Energy-saving system for urea pyrolysis ammonia production based on flue gas waste heat utilization

Abstract

The application discloses a urea pyrolysis ammonia production energy-saving system based on flue gas waste heat utilization, which comprises a flue gas heat exchange subsystem, a urea pyrolysis ammonia production subsystem and a control unit; the flue gas heat exchange subsystem comprises a high-temperature flue gas heat exchanger, a low-temperature flue gas heat exchanger, a first independent circulation loop for circulating a first heat exchange working medium and connecting a working medium side of the high-temperature flue gas heat exchanger with a heat supply unit of a urea pyrolysis furnace and a second independent circulation loop for circulating a second heat exchange working medium and connecting a working medium side of the low-temperature flue gas heat exchanger with a urea solution preheater. The high-temperature and low-temperature flue gas heat exchangers are arranged in series, and the independent first and second circulation loops are matched, so that the high, medium and low grade waste heat of flue gas is recovered and accurately utilized in stages, the high-temperature heat is directly used for high-temperature pyrolysis of urea, and the low-temperature heat is used for preheating of urea solution, thereby reducing the dependence on traditional electric or steam heat sources, reducing the energy consumption of system operation and achieving outstanding energy-saving effect.

Description

Technical Field

[0001] This application belongs to the field of energy-saving ammonia production technology, specifically involving an energy-saving system for urea pyrolysis ammonia production based on the utilization of waste heat from flue gas. Background Technology

[0002] In the flue gas treatment processes of combustion equipment such as thermal power plants and industrial boilers, selective catalytic reduction (SCR) technology is the mainstream method for reducing nitrogen oxide (NOx) emissions. Urea pyrolysis, due to its high safety and convenient storage and transportation, has become the main method for reducing ammonia emissions in SCR denitrification systems. Preparation process. However, traditional urea pyrolysis ammonia production systems typically use electric heaters or steam as heat sources to heat the urea solution to a high temperature of 350℃-450℃ to achieve complete pyrolysis. This process consumes a huge amount of energy, increases the plant's power consumption or steam consumption, and results in high operating costs.

[0003] To reduce energy consumption, existing technologies have attempted to utilize waste heat from flue gas for urea pyrolysis. For example, some solutions directly install heat exchangers in the flue to heat air or the medium before pyrolysis. However, these solutions often have the following drawbacks: First, they can only recover a portion of the sensible heat from the flue gas, resulting in insufficient utilization of low- and medium-grade waste heat and limited overall energy-saving efficiency. Second, they do not fully consider the acid dew point corrosion problem that may occur in the heat exchangers due to flue gas components (especially sulfur oxides) at low temperatures, affecting the long-term reliable operation of the system.

[0004] Therefore, how to construct a system that can efficiently, safely, and adaptably utilize waste heat from flue gas for urea pyrolysis to produce ammonia under all operating conditions, while maximizing the recovery of waste heat from flue gas and ensuring the stability, reliability, and equipment safety of the urea pyrolysis process, has become a pressing technical problem to be solved in this field. Summary of the Invention

[0005] This application provides an energy-saving system for urea pyrolysis to ammonia production based on the utilization of waste heat from flue gas, aiming to solve the problems that existing technologies may have with insufficient utilization of medium and low grade waste heat, limited overall energy-saving efficiency, and other issues.

[0006] An energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization includes:

[0007] Flue gas heat exchange subsystem, urea pyrolysis ammonia production subsystem and control unit;

[0008] The flue gas heat exchange subsystem includes:

[0009] A high-temperature flue gas heat exchanger is installed in the flue upstream of the SCR reactor;

[0010] A low-temperature flue gas heat exchanger is connected in series in the flue downstream of the high-temperature flue gas heat exchanger;

[0011] The first independent circulation loop, with the first heat exchange working medium circulating, connects the working medium side of the high-temperature flue gas heat exchanger to the heating unit of the urea pyrolysis furnace.

[0012] The second independent circulation loop, with the second heat exchange working fluid circulation, connects the working fluid side of the low-temperature flue gas heat exchanger to the urea solution preheater.

[0013] The urea pyrolysis ammonia production subsystem includes: a urea solution storage tank, a urea solution delivery pump, a urea solution preheater, a urea pyrolysis furnace, and a mixed gas delivery pipeline; the urea solution preheater uses heat provided by the second independent circulation loop to preheat the urea solution; the urea pyrolysis furnace is equipped with the heating unit inside, which pyrolyzes the preheated urea solution into a mixed gas of ammonia and carbon dioxide; the mixed gas delivery pipeline delivers the mixed gas to the inlet flue of the SCR reactor;

[0014] The control unit monitors and adjusts the system operating parameters, and performs temperature control of the urea pyrolysis furnace, protection control of the low-temperature flue gas heat exchanger wall temperature, and coordinated switching control between the main heat source and the auxiliary heat source.

[0015] Optionally, the first independent circulation loop uses synthetic heat transfer oil or molten salt as the first heat exchange medium; the second independent circulation loop uses water or ethylene glycol aqueous solution as the second heat exchange medium.

[0016] Optionally, a three-way mixing valve is provided on the second independent circulation loop to adjust the inlet temperature of the second heat exchange medium entering the low-temperature flue gas heat exchanger, so as to ensure that the metal wall temperature of the low-temperature flue gas heat exchanger is higher than the acid dew point temperature of the flue gas.

[0017] Optionally, the system further includes an auxiliary electric heater;

[0018] The auxiliary electric heater is installed in series on the pipeline of the first independent circulation loop, located between the outlet of the high-temperature flue gas heat exchanger and the inlet of the urea pyrolysis furnace heating unit, for directly heating the first heat exchange medium flowing through it; or the heating element of the auxiliary electric heater is directly built into the reaction chamber of the urea pyrolysis furnace or near the heat exchange coil / jacket.

[0019] Optionally, the control unit is configured to execute control logic for at least one of the following operating modes:

[0020] Cold start / pure auxiliary heating mode: When insufficient waste heat of flue gas is detected, the auxiliary electric heater is started and the first independent circulation loop is controlled to circulate until the temperature of the pyrolysis furnace reaches the set value;

[0021] Hybrid heating transition mode: Real-time calculation of the heat that the waste heat of the flue gas can provide, and dynamic adjustment of the power of the auxiliary electric heater to make up for the difference in the real-time total heat demand of the pyrolysis furnace;

[0022] Waste heat-dominated mode: When the waste heat of the flue gas can meet the needs of the pyrolysis furnace on its own, the power of the auxiliary electric heater is reduced to standby mode, and the temperature of the pyrolysis furnace is controlled entirely by adjusting the flow rate of the working fluid in the first independent circulation loop.

[0023] Optionally, the control unit has a built-in urea pyrolysis furnace temperature main control loop. This loop adopts a cascade control structure, with the temperature of the pyrolysis furnace reaction zone as the main controlled variable. Its output is used to distribute the load between the first independent loop and the second independent loop.

[0024] Optionally, the control unit has a built-in low-temperature flue gas heat exchanger wall temperature protection control loop. This loop calculates the acid dew point temperature online based on real-time monitoring of flue gas composition and temperature, and controls the inlet temperature of the second heat exchange medium entering the low-temperature flue gas heat exchanger by adjusting the opening of the three-way mixing valve in the second independent circulation loop, so as to ensure that the minimum metal wall temperature is higher than the acid dew point temperature plus a safety margin.

[0025] Optionally, the heat transfer areas of the high-temperature flue gas heat exchanger and the low-temperature flue gas heat exchanger are determined by matching calculation and optimization based on the heat load, working temperature range of the working fluid, logarithmic mean temperature difference and total heat transfer coefficient under the system design reference conditions, and are verified under different host load conditions.

[0026] Optionally, the control unit is configured to: determine the flue gas separation temperature point between the high-temperature flue gas heat exchanger and the low-temperature flue gas heat exchanger using an optimization algorithm based on the flue gas parameter curves of the target host or boiler under different loads and the urea pyrolysis requirements, so as to optimize the cascade utilization of flue gas waste heat.

[0027] Optionally, the control unit is pre-set with multi-level safety interlock logic, including: interlocking to shut down the urea solution delivery pump when the pyrolysis furnace temperature exceeds the limit, limiting or cutting off the waste heat recovery of flue gas when the wall temperature of the low-temperature heat exchanger is close to the safety boundary, and interlocking to shut down the auxiliary electric heater when the working fluid flow or pressure of the first circulation loop is abnormal.

[0028] Compared with the prior art, this application has at least the following beneficial effects:

[0029] This application achieves the tiered recovery and precise utilization of high, medium, and low-grade waste heat from flue gas by setting up high-temperature and low-temperature flue gas heat exchangers in series and matching them with independent first and second circulation loops. The high-temperature heat is directly used for the high-temperature pyrolysis of urea, and the low-temperature heat is used for the preheating of urea solution, which reduces the dependence on traditional electric or steam heat sources, reduces the system's operating energy consumption, and has outstanding energy-saving effect.

[0030] To address the issue of condensation and corrosion of sulfur-containing flue gas at low temperatures, a wall temperature control strategy based on a three-way mixing valve was designed in the second independent circulation loop. By dynamically adjusting the inlet temperature of the working fluid entering the low-temperature heat exchanger, the wall temperature of the heat exchanger is ensured to always be higher than the acid dew point of the flue gas, fundamentally avoiding low-temperature acid corrosion and ensuring the long-term safe and stable operation of the core equipment.

[0031] The control unit of this application integrates a multi-mode intelligent control algorithm, which can automatically and smoothly switch between modes such as pure auxiliary heating, mixed heating, and flue gas waste heat dominance according to flue gas conditions, main unit load, and pyrolysis requirements. Under conditions of insufficient flue gas waste heat, such as during startup or low load, the precise compensation of the auxiliary electric heater ensures that the urea pyrolysis temperature remains stable within the optimal reaction range, thus guaranteeing the continuity and stability of ammonia supply and improving the adaptability and reliability of the entire SCR denitrification system. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the module connection of the urea pyrolysis ammonia production energy-saving system based on flue gas waste heat utilization provided in this application. Detailed Implementation

[0033] 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.

[0034] The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization provided in this application includes:

[0035] This includes a flue gas heat exchange subsystem, a urea pyrolysis ammonia production subsystem, and a control unit;

[0036] The flue gas heat exchange subsystem includes: a high-temperature flue gas heat exchanger, installed in the flue upstream of the SCR reactor; a low-temperature flue gas heat exchanger, connected in series in the flue downstream of the high-temperature flue gas heat exchanger; a first independent circulation loop, using a first heat exchange medium, connecting the working medium side of the high-temperature flue gas heat exchanger to the heating unit of the urea pyrolysis furnace; and a second independent circulation loop, using a second heat exchange medium, connecting the working medium side of the low-temperature flue gas heat exchanger to the urea solution preheater.

[0037] The high-temperature flue gas heat exchanger is specifically installed in the vertical or horizontal flue section upstream of the inlet of the selective catalytic reduction (SCR) reactor. This heat exchanger preferably employs a counter-current shell-and-tube or plate structure, and its heat exchange surface material must withstand the high temperature of the flue gas and any corrosive components it may contain. Its shell side is connected to the main flue gas passage, while its tube side is connected to the first independent circulation loop. This first independent circulation loop uses a first heat exchange medium, preferably a synthetic heat transfer oil or molten salt medium with a high boiling point, good thermal stability, and suitable specific heat capacity. The loop is equipped with a high-temperature working medium circulation pump, an expansion tank to compensate for changes in the working medium's volume, and necessary safety valves and instruments. This circuit forms a closed high-temperature heat transfer path: after the first heat exchange medium absorbs heat from the flue gas in the tube side of the high-temperature flue gas heat exchanger, it is driven by the high-temperature medium circulation pump to be transported to the heating unit of the urea pyrolysis furnace (e.g., coils or jackets wrapped around the furnace body or built into the reaction chamber), releasing heat to supply the urea pyrolysis reaction. The cooled medium returns to the high-temperature flue gas heat exchanger to complete the cycle.

[0038] The low-temperature flue gas heat exchanger is installed in series in the flue downstream of the high-temperature flue gas heat exchanger, specifically after the induced draft fan and before entering subsequent environmental protection facilities (such as a desulfurization tower). This heat exchanger is used to recover residual low-to-medium grade waste heat in the flue gas after high-temperature heat exchange. It also adopts a shell-and-tube structure, with flue gas flowing through the shell side and the tube side connected to the second independent circulation loop. This second independent circulation loop uses a second heat exchange medium, preferably water or an aqueous ethylene glycol solution. The loop includes a low-temperature medium circulation pump, a pressure regulating device, and a three-way valve for adjusting the medium temperature. This loop forms a closed low-temperature heat transfer path: after the second heat exchange medium is heated in the tube side of the low-temperature flue gas heat exchanger, it is transported by the low-temperature medium circulation pump to the urea solution preheater (usually a plate or shell-and-tube heat exchanger), where heat is transferred to the urea solution to be treated, giving it preliminary preheating. The cooled second heat exchange medium returns to the low-temperature flue gas heat exchanger.

[0039] To ensure long-term reliable system operation, and especially to prevent acid corrosion caused by condensation of sulfur-containing flue gas on the surface of the low-temperature heat exchanger, the second independent circulation loop is designed with a wall temperature control strategy. By adjusting the opening of the three-way mixing valve in the loop, the inlet temperature of the second heat exchange medium entering the tube side of the low-temperature flue gas heat exchanger is controlled, thereby ensuring that the temperature of the metal wall surface of the heat exchanger in contact with the flue gas is higher than the acid dew point temperature of the flue gas under any operating conditions. In addition, both independent circulation loops are equipped with bypasses and corresponding regulating valves to flexibly adjust the flow rate of the working medium through the heat exchanger when the main unit or boiler load changes, thereby optimizing waste heat recovery efficiency and protecting the equipment while ensuring the heat required for urea pyrolysis.

[0040] The specific calculation method for matching the heat transfer area of ​​high-temperature flue gas heat exchangers and low-temperature flue gas heat exchangers includes the following steps:

[0041] First, determine the system design baseline operating condition. Take a typical load point of the main unit or boiler (e.g., 75%-100% of the rated load) as the design operating condition, and obtain key parameters such as the mass flow rate, temperature, and composition of the flue gas before the SCR reactor inlet under this operating condition (especially the content of water vapor and sulfur oxides to calculate the acid dew point).

[0042] Secondly, calculate the total heat load required for the urea pyrolysis to ammonia production subsystem ( The load consists of two parts: first, the sensible heat required to heat a urea solution at a specified flow rate (determined based on the ammonia-nitrogen molar ratio required for denitrification) from the initial temperature to the pyrolysis reaction temperature (usually 350℃-450℃); and second, the reaction heat required for the complete decomposition of urea into ammonia and carbon dioxide at the pyrolysis reaction temperature. This serves as the core basis for subsequent heat exchanger design.

[0043] Then, the design calculations for the high-temperature flue gas heat exchanger and the first independent circulation loop are carried out.

[0044] Determine the heat load borne by the high-temperature circuit ( In principle, It should be equal to or slightly greater than the effective heat supply required by the urea pyrolysis furnace heating unit. This heat supply must take into account the heat loss from the piping between the high-temperature flue gas heat exchanger and the pyrolysis furnace.

[0045] Selecting the first heat exchange medium and determining its operating temperature range: Based on the heat source temperature required by the pyrolysis furnace (i.e., the temperature at which the first heat exchange medium enters the heating unit). Based on the thermal stability of the working fluid, the target temperature at the outlet of the high-temperature flue gas heat exchanger is determined. Simultaneously, considering system economy and safety, the minimum allowable temperature of the working fluid at the heat exchanger inlet is set. ;

[0046] Calculate the required mass flow rate of the first heat exchange medium ( According to the formula Perform calculations, where is the specific heat capacity of the first heat exchange medium at the average temperature;

[0047] Calculate the heat transfer area of ​​the high-temperature flue gas heat exchanger ( a. Flue gas parameters based on design conditions (mass flow rate, temperature entering the high-temperature heat exchanger) ) and the calculation from the previous step and Perform heat balance calculations on the flue gas side and the working fluid side to determine the temperature at which the flue gas leaves the high-temperature heat exchanger. ;

[0048] b. Calculate the logarithmic mean temperature difference based on the flow arrangement of the flue gas and the first heat exchange medium within the heat exchanger (e.g., countercurrent). );

[0049] c. According to the formula Calculate the required heat transfer area.

[0050] Among them, the overall heat transfer coefficient It needs to be estimated or determined through empirical correlation based on the selected heat exchanger type (such as shell and tube), material, physical properties of flue gas and working fluid, preset flow rate and possible fouling thermal resistance.

[0051] Next, design calculations were performed for the low-temperature flue gas heat exchanger and the second independent circulation loop.

[0052] Determine the heat load borne by the low-temperature circuit ( This load is equal to the heat required to preheat the urea solution from its initial temperature to a certain intermediate target temperature. This intermediate target temperature needs to be determined in conjunction with the thermal integration optimization of the entire system, usually aiming to minimize the heating load entering the high-temperature circuit, but is limited by the corrosion protection requirements of the low-temperature heat exchanger;

[0053] Parameter determination under corrosion-resistant wall temperature constraints: To ensure that the metal wall temperature of the low-temperature flue gas heat exchanger is higher than the acid dew point of the flue gas, the inlet temperature of the second heat exchange medium entering the heat exchanger needs to be determined. Set a lower limit value. Typically, the temperature needs to be at least 10°C higher than the acid dew point temperature. This temperature is regulated and controlled by a three-way mixing valve in the second circulation loop by mixing with a cooler working fluid from the preheater outlet.

[0054] Calculate the mass flow rate of the second heat exchange medium ( ) and the heat transfer area of ​​the low-temperature heat exchanger ( a. Set the target temperature at which the second heat exchange medium leaves the cryogenic heat exchanger. This temperature is related to the design of the urea solution preheater;

[0055] b. Calculation ;

[0056] c. The state of the flue gas at the outlet of the high-temperature heat exchanger ( (At this point, the flue gas mass flow rate may need to be corrected due to factors such as air extraction) is used as the inlet condition for the low-temperature heat exchanger. Heat balance calculations are performed to determine the final temperature of the flue gas leaving the low-temperature heat exchanger. ;

[0057] d. Calculate the logarithmic mean temperature difference within the low-temperature heat exchanger ( );

[0058] e. Calculation The estimation of K_low needs to take into account its different operating temperature ranges and working fluid characteristics;

[0059] Finally, system compatibility verification and iterative optimization were performed. The calculated heat transfer areas of the high and low temperature heat exchangers, along with their corresponding flue gas outlet temperatures and working fluid parameters, were substituted into other major load conditions (such as 50% load and 100% load) for verification calculations. Verification was conducted under different operating conditions:

[0060] Can the high-temperature circuit provide enough heat to maintain the temperature of the urea pyrolysis reaction?

[0061] Does the wall temperature of the low-temperature flue gas heat exchanger always meet the corrosion prevention requirements?

[0062] Does the area ratio of the two heat exchangers ensure that the cascade utilization of flue gas waste heat is relatively balanced and efficient over a wide load range?

[0063] If the verification results are not ideal, the initial design parameters need to be adjusted (such as the heat load distribution of high and low temperature circuits, the working temperature range of the working fluid, and even the type of heat exchanger). The above calculation process is repeated until a heat transfer area matching scheme that can meet the urea pyrolysis requirements, ensure equipment safety, and is economical and reasonable under all major operating conditions is obtained.

[0064] Furthermore, based on the typical flue gas temperature curves of different main unit or boiler models, the flue gas separation temperature point between the high-temperature flue gas heat exchanger and the low-temperature flue gas heat exchanger (i.e., the flue gas temperature at the outlet of the high-temperature heat exchanger / the inlet of the low-temperature heat exchanger) is optimized and determined. The specific steps of this optimization method include:

[0065] Step 1: Basic Data Acquisition and Operating Condition Definition. Collect flue gas parameter curves of the target main unit or boiler at multiple typical load points (e.g., 25% MCR, 50% MCR, 75% MCR, 100% MCR). Key data include: flue gas mass flow rate, flue gas temperature, and flue gas composition (especially...) at each load point. , SOx content (used to calculate acid dew point temperature), and flue gas pressure. Simultaneously, the ammonia demand curve for the urea pyrolysis ammonia production subsystem is determined across the entire load range, thereby identifying its required heat load (SOx content, used to calculate acid dew point temperature), and flue gas pressure. The relationship between load and host load.

[0066] Step 2: Establish a system thermodynamic model and set constraints. Establish a steady-state thermodynamic model including high- and low-temperature heat exchangers, the first and second independent circulation loops, the urea solution preheater, and the pyrolysis furnace. This model should be able to adapt to the input initial flue gas parameters and the set segmentation temperature points (…). The design parameters of the working fluid in the two circulation loops (such as operating temperature range and flow rate) are used to calculate key performance indicators such as temperature at each node, heat load distribution, and heat exchanger approach temperature difference.

[0067] Core constraints are set as follows: a. Urea pyrolysis temperature constraint: Under any load, the heat supplied to the pyrolysis furnace by the first independent circulation loop must be sufficient to heat the specified flow rate of urea solution to the reaction temperature range required for its complete pyrolysis (e.g., 350-450°C) and maintain this temperature; b. Low-temperature heat exchanger corrosion constraint: To ensure that the metal wall temperature of the low-temperature flue gas heat exchanger is higher than the acid dew point of the flue gas, the inlet temperature of the second heat exchange medium entering the tube side of the heat exchanger must be controlled ( ). The minimum allowable value is determined by adding a safety margin (usually ≥10℃) to the calculated flue gas acid dew point temperature. This constraint directly limits the lower limit of the grade of heat that the low-temperature heat exchanger can recover; c. Minimum heat transfer temperature difference constraint for heat exchangers: In high- and low-temperature heat exchangers, the minimum temperature difference (pinch point temperature difference) between the flue gas and the corresponding working fluid must be greater than the minimum allowable value for technical and economic reasons (usually 20-50℃) to avoid excessive heat exchange area requirements leading to a surge in equipment costs;

[0068] Step 3: Divide the temperature points ( The optimization search, based on the above model and constraints, will segment the temperature points. As the core optimization variable, iterative or numerical search methods (such as the golden section method, gradient-based methods, or ergonomic methods) are used for optimization. The objective function of optimization is a combination of multiple objectives, which usually requires trade-offs:

[0069] Technical performance objectives: Maximize the net heat gain of the entire waste heat recovery system (i.e., the total heat effectively used for urea pyrolysis and preheating), or maximize the system's adaptability over a wide load range.

[0070] Economic objective: Minimize the total system cost (including initial investment and operating costs of high- and low-temperature heat exchangers) over its lifecycle. The cost of the heat exchanger is primarily related to its heat transfer area, which is determined by the heat load and the logarithmic mean temperature difference. This directly affects the allocation of both.

[0071] Specifically, the process of optimizing the search is as follows:

[0072] a. Select a representative load for the design (usually the load point where the unit operates for the longest time, such as 75%-85% MCR);

[0073] b. Under this design load, a given The initial assumed values;

[0074] c. Run the system thermodynamic model and perform heat balance calculations: The high-temperature flue gas heat exchanger needs to cool the flue gas from the inlet temperature to... The released heat is absorbed by the first circulating working fluid. Calculate whether this heat is sufficient to meet the heat requirements of the urea pyrolysis furnace under this load. If insufficient, adjustments and improvements are needed. (This means recovering more heat from the high-temperature heat exchanger), but this will cause the initial temperature of the flue gas entering the low-temperature heat exchanger to rise; the receiving temperature of the low-temperature flue gas heat exchanger is... The flue gas, while meeting corrosion resistance constraints (through adjustment) Under the constraints of achieving minimum heat transfer temperature difference and minimum heat transfer, calculate the maximum possible heat recovery. ), used for preheating urea solution;

[0075] d. Verify overall system compatibility: Check here. Next, is the heat provided in the first cycle precisely matched or slightly greater? Meanwhile, it is necessary to check whether the heat recovered in the second cycle is sufficient to preheat the urea solution to a reasonable temperature. Furthermore, it is required to verify whether the wall temperature constraints of the cryogenic heat exchanger can still be met at other non-design load points (especially low load points).

[0076] e. Evaluate the objective function: Calculate the current... The technical performance indicators (such as total waste heat recovery rate) and economic indicators (such as estimated total area cost of heat exchangers) are as follows.

[0077] f. Systematic change Repeat step ce for each value until the optimal value of the comprehensive objective function is found (e.g., maximizing the total waste heat recovery rate or minimizing the total cost while satisfying all constraints). The value is the optimized segmentation temperature point;

[0078] Step 4: Consideration of dynamic adjustment mechanism. Due to real-time changes in the main unit load, the flue gas parameter curves change accordingly. A fixed optimal split temperature point may only be optimal for the design conditions. Therefore, the concept of dynamically setting the split temperature point can be further introduced into the system control logic. That is, based on the real-time monitored flue gas flow rate, temperature, and urea demand, the working fluid flow distribution of the two circulation loops can be finely adjusted within a certain range through a simplified online model or a preset lookup table. This effectively achieves dynamic optimization of the ratio of flue gas heat recovery between the high and low temperature heat exchangers, ensuring that the system always approaches the optimal operating state under the current conditions.

[0079] The urea pyrolysis ammonia production subsystem includes: a urea solution storage tank and a delivery pump; a urea solution preheater for preheating the urea solution using heat provided by the second independent circulation loop; a urea pyrolysis furnace with an internal heating unit connected to the first independent circulation loop for pyrolyzing the preheated urea solution into a mixture of ammonia and carbon dioxide; and a mixed gas delivery pipeline for delivering the mixed gas to the inlet flue of the SCR reactor.

[0080] Specifically, the urea solution storage tank is used to store urea aqueous solutions of a predetermined concentration (typically 40% or 50% by weight). The tank is equipped with electric or steam heating and insulation to prevent crystallization of the solution at low temperatures; it is also equipped with a level gauge, temperature sensor, and venting device. The tank outlet is connected to a urea solution delivery pump via a pipeline.

[0081] The urea solution delivery pump is a precision metering pump resistant to urea corrosion. Its function is to precisely control and deliver a predetermined flow rate of urea solution to the subsequent preheating and pyrolysis units based on the real-time ammonia demand of the SCR system. This pump is typically frequency-controlled, and its flow signal is interlocked with the upstream NOx concentration monitoring signal and flue gas flow signal to achieve on-demand liquid supply.

[0082] The urea solution preheater is essentially a liquid-liquid heat exchanger, preferably a plate heat exchanger to achieve higher heat transfer efficiency. The preheater has two independent channels: one channel (primary side) flows with a second heat exchange medium (such as water or ethylene glycol solution) heated by the waste heat of the low-temperature flue gas from the second independent circulation loop; the other channel (secondary side) flows with cold urea solution from the delivery pump. The two channels exchange heat in counter-current or cross-current flow, allowing the urea solution to receive an initial temperature rise before entering the high-temperature pyrolysis furnace. Setting the preheating temperature is crucial, requiring a balance between maximizing energy efficiency (reducing the high-temperature heat load) and preventing premature decomposition of the urea solution in the preheater due to excessively high local temperatures, which could lead to the formation of polymers such as biuret and cause blockage. A temperature sensor is installed at the preheater outlet; its signal is used to adjust the flow rate or temperature of the working medium in the second independent circulation loop to stabilize the preheating effect.

[0083] The urea pyrolysis furnace is the reaction equipment of this subsystem. Its main body is a sealed pressure vessel or tubular reactor lined with refractory material. Key components inside the furnace are the built-in heat exchange coils or the jacket surrounding the reaction chamber, which constitute the heat-consuming end of the first independent circulation loop. The first heat exchange medium (high-temperature heat transfer oil or molten salt) from the high-temperature flue gas heat exchanger flows through the coils or jacket, continuously providing a uniform and controllable high-temperature heat source to the furnace. The preheated urea solution is sprayed into the reaction chamber of the pyrolysis furnace through one or more atomizing nozzles. At the set pyrolysis temperature (usually strictly controlled between 350°C and 450°C), the urea droplets rapidly evaporate and undergo a decomposition reaction, generating ammonia gas (…). ) and carbon dioxide ( The mixture is mainly composed of gases, and may also produce a small amount of isocyanate (HNCO), which will be completely hydrolyzed in the subsequent high-temperature environment. and The internal design of the pyrolysis furnace must ensure sufficient residence time and good temperature field uniformity to guarantee complete urea conversion. Multiple thermocouples are installed on the furnace body for real-time monitoring and precise control of the reaction temperature; a pressure safety valve and a discharge port are also provided.

[0084] The mixed gas delivery pipeline, made of ammonia-resistant steel (such as stainless steel), leads out from the outlet of the pyrolysis furnace. This pipeline must be fully insulated and typically equipped with electric heating tape to prevent the high-temperature mixed gas from cooling during transport, which could cause ammonia to react with any remaining incompletely hydrolyzed HNCO to form solid deposits (such as ammonium cyanurate), thus preventing pipeline blockage. The delivery pipeline ultimately connects to the ammonia injection grid (AIG) at the SCR reactor inlet flue, uniformly injecting the ammonia-mixed gas that meets the temperature, pressure, and flow requirements into the flue gas.

[0085] The control unit is used to monitor and adjust the system operating parameters. The control unit adopts a distributed control system architecture and is connected to sensors and actuators arranged at key nodes of the system through a high-speed data bus to realize real-time monitoring and precise control of the operating status.

[0086] The control unit continuously monitors the following key parameters:

[0087] On the flue gas side, dustproof and wear-resistant thermocouples are installed before and after the high-temperature flue gas heat exchanger, and after the low-temperature flue gas heat exchanger, to measure the flue gas temperature in real time; the flue gas flow rate is measured via a differential pressure transmitter; and SOx data are received from the flue gas analyzer. , Content signal, used for online calculation of real-time acid dew point temperature;

[0088] In the first independent circulation loop, thermocouples are arranged at the inlet and outlet of the working fluid in the high-temperature flue gas heat exchanger, the inlet and outlet of the urea pyrolysis furnace heating unit, and before and after the auxiliary electric heater (if installed) to measure the working fluid temperature; a mass flow meter is used to measure the working fluid flow rate; and a pressure sensor monitors the loop pressure.

[0089] The second independent circulation loop is equipped with resistance temperature detectors (such as PT100) to measure temperature at the working fluid inlet and outlet of the low-temperature flue gas heat exchanger, the heat medium side inlet and outlet of the urea solution preheater, and the three-way mixing valve; and electromagnetic flow meters to measure flow rate.

[0090] The urea pyrolysis ammonia production subsystem is equipped with thermocouples at multiple points (such as the upper, middle, and lower parts) of the urea solution preheater outlet, the urea pyrolysis furnace reaction chamber, and the pyrolysis gas outlet; the urea solution delivery pipeline is equipped with a mass flow meter; and the pyrolysis furnace is equipped with a pressure transmitter.

[0091] The control unit incorporates multiple parallel and seamlessly switchable control algorithm modules to achieve the core objective, specifically including:

[0092] The main temperature control loop for the urea pyrolysis furnace is based on the temperature of a key measuring point in the middle of the reaction zone of the pyrolysis furnace. As the master controlled variable, its setpoint ( The temperature is set within the range of 350℃ to 450℃ based on the urea solution concentration and the required conversion rate. This circuit employs a cascade control structure, as detailed below:

[0093] The main controller (PID1) according to and The deviation is calculated, and the output is the required total heat load ( ).

[0094] The secondary controller (feedforward-feedback composite) will The load is allocated to a first independent circulation loop (flue gas waste heat + auxiliary electric heating) and a second independent circulation loop (preheating load). For the first loop, its output is the first heat exchange medium flow rate setpoint and / or the auxiliary electric heater power setpoint. This allocation logic integrates the aforementioned operating mode switching strategy.

[0095] The low-temperature flue gas heat exchanger wall temperature protection control loop is a high-priority constraint control loop. Its calculation module calculates the flue gas composition (SOx, SOx, SOx) in real time based on the measured flue gas composition. The acid dew point temperature of the current flue gas is calculated online using empirical formulas (such as the K. Kiiski formula) based on the given temperature and temperature. );

[0096] The control module ensures the lowest possible metal wall temperature of the low-temperature flue gas heat exchanger. ) higher than ( ) as the goal ( For safety margin, it is usually ≥10℃. Because direct measurement of wall temperature is difficult, By establishing a simplified heat transfer model, the temperature of the second heat transfer medium entering the heat exchanger ( Based on the calculations using the flue gas temperature, the core control action is to adjust the three-way mixing valve, dynamically setting and maintaining its position. Above the minimum allowable value, even if this may cause the preheating temperature to temporarily deviate from the set value;

[0097] The parameters of the controller (such as PID1) are not fixed, but are called from a preset parameter table according to the current host load (%MCR) and system operating mode (such as pure electric heating, hybrid heating) to adapt to the changes in the dynamic characteristics of the object;

[0098] The output commands from the control unit directly drive the following actuators:

[0099] Adjust the speed of the working fluid circulation pumps in the first and second independent circulation loops to change the working fluid flow rate;

[0100] The opening of the bypass valve in the first loop is controlled to regulate the flow rate through the high-temperature flue gas heat exchanger; the opening of the three-way mixing valve in the second loop is controlled to regulate... Control the regulating valves at each stage of the heat medium inlet of the urea solution preheater;

[0101] If an auxiliary electric heater is provided, control the switching on and off of its heating element or continuously adjust its power;

[0102] The speed of the urea solution delivery pump is adjusted, and its set value is derived from the calculation of the ammonia-nitrogen molar ratio required for SCR denitrification and is coordinated with the temperature control of the pyrolysis furnace.

[0103] Furthermore, the control unit has preset multi-level safety interlocking logic, for example:

[0104] If the temperature of the pyrolysis furnace is below the lower safety limit (e.g., 300℃) or exceeds the upper limit (e.g., 500℃), the urea solution delivery pump should be immediately stopped and an alarm should be triggered.

[0105] If the calculated wall temperature of the low-temperature heat exchanger is close to the safety margin boundary, wall temperature protection control should be implemented first, and the waste heat recovery of flue gas leading to the heat exchanger should be limited or cut off.

[0106] If the working fluid flow rate or pressure in the first circulation loop is detected to be too low or abnormal, the auxiliary electric heater will be shut down by interlock.

[0107] All critical control valves, pumps, and heaters are equipped with "fault position" status feedback. Once the control unit detects a fault, it will automatically execute the preset fail-safe procedure.

[0108] The system also has a startup and low-load operation strategy that includes an auxiliary electric heater and its intelligent switching control logic. When the waste heat from the flue gas is insufficient as the main heat source, the auxiliary electric heater is automatically introduced to supplement the heat, and the main heat source is smoothly switched back to waste heat from the flue gas as the main load increases.

[0109] Specifically, we will first provide an integrated solution for the auxiliary electric heater, which includes the following two solutions:

[0110] Option A: Integrated into the first independent circulation loop. In this option, the auxiliary electric heater is installed in series as an independent shell-and-tube or sleeve-type heating unit on the pipeline of the first independent circulation loop, located between the outlet of the high-temperature flue gas heat exchanger and the inlet of the urea pyrolysis furnace heating unit. Its electric heating element (such as resistance wire or heating rod) is immersed in the flowing first heat exchange medium (heat transfer oil or molten salt) to directly heat the medium;

[0111] Option B: Integrated inside the urea pyrolysis furnace. In this option, the auxiliary electric heating element is directly built into the reaction chamber of the urea pyrolysis furnace or near the heat exchange coil / jacket. The electric heating element can adopt a radiant tube type or a sheathed type structure, and its surface temperature is strictly controlled by the furnace temperature monitoring system to avoid local overheating that could exacerbate the production of urea pyrolysis byproducts.

[0112] The rated heating power of the auxiliary electric heater ( Based on the system thermodynamic model and operating condition analysis, the following details are determined:

[0113] The lower power limit must be sufficient to heat the specified minimum flow rate of urea solution (corresponding to the minimum safe ammonia injection rate) from the preheater outlet temperature to the temperature required for complete pyrolysis under the worst-case conditions (i.e., the initial stage of cold start-up) without any waste heat input from flue gas, and to compensate for all heat losses of the system under this condition.

[0114] Power limits and tiers: Generally, Designed for the total heat load required by the pyrolysis furnace under the rated load of the main unit (100% MCR) The power output can be 30% to 50% of the rated power. Multiple heating elements can be used to achieve graded power adjustment (e.g., 30%, 60%, 100%), thereby improving the electrothermal conversion efficiency and control accuracy under partial load.

[0115] The control unit automatically manages the coordination and switching between the main heat source (flue gas waste heat) and the auxiliary heat source (electric heater) based on preset logical criteria and algorithms. The control logic is divided into the following modes:

[0116] Mode 1: Cold Start / Pure Auxiliary Heating Mode

[0117] The entry condition is that the system start command is issued and the temperature of the flue gas at the inlet of the high-temperature flue gas heat exchanger is detected to be lower than the minimum start-up temperature threshold required by the first heat exchange medium (e.g., lower than 150°C), or the temperature of the working medium in the first circulation loop is lower than the safe feeding temperature of the pyrolysis furnace.

[0118] The control actions are as follows: turn on the auxiliary electric heater to the preset start-up power (e.g., 50% P_aux); start the first independent circulation loop circulation pump to circulate the working fluid; when the temperature inside the pyrolysis furnace is detected to reach the lower limit of the safe injection temperature of the urea solution (e.g., 280℃), start the urea solution delivery pump to supply the solution at a low flow rate; and adjust the output power of the auxiliary electric heater according to the deviation between the actual temperature inside the pyrolysis furnace and the set value, gradually stabilizing the temperature of the pyrolysis furnace at the target value (e.g., 350℃).

[0119] Mode 2: Hybrid Heating Transition Mode

[0120] The entry conditions are that the main unit load increases, the outlet working fluid temperature of the high-temperature flue gas heat exchanger continues to rise, and the heat it provides is sufficient to share part of the heat load.

[0121] The control action involves the control unit calculating in real time the theoretical heat that the waste heat from the flue gas can provide (based on flue gas temperature, flow rate, and parameters of the first circulating working fluid), and employing a heat differential control strategy: the instantaneous power setpoint of the auxiliary electric heater ( = Real-time total heat demand of pyrolysis furnace ( - Waste heat from flue gas can provide heat in real time ( With appropriate margin, the heating elements can be continuously adjusted (if controlled by a power regulator) or switched on and off in stages to ensure that... The above heat difference is dynamically tracked to ensure the temperature of the pyrolysis furnace remains stable;

[0122] In this mode, the proportion of waste heat contribution from flue gas gradually increases, while the proportion of electric auxiliary heating gradually decreases.

[0123] Mode 3: Standby mode with flue gas waste heat as the primary / auxiliary heat source

[0124] The entry condition is when the waste heat from the flue gas can be detected to provide more heat than the current total heat demand of the pyrolysis furnace. When a certain proportion (e.g., 105%) is reached, and this state is maintained stably for a period of time;

[0125] The control actions include reducing the power of the auxiliary electric heater to zero or maintaining a minimum antifreeze / standby power, and controlling the temperature of the pyrolysis furnace entirely by adjusting the flow rate of the working fluid in the first independent circulation loop (e.g., through a bypass valve).

[0126] The auxiliary electric heater remains powered on and ready to operate, while the control unit continuously monitors key parameters.

[0127] The switching between modes is equipped with hysteresis and delay to prevent frequent switching due to small fluctuations in parameters;

[0128] In any mode, if the temperature of the pyrolysis furnace is lower than the lower limit of safe operation or exceeds the upper limit, an advanced alarm will be triggered and preset protective actions will be executed (such as stopping the urea pump, increasing or cutting off the heating power).

[0129] The control logic communicates with the main unit load signal and the DCS system to achieve predictive control. For example, if it is anticipated that the main unit will significantly reduce its load, the auxiliary heating power can be increased slowly in advance.

[0130] 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. An energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization, characterized in that, include: Flue gas heat exchange subsystem, urea pyrolysis ammonia production subsystem and control unit; The flue gas heat exchange subsystem includes: A high-temperature flue gas heat exchanger is installed in the flue upstream of the SCR reactor; A low-temperature flue gas heat exchanger is connected in series in the flue downstream of the high-temperature flue gas heat exchanger; The first independent circulation loop, with the first heat exchange working medium circulating, connects the working medium side of the high-temperature flue gas heat exchanger to the heating unit of the urea pyrolysis furnace. The second independent circulation loop, with the second heat exchange working fluid circulation, connects the working fluid side of the low-temperature flue gas heat exchanger to the urea solution preheater. The urea pyrolysis ammonia production subsystem includes: a urea solution storage tank, a urea solution delivery pump, a urea solution preheater, a urea pyrolysis furnace, and a mixed gas delivery pipeline; the urea solution preheater uses heat provided by the second independent circulation loop to preheat the urea solution; the urea pyrolysis furnace is equipped with the heating unit inside, which pyrolyzes the preheated urea solution into a mixed gas of ammonia and carbon dioxide; the mixed gas delivery pipeline delivers the mixed gas to the inlet flue of the SCR reactor; The control unit monitors and adjusts the system operating parameters, and performs temperature control of the urea pyrolysis furnace, protection control of the low-temperature flue gas heat exchanger wall temperature, and coordinated switching control between the main heat source and the auxiliary heat source.

2. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The first independent circulation loop uses synthetic heat transfer oil or molten salt as the first heat exchange medium; the second independent circulation loop uses water or ethylene glycol aqueous solution as the second heat exchange medium.

3. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, A three-way mixing valve is provided on the second independent circulation loop to adjust the inlet temperature of the second heat exchange medium entering the low-temperature flue gas heat exchanger, so as to ensure that the metal wall temperature of the low-temperature flue gas heat exchanger is higher than the acid dew point temperature of the flue gas.

4. The urea pyrolysis ammonia production energy-saving system based on flue gas waste heat utilization according to claim 1, characterized in that, The system also includes an auxiliary electric heater; The auxiliary electric heater is installed in series on the pipeline of the first independent circulation loop, located between the outlet of the high-temperature flue gas heat exchanger and the inlet of the urea pyrolysis furnace heating unit, for directly heating the first heat exchange medium flowing through it; or the heating element of the auxiliary electric heater is directly built into the reaction chamber of the urea pyrolysis furnace or near the heat exchange coil / jacket.

5. The urea pyrolysis ammonia production energy-saving system based on flue gas waste heat utilization according to claim 4, characterized in that, The control unit is configured to execute control logic in at least one of the following operating modes: Cold start / pure auxiliary heating mode: When insufficient waste heat of flue gas is detected, the auxiliary electric heater is started and the first independent circulation loop is controlled to circulate until the temperature of the pyrolysis furnace reaches the set value; Hybrid heating transition mode: Real-time calculation of the heat that the waste heat of the flue gas can provide, and dynamic adjustment of the power of the auxiliary electric heater to make up for the difference in the real-time total heat demand of the pyrolysis furnace; Waste heat-dominated mode: When the waste heat of the flue gas can meet the needs of the pyrolysis furnace on its own, the power of the auxiliary electric heater is reduced to standby mode, and the temperature of the pyrolysis furnace is controlled entirely by adjusting the flow rate of the working fluid in the first independent circulation loop.

6. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The control unit has a built-in urea pyrolysis furnace temperature main control loop. This loop adopts a cascade control structure, with the temperature of the pyrolysis furnace reaction zone as the main controlled variable. Its output is used to distribute the load between the first independent loop and the second independent loop.

7. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The control unit has a built-in low-temperature flue gas heat exchanger wall temperature protection control loop. This loop calculates the acid dew point temperature online based on real-time monitoring of flue gas composition and temperature, and controls the inlet temperature of the second heat exchange medium entering the low-temperature flue gas heat exchanger by adjusting the opening of the three-way mixing valve in the second independent circulation loop, so as to ensure that the minimum metal wall temperature is higher than the acid dew point temperature plus a safety margin.

8. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The heat transfer areas of the high-temperature flue gas heat exchanger and the low-temperature flue gas heat exchanger are determined by matching calculation and optimization based on the heat load, working temperature range of the working fluid, logarithmic mean temperature difference and total heat transfer coefficient under the system design benchmark conditions, and are verified under different host load conditions.

9. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The control unit is configured to: determine the flue gas separation temperature point between the high-temperature flue gas heat exchanger and the low-temperature flue gas heat exchanger based on the flue gas parameter curves of the target host or boiler under different loads and the urea pyrolysis requirements, so as to optimize the cascade utilization of flue gas waste heat.

10. The energy-saving system for urea pyrolysis to ammonia production based on flue gas waste heat utilization according to claim 1, characterized in that, The control unit is pre-programmed with multi-level safety interlock logic, including: interlocking to shut down the urea solution delivery pump when the pyrolysis furnace temperature exceeds the limit; limiting or cutting off the waste heat recovery of flue gas when the wall temperature of the low-temperature heat exchanger approaches the safety boundary; and interlocking to shut down the auxiliary electric heater when the working fluid flow or pressure of the first circulation loop is abnormal.

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