Experimental and Numerical Study of the Effect of High Steam

Article pubs.acs.org/EF

Experimental and Numerical Study of the Effect of High Steam Concentration on the Oxidation of Methane and Ammonia during Oxy-Steam Combustion Yizhuo He, Chun Zou,* Yu Song, Wuzhong Chen, Huiqiao Jia, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China S Supporting Information *

ABSTRACT: The effect of high H2O concentration during oxy-steam combustion on the oxidation of methane and ammonia was investigated both experimentally and numerically. Comparison experiments between O2/N2 and O2/H2O atmosphere were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 to 1773 K. Experimental results showed that the presence of high H2O concentration dramatically suppressed CO formation at temperatures above 1300 K. High H2O concentrations inhibited NO formation under stoichiometric and fuel-lean conditions but enhanced NO formation under fuel-rich conditions. The chemical kinetic mechanism, which was hierarchically structured and updated, satisfactorily reproduced the main characteristics of CO and NO formation. High H2O concentrations significantly alter the structure of radical pool and subsequently the formation of CO and NO. Ultralow CO concentrations above 1300 K are attributed to the enhancement of CO + OH ⇄ CO2 + H by high OH radical concentrations. NO suppressions under stoichiometric and fuel-lean conditions are caused by strong suppression of NH2 + O ⇄ H + HNO in the pathway NH2 → HNO → NO. This suppression is due to the lack of O radicals. By contrast, NO enhancement under fuel-rich conditions is caused by the significant enhancement of NH2 + OH ⇄ NH + H2O in the pathway NH2 → NH → HNO → NO. This enhancement is due to the fairly high OH concentration in the O2/H2O atmosphere. dilution, reduction of N2, and thermal effects. Mazas et al.12 found that the effect of steam addition (0−40% in volume) under lean and near-stoichiometric conditions is greater than that under rich conditions. In the study of Abián et al.13 using Glarborge’s mechanism14 with minor modifications and updates, the group observed that steam addition (0.1−32.1% in volume) enhances CO oxidation. The studies above focus mainly on the effects of steam at a mole fraction of less than 50% as an additive on combustion. However, the effects of high steam concentrations (99.35, 99.13, and 96.86% for fuel-rich, stoichiometric, and fuel-lean conditions in the study, respectively) on the oxidation of fuel and ammonia during oxy-steam combustion remain poorly understood. In this article, a systematic experimental study of the oxidation of methane and ammonia in an O2/H2O atmosphere at 1 atm over a wide range of equivalence ratios (i.e., 0.2, 1.0, and 1.6) and within the reaction temperature range of 973−1773 K was performed. The study was conducted in a laboratory plugflow reactor. Ammonia was adopted because hydrogen cyanide (HCN) and ammonia (NH3) are the main precursors of NO in coal combustion. Moreover, the production of NH3 is considerably greater than that of HCN in the case of a highsteam atmosphere.15 A chemical kinetic model with 168 species and 1208 reactions was hierarchically structured and updated, and new experimental data are analyzed in terms of this model to evaluate the oxidation mechanisms of methane and ammonia in an oxy-steam atmosphere.

1. INTRODUCTION Global warming caused by greenhouse gases is considered a worldwide issue. Carbon dioxide (CO2) is the primary greenhouse gas emitted through the combustion of fossil fuels to supply energy.1 Oxy-fuel combustion is a promising technology for CO2 abatement in coal-fired power generation.2,3 In the oxysteam combustion process proposed by Carlos4 and Seepana,5 steam, instead of N2 or CO2, is used to moderate the high temperature produced by fuel combustion with oxygen. This combustion process is regarded as a next-generation oxy-fuel combustion technology because it features a compact system, ease of operation, small geometric size, low emissions, and energy savings. Richards et al.6 compared the combustion performance of O2/CO2 combustion to O2/H2O combustion by using numerical calculations and experiments. The team claimed that CO levels in O2/CO2 combustion are higher than those in O2/H2O combustion. Zou et al.7,8 studied the effects of steam on temperature and NO formation during oxy-steam combustion by numerical simulation. The group found that steam can change the oxidation pathway of fuel and nitrogen-containing compounds in O2/H2O atmosphere. However, these numerical results were not directly validated by experiments. Seiser et al.9 suggested that water addition (0−15% in volume) favors extinction and inhibits ignition because of the chaperon efficiency of steam. Experimental and computational results by Cong and Dagaut10,11 show that steam addition (0−20% in volume) inhibits oxidation of methane because of the enhancement of the three-body reaction, H + O2 + M ⇄ HO2 + M, the competition between reactions H2O + O ⇄ OH + OH and CH4 + O ⇄ CH3 + OH, and reduced NOx formation due to © 2016 American Chemical Society

Received: April 25, 2016 Revised: July 15, 2016 Published: August 9, 2016 6799

DOI: 10.1021/acs.energyfuels.6b00993 Energy Fuels 2016, 30, 6799−6807

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Energy & Fuels

a resolution of 8 cm−1 and scanning speed of 10 scans per second. The uncertainty of this on-line measurement is estimated as ±1%. In all experiments, the total flow rate was kept constant at 1 L/min (standard temperature and pressure) to minimize the axial dispersion of the reactants and conform to a reasonable plug-flow approximation, which has been discussed and validated by Skjøth-Rasmussen and Glarborg et al.16,17 In addition, the reactants were highly diluted in nitrogen or steam to diminish the impact of temperature increases. The methane concentration was approximately 2500 ppm, the ammonia concentration was around 500 ppm, and the oxygen concentration was varied according to the designed equivalence ratios. Considering that the rich and lean equivalence ratios can be applied in staged combustion, a wide range of equivalence ratios was chosen to validate the availability of the proposed detailed chemical mechanism in this work. Three equivalence ratios, namely, 0.2, 1.0, and 1.6, were considered to represent fuel-lean, stoichiometric, and fuel-rich conditions, respectively. Although water has an impact on the combustion behavior, very small amount of oxygen atoms coming from steam participate in oxidation reactions, and the amount is very difficult to determine. The overwhelming proportion of steam plays a role of diluent gas, similar to CO2 in O2/CO2 atmospheres, in moderating the high temperature in the combustion system. Hence, the oxygen atom in steam is not considered in the calculation of equivalence ratio. The equivalence ratio was calculated according to the oxidation reaction as follows:

2. EXPERIMENTAL SECTION The experimental apparatus referred to in the study of Glarborg16 is schematically depicted in Figure 1. The apparatus consists of a gas

Figure 1. Schematic diagram of the experimental apparatus. feeding system, a steam generating system, a flow reactor, and a gas analysis system. The flow reactor, which features a 12 mm internal diameter and 1100 mm length according to Kristensen et al.,17 was designed for homogeneous gas-phase reactions. It was made of alumina in order to eliminate the catalytic effect. The reactor was heated by an electrically heated oven, which allows temperatures of up to 1800 K. The temperature profile along the reactor was measured using a type S thermocouple under inert conditions (1 L/min N2). The uncertainty of the temperature measurement is ±4 K. Typical temperature profiles are displayed in Figure 2, which indicates that the isothermal reaction zone is about 700 mm. The temperature of the isothermal zone is regarded as the reaction temperature in the paper. High-purity gases (99.99%) were separately supplied from gas cylinders and regulated accurately by mass flow controllers. Distilled water was injected into the evaporation chamber at a constant temperature of 473 K by a high-accuracy syringe pump. Oxygen was used to transport the water vapor and mixed with methane and ammonia in the mixer, in which reactants are premixed sufficiently prior to entering the reactor. To avoid steam condensation, the pipeline connecting the evaporation chamber, the mixer, and the reactor was heated by a heating tape wound around the pipeline to maintain a constant temperature of 423 K. At the outlet of the reactor, the product gas was rapidly cooled down by a water cooler. In the case of steam, argon (Ar) of equal volume to nitrogen in the corresponding case of O2/N2 was injected into the product gas to compensate for volume loss because of steam condensation. CO and NO in the product gas were measured on-line by Fourier transform infrared spectroscopy (GASMET-DX4000) with

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 conducted 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 φ

CH4 (ppm)

NH3 (ppm)

O2 (ppm)

N2 or H2O(g) (%)

1.6 1.0 0.2

2508 2506 2511

503 505 505

3533 5642 28378

99.35 99.13 96.86

3. MODELING Numerical calculations applying full experimental temperature profiles through the flow reactor were carried out by the plugflow reactor (PFR) module from CHEMKIN-PRO. The improved mechanism used in this work was hierarchically structured

Figure 2. Temperature profiles within the reactor. 6800


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Energy & Fuels Table 2. Reactions Updated and Modified in the Present Worka

a

no.

reaction

R1 R7 R14 R19 R22 R29 R30 R826 R829 R830 R831 R832 R833 R839 R845 R846 R847 R851 R853 R854 R858 R859 R860 R862 R863 R867

H + O2 ⇄ O + OH H + O2 + M ⇄ HO2 + M OH + OH ⇄ H2O + O OH + HO2 ⇄ H2O + O2 H2O2+M ⇄ 2OH + M H2O + H2O ⇄ OH + H + H2O OH + H2 ⇄ H + H2O NH2 + O ⇄ H + HNO NH2 + OH ⇄ NH + H2O NH2 + HO2 ⇄ H2NO + OH NH2 + HO2 ⇄ NH3 + O2 NH2 + O2 ⇄ H2NO + O NH2 + O2 ⇄ HNO + OH NH2 + NO ⇄ N2 + H2O NH + O ⇄ NO + H NH + OH ⇄ HNO + H NH + OH ⇄ N + H2O NH + NH ⇄ NH2 + N NH + NO ⇄ N2O + H NH + NO ⇄ N2 + OH N + O2 ⇄ NO + O N + NO ⇄ N2 + O NNH ⇄ N2 + H NNH + O ⇄ N2O + H NNH + O ⇄ N2 + OH NNH + O2 ⇄ N2 + HO2

A 1.04 5.59 3.57 2.89 8.59 1.00 4.38 3.60 2.31 2.90 1.65 2.60 2.90 1.30 4.00 2.24 1.12 5.70 1.80 2.70 1.42 3.30 1.00 1.90 1.20 5.60

× × × × × × × × × × × × × × × × × × × × × × × × × ×

1014 1013 104 1013 1014 1026 1013 1013 106 1017 104 1011 10−2 1016 1013 1014 107 10−1 1014 1012 1013 1012 109 1014 1013 1014

β

Ea

source

0.0 0.2 2.4 0.0 0.0 −2.4 0.0 0.0 1.9 −1.3 1.6 0.5 3.8 −1.2 0.0 −0.4 1.7 3.9 −0.4 −0.1 0.0 0.3 0.0 −0.3 0.1 −0.4

15286 0 −2111 −500 48560 120160 6990 0 −217 1248 2027 29050 18185 0 0 −46 −576 342 −244 −512 10600 0 0 −22 −217 −13

19 20, 21 24 23 22 26 25 29 0.7 × ref 27 30 30 28 28 28 GRI3.0 0.7 × ref 27 0.7 × ref 27 27 28 28 31 32 28 28 28 28

Rate constants are expressed as k = ATβ exp(−Ea/RT) with units of calories, cm3, mole, and second.

and improve model agreement under O2/H2O. The detailed modifications and updates are listed in Table 2, and the full mechanism is available in the Supporting Information. In addition, Figure 3 displays the validation of present mechanism against previous experimental data.33,12,34−37 It can be found that the calculation results are in good agreement with experimental results and a better agreement than GRI 3.0 is obtained.

based on the comprehensive oxidation mechanism previously developed for hydrogen,16 C1−C2 hydrocarbons,18 and nitrogencontaining species (HCN and NH3), as well as the interactions of these components.16 The mechanism consisted of 170 species and 1208 reactions. Applied updates and modifications were focused on the key reactions which exert significant influence in O2/H2O cases, such as key chain-branching reactions, three-body reactions of H2O, reactions in which H2O participates, and nitrogencontaining radical-related reactions. For H2/O2 subset, the key chain-branching reaction H + O2 ⇄ OH + O was updated by the value reported by Hong et al.19 The three-body reactions related to H2O were updated due to the chaperon effect in present mechanism. The parameters of H+O2 + M ⇄ HO2 + M were referenced from the results of Bates et al.20 and Michael et al.,21 and those of H2O2 + M ⇄ 2OH + M were referenced from Hong et al.22 The parameters of three reactions H2O participates in, OH + HO2 ⇄ H2O + O2, OH + OH ⇄ H2O + O, and OH + H2 ⇄ H + H2O, which determine the radical pool structure to some extent, were updated according to the recent studies.23−25 Moreover, a new reaction H2O + H2O ⇄ OH + H + H2O proposed by Srinivasan and Michael26 was adopted in the current mechanism. For the nitrogen-containing subset, Klippenstein et al.27,28 investigated NH2OH decomposition and subsequent reactions, thermal DeNOx, and NNH mechanism for NO formation; hence, we updated the NH-, NH2-, and NNH-related reactions according to their results. Among these reactions, given the ultrahigh OH radical concentration in H2O/O2 cases, the rate constants of NH2 + OH ⇄ NH + H2O, NH + OH ⇄ HNO + H, and NH + OH ⇄ N + H2O were reduced by 30% based on those determined by Klippenstein et al.27 to reduce the reactivity

4. RESULTS AND DISCUSSION Figure 4 compares the experimental and modeling results of the CO and NO profiles as a function of temperature at various atmospheres (N2 or H2O) and equivalence ratios (fuel-rich, stoichiometric, and fuel-lean conditions). Closed symbols and solid lines denote experimental and modeling results in N2, whereas open symbols and dashed lines denote those in H2O. For fuel-rich conditions, as shown in Figure 4a, CO formation in the N2 case is initiated at 1113 K, peaks (1944 ppm) at 1313 K, and then reaches a plateau (about 1680 ppm). The NO concentration increases gradually up to 30 ppm at 1613 K and then remains nearly constant. In the case of H2O, CO formation begins at 1053 K, reaches a peak (570 ppm) at 1313 K, and then decreases gradually to a constant value of 60 ppm above 1533 K. The NO concentration increases gradually up to 90 ppm at 1633 K and then remains approximately unchanged. Under the stoichiometric conditions shown in Figure 4b, the CO concentration in N2 starts to rise at 1113 K, exhibits a peak at 1253 K, and then declines gradually. NO formation sharply increases to 148 ppm at 1273 K and then increases slowly to a constant value of 228 ppm above 1573 K. CO formation in H2O is identical to that in N2, and a peak value of 461 ppm 6801


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Figure 3. Validation of present mechanism against previous experimental data.

On the basis of a comparison of the results obtained from the two atmospheres, several key points should be emphasized as follows: (1) The onset temperature of CO formation, i.e., CH4 oxidation in H2O, is lower than that in N2. (2) The temperature at which CO production peaks is the same as the onset temperature of NO formation in both N2 and H2O atmospheres, consistent with a previous investigation on oxy-fuel combustion.16 (3) The peak of CO concentration is slightly higher in O2/H2O atmosphere than that in O2/N2 atmosphere under stoichiometric and fuel-lean conditions, whereas the opposite result is observed under fuel-rich conditions; the

appears at 1273 K. The NO concentration increases gradually to a stable value of 160 ppm above 1573 K. Under fuel-lean conditions, as demonstrated in Figure 4c, the CO concentration exhibits a maximum value of 1006 ppm at 1093 K but is not detected above 1253 K in the N2 case. The NO concentration increases gradually up to 276 ppm at 1353K and then approximately levels off. In H2O, the CO concentration peaks (765 ppm) at 1133 K and then decreases to zero above 1253 K. The NO concentration starts to rise at 1133 K and increases to a steady value of 160 ppm above 1453 K. 6802


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proposed in the present study can be employed to analyze the oxidation of CH4 and NH3 during conventional and oxy-steam conditions. The radical pool has been comprehensively identified as a key participant in the oxidation of fuel and the formation of pollutants; thus, it is necessary to examine the radical pool structure in the present system. Figure 5 displays the profiles of

Figure 4. Experimental and modeling results for different equivalence ratios, namely, fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions, as functions of temperature.

Figure 5. Comparison of profiles of H, O, and OH mole fractions between N2 and H2O atmospheres for fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions at 1673 K.

presence of high H2O concentrations dramatically suppresses CO formation above 1300 K. (4) The presence of high H2O concentrations inhibits NO formation under stoichiometric and fuel-lean conditions but enhances NO formation under fuelrich conditions. As seen in Figure 4, the chemical kinetic model satisfactorily predicted the main characteristics of CO and NO formation observed in experiments in both N2 and H2O atmospheres, although certain discrepancies related to specific values exist, especially under fuel-lean conditions. Meanwhile, it demonstrates a better agreement than GRI 3.0. Hence, the mechanism

H, O, and OH mole fractions in N2 and H2O atmosphere under fuel-rich, stoichiometric, and fuel-lean conditions at 1673 K. As can be seen from Figure 5, under all three conditions, OH radicals are considerably higher and H and O radicals are markedly lower in H2O than in N2. This higher production is due to the presence of high H2O concentrations in oxy-steam combustion, which substantially facilitate the reactions R30 (H + H2O ⇄ OH + H2) and R14 (O + H2O ⇄ OH + OH) to produce a large amount of OH radical and consume large amounts of H and O radical. Comparing the radical pool structures at different equivalence ratios, we can see that the 6803


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Energy & Fuels effect of equivalence ratio on the radical pool structure in O2/H2O is substantially lower than that under O2/N2 conditions because the overwhelming superiority of R14 and R30 renders OH dominant in the radical pool. However, in the O2/N2 atmosphere, increasing O2 concentrations from fuel-rich to fuel-lean conditions strengthens R1 (H + O2 ⇄ O + OH), resulting in an increase in the O and OH radical concentrations. Therefore, the concentration of OH is substantially higher in the O2/H2O atmosphere than in the O2/N2 atmosphere under fuel-rich conditions, whereas the concentration of O is considerably lower than those under stoichiometric and fuel-lean conditions. In the present work, the overall production rate, which is abbreviated OPR, is introduced and defined as follows: OPR i , j =

∫0

case. This larger difference is due to the chaperon effect of H2O, which enhances R36. Thus, the peak CO concentration in the H2O atmosphere is slightly higher in comparison with that in the N2 atmosphere. When the temperature exceeds 1300 K, OPRCO,R33 increases more rapidly than OPRCO,R36 with increasing temperature because of the high OH concentration in the H2O case, resulting in considerably lower CO concentrations in the case of H2O than that in the case of N2. In addition, R37 (HCO + O2 ⇄ CO + HO2) is another important reaction contributing to the formation of CO under fuel-lean conditions especially in O2/N2 cases, due to the presence of high O2 concentration, and it is the reason for that the fuel-lean condition has the larger CO peak concentration at lower temperature than that of stoichiometric condition. For NH3 conversion, the main pathways of NO formation from NH3 during air and oxy-steam combustion are illustrated in Figure 7. NH3 is primarily converted into NH2 by hydrogen abstraction reaction. Then, NH2 is oxidized to NO through four main pathways: (a) NH2 → HNO → NO, (b) NH2 → NH → NO, (c) NH2 → NH → HNO → NO, and (d) NH2 → NH → N → NO. For the channel of NH2 → NH, R825 (NH2 + H ⇄ NH + H2), the main reaction of NH formation in the N2 case, is inhibited because of the lack of H radicals in the case of H2O. Instead, NH2 is mainly converted into NH through R829 (NH2 + OH ⇄ NH + H2O) at high temperatures because of high OH concentrations in the H2O case. This reaction is extremely slow at low temperatures, thus suppressing NH3 oxidation and resulting in the gradual increase in NO formation with increasing temperature in the H2O case, as shown in Figure 4. For HNO radical production (NH2 → HNO and NH → HNO), R826 (NH2 + O ⇄ H + HNO), which is dominant in the N2 case, is suppressed because of lower O concentrations in the H2O case; by contrast, an abundance of OH radicals considerably facilitates R846 (NH + OH ⇄ HNO + H). Thus, NH2 → NH → HNO is an important channel for HNO formation in O2/H2O atmospheres. For the channel of HNO → NO, R773 (HNO + H ⇄ NO + H2) and R774 (HNO + O ⇄ NO + OH), which are the predominant reactions for NO formation in N2 cases, are suppressed because of the lack of O and H radicals in the case of H2O. Moreover, R775 (HNO + OH ⇄ NO + H2O) is remarkably strengthened because of high OH concentration in the case of H2O. Hence, pathway c is markedly dominant in O2/ H2O atmospheres, whereas pathway a is inhibited because of the suppression of NH2 → HNO. For the NH → NO channel, the lack of O radical weakens R845 (NH + O ⇄ NO + H) leading to the weakness of pathway b in O2/H2O atmospheres. Although R844 (NH + H ⇄ N + H2) is suppressed by low H concentrations in the O2/H2O atmosphere, sufficient OH radicals result in enhancement of the reaction R857 (N + OH ⇄ NO + H). Thus, pathway d is promoted slightly in O2/H2O atmospheres in comparison with that in O2/N2 atmospheres. To evaluate the importance of various channels of NO formation from NH3, we introduced the conversion rate of nitrogen to evaluate the conversion of NH3 to NO quantitatively, as defined as

l

ωi , j dx

(3)

where i designates species, j designates elementary reaction, ωi,j designates the mole production rate of species i through elementary reaction j, and l is the length of reaction zone. According to the analysis of OPRCO, R36 (HCO + M ⇄ H + CO + M) is the most important reaction in CO production while R33 (CO + OH ⇄ CO2 + H) shows the largest CO consumption rate in both O2/N2 and O2/H2O atmospheres. As a consequence, R36 and R33 predominantly determine the amount of CO produced in each reaction system. Figure 6 shows

Figure 6. Comparison of the overall production rates of R33 and R36 in N2 and H2O atmospheres under fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions.

OPRCO,R33 and OPRCO,R36 at three temperatures under fuelrich, stoichiometric, and fuel-lean conditions. The three temperatures include a temperature that is 20 K lower than that corresponding to the CO peak concentration, the temperature corresponding to the CO peak concentration, and 1673 K for each case. In the fuel-rich case, at the temperature corresponding to the CO peak, OPRCO,R36 is considerably larger than OPRCO,R33 in the N2 case, whereas OPRCO,R36 is smaller than OPRCO,R33 in the H2O case because of the substantially higher OH concentration in the H2O case, as shown in Figure 5a. Therefore, the CO peak in the H2O case is markedly lower than that in the N2 case, as displayed in Figure 4. Under stoichiometric and fuel-lean conditions, OPRCO,R36 is larger than OPRCO,R33 in both atmospheres at the temperature corresponding to the CO peak, but the difference between OPRCO,R33 and OPRCO,R36 in the H2O case is slightly larger than that in the N2

CR NO =

MNO MN

(4)

where MN is the mass of N in NH3 and MNO is the mass of N in NO. 6804


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Figure 7. Main pathways of NH3 conversion for fuel-rich (a), stoichiometric (b), and fuel-lean (c) conditions in O2/N2 (left) and O2/H2O (right) atmospheres.

lower in the O2/H2O atmosphere than that in the O2/N2 atmosphere. Under fuel-lean conditions, the reduction of CRNO through pathway a is markedly larger than the increase in CRNO through pathway c because the concentration of O is immensely lower in the O2/H2O atmosphere than that in the O2/N2 atmosphere. Therefore, NO formation is substantially lower in the O2/H2O atmosphere than that in the O2/N2 atmosphere.

Figure 8 displays the conversion rate (CRNO) obtained through each pathway. This value is calculated on the basis of the ORP of all the corresponding elementary reactions along the pathways. As discussed above, pathway c is tremendously predominant in O2/H2O atmospheres, accounting for CRNO of 12.2, 23.7, and 24.0% under fuel-rich, stoichiometric, and fuel-lean conditions, respectively. Under fuel-rich conditions, given that the concentration of OH is substantially higher in the O2/H2O atmosphere than that in the O2/N2 atmosphere, the increase in CRNO through pathway c is 11.1% in the O2/H2O atmosphere relative to that in the O2/N2 atmosphere, whereas the drop in CRNO through pathway a is 2.0%. Therefore, high H2O concentrations enhance NO formation under fuel-rich conditions. Under stoichiometric conditions, the increase in CRNO through pathway c is slight lower than the drop of CRNO through pathway a in the O2/H2O atmosphere; thus, NO formation is slight

5. CONCLUSIONS The effect of high H2O concentration during oxy-steam combustion on the oxidation of methane and ammonia was investigated both experimentally and numerically. Comparison experiments between O2/N2 and O2/H2O atmospheres were performed in a flow reactor at atmospheric pressure, covering equivalence ratios from fuel-rich to fuel-lean, and a temperature range of 973−1773 K. The chemical kinetic mechanism, which 6805


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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the general program (No. 51176055) of the National Natural Science Foundation of China.

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Figure 8. Comparison of the elemental N conversion rates (CRNO) obtained through each pathway at 1673 K in O2/N2 and O2/H2O atmospheres.

involved 168 species and 1208 reactions, was hierarchically structured and updated based on recent works. Experimental results show that the onset temperature of CO formation, that is, CH4 oxidation in O2/H2O, is lower than that in O2/N2. The peak CO concentration is slightly larger in O2/ H2O than in O2/N2 under stoichiometric and fuel-lean conditions but considerably lower under fuel-rich conditions. The presence of high H2O concentrations markedly suppresses CO formation above 1300 K. For NO formation, high H2O concentrations inhibit NO formation under stoichiometric and fuel-lean conditions but enhance NO formation under fuel-rich conditions. The chemical kinetic model satisfactorily reproduced the main characteristics of CO and NO formation. Thus, the underlying mechanism can be interpreted through production rate and pathway analyses. High H2O concentrations significantly enhance H + H2O ⇄ OH + H2 and O + H2O ⇄ OH + OH, leading to substantially higher concentrations of OH and markedly lower O concentrations in the O2/H2O atmosphere than those in the O2/N2 atmosphere. The chaperon effect of H2O on enhancing HCO + M ⇄ H + CO + M is responsible for the slightly larger CO peak concentration in O2/H2O than that in O2/N2 under stoichiometric and fuel-lean conditions. Ultralow CO concentrations in the O2/H2O atmosphere under fuel-rich conditions are attributed to the enhancement of CO + OH ⇄ CO2 + H by high OH radical concentrations. The presence of high H2O concentration inhibits NO formation under stoichiometric and fuel-lean conditions because of the strong suppression of NH2 + O ⇄ H + HNO in the pathway NH2 → HNO → NO. This suppression is due to the lack of O radicals. By contrast, high H2O concentrations enhance NO formation under fuel-rich conditions because of the significant enhancement of NH2 + OH ⇄ NH + H2O in the pathway NH2 → NH → HNO → NO. This enhancement is due to the fairly high OH concentration in the O2/H2O atmosphere.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00993. Full mechanism used in this work (TXT) 6806


Article

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