Numerical simulation of selective catalytic reduction of NO and SO2 oxidation in monolith catalyst

Chemical Engineering Journal 361 (2019) 874–884

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Numerical simulation of selective catalytic reduction of NO and SO2 oxidation in monolith catalyst

T

Chenghang Zheng, Lifeng Xiao, Ruiyang Qu, Shaojun Liu, Qi Xin, Peidong Ji, Hao Song, ⁎ Weihong Wu, Xiang Gao State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou 310027, China

H I GH L IG H T S

SCR reaction occurred within a thin layer (ca. 0.2 mm) in the wall near the catalyst channel. • The SO oxidation occurred in the entire catalyst wall. • The effects of the operating parameters were investigated synthetically. • The • A strategy for catalyst design, i.e., applying thin-walled design was proposed. 2

A R T I C LE I N FO

A B S T R A C T

Keywords: Selective catalytic reduction (SCR) SO2 oxidation Numerical simulation Monolith honeycomb catalyst

A three-dimensional model that combined the selective catalytic reduction (SCR) of NO with ammonia and SO2 oxidation reactions over monolith honeycomb catalyst was established to study the effects of catalyst structure and operating parameters on NO reduction and SO2 oxidation. The model was to proved to be valid by experimental data. Simulation results showed that the SCR reaction only took place within a thin layer (ca. 0.2 mm) of the catalyst wall surface, whereas SO3 generated via SO2 oxidation accumulated throughout the entire wall. The effects of typical operating parameters, i.e., space velocity, temperature, feed concentrations of NO, NH3 and SO2 were studied. Temperature and gas velocity were found to be the major influencing factors on SO2 oxidation. A strategy for monolith catalyst struction optimization, i.e., using thin-walled catalyst was proposed on the basis of these results.

1. Introduction The selective catalytic reduction (SCR) of NOx with NH3 over monolith catalysts is one of the major technologies to control the emissions of nitrogen oxides in the flue gases from stationary sources such as coal-fired power plants [1–3]. In the treatment of SO2-containing flue gases, undesired oxidation of SO2 occurs simultaneously along with the NO reduction in the SCR unit [4–6]. Accordingly, the generated SO3 would form sulfuric acid [3,7–9] and ammonium bisulfate (NH4HSO4) [10–13] by reacting with water and unconverted ammonia in the flue gases, which could cause pressure drop and corrosion of downstream of the SCR unit [12–15]. Therefore, it is important to inhibit SO2 oxidation while maintaining sufficient NO removal efficiency. Monolith honeycomb catalyst is one of the main commercial SCR

⁎

catalysts [16–18], consisting of homogeneous mixtures of vanadia pentoxide and tungsta (or molybdena) as a promoter of the SCR reactions, supported on high surface area anatase TiO2. In our previous study [19], V2O5 is the active component, not only enhancing the DeNOx efficiency but also improving the oxidation of SO2. The oxidation rate of SO2 on the V2O5/TiO2 catalyst has a linear relationship with V2O5 loading [20]. Moreover, the addition of WO3, which typically acts as the promoter for the SCR catalyst, could promote the adsorption of SO2 and increase the oxidation rate of SO2 [21]. On the other hand, the removal of NOx and the oxidation of SO2 are affected by the monolith catalyst structure. For the SO2 oxidation reaction, Svachula et al. [22] reported that the rate of SO2 oxidation depends linearly on the catalyst wall thickness. Moreover, For the SCR reaction, several simulation models [23–25] describe reactors and the reactions in monolith catalyst to obtain insights into the effect of the

Corresponding author. E-mail address: [email protected] (X. Gao).

https://doi.org/10.1016/j.cej.2018.12.150 Received 30 June 2018; Received in revised form 22 December 2018; Accepted 27 December 2018 Available online 27 December 2018 1385-8947/ © 2018 Published by Elsevier B.V.

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catalyst structure. Furthermore, a model of Fe/Cu-SCR catalysts [26] was built by Shakya to simulate the coupling of the diffusion and reaction of SCR while Millo et al. [27] established a simulation model on SCR coated on a filter catalyst to investigate its catalytic properties. Roduit et al. [28] used numerical simulation methods to study the flow, mass transfer, and chemical reaction processes inside a honeycomb catalyst. The finite-difference method was used to discretize the control equation in the catalyst wall, and the distribution of NO concentration in the catalyst wall was obtained. The kinetic equations of the SCR reaction and the SO2 oxidation reaction was established by Orsenigo [29,30] to study the effects of flue gas parameters. However, a comprehensive study that combines the selective catalytic NO reduction and SO2 oxidation in monolith catalysts typically used in power plants under typical operating parameter is still lacking. The objective of this study was to apply a combination of modeling and experiments to provide insights into the aforementioned reactions in monolith catalysts. A 3D monolith catalyst model was established. The porous medium approach [31], which described the flow field and the effects of chemical reactions simultaneously, was used in consideration of the external and internal mass transfer of the catalyst wall. The effects of catalyst structure and operating parameters on the reactions were investigated. This study is beneficial to the design of monolith honeycomb catalysts.

where ri corresponds to the species reaction rate, which is a function of the reaction rates and the reaction stoichiometry; ci is the species concentration; and De is the diffusion coefficient. The initial and boundary conditions are defined as follows: At the inlet y = 0, the following boundary conditions were applied: u = u 0 , ci = ci0. (4) At the gas solid interface, i.e., at x = 4 mm, the following boundary conditions were applied: ∂ci ∂c u = 0, De ∂x = Dg ∂xi , (5) where De is the diffusion coefficient in the catalyst, and Dg is the diffusion coefficient in the gas phase. 2.2. Transport phenomena in the catalyst A uniform and porous monolith catalyst was assumed on the basis of the porous medium approach [31]. Diffusion coefficient and gas viscosity were dependent on pressure and temperature [35]. The potential of the collision between gas molecules and wall was high in the catalyst; therefore, the Knudsen flow described such a situation, and the diffusivity coefficient of the species is given by the following equations: The diffusion coefficient in the catalyst is

De =

2. Modeling and methodology

Dg = 2.695 × 10−3

A 3D two-phase model was used to describe a honeycomb-structured monolith catalyst. A quarter of monolith V2O5/TiO2 catalyst model was established due to the space axial symmetry problem [32]. A monolith catalyst with a length of 200 mm, a square channel structure of 5 holes × 5 holes, an internal diameter of 18 mm, and a wall thickness of 1.2 mm, was used. The model accounted for convection in the axial direction, gas-to-solid external transport, and diffusion and catalyst surface reaction within the catalyst wall and its structural diagram is showed in the Fig. 1. Furthermore, the model was based on the assumption that the distributions of the velocity, temperature, and concentration of reactants at the entrance of the monolith catalyst were uniform. In addition, the gas flow in the honeycomb channel was assumed to be turbulent. Heat transfer in the catalyst was ignored [33] because the concentrations of reactants were extremely low and the energy accumulation was insignificant. The species diffusion in the gas phase and in the catalyst wall followed Fick’s law. The model was made up of mass and momentum balances, which contemplated all the chemical kinetics and mass transport properties. For the flow pattern, a turbulence model called κ-ε model was selected, and the SIMPLE algorithm [34] was used to solve the pressure–velocity coupling problem in the momentum equations. The governing equations are given as follows: The continuity equation is

∂u 2 + ρ (u·∇u) = ∇ ·⎧−p + (μ + μt ) ⎡(∇u + ∇uT ) − ∇·uI⎤ ⎫ , ⎨ ∂t 3 ⎣ ⎦⎬ ⎩ ⎭

T 3 (Mi + M0)/(2 × 103Mi M0) , pσi σ0 ΩD

(7)

2.3. Kinetic models The kinetic models of the SCR reaction [33] and undesired oxidation of SO2 [36] for the monolith catalyst have been reported. The effect of ammonia adsorption on SCR reaction has been considered [37]. The reactions along with rate expressions used in the model are given by the following equations: 0 rNO = kCNO θNH3 = kCNO (1 − XNO ) θNH3,

θNH3 =

KCNH3 , 1 + KCNH3

(8)

(9)

0 CNH3 = CNO (α − XNO ),

(10)

rSO2 = kSO2 CSO2,

(11)

where k is the intrinsic reaction rate constant; Ci is the concentration of i species, i = NO, NH3 and SO2; θNH3 is the coverage of NH3 on the catalyst surface; XNO is the NO conversion and K is the NH3 adsorption equilibrium constant on the catalyst surface and is only correlative to temperature. The k and K of the corresponding temperature were obtained according to the NO removal efficiency of the catalyst at different ammonia nitrogen ratios and temperatures and the SO2 oxidation efficiency of the catalyst under different temperatures to obtain the kinetic parameters of the two reactions. The relevant parameters were obtained according to Arrhenius’s law as follows:

(1)

(2)

where ρ, u, and μ are the local density, velocity, and viscosity, respectively; p is the system pressure; and I is the turbulence intensity. The mass transfer convection diffusion equation is

∂ρci ∂ ⎛ ∂c ρu ci − De i ⎞ = ri, + ∂t ∂x ⎝ ∂x ⎠

(6)

where ε and τ are the porosity and tortuosity of the catalyst, which are 4 and 0.4 in this study, respectively; Mi and Mo are the molar masses; and σi and σ0 are the diameters of the gas species. p is the pressure, and ΩD and ΩV are the collision integrals [35].

The momentum transport equation is

ρ

T . Mi

The diffusion coefficient in the gas phase is

2.1. Reactor model

∂ρ + ∇ ·(ρu) = 0. ∂t

ε × 9700 × r¯ × τ

(3)

i: NO,NH3 ,SO2 ,SO3 , 875

−E k = A exp ⎛ a ⎞, ⎝ RT ⎠

(12)

ΔHads ⎞, K = Aads exp ⎛ ⎝ RT ⎠

(13)

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Fig. 1. The schematic of catalyst structure.

where Ea is the activation energy, A is the preexponential factor, ΔHads is the heat of NH3 adsorption, and Aads is the preexponential factor of the NH3 adsorption equation. Combined with the previous findings of our research group [19], experiments were performed to acquire the kinetic parameters of the catalyst. In the experiments, 0.05 g of the catalyst was loaded on a fixed bed under flue gas that contained NO, NH3, SO2, O2, and N2 as balance. The total flue gas flow was 1200 ml/min. The experiments were performed at several temperature conditions (320 °C–400 °C) with two different ammonia nitrogen molar ratios (NH3/NO = 0.3, 0.75). The feed concentration of NO was 1000 ppm, the feed concentrations of NH3 were 300 and 750 ppm, the concentration of O2 was 5%, and N2 was the balance. In the SO2 oxidation reaction, the kinetic parameters were obtained by the experiments under several temperature conditions (320 °C–400 °C). The data measured by the catalyst experiments in different ammonia nitrogen ratios and temperatures were taken into rate expressions. k and K at different temperatures could then be calculated. Combined with the Arrhenius’s law of the reactions, the rate expressions could be given as

XNO =

XSO2 =

(

1.532 × 105 RT

⎜

⎟

(

)C )C

RT

⎜

7.233 × 10 4 ⎞ CSO2. RT ⎠

× 100% (17)

3.1. Model validation We started from the comparison between the simulated results and experimental data to validate the model, as shown in Figs. 2 and 3. In Fig. 2, the effects of temperature, gas hourly space velocity (GHSV) and inlet NO concentration on the NO conversion were investigated. It can be found that the simulation results correlated well with the experimental data, indicating that the model established for SCR reaction over the catalyst could simulate the experimental conditions well. Besides, both results suggested that in the temperature range from 320 °C to 400 °C, the NO conversion increased with increasing temperature and lower GHSV. While the inlet NO concentration slightly affected the NO conversion. Fig. 3 shows the comparison between the simulated data and experimental results for SO2 oxidation over the catalyst at different temperatures. The same trends were observed as well, suggesting that the simulation model was in good agreement with the experimental results for the SO2 oxidation reaction. Both results also showed that the SO2 oxidation efficiency over the catalyst increased obviously with increasing temperature in the range from 300 °C to 420 °C. Therefore, according to Figs. 2 and 3, the model we established for the SCR reaction and SO2 oxidation over the monolith catalyst was valid to simulate these two reaction processes.

CNO,

NH3

(14)

rSO2 = 7.46 × 10 4 exp ⎛− ⎝

in CSO 2

3. Results and discussion

NH3

1.532 × 105

in out CSO − CSO 2 2

(16)

in is where, XNO is the NO conversion; XSO2 is the SO2 conversion; CNO the concentration of NO at the inlet and Ciout is the concentration of i species, i = NO, SO2 and SO3 at outlet.

rNO = kCNO θNH3 = 3.17 2.55 × 10−10 exp 5.038 × 10 4 ⎞ × 106 exp ⎛− RT ⎝ ⎠ 1 + 2.55 × 10−10 exp

in out CNO − CNO × 100% in CNO

⎟

(15)

In the other hand, the NO removal efficiency and SO2 conversion rate are significant to evaluate the activity and stability of catalysts. The definition of NO removal efficiency and SO2 conversion rate could be given as 876

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80

5 SO2 Conversion (%)

6

NO Conversion (%)

(a) 100

60 Experimental results Simulation results

40 20 0

320

340

360

380

2

0

300

330

360

390

420

Temperature ( C) Fig. 3. Experimental and simulation results of SO2 conversion under different operating temperatures.

80 NO Conversion (%)

Simulation results

400

(b) 100

for conventional coal-fired power plants. Fig. 4(b) depicts that with the increase in distance from the inlet, the NO concentration decreased gradually from 499 ppm to 118 ppm at the axisymmetrical line of the channel. While at the corner of the monolith catalyst, the NO concentration decreased significantly from 315 ppm to 32 ppm with a sharp drop close to the inlet. This trend was similar to that at the channel wall. Fig. 5 shows the simulated distribution of NO concentration within transverse sector of the channel of the monolith catalyst. In the radial direction of the channel, the highest NO concentration appeared at the central axis in the channel of the honeycomb catalyst, and the lowest NO concentration distributed near the wall. This was due to the SCR reaction at the wall, thereby lowering the NO concentration. A sharp decline of NO concentration was observed inside the wall. At position y = 10 mm and z = 0 mm, the concentration of NO showed a dramatical decrease from 262 ppm at x = 4.0 mm to 2 ppm at x = 4.2 mm. Thus, the sharp decrease of NO concentration within the wall indicated that the SCR reaction occurred mainly at a thin layer (0.2 mm) of the wall near the channel. Similar results were also obtained over other transverse sectors. This result indicated that the reaction rates of NO and NH3 inside the wall were much higher than their diffusion rates. As a result, NO and NH3 would be rapidly consumed before diffusing to the wall center. Most of the catalyst wall bulk was not involved in the reaction. Then we performed the SO2 oxidation over the monolith catalyst using this model. The SO3 concentration distribution along the axial direction is shown in Fig. 6. It differed a lot from the result of NO concentration distribution. Since the reaction rate of homogeneous SO2 oxidation without the catalyst was negligible in the testing temperature range [19], the SO3 concentration in the central axis of the catalyst channel was the lowest. While the highest SO3 concentration was observed inside the catalyst wall. As shown in Fig. 6(b), when the distance from the inlet increased, the SO3 concentration at the wall intersection increased significantly from 28 ppm to 127 ppm. Similar trends were also observed at the wall center and corner. At the outlet, a sharp decline in SO3 concentration was observed, probably due to the structure of the wall at the outlet which intensified the effect of the mixing of the flue gases thus decreasing the SO3 concentration. The SO3 concentration distribution in the transverse sector was shown in Fig. 7. An obvious increase of SO3 concentration was observed within the entire catalyst wall. The SO3 concentration increased significantly from 14 ppm to 62 ppm at position y = 100 mm. The SO3 concentration in the wall intersection was found to be higher than that

60 Experimental results Simulation results

40 20 0 1800

3600 5400 7200 Gas Hourly Space Velocity (h -1)

9000

(c) 100 NO Conversion (%)

Experimental results

3

1 Temperature ( C)

80 60 Experiment results Simulation results

40 20 0

4

400

600

800

1000

Inlet NO Concentration (ppm) Fig. 2. Experimental and simulation results of monolith catalyst: (a) NO conversion under different operating temperatures, (b) NO conversion under different gas hourly space velocity (GHSV), (c) NO conversion under different inlet NO concentration;

3.2. Effect of structure of monolith honeycomb catalyst on reaction In this section, the model was used to study the effect of monolith catalyst structure on the reactions. The NO concentration distribution within the wall and channel along the axial direction of the monolith catalyst was investigated and the results are shown in Fig. 4. The operating parameters used here were selected based on the typical values

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Fig. 4. Concentration distribution of NO along the axial direction of the monolith catalyst: (a) monolith catalyst profile, (b) NO concentration in axis direction. Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm.

resulted in the decrease in NO concentration and the increase in SO3 concentration, respectively. The reason was that the increasing temperature resulted in increasing reaction rates of NO reduction and SO2 oxidation. When the temperature was higher than 340°, the NO concentration almost did not change with the temperature, while the SO3 concentration still increased greatly with increasing temperature. Fig. 9 shows the results of NO and SO2 conversion at the reaction temperatures ranging from 300 °C to 400 °C. Temperature is a key parameter that determines the overall performance. As shown in Fig. 9(a), the NO concentration varied slightly whereas the SO3 concentration obviously enhanced with increasing temperature. As a result, the NO and SO2 conversion varied with temperature, as observed in Fig. 9(b). Below 380 °C, the NO conversion slightly improved with increasing temperature. When the temperature exceeded 380 °C, the NO conversion remained constant. While in the case of SO2 conversion, a higher temperature led to a higher SO2 conversion. At 340 °C, a typical temperature in the SCR unit in coal-fired power plants, the NO conversion was 84.31% and the SO2 conversion was 1.19%, which was in agreement with our expectations, i.e. low SO2 conversion and high NO conversion. Gas hourly space velocity (GHSV) is also a key operating parameter influencing the NO reduction and SO2 oxidation over the monolith catalyst. Fig. 10 shows the effect of GHSV on NO and SO2 conversions. As shown in Fig. 10(a), the NO concentration gradually decreased along the axisymmetrical line of channel with increasing GHSV. In the radial direction, NO concentration increased as GHSV increased, whereas the

at other positions. These results suggested that the reaction rate of the SO2 oxidation inside the catalyst wall was slower than the diffusion rate of SO2. In addition, the produced SO3 would diffuse from the wall surface to the channel. A distinct gradient SO3 concentration in the channel was formed, where the SO3 concentration increased from 4 ppm at y = 10 mm to 14 ppm at position y = 190 mm.

3.3. Effects of operating parameters on reaction The operating parameters would obviously influence both the NO reduction and SO2 oxidation performances of the monolith catalyst [1,4]. Here, we performed a systematic simulation to investigate the effects of operation parameters to gain further insight into the NO reduction and SO2 oxidation processes in the monolith catalyst. The concentrations of NO and SO3 at different directions are shown in Fig. 8. Fig. 8(a) shows the NO concentration distribution along with the axisymmetrical line of channel. Specifically, the NO concentration presented a similar decreasing trend at different temperatures along with the axisymmetrical line which was because the NO reduction only took place inside the wall and the catalytic activity increased with the increase of temperature. When the temperature increased, the outlet NO concentration at the axis direction slightly decreased. Differing from NO distribution, the SO3 concentration increased rapidly along the axisymmetrical line, as shown in Fig. 8(c). At the radical direction, the distributions of NO and SO3 showed similar trends at different temperatures, similar to previous results. The increase of temperature

Fig. 5. Concentration distribution of NO in transverse sector of the monolith catalyst: (a) monolith catalyst transverse sector profile, (b) NO concentration in radial direction. Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm. 878

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Fig. 6. Distribution of SO3 concentration along the axial direction in honeycomb catalyst: (a) monolith catalyst profile, (b) SO3 concentration in axis direction. Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm.

directions are separately illustrated in Fig. 12(c) and (d), where the SO3 concentration decreased with increasing NO feed concentration. Le Chatelier’s principle states that changing the reactant concentration would shift the equilibrium to the side that would reduce that change in concentration. Comparison of different inlet NO and NH3 concentrations demonstrated that high NO and NH3 concentrations led to a high NO concentration and a low SO3 concentration in the catalyst. Fig. 13(a) presents that the NO and NH3 feed concentrations greatly affected the distributions of NO concentration and SO3 concentration. Fig. 13(b) shows that as the feed concentrations of NO and NH3 increased, the NO conversion increased, whereas the SO2 conversion obviously decreased. Under this operating parameter, the NO and SO2 conversion was acceptable for industrial use under different NO and NH3 feed concentrations. As the sulfur content of coal used in power plants varied greatly, thus a significant difference in the SO2 inlet concentration would exist in the flue gas generated via coal combustion. We performed a simulation with different SO2 feed concentrations to study the influence of the inlet flue gas of the SO2 concentration. As shown in Fig. 14, with different SO2 feed concentrations, the SO3 concentration gradually increased along the axisymmetrical line of channel. In the radial direction, the concentration increase tendency was similar with that in the channel, while the increase in SO2 concentration inside the wall led to higher SO3 concentration. Fig. 15(a) depicts that the gradient of NO concentration was varied slightly with different SO2 concentrations, whereas the SO3

NO concentration greatly decreased in a thin layer of wall near the channel, as mentioned previously and as shown in Fig. 10(b). Comparison of the concentration distribution under different GHSVs implied that the increase in GHSV resulted in the decrease of both NO concentration and SO3 concentration. A higher GHSV led to a shorter residence time for the species contacting with the catalyst, thus lowering the conversions. Fig. 11(a) demonstrates that the GHSV greatly affected the NO and SO3 concentration distribution. Both NO concentration and SO3 concentration increased gradually with increasing GHSV. The NO conversion exhibited an approximatively linear decrease under the testing GHSV conditions, in. Fig. 11(b) indicates that the SO2 conversion decreased significantly from 2.59% to 0.70% as the GHSV increased from 4000 h−1 to 12,000 h−1. As discussed above, the residence time was shortened with the increase of GHSV, which was unconducive to the diffusion and adsorption of the reactants in the catalyst pores and resulted in a decrease in the reaction rate. Therefore, a higher GHSV would lead to a declined NO removal efficiency and SO2 conversion. Another simulation was performed to study the influence of inlet flue gas concentration. As shown in Fig. 12(a), under different inlet NO and NH3 concentration conditions, the tendency of concentration variation was similar along with the axisymmetrical line of channel. In the radial direction, the concentration decline tendency was similar in the channel, as mentioned previously. Inside the wall, the NO concentration declined rapidly within a thin wall, as shown in Fig. 12(b). The SO3 concentrations along with the axisymmetrical line and in the radial

Fig. 7. Concentration of SO3 and SO2 in the transverse sector of the honeycomb catalyst: (a) monolith catalyst transverse sector profile, (b) SO3 concentration in radial direction, (c) SO2 concentration in radial direction; Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm. 879

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Fig. 8. Simulated results on effect of temperature on concentration distributions of NO and SO3: (a) NO concentration along with the axisymmetrical line of channel; (b) NO concentration in the radial direction at the position y = 100 mm; (c) SO3 concentration along with the axisymmetrical line of channel; (d) SO3 concentration in radial direction at the position y = 100 mm. Reaction conditions: GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm.

concentration differed. Fig. 15(b) shows the relation between the feed concentration of SO2 and the conversion of NO and SO2. The NO conversion was slightly affected by the inlet SO2 concentration, whereas the SO2 conversion increased from 1.15% to1.18% when the inlet SO2 concentration increased from 250 ppm to 500 ppm. Further increasing the inlet SO2 concentration, the SO2 conversion slightly decreased.

3.4. Enlightenment The comprehensive understanding of NO reduction and SO2 oxidation over the monolith catalyst could provide theoretical guidance for the monolith catalyst structure optimization, which helped to inhibit SO2 oxidation while maintaining sufficient NO removal efficiency over the monolith catalyst. Since results suggested that NO and NH3

(b) 100

(a)

6

4 60

40

NO conversion SO2 conversion 2

SO 2 conversion (%)

NO conversion (%)

80

20

0 320

340

360

380

0 400

Fig. 9. (a) Simulated results of concentration profile; (b) NO and SO2 conversion under different operating temperatures Reaction conditions: GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm. 880

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(b)

GHSV=4000h-1 -1 GHSV=6000h GHSV=8000h-1 GHSV=10000h-1 -1 GHSV=12000h

500 400 300 200 100 0

0

50

100

150

Channel

400 NO Concentration (ppm)

NO Concentration (ppm)

(a)

300

200

100

0

200

0

SO3 Concentration (ppm)

10 -1

GHSV=4000h GHSV=6000h-1 -1 GHSV=8000h -1 GHSV=10000h GHSV=12000h-1

8

6

4 2

0

0

50

100

150

1

2

3

4

Distance from the Central Axis of Channel (mm)

(d)

25

SO3 Concentration (ppm)

Distance from Inlet (mm)

(c)

Wall GHSV=8000h-1 GHSV=10000h-1 GHSV=12000h-1

GHSV=4000h-1 -1 GHSV=6000h

20

Wall

Channel -1

GHSV=4000h GHSV=6000h-1 -1 GHSV=8000h -1 GHSV=10000h GHSV=12000h-1

15

10 5

0

200

0

Distance from Inlet (mm)

1

2

3

4

Distance from the Central Axis of Channel (mm)

Fig. 10. Simulation results on effect of GHSV on concentration distribution of NO and SO3: (a) NO concentration along with the axisymmetrical line of channel; (b) NO concentration in radial direction at the position y = 100 mm; (c) SO3 concentration along with the axisymmetrical line of channel; (d) SO3 concentration in radial direction at the position y = 100 mm. Reaction conditions: T = 340 °C, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm.

catalyst with tunable partial concentration of active component could be manufactured in future. In this monolith catalyst, the concentration of active component at the position such as intersection of catalyst wall should be decreased to reduce the SO2 oxidation since high concentration of active component is an important cause of undesirable SO2 oxidation.

(a)

NO conversion (%)

(b) 100

3

90 2 80 1 70

60 4000

SO 2 conversion (%)

were already reacted at a thin layer (0.2 mm) near the catalyst wall surface while SO2 oxidation took place in the entire catalyst wall, a rational design strategy for SCR catalyst with low SO2 oxidation is proposed, in which the thickness of catalyst wall was suggested to reduce to inhibit SO2 oxidation and maintain sufficient NO removal efficiency. Moreover, as advanced material manufacturing technology such as 3D printing technology is developing [38], a new monolith

NO SO2

6000

8000

10000

0 12000

-1

GHSV (h )

Fig. 11. (a) Simulation results of concentration profile; (b) NO and SO2 conversion under different GHSVs Reaction conditions: T = 340 °C, [NO] = 500 ppm, [NH3] = 500 ppm, [SO2] = 500 ppm. 881

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(b)

700 NO= 300ppm NO= 400ppm NO= 500ppm NO= 600ppm NO= 700ppm

600 500 400 300 200

Channel

250 200 150 100 50

0

50

100

150

0

200

0

(d) SO3 Concentration (ppm)

SO3 Concentration (ppm)

4

NO= 300ppm NO= 400ppm NO= 500ppm NO= 600ppm NO= 700ppm

3

2

1

0

0

50

100

150

1

2

3

4

Distance from the Central Axis of Channel (mm)

Distance from Inlet (mm)

(c)

Wall

NO= 300ppm NO= 400ppm NO= 500ppm NO= 600ppm NO= 700ppm

300

100 0

400 350

NO Concentration (ppm)

NO Concentration (ppm)

(a)

200

20

Channel

Wall

NO= 300ppm NO= 400ppm NO= 500ppm NO= 600ppm NO= 700ppm

15

10

5

0

0

Distance from Inlet (mm)

1

2

3

4

Distance from the Central Axis of Channel (mm)

(b) 85

1.5

84

1.4

83

1.3

82

1.2

NO conversion (%)

(a)

81

80 300

SO 2 conversion (%)

Fig. 12. Simulation results on effects of NO and NH3 feed concentrations on concentration distributions of NO and SO3: (a) NO concentration along with the axisymmetrical line of channel; (b) NO concentration in radial direction at the position y = 100 mm; (c) SO3 concentration along with the axisymmetrical line of channel; (d) SO3 concentration in radial direction at the position y = 100 mm Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [SO2] = 500 ppm.

1.1

NO SO3

1.0

400

500

600

700

Inlet NO/NH3 concentration (ppm)

Fig. 13. (a) Simulation results of concentration profile; (b) effects of NO and NH3 inlet concentrations on conversion of NO and SO2 Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [SO2] = 500 ppm.

4. Conclusions

Most of the bulk catalyst walls were not involved in the SCR reaction, whereas the SO2 oxidation took place in the entire catalyst wall. The effects of operation parameters, including reaction temperature, GHSV, NO, NH3 and SO2 feed concentrations, on the performances of NO reduction and SO2 oxidation were investigated. The results showed that these operating parameters had profound effects on the NO reduction and SO2 oxidation, which provided valuable information for optimizing the operating parameters in coal-fired power plants and monolith

A 3D model was established to systematically investigate NO reduction and SO2 oxidation over honeycomb catalyst. The simulation results from the model was in good agreement with the experimental data. Several conclusions could be drawn. The simulation results showed that the SCR reaction occurred only within a thin layer (ca. 0.2 mm) near the monolith catalyst wall surface. 882

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Fig. 14. Simulated results on effect of SO2 feed concentration on concentration distributions of NO and SO3: (a) NO concentration along with the axisymmetrical line of channel; (b) NO concentration in radial direction at the position y = 100 mm; (c) SO3 concentration along with the axisymmetrical line of channel; (d) SO3 concentration in radial direction at the position y = 100 mm. Reaction conditions: T = 340 °C, GHSV = 8000 h−1, [NO] = 500 ppm, [NH3] = 500 ppm.

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catalyst design. The results obtained in this paper indicated that the optimization of the operating parameters and catalyst structure were essential to maintain high NO reduction and low SO2 oxidation performances. Hence a catalyst design strategy for low SO2 conversion was proposed, i.e. the preparation of a thin-walled catalyst, which could not only result in a high efficiency of NO reduction but also inhibit SO2 oxidation.

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