which are main

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precursors of NO in the coal combustion, to NO become the focus of researches is this

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area. Giménez-López et al.23 studied the oxidation of HCN in O2/CO2 atmospheres in a

3


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flow reactor both experimentally and numerically. It was found that CO2 + H = CO + OH

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competes with O2 + H = O + OH, reducing the formation of chain carriers, which clearly

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leads to inhibiting HCN oxidation. In the experimental and numerical investigations of

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ammonia oxidation in O2/CO2 atmospheres, both Mendiara et al.24 and Watanabe et al.25

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concluded that the increased OH/H ratio and high CO levels increase the probability of

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forming N2 instead of NO.

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The review of literature above is mainly concerned in CO2 addition and O2/CO2

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combustion atmospheres. In fact, steam is also a main composition in flue gases during

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oxy-fuel combustion.26 Therefore, oxy-fuel combustion with H2O addition has drawn

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researchers’ attentions recently. The effects of steam addition on coal gasification,27 coal

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ignition28-30 and methane oxidation31 during oxy-fuel combustion have been investigated,

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however, little information is available for the effects of steam addition on NO formation

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during oxy-fuel combustion.

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In this work, a systematic experimental study of the oxidation of methane and

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ammonia in O2/CO2/H2O atmospheres was carried out at atmospheric pressure with

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equivalence ratios ranging from fuel-lean to fuel-rich conditions (i.e. 0.2, 1.0, 1.6) and

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temperature from 973 to 1773 K. The experiments were performed in a laboratory plug

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flow reactor. Ammonia was chosen because it is the main N-compound in the

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devolatilization of biomass and low-rank coal,5,32 and its production is greater than that of

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HCN in the presence of steam.27,33 The experimental results are analyzed in terms of a

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chemical kinetic model with 168 species and 1208 reactions, which was hierarchically

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structured and updated in our previous work.34 In addition, the effects of H2O

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concentration on NO formation from NH3 were also addressed.

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2. Experimental and Modeling

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Fig. 1 schematically demonstrates the experimental apparatus used in the present

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work. The apparatus is comprised of a gas feeding system, a VDM (Vapor Delivery

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Module) system, a flow reactor, and a gas composition test system.

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The flow reactor with an internal diameter of 12 mm and a length of 1100 mm was

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constructed according to Skjøth-Rasmussen et al.35 for homogeneous gas-phase reactions.

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Alumina was chosen as the material of the flow reactor in order to avoid the catalytic

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effect on experimental results. The flow reactor was heated using an electrically heated

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oven, which allows the maximum temperature up to 1800 K. The temperature profiles

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within the reactor were measured using a type S thermocouple under inert conditions (1

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L/min CO2). The uncertainty of the temperature measurement is ±4 K. Typical

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temperature profiles are demonstrated in Fig. 2, which implies that the length of

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isothermal reaction zone is approximately 700 mm. The temperature of the isothermal

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zone is referred to as the reaction temperature in this work.

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High-purity gases (99.99%) were separately supplied from gas cylinders and the

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flow rates were controlled precisely by mass flow controllers. The Bronkhorst VDM

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(Vapor Delivery Module) system, a compact integrated system to realize mass flow

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control of vapor, was used to add H2O vapor to O2/CO2 atmospheres. The VDM system

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was intended to generate a predefined vapor flow, accruing from an accurately controlled

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distilled water mass flow injected into an accurately controlled carrier gas (CO2) flow

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with subsequent evaporation inside a temperature controlled chamber. This predefined

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vapor flow was preliminarily blended with the oxygen and remaining carbon dioxide in a

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isothermal chamber at 473 K. Then the total flow was premixed sufficiently with

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methane and ammonia in the mixer prior to entering the reactor. In order to avoid steam

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condensation, the heating tapes were used to heat the pipeline connecting the isothermal

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chamber, the mixer and the reactor to maintain the temperature at a constant value of 423

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K.

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A water cooler was installed at the outlet of the reactor to cool down the product gas

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rapidly. In the case of steam addition, argon (Ar) of equal volume to steam was fed into

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the product gas to compensate for volume loss due to steam condensation. The

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concentrations of CO and NO in the product gas were measured on-line using Fourier

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transform infrared spectroscopy (GASMET-DX4000) with a resolution of 8 cm−1 and

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scanning speed of 10 scans per second. The uncertainty of the measurement is estimated

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as ±1%.

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In order to minimize the axial dispersion of the reactants and comply with a

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reasonable plug-flow approximation, the total flow rate for all experiments was kept

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constant at 1 L/min (Standard Temperature and Pressure), which has been investigated

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and validated by Skjøth-Rasmussen35 and Glarborg et.al.24 Also, the reactants were

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highly diluted with carbon dioxide or carbon dioxide and steam to minimize the influence

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of temperature rise due to chemical reaction. The methane concentration was about 2500

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ppm, the ammonia concentration was approximately 500 ppm, and the oxygen

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concentration was calculated according to the defined equivalence ratios. In consideration

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of the application of rich and lean equivalence ratios in staged combustion, the

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experiments were conducted covering a wide range of equivalence ratios to investigate

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the effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel

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combustion. Three equivalence ratios, i.e. 0.2, 1.0 and 1.6, were chosen on behalf of

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fuel-lean, stoichiometric, and fuel-rich conditions, respectively. And the calculations of

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the equivalence ratios were based on the oxidation reaction as follows:

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xCH4 + yNH3 + (2x+1.25y)O2 → xCO2 + yNO + (2x+1.5y)H2O

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ϕ=

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Experiments were carried out in the temperature range of 973–1773 K at intervals of

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(CH 4 + NH 3 ) / O2 [(CH 4 + NH3 ) / O2 ] stoic

(1) (2)

20 K. Detailed experimental conditions are listed in Table 1.

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Numerical predictions adopting full experimental temperature profiles within the

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flow reactor were performed using the plug flow reactor (PFR) module in conjunction

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with CHEMKIN-PRO. The mechanism adopted in this work was hierarchically

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established for the oxidation of methane and ammonia in O2/H2O atmospheres in our

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previous study34, containing the comprehensive oxidation mechanism for hydrogen,

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C1–C2 hydrocarbons, nitrogen-containing species (HCN, NH3) and the interactions of

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these compositions. The mechanism contained 170 species and 1208 reactions. The more

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details and validations of this mechanism can be found elsewhere34.

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3．．Results and discussion

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Fig. 3 shows the comparisons between the experimental and numerical results of the

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CO and NO profiles as a function of reaction temperature at two different atmospheres

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(O2/CO2 and O2/CO2/H2O) and equivalence ratios (fuel-rich, stoichiometric, and fuel-lean

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conditions). Closed symbols and solid lines represent experimental and numerical results

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in CO2 atmospheres, whereas open symbols and dashed lines represent those in CO2/H2O 7


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atmospheres.

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Under fuel-rich conditions, as displayed in Fig. 3a, the CO formation in CO2 case

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occurs at 1113 K, increases sharply to peak (5985 ppm) at 1393 K, and then increases

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again moderately with temperature above 1453 K. The NO formation increases extremely

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slowly at the beginning until it steps to 56 ppm at 1413 K and then increases very slowly

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up to 108 ppm at 1773 K. In CO2/H2O case, CO formation initiates at 1093 K, goes up to

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peak (3830 ppm) at 1373 K, and then increases slightly with temperature above 1393 K.

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Similar to that in CO2 case, the NO concentration jumps to 78 ppm at 1393 K and then

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increases gradually up to 166 ppm at 1773 K.

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Under stoichiometric conditions exhibited in Fig. 3b, the CO formation in CO2 case

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starts to rise at 1133 K, exhibits a peak (3104 ppm) at 1333 K, and then increase again

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gradually with reaction temperature above 1393 K. The NO formation suddenly steps to

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85 ppm at 1353 K and then increases slightly with reaction temperature up to 142 ppm at

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1773 K. In CO2/H2O case, CO formation exhibits a semblable tendency with a peak value

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of 3088 ppm at 1313 K. The NO concentration jumps to 100 ppm at 1333 K, and then

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increases gradually to a steady value of 191 ppm above 1633 K.

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Under fuel-lean conditions, as shown in Fig. 3c, the CO formation increases and

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decreases dramatically exhibiting a maximum value of 353 ppm at 1113 K, and then

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increases again with temperature in CO2 case. The NO formation increases gradually up

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to 199 ppm at 1453 K and then nearly levels off. In CO2/H2O case, the CO formation

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demonstrates a maximum value of 673 ppm at 1113 K, decreases to zero and remains

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undetected until it increases again extremely slowly with temperature above 1453 K. The

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NO formation increases gradually up to 242 ppm at 1533 K and then remains nearly

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Energy & Fuels

unchanged.

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Synthesizing the comparisons of the results observed in both CO2 and CO2/H2O

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atmospheres, the effects of H2O addition on the oxidation of methane and ammonia

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during oxy-fuel combustion can be summarized as follows: (1) it shifts the onset

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temperature of oxidation to the lower values; (2) it inhibits CO formation significantly; (3)

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it enhances NO formation markedly.

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It can be seen from Fig. 3 that the chemical kinetic model satisfactorily reproduces

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the main features of CO and NO formation measured in experiments in both CO2 and

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CO2/H2O atmospheres, although tolerant deviations exist especially under fuel-lean

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conditions. Hence, the mechanism proposed previously is appropriate for revealing the

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effects of H2O addition on the ammonia oxidation during oxy-fuel combustion of

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methane.

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In the process of fuel oxidation and pollutants formation the radical pool has been

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generally considered as a critical participant, hence, it is always dispensable to analyze

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the radical pool structure at first. Fig. 4 compares the H, O and OH mole fractions

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profiles in CO2 and CO2/H2O atmospheres under fuel-rich, stoichiometric and fuel-lean

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conditions at 1673 K. As shown in Fig. 4, OH radicals are consistently predominant in

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radical pool under all three conditions for both CO2 and CO2/H2O atmospheres. It also

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can be seen that OH radicals in CO2/H2O atmospheres are much higher while H and O

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radicals are nearly tantamount compared with those in CO2 atmospheres for different

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equivalence ratios. In CO2 cases, R33 (H + CO2 = CO + OH) competes for H radicals

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with the main chain branching reaction R1 (H + O2 = O + OH) leading to the dominant

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position of OH radicals. However, in CO2/H2O cases, the presence of H2O substantially

9


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enhances the reactions R30 (H + H2O = OH + H2) and R14 (O + H2O = OH + OH),

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yielding a large amount of OH radicals and simultaneously consuming large amounts of

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H and O radicals. Hence, OH radicals are far higher in CO2/H2O atmospheres than those

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in CO2 atmospheres. It also can be found that O radicals increase to some extent under

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fuel-lean conditions for both CO2 and CO2/H2O atmospheres. And this can be attributed

210

to the fact that the increasing O2 concentration strengthens R1 (H + O2 = O + OH)

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subsequently producing more O radicals under fuel-lean conditions.

212 213 214

In order to clarify the NO formation mechanism intuitively, the OPR, which is short for the overall production rate, is introduced and defined as follows: l

OPRi , j = ∫ ωi , j dx

(3)

0

215

where i denotes species, j denotes elementary reaction, ωi,j means the mole production

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rate of species i through elementary reaction j and l is the length of reaction zone.

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Fig. 5 compares the OPRNO of importantly contributing reaction steps between CO2

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and CO2/H2O atmospheres covering from fuel-rich to fuel-lean conditions. The top ten

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reactions contributing to NO formation for each case are extracted to be synthesized in

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Fig. 5. It is compelling that significant differences exist in common among fuel-rich,

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stoichiometric and fuel-lean conditions between CO2 and CO2/H2O atmospheres, which

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are summarized as follows: (1) the contribution to NO of R755 (HNO + OH = NO + H2O)

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is enhanced vastly in CO2/H2O cases; (2) the OPR of R857 (N + OH = NO + H) is

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strengthened dramatically in CO2/H2O cases; (3) the NCO-related reactions R1005 (NCO

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+ O = NO + CO) and R1009 (NCO + O2 = NO + CO2), which are dominant in CO2

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atmospheres, are suppressed sharply in CO2/H2O cases. It can be seen that the presence of

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H2O alters the structure of radical pool, resulting in a remarkable variation of oxidation 10


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pathway of NH3. Hence, it is indispensable to carry out an elaborate analysis on the

229

oxidation pathways of NH3.

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Fig. 6 compares the critical NO formation pathways between the CO2 and CO2/H2O

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atmospheres. As demonstrated in Fig. 6, NH3 is initially converted into NH2 through

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hydrogen abstraction reaction. Then, the pathways from NH2 to NO can be generally

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divided into five main pathways as follows: (a) NH2 → HNO → NO; (b) NH2 → NH →

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NO; (c) NH2 → NH → HNO → NO; (d) NH2 → NH → N → NO; and (e) NH2 →

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CHxNHy/HNCO → NCO → NO.

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For NH2 conversion, R829 (NH2 + OH = NH + H2O) is enhanced by the abundant

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OH radicals in CO2/H2O atmospheres, implying that NH2 converts more to NH radicals

238

instead of reacting with other species. Correspondingly, the channel of NH2 → CHxNHy

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through R1090 (CH3 + NH2 = CH3NH2) and R1091 (CH3 + NH2 = CH2NH2 + H) is

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weakened dramatically and ultimately the channel of NH2 → CHxNHy/HNCO → NCO is

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suppressed. Meanwhile, the promotion of the channel of NH2 → NH in CO2/H2O

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atmospheres alters the availability of NH2 and NH radicals, which is the reason for the

243

change of main NO reduction reactions R839 (NH2 + NO = N2 + H2O), R840 (NH2 + NO

244

= NNH + OH) and R853 (NH + NO = N2O + H) as displayed in Fig. 5. In addition, it is

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noteworthy that the channel of NH2 → HNCO is reversed in CO2/H2O atmospheres. The

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ultrahigh CO concentration in CO2 atmospheres promotes HNCO formation through

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-R978 (NH2 + CO = HNCO + H). However, H2O addition decreases the CO

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concentration sharply enough to reverse -R978 (NH2 + CO = HNCO + H) in CO2/H2O

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atmospheres. Therefore, the channel of NH2 → HNCO → NCO is forbidden in CO2/H2O

250

atmospheres.

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For NH conversion, the channels of NH → HNO and NH → N are facilitated

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through R846 (NH + OH = HNO + H) and R847 (NH + OH = N + H2O) , yielding larger

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amount of HNO and N radicals in CO2/H2O atmospheres. This can be attributed to more

254

sufficient NH and OH radicals in CO2/H2O atmospheres compared with those in CO2

255

atmospheres. Subsequently, sufficient HNO, N and OH radicals strengthen R775 (HNO +

256

OH = NO + H2O) and R857 (N + OH = NO + H) as shown in Fig. 5 , leading to the

257

enhancement of pathway (c) NH2 → NH → HNO → NO and (d) NH2 → NH → N →

258

NO in CO2/H2O atmospheres.

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For NCO conversion, NCO is almost all derived from the channel of NH2 →

260

CHxNHy/HNCO → NCO. As mentioned above, the channel of NH2 → CHxNHy/HNCO

261

→ NCO is remarkably suppressed in CO2/H2O atmospheres, leading to the suppression

262

of NCO formation. Hence, R1005 (NCO + O = NO + CO) and R1009 (NCO + O2 = NO

263

+ CO2) are inhibited dramatically for NO formation, as show in Fig. 5, implying that

264

pathway (e) NH2 → CHxNHy/HNCO → NCO → NO is vastly demoted in CO2/H2O

265

atmospheres.

266

In order to reveal the NO formation mechanism quantitatively from pathway point

267

of view, the conversion rate of nitrogen was introduced to evaluate the conversion of NH3

268

to NO, which is defined as:

269 270

CRNO =

M NO MN

(4)

where MNO designates the mass of N in NO and MN represents the mass of N in NH3.

271

The conversion rate (CRNO) of each pathway, which is obtained based on the ORP of

272

all the relevant elementary reactions along the NO formation pathways, is compared

273

between CO2 and CO2/H2O atmospheres demonstrated in Fig. 7. It can be seen from Fig. 12


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7 that pathway (e) NH2 → CHxNHy/HNCO → NCO → NO and (c) NH2 → NH → HNO

275

→ NO are dominant for NO formation in CO2 and CO2/H2O atmosphere, respectively.

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Although pathway (a) NH2 → HNO → NO is fairly critical under fuel-lean condition,

277

which is attributed to that the extremely high O2 concentration enhances the channel of

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NH2 → HNO through R833 (NH2 + O2 = HNO + OH), the effect of H2O addition on

279

pathway (a) is limited as shown in Fig. 7. As discussed in the previous section, the H2O

280

addition in oxy-fuel combustion strengthens pathway (c) NH2 → NH → HNO → NO

281

significantly reflecting in that the increase value of CRNO through pathway (c) is 12.4%,

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12.8% and 9.3% under fuel-rich, stoichiometric, and fuel-lean conditions, respectively. It

283

also enhances pathway (d) NH2 → NH → N → NO reflecting in that the increase value of

284

CRNO through pathway (d) is 4.1%, 4.5% and 2.8% under fuel-rich, stoichiometric, and

285

fuel-lean conditions, respectively. Meanwhile, the H2O addition weakens pathway (e)

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NH2 → CHxNHy/HNCO → NCO → NO markedly embodied in that the decline value of

287

CRNO through pathway (e) is 10.8%, 10.6% and 4.3% under fuel-rich, stoichiometric, and

288

fuel-lean conditions, respectively. It can be found that the increase of pathway (c) and (d)

289

is always superior to the decline of pathway (e) in CO2/H2O atmospheres. Hence, H2O

290

addition in oxy-fuel combustion enhances NO formation during the oxidation of methane

291

and ammonia.

292

In addition, the effect of H2O concentration on NO formation will be discussed in

293

the following section. Fig. 8 shows the comparisons between the experimental and

294

numerical results of the CO and NO profiles as a function of H2O concentration in

295

CO2/H2O atmospheres at different equivalence ratios (fuel-rich, stoichiometric, and

296

fuel-lean conditions). As displayed in Fig. 8, the increasing H2O concentration inhibits

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297

the CO formation and enhances the NO formation. Meanwhile, the prediction results are

298

in good agreements with the experiments data, and this offers a further validation for the

299

applicability of the present mechanism. Fig. 9 demonstrates that the variations of the

300

conversion rate (CRNO) of each pathway with the increasing H2O concentration (5%, 15%

301

and 30%) at different equivalence ratios (fuel-rich, stoichiometric and fuel-lean

302

conditions) in CO2/H2O atmospheres. It indicates that pathway (c) NH2 → NH → HNO

303

→ NO and (d) NH2 → NH → N → NO are dramatically strengthened with the increasing

304

H2O concentration while pathway (e) NH2 → CHxNHy/HNCO → NCO → NO is

305

evidently suppressed. Specially, pathway (a) NH2 → HNO → NO also decreases with the

306

increasing H2O concentration under fuel-lean conditions. However, the increase of

307

pathway (c) and (d) with the increasing H2O concentration is always overwhelmingly

308

dominant in CO2/H2O atmospheres which in responsible for the increasing NO formation.

309

The fundamental reason for this is that the increasing H2O concentration further enhances

310

the reactions R30 (H + H2O = OH + H2) and R14 (O + H2O = OH + OH) yielding a

311

larger amount of OH radicals, which amplifies the effect of H2O addition on the

312

oxidation of methane and ammonia during oxy-fuel combustion.

313 314

5. Conclusions

315

The effect of H2O addition on the oxidation of methane and ammonia during

316

oxy-fuel combustion was investigated both experimentally and numerically. Comparison

317

experiments between CO2 and CO2/H2O atmospheres were accomplished in a flow

318

reactor at atmospheric pressure with equivalence ratios ranging from fuel-rich to

319

fuel-lean and temperature from 973 K to 1773 K.

14


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The comparative experiments results indicate that the effects of H2O addition on the

321

oxidation of methane and ammonia during oxy-fuel combustion shift the onset

322

temperature of oxidation to the lower values, inhibit CO formation significantly and

323

enhance NO formation markedly. The underlying mechanisms have been revealed using

324

a detailed chemical kinetic mechanism in the production rate and pathway analysis point

325

of view.

326

The H2O addition substantially enhances the reactions H + H2O = OH + H2 and O +

327

H2O = OH + OH, leading to far higher OH radical concentrations in the CO2/H2O

328

atmospheres than those in the CO2 atmospheres. The ultrahigh OH radical concentrations

329

dramatically enhance the reactions between OH and amine radicals (NH2, NH, and N),

330

resulting in the significant enhancement of pathway NH2 → NH → HNO → NO and NH2

331

→ NH → N → NO in CO2/H2O atmospheres. NH2 radicals are converted far more to NH

332

radicals inhibiting the channel of NH2 → CHxNHy. Meanwhile, the channel of NH2 →

333

HNCO → NCO is forbidden in CO2/H2O atmospheres by reversing NH2 + CO = HNCO

334

+ H. Consequently, pathway NH2 → CHxNHy/HNCO → NCO → NO is vastly demoted

335

in CO2/H2O atmospheres. However, the increase of pathway NH2 → NH → HNO → NO

336

and NH2 → NH → N → NO is always much more than the decline of pathway NH2 →

337

CHxNHy/HNCO → NCO → NO in CO2/H2O atmospheres. Hence, H2O addition in

338

oxy-fuel combustion enhances NO formation during the oxidation of methane and

339

ammonia. In addition, the effects of H2O addition become stronger on enhancing NO

340

formation with the increasing H2O concentration in CO2/H2O atmospheres by further

341

amplifying the amount of OH radicals.

342

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343 344 345

Acknowledgements This work was supported by the National Key Research & Development Special Project (No. 2016YFB0600801) of the National Natural Science Foundation of China.

346 347

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Figure captions

460

Fig. 1. Schematic diagram of the experimental apparatus.

461

Fig. 2. Temperature profiles within the reactor.

462

Fig. 3. Experimental data and numerical predictions for different equivalence ratios

463

(fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions) as functions of reaction

464

temperature.

465

Fig. 4. Comparison of H, O, and OH mole fraction profiles between O2/CO2 and

466

O2/CO2/H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions

467

at 1673 K.

468

Fig. 5. Comparison of OPRNO between O2/CO2 and O2/CO2/H2O atmospheres for

469

fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K.

470

Fig. 6. Comparison of the NO formation pathways between the O2/CO2 (a) and

471

O2/CO2/H2O (b) atmospheres for stoichiometric condition at 1673 K.

472

Fig. 7. Comparison of the elemental N conversion rates (CRNO) obtained through each

473

pathway at 1673 K in O2/CO2 and O2/CO2/H2O atmospheres.

474

Fig. 8. Experimental data and numerical predictions for different equivalence ratios

475

(fuel-rich, stoichiometric, and fuel-lean conditions) as functions of H2O concentration at

476

1673 K in O2/CO2/H2O atmospheres.

477

Fig. 9. Comparison of the elemental N conversion rates (CRNO) among different H2O

478

concentrations at 1673 K in O2/CO2/H2O atmospheres.

479 480

Table captions

481

Table 1. Experimental conditions in the present work.

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Fig. 1. Schematic diagram of the experimental apparatus. 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 22


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Fig. 2. Temperature profiles within the reactor. 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 23


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a

b

c

Fig. 3. Experimental data and numerical predictions for different equivalence ratios (fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions) as functions of reaction temperature. 517

24


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Energy & Fuels

a

b

c

Fig. 4. Comparison of H, O, and OH mole fraction profiles between O2/CO2 and O2/CO2/H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K. 518 519 520 521 522 523 524 525 25


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526

a

b

c

Fig. 5. Comparison of OPRNO between O2/CO2 and O2/CO2/H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K. 527 528 529 530 26


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a

b

Fig. 6. Comparison of the NO formation pathways between the O2/CO2 (a) and O2/CO2/H2O (b) atmospheres for stoichiometric condition at 1673 K. 532 533 534 535 536 537 27


Energy & Fuels

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Fig. 7. Comparison of the elemental N conversion rates (CRNO) obtained through each pathway at 1673 K in O2/CO2 and O2/CO2/H2O atmospheres. 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 28


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Fig. 8. Experimental data and numerical predictions for different equivalence ratios (fuel-rich, stoichiometric, and fuel-lean conditions) as functions of H2O concentration at 1673 K in O2/CO2/H2O atmospheres. 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570

29


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

Fig. 9. Comparison of the elemental N conversion rates (CRNO) among different H2O concentrations at 1673 K in O2/CO2/H2O atmospheres. 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 30


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589 φ 1.6 1.0 0.2

Table 1. Experimental conditions in the present work. CH4/ppm NH3/ppm O2/ppm CO2/% H2O/% 2511 507 3531 99.35-H2O% 1,5,10,15,20,25,30 2505 505 5647 99.13-H2O% 1,5,10,15,20,25,30 2499 503 28276 96.86-H2O% 1,5,10,15,20,25,30

590

31


Experimental and Numerical Study of the Effects of Steam Addition on

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