Alkali furnace low NOx emission combustion control method based on flue gas recirculation

Abstract

The invention discloses an alkali furnace low NOx emission combustion control method based on flue gas recirculation. According to the method, part of dedusted flue gas is conveyed to a tertiary air pipe through an induced draft fan and mixed with air to form air-flue gas mixed gas, and the air-flue gas mixed gas is injected into a hearth from a tertiary air distribution port of the alkali furnace. According to the method, a geometric structure model and a numerical calculation model of the alkali furnace are established, and a temperature field, a concentration field and heat efficiency in the furnace are obtained through CFD calculation; by adjusting different flue gas recirculation rates, the influence of the flue gas recirculation rate on the temperature field, the concentration field and the heat efficiency in the furnace is simulated, and the optimized flue gas recirculation rate is obtained. The oxygen content of tertiary air can be reduced, generation of NOx in a high-temperature combustion area is reduced, pollutant emission is reduced, and meanwhile, the temperature of the lower portion of the superheater can be reduced and high-temperature thermal stress corrosion can be relieved through the increased recirculating flue gas flow.

Description

Technical Field

[0001] This invention relates to the field of black liquor combustion technology in alkali recovery furnaces, and in particular to a low-NOx emission combustion control method for alkali furnaces based on flue gas recirculation. Background Technology

[0002] An alkali recovery boiler, or alkali furnace for short, is a special type of boiler used in pulp production lines. It plays a crucial role in recovering alkali from black liquor combustion, generating steam to produce electricity, and maintaining the power balance of pulp production. Currently, the emission standard for NOx (nitrogen oxides) from alkali furnace flue gas is no higher than 200 mg / m³. 3 .

[0003] Currently, under conditions of good operation control and stable combustion load, the NOx emission concentration of alkali furnaces is generally between 200 and 300 mg / m³. 3 The level of NOx emissions is high. Therefore, additional denitrification methods are usually required to meet NOx emission requirements. These methods mainly include: 1. Modifying the boiler body to low-NOx combustion and adding a selective non-catalytic reduction (SCR) denitrification system; 2. Injecting denitrification agents into the alkali furnace; 3. Using ozone or chlorine dioxide oxidation for denitrification; 4. Electron beam electrostatic precipitator denitrification; 5. Low-temperature SCR denitrification. However, the above methods generally suffer from drawbacks such as complex processes, difficult control, severe equipment corrosion, and easy catalyst deactivation. In addition, the bottom of the alkali furnace superheater often faces overheating, which further aggravates the corrosion of the heat exchange surface. Therefore, the alkali furnace needs frequent shutdowns for maintenance and replacement of corroded heat exchange surfaces during actual operation.

[0004] Therefore, how to effectively reduce NOx formation in the furnace and lower the furnace outlet temperature is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] To overcome the problems existing in the background technology, this invention provides a low-NOx emission combustion control method for alkali furnaces based on flue gas recirculation. By rationally controlling the flow rate of the recycled flue gas, the technical effects of reducing the flue gas temperature at the furnace outlet, reducing NOx formation, and improving thermal efficiency are achieved.

[0006] The technical solution adopted by this invention to solve its technical problem is:

[0007] A method for controlling low NOx emissions from an alkali furnace based on flue gas recirculation involves mixing a portion of the flue gas after dust removal from the alkali furnace with tertiary air and then feeding it into the tertiary air inlet of the alkali furnace for recirculation. This method aims to reduce the oxygen content in the high-temperature combustion zone of the tertiary air inside the furnace, reduce the generation of nitrogen oxides, lower the flue gas temperature at the furnace outlet, and improve thermal efficiency.

[0008] Furthermore, the recirculation ratio of the flue gas after dust removal from the alkali furnace is controlled by adjusting the opening of the flow control valve in the pipeline and the pressurization of the blower.

[0009] Furthermore, part of the flue gas after dust removal from the alkali furnace is transported to the flue gas cooler of the alkali furnace through a flow control valve, and another part is transported to the tertiary air inlet of the alkali furnace through a flow control valve and a fan in the recirculation branch pipeline. After mixing with the tertiary air, it is used for flue gas recirculation. The tertiary air delivery pipeline is equipped with a flow control valve and a fan for adjusting and controlling the flow rate.

[0010] Furthermore, the ratio of flue gas used for recirculation after dust removal in the alkali furnace is controlled by the flow control valve at the inlet of the flue gas cooler of the alkali furnace, the flow control valve and fan on the tertiary air delivery pipeline, and the flow control valve and fan on the branch pipeline of the recirculation pipeline.

[0011] Furthermore, the recirculation rate of the flue gas after dust removal from the alkali furnace is 10% to 30%.

[0012] Furthermore, by establishing numerical models of the combustion process in the furnace, the heat exchange process in the superheater and economizer of the alkali furnace, the combustion and flow heat exchange in the alkali furnace are simulated to obtain the distribution of temperature field, velocity field, concentration field and heat exchanger heat exchange capacity in the furnace. By adjusting different flue gas recirculation ratios, the influence of different flue gas recirculation ratios on the temperature field, concentration field and thermal efficiency in the furnace is simulated, and the recirculation ratio of flue gas after dust removal in the alkali furnace is optimized. The specific steps are as follows:

[0013] S1. Establish the geometric structure model and numerical calculation model of the alkali furnace, and obtain the temperature field, concentration field and thermal efficiency inside the furnace through CFD fluid simulation calculation.

[0014] S2, adjust the flue gas recirculation ratio, simulate the effects of different flue gas recirculation ratios on the furnace temperature field, concentration field and thermal efficiency, and optimize the flue gas recirculation ratio.

[0015] Furthermore, step S1 includes:

[0016] S11, the flow of gas inside the alkali furnace is modeled using the continuity equation, momentum conservation equation, and energy conservation equation; the high-speed jet air supply of the primary, secondary, and tertiary air in the alkali furnace is simulated using an achievable k-ε turbulence model to simulate jet propagation; the interaction between the achievable k-ε turbulence model and the chemical reaction is modeled using a finite-rate / eddy-dissipation model, and a coupled solution method is used for pressure-velocity coupling solution;

[0017] S12, the discrete coordinate method is used to simulate the radiative heat transfer of the alkali furnace, and a vortex dissipation model is established to simulate the gas phase combustion model of the alkali furnace gas. The gas phase combustion model uses component transport to calculate the gas phase reaction.

[0018] Furthermore, step S2 includes:

[0019] S21, calculate the proportion of flue gas used for recirculation after dust removal in the alkali furnace, using the following iterative calculation:

[0020]

[0021] In the formula: c i.mix The concentration of components in the air-flue gas mixture, expressed in vol%; c i.air The concentration of tertiary air components, in vol%; c i.flue The concentration of flue gas components, in vol%; m air The third-order air mass flow rate is expressed in kg / s and m³. flue T represents the mass flow rate of recirculated flue gas, expressed in kg / s. mix Temperature after mixing, in K; T air Air temperature, unit K; T flue Flue gas temperature, in K;

[0022] S22, the DEFINE_PROFILE macro is used to obtain the component ratio at the outlet boundary of the alkali furnace flue gas, and the mass flow rate and outlet temperature of each component in the circulating flue gas are calculated accordingly. The composition and temperature of the tertiary air inlet are calculated according to the iterative calculation formula in step S21, which are used as the inlet boundary conditions of the tertiary air.

[0023] The beneficial effects of this invention are as follows: By mixing a portion of the flue gas from the alkali furnace combustion dust removal process with tertiary air before feeding it into the alkali furnace for flue gas recirculation, the oxygen content in the high-temperature combustion zone within the furnace is reduced, thus decreasing the generation of nitrogen oxides and lowering the flue gas temperature at the furnace outlet. Furthermore, the increased flue gas flow rate enhances convective heat transfer, improving the thermal efficiency of the alkali furnace. This invention, through the adjustment and control of various flow control valves, allows for flexible adjustment of the recirculated flue gas flow rate according to the operating conditions of the alkali furnace, increasing adaptability to different black liquor types and flow rates. Compared to existing technologies, this invention employs a source-level control solution, reducing or eliminating the need for a downstream denitrification system, significantly lowering system investment and maintenance costs.

[0024] This invention establishes a geometric and numerical model of an alkali furnace, and uses CFD simulation to obtain the furnace's temperature field, concentration field, and thermal efficiency. By adjusting different flue gas recirculation ratios, the impact of the recirculation ratio on the furnace's temperature field, concentration field, and thermal efficiency is simulated, resulting in an optimized recirculation ratio. This invention can reduce the oxygen content of tertiary air, decrease NOx formation in the high-temperature combustion zone, and reduce pollution emissions. Simultaneously, the increased recirculated flue gas flow helps lower the temperature at the bottom of the superheater, mitigating high-temperature thermal stress corrosion.

[0025] This invention allows for flexible adjustment of the flue gas recirculation ratio of different alkali furnaces through the regulation and setting of various valves and fans in the pipeline, in order to match different types of black liquor and different operating conditions. It is flexible, convenient and highly applicable, and adopts a technical solution that controls the source. It has broad application prospects in the field of black liquor combustion technology in alkali recovery furnaces. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a combustion device system according to an embodiment of the present invention;

[0027] Figure 2 This is a temperature distribution cloud map of the central cross section of the alkali furnace in an embodiment of the present invention;

[0028] Figure 3 This is a temperature distribution cloud map of the outlet section of the alkali furnace in an embodiment of the present invention.

[0029] Explanation of the reference numerals in the figure:

[0030] 1-Alkali furnace; 2-Flue gas dust collector; 3-Cooler flow control valve; 4-Flue gas cooler; 5-Chimney; 6-Recirculation flow control valve; 7-Recirculation fan; 8-Tertiary air flow control valve; 9-Tertiary air fan; 10-Tertiary air inlet; 11-Tertiary air duct; 12-Recirculation pipeline; 13-Flue gas dust collector output pipeline. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0032] Example 1

[0033] like Figure 1 As shown, the low NOx emission combustion control system for alkali furnace based on flue gas recirculation adopted in this embodiment includes an alkali furnace 1, a flue gas dust collector 2, a flue gas dust collector output pipe 13, a flue gas cooler 4, and a cooler flow control valve 3.

[0034] The alkali furnace 1 is equipped with a tertiary air inlet 10 and a tertiary air duct 11 connected thereto. The front end of the tertiary air duct 11 is equipped with a tertiary air flow control valve 8 and a tertiary air blower 9 in sequence. The tail flue of the alkali furnace 1 is equipped with an economizer. The flue gas dust collector 2 is connected to the flue gas outlet of the economizer of the alkali furnace 1. The output pipe 13 of the flue gas dust collector is connected to the flue gas cooler 4 through the cooler flow control valve 3. The outlet of the flue gas cooler 4 is connected to a chimney 5.

[0035] The flue gas dust collector output pipe 13 is provided with a bypass recirculation pipe 12, which is connected to the tertiary air inlet 10 of the tertiary air pipe 11. The recirculation pipe 12 is provided with a recirculation flow control valve 6 and a recirculation fan 7 for adjusting and controlling the recirculation ratio of the flue gas after dust removal in the alkali furnace 1.

[0036] The cooler flow control valve 3, recirculation flow control valve 6, recirculation fan 7, tertiary air flow control valve 8, and tertiary air fan 9 are used for the linkage adjustment and control of the recirculation ratio of the flue gas after dust removal from the alkali furnace 1.

[0037] The bypass interface of the recirculation pipe 12 on the flue gas dust collector output pipe 13 is located at the front end of the inlet of the cooler flow control valve 3.

[0038] Ultrasonic flow meters are installed on the recirculation pipe 12 and the tertiary air pipe 11, respectively.

[0039] The alkali furnace 1 is also equipped with a primary air duct and a secondary air duct.

[0040] Implementation process: The high-temperature flue gas generated by combustion in the lower furnace of the alkali furnace 1 flows through the upper superheater, boiling tube screen, and economizer before flowing to the flue gas dust collector 2. The flue gas dust collector 2 captures most of the fly ash in the flue gas. Subsequently, most of the flue gas flows through the flue gas dust collector output pipe 13, exchanges heat with the boiler feedwater in the flue gas cooler 4, and is then sent into the chimney 5 to be discharged into the atmosphere. At the same time, another part of the flue gas is sent through the recirculation pipe 12 as recirculated flue gas, passes through the recirculation flow control valve 6 and the recirculation fan 7 to the tertiary air pipe 11, mixes with the air sent by the tertiary air fan 9, and then enters the tertiary air inlet 10.

[0041] Regulating and controlling the circulating flue gas flow rate: Open the recirculation fan 7 and the recirculation flow control valve 6, and adjust the recirculation flue gas flow rate in the flue gas dust collector output pipe 13 by adjusting the opening of the cooler flow control valve 3 and the recirculation flow control valve 6. Simultaneously, adjust the opening of the tertiary air flow control valve 8 and the tertiary air fan 9 to maintain a stable flue gas flow rate after dust removal. The ratio of flue gas after dust removal in the alkali furnace 1 used for recirculation is controlled by adjusting the cooler flow control valve 3, the recirculation flow control valve 6, the recirculation fan 7, the tertiary air flow control valve 8, and the tertiary air fan 9 at the inlet of the flue gas cooler 4 of the alkali furnace 1.

[0042] If flue gas recirculation is not required, keep cooler flow control valve 3 and tertiary air flow control valve 8 fully open, and shut down recirculation fan 7 and recirculation flow control valve 6.

[0043] In this embodiment, the alkali furnace 1 has a processing load of 2200 tds / day. An ultrasonic flow meter is used to monitor the flue gas flow rate of the recirculation pipeline 12. The recirculation ratio of the flue gas after dust removal is controlled to 10% by adjusting the cooler flow control valve 3, the recirculation flow control valve 6, the recirculation fan 7, the tertiary air flow control valve 8, and the tertiary air fan 9 in a linked manner.

[0044] Numerical models of the combustion process in the furnace, superheater, and economizer of alkali furnace 1 were established to simulate the combustion and flow heat transfer in alkali furnace 1. The temperature, velocity, and concentration field distributions within the furnace, as well as the heat exchange capacity of the heat exchangers, were obtained through simulation. By adjusting different flue gas recirculation ratios, the effects of different recirculation ratios on the temperature, concentration, and thermal efficiency within the furnace were simulated. The optimal recirculation ratio of the flue gas after dust removal in alkali furnace 1 was then determined. The specific steps are as follows:

[0045] S1. Establish the geometric structure model and numerical calculation model of alkali furnace 1, and obtain the temperature field, concentration field and thermal efficiency inside the furnace through CFD fluid simulation calculation.

[0046] S2, adjust the flue gas recirculation ratio, simulate the effect of different flue gas recirculation ratios on the furnace temperature field, concentration field and thermal efficiency, and optimize the flue gas recirculation ratio.

[0047] Step S1 includes:

[0048] S11, the flow of gas inside alkali furnace 1 is modeled using the continuity equation, momentum conservation equation, and energy conservation equation; the high-speed jet air supply of primary, secondary, and tertiary air in alkali furnace 1 is simulated using an achievable k-ε turbulence model to model jet propagation; the interaction between the achievable k-ε turbulence model and the chemical reaction is modeled using a finite-rate / eddy-dissipation model, and a coupled solution method is used for pressure-velocity coupling solution;

[0049] S12, the discrete coordinate method is used to simulate the radiative heat transfer of alkali furnace 1, and a vortex dissipation model is established to simulate the gas phase combustion model of the gas in alkali furnace 1. The gas phase combustion model uses component transport to calculate the gas phase reaction.

[0050] Step S2 includes:

[0051] S21, calculate the proportion of flue gas used for recirculation after dust removal in alkali furnace 1, using the following iterative calculation:

[0052]

[0053] In the formula: c i.mix The concentration of components in the air-flue gas mixture, expressed in vol%; c i.air The concentration of tertiary air components, in vol%; c i.flue The concentration of flue gas components, in vol%; m air The third-order air mass flow rate is expressed in kg / s and m³. flue T represents the recirculated flue gas mass flow rate, expressed in kg / s. mix Temperature after mixing, in K; T air Air temperature, unit K; T flue Flue gas temperature, unit K;

[0054] S22, the DEFINE_PROFILE macro is used to obtain the component ratio of the flue gas outlet boundary of the alkali furnace 1, and the mass flow rate and outlet temperature of each component in the circulating flue gas are calculated accordingly. The composition and temperature of the tertiary air inlet are calculated according to the iterative calculation formula in step S21, which are used as the inlet boundary conditions of the tertiary air.

[0055] Actual verification results show that the oxygen content in the high-temperature zone of the tertiary air area of ​​this embodiment is reduced by 25.0%, thus reducing the generation of nitrogen oxides. Area averaging of NOx concentration at the flue gas outlet cross-section shows a 13.0% reduction in NOx emission concentration. The flue gas outlet temperature in the furnace decreased from 1412K to 1342K, alleviating thermal stress corrosion of the superheater. A 10% increase in flue gas flow rate enhanced convective heat transfer, resulting in a 3.6% increase in total heat transfer and a 3.6% increase in indicated thermal efficiency.

[0056] like Figure 2 and Figure 3 The temperature distribution cloud maps of the central section and furnace outlet section of alkali furnace 1 are shown under different flue gas reuse rates, demonstrating the significant effect of flue gas reuse on reducing the average furnace outlet temperature and improving temperature uniformity.

[0057] Example 2

[0058] The combustion control system of the alkali furnace in Example 1 is used to control the combustion of alkali furnace 1. In this example, the processing load of alkali furnace 1 is 2200 tds / day. The combustion control system of alkali furnace 1 in Example 1 is still used to control the combustion of alkali furnace 1. The flue gas flow rate of the recirculation pipeline 12 is monitored by an ultrasonic flow meter. The recirculation ratio of the flue gas after dust removal is controlled to be 20% by adjusting the flow control valve 3 of the cooler, the flow control valve 6 of the recirculation, the recirculation fan 7, the tertiary air flow control valve 8, and the tertiary air fan 9.

[0059] In this embodiment, the control parameters are still based on the system and method of embodiment 1. A numerical model of the combustion process in the furnace, the heat exchange process of the superheater and the economizer of the alkali furnace 1 is established. The combustion and flow heat exchange of the alkali furnace 1 are simulated to obtain the distribution of the temperature field, velocity field, concentration field and heat exchanger in the furnace. By adjusting different flue gas recirculation ratios and mixing them with tertiary air, the influence of the flue gas recirculation ratio on the temperature field, concentration field and thermal efficiency in the furnace is simulated.

[0060] Actual verification results show that the oxygen content in the high-temperature zone of the tertiary air section of alkali furnace 1 decreased by 40%, reducing the generation of nitrogen oxides. Area averaging of NOx concentration at the flue gas outlet section showed a 24.6% reduction in NOx emission concentration. The flue gas outlet temperature in the furnace decreased from 1412K to 1280K, alleviating thermal stress corrosion of the superheater. A 20% increase in flue gas flow rate enhanced convective heat transfer, resulting in a 4.3% increase in total heat transfer and a 4.3% increase in indicated thermal efficiency.

[0061] Reference Figure 2 and Figure 3 The images show temperature distribution cloud maps of the central section and furnace outlet section of alkali furnace 1 under different flue gas reuse rates, demonstrating the significant effect of flue gas reuse on reducing the average furnace outlet temperature and improving temperature uniformity.

[0062] Example 3

[0063] The combustion control of alkali furnace 1 was simulated using the combustion control system of alkali furnace 1 in Example 1. In this example, the processing load of alkali furnace 1 is 2200 tds / day. The combustion control system of alkali furnace 1 in Example 1 was still used to control the combustion of alkali furnace 1. The flue gas flow rate of recirculation pipe 12 was monitored by ultrasonic flow meter. The recirculation ratio of flue gas after dust removal was controlled to 30% by adjusting the flow control valve 3 of cooler, the flow control valve 6 of recirculation, the recirculation fan 7, the tertiary air flow control valve 8, and the tertiary air fan 9.

[0064] In this embodiment, the control parameters are still based on the system and method of embodiment 1. A numerical model of the combustion process in the furnace, the heat exchange process of the superheater and the economizer of the alkali furnace 1 is established. The combustion and flow heat exchange of the alkali furnace 1 are simulated to obtain the distribution of the temperature field, velocity field, concentration field and heat exchanger in the furnace. By adjusting different flue gas recirculation ratios and mixing them with tertiary air, the influence of the flue gas recirculation ratio on the temperature field, concentration field and thermal efficiency in the furnace is simulated.

[0065] Actual verification results show that the oxygen content in the high-temperature zone of the tertiary air section of alkali furnace 1 decreased by 50%, reducing the generation of nitrogen oxides. Area averaging of NOx concentration at the flue gas outlet section showed a 33.3% reduction in NOx emission concentration. The flue gas outlet temperature in the furnace decreased from 1412K to 1228K, alleviating thermal stress corrosion of the superheater. A 30% increase in flue gas flow enhanced convective heat transfer, increasing the total heat transfer capacity of the heat exchanger by 1.8% and the indicated thermal efficiency by 1.8%.

[0066] pass Figure 2 and Figure 3 The temperature distribution cloud maps of the central section and furnace outlet section of alkali furnace 1 are shown under different flue gas reuse rates, demonstrating the significant effect of flue gas reuse on reducing the average furnace outlet temperature and improving temperature uniformity.

[0067] Based on the actual verification in Examples 1-3, by mixing the low-dust-content flue gas at the dust collector inlet with the tertiary air, the aim is to increase the tertiary air volume and reduce the oxygen content of the tertiary air. This can reduce the furnace outlet temperature by 70℃ to 180℃, reduce NOx generation by 13.0% to 33.3%, and simultaneously increase the flue gas flow rate, which helps to improve the convective heat transfer coefficient on the flue gas side and increase the thermal efficiency of the alkali furnace by 1.8% to 4.3%.

[0068] This invention allows for flexible adjustment of the flue gas recirculation rate and the air-to-flue gas ratio of the tertiary air in different alkali furnaces through the adjustment and setting of various valves and fans in the pipeline, in order to match different types of black liquor and different operating conditions. It is flexible, convenient and highly applicable.

Claims

1. A method for controlling low NOx emissions from an alkali furnace based on flue gas recirculation, characterized in that, Part of the flue gas after dust removal from the alkali furnace is mixed with tertiary air and then fed into the tertiary air inlet of the alkali furnace for recirculation. This is used to reduce the oxygen content in the high-temperature combustion zone of the tertiary air in the furnace, reduce the generation of nitrogen oxides, reduce the flue gas temperature at the furnace outlet, and improve thermal efficiency.

2. The method for low NOx emission combustion control in an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, The recirculation rate of the flue gas after dust removal from the alkali furnace is controlled by adjusting the opening of the flow control valve in the pipeline and the pressurization of the fan.

3. The method for low NOx emission combustion control in an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, Part of the flue gas after dust removal from the alkali furnace is transported to the flue gas cooler of the alkali furnace through a flow control valve, and the other part is transported to the tertiary air inlet of the alkali furnace through a flow control valve and a fan in the recirculation branch pipeline. After mixing with the tertiary air, it is used for flue gas recirculation. The tertiary air delivery pipeline is equipped with a flow control valve and a fan for adjusting and controlling the flow rate.

4. The method for low NOx emission combustion control in an alkali furnace based on flue gas recirculation according to claim 3, characterized in that, The recirculation rate of the flue gas after dust removal in the alkali furnace is controlled by the flow control valve at the inlet of the flue gas cooler of the alkali furnace, the flow control valve and fan on the tertiary air delivery pipeline, and the flow control valve and fan on the branch pipeline of the recirculation pipeline.

5. The method for low NOx emission combustion control in an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, The recirculation rate of the flue gas after dust removal from the alkali furnace is 10% to 30%.

6. The method for low NOx emission combustion control of an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, Numerical models of the combustion process in the furnace, superheater, and economizer of the alkali furnace were established to simulate combustion and flow heat transfer, obtaining the distribution of temperature, velocity, and concentration fields within the furnace, as well as the heat exchange capacity of the heat exchangers. By adjusting different flue gas recirculation ratios, the effects of different recirculation ratios on the temperature, concentration, and thermal efficiency within the furnace were simulated, optimizing the recirculation ratio of flue gas after dust removal in the alkali furnace. The specific steps are as follows: S1. Establish the geometric structure model and numerical calculation model of the alkali furnace, and obtain the temperature field, concentration field and thermal efficiency inside the furnace through CFD fluid simulation calculation. S2, adjust the flue gas recirculation ratio, simulate the effect of different flue gas recirculation ratios on the furnace temperature field, concentration field and thermal efficiency, and optimize the flue gas recirculation ratio.

7. The method for low NOx emission combustion control of an alkali furnace based on flue gas recirculation according to claim 6, characterized in that, Step S1 includes: S11, the flow of gas inside the alkali furnace is modeled using the continuity equation, momentum conservation equation, and energy conservation equation; the high-speed jet air supply of the primary, secondary, and tertiary air in the alkali furnace is simulated using an achievable k-ε turbulence model to simulate jet propagation; the interaction between the achievable k-ε turbulence model and the chemical reaction is modeled using a finite-rate / eddy-dissipation model, and a coupled solution method is used for pressure-velocity coupling solution; S12, the discrete coordinate method is used to simulate the radiative heat transfer of the alkali furnace, and a vortex dissipation model is established to simulate the gas phase combustion model of the alkali furnace gas. The gas phase combustion model uses component transport to calculate the gas phase reaction.

8. The method for low NOx emission combustion control of an alkali furnace based on flue gas recirculation according to claim 6, characterized in that, Step S2 includes: S21, calculate the proportion of flue gas used for recirculation after dust removal in the alkali furnace, using the following iterative calculation: In the formula: c i.mix The concentration of components in the air-flue gas mixture, expressed in vol%; c i.air The concentration of tertiary air components, in vol%; c i.flue The concentration of flue gas components, in vol%; m air The third-order air mass flow rate is expressed in kg / s and m³. flue T represents the recirculated flue gas mass flow rate, expressed in kg / s. mix Temperature after mixing, in K; T air Air temperature, unit K; T flue Flue gas temperature, unit K; S22, the DEFINE_PROFILE macro is used to obtain the component ratio at the outlet boundary of the alkali furnace flue gas, and the mass flow rate and outlet temperature of each component in the circulating flue gas are calculated accordingly. The composition and temperature of the tertiary air inlet are calculated according to the iterative calculation formula in step S21, which are used as the inlet boundary conditions of the tertiary air.

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