A gas turbine flue gas alkaline denitrification system and control method

Abstract

This invention discloses a gas turbine flue gas denitrification system and control method. The system includes a heat exchanger, an oxidation device, a reaction unit, and a chimney connected sequentially along the flue gas flow direction. The core of the system lies in the control method: by acquiring the NO and NO2 concentrations at the outlet of the oxidation device, the degree of flue gas oxidation is calculated; simultaneously, the measured NOx concentration at the chimney outlet is acquired and compared with a set value. Based on the comparison result, the dosage of oxidizing agent in the oxidation device is dynamically adjusted, thereby controlling the degree of flue gas oxidation and ultimately stabilizing the emission concentration within a preset range. This invention achieves flexible and precise control of NOx emission concentration from gas turbines, effectively reducing the consumption of oxidant and alkali while ensuring compliance with emission standards, improving operational economy, and further addressing the need for lower NOx emission requirements throughout the entire operating cycle of gas turbine units. Furthermore, by correlating the degree of oxidation with the actual removal efficiency of the reactor, the operating status of the reactor can be evaluated, and regeneration and maintenance cycles can be optimized.

Description

Technical Field

[0001] This invention belongs to the field of flue gas denitrification technology, specifically relating to a gas turbine flue gas denitrification system and control method. Background Technology

[0002] With the increasing installed capacity of gas-fired power generation, the issue of nitrogen oxide (NOx) emission control has become increasingly prominent. During stable operation, gas-fired units can achieve NOx emission standards (e.g., below 35 mg / m³) through technologies such as low-NOx combustion. However, during start-up and shutdown, NOx concentrations can easily exceed the limits for short periods, causing "yellow smoke" from the chimney. This is a common environmental problem faced by gas-fired power plants and urgently needs to be addressed. Furthermore, with increasingly stringent environmental standards, gas-fired units may face even stricter NOx emission limits in the future.

[0003] To address this challenge, existing technical solutions all have significant limitations: 1) Selective Catalytic Reduction (SCR) technology retrofitting is costly and has poor adaptability: Although the technology is mature, for existing gas turbine units, the space inside the tail-end waste heat boiler is small, making it difficult and costly to retrofit with an SCR reactor. At the same time, the high humidity of gas turbine flue gas places more stringent requirements on the performance and service life of the catalyst, further increasing the technical complexity and operating costs.

[0004] 2) Other denitrification technologies are insufficient to meet the retrofitting needs of existing generating units: Dry / semi-dry denitrification: usually involves injecting alkaline adsorbents or slurries into flue gas. It has problems such as system complexity, by-product treatment, and potential equipment corrosion or blockage. In addition, it has strict requirements for operating conditions (such as temperature window). Its applicability under gas flue gas conditions needs to be verified.

[0005] Traditional wet denitrification: Although it has a high removal efficiency, it usually requires a large spray tower and a large amount of circulating liquid. The system is huge, energy-intensive, and produces wastewater that needs further treatment, posing a risk of secondary pollution and high investment and operating costs.

[0006] Therefore, for existing gas turbine units, there is an urgent need to develop a new type of denitrification system and intelligent control method that has relatively low retrofit costs, strong spatial adaptability, flexible and efficient operation, and can effectively control NOx emissions during start-up and shutdown, avoid "yellow smoke," and meet the lower emission requirements of gas turbine units throughout their entire operating cycle. Summary of the Invention

[0007] To address the above problems, the present invention aims to provide a gas turbine flue gas denitrification system and control method.

[0008] The specific technical solution is as follows: A gas turbine flue gas denitrification system includes a heat exchanger, an oxidation device, a reaction unit, and a chimney connected in sequence along the flue gas flow direction; The reaction unit includes a reactor filled with bed packing, an absorbent preparation device for supplying alkaline solution to the reactor, a rich solution circulation device, and a liquid film maintenance device located in the reactor. The rich liquor circulation device includes a rich liquor storage tank for collecting and storing the accumulated water from the bottom of the reactor; The liquid film maintaining device is used to supply a gas-liquid two-phase flow to the bed packing to maintain its surface wettability, and its liquid phase supply source can be selectively from a rich liquid storage tank or demineralized water.

[0009] A control method for a gas turbine flue gas denitrification system includes the following steps: S1. After being cooled by a heat exchanger, the flue gas from the gas turbine enters the oxidation unit. By introducing oxidizing substances into the oxidation unit, some of the NO in the flue gas is oxidized to NO2. S2. The oxidized flue gas enters the reactor and comes into contact with the alkaline solution on the surface of the moist bed packing to carry out the denitrification reaction; S3. Monitor the NO concentration X and NO2 concentration Y at the outlet of the oxidation unit in real time, and calculate the degree of oxidation Y / (X+Y). S4. Obtain the measured value N of NOx concentration at the chimney outlet, compare the measured value N with the preset value Z, and adjust the amount of oxidant added to the oxidation device according to the comparison result to control the degree of oxidation, thereby controlling the NOx emission concentration at the chimney outlet within the preset range.

[0010] Furthermore, in step S4, When N < ZC, the oxidation device is controlled to reduce the addition of oxidizing substances until it is shut down; When ZC≤N≤Z+C, maintain the current parameters of the oxidation unit. When N > Z + C, the oxidation device is controlled to increase the addition of oxidizing substances until N ≤ Z + C or the degree of oxidation reaches the preset upper limit. When the degree of oxidation reaches the preset upper limit, but N > Z + C, alkali solution is added to the reactor and the reactor is reminded to regenerate. C is the deviation value, which is determined by testing and the accuracy of online instruments.

[0011] Furthermore, when N > Z + C, the increase in oxides conforms to the following formula: A = (NZ) × η / β In the formula, A is the increase in oxides, ppm; N is the NOx concentration detected online at the chimney, ppm; Z is the preset value of chimney concentration, ppm; η is the test coefficient of the oxidation device, that is, the oxidation efficiency corresponding to the oxidizing substances in the oxidation device; β is the NO2 removal efficiency.

[0012] Preferably, when the oxidation degree is 0%~60%, the initial α=0%~15% and β=60%~95%; when the oxidation degree is 60%~80%, the initial α=5%~20% and β=70%~97%; when the oxidation degree is 80%~100%, the initial α=10%~20% and β=70%~100%. The initial α and β values ​​are determined by system debugging. After each subsequent system start-up and shutdown (the denitrification system starts and stops following the gas turbine unit, i.e., when the gas turbine unit starts, the denitrification system starts; when the predicted NOx concentration in the chimney flue gas is M=X·(1-α)+Y·(1-β), the gas turbine unit stops and the denitrification system stops), if |MN|≥C, then after confirming that there are no abnormalities in the online meters, refresh the device β, where β is the average value during operation, β=(Yy) / Y.

[0013] Preferably, when β = (Yy) / Y ≤ 70%, a reminder to regenerate the reactor or add alkali solution can be issued. If β is below 70%, but the oxidation degree has not reached the upper limit and the emission meets the standard (N ≤ Z+C), this situation may be due to a high preset value Z. In this case, only a reminder to regenerate or add alkali solution is issued, which is an option for professionals to choose based on actual needs and is not mandatory. However, when N > Z+C, alkali solution must be added to the reactor and a reminder to regenerate the reactor must be issued.

[0014] Furthermore, both the rich solution storage tank and the return pipeline connecting the rich solution storage tank to the reactor are equipped with pH detection devices. When the pH value detected by the pH detection device is not higher than 8.5, the rich solution is discharged and the reactor is regenerated with alkali solution.

[0015] Furthermore, the ratio of gas volumetric flow rate to bed volume within the reaction unit is ≤13000 h⁻¹. -1 .

[0016] Furthermore, the ratio of the liquid supply rate of the liquid film maintaining device to the flue gas volumetric flow rate is 0.01-0.02 kg / m³. 3 .

[0017] The beneficial effects of this invention are as follows: 1) This invention achieves flexible control of NOx emission concentration by controlling the degree of oxidation of flue gas, thereby reducing energy and material consumption; 2) This invention manages the effective denitrification cycle of the reactor by synergistically managing oxidation degree and denitrification effect, and manages the effective denitrification cycle of the reactor by monitoring the pH of the reflux liquid.

[0018] 3) This invention can effectively control NOx emissions during start-up and shutdown and avoid "yellow smoke". It can also be applied to denitrification throughout the entire operation cycle of gas turbine units to achieve lower nitrogen oxide emission requirements. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a gas turbine flue gas denitrification system; Figure 2 This is a schematic diagram of the reaction unit; Figure 3 This is a schematic diagram of the reactor; Figure 4 This is a schematic diagram of NOx concentration control methods; Figure 5 This is a schematic diagram of a rich liquid storage tank. Detailed Implementation

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] like Figure 1 As shown, a gas turbine flue gas denitrification system includes a heat exchanger, an oxidation device, a reaction unit, and a chimney connected sequentially along the flue gas flow direction. The flue gas from the gas turbine unit is discharged from a waste heat boiler, pressurized by an induced draft fan, and then enters the tubular heat exchanger. The flue gas temperature is reduced to 95℃~98℃ before entering the oxidation device. NO in the flue gas fully mixes and reacts with the oxidizing substances produced by the oxidation device to form NO2. The flue gas with altered composition then enters the reaction unit, where it undergoes a denitrification reaction after contacting an alkaline solution in the reactor. The purified flue gas is then discharged through the chimney.

[0022] The oxidation unit is a high-voltage discharge ozone generator that produces ozone from oxygen. The ozone generator can control the flue gas oxidation degree (NO2 / NOx ratio) within the range of 0% to 100%. The setpoint for the NOx concentration in the chimney exhaust is Z ppm, the detected NO concentration at the oxidation unit outlet is X ppm, the detected NO2 concentration is Y ppm, the NO removal efficiency is α, the NO2 removal efficiency is β, the predicted NOx concentration in the chimney exhaust is M = X·(1-α) + Y·(1-β), and the online NOx concentration detected at the chimney is N ppm, where the NO concentration is x ppm and the NO2 concentration is y ppm. When N < ZC, the oxidation unit is controlled to reduce the addition of oxidizing agents until it is shut down. When ZC≤N≤Z+C, the oxidation device operates at its current parameters. When N>Z+C, the oxidation device is controlled to increase the addition of oxidizing agents until N≤Z+C or the oxidation degree reaches the preset upper limit. The increase in oxides conforms to the following formula: A=(NZ)×η / β, where A is the increase in oxides, ppm; N is the NOx concentration detected online at the chimney, ppm; Z is the preset chimney concentration, ppm; η is the experimental coefficient of the oxidation device, i.e., the oxidation efficiency corresponding to the oxidizing agent in the oxidation device; β is the NO2 removal efficiency under the current flue gas oxidation degree range. When the oxidation degree reaches the preset upper limit, but N>Z+C, alkali solution is added to the reactor and the reactor is reminded to regenerate; C is the deviation value, determined by experiment and online instrument accuracy. In this embodiment, C is taken as 5.

[0023] like Figure 2 and Figure 3 As shown, the reaction unit includes a reactor filled with bed packing, an absorbent preparation device for supplying alkaline solution to the reactor, a rich solution circulation device, and a liquid film maintenance device located within the reactor. The reactor is a bed reactor, using activated carbon fiber as the bed packing material and fixed with metal wire mesh. In this embodiment, the gas volumetric flow rate to bed volume ratio within the reaction unit is 13000 h⁻¹. -1 The reactor contains a liquid film maintenance device that maintains bed moisture by atomizing liquid droplets through a gas-liquid two-phase flow nozzles. The gas phase is compressed air. The liquid phase source is demineralized water or a rich solution storage tank. Preferably, rich solution is used when there is solution in the rich solution tank; otherwise, demineralized water is used. The ratio of liquid supply rate to flue gas flow rate of the liquid film maintenance device is 0.01 kg / m³. 3 ~0.02kg / m 3 The reactor has an online liquid level monitoring device at the bottom to monitor the water accumulation at the bottom of the reactor.

[0024] The absorbent preparation device includes an alkali transfer pump and an alkali storage tank. It is prepared by diluting industrial alkali, and the alkali storage tank is equipped with a level gauge.

[0025] The rich solution circulation system includes a rich solution storage tank, an external discharge pump, and a rich solution pump. The solution in the rich solution storage tank originates from the return flow of water accumulated at the bottom of the reactor after unit shutdown or reactor shutdown, or from the return flow of water accumulated during reactor regeneration and cleaning. When the solution level in the rich solution storage tank falls below a certain level, the reactor's water supply is switched to demineralized water.

[0026] like Figure 5 As shown, both the reflux pipe and the main body of the rich solution storage tank are equipped with online pH meters to monitor the pH of the reflux liquid and the pH of the solution in the storage tank. When the pH of the reflux liquid is ≤8.5, the solution in the rich solution storage tank can be discharged and the reactor can be regenerated by alkaline spraying.

[0027] NOx concentration control methods such as Figure 4 As shown, it includes the following steps: S1. After being cooled by a heat exchanger, the flue gas from the gas turbine enters the oxidation unit. By introducing oxidizing substances into the oxidation unit, some of the NO in the flue gas is oxidized to NO2. S2. The oxidized flue gas enters the reactor and comes into contact with the alkaline solution on the surface of the moist bed packing to carry out the denitrification reaction; S3. Monitor the NO concentration X and NO2 concentration Y at the outlet of the oxidation unit and the NO concentration x and NO2 concentration y at the outlet of the chimney in real time, and calculate the degree of oxidation Y / (X+Y). S4. Obtain the measured NOx concentration N at the chimney outlet, compare the measured value N with the preset value Z, and adjust the amount of oxidant added to the oxidation unit according to the comparison result to control the degree of oxidation, thereby controlling the NOx emission concentration at the chimney outlet within the preset range. When N < ZC, control the oxidation unit to reduce the amount of oxidant added until it is shut down; when ZC ≤ N ≤ Z+C, maintain the current parameters of the oxidation unit; when N > Z+C, control the oxidation unit to increase the amount of oxidant added until N ≤ Z+C or the degree of oxidation reaches the preset upper limit value. When the degree of oxidation reaches the preset upper limit value, but N > Z+C, add alkali solution to the reactor and remind the reactor to regenerate.

[0028] Example 1 use Figure 1 and Figure 2 The system is configured with an oxidation degree of 0%–60%, initial α=0%, β=95%; for oxidation degrees of 60%–80%, initial α=5%, β=97%; and for oxidation degrees of 80%–100%, initial α=15%, β=98%. η is set to 1.01. The ratio of liquid supply to flue gas volume for the liquid film maintaining device is 0.01 kg / m³. 3 ~0.02kg / m 3 .

[0029] The online instrument obtained NO and NO2 measurement values ​​X=10.2 and Y=10.1, and calculated the predicted value M=10.7. The set control value Z=5.0, and the actual values ​​at the chimney outlet were x=10.0, y=0.5, and N=10.5. The actual NOx (i.e., N) > the set value Z+5, and the flue gas oxidation degree <95% (the preset upper limit of oxidation degree is 95%). After increasing the ozone injection by 5.8 ppm, the online instrument monitored the changes in flue gas composition X=4.1 and Y=16.1, and calculated the predicted value M=4.4. The actual values ​​at the chimney outlet were x=3.5, y=0.5, and N=4.0. At this point, (Z-5) < N ≤ (Z+5), and the system parameters were maintained. At this point, |MN| < 5, and β was not refreshed. The results are shown in Table 1.

[0030] Table 1. Control effect of the denitrification system in Example 1 (unit: ppm) .

[0031] Example 2 The online instrument obtained NO and NO2 measurement values ​​X=15.2 and Y=1.2, and calculated the predicted value M=15.3. The set control value Z=10.0. The actual values ​​at the chimney outlet were x=15.1, y=0.9, and N=16.0. Since the actual NOx (i.e., N) > the set value Z+5, the flue gas oxidation degree <95%. After increasing the ozone injection by 6.4 ppm, the online instrument monitored the flue gas composition changes as X=10.0 and Y=5.0, and calculated the predicted value M=10.3. The actual values ​​at the chimney outlet were x=9.5, y=0.2, and N=9.7. At this point, (Z-5) < N ≤ (Z+5), and the system parameters were maintained. Since |MN| < 5, β was not refreshed. The results are shown in Table 2.

[0032] Table 2. Control effect of the denitrification system in Example 2 (unit: ppm) .

Claims

1. A gas turbine flue gas denitrification system, characterized in that, It includes a heat exchanger, an oxidation device, a reaction unit, and a chimney connected in sequence along the flue gas flow direction; The reaction unit includes a reactor filled with bed packing, an absorbent preparation device for supplying alkaline solution to the reactor, a rich solution circulation device, and a liquid film maintenance device located in the reactor. The rich liquor circulation device includes a rich liquor storage tank for collecting and storing the accumulated water from the bottom of the reactor; The liquid film maintaining device is used to supply a gas-liquid two-phase flow to the bed packing to maintain its surface wettability, and its liquid phase supply source can be selectively from a rich liquid storage tank or demineralized water.

2. A control method for a gas turbine flue gas denitrification system as described in claim 1, characterized in that, Includes the following steps: S1. After being cooled by a heat exchanger, the flue gas from the gas turbine enters the oxidation unit. By introducing oxidizing substances into the oxidation unit, some of the NO in the flue gas is oxidized to NO2. S2. The oxidized flue gas enters the reactor and comes into contact with the alkaline solution on the surface of the moist bed packing to carry out the denitrification reaction; S3. Monitor the NO concentration X and NO2 concentration Y at the outlet of the oxidation unit in real time, and calculate the degree of oxidation Y / (X+Y). S4. Obtain the measured value N of NOx concentration at the chimney outlet, compare the measured value N with the preset value Z, and adjust the amount of oxidant added to the oxidation device according to the comparison result to control the degree of oxidation, thereby controlling the NOx emission concentration at the chimney outlet within the preset range.

3. The control method as described in claim 2, characterized in that, In step S4, When N < ZC, the oxidation device is controlled to reduce the addition of oxidizing substances until it is shut down; When ZC≤N≤Z+C, maintain the current parameters of the oxidation unit. When N > Z + C, the oxidation device is controlled to increase the addition of oxidizing substances until N ≤ Z + C or the degree of oxidation reaches the preset upper limit. When the degree of oxidation reaches the preset upper limit, but N > Z + C, alkali solution is added to the reactor and the reactor is reminded to regenerate. C is the deviation value, which is determined by testing and the accuracy of online instruments.

4. The control method as described in claim 3, characterized in that, When N > Z + C, the increase in oxide content conforms to the following formula: A = (NZ) × η / β In the formula, A is the increase in oxides, ppm; N is the NOx concentration detected online at the chimney, ppm; Z is the preset value of chimney concentration, ppm; η is the test coefficient of the oxidation device, that is, the oxidation efficiency corresponding to the oxidizing substances in the oxidation device; β is the NO2 removal efficiency.

5. The control method as described in claim 3, characterized in that, Both the rich solution storage tank and the return pipeline connecting the rich solution storage tank to the reactor are equipped with pH detection devices. When the pH value detected by the pH detection device is not higher than 8.5, the rich solution is discharged and the reactor is regenerated with alkali solution.

6. The control method as described in claim 3, characterized in that, The ratio of gas volumetric flow rate to bed volume in the reaction unit is ≤13000 h⁻¹ -1 .

7. The control method as described in claim 3, characterized in that, The ratio of the liquid supply rate to the flue gas volumetric flow rate of the liquid film maintaining device is 0.01-0.02 kg / m³. 3 .

Images