Method for evaluating hydrothermal aging performance of selective catalytic reduction reactor in different temperature ranges

Abstract

The invention discloses a method for evaluating the hydrothermal aging performance of a selective catalytic reduction reactor in different temperature ranges, a plurality of SCR (selective catalytic reduction) single channels are arranged in the reactor, and a channel in a square cylindrical structure defined by a catalytic coating is a main channel; the method comprises the following steps: firstly, establishing an SCR single-channel mathematical model, giving limiting conditions, forming a closed equation set, and then expanding hydrothermal aging parameters based on the model to evaluate the hydrothermal aging performance of the reactor in different temperature ranges. According to the method, the nitrogen oxide conversion rate of the reactor under different hydrothermal aging degrees can be obtained, the model is a theoretical model, the evaluation cost of the reactor is low, and the efficiency is high. The quantitative characterization of the influence of the mild hydrothermal aging on the distribution of the active sites of the catalyst is realized by defining relative concentrations of a Bronsted acid site, a copper site and a physical adsorption site, and the severe hydrothermal aging performance of the reactor is evaluated by defining the total concentration of the standardized hydrothermal aging catalytic active sites. And accurate regulation and control indexes are provided for the design of an anti-aging catalyst.

Description

Technical Field

[0001] The invention belongs to the technical field of diesel engine exhaust treatment, and in particular relates to a temperature domain evaluation method for the hydrothermal aging performance of a selective catalytic reduction reactor. Background Art

[0002] Heavy-duty commercial vehicles have large load capacities, long operating hours, high requirements for stable operation quality, and some operating conditions are very harsh. It is difficult for new energy heavy-duty commercial vehicles to meet the needs of the commercial transportation market in a short period of time. Diesel engines have become the main source of power in the commercial transportation field due to their high fuel economy, low carbon dioxide emissions, and strong torque output. For long-distance transportation and heavy-load vehicles, diesel engines can significantly reduce operating costs, while reducing carbon emissions while ensuring power, meeting the current dual needs of environmental protection and economy. These advantages have enabled diesel engines to dominate the commercial vehicle market, especially in industries such as logistics, freight, and public transportation. However, although the current heavy-duty commercial vehicles such as heavy trucks and large buses account for a small proportion of the total ownership, their pollutants, particulate nitrogen oxides (NO X , nitrogen oxides) emissions account for a high proportion in the transportation sector. Therefore, heavy-duty commercial vehicles have become the focus of pollution reduction and carbon reduction in the transportation sector.

[0003] Currently, the selective catalytic reduction (SCR) technology based on copper-based molecular sieve catalysts is widely considered to be the most effective method for reducing nitrogen oxides (NO X ) is one of the technologies for reducing emissions.

[0004] Among them, Cu-SSZ-13 molecular sieve has become the current mainstream commercial catalyst due to its unique CHA topology, high specific surface area and regular pores, showing excellent NH3-SCR activity, N2 selectivity and durability. Durability is a major indicator used to evaluate the performance of a catalyst. With the increase in the mileage of heavy-duty commercial vehicles powered by diesel engines, the performance of the molecular sieve catalyst of the selective catalytic reduction device has gradually deteriorated, resulting in reduced reaction activity and conversion efficiency. At this stage, SCR technology can be used to reduce NO in the exhaust gas of heavy-duty commercial vehicles powered by diesel engines. X Emissions are reduced by over 90%. As emissions regulations become increasingly stringent, SCR catalysts must maintain high and stable conversion efficiencies over the long term, placing higher demands on their durability. Hydrothermal aging mechanisms and anti-aging strategies are key to developing efficient and durable SCR catalysts.

[0005] We need to find a low-cost and reliable method to evaluate the hydrothermal stability of SCR catalysts under different hydrothermal aging temperature ranges in order to understand the treatment effect of SCR catalysts on diesel engine exhaust during use. Summary of the Invention

[0006] In view of the above problems existing in the prior art, the purpose of the present invention is to provide a temperature domain evaluation method for the hydrothermal aging performance of a selective catalytic reduction reactor, and to establish a mathematical model of a single channel of SCR. The nitrogen oxide conversion rate can be obtained through this model. The mathematical model is a theoretical model with low cost and high efficiency for reactor evaluation. By defining the relative concentrations of Brønsted acid sites, copper sites and physical adsorption sites, a quantitative characterization of the effect of mild hydrothermal aging on the distribution of catalyst active sites is achieved, providing precise control indicators for the design of anti-aging catalysts. The standardized total concentration of hydrothermal aging catalytic active sites is used to quantify the degree of severe hydrothermal aging, and a relationship between the degree of aging and NO is established. x Direct correlation model of the conversion rate. The total concentration of active sites of hydrothermal aging catalysis normalized by core parameters and NO x The mapping relationship of conversion rate is used to establish the rapid aging evaluation standard.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: A method for evaluating the hydrothermal aging performance of a selective catalytic reduction reactor in different temperature domains is provided. The reactor is internally provided with multiple SCR single channels, each of which is a square cylindrical structure. The sidewalls of the square cylindrical structure are catalytic coatings formed by a catalyst. The channels within the square cylindrical structure formed by the catalytic coatings are main channels. The catalytic coatings have numerous pores within them, and ammonia molecules are adsorbed on the active sites of the catalysts to perform selective catalytic reduction reactions. First, a mathematical model of a single-channel SCR is established. Then, based on the model, hydrothermal aging parameters are expanded to evaluate the hydrothermal aging performance of the reactor in different temperature domains. The method for establishing the mathematical model includes the following steps: Step S1: Establishing a mathematical model of a single-channel SCR, wherein the equation group corresponding to the model includes a component balance equation of gas-phase substances in the main channel, a momentum balance equation of gas-phase substances in the main channel, an enthalpy balance equation of gas-phase substances in the main channel, a gas-phase component balance equation in the catalytic coating, a gas-phase enthalpy balance equation in the catalytic coating, an NH3 coverage balance equation in the catalytic coating, a component balance equation at the gas-coating interface, and an enthalpy balance equation at the gas-coating interface; Step S2: Given the initial conditions and boundary conditions of the SCR single-channel model, and based on the characteristics of the SCR single channel, define the values ​​or expressions of some unknown parameters in the equation group in step S1, and substitute the values ​​or expressions of the defined parameters into each equation in step S1 to obtain a closed set of equations corresponding to the SCR single-channel model.

[0008] When evaluating the hydrothermal aging performance of the reactor in different temperature domains, the evaluation was carried out from two states: mild hydrothermal aging and severe hydrothermal aging. The hydrothermal aging temperature corresponding to mild hydrothermal aging was 550°C~750°C, and the hydrothermal aging temperature corresponding to severe hydrothermal aging was above 750°C. During mild hydrothermal aging, the aging performance of the reactor was evaluated by the Brønsted acid site concentration, copper site concentration and physical adsorption site concentration. During severe hydrothermal aging, the aging performance of the reactor was evaluated by the standardized total concentration of hydrothermal aging catalytic active sites.

[0009] Calculate the relative Brønsted acid site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ; ; ; is the normalized Brønsted acid site concentration, is the Brønsted acid site concentration, is the Brønsted acid site concentration of the catalyst without hydrothermal aging, k a is the mild hydrothermal aging Arrhenius kinetic pre-exponential factor, Ea is the activation energy; R is the ideal gas constant, c is a constant, is the total concentration of catalytic active sites after hydrothermal aging.

[0010] Calculate the relative copper site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ; ; is the copper site concentration, is the concentration of copper sites when the Brønsted acid site concentration is 0. For a specific catalyst, is a constant and a is the coefficient of the linear function of the copper site concentration and the Brønsted acid site concentration.

[0011] Calculate the relative physical adsorption site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ; ; ; is the concentration of physical adsorption sites.

[0012] The global or local performance evaluation method for severe hydrothermal aging is: Calculation of the normalized total concentration of hydrothermal aging catalytic active sites , obtain different temperatures and NO X Conversion rate or NO in catalytic coating X Concentration distribution, where: ; is the total concentration of catalytic active sites in the non-hydrothermal aged state.

[0013] In step S2, the defined unknown parameters include: the mass transfer coefficient of any component on the surface of the catalytic coating, the diffusion coefficient of any component in the main channel and in the catalytic coating respectively, the specific surface area of ​​the catalyst in the catalytic coating, the reaction rate expression of each chemical reaction involved in the SCR single channel, and the total concentration of hydrothermally aged catalytic active sites in the catalytic coating.

[0014] The component balance equation of the gas phase in the main channel is: ; Wherein, t represents time; z represents the axial direction; ρ represents the exhaust density, i.e., the gas density in the main channel; v Indicates the exhaust velocity, that is, the gas velocity in the main channel; y i represents the mass fraction of component i in the main channel; D i represents the diffusion coefficient of component i in the main channel; The momentum balance equation of the gas phase in the main channel is: ; Wherein, μ represents dynamic viscosity; p represents pressure; The enthalpy balance equation of the gas phase in the main channel is: ; Among them, c p Represents the specific heat capacity of the gas in the main channel; λ g represents the thermal conductivity of the gas in the main channel; T g Indicates the gas temperature in the main channel; The gas phase component balance equation in the catalytic coating is: ; in, ρw is the density of the gas in the catalytic coating; x represents the radial direction; y w,i is the mass fraction of component i in the catalytic coating; D w,i is the diffusion coefficient of component i in the catalytic coating; a c is the specific surface area of ​​the catalyst in the catalytic coating; γ i,j is the stoichiometric ratio of component i in chemical reaction j; M i is the molar mass of component i; T w is the gas temperature in the catalytic coating; r j (T w ) is the chemical reaction j and the gas temperature in the coating is T w The reaction rate when Gas phase enthalpy balance equation in catalytic coating: ; Among them, λ w is the thermal conductivity of the gas in the catalytic coating; ΔH m，j is the standard formation enthalpy of chemical reaction j; The NH3 coverage balance equation in the catalytic coating is: ; Among them, θ NH3 is the coverage of adsorbed NH3 on the reaction surface of the catalytic coating; γ NH3,j is the stoichiometric ratio of NH3 in chemical reaction j; The component balance equation at the gas phase-coating interface is: ; Among them, k m,i is the mass transfer coefficient of component i on the coating surface; The enthalpy balance equation at the gas-coating interface is: ; Among them, d h is the hydraulic diameter of the main channel, k h is the heat transfer coefficient at the gas-coating interface.

[0015] The initial conditions of the SCR single-channel model are: ; ; ; The boundary conditions are: ; ; ; ; in, T in represents the temperature at the reactor inlet, T 0 is the initial temperature, p atm is the atmospheric pressure under standard conditions, v in is the air flow velocity at the reactor inlet, v 0 is the initial air flow velocity, p out is the pressure at the reactor outlet, L is the main channel length, which is the axial length; δ w is the thickness of the catalytic coating; is the coverage of the initially adsorbed NH3 on the reaction surface of the catalytic coating.

[0016] The nitrogen oxides in diesel exhaust include nitrogen monoxide and nitrogen dioxide. The nitrogen oxide conversion rate is ; ; in, and The NO at the reactor inlet and outlet are X concentration.

[0017] The beneficial effects of the present invention are: (1) A mathematical model of a single-channel SCR reactor was established, through which the nitrogen oxide conversion rate can be calculated, thereby evaluating or assessing the emission reduction performance of the reactor.

[0018] (2) The SCR single-channel model takes into account the influence of the degree of hydrothermal aging of the catalyst, and the evaluation of the emission reduction performance of the reactor is more accurate.

[0019] (3) The established theoretical model can be used to capture the mechanism of hydrothermal aging and its corresponding impact on catalyst performance, and can more accurately describe the influence of aging temperature and aging time on SCR performance during the hydrothermal aging process of copper-based molecular sieve catalysts.

[0020] (4) The mathematical model is a theoretical model, which has low cost and high efficiency in reactor evaluation.

[0021] (5) By defining the relative concentrations of Brønsted acid sites, copper sites, and physical adsorption sites, the effect of mild hydrothermal aging on the distribution of active sites of the catalyst can be quantitatively characterized, providing precise control indicators for the design of anti-aging catalysts.

[0022] (6) The degree of severe hydrothermal aging was quantified using the standardized total concentration of hydrothermal aging catalytic active sites, and the relationship between the degree of aging and NO was established. X Direct correlation model of the conversion rate. The total concentration of active sites of hydrothermal aging catalysis normalized by core parameters and NO X The mapping relationship of conversion rate is used to establish the rapid aging evaluation standard.

[0023] (7) The model can simulate the axial / radial concentration field distribution and accurately locate NO in the aged catalyst. X The "dead zone" of insufficient reaction guides the optimization of catalyst structure.

[0024] (8) The NH3 coverage equation is coupled with the active site concentration parameter to achieve dynamic early warning of the ammonia escape risk of aging catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the SCR single-channel model of the present invention.

[0026] Figure 2 is the relationship between the relative Brønsted acid site concentration and the hydrothermal aging time and temperature.

[0027] Figure 3 is the relationship between the relative copper site concentration and the hydrothermal aging time and temperature.

[0028] Figure 4 It is the relationship between the relative physical adsorption site concentration and the hydrothermal aging time and temperature.

[0029] Figure 5 is NO at different temperatures and aging levels X Conversion rate.

[0030] Figure 6 is Ω Total_norm NO concentration distribution in the catalytic coating when η is 1.0 and the temperature is 100°C.

[0031] Figure 7 is Ω Total_norm NO concentration distribution in the catalytic coating when the value is 1.0 and the temperature is 300℃.

[0032] Figure 8 is Ω Total_norm NO concentration distribution in the catalytic coating when the value is 1.0 and the temperature is 350℃.

[0033] Figure 9 is Ω Total_norm NO concentration distribution in the catalytic coating when the value is 1.0 and the temperature is 400℃.

[0034] Figure 10 is Ω Total_normNO concentration distribution in the catalytic coating when the value is 1.0 and the temperature is 500℃.

[0035] Figure 11 is Ω Total_norm NO concentration distribution in the catalytic coating when the pressure is 0.8 and the temperature is 300℃.

[0036] Figure 12 is Ω Total_norm NO concentration distribution in the catalytic coating when the value is 0.8 and the temperature is 350℃.

[0037] Figure 13 is Ω Total_norm NO concentration distribution in the catalytic coating when the pressure is 0.8 and the temperature is 400℃.

[0038] Figure 14 is Ω Total_norm NO concentration distribution in the catalytic coating when the pressure is 0.6 and the temperature is 300℃.

[0039] Figure 15 is Ω Total_norm NO concentration distribution in the catalytic coating when the value is 0.6 and the temperature is 350℃.

[0040] Figure 16 is Ω Total_norm NO concentration distribution in the catalytic coating when the pressure is 0.6 and the temperature is 400℃. DETAILED DESCRIPTION

[0041] The following describes the specific embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0042] For ease of description, spatially relative terms such as "above," "above," "on the upper surface of," and "upper" may be used herein to describe the spatial positional relationship of a device or feature to other devices or features as shown in the figures. It should be understood that spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in a drawing is inverted, a device described as "above" or "on top of" another device or structure would then be positioned as "below" or "below" the other device or structure. Thus, the exemplary term "above" can include both the "above" and "below" orientations. The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatially relative descriptions used herein should be interpreted accordingly.

[0043] A method for evaluating the hydrothermal aging performance of a selective catalytic reduction reactor (SCR) in a temperature domain is disclosed. The reactor can be considered as having multiple SCR single channels. Diesel engine exhaust enters the reactor and, as a reactant, undergoes catalytic reduction through the SCR single channels, with the resulting product exiting the reactor.

[0044] First, a mathematical model of a single-channel SCR is established, and then the hydrothermal aging parameters are expanded based on the model to evaluate the hydrothermal aging performance of the reactor in different temperature domains.

[0045] The schematic diagram of the SCR single-channel model is as follows Figure 1 As shown, the SCR single channel has a square cylindrical structure. The sidewalls of the square cylindrical structure are formed by a catalytic coating formed by a catalyst. The channel within the square cylindrical structure formed by the catalytic coating is the main channel. The catalytic coating has numerous pores within it, and ammonia molecules are adsorbed on the active sites of the catalyst. In other words, the SCR single channel comprises the main channel and the catalytic coating that surrounds the main channel. Figure 1 X and Z represent the radial and axial directions of the main channel respectively. The mass transfer process of reactants and products in the main channel and catalytic coating is divided into two types: intra-phase mass transfer and inter-phase mass transfer. There are a lot of nitrogen oxides (NO X ) and oxygen (O2) enter the catalytic coating, a mass transfer process known as interphase mass transfer. The reaction products, including water and nitrogen, are initially generated within the catalytic coating and transported through the coating's porous structure. These products diffuse outward from the active sites within the coating through the pores of the catalytic coating, a mass transfer process known as intraphase mass transfer. The coating's pore structure determines the speed and efficiency of product transfer, ensuring that the products quickly leave the reaction sites and reach the coating surface. After H2O and N2 pass through the catalytic coating via intraphase mass transfer and reach the surface, they enter the interface between the gas and solid phases—the gas-phase channels within the reactor. During this process, the products move from the microscopic pores of the coating into the macroscopic channels, and then from the solid-phase coating into the gas phase via interphase mass transfer. The entire process involves complex multiple mass transfer and reaction mechanisms, including axial convection and diffusion in the gas phase, mass transfer at the gas-solid interface, and radial diffusion and catalytic reactions within the catalytic coating. These various processes interact to ensure that the reactants enter the catalytic coating from the gas phase and efficiently complete the chemical conversion within the coating to produce the target products.

[0046] SCR reactors have the following structural characteristics: parallel, small main channels, a large main channel length-to-width ratio, negligible heat losses, and uniform radial flow distribution. It is reasonable to assume that all individual SCR channels exhibit consistent flow and reaction behavior within the reactor. This assumption simplifies analysis of reactor performance and provides consistent expectations of overall system behavior.

[0047] Specific assumptions include the following: A1) In the model, axial diffusion effects within the main channels and within the catalyst coating are neglected. This assumption simplifies the mathematical description of gas transport and reaction processes, as axial diffusion typically contributes little to overall mass transfer in real reactors. By neglecting this factor, the analysis can focus on dominant processes, such as convection and radial diffusion, thereby reducing computational complexity.

[0048] A2) The model assumes laminar gas flow within the main channel, meaning there is no significant turbulence. This assumption, based on the main channel geometry and inlet conditions, simplifies the fluid dynamics analysis and provides more accurate mass transfer and reaction calculations.

[0049] A3) In the model, the gas is evenly distributed at the inlet of the SCR single channel, that is, the gas flow rate, velocity, temperature and concentration at the inlet of each main channel are the same.

[0050] A4) The model assumes that the catalyst active material and the coating are uniformly distributed in all single channels, and the porous medium of the coating is also uniform and isotropic.

[0051] The A5 model divides the catalyst coating area into high-permeability and low-permeability regions. In the high-permeability region, gas mass transfer is primarily convection, meaning that gas can quickly pass through the catalyst pores and enter the reaction zone. In the low-permeability region, mass transfer is primarily diffusion, with gas molecules moving slowly through smaller pores. Convection is not considered in the mass transfer within the copper-based molecular sieve catalyst coating.

[0052] A6) Nuclear magnetic resonance experiments show that there is no evidence of dealumination of copper-based molecular sieve catalysts at mild aging temperatures, indicating that the performance degradation of copper-based molecular sieve catalysts at mild aging temperatures can be assumed to be due to the reduction of their active sites. Therefore, the effect of mild hydrothermal aging can be reflected by changes in the concentration of molecular sieve active sites.

[0053] Based on the above six assumptions, the modeling method of the SCR single-channel mathematical model includes the following steps: Step S1: Establish a mathematical model of a single channel of SCR. The corresponding set of equations of the model includes the component balance equation of the gas phase species in the main channel, the momentum balance equation of the gas phase species in the main channel, the enthalpy balance equation of the gas phase species in the main channel, the gas phase component balance equation in the catalytic coating, the gas phase enthalpy balance equation in the catalytic coating, the NH3 coverage balance equation in the catalytic coating, the component balance equation at the gas phase-coating interface, and the enthalpy balance equation at the gas phase-coating interface.

[0054] In this embodiment, each equation of the equation group is as follows: 1) Component balance equation of gas phase in the main channel (1) Wherein, t represents time; z represents the axial direction; ρ Indicates the exhaust density, that is, the gas density in the main channel; v Indicates the exhaust velocity, that is, the gas velocity in the main channel; y i represents the mass fraction of component i in the main channel; D i represents the diffusion coefficient of component i in the main channel.

[0055] 2) Momentum balance equation of gas phase in the main channel (2) Where μ represents dynamic viscosity and p represents pressure.

[0056] 3) Enthalpy balance equation of gas phase substances in the main channel (3) Among them, c p Represents the specific heat capacity of the gas in the main channel; λ g represents the thermal conductivity of the gas in the main channel; T g Indicates the gas temperature in the main channel.

[0057] 4) Balance equation of gas phase components in catalytic coating (4) in, ρ w is the density of the gas in the catalytic coating; x represents the radial direction; y w,i is the mass fraction of component i in the catalytic coating; D w,i is the diffusion coefficient of component i in the catalytic coating; a c is the specific surface area of ​​the catalyst in the catalytic coating; γ i,j is the stoichiometric ratio of component i in chemical reaction j; M i is the molar mass of component i; T w is the gas temperature in the catalytic coating; r j (T w ) is the chemical reaction j and the gas temperature in the coating is T w The reaction rate when .

[0058] 5) Gas phase enthalpy balance equation in catalytic coating (5) Among them, λ w is the thermal conductivity of the gas in the catalytic coating; ΔH m，j is the standard enthalpy of formation of chemical reaction j.

[0059] 6) NH3 coverage equilibrium equation in catalytic coating (6) in, is the total concentration of hydrothermal aging catalytic active sites; θ NH3 is the coverage of adsorbed NH3 on the coating reaction surface; γ NH3,j is the stoichiometric ratio of NH3 in chemical reaction j.

[0060] 7) Component balance equation at the gas-coating interface (7) in, k m,i is the mass transfer coefficient of component i on the coating surface. h is the main channel width.

[0061] 8) Enthalpy balance equation at the gas-coating interface (8) Among them, k h is the heat transfer coefficient at the gas-coating interface.

[0062] Step S2: Given the initial conditions and boundary conditions of the SCR single-channel model, and based on the characteristics of the SCR single channel, define the values ​​or expressions of some unknown parameters in the equation group in step S1, and substitute the values ​​or expressions of the defined parameters into each equation in step S1 to obtain a closed set of equations corresponding to the SCR single-channel model.

[0063] First, the initial conditions and boundary conditions of the SCR single-channel model are given in Equations (9) and (10), respectively: ; ; (9) ; ; ; (10) in, T in represents the temperature at the reactor inlet, T 0 is the initial temperature, p atm is the atmospheric pressure under standard conditions, v inis the air flow velocity at the reactor inlet, v 0 is the initial air flow velocity, p out is the pressure at the reactor outlet, L is the main channel length, which is the axial length; δ w is the coating thickness; is the coverage of the initially adsorbed NH3 on the coating reaction surface.

[0064] In the NH3-SCR reaction, the Eley-Rideal (ER) mechanism is applied to the SCR kinetic model.

[0065] Among them, it is believed that NH3 is adsorbed on the catalytic surface and NO in the gas phase X A reaction occurs.

[0066] The ammonia adsorption reaction is: (11) The ammonia desorption reaction is: (12) The nitric oxide oxidation reaction is: (13) The ammonia oxidation reaction is: (14) The standard SCR reaction is: (15) The fast SCR reaction is: (16) The slow SCR reaction is: (17) Among them, S represents the catalytic active site. In the adsorption process of NH3, studies have shown that this process belongs to the non-activation adsorption type, which means that its adsorption reaction does not require additional activation energy.

[0067] The Temkin relationship takes into account the change in adsorption heat and is applicable to desorption behavior under surface heterogeneity conditions.

[0068] In the desorption process of NH3, the activation energy (E des ) can be described by Temkin expression (18).

[0069] (18) Among them, E 0 des is the initial desorption activation energy; α is the coefficient; θ NH3is the coverage of adsorbed NH3 on the coating reaction surface.

[0070] The Arrhenius law describes the effect of temperature on the rate of a chemical reaction. Specifically, the rate of a reaction increases with increasing temperature, a phenomenon that can be quantitatively described using the Arrhenius equation.

[0071] Arrhenius's law shows that there is an exponential relationship between the reaction rate and the activation energy and absolute temperature of the reactants. The reaction rate expressions represented by Equations (19) to (25) are all expressions that conform to this law for this model.

[0072] The ammonia adsorption reaction rate expression is: (19) The expression for the ammonia desorption reaction rate is: (20) The reaction rate expression of nitric oxide oxidation is: (twenty one) The reaction rate expression of ammonia oxidation is: (twenty two) The standard SCR reaction rate expression is: (twenty three) The fast SCR reaction rate expression is: (twenty four) The slow SCR reaction rate expression is: (25) Among them, r ads and k ads is the ammonia adsorption reaction rate and rate constant; r des and k des is the ammonia desorption reaction rate and rate constant; r NO and k NO is the NO oxidation reaction rate and rate constant; r NH3 and k NH3 is the NH3 oxidation reaction rate and rate constant; r stand and k stand is the standard SCR reaction rate and rate constant; r rapid and k rapid is the fast SCR reaction rate and rate constant; r slow and k slow is the slow SCR reaction rate and rate constant; c NH3 、c NO、 c NO2and c O2 are the molar concentrations of NH3, NO, NO2, and O2, respectively; is the maximum coverage of NH3 on the coating reaction surface; is the critical coverage of NH3 on the coating reaction surface; is the reaction equilibrium constant.

[0073] The parameters of the kinetic model for the fresh catalyst (i.e., catalyst that has not been hydrothermally aged) are shown in Table 1.

[0074] Table 1 Parameters of the kinetic model for fresh catalyst

[0075] Nitrogen oxides in diesel engine exhaust mainly include nitrogen monoxide and nitrogen dioxide. The nitrogen oxide conversion rate is the performance index of this model, which can be expressed as: (26) in, and The NO at the reactor inlet and outlet are X concentration.

[0076] From the above, after obtaining the closed equation group corresponding to the SCR single-channel model, the nitrogen oxide concentration at the outlet can be calculated, and then the nitrogen oxide conversion rate can be obtained. The above conversion rate reflects the emission reduction performance of the reactor.

[0077] If not specified, the simulated inlet gas composition includes 900ppm NO, 100ppm NO2, 1000ppm NH3, 5% O2 and 7% H2O, with a space velocity of ,The main structure and material parameters of SCR are shown in Table 2.

[0078] Table 2 Main structure and material parameters of SCR

[0079] The above has completed the establishment of the SCR single-channel mathematical model.

[0080] Next, the hydrothermal aging performance of the reactor was evaluated in three different temperature domains: mild hydrothermal aging, global performance under severe hydrothermal aging, and local performance under severe hydrothermal aging.

[0081] In this embodiment, the copper-based molecular sieve is Cu-SSZ-13.

[0082] The realization of evaluating the hydrothermal aging performance of the reactor in the temperature domain is based on the establishment of the SCR single-channel mathematical model, and mainly applies the results of step S2 from three aspects: direct application of the equation group, parameterized expansion, and realization of temperature domain evaluation.

[0083] Direct application of the equation system refers to using the closed equation system established in step S2 (including eight equations, including composition balance, momentum balance, and enthalpy balance) as the basis for calculation or analysis during the temperature domain evaluation. For example, NOx concentration can be obtained by solving equations (1)-(8) and calculating the conversion rate. The NH3 coverage balance equation, equation (6), can be used to analyze the effects of aging on adsorption sites.

[0084] The parameterization expansion mainly refers to the quantification of active sites. Specifically, the hydrothermal aging parameters are expanded based on the kinetic parameters of step S2 (as shown in Table 1): the standardized concentrations of Brønsted acid sites, copper sites, and physical adsorption sites are defined by equations (27)-(39), and the active site model of step S2 (Ω, ) is associated with the aging conditions (temperature, time). The aging rate function, i.e., Equation (28), is derived using the Arrhenius equation of step S2, i.e., Equations (19)-(25).

[0085] The implementation of the temperature domain evaluation is to simulate the performance under different aging conditions through the equation group of step S2.

[0086] The temperature domain evaluation transforms the theoretical model of step S2 into a tool for quantitatively evaluating aging performance through parameter expansion (aging time / temperature) and scenario subdivision (mild / severe hydrothermal aging).

[0087] The hydrothermal aging of Cu-SSZ-13 catalyst can be divided into two states: mild and severe. In the mild aging state (from 550°C to 750°C), there are two major structural changes, including the dialumination of zeolite and the redistribution of exchanged Cu species, but NO X The conversion rate remains almost unchanged. In the "severe" aging state (above 750°C), almost no Brønsted acid sites remain in the zeolite and the catalyst structure begins to collapse, resulting in a decrease in NOx conversion, oxidation function and NH3 storage.

[0088] Four NH3 adsorption sites were found in Cu-SSZ-13, including physical adsorption sites, two exchange copper sites (ZCuOH and Z2Cu) and Brønsted acid sites. Two different copper sites are responsible for low-temperature NH3 storage, and Brønsted acid sites are responsible for high-temperature NH3 storage. The total NH3 storage capacity remains unchanged as mild hydrothermal aging proceeds. The decrease in the number of Brønsted acid sites in Cu-SSZ-13 during mild hydrothermal aging is not due to dealumination, but rather to the transformation of copper sites. It is worth noting that the total number of active sites remains unchanged during mild hydrothermal aging, and this transformation hardly changes the total NH3 storage capacity, NO X The conversion efficiency is almost unaffected, but the proportion of active sites of different types changes.

[0089] For Brønsted acid sites, the normalized Brønsted acid site concentration of the hydrothermally aged catalyst relative to the fresh catalyst can be defined as follows: (27) Where, is the normalized Brønsted acid site concentration, is the Brønsted acid site concentration, is the Brønsted acid site concentration of the un-hydrothermally aged catalyst.

[0090] The reactor model is based on the assumption that the mild hydrothermal aging process can be considered as a chemical reaction whose rate can be expressed using Arrhenius kinetics. That is, the extent of mild hydrothermal aging is calculated using the loss of catalytic active sites, which is the aging temperature (T a ) and aging time (t a ). The integral form of the mild hydrothermal aging calculation function is shown in Equation (28).

[0091] (28) Where k a is the Arrhenius kinetic pre-exponential factor for mild hydrothermal aging; Ea is the activation energy; R is the ideal gas constant, and c is a constant.

[0092] Defining relative Brønsted acid site concentrations : (29) Where, is the total concentration of catalytic active sites after hydrothermal aging.

[0093] Substituting equations (27) and (28) into (29), we can obtain: (30) The relationship between the relative Brønsted acid site concentration and the hydrothermal aging time and temperature can be calculated as follows: Figure 2 Brønsted acid sites are the main sites for NH3 storage at high temperatures. The lower the temperature, the smaller the NH3 storage capacity, especially at >300°C for reactant coverage (θ NH3 ) is more unstable. The decrease will lead to the NO X The conversion rate decreases and the peak temperature shifts to a higher temperature. The catalyst exhibits slower activity decay during mild hydrothermal aging, thereby extending the reactor life.

[0094] The copper site concentration can be described as a linear function of the Brønsted acid site concentration. This linear relationship indicates that the NH3 storage capacity on the copper site is unique for a given concentration of Brønsted acid sites. The implication of the calculations shown here is that for a known temperature exposure time, the equivalent hydrothermal aging can be estimated, and for a known hydrothermal aging, the copper and Brønsted acid site concentrations can be predicted. These predictions can be further used to calculate total NH3 storage and site-specific storage.

[0095] (31) in, is the copper site concentration. is the concentration of copper sites when the Brønsted acid site concentration is 0. For a specific catalyst, is a constant. a is the coefficient of the linear function of the copper site concentration and the Brønsted acid site concentration.

[0096] Defining relative copper site concentrations : (32) Substituting into equations (27), (28) and (31), we can obtain: (33) The relationship between the relative copper site concentration and the hydrothermal aging time and temperature can be calculated as follows: Figure 3As shown in Figure 3 , copper sites dominate low-temperature NH3 storage, and their increased concentration partially compensates for the loss of high-temperature storage capacity at Brønsted acid sites. The increased copper site concentration compensates for the decreased reaction rate due to the reduction of Brønsted acid sites. The increased copper site concentration is a hallmark of mild hydrothermal aging of Cu-SSZ-13, reflecting the thermodynamic stabilization of copper species. While NH3 storage and high-temperature SCR activity can be partially maintained, reactor performance must be comprehensively evaluated in conjunction with Brønsted acid site attenuation.

[0097] For the physical adsorption sites, combined with formula (31), the physical adsorption sites are calculated as shown in formula (34).

[0098] (34) is the concentration of physical adsorption sites.

[0099] Assumptions: (35) but: (36) Defining relative physical adsorption site concentrations : (37) Substituting into equations (27), (28) and (36), we can obtain: (38) The relationship between the relative physical adsorption site concentration and the hydrothermal aging time and temperature can be calculated as follows: Figure 4 The concentration of physical adsorption sites is positively correlated with the concentration of Brønsted acid sites, which is reduced due to the migration of copper sites, resulting in Synchronous decay. Although physical adsorption sites are not active centers, they serve as temporary storage reservoirs for NH3. Their reduction will weaken the catalyst's NH3 buffering capacity in the low-temperature range, resulting in insufficient NH3 supply under transient conditions. The rapid decay of (especially at >600°C) is an early sign of framework dealumination, indicating the subsequent acceleration of copper site aggregation and acid site loss, and early intervention is required to avoid a cliff-like drop in performance. The decrease in the concentration of physical adsorption sites is a structural indicator of mild hydrothermal aging of Cu-SSZ-13, reflecting the degradation of the integrity of the molecular sieve framework. Although it does not directly participate in the SCR reaction, its decay will indirectly reduce low-temperature activity by weakening NH3 storage and mass transfer, and indicate a more serious aging risk. It needs to be combined with and Comprehensive assessment of catalyst status based on changes.

[0100] The global performance analysis of severe hydrothermal aging is as follows:

[0101] Under severe hydrothermal aging conditions, hydrothermal aging significantly reduces the total concentration of hydrothermally aged catalytic active sites of the Cu-SSZ-13 catalyst by destroying the molecular sieve framework structure, reducing Brønsted acid sites, changing the distribution and aggregation state of Cu species, etc. It is still unknown how the aging temperature (above 750°C) and aging time quantitatively affect the concentration of active sites. According to site theory, the adsorption of NH3 on the catalyst is related to the distribution of Brønsted acid sites. With the increase of aging temperature, the activity of Brønsted sites decreases, the proportion of deactivated sites increases, the coverage of the aged catalyst decreases significantly, the adsorption capacity for NH3 decreases, and the amount of adsorbed NH3 decreases, but its basic reaction mechanism is not changed. Therefore, the present invention uses a standardized total concentration of hydrothermally aged catalytic active sites It is used to express the ratio of the total concentration of catalytic active sites of the catalyst after hydrothermal aging to that of the catalyst in the fresh state, which represents the aging degree of the catalyst.

[0102] (39) Figure 5 In the diagram, the two curves are the display of the cloud map under specific conditions, including The curve graph and Graph of the curve.

[0103] like Figure 5 As shown in the figure, the molecular sieve catalysts with various ammonia storage capacities have NO X The conversion rate increases, especially at 200~300℃, the catalytic activity increases significantly, and reaches the maximum NO at 300~400℃. X The conversion rate, then with the increase of temperature, NO X The conversion rate dropped significantly. This is because when the temperature is low, the molecular energy is low, resulting in a lower chemical reaction rate. X It cannot fully react with NH3; as the temperature rises, the catalyst becomes active and the SCR reaction proceeds rapidly. X Reacts fully with NH3 in a short time to reach the highest NO X When the temperature is further increased to above 400℃, the non-selective oxidation of NH3 makes NO X The conversion rate has decreased.

[0104] However, with decreases, the aging degree of the catalyst increases, and NO X The conversion rate decreases, NO X The peak conversion rate of NO2 moves toward high temperature, which is due to the decrease in ammonia storage capacity, resulting in a decrease in the total concentration of hydrothermal aging catalytic active sites. XThe conversion rate decreases, and higher temperature is required to accelerate the SCR reaction and increase NO X Conversion rate. Moreover, at high temperature, NO on the catalyst with large degree of aging X Conversion rates dropped rapidly.

[0105] The local performance analysis of severe hydrothermal aging is as follows: In order to improve the catalytic efficiency of aged catalysts in SCR reaction, the effect of aging degree on SCR reaction was studied. The simulated inlet gas components included 1000ppm NO, 1000ppm NH3, 5% O2 and 7% H2O. Total_norm The concentration distribution of NO in the catalytic coating at and temperature are shown as follows: Figure 6-16 As shown in the figure, since there is no NO2 in the intake air and the amount of NO2 generated is very small, the small amount of NO2 generated will also be consumed, so the concentration distribution of NO2 is not considered here.

[0106] (1) Below 350°C, the overall NO concentration in the catalytic coating decreases with increasing temperature, but increases above 350°C. This is due, on the one hand, to the equilibrium limitation of NO oxidation, which shifts toward NO2 decomposition as temperature increases; and, on the other hand, to the fact that after 350°C, the SCR reaction slows down, resulting in less NO being consumed.

[0107] (2) At different temperatures, the concentration at the inlet is high and the concentration at the outlet is low. The NO concentration decreases continuously along the axial airflow direction.

[0108] (3) In the radial direction, below 350°C, the NO concentration decreases continuously from the gas-solid interface to the interior of the catalytic coating. Above 350°C, the NO concentration at the rear end of the catalytic coating is lower than the gas-solid interface. The concentration decreases in the low-temperature region because NO is rapidly consumed in the near-interface region when it diffuses from the gas phase into the interior of the coating (dominated by the SCR reaction). Due to the reactant concentration gradient and diffusion resistance in the internal region, the NO penetration depth is limited, resulting in a concentration decrease from the surface to the inside. Moreover, the higher the temperature, the lower the NO concentration region at the rear end extends into the interior of the catalytic coating. The expansion of the low-concentration region in the high-temperature section is due, on the one hand, to the decrease in the SCR reaction rate at high temperatures, which slows down the NO consumption rate, causing the reaction region to extend deeper into the coating, resulting in the expansion of the low-concentration region in the interior. On the other hand, the high temperature exacerbates the temperature gradient inside the coating, and the thermal migration effect promotes the diffusion of NO to the high-temperature region (such as the center of the coating), further expanding the scope of the low-concentration region in the interior.

[0109] (4) With the normalized total concentration of catalytic active sites Ω Total_normAs the NH3 adsorption capacity decreases, the standard SCR reaction slows down, and the NO concentration in the coating increases. The overall concentration change of NO in the catalytic coating becomes sluggish. The sluggish response of the overall NO concentration is due to the weakening of the reaction kinetics. When Ω Total_norm At lower temperatures, the diffusion-reaction competition within the catalytic coating is unbalanced: the NO mass transfer rate from the gas phase to the interior is relatively higher than the restricted surface reaction rate, resulting in a gentler concentration gradient and a more uniform overall distribution. This phenomenon is more pronounced at low temperatures, where the SCR reaction is already slow. The reduced number of active sites further amplifies the kinetic inertia, making the system less sensitive to changes in operating conditions.

[0110] (5) The critical temperature of the NO concentration reduction point at the rear end increases. A higher critical temperature is required to significantly reduce the NO concentration at the rear end of the coating (near the outlet area), which is the result of the catalyst activity window shift. Total_norm After the reduction, the reaction rate per unit area decreases, and the temperature must be increased to compensate for the kinetic loss: high temperatures (>350°C) can temporarily enhance the reactivity of oxygen active sites, promoting NO adsorption and conversion, but this is only effective under more stringent conditions. While increasing the temperature accelerates the reaction, it also exacerbates side reactions, further delaying the effective consumption of NO in the back-end, pushing up the critical temperature threshold.

[0111] In summary, a temperature-domain evaluation method for the hydrothermal aging performance of a selective catalytic reduction reactor based on Cu-SSZ-13 was proposed based on its different hydrothermal aging characteristics at different hydrothermal aging temperatures.

[0112] Finally, it is necessary to explain here that the above embodiments are only used to further illustrate the technical solution of the present invention in detail and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by technicians in this field based on the above content of the present invention all fall within the scope of protection of the present invention.

Claims

1. A method for evaluating the hydrothermal aging performance of a selective catalytic reduction reactor in different temperature ranges, characterized in that: The reactor is provided with a plurality of SCR single channels, each of which is a square cylindrical structure. The sidewall of the square cylindrical structure is a catalytic coating formed by a catalyst. The channel within the square cylindrical structure surrounded by the catalytic coating is a main channel. The catalytic coating has many pores inside. Ammonia molecules are adsorbed on the active sites of the catalyst and a selective catalytic reduction reaction is performed. First, a mathematical model of a single-channel SCR is established. Then, based on the model, hydrothermal aging parameters are expanded to evaluate the hydrothermal aging performance of the reactor in different temperature domains. The establishment of the mathematical model includes the following steps: Step S1: Establishing a mathematical model of a single-channel SCR, wherein the equation group corresponding to the model includes a component balance equation of gas-phase substances in the main channel, a momentum balance equation of gas-phase substances in the main channel, an enthalpy balance equation of gas-phase substances in the main channel, a gas-phase component balance equation in the catalytic coating, a gas-phase enthalpy balance equation in the catalytic coating, an NH3 coverage balance equation in the catalytic coating, a component balance equation at the gas-coating interface, and an enthalpy balance equation at the gas-coating interface; Step S2: Given the initial conditions and boundary conditions of the SCR single-channel model, a closed set of equations corresponding to the SCR single-channel model is calculated; When evaluating the hydrothermal aging performance of the reactor in different temperature domains, the reactor is evaluated in two states: mild hydrothermal aging and severe hydrothermal aging. The hydrothermal aging temperature corresponding to mild hydrothermal aging is 550°C to 750°C, and the hydrothermal aging temperature corresponding to severe hydrothermal aging is above 750°C. During mild hydrothermal aging, the aging performance of the reactor is evaluated by the concentration of Brønsted acid sites, copper sites, and physical adsorption sites. During severe hydrothermal aging, the aging performance of the reactor is evaluated by the standardized total concentration of hydrothermal aging catalytic active sites. Calculate the relative Brønsted acid site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ； ； ； is the normalized Brønsted acid site concentration, is the Brønsted acid site concentration, is the Brønsted acid site concentration of the catalyst without hydrothermal aging, k a is the mild hydrothermal aging Arrhenius kinetic pre-exponential factor, Ea is the activation energy; R is the ideal gas constant, c is a constant, is the total concentration of catalytic active sites after hydrothermal aging; Calculate the relative copper site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ； ； is the copper site concentration, is the concentration of copper sites when the Brønsted acid site concentration is 0. For a specific catalyst, is a constant, a is the coefficient of the linear function of the copper site concentration and the Brønsted acid site concentration; Calculate the relative physical adsorption site concentration and hydrothermal aging time t a and aging temperature T a relationship, where: ； ； ； is the concentration of physical adsorption sites.

2. The evaluation method according to claim 1, wherein: The global or local performance evaluation method for severe hydrothermal aging is to calculate the normalized total concentration of hydrothermally aged catalytic active sites. , obtain different temperatures and NO X Conversion rate or NO in catalytic coating X Concentration distribution, where: ； is the total concentration of catalytic active sites in the non-hydrothermal aged state.

3. The evaluation method according to claim 1 or 2, characterized in that: In step S2, given the initial conditions and boundary conditions, the values ​​or expressions of some unknown parameters in the equation group in step S1 are defined according to the characteristics of the SCR single channel, and the values ​​or expressions of the defined unknown parameters are substituted into each equation in step S1 to obtain a closed set of equations corresponding to the SCR single channel model. The defined unknown parameters include: the mass transfer coefficient of any component on the surface of the catalytic coating, the diffusion coefficient of any component in the main channel and the catalytic coating respectively, the specific surface area of ​​the catalyst in the catalytic coating, the reaction rate expression of each chemical reaction involved in the SCR single channel, and the total concentration of hydrothermal aging catalytic active sites in the catalytic coating.

4. The evaluation method according to claim 1 or 2, wherein: The component balance equation of the gas phase in the main channel is: ； Wherein, t represents time; z represents the axial direction; ρ represents the exhaust density, i.e., the gas density in the main channel; v Indicates the exhaust velocity, that is, the gas velocity in the main channel; y i represents the mass fraction of component i in the main channel; D i represents the diffusion coefficient of component i in the main channel; The momentum balance equation of the gas phase in the main channel is: ； Wherein, μ represents dynamic viscosity; p represents pressure; The enthalpy balance equation of the gas phase in the main channel is: ； Among them, c p Represents the specific heat capacity of the gas in the main channel; λ g represents the thermal conductivity of the gas in the main channel; T g Indicates the gas temperature in the main channel; The gas phase component balance equation in the catalytic coating is: ； in, ρ w is the density of the gas in the catalytic coating; x represents the radial direction; y w,i is the mass fraction of component i in the catalytic coating; D w,i is the diffusion coefficient of component i in the catalytic coating; a c is the specific surface area of ​​the catalyst in the catalytic coating; γ i,j is the stoichiometric ratio of component i in chemical reaction j; M i is the molar mass of component i; T w is the gas temperature in the catalytic coating; r j (T w ) is the chemical reaction j and the gas temperature in the coating is T w The reaction rate when Gas phase enthalpy balance equation in catalytic coating: ； Among them, λ w is the thermal conductivity of the gas in the catalytic coating; ΔH m，j is the standard formation enthalpy of chemical reaction j; The NH3 coverage balance equation in the catalytic coating is: ； Among them, θ NH3 is the coverage of adsorbed NH3 on the reaction surface of the catalytic coating; γ NH3,j is the stoichiometric ratio of NH3 in chemical reaction j; The component balance equation at the gas phase-coating interface is: ； Among them, k m,i is the mass transfer coefficient of component i on the coating surface; The enthalpy balance equation at the gas-coating interface is: ； Among them, d h is the hydraulic diameter of the main channel, k h is the heat transfer coefficient at the gas-coating interface.

5. The evaluation method according to claim 4, wherein: The initial conditions of the SCR single-channel model are: ； ； ； The boundary conditions are: ； ； ； ； in, T in represents the temperature at the reactor inlet, T 0 is the initial temperature, p atm is the atmospheric pressure under standard conditions, v in is the air flow velocity at the reactor inlet, v 0 is the initial air flow velocity, p out is the pressure at the reactor outlet, L is the main channel length, which is the axial length; δ w is the thickness of the catalytic coating; is the coverage of the initially adsorbed NH3 on the reaction surface of the catalytic coating.

6. The evaluation method according to claim 1, wherein: The nitrogen oxides in diesel exhaust include nitrogen monoxide and nitrogen dioxide. The nitrogen oxide conversion rate is ; ； in, and The NO at the reactor inlet and outlet are X concentration.

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