Numerical calculation method for predicting NOx emission in coal-ammonia mixed combustion

Abstract

The invention discloses a numerical calculation method for predicting NOx emission in coal-ammonia blending combustion, and belongs to the technical field of coal-ammonia co-combustion numerical simulation. The method comprises the following steps: establishing a gas-solid two-phase control equation, and representing differential diffusion through a non-unity Lewis number; discretizing the processing model and coupling a speed-pressure field through a PIMPLE algorithm; cP-DNS simulation considering differential diffusion and NOx generation is carried out; the method comprises the following steps: calculating a universal mixing score matched with coal ammonia mixing combustion based on a CP-DNS data set, incorporating difference diffusion in a small flame model through a non-unity constant Lewis number, generating a small flame lookup table containing difference diffusion, evaluating model performance through prior analysis, combining a reaction path, a chemical time scale and budget analysis, and calculating a coal ammonia mixing score matched with the coal ammonia mixing combustion. A NOx generation mechanism is defined to support emission prediction. According to the invention, the influence of differential diffusion and multi-dimensional diffusion on NOx generation can be fully captured.

Description

Technical Field

[0001] This invention belongs to the field of numerical simulation technology for coal-ammonia co-combustion, specifically relating to a method for predicting NO in coal-ammonia co-combustion. x Numerical calculation methods for emissions. Background Technology

[0002] Coal-ammonia co-combustion, as a low-carbon emission technology, has significant advantages in reducing CO2 emissions, while the addition of ammonia alters NO2 emissions. x Generation mechanism, prediction of NO x Emissions are crucial for the industrial application of this technology. In coal-ammonia co-combustion systems, large molecules in volatiles coexist with small molecules such as NH3 and H2, resulting in significant differential diffusion effects that influence combustion characteristics and NO emissions. x The generation process has a significant impact. Carrier phase direct numerical simulation (CP-DNS) can resolve all spatiotemporal scales of turbulence and chemistry, providing high-precision datasets for combustion mechanism research; the small flame model can consider detailed chemical reaction mechanisms while ensuring computational efficiency, making it an effective tool for studying complex combustion processes. The differential diffusion model applicable to gas-phase combustion is extended to the coal-ammonia blended combustion system, and the differential diffusion is characterized by non-unity Lewis numbers in the gas-solid two-phase governing equations and the small flame model, respectively, to investigate NO. x Generation mechanism, improving model building assumptions and enhancing NO x The reliability of emissions prediction is of great importance. Summary of the Invention

[0003] This invention provides a method for predicting NO in coal-ammonia blended combustion. x Numerical methods for calculating emissions extend the differential diffusion model of gas-phase combustion to coal-ammonia blended combustion systems. Differential diffusion is characterized in both the gas-solid two-phase governing equations and the small flame model. Based on a high-precision dataset obtained from CP-DNS, multi-dimensional analysis is used to explore NO emissions. x Generation mechanism and model construction hypotheses.

[0004] The technical solution of the present invention is as follows:

[0005] A method for predicting NO in coal-ammonia blended combustion x The numerical calculation method for emissions includes the following steps:

[0006] Step 1: Characterize the differential diffusion effect in the gas-solid two-phase model using the non-uniform Lewis number and establish the governing equations;

[0007] Step 2: Couple the pressure-velocity field and conduct direct numerical simulation to obtain combustion dataset, thereby generating CP-DNS data; CP-DNS represents direct numerical simulation of the carrier phase.

[0008] Step 3: Based on CP-DNS data, calculate the universal mixing fraction Z applicable to coal-ammonia blended combustion using the Bilger formula. Based on the universal mixing fraction Z, solve the non-premixed small flame equation using a non-unity constant Lewis number to generate a small flame lookup table with differential diffusion and construct a small flame model.

[0009] Step 4: By comparing the predicted values ​​of the small flame model with differential diffusion data through prior analysis, the predictive effectiveness of temperature and composition is evaluated; combined with reaction pathway analysis, chemical timescale analysis, and budget analysis, the NO content is determined. x The formation mechanism of NO produced by the co-combustion of coal and ammonia x Emissions forecasting.

[0010] The beneficial effects of this invention are as follows:

[0011] 1. The gas-phase differential diffusion model is extended to the coal-ammonia blended combustion system. The differential diffusion effect is captured by non-unity Lewis numbers in the gas-solid two-phase control equation and small flame model, which is adapted to the complex characteristics of coal-ammonia blended combustion.

[0012] 2. Based on the high-precision CP-DNS dataset considering differential diffusion, a universal mixing fraction adapted to coal-ammonia blended combustion is calculated using the Bilger formula. A small flame model incorporating differential diffusion is then constructed to provide a basis for NO... x This provides a solid foundation for the study of formation mechanisms and emission prediction;

[0013] 3. Combining prior analysis, reaction pathway analysis, chemical timescale analysis, and budget analysis, a systematic investigation of NO was conducted. x The generation mechanism and model construction assumptions, balancing computational accuracy and efficiency, provide a basis for the optimized design of coal-ammonia co-fired burners and NO production. x Emissions forecasting provides reliable numerical tools. Attached Figure Description

[0014] Figure 1 This invention provides a method for predicting NO in coal-ammonia blended combustion. x A flowchart of the numerical calculation method for emissions;

[0015] Figure 2 This is a schematic diagram of the CP-DNS calculation domain setting for coal-ammonia blended combustion according to the present invention;

[0016] Figure 3 Figure 1 shows the prior analysis results of the differential diffusion small flame model of this invention - temperature;

[0017] Figure 4 The graph shows the prior analysis results of the differential diffusion small flame model of this invention - H2O mass fraction;

[0018] Figure 5 The prior analysis results of the differential diffusion small flame model of the present invention are shown in the figure - C6H6O2 mass fraction;

[0019] Figure 6 The prior analysis results of the differential diffusion small flame model of the present invention are shown in the figure - H2 mass fraction;

[0020] Figure 7 The graph shows the prior analysis results of the differential diffusion small flame model of this invention - NO mass fraction;

[0021] Figure 8 The graph shows the prior analysis results of the differentially diffusing small flame model of this invention - HCN mass fraction;

[0022] Figure 9 The diagram shows the budget analysis of the Lagrange-like transient equations of this invention.

[0023] Figure 10 This is a budget analysis diagram of the generalized small flame equation of the present invention. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. However, the following embodiments are only for explaining the present invention, and the scope of protection of the present invention should include all the contents of the claims. Moreover, through the description of the following embodiments, those skilled in the art can fully implement all the contents of the claims of the present invention.

[0025] Example:

[0026] This embodiment takes the co-combustion of coal and ammonia in a turbulent mixing layer as the research object, and describes in detail the implementation process of the present invention (see...). Figure 1 ):

[0027] Step 1, establish the relevant mathematical model:

[0028] 1. Gas-Solid Two-Phase Governing Equation: The gas-phase scalar transport equation takes the following form, encompassing species mass fraction, total enthalpy, and various mixture fractions. Differential diffusion effects in this governing equation are characterized by a non-unity constant Lewis number. Mature models are used to calculate the volatile matter leaching and coke surface reaction rates, with the corresponding gas-solid two-phase coupling source terms incorporated into the governing equation. The general form of the gas-phase scalar transport equation for the gas-solid two-phase governing equation is:

[0029] ,

[0030] in, For general scalars, corresponding to species mass fractions Total enthalpy , or ; For gas phase density, Euler time, For directional fluid velocity, The index of three-dimensional spatial coordinates representing the Cartesian coordinate system ( Corresponding to direction), For gas phase thermal conductivity, The non-unity constant Lewis number characterizes the differential diffusion effect in the governing equation; For gas-specific heat capacity, This term represents the chemical reaction rate; only the equation for the mass fraction of species contains this term. It is a gas-solid two-phase coupled source term, originating from volatile matter and coke surface reaction.

[0031] 2. Extension of the Differential Diffusion Model: The differential diffusion model applicable to gas-phase combustion is extended to coal-ammonia blended combustion systems to explore the role of differential diffusion in coal-ammonia blended combustion and its effect on NO. x The impact on predictive performance; this extended model is also based on the non-unity Lewis number principle, ensuring consistency with the diffusion characterization logic of the gas-solid two-phase control equation.

[0032] Step 2, Numerical Discretization and Iterative Solution:

[0033] 1. Mesh Generation: The finite volume method was used to discretize the governing equations. Spatial derivatives were processed using a cubic discretization scheme, and temporal derivatives were discretized using an implicit scheme. A cuboid computational domain was used, with flow, transverse, and spanwise dimensions of 59.7 mm × 59.7 mm × 14.9 mm, a mesh resolution of 384 × 384 × 96, and a uniform mesh spacing of 155.5 μm. This ensured that the multi-scale characteristics of turbulent vortices and the flame front were fully analyzed. The boundary conditions of the computational domain were set to a coal / ammonia / air mixture in the upper laminar flow. , , Temperature 600K, speed 15m / s These represent the mass fractions of NH3, O2, and N2, respectively; the lower laminar flow is hot air at a temperature of 1300 K and a velocity of -15 m / s; the coal powder particles have a diameter of 25 μm and a number density of 1 × 10⁻⁶. 11 The particle count is approximately 1 m³, with initial velocity and temperature consistent with the gas phase. The x and z directions have periodic boundaries, while the y direction has a zero-gradient boundary. Initial turbulent disturbances are generated using a digital filtering method, and their intensity is... Length scale . Figure 2 This is a schematic diagram of the CP-DNS calculation domain setting for coal-ammonia blended combustion according to the present invention.

[0034] 2. Discretization and Solution: The spatial derivative is discretized using a cubic scheme, and the time integral is solved using an implicit scheme. After generating the nonlinear algebraic equation system, iterative solutions are obtained at each time step to ensure computational stability.

[0035] Step 3, CP-DNS solution and dataset acquisition:

[0036] 1. Numerical Solution: A low Mach number finite volume solver based on OpenFOAM is employed. A hybrid algorithm combining pressure implicit operator partitioning and pressure-coupled equations (PIMPLE) is used to couple the velocity-pressure field, and OpenSMOKE++ is used to solve for detailed chemical reaction mechanisms. The solution also considers differential diffusion and NO... x The generated CP-DNS simulation continues until it reaches a stable state.

[0037] 2. Dataset Acquisition: Extract a 3D dataset containing parameters such as temperature, mass fraction of each component, and mixture fraction.

[0038] Step 4, Small Flame Model Construction and Mechanism Analysis:

[0039] 1. Universal Mixing Fraction Calculation and Lookup Table Generation: Based on the CP-DNS dataset, the universal mixing fraction Z applicable to coal-ammonia blended combustion is calculated using the Bilger formula.

[0040] The universal mixing fraction Z applicable to coal-ammonia blended combustion is calculated based on the Bilger formula:

[0041] ,

[0042] ,

[0043] in, and respectively fuel side and oxidizer side value, It represents the mixed fraction of elements C, H, and O. Molecular weights of elements C, H, and O. Mass fraction of fuel-side components. Calculated using a three-mixed fractional model:

[0044] ,

[0045] ,

[0046] ,

[0047] These are the gas masses from ammonia, volatile matter, and coke tail gas, respectively. , , These are components from ammonia stream, volatile matter, and coke tail gas. The mass fraction; , , These represent the mixed fractions of ammonia, volatile matter, and coke tail gas, respectively. To determine the proportion of coke tail gas, the mixing relationship between coke tail gas and volatile matter is quantified; The coal-ammonia mixture ratio distinguishes the dominant combustion components of coal and ammonia. General mixture fraction. The specific calculation formula is as follows:

[0048] .

[0049] in, Representative components in the combustion system Quantity, Components mass fraction, Components The number of C, H, and O atoms in the middle. Representative components The molecular weight.

[0050] Prior analysis compares small flame model predictions with CP-DNS data, focusing on assessing temperature, major components, and NO. x Predictive performance; the Small Flame Lookup Table (FLT) was generated using FlameMaster software. For progress variables, These represent the mass fractions of CO2, H2O, and H2, respectively. For normalized total enthalpy, For total enthalpy in the gas phase, and Total enthalpy of the gas phase The minimum and maximum total enthalpy. Combining the manifold coordinates above, differential diffusion is incorporated into the small flame model using a non-unity constant Lewis number. The equations for the unpremixed small flame are then solved, generating a small flame lookup table containing differential diffusion:

[0051] The non-premixed small flame equations include a species transport equation and an energy equation, which are as follows:

[0052] ,

[0053] ,

[0054] in, For gas phase density, For scalar dissipation rate, Representative components Lewis number, Components The reaction rate, For the heat release rate, For gas-specific heat capacity, Components Specific heat capacity, The average molecular weight of the mixture. The temperature is the gas phase temperature. Calculations were performed using a detailed chemical reaction mechanism involving 129 components and 1664 elementary reactions.

[0055] 2. Prior Analysis: Manifold coordinates were extracted from the CP-DNS data, and the predicted values ​​of the small flame model with differential diffusion were compared with the CP-DNS data (see...). Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 and Figure 8 (In the figure, FLT-DD represents the flame surface lookup table with differential diffusion). The results show that the prediction of temperature and major components is good, but the prediction of NO mass fraction has a deviation.

[0056] 3. Reaction Path Analysis: Quantifying NO by calculating the nitrogen atom flux ratio. x Generation path:

[0057] ,

[0058] In the formula, This is used to define the reaction pathway flux ratio of nitrogen atoms from component P to component Q. Among them, The total number of elementary reactions. Total number of species; For flame index, , Representing C respectively 16 H 10 The mass fractions of CO and NH3 Represents the gradient symbol; The mass fraction of O2; For the reaction The flux of nitrogen atoms between species P and Q NH3 and components The nitrogen atom flux between them. Define the premixed combustion zone ( ) and non-premixed combustion zone ( The net NH3 consumption rate of ) was 0.121 kg / (m³). s) and 0.145kg / (m The corresponding net NO flux was 0.001 kg / (m³). s) and 0.016kg / (m (s) The premixed combustion mode has no significant impact on overall NO generation.

[0059] 4. Chemical timescale analysis: through Estimate, Used to calculate species Chemical characteristics timescale; This refers to the gas phase density. For species mass fraction, For species The net reaction rate, Small positive numbers are used to verify the applicability of the steady-state small flame hypothesis; NO, NO2, etc. x The chemical timescale of the species is much smaller than that of the main components such as CO2 and H2O, which verifies the applicability of the steady-state small flame hypothesis.

[0060] 5. Budget Analysis: Budget analysis includes the Lagrange transient equations and the generalized small flame equations, as well as Lagrange-like transient terms. and Euler transient term The relationship between general scalars for:

[0061] ,

[0062] In the formula, Represents the Euler transient term. For Lagrange transient terms, For mixed fractions Transient contribution of the field For the small flame coordinates along the mixed fraction The convection term of the isosurface; where, Euler time, For Lagrange-like time, For gas phase density, For fluid velocity vector, for The unit normal vector of the isosurface. Represents the gradient symbol.

[0063] The budget equation for the generalized small flame equation is:

[0064] ,

[0065] For the fraction of the mixture The normal diffusion term in the gradient direction. For along The multidimensional diffusion term of the isosurface is ignored in the one-dimensional small flame equation. For the differential diffusion term, For source terms, For the correction term; budgetary analysis of the Lagrange transient term shows that the Euler transient term is mainly balanced by the transient contributions of the mixed fractional field (see...). Figure 9 Although the Lagrange transient term is crucial for NO, it can be partially considered in prior analysis because the popular coordinates are derived from DNS data, and this term has been incorporated into the DNS calculation process; budgetary analysis of the generalized small flame equation shows that the one-dimensional small flame model's neglect of multidimensional diffusion effects is the main factor leading to the NO prediction bias in the non-premixed combustion zone (see...). Figure 10 (The gray area in the diagram represents the reaction zone).

[0066] This embodiment demonstrates that, by characterizing the differential diffusion effect in the gas-solid two-phase control equation and the small flame model, and combining this with analysis of the CP-DNS high-precision dataset, the present invention can effectively investigate NO. x The generation mechanism and model construction assumptions are based on the following: NO generated by coal-ammonia blending combustion x It provides reliable numerical tools and theoretical support for emission prediction and combustion system optimization design.

[0067] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined in this invention may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this invention.

Claims

1. A numerical calculation method for predicting NOx emission in coal-ammonia co- combustion, characterized by, x The method comprises the following steps: ​ Step 1, difference diffusion effect is characterized in gas-solid two-phase model by non-unity Lewis number, and control equation is established; Step 2, pressure-velocity field is coupled, direct numerical simulation is carried out to obtain combustion data set, and CP-DNS data are generated; CP-DNS represents carrier phase direct numerical simulation; Step 3, based on CP-DNS data, general mixing fraction Z suitable for coal-ammonia mixed combustion is calculated by Bilger formula, and based on the general mixing fraction Z, non-premixed small flame equation is solved by non-unity constant Lewis number, small flame lookup table containing difference diffusion is generated, and a small flame model is constructed; Step 4, by priori analysis, compare the small flame model prediction value with the CP-DNS data, evaluate the prediction effect of temperature and components; combine reaction path analysis, chemical time scale analysis and budget analysis, clarify the NO x generation mechanism, realize the NO x emission prediction of coal-ammonia mixed combustion.

2. A method for predicting NO in coal-ammonia blended combustion according to claim 1. x The numerical calculation method for emissions is characterized by, In step 1, the general form of the gas phase scalar transport equation of the control equation is: ， wherein, is the mass fraction of the species i, is the total enthalpy, , or ; is the gas phase density, is the Euler time, is the directional fluid velocity, represents the three-dimensional spatial coordinate index of the Cartesian coordinate system, respectively correspond to directions, is the gas phase thermal conductivity, is the non-unity constant Lewis number, representing the differential diffusion effect in the governing equations; is the gas phase specific heat capacity, is the chemical reaction rate, is the gas-solid two-phase coupling source term.

3. A method for predicting NO in coal-ammonia blended combustion according to claim 2. x The numerical calculation method for emissions is characterized by, In step 3, the general mixing fraction Z suitable for coal-ammonia mixed combustion is calculated based on the Bilger formula: ， ， wherein , , is the mixing fraction of the elements C, H, O, , , is the molecular weight of the elements C, H, O; and are the values of the fuel side and the oxidizer side value, respectively; Fuel side component mass fraction Calculated by ternary mixing fraction model: ， ， ， wherein, , , are the mass fractions of the components in the ammonia stream, the mass fractions of the components in the volatiles, and the mass fractions of the components in the coke tail gas, respectively; , , are the gas masses of the ammonia, the volatiles, and the coke tail gas, respectively, , , are the mixing fractions of the ammonia, the volatiles, and the coke tail gas, respectively; is the coke tail gas proportion; is the coal-ammonia mixing ratio.​​​ 4. A method for predicting NO in coal-ammonia blended combustion according to claim 3. x The numerical calculation method for emissions is characterized by, Difference diffusion is included in the small flame model by non-unity constant Lewis number, and a small flame lookup table is generated by solving the non-premixed small flame equation, the non-premixed small flame equation includes species transport equation and energy equation, respectively: ， ， where is the gas phase density, is the mass fraction of component , is the scalar dissipation rate, represents the Lewis number of component , is the reaction rate of component , is the heat release rate, is the gas phase temperature, represents the number of components in the combustion system; , is the gas phase specific heat capacity, is the specific heat capacity of component , is the average molecular weight of the mixture, Z is the mixture fraction, and was calculated using a detailed chemical reaction mechanism containing 129 species and 1664 elementary reactions.

5. A method for predicting NO in coal-ammonia blended combustion according to claim 4. x The numerical calculation method for emissions is characterized by, The prior analysis in step 4 compares the predicted values ​​of the small flame model with differential diffusion with CP-DNS data, focusing on assessing temperature, composition, and NO. x The predictive effect; For progress variables, , , These represent the mass fractions of CO2, H2O, and H2, respectively. For normalized total enthalpy, For total enthalpy in the gas phase, and Total enthalpy of the gas phase The minimum and maximum total enthalpy.

6. A method for predicting NO in coal-ammonia blended combustion according to claim 1. x The numerical calculation method for emissions is characterized by, In step 1, the control equation is discretized by finite volume method, the spatial derivative is processed by cubic discrete format, and the time derivative is discretized by implicit format; the grid division adopts a cuboid calculation domain, the flow direction, transverse direction and spanwise size are 59.7mm*59.7mm*14.9mm respectively, the grid resolution is 384*384*96, and the uniform grid spacing is 155.5μm.

7. A method for predicting NO in coal-ammonia blended combustion according to claim 1. x The numerical calculation method for emissions is characterized by, Boundary conditions in step 2 are set as: the upper stream is coal / ammonia / air mixture, , , , temperature 600 K, velocity 15 m / s, , , represent the mass fraction of NH3, O2, N2, respectively; the lower stream is hot air, temperature 1300 K, velocity -15 m / s; the diameter of coal powder particle is 25 μm, the number density is 1 x 10 11 particles / m³, the initial velocity and temperature are consistent with the gas phase; the x and z directions are periodic boundary, and the y direction is zero gradient boundary.

8. A method for predicting NO in coal-ammonia blended combustion according to claim 7. x The numerical calculation method for emissions is characterized by, Reaction path analysis in Step 4 quantifies NO by calculating the ratio of nitrogen atom flux x Path of generation: ， wherein, for defining the reaction path flux ratio of nitrogen atoms from species P to species Q; wherein, is the total number of elementary reactions, is the total number of species; is the flame index, represents the gradient sign, , , , represent the mass fractions of C 16 H 10 , CO, NH3, respectively; is the mass fraction of O2; is the flux of nitrogen atoms between species P and species Q in the reaction , is the flux of nitrogen atoms between NH3 and species .

9. A method for predicting NO in coal-ammonia blended combustion according to claim 1. x The numerical calculation method for emissions is characterized by, The chemical timescale in step 4 is estimated by , The chemical characteristic timescale for the species ; The gas phase density; The mass fraction of the species , The net reaction rate of the species , is positive, verifying the applicability of the steady-state small flame assumption.

10. A method for predicting NO in coal-ammonia blended combustion according to claim 9. x The numerical calculation method for emissions is characterized by, The budget analysis in Step 4 employs a Lagrangian transient equation and a generalized small flame budget equation, a relationship between the Lagrangian transient term and the Eulerian transient term with respect to a generic scalar is: ， wherein represents the Euler transient term, is a Lagrangian-like transient term, is a mixing fraction field, is a small flame coordinate along the mixing fraction contour surface; wherein, is the Euler time, is a Lagrangian-like time, is the gas phase density, is the fluid velocity vector, is is the unit normal vector of the contour surface, represents the gradient sign; The budget equation of the generalized small flame equation is: ， where, is the normal diffusion term along the mixture fraction gradient direction, is the multi-dimensional diffusion term along the iso-surface, which is neglected in one-dimensional small flamelet equations, is the differential diffusion term, is the source term, is the correction term.

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