SCR (Selective Catalytic Reduction) denitration process based on urea pyrolysis ammonia preparation

Abstract

The invention relates to the technical field of SCR (Selective Catalytic Reduction) denitration, in particular to an SCR denitration process for preparing ammonia based on urea pyrolysis. Comprising the following steps that urea with the concentration of 40%-50% serves as a raw material, after being heated through waste heat of flue gas at an SCR outlet, the urea enters a first-stage pyrolysis zone, the temperature is controlled to be 440-460 DEG C, the urea stays for 8-10 seconds, the urea enters a second-stage deep decomposition zone, the temperature is kept to be 645-660 DEG C, and the urea stays for 5-8 seconds; tail gas generated in the step (1) enters an SCR reactor, the temperature ranges from 180 DEG C to 410 DEG C, an MVGC-NTA catalyst is arranged in the SCR reactor, the air speed is controlled to range from 3000 h <-1 > to 3500 h <-1 >, and SCR denitration is conducted. The urea decomposition rate is increased through the two-stage pyrolysis process, the four-layer gradient coating catalyst is matched, wide-temperature efficient denitration is achieved, ammonia escape is low, toxicity resistance is high, the preparation process is energy-saving, and the service life of the catalyst is long.

Description

Technical Field

[0001] The present invention relates to the technical field of SCR denitration, and in particular to an SCR denitration process based on urea pyrolysis to produce ammonia. Background Art

[0002] Selective catalytic reduction (SCR) denitrification technology is a core method for industrial flue gas treatment. It uses a catalyst to reduce nitrogen oxides (NOx) to harmless nitrogen and water. The SCR process, which produces ammonia through urea pyrolysis, is widely used in coal-fired power plants and waste incineration due to its high safety and convenient transportation and storage. This process uses urea as the ammonia source. The ammonia generated through pyrolysis then reacts with the flue gas on the catalyst surface. Its key performance relies on the catalyst performance and the efficiency of the urea pyrolysis.

[0003] The current mainstream SCR catalyst is mainly based on the VW-Ti system, and the carrier is mostly cordierite or cordierite-silicon carbide composite materials. However, this type of catalyst has significant technical bottlenecks: first, the temperature window is narrow. At low temperatures, the V2O5 active sites are deactivated, and at high temperatures, the TiO2 lattice distortion leads to sintering, limiting its application in flue gas temperature fluctuations. Second, it has poor resistance to poisoning. SO2 in flue gas reacts with alkali metals (such as Na + , K + ) It is easy to react with active components to form sulfates or solid solutions, resulting in catalyst passivation and a lifespan of only 2-3 years; thirdly, the urea pyrolysis efficiency is insufficient. The traditional single-stage pyrolysis has a low ammonia yield at 400-500°C, and the by-product HNCO can easily cause ammonia escape to exceed the standard. At the same time, the generation of N2O exacerbates the greenhouse effect.

[0004] Furthermore, the carrier pretreatment process is simple. Traditional acid etching only increases surface roughness without forming nano-scale anchoring structures, resulting in insufficient coating bonding strength. Catalyst coatings are mostly single active layers, failing to balance wide-temperature activity with toxicity resistance. The active component loading capacity is limited, further hampering denitrification efficiency. Therefore, developing an SCR process that combines wide-temperature activity, long life, and an efficient pyrolysis pathway has become an urgent challenge for the industry. Summary of the Invention

[0005] In view of the above deficiencies in the prior art, the purpose of the present invention is to provide an SCR denitrification process based on urea pyrolysis to produce ammonia, which improves the urea decomposition rate through a secondary pyrolysis process and is matched with a four-layer gradient coating catalyst to achieve wide temperature and high-efficiency denitrification, low ammonia escape, strong resistance to poisoning, energy-saving preparation process and long catalyst life.

[0006] The present invention is achieved by adopting the following technical solutions: The SCR denitrification process based on urea pyrolysis to produce ammonia comprises the following steps: (1) Urea with a concentration of 40-50% is used as raw material. After being heated by the waste heat of the flue gas at the SCR outlet, it enters the primary pyrolysis zone with a controlled temperature of 440-460°C. After staying for 8-10 seconds, it enters the secondary deep decomposition zone with a temperature maintained at 645-660°C and a stay of 5-8 seconds. (2) The tail gas generated in step (1) enters the SCR reactor at a temperature of 180-410°C. The SCR reactor is equipped with an MVGC-NTA catalyst and the air velocity is controlled at 3000-3500 h -1 , perform SCR denitrification; The MVGC-NTA catalyst is based on a cordierite-silicon carbide composite honeycomb body, and is coated with an anti-poisoning armor layer, a low-temperature active layer, a medium-temperature main effect layer, and a high-temperature stable layer in sequence on the outside; the anti-poisoning armor layer is a Nb2O5-Fe2O3 / TiO2 layer; the high-temperature stable layer is a Ru / WO3-TiO2 layer; the medium-temperature main effect layer is a Ni( （0.4-0.6） Mn （0.4-0.6） ) 3Al1-LDOs@V2O5; the low-temperature active layer is Mn （0.7-0.9） Ce （0.1-0.3） O x / TiO2 layer.

[0007] The preparation method of the MVGC-NTA catalyst comprises the following steps: a. Niobium oxalate, ferric nitrate and tetrabutyl titanate were mixed and formed into a gel at 80-82 ° C. The pretreated cordierite - silicon carbide composite honeycomb support was immersed in the sol, dried at 120-125 ° C, and finally calcined at 580-600 ° C for 5-6 hours to obtain a pretreated body A; b. Take Mn （0.7-0.9） Ce （0.1-0.3） O x / TiO2 powder, by ultrasonic spray pyrolysis of the precursor solution, the atomized solution is passed into the reaction furnace, the product is collected, and the product is applied to the pre-treated body A using fluidized bed vapor deposition method, and finally subjected to high temperature treatment to obtain the pre-treated body B; c. Synthesize LDHs precursor by co-precipitation method. 2+ / Mn 2+ / Al 3+ The mixed solution is adjusted to pH 10, aged, and then calcined to obtain LDOs. The LDOs carrier is then immersed in an ammonium metavanadate solution, ultrasonically vibrated, and then coated on the pre-treated body B and dried to obtain a pre-treated body C. d. The WO3 was loaded by impregnation with ammonium paratungstate and calcined; by impregnating an equal volume of RuCl3 solution, reducing the solution, and then spraying the solution on the pretreated body C. After spraying, the solution was infrared dried and then reduced to obtain a pretreated body D. e. The pretreated body D was calcined at 250°C for 2 hours, then heated to 550°C for 6 hours, and finally quenched at 800°C to obtain the MVGC-NTA catalyst.

[0008] The molar ratio of Nb:Fe:Ti is (0.8-1.2):(1.8-2.2):100; the loading amount of Ru is 0.03-0.07wt%; the molar ratio of Ni:Mn:Al is (0.4-0.6):(0.4-0.6):1; and the molar ratio of Mn:Ce is (0.7-0.9):(0.1-0.3).

[0009] The pretreatment method of the cordierite-silicon carbide composite honeycomb carrier is as follows: using nitric acid aqueous solution as the treatment solution, and simultaneously performing ultrasonic-assisted treatment on the gaps of the cordierite-silicon carbide composite honeycomb carrier to form micro-pits with a depth of 5-10 μm on the surface of the substrate; placing it in a mixed electrolyte of H2SO4 and Na2SiO3 for plasma electrolytic oxidation to generate a vertical array of anatase-type TiO2 nanosheets; then placing it in a high-pressure reactor containing a solution of titanyl sulfate and ammonium fluoride, and treating it at a constant temperature of 180-185°C for 6-8 hours to obtain the product.

[0010] In the step a, after the cordierite-silicon carbide composite honeycomb carrier is immersed in the sol, it is maintained at a vacuum of -0.08 MPa for 5-8 minutes, and then the excess sol is blown away with compressed air at 0.3-0.5 MPa to control the coating thickness to 80±5 μm; the heating rate is 2°C / min.

[0011] In the step d, the calcination conditions are 450-460°C for 3-4 hours; the reduction conditions are 280-300°C for 2-3 hours in a H2 / N2 (5:95) atmosphere, and the obtained suspension particle size D50 = 1.2 μm.

[0012] In the step c, the preparation method of the LDHs precursor is: dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, adding them and a 1.6 mol / L NaOH solution into a reactor simultaneously, and stirring to obtain the LDHs precursor; the calcination conditions are 500-520°C for 3-5 hours.

[0013] In the step b, before ultrasonic spray pyrolysis, Mn(NO3)2·4H2O and Ce(NO3)3·6H2O need to be dissolved in deionized water, TiO2 and PEG-400 are added, and a uniform suspension is obtained after ball milling.

[0014] Collaborative exhaust gas purification: catalytic hydrolysis of HNCO / HCN in the 200-220°C area in front of the SCR.

[0015] Rapid cooling treatment: After keeping at 800℃, cool to room temperature with nitrogen blast (rate 50℃ / min).

[0016] PEG-400 pyrolysis: After the low-temperature layer is coated, PEG-400 is removed by pre-calcination at 300℃ / 1h, and then activated at 550℃.

[0017] During the preparation of the MVGC-NTA catalyst, the support must first be pretreated. A nitric acid solution is used as the treatment solution, while ultrasonically assisted treatment is performed on the interstitial spaces of the cordierite-silicon carbide composite honeycomb support, forming micro-pits 5-10 μm deep on the substrate surface. The substrate is then subjected to plasma electrolytic oxidation in a mixed electrolyte of H₂SO₄ and Na₂SiO₃ at 120V for 20 minutes, generating vertical arrays of anatase-type TiO₂ nanosheets. The substrate is then placed in an autoclave containing a solution of titanyl sulfate and ammonium fluoride and treated at 180-185°C for 6-8 hours to obtain the final product. The cordierite-silicon carbide composite honeycomb support consists of a cordierite (2MgO·2Al₂O₃·5SiO₂) matrix with a 30wt% silicon carbide (SiC) reinforcement phase, a pore density of 600 cpsi, and a wall thickness of 0.2 mm.

[0018] After the carrier is pretreated, it enters the gradient coating process stage, in which a four-layer structure is constructed from the inside out.

[0019] The first, innermost layer is the anti-poisoning armor layer, made from a Nb2O5-Fe2O3 / TiO2 composite sol. The composite sol is prepared via a sol-gel process: niobium oxalate, ferric nitrate, and tetrabutyl titanate are mixed and gelled at 80-82°C, then dried at 120-125°C and calcined at 580-600°C for 5-6 hours. During coating, the substrate is immersed in the sol and held at a vacuum of -0.08 MPa for 5-8 minutes. Excess sol is then blown away with compressed air at 0.3-0.5 MPa, maintaining a coating thickness of 80 ± 5 μm. Finally, the temperature is raised to 120-125°C at a rate of 2°C / min.

[0020] The second layer is the low temperature active layer, the material is Mn （0.7-0.9） Ce （0.1-0.3） O xTo prepare the TiO2 / TiO2 powder, Mn(NO3)2•4H2O and Ce(NO3)3•6H2O were dissolved in deionized water, TiO2 and PEG-400 were added, and a uniform suspension was obtained after ball milling. The precursor solution was then pyrolyzed by ultrasonic spraying. The atomized solution was passed into a 500°C reactor, and the product was collected and calcined at 550°C for 4 hours. The coating was prepared using fluidized bed vapor deposition. The carrier was preheated to 200°C, the powder fluidization velocity was controlled at 0.8m / s, and deposition was performed for 15 minutes to achieve a coating thickness of 150±5μm. Finally, the coating was treated at 550°C for 1 hour under a nitrogen atmosphere.

[0021] The third layer is the medium temperature main effect layer, the material is Ni （0.4-0.6） Mn （0.4-0.6） Al1-LDOs@V2O5 slurry. First, the LDHs precursor was synthesized by co-precipitation method, and Ni 2+ / Mn 2+ / Al 3+ The mixture was adjusted to pH 10, aged at 60°C for 12 hours, and then calcined at 500-520°C for 3-5 hours to produce LDOs. The LDOs support was then immersed in a 0.1 mol / L ammonium metavanadate solution. V2O5 was intercalated into the interlayer gaps using 40kHz ultrasonic oscillation. Spin coating was performed at 800 rpm, with a coating thickness of 120±10 μm. After coating, a step-drying process was performed, first at 80°C for 1 hour and then at 120°C for 2 hours. The LDH precursor was prepared by dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, and adding the solution and 1.6 mol / L NaOH solution to a reactor. After stirring, the LDH precursor was obtained. Ammonium metavanadate is hydrolyzed to generate a V2O5 precursor, which is embedded into the gaps between LDOs layers with the assistance of ultrasound and converted into V2O5 by high-temperature calcination.

[0022] The outermost layer is a high-temperature stable layer made of a Ru / WO3-TiO2 suspension. The WO3 is first loaded by impregnation with ammonium paratungstate and calcined at 450-460°C for 3-4 hours. Ru is impregnated with an equal volume of RuCl3 solution and then reduced at 280-300°C for 2-3 hours in a H2 / N2 (5:95) atmosphere. The resulting suspension has a particle size of D50 = 1.2μm, which is then mixed with the TiO2. Coating is performed using an equal volume spraying method at a pressure of 0.15MPa and a nozzle diameter of 0.3mm. After spraying, the Ru nanoclusters are dried at 150°C for 30 minutes using infrared technology to ensure a particle size of ≤2nm.

[0023] After gradient coating is complete, the overall sintering stage begins. The coating is performed in the order of cordierite carrier, anti-poisoning armor layer, high-temperature stabilization layer, medium-temperature main effect layer, and low-temperature active layer, with each layer coating 150g / L of carrier. Sintering is carried out in three stages: the first is a 250°C hold for 2 hours to remove the binder; the second is a 6-hour calcination at 550°C to promote crystalline phase formation; the third is a rapid cooling at 800°C to enhance the mechanical strength of the catalyst, thus completing the preparation of the MVGC-NTA catalyst.

[0024] This process utilizes a two-stage pyrolysis process: the primary pyrolysis zone (440-460°C, 8-10 seconds) utilizes waste heat from the SCR exhaust to preheat a 40-50% urea solution, promoting the decomposition of urea into NH3 and HNCO through molecular vibration. The secondary deep decomposition zone (645-660°C, 5-8 seconds) activates the HNCO hydrolysis reaction (HNCO + H2O → NH3 + CO2) at high temperatures, significantly increasing the overall ammonia yield. This process utilizes a temperature gradient to regulate the pyrolysis path, avoiding ammonia slip caused by HNCO accumulation in traditional single-stage pyrolysis. It also reduces energy consumption by recovering waste heat from the flue gas.

[0025] Achieve technological breakthroughs through triple innovation of “materials-structure-process”. 3+ / Mn 4+ The redox cycle promotes the conversion of NO to NO2, Ce 4+ The oxygen storage capacity improves the low-temperature oxygen mobility and jointly ensures low-temperature activity; the medium-temperature layer LDOs layered structure provides Brønsted acid sites to adsorb NH3, and the core-shell structure inhibits NH3 over-penetration, optimizing the medium-temperature catalytic effect; the high-temperature layer Ru nanoclusters activate lattice oxygen, inhibit V2O5 phase change sintering, and maintain high-temperature stability; the armor layer Nb 5+ Occupy Ti 4+ The LDOs form Nb-O-Ti bonds at the sites, blocking SO2 adsorption sites and enhancing resistance to poisoning. During the preparation process, ultrasonic spray pyrolysis instantly dries the droplets to form a porous Mn-Ce solid solution with a specific surface area >180 m² / g. The LDOs core-shell structure prevents V2O5 aggregation through the layer confinement effect, controlling the acid site density. Plasma electrolytic oxidation creates nanosheet arrays on the TiO2 surface, increasing the coating's adhesion by 300%. A gradient coating sequence prevents high-temperature sintering from destroying the low-temperature active phase, ensuring synergistic function among the layers. Ru nanoclusters in the high-temperature layer activate lattice oxygen at 450°C, absorbing heat and cooling the flue gas by 50±5°C.

[0026] Compared with the prior art, the present invention has the following advantages: (1) The MVGC-NTA catalyst prepared in this invention achieves ultra-wide temperature window catalysis, maintaining high efficiency throughout the entire process from 170-410°C, with a NOx conversion rate of >92%, completely resolving the problem of poor temperature adaptability of traditional catalysts. At the same time, ammonia slip is <0.8ppm and N2O is <5ppm, an 80% decrease compared to traditional catalysts, significantly reducing the risk of secondary pollution. It has significant resistance to sulfur and alkali metal poisoning, an annual attenuation rate of <2%, and a service life of up to 10 years, more than three times the life of traditional catalysts, significantly reducing replacement costs and maintenance workload.

[0027] (2) This scheme constructs a synergistic system of "anti-poisoning - high temperature stability - medium temperature main effect - low temperature activity" through a four-layer gradient coating design and carrier pretreatment process. In the urea pyrolysis ammonia production process, the two-stage pyrolysis process improves the urea decomposition rate, reduces the generation of HNCO byproducts from the source, and provides a high-purity ammonia source for SCR denitrification; the urea solution is preheated using the waste heat of the SCR outlet flue gas, the first pyrolysis zone completes the initial decomposition, and the second deep decomposition zone further improves the yield, which is more energy-efficient than traditional electric heating methods. DETAILED DESCRIPTION

[0028] In order to make the purpose and technical solution of the present invention more clear, the present invention is further described in detail below.

[0029] Test method: Activity test: A fixed-bed reactor was used to simulate actual SCR denitrification conditions. The pretreated cordierite-silicon carbide carrier-loaded MVGC-NTA catalyst was placed in the reactor and introduced with simulated flue gas consisting of 1000ppm NO, 1000ppm NH3, and 5% O2, with the balance gas being N2. The air velocity was set to 3500h⁻¹, simulating the flue gas environment of waste incineration and power plants. The temperature was programmed to increase at a rate of 10°C / min within the 150-500°C temperature range. A Fourier transform infrared spectrometer (FT-IR) was used to monitor the concentrations of gases such as NO, NO2, NH3, and N2O at the reactor inlet and outlet online, according to the formula: , calculate the NOx conversion rate at different temperatures and evaluate the activity performance of the catalyst in a wide temperature range.

[0030] Poisoning resistance test: This simulates the sulfur and alkali metal poisoning environments found in actual industrial flue gas. For the sulfur poisoning test, 1000 ppm of SO₂ is introduced into the simulated flue gas. After 500 hours of operation, the catalyst activity is monitored. X-ray photoelectron spectroscopy (XPS) is used to analyze the valence and chemical composition of the catalyst surface elements to determine the adsorption form of SO₂ on the catalyst surface and the reaction products. For the alkali metal poisoning test, 1000 ppm of Na⁺ or K⁺ (introduced in the form of sodium or potassium salt aerosols) is added to the simulated flue gas. After a certain period of operation, the activity change is similarly monitored. Inductively coupled plasma optical emission spectrometry (ICP-OES) is used to analyze the alkali metal content on the catalyst surface to assess poisoning resistance.

[0031] Ammonia escape: GB / T16157-1996; Urea pyrolysis efficiency test method: Fourier transform infrared spectrometer (FT-IR) was used to monitor the gas components at the outlet of the first pyrolysis zone (440-460℃) and the outlet of the second deep decomposition zone (645-660℃) in real time, and the concentrations of NH3, HNCO, and CO2 were quantitatively analyzed.

[0032] Calculation formula: Urea decomposition rate = (NH3 molar amount generated / theoretical NH3 molar amount) × 100% HNCO conversion rate = ((inlet HNCO - outlet HNCO) / inlet HNCO) × 100% Catalyst mechanical strength and life test method: Peel strength: According to ASTM D4541 standard, use a hydraulic puller to test the bonding strength between the coating and the carrier.

[0033] Crushing strength: According to GB / T1964-1996, the crushing strength of the catalyst monomer under axial pressure is tested (unit: N / cm).

[0034] Accelerated aging: Continuously operate for 2000 hours in a simulated flue gas containing 1000ppm SO2 + 100ppm Na⁺, and test the NOx conversion rate decay rate every 500 hours.

[0035] Process energy consumption comparison test method: Comparison of energy consumption between the two-stage pyrolysis process and the traditional electric heating single-stage pyrolysis (500°C): Record the actual energy consumption of the two-stage pyrolysis zone (kW·h / t urea) Calculate the SCR outlet flue gas waste heat recovery efficiency (%) The following are some of the raw material amounts and manufacturers in each embodiment: Nitric acid (10%): 5 L, Sinopharm Chemical Reagent Co., Ltd.; Na2SiO3: 85g, Aladdin Holding Group Co., Ltd., ≥99% electrolyte doping; Titanyl sulfate: 210 g, TCI (Shanghai) Chemical Industry Development Co., Ltd. Ammonium fluoride: 37 g, Shanghai MacLean Biochemical Technology Co., Ltd., ≥98% crystal face orientation growth; TiO2 (anatase): Panzhihua Iron and Steel Group Titanium Co., Ltd., ≥99%; PEG-400: Sinopharm Chemical Reagent Co., Ltd.

[0036] Example 1 Amount of each raw material added: Armor layer: Niobium oxalate: 2.52g; Ferric nitrate (Fe(NO3)3·9H2O): 15.8g; Tetrabutyl titanate: 612g; High temperature layer: ammonium paratungstate: 16.4g; RuCl3: 0.065g; TiO2 (anatase): 135g; Mesothermal layer: Ni(NO3)2·6H2O: 158.6g; Mn(NO3)2·4H2O: 227.2g; Al(NO3)3·9H2O: 188.6g; Ammonium metavanadate: 19.3g; Low temperature layer: Mn(NO3)2·4H2O: 9.55g; Ce(NO3)3·6H2O: 4.09g; TiO2 (anatase): 136.4g; PEG-400: 3.0g.

[0037] The preparation process of the MVGC-NTA catalyst begins with pretreatment of the support. A nitric acid aqueous solution is used as the treatment solution, while ultrasonically assisted treatment is applied to the interstitial spaces of the cordierite-silicon carbide composite honeycomb support, forming micro-pits 5 μm deep on the substrate surface. The substrate is then subjected to plasma electrolytic oxidation in a mixed electrolyte of H₂SO₄ and Na₂SiO₃ at 120 V for 20 minutes, generating vertical arrays of anatase-type TiO₂ nanosheets. The substrate is then placed in an autoclave containing a solution of titanyl sulfate and ammonium fluoride and treated at 180°C for 6 hours to obtain the final product. The cordierite-silicon carbide composite honeycomb support consists of a cordierite (2MgO·2Al₂O₃·5SiO₂) matrix with a 30 wt% silicon carbide (SiC) reinforcement phase, a pore density of 600 cpsi, and a wall thickness of 0.2 mm.

[0038] After the carrier is pretreated, it enters the gradient coating process stage, in which a four-layer structure is constructed from the inside out.

[0039] The first, innermost layer is the anti-poisoning armor layer, made from a Nb2O5-Fe2O3 / TiO2 composite sol. The composite sol is prepared via a sol-gel process by mixing niobium oxalate, ferric nitrate, and tetrabutyl titanate (molar ratio of Nb:Fe:Ti = 0.8:1.8:100). The mixture is gelled at 80°C, dried at 120°C, and finally calcined at 580°C for 5 hours. During coating, the substrate is immersed in the sol and held at a vacuum of -0.08 MPa for 5 minutes. Excess sol is then blown off with 0.3 MPa compressed air to a controlled coating thickness of 75 μm. Finally, the temperature is increased to 120°C at a rate of 2°C / min.

[0040] The second layer is the low temperature active layer, the material is Mn 0.7 Ce 0.3 O x / TiO2 powder (Mn:Ce molar ratio of 0.7:0.3) was prepared by dissolving Mn(NO3)2•4H2O and Ce(NO3)3•6H2O in deionized water, adding TiO2 and PEG-400, and ball milling to obtain a uniform suspension. The precursor solution was then pyrolyzed by ultrasonic spraying, and the atomized solution was passed into a 500°C reactor. The product was collected and calcined at 550°C for 4 hours. Fluidized bed vapor deposition was used for coating: the carrier was preheated to 200°C, the powder fluidization velocity was controlled at 0.8m / s, and deposition was performed for 15 minutes to achieve a coating thickness of 145μm. Finally, the coating was treated at 550°C for 1 hour under a nitrogen atmosphere.

[0041] The third layer is the medium temperature main effect layer, the material is (Ni 0.4 Mn 0.6 )3Al1-LDOs@V2O5 slurry (Ni:Mn:Al molar ratio is 0.4:0.6:1). First, LDHs precursor was synthesized by co-precipitation method, and Ni 2+ / Mn 2+ / Al 3+The mixture was adjusted to pH 10, aged at 60°C for 12 hours, and subsequently calcined at 500°C for 3 hours to produce LDOs. The LDOs support was then immersed in a 0.1 mol / L ammonium metavanadate solution, and 40 kHz ultrasonic oscillation assisted the intercalation of V2O5 into the interlayer gaps. Spin coating was performed at 800 rpm, with a coating thickness of 110 μm. After coating, a step-drying process was performed, first at 80°C for 1 hour and then at 120°C for 2 hours. The LDH precursor was prepared by dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, and then adding the solution and a 1.6 mol / L NaOH solution to a reactor. After stirring, the LDH precursor was obtained. Ammonium metavanadate was hydrolyzed to generate a V2O5 precursor, which was embedded into the gaps between LDOs layers with the assistance of ultrasound and converted into V2O5 by calcination at 500℃.

[0042] The outermost layer is a high-temperature stable layer, made from a Ru / WO3-TiO2 suspension. WO3 is first impregnated with ammonium paratungstate and calcined at 450°C for 3 hours. Ru is impregnated with an equal volume of RuCl3 solution and then reduced at 280°C for 2 hours in a H2 / N2 (5:95) atmosphere. The resulting suspension has a particle size of D50 = 1.2μm, which is then mixed with TiO2. Coating is performed using an equal volume spraying method at a pressure of 0.15MPa and a nozzle diameter of 0.3mm. After spraying, the Ru nanoclusters are infrared dried at 150°C for 30 minutes to ensure a particle size of 2nm. The Ru loading is 0.03wt% / WO3-TiO2.

[0043] After gradient coating is complete, the overall sintering stage begins. Application is performed in the order of cordierite support, anti-poisoning armor layer, high-temperature stabilization layer, medium-temperature main effect layer, and low-temperature active layer, with each layer coating 150g / L of support. Sintering is performed in three stages: the first is held at 250°C for two hours; the second is heated to 550°C and calcined for six hours; and the third is quenched at 800°C to produce the MVGC-NTA catalyst. The coating weight is 130g / L / layer.

[0044] The SCR denitrification process based on urea pyrolysis to produce ammonia includes the following steps: (1) Urea with a concentration of 40% is used as raw material. After being heated by the waste heat of the flue gas at the SCR outlet, it enters the primary pyrolysis zone with a controlled temperature of 450°C. After staying for 8 seconds, it enters the secondary deep decomposition zone with a temperature maintained at 650°C and staying for 5 seconds. (2) The tail gas generated in step (1) enters the SCR reactor at a temperature of 210°C. The SCR reactor is equipped with an MVGC-NTA catalyst and the air velocity is controlled at 3000 h -1 , carry out SCR denitrification.

[0045] Example 2 Amount of each raw material added: Armor layer: Niobium oxalate: 3.04g; Ferric nitrate (Fe(NO3)3·9H2O): 19.2g; Tetrabutyl titanate: 615g; High temperature layer: ammonium paratungstate: 16.4g; RuCl3: 0.108g; TiO2 (anatase): 135g; Mesothermal layer: Ni(NO3)2·6H2O: 218.2g; Mn(NO3)2·4H2O: 208.3g; Al(NO3)3·9H2O: 103.8g; Ammonium metavanadate: 19.3g; Low temperature layer: Mn(NO3)2·4H2O: 13.33 g; Ce(NO3)3·6H2O: 3.33 g; TiO2 (anatase): 133.3 g; PEG-400: 3.0 g.

[0046] The preparation process of the MVGC-NTA catalyst begins with pretreatment of the support. A nitric acid solution is used as the treatment solution, while ultrasonically assisted treatment is applied to the interstitial spaces of the cordierite-silicon carbide composite honeycomb support, forming micro-pits 8 μm deep on the substrate surface. The substrate is then subjected to plasma electrolytic oxidation in a mixed electrolyte of H₂SO₄ and Na₂SiO₃ at 120 V for 20 minutes, generating vertical arrays of anatase-type TiO₂ nanosheets. The substrate is then placed in an autoclave containing a solution of titanyl sulfate and ammonium fluoride and treated at 182°C for 7 hours to obtain the final product. The cordierite-silicon carbide composite honeycomb support consists of a cordierite (2MgO·2Al₂O₃·5SiO₂) matrix with a 30 wt% silicon carbide (SiC) reinforcement phase, a pore density of 600 cpsi, and a wall thickness of 0.2 mm.

[0047] After the carrier is pretreated, it enters the gradient coating process stage, in which a four-layer structure is constructed from the inside out.

[0048] The first, innermost layer is the anti-poisoning armor layer, made from a Nb2O5-Fe2O3 / TiO2 composite sol. The composite sol is prepared via a sol-gel process by mixing niobium oxalate, ferric nitrate, and tetrabutyl titanate (molar ratio of Nb:Fe:Ti = 1:2:100). The mixture is gelled at 82°C, dried at 125°C, and finally calcined at 600°C for 6 hours. During coating, the substrate is immersed in the sol and held at a vacuum of -0.08 MPa for 8 minutes. Excess sol is then blown off with 0.5 MPa compressed air to a controlled coating thickness of 80 μm. Finally, the temperature is increased to 125°C at a rate of 2°C / min.

[0049] The second layer is the low temperature active layer, the material is Mn 0.8Ce 0.2 O x / TiO2 powder (Mn:Ce molar ratio of 0.8:0.2) was prepared by dissolving Mn(NO3)2•4H2O and Ce(NO3)3•6H2O in deionized water, adding TiO2 and PEG-400, and ball milling to obtain a uniform suspension. The precursor solution was then pyrolyzed by ultrasonic spraying, and the atomized solution was passed into a 500°C reactor. The product was collected and calcined at 550°C for 4 hours. Fluidized bed vapor deposition was used for coating: the carrier was preheated to 200°C, the powder fluidization velocity was controlled at 0.8 m / s, and deposition was performed for 15 minutes to achieve a coating thickness of 150 μm. Finally, the coating was treated at 550°C for 1 hour under a nitrogen atmosphere.

[0050] The third layer is the medium temperature main effect layer, the material is (Ni 0.5 Mn 0.5 )3Al1-LDOs@V2O5 slurry (Ni:Mn:Al molar ratio is 0.5:0.5:1). First, LDHs precursor was synthesized by co-precipitation method, and Ni 2+ / Mn 2+ / Al 3+ The mixture was adjusted to pH 10, aged at 60°C for 12 hours, and subsequently calcined at 520°C for 5 hours to produce LDOs. The LDOs support was then immersed in a 0.1 mol / L ammonium metavanadate solution, and 40 kHz ultrasonic oscillation assisted the intercalation of V2O5 into the interlayer gaps. Spin coating was performed at 800 rpm, with a coating thickness controlled at 120 ± 10 μm. After coating, a step-drying process was performed, first at 80°C for 1 hour and then at 120°C for 2 hours. The LDH precursor was prepared by dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, and adding the solution and 1.6 mol / L NaOH solution to a reactor. After stirring, the LDH precursor was obtained.

[0051] The outermost layer is a high-temperature stable layer, made from a Ru / WO3-TiO2 suspension. WO3 is first impregnated with ammonium paratungstate and calcined at 450°C for 3 hours. Ru is impregnated with an equal volume of RuCl3 solution and then reduced at 290°C for 3 hours in a H2 / N2 (5:95) atmosphere. The resulting suspension has a particle size of D50 = 1.2μm, which is then mixed with TiO2. Coating is performed using an equal volume spraying method at a pressure of 0.15MPa and a nozzle diameter of 0.3mm. After spraying, the Ru nanoclusters are infrared dried at 150°C for 30 minutes to ensure a particle size of 2nm. The Ru loading is 0.05wt% / WO3-TiO2.

[0052] After gradient coating is complete, the overall sintering stage begins. Application is performed in the order of cordierite support, anti-poisoning armor layer, high-temperature stabilization layer, medium-temperature main effect layer, and low-temperature active layer, with each layer coating 150g / L of support. Sintering is performed in three stages: the first is held at 250°C for two hours; the second is calcined at 550°C for six hours; and the third is quenched at 800°C to produce the MVGC-NTA catalyst. The coating rate is 150g / L / layer.

[0053] The SCR denitrification process based on urea pyrolysis to produce ammonia includes the following steps: (1) Urea with a concentration of 45% is used as raw material. After being heated by the waste heat of the flue gas at the SCR outlet, it enters the primary pyrolysis zone with a controlled temperature of 450°C. After staying for 10 seconds, it enters the secondary deep decomposition zone with a temperature maintained at 650°C and staying for 8 seconds. (2) The tail gas generated in step (1) enters the SCR reactor at a temperature of 330°C. The SCR reactor is equipped with an MVGC-NTA catalyst and the air velocity is controlled at 3300 h -1 , carry out SCR denitrification.

[0054] Example 3 Amount of each raw material added: Armor layer: Niobium oxalate: 3.56g; Ferric nitrate (Fe(NO3)3·9H2O): 22.6g; Tetrabutyl titanate: 618g; High temperature layer: ammonium paratungstate: 16.4g; RuCl3: 0.151g; TiO2 (anatase): 135g; Mesothermal layer: Ni(NO3)2·6H2O: 277.8g; Mn(NO3)2·4H2O: 189.5g; Al(NO3)3·9H2O: 188.6g; Ammonium metavanadate: 19.3g; Low temperature layer: Mn(NO3)2·4H2O: 17.14 g; Ce(NO3)3·6H2O: 1.9 g; TiO2 (anatase): 130.0 g; PEG-400: 3.0 g.

[0055] The preparation process of the MVGC-NTA catalyst begins with pretreatment of the support. A nitric acid aqueous solution is used as the treatment solution, while ultrasonically assisted treatment is performed on the interstitial spaces of the cordierite-silicon carbide composite honeycomb support, forming micro-pits 10 μm deep on the substrate surface. The substrate is then subjected to plasma electrolytic oxidation in a mixed electrolyte of H₂SO₄ and Na₂SiO₃ at 120 V for 20 minutes, generating vertical arrays of anatase-type TiO₂ nanosheets. The substrate is then placed in an autoclave containing a solution of titanyl sulfate and ammonium fluoride and treated at 185°C for 8 hours to obtain the final product. The cordierite-silicon carbide composite honeycomb support consists of a cordierite (2MgO·2Al₂O₃·5SiO₂) matrix with a 30 wt% silicon carbide (SiC) reinforcement phase, a pore density of 600 cpsi, and a wall thickness of 0.2 mm.

[0056] After the carrier is pretreated, it enters the gradient coating process stage, in which a four-layer structure is constructed from the inside out.

[0057] The first, innermost layer is the anti-poisoning armor layer, made from a Nb2O5-Fe2O3 / TiO2 composite sol. The composite sol is prepared via a sol-gel process by mixing niobium oxalate, ferric nitrate, and tetrabutyl titanate (molar ratio of Nb:Fe:Ti = 1.2:2.2:100). The mixture is gelled at 82°C, dried at 125°C, and finally calcined at 600°C for 6 hours. During coating, the substrate is immersed in the sol and held at a vacuum of -0.08 MPa for 8 minutes. Excess sol is then blown off with 0.5 MPa compressed air to a controlled coating thickness of 85 μm. Finally, the temperature is increased to 125°C at a rate of 2°C / min.

[0058] The second layer is the low temperature active layer, the material is Mn 0.9 Ce 0.1 O x The crystalline TiO2 powder (Mn:Ce molar ratio of 0.9:0.1) was prepared by dissolving Mn(NO3)2•4H2O and Ce(NO3)3•6H2O in deionized water, adding TiO2 and PEG-400, and ball milling to obtain a uniform suspension. The precursor solution was then subjected to ultrasonic spray pyrolysis. The atomized solution was passed into a 500°C reactor, and the product was collected and calcined at 550°C for 4 hours. Coating was performed using fluidized bed vapor deposition. The support was preheated to 200°C, the powder fluidization velocity was controlled at 0.8 m / s, and deposition was performed for 15 minutes to achieve a coating thickness of 155 μm. Finally, the coating was treated at 550°C for 1 hour under a nitrogen atmosphere. The high Mn content (Mn:Ce = 0.9:0.1) enhances oxidation capacity, compensating for the oxygen storage loss caused by the low Ce content.

[0059] The third layer is the medium temperature main effect layer, the material is (Ni -0.6 Mn 0.4) The molar ratio of Ni:Mn:Al in 3Al1-LDOs@V2O5 slurry is 0.6:0.4:1. First, the LDHs precursor is synthesized by co-precipitation method. 2+ / Mn 2+ / Al 3+ The mixture was adjusted to pH 10, aged at 60°C for 12 hours, and subsequently calcined at 520°C for 5 hours to produce LDOs. The LDOs support was then immersed in a 0.1 mol / L ammonium metavanadate solution, and 40 kHz ultrasonic oscillation assisted the intercalation of V2O5 into the interlayer gaps. Spin coating was performed at 800 rpm, with a coating thickness of 130 μm. After coating, a step-drying process was performed, first at 80°C for 1 hour and then at 120°C for 2 hours. The LDH precursor was prepared by dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, and adding the solution and 1.6 mol / L NaOH solution to a reactor. After stirring, the LDH precursor was obtained.

[0060] The outermost layer is a high-temperature stable layer, made from a Ru / WO3-TiO2 suspension. WO3 is first impregnated with ammonium paratungstate and calcined at 460°C for 4 hours. Ru is impregnated with an equal volume of RuCl3 solution and then reduced at 300°C for 3 hours in a H2 / N2 (5:95) atmosphere. The resulting suspension has a particle size of D50 = 1.2μm, which is then mixed with TiO2. Coating is performed using an equal volume spraying method at a pressure of 0.15MPa and a nozzle diameter of 0.3mm. After spraying, the Ru nanoclusters are infrared-dried at 150°C for 30 minutes to ensure a particle size of 2nm. The Ru loading is 0.07wt% / WO3-TiO2.

[0061] After gradient coating is complete, the overall sintering stage begins. Application is performed in the order of cordierite support, anti-poisoning armor layer, high-temperature stabilization layer, medium-temperature main effect layer, and low-temperature active layer, with each layer coating 150g / L of support. Sintering is performed in three stages: the first is held at 250°C for two hours; the second is heated to 550°C and calcined for six hours; and the third is quenched at 800°C to produce the MVGC-NTA catalyst. The coating weight is 130g / L / layer.

[0062] The SCR denitrification process based on urea pyrolysis to produce ammonia includes the following steps: (1) Urea with a concentration of 50% is used as the raw material. After being heated by the waste heat of the flue gas at the SCR outlet, it enters the primary pyrolysis zone with a controlled temperature of 460°C. After staying for 10 seconds, it enters the secondary deep decomposition zone with a temperature maintained at 650°C and staying for 8 seconds. (2) The tail gas generated in step (1) enters the SCR reactor at a temperature of 400°C. The SCR reactor is equipped with an MVGC-NTA catalyst and the air velocity is controlled at 3500 h -1 , carry out SCR denitrification.

[0063] Comparative Example 1 Compared with Example 2, the armor layer was not coated, and the coating amount of the medium-temperature layer was increased by 20%.

[0064] Comparative Example 2 Compared with Example 1, Ce is not used in the low-temperature active layer, and the amount of Mn used is the sum of Ce and Mn.

[0065] Comparative Example 3 Compared with Example 3, no Ru loading was performed and the amount of WO3 increased by 50%.

[0066] Comparative Example 4 Compared with Example 2, the carrier was pretreated and the calcination time was extended by 50%.

[0067] The basic test data of Examples 1-3 and Comparative Examples 1-4 are shown in Table 1.

[0068] The test data of Examples 1-3 and Comparative Examples 1-4 are shown in Table 2.

[0069] Table 1: Basic test data of Examples 1-3 and Comparative Examples 1-4

[0070] It can be seen from the above experimental data that the low-temperature activity is significantly improved when Mn:Ce>0.8:0.2. For example, the low-temperature activity of Example 3 at 180°C reaches 95.3%, which verifies the promoting effect of Ce element on low-temperature catalysis. Ru loading>0.05wt% can ensure high-temperature activity>94% at 450°C. After the Ru loading is cancelled in Example 3, the high-temperature activity drops to 82.4%, highlighting the key role of Ru in high-temperature stability. The lack of armor layer (such as Example 1) or the unpretreatment of the carrier (such as Example 4) will cause the sulfur / alkali metal resistance to drop by>30%, which fully demonstrates the importance of anti-poisoning armor layer and carrier pretreatment process to improve the durability of the catalyst. Among the various embodiments, Example 2 has the best comprehensive performance, with low-temperature activity of 92.5%, high-temperature activity of 94.8%, SO2 tolerance of 98.2%, ammonia escape of 0.7ppm, and resistance to Na + It reached 96%, demonstrating the superiority and reliability of this technical solution.

[0071] Table 2: Test data of Examples 1-3 and Comparative Examples 1-4

[0072] Table 2 shows that Example 3 achieved a secondary decomposition rate of 99.7%, with only 2.7 ppm of residual HNCO, demonstrating that high-temperature deep decomposition effectively suppresses byproducts. Comparative Example 2 (no Ce) exhibited the lowest primary decomposition rate (75.3%) due to insufficient Mn oxidation capacity, with a high residual HNCO of 35.6 ppm. Support pretreatment (Examples 1-3) resulted in peel strengths exceeding 8.7 MPa, far exceeding Comparative Example 4 (untreated, 4.3 MPa). The absence of the armor layer (Comparative Example 1) resulted in a 24% decrease in crush strength and a 28.5% activity reduction after accelerated aging.

Claims

1. An SCR denitrification process based on urea pyrolysis to produce ammonia, characterized in that: The following steps are involved: (1) Urea with a concentration of 40-50% is used as raw material. After being heated by the waste heat of the flue gas at the SCR outlet, it enters the primary pyrolysis zone with a controlled temperature of 440-460°C. After staying for 8-10 seconds, it enters the secondary deep decomposition zone with a temperature maintained at 645-660°C and a stay of 5-8 seconds. (2) The tail gas generated in step (1) enters the SCR reactor at a temperature of 180-410°C. The SCR reactor is equipped with an MVGC-NTA catalyst and the air velocity is controlled at 3000-3500 h -1 , perform SCR denitrification; The MVGC-NTA catalyst is based on a cordierite-silicon carbide composite honeycomb body, and is coated with an anti-poisoning armor layer, a low-temperature active layer, a medium-temperature main effect layer, and a high-temperature stable layer in sequence on the outside; the anti-poisoning armor layer is a Nb2O5-Fe2O3 / TiO2 layer; the high-temperature stable layer is a Ru / WO3-TiO2 layer; the medium-temperature main effect layer is (Ni （0.4-0.6） Mn （0.4-0.6） ) 3Al1-LDOs@V2O5; the low-temperature active layer is Mn （0.7-0.9） Ce （0.1-0.3） O x / TiO2 layer.

2. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 1, characterized in that: The preparation method of the MVGC-NTA catalyst comprises the following steps: a. Niobium oxalate, ferric nitrate and tetrabutyl titanate were mixed and formed into a gel at 80-82 ° C. The pretreated cordierite - silicon carbide composite honeycomb support was immersed in the sol, dried at 120-125 ° C, and finally calcined at 580-600 ° C for 5-6 hours to obtain a pretreated body A; b. Take Mn （0.7-0.9） Ce （0.1-0.3） O x / TiO2 powder, by ultrasonic spray pyrolysis of the precursor solution, the atomized solution is passed into the reaction furnace, the product is collected, and the product is applied to the pre-treated body A using fluidized bed vapor deposition method, and finally subjected to high temperature treatment to obtain the pre-treated body B; c. Synthesize LDHs precursor by co-precipitation method. 2+ / Mn 2+ / Al 3+ The mixed solution is adjusted to pH 10, aged, and then calcined to obtain LDOs. The LDOs carrier is then immersed in an ammonium metavanadate solution, ultrasonically vibrated, and then coated on the pre-treated body B and dried to obtain a pre-treated body C. d. The WO3 was loaded by impregnation with ammonium paratungstate and calcined; by impregnating an equal volume of RuCl3 solution, reducing the solution, and then spraying the solution on the pretreated body C. After spraying, the solution was infrared dried and then reduced to obtain a pretreated body D. e. The pretreated body D was calcined at 250°C for 2 hours, then heated to 550°C for 6 hours, and finally quenched at 800°C to obtain the MVGC-NTA catalyst.

3. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, It is characterized in that Nb: The molar ratio of Fe:Ti is (0.8-1.2):(1.8-2.2):100; the loading amount of Ru is 0.03-0.07wt%.

4. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, characterized in that: The pretreatment method of the cordierite-silicon carbide composite honeycomb carrier is as follows: using nitric acid aqueous solution as the treatment solution, and simultaneously performing ultrasonic-assisted treatment on the gaps of the cordierite-silicon carbide composite honeycomb carrier to form micro-pits with a depth of 5-10 μm on the surface of the substrate; placing it in a mixed electrolyte of H2SO4 and Na2SiO3 for plasma electrolytic oxidation to generate a vertical array of anatase-type TiO2 nanosheets; then placing it in a high-pressure reactor containing a solution of titanyl sulfate and ammonium fluoride, and treating it at a constant temperature of 180-185°C for 6-8 hours to obtain the product.

5. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, characterized in that: In the step a, after the cordierite-silicon carbide composite honeycomb carrier is immersed in the sol, it is maintained at a vacuum of -0.08 MPa for 5-8 minutes, and then the excess sol is blown away with compressed air at 0.3-0.5 MPa to control the coating thickness to 80±5 μm; the heating rate is 2°C / min.

6. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, characterized in that: In the step d, the calcination conditions are 450-460°C for 3-4 hours; the reduction conditions are 280-300°C for 2-3 hours in a H2 / N2 (5:95) atmosphere, and the obtained suspension particle size D50 = 1.2 μm.

7. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, characterized in that: In the step c, the preparation method of the LDHs precursor is: dissolving Ni(NO3)2·6H2O, Mn(NO3)2·4H2O, and Al(NO3)3·9H2O in deionized water, stirring until clear, adding them and a 1.6 mol / L NaOH solution into a reactor simultaneously, and stirring to obtain the LDHs precursor; the calcination conditions are 500-520°C for 3-5 hours.

8. The SCR denitrification process based on urea pyrolysis to produce ammonia according to claim 2, characterized in that: In the step b, before ultrasonic spray pyrolysis, Mn(NO3)2·4H2O and Ce(NO3)3·6H2O need to be dissolved in deionized water, TiO2 and PEG-400 are added, and a uniform suspension is obtained after ball milling.

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