Method for controlling nitrogen oxides in the initial stage of a circulating fluidized bed boiler

Abstract

The present application relates to circulating fluidized bed production technical field, more specifically, it relates to a kind of circulating fluidized bed boiler and furnace initial nitrogen oxide control method, the fuel in coal hopper before fluidized bed boiler ignition is coal material and slag block mixture material. Boiler ignition and furnace after the first hour, nitrogen oxide emission value can be controlled in 50mg / Nm 3 NO X Emission value is significantly controlled after optimization, can fully meet the examination requirements of environmental protection electricity price. Reduce the amount of ammonia water, prevent the effect of air preheater ash deposition in tail flue, ensure that the boiler long period, full load safe and stable operation is remarkable.

Description

Technical Field

[0001] This invention relates to the field of circulating fluidized bed production technology, and more specifically, to a method for controlling nitrogen oxide emissions during the initial stage of circulating fluidized bed boiler operation. Background Technology

[0002] During combustion, circulating fluidized bed boilers produce nitrogen oxides. To control nitrogen oxide emissions from circulating fluidized bed boilers, existing technologies often introduce ammonia gas into the tail end of the furnace or into a cyclone separator, utilizing the selective reduction reaction of the NH2 groups produced by decomposition.

[0003] It reacts with NO and N2O to reduce nitrogen oxide emissions. However, within several hours after boiler ignition and commissioning, even with the ammonia injection rate adjusted to the maximum, it is impossible to control nitrogen oxide emissions within the ultra-clean emission level (≤50mg / Nm³). 3 Within a certain range, this seriously affects the assessment of environmental protection electricity prices.

[0004] Existing technologies also increase the thickness of the bottom material before ignition, so that the material layer thickness after the boiler is connected to the furnace is at a higher level, thereby allowing the temperature of the reaction zone to rise to the optimal reaction temperature in a shorter time. However, this approach has certain drawbacks. Because the material layer thickness is already high before ignition, more diesel fuel is consumed during the ignition process. Also, because the material layer thickness is high, the fan speed needs to be increased to ensure that the fluidized bed boiler reaches the fluidization state, which increases the plant power consumption required during the ignition process. Therefore, although this method can effectively reduce the amount of ammonia water used after the boiler is connected to the furnace, it is not economical.

[0005] Therefore, it is necessary to provide an economical and improved measure for controlling nitrogen oxide emissions from circulating fluidized bed boilers. Summary of the Invention

[0006] The present invention aims to provide a method for controlling nitrogen oxide emissions in the initial stage of ignition and commissioning of a circulating fluidized bed boiler, which can more effectively control nitrogen oxide emissions while reducing the amount of ammonia used in the initial stage after the boiler is ignited and commissioned.

[0007] The technical solution of the present invention to achieve the above objectives is as follows:

[0008] A method for controlling nitrogen oxide emissions during the initial stage of a circulating fluidized bed boiler, wherein the fuel in the coal hopper before ignition of the fluidized bed boiler is a mixture of coal and slag.

[0009] Preferably, the mass ratio of coal to slag is 2:1.

[0010] Preferably, the slag is slag with a particle size of 0-10 mm and a carbon content of less than 3%.

[0011] Preferably, the mixture is fed in a single operation.

[0012] Furthermore, it also includes the following steps:

[0013] S1. Preparations before ignition: Fill the coal hopper with fuel, prepare the electrical instrumentation and control system, fill the boiler with water, ensure the boiler inlet and outlet valves are open correctly, and ensure the circulating pump is running normally.

[0014] S2, Ignition and Operation:

[0015] (1) Preheating: Lay the bottom material at the bottom of the boiler, put in the ignition oil gun and preheat until the average bottom material temperature reaches more than 400℃ and is in a fluidized state;

[0016] (2) Coal feeding: The fuel in the coal hopper is added to the system for combustion by the coal feeder. The fluidization air volume is controlled, the bed temperature rises steadily, and the temperature is maintained at 850℃-900℃. Ignition is successful.

[0017] S3. Boiler Connection: Once the boilers meet the conditions for connection, prepare for connection.

[0018] Preferably, the bottom material in step S1 is fluidized bed slag that has not been saturated with water or become damp.

[0019] Preferably, the bottom material of the fluidized bed boiler in step S1 is slag with a particle size of 0-10 mm and a carbon content of less than 3%; the bottom material thickness is 800-900 mm.

[0020] Preferably, the base material in step S1 is laid evenly.

[0021] Preferably, the coal feeder in step S2 uses pulse coal feeding to add fuel to the system.

[0022] Preferably, after successful ignition in step S2, the fuel added to the coal hopper is coal.

[0023] The beneficial effects of this invention are:

[0024] This invention provides a method for controlling nitrogen oxide emissions during the initial stage of boiler commissioning in a circulating fluidized bed boiler. Before ignition, the fuel in the coal hopper of the fluidized bed boiler is a mixture of coal and slag. Within the first hour after boiler commissioning, nitrogen oxide emissions can be controlled to 50 mg / Nm³. 3 Within acceptable limits. Thermal nitrogen oxide emissions were significantly controlled after optimization, fully meeting the environmental electricity price assessment requirements, while simultaneously reducing energy consumption and improving economic efficiency. The reduction in ammonia water consumption significantly prevented ash accumulation in the tail flue air preheater, ensuring the boiler's long-term, full-load, safe, and stable operation. Attached Figure Description

[0025] Figure 1 Comparison chart of fuel consumption before and after ignition of slag blocks Detailed Implementation

[0026] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The specific embodiments of the present invention will be described in further detail below with reference to the examples. These examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0028] A method for controlling nitrogen oxide emissions during the initial stage of a circulating fluidized bed boiler is disclosed. Before ignition, the fuel in the coal hopper of the fluidized bed boiler is a mixture of coal and slag. The mass ratio of coal to slag is 2:1. The slag consists of slag with a particle size of 0-10 mm and a carbon content of less than 3%. The mixture is fed in a single operation.

[0029] Based on the above embodiments, a method for controlling nitrogen oxide emissions during the initial stage of parallel operation of a circulating fluidized bed boiler further includes the following steps:

[0030] S1. Preparations before ignition: Fill the coal hopper with fuel, prepare the electrical instrumentation and control system, fill the boiler with water, ensure the boiler inlet and outlet valves are open correctly, and ensure the circulating pump is running normally.

[0031] S2, Ignition and Operation:

[0032] (1) Preheating: Lay bottom material and ignition material at the bottom of the boiler, ignite the ignition material to preheat until the average bottom material temperature reaches above 400℃ and is in a fluidized state; the ignition material is bottom ash that has been screened.

[0033] (2) Coal feeding: The fuel in the coal hopper is added to the system for combustion by the coal feeder. The fluidization air volume is controlled, the bed temperature rises steadily, and the temperature is maintained at 850℃-900℃. Ignition is successful.

[0034] S3. Boiler Connection: Once the boilers meet the conditions for connection, prepare for connection.

[0035] In some embodiments of this application, the bottom material in step S1 is fluidized bed slag that has not been saturated with water or become damp.

[0036] In some embodiments of this application, the thickness of the bottom material of the fluidized bed boiler in step S1 is 600-900 mm.

[0037] In some embodiments of this application, the base material described in step S1 is laid evenly.

[0038] In some embodiments of this application, the coal feeder in step S2 uses pulse feeding to add fuel to the system. Four coal feeders take turns feeding the lowest amount of coal.

[0039] In some embodiments of this application, after successful ignition in step S2, the fuel added to the coal hopper is coal.

[0040] In this embodiment of the invention, before ignition, the fuel in the coal hopper is a mixture of coal and slag, with 40 tons of coal and 20 tons of slag, which is burned within about 5 hours after coal is added; the calorific value of the coal is about 4000 kcal; when the average bed temperature is 400°C, the fuel in the coal hopper is added to the system. At this time, the temperature is far from the ignition temperature of the coal, and only a small amount of volatile matter in the coal participates in combustion. At this stage, the bed temperature is still mainly increased by oil gun ignition.

[0041] The reasons for the excessive nitrogen oxide emissions and excessive ammonia injection in the initial stage after boiler ignition and commissioning are analyzed as follows:

[0042] The circulating fluidized bed boiler is model SG-240 / 9.81-M257, a high-temperature, high-pressure, single-drum, horizontally mounted, single-furnace, natural circulation, all-steel frame, π-type arrangement circulating fluidized bed boiler. The air preheater is a tubular type. The denitrification system consists of an ammonia storage system, an ammonia circulation and delivery system, a distribution module system, an injection system, a compressed air system, and a control system. Ammonia tank trucks deliver 20% ammonia water to the denitrification ammonia storage tank via an ammonia water unloading pump. The ammonia water is then injected into the furnace through a horizontal flue or wall-mounted injectors after passing through an ammonia circulation pump and metering and distribution devices.

[0043] The denitrification system uses SNCR (Selective Non-Catalytic Reduction), injecting 20% ​​ammonia water as a reducing agent into the boiler furnace to react with NO. X Selective reactions can be performed without a catalyst, therefore a reducing agent must be added in the high-temperature zone. The reducing agent is injected into the furnace at a temperature of 850–1100°C, where it rapidly decomposes into NH3, which reacts with NO in the flue gas. X The reaction produces nitrogen, carbon dioxide and water. The main reactions are: 4NO + 4NH3 + O2 → 4N2 + 6H2O; 6NO2 + 8NH3 → 7N2 + 12H2O.

[0044] NO in the actual production process of circulating fluidized bed boilers X The emission mechanism is as follows: When the boiler is in combustion operation, NO will be generated inside. X NO XBased on their different properties, nitrogen oxides can be divided into two types: fuel-type nitrogen oxides and thermal nitrogen oxides. To control the formation of thermal nitrogen oxides, the main approach is to focus on the temperature of the reaction zone. In the combustion process of a fluidized bed boiler, the emission of nitrogen oxides is related to the temperature of the reaction zone. Experiments show that the temperature of the reaction zone is most suitable when it is between 850℃ and 900℃, which can effectively reduce nitrogen oxide emissions.

[0045] In the initial stage after ignition and boiler connection, the bed temperature is too high due to the thin bed material and insufficient circulating ash in the fluidized bed boiler. Excessive bed temperature can easily cause coking on the bed surface of the fluidized bed boiler, which seriously affects the safe operation of the boiler. Therefore, in order to control the bed temperature in the initial stage of boiler connection, the boiler load is generally controlled at 150t / h. However, at low load, the temperature of the horizontal flue (denitrification ammonia water reaction zone) is low, generally around 750℃. At this time, the horizontal flue temperature deviates significantly from the optimal reaction temperature of 850℃, resulting in low denitrification efficiency. In some cases, even with the ammonia injection rate adjusted to the maximum, nitrogen oxide emissions cannot be effectively controlled.

[0046] In the initial stage of ignition and commissioning of the fluidized bed boiler, when the horizontal flue temperature was below 800℃, even with the ammonia injection rate adjusted to the maximum of 1050 L / h, the hourly average emission of nitrogen oxides still could not meet the requirements for ultra-clean emissions. As the temperature of the horizontal flue continued to rise until it reached 850℃, the ammonia injection rate was reduced to more than half of the maximum ammonia injection rate, and at this time the hourly average emission of nitrogen oxides was only 28, which fully met the requirements for ultra-clean emissions, as shown in Table 1. At the same time, it is clear from Table 1 that as the temperature of the boiler's horizontal flue gradually increased, the nitrogen oxide emission value gradually decreased. When the temperature of the horizontal flue (denitrification reaction zone) reached about 850℃, the ammonia water consumption had been reduced to less than half of the initial consumption, and the nitrogen oxide emission value at this time was far below the requirements for ultra-clean emissions. It can be concluded that the main reason for the excessive ammonia water consumption and the difficulty in controlling nitrogen oxide emissions in the initial stage of ignition and commissioning was the low temperature of the reaction zone.

[0047] Table 1

[0048]

[0049]

[0050] Before optimization measures were implemented, it typically took 6 to 7 hours after boiler ignition and commissioning to bring nitrogen oxide emissions within the ultra-clean emission requirements. In extreme cases, it could take up to 17 hours after commissioning, as shown in Table 2. However, the environmental electricity price assessment rules stipulate that the cumulative exceedance of the hourly average values ​​of sulfur dioxide, particulate matter, and nitrogen oxides cannot exceed 1% of the boiler's operating hours in the current quarter. Even assuming a full boiler cycle (90 days) in the current quarter, the maximum exceedance for these three indicators is 21 hours. If nitrogen oxides can only be controlled within 17 hours after commissioning, it will be difficult to secure environmental electricity prices for that quarter. Failure to meet the quarterly environmental electricity price assessment for a single boiler can result in losses of nearly one million yuan. With current adjustment methods, achieving ultra-clean nitrogen oxide emissions in the initial stage after boiler ignition and commissioning remains challenging. This significantly impacts the economic efficiency and safety of the manufacturer's equipment operation.

[0051] Table 2

[0052]

[0053]

[0054] In some embodiments of this application, in response to the above analysis of the reasons for the excessive nitrogen oxide emissions and excessive ammonia injection in the initial stage after boiler ignition and connection, the countermeasure adopted is to mix a certain proportion of slag blocks into the coal when feeding coal into the coal hopper before ignition.

[0055] Controlled variables: During the process of adding bottom material before ignition, the thickness of the bottom material should be strictly controlled at 850mm to avoid the impact caused by uneven thickness of the bottom material before ignition.

[0056] Before ignition, a certain proportion of slag is mixed into the coal during coal feeding in the coal hopper, using a coal-to-slag ratio of 2:1. For coals with high ash content, the amount of slag can be slightly reduced. The advantage of this method is that before coal feeding, the boiler bed temperature rises faster due to the relatively low bed thickness, resulting in relatively less diesel consumption. Furthermore, because of the low bed thickness, the fan speed does not need to be increased to a high level, saving diesel consumption and plant power during ignition. After the coal feeding conditions are met, the coal mixed with a certain proportion of slag enters the furnace, causing the bed thickness to rise to a higher level in a short time. During this stage, oil consumption increases significantly due to the increased bed thickness. However, the time required from reaching the average bed temperature to withdrawing the oil gun is not very long. After coal feeding, the oil pressure of the ignition oil is appropriately reduced according to the bed temperature, so the amount of diesel consumed is limited. To more intuitively compare the fuel consumption of the traditional ignition method and the slag-optimized ignition method, the fuel consumption before and after optimization was reviewed and compared.

[0057] Depend on Figure 1It is evident that after optimizing the ignition method with co-fired slag blocks, the fuel consumption did not increase significantly, and was even slightly less than before optimization. Of course, the difference in equipment condition affects the fuel consumption during ignition, but based on the current data analysis, the co-fired slag blocks have a very small impact on the fuel consumption during the ignition process, which can be ignored.

[0058] By controlling the aforementioned variables and adding slag, nitrogen oxide emissions can be controlled at 50 mg / Nm³ within the first hour after boiler ignition and commissioning. 3 Within [a certain range]; the ammonia water consumption after boiler ignition and connection was significantly lower than before optimization.

[0059] Comparing Table 3, the time required to control NOx emissions within ultra-clean emission levels before and after optimization shows that before optimization, NOx emissions exceeded the standard a total of 21 times within 10 hours after boiler commissioning. After optimization, NOx emissions exceeded the standard only once, and NOx emissions could be controlled to 50 mg / Nm³ within the first hour after boiler ignition and commissioning. 3 NOx emissions have been significantly controlled after optimization, fully meeting the requirements for environmental protection electricity pricing. After one year of operation, this alone can bring economic benefits of more than 7.1 million yuan.

[0060] Table 3

[0061]

[0062] Comparing the ammonia water consumption in Table 4 before and after optimization within 10 hours of boiler co-operation, it can be seen that the total ammonia water consumption of the three boilers was 25,517 L before optimization, while after optimization, the ammonia water consumption was only 11,155 L, a decrease of 56%. Combining Tables 3 and 4, with the ammonia water consumption reduced by 56%, the number of hours exceeding NOx emission standards decreased by 95.2%, demonstrating significant effectiveness. While ensuring compliance with environmental emission standards, it also brings certain economic benefits. For example, in a specific application case: with 7 boiler ignitions per year, the cumulative reduction in ammonia water consumption within 10 hours of boiler co-operation after optimization is approximately 40 tons. With the current market price of 700 yuan per ton of ammonia water, this translates to a reduction of approximately 28,000 yuan in ammonia water consumption annually. The reduction in ammonia water consumption is particularly significant in preventing ash accumulation in the tail flue air preheater and ensuring the long-term, full-load, safe, and stable operation of the boiler.

[0063] Table 4

[0064]

[0065] By adding slag blocks during coal feeding, the air chamber pressure is already at a high level when the boiler is connected to the grid, and the bed temperature is effectively controlled. This gives the boiler a high load-bearing capacity. As shown in Table 5, the main steam flow rate of the optimized boiler is about 50% higher than that before optimization when the boiler is connected to the grid. Within 24 hours after the boiler is connected to the grid, the optimized boiler can produce about 2,731 tons more steam than before optimization. This translates to an additional 682,000 kilowatt-hours of electricity. Each kilowatt-hour can generate a profit of about 80 yuan. In other words, after optimizing the boiler, an additional 1,590,000 kilowatt-hours of electricity can be generated annually, generating a profit of about 127,000 yuan.

[0066] Table 5

[0067]

[0068]

[0069] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for controlling nitrogen oxide emissions during the initial stage of parallel operation of a circulating fluidized bed boiler, characterized in that: Before the fluidized bed boiler is ignited, the fuel in the coal hopper is a mixture of coal and slag. It also includes the following steps: S1. Preparations before ignition: Fill the coal hopper with fuel, prepare the electrical instrumentation and control system, fill the boiler with water, ensure the boiler inlet and outlet valves are open correctly, and ensure the circulating pump is running normally. S2, Ignition and Operation: (1) Preheating: Lay the bottom material at the bottom of the boiler, put in the ignition oil gun and preheat until the average bottom material temperature reaches more than 400℃ and is in a fluidized state; (2) Coal feeding: The fuel in the coal hopper is added to the system for combustion by the coal feeder. The fluidization air volume is controlled, the bed temperature rises steadily, and the temperature is maintained at 850℃-900℃. Ignition is successful. S3. Boiler Connection: Once the boilers meet the conditions for connection, prepare for connection. The bottom material mentioned in step S1 is fluidized bed slag that has not been soaked in water or become damp; The bottom material of the fluidized bed boiler in step S1 is slag with a particle size of 0-10 mm and a carbon content of less than 3%; the bottom material thickness is 800-900 mm. The base material described in step S1 is laid evenly; The mass ratio of coal to slag is 2:1; The slag lumps are slag with a particle size of 0-10 mm and a carbon content of less than 3%. The mixture is fed in a single operation. The coal feeder described in step S2 uses pulse coal feeding to add fuel to the system.

2. The method for controlling nitrogen oxide emissions during the initial stage of a circulating fluidized bed boiler as described in claim 1, characterized in that: After successful ignition in step S2, the fuel added to the coal hopper is coal.

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