Experimental and Kinetic Study on the Influence of CaO on the N2O+

Feb 9, 2015 - Energy Fuels , 2015, 29 (3), pp 1905–1912. DOI: 10.1021/ef502512f ... [email protected]. Cite this:Energy Fuels 29, 3, 1905-1912 ...
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Experimental and Kinetic Study on the Influence of CaO on the N2O + NH3 + O2 System Shi-long Fu, Qiang Song,* and Qiang Yao Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: N2O is a non-negligible pollutant emitted from circulating fluidized bed (CFB) boilers, which may be reduced by NH3. The influence of bed material (mainly CaO) on the N2O + NH3 + O2 system was investigated using a fixed bed reactor in the temperature range from 873 to 1123 K. Experimental results showed that CaO catalyzed N2O decomposition and the reaction was first-order with respect to N2O. O2 had no effect on N2O decomposition. In the CaO-involved N2O + NH3 system, NH3 did not influence N2O conversion, while N2O promoted NH3 conversion and oxidized NH3 partly to NO. In the CaOinvolved N2O + NH3 + O2 system, O2 greatly enhanced NH3 oxidation and the effect of N2O was negligible. H2O produced in NH3 oxidation inhibited N2O decomposition. The mechanism study showed that adsorbed N2O formed cis-dimer nitroso, and the dissociation of cis-dimer nitroso was the rate-controlling step of N2O conversion. The cis-dimer nitroso dissociated to N2 and peroxide ions. Peroxide ions donated electrons to form free radical oxygen, and free radical oxygen accelerated NH3 dissociation to NH2. NH2 reacted with free radical oxygen or NO to produce NO or N2 or directly dissociated to N2 and H2. NO selectivity was determined by the three reaction routes. A kinetic model was established and well-described the CaO-involved N2O + NH3 + O2 system. al.20 studied N2O decomposition on a Na−CaO catalyst and indicated that the coordinately unsaturated lattice oxygen (O2−) on the CaO surface is the active site for N2O decomposition. This coordinately unsaturated lattice oxygen donated electrons to the adsorbed N2O. Hussain et al.21 obtained similar results and proposed the reaction process of N2O decomposition, as shown in eqs 1 and 2. However, the intermediate products and reaction routes of N2O conversion on the CaO surface were not clear, and the rate-controlling step of N2O conversion was not indicated in the reported studies.

1. INTRODUCTION N2O is an important air pollutant that causes a greenhouse effect and destroys the ozone layer.1−3 Fossil fuel combustion is one of the sources of N2O production. Circulating fluidized bed (CFB) boilers have been widely used because of their good fuel flexibility and low SO2 emission.3,4 However, the combustion temperature of CFB boilers is low (usually below 1173 K), and N2O emission from the boilers can be as high as a few hundred parts per million (ppm), which is far higher than other combustion technologies.5−7 Thus, N2O emission from CFB boilers must be controlled. CFB boilers have high concentrations of circulating bed materials. CaO, which is used for SO2 removal, is one of the active components of circulating bed materials.8,9 Barisic et al.10 and Liu and Gibbs11 found that circulating bed materials had a catalytic effect on N2O decomposition, which increased with increasing the CaO content. Hou et al.12 studied the catalytic effect of different components of circulating bed materials on N2O decomposition and sorted the effect of each component in the following order: CaO > Fe2O3 > Al2O3 > SiO2. Pilot- and full-scale experiments showed that CaO catalyzed the decomposition of N2O and decreased N2O emission.10,11,13−15 Sasaoka et al.16 and Shimizu and Inagaki17 studied the effect of the reaction atmosphere on CaO-catalyzed N2O decomposition and found that H2O inhibited the catalytic effect of CaO. Kantorovich and Gillan18 studied N2O decomposition on the CaO surface and found that N2O initially decomposed to N2 and peroxide ion with the electrons from lattice oxygen. Peroxide ions donated electrons and subsequently dimerized to O2. The study conducted by Satsuma et al.19 showed that N2O decomposition on the CaO surface is a structure-sensitive reaction that requires coordinately unsaturated sites. Golden et © XXXX American Chemical Society

2− N2O + [2e]surf. → N2 + Osurf.

(1)

− 2O2surf. → O2 + [4e]surf.

(2)

The selective non-catalytic reduction (SNCR) technology is of low cost and high deNOx efficiency when applied for CFB boilers because of their good reactant mixing condition and appropriate temperature range. During the SNCR deNOx process, N2O can be reduced to N2 by NH3 in the gas phase,22−25 and the simultaneous removal of NO and N2O can be realized.26 Previous studies23,24 showed that the stoichiometric ratio of NH3 to N2O, temperature, and O2 concentration all influence N2O removal efficiency. In the SNCR deNOx process, CaO among the circulating bed materials catalyzes NH3 oxidation to NO and, thus, decreases deNOx efficiency.27 Lu et al.28 found that N2O emission also increased when NH3 was injected into a CFB boiler. Hou et al.24 reported that NO was produced in the N2O + NH3 system in the presence of Received: November 8, 2014 Revised: January 5, 2015

A

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typical temperature range for the SNCR process was from 1173 to 1373 K. Below 1173 K, the gas-phase reaction degree was low. In this work, the pre-experiments showed that both N 2 O and NH 3 conversions were below 3.0% at 1123 K. Therefore, the experiments were conducted in the temperature range from 873 to 1123 K, in which the gas-phase reactions can be ignored. Thus, the results obtained in this study represented the gas−solid reaction on the CaO surface. Fourier transform infrared spectroscopy (FTIR, Nicolet 6700) was used to measure the composition of the exhaust gas. FTIR was calibrated using the method presented by Li;29 the measuring error of this technique was less than 1.0%. A diffuse reflectance kit accessory (Harrick) was used in the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments to observe N2O adsorption species on the CaO surface. 2.3. Method. N2O and NH3 conversions (αN2O and αNH3) were calculated using eqs 3 and 4, respectively. The products of N2O decomposition on the CaO surface were N2 and O2. NH3 was partly oxidized to NO. NO2 and N2O were not produced during the NH3 oxidation. Thus, NO selectivity (SNO) can be calculated as eq 5.

circulating bed materials. However, no further studies were reported on the CaO-involved N2O + NH3 + O2 system. The studies cited above manifest that CaO not only catalyzes N2O decomposition but also influences N2O removal by NH3. The performance and mechanism of CaO-catalyzed N2O conversion and the kinetic model for the CaO-involved N2O + NH3 + O2 system are very important for the development of N2O control technology for CFB boilers. In this study, the influence of CaO on the N2O + NH3 + O2 system was investigated using a fixed bed reactor. The catalytic mechanism of CaO was analyzed, and a kinetic model was proposed to describe the CaO-involved N2O + NH3 + O2 system.

2. EXPERIMENTAL SECTION 2.1. Samples. All CaO samples used in this study were analytically pure. The properties of the CaO samples are listed in Table 1.

Table 1. Properties of CaO Samples CaO content (%) average particle diameter (μm) specific area (m2/g) average pore volume (cm3/g) average pore diameter (nm) particle density (kg/m3)

99.7 138.4 33.2 0.083 10.0 2720

α N2O =

α NH3 =

2.2. Setup and Gas Analysis. The fixed bed experimental system is shown in Figure 1. The quartz reactor consisted of external and internal sections. The internal section was 10 mm in diameter and 25 mm in length. CaO samples were distributed on the analytically pure quartz wool and placed at the bottom of the internal section. The quartz reactor was inserted into the electric furnace, and the internal section was placed at the constant temperature zone. The reactant gases (NH3, N2O, O2, and Ar as balance) were fed into the reactor from different inlets, and the total flow rate was 750 N mL min−1. The

S NO =

C N2O,in − C N2O,out C N2O,in C NH3,in − C NH3,out C NH3,in

C NO,out C NH3,in − C NH3,out

× 100% (3)

× 100% (4)

× 100% (5)

where CN2O,in and CN2O,out are the inlet and outlet concentrations of N2O, respectively, CNH3,in and CNH3,out are the inlet and outlet concentrations of NH3, respectively, and CNO,out is the outlet concentration of NO.

Figure 1. Schematic of the fixed bed experimental system. B

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3.2. Influence of CaO on the N2O + NH3 System. N2O conversion in the CaO-involved N2O + NH3 system as a function of the temperature is shown in Figure 4a. N2O

3. RESULTS AND DISCUSSION 3.1. Influence of CaO on N2O Decomposition. The influence of CaO on N2O decomposition as a function of the temperature is shown in Figure 2. The influence of O2 on the

Figure 2. Effect of CaO on N2O decomposition as a function of the temperature (50 mg of CaO, 500 ppm of N2O, and 2.0% O2).

catalytic effect of CaO is also shown in Figure 2. As mentioned in section 2.2, the gas-phase N2O decomposition can be ignored in the studied temperature range. The addition of CaO catalyzed N2O decomposition of N2O, and the products formed were N2 and O2. No other products were observed in the experiment. In the absence of O2, N2O conversion increased from 1.5% at 873 K to 54.7% at 1123 K. N2O conversion was not influenced by O2, indicating that O2 was not involved in the rate-controlling step of N2O conversion and did not compete with N2O at the same active sites on the CaO surface. The influence of CaO on N2O decomposition as a function of the N2O inlet concentration is shown in Figure 3. Increasing

Figure 4. Effect of CaO on the N2O + NH3 system as a function of the temperature: (a) N2O conversion and (b) NH3 conversion and NO selectivity (50 mg of CaO, 500 ppm of N2O, and 1000 ppm of NH3).

conversion without NH3 is also shown in Figure 4a. As mentioned in section 2.2, the gas-phase reactions between N2O and NH3 can be ignored in the studied temperature range. In the CaO-involved N2O + NH3 system, N2O conversion increased from 2.1% at 873 K to 55.0% at 1123 K. The results were comparable to the results obtained from N 2 O decomposition. Experimental results demonstrated that NH3 had no effect on N2O conversion on the CaO surface. NH3 conversion and NO selectivity in the CaO-involved N2O + NH3 system are shown in Figure 4b. In the absence of N2O, NH3 conversion increased from 1.0% at 873 K to 15.0% at 1123 K and the products of CaO-catalyzed NH 3 decomposition were N2 and H2.27 In the presence of N2O, NH3 conversion was enhanced and increased from 1.9% at 873 K to 25.5% at 1123 K. Part of NH3 was oxidized to NO and H2O, which were detected by FTIR. NO selectivity decreased from 52.6% at 873 K to 20.4% at 1123 K. The rate-controlling step of NH3 conversion is the dissociation of NH3 to NH2. On the basis of the studies conducted by Golden et al.20 and Hussian et al.,21 N2O first dissociated to N2 and peroxide ion on the CaO surface. The produced peroxide ions donated electrons and formed free radical oxygen. Free radical oxygen accelerated NH3 dissociation to NH2; thus, NH3 conversion

Figure 3. Effect of CaO on N2O decomposition as a function of the N2O inlet concentration (50 mg of CaO, 100−1000 ppm of N2O, and 1123 K).

the N2O inlet concentration from 100 to 1000 ppm did not significantly affect the N2O conversion, which was maintained at approximately 56.0%. Results showed that CaO-catalyzed N2O decomposition was first-order with respect to the N2O concentration, which indicated that the adsorption equilibrium constant of N2O on the CaO surface was minute. C

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Energy & Fuels increased. NH2 can be oxidized by free radical oxygen to form NO or reduce NO to form N2.27 If NH3 was converted only by this way, the ratio of consumed N2O to consumed NH3 should be at least 1.5. However, the ratio was less than 1.5; e.g., it was 0.8 at 873 K and 1.1 at 1123 K. It meant that CaO-catalyzed NH3 decomposition also existed in this process, and the products were N2 and H2.27 The decrease in NO selectivity indicated that the increase in the NO production rate was lower than that in N2 production with increasing the temperature. N2O and NH3 conversions and NO selectivity as a function of the N2O inlet concentration are shown in Figure 5. With the

Figure 5. Effect of CaO on the N2O + NH3 system as a function of the N2O inlet concentration (50 mg of CaO, 100−1000 ppm of N2O, 1000 ppm of NH3, and 1123 K). Figure 6. Effect of CaO on the N2O + NH3 + O2 system as a function of the temperature: (a) N2O conversion and (b) NH3 conversion and NO selectivity (50 mg of CaO, 500 ppm of N2O, 1000 ppm of NH3, and 1.5% O2).

N2O inlet concentration increasing from 100 to 1000 ppm, N2O conversion was not significantly affected and maintained at around 56.0%, whereas NH3 conversion and NO selectivity increased from 5.9 and 15.3 to 50.0 and 31.0%, respectively. Results showed that NH3 had no effect on N2O conversion. Moreover, CaO-catalyzed N2O decomposition remained firstorder with respect to N2O concentration in the presence of NH3. Increasing the N2O inlet concentration produced more free radical oxygen and accelerated the dissociation of NH3 and oxidation of NH2. Thus, NH3 conversion and NO selectivity increased. NO was produced in NH2 oxidation and also consumed in the reaction with NH2. When the N2O concentration was low, the consumption of NO in the reaction with NH2 was small; thus, NO selectivity increased fast with increasing the N2O concentration. When the N2O concentration was high, more NO was formed on the CaO surface and then more NO was consumed upon reaction with NH2. Thus, NO selectivity increased slowly and gradually reached a constant value. 3.3. Influence of CaO on the N2O + NH3 + O2 System. In the CaO-involved N2O + NH3 + O2 system, N2O decomposition and NH3 oxidation simultaneously occurred on the CaO surface. N2O conversion as a function of the temperature is shown in Figure 6a. N2O conversion increased from 0.6% at 873 K to 50.0% at 1123 K, which was lower compared to the results obtained under other atmospheric conditions. NH3 conversion and NO selectivity in the CaOinvolved N2O + NH3 + O2 system as well as those in CaOcatalyzed NH3 oxidation are shown in Figure 6b. NH3 conversion and NO selectivity were basically the same in the absence or presence of N2O, which indicated that NH3 conversion was dominated by its reaction with O2. Because

NH3 did not inhibit N2O decomposition on the CaO surface (Figure 4a), the inhibitory effect on N2O decomposition should be caused by the reaction products. H2O was found to be produced in the catalytic oxidation of NH3. Only NO and H2O were found by FTIR in the product gas. NO had no influence on N2O decomposition according to our pre-experiment on the CaO-involved N2O + NO system; therefore, it must be H2O that inhibited N2O decomposition. H2O competed with N2O in adsorption on the same active sites on the CaO surface and inhibited N2O decomposition.16,17 H2O was also produced in the CaO-involved N2O + NH3 system but did not influence N2O decomposition because of its small amount. In the CaOinvolved N2O + NH3 system, the H elements in NH3 bonded with the free radical oxygen to form H2O or dissociated to form H2. The maximum of H2O formation can be estimated by NH3 conversion. In the CaO-involved N2O + NH3 + O2 system, NH3 was oxidized mainly by O2 and NH3 decomposition to N2 and H2 was negligible because of the strong oxidizing ability of O2.27 Thus, all of the H elements in the consumed NH3 were converted to H2O, and the amount of H2O can be calculated by NH3 conversion. H2O produced in the CaO-involved N2O + NH3 + O2 system was 30 times of that in the absence of O2 at 873 K. This ratio decreased with the increasing temperature and was still 5.8 at 1123 K. It meant that much more H2O was produced in the CaO-involved N2O + NH3 + O2 system than in the CaO-involved N2O + NH3 system. This was the reason why H2O showed an apparent inhibitory effect on N2O D

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on N2O conversion in the experiments, the consumption of free oxygen in NH3 oxidation would not inhibit N2O conversion. NH2 produced in NH3 dissociation had three conversion routes: oxidized by free radical oxygen to form NO (shown in eq 11), oxidized by the produced NO to form N2 (shown in eq 12), and dissociating into N2 and H2 (shown in eq 13).27,30 NO selectivity was determined by the competition of the three conversion routes. In the CaO-involved N2O + NH3 + O2 system, NH3 conversion and NO selectivity were not influenced by N2O and mainly dominated by O2 in the experiments. The experimental results indicated that N2O did not participate in the NH3 oxidation in the presence of O2 and the reducing radicals formed during NH3 dissociation, such as NH2 and NH, cannot reduce N2O to N2 either. However, H2O produced in NH3 oxidation competed with N2O at the same kind of active sites on the CaO surface and inhibited N2O decomposition. Thus, the influence of H2O must be considered when calculating N2O conversion on the CaO surface.

decomposition in the CaO-involved N2O + NH3 + O2 system, while it had little influence on the CaO-involved N2O + NH3 system. 3.4. Mechanism of CaO Catalysis in the N2O + NH3 + O2 System. N2O adsorption species on the CaO surface are shown in Figure 7. An obvious adsorption band of cis-dimer

CaO

NH3(ad) ⎯⎯⎯→ NH 2(ad) + H(ad)

(9)

CaO

NH3(ad) + O(ad) ⎯⎯⎯→ NH 2(ad) + OH(ad)

(10)

CaO

NH 2(ad) + 2O(ad) ⎯⎯⎯→ NO(ad) + H 2O(ad)

Figure 7. DRIFTS of N2O adsorption species on the surface of CaO (500 ppm of N2O, 200 N mL min−1, and 323 K).

CaO

NH 2(ad) + NO(ad) ⎯⎯⎯→ N2(g) + H 2O(ad)

nitroso was observed at 1380 cm−1 after exposure to N2O and became stronger with increasing time. Weak bands of nitro at 1550 cm−1 and nitrate at 1060 cm−1 were also observed and kept nearly constant with increasing time. The presence of the adsorption band of nitrate verified the production of calcium nitrate in the previous study.16 The DRIFTS experiment showed that the main adsorption species of N2O on the CaO surface was cis-dimer nitroso. The two N atoms in N2O interacted with the coordinately unsaturated lattice oxygen on the CaO surface19,20 to form the adsorbed N2O molecule, as shown in eq 6. The neighboring adsorbed N2O molecules dimerized to form cis-dimer nitroso. The cis-dimer nitroso dissociated to N2 and peroxide ion, and the peroxide ion donated electrons to form free radical oxygen. The overall reaction of the above process is shown in eq 7. Two free radical oxygen atoms subsequently dimerized to form O2, as shown in eq 8. The strength of the cis-dimer nitroso adsorption was enhanced with increasing time, which indicated that the dissociation of cis-dimer nitroso was the rate-controlling step of N2O conversion on the CaO surface. (6) (7)

CaO

2O(ad) ⎯⎯⎯→ O2(g)

(13)

4. MODELING 4.1. CaO-Catalyzed N2O Decomposition. The influence of diffusion on the reaction process was considered and calculated in the kinetic model as a previous study.27 In CaO-catalyzed N2O decomposition, dissociation of cisdimer nitroso was the rate-controlling step of N2O conversion. The reaction rate of N2O can be expressed by eq 14 dC N2O = −ṁ CaOsCaOηN Okde, θθN2O 2 (14) dτ where ṁ CaO (kg/m3) is the mass concentration of the catalyst bed, sCaO (m2/kg) is the specific surface area of the CaO sample, ηN2O is the effectiveness factor of N2O, kde,θ (mol m−2 s−1) is the reaction rate constant, and θN2O is the coverage of N 2O on the CaO surface [θ N2O = Kads,N2OCN2O/(1 + Kads,N2OCN2O), where Kads,N2O (m3/mol) is the adsorption equilibrium constant of N2O on the CaO surf CaO-catalyzed N2O decomposition was first-order with respect to the N2O concentration, which indicated that Kads,N2OCN2O is far less than 1 and θN2O ≈ Kads,N2OCN2O. Thus, eq 14 can be transformed to eq 15. dC N2O = −ṁ CaOsCaOηN Okde,CC N2O 2 (15) dτ where kde,C (m/s) is the reaction rate constant (kde,C = kde,θKads,N2O). 4.2. CaO-Involved N2O + NH3 System. In the CaOinvolved N2O + NH3 reaction, the conversion of N2O was not influenced by NH3 and the small amount of H2O produced in the reactions. Thus, the reaction rate of N2O can be expressed

CaO

N2O(ad) ⎯⎯⎯→ N2(g) + O(ad)

(12)

CaO

2NH 2(ad) ⎯⎯⎯→ N2(g) + 2H 2(g)

CaO

N2O(g) ⎯⎯⎯→ N2O(ad)

(11)

(8) 27

As NH3 adsorbed on the Lewis acid sites of CaO, no adsorption competition between NH3 and N2O occurred on the CaO surface in the CaO-involved N2O + NH3 reaction. The rate-controlling step of NH3 conversion was hydrogen abstraction from adsorbed NH3 to form NH2.27 This process occurred through two reactions: catalyzed by CaO (shown in eq 9) or oxidized by free radical oxygen produced in N2O decomposition (shown in eq 10). Thus, NH3 conversion increased in the presence of N2O. Because O2 showed no effect E

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Figure 9. Comparison between experimental and simulation results during the CaO-involved N2O + NH3 system at (a) different temperatures and (b) different N2O inlet concentrations.

Figure 8. Comparison between experimental and simulation results during CaO-catalyzed N2O decomposition at (a) different temperatures and (b) different N2O inlet concentrations.

by eq 15. NH3 dissociation to NH2 was the rate-controlling step of NH3 conversion and occurred through eqs 9 and 10. Thus, the reaction rate of NH3 can be expressed as eq 16 dC NH3 dτ

= −ṁ CaOsCaOηNH θNH3(k NH3→ NH2,N2OC N2O 3

+ k NH3→ NH2,de)

(16) −2 −1 27

where KNH3 → NH2,N2O (m/s) and KNH3 → NH2,de (mol m s ) are the reaction rate constants of NH3 dissociation to NH2 catalyzed by CaO or oxidized by free radical oxygen, respectively, and θNH3 is the coverage of NH3 on the CaO surface [θNH3 = Kads,NH3CNH3/(1 + Kads,NH3CNH3), where Kads,NH3 (m3/mol) is the adsorption equilibrium constant of NH3 and CNH3 (mol/m3) is the concentration of NH3]. Once NH2 was formed, there existed three further reaction routes for NH2: oxidation to NO, reducing NO to form N2, and dissociation into N2. The reaction rate constant of NH2 oxidation to NO was nearly 3 times larger than that of NO reaction with NH2 at 1123 K.27 The ratio got even larger when the temperature decreased. In addition, the ratio of the N2O concentration to the NO concentration was larger than 10 in these experiments. Therefore, the reaction rate of NO production by NH2 oxidation was more than 30 times faster than that of NO consumption by the NH2 + NO reaction,

Figure 10. Comparison between experimental and simulation results during the CaO-involved N2O + NH3 + O2 system.

which meant that the influence of NO consumption by the NH2 + NO reaction on the NO concentration can be neglected. N2 selectivity was far larger than NO selectivity in NH3 conversion, which meant that the influence of the NH2 + NO reaction on N2 production can also be neglected. Therefore, the reaction model for the CaO-involved N2O + NH3 system can be simplified as just two reactions: NH3 oxidation to NO and NH3 dimerization to N2. NO selectivity can be expressed as eq 17 F

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k1θNH3θN2O k1θNH3θN2O + k 2θNH3θNH3

k12C N2O

=

Figures 8, 9, and 10 showed the comparison between the experimental and simulation results of CaO-catalyzed N2O decomposition and CaO-involved N2O + NH3 and N2O + NH3 + O2 systems, respectively. Simulation results were in good agreement with experimental data in the studied temperature and N2O and NH3 concentrations, which demonstrated that the proposed kinetic model well-described the influence of CaO on the N2O + NH3 + O2 system.

k12C N2O + θNH3 (17)

−2 −1

−2 −1

where k1 (mol m s ) and k2 (mol m s ) are the reaction rate constants of NH3 oxidation to NO and dimerization to N2, respectively, and k12 = Kads,N2O(k1/k2) = Kads,N2O(A1/A2) exp(−(E1 − E2)/RT). The reaction rate of NO can be expressed as eq 18. dC NH3 dC NH3 k12C N2O dC NO =− S NO = − dτ dτ dτ k12C N2O + θNH3

5. CONCLUSION CaO exhibited a catalytic effect on N2O decomposition to N2 and O2, and the reaction was first-order with respect to the N2O concentration. O2 had no effect on N2O decomposition. In the CaO-involved N2O + NH3 system, NH3 had no effect on N2O conversion. N2O promoted NH3 conversion on the CaO surface and subsequently oxidized NH 3 to NO. NH 3 conversion increased with increasing the temperature and N2O concentration. NO selectivity decreased with increasing the temperature but increased with increasing the N2O concentration. In the CaO-catalyzed N2O + NH3 + O2 system, N2O had no effect on NH3 conversion and NO selectivity; however, N2O decomposition was inhibited by the H2O produced in NH3 oxidation. Mechanism analysis showed that the rate-controlling step of N2O conversion on the CaO surface was the dissociation of cisdimer nitroso produced by the adsorbed N2O to form N2 and peroxide ions. Peroxide ions donated electrons to form free radical oxygen and subsequently dimerized to O2. Free radical oxygen accelerated the dissociation of NH3 to NH2 and increased NH3 conversion. NH2 reacted with free radical oxygen to produce NO and H2O, reacted with NO to produce N2, or dimerized to N2H4 and subsequently dissociated to form N2. NO selectivity was determined by the competition of the three routes of NH2 conversion. In the presence of O2, O2 substituted the role of N2O and N2O conversion was inhibited by the produced H2O through competitive adsorption. A kinetic model was established on the basis of the mechanism analysis to predict the influence of CaO on N2O decomposition and N2O + NH3 and N2O + NH3 + O2 systems. Simulation results were in good agreement with experimental data, which demonstrates the accuracy of the proposed mechanism and kinetic model.

(18)

4.3. CaO-Involved N2O + NH3 + O2 System. In the CaOinvolved N2O + NH3 + O2 system, NH3 oxidation was not influenced by N2O because of the presence of O2; thus, the reaction rates of NH3 consumption and NO formation can be calculated as the previous study.27 N2O decomposition was inhibited by the H2O produced in NH3 oxidation. In the presence of H2O, the coverage of N2O on the CaO surface can be expressed as eq 19 θN2O =

K ads,N2OC N2O 1 + K ads,N2OC N2O + K ads,H2OC H2O

(19)

where Kads,H2O is the adsorption equilibrium constant of H2O on the CaO surface and CH2O (mol/m3) is the concentration of H2O. Because Kads,N2OCN2O is far less than 1, eq 19 can be simplified to eq 20. θN2O =

K ads,N2O 1 + K ads,H2OC H2O

C N2O

(20)

The reaction rate of N2O decomposition in the presence of H2O can be obtained by combining eqs 14 and 20 to come up with eq 21. dC N2O dτ

= −ṁ CaOsCaOηN O 2

kde,C 1 + K ads,H2OC H2O

C N2O

(21)

In the CaO-involved N2O + NH3 + O2 system, all H elements in NH3 are converted to H2O. Thus, the H2O concentration can be calculated as a result of the consumption of NH3. The reactor used in the experiment was a plug flow integral reactor. Data obtained at different temperatures were used to correlate the reaction rate constants and the adsorption equilibrium constant of H2O on the CaO surface. The pre-exponential factors and activation energies were obtained by double-log transformation. The obtained kinetic parameters are listed in Table 2. E/R is the ratio of the activation energy and the gas constant, and A is the pre-exponential factor.ace and CN2O (mol/m3) is the concentration of N2O].

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ASSOCIATED CONTENT

S Supporting Information *

Calculation of the effectiveness factor. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Table 2. Kinetic Parameters of the Reactions

The authors declare no competing financial interest.

rate constant

E/R (K)

A

kde,C (m/s) kNH3 → NH2,N2O (m/s)

15885 20436

65.6 m/s 9874.3 m/s

k12 (m3/mol) Kads,H2O (m3/mol)

−3079 −957

0.8 m3/mol 2.5 m3/mol

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ACKNOWLEDGMENTS This work was supported by the fund from the National Key Technologies Research and Development Program (2011BAE29B03) and the “111” Project (B13001). G

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DOI: 10.1021/ef502512f Energy Fuels XXXX, XXX, XXX−XXX