1. Introduction
Due to the high energy consumption, the environmental pollution caused by the use of non-renewable energy needs to be solved urgently. Diesel vehicle exhaust contains a large amount of nitrogen oxide, which poses threats to human health and the environment. As one of the most mature and widest used technology for denitrification, selective catalytic reduction with ammonia (NH
3-SCR) has been studied in various aspects. According to the different reaction rate, NH
3-SCR can be divided into three types: standard SCR (Equation (1)), fast SCR (Equation (2)) and NO
2-SCR (Equation (3)):
The reason why the fast SCR has a faster reaction rate than the standard SCR is that the oxidation ability of NO
2 is stronger than that of O
2. However, the share of NO
2 in diesel exhaust is about 5%, so NO
2-SCR becomes known as slow SCR. Therefore, the ratio of NO
2 in the feed gases is very important. Also, some researches on the influence of NO
2 on Cu-SSZ-13 [
1], Cu-SSZ-39 [
2], Cu-T3 [
3] etc. indicate that NO
2 can result in the promotion of NO
x conversion.
Many kinds of catalysts have been found to be used in NH
3-SCR because of the active sites on their surface that can adsorb NO
x in vehicle exhaust. As so far, catalysts applied to NH
3-SCR are generally used at medium and high temperature, and have poor water and sulfur resistance, such as commercial V
2O
5-TiO
2. Hence, it is necessary to explore replaceable catalysts. Generally speaking, Cu-zeolite catalyst has better low-temperature performance and higher NH
3-storage capacity [
4]. Although a large number of experimental studies aimed to Cu-zeolite catalysts, most of them focused on cations doping, selection of molecular sieve carriers or the comparison of topological structures [
5,
6,
7]. As to the simulation researches, the kinetic models for NH
3-SCR over vanadium-based catalysts only involved standard SCR and ammonia oxidation [
8,
9]. To zeolite catalyst, however, most researches were concerned about powdered catalysts or reaction in single channel model [
10,
11,
12], which is not in line with the actual situation. Only a few of researches investigated the NH
3-SCR in a monolithic reactor using Cu-zeolite as catalysts [
13,
14].
Among the zeolites, the chabazite (CHA) has a large surface area and high porosity as a result of microporous topological structure, which makes them one of the most popular NH
3-SCR candidates for the catalyst carrier. Thus, Cu-CHA catalysts are more attractive. Many experimental studies have proved that water has a positive influence on the denitrification efficiency of Cu-CHA [
15,
16,
17] and drawn a common conclusion that Cu-CHA has an excellent hydrothermal stability. Contrary to other metals or their oxide catalysts, water can increase the NO conversion on Cu-CHA, which makes Cu-CHA a quite suitable catalyst for NH
3-SCR. However, few studies have been done on water effect, especially in simulation studies using a global kinetic mechanism. Fahami, et al. [
18] explored the effects of gas hourly space velocity (GHSV), NO, O
2, NO
2 and H
2O concentration on NO conversion with a detailed kinetic mechanism for NO oxidation on Cu-CHA at low and medium temperature (150–350 °C). In their studies, the experimental data and simulation results fit well and the parameters of the mathematical model in each chemical reaction step were illustrated. They also found that increasing GHSV and the concentration of H
2O and NO
2 could inhibit NO oxidation, while O
2 could promote it. Active sites may perform diversely at different reaction temperatures. Olsson, et al. [
11] established a multi-sites kinetic model containing NH
3 storage, release and oxidation, standard SCR and N
2O formation in 100–600 °C with 5% H
2O in feed gases, but fast SCR and NO
2 SCR were not included. The results showed that different NH
3 adsorption temperatures corresponded to different sites. They also suggested that standard SCR mainly took place on site 1 and NH
3 oxidation was major on site 2. Besides, Gao, et al. [
19] built a comprehensive model with two adsorption sites and global kinetic reactions, which was a rare model including low temperature H
2O storage on site 2.
In summary, most of the research focused on catalysts themselves. However, the structural design of a NH3-SCR monolith catalyst reactor is lack of the theoretical support. Thus, a global kinetic mechanism is used to simulate the chemical reactions of NOx over Cu-based monolith catalyst and study the influence of different operating conditions, such as inlet velocity of reactants, the length of channels etc., providing reference for engineering design and evaluating the performance of Cu-CHA monolith catalyst. When the reactant gases flow through the porous monolith catalyst, the selective catalytic reactions occur on the support surface accompanied by heat and mass transfer. Hence, a mathematical model for predicting heterogeneous reactions, flow, mass and heat transfer characteristics of NOx/NH3/H2O/N2 mixture flowing in a honeycomb cordierite ceramic monolith with Cu-CHA washcoating is established by commercial software COMSOL Multiphysics. And the chemical kinetic model includes ammonia oxidation, nitrogen oxidation, standard, fast and slow SCR reactions, NH4NO3 and N2O formation and decomposition. Also the presence of H2O in feed gases impacts on the denitrification efficiency of Cu-CHA is studied.
3. Simulation Methods
The geometric model of honeycomb monolith is established, as shown in
Figure 13. In order to reduce running memory and save simulation time, only 1/8 of the reactor is used as the computational domain. And structural physical parameters of Cu-CHA honeycomb cordierite ceramic monolith catalyst is displayed in
Table 3. The substrate is a porous media with the porosity of 0.4. The powder Cu-CHA catalyst is washcoated on the walls of small channels.
A three-dimension model was used to study the performance of Cu-CHA catalysts in NH3-SCR. The model involved fluid flow in honeycomb pores, diffusion of gases, chemical reaction and heat transfer. In general, the standard SCR reaction occurring above 200 °C conforms to Eley-Rideal mechanism. Thus, Eley-Rideal mechanism model is utilized to simulate the SCR reactions of NO over Cu-CHA. Some assumptions are made in order to simplify the calculation and model.
The inlet velocity and concentration of mixture, as well as inlet temperature, etc. uniformly distribute;
The reactions in each channel of the Cu-CHA monolithic catalytic reactor are exactly the same;
The mass transfer conforms to Fick’s law.
The feed gases flowing in every small channel of the monolith catalyst can be divided into two parts, mainstream flow in central areas and seepage in porous areas near wall. The Naiver-Stokes (N-S) equation is used to describe the mainstream flow and the Brinkman equation is used in porous areas in the meantime:
Here, ρ is fluid density, kg/m3, u is velocity of fluid, m/s, p is pressure, Pa, μ is dynamic viscosity, N·s/m2, εp is porosity of porous area near wall, I is unit tensor and κ is permeability.
Then, mass transfer equation is written as:
In the equation above, DF,j is the diffusion coefficient of the reaction gases, m2/s, Ci is the concentration of reactant i, mol/m3, Ri is the rate of chemical reaction. DF,j is a value related to the collision integral ΩD, the characteristic length of the potential σ (m), the minimum potential energy ε/kb (K) and dipole moment μD (D). These parameters can be got directly in the CHEMKIN.
Heat transfer, chemical reactions and thermal boundary conditions can change the internal temperature of the catalyst. So, the energy equation is given by:
In this equation, CP,L is heat capacity of gases, J/(kg·K), k is thermal conductivity, W/(m·K), Q is heat source, W/m3.
Some boundary conditions are set as follow. The temperature for inlet feed mixture is in the range of 150–375 °C and ambient temperature is 50 °C. The mixture gases get into the reactor at a flow rate of 3000 mL/min with CNH3 = CNO = 750 ppm, CO2 = 6%, CH2O = 5% and CNO2 = CN2 =0 ppm. The outlet pressure is 1 atm. Also, convective heat transfer around the side and the outlet walls in the model are considered and the convective heat transfer coefficients h are 25 W/(m2·K) and 10 W/(m2·K) respectively.
It is well known that water can affect the catalytic efficiency of catalysts. As mentioned above, some experimental researches have proved that H
2O has an advantage in NO conversion of NH
3-SCR on Cu-CHA catalyst. In order to investigate the influence of H
2O on the NO conversion, two kinds of global kinetic models are built. The model without H
2O in feed gases comes from De-La-Torre, et al. [
25], and the model with H
2O originates from Metkar, et al. [
26]. Both models use one single site and apply Arrhenius to define the rate equations.
Table 4 and
Table 5 show the reactions with related parameters of two models respectively.
Here, in
Table 4 and
Table 5,
Aif is the forward pre-exponential factor and
Aib is the backward pre-exponential factor,
Eif is the forward activation energy and
Eib is the backward activation energy. In
Table 4,
A2b =
A2f/(8.61 × 10
−4 m
1.5·mol
−0.5),
E2b = 57.28 kJ/mol.
Ci means the concentration of reactant
i. Besides,
KNH3 is equilibrium constant of NH
3 adsorption and desorption while
θNH3 is the adsorption rate of NH
3. In
Table 5,
K* is the suppressing co-efficiency of NH
3 adsorption, which is equal to zero in Cu-CHA catalyst.
5. Conclusions
A three dimensionally numerical model including global kinetic chemical reactions, fluid dynamics, heat and mass transfer was built for a Cu-CHA catalyst reactor by COMSOL Multiphysics with 32,300 grid numbers. And several structural and operating conditions of NH3-SCR system were researched respectively to investigate the influence on the performance of Cu-CHA catalyst.
The presence of H2O in feed gases can increase the denitrification efficiency of Cu-CHA catalyst at medium temperature range (250–375 °C) and improves the NH3 conversion especially at low temperature range (150–250 °C). It inhibits NH3 oxidation, increases Cu reducibility and Bronsted acidity, causing higher NO conversion for Cu-CHA than that without water.
The CO2 has almost no influence on catalytic performance of Cu-CHA catalyst, possibly because the existence of NO and NO2 in reactant have higher oxidation ability than O2 and concentration of reactants is low. By ascending the ratio of NO2 to NOx, the outlet CNO declines, CN2 firstly increases and then decreases and CN2O continues to rise. The balanced point is x0 = 0.5, where fast SCR prevails and Cu-CHA catalyst performs high NO conversion and yield of N2, extremely low output of N2O as well. With the ratio of NH3 to NOx (ANR) increase, outlet CNO gradually declines while CNH3 rapidly grows when ANR > 1. The appropriate value of ANR is approximately 1.1, where Cu-CHA catalyst has great denitrification efficiency and low NH3 escape.
Increasing inlet flow velocity leads to worse denitrification performance of Cu-CHA catalyst, which may be due to insufficient react time between feed gases and catalyst. As the length of channels increases by 20 mm, the denitrification efficiency is enhanced by about 10% evenly, owing to providing more contact areas. In addition, the change of cross area of channels and wall thickness have great influence on NO conversion by affecting WHSV in reaction system. Bigger cross area causes larger WHSV, resulting in lower NO abatement efficiency. In contrast, thicker wall of channels brings lower WHSV, leading to longer residence time of exhaust gases and higher NO conversion of the reactor. However, the effect of WHSV is finite which may because the total mass of Cu-CHA catalyst is fixed.