A system and method for controlling nitrogen oxide emissions from circulating fluidized bed boilers using calcium sulfate.

Abstract

This invention provides a system and method for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate. The system includes: a combustion chamber, divided into a dense phase zone and a dilute phase zone in the height direction; the dense phase zone is equipped with a fuel inlet, a desulfurizing agent inlet, and multiple calcium sulfate inlets; the dense phase zone has a first air outlet; the dilute phase zone has multiple second air outlets; and a gas-solid separator, connected to the dilute phase zone at the top and to a return valve at the bottom via a lower riser, the return valve being connected to the dense phase zone via a return inclined leg. This invention, by rationally adjusting the primary and secondary air ratio, controls the dense phase zone to have a strongly reducing atmosphere, which is beneficial for suppressing NOx formation. Simultaneously, the CaS fixation reaction under a reducing atmosphere also helps improve the utilization rate of the desulfurizing agent. Furthermore, by providing calcium sulfate through multiple calcium sulfate inlets, the conversion rate of calcium sulfate to calcium sulfide under a reducing atmosphere can be increased, thereby exerting a strong reducing effect on NOx and effectively reducing nitrogen oxide emissions.

Description

Technical Field

[0001] This invention relates to the field of pollutant control in circulating fluidized bed (CFB) boilers, specifically to a system and method for controlling nitrogen oxide emissions from CFB boilers using calcium sulfate. Background Technology

[0002] The use of traditional fossil fuels will gradually tighten, and the control of pollutant emissions will move towards higher standards, stricter requirements, and more refined implementation. Circulating fluidized bed (CFB) boilers, with their wide fuel adaptability, high combustion efficiency, and dual advantages of efficient in-furnace desulfurization and low initial NOx emissions, have seen rapid development and application both domestically and internationally in recent years. However, with the introduction of ultra-low emission standards, their advantages are no longer as prominent. Deeply exploring the potential of low-cost in-furnace pollutant emission control in CFB boilers has become the driving force for their development.

[0003] Generally, CFB boilers use in-furnace desulfurization agents (such as limestone) to absorb sulfur oxides produced during fuel combustion, but this method often causes changes in NOx. Because the excess CaO from the limestone desulfurization process has a strong catalytic effect on the conversion of volatile nitrogen in solid fuels such as coal into NOx, adding limestone to reduce SO2 emissions usually leads to an increase in nitrogen oxide emissions. To meet ultra-low NOx emission requirements, industrial boilers typically employ a two-stage denitrification process. This involves first using low-NOx combustion technology to reduce the initial NOx emission concentration, and then combining this with SCR (Selective Catalytic Reduction) or SNCR (Selective Non-Catalytic Reduction) technology for deep denitrification of the flue gas. However, installing a flue gas denitrification device in a circulating fluidized bed not only increases the floor space, investment costs, and operational difficulties for personnel, but also weakens the naturally low NOx emission advantage of CFB boilers, reducing their market competitiveness.

[0004] Therefore, in order to resolve the contradiction between efficient desulfurization and denitrification in circulating fluidized bed boilers and meet increasingly stringent pollutant emission standards, it is necessary to optimize the in-furnace calcium injection method, develop a new and efficient low-NOx combustion technology, and explore a new path to achieve ultra-low emissions of NOx and SO2. Summary of the Invention

[0005] To address the above problems, this invention provides a system and method for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, which can achieve deep NOx emission reduction.

[0006] This invention provides a system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, comprising: a combustion chamber, which is divided into a dense phase zone at the bottom and a dilute phase zone at the top in the height direction; the dense phase zone is provided with a fuel inlet connected to a fuel bin, a desulfurizing agent inlet connected to a desulfurizing agent bin, and multiple calcium sulfate inlets connected to a calcium sulfate bin; the dense phase zone is provided with a first air outlet; the dilute phase zone is provided with multiple second air outlets; and a gas-solid separator, the upper part of which is connected to the dilute phase zone, and the lower part of which is connected to a return valve via a lower riser, the return valve being connected to the dense phase zone via a return inclined leg.

[0007] The system for controlling nitrogen oxide emissions from circulating fluidized bed boilers using calcium sulfate is simple in structure, requires less space, has lower cost, and is easy to operate; it can achieve deep reduction of nitrogen oxide emissions and decouple the contradiction between efficient desulfurization and denitrification in the circulating fluidized bed boiler.

[0008] In the optional technical solution of the present invention, a dust collector and a fly ash collection device are also included. The dust collector is connected to the gas-solid separator, and the fly ash collection device is connected to the outlet of the dust collector. The fly ash collection device is used to collect the dust separated by the gas-solid separator.

[0009] According to this technical solution, the dust collector can purify the flue gas separated by the gas-solid separator, reducing the pollution of the flue gas to the environment; the fly ash collection device collects dust containing a large amount of calcium sulfate, and by partially replacing calcium sulfate with dust in the dense phase zone, the amount of calcium sulfate used is saved, the calcium sulfate is reused, and the system's economy is improved.

[0010] In an optional technical solution of the present invention, the fuel tank and the dense phase zone are connected through a fuel feed pipe, the desulfurizer tank and the dense phase zone are connected through a desulfurizer feed pipe, the fuel feed pipe is provided with a first calcium sulfate inlet, the desulfurizer feed pipe is provided with a second calcium sulfate inlet, the return valve is provided with a third calcium sulfate inlet, and the dense phase zone of the combustion chamber is provided with a fourth calcium sulfate inlet.

[0011] According to this technical solution, the multi-point feeding method of calcium sulfate improves the uniformity of calcium sulfate distribution in the dense phase region and allows calcium sulfate to fully contact and mix with fuel, desulfurizing agent and circulating materials, which is beneficial to improving denitrification efficiency.

[0012] The present invention also provides a method for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, comprising the following steps:

[0013] Preliminary nitrogen oxide emission reduction steps: Fuel is fed into the dense phase zone, and air is supplied into the combustion chamber through the first and second air inlets according to the specified primary and secondary air supply ratio, so that the fuel is burned in stages in the combustion chamber, forming a reducing atmosphere in the dense phase zone and an oxidizing atmosphere in the dilute phase zone; the nitrogen element in the fuel is first oxidized into nitrogen oxides in the dense phase zone, and then reduced into N2.

[0014] The deep reduction steps for nitrogen oxides: Calcium sulfate enters the dense phase zone from the calcium sulfate silo through multiple calcium sulfate inlets. Under the strong reducing atmosphere in the dense phase zone, it decomposes to generate calcium sulfide, which is used to reduce NOx generated during fuel combustion, thereby achieving deep reduction of nitrogen oxides.

[0015] This invention, based on the use of primary and secondary air staged combustion technology to suppress NOx generation and in-furnace calcium injection technology to reduce SO2 emissions, sets up multiple calcium sulfate inlets in the dense phase zone of the furnace. By rationally adjusting the ratio of primary and secondary air, the entire dense phase zone of the furnace is made into a strongly reducing atmosphere, which promotes the thermal decomposition of calcium sulfate in the reducing atmosphere to generate calcium sulfide, which has a strong reducing effect on NOx. This decouples the contradiction between in-furnace desulfurization and nitrogen oxide emission reduction, and achieves ultra-low emissions of both NOx and SO2 from the circulating fluidized bed boiler.

[0016] In the optional technical solution of the present invention, in the preliminary nitrogen oxide emission reduction step: the primary air supply ratio is 40-60%, the excess air coefficient in the dense phase zone is less than 0.9, the CO concentration is greater than 5000ppm, and the excess air coefficient in the dilute phase zone is 1.1-1.3.

[0017] In an optional technical solution of the present invention, the desulfurizing agent inlet is located above the fuel inlet, and the temperature in the combustion chamber is uniform and maintained at 800-950°C.

[0018] According to this technical solution, the temperature and location are conducive to ensuring the desulfurization reaction proceeds and improving desulfurization efficiency.

[0019] In the optional technical solutions of the present invention, calcium sulfate is fed into the dense phase region from multiple points at different heights. The feeding methods include: feeding it through the fuel inlet after mixing with fuel; feeding it through the desulfurizer inlet after mixing with desulfurizing agent; feeding it from the outlet of the return inclined leg; and feeding it directly into one or more of the dense phase regions.

[0020] According to this technical solution, the multi-point feeding method of calcium sulfate improves the uniformity of calcium sulfate distribution in the dense phase region and allows calcium sulfate to fully contact and mix with fuel, desulfurizing agent and circulating materials, which is beneficial to improving denitrification efficiency.

[0021] In the optional technical solution of the present invention, when the sulfur content in the fuel is greater than 3%, the amount of calcium sulfate fed is 0; when the sulfur content in the fuel is less than 3%, it needs to be adjusted according to the original NO emission concentration control target to determine the amount of calcium sulfate added.

[0022] According to this technical solution, the amount of calcium sulfate added varies depending on the sulfur content of the fuel, which improves the accuracy of nitrogen oxide emission reduction and helps to improve the efficiency of nitrogen oxide removal.

[0023] In an optional technical solution of the present invention, the method further includes measuring the calcium sulfate content in the dust separated by the gas-solid separator, and after measuring the calcium sulfate content in the dust, adding a portion of the dust to the dense phase zone in place of the calcium sulfate.

[0024] According to this technical solution, it is beneficial to realize the reuse of calcium sulfate, save the amount of calcium sulfate used, reduce costs, and improve the economic efficiency of the system.

[0025] The present invention has the following advantages and outstanding technical effects:

[0026] ① By reasonably adjusting the ratio of primary and secondary air through the primary and secondary air outlets, the dense phase zone can be controlled to be a strongly reducing atmosphere, which is conducive to suppressing the formation of NOx. At the same time, the sulfur fixation reaction of CaS generated under the reducing atmosphere can improve the utilization rate of desulfurizing agent and reduce the amount of desulfurizing agent used.

[0027] ② By introducing calcium sulfate into the dense phase region through multiple calcium sulfate inlets, the influence of the alternating redox atmosphere on the decomposition reaction of calcium sulfate under the bubbling operation in the dense phase region can be reduced, the conversion rate of calcium sulfate to calcium sulfide under the reducing atmosphere can be improved, and thus exert a strong reducing effect on NOx, achieving deep removal of nitrogen oxides.

[0028] ③ This invention comprehensively utilizes and optimizes multiple pollutant emission reduction technologies, such as primary and secondary air staged combustion, in-furnace calcium injection desulfurization, and calcium sulfate removal of nitrogen oxides, which can achieve synergistic removal of SO2 and NOx in the furnace of circulating fluidized bed boilers, thus improving the economy and environmental protection of circulating fluidized bed combustion. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, as described in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram of the process for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, as described in an embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram illustrating the effect of CaSO4 on NO emission concentration under different CO concentrations.

[0032] Figure 4 This is a schematic diagram of XRD detection of the reaction products of CaSO4 in the CO and NO reaction system.

[0033] Figure 5 This is a schematic diagram showing the effect of limestone on NO and SO2 concentrations in typical areas of a 20kW CFB boiler.

[0034] Figure label:

[0035] 1-Combustion chamber; 2-Dense phase zone; 3-Dilute phase zone; 4-Gas-solid separator; 5-Riser; 6-Return valve; 7-Return inclined leg; 8-Air distribution plate; 9-Heating surface; 10-Dust collector; 11-Fly ash collection device; 12-Induced draft fan; 13-Flue gas outlet; 14-Forced draft fan; 15-First air outlet; 16-Primary air chamber; 17-Second air outlet; 18-Fuel bin; 19-Desulfurizing agent bin; 20-Calcium sulfate bin; 21-First calcium sulfate inlet; 22-Second calcium sulfate inlet; 23-Third calcium sulfate inlet; 24-Fourth calcium sulfate inlet. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0037] like Figure 1 As shown, the present invention provides a system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, comprising: a combustion chamber 1 and a gas-solid separator 4. The combustion chamber 1 is divided into a dense phase zone 2 at the lower part and a dilute phase zone 3 at the upper part in the height direction. The dense phase zone 2 is provided with a fuel inlet connected to a fuel bin 18, a desulfurizing agent inlet connected to a desulfurizing agent bin 19, and multiple calcium sulfate inlets connected to a calcium sulfate bin 20. The dense phase zone 2 is provided with a first air outlet 15. The dilute phase zone 3 is provided with multiple second air outlets 17. The upper part of the gas-solid separator 4 is connected to the dilute phase zone 3, and the lower part of the gas-solid separator 4 is connected to a return valve 6 through a lower riser 5. The return valve 6 is connected to the dense phase zone 2 through a return inclined leg 7.

[0038] Specifically, the combustion chamber 1 is also equipped with an air distribution plate 8, which is located above the first air inlet 15. The area of ​​the air distribution plate 8 below the dense phase zone 2 is the primary air chamber 16. Furthermore, it also includes a blower 14. The primary and secondary air are respectively sent to the first air inlet 15 and the second air inlet 17 through the blower 14. The second air inlet 17 is located in the transition zone between the dense phase zone and the dilute phase zone of the secondary air inlet, at a position slightly below the middle of the furnace. The first air inlet 15 is located in the lower part of the furnace of the combustion chamber 1.

[0039] In a preferred embodiment of the present invention, the invention further includes a dust collector 10 and a fly ash collection device 11. The dust collector 10 is connected to the gas-solid separator 4, and the fly ash collection device 11 is connected to the outlet of the dust collector 10. The fly ash collection device 11 is used to collect the dust separated by the gas-solid separator 4. Furthermore, the invention includes an induced draft fan 12, which is connected to the dust collector 10. The flue gas purified by the dust collector 10 is discharged through the flue gas outlet 13.

[0040] In a preferred embodiment of the present invention, the fuel tank 18 is connected to the dense phase zone 2 via a fuel feed pipe, the desulfurizer tank 19 is connected to the dense phase zone 2 via a desulfurizer feed pipe, the fuel feed pipe is provided with a first calcium sulfate inlet 21, the desulfurizer feed pipe is provided with a second calcium sulfate inlet 22, the return valve 6 is provided with a third calcium sulfate inlet 23, and the dense phase zone 2 of the combustion chamber 1 is provided with a fourth calcium sulfate inlet 24.

[0041] The above describes the structure of the system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate according to the present invention. The following describes its specific workflow: Fuel in fuel bin 18 enters combustion chamber 1 through fuel feed pipe. Primary and secondary air are staged for combustion. Primary air flows from primary air chamber 16 at the bottom of dense phase zone 2 through air distribution plate 8 into dense phase zone 2. Secondary air is sent to dilute phase zone 3 through multiple paths 17. After fuel combustion, the flue gas passes through dense phase zone 2, dilute phase zone 3, gas-solid separator 4, and heating surface 9 in sequence before entering dust collector 10. Fly ash is stored in fly ash collection device 11. The purified flue gas is led to flue gas outlet 13 by induced draft fan 12. Desulfurizing agent bin 19 sprays desulfurizing agent into dense phase zone 2 of the furnace through feed pipe. Calcium sulfate is sent into dense phase zone 2 through calcium sulfate first feed inlet 21, calcium sulfate second feed inlet 22, calcium sulfate third feed inlet 23, and calcium sulfate fourth feed inlet 24.

[0042] like Figure 2 As shown, corresponding to the system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate according to the present invention, the present invention further provides a method for using the above-described system for controlling nitrogen oxide emissions from a circulating fluidized bed boiler using calcium sulfate, comprising the following steps:

[0043] Preliminary nitrogen oxide emission reduction steps: Fuel is fed into the dense phase zone 2, and air is supplied into the combustion chamber 1 through the first air inlet 15 and the second air inlet 17 according to the prescribed primary and secondary air supply ratio. This allows the fuel to undergo staged combustion in the combustion chamber 1, forming a reducing atmosphere in the dense phase zone 2 and an oxidizing atmosphere in the dilute phase zone 3. Specifically, due to incomplete combustion of the fuel in the dense phase zone 2, a reducing atmosphere with rich fuel, low oxygen content, and high carbon monoxide concentration is formed in the lower dense phase zone 2 of the furnace. After secondary air supplementation, an oxidizing atmosphere with higher oxygen content and lower carbon monoxide content is formed in the upper dilute phase zone 3 of the furnace. Nitrogen in the fuel is first oxidized to nitrogen oxides in the dense phase zone 2, and then reduced to N2 by the higher concentrations of CO and coke in the dense phase zone 2. Sulfur in the fuel is oxidized to SO2 during fuel combustion.

[0044] Sulfur oxide removal steps: Desulfurizing agent is added from desulfurizing agent tank 19 to dense phase zone 2 according to a certain calcium-to-sulfur ratio. The desulfurizing agent flows sequentially through dense phase zone 2 and dilute phase zone 3 with the flue gas in the combustion chamber, fixing the SO2 generated during combustion and achieving in-furnace desulfurization. The main reactions occurring in this process are:

[0045] CaCO3→CaO+CO2 R1

[0046] CaO + 0.5O₂ + SO₂ → CaSO₄ (R₂)

[0047] CaO + 3CO + SO2 → CaS + 3CO2 (R3)

[0048] The deep reduction process for nitrogen oxides involves calcium sulfate entering the dense phase zone 2 of combustion chamber 1 from calcium sulfate reservoir 20 via multiple calcium sulfate inlets. Under the strong reducing atmosphere of dense phase zone 2, calcium sulfate decomposes to form calcium sulfide, which is used to reduce NOx generated during fuel combustion, thus achieving deep reduction of nitrogen oxide emissions. The main reactions occurring in this process are:

[0049] CaSO4 + 4CO → CaS + 4CO2 (R4)

[0050] CaSO4 + 2C → CaS + 2CO2 (R5)

[0051] CaS+6NO+2CO→CaSO4+2CO2+3N2 R6

[0052] CaS+4NO+CO→CaO+SO2+2N2+CO2 R7

[0053] 2NO + 2CON + 2CO2 R8

[0054] This invention, based on the use of primary and secondary air staged combustion technology to suppress NOx generation and in-furnace calcium injection technology to reduce SO2 emissions, sets up multi-stage calcium sulfate inlets in the dense phase zone 2 of the furnace. The amount of calcium sulfate added is determined according to the fuel properties, and by reasonably adjusting the ratio of primary and secondary air, the entire dense phase zone 2 of the furnace is made into a strongly reducing atmosphere, which promotes the thermal decomposition of calcium sulfate in the reducing atmosphere to generate calcium sulfide, which has a strong reducing effect on NOx. This decouples the contradiction between in-furnace desulfurization and nitrogen oxide emission reduction, and is conducive to achieving the original ultra-low emissions of NOx and SO2 in circulating fluidized bed boilers.

[0055] In a preferred embodiment of the present invention, in the preliminary nitrogen oxide emission reduction step: the primary air supply ratio is 40-60%, preferably around 50%, to facilitate the formation of an alternating redox atmosphere in the combustion chamber 1; the excess air coefficient of the dense phase zone 2 is less than 0.9, the CO concentration is greater than 5000ppm, and the excess air coefficient of the dilute phase zone 3 is 1.1-1.3.

[0056] In a preferred embodiment of the present invention, the desulfurizing agent inlet is located above the fuel inlet. To ensure that the desulfurization reaction is in the optimal temperature range, the temperature in the combustion chamber 1 is uniform and maintained at 800-950°C.

[0057] In a preferred embodiment of the present invention, calcium sulfate is fed into the dense phase zone 2 from multiple points at different heights. The feeding methods include: feeding it through the fuel inlet after mixing with fuel; feeding it through the desulfurizer inlet after mixing with desulfurizer; feeding it from the outlet of the return material inclined leg 7; and feeding it directly into the dense phase zone 2 of the furnace, or one or more of these methods.

[0058] In a preferred embodiment of the present invention, reactions R6 and R7, which reduce NO with calcium sulfide, are more likely to occur at low temperatures. Therefore, the furnace temperature should be appropriately reduced while ensuring boiler operating efficiency.

[0059] In a preferred embodiment of the present invention, when the sulfur content in the fuel is greater than 3%, the amount of calcium sulfate fed is 0, and deep removal of NOx can be achieved solely through the large amount of calcium sulfate and calcium sulfide generated in the sulfur fixation reaction of the desulfurizing agent in the sulfur oxide removal step; when the sulfur content in the fuel is less than 3%, an adjustment experiment is conducted according to the original NO emission concentration control target to determine the amount of calcium sulfate added.

[0060] In a preferred embodiment of the present invention, since a large amount of calcium sulfate is present in the dust, the method further includes measuring the calcium sulfate content in the dust separated by the gas-solid separator 4, and after measuring the calcium sulfate content in the dust, adding a portion of the dust to the dense phase zone 2 to replace the industrial calcium sulfate, based on the actual situation.

[0061] The present invention has the following advantages and outstanding technical effects:

[0062] ① By reasonably adjusting the ratio of primary and secondary air, controlling the dense phase zone 2 to be a strongly reducing atmosphere is beneficial to suppressing the formation of NOx. At the same time, the sulfur fixation reaction of CaS generated under the reducing atmosphere can improve the utilization rate of desulfurizing agent and reduce the amount of desulfurizing agent used.

[0063] ② By introducing calcium sulfate into the dense phase zone 2 through a multi-stage combination method, the influence of the alternating redox atmosphere on the decomposition reaction of calcium sulfate under the bubbling operation of the dense phase zone 2 can be reduced, the conversion rate of calcium sulfate to calcium sulfide under the reducing atmosphere can be improved, and thus exert a strong reducing effect on NOx, achieving deep removal of nitrogen oxides.

[0064] ③ This invention comprehensively utilizes and optimizes multiple pollutant emission reduction technologies, such as primary and secondary air staged combustion, in-furnace calcium injection desulfurization, and calcium sulfate removal of nitrogen oxides, which can achieve synergistic removal of SO2 and NOx in the furnace of circulating fluidized bed boilers, thus improving the economy and environmental protection of circulating fluidized bed combustion.

[0065] Example 1

[0066] For high-sulfur fuels (typically with a sulfur content greater than 3%, such as high-sulfur coal and high-sulfur petroleum coke), during normal operation of a circulating fluidized bed furnace, due to the high sulfur content of these fuels, a large amount of limestone is continuously fed into the furnace along with the fuel to ensure desulfurization efficiency. While the limestone undergoes a sulfur fixation reaction, large amounts of CaSO4 and CaS are continuously generated under the alternating redox atmosphere within the furnace. CaSO4 is returned to the dense phase zone 2 of the furnace via a return feed cycle, where it further decomposes into CaS under the strong reducing atmosphere. Additionally, with staged combustion using primary and secondary air, a significant portion of the limestone sulfur fixation process directly generates CaS under the localized reducing atmosphere in combustion chamber 1. Since CaS not only catalyzes the reduction reaction of CO to NO but also directly participates in the chemical reaction of NO decomposition as a reactant, the generation of large amounts of CaS in the furnace will significantly suppress NOx emissions. Therefore, when high-sulfur fuel is used in CFB boilers, by reasonably adjusting the calcium-to-sulfur ratio of the desulfurizing agent, adjusting the ratio of primary and secondary air, strengthening the reducing atmosphere in the furnace, and promoting the decomposition of a large amount of sulfur-fixing product CaSO4 to generate CaS, deep synergistic removal of NOx and SO2 can be achieved without the addition of calcium sulfate.

[0067] Example 2

[0068] For non-high-sulfur fuels (sulfur content usually less than 3%), during normal operation of the circulating fluidized bed, according to the original NO emission concentration control target during fuel combustion, calcium sulfate is added to the dense phase zone 2 in a certain proportion through one or more methods, such as mixing with fuel and feeding it through the fuel inlet, mixing with desulfurizing agent and feeding it through the desulfurizing agent inlet, feeding it from the return material inclined leg 7 with the circulating material, or directly feeding it into the dense phase zone 2 of the furnace. At the same time, by reasonably reducing the proportion of primary air, the reducing atmosphere in the dense phase zone 2 is strengthened, promoting the decomposition and conversion of calcium sulfate into calcium sulfide, thereby achieving deep removal of nitrogen oxides in the CFB boiler furnace.

[0069] The effectiveness of the implementation plan will be verified through specific test examples below.

[0070] Test Example 1

[0071] A vertical tube furnace was used to verify the decomposition of CaSO4 into CaS under a reducing atmosphere, which then reduced NO. The furnace temperature was set to 900℃. 200 mg of CaSO4 sample was fed into the furnace each time, with a total NO and CO gas flow rate of 1 L / min. The initial NO concentration was set to 2500 ppm. By varying the CO concentration, the effect of the CaSO4 decomposition process on the NO concentration was observed. The experimental results are shown below. Figure 3 As shown in the figure, when the CO concentration was 2500 ppm, adding CaSO4 to the furnace had little effect on NO, with the NO concentration decreasing by only 130 ppm. With increasing CO concentration, CaSO4 showed a more pronounced inhibitory effect on NO. When the CO concentration was 5000 ppm and 9000 ppm, adding the same mass of CaSO4 resulted in NO reduction rates of 51% and 72%, respectively. After 1 hour of reaction, the samples were removed for XRD analysis, and the results are shown in the figure. Figure 4 As shown, when the CO concentration is greater than 5000 ppm, most of CaSO4 has decomposed into CaS. Therefore, under high CO concentrations, CaSO4 can readily decompose into CaS and thus exert a strong reducing effect on NO. In staged combustion using a circulating fluidized bed, by rationally adjusting the primary and secondary air ratio, the dense phase zone 2 of the furnace can provide suitable temperature and atmospheric conditions for the decomposition of CaSO4. Therefore, when industrial CFB boilers burn low-sulfur fuels, by supplementing the dense phase zone 2 with a certain mass of CaSO4 and rationally adjusting the primary and secondary air ratio, the decomposition of calcium sulfate in the dense phase zone 2 can be enhanced, which will help achieve deep reduction of nitrogen oxides in the CFB furnace.

[0072] Test Example 2

[0073] Industrial operating conditions were simulated in a 20kW circulating fluidized bed hot test bench. Experiments were conducted on the addition and shutdown of limestone during the co-firing of petroleum coke and coal. The changes in NO concentration in typical areas of the furnace were simultaneously monitored along the furnace height using an MCA14m mobile high-temperature infrared multi-component gas analyzer. The flue gas sampling locations are shown in Table 1 below.

[0074] Table 1 Sampling locations for a 20kW circulating fluidized bed boiler

[0075]

[0076]

[0077] The mass ratio of petroleum coke (6.92% sulfur content) to bituminous coal (0.53% sulfur content) in the mixed fuel was 3:1. Limestone was added at a calcium-to-sulfur ratio of 2:1. The furnace temperature was maintained at 900℃, the primary air ratio was 50%, and the excess air coefficient at the furnace outlet was 1.2. The effects of limestone on NO and SO2 concentrations in typical zones within the furnace of a 20kW CFB boiler are shown below. Figure 5 As shown in the figure, when using high-sulfur fuel in a circulating fluidized bed furnace, the in-furnace calcium injection method for desulfurization significantly inhibits NO emissions. The specific changes in NO concentration along the furnace height before and after limestone addition are as follows: NO concentration slightly increases in the dense phase zone 2, while a significant decrease occurs in the transition zone near the second air inlet 17, from 336 mg / m³. 3 Decreased to 231 mg / m 3 The NO emission concentration decreased by 32% initially, but the rate of decrease slowed down with increasing height, eventually reaching a low of 212 mg / m³ at the boiler tail end. 3 Reduced to 156 mg / m 3 The reduction is still around 26%. Therefore, when using high-sulfur fuel in CFB boilers, by reasonably adjusting the calcium-to-sulfur ratio of the desulfurizing agent, the synergistic removal of NO and SO2 can be achieved without the need to add additional calcium sulfate.

[0078] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A system for controlling the emission of nitrogen oxides from a circulating fluidized bed boiler with calcium sulphate, characterized in that, The system comprises: a combustion chamber, which is divided into a dense phase zone at the lower part and a dilute phase zone at the upper part in the height direction; the dense phase zone is provided with a fuel feeding inlet connected with a fuel bin, a desulfurizing agent feeding inlet connected with a desulfurizing agent bin, and multiple calcium sulfate feeding inlets connected with a calcium sulfate bin; the dense phase zone is provided with a first air feeding inlet; the dilute phase zone is provided with multiple second air feeding inlets; the air feeding ratio of the primary air is 40-60%, the excess air coefficient of the dense phase zone is less than 0.9, the CO concentration is greater than 5000 ppm, and the excess air coefficient of the dilute phase zone is 1.1-1.3; a gas-solid separator, the upper part of which is connected with the dilute phase zone, and the lower part of which is connected with a back feeding valve through a lower vertical pipe, and the back feeding valve is connected with the dense phase zone through a back feeding inclined leg, the fuel bin is communicated with the dense phase zone through a fuel feeding pipe, the desulfurizing agent bin is communicated with the dense phase zone through a desulfurizing agent feeding pipe, the fuel feeding pipe is provided with a calcium sulfate first feeding inlet, the desulfurizing agent feeding pipe is provided with a calcium sulfate second feeding inlet, the back feeding valve is provided with a calcium sulfate third feeding inlet, and the dense phase zone of the combustion chamber is provided with a calcium sulfate fourth feeding inlet, the calcium sulfate is fed into the dense phase zone from different heights and multiple points in multiple ways, including being mixed with the fuel and fed into the dense phase zone through the fuel feeding inlet, being mixed with the desulfurizing agent and fed into the dense phase zone through the desulfurizing agent feeding inlet, being fed into the dense phase zone from the outlet of the back feeding inclined leg, and being directly fed into the dense phase zone, and the addition amount of the calcium sulfate is determined according to the sulfur content of the fuel, the calcium sulfate is fed into the dense phase zone from the calcium sulfate bin through the multiple calcium sulfate feeding inlets, and decomposes to generate calcium sulfide in the strong reducing atmosphere of the dense phase zone, so as to reduce the NOx generated in the fuel combustion process.

2. A system for controlling the emission of nitrogen oxides from a circulating fluidized bed boiler with calcium sulphate according to claim 1, characterized in that, It also comprises a dust remover and a fly ash collecting device, the dust remover is connected with the gas-solid separator, the fly ash collecting device is connected with the outlet of the dust remover, and the fly ash collecting device is used to collect the dust ash separated from the gas-solid separator.

3. A method for controlling the emission of nitrogen oxides from a circulating fluidized bed boiler with calcium sulphate, characterized in that, The system for controlling the emission of nitrogen oxides of a circulating fluidized bed boiler by using calcium sulfate according to claim 1 or 2 comprises the following steps: a preliminary emission reduction step of nitrogen oxides: the fuel is fed into the dense phase zone, air is fed into the combustion chamber through the first air feeding inlet and the second air feeding inlet according to a specified primary air / secondary air ratio, the fuel is combusted in stages in the combustion chamber, a reducing atmosphere is formed in the dense phase zone, and an oxidizing atmosphere is formed in the dilute phase zone; the nitrogen element in the fuel is first oxidized into nitrogen oxides in the dense phase zone, and then reduced into N2, and the sulfur element in the fuel is oxidized into SO2 in the fuel combustion process; a deep emission reduction step of nitrogen oxides: the calcium sulfate is fed into the dense phase zone from the calcium sulfate bin through the multiple calcium sulfate feeding inlets, and decomposes to generate calcium sulfide in the strong reducing atmosphere of the dense phase zone, so as to reduce the NOx generated in the fuel combustion process, and realize the deep emission reduction of nitrogen oxides.

4. A method for controlling the emission of nitrogen oxides from a circulating fluidized bed boiler with calcium sulphate according to claim 3, characterized in that, The desulfurizing agent feeding inlet is located above the fuel feeding inlet, and the temperature in the combustion chamber is uniform and maintained at 800-950℃.

5. The method of controlling nitrogen oxides emissions from a circulating fluidized bed boiler with calcium sulphate according to claim 3, characterized in that, When the sulfur content in the fuel is greater than 3%, the input amount of calcium sulfate is 0, and when the sulfur content in the fuel is less than 3%, the input amount of calcium sulfate is adjusted according to the original emission concentration control target of NO to determine the input amount of calcium sulfate.

6. The method of controlling nitrogen oxides emissions from a circulating fluidized bed boiler with calcium sulphate according to claim 3, characterized in that, Further comprising determining the content of calcium sulfate in the dust removed by the gas-solid separator, and after determining the content of calcium sulfate in the dust, replacing the calcium sulfate with part of the dust in the dense phase zone.

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