Chain boiler thermal state simulation method and system

Abstract

The invention discloses a chain boiler thermal state simulation method and system, and relates to the technical field of gas-solid two-phase combustion process numerical solution, and the method comprises the following steps: carrying out mesh generation on a three-dimensional calculation model of a chain boiler to obtain a mesh model; the grid model is loaded with a self-defined function for numerical simulation, the self-defined function defines that drag force borne by the coal particles larger than the critical particle size in the X direction, the Y direction and the Z direction is zero, and the coal particles smaller than the critical particle size are obtained after the coal particles advance along with the fire grate and are combusted; coal particles smaller than the critical particle size are blown by air at the bottom of the fire grate, and coal ash is discharged from an outlet above the hearth; according to the method, numerical simulation is adopted to simulate the real conditions that fuel burns on the bottom fire grate along with advancing of the fire grate and part of small particles are blown up by bottom air and burns in a hearth, the numerical calculation result better conforms to the actual operation condition of the boiler, and the simulation result is more real and credible.

Description

Technical Field

[0001] The present invention relates to the technical field of numerical solution of gas-solid two-phase combustion processes, and particularly to a method and system for hot-state simulation of chain boilers. Background Art

[0002] At present, more than 80% of industrial boilers in China are coal-fired boilers. Among the currently used coal-fired industrial boilers, most are traditional stoker boilers, and the chain boiler is one of the most common types of stoker boilers. During the operation of the chain boiler, the combustible content in the slag and fly ash is relatively high, resulting in the actual operation efficiency of the boiler being lower than the designed value.

[0003] The main reasons for the low actual operation efficiency of the chain boiler are low boiler operation load, large mechanical incomplete combustion loss, large excess air coefficient at the tail, and high flue gas temperature, etc. These problems are closely related to the stoker combustion mode. Since raw coal that has not been further crushed is burned, the particle size of the coal is often relatively large, even reaching the centimeter level. As the particle size increases, the combustion of coal particles undergoes a completely different process from that of pulverized coal combustion. There are also heat transfer, mass transfer, and chemical reaction processes inside the large coal particles. At the same time, the structural changes of the coal particles have a direct impact on the diffusion rate of oxygen and other gaseous products, the combustion rate, and the change of the bed height during the combustion process of the coal particles. Therefore, the combustion becomes more complex. For this reason, strengthening the research on the combustion characteristics of chain boilers and developing a numerical calculation model for stratified combustion are of great significance for improving the design of chain boilers, increasing the operation efficiency, and reducing pollutant emissions.

[0004] The currently common hot-state simulation method for chain boilers is to separately model the bed and the furnace to obtain a simplified model. Among them, the bed is calculated using FLIC software or Fluent, and the furnace adopts a gas-phase combustion model, assuming no relative slip between the fuel and the grate, and the coal blocks are relatively stationary. However, in actual situations, the coal blocks have different movement conditions during combustion. The simplified model ignores the combustion conditions of different particle fuels, does not conform to the actual combustion situation, and cannot restore the true combustion situation inside the chain boiler. Summary of the Invention

[0005] Based on the defects existing in the above-mentioned prior art, the present invention provides a method and system for hot-state simulation of chain boilers, which solves the problem that the existing simplified model ignores the combustion conditions of different particle fuels, does not conform to the actual combustion situation, and cannot restore the true combustion situation inside the chain boiler.

[0006] The present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a method for hot-state simulation of chain boilers, including the following steps:

[0008] Establish a three-dimensional physical model of the chain boiler, and set different air inlets in its grate area;

[0009] A computational model for simulating the flow field and temperature field of a boiler is established inside a three-dimensional physical model to obtain a three-dimensional computational model of a chain boiler, and the three-dimensional computational model is meshed to obtain a mesh model;

[0010] Set the initial boundary conditions, simulation calculation parameters, chemical reaction equations involved in the internal components of the boiler, and corresponding chemical reaction coefficients of the mesh model;

[0011] A custom function is loaded onto the mesh model for numerical simulation to obtain the internal operating parameters of the chain furnace; wherein, the custom function defines that the drag force on coal particles larger than the critical particle size is zero in the X, Y, and Z directions, the coal particles larger than the critical particle size follow the grate forward and burn to obtain coal particles smaller than the critical particle size; the coal particles smaller than the critical particle size are blown up by the air entering through different air inlets of the grate, burn in the furnace and the coal ash is discharged from the outlet above the furnace; the critical particle size is obtained through the air velocity.

[0012] Preferably, the computational model for simulating the flow field and temperature field of the boiler is established inside the three-dimensional physical model, wherein, the realizable k-ε turbulent flow model is selected to simulate the flow field inside the furnace, and the coke combustion model uses the shrinking core model based on ash layer accumulation and the species transport model as the gas-phase combustion model.

[0013] Preferably, the initial boundary conditions include the composition and physical state of the fuel, the feeding rate, the primary air volume, and the wall temperature.

[0014] Preferably, the critical particle size is obtained through the air velocity, specifically including the following steps:

[0015] Obtain the air velocity, and the air velocity is specifically as follows:

[0016]

[0017] In the formula, U is the air velocity, F air is the inlet air mass flow rate, S is the inlet cross-sectional area, ρ G is the inlet air density;

[0018] Obtain the minimum fluidization velocity, and the minimum fluidization velocity is specifically as follows:

[0019]

[0020] In the formula, U mf is the minimum fluidization velocity, μ G is the viscosity coefficient of the air, N Re,mf is the Reynolds number at the minimum fluidization velocity, N Aris the Archimedes number, d p is the particle size, a1 and a2 are semi-empirical values, g is the local acceleration of gravity, ρ p is the apparent density of the particles;

[0021] When the air velocity is equal to the minimum fluidization velocity, the critical particle size is obtained, and the critical particle size is specifically as follows:

[0022]

[0023] In the formula, d p,lj is the critical particle size.

[0024] In a second aspect, the present invention provides a hot-state simulation system for a chain boiler, including:

[0025] A building module, configured to build a three-dimensional physical model of the chain boiler and set different air inlets for its grate area;

[0026] A meshing module, configured to build a calculation model for simulating the flow field and temperature field of the boiler inside the three-dimensional physical model, obtain a three-dimensional calculation model of the chain boiler, and perform mesh meshing on the three-dimensional calculation model to obtain a mesh model;

[0027] A setting module, configured to set the initial boundary conditions, simulation calculation parameters, chemical reaction equations involved in the internal components of the boiler, and corresponding chemical reaction coefficients of the mesh model;

[0028] A simulation module, configured to perform numerical simulation on the mesh model by loading a custom function to obtain the internal operation parameters of the chain furnace; wherein, the custom function defines that the drag force on coal particles larger than the critical particle size is zero in the X, Y, and Z directions, the coal particles larger than the critical particle size follow the grate forward and burn to obtain coal particles smaller than the critical particle size; the coal particles smaller than the critical particle size are blown up by the air entering through different air inlets of the grate, burn in the furnace and discharge the coal ash from the outlet above the furnace; the critical particle size is obtained through the air velocity.

[0029] Compared with the prior art, at least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects:

[0030] The present invention first establishes a three-dimensional physical model of a chain boiler, sets different air inlets, and introduces a custom function. This custom function defines that the drag forces on coal particles larger than the critical particle size are zero in the X, Y, and Z directions. The coal particles larger than the critical particle size follow the grate and burn, resulting in coal particles smaller than the critical particle size. The coal particles smaller than the critical particle size are blown up by the air entering through different air inlets of the grate, burn in the furnace, and the coal ash is discharged from the outlet above the furnace. The present invention uses numerical simulation to simulate the actual situation where the fuel burns while advancing with the grate at the bottom, and some smaller particles are blown up by the bottom air and burn in the furnace. The numerical calculation results are more in line with the actual operation of the boiler, and the simulation results are more realistic and reliable. Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 It is a schematic flow chart of a hot state simulation method for considering the actual particle movement mode of a chain boiler according to the present invention;

[0033] Figure 2 It is a three-dimensional physical model diagram of the chain furnace according to the present invention;

[0034] Figure 3 It is a schematic diagram of the division of the chain grate according to the present invention;

[0035] Figure 4 It is a schematic diagram of the grid model of the chain boiler in the embodiment of the present invention;

[0036] Figure 5 It is a temperature field cloud diagram of the central longitudinal section of the furnace under different air pressure conditions calculated by adopting the solution of the present invention in the embodiment;

[0037] Among them, Figure 5 (a) of : Temperature field cloud diagram of the central longitudinal section of the furnace under the air pressure condition of 101325Pa, Figure 5 (b) of : Temperature field cloud diagram of the central longitudinal section of the furnace under the air pressure condition of 77273Pa, Figure 5 (c) of : Temperature field cloud diagram of the central longitudinal section of the furnace under the air pressure condition of 65200Pa;

[0038] Figure 6 It is a diagram of the movement trajectory of particles in the furnace calculated by adopting the solution of the present invention in the embodiment;

[0039] Figure 7 The burnout conditions of some grate coal particles calculated by the solution of the present invention. Specific embodiments

[0040] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] The common simulation method for chain boilers is to model the bed and the furnace separately. The bed is calculated using FLIC software or Fluent, and the furnace adopts a gas-phase combustion model. It is assumed that there is no relative slip between the fuel and the grate, and the coal blocks are relatively stationary. The simplified model ignores the situation where smaller particles are blown up and cannot restore the actual combustion situation in the chain boiler. It cannot handle complex conditions such as high-altitude environments or the combustion of solid fuel blended with gas; the influence of fly ash in the furnace on radiation is not considered, etc.

[0042] To solve the existing technical problems, referring to Figure 1 , the present invention proposes a hot-state simulation method considering the actual particle movement mode of a chain boiler. The core lies in using a simulation platform to achieve functions and changing the movement mode of fuel particles in the chain furnace: the fuel enters the grate from the inlet, and it is set that the drag forces on larger coal particles in the X, Y, and Z directions are all zero, and an initial velocity along the grate direction is set, so as to ensure that this part of the coal particles move horizontally and uniformly, and burn while the grate advances; smaller particles are normally stressed and are blown up by the air at the bottom of the grate, can burn in the furnace and finally discharge coal ash from the upper outlet of the furnace. As the combustion progresses, the density of the large particles originally moving on the grate decreases, and the small particles generated by cracking will also be blown up and continue to react in the furnace, which conforms to the actual situation of the operation of the chain boiler. The basis for distinguishing whether particles can be blown up by the bottom air is the critical particle size calculated based on the bottom air flow velocity as the minimum fluidization velocity. Particles smaller than this critical particle size will be blown up.

[0043] Among them, the calculation process of the critical particle size is as follows:

[0044] Inlet air density:

[0045]

[0046] In the formula, T G is the inlet air temperature, with the unit of K.

[0047] Inlet air flow velocity:

[0048]

[0049] In the formula, F air is the inlet air mass flow rate, with the unit of kg / s; S is the inlet cross-sectional area, with the unit of m 2 .

[0050] Minimum fluidization velocity:

[0051]

[0052] In the formula, μ G is the viscosity coefficient of air, with the unit of Pa·s; d p is the particle diameter, with the unit of m; a1 and a2 are semi-empirical values, generally taking a1 = 27.2 and a2 = 0.0408; g is the local acceleration of gravity, with the unit of m / s 2 ; ρ p is the apparent density of particles, with the unit of kg / m 3 .

[0053] When the superficial gas velocity U is greater than the minimum fluidization velocity U mf , the particles reach the fluidized state and are blown up. Therefore, when the inlet gas velocity is equal to the minimum fluidization velocity, that is, by setting the critical particle diameter d p,lj can be calculated.

[0054] Specifically, a hot-state simulation method for a chain boiler according to the present invention includes the following steps:

[0055] Step 1: Establish a physical model of the chain boiler.

[0056] Referring to Figure 2 , establish a physical model of the chain boiler, process the physical model, divide the grate part into multiple different regions, arrange the air inlets intermittently and evenly along the furnace depth direction to simulate the pores of the real grate. Due to considering the difficulty of division, each primary air inlet is divided into 4 small air inlets for air intake, and the air volume is evenly distributed. Referring to Figure 3 , the grate surface is arranged as wall-inlet1-wall-inlet2-wall-inlet3-wall-inlet4-wall along the furnace depth direction, and the grate ventilation cross-sectional ratio f tf is 25%.

[0057] Step 2: Establish a three-dimensional calculation model for simulating the flow field and temperature field based on the chain boiler; considering the reliability of the calculation model, select the realizable k-ε turbulent flow model to simulate the flow field in the furnace, adopt the shrinking core model based on ash layer accumulation for the coke combustion model, and the species transport model as the gas-phase combustion model.

[0058] Among them, the coke combustion model adopts a shrinking core model based on ash layer accumulation, uses a finite rate / eddy dissipation and kinetics / diffusion controlled reaction rate model, and calls the DEFINE_PR_RATE macro to customize the surface reaction rate of coke combustion. Due to the high ash content of coal in China and the large diameter of coal particles used in chain coal-fired boilers, the internal heat conduction of particles is slow. When the surface coke is burned out to form an ash layer, the internal carbon core has not been completely burned out and may even still be raw coal. When the ash layer is thick, it will significantly hinder the diffusion of oxygen to the carbon core. Considering that the carbon core continuously shrinks and the ash layer continuously thickens during the actual combustion process of carbon particles, the oxygen is affected by the increasing ash layer resistance during the diffusion process, resulting in a slowdown in the combustion rate.

[0059] Step 3: Mesh the computational model to obtain the corresponding mesh model, as Figure 4 shown.

[0060] Step 4: Set the initial boundary conditions of the mesh model. The initial boundary conditions include the composition and physical state of the fuel, the feeding rate, the primary air volume, the wall temperature, etc.

[0061] Specifically, for the setting of the initial boundary conditions, the primary air inlet is a mass inlet, and the excess air coefficient and the inlet air mass flow rate are set according to the actual situation. The outlet above the furnace is set as a negative pressure outlet, the water-cooled wall is a constant temperature wall, and the furnace arch is set as adiabatic. The DPM boundary types at the outlet above the furnace and the outlet at the tail of the grate are both set to escape, so that the particle phase automatically escapes when it moves to the outlet, avoiding the accumulation of coal ash.

[0062] Step 5: Set the simulation calculation parameters according to the actual operating conditions inside the boiler, and add the chemical reaction equations and corresponding chemical reaction coefficients involved in the components inside the boiler (the chemical reaction equations and corresponding chemical reaction coefficients for the reaction of the volatile matter generated by coal with oxygen need to be set in the boiler simulation).

[0063] The simulation calculation parameters are shown in Tables 1 and 2 below:

[0064] Table 1 Proximate and ultimate analysis of coal

[0065]

[0066] Table 2 Simulation calculation parameter table

[0067] Input data <![CDATA[Mass flow rate / kg·s -1 > Temperature / K Coal feeding amount 0.443 353 Primary air 4.616 473 Water wall / 600

[0068] Step 6: Based on the mesh model with the initial boundary conditions set, load the UDF (User Defined Function), and use the CFD numerical simulation algorithm to simulate the three-dimensional flow field, temperature field, and component field, etc., and calculate the internal operating parameters of the chain boiler. The operating parameters include the temperature field distribution of the chain furnace (Figure 5 ) Particle motion trajectory Figure 6 ) and the burnout condition of the fuel Figure 7 ).

[0069] The above hot-state simulation method considering the actual particle motion mode of a chain boiler uses the CFD numerical simulation algorithm to simulate the real situation of the fuel burning while advancing with the grate on the bottom grate and some smaller particles being blown up by the bottom air and burning in the furnace. It also improves the coke combustion model and is applicable to the plateau environment, providing technical guidance for the optimization of industrial chain boilers. Different from the existing cold-state simulation, the simulation of the present invention belongs to hot-state simulation, and the numerical calculation results are more in line with the actual operation of the boiler, and the simulation results are more real and reliable.

[0070] Based on the same concept, the present invention also provides a hot-state simulation system for a chain boiler, including a building module, a meshing module, a setting module, and a simulation module.

[0071] The building module is used to build a three-dimensional physical model of the chain boiler and set different air inlets in its grate area.

[0072] The meshing module is used to build a calculation model for simulating the boiler flow field and temperature field inside the three-dimensional physical model, obtain a three-dimensional calculation model of the chain boiler, and perform grid meshing on the three-dimensional calculation model to obtain a grid model.

[0073] The setting module is used to set the initial boundary conditions of the grid model, simulation calculation parameters, chemical reaction equations involved in the internal components of the boiler, and corresponding chemical reaction coefficients.

[0074] The simulation module is used to perform numerical simulation on the grid model by loading a custom function to obtain the internal operation parameters of the chain furnace; wherein, the custom function defines that the drag force on coal particles larger than the critical particle size is zero in the X, Y, and Z directions, and the coal particles larger than the critical particle size follow the grate forward and burn to obtain coal particles smaller than the critical particle size; the coal particles smaller than the critical particle size are blown up by the air entering from different air inlets of the grate, burn in the furnace and discharge coal ash from the outlet above the furnace; the critical particle size is obtained through the air velocity.

[0075] The present invention is convenient for operating under conditions such as air pressure change, co-firing conditions, and adjustment of secondary air, is applicable to complex conditions such as the plateau environment and the combustion of solid fuel blended with gas, can simulate the actual combustion situation of the fuel in the chain furnace under low air pressure conditions, and provides guidance for the design and operation parameter optimization of plateau boilers.

[0076] The present invention uses the CFD numerical simulation algorithm to simulate the real situation of fuel burning while advancing with the grate on the bottom grate, and some smaller particles being blown up by the bottom air and burning in the furnace. The coke combustion model is improved and applicable to the plateau environment, providing technical guidance for the optimization of industrial chain boilers. Different from the cold-state simulation of the prior art, the simulation of the present invention belongs to the hot-state simulation, and the numerical calculation results are more in line with the actual operation of the boiler, and the simulation results are more real and reliable.

[0077] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0078] Obviously, those skilled in the art can make various changes and deformations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and deformations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and deformations.

Claims

1. A hot-state simulation method for a chain boiler, characterized in that It includes the following steps: Establish a three-dimensional physical model of a chain boiler, and set different air inlets in its grate area; Establish a calculation model for simulating the boiler flow field and temperature field inside the three-dimensional physical model, obtain the three-dimensional calculation model of the chain boiler, and perform grid meshing on the three-dimensional calculation model to obtain a grid model; Set the initial boundary conditions, simulation calculation parameters, chemical reaction equations involved in the components inside the boiler, and corresponding chemical reaction coefficients of the grid model; Perform numerical simulation on the grid model by loading a user-defined function to obtain the internal operating parameters of the chain furnace; wherein, the user-defined function defines that the drag force on coal particles larger than the critical particle size is zero in the X, Y, and Z directions, the coal particles larger than the critical particle size follow the grate forward and burn to obtain coal particles smaller than the critical particle size; the coal particles smaller than the critical particle size are blown up by the air entering through different air inlets of the grate, burn in the furnace and discharge coal ash from the outlet above the furnace; the critical particle size is obtained through the air velocity.

2. The hot state simulation method of a chain boiler according to claim 1, characterized in that The establishment of a calculation model for simulating the boiler flow field and temperature field inside the three-dimensional physical model, wherein, the realizable k-ε turbulent flow model is selected to simulate the flow field in the furnace, and the coke combustion model adopts the shrinking core model based on ash layer accumulation and the species transport model as the gas-phase combustion model.

3. A hot-state simulation method for a chain boiler according to claim 1, characterized in that, The initial boundary conditions include the composition and physical state of the fuel, the feeding rate, the primary air volume, and the wall temperature.

4. A hot state simulation method for a chain boiler according to claim 1, characterized in that, The critical particle size is obtained through the air velocity, specifically including the following steps: Obtain the air velocity, and the air velocity is specifically as follows: where U is the air velocity, F air is the inlet air mass flow rate, S is the inlet cross-sectional area, ρ G is the inlet air density; Obtain the minimum fluidization velocity, and the minimum fluidization velocity is specifically as follows: Wherein, U mf is the minimum fluidization velocity, μ G is the viscosity coefficient of air, N Re,mf is the Reynolds number at the minimum fluidization velocity, N Ar is the Archimedes number, d p is the particle size, a1 and a2 are semi-empirical values, g is the local acceleration of gravity, ρ p is the apparent density of the particles; When the air velocity is equal to the minimum fluidization velocity, obtain the critical particle size, and the critical particle size is specifically as follows: where d p,lj is the critical particle size.

5. A hot-state simulation system for a chain boiler, characterized in that, It includes: A building module for building a three-dimensional physical model of a chain boiler and setting different air inlets in its grate area; A meshing module for establishing a calculation model for simulating the boiler flow field and temperature field inside the three-dimensional physical model, obtaining the three-dimensional calculation model of the chain boiler, and performing grid meshing on the three-dimensional calculation model to obtain a grid model; A setting module for setting the initial boundary conditions, simulation calculation parameters, chemical reaction equations involved in the components inside the boiler, and corresponding chemical reaction coefficients of the grid model; A simulation module for performing numerical simulation on the grid model by loading a user-defined function to obtain the internal operating parameters of the chain furnace; wherein, the user-defined function defines that the drag force on coal particles larger than the critical particle size is zero in the X, Y, and Z directions, the coal particles larger than the critical particle size follow the grate forward and burn to obtain coal particles smaller than the critical particle size; the coal particles smaller than the critical particle size are blown up by the air entering through different air inlets of the grate, burn in the furnace and discharge coal ash from the outlet above the furnace; the critical particle size is obtained through the air velocity.

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