Experimental and Numerical Study of the Effects of Steam Addition on

Aug 4, 2017 - Comparison experiments between O2/CO2 and O2/CO2/H2O atmospheres were conducted in a flow reactor at atmospheric pressure with equivalen...
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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 State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, P. R. China ABSTRACT: The effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion was investigated both experimentally and numerically. Comparison experiments between O2/CO2 and O2/CO2/H2O atmospheres were conducted in a flow reactor at atmospheric pressure with equivalence ratios ranging from fuel-rich to fuel-lean and temperature from 973 to 1773 K. The experimental results indicate that the effects of H2O addition shift the onset temperature of oxidation to the lower values, inhibit CO formation significantly, and enhance NO formation remarkably. The chemical kinetic mechanism, which was hierarchically structured and updated in our previous work, captured the main characteristics of CO and NO formation satisfactorily. The presence of H2O leads to far higher OH radical concentrations in the CO2/H2O atmospheres. The ultrahigh OH radical concentrations dramatically enhance the reactions between OH and amine radicals, resulting in the significant enhancement of pathways NH2 → NH → HNO → NO and NH2 → NH → N → NO in CO2/H2O atmospheres. Meanwhile, NH2 → CHxNHy/HNCO → NCO → NO is vastly demoted in CO2/H2O atmospheres. The increase in pathways NH2 → NH → HNO → NO and NH2 → NH → N → NO is always much more than the decline in pathway NH2 → CHxNHy/ HNCO → NCO → NO. Hence, H2O addition in oxy-fuel combustion enhances NO formation during the oxidation of methane and ammonia. In addition, the effects of H2O addition become stronger on enhancing NO formation with the increasing H2O concentration in CO2/H2O atmospheres by further amplifying the amount of OH radicals.

1. INTRODUCTION The environmental crisis caused by greenhouse gases has drawn extensive attention in the international community in recent decades. Carbon dioxide (CO2) emitted from the combustion of fossil fuels is the primary greenhouse gas at the present stage.1,2 In response to the emission challenges, oxy-fuel combustion has been comprehensively considered as a promising alternative technology for carbon capture and storage.3−8 Oxy-fuel combustion implies that the recycled flue gases are used to moderate the high temperature generated by fuel combustion with pure oxygen, instead of air, ensuring that the CO2 volume concentration exceeds 90% in the exhaust gas, which is almost sequestration-ready. It also has been found that NO emissions can be reduced during oxy-fuel combustion compared with conventional air combustion for coal;9−14 hence, the mechanisms and characteristics of NO formation impacted by CO2 have attracted considerable attention in recent years.15−18 Because the nitrogen chemistry mechanisms for coal combustion are extremely complicated, homogeneous gas-phase chemistry was chosen to provide a theoretical foundation for the underlying mechanisms in many investigations. Kim et al.19 numerically studied the effect of CO2 addition on NO emission in H2/N2 laminar diffusion flame, and they pointed out that the C-related reactions affect the production of prompt NO in the case of CO2 addition. Park et al.20 numerically investigated the chemical effect of CO2 dilution on NO emission characteristic in methane−air counterflow diffusion flame, and results showed that the mole production rates of nitrogenous species are prevented considerably. Park et al.21 also computationally © 2017 American Chemical Society

examined NO emission behavior in methane oxy-fuel combustion recirculated with CO2, and they pointed out that the chemical effects of recirculated CO2 not only reduce the formation and destructions of NO through the Fenimore mechanism but also suppress the NO formation through the thermal mechanism. The chemical effects of CO2 addition in ethylene diffusion flame were demonstrated by Liu et al.,22 and numerical results showed that CO2 reduces NO emissions and the reaction CO2 + H = CO + OH is primarily responsible for the chemical effects of CO2 addition. Meanwhile, due to the absence of N2 in oxy-fuel combustion, NO emissions almost completely derive from fuel-NO. Hence, the conversions of hydrogen cyanide and ammonia, which are main precursors of NO in the coal combustion, to NO become the focus of researches is this area. Giménez-López et al.23 studied the oxidation of HCN in O2/CO2 atmospheres in a flow reactor both experimentally and numerically. It was found that CO2 + H = CO + OH competes with O2 + H = O + OH, reducing the formation of chain carriers, which clearly leads to inhibiting HCN oxidation. In the experimental and numerical investigations of ammonia oxidation in O2/CO2 atmospheres, both Mendiara et al.24 and Watanabe et al.25 concluded that the increased OH/H ratio and high CO levels increase the probability of forming N2 instead of NO. The review of literature above is mainly concerned with CO2 addition and O2/CO2 combustion atmospheres. In fact, steam Received: May 31, 2017 Revised: July 17, 2017 Published: August 4, 2017 10093

DOI: 10.1021/acs.energyfuels.7b01550 Energy Fuels 2017, 31, 10093−10100

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Energy & Fuels is also a main composition in flue gases during oxy-fuel combustion.26 Therefore, oxy-fuel combustion with H2O addition has drawn researchers’ attentions recently. The effects of steam addition on coal gasification,27 coal ignition,28−30 and methane oxidation31 during oxy-fuel combustion have been investigated; however, little information is available for the effects of steam addition on NO formation during oxy-fuel combustion. In this work, a systematic experimental study of the oxidation of methane and ammonia in O2/CO2/H2O atmospheres was carried out at atmospheric pressure with equivalence ratios ranging from fuel-lean to fuel-rich conditions (i.e., 0.2, 1.0, and 1.6) and temperature from 973 to 1773 K. The experiments were performed in a laboratory plug flow reactor. Ammonia was chosen because it is the main N-compound in the devolatilization of biomass and low-rank coal,5,32 and its production is greater than that of HCN in the presence of steam.27,33 The experimental results are analyzed in terms of a chemical kinetic model with 168 species and 1208 reactions, which was hierarchically structured and updated in our previous work.34 In addition, the effects of H2O concentration on NO formation from NH3 were also addressed.

Figure 2. Temperature profiles within the reactor. generate a predefined vapor flow, accruing from an accurately controlled distilled water mass flow injected into an accurately controlled carrier gas (CO2) flow with subsequent evaporation inside a temperature controlled chamber. This predefined vapor flow was preliminarily blended with the oxygen and remaining carbon dioxide in a isothermal chamber at 473 K. Then the total flow was premixed sufficiently with methane and ammonia in the mixer prior to entering the reactor. In order to avoid steam condensation, heating tapes were used to heat the pipeline connecting the isothermal chamber, the mixer, and the reactor to maintain the temperature at a constant value of 423 K. A water cooler was installed at the outlet of the reactor to cool the product gas rapidly. In the case of steam addition, argon (Ar) of equal volume to steam was fed into the product gas to compensate for volume loss due to steam condensation. The concentrations of CO and NO in the product gas were measured online using Fourier transform infrared spectroscopy (GASMET-DX4000) with a resolution of 8 cm−1 and scanning speed of 10 scans/s. The uncertainty of the measurement is estimated as ±1%. In order to minimize the axial dispersion of the reactants and comply with a reasonable plug-flow approximation, the total flow rate for all experiments was kept constant at 1 L/min (standard temperature and pressure), which has been investigated and validated by Skjøth-Rasmussen35 and Glarborg et.al.24 Also, the reactants were highly diluted with carbon dioxide or carbon dioxide and steam to minimize the influence of temperature rise due to chemical reaction. The methane concentration was about 2500 ppm, the ammonia concentration was approximately 500 ppm, and the oxygen concentration was calculated according to the defined equivalence ratios. In consideration of the application of rich and lean equivalence ratios in staged combustion, the experiments were conducted covering a wide range of equivalence ratios to investigate the effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion. Three equivalence ratios, i.e., 0.2, 1.0, and 1.6, were chosen on behalf of fuel-lean, stoichiometric, and fuel-rich conditions, respectively. The calculations of the equivalence ratios were based on the oxidation reaction as follows:

2. EXPERIMENTAL AND MODELING Figure 1 schematically demonstrates the experimental apparatus used in the present work. The apparatus is comprised of a gas feeding

Figure 1. Schematic diagram of the experimental apparatus. system, a VDM (vapor delivery module) system, a flow reactor, and a gas composition test system. The flow reactor with an internal diameter of 12 mm and a length of 1100 mm was constructed according to Skjøth-Rasmussen et al.35 for homogeneous gas-phase reactions. Alumina was chosen as the material of the flow reactor in order to avoid the catalytic effect on experimental results. The flow reactor was heated using an electrically heated oven, which allows for a maximum temperature up to 1800 K. The temperature profiles within the reactor were measured using a type S thermocouple under inert conditions (1 L/min CO2). The uncertainty of the temperature measurement is ±4 K. Typical temperature profiles are demonstrated in Figure 2, which implies that the length of isothermal reaction zone is approximately 700 mm. The temperature of the isothermal zone is referred to as the reaction temperature in this work. High-purity gases (99.99%) were separately supplied from gas cylinders, and the flow rates were controlled precisely by mass flow controllers. The Bronkhorst VDM system, a compact integrated system to realize mass flow control of vapor, was used to add H2O vapor to O2/CO2 atmospheres. The VDM system was intended to

xCH4 + y NH3 + (2x + 1.25y)O2 → xCO2 + y NO + (2x + 1.5y)H 2O

φ=

(1)

(CH4 + NH3)/O2 [(CH4 + NH3)/O2 ] stoic

(2)

Experiments were carried out in the temperature range of 973− 1773 K at intervals of 20 K. Detailed experimental conditions are listed in Table 1.

Table 1. Experimental Conditions in the Present Work

10094

φ

CH4/ ppm

NH3/ ppm

O2/ ppm

CO2/%

H2O/%

1.6 1.0 0.2

2511 2505 2499

507 505 503

3531 5647 28276

99.35-H2O% 99.13-H2O% 96.86-H2O%

1,5,10,15,20,25,30 1,5,10,15,20,25,30 1,5,10,15,20,25,30

DOI: 10.1021/acs.energyfuels.7b01550 Energy Fuels 2017, 31, 10093−10100

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Energy & Fuels Numerical predictions adopting full experimental temperature profiles within the flow reactor were performed using the plug flow reactor (PFR) module in conjunction with CHEMKIN-PRO. The mechanism adopted in this work was hierarchically established for the oxidation of methane and ammonia in O2/H2O atmospheres in our previous study,34 containing the comprehensive oxidation mechanism for hydrogen, C1−C2 hydrocarbons, nitrogen-containing species (HCN, NH3), and the interactions of these compositions. The mechanism contained 170 species and 1208 reactions. Additional details and validations of this mechanism can be found elsewhere.34

3. RESULTS AND DISCUSSION Figure 3 shows the comparisons between the experimental and numerical results of the CO and NO profiles as a function of reaction temperature at two different atmospheres (O2/CO2 and O2/CO2/H2O) and equivalence ratios (fuel-rich, stoichiometric, and fuel-lean conditions). Closed symbols and solid lines represent experimental and numerical results in CO2 atmospheres, whereas open symbols and dashed lines represent those in CO2/H2O atmospheres. Under fuel-rich conditions, as displayed in Figure 3a, the CO formation in the CO2 case occurs at 1113 K, increases sharply to peak (5985 ppm) at 1393 K, and then increases again moderately with temperature above 1453 K. The NO formation increases extremely slowly at the beginning until it steps to 56 ppm at 1413 K and then increases very slowly up to 108 ppm at 1773 K. In the CO2/H2O case, CO formation initiates at 1093 K, goes up to peak (3830 ppm) at 1373 K, and then increases slightly with temperature above 1393 K. Similar to that in the CO2 case, the NO concentration jumps to 78 ppm at 1393 K and then increases gradually up to 166 ppm at 1773 K. Under stoichiometric conditions exhibited in Figure 3b, the CO formation in the CO2 case starts to rise at 1133 K, exhibits a peak (3104 ppm) at 1333 K, and then increase again gradually with reaction temperature above 1393 K. The NO formation suddenly steps to 85 ppm at 1353 K and then increases slightly with reaction temperature up to 142 ppm at 1773 K. In the CO2/H2O case, CO formation exhibits a semblable tendency with a peak value of 3088 ppm at 1313 K. The NO concentration jumps to 100 ppm at 1333 K and then increases gradually to a steady value of 191 ppm above 1633 K. Under fuel-lean conditions, as shown in Figure 3c, the CO formation increases and decreases dramatically exhibiting a maximum value of 353 ppm at 1113 K and then increases again with temperature in the CO2 case. The NO formation increases gradually up to 199 ppm at 1453 K and then nearly levels off. In the CO2/H2O case, the CO formation demonstrates a maximum value of 673 ppm at 1113 K, decreases to zero, and remains undetected until it increases again extremely slowly with temperature above 1453 K. The NO formation increases gradually up to 242 ppm at 1533 K and then remains nearly unchanged. Synthesizing the comparisons of the results observed in both CO2 and CO2/H2O atmospheres, the effects of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion can be summarized as follows: (1) it shifts the onset temperature of oxidation to the lower values; (2) it inhibits CO formation significantly; and (3) it enhances NO formation markedly. It can be seen from Figure 3 that the chemical kinetic model satisfactorily reproduces the main features of CO and NO formation measured in experiments in both CO2 and CO2/ H2O atmospheres, although tolerant deviations exist especially under fuel-lean conditions. Hence, the mechanism proposed

Figure 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.

previously is appropriate for revealing the effects of H2O addition on the ammonia oxidation during oxy-fuel combustion of methane. In the process of fuel oxidation and pollutants formation, the radical pool has been generally considered as a critical participant; hence, it is always dispensable to analyze the radical pool structure at first. Figure 4 compares the H, O, and OH mole fractions profiles in CO2 and CO2/H2O atmospheres under fuel-rich, stoichiometric, and fuel-lean conditions at 1673 10095

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Energy & Fuels OPR i , j =

∫0

l

ωi , j dx

(3)

where i denotes species, j denotes elementary reaction, ωi,j means the mole production rate of species i through elementary reaction j, and l is the length of reaction zone. Figure 5 compares the OPRNO of importantly contributing reaction steps between CO2 and CO2/H2O atmospheres

Figure 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.

K. As shown in Figure 4, OH radicals are consistently predominant in the radical pool under all three conditions for both CO2 and CO2/H2O atmospheres. It also can be seen that OH radicals in CO2/H2O atmospheres are much higher, while H and O radicals are nearly tantamount compared with those in CO2 atmospheres for different equivalence ratios. In CO2 cases, R33 (H + CO2 = CO + OH) competes for H radicals with the main chain branching reaction R1 (H + O2 = O + OH) leading to the dominant position of OH radicals. However, in CO2/ H2O cases, the presence of H2O substantially enhances the reactions R30 (H + H2O = OH + H2) and R14 (O + H2O = OH + OH), yielding a large amount of OH radicals and simultaneously consuming large amounts of H and O radicals. Hence, OH radicals are far higher in CO2/H2O atmospheres than those in CO2 atmospheres. It also can be found that O radicals increase to some extent under fuel-lean conditions for both CO2 and CO2/H2O atmospheres. This can be attributed to the fact that the increasing O2 concentration strengthens R1 (H + O2 = O + OH) subsequently producing more O radicals under fuel-lean conditions. 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:

Figure 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.

covering from fuel-rich to fuel-lean conditions. The top ten reactions contributing to NO formation for each case are extracted to be synthesized in Figure 5. It is compelling that significant differences exist in common among fuel-rich, stoichiometric, and fuel-lean conditions between CO2 and 10096

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in Figure 5. In addition, it is noteworthy that the channel of NH2 → HNCO is reversed in CO2/H2O atmospheres. The ultrahigh CO concentration in CO2 atmospheres promotes HNCO formation through −R978 (NH2 + CO = HNCO + H). However, H2O addition decreases the CO concentration sharply enough to reverse −R978 (NH2 + CO = HNCO + H) in CO2/H2O atmospheres. Therefore, the channel of NH2 → HNCO → NCO is forbidden in CO2/H2O atmospheres. For NH conversion, the channels of NH → HNO and NH → N are facilitated through R846 (NH + OH = HNO + H) and R847 (NH + OH = N + H2O), yielding a larger amount of HNO and N radicals in CO2/H2O atmospheres. This can be attributed to more sufficient NH and OH radicals in CO2/H2O atmospheres compared with those in CO2 atmospheres. Subsequently, sufficient HNO, N, and OH radicals strengthen R775 (HNO + OH = NO + H2O) and R857 (N + OH = NO + H) as shown in Figure 5, leading to the enhancement of pathways (c) NH2 → NH → HNO → NO and (d) NH2 → NH → N → NO in CO2/H2O atmospheres. For NCO conversion, NCO is almost all derived from the channel of NH2 → CHxNHy/HNCO → NCO. As mentioned above, the channel of NH2 → CHxNHy/HNCO → NCO is remarkably suppressed in CO2/H2O atmospheres, leading to the suppression of NCO formation. Hence, R1005 (NCO + O = NO + CO) and R1009 (NCO + O2 = NO + CO2) are inhibited dramatically for NO formation, as shown in Figure 5, implying that pathway (e) NH2 → CHxNHy/HNCO → NCO → NO is vastly demoted in CO2/H2O atmospheres. In order to reveal the NO formation mechanism quantitatively from the pathway point of view, the conversion rate of nitrogen was introduced to evaluate the conversion of NH3 to NO, which is defined as

CO2/H2O atmospheres, which are summarized as follows: (1) the contribution to NO of R755 (HNO + OH = NO + H2O) is enhanced vastly in CO2/H2O cases; (2) the OPR of R857 (N + OH = NO + H) is strengthened dramatically in CO2/H2O cases; and (3) the NCO-related reactions R1005 (NCO + O = NO + CO) and R1009 (NCO + O2 = NO + CO2), which are dominant in CO2 atmospheres, are suppressed sharply in CO2/ H2O cases. It can be seen that the presence of H2O alters the structure of radical pool, resulting in a remarkable variation of oxidation pathway of NH3. Hence, it is indispensable to carry out an elaborate analysis on the oxidation pathways of NH3. Figure 6 compares the critical NO formation pathways between the CO2 and CO2/H2O atmospheres. As demon-

CR NO =

MNO MN

(4)

where MNO designates the mass of N in NO and MN represents the mass of N in NH3. The conversion rate (CRNO) of each pathway, which is obtained based on the ORP of all of the relevant elementary reactions along the NO formation pathways, is compared between CO2 and CO2/H2O atmospheres demonstrated in Figure 7. It can be seen from Figure 7 that pathways (e) NH2 → CHxNHy/HNCO → NCO → NO and (c) NH2 → NH →

Figure 6. Comparison of the NO formation pathways between the O2/CO2 (a) and O2/CO2/H2O (b) atmospheres for stoichiometric conditions at 1673 K.

strated in Figure 6, NH3 is initially converted into NH2 through hydrogen abstraction reaction. Then, the pathways from NH2 to NO can be generally divided into five main pathways as follows: (a) NH2 → HNO → NO; (b) NH2 → NH → NO; (c) NH2 → NH → HNO → NO; (d) NH2 → NH → N → NO; and (e) NH2 → CHxNHy/HNCO → NCO → NO. For NH2 conversion, R829 (NH2 + OH = NH + H2O) is enhanced by the abundant OH radicals in CO2/H2O atmospheres, implying that NH2 converts more to NH radicals instead of reacting with other species. Correspondingly, the channel of NH2 → CHxNHy through R1090 (CH3 + NH2 = CH3NH2) and R1091 (CH3 + NH2 = CH2NH2 + H) is weakened dramatically, and ultimately the channel of NH2 → CHxNHy/HNCO → NCO is suppressed. Meanwhile, the promotion of the channel of NH2 → NH in CO2/H2O atmospheres alters the availability of NH2 and NH radicals, which is the reason for the change of main NO reduction reactions R839 (NH2 + NO = N2 + H2O), R840 (NH2 + NO = NNH + OH), and R853 (NH + NO = N2O + H) as displayed

Figure 7. Comparison of the elemental N conversion rates (CRNO) obtained through each pathway at 1673 K in O2/CO2 and O2/CO2/ H2O atmospheres. 10097

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Energy & Fuels HNO → NO are dominant for NO formation in CO2 and CO2/H2O atmosphere, respectively. Although pathway (a) NH2 → HNO → NO is fairly critical under fuel-lean conditions, which is attributed to the fact that the extremely high O2 concentration enhances the channel of NH2 → HNO through R833 (NH2 + O2 = HNO + OH), the effect of H2O addition on pathway (a) is limited as shown in Figure 7. As discussed in the previous section, the H2O addition in oxy-fuel combustion strengthens pathway (c) NH2 → NH → HNO → NO significantly reflecting in that the increase values of CRNO through pathway (c) are 12.4%, 12.8%, and 9.3% under fuelrich, stoichiometric, and fuel-lean conditions, respectively. It also enhances pathway (d) NH2 → NH → N → NO reflecting in that the increase values of CRNO through pathway (d) are 4.1%, 4.5%, and 2.8% under fuel-rich, stoichiometric, and fuellean conditions, respectively. Meanwhile, the H2O addition weakens pathway (e) NH2 → CHxNHy/HNCO → NCO → NO markedly embodied by the fact that the declining values of CRNO through pathway (e) are 10.8%, 10.6%, and 4.3% under fuel-rich, stoichiometric, and fuel-lean conditions, respectively. It can be found that the increase of pathways (c) and (d) is always superior to the decline of pathway (e) in CO2/H2O atmospheres. Hence, H2O addition in oxy-fuel combustion enhances NO formation during the oxidation of methane and ammonia. In addition, the effect of H2O concentration on NO formation will be discussed in the following section. Figure 8

Figure 9. Comparison of the elemental N conversion rates (CRNO) among different H2O concentrations at 1673 K in O2/CO2/H2O atmospheres.

tration while pathway (e) NH2 → CHxNHy/HNCO → NCO → NO is evidently suppressed. Specially, pathway (a) NH2 → HNO → NO also decreases with the increasing H2O concentration under fuel-lean conditions. However, the increase of pathways (c) and (d) with the increasing H2O concentration is always overwhelmingly dominant in CO2/H2O atmospheres which is responsible for the increasing NO formation. The fundamental reason for this is that the increasing H2O concentration further enhances the reactions R30 (H + H2O = OH + H2) and R14 (O + H2O = OH + OH) yielding a larger amount of OH radicals, which amplifies the effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion.

4. CONCLUSIONS The effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion was investigated both experimentally and numerically. Comparison experiments between CO2 and CO2/H2O atmospheres were accomplished in a flow reactor at atmospheric pressure with equivalence ratios ranging from fuel-rich to fuel-lean and temperatures from 973 to 1773 K. The comparative experimental results indicate that the effects of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion shift the onset temperature of oxidation to the lower values, inhibit CO formation significantly, and enhance NO formation markedly. The underlying mechanisms have been revealed using a detailed chemical kinetic mechanism in the production rate and pathway analysis point of view. The H2O addition substantially enhances the reactions H + H2O = OH + H2 and O + H2O = OH + OH, leading to far higher OH radical concentrations in the CO2/H2O atmospheres than those in the CO2 atmospheres. The ultrahigh OH radical concentrations dramatically enhance the reactions between OH and amine radicals (NH2, NH, and N), resulting in the significant enhancement of pathway NH2 → NH → HNO → NO and NH2 → NH → N → NO in CO2/H2O atmospheres. NH2 radicals are converted far more to NH radicals inhibiting the channel of NH2 → CHxNHy. Meanwhile, the channel of NH2 → HNCO → NCO is forbidden in CO2/ H2O atmospheres by reversing NH2 + CO = HNCO + H. Consequently, pathway NH2 → CHxNHy/HNCO → NCO → NO is vastly demoted in CO2/H2O atmospheres. However, the increase of pathways NH2 → NH → HNO → NO and NH2 → NH → N → NO is always much more than the decline of pathway NH2 → CHxNHy/HNCO → NCO → NO in CO2/

Figure 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.

shows the comparisons between the experimental and numerical results of the CO and NO profiles as a function of H2O concentration in CO2/H2O atmospheres at different equivalence ratios (fuel-rich, stoichiometric, and fuel-lean conditions). As displayed in Figure 8, the increasing H2O concentration inhibits the CO formation and enhances the NO formation. Meanwhile, the prediction results are in good agreement with the experiments data, and this offers a further validation for the applicability of the present mechanism. Figure 9 demonstrates that the variations of the conversion rate (CRNO) of each pathway with the increasing H2O concentration (5%, 15%, and 30%) at different equivalence ratios (fuel-rich, stoichiometric, and fuel-lean conditions) in CO2/ H2O atmospheres. It indicates that pathways (c) NH2 → NH → HNO → NO and (d) NH2 → NH → N → NO are dramatically strengthened with the increasing H2O concen10098

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Energy & Fuels

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H2O atmospheres. Hence, H2O addition in oxy-fuel combustion enhances NO formation during the oxidation of methane and ammonia. In addition, the effects of H2O addition become stronger on enhancing NO formation with the increasing H2O concentration in CO2/H2O atmospheres by further amplifying the amount of OH radicals.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 2787542417-8314. Fax: +86 2787545526. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chun Zou: 0000-0002-7734-515X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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

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DOI: 10.1021/acs.energyfuels.7b01550 Energy Fuels 2017, 31, 10093−10100

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DOI: 10.1021/acs.energyfuels.7b01550 Energy Fuels 2017, 31, 10093−10100