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Comparison of the reburning chemistry in O2/N2, O2/CO2 and O2/H2O atmospheres Yizhuo He, Jianghui Luo, Yangguang Li, Huiqiao Jia, Feng Wang, Chun Zou, and Chuguang Zheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01797 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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Energy & Fuels

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Comparison of the reburning chemistry in

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O2/N2, O2/CO2 and O2/H2O atmospheres

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Yizhuo He, Jianghui Luo, Yangguang Li, Huiqiao Jia, Feng Wang, Chun Zou*,

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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 87542417-8314; fax: +86 87545526.

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

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Abstract: The reburning chemistry in oxy-fuel and oxy-steam combustion of methane

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was investigated both experimentally and numerically. Comparison experiments in

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O2/N2, O2/CO2 and O2/H2O atmospheres were performed in a flow reactor at

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atmospheric pressure with equivalence ratio ranging from fuel-rich to fuel-lean and

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temperature from 973 K to 1773 K. Experimental results showed that compared with

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N2 and CO2 atmospheres NO reduction observed in H2O atmosphere is the lowest

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under fuel-rich and stoichiometric condition, while it is the highest under fuel-lean

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condition. The NO reduction intensity in CO2 atmosphere lies between N2 and H2O

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atmosphere under fuel-rich and fuel-lean condition, however, it is the highest under

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stoichiometric conditions. A chemical kinetic mechanism, which was hierarchically

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structured and updated in our previous work, captured the main characteristics and

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quantity of CO and NO formation satisfactorily even under fuel-lean conditions.

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According to the analysis from a chemical kinetic point of view, CO2 and H2O exert

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significant impacts on altering the radical pool structure to OH dominant subsequently

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varying the availability of hydrocarbon radical as a reducing agent, which is the 1

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primary reason for the different degrees of NO reduction under fuel-rich,

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stoichiometric and fuel-lean conditions. In addition, CO2 and H2O also impact the NO

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reduction by nitrogen-containing radicals. For CO2 atmosphere, NCO radical always

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occupies an overwhelmingly dominant position in NO reduction due to HCN →

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CH3CN → CH2CN → CN → NCO and HNCO → NCO channel is amplified

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substantially. For H2O atmosphere, under fuel-rich and stoichiometric conditions, NH2

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and NH radical are dominant due to the enhancement of NCO → HNCO → NH2 →

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NH channel. Under fuel-lean conditions, NCO radical is dominant due to the strength

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of HNCO → NCO channel.

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Keywords: Oxy-fuel combustion; Oxy-steam combustion; Reburning chemistry;

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Plug-flow reactor; NO

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1．．Introduction

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The climate change synthesis report of Intergovernmental Panel on Climate

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Change (IPCC) indicated that carbon dioxide (CO2) emission from fossil fuel

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combustion is the primary reason for global warming.1 In response to environmental

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crisis, oxy-fuel combustion has been comprehensively considered as one of the most

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promising options for CO2 capture and sequestration, due to its feasibility to produce

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sequestration-ready exhaust gas and flexibility of being implemented in existing

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equipment with modification.2-7

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In oxy-fuel combustion, fuel combustion occurs in mixtures of oxygen and

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recycled flue gas instead of air, ensuring that the CO2 volume concentration exceeds

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90% in the exhaust gas, which is almost sequestration-ready. Carlos8 and Seepana9

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proposed oxy-steam combustion technology, in which steam, instead of recycled flue 2


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gas, is used to moderate the high temperature generated by fuel combustion with

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oxygen. Due to its advantages, such as a compact system, ease of operation, small

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geometric size, low emissions, and energy savings, oxy-steam combustion technology

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is regarded to be an alternative for the next-generation oxy-fuel combustion

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technology.

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Due to the absence of nitrogen, fuel-N is the main source of NO formation

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during oxy-fuel and oxy-steam combustion. Hence, many researchers have paid

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attentions to the mechanism of fuel-N evolution. Investigations on NO emission for

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solid fuels10-15 in oxy-fuel combustion have indicated that NO emission can be

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reduced during oxy-fuel combustion compared with conventional air combustion.

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Despite gas-phase chemistry, heterogeneous reaction and mixing should be comprised

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in a complete combustion process for solid fuel, gas-phase chemistry plays a critical

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role in NO formation and reduction. Hence, it is indispensable to investigate

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gas-phase chemistry separately to provide theoretical foundation for the underlying

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mechanisms. Giménez-López et al.16 studied the oxidation of HCN in O2/CO2

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atmospheres in a flow reactor both experimentally and numerically. They indicated

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that the inhibition of HCN oxidation is due to the impact of CO2 on radical pool

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structure. In the experimental and numerical investigations of ammonia oxidation

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during oxy-fuel combustion, both Mendiara et al.17 and Watanabe et al.18 indicated

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

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N2 instead of NO. He et al.19 experimentally studied the oxidation of ammonia during

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oxy-steam combustion and they hierarchically structured a detailed chemical kinetic

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mechanism, which captures the experimental data satisfactorily.

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In spite of the advantage of oxy-fuel combustion in NOx emission, a NOx

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reduction technology is still essential in practical combustion equipment to implement 3


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a further reduction. The NOx-control technologies in oxy-fuel process have been

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reviewed by Normann et al.,20 they indicated that fuel-staging (reburning) technology

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is a promising option for both conventional air and oxy-fuel combustion system

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among the primary measures. Hence, many investigations have focused on gas-phase

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reburning chemistry to reveal the underlying mechanism. Extensive studies have been

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carried out on reburning mechanism in air combustion,21-29 typically, Glarborg and

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Miller27,28 updated and structured a detailed chemical kinetic modeling, which

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predicts the experimental very well in flow reactors, to reveal the reburning

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mechanism for C1 and C2 hydrocarbon. Dagaut et al.29 further investigated the kinetics

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of C1 to C4 hydrocarbon/NO interactions in relation with reburning mechanism. There

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are also recent studies for reburning mechanism in oxy-fuel combustion. Watanabe et

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al.30 compared NO reduction mechanisms between staged O2/CO2 combustion and air

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combustion, results showed that a significant reduction in NO emission could be

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achieved by staged combustion in O2/CO2 combustion. Giménez-López et al.31

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experimentally examined the NO reduction by reburning under oxy-fuel conditions in

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a flow reactor and they provided a wide amount of experimental data on reburning.

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Normann et al.32 numerically investigated reburning reduction of NO in oxy-fuel

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combustion using a plug-flow reactor and they demonstrated that CO2 changes the

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radial pool altering the nitrogen chemistry by impacting the formation and oxidation

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of hydrocarbon radicals. An experimental and numerical study on reburning chemistry

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in oxy-fuel combustion of methane was carried out by Mendiara and Glarborg33 in a

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flow reactor and they structured a detailed chemistry kinetic mechanism, but there

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were obvious defects in predicting NO reduction under fuel-lean conditions. In

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addition, little information is available for the reburning chemistry in oxy-steam

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combustion. 4


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The objective of this work is to reveal the reburning mechanism in oxy-steam

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combustion. In the present work, a systematic experimental study of the gas-phase

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NO reduction by methane in O2/N2, O2/CO2, O2/H2O atmospheres at 1 atm, over a

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wide range of equivalence ratios (i.e., 0.2, 1.0, 1.6), and within the temperature range

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of 973–1773 K was performed. The study was conducted in a laboratory plug flow

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reactor. A chemical kinetic model with 170 species and 1208 reactions, which was

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hierarchically structured and updated, was adopted and new experimental data are

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analyzed in terms of this model to evaluate the detailed reburning mechanism of NO

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interaction with methane in oxy-fuel and oxy-steam atmosphere.

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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 comprise a gas supplying system, a steam producing system, a

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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

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was constructed according to Skjøth-Rasmussen et al.34 for homogeneous gas-phase

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

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catalytic effect on experiments. The flow reactor was heated using molybdenum bars

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in a five-zone temperature controlled electrically heated oven, which allows the

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maximum temperature up to 1800 K. The temperature profiles within the reactor were

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measured using a type S thermocouple under inert conditions (1 L/min CO2). The

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measurement error for the temperature is ±4 K. Typical temperature profiles are

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demonstrated in Fig. 2, which implies that the length of isothermal reaction zone is

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approximately 700 mm. The temperature of the isothermal zone is referred to as the

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reaction temperature in this work. 5


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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. Distilled water was

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supplied into the homothermal evaporation chamber at 473 K employing a

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high-accuracy syringe pump. This predefined steam flow was preliminarily blended

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with the oxygen in the evaporation chamber. Then the total flow was premixed

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sufficiently with methane in the mixer prior to entering the reactor. In order to avoid

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

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homothermal chamber, the mixer, and the reactor to maintain the temperature at a

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constant of 423 K.

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

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

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

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

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Fourier transform infrared spectroscopy (GASMET-DX4000) with a resolution of 8

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cm−1 and scanning speed of 10 scans per second. The measurement error 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

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investigated and validated by Skjøth-Rasmussen34 and Mendiara et.al.17 Also, the

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reactants were highly diluted in nitrogen, carbon dioxide or steam to minimize the

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influence of temperature rise due to chemical reaction. The methane concentration

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was about 2500 ppm, the nitric oxide concentration was approximately 1000 ppm, and

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the oxygen concentration was calculated according to the defined equivalence ratios.

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Three equivalence ratios, i.e. 0.2, 1.0, and 1.6, were chosen on behalf of fuel-lean, 6


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stoichiometric, and fuel-rich conditions, respectively. Experiments were carried out in

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the temperature range of 973–1773 K at intervals of 20 K. Detailed experimental inlet

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parameters 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 in our previous study,19 containing the comprehensive oxidation

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mechanism for hydrogen, C1–C2 hydrocarbons, nitrogen-containing species (HCN,

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NH3) and the interactions of these compositions. The mechanism contained 170

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species and 1208 reactions. The more details and validations of this mechanism can

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be found elsewhere.19

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3．．Results and Discussion

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

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the CO and NO profiles for CH4 oxidation in the presence of NO as a function of

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reaction temperature in diverse atmospheres (N2 or CO2 or H2O) and equivalence

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ratios (fuel-rich, stoichiometric, and fuel-lean conditions). Solid symbols and solid

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lines represent experimental and numerical results in N2, half solid symbols and grey

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lines represent those in CO2, whereas open symbols and dashed lines represent those

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in H2O.

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Under fuel-rich conditions, as displayed in Fig. 3a, the CO formation in N2 case

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occurs below 973 K, increases to peak (1617 ppm) at 1293 K, and then nearly

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stabilizes. The peak of CO formation is accompanied by a sharp increase of NO

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reduction to 435 ppm and then the NO concentration nearly keeps steady at 450 ppm.

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In CO2, CO formation also initiates below 973 K, goes up to a higher peak (4435 ppm) 7


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at 1373 K, and then increases gradually with temperature above 1453 K. NO

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reduction initiates at 1173 K, increases significantly at 1393 K with a NO value of

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381 ppm and then the NO concentration increases gradually up to 622 ppm at 1773 K.

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In H2O, CO formation exhibits a maximum value of 566 ppm at 1313 K and then

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keeps unchanged at a value of 20 ppm. NO reduction reaches a peak (665 ppm) and

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then weakens to a constant value of 830 ppm.

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Under stoichiometric conditions exhibited in Fig. 3b, the CO concentration in N2

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initiates below 973 K, goes up to maximum (485 ppm) at 1233 K, and then reaches a

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plateau at 85 ppm. The peak of CO concentration is in consistence with a sharp

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increase of NO reduction to 659 ppm and then the NO concentration increases slowly

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to a steady value of 776 ppm. In CO2, the maximum CO concentration is 2814 ppm at

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1333 K, and then CO formation increases gradually with temperature up to 3536 ppm

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at 1773 K. NO concentration starts to drop at 1133 K, goes down to a minimum value

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of 500 ppm at 1353 K and then increases gradually to stabilize at 740 ppm at 1653 K.

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In H2O, CO formation demonstrates a peak value of 460 ppm at 1253 K and then

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remains unchanged at a value of 12 ppm. NO concentration reaches a bottom (741

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ppm) at 1273 K and then goes up to a constant value of 900 ppm.

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

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value of 516 ppm at 973 K and the decreases to zero above 1273 K. NO concentration

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is 924 ppm at 973 K and then increase to a steady level of 971 ppm in N2 cases. In

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CO2, CO concentration starts at 660 ppm, goes down to 2 ppm and then increases up

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to 242 ppm at 1773 K. NO concentration remains nearly unchanged at 932 ppm. In

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H2O, CO concentration initiates at 423 ppm, declines to zero at 1113 K and then

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cannot be detected again. NO concentration at 973 K is 857 ppm and it increases to

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stabilize at a value of 915 ppm. 8


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Synthesizing the comparisons of the experimental results observed in N2, CO2

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and H2O atmospheres, there are common phenomena that the temperature of CO

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concentration peak is in consistence with a vast increase of NO reduction; the onset

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temperature of NO reduction is N2, H2O and CO2 atmosphere in an ascending

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sequence; and this temperature is lowered by an increasing O2 concentration for all

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the three atmospheres. Besides, compared with N2 and CO2 atmospheres, NO

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reduction observed in H2O atmosphere is the lowest under fuel-rich and

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stoichiometric conditions, while it is the highest under fuel-lean conditions. The NO

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reduction intensity in CO2 atmosphere lies between N2 and H2O atmospheres under

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fuel-rich and fuel-lean conditions, however, it is highest under stoichiometric

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conditions.

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

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reproduces the main features of CO and NO formation measured in experiments in N2,

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CO2 and H2O atmospheres, although tolerant deviations exist especially under

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fuel-lean conditions. Hence, the mechanism proposed previously is appropriate for

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revealing the NO reduction mechanisms for N2, CO2 and H2O atmospheres in the

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chemical kinetic point of view.

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In the process of fuel oxidation and reduction the radical pool has been generally

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considered as a critical participant, hence, it is always indispensable to analyze the

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

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profiles in N2, CO2 and H2O atmospheres under fuel-rich, stoichiometric, and

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fuel-lean conditions at 1673 K. As displayed in Fig. 4, for CO2 atmosphere, OH

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radicals are dominant in the radical pools under all three equivalent ratios. It can be

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attributed to that R33 (H + CO2 = CO + OH) competes for H radicals with the main

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chain branching reaction R1 (H + O2 = O +OH), leading to the dominant position of 9


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OH radicals. For H2O atmosphere, OH radicals are overwhelmingly predominant in

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the radical pools under all three equivalent ratios. This high production is caused by

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that the presence of H2O substantially enhances the reactions R30 (H + H2O = OH +

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H2) and R14 (O + H2O = OH +OH), yielding a large amount of OH radicals and

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simultaneously consuming large amounts of H and O radicals. In regard to the effect

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of equivalence ratio on radical pool structure, it can be found that the effect of

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equivalence ratio is strong in N2 atmosphere, while limited in CO2 and H2O

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atmospheres. This can be attributed to that from fuel-rich to fuel-lean conditions the

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increasing O2 concentration enhances R1 (H + O2 = O + OH), leading to a gradual

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increase of the O and OH radicals. Hence, the concentrations of O and OH radicals

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increase up to be higher than that of H radical in N2 atmosphere under fuel-lean

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conditions. However, for CO2 atmosphere, although R1 (H + O2 = O + OH) is

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enhanced by the increasing O2 concentration, the contribution of R33 (H + CO2 = CO

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+ OH) to OH production is still dominant in radical pool. As for H2O atmosphere, the

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impact of the equivalence ratio on the radical pool structure is extremely restricted

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due to the overwhelming contribution of R14 (O + H2O = OH +OH) and R30 (H +

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H2O = OH + H2) to OH radicals.

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

OPRi , j = ∫ ωi , j dx

(1)

0

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

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

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reaction zone.

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Fig.5 compares the pathways of NO conversion in methane oxidation in N2, CO2

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and H2O atmosphere by synthesizing the OPRis of all the corresponding elementary 10


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reactions along the pathways, taking the fuel-lean conditions as a typical case. As

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shown in Fig.5, the pathways of nitric oxide reburn chemistry are quite sophisticated.

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Nitric oxide is reduced through the reducing reactions of NO with hydrocarbon

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radical, NCO or amine radical. Although all these reactions occur simultaneously

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during the reburn chemistry, it can be divided into two sections artificially in the

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pathway point of view throughout this article in order to make the discussions concise

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and incisive. First, hydrocarbon radicals react with NO generating to HCN, which is

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the dominant intermediate nitrogen species during NO reduction, and then HCN

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converts to four critical nitrogen-containing radicals including NCO, NH2, NH and N

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through a series of chemical reactions. Second, the critical nitrogen-containing

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radicals interact with NO reduced to N2 ultimately.

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Fig. 6 demonstrates the OPRNOs of critical reactions between nitric oxide and

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hydrocarbon radicals in the N2, CO2 and H2O atmospheres for fuel-rich,

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stoichiometric and fuel-lean conditions at 1673 K. In order to demonstrate a

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convincing elucidation, the evolution path of hydrocarbon radicals during methane

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oxidation in N2, CO2 and H2O atmosphere are compared in Fig. 7. It can be seen from

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Fig.6 that the total amount of NO reduction in different atmospheres through reactions

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between nitric oxide and hydrocarbon radicals are in well agreement with the NO

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reduction observed in experiments, however, the importantly contributing reactions

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vary with diverse atmospheres and equivalence ratios. Under fuel-rich conditions, the

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reactions between NO and hydrocarbon radicals are all quite strong in N2 atmosphere

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especially R1145 (HCCO + NO = HCNO + CO). As for CO2 atmosphere, OPRNO of

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R1104 (CH3 + NO = HCN + H2O) and R1105 (CH3 + NO = H2CN + OH) are higher,

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while OPRNO of R1145 (HCCO + NO = HCNO + CO) is lower than that in N2

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atmosphere. These can be attributed to the change of radical pool structure in CO2 11


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atmosphere. Because C2 branch relies more on H and O radicals than C1 branch, C2

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branch is suppressed in CO2 atmosphere as shown in Fig. 7. Subsequently, the

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availability of CH3, which is the initial radical for C1 branch, is strengthened as a

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reducing agent. However, the positive effect of CH3 radical cannot compensate the

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negative effect of C2 branch under fuel-rich conditions, leading to the smaller

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magnitude of total NO reduction by hydrocarbon radicals. Regarding to H2O

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atmosphere, the ultrahigh concentration of OH radical enhances the oxidation of

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hydrocarbon radicals to CH2OH, CH2O and CO as shown in Fig. 7, suppressing their

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availability for NO reduction dramatically. Consequently, all the corresponding

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reactions shown in Fig. 6 are much weaker in comparisons with those in N2 and CO2

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atmospheres.

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Under stoichiometric conditions, the reactions between NO and hydrocarbon

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radicals are still strong in N2 atmosphere, but they are evidently weakened compared

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to those under fuel-rich conditions. In N2 atmosphere, the increasing O2 concentration

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promotes the formation of O radical as mentioned above. Then the higher O radical

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and O2 levels facilitate the oxidation of hydrocarbon radicals mainly through CH3 →

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CH2O shown in Fig. 7 weakening their removal of NO. For CO2 atmosphere, R33 (H

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+ CO2 = CO + OH) competes with R1 (H + O2 = O + OH) ensuring the stability of the

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radical pool structure. This suppresses the production of O radical and subsequently

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the CH3 → CH2O maintaining the availability of hydrocarbon radical. Hence, the

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OPRNO values of corresponding reactions are nearly unchanged in comparison with

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those under fuel-rich conditions. In this case, the positive effect of CH3 radical

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surpasses the negative effect of C2 branch resulting in the highest value of total NO

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reduction by hydrocarbon radicals among the three atmospheres. With respect to the

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H2O atmosphere, similar to CO2 atmosphere but R14 (O + H2O = OH + OH) and R30 12


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(H + H2O = OH + H2) maintain the structure of radical pool mightily holding the

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OPRNO values of corresponding reactions steady, which are still much lower in

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comparisons with those in N2 and CO2 atmospheres.

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Under fuel-lean conditions, the higher O2 levels exert further influence on

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promoting the formation of O radical and the oxidation of hydrocarbon radicals

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inhibiting the NO reduction. This impact is so strong in N2 atmosphere that the O

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radical is dominant in the radical pool as discussed above, causing a vast decline of

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the reactions between NO and hydrocarbon radicals. Hence, the NO reduction by

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hydrocarbon radicals is weakest in N2 atmosphere under fuel-lean conditions. As for

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CO2 atmosphere, although R33 (H + CO2 = CO + OH) is not strong enough to inhibit

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the evident rise of O radical, it ensures the leading position of OH radical in the

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radical pool. Hence, OPRNO values of corresponding reactions are a little higher than

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those in N2 atmosphere. In regard to H2O atmosphere, R30 (H + H2O = OH + H2)

313

competes with R1 (H + O2 = O + OH) suppressing the production of O radical,

314

simultaneously, R14 (O + H2O = OH +OH) converts the O radical generated through

315

R1 to OH radical. Therefore, these two reactions can maintain the overwhelmingly

316

predominant status of OH radical in the radical in despite of the sufficient O2

317

concentration. Consequently, the availability of hydrocarbon radicals influenced by

318

O2 concentration is diminished to minimum in H2O atmosphere and the OPRNO values

319

of corresponding reactions are superior to those in N2 and CO2 atmospheres.

320

It can be seen from Fig. 6 that nitric oxide interacts with hydrocarbon radicals

321

generating HCN, HCNO and H2CN. According to the numerical prediction, both

322

HCNO and H2CN are almost completely converted to HCN. Then HCN is ultimately

323

converted to four critical nitrogen-containing radicals including NCO, NH2, NH and

324

N through a series of complicated pathways, as displayed in Fig. 5. This implies that 13


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

325

the total amount of NCO, NH2, NH and N are directly determined by the amount of

326

NO reduced by hydrocarbon radicals. These four critical nitrogen-containing radicals

327

react with NO converted to N2, N2O and NNH, and N2O and NNH are almost totally

328

converted to N2 through R812 (N2O + M = N2 + O + M), R814 (N2O + H N2 +

329

OH), R860 (NNH = N2 + H) and R861 (NNH + H = N2 + H2). Hence, the reactions

330

between the four nitrogen-containing radicals and nitric oxide can be considered as

331

another key point for NO reduction, where further insight should be given to in the

332

following section.

333

Fig. 8 exhibits the OPRNOs of critical reactions between nitric oxide and

334

nitrogen-containing radicals in the N2, CO2 and H2O atmospheres for fuel-rich,

335

stoichiometric and fuel-lean conditions at 1673 K. As seen from Fig. 8, the total

336

amount of NO reduction in different atmospheres through reactions between nitric

337

oxide and nitrogen-containing radicals are in consistence with the amount of NO

338

reduction observed in experiments, however, the importantly contributing reactions to

339

NO reduction are different at diverse atmospheres and equivalence ratios. Under

340

fuel-rich conditions, N and NH radicals exhibit overwhelming dominance in NO

341

reduction in N2 atmosphere through R859 (N + NO = N2 + O) and R853 (NH + NO =

342

N2O + H). This can be attributed to that the high H radical concentration promotes

343

NCO → NH → N channel shown in Fig. 5a by enhancing R1004 (NCO + H = CO +

344

NH) and R844 (NH + H = N + H2). For CO2 atmosphere, NCO radical occupies an

345

overwhelmingly dominant position in NO reduction through R1010 (NCO + NO =

346

N2O + CO) and R1011 (NCO + NO = N2 + CO2). This is because that the lack of H

347

radical suppresses the NCO → NH → N channel dramatically. And the HCN →

348

CH3CN → CH2CN → CN → NCO channel is amplified substantially by sufficient

349

OH radical and the promotion of R1083 (CN + CO2 = NCO + CO). Meanwhile, R983 14


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350

(HNCO + OH = NCO + H2O) is strengthened by the sufficient OH radical leading to

351

the promotion of HNCO converting to NCO. Hence, the availability of NCO radical

352

for NO reduction is superior in CO2 atmosphere. Regarding H2O atmosphere, NH2

353

and NH radical are dominant, which can be attributed to the enhancement of NCO →

354

HNCO channel. The presence of high H2O concentration facilitates R983 (NCO +

355

H2O = HNCO + OH), which is responsible for the enhancement of NCO → HNCO

356

channel. Therefore, more NCO radical is converted to NH2 and NH radical through

357

NCO → HNCO → NH2 → NH channel.

358

Under stoichiometric conditions, N and NH radicals are still dominant in NO

359

reduction in N2 atmosphere, but R859 (N + NO = N2 + O) and R853 (NH + NO =

360

N2O + H) are weakened dramatically in comparison with those under fuel-rich

361

conditions. On one side, the reactions between NO and hydrocarbon radicals are

362

evidently

363

nitrogen-containing radicals. On the other side, the decreasing H radical composition

364

in radical pool is adverse to NCO → NH → N channel through R1004 (NCO + H =

365

CO + NH) and R844 (NH + H = N + H2). For CO2 and H2O atmospheres, the stability

366

of the radical pool structure results in the steady OPRNO values of corresponding

367

reactions in comparison with those under fuel-rich conditions.

weakened

as

discussed

above

shrinking

the

total

amount

of

368

Under fuel-lean conditions, it is quite different from fuel-rich and stoichiometric

369

conditions that NCO radical is predominant in NO reduction for all the three

370

atmospheres as shown in Fig. 8c. As discussed above, the NCO → NH → N and

371

HNCO → NH2 → NH channels are positively sensitive to H radical, and the HNCO

372

→ NCO channel is positively sensitive to OH radical, therefore, the radical pool

373

structure under fuel-lean conditions is beneficial to the accumulation of NCO radical.

374

Especially, it is worth emphasizing that the ultrahigh OH radical in H2O atmosphere 15


Energy & Fuels

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375

composition reverses R983 (NCO + H2O= HNCO + OH) subsequently the NCO →

376

HNCO channel, which is responsible for the dominant position of NCO radical under

377

fuel-lean conditions. In addition, the total amount of nitrogen-containing radicals is

378

determined by the total amount of NO reduced by hydrocarbon radicals, hence, the

379

total amount of NO reduced by nitrogen-containing radicals is H2O, CO2 and N2

380

atmosphere in a descending order.

381

In addition, in order to give a further insight into reburning chemistry under

382

reducing conditions, it is necessary to discuss the effect of equivalence ratio on

383

reburning chemistry. Fig. 9 shows the comparisons between the experimental and

384

numerical results of the CO and NO profiles as a function of equivalence ratio at 1673

385

K in the N2, CO2 and H2O atmospheres. As displayed in Fig. 9, the prediction results

386

are in good agreements with the experiments data, which offers a further validation

387

for the applicability of the present mechanism. It can been found from Fig.9 that NO

388

reduction intensity observed in H2O and CO2 atmosphere increase slightly with the

389

increasing equivalence ratio, much slower than the increase in N2 atmosphere, which

390

means the equivalence ratio exerts limited effect on NO reduction in H2O and CO2

391

atmosphere while evident effect on that in N2 atmosphere under reducing conditions.

392

NO reduction in H2O atmosphere is always the lowest under reducing conditions. The

393

NO reduction intensity in CO2 atmosphere is the highest at first (1.0-1.2), then it lies

394

between N2 and H2O atmospheres. These imply H2O alters the radical pool structure

395

shrinking the availability of hydrocarbon radical as a reducing agent dramatically,

396

which is hardly affected by O2 concentration. However, this shrink effect of CO2

397

depends on O2 concentration. When the O2 is sufficient enough even under reducing

398

conditions, the suppression effect of CO2 on O radical production and subsequently

399

CH3 → CH2O channel is more dominant than the shrink effect, which is responsible 16


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Energy & Fuels

400

for the highest NO reduction (equivalence ratio 1.0-1.2) in CO2 atmosphere.

401 402

5. Conclusions

403

The reburning chemistry in oxy-fuel and oxy-steam combustion of methane was

404

investigated both experimentally and numerically. Comparison experiments in O2/N2,

405

O2/CO2 and O2/H2O atmospheres were performed in a flow reactor at atmospheric

406

pressure with equivalence ratio ranging from fuel-rich to fuel-lean and temperature

407

from 973 K to 1773 K.

408

Experimental results showed that the onset temperature of NO reduction is N2,

409

H2O and CO2 atmosphere in an ascending sequence, the temperature of CO

410

concentration peak is in consistence with a vast increase of NO reduction. Compared

411

with N2 and CO2 atmospheres, NO reduction observed in H2O atmosphere is the

412

lowest under fuel-rich and stoichiometric condition, while it is the highest under

413

fuel-lean condition. The NO reduction intensity in CO2 atmosphere lies between N2

414

and H2O atmosphere under fuel-rich and fuel-lean conditions, however, it is the

415

highest under stoichiometric conditions.

416

The numerical results using the detailed chemical kinetic mechanism captured

417

the main characteristics and quantity of CO and NO formation satisfactorily even

418

under fuel-lean conditions. The numerical results showed that CO2 and H2O exert

419

significant impacts on altering the radical pool structure to OH dominant and the

420

impact of H2O is much stronger than that of CO2. Under fuel-rich conditions, the

421

alteration of the radical pool structure shrinks the availability of hydrocarbon radical

422

as a reducing agent in both CO2 and H2O atmospheres. Meanwhile, the shrink is far

423

more remarkable in H2O atmosphere. Under stoichiometric conditions, the presence

424

of CO2 suppresses the production of O radical, which is responsible for the 17


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

425

enhancement of NO reduction in CO2 atmosphere. For H2O atmosphere, the ultrahigh

426

OH radical results in that the availability of hydrocarbon radical as a reducing agent

427

still much lower in comparisons with those in N2 and CO2 atmospheres.

428

Meanwhile, regarding the nitrogen-containing radicals under both fuel-rich and

429

stoichiometric conditions, in CO2 atmosphere, NCO radical occupies an

430

overwhelmingly dominant position in NO reduction due to HCN → CH3CN →

431

CH2CN → CN → NCO and HNCO → NCO channel is amplified substantially. In

432

H2O atmosphere, NH2 and NH radical are dominant due to the enhancement of NCO

433

→ HNCO → NH2 → NH channel.

434

Under fuel-lean conditions, the higher O2 levels exert further influence on

435

promoting the formation of O radical and the oxidation of hydrocarbon radicals

436

inhibiting the NO reduction. CO2 and H2O alter the radical pool structure diminishing

437

the influence of high O2 levels. Likewise, the diminishment by H2O is much stronger

438

than that by CO2. In this case, NCO radical occupies an overwhelmingly dominant

439

position in NO reduction in both CO2 and H2O atmosphere due to the strength of

440

HNCO → NCO channel.

441 442

Acknowledgments

443

This work was supported by the National Key Research & Development Special

444

Project (No. 2016YFB0600801) of the National Natural Science Foundation of China.

445 446

References

447

(1) Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.; Tignor,

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M.; Miller, H. IPCC, Climate change 2007: the physical science basis. Contribution of

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climate change. Cambridge University Press, Cambridge, United Kingdom and New

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York, NY, USA: 2007.

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(4) Buhre, B.; Elliott, L.; Sheng, C.; Gupta, R.; Wall, T., Oxy-fuel combustion

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(5) Scheffknecht, G.; Al-Makhadmeh, L.; Schnell, U.; Maier, J., Oxy-fuel coal

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(9) Seepana, S.; Jayanti, S., Steam-moderated oxy-fuel combustion. Energy

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(10) Andersson, K.; Normann, F.; Johnsson, F.; Leckner, B., NO emission during 19


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oxy-fuel combustion of lignite. Industrial & Engineering Chemistry Research 2008,

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Formation and Reduction Mechanisms in O2/CO2 Combustion. Energy & Fuels 2012,

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Methane and Ammonia during Oxy-Steam Combustion. Energy & Fuels 2016, 30, (8),

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6799-6807.

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2009, 35, (5), 385-397.

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concentration on NOx reduction in the natural gas reburning process. Industrial &

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engineering chemistry research 1994, 33, (11), 2846-2852.

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(23) Bilbao, R.; Alzueta, M. U.; Millera, A., Experimental study of the influence of

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the operating variables on natural gas reburning efficiency. Industrial & engineering

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chemistry research 1995, 34, (12), 4531-4539.

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(24) Bilbao, R.; Alzueta, M. U.; Millera, A.; Duarte, M., Simplified kinetic model of

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the chemistry in the reburning zone using natural gas. Industrial & engineering

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chemistry research 1995, 34, (12), 4540-4548.

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(25) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K., Low temperature interactions

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between hydrocarbons and nitric oxide: an experimental study. Combustion and flame

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1997, 109, (1), 25-36.

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(26) Dagaut, P.; Lecomte, F.; Chevailler, S.; Cathonnet, M., Experimental and detailed

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kinetic modeling of nitric oxide reduction by a natural gas blend in simulated

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reburning conditions. Combustion science and technology 1998, 139, (1), 329-363.

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(27) Prada, L.; Miller, J., Reburning using several hydrocarbon fuels: a kinetic

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modeling study. Combustion science and technology 1998, 132, (1-6), 225-250. 21


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(28) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A., Kinetic modeling

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of hydrocarbon/nitric oxide interactions in a flow reactor. Combustion and flame 1998,

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115, (1), 1-27.

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(29) Dagaut, P.; Luche, J.; Cathonnet, M., The kinetics of C1 to C4 hydrocarbons/NO

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28, (2), 2459-2465.

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mechanisms in staged O2/CO2 combustion. Combustion and flame 2011, 158, (7),

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1255-1263.

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experimental parametric study of gas reburning under conditions of interest for

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oxy-fuel combustion. Fuel Processing Technology 2011, 92, (3), 582-589.

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Engineering Chemistry Research 2010, 49, (19), 9088-9094.

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Energy & Fuels 2009, 23, (7), 3565-3572.

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(34) Skjøth-Rasmussen, M. S.; Glarborg, P.; Østberg, M.; Johannessen, J.; Livbjerg,

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546 547 548 549 22


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Energy & Fuels

550

Figure captions

551

Fig. 1. Schematic diagram of the experimental apparatus.

552

Fig. 2. Typical temperature profiles within the reactor.

553

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

554

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

555

temperature.

556

Fig. 4. Comparisons of H, O, and OH mole fraction profiles in the N2, CO2 and H2O

557

atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673

558

K.

559

Fig. 5. Comparison of the NO conversion pathways in N2 (a), CO2 (b) and H2O (c)

560

atmospheres for fuel-rich condition at 1673 K.

561

Fig. 6. OPRNO of critical reactions between nitric oxide and hydrocarbon in the N2,

562

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

563

conditions at 1673 K.

564

Fig. 7. Comparison of the methane oxidation pathways in the N2 (a), CO2 (b) and H2O

565

(c) atmospheres for stoichiometric condition at 1673 K.

566

Fig. 8. OPRNO of critical reactions between nitric oxide and nitrogen-containing

567

radicals in the N2, CO2 and H2O atmospheres for fuel-rich (a), stoichiometric (b) and

568

fuel-lean (c) conditions at 1673 K.

569

Fig. 9. Experimental data and numerical predictions as functions of equivalence ratio

570

at 1673 K in the N2, CO2 and H2O atmospheres.

571 572

Table captions

573

Table 1. Experimental conditions in the present work.

574 23


Energy & Fuels

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575

Fig. 1. Schematic diagram of the experimental apparatus. 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 24


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Energy & Fuels

Fig. 2. Typical temperature profiles within the reactor. 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 25


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

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. 622 623 624 625 626 26


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Energy & Fuels

a

b

c

Fig. 4. Comparisons of H, O, and OH mole fraction profiles in the N2, CO2 and H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K. 627 628 629 630 631 632 633 27


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

a

b

c

Fig. 5. Comparison of the NO conversion pathways in N2 (a), CO2 (b) and H2O (c) atmospheres for fuel-rich condition at 1673 K. 634 635 636 637 638 639 640

28


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Energy & Fuels

a

b

c

Fig. 6. OPRNO of critical reactions between nitric oxide and hydrocarbon in the N2, CO2 and H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K. 641 642 643 644 645 646 647 648 649 650 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

a

Page 30 of 33

b

c

Fig. 7. Comparison of the methane oxidation pathways in the N2 (a), CO2 (b) and H2O (c) atmospheres for stoichiometric condition at 1673 K. 651 652 653 654 655 656

30


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Energy & Fuels

a

b

c

Fig. 8. OPRNO of critical reactions between nitric oxide and nitrogen-containing radicals in the N2, CO2 and H2O atmospheres for fuel-rich (a), stoichiometric (b) and fuel-lean (c) conditions at 1673 K. 657 658 659 660 661 662 663 664 665 666 31


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. Experimental data and numerical predictions as functions of equivalence ratio at 1673 K in the N2, CO2 and H2O atmospheres. 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687

32


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Energy & Fuels

688 689

φ 1.6 1.0 0.2

CH4/ppm 2511 2505 2499

Table 1. Experimental conditions in the present work. NO/ppm O2/ppm N2 or CO2 or H2O(g)/% Residence time/s 1006 3147 99.34 2038/T(K) 1003 5062 99.15 2038/T(K) 998 24977 97.15 2038/T(K)

690

33


CO2 and O2

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