SNCR (selective non-catalytic reduction) denitration agent reduction method for large circulating fluidized bed boiler

Abstract

The invention discloses an SNCR (selective non-catalytic reduction) denitration agent reduction method for a large-scale circulating fluidized bed boiler, which comprises the following steps of: adjusting primary and secondary air rates, adjusting upper and lower secondary air and adjusting flue gas recirculation in the operation process of the large-scale circulating fluidized bed boiler in a boiler starting period, a load increasing period, a load stabilizing period and a load reducing period; and the average oxygen content of the boiler is controlled to be 2.5-3.5%, and the single-point oxygen content is not allowed to be lower than 2%. And the original emission of boiler nitrogen oxide indexes is reduced, so that the input amount of the required denitration agent is reduced.

Description

Technical Field

[0001] This invention belongs to the technical field of thermal power generation and relates to a method for reducing the amount of SNCR denitrification agent in large circulating fluidized bed boilers. Background Technology

[0002] The 300MW (CFB) large-scale circulating fluidized bed boiler employs SNCR denitrification technology. The denitrification agent is a mixture of 20% ammonia water and dilution water at a 1:3 ratio, which is then injected into the boiler separator inlet flue and the dense phase zone of the furnace for denitrification. Under the trend of grid spot trading, the unit's annual load rate is low, leading to a decrease in boiler evaporation, a reduction in furnace bed temperature, excess oxygen, and a significant increase in raw nitrogen oxide emissions. In addition, the need to balance ultra-low sulfur dioxide emission targets has increased the amount of limestone powder added to the furnace, further increasing the difficulty of controlling ultra-low nitrogen oxide emissions. This has resulted in long-term problems such as high denitrification agent dosage, increased ammonia escape, and corrosion of the tail heating surfaces and air preheater pipelines, seriously affecting the boiler's combustion economy and safety. Summary of the Invention

[0003] To address the above problems, this invention proposes a method for reducing the amount of SNCR denitrification agent in large-scale circulating fluidized bed boilers, which effectively solves the problems in the prior art.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler includes:

[0006] Boiler start-up period:

[0007] S11. After starting coal feeding, reduce the ratio of primary and secondary air volume to maintain the boiler operating at low oxygen levels.

[0008] S12. When the bed temperature inside the furnace reaches 650℃ or above, start the denitrification system and add denitrification agent into the furnace.

[0009] S13. When the flue gas temperature at the boiler separator outlet reaches 730℃ or above, the denitrification system switches to the separator to add denitrification agent.

[0010] Load increase period:

[0011] S31. When the evaporation rate reaches 430t / h or more, the flue gas recirculation system shall be shut down.

[0012] S32 and nitrogen oxide levels were reduced to below 45 mg / Nm³. 3 At the same time, open the left and right upper and lower secondary main air and small air dampers to increase the output of the secondary air fan and maintain the boiler oxygen content at 2.5~3.5%, and the oxygen content at a single point shall not be lower than 2%;

[0013] S33 and nitrogen oxide levels rose to over 45 mg / Nm³. 3 Then, the flue gas recirculation system should be put back into operation to maintain the boiler oxygen content at 2.5-3.5%, and the oxygen content at any single point should not be lower than 2%.

[0014] Stable load period:

[0015] S41. Adjust the opening of the coal feeding pipe gate of the boiler coal feeding system, control the bed temperature to 730-930℃, control the temperature deviation between the two sides of the bed to 0-30℃, the temperature difference of the lower bed at each point on the same side to 0-50℃, and the temperature of the separator outlet shall not exceed 1030℃.

[0016] Load reduction period:

[0017] S51. When the evaporation rate reaches below 430t / h, the recirculation fan shall be activated to maintain the oxygen content at a single point in the boiler at no less than 2%.

[0018] S52. Adjust the ratio of primary and secondary air volume, reduce the primary air volume and the total secondary air volume, reduce the output of the secondary fan, and adjust the pressure of the secondary air main pipe.

[0019] S53. After the boiler oxygen level stabilizes, put the flue gas recirculation system into operation and adjust the average oxygen level of the boiler to 2.5-3.5%. The oxygen level at any single point must not be lower than 2%.

[0020] Optionally, the boiler startup process further includes:

[0021] S14. Adjust the opening of the regulating valve on the separator branch pipe to make the flow rate of the separator branch pipe the same.

[0022] Optionally, it also includes the period before the load increase:

[0023] S21. Adjust the amount of limestone and ammonia water injected, monitor the change curves of the three pollutant parameters, and control the instantaneous value and hourly average value of pollutant emission concentration to be below 80% of the standard limit.

[0024] When sulfur dioxide and nitrogen oxides in flue gas show a downward trend, increase the air supply and coal feed, and control the oxygen content in the flue gas to 2.5%~3.5%.

[0025] Optionally, the load reduction period further includes:

[0026] S54, nitrogen oxide levels are below 45 mg / Nm³ 3 At this time, reduce the opening of the flue gas recirculation valve to the primary air fan inlet, decrease the operating frequency of the recirculation fan, and control the average oxygen content of the boiler to 2.5~3.5%;

[0027] S55. Without shutting down the flue gas recirculation system, open the left and right upper secondary main air dampers and small air dampers to increase the pressure of the secondary air header and control the average oxygen content of the boiler to 2.5~3.5%.

[0028] Optionally, the load stabilization period further includes:

[0029] S42. When using calcium injection for desulfurization, reduce the primary air volume, control the oxygen content to 2.8%~3.5%, and put it into the flue gas recirculation system.

[0030] S43. Adjust the ratio of ammonia water to dilution water in the denitrification system to 1:3, increase the outlet pressure of the dilution water pump, and improve the atomization effect.

[0031] Optionally, it also includes a coal feeding system failure combustion adjustment period: control the boiler bed temperature rise rate to be no more than ±3℃ / min, put the flue gas recirculation system into operation, fully open the flue gas recirculation valve at the inlet of the primary air fan on the operating side of one of the two coal feeding systems, and close the flue gas recirculation valve at the inlet of the primary air fan on the operating side of the other coal feeding system to 10~20%.

[0032] Optionally, S12 further includes adjusting the amount of denitrifying agent added to the furnace according to the changes in nitrogen oxide index, and stopping the operation of the denitrification system when the nitrogen oxide index does not show a downward trend after the denitrifying agent is added.

[0033] Compared with the prior art, the present invention has the following beneficial effects: during the operation of a large circulating fluidized bed boiler, by adjusting the primary and secondary air rates, upper and lower secondary air, and flue gas recirculation at different times, the original emissions of nitrogen oxides in the boiler can be reduced, and the required amount of denitrification agent can be reduced. Detailed Implementation

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0035] This invention discloses a method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler, comprising the following steps.

[0036] Boiler start-up period:

[0037] S11. After starting coal feeding, reduce the ratio of primary and secondary air volume to maintain the boiler operating at low oxygen levels.

[0038] S12. When the bed temperature inside the furnace reaches 650℃ or above, start the denitrification system and add denitrification agent into the furnace.

[0039] S13. When the flue gas temperature at the boiler separator outlet reaches 730℃ or above, the denitrification system switches to the separator to add denitrification agent.

[0040] Load increase period:

[0041] S31. When the evaporation rate reaches 430t / h or more, the flue gas recirculation system shall be shut down.

[0042] S32 and nitrogen oxide levels were reduced to below 45 mg / Nm³. 3 At the same time, open the left and right upper and lower secondary main air and small air dampers to increase the output of the secondary air fan and maintain the boiler oxygen content at 2.5~3.5%, and the oxygen content at a single point shall not be lower than 2%;

[0043] S33 and nitrogen oxide levels rose to over 45 mg / Nm³. 3 Then, the flue gas recirculation system should be put back into operation to maintain the boiler oxygen content at 2.5-3.5%, and the oxygen content at any single point should not be lower than 2%.

[0044] Stable load period:

[0045] S41. Adjust the opening of the coal feeding pipe gate of the boiler coal feeding system, control the bed temperature to 730-930℃, control the temperature deviation between the two sides of the bed to 0-30℃, the temperature difference of the lower bed at each point on the same side to 0-50℃, and the temperature of the separator outlet shall not exceed 1030℃.

[0046] Load reduction period:

[0047] S51. When the evaporation rate reaches below 430t / h, the recirculation fan shall be activated to maintain the oxygen content at a single point in the boiler at no less than 2%.

[0048] S52. Adjust the ratio of primary and secondary air volume, reduce the primary air volume and the total secondary air volume, reduce the output of the secondary fan, and adjust the pressure of the secondary air main pipe.

[0049] S53. After the boiler oxygen level stabilizes, put the flue gas recirculation system into operation and adjust the average oxygen level of the boiler to 2.5-3.5%. The oxygen level at any single point must not be lower than 2%.

[0050] Specifically, during the operation of a large circulating fluidized bed boiler, the original emissions of nitrogen oxides from the boiler are reduced by adjusting the primary and secondary air rates, upper and lower secondary air, and flue gas recirculation at different times, thereby reducing the amount of denitrification agent required.

[0051] In some feasible embodiments, this example uses two 300MW (CFB) circulating fluidized bed boilers. The maximum continuous evaporation capacity of the boilers is 1025t / h. The rated primary air volume in the boiler system is 390,000 Nm3 / h, the rated secondary air volume is 410,000 Nm3 / h, and the rated secondary air header pressure is 11 kPa. Two coal feeding systems are provided. In S52, the adjustment of the primary and secondary air volume ratio specifically involves reducing the primary air volume to 200,000 Nm3 / h to 220,000 Nm3 / h, reducing the total secondary air volume on the left, right, upper, and lower sides to 190,000 Nm3 / h to 210,000 Nm3 / h, and adjusting the secondary air header pressure to 5 kPa to 5.5 kPa.

[0052] In some feasible ways, the sulfur content of the coal fed into the boiler needs to be controlled during boiler startup. The sulfur content of the coal fed into the boiler should be less than 0.3% and the ash content should be ≤65% on an as-received basis. At the same time, a certain minimum critical fluidizing air volume should be ensured to reduce the excess oxygen in the lower part of the furnace, increase the operating bed temperature in the lower part of the boiler at low load, and reduce the original nitrogen oxide emissions.

[0053] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, the boiler startup process also includes S14, adjusting the opening of the separator branch pipe regulating valve to make the flow rate of each separator branch pipe the same. By adjusting the flow rate of each separator branch pipe to be the same, the denitrification agent flow rate of each separator is ensured to be uniform. At the same time, the amount of desulfurization input in the furnace can be reduced according to the operation of the desulfurization tower.

[0054] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, S12 further includes adjusting the amount of denitrification agent added in the furnace according to the changes in nitrogen oxide index, and stopping the operation of the denitrification system when the nitrogen oxide index does not show a downward trend after the denitrification agent is added.

[0055] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, it also includes the following before load increase:

[0056] S21. Adjust the amount of limestone and ammonia water injected, monitor the change curves of the three pollutant parameters, and control the instantaneous value and hourly average value of pollutant emission concentration to be below 80% of the standard limit.

[0057] When sulfur dioxide and nitrogen oxides in flue gas show a decreasing trend, increase the air supply and coal feed rate, and control the oxygen content in the flue gas to 2.5%~3.5%;

[0058] In some feasible ways, to facilitate the control of pollutant emission concentrations, the converted values ​​of boiler environmental protection indicators are reduced, and the emission concentrations of the three pollutants are kept stable.

[0059] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, the load reduction period also includes:

[0060] S54, nitrogen oxide levels are below 45 mg / Nm³ 3 At this time, reduce the opening of the flue gas recirculation valve to the primary air fan inlet, decrease the operating frequency of the recirculation fan, and control the average oxygen content of the boiler to 2.5~3.5%;

[0061] S55. Without shutting down the flue gas recirculation system, open the left and right upper secondary main air dampers and small air dampers to increase the pressure of the secondary air header and control the average oxygen content of the boiler to 2.5~3.5%.

[0062] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, the load stabilization period also includes:

[0063] S42. When using calcium injection for desulfurization, reduce the primary air volume, control the oxygen content to 2.8%~3.5%, and put it into the flue gas recirculation system.

[0064] S43. Adjust the ratio of ammonia water to dilution water in the denitrification system to 1:3, increase the outlet pressure of the dilution water pump, and improve the atomization effect.

[0065] It should be understood that when calcium is injected into the furnace for desulfurization, nitrogen oxides will increase. By adjusting the boiler combustion, the primary air volume is reduced to 100,000 Nm3 / h to 110,000 Nm3 / h, the oxygen content is controlled at 2.8% to 3.5%, and flue gas recirculation is put into operation to reduce the generation of nitrogen oxides in the furnace. In conjunction with the operation of the denitrification system, the ratio of ammonia water to dilution water is adjusted to 1:3, and the outlet pressure of the dilution water pump is increased to achieve better atomization effect, thereby reducing the amount of denitrification agent to be added.

[0066] As a specific implementation of the SNCR denitrification agent reduction method for large circulating fluidized bed boilers provided in the application, it also includes a coal feeding system failure combustion adjustment period: controlling the boiler bed temperature rise rate to be no more than ±3℃ / min, putting the flue gas recirculation system into operation, with the flue gas recirculation valve at the inlet of the primary air fan on the operating side of one coal feeding system fully open, and the flue gas recirculation valve at the inlet of the primary air fan on the operating side of the other coal feeding system closed to 10~20%.

[0067] It should be understood that by adjusting the coal feeding system, it is beneficial to average the oxygen content on both sides of the boiler, reduce the generation of nitrogen oxide emissions caused by the significant uneven oxygen content on the left and right sides of the furnace after the coal feeding system is shut down, thereby reducing the amount of denitrification agent required.

[0068] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler, characterized in that, include: Boiler start-up period: S11. After starting coal feeding, reduce the ratio of primary and secondary air volume to maintain the boiler operating at low oxygen levels. S12. When the bed temperature inside the furnace reaches 650℃ or above, start the denitrification system and add denitrification agent into the furnace. S13. When the flue gas temperature at the boiler separator outlet reaches 730℃ or above, the denitrification system switches to the separator to add denitrification agent. Load increase period: S31. When the evaporation rate reaches 430t / h or more, the flue gas recirculation system shall be shut down. S32 and nitrogen oxide levels were reduced to below 45 mg / Nm³. 3 At the same time, open the left and right upper and lower secondary main air and small air dampers to increase the output of the secondary air fan and maintain the boiler oxygen content at 2.5~3.5%, and the oxygen content at a single point shall not be lower than 2%; S33 and nitrogen oxide levels rose to over 45 mg / Nm³. 3 Then, the flue gas recirculation system should be put back into operation to maintain the boiler oxygen content at 2.5-3.5%, and the oxygen content at any single point should not be lower than 2%. Stable load period: S41. Adjust the opening of the coal feeding pipe gate of the boiler coal feeding system, control the bed temperature to 730-930℃, control the temperature deviation between the two sides of the bed to 0-30℃, the temperature difference of the lower bed at each point on the same side to 0-50℃, and the temperature of the separator outlet shall not exceed 1030℃. Load reduction period: S51. When the evaporation rate reaches below 430t / h, the recirculation fan shall be activated to maintain the oxygen content at a single point in the boiler at no less than 2%. S52. Adjust the ratio of primary and secondary air volume, reduce the primary air volume and the total secondary air volume, reduce the output of the secondary fan, and adjust the pressure of the secondary air main pipe. S53. After the boiler oxygen level stabilizes, put the flue gas recirculation system into operation and adjust the average oxygen level of the boiler to 2.5-3.5%. The oxygen level at any single point must not be lower than 2%.

2. The method for reducing the amount of SNCR denitrification agent in a large-scale circulating fluidized bed boiler according to claim 1, characterized in that: The boiler startup process also includes: S14. Adjust the opening of the regulating valve on the separator branch pipe to make the flow rate of the separator branch pipe the same.

3. The method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler according to claim 1, characterized in that: Also includes before the load increase: S21. Adjust the amount of limestone and ammonia water injected, monitor the change curves of the three pollutant parameters, and control the instantaneous value and hourly average value of pollutant emission concentration to be below 80% of the standard limit. When sulfur dioxide and nitrogen oxides in flue gas show a downward trend, increase the air supply and coal feed, and control the oxygen content in the flue gas to 2.5%~3.5%.

4. The method for reducing the amount of SNCR denitrification agent in a large-scale circulating fluidized bed boiler according to claim 1, characterized in that: The load reduction period also includes: S54, nitrogen oxide levels are below 45 mg / Nm³ 3 At this time, reduce the opening of the flue gas recirculation valve to the primary air fan inlet, decrease the operating frequency of the recirculation fan, and control the average oxygen content of the boiler to 2.5~3.5%; S55. Without shutting down the flue gas recirculation system, open the left and right upper secondary main air dampers and small air dampers to increase the pressure of the secondary air header and control the average oxygen content of the boiler to 2.5~3.5%.

5. The method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler according to claim 1, characterized in that: The load stabilization period also includes: S42. When using calcium injection for desulfurization, reduce the primary air volume, control the oxygen content to 2.8%~3.5%, and put it into the flue gas recirculation system. S43. Adjust the ratio of ammonia water to dilution water in the denitrification system to 1:3, increase the outlet pressure of the dilution water pump, and improve the atomization effect.

6. The method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler according to claim 1, characterized in that: It also includes the coal feeding system failure combustion adjustment period: control the boiler bed temperature rise rate to be no more than ±3℃ / min, put the flue gas recirculation system into operation, fully open the flue gas recirculation valve at the inlet of the primary air fan on the operating side of one of the two coal feeding systems, and close the flue gas recirculation valve at the inlet of the primary air fan on the operating side of the other coal feeding system to 10~20%.

7. The method for reducing the amount of SNCR denitrification agent in a large circulating fluidized bed boiler according to claim 1, characterized in that: S12 further includes adjusting the amount of denitrifying agent added to the furnace according to the changes in nitrogen oxide index. If the nitrogen oxide index does not show a downward trend after the denitrifying agent is added, the denitrification system is stopped.