A low NOx emission combustion control system for soda furnaces based on flue gas recirculation

Abstract

The utility model relates to a kind of low NOx emission combustion control system of soda furnace based on flue gas recirculation, including, soda furnace, flue gas dust collector, flue gas dust collector output pipeline, flue gas cooler, cooler flow control valve;The flue gas dust collector output pipeline is equipped with bypass recirculation pipeline connection to the tertiary air line of the tertiary air import, part of flue gas after dust removal is transported to tertiary air distribution duct by induced draft fan, air mixture is formed after mixing with air, and it is injected into hearth from soda furnace tertiary air distribution port.The utility model adopts the solution from front end source control, can reduce the oxygen content of tertiary air, reduce NOx generation in high-temperature combustion zone, reduce pollution emission, at the same time, the increased recirculation flue gas flow helps to reduce the temperature of lower part of superheater, relieve high-temperature thermal stress corrosion.

Description

Technical Field

[0001] This utility model relates to the field of black liquor combustion technology in alkali recovery furnaces, and in particular to a low-NOx emission combustion control system 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 operational 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. Utility Model Content

[0005] To overcome the problems existing in the background technology, this utility model provides a low-NOx emission combustion control system 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 utility model to solve its technical problem is:

[0007] A low-NOx emission combustion control system for an alkali furnace based on flue gas recirculation includes an alkali furnace, a flue gas dust collector, a flue gas dust collector output pipe, a flue gas cooler, and a cooler flow control valve.

[0008] The alkali furnace is equipped with a tertiary air inlet and a tertiary air duct connected thereto. A tertiary air flow control valve and a tertiary air blower are sequentially installed at the front end of the tertiary air duct. An economizer is installed in the tail flue of the alkali furnace. The flue gas dust collector is connected to the flue gas outlet of the economizer of the alkali furnace. The output pipe of the flue gas dust collector is connected to the flue gas cooler through a cooler flow control valve. A chimney is connected to the outlet of the flue gas cooler.

[0009] The flue gas dust collector output pipe is provided with a bypass recirculation pipe, which is connected to the tertiary air inlet of the tertiary air pipeline. The recirculation pipe is equipped with a recirculation flow control valve and a recirculation fan for adjusting and controlling the recirculation ratio of the flue gas after dust removal from the alkali furnace.

[0010] Furthermore, the cooler flow control valve, recirculation flow control valve, recirculation fan, tertiary air flow control valve, and tertiary air fan are used to adjust and control the ratio of flue gas recirculated after dust removal from the alkali furnace.

[0011] Furthermore, ultrasonic flow meters are installed on the recirculation pipeline and the tertiary air pipeline respectively.

[0012] Furthermore, the bypass interface of the recirculation pipeline on the flue gas dust collector output pipeline is located at the inlet of the cooler flow control valve.

[0013] Furthermore, the cooler flow control valve, recirculation flow control valve, and tertiary air flow control valve are electrically adjustable valves.

[0014] Furthermore, the recirculation fan and the tertiary air fan are connected to a frequency converter.

[0015] Furthermore, the alkali furnace is also equipped with primary air ducts and secondary air ducts.

[0016] 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, contributing to improved thermal efficiency of the alkali furnace. This invention allows for flexible adjustment of the recirculated flue gas flow rate based on the operating conditions of the alkali furnace through the regulation of various flow control valves, increasing adaptability to different black liquor types and flow rates. This invention employs a technical solution controlling the combustion source, reducing or eliminating the need for a downstream denitrification system to reduce emissions, significantly lowering system investment and maintenance costs.

[0017] This system can establish a geometric and numerical model of the alkali furnace, and obtain the furnace temperature field, concentration field, and thermal efficiency through Fluent simulation. By adjusting different flue gas recirculation ratios, the system simulates the impact of the flue gas recirculation ratio on the furnace temperature field, concentration field, and thermal efficiency, thus obtaining an optimized flue gas recirculation ratio. This invention can reduce the oxygen content of the tertiary air, reduce NOx formation in the high-temperature combustion zone, and reduce pollution emissions. At the same time, the increased recirculated flue gas flow helps to reduce the temperature at the bottom of the superheater and alleviate high-temperature thermal stress corrosion.

[0018] The present invention comprises a cooler flow control valve, a recirculation flow control valve, a recirculation fan, a tertiary air flow control valve, and a tertiary air blower, forming a linkage control system. This system can flexibly adjust the flue gas recirculation volume and the air-to-flue gas ratio of the tertiary air for different alkali furnaces to match different types of black liquor and different operating conditions. It is flexible, convenient, and highly applicable. Compared with existing technologies that address back-end denitrification, this invention adopts a front-end source control solution and has broad application prospects in the field of black liquor combustion technology in alkali recovery furnaces. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the combustion device system of this utility model;

[0020] Figure 2 This is a temperature distribution cloud map of the central cross section of the alkali furnace according to an embodiment of this utility model;

[0021] Figure 3 This is a temperature distribution cloud map of the outlet section of the alkali furnace in an embodiment of this utility model.

[0022] Components and their numbers shown in the picture:

[0023] 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

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0025] Example 1

[0026] like Figure 1 As shown, a low NOx emission combustion control system for an alkali furnace based on flue gas recirculation 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.

[0027] 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.

[0028] 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.

[0029] 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 in the alkali furnace 1.

[0030] Ultrasonic flow meters are installed on the recirculation pipe 12 and the tertiary air pipe 11, respectively.

[0031] 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.

[0032] The cooler flow control valve 3, the recirculation flow control valve 6, and the tertiary air flow control valve 8 are electrically adjustable valves.

[0033] The recirculation fan 7 and the tertiary air fan 9 are connected to a frequency converter.

[0034] The alkali furnace 1 is also equipped with a primary air duct and a secondary air duct.

[0035] 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.

[0036] Regulating and controlling the circulating flue gas flow rate: Open the flue gas recirculation fan 7 and the recirculation flow control valve 6, and adjust the recirculated 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 from the alkali furnace 1 used for recirculation after dust removal 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.

[0037] 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.

[0038] 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.

[0039] Numerical models of the combustion process in the furnace, superheater, and economizer of the alkali furnace 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 these 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:

[0040] 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.

[0041] 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.

[0042] Step S1 includes:

[0043] 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;

[0044] 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.

[0045] Step S2 includes:

[0046] S21, calculate the proportion of flue gas used for recirculation after dust removal in alkali furnace 1, using the following iterative calculation:

[0047]

[0048] 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;

[0049] 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.

[0050] 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.

[0051] 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.

[0052] Example 2

[0053] The combustion control system of the alkali furnace in Example 1 is still 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. An ultrasonic flow meter is used to monitor the flue gas flow of the recirculation pipeline 12. The recirculation ratio of the flue gas after dust removal is controlled to be 20% by adjusting 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 in a linkage manner.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] Example 3

[0058] The combustion control system of the alkali furnace in Example 1 is still used to simulate the actual combustion control 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 30% by adjusting the flow control valve 3 of the cooler, the flow control valve 6 of the recirculation pipeline, 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 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%.

[0061] 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.

[0062] Based on the actual verification in Examples 1-3, by mixing the low-dust-content flue gas at the outlet of flue gas dust collector 2 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 alkali furnace 1 by 1.8% to 4.3%.

[0063] 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 low NOx emission combustion control system for an alkali furnace based on flue gas recirculation, comprising: 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); Its features are, The alkali furnace (1) is provided with a tertiary air inlet (10) and a tertiary air duct (11) connected thereto. The front end of the tertiary air duct (11) is provided 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 provided 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). 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).

2. The low-NOx emission combustion control system for an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, 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 to adjust and control the ratio of flue gas recirculated after dust removal from the alkali furnace (1).

3. The low-NOx emission combustion control system for an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, Ultrasonic flow meters are installed on the recirculation pipe (12) and the tertiary air pipe (11).

4. The low-NOx emission combustion control system for an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, The cooler flow control valve (3), recirculation flow control valve (6) and tertiary air flow control valve (8) are electrically adjustable valves.

5. A low-NOx emission combustion control system for an alkali furnace based on flue gas recirculation according to claim 1, characterized in that, The recirculation fan (7) and the tertiary air fan (9) are connected to a frequency converter.

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