A numerical simulation-based optimization design method for black liquor combustion alkali furnace

Abstract

The application discloses a kind of black liquor combustion alkali furnace optimization design method based on numerical simulation, the present application is carried out geometric modeling to the alkali furnace in design stage, is combined with alkali furnace combustion, NOx generation and flue gas-water / steam heat exchange model, the temperature field, velocity field and concentration field distribution law in the alkali furnace and the thermal efficiency of alkali furnace are obtained by CFD numerical simulation.Thereby the rationality of current design can be evaluated, and the optimized alkali furnace design scheme can be obtained by iterative optimization, to reduce the operation and maintenance cost in later period.In addition to in optimization design stage, the present technical method can also be used to existing alkali furnace, to carry out fault diagnosis and evaluate the effectiveness of technical transformation scheme.

Description

A Numerical Simulation-Based Optimization Design Method for Black Liquor Combustion Alkali Furnace Technical Field

[0001] This invention relates to the field of black liquor combustion technology in alkali recovery furnaces, and in particular to an optimized design method for black liquor combustion alkali furnaces based on numerical simulation. Background Technology

[0002] An alkali furnace is a specialized piece of equipment used in paper pulping plants. Its fuel is black liquor produced in the pulping process. The black liquor is burned in the alkali furnace to generate steam, utilizing energy while recovering alkali salts. The design and operation of the alkali furnace directly impacts the profitability of the pulping plant.

[0003] Currently, traditional alkali furnace design methods suffer from low energy efficiency, overheating, and excessive NOx concentration. Overheating at the furnace outlet exacerbates thermal stress corrosion on the superheater walls, leading to frequent tube ruptures. Excessive NOx formation often necessitates ammonia injection for reduction, which not only increases investment but also exacerbates corrosion and scaling on heat exchange surfaces. Inadequate initial design results in frequent shutdowns and modifications during the construction and operation phases, increasing investment costs and negatively impacting the company's production and operations.

[0004] Traditional alkali furnace design methods are increasingly inadequate for stringent energy conservation and environmental protection requirements, thus limiting the development of pulping enterprises. Relying solely on experience is insufficient to solve problems, while experiments are costly and time-consuming, and some crucial parameters cannot be obtained experimentally. With the development of simulation technology and the improvement of computational examples, the advantages of numerical simulation-assisted optimization design are becoming increasingly significant. Summary of the Invention

[0005] Given the complexity of black liquor combustion alkali furnaces, it is difficult to obtain an optimal design using only traditional design methods. Therefore, this invention proposes a numerical simulation-based optimization design method for black liquor combustion alkali furnaces. By geometrically modeling the furnace during the design phase and combining models of alkali furnace combustion, NOx generation, and flue gas-water / steam heat exchange, CFD numerical simulation is used to obtain the internal temperature, velocity, and concentration field distributions, as well as the furnace's thermal efficiency. This allows for the evaluation of the rationality of the current design, and through iterative optimization, an optimized alkali furnace design scheme can be obtained. Besides optimization during the design phase, this technology can also be applied to existing alkali furnaces for fault diagnosis and evaluation of the effectiveness of retrofitting solutions.

[0006] To achieve the above objectives, this invention provides a method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation, comprising the following steps:

[0007] S1. Perform elemental or industrial analysis on the black liquor solids to obtain the components of volatiles, fixed carbon, sulfates, and sulfides required for CFD simulation. Set the black liquor solids to consist of volatiles and coke after volatiles are released, with the coke composed of C and inorganic salts. Based on the elemental analysis of the black liquor solids, use the chemical elemental balance method to obtain the molar number of each component and the elemental composition of the volatiles per kilogram of solids. Based on the measured calorific value of the black liquor solids, determine the calorific value of the volatiles according to the energy balance principle.

[0008] S2. Set the daily processing capacity of black liquor solids. Calculate the theoretical air volume required for combustion based on the volatile matter and C content in the black liquor solids. Multiply the theoretical air volume by the set excess air coefficient to determine the actual air distribution volume.

[0009] S3. Conduct preliminary design of the alkali furnace type based on design conditions and boiler specifications;

[0010] S4. Based on the preliminary design scheme of the alkali furnace, establish a model of the black liquor combustion alkali furnace, and perform CFD simulation on the combustion process and heat exchange process to obtain temperature field, velocity field and concentration field data in the combustion furnace and flue gas-water heat exchange area.

[0011] S5. Based on the temperature field, velocity field, and concentration field distribution within the alkali furnace, and the heat absorption of the superheater and economizer, an optimization analysis is performed. The specific steps are as follows:

[0012] When the NOx concentration at the furnace outlet meets national emission standards, the furnace outlet temperature meets the maximum design temperature limit, and the thermal efficiency meets the design parameter requirements, then the alkali furnace design is reasonable and meets the design requirements; among these, the maximum design temperature is <1000℃, and the design thermal efficiency is 75%-85%.

[0013] If the furnace outlet temperature is higher than the design parameters and the NOx emission concentration is higher than the national emission standards in the simulation results, the tertiary air distribution volume should be increased and the simulation should continue until the furnace outlet temperature and NOx concentration meet the design requirements.

[0014] If the thermal efficiency is less than the design parameter requirements, the heat exchange area of ​​the heat exchanger is increased and the simulation continues until the thermal efficiency meets the design requirements, and the optimized design parameters of the black liquor alkali furnace are obtained. The heat exchanger includes a superheater, an economizer, and a boiling tube screen.

[0015] Furthermore, step S4 specifically includes the following steps:

[0016] S41. Establish the geometric model of the alkali furnace;

[0017] S42. Grid the combustion zone and the flow heat transfer zone;

[0018] S43. Establish the conservation equations for mass, momentum, and energy;

[0019] S44. Set the solution parameters for the gas-phase combustion model and the flow heat transfer model; the gas-phase combustion model uses the eddy dissipation model, and the gas-phase reaction uses the component transport model; Turbulence model simulates jet propagation; since radiation dominates in the alkali furnace, P1 radiation model is used to calculate the heat transfer rate between gas and particles; the volume fraction of black liquor in the boiler is much less than 10%, so the mutual collision between black liquor particles can be ignored, and DPM model is used to simulate the interaction between black liquor particles and fluid; built-in NOx generation model is used to simulate NOx generation.

[0020] S45. Set boundary conditions for black liquor inlet, air inlet, and flue gas outlet.

[0021] S46. Numerical solution is used to obtain the temperature field, velocity field and concentration field distribution inside the alkali furnace;

[0022] S47. Calculate the heat absorption of the heat exchange wall and calculate the thermal efficiency of the alkali furnace.

[0023] Furthermore, the mass, momentum, and energy conservation equations established in step S43 are as follows:

[0024] mass conservation equation:

[0025]

[0026] In the formula, S m A source or sink for chemical reactions;

[0027] Momentum conservation equation:

[0028]

[0029] In the formula, For stress tensor, External volume force;

[0030] Energy conservation equation:

[0031]

[0032] In the formula, k eff To account for the effective thermal conductivity of the fluid after considering turbulence effects, This is due to energy dissipation caused by fluid viscosity.

[0033] Furthermore, the method used in step S44 The turbulence model calculation formula is as follows:

[0034] ;

[0035] ;

[0036] In the formula, , , ;

[0037] In the formula, G k G is the turbulent kinetic energy caused by the average velocity gradient. b Y is the turbulent kinetic energy generated by buoyancy. M The effect of wave expansion in compressible flow on the overall dissipation rate. It is a constant. Let be the Prandtl number for turbulence corresponding to k, where The constants are taken as 1.9, 1.0, 1.2, 1.44, and 0.3 respectively;

[0038] Formula for calculating turbulent viscosity: ;

[0039] ;

[0040] ;

[0041] In the formula, , The turbulent kinetic energy generated by the average velocity gradient, The turbulent kinetic energy generated by buoyancy. The effect of wave expansion in compressible flow on the overall dissipation rate. It is a constant. For corresponding to k and The Prandtl number for turbulence.

[0042] Furthermore, the radiative heat flux q of the P1 radiation model used in step S44... r Calculate using the following formula:

[0043]

[0044] In the formula, a is the absorption coefficient, in l / m; G is the scattering coefficient, in units of 1 / m; G is the incident radiation intensity, in units of W / m. 2 C represents the coefficients of the linear anisotropic phase function.

[0045] Furthermore, in step S44, the DPM model equations used are as follows:

[0046]

[0047] In the formula, The pulling force exerted on the black liquid;

[0048] F in the formula D The formula for calculation is: .

[0049] Furthermore, in step S44, the gas-phase combustion model adopts a component transport model and combines it with an Eddy-Dissipation model to simulate the interaction between turbulence and chemical reactions.

[0050] The component transport equations are as follows:

[0051]

[0052] In the formula, R k S represents the net rate of chemical reaction, expressed in m / s. k The additional generation rate caused by the discrete phase and the custom source phase, in m / s.

[0053] Furthermore, in step S47, the calculation conditions for the heat absorption of the heat exchange wall are set as follows:

[0054] The heat pipes are set as tube banks; the thermal boundary conditions are: the boiling tube screen is set to only undergo phase change, and the inner wall surface of the tube bank is set as an isothermal boundary condition.

[0055] The thermal boundary conditions for the economizer and superheater tube bank of the alkali furnace are set as convective boundary conditions, and the water / steam temperature and water / steam side heat transfer coefficient in the tube bank are set as boundary conditions. The wall heat flux and wall temperature are solved in this way. Finally, the heat transfer Q is obtained by integration, and the boiler thermal efficiency is calculated.

[0056] Furthermore, in step S2, the excess air coefficient is 1.1 to 1.3.

[0057] The beneficial effects of this invention are as follows: By geometrically modeling the alkali furnace during the design phase, and combining it with models of alkali furnace combustion, NOx generation, and flue gas-water / steam heat exchange, this invention uses CFD numerical simulation to obtain the distribution patterns of the temperature, velocity, and concentration fields inside the alkali furnace, as well as the furnace's thermal efficiency. This allows for the evaluation of the rationality of the current alkali furnace design. Through iterative optimization, an optimized alkali furnace design scheme can be obtained, reducing subsequent operation and maintenance costs. Besides the optimization design phase, this technical method can also be applied to existing alkali furnaces for fault diagnosis and evaluation of the effectiveness of technical modification schemes. Attached Figure Description

[0058] Figure 1 is a flowchart of the method of the present invention;

[0059] Figure 2 is a schematic diagram of the geometric model and mesh division of the black liquor combustion alkali furnace in an embodiment of the present invention;

[0060] Figure 3 is a schematic diagram of the simulation results of black liquor combustion alkali furnace according to an embodiment of the present invention;

[0061] Figure 4 is a temperature cloud map of the cross section of the alkali furnace in an embodiment of the present invention;

[0062] Figure 5 is a schematic diagram of three wind changes according to an embodiment of the present invention. Detailed Implementation

[0063] The specific embodiments of the present invention will now be described with reference to Figures 1 to 5.

[0064] This embodiment describes the operational optimization of an alkali furnace with a daily black liquor solids processing capacity of 2200 tds / d. Obviously, the described embodiment is only a part of the embodiments of the present invention. However, the present invention is not limited to the above-described embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

[0065] As shown in Figure 1, an optimization design method for a black liquor combustion alkali furnace based on numerical simulation includes the following steps:

[0066] Step S1: Determine the content of black liquor solids components

[0067] The pulp used in this embodiment is wood pulp black liquor, with a black liquor concentration of 85% and a higher heating value of 12.6 MJ / kg. The chemical elemental analysis (%) of the solids is shown in Table 1.

[0068] Table 1 Elemental analysis of solids in wood pulp black liquor

[0069]

[0070] The composition of black liquor solids was estimated using the chemical elemental balance method. Volatile matter was assumed to consist of C, H, O, N, and S elements, without considering specific molecular composition. The total chemical formula of the volatile matter is: After volatile matter analysis, the composition of the coke is: fixed carbon, Na₂SO₄, Na₂CO₃, NaCl, K₂CO₃, and NaCNO₃. The heat generated by black liquor combustion comes from the combustion of coke and volatile matter. The molar amounts of the above substances are determined using the law of conservation of elements. The volatile matter mass fraction is set at 30%, the molar mass of volatile matter is 50 g / mol, and the S and N contents in the volatile matter account for 50% and 0.23% of the total S and N contents of the black liquor solids, respectively. Based on this, the mass fractions of each component are shown in Table 2.

[0071] Table 2. Estimated composition of black liquor solids

[0072]

[0073] The calorific value of black liquor solids is contributed by volatile matter and coke. The calorific value of coke is 32.79 MJ / kg, and the calorific value of volatile matter is calculated to be 23.59 MJ / kg.

[0074] Step S2: Determine the air distribution and fuel inlet parameters based on the design or operating parameters of the alkali furnace. For the alkali furnace in this embodiment, the excess air coefficient is set to 1.2, and the mass flow rates of the primary air, lower secondary air, upper secondary air, and tertiary air are 29.412 kg / s, 25.675 kg / s, 19.175 kg / s, and 37.375 kg / s, respectively. The inlet temperature is 170℃ for all of them. The black liquor entering the furnace has a moisture content of 15% and an inlet temperature of 135℃.

[0075] Step S3: In the preliminary design of this embodiment, the alkali furnace is a single-drum, natural circulation, low-odor alkali recovery boiler. The furnace is surrounded by membrane water-cooled walls. After the furnace outlet, there are stage II superheaters, stage III superheaters, stage IV superheaters, stage IB superheaters, stage IA superheaters, and evaporator tube panels. Two stages of economizers are arranged at the tail end.

[0076] Step S4: Establish a model based on the design parameters, build a model of the black liquor combustion alkali furnace, and perform CFD simulation on the combustion and heat exchange processes to obtain temperature, velocity, and concentration field data within the combustion furnace and the flue gas-water heat exchange area; this includes the following steps:

[0077] S41: Establish the geometric model of the alkali furnace;

[0078] S42: Mesh the combustion zone and the flow heat transfer zone as shown in Figure 2;

[0079] S43: Establish the conservation equations for mass, momentum, and energy;

[0080] S44: Solution settings for the gas-phase combustion model and the flow heat transfer model; wherein, the gas-phase combustion model adopts the eddy dissipation model, and the gas-phase reaction adopts the component transport model; Turbulence model simulates jet propagation; since radiation dominates in the alkali furnace, P1 radiation model is used to calculate the heat transfer rate between gas and particles; the volume fraction of black liquor in the boiler is much less than 10%, so the mutual collision between black liquor particles can be ignored, and DPM model is used to simulate the interaction between black liquor particles and fluid; built-in NOx generation model is used to simulate NOx generation, NOx model in component transport is turned on, three types of NOx products are considered: rapid type, thermal type and fuel type, fuel N content is set to account for 0.23% of the total content, and numerical solution is performed.

[0081] S45: Set boundary conditions for black liquor inlet, air inlet, and flue gas outlet; set boundary conditions, using mass inlet boundary conditions for fuel and air inlets, and mass outlet boundary conditions for outlets; use radiator models for superheater, boiling tube screen, and economizer, and set the surface temperature and the power-law correlation between the fitted flue gas velocity and heat transfer coefficient; set porous media models in the fluid domains of superheater, boiling tube screen, and economizer, and set the porosity, viscous resistance, and inertial resistance coefficients.

[0082] S46: Numerical solution to obtain the temperature field, velocity field and concentration field distribution inside the alkali furnace;

[0083] S47: Calculate the heat absorption of the heat exchanger walls and the thermal efficiency of the alkali furnace; perform data post-processing to obtain the temperature field inside the furnace, as shown in part a of Figure 3, and the NOx concentration field data, as shown in part b of Figure 3. The average furnace outlet temperature is 1412K, the design calculated value is 1339K, with an error of 5.44%. The NOx concentration at the flue gas outlet is 163.5ppm. The total heat absorption of the superheater, boiling tube screen, and economizer is calculated to be 128.62MW, and the thermal efficiency of the alkali furnace is calculated to be 40.1% (total heat absorption / total fuel calorific value). The design heat absorption is 116.32MW, with an error of 10.57%, meeting the calculation requirements for engineering applications.

[0084] The temperature field distribution revealed a high-temperature region below the front wall superheater. Actual operation monitoring also showed that the temperature below the front wall superheater was too high, causing thermal stress corrosion on the heat exchange surface.

[0085] Step S5: For the furnace outlet temperature exceeding the design parameters in the simulation results, air distribution optimization is performed to reduce the furnace outlet temperature. With other conditions unchanged, the tertiary air volume is increased by 10%, 20%, 30%, 40%, and 50%, and steps S3 and S4 are repeated respectively to obtain the central cross-section of the alkali furnace under different tertiary air volumes, as shown in part a of Figure 4, and the temperature cloud map of the furnace outlet cross-section, as shown in part b of Figure 4. When the air volume is increased by more than 30%, the temperature in the high-temperature region of the furnace outlet decreases significantly, with an average temperature drop of 70-110℃. The corresponding changes in total heat exchange and NOx emission concentration are shown in parts a and b of Figure 5, respectively. Due to the increase in tertiary air volume, the heat transfer temperature difference decreases, resulting in a slight decrease in overall heat exchange. Increasing the tertiary air volume is beneficial for reducing NOx emission concentration. The simulation results show that increasing the tertiary air volume has a significant effect on improving the overheating in the area below the superheater and alleviating thermal stress corrosion.

[0086] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation, characterized in that, Includes the following steps: S1. Perform elemental or industrial analysis on the black liquor solids to obtain the components of volatiles, fixed carbon, sulfates, and sulfides required for CFD simulation. Assume the black liquor solids consist of volatiles and the coke produced after volatiles are released, with the coke composed of carbon and inorganic salts. Based on the elemental analysis of the black liquor solids, use the chemical elemental balance method to obtain the molar number of each component per kilogram of solids and the elemental composition of the volatiles. Determine the calorific value of the volatiles based on the measured combustion calorific value of the black liquor solids and the energy balance principle. S2. Set the daily processing capacity of the black liquor solids. Calculate the theoretical air volume required for combustion based on the volatiles and carbon content in the black liquor solids. Multiply the theoretical air volume by the set excess air coefficient to determine the actual air distribution volume. S3. Conduct a preliminary design of the alkali furnace type based on the design conditions and boiler specifications. S4. Based on the preliminary design scheme of the alkali furnace, establish a model of the black liquor combustion alkali furnace and conduct CFD simulation of the combustion and heat exchange processes. Simulations are performed to obtain temperature, velocity, and concentration field data within the combustion furnace and the flue gas-water heat exchange area. S5. Based on the temperature, velocity, and concentration field distributions within the alkali furnace, and the heat absorption of the superheater and economizer, optimization analysis is conducted. The specific steps are as follows: If the NOx concentration at the furnace outlet meets national emission standards, the furnace outlet temperature meets the maximum design temperature limit, and the thermal efficiency meets the design parameter requirements, then the alkali furnace design is reasonable and meets the design requirements. If the furnace outlet temperature is higher than the design parameters and the NOx emission concentration is higher than national emission standards, the tertiary air distribution volume is increased, and the simulation continues until the furnace outlet temperature and NOx concentration meet the design requirements. If the thermal efficiency is lower than the design parameter requirements, the heat exchange area of ​​the heat exchanger is increased, and the simulation continues until the thermal efficiency meets the design requirements, thus obtaining the optimized design parameters for the black liquor alkali furnace.

2. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 1, characterized in that, Step S4 specifically includes the following steps: S41, establishing a geometric model of the alkali furnace; S42, meshing the combustion zone and the flow heat transfer zone; S43, establishing mass, momentum, and energy conservation equations; S44, setting the solution parameters for the gas-phase combustion model and the flow heat transfer model; wherein, the gas-phase combustion model adopts the eddy dissipation model, and the gas-phase reaction adopts the component transport model; The turbulence model simulates jet propagation; the P1 radiation model is used to calculate the heat transfer rate between gas and particles; the DPM model is used to simulate the interaction between black liquor particles and fluid, and the built-in NOx generation model is used to simulate NOx generation; S45, set the boundary conditions for black liquor inlet, air inlet, and flue gas outlet; S46, obtain the temperature field, velocity field, and concentration field distribution inside the alkali furnace through numerical solution; S47, calculate the heat absorption of the heat exchange wall and calculate the thermal efficiency of the alkali furnace.

3. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 2, characterized in that, The steps used in step S44 The turbulence model calculation formula is as follows: ； In the formula, ， ， In the formula, G k G is the turbulent kinetic energy caused by the average velocity gradient. b Y is the turbulent kinetic energy generated by buoyancy. M The effect of wave expansion in compressible flow on the overall dissipation rate. Let be the Prandtl number for turbulence corresponding to k and ε, where Using constants of 1.9, 1.0, 1.2, 1.44, and 0.3 respectively; the formula for calculating turbulent viscosity is: ； ； In the formula, 。 4. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 2, characterized in that, The radiative heat flux q of the P1 radiation model used in step S44 r Calculate using the following formula: In the formula, a is the absorption coefficient, in l / m; G is the scattering coefficient, in units of 1 / m; G is the incident radiation intensity, in units of W / m. 2 ; C represents the coefficients of the linear anisotropic phase function.

5. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 2, characterized in that, In step S44, the DPM model equations used are as follows: In the formula, The force F is the drag force on the black liquor; D The formula for calculation is: 。 6. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 2, characterized in that, In step S44, the gas-phase combustion model adopts a component transport model and combines it with an eddy dissipation model to simulate the interaction between turbulence and chemical reactions.

7. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 6, characterized in that, The component transport model equations are as follows: In the formula, R k S represents the net rate of chemical reaction, expressed in m / s. k The additional generation rate caused by the discrete phase and the custom source phase, in m / s.

8. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 2, characterized in that, In step S47, the heat absorption calculation conditions of the heat exchange wall are set as follows: the heat pipes are set as tube banks; the thermal boundary conditions are: the boiling tube screen is set to only undergo phase change, and the inner wall of the tube bank is set as isothermal boundary conditions; the thermal boundary conditions of the economizer and superheater tube banks of the alkali furnace are set as convective boundary conditions, and the temperature of water / steam in the tube bank and the heat transfer coefficient on the water / steam side are set as boundary conditions, so as to solve the wall heat flow and wall temperature; finally, the heat exchange Q is obtained by integration, and the boiler thermal efficiency is calculated.

9. The method for optimizing the design of a black liquor combustion alkali furnace based on numerical simulation according to claim 1, characterized in that, In step S2, the excess air coefficient is 1.1 to 1.3.

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