A low-nitrogen method for mixed combustion of biomass and coal powder in cooperation with fuel-air staging

Abstract

This invention provides a low-NOx method for biomass-pulverized coal co-combustion with staged fuel-air coordination, belonging to the field of boiler low-NOx combustion control technology. It can significantly alleviate or solve the problems of high NOx emissions, uncoordinated combustion organization, and insufficient adaptability in existing biomass-pulverized coal co-combustion processes. The method includes: pre-treating the biomass feedstock and feeding it into the boiler furnace re-combustion zone via an independent conveying system; forming a main combustion zone, a re-combustion zone, and a burnout zone within the furnace along the flue gas flow direction; implementing oxygen-deficient combustion in the main combustion zone, injecting biomass fuel into the re-combustion zone for reduction reaction, and introducing secondary air in stages into the burnout zone; simultaneously monitoring the combustion state parameters in the furnace online and dynamically adjusting the biomass fuel supply, injection location, and air volume distribution based on the monitoring results to achieve low NOx emission control.

Description

Technical Field

[0001] This invention belongs to the field of boiler low-NOx combustion control technology, specifically relating to a low-NOx method for biomass-pulverized coal co-combustion with fuel-air staged synergy. Background Technology

[0002] In existing large-scale coal-fired power units, replacing some fossil fuels with biomass co-firing can effectively reduce coal consumption and carbon dioxide emissions, making it one of the important technological pathways for coal-fired power plants to achieve low-carbon transformation. Meanwhile, nitrogen oxides (NOx), a key pollutant for emission control in coal-fired boilers, are influenced by factors such as fuel composition, combustion temperature, local oxygen concentration, and residence time. Under the condition of biomass co-firing in boilers, existing low-NOx control strategies designed for pure coal combustion often fail to achieve stable and ideal results. Therefore, there is an urgent need to develop new source emission reduction methods suitable for biomass / pulverized coal co-combustion conditions.

[0003] Currently, the co-combustion of biomass and pulverized coal has been applied in some power plant boilers, but there are significant differences in the physicochemical properties of biomass and coal. Biomass typically has characteristics such as high volatile matter, low ignition temperature, rapid combustion reaction, and complex ash characteristics, and its fuel nitrogen occurrence form is also different from that of coal. If conventional pulverized coal combustion organization is still used during co-combustion, problems such as unreasonable oxygen distribution in different areas of the furnace and mismatch between temperature and concentration fields can easily occur. On the one hand, the rapid release of volatile matter from biomass will change the reaction atmosphere and combustion rate in the main combustion zone; on the other hand, fuel nitrogen is more easily converted into NOx under unsuitable oxidation conditions, leading to increased or more volatile NOx emissions at the boiler outlet. Furthermore, high-proportion biomass co-combustion may also bring new impacts on combustion stability, burnout effect, and boiler operation regulation.

[0004] Existing technologies for NOx control typically employ low-NOx burners, staged air combustion, flue gas recirculation, and technologies such as SCR and SNCR. Low-NOx burners and staged air combustion are primarily designed for pulverized coal combustion and are insufficiently adaptable to changes in fuel reaction characteristics under biomass co-firing conditions, making it difficult to simultaneously achieve low NOx formation and efficient combustion. While end-of-pipe denitrification technologies such as SNCR and SCR offer high denitrification efficiency, their system investment and operating costs are high. Furthermore, under biomass co-firing conditions, alkali metals and alkaline earth metals in the ash may cause slagging, ash accumulation, or catalyst poisoning, affecting the long-term stable operation of the plant. In particular, existing technologies generally lack integrated design solutions that coordinate the inherent reduction characteristics of biomass with in-furnace fuel and air staging, making it difficult to simultaneously enhance NOx formation inhibition and reduction of already generated NOx from the source of the combustion process.

[0005] To address this, a low-NOx method for biomass-coal pulverized coal co-combustion with staged fuel-air synergy is proposed. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the prior art, and to provide a low-NOx method for biomass-coal powder co-combustion with fuel-air staged synergy.

[0007] This invention provides a low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy, comprising the following steps: S1: The biomass raw materials are pretreated and transported to the burner position in the recombustion zone of the coal-fired boiler furnace through a biomass fuel metering and conveying system that is independent of the original coal powder preparation and conveying system of the coal-fired boiler; wherein the burner arrangement area of ​​the coal-fired boiler furnace is divided into the main combustion zone, the recombustion zone and the burnout zone in sequence along the flue gas flow direction. S2: Inject pulverized coal or a mixture of pulverized coal and a portion of the biomass fuel into the main combustion zone, and introduce primary air in conjunction with the pulverized coal or the mixed fuel into the main combustion zone to keep the main combustion zone in an oxygen-deficient combustion state, thereby suppressing the generation of nitrogen oxides. S3: The biomass fuel is injected into the re-combustion zone, causing the biomass fuel to undergo pyrolysis and gasification reactions in the re-combustion zone to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides in the flue gas from the main combustion zone to reduce the nitrogen oxides to nitrogen. S4: Secondary air is introduced into the burnout zone in stages to further burn the unburned products generated in the reburning zone; S5: Online monitoring of the combustion state parameters in the furnace during the combustion process of the coal-fired boiler, and dynamic adjustment of the biomass fuel supply, biomass fuel injection position and staged air volume distribution in the reburning zone based on the monitoring results, so as to achieve low nitrogen oxide emission control during the biomass-coal powder mixed combustion process.

[0008] Furthermore, in step S1, the biomass raw material is crushed and ground to form powdered biomass fuel with a particle size of less than 1 mm, or it is transported while maintaining the particle shape; the pretreated biomass fuel is stored in an independent silo and transported to the reburning zone through an independent metering and conveying system.

[0009] Specifically, in step S2, 70% to 85% of the total amount of fuel fed into the boiler furnace for staged combustion, or a mixture of pulverized coal and some biomass, is fed into the main combustion zone. Primary air equivalent to 70% to 80% of the theoretical air volume required for complete combustion of the pulverized coal or the mixed fuel is also fed into the main combustion zone, so that the excess air coefficient of the main combustion zone is controlled at 0.8 to 0.9.

[0010] Specifically, in step S3, biomass fuel accounting for 10% to 25% of the total amount of fuel fed into the boiler furnace for staged combustion is injected into the reburning zone. The reburning zone is located in the flue gas flow area after the main combustion zone and is equipped with a corresponding biomass reburning burner.

[0011] Preferably, in step S3, the reburning zone is not provided with an independent combustion air supply. The biomass fuel injected into the reburning zone undergoes pyrolysis and gasification to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides from the flue gas in the main combustion zone.

[0012] Specifically, in step S4, the burnout zone is provided with multi-layer secondary air nozzles. Air accounting for 20% to 30% of the total air fed into the boiler furnace for staged combustion is fed into the burnout zone through the multi-layer secondary air nozzles in stages, so as to promote the reaction of CO and residual carbon generated in the reburning zone and inhibit the generation of new thermal nitrogen oxides.

[0013] Furthermore, in step S5, when the CO concentration at the cross section through which the flue gas flows in the reburning zone is detected to be higher than a preset value, the secondary air volume supplied through the preset secondary air nozzle in the burnout zone is increased to promote the reaction of reduction reaction products and reduce the risk of boiler efficiency decline and CO emission exceeding the standard.

[0014] Furthermore, in step S5, the online monitoring includes: monitoring the nitrogen oxide concentration and oxygen concentration at the boiler furnace outlet, monitoring the CO concentration and temperature at the cross section through which the flue gas flows in the reburning zone, and monitoring the flue gas temperature in the area corresponding to the main combustion zone.

[0015] Furthermore, the dynamic adjustment is a closed-loop feedback adjustment based on the control unit; the control unit adjusts the biomass fuel supply, biomass fuel injection position, and staged air volume distribution in the reburning zone according to the combustion state parameters obtained from online monitoring; when the nitrogen oxide concentration at the boiler furnace outlet is detected to be higher than the target value, the amount of biomass fuel supplied to the reburning zone is increased and / or the biomass fuel injection position is adjusted to enhance the reducing atmosphere; when the nitrogen oxide concentration at the boiler furnace outlet is detected to be lower than the target value, the amount of biomass fuel supplied to the reburning zone is reduced and / or the biomass fuel injection position is adjusted.

[0016] Specifically, in step S5, the proportion of primary air and the fineness of pulverized coal fed into the main combustion zone are adjusted according to the temperature monitoring results of the main combustion zone, so as to regulate the combustion temperature of the main combustion zone and reduce the generation of thermal nitrogen oxides.

[0017] The beneficial effects of this invention are as follows: Compared with existing technologies, the present invention has at least the following beneficial effects: The present invention constructs a main combustion zone, a recombustion zone, and a burnout zone within the boiler furnace, and coordinates fuel staging and air staging. This ensures that the main combustion zone is in an oxygen-deficient combustion state to suppress nitrogen oxide generation. The recombustion zone utilizes reducing intermediate products from biomass fuel pyrolysis and gasification to reduce nitrogen oxides to nitrogen. The burnout zone promotes further reaction of incompletely combusted products through the staged introduction of secondary air, thereby achieving source suppression and process reduction of nitrogen oxides. Simultaneously, the present invention employs an independent biomass fuel delivery system, facilitating precise adjustment of the biomass fuel supply and injection location, which is beneficial for forming a stable recombustion reducing atmosphere. Furthermore, the present invention monitors in-furnace combustion state parameters such as NOx, O2, CO, and temperature online, and implements closed-loop feedback regulation based on the control unit. This allows for dynamic optimization of fuel quantity, injection location, and air volume distribution according to load changes and fuel fluctuations, thereby ensuring boiler combustion efficiency and operational stability while achieving stable and low-level nitrogen oxide emissions and reducing the load and operating costs of subsequent flue gas denitrification systems. Attached Figure Description

[0018] Figure 1 This is a flowchart illustrating the steps of a low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy, according to a specific embodiment of the present invention. Detailed Implementation

[0019] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] like Figure 1 As shown in the figure, a low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy is provided by a specific embodiment of the present invention, comprising the following steps: S1: Biomass raw materials are pretreated and biomass fuel is transported to the burner position in the recombustion zone of the coal-fired boiler furnace through a biomass fuel metering and conveying system that is independent of the original coal powder preparation and conveying system of the coal-fired boiler. The burner arrangement area in the coal-fired boiler furnace is divided into the main combustion zone, the recombustion zone and the burnout zone in sequence along the flue gas flow direction. S2: Inject pulverized coal or a mixture of pulverized coal and some biomass fuel into the main combustion zone, and introduce primary air in conjunction with the pulverized coal or the mixture into the main combustion zone to keep the main combustion zone in an oxygen-deficient combustion state in order to suppress the generation of nitrogen oxides. S3: The biomass fuel is injected into the reburning zone, causing the biomass fuel to undergo pyrolysis and gasification reactions in the reburning zone to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides in the flue gas from the main combustion zone to reduce the nitrogen oxides to nitrogen. S4: Secondary air is introduced into the burnout zone in stages to further burn the unburned products generated in the reburning zone; S5: Online monitoring of the combustion state parameters in the furnace during the combustion process of the coal-fired boiler, and dynamic adjustment of the biomass fuel supply, biomass fuel injection position and staged air volume distribution in the reburning zone based on the monitoring results, so as to achieve low nitrogen oxide emission control during the biomass-coal powder mixed combustion process.

[0021] In one embodiment, the combustion state parameters in the furnace include at least the NOx / O2 ratio at the furnace outlet, the CO / temperature ratio at the corresponding cross-section of the reburning zone, and the temperature of the main combustion zone. The method reconstructs the combustion organization in the furnace by combining staged fuel supply with staged air supply, so that pulverized coal mainly completes initial combustion and nitrogen oxide generation control in the main combustion zone, and biomass fuel mainly plays a reducing role in the reburning zone. The staged air supply in the burnout zone further reacts the incompletely combusted products, thereby achieving source reduction of nitrogen oxides while ensuring boiler combustion efficiency.

[0022] Specifically, biomass fuel is not transported through the original pulverized coal preparation and conveying system of the coal-fired boiler, but is sent to the corresponding burner position in the reburning zone through independently set silos, metering devices and conveying channels, so as to ensure the independence, adjustability and stability of biomass fuel supply.

[0023] Furthermore, the main combustion zone, recombustion zone, and burnout zone do not necessarily need to be separated by physical partitions. Instead, they are functional zones formed in the boiler furnace based on the burner arrangement, fuel injection location, air supply location, and flue gas flow path. These zones together constitute a synergistic reaction space for achieving low-NOx combustion.

[0024] Furthermore, under different boiler structures and different load conditions, the specific locations of the main combustion zone, recombustion zone, and burnout zone can be adaptively arranged according to the furnace elevation, burner operation layer, and flue gas residence characteristics, but their functional division remains the same: three stages: main combustion, recombustion, and burnout.

[0025] Based on the above basic implementation method, in step S1, the biomass raw material is crushed and ground to form powdered biomass fuel with a particle size of less than 1 mm, or it is transported while maintaining the particle shape; the pretreated biomass fuel is stored in an independent silo and transported to the reburning zone through an independent metering and conveying system.

[0026] Furthermore, the biomass raw materials can be straw-based biomass, wood-based biomass, or other biomass raw materials suitable for co-firing in boilers; the pretreatment method can be selected according to the initial particle size, moisture content, and transportation method of the raw materials so that the treated biomass fuel meets the requirements for independent transportation and injection into the reburning zone.

[0027] In one specific embodiment, in step S2, 70% to 85% of the total amount of fuel fed into the boiler furnace for staged combustion, or a mixture of pulverized coal and some biomass, is fed into the main combustion zone. Primary air equivalent to 70% to 80% of the theoretical air volume required for complete combustion of pulverized coal or mixed fuel is also fed into the main combustion zone, so that the excess air coefficient of the main combustion zone is controlled at 0.8 to 0.9.

[0028] In this embodiment, by controlling the amount of fuel and the amount of primary air supplied in the main combustion zone, the main combustion zone is kept in an oxygen-deficient combustion state, thereby reducing the generation rate of fuel-type nitrogen oxides. At the same time, the main combustion zone still maintains the necessary combustion intensity to provide the flue gas basis and temperature conditions for the nitrogen oxide reduction reaction in the subsequent reburning zone.

[0029] Furthermore, the main combustion zone can be injected with only pulverized coal, or with a mixture of pulverized coal and some biomass fuel. When using a mixed fuel, the proportion of biomass entering the main combustion zone can be adjusted according to the boiler load, biomass blending ratio, and combustion stability requirements.

[0030] In another specific embodiment, in step S3, biomass fuel accounting for 10% to 25% of the total amount of fuel fed into the boiler furnace for staged combustion is injected into the reburning zone. The reburning zone is located in the flue gas flow area after the main combustion zone and is equipped with a corresponding biomass reburning burner. In step S3, the reburning zone is not provided with an independent combustion air supply. The biomass fuel injected into the reburning zone undergoes pyrolysis and gasification to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides from the flue gas in the main combustion zone.

[0031] In this embodiment, biomass fuel is mainly used as a reburning reducing agent in the reburning zone, rather than as a primary fuel. Taking advantage of the rapid volatilization of biomass and its tendency to form reducing intermediates, biomass fuel generates reducing components such as CO, H2, and hydrocarbon free radicals in a low-oxygen environment, thereby promoting the conversion of nitrogen oxides into nitrogen.

[0032] Specifically, the biomass reburner corresponding to the reburning zone can be set up as one or more operating layers and connected to an independent metering and delivery system so as to adjust the amount and position of biomass fuel injection based on online monitoring results.

[0033] Furthermore, when the boiler load changes, the properties of biomass fuel fluctuate, or the level of nitrogen oxides generated in the furnace changes, the intensity and spatial distribution of the reducing atmosphere in the reburning zone can be changed by adjusting the amount of biomass fuel fed into the reburning zone and the corresponding burner operation position, thereby improving the low-NOx control effect.

[0034] In another specific embodiment, in step S4, the burnout zone is provided with multi-layer secondary air nozzles. Air accounting for 20% to 30% of the total air fed into the boiler furnace for staged combustion is fed into the burnout zone through these multi-layer secondary air nozzles in stages. This promotes further reaction of CO and residual carbon generated in the reburning zone and inhibits the formation of new thermal nitrogen oxides. In step S5, when the CO concentration at the cross-section through which the flue gas flows in the reburning zone is detected to be higher than a preset value, the secondary air volume fed into the burnout zone through the preset secondary air nozzles is increased to promote further reaction of the reduction reaction products and reduce the risk of boiler efficiency decline and excessive CO emissions. The staged air volume allocation includes the primary air ratio in the main combustion zone and the air volume allocation of each secondary air nozzle in the burnout zone. The total fuel volume refers to the total amount of pulverized coal and biomass fed into the main combustion zone and reburning zone for staged combustion in the furnace. The total air volume refers to the total air volume fed into the main combustion zone and burnout zone for staged combustion. The theoretical air volume refers to the amount of air required for the complete combustion of the corresponding portion of fuel in the main combustion zone.

[0035] Furthermore, the multi-layer secondary air nozzles can be arranged at intervals along the flue gas flow direction, so that the air entering the burnout zone comes into contact with the incomplete combustion products in stages, avoiding excessively high oxygen concentration or temperature in local areas, thus taking into account both the burnout effect and the thermal nitrogen oxide suppression effect.

[0036] Specifically, the preset secondary air nozzle can be a secondary air nozzle located near the reburning zone within the burnout zone, or a secondary air nozzle determined according to the actual structure of the boiler to preferentially enhance the burnout reaction; when an increase in CO concentration is detected, the subsequent oxidation conditions of the products after the reduction reaction can be improved by preferentially increasing the air supply of the preset secondary air nozzle.

[0037] In another specific embodiment, in step S5, online monitoring includes: monitoring the concentration of nitrogen oxides and oxygen at the boiler furnace outlet, monitoring the CO concentration and temperature at the cross section through which the flue gas flows in the reburning zone, and monitoring the flue gas temperature in the area corresponding to the main combustion zone; the cross section or area can be determined according to the corresponding burner elevation, flue gas flow direction, and measuring point arrangement; the target value and preset value can be preset according to the emission control requirements of the boiler unit, boiler load level, biomass co-firing ratio, fuel properties, and boiler operating experience, and can be corrected as the operating conditions change. The target value of nitrogen oxide concentration at the boiler furnace outlet is used to characterize the nitrogen oxide control target at the boiler outlet. The preset value of CO concentration at the cross section through which the flue gas flows in the reburning zone is used to characterize the burnout control threshold of the products after the reduction reaction in the reburning zone. The temperature monitoring results of the main combustion zone can be compared with the preset temperature control range to determine whether the combustion intensity of the main combustion zone meets the requirements for suppressing the generation of thermal nitrogen oxides and maintaining the conditions for the reduction reaction in the reburning zone. The control unit is connected to the online monitoring system, the biomass fuel metering and conveying system, the biomass reburner operation layer adjustment mechanism, the primary air adjustment mechanism, the secondary air adjustment mechanism, and the pulverized coal fineness adjustment mechanism. It is used to receive the combustion state parameters in the furnace collected by the online monitoring system and generate corresponding control commands based on the deviation between each monitoring parameter and the corresponding target value, preset value, or control range. The control commands include at least: commands for adjusting the biomass fuel supply to the reburning zone, commands for adjusting the biomass fuel injection position, commands for adjusting the air volume of each secondary air nozzle in the burnout zone, and commands for adjusting the primary air ratio and pulverized coal fineness in the main combustion zone. After receiving online monitoring data, the control unit comprehensively judges the nitrogen oxide concentration and oxygen concentration at the boiler furnace outlet, the CO concentration and temperature at the cross-section through which the flue gas flows in the reburning zone, and the flue gas temperature in the corresponding area of ​​the main combustion zone. When the nitrogen oxide concentration at the boiler furnace outlet is higher than the target value, the control unit outputs control commands to increase the biomass fuel supply to the reburning zone and / or adjust the corresponding biomass reburner operating layer. When the CO concentration at the cross-section through which the flue gas flows in the reburning zone is higher than a preset value, the control unit outputs control commands to increase the air volume of the preset secondary air nozzle in the burnout zone. When the temperature in the main combustion zone deviates from the preset temperature control range, the control unit outputs control commands to adjust the primary air ratio and / or pulverized coal fineness in the main combustion zone. Through the above monitoring, judgment, and execution processes, a closed-loop control link is formed for the boiler's low-NOx co-fired operation.

[0038] Furthermore, online monitoring can be achieved through a continuous online monitoring system, which is set at the corresponding monitoring location in the boiler flue or furnace to acquire key parameters reflecting the combustion status and pollutant generation in the furnace in real time.

[0039] Specifically, the nitrogen oxide concentration and oxygen concentration at the furnace outlet are used to reflect the overall combustion and denitrification effect; the CO concentration and temperature at the cross section through which the flue gas flows in the reburning zone are used to reflect the intensity of the reburning reduction reaction and the burnout requirement; and the flue gas temperature in the corresponding area of ​​the main combustion zone is used to reflect the combustion intensity and thermal nitrogen oxide generation tendency in the main combustion zone.

[0040] In another specific embodiment, dynamic adjustment is based on closed-loop feedback regulation of the control unit. The control unit adjusts the biomass fuel supply to the reburning zone, the biomass fuel injection position, and the staged air volume distribution according to the combustion state parameters obtained from online monitoring. When the nitrogen oxide concentration at the boiler furnace outlet is higher than the target value, the amount of biomass fuel supplied to the reburning zone is increased and / or the biomass fuel injection position is adjusted to enhance the reducing atmosphere. When the nitrogen oxide concentration at the boiler furnace outlet is lower than the target value, the amount of biomass fuel supplied to the reburning zone is reduced and / or the biomass fuel injection position is adjusted. The target value can be preset according to boiler emission requirements, load conditions, biomass co-firing ratio, and operating experience. The control unit can be implemented by DCS, PLC, or industrial control computer and connected to the monitoring system, metering and conveying system, and damper actuator.

[0041] Furthermore, the control unit can also receive boiler load commands and, in conjunction with the combustion status parameters obtained from online monitoring, make a comprehensive judgment on fuel supply and air volume distribution, thereby enabling the method to adapt to the operational needs under different load levels and biomass co-firing conditions.

[0042] Furthermore, the adjustment of the biomass fuel injection position can be achieved by adjusting the operation layer of the corresponding biomass reburner, thereby changing the spatial distribution of the reburning reaction zone in the furnace and improving the matching degree between the reburning reduction effect and the flue gas flow field and temperature field.

[0043] In one specific implementation, in step S5, the proportion of primary air and the fineness of pulverized coal fed into the main combustion zone are adjusted according to the temperature monitoring results of the main combustion zone, so as to regulate the combustion temperature of the main combustion zone, reduce the generation of thermal nitrogen oxides, and maintain the reduction reaction conditions in the reburning zone.

[0044] Specifically, when the temperature in the main combustion zone is too high, the temperature level in the main combustion zone can be reduced by adjusting the primary air ratio and / or the fineness of the pulverized coal to suppress the generation of thermal nitrogen oxides; when the temperature in the main combustion zone is too low and affects the combustion stability or subsequent reburning reaction conditions, the primary air ratio and / or the fineness of the pulverized coal can be adjusted accordingly to restore a suitable combustion state in the main combustion zone.

[0045] Furthermore, by coordinating and controlling parameters such as the temperature of the main combustion zone, the CO concentration in the recombustion zone, the nitrogen oxide concentration and oxygen concentration at the furnace outlet, the boiler can maintain a low-NOx, stable and efficient operating state under a high proportion of biomass co-firing conditions.

[0046] In one specific implementation, taking the operation of a large coal-fired boiler co-fired with biomass as an example, the low-NOx method of biomass-coal powder co-fired based on fuel and air staged synergy provided by this invention is used for operation control. The method includes the following steps: First, the biomass raw materials are pretreated. In this embodiment, one of straw-type biomass and wood-type biomass is selected as the reburning fuel. After being processed by a dedicated crushing and grinding system, it forms powdered biomass fuel with a particle size of less than 1 mm, which is stored in an independent silo; the coal powder is supplied through the boiler's original pulverizing and conveying system. Subsequently, the powdered biomass fuel is transported to the biomass reburning burner position in the corresponding reburning zone of the boiler furnace through an independent metering and conveying system, so that the biomass fuel conveying channel is independent of the original coal powder preparation and conveying system; second, in the boiler furnace burner arrangement area, the main combustion zone, reburning zone and burnout zone are formed sequentially according to the flue gas flow path. The system comprises three main combustion zones: the main combustion zone for initial combustion of pulverized coal and suppression of initial nitrogen oxide formation; the recombustion zone for injecting biomass fuel and creating a reducing atmosphere; and the burnout zone for promoting further reaction of incompletely combusted products through staged air supply. During the main combustion zone operation, pulverized coal, or a mixture of pulverized coal and some biomass fuel, accounting for 80% of the total fuel fed into the boiler furnace for staged combustion, is supplied. Simultaneously, primary air, equivalent to 75% of the theoretical air volume required for complete combustion of this portion of fuel, is supplied to the main combustion zone, maintaining the excess air coefficient at 0.85. Through this control, the main combustion zone maintains an oxygen-deficient combustion state, thereby suppressing nitrogen oxide formation and providing suitable flue gas composition and temperature conditions for the subsequent reduction reaction in the recombustion zone. During the recombustion zone operation, biomass fuel, accounting for 15% of the total fuel fed into the boiler furnace for staged combustion, is injected into the recombustion zone through an independent metering and delivery system. The recombustion zone is equipped with a biomass recombustion burner and does not have an independent combustion air supply. Biomass fuel injected into the reburning zone undergoes pyrolysis and gasification in the low-oxygen, high-temperature environment formed by the flue gas in the main combustion zone, generating reducing intermediate products including CO, H2, and hydrocarbon free radicals. These reducing intermediate products react with nitrogen oxides from the flue gas in the main combustion zone, converting the nitrogen oxides into nitrogen. During the burnout zone operation phase, the burnout zone is equipped with multi-layer secondary air nozzles, which deliver 25% of the total air supplied to the boiler furnace for staged combustion into the burnout zone in stages. This promotes further reaction of CO and residual char generated in the reburning zone and inhibits the formation of new thermal nitrogen oxides.The multi-layer secondary air nozzles can be arranged at intervals along the flue gas flow direction, so that the air entering the burnout zone comes into contact with the incompletely combusted products in stages, thereby taking into account both the burnout effect and the thermal nitrogen oxide suppression effect. In the online monitoring and dynamic adjustment stage, a continuous online monitoring system is set at the corresponding positions of the boiler flue and furnace to monitor the nitrogen oxide and oxygen concentrations at the boiler furnace outlet, the CO concentration and temperature at the cross section through which the flue gas flows in the reburning zone, and the flue gas temperature in the corresponding area of ​​the main combustion zone in real time. A control unit is established to perform closed-loop feedback adjustment on the biomass fuel supply to the reburning zone, the biomass fuel injection position, and the staged air volume distribution based on the combustion state parameters obtained from the online monitoring.

[0047] In this embodiment, when the nitrogen oxide concentration at the boiler furnace outlet is detected to be higher than the target value, the control unit increases the amount of biomass fuel supplied to the reburning zone and / or adjusts the operation position of the biomass reburner to enhance the reducing atmosphere in the reburning zone. When the nitrogen oxide concentration at the boiler furnace outlet is detected to be lower than the target value, the control unit reduces the amount of biomass fuel supplied to the reburning zone and / or adjusts the biomass fuel injection position accordingly. Simultaneously, when the CO concentration at the cross-section through which the flue gas flows in the reburning zone is detected to be higher than a preset value, the control unit increases the secondary air volume supplied through the preset secondary air nozzle in the burnout zone to promote further reaction of the reduction products and reduce the risk of increased CO emissions and decreased boiler efficiency.

[0048] Specifically, when the temperature in the main combustion zone is detected to be too high, the control unit reduces the proportion of primary air supplied to the main combustion zone and / or adjusts the fineness of pulverized coal to lower the combustion temperature and suppress the formation of thermal nitrogen oxides. When the temperature in the main combustion zone is detected to be too low and affects the subsequent reburning reaction conditions, the control unit adjusts the proportion of primary air and / or the fineness of pulverized coal accordingly to restore a suitable combustion state in the main combustion zone. Through the coordinated feedback regulation of fuel quantity, injection location, and air volume distribution, the boiler can continuously maintain a low-NOx, stable, and efficient operating state under biomass co-firing conditions.

[0049] To aid in a better understanding of the present invention, a more comprehensive and specific embodiment is described, in which the present invention provides a low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy, comprising the following steps: S1: Biomass raw materials are pretreated and biomass fuel is transported to the burner position in the recombustion zone of the coal-fired boiler furnace through a biomass fuel metering and conveying system that is independent of the original coal powder preparation and conveying system of the coal-fired boiler. The burner arrangement area in the coal-fired boiler furnace is divided into the main combustion zone, the recombustion zone and the burnout zone in sequence along the flue gas flow direction. S2: Inject pulverized coal or a mixture of pulverized coal and some biomass fuel into the main combustion zone, and introduce primary air in conjunction with the pulverized coal or the mixture into the main combustion zone to keep the main combustion zone in an oxygen-deficient combustion state in order to suppress the generation of nitrogen oxides. S3: The biomass fuel is injected into the reburning zone, causing the biomass fuel to undergo pyrolysis and gasification reactions in the reburning zone to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides in the flue gas from the main combustion zone to reduce the nitrogen oxides to nitrogen. S4: Secondary air is introduced into the burnout zone in stages to further burn the unburned products generated in the reburning zone; S5: Online monitoring of combustion state parameters in the furnace during the combustion process of coal-fired boilers, and dynamic adjustment of biomass fuel supply, biomass fuel injection location and staged air volume distribution in the reburning zone based on the monitoring results, so as to achieve low nitrogen oxide emission control during the biomass-coal pulverized coal mixed combustion process.

[0050] In this embodiment, in step S1, the biomass raw material is crushed and ground to form powdered biomass fuel with a particle size of less than 1 mm, or it is transported while maintaining its particle shape; the pretreated biomass fuel is stored in an independent silo and transported to the recombustion zone through an independent metering and conveying system; in step S2, 70% to 85% of the total amount of fuel fed into the boiler furnace for staged combustion, or a mixture of pulverized coal and some biomass, is fed into the main combustion zone, and primary air equivalent to 70% to 80% of the theoretical air volume required for complete combustion of pulverized coal or mixed fuel is fed into the main combustion zone, so that the excess air coefficient of the main combustion zone is controlled at 0.8 to 0.9; in step S3, 10% to 25% of the total amount of biomass fuel fed into the boiler furnace for staged combustion is injected into the recombustion zone, which is located in the flue gas flow area after the main combustion zone, and the recombustion zone is set up within the flue gas flow area after the main combustion zone, and the primary air flow is controlled within the flue gas flow area. A biomass reburner should be installed. In step S3, no independent combustion air supply is provided in the reburning zone. The biomass fuel injected into the reburning zone undergoes pyrolysis and gasification to generate reducing intermediate products. These reducing intermediate products react with nitrogen oxides from the flue gas in the main combustion zone. In step S4, the burnout zone is equipped with multi-layer secondary air nozzles. Air accounting for 20% to 30% of the total air fed into the boiler furnace for staged combustion is sent to the burnout zone through the multi-layer secondary air nozzles in stages to promote further reaction of CO and residual carbon generated in the reburning zone and inhibit the generation of new thermal nitrogen oxides. In step S5, when the CO concentration at the cross-section through which the flue gas flows in the reburning zone is detected to be higher than a preset value, the secondary air volume supplied through the preset secondary air nozzles in the burnout zone is increased to promote further reaction of the reduction products and reduce the risk of boiler efficiency decline and CO emission exceeding the standard.

[0051] Specifically, in step S5, online monitoring includes: monitoring the nitrogen oxide concentration and oxygen concentration at the boiler furnace outlet, monitoring the CO concentration and temperature at the cross-section through which the flue gas flows in the reburning zone, and monitoring the flue gas temperature in the corresponding area of ​​the main combustion zone; dynamic adjustment is a closed-loop feedback adjustment based on the control unit; the control unit adjusts the biomass fuel supply, biomass fuel injection position, and staged air volume distribution in the reburning zone according to the combustion state parameters obtained from online monitoring; when the nitrogen oxide concentration at the boiler furnace outlet is higher than the target value, the amount of biomass fuel supplied to the reburning zone is increased and / or the biomass fuel injection position is adjusted to enhance the reducing atmosphere; when the nitrogen oxide concentration at the boiler furnace outlet is lower than the target value, the amount of biomass fuel supplied to the reburning zone is reduced and / or the biomass fuel injection position is adjusted; in step S5, based on the temperature monitoring results of the main combustion zone, the proportion of primary air and the fineness of pulverized coal supplied to the main combustion zone are adjusted to regulate the combustion temperature of the main combustion zone, reduce the generation of thermal nitrogen oxides, and maintain the reduction reaction conditions in the reburning zone.

[0052] In summary, the embodiments disclosed herein have at least the following technical effects: This invention constructs a main combustion zone, a recombustion zone, and a burnout zone within the boiler furnace, and coordinates fuel grading and air grading to maintain an oxygen-deficient combustion state in the main combustion zone, create a reducing atmosphere in the recombustion zone, and achieve graded oxygen replenishment in the burnout zone. This enables the suppression of nitrogen oxide generation at the combustion source and promotes the conversion of generated nitrogen oxides into nitrogen, significantly improving the low-nitrogen control effect under biomass-coal pulverized coal co-combustion conditions. This invention utilizes the reducing intermediate products such as CO, H2 and hydrocarbon free radicals generated after the pyrolysis and gasification of biomass fuel in the reburning zone to react with nitrogen oxides in the flue gas of the main combustion zone, giving full play to the reducing advantages of biomass as a reburning fuel and achieving the synergistic unity of biomass resource utilization and pollutant control. This invention employs a biomass fuel delivery channel independent of the original pulverized coal system, which can quantitatively and controllably deliver biomass fuel to the corresponding burner position in the reburning zone, providing a reliable basis for the staged supply of biomass fuel and the reduction reaction in the reburning zone, and is conducive to improving the adjustability and stability of system operation. This invention promotes the further reaction of incompletely combusted products generated in the reburning zone by setting up multiple layers of secondary air nozzles and staged air supply in the burnout zone, reducing the risk of increased CO and decreased combustion efficiency caused by excessively strong reducing atmosphere, while avoiding the generation of new thermal nitrogen oxides due to local high temperature, thus balancing low nitrogen emissions and boiler combustion efficiency. This invention monitors NOx and oxygen concentrations at the furnace outlet, CO concentration and temperature at the corresponding cross section of the reburning zone, and temperature of the main combustion zone online, and implements closed-loop feedback regulation based on the control unit. This enables the system to dynamically adjust the biomass fuel supply, injection location, and staged air volume distribution according to boiler load changes and fuel fluctuations, thereby enhancing the system's adaptability to different operating conditions.

[0053] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A low-NOx method for biomass-coal pulverized coal co-combustion with staged fuel-air synergy, characterized in that, Includes the following steps: S1: The biomass raw materials are pretreated and transported to the burner position in the recombustion zone of the coal-fired boiler furnace through a biomass fuel metering and conveying system that is independent of the original coal powder preparation and conveying system of the coal-fired boiler; wherein the burner arrangement area of ​​the coal-fired boiler furnace is divided into the main combustion zone, the recombustion zone and the burnout zone in sequence along the flue gas flow direction. S2: Inject pulverized coal or a mixture of pulverized coal and a portion of the biomass fuel into the main combustion zone, and introduce primary air in conjunction with the pulverized coal or the mixed fuel into the main combustion zone to keep the main combustion zone in an oxygen-deficient combustion state, thereby suppressing the generation of nitrogen oxides. S3: The biomass fuel is injected into the re-combustion zone, causing the biomass fuel to undergo pyrolysis and gasification reactions in the re-combustion zone to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides in the flue gas from the main combustion zone to reduce the nitrogen oxides to nitrogen. S4: Secondary air is introduced into the burnout zone in stages to further burn the unburned products generated in the reburning zone; S5: Monitor the combustion state parameters in the furnace during the combustion process of the coal-fired boiler online, and dynamically adjust the biomass fuel supply, biomass fuel injection position and staged air volume distribution in the reburning zone according to the monitoring results.

2. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S1, the biomass raw material is crushed and ground to form powdered biomass fuel with a particle size of less than 1 mm, or it is transported while maintaining the particle shape; the pretreated biomass fuel is stored in an independent silo and transported to the reburning zone through an independent metering and conveying system.

3. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S2, 70% to 85% of the total amount of fuel fed into the boiler furnace for staged combustion, or a mixture of pulverized coal and some biomass, is fed into the main combustion zone. Primary air equivalent to 70% to 80% of the theoretical air volume required for complete combustion of the pulverized coal or the mixed fuel is also fed into the main combustion zone, so that the excess air coefficient of the main combustion zone is controlled at 0.8 to 0.

9.

4. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S3, biomass fuel accounting for 10% to 25% of the total amount of fuel fed into the boiler furnace for staged combustion is injected into the reburning zone. The reburning zone is located in the flue gas flow area after the main combustion zone and is equipped with a biomass reburning burner.

5. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S3, the re-combustion zone is not provided with an independent combustion air supply. The biomass fuel injected into the re-combustion zone undergoes pyrolysis and gasification to generate reducing intermediate products. The reducing intermediate products react with nitrogen oxides from the flue gas in the main combustion zone.

6. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S4, the burnout zone is provided with multi-layer secondary air nozzles. Air accounting for 20% to 30% of the total air fed into the boiler furnace for staged combustion is fed into the burnout zone through the multi-layer secondary air nozzles in stages to promote the reaction of CO and residual carbon generated in the reburning zone and inhibit the formation of new thermal nitrogen oxides.

7. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 6, characterized in that, In step S5, when the CO concentration at the cross section through which the flue gas flows in the reburning zone is detected to be higher than a preset value, the secondary air volume supplied through the preset secondary air nozzle in the burnout zone is increased to promote the reaction of reduction reaction products and reduce the risk of boiler efficiency decline and CO emission exceeding the standard.

8. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, In step S5, the online monitoring includes: monitoring the nitrogen oxide concentration and oxygen concentration at the boiler furnace outlet, monitoring the CO concentration and temperature at the cross section through which the flue gas flows in the reburning zone, and monitoring the flue gas temperature in the area corresponding to the main combustion zone.

9. The low-NOx method for biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to claim 1, characterized in that, The dynamic adjustment is a closed-loop feedback adjustment based on the control unit; the control unit adjusts the biomass fuel supply, biomass fuel injection position and staged air volume distribution in the reburning zone according to the combustion state parameters obtained by online monitoring; when the nitrogen oxide concentration at the boiler furnace outlet is detected to be higher than the target value, the amount of biomass fuel supplied to the reburning zone is increased and / or the biomass fuel injection position is adjusted to enhance the reducing atmosphere; When the nitrogen oxide concentration at the boiler furnace outlet is detected to be lower than the target value, the amount of biomass fuel sent into the reburning zone is reduced and / or the biomass fuel injection position is adjusted.

10. The method for low-NOx biomass-coal pulverized coal co-combustion with fuel-air staged synergy according to any one of claims 1 to 9, characterized in that: In step S5, the proportion of primary air and the fineness of pulverized coal supplied to the main combustion zone are adjusted according to the temperature monitoring results of the main combustion zone, so as to regulate the combustion temperature of the main combustion zone and reduce the generation of thermal nitrogen oxides.

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