Method for preparing ammonia by hydrolysis of urea and its application in coal-fired power plant scr denitration

Abstract

This application discloses a method for producing ammonia through urea hydrolysis and its application in SCR denitrification in coal-fired power plants. The method includes: mixing a urea solution with a composite catalyst to form a slurry; feeding the slurry into a hydrolysis reactor and carrying out a hydrolysis reaction at a temperature of 90°C to 110°C and a pressure of 0.2 MPa to 0.4 MPa to generate ammonia-containing gas; wherein the heat source for the hydrolysis reaction is preferably waste heat from flue gas; the composite catalyst comprises metal oxides and immobilized urease. This application has at least the following beneficial effects: the hydrolysis reaction is carried out under low temperature and low pressure conditions of 90°C to 110°C and 0.2 MPa to 0.4 MPa, reducing energy consumption requirements, and the preferred use of waste heat from flue gas as the main heat source further improves energy utilization efficiency and reduces operating costs.

Description

Technical Field

[0001] This application belongs to the field of urea ammonia production technology, specifically relating to a method for producing ammonia by urea hydrolysis and its application in SCR denitrification in coal-fired power plants. Background Technology

[0002] With increasingly stringent environmental protection requirements, coal-fired power plants commonly employ selective catalytic reduction (SCR) technology for flue gas denitrification. SCR denitrification requires stable and reliable ammonia (NH3) as a reducing agent. Currently, ammonia sources mainly include liquid ammonia, ammonia water, and ammonia produced through urea pyrolysis or hydrolysis. However, liquid ammonia storage and use pose safety risks; ammonia water transportation and storage costs are high; and traditional urea pyrolysis or hydrolysis processes suffer from high energy consumption, high reaction temperatures, easy catalyst deactivation, and slow system response.

[0003] Existing urea hydrolysis technologies mostly employ high temperature and high pressure conditions (typically >130℃, >0.5MPa), resulting in high energy consumption. Furthermore, the catalysts are not sufficiently active at low temperatures, leading to slow system start-up and poor load tracking capabilities. In addition, the catalysts in traditional processes are difficult to recover and reuse, resulting in increased operating costs and the risk of secondary pollution.

[0004] Therefore, there is an urgent need to develop a method for producing ammonia from urea through hydrolysis at lower temperatures and pressures, and to integrate it into the SCR denitrification system of coal-fired power plants. Summary of the Invention

[0005] This application provides a method for producing ammonia by urea hydrolysis and its application in SCR denitrification in coal-fired power plants, aiming to solve the problems of slow start-up, poor load tracking capability, and difficulty in catalyst recovery and utilization in existing technology systems.

[0006] In a first aspect, a method for producing ammonia by hydrolysis of urea, the method comprising:

[0007] The urea solution is mixed with the composite catalyst to form a slurry;

[0008] The slurry is fed into a hydrolysis reactor and hydrolyzed at a temperature of 90°C to 110°C and a pressure of 0.2 MPa to 0.4 MPa to generate ammonia-containing gas.

[0009] The heat source for the hydrolysis reaction is preferably the waste heat from the flue gas.

[0010] The composite catalyst comprises metal oxides and immobilized urease.

[0011] Optionally, in the composite catalyst, the dry basis mass ratio of the metal oxide to the immobilized urease is 7:3 to 8:2.

[0012] Optionally, the metal oxide is Nanowires or Nanoparticles;

[0013] The immobilized urease is a urease immobilized on aldehyde-modified chitosan microspheres using glutaraldehyde as a crosslinking agent.

[0014] Optionally, the amount of the composite catalyst added is 5% to 8% of the total mass of the urea solution.

[0015] Optionally, the waste heat from the flue gas comes from the tail flue after the air preheater of the coal-fired boiler, and indirectly provides heat for the hydrolysis reaction through an intermediate heat medium circulation loop.

[0016] Optionally, the method further includes filtering the residual liquid generated after the hydrolysis reaction to recover the composite catalyst therein and return it to the hydrolysis reactor for recycling.

[0017] Secondly, a urea hydrolysis ammonia production system for implementing the urea hydrolysis ammonia production method includes:

[0018] Urea solution storage tank, metering pump, composite catalyst dosing device, static mixer;

[0019] The low-temperature hydrolysis reactor is made of 316L stainless steel, with an anti-caking nano-coating on the inner wall and a swirl distribution plate.

[0020] The heat source supply unit includes a flue gas waste heat recovery subsystem and a low-parameter steam supply subsystem;

[0021] The intelligent control unit is configured to execute the fuzzy PID algorithm;

[0022] Gas-liquid separator, ammonia buffer tank;

[0023] And an automatic backwash filter for catalyst recovery.

[0024] Optionally, the execution of the fuzzy PID algorithm includes: using the unit load and SCR inlet NOx concentration as feedforward signals, obtaining the required ammonia setpoint through a calculation model; using the error between the actual ammonia production and the setpoint and the error change rate as input variables, dynamically adjusting the proportional coefficient, integral time, and derivative time of the PID controller through a fuzzy inference rule base; and outputting control signals to the frequency converter of the urea solution feed pump and the regulating valve of the heat source pipeline.

[0025] Thirdly, a coal-fired power plant SCR denitrification system integrates the aforementioned urea hydrolysis ammonia production system. The ammonia gas generated by this system is buffered and stabilized before being transported to the ammonia injection grid of the SCR reactor.

[0026] Compared with the prior art, this application has at least the following beneficial effects:

[0027] This application conducts the hydrolysis reaction under low temperature and low pressure conditions of 90℃ to 110℃ and 0.2MPa to 0.4MPa, which reduces energy consumption requirements. Furthermore, it preferably uses waste heat from flue gas as the main heat source, which further improves energy utilization efficiency and reduces operating costs.

[0028] This application employs a composite catalyst composed of metal oxide and immobilized urease. In the low-temperature stage, urease dominates catalysis, significantly reducing the reaction activation energy and achieving rapid start-up. In the high-temperature stage, the metal oxide maintains catalytic activity, ensuring efficient reaction under all operating conditions.

[0029] This application utilizes an automatic backwashing filter to recover the composite catalyst from the residual liquid and return it to the reactor for recycling, thereby reducing catalyst consumption costs and solid waste emissions, which meets the requirements of green processes. Attached Figure Description

[0030] Figure 1 A schematic diagram of the process for producing ammonia from urea hydrolysis provided in this application; Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments.

[0032] The method for producing ammonia from urea by hydrolysis provided in this application includes the following steps:

[0033] A urea solution with a mass fraction of 40% to 50% is mixed with a composite catalyst in a static mixer to form a homogeneous slurry. The amount of composite catalyst added is 5% to 8% of the total mass of the urea solution. The composite catalyst is composed of metal oxide and immobilized urease in a dry basis mass ratio of 7:3 to 8:2, preferably 75:25. In this ratio, the metal oxide constitutes the main framework of the catalyst, ensuring the stability of the structure and its persistence at high temperatures, while the immobilized urease is uniformly dispersed therein, providing sufficient low-temperature active sites.

[0034] Among them, metal oxides are preferably those with a high oxygen vacancy concentration. Nanowires, or those with high specific surface area and abundant surface hydroxyl groups Nanoparticles, and can be doped or (Doping amount 1-5wt%) to further enhance the stability of oxygen vacancies and catalytic activity; immobilized urease is fixed to aldehyde-modified chitosan microspheres or mesoporous structures via glutaraldehyde cross-linking. Urease on the carrier;

[0035] To increase the contact area, the metal oxide morphology should be nanowires with a length of 200-500 nm. ) or nanoparticles with a particle size of 50-100 nm ( );

[0036] During system startup or low-load phases, the reaction temperature is relatively low (e.g., 90-110℃). At this time, immobilized urease plays a key role. Its binuclear nickel active center can efficiently and specifically catalyze the breaking of the CN bond in the urea molecule, drastically reducing the activation energy of the reaction from 113 kJ / mol in conventional hydrolysis to below 30 kJ / mol, achieving rapid reaction startup and high reaction rate at low temperatures. As the reaction proceeds and the temperature rises, the role of metal oxides becomes increasingly prominent. oxygen vacancies and The surface alkaline sites (-OH) can adsorb and activate water molecules, generating highly reactive ·OH radicals. Simultaneously, they adsorb urea molecules through Lewis acid sites, jointly promoting the hydrolysis reaction. These sites provide stable catalytic activity at high temperatures, compensating for the decreased activity of urease near its inactivation temperature.

[0037] The slurry is pumped into a low-temperature hydrolysis reactor. The inner wall of the reactor is coated with a fluorosilane-modified nanocomposite coating with a contact angle greater than 150°, and the reactor is equipped with 2 to 3 stages of swirling distribution plates, including an inlet stage located below the feed inlet and a main stage located in the middle of the reactor.

[0038] The swirl distribution plate includes a large-angle, open-pore inlet stage located below the feed inlet and a small-angle, dense-pore main stage located in the middle of the reactor.

[0039] In the hydrolysis reactor, the reaction temperature is controlled at 90℃ to 110℃ and the reaction pressure is controlled at 0.2MPa to 0.4MPa for hydrolysis reaction.

[0040] The heat source for the hydrolysis reaction is preferentially the waste heat of flue gas from the tail flue of the air preheater, which is indirectly provided by a circulation loop formed by heating the intermediate heat medium through a corrosion-resistant heat exchanger; when the system is started or the waste heat of the flue gas is insufficient, steam extracted from the turbine with an absolute pressure of 0.3MPa to 0.5MPa is used as an auxiliary heat source.

[0041] Through the intelligent control unit, based on the unit load signal and the NOx concentration signal at the SCR reactor inlet, the speed of the urea solution feed pump and the valve opening of the heat source supply pipeline are dynamically adjusted using the fuzzy PID algorithm, so that the system ammonia production response time is less than 1 minute.

[0042] The execution of the fuzzy PID algorithm includes: using the unit load and SCR inlet NOx concentration as feedforward signals, the demand ammonia setpoint is obtained through a calculation model;

[0043] Using the error between the actual ammonia production and the set value, and the rate of change of the error as input variables, the proportional coefficient, integral time, and derivative time of the PID controller are dynamically adjusted through a fuzzy inference rule base.

[0044] The inverter that outputs control signals to the urea solution feed pump and the regulating valve in the heat source pipeline.

[0045] After the reaction products are separated into gas and liquid, the resulting ammonia is sent to the SCR denitrification system, and the residual liquid phase is recycled by an automatic backwashing filter to recover more than 95% of the composite catalyst and returned to the reactor for recycling.

[0046] Specifically, the control objective of the fuzzy PID algorithm is to dynamically adjust the urea feed rate (F_urea) and steam supply rate (F_steam) so that the actual ammonia production can quickly and accurately track the ammonia demand of the unit, and maintain the internal temperature (T_reactor) and pressure (P_reactor) of the reactor within the set range.

[0047] The computational model of the fuzzy PID algorithm is as follows:

[0048]

[0049] Where k and a are unit characteristic coefficients based on historical data regression, and f() is a feedforward compensation function based on NOx concentration and flue gas flow. This model can predict changes in ammonia demand in advance.

[0050] The reasoning mechanism of the fuzzy PID algorithm includes the following steps:

[0051] The input variables are fuzzified, and the error (e) is: e(t) = SP_NH3 - actual ammonia production (calculated through the reactor pressure and temperature inversion model).

[0052] Error rate of change (ec): ec(t) = de(t) / dt;

[0053] Convert the precise values ​​of e and ec into fuzzy linguistic variables such as "negative large (NB)", "negative small (NS)", "zero (ZO)", "positive small (PS)" and "positive large (PB)";

[0054] To build a fuzzy rule base, the following is a specific example:

[0055] IF e is PB AND ec is ZO, THEN ΔKp is PB, ΔKi is NB, ΔKd is PS.

[0056] (Explanation: When the actual ammonia production is far lower than the demand and the error has not decreased, the proportional action Kp should be increased significantly, the integral action Ki should be canceled to prevent overshoot, and the derivative action Kd should be appropriately strengthened to suppress overshoot.)

[0057] The rule base contains approximately 20-50 similar empirical rules, which encapsulate expert operational knowledge.

[0058] The output and defuzzification process involves the dynamic correction values ​​ΔKp, ΔKi, and ΔKd of the PID parameters. Through fuzzy inference, a fuzzy set of outputs is obtained, which is then converted into precise PID parameter values ​​using defuzzification methods such as the centroid method.

[0059] The final output is Where Kp', Ki', and Kd' are parameters that are updated in real time.

[0060] In one embodiment, a urea hydrolysis ammonia production system is also provided for implementing the above method, comprising:

[0061] Urea solution storage tank, metering pump, composite catalyst dosing device, static mixer;

[0062] The low-temperature hydrolysis reactor is made of 316L stainless steel, with an anti-crystallization nano-coating on the inner wall and a swirl distribution plate.

[0063] The heat source supply unit includes a flue gas waste heat recovery subsystem and a low-parameter steam supply subsystem;

[0064] The intelligent control unit is configured to execute the fuzzy PID algorithm;

[0065] Gas-liquid separator, ammonia buffer tank;

[0066] And an automatic backwashing filter for catalyst recovery;

[0067] The heat source of the flue gas waste heat recovery subsystem comes from a portion of the flue gas extracted from the tail flue after the air preheater (flue gas temperature is usually 120-150℃). Its heat exchange method adopts corrosion-resistant fluoroplastic heat exchanger (anti-acid dew point corrosion) or stainless steel plate heat exchanger to establish an intermediate heat medium circulation loop.

[0068] The specific process is as follows: the intermediate heat medium (such as high-temperature heat transfer oil or demineralized water) is heated to 110-130℃ in the flue gas heat exchanger. The high-temperature heat medium is pumped to the jacket or built-in coil of the hydrolysis reactor as the main heat source to maintain the reaction temperature (90-110℃). The heat medium after heat release (about 90-100℃) returns to the flue gas heat exchanger to complete the cycle.

[0069] The heat source for the low-parameter steam supply subsystem comes from low-grade steam (0.3-0.5 MPa, absolute pressure, 140-160℃) extracted from the low-pressure cylinder of the steam turbine. Its usage strategy includes:

[0070] When the system starts up: when the waste heat of the flue gas is insufficient or not in use, the extracted steam is used to quickly heat the system to the operating temperature by passing through another set of coils in the reactor or as a backup heat source for the heat transfer medium heater.

[0071] When operating under variable load: When the unit load fluctuations cause the waste heat of the flue gas to be unstable, the extracted steam serves as a peak-shaving heat source, and the heat is precisely supplemented through the proportional regulating valve to ensure the stability of the reaction temperature;

[0072] The swirl distribution plate is set in multiple stages. Specifically, in the reactor, 2-3 stages of swirl distribution plates are set along the material flow direction (from the feed inlet to the gas phase outlet). The first stage (inlet stage) is located below the feed inlet and adopts a large inclination angle and open porosity design. Its main function is to crush droplets, initially distribute them evenly, and generate a strong swirling flow to prevent material from accumulating near the inlet. The second stage (main stage) is located in the middle of the reactor and adopts a small inclination angle and dense open porosity design. Its main function is to maintain and enhance the swirl across the entire cross-section, eliminate dead zones, ensure the uniformity of the temperature and concentration fields, and avoid local supersaturation crystallization.

[0073] The anti-crystallization nano-coating adopts a two-layer composite structure of bottom layer + top layer. The bottom layer is a nickel-based alloy coating with high adhesion, which is prepared by high-velocity flame spraying (HVOF) to provide excellent substrate adhesion and corrosion resistance barrier. The top layer is a fluorosilane-modified nano-ceramic-polymer composite coating, which is prepared by sol-gel method.

[0074] In one embodiment, a coal-fired power plant SCR denitrification system is also provided, which integrates the above-mentioned urea hydrolysis ammonia production system. The ammonia gas generated by the system is buffered and stabilized before being transported to the ammonia injection grid of the SCR reactor.

[0075] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for producing ammonia by urea hydrolysis, characterized in that, The method includes: The urea solution is mixed with the composite catalyst to form a slurry; The slurry is fed into a hydrolysis reactor and hydrolyzed at a temperature of 90°C to 110°C and a pressure of 0.2 MPa to 0.4 MPa to generate ammonia-containing gas. The heat source for the hydrolysis reaction is preferably the waste heat from the flue gas. The composite catalyst comprises metal oxides and immobilized urease.

2. The method for producing ammonia by urea hydrolysis according to claim 1, characterized in that, In the composite catalyst, the dry basis mass ratio of the metal oxide to the immobilized urease is 7:3 to 8:

2.

3. The method for producing ammonia by urea hydrolysis according to claim 1, characterized in that, The metal oxide is Nanowires or Nanoparticles; The immobilized urease is a urease immobilized on aldehyde-modified chitosan microspheres using glutaraldehyde as a crosslinking agent.

4. The method for producing ammonia by urea hydrolysis according to claim 1, characterized in that, The amount of the composite catalyst added is 5% to 8% of the total mass of the urea solution.

5. The method for producing ammonia by urea hydrolysis according to claim 1, characterized in that, The waste heat from the flue gas comes from the tail flue after the air preheater of the coal-fired boiler, and indirectly provides heat for the hydrolysis reaction through an intermediate heat medium circulation loop.

6. The method for producing ammonia by urea hydrolysis according to claim 1, characterized in that, The method further includes filtering the residual liquid generated after the hydrolysis reaction to recover the composite catalyst therein and return it to the hydrolysis reactor for recycling.

7. A urea hydrolysis ammonia production system, used to implement the urea hydrolysis ammonia production method according to any one of claims 1-6, characterized in that, include: Urea solution storage tank, metering pump, composite catalyst dosing device, static mixer; The low-temperature hydrolysis reactor is made of 316L stainless steel, with an anti-caking nano-coating on the inner wall and a swirl distribution plate. The heat source supply unit includes a flue gas waste heat recovery subsystem and a low-parameter steam supply subsystem; The intelligent control unit is configured to execute the fuzzy PID algorithm; Gas-liquid separator, ammonia buffer tank; And an automatic backwash filter for catalyst recovery.

8. The urea hydrolysis ammonia production system according to claim 7, characterized in that, The execution of the fuzzy PID algorithm includes: using the unit load and SCR inlet NOx concentration as feedforward signals, the required ammonia setpoint is obtained through a calculation model; using the error between the actual ammonia production and the setpoint and the error change rate as input variables, the proportional coefficient, integral time, and derivative time of the PID controller are dynamically adjusted through a fuzzy inference rule base; and outputting control signals to the frequency converter of the urea solution feed pump and the regulating valve of the heat source pipeline.

9. A SCR denitrification system for a coal-fired power plant, characterized in that, The system integrates the urea hydrolysis ammonia production system as described in claim 7, wherein the ammonia gas produced by the system is buffered and stabilized before being transported to the ammonia injection grid of the SCR reactor.

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