New Insights into the Heterogeneous Reduction ... - ACS Publications

Article pubs.acs.org/IECR

New Insights into the Heterogeneous Reduction Reaction between NO and Char-Bound Nitrogen Hai Zhang, Xiumin Jiang,* Jiaxun Liu, and Jun Shen Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: New insights into the heterogeneous reduction reaction between NO and char-bound nitrogen [char(N)] were obtained by a combination of density functional theory (DFT) and conventional transition state theory (TST). A detailed description of the nature of NO chemisorption is reported based on the HOMO and LUMO, Mulliken atomic charges, and spin densities. It is suggested that, during chemisorption, char(N) contributes electrons to NO. The seven most stable structures (I− VII) resulting from NO chemisorption were identified, and the exothermicity was found to increase in the order III < V < IV < VI < VII< II < I. This finding is reasonable considering the fact that the HOMO of char(N) is predominantly reflected in the active C(2) atom and the LUMO of NO is mainly concentrated on the N(8) atom. Three stepwise reactions leading to N2 formation have been characterized with low energetic penalty acceptable for occurring at the practical heterogeneous combustion temperature. The highest energy penalty was calculated to be 270.794 kJ/mol. A kinetic similarity over the temperature range of 850−1000 K between the rate-limiting step and char gasification was found (10−3−100 compared to 10−4−10−1 s−1, respectively). By comparison with previous experiments, the calculated results were validated, and on the basis of these results, the reburning of superfine bituminous coal is recommended.

1. INTRODUCTION In China, coal is an attractive energy source. Among the most important environmental problems associated with coal combustion is the emission of nitric oxide (NO), which is known to result in acid rain, tropospheric destruction, and photochemical smog.1,2 Considerable attention has been focused on NO reduction over the past several decades. During coal combustion, homogeneous and heterogeneous reduction can occur.3 The major homogeneous reduction channels are through reactions with hydrocyanic acid (HCN) and ammonia (NH3) as intermediates, whereas heterogeneous reduction takes place between char and NO. NO reduction by volatile N (HCN and NH3) can readily be restricted by air staging, fuel staging, use of low-NOx burners, and so on.4−7 In contrast, NO reduction by char is far more difficult to control, because it is a gas−solid two-phase reaction. Extensive experimental efforts under different combustion conditions, mainly using fixed-bed,8,9 entrained-flow,10 and fluidized-bed11−13 reactors, have been made to describe the NO heterogeneous reduction reactions as follows 2Cac + NO → C(N) + C(O)

(R1)

Cac + 2NO → N2 + CO2

(R2)

C(N) + NO → N2 + C(O) or CO

(R3)

different mechanisms have been proposed: (1) At low temperatures (T < 250 °C), a certain amount of NO is trapped, creating C(N) and C(O) species (see reaction R1). (2) At moderate temperatures (250 °C < T < 750 °C), NO chemisorption and char gasification by NO take place together, and new active sites formed from the decomposition of C(O) complexes C(O) → CO + Cac

strongly contribute to the NO reduction. (3) At high temperatures (T > 750 °C), char gasification is severe, and CO2 and N2 are the main products. Notwithstanding the long history of experimental efforts, the heterogeneous reduction mechanism is still not clear because of numerous reactions occurring at the gas−solid interface. Quantum chemistry calculations are increasingly being called upon to further elucidate reduction mechanisms. The first attempt to investigate NO chemisorption on zigzag and armchair models of char using ab initio molecular orbital theory was made by Kyotani and Tomita.23 O-down, N-down, and side-on chemisorption configurations were examined, and the exothermicity was found to increase in the order O-down < N-down < side-on. Montoya et al.24 carried out density functional theory (DFT) calculations to investigate the pathways for the interaction between NO and char(N). They proposed that the main channel for N2 formation was direct nitrogen−nitrogen interaction between char-bound nitrogen and NO molecule. Zhu et al.25 considered the reason why the presence of oxygen reinforces the NO−char reaction while

where Cac represents the active sites, C(N) represents charbound nitrogen, and C(O) represents char-bound oxygen. Aided by new experimental methods, including temperatureprogrammed desorption (TPD),14,15 temperature-programmed reduction (TPR),16,17 and isotopically labeled NO,18,19 and well-established analysis techniques, such as X-ray photoelectron spectroscopy (XPS)20,21 and Fourier transform infrared (FTIR) spectroscopy,9,22 knowledge about NO heterogeneous reduction has been widely promoted. Three © 2014 American Chemical Society

(R4)

Received: Revised: Accepted: Published: 6307

November 19, 2013 February 12, 2014 March 20, 2014 March 20, 2014 dx.doi.org/10.1021/ie403920j | Ind. Eng. Chem. Res. 2014, 53, 6307−6315

Industrial & Engineering Chemistry Research

Article

depressing the N2O−char reaction. The contradiction is mainly due to the different modes of chemisorption of NO and N2O. Zhu and Lu26 provided new insights into the NO−char reaction and found that the kinetic behaviors are principally influenced by the surface nitrogen groups. More recently, based on two NO molecules and a zigzag model containing four active sites, systematic routes including transition states and intermediates were proposed by Sander et al.27 A large negative value of ΔE was found for NO side-on chemisorption on the zigzag edge of graphite, indicating easy generation of N2. Two char models, together with NO dimerization, were selected as heterogeneous reduction primitive reactants by Arenillas et al.,28 who chiefly studied the effect of temperature. They suggested that, at low temperature, N2 is the main reduction product, whereas at high temperature, emissions of N2 and CO2 occur simultaneously. To summarize, concerning the reaction between NO and char(N), no routes have been determined connecting the reactants to intermediates leading eventually to the products, so it is difficult to draw any explicit conclusions. Moreover, the reported works do not quantify the energy differences between different electronic spin states, which are of great importance because the energy profiles and reaction mechanisms in different states might be different.29,30 In an effort to comprehend the heterogeneous reduction reaction between NO and char(N) in greater detail, we performed rigorous DFT calculations at the B3LYP/6-31G(d) level of theory. Our first efforts focused on the nature of NO chemisorption (electrontransfer characteristics), as well as the chemisorption enthalpy. Next, based on rigorous calculations, a reaction path comprising three stepwise reactions is proposed. Thermodynamic and kinetic considerations of the proposed path are also discussed. Then, comparisons with experimental data from the available literature were carried out to validate our theoretical results. Finally, we make recommendations on how fuel-staging combustion processes should be improved by our calculated results. In this work, we have gone beyond the results obtained by earlier researchers.

Figure 1. Geometric configurations of (a) char(N) and (b) NO models. Bond lengths are given in angstroms.

molecule can be chemisorbed. The generation of carbon radical sites has to be attributed to the high heterogeneous combustion temperature and the presence of gas-phase species, where thermal decomposition and radical abstraction take place easily. (3) Other edge carbon atoms are saturated with H atoms because it has been demonstrated clearly that H-atom saturation is a good alternative as a boundary for char and/or char(N) surfaces.23−30,34 2.2. Theoretical Method. The B3LYP/6-31G(d) level of theory was utilized for global optimizations of all reactants, transition states, intermediates, and products involved in the reaction between NO and char(N). Further information about this method can be found elsewhere.36−38 Restricted wave function calculations were employed for singlets, and unrestricted wave function calculations were employed for doublets, triplets, and quartets. Vibrational analysis at the evaluated temperature of 298 K was applied, with equilibrium structures characterized by all real harmonic vibrational frequencies and transition-state structures characterized with one and only one imaginary vibrational frequency. At this level of theory, minimum-energy paths from each transition state were monitored by intrinsic reaction coordinate (IRC) calculations to verify that they truly connected the corresponding energy minima. The number of points and step size along the reaction path had their default values. The electronic ground state of each given model was acquired from singlepoint energy calculations. It should be kept in mind that, unless otherwise indicated, all energies are reported in enthalpy rather than electronic energies. The thermal corrections to enthalpy were considered. All calculations were performed with the Gaussian 09 suite of programs.39 Conventional transition state theory (TST) 40,41 was employed to estimate the thermal rate constants for the reactions involved in NO heterogeneous reduction in the temperature range of 300−1800 K according to the equation

2. COMPUTATIONAL DETAILS 2.1. Physical Model. Existing experimental evidence31 shows that char obtained from coal pyrolysis is stacked in armchair or zigzag configurations with three to seven benzene rings. Information on the fate of organically bound nitrogen residing in char has been provided by XPS analysis.32 This surface analytic technique identified pyridinic, pyrrolic, and quaternary complexes as the most important nitrogencontaining functionalities in char. However, under severe pyrolysis conditions (i.e., T > 1273 K), pyridinic-N located at the edge of the graphene layer is the most abundant nitrogen functional form.33 Therefore, a substratal zigzag plane configuration containing pyridinic-N (shown in Figure 1a, with some numbered atoms) was used in this work to model the char-bound nitrogen. Previous studies24,26,34 showed that good agreement between experimental phenomena and theoretical results can be achieved using this model. Three points regarding this char(N) model should be noted: (1) Char reactivity depends to a great degree on the nature of the active site rather than the model size.35 In addition, the calculation time appears prohibitively long for large molecular models. Therefore, we chose a model consisting of the five sixmembered rings. (2) On the upper side, edge atoms [C(2) and C(6)] are unsaturated to represent the active sites where a NO

kTST = 6308

QTS(T ) ⎛ −E ⎞ kBT exp⎜ a ⎟ ⎝ RT ⎠ h Q A(T ) Q B(T )

(1)

dx.doi.org/10.1021/ie403920j | Ind. Eng. Chem. Res. 2014, 53, 6307−6315


Article

At the moment that the evaluated thermal rate constants were acquired, they were fitted to the Arrhenius expression ⎛ −E ⎞ k(T ) = A exp⎜ a ⎟ ⎝ RT ⎠

(2)

by a linear least-squares fitting algorithm.

3. RESULTS AND DISCUSSION 3.1. Information on the Geometries and Energies of the Reactants. Information on the geometries and energies of the reactants was the starting pointing for determining the mechanism of NO chemisorption and subsequent heterogeneous reduction. The optimized configurations of char(N) and NO are presented in Figure 1, where some numbered atoms and important bond distances are included. Dihedral angles involved in char(N) are either 0° or 180°, showing its planar characteristics. Even though a N atom is trapped in the char surface, all of the calculated C−H and C−C distances and C− C−H and C−C−C angles were consistent with the experimental results obtained for graphite.42 The N(8)−O(9) bond length was found to be 1.159 Å, showing good agreement with previous theoretical results (1.15 Å).27 The energies of the reactants on the doublet and quartet surfaces and the corresponding doublet−quartet splitting energies are listed in Table 1. For char(N), the doublet lies 11.124 kJ/mol above the

Figure 2. HOMOs and LUMOs of char(N) and NO.

Table 1. Reactant Energies on the Doublet and Quartet Surfaces and Corresponding Doublet−Quartet Splitting Energies (Hartreea)

a

species

doublet

quartet

splitting energy

char(N) NO

−745.138487 −129.880313

−745.142724 −129.658226

0.004237 0.222087

1 hartree = 627.51 kcal/mol = 2625.50 kJ/mol.

quartet, predicting quartet to be the electronic ground state. On the contrary, the doublet of NO is the most stable, with the quartet located 583.089 kJ/mol above it. 3.2. NO Chemisorption Mechanisms. 3.2.1. Nature of Chemisorption. An intuitive understanding of NO chemisorption on char and char(N) surfaces has been well established,23−27 where only chemisorption enthalpy is considered. Fundamental information on the nature of chemisorption, in particular the electron-transfer characteristics, is provided in this section. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were examined because they are considered to provide an important indication of chemical reactivity.43 The HOMO is the orbital that largely plays the role of electron donor, whereas the LUMO is the orbital that primarily acts as an electron acceptor. The shapes of the HOMOs and LUMOs of char(N) and NO, as well as their homologous energies, are shown in Figure 2. The char(N) (HOMO)−NO (LUMO) gap is 0.0778 eV, whereas the NO (HOMO)−char(N) (LUMO) gap is 0.1048 eV, suggesting that char(N) donates electrons to NO rather than NO donating to char(N). Mulliken atomic charges (see Figure 3a) and spin densities (Figure 3b) were also investigated to corroborate the electron-transfer characteristics. As expected, the greater spin density contributions were found to come from the surface active carbon atoms, namely, C(2) and C(6). The spin densities of C(2) and C(6) were found to be close to +1,

Figure 3. (a) Mulliken atomic charges and (b) spin densities of char(N).

confirming that they each have a free electron. These results, together with their small negative-charge characters [−0.011 for C(2) and −0.013 for C(6)], show that the electronconfinement capabilities of the surface active carbon atoms are so small they can easily donate electrons to NO. This welldefined nature of chemisorption is of great help in the following study of the structures of chemisorbed NO. As described above, the active sites on the char(N) surface make great contributions to NO chemisorption because of their small electron-confinement capabilities. It is therefore advisable to introduce a certain amount of oxygen into the heterogeneous reduction zone. More active sites will be created by C(O) 6309



Article

desorption,15 and these, in turn, will enhance the NO reduction. 3.2.2. Seven Chemisorbed Structures. The chemisorption of gas species on carbonaceous materials mainly occurs on the surface with unsaturated carbon atoms. Therefore, N-down, Odown, and side-on configurations for NO chemisorption on char(N) surface active sites are taken into account in this section. Seven stable triplet structures (I−VII) resulting from NO chemisorption are presented in Figure 4, where important

139.894, 116.023, 117.536, 193.213, 177.738, 52.796, and 58.522 kJ/mol, respectively. All of the located triplet energies were lower than the singlet energies, suggesting that triplets are the electronic ground states. N-down chemisorption approach was found to lead to three stable structures, denoted as structures I−III. The most energetically stable products after O-down interaction with char(N) surface were found to be structures IV and V (see Figure 4). In terms of side-on chemisorption, our theoretical calculations showed structures VI and VII to be the most stable. As can be seen from Figure 5, all seven NO chemisorption processes were identified as exothermic, that is, ΔH was calculated to be negative (in the range from −57.302 to −235.140 kJ/mol). This is a sizable driving force for the subsequent NO heterogeneous reduction. The process of creating structure III had the smallest chemisorption enthalpy of −57.302 kJ/mol, showing good agreement with the reported study. 24 This interesting finding can be explained by considering the fact that C(2) makes a large contribution to the HOMO whereas N(4) is unrelated to the HOMO (see Figure 1 for atom numbering). The N-down chemisorption process was found to be exothermic, amounting to −235.140 and −217.641 kJ/mol for structures I and II, respectively. The O-down chemisorption process was found to be exothermic, amounting to −84.478 and −62.763 kJ/mol for structures IV and V, respectively. Chemisorbed structures IV and V were found to be less stable than the structures I and II, which is consistent with previous theoretical data reported by Kyotani and Tomita,23 who found that the N-down approach gives more stable structures than the O-down approach. N(8) was found to be a more active site for chemisorption than O(9), because it can easily gain electrons from char(N) due to its electron-deficient character. In addition, the LUMO of separated NO is mostly reflected in the N atom. The side-on interactions of NO molecule with char(N) surface leading to structures VI and VII were found to be exoergic with 123.060 and 183.452 kJ/mol, respectively. We paid particular attention to chemisorbed structure VII because the direct nitrogen−nitrogen interaction between the charsurface nitrogen and the NO molecule can ultimately lead to N2 formation. 3.3. NO Heterogeneous Reduction Mechanisms. Having studied the NO chemisorption mechanisms, we next investigated the further NO heterogeneous reduction mechanisms. The energy differences between the singlets and triplets (see Figure 5) indicate that the triplets are the electronic ground states. Therefore, all heterogeneous reduction calculations were performed on the triplets. Selected bond distances for all species discussed below are presented in Figure 6. The energetics of NO heterogeneous reduction to afford free N2 was calculated and is depicted in Figure 7. Table 2 lists the imaginary frequencies and vibrational bonds for each transition state. Panels a−c of Figure 8 show the IRC curves of TS1, TS2, and TS3, respectively. For the sake of brevity, the energies reported in Figure 8 are electronic energies. Because the aim of IRC calculations is to confirm the appropriate connection between transition states and associated equilibrium structures, we would not pursue the enthalpy values further by frequency calculations on each point along the minimum-energy path. 3.3.1. N(8)−O(9) Bond Dissociation. NO chemisorption induces N(8)−O(9) bond elongation from 1.159 to 1.445 Å, indicating that the bond strength is weakened. The bond will be further weakened in the subsequent heterogeneous reduction

Figure 4. Stable triplet structures after NO chemisorption on the char(N) surface. Bond lengths are given in angstroms.

bond distances are also indicated. Corresponding chemisorption enthalpies, as well as singlet−triplet splitting energies, are schematically depicted in Figure 5. The singlet−triplet splitting energies of chemisorbed structures I−VII were found to be

Figure 5. Schematic energy profiles for seven chemisorbed structures at 298 K, relative to the doublet NO and quartet char(N). 6310



Article

Å, as shown in Figure 6. At the same time, the N(4)−N(8) and C(2)−O(9) bond distances, which were 1.376 and 1.374 Å in VII, decreased to 1.305 and 1.309 Å in TS1 and decreased to 1.268 and 1.252 Å in VIII (Figure 6), indicating that both bonds were strengthened. Our DFT calculations show an energy penalty of 51.862 kJ/mol and an endothermicity of 4.390 kJ/mol for this step. 3.3.2. Synchronous N(4)−C(3) and C(2)−C(1) Bond Dissociations. Through a number of structure conjectures, the transition state for synchronous N(4)−C(3) and C(2)− C(1) bond dissociations was confirmed and labeled as TS2, having only one imaginary frequency of −542.18 cm−1 (Table 2). IRC analysis (see Figure 8b) showed TS2 to be a first-order saddle point on the minimum-energy path, and geometric optimizations of REP and FEP yielded structures VIII and IX, respectively. As visualized in Figure 6, the N(4)−C(3) bond length changed from 1.400 to 2.246 to 2.940 Å, whereas the C(2)−C(1) bond length changed from 1.458 to 1.636 to 2.438 Å. The activation energy was found to amount to 270.794 kJ/ mol, a quite large value far different from that of the N(8)− O(9) dissociation step (51.862 kJ/mol). This indicates that this process is unlikely to take place at the temperature required for N(8)−O(9) bond cleavage. IX afforded by stable species VIII, and transition state TS2 was located at an energy of 88.721 kJ/ mol, which is 84.331 kJ/mol above that of VIII, indicating moderate endothermicity. 3.3.3. N(4)−C(5) Bond Dissociation. Starting from structure IX, the evolution through TS3 that has only one imaginary frequency of −404.53 cm−1 (see Table 2) belonging to the N(4)−C(5) bond dissociation could result in the generation of structure X and N2. The N(4)−C(5) bond is elongated from 1.454 to 1.864 Å and to infinity (Figure 6). The energetics of this bond dissociation was calculated, as shown in Figure 7. The product was found to be much more stable than the reactant by 295.907 kJ/mol. The energy barrier of TS3, computed with respect to structure IX, is 30.540 kJ/mol, whereas that computed with respect to structure X and N2 is as large as 326.447 kJ/mol, indicating that TS3 is relatively close to the structure IX. This can be confirmed by the minimum-energy path (see Figure 8c): The energy in the reverse direction smoothly approaches its trimmed value, whereas the energy in the forward direction descends quickly in a pronounced way, and more points and/or a different step size is required to reach its stationary value. 3.4. Thermodynamics and Kinetics of the NO Heterogeneous Reduction Path. After chemisorption, the reaction

Figure 6. Optimized geometries of stable species and transition states involved in NO heterogeneous reduction. Bond lengths are given in angstroms.

Figure 7. Schematic energy profiles for NO heterogeneous reduction reactions at 298 K, relative to the triplet VII.

Table 2. Imaginary Frequencies and Vibrational Bonds for the Three Transition States transition state

imaginary frequency (cm−1)

vibrational bond(s)

TS1 TS2 TS3

−919.68 −542.18 −404.53

N(8)−O(9) N(4)−C(3) and C(2)−C(1) N(4)−C(5)

TS1

TS2

TS3

VII ⎯⎯⎯→ VIII ⎯⎯⎯→ IX ⎯⎯⎯→ X + N2

takes place, providing possibilities for reducing NO emissions. This proposed reaction is especially significant for fuel-staging combustion due to the creation of a fuel-rich zone. The measured overall ΔH value is −207.186 kJ/mol. This means that the NO heterogeneous reduction process is energetically accessible. The stable five-membered ring structure X might be responsible for the large negative ΔH value. The energy barrier increases in the order TS3 (30.540 kJ/mol) < TS1 (51.862 kJ/ mol) < TS2 (270.794 kJ/mol), meaning the NO heterogeneous reduction is mostly controlled by the reaction

reaction. Going from structure VII to VIII, one transition-state structure (TS1) is characterized. From the vibration mode associated with the imaginary frequency of −919.68 cm−1 (see Table 2), we found that the motion mainly belongs to N(8)− O(9) bond elongation. IRC calculations starting from TS1 were carried out (see Figure 8a). Although no equilibrium structure was at hand, the reverse ending point (REP) and forward ending point (FEP) enabled us to predict its accuracy. The N(8)−O(9) bond length changed from 1.445 to 1.828 to 2.357

TS2

VIII ⎯⎯⎯→ IX 6311



Article

Figure 8. IRC curves for (a) TS1, (b) TS2, and (c) TS3. Energies are given in electronic energies and relative to the transition state. REP and FEP indicate the reverse ending point and the forward ending point, respectively.

This highest energy penalty might be induced by the stabilization of sp2 hybridized orbitals. Moderate oxygen should be added considering the fact that oxygen residing on the char surface can activate the neighboring atom.15 The chemically bound nitrogen might be exposed by severe char gasification, decreasing the NO reduction activation energy (270.794 kJ/ mol in this work vs 151.5 kJ/mol in ref 27). Thermal rate constants for the three stepwise reactions were evaluated over the temperature range of 300−1800 K to investigate the kinetics. Arrhenius curves of the calculated thermal rate constants are given in Figure 9. Table 3 lists the rate constants at different temperatures. The fitted Arrhenius expressions for the reactions TS1

VII ⎯⎯⎯→ VIII TS2

VIII ⎯⎯⎯→ IX Figure 9. Arrhenius curves of the calculated thermal rate constants for three stepwise reactions.

and TS3

IX ⎯⎯⎯→ X + N2

TS2

TS1

TS3

VIII ⎯⎯⎯→ IX < VII ⎯⎯⎯→ VIII < IX ⎯⎯⎯→ X + N2

are 4.344 × 1013 exp(−6939/T), 8.428 × 1014 exp(−32780/T), and 7.565 × 1013 exp(−4270/T) s−1, respectively. The calculated reaction rate of the rate-limiting step is at least 5 orders of magnitude lower than the rates of the other two reactions (see Table 3). It can be concluded from the results presented in Figure 9 that the rate constants follow the same order for all of the evaluated temperatures

This tendency is mainly attributed to the dissimilarity in activation energies. The kinetic information delineated here is available for the modeling of NO, N2O, and other gas emissions, as employed by Desroches-Ducarne et al.44 and Liu and Gibbs.45 6312



Article

Table 3. Thermal Rate Constants (s−1) for Three Stepwise Reactions at Different Temperatures (K) temperature (K) 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

TS1

VII ⎯⎯⎯→ VIII 5.07 1.26 3.62 3.52 1.83 6.40 1.72 3.84 7.46 1.31 2.12 3.22 4.65 6.45 8.63 1.12

× × × × × × × × × × × × × × × ×

103 106 107 108 109 109 1010 1010 1010 1011 1011 1011 1011 1011 1011 1012

TS2

VIII ⎯⎯⎯→ IX 3.83 2.14 2.52 1.35 3.29 1.16 1.13 4.45 9.08 1.13 9.57 6.02 2.98 1.21 4.19 1.27

× × × × × × × × × × × × × × × ×

10−33 10−21 10−14 10−9 10−6 10−3 10−1 100 101 103 103 104 105 106 106 107

4. CONCLUSIONS With the help of density functional theory and conventional transition state theory, an in-depth theoretical study of the heterogeneous reduction reaction between NO and char(N) has been carried out. Conclusions arising from the results described herein can be listed as follows: (1) The electron transition from char(N) to NO occurs during low-temperature chemisorption according to the gap between the HOMO and LUMO, which is induced by small electron confinement capability of the surface active C(2) atom. (2) The smallest chemisorption enthalpy (−57.302 kJ/mol) of structure III suggests that the N(4) atom is not a susceptible site for chemisorption. This is in accordance with the fact that the HOMO is predominantly reflected in the active C(2) atom, whereas the N(4) atom contributes nothing to the HOMO. (3) Apart from structure III, the exothermicity increases in the order: V < IV < VI < VII < II < I, indicating that the adsorbed NO molecule is prone to bond with the surface C(2) atom in the N-down configuration. A possibility is that N(8) atom in the separated NO molecule is electron-deficient. (4) From the calculations focusing on the N2 formation mechanisms, we corroborate that it can be released at the char(N) interface by direct nitrogen−nitrogen interaction. The reduction path

TS3

IX ⎯⎯⎯→ X + N2 6.45 1.73 1.31 5.23 1.44 3.13 5.81 9.63 1.47 2.11 2.88 3.77 4.80 5.95 7.22 8.60

× × × × × × × × × × × × × × × ×

107 109 1010 1010 1011 1011 1011 1011 1012 1012 1012 1012 1012 1012 1012 1012

3.5. Reliability of the Theoretical Results. It has been widely documented8,9,18,19,30,46,47 that char gasification takes place at temperatures between 850 and 1000 K. In this temperature range, the experimental gasification reaction rate is roughly in the range of 10−4−10−1 s−1. Char gasification starts to limit the heterogeneous reduction owing to its low reaction rate. As the temperature increases, gasification becomes increasingly intense. Surface nitrogen complexes [char(N)] are generated, and they were verified to be real intermediates in the heterogeneous reduction. In this work, three stepwise reactions with the highest energy barrier of 270.794 kJ/mol were found to produce separated N2. The thermal rate constant (see Table 3) for the rate-limiting step over the temperature range of 850−1000 K was found to be in the range of 10−3−100 s−1, which is similar to that of the char gasification reaction from the kinetic point of view. One important conclusion stemming from the kinetics comparisons is, that above 1000 K, reaction R3 dominates the N2 releasing channel. Good agreement can be seen between our theoretical results and previous experimental observations. 3.6. Suggestions on Fuel-Staging Combustion. The aim of fuel-staging combustion is mostly to enhance NO reduction in the fuel-rich zone. Previously generated NO will be reduced to N2 by (1) CHi radicals from the rapid pyrolysis of reburning fuel and (2) the nitrogen residing on char. Heterogeneous reduction plays a more crucial role in NO emissions than homogeneous reduction for bituminous coal reburning.48 Heterogeneous NO reduction in the reburning zone will benefit significantly from superfine bituminous coal, which has a large specific surface area. The char surface energy, increasing with the specific surface area,49 will capture more NO. As a consequence, more energy will be released (as described in section 3.2), favoring the subsequent N2 formation. On the other hand, NO will easily penetrate into the inner pores of the char because of the relatively smooth surface and simplified pore structure. Additionally, considering the fact that the highest energy penalty of NO reduction amounts to 270.794 kJ/mol, the combination of superfine particles and bituminous coal reburning is also preferable because more char(N) will convert to volatile N.50

TS1

TS2

TS3

VII ⎯⎯⎯→ VIII ⎯⎯⎯→ IX ⎯⎯⎯→ X + N2

can be divided into three steps: (1) N(8)−O(9) bond dissociation, (2) synchronous N(4)−C(3) and C(2)−C(1) bond dissociations, and (3) N(4)−C(5) bond dissociation. The highest energy barrier (270.794 kJ/mol) appears at the second step. (5) It is interesting to note that, in the temperature range of 850−1000 K, the calculated thermal rate constant for the ratelimiting step is kinetically similar to that of the char gasification. Our theoretical results provide evidence to support the conclusion that the interaction between NO and char(N) is the main N2-releasing channel at high temperature. (6) Based on our theoretical results and previous experimental findings, the reburning of superfine bituminous coal is recommended. The present work contributes to knowledge about NO heterogeneous reduction and can account for previous experimental observations. However, NO and char(N) heterogeneous interaction occurs with the presence of oxygen, and its presence can largely increase the reduction rate. This will be a subject of future research.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-34205681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51306116 and 51376131).

■

REFERENCES

(1) Li, F. X.; Fan, L. S. Clean coal conversion processesprogress and challenges. Energy Environ. Sci. 2008, 1, 248.

6313



Article

(2) Burkinshaw, J. R.; Smoot, L. D.; Hedman, P. O.; Blackham, A. U. Analysis of sulfur and nitrogen pollutants in three-phase coal combustion effluent samples. Ind. Eng. Chem. Res. 1983, 22, 292. (3) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Fuel nitrogen in solid fuel fired systems. Prog. Energy Combust. Sci. 2003, 29, 89. (4) Spliethoff, H.; Greul, U.; Rüdiger, H.; Hein, K. R. G. Basic effects on NOx emissions in air staging and reburning at a bench-scale test facility. Fuel 1996, 75, 560. (5) Zabetta, E. C.; Hupa, M.; Saviharju, K. Reducing NOx emissions using fuel staging, air staging, and selective noncatalytic reduction in synergy. Ind. Eng. Chem. Res. 2005, 44, 4552. (6) Smoot, L. D.; Hill, S. C.; Xu, H. NOx control through reburning. Prog. Energy Combust. Sci. 1998, 24, 385. (7) Chen, Z. C.; Li, Z. Q.; Zhu, Q. Y.; Jing, J. P. Gas/particle flow and combustion characteristics and NOx emissions of a new swirl coal burner. Energy 2011, 36, 709. (8) Chambrion, P.; Kyotani, T.; Tomita, A. Role of N-containing surface species on NO reduction by carbon. Energy Fuels 1998, 12, 416. (9) Ashman, P. J.; Haynes, B. S.; Nicholls, P. M.; Nelson, P. F. Interactions of gaseous NO with char during the low-temperature oxidation of coal chars. Proc. Combust. Inst. 2000, 28, 2171. (10) Nelson, P. F.; Nancarrow, P. C.; Bus, J.; Prokopiuk, A. Fractional conversion of char N to NO in an entrained flow reactor. Proc. Combust. Inst. 2002, 29, 2267. (11) Arenillas, A.; Rubiera, F.; Pis, J. J. Nitric oxide reduction in coal combustion: Role of char surface complexes in heterogeneous reactions. Environ. Sci. Technol. 2002, 36, 5498. (12) Chan, L. K.; Sarofim, A. F.; Beér, J. M. Kinetics of the NO− carbon reaction at fluidized bed combustor conditions. Combust. Flame 1983, 52, 37. (13) Mirasol, J. R.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. NO and N2O decomposition over coal char at fluidized-bed combustion conditions. Combust. Flame 1994, 99, 499. (14) Teng, H.; Suuberg, E. M. Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption. J. Phys. Chem. 1993, 97, 478. (15) Yang, J.; Mestl, G.; Herein, D.; Schlögl, R.; Find, J. Reaction of NO with carbonaceous materials 2. Effect of oxygen on the reaction of NO with ashless carbon black. Carbon 2000, 38, 729. (16) Illán-Gómez, M. J.; Linares-Solano, A.; Salinas-Martínez de Lecea, C. NO reduction by activated carbons. 1. The role of carbon porosity and surface area. Energy Fuels 1993, 7, 146. (17) Illán-Gómez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartínez de Lecea, C. NO reduction by activated carbons. 7. Some mechanistic aspects of uncatalyzed and catalyzed reaction. Energy Fuels 1996, 10, 158. (18) Chambrion, P.; Orikasa, H.; Suzuki, T.; Kyotani, T.; Tomita, A. A study of the C−NO reaction by using isotopically labelled C and NO. Fuel 1997, 76, 493. (19) Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms. Fuel 2007, 86, 41. (20) García, P.; Espinal, J. F.; Salinas-Martínez de Lecea, C.; Mondragón, F. Experimental characterization and molecular simulation of nitrogen complexes formed upon NO−char reaction at 270°C in the presence of H2O and O2. Carbon 2004, 42, 1507. (21) Xiao, B.; Boudou, J. P.; Thomas, K. M. Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres. Langmuir 2005, 21, 3400. (22) Park, D. C.; Day, S. J.; Nelson, P. F. Formation of N-containing gas-phase species from char gasification in steam. Fuel 2008, 87, 807. (23) Kyotani, T.; Tomita, A. Analysis of the reaction of carbon with NO/N2O using ab initio molecular orbital theory. J. Phys. Chem. B 1999, 103, 3434. (24) Montoya, A.; Truong, T. N.; Sarofim, A. F. Application of density functional theory to the study of the reaction of NO with charbound nitrogen during combustion. J. Phys. Chem. A 2000, 104, 8409.

(25) Zhu, Z. H.; Finnerty, J.; Lu, G. Q.; Yang, R. T. Opposite roles of O2 in NO− and N2O−carbon reactions: An ab initio study. J. Phys. Chem. B 2001, 105, 821. (26) Zhu, Z. H.; Lu, G. Q. New insights into NO−carbon and N2O− carbon reactions from quantum mechanical calculations. Energy Fuels 2003, 17, 1057. (27) Sander, M.; Raj, A.; Inderwildi, O.; Kraft, M.; Kureti, S.; Bockhorn, H. The simultaneous reduction of nitric oxide and soot in emissions from diesel engines. Carbon 2009, 47, 866. (28) Arenillas, A.; Arias, B.; Rubiera, F.; Pis, J. J.; López, R.; Campomanes, P.; Pevida, C.; Menéndez, M. I. Heterogeneous reaction mechanisms of the reduction of nitric oxide on carbon surfaces: A theoretical analysis. Theor. Chem. Acc. 2010, 127, 95. (29) Montoya, A.; Mondragón, F.; Truong, T. N. First-principles kinetics of CO desorption from oxygen species on carbonaceous surface. J. Phys. Chem. A 2002, 106, 4236. (30) Frankcombe, T. J.; Smith, S. C. On the microscopic mechanism of carbon gasification: A theoretical study. Carbon 2004, 42, 2921. (31) Perry, S. T.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Solid-state 13C NMR characterization of matched tars and chars from rapid coal devolatilization. Proc. Combust. Inst. 2000, 28, 2313. (32) Li, C. Z.; Buckley, A. N.; Nelson, P. F. Effects of temperature and molecular mass on the nitrogen functionality of tars produced under high heating rate conditions. Fuel 1998, 77, 157. (33) Pels, J. R.; Kapteijn, F.; Moulijin, J. A.; Zhu, Q.; Thomas, K. M. Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis. Carbon 1995, 33, 1641. (34) Espinal, J. F.; Truong, T. N.; Mondragón, F. Mechanisms of NH3 formation during the reaction of H2 with nitrogen containing carbonaceous materials. Carbon 2007, 45, 2273. (35) Montoya, A.; Truong, T. T. T.; Mondragón, F.; Truong, T. N. CO desorption from oxygen species on carbonaceous surface: 1. Effects of the local structure of the active site and the surface coverage. J. Phys. Chem. A 2001, 105, 6757. (36) Becke, A. D. A new mixing of Hartree−Fock and local density functional theories. J. Chem. Phys. 1993, 98, 5648. (37) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti correction-energy formula into a functional of electron density. Phys. Rev. B 1988, 37, 785. (38) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results obtained with the correlation energy density functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (40) Eyring, H. The activated complex in chemical reactions. J. Chem. Phys. 1935, 3, 107. (41) Mora-Diez, N.; Alvarez-Idaboy, J. R.; Boyd, R. J. A quantum chemical and TST study of the OH hydrogen-abstraction reaction from substituted aldehydes, FCHO and ClCHO. J. Phys. Chem. A 2001, 105, 9034. (42) Chen, N.; Yang, R. T. Ab-initio molecular orbital calculation on graphite, selection of molecular system and model chemistry. Carbon 1998, 36, 1061. 6314



Article

(43) Ruiz-Morales, Y. HOMO−LUMO gap as an index of molecular size and structure for polycyclic aromatic hydrocarbons (PAHs) and asphaltenes: A theoretical study. I. J. Phys. Chem. A 2002, 106, 11283. (44) Desroches-Ducarne, E.; Dolignier, J. C.; Marty, E.; Martin, G.; Delfosse, L. Modelling of gaseous pollutants emissions in circulating fluidized bed combustion of municipal refuse. Fuel 1998, 77, 1399. (45) Liu, H.; Gibbs, B. M. Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors. Fuel 2002, 81, 271. (46) Suzuki, T.; Kyotani, T.; Tomita, A. Study on the carbon−nitric oxide reaction in the presence of oxygen. Ind. Eng. Chem. Res. 1994, 33, 2840. (47) Ma, M. C.; Brown, T. C.; Haynes, B. S. Evaluation of thermaldesorption spectra for heterogeneous surfaces: Application to carbon surface oxides. Surf. Sci. 1993, 297, 312. (48) Chen, W. Y.; Ma, L. Effect of heterogeneous mechanism during reburning of nitrogen oxide. AIChE J. 1996, 42, 1968. (49) Burdukov, A. P.; Konovalov, V. V.; Yusupov, T. S. Prospects for use of micronized coal in power industry. Therm. Sci. 2002, 6, 29. (50) Feng, J.; Li, W. Y.; Xie, K. C.; Liu, M. R.; Li, C. Z. Studies of the release rule of NOx precursors during gasification of coal and its char. Fuel Process. Technol. 2003, 84, 243.

6315


New Insights into the Heterogeneous Reduction ... - ACS Publications

Recommend Documents