A circulating fluidized bed boiler low load full working condition denitration coordinated control system

Abstract

The present application relates to circulating fluidized bed boiler flue gas treatment technical field, specifically for a kind of circulating fluidized bed boiler low load full operating condition denitration coordination and control system, by setting up scalable electric heating unit on the connecting flue from separator to tail flue, it can be in the low load of boiler and the stage of starting quickly the temperature of flue gas is promoted to the effective working temperature (>=280 DEG C) of SCR denitration device, and when the inlet smoke temperature of SCR denitration device is too low, most or all of the working medium of superheater and economizer as two-stage heating surface can be switched to bypass, so that the heat absorption of heating surface tends to zero, eliminate the temperature drop problem of flue gas before reaching SCR device, ensure that the inlet smoke temperature of SCR is stably above 280 DEG C, so that SCR denitration system can be normally put into operation in full load range, fundamentally solve the problem that NOx emission cannot be controlled at low load stage.

Description

Technical Field

[0001] This invention relates to the field of flue gas treatment technology for circulating fluidized bed boilers, specifically to a denitrification synergistic control system for circulating fluidized bed boilers under low load and full operating conditions. Background Technology

[0002] Circulating fluidized bed (CFB) boilers are widely used in the power and industrial heating sectors due to their advantages such as wide fuel adaptability, high combustion efficiency, and good desulfurization efficiency. During startup and low-load operation, the flue gas temperature at the separator outlet of a CFB boiler is typically only 150℃–200℃, far below the effective operating temperature window required by SCR denitrification devices (generally 300℃–400℃). Although the flue gas temperature can be raised to above 280℃ using flue gas temperature regulating devices (such as electric heating devices), the flue gas must still flow through a superheater and economizer before reaching the SCR denitrification device. The temperature of the flue gas typically drops by 30℃–80℃ after passing through the superheater, and then drops another 20℃–50℃ after passing through the economizer. This means that even if the flue gas temperature regulation device has heated the flue gas to 280°C, after passing through two stages of heating surfaces, the flue gas temperature reaching the SCR inlet may drop to 150°C to 200°C, which is still lower than the minimum effective operating temperature of the SCR catalyst (generally 280°C to 300°C). The SCR catalyst activity is insufficient, the denitrification efficiency drops sharply and cannot meet the emission requirements, and the NOx emission is difficult to meet the ultra-low emission requirements.

[0003] To address the denitrification problem during low-load periods, the following solutions are typically employed: (1) Flue gas recirculation: High-temperature flue gas is drawn back and mixed with low-temperature flue gas. However, there is no high-temperature flue gas to be circulated during the start-up phase of the CFB boiler. Moreover, this scheme has high energy consumption and complex control.

[0004] (2) Economizer water bypass: Only the economizer is equipped with a water bypass, but the superheater continues to absorb heat, and the water in the economizer cannot be drained, so the problem of flue gas temperature drop is not fundamentally solved.

[0005] (3) Delay the commissioning of SCR: wait for the boiler load to increase before putting SCR into operation, but this contradicts the trend of gradually canceling the NOx emission exemption policy.

[0006] Therefore, there is an urgent need to develop a device that can rapidly and accurately increase the flue gas temperature in the flue section between the separator outlet and the SCR inlet, so that the SCR denitrification system can enter the effective operating temperature range even at low loads. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a low-load, full-condition denitrification coordinated control system for circulating fluidized bed boilers. This system can rapidly raise the flue gas temperature from the separator outlet to the tail flue connection section to the effective operating temperature of the SCR device during boiler startup and low-load operation. In conjunction with the medium bypass flow control system of the superheater and economizer, it can achieve effective NOx removal during this stage.

[0008] The technical solution is as follows: a low-load, full-condition denitrification and synergistic control system for a circulating fluidized bed boiler, comprising a separator, a connecting flue, and a tail flue. A superheater, an economizer, and an SCR denitrification device are sequentially arranged within the tail flue. The connecting flue is characterized by having a retractable electric heating unit installed within it. The superheater is connected to a superheater working fluid bypass system, which includes a superheater main pipeline, a superheater bypass pipeline, a superheater main pipeline shut-off valve and a superheater main pipeline regulating valve, and a superheater bypass shut-off valve and a superheater bypass regulating valve, all installed on the superheater bypass pipeline. The economizer is connected to an economizer working fluid bypass system, which includes an economizer main pipeline, an economizer bypass pipeline, and an economizer main pipeline shut-off valve and an economizer main pipeline regulating valve. The system includes a drain valve, an economizer bypass shut-off valve, and an economizer bypass regulating valve installed on the economizer bypass pipeline. When the outlet flue gas temperature of the separator is below 280°C, the electric heating unit extends into the connecting flue to heat the flue gas. When the inlet flue gas temperature of the SCR denitrification device is below the lower limit, the superheater bypass shut-off valve and the economizer bypass shut-off valve are opened, and the superheater main regulating valve and the economizer main regulating valve are closed or shut off, so that the working fluid flow rate in the superheater and the economizer approaches zero. When the inlet flue gas temperature of the SCR denitrification device returns to the set upper limit, the superheater bypass shut-off valve and the economizer bypass shut-off valve are closed, the superheater main shut-off valve and the economizer main shut-off valve are opened, and the superheater main regulating valve and the economizer main regulating valve are opened at a preset speed.

[0009] A further feature is that the electric heating unit includes heating gun bodies arranged in an array, and the end of the heating gun body is equipped with a heating wire and covered with a high-temperature resistant ceramic insulation layer. The heating wire is made of nickel-chromium alloy or iron-chromium-aluminum alloy, and the high-temperature resistant ceramic insulation layer is made of alumina ceramic or silicon nitride ceramic. The heating gun body penetrates through the top of the connecting flue and is exposed at the end of the heating gun body in the connecting flue, and is connected to an electric lifting drive mechanism. The diameter of the superheater bypass pipe and the economizer bypass pipe shall not be less than 70% of the diameter of the corresponding main pipe to ensure that the bypass can carry the full working fluid flow.

[0010] By adopting this invention, a retractable electric heating unit can be installed on the connecting flue from the separator to the tail flue, which can quickly raise the flue gas temperature to the effective operating temperature (≥280℃) of the SCR denitrification device during the boiler's low load and start-up phases. Furthermore, when the inlet flue gas temperature of the SCR denitrification device is too low, most or all of the working fluid of the superheater and economizer, which serve as the two-stage heating surfaces, can be switched to the bypass, making the heat absorption of the heating surfaces approach zero. This eliminates the problem of temperature drop of the flue gas before reaching the SCR device, ensuring that the SCR inlet flue gas temperature is stable above 280℃. This allows the SCR denitrification system to operate normally within the full load range, fundamentally solving the problem of uncontrollable NOx emissions during the low load phase. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the system of the present invention; Figure 2 for Figure 1 Top view (electric heating unit 7 is hidden); Figure 3 A schematic diagram showing the connection of an electric lifting drive mechanism to the electric heating unit. Detailed Implementation

[0012] See Figures 1 to 3 As shown, a low-load, full-condition denitrification coordinated control system for a circulating fluidized bed boiler includes a separator 1, a connecting flue 2, and a tail flue 3. A superheater 4, an economizer 5, and an SCR denitrification device 6 are sequentially arranged in the tail flue 3. A retractable electric heating unit 7 is installed in the connecting flue 2. The electric heating unit 7 includes heating gun bodies 7-1 arranged in an array. Heating wires 7-2 are installed at the ends of the heating gun bodies 7-1 and covered with a high-temperature resistant ceramic insulation layer. The heating wires are made of nickel-chromium alloy or iron-chromium-aluminum alloy. The thickness of the high-temperature resistant ceramic insulation layer is 3mm to 8mm, and the material is alumina ceramic or silicon nitride ceramic, with a temperature resistance of not less than 1200℃.

[0013] A mounting hole 7-3 is opened at the top of the connecting flue 2. A high-temperature resistant sealing structure is set at the mounting hole 7-3. The heating gun body 7-1 penetrates the top of the connecting flue and is exposed at the end of the heating gun body in the connecting flue. It is connected to an electric lifting drive mechanism 7-4. A high-temperature resistant sealing layer 7-5 is set at the penetration point. An electric push rod device can be used, which can be remotely driven under the control of the DCS system. When in use, the heating gun body 7-1 extends into the connecting flue 2 to heat the flue gas. When out of use, it is removed from the connecting flue 2. This avoids the continuous scouring and oxidation of the heating element by the high-temperature flue gas, significantly extends the service life of the equipment, and reduces the flue pressure loss during high load periods.

[0014] Taking a 300MW circulating fluidized bed boiler as an example. During the boiler start-up phase, the flue gas temperature at the separator outlet is about 50℃, and the flue gas volume is about 100,000 Nm³ / h (standard conditions). The flue gas temperature needs to be increased to 280℃ to meet the minimum operating temperature requirements of the SCR denitrification unit.

[0015] Step 1: Calculate the required heating power Qheat Flue gas mass flow rate: Qm = 100000 × 1.3 / 3600 ≈ 36.11 kg / s Specific heat capacity of flue gas at constant pressure: Cp = 1.1 kJ / (kg·℃) Target temperature rise: ΔT = 280 − 50 = 230 ℃ Required heating power: Qheat = Qm × Cp × ΔT = 36.11 × 1.1 × 230 = 9135.83 kW Step 2: Calculate the number N of electric heating units. A single electric heating gun with a rated power of P = 100 kW and a heating efficiency of η = 0.85 was selected. N=9135.83 / (100×0.85)≈107.5 A 300MW circulating fluidized bed boiler typically has three connecting flues, with 36 electric heating guns installed in each flue. The electric heating guns are arranged in a 6-row × 6-column array along the top of each flue.

[0016] For a 135MW class CFB boiler, the flue gas volume during the start-up phase is approximately 50,000 Nm³ / h, and the flue gas temperature is approximately 50℃, with a target temperature rise to 280℃. Using the same calculation method, approximately 72 heating guns with a single power of 75kW are required. There are two flue ducts, and the electric heating guns at the top of each flue are arranged in 6 rows × 6 columns.

[0017] Operating Process: During boiler ignition and startup, when the DCS system detects that the flue gas temperature at the separator outlet is below 280℃, it automatically issues a command, and the electric lifting drive mechanism controls the electric heating unit to extend into the connecting flue gas duct to begin heating the flue gas. When the flue gas temperature reaches above 280℃, the SCR denitrification device starts operating. As the boiler load increases and the flue gas temperature naturally rises to the effective operating range of the SCR, the DCS system removes the electric heating unit from the flue gas duct and enters standby mode to avoid unnecessary energy consumption and equipment damage.

[0018] The superheater 4 is connected to the superheater working fluid bypass system, which includes the superheater main pipeline 8, the superheater bypass pipeline 9, the superheater main pipeline shut-off valve 10 and the superheater main pipeline regulating valve 11 installed on the superheater main pipeline 8, and the superheater bypass shut-off valve 12 and the superheater bypass regulating valve 13 installed on the superheater bypass pipeline 9. The economizer 5 is connected to the economizer working fluid bypass system, which includes the economizer main pipeline 14, the economizer bypass pipeline 15, the economizer main pipeline shut-off valve 16 and the economizer main pipeline regulating valve 17 and the drain valve 18 installed on the economizer main pipeline 14, and the economizer bypass shut-off valve 19 and the economizer bypass regulating valve 20 installed on the economizer bypass pipeline 15.

[0019] The superheater main circuit shut-off valve 10 and the economizer main circuit shut-off valve 16 remain fully open under normal operating conditions. When the inlet flue gas temperature of the SCR denitrification unit 6 is lower than the lower limit (e.g., 280℃), open the superheater bypass shut-off valve 12 and the economizer bypass shut-off valve 19, and close the superheater main circuit regulating valve 11 and the economizer main circuit regulating valve 17 at a preset speed (e.g., 30s-60s), or close the superheater main circuit shut-off valve 10 and the economizer main circuit shut-off valve 16. This brings the working fluid flow rate in the superheater 4 and economizer 5 close to zero, and the heat absorption of the heating surface also close to zero, thereby eliminating the temperature drop of the flue gas when passing through the superheater 4 and economizer 5, and ensuring that the inlet flue gas temperature of the SCR denitrification unit 6 meets its minimum operating temperature requirement. When the economizer main circuit shut-off valve 16 needs to be completely closed, the drain valve 18 needs to be opened simultaneously.

[0020] When the inlet flue gas temperature of the SCR denitrification unit 6 returns to the set upper limit (e.g., 300℃), the superheater bypass shut-off valve 12 and the economizer bypass shut-off valve 19 are closed, the superheater main shut-off valve 10 and the economizer main shut-off valve 16 are opened, and the superheater main regulating valve 11 and the economizer main regulating valve 17 are opened at the preset speed.

[0021] The diameter of the superheater bypass pipeline and the economizer bypass pipeline shall not be less than 70% of the diameter of the corresponding main pipeline to ensure that the bypass can carry the full working fluid flow.

[0022] The following are specific examples to illustrate this.

[0023] Taking a 135MW circulating fluidized bed boiler as an example. During the boiler start-up phase, the flue gas temperature at the separator outlet is approximately 50℃, which is then heated to 280℃ by the electric heating unit. The flue gas then flows sequentially through the superheater and economizer before reaching the SCR denitrification unit.

[0024] The flue gas temperature drop during normal operation (without bypass) is shown in the table below. The inlet flue gas temperature of the SCR denitrification unit is only 185℃, which is far below the minimum operating temperature of 280℃, making it unable to effectively denitrify.

[0025] Then, the superheater working fluid bypass system and the economizer working fluid bypass system are put into operation. The superheater bypass shut-off valve 12 and the economizer bypass shut-off valve 19 are opened, the superheater main circuit regulating valve 11 and the economizer main circuit regulating valve 17 are closed, the superheater main circuit shut-off valve 10 and the economizer main circuit shut-off valve 16 are closed, and the drain valve 18 is opened to make the working fluid flow in the superheater 4 and the economizer 5 approach zero. The superheater bypass regulating valve 13 and the economizer bypass regulating valve 20 are opened to allow all the working fluid to flow through the bypass.

[0026] The temperature drop of the flue gas at this time is shown in the table below. The inlet flue gas temperature of the SCR denitrification unit reaches 275℃, which basically meets the working temperature requirements and can be put into normal operation for denitrification.

[0027] The control of the active regulating valve and the bypass regulating valve is as follows, taking the superheater as an example, setting the total working fluid flow rate as Q0. During the switching process, the main regulating valve closes slightly and the bypass regulating valve opens slightly simultaneously, maintaining a constant total flow rate and avoiding sudden changes in flow. The switching time is set to 30-60 seconds, and the rate can be adjusted by the DCS system according to actual operating conditions. When the bypass regulating valve is fully open but the flow rate is still insufficient (e.g., due to bypass pipe diameter limitations), the main shut-off valve is directly closed to ensure that all working fluid flows through the bypass.

[0028] Taking a 120t / h CFB boiler superheater as an example, the design of the superheater bypass piping is as follows: This invention employs a synergistic scheme of "front-end electric heating actively raising flue gas temperature + rear-end dual-stage working fluid bypass locking flue gas temperature." By simultaneously setting up a working fluid bypass system for both the superheater and economizer, when the electric heating unit is activated, all working fluids on both stages of the heating surface are switched to the bypass, making the heat absorption of the heating surface approach zero. This fundamentally eliminates the temperature drop problem of flue gas before reaching the SCR unit, ensuring that the SCR inlet flue gas temperature remains stable above 280℃. Both the main line and the bypass are equipped with shut-off valves and regulating valves, enabling smooth switching of the main line and bypass flow rates, avoiding water hammer or thermal stress caused by sudden changes in working fluid flow, and protecting the heating surface safety. This invention constitutes a complete low-load denitrification solution, enabling CFB boilers to achieve effective NOx control during startup and low-load phases, meeting increasingly stringent emission policy requirements.

Claims

1. A low-load, full-condition denitrification synergistic control system for a circulating fluidized bed boiler, comprising a separator, a connecting flue, and a tail flue, wherein a superheater, an economizer, and an SCR denitrification device are sequentially arranged within the tail flue, characterized in that, A retractable electric heating unit is installed inside the connecting flue. The superheater is connected to a superheater working fluid bypass system, which includes a superheater main pipeline, a superheater bypass pipeline, a superheater main pipeline shut-off valve and a superheater main pipeline regulating valve, and a superheater bypass pipeline shut-off valve and a superheater bypass pipeline regulating valve. The economizer is connected to an economizer working fluid bypass system, which includes an economizer main pipeline, an economizer bypass pipeline, an economizer main pipeline shut-off valve, an economizer main pipeline regulating valve and a drain valve, and an economizer bypass pipeline shut-off valve and a economizer bypass pipeline regulating valve. When the separation... When the outlet flue gas temperature of the device is below 280°C, the electric heating unit extends into the connecting flue to heat the flue gas; when the inlet flue gas temperature of the SCR denitrification device is below the lower limit, the superheater bypass shut-off valve and the economizer bypass shut-off valve are opened, and the superheater main circuit regulating valve and the economizer main circuit regulating valve are closed or closed, so that the working fluid flow rate in the superheater and the economizer approaches zero; when the inlet flue gas temperature of the SCR denitrification device recovers to the set upper limit, the superheater bypass shut-off valve and the economizer bypass shut-off valve are closed, the superheater main circuit shut-off valve and the economizer main circuit shut-off valve are opened, and the superheater main circuit regulating valve and the economizer main circuit regulating valve are opened at a preset speed.

2. The circulating fluidized bed boiler low-load full-condition denitrification synergistic control system according to claim 1, characterized in that, The electric heating unit includes heating gun bodies arranged in an array, with heating wires installed at the ends of the heating gun bodies and covered with a high-temperature resistant ceramic insulation layer.

3. The circulating fluidized bed boiler low-load full-condition denitrification synergistic control system according to claim 2, characterized in that, The heating wire is made of nickel-chromium alloy or iron-chromium-aluminum alloy, and the high-temperature resistant ceramic insulation layer is made of alumina ceramic or silicon nitride ceramic.

4. The circulating fluidized bed boiler low-load full-condition denitrification synergistic control system according to claim 2, characterized in that, The heating gun body extends through the top of the connecting flue and is exposed at the end of the heating gun body in the connecting flue, and is connected to an electric lifting drive mechanism.

5. The circulating fluidized bed boiler low-load full-condition denitrification synergistic control system according to claim 1, characterized in that, The diameter of the superheater bypass pipe and the economizer bypass pipe shall not be less than 70% of the diameter of the corresponding main pipe to ensure that the bypass can carry the full working fluid flow.

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