A large diameter chimney traceable flue gas flow measurement system and method

Abstract

The application discloses a large-diameter chimney traceable flue gas flow measuring system and method, and belongs to the field of flue gas flow measurement. The system comprises: a tracer gas cylinder for storing a tracer; a flow controller for controlling the output flow of the tracer; a tracer injection probe, the tracer output from the flow controller is injected into the upstream position of the chimney pipeline through the tracer injection probe to mix with the flue gas; a tracer sampling probe installed at the downstream position of the chimney pipeline, the mixed gas sample is extracted through an air pump; and a gas analyzer for analyzing the obtained gas sample and calculating the flue gas volume flow in the chimney pipeline. The application directly calculates the flue gas volume flow in the chimney by monitoring the concentration change of the tracer in the pipeline flow process, so as to improve the accuracy of the flow measurement, and the measurement result has traceability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of flue gas flow measurement, and in particular to a large-diameter chimney traceable flue gas flow measurement system and method. BACKGROUND

[0002] The carbon dioxide emissions of the thermal power industry account for about 40% of the total carbon dioxide emissions, so it is of great practical significance to achieve accurate measurement of carbon emissions in the thermal power industry. The largest error in the measurement of carbon emissions of thermal power enterprises comes from flue gas flow measurement. The commonly used flue gas flow measurement methods at present include differential pressure method, ultrasonic method, etc., which are all based on the principle of velocity-area method. These methods have very high requirements for the length of the straight pipe section at the installation position of the flowmeter in the chimney flue gas flow measurement, often requiring a straight pipe section of 10-20 times the pipe diameter length upstream. However, in actual application, due to factors such as site and economy, the installation position of the flowmeter often cannot meet the above requirements, resulting in that the flue gas in the pipe cannot fully develop, and flow states such as strong turbulence, backflow, asymmetric flow and secondary flow occur. Combined with the large diameter of the chimney of the thermal power plant, the flow velocity measured by the above measurement methods is difficult to represent the cross-sectional average flow velocity, thereby a large flow measurement error occurs. According to research, the complex flow characteristics of flue gas can make the measurement error of the conventional flow measurement method reach 25% or even more. In addition, in a large-diameter chimney, it is difficult to ensure the measurement accuracy and stability of the flowmeter during long-term operation, and there is a lack of standard devices for calibrating the flue gas flow of large-diameter chimneys and a perfect value traceability system in China, which further limits the improvement of the accuracy of flue gas flow measurement. SUMMARY

[0003] To at least partially solve one of the technical problems existing in the prior art, the purpose of the present application is to provide a large-diameter chimney traceable flue gas flow measurement system and method based on tracer gas dilution method.

[0004] The first technical solution adopted by the present application is:

[0005] A large-diameter chimney traceable flue gas flow measurement system, comprising:

[0006] A tracer gas cylinder for storing a tracer;

[0007] A flow controller for controlling the output flow of the tracer;

[0008] A tracer injection probe, the tracer output from the flow controller is injected into the upstream position of the chimney pipe through the tracer injection probe to mix with the flue gas;

[0009] A tracer sampling probe installed at the downstream position of the chimney pipe to extract the mixed gas sample through a gas pump;

[0010] A gas analyzer is used to analyze the obtained gas sample for calculating the flue gas volume flow in the chimney pipe.

[0011] Further, the calculation formula of the flue gas volume flow is as follows:

[0012]

[0013] In the formula, X T,1 is the concentration of the injected tracer, V T,1 is the volume flow of the injected tracer; X T,D is the concentration of the tracer in the gas sample collected downstream; X T,U is the concentration of the tracer measured upstream of the injection point.

[0014] Further, the tracer is sulfur hexafluoride (SF6), and the tracer dilution ratio is 1:500,000.

[0015] Further, the tracer is injected into the chimney pipe by means of flue injection or chimney injection; and / or,

[0016] The tracer sampling probe adopts a three-point sampling method.

[0017] The second technical solution adopted by the present application is:

[0018] A traceable flue gas flow measurement method for a large-diameter chimney, comprising the following steps:

[0019] A physical model of the chimney is built, and the physical model is meshed, and the model is simulated;

[0020] The final tracer type is determined according to the mixing effect of the tracer and the flue gas, the gas detection level and the cost;

[0021] The tracer dilution ratio is determined according to the mixing effect of the gas;

[0022] The injection section of the tracer is determined according to the mixing effect of the gas;

[0023] A flue gas flow measurement system is built according to the information determined by simulation, for measuring the flue gas flow.

[0024] Further, the physical model of the chimney is built, and the physical model is meshed, and the model is simulated, comprising:

[0025] The chimney diameter is determined, and the chimney model is built;

[0026] The model is meshed using Fluent meshing, and the grid near the tracer injection port is encrypted;

[0027] Assuming that the flow of flue gas in the chimney pipe is steady, incompressible flow, and using the standard k-ε turbulence model;

[0028] Through the component transport model in the FLUENT numerical simulation software, the diffusion of the tracer and the mixing with the flue gas in the transportation process are reflected, and the material transport equation and the mass diffusion equation are:

[0029]

[0030] In the formula, R i represents the net generation rate of the chemical reaction substance; S i represents the dispersion term; Sc t represents the Schmidt number of turbulence; ρ represents the density of the mixed fluid, and the unit is kg / m 3 , Y i represents the mass fraction of component i, represents the velocity vector of the mixed fluid, and the unit is m / s, D i,m represents the molecular diffusion coefficient of component i in the mixture, and the unit is m 2 / s, μ t represents the turbulent viscosity of the mixture.

[0031] Further, the final tracer type is determined according to the mixing effect of the tracer and the flue gas, comprising:

[0032] Under the preset conditions, the mixing effects of a plurality of tracers with flue gas in the chimney are simulated respectively, and the main factors affecting the mixing effect of the tracer with the flue gas are obtained;

[0033] The final tracer type is selected in combination with the simulation results, the gas detection level and the cost.

[0034] Further, the tracer dilution ratio is determined according to the mixing effect of the gas, comprising:

[0035] The flow rate of flue gas in the chimney is set;

[0036] A plurality of tracer dilution ratios are set, and the injection speed of the tracer affected by the tracer dilution ratio is simulated respectively;

[0037] According to the simulation results, the influence of the tracer dilution ratio on the mixing effect of the tracer with the flue gas is analyzed.

[0038] Further, the injection section of the tracer is determined according to the mixing effect of the gas, comprising:

[0039] Two kinds of tracer injection sections of chimney injection and flue injection are set, and the influences of chimney and flue section injection on the mixing of the tracer with the flue gas and the flow measurement error are simulated respectively.

[0040] According to the simulation result and the flow measurement error, the final injection section of the tracer is selected.

[0041] Further, the large-diameter chimney traceable flue gas flow measurement method further comprises a tracer sampling setting step.

[0042] Three sampling sections are set at the downstream of the pipeline at a distance of 3D, 8D and 12D from the tracer injection position; wherein, when the flue is injected, the upper plane of the flue before the chimney inlet is defined as 0D, 3D and 8D are close to the two original measurement sections of the chimney of the power plant, and D represents the diameter of the chimney pipeline.

[0043] The beneficial effects of the present application are: the present application provides a tracer gas dilution method applied to a large-diameter chimney flue gas flow measurement system, the volume flow of the flue gas in the chimney is directly calculated by monitoring the concentration change of the tracer in the pipeline flow process, so as to improve the accuracy of the flow measurement, and the measurement result has traceability. In addition, the measurement accuracy mainly depends on the mixing of the tracer and the flue gas, and the strong turbulence and various complex flow characteristics of the flue gas in the chimney flow process can well promote the mixing of the tracer and the flue gas, so the best scheme for implementing the method is studied based on the numerical simulation method, so as to achieve the effect of greatly reducing the required straight pipe length and increasing the flow measurement accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following introduces the drawings of the related technical solutions in the embodiments of the present application or the prior art. It should be understood that the drawings in the following introduction are only for the convenience of clearly describing part of the embodiments of the technical solutions of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0045] Figure 1 is a flue-chimney three-dimensional model diagram in the embodiments of the present application;

[0046] Figure 2 is a schematic diagram of a large-diameter chimney traceable flue gas flow measurement system in the embodiments of the present application;

[0047] Figure 3 is a schematic diagram of a chimney model and a straight pipe model in the embodiments of the present application; wherein, Figure 3 (a) in the figure is a schematic diagram of a chimney model, Figure 3 (b) in the figure is a schematic diagram of a straight pipe model;

[0048] Figure 4 is a schematic diagram of section grid division in the embodiments of the present application;

[0049] Figure 5 is a schematic diagram of the relative standard deviation of cross-sectional tracer concentration versus chimney height in an embodiment of the present application; wherein, Figure 5 (a) in is a schematic diagram of mixing effect in chimney model, Figure 5 (b) in is a schematic diagram of mixing effect in straight pipe model;

[0050] Figure 6 is a schematic diagram of two tracer injection schemes in an embodiment of the present application; wherein, Figure 6 (a) in is a schematic diagram of chimney injection, Figure 6 (b) in is a schematic diagram of flue injection;

[0051] Figure 7 is a schematic diagram of six tracer sampling schemes in an embodiment of the present application;

[0052] Figure 8 is a comparative diagram of mixing effect of tracer and flue gas when tracer is injected into chimney and flue respectively in a certain working condition in an embodiment of the present application (D represents chimney pipe diameter in the diagram);

[0053] Figure 9 is a schematic diagram of flue gas flow measurement error of each sampling scheme when tracer is injected into chimney and flue in three working conditions in an embodiment of the present application; wherein, Figure 9 (a) in is a schematic diagram of flue gas flow measurement error under 60% load, Figure 9 (b) in is a schematic diagram of flue gas flow measurement error under 80% load, Figure 9 (c) in is a schematic diagram of flue gas flow measurement error under 100% load.

[0054] Figure 2 Reference signs of are as follows: 1-tracer gas cylinder; 2-Ethernet; 3-flow controller; 4-tracer injection probe; 5-chimney pipe; 6-tracer sampling probe; 7-gas pump; 8-gas analyzer; 9-data acquisition and control device. DETAILED DESCRIPTION

[0055] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference signs represent the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are only used to explain the present application, and cannot be understood as a limitation of the present application. For the step numbers in the following embodiments, they are only set for the convenience of explanation, and the order between the steps is not limited in any way, and the execution order of each step in the embodiments can be adaptively adjusted according to the understanding of those skilled in the art.

[0056] In the description of the present application, it should be understood that the orientation description, such as the orientation or position relationship indicated by up, down, front, back, left, right and the like, is based on the orientation or position relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0057] In the description of the present application, one or more is understood as one or more, more than two is understood as more than two, greater than, less than, more than, etc. are understood as not including the number, above, below, etc. are understood as including the number. If the first, second is described, it is only for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features or the sequence of indicated technical features.

[0058] In the description of the present application, unless otherwise explicitly limited, the words such as setting, installing, connecting and the like should be broadly understood, and those skilled in the art can reasonably determine the specific meaning of the above words in the present application in combination with the specific content of the technical solution.

[0059] In order to solve the existing technical problems, the present application applies the tracer gas dilution method to the measurement of flue gas flow of large-diameter chimneys, and studies the best scheme for implementing the method based on numerical simulation method. The tracer gas dilution method can overcome the flow measurement error caused by uneven flow distribution, and the measurement result has traceability. Its application to the measurement of flue gas flow of large-diameter chimneys or as a kind of on-site checking method has good application prospect. The tracer gas dilution method has the following advantages: (1) it does not need to measure the pipe cross-sectional area, so it can avoid the flow measurement error caused by size error; (2) it can overcome the problem that the flow measurement result is overestimated due to wall effect; (3) the concentration of the injected tracer is known, and the determination of flue gas volume flow only needs to measure the volume flow of the tracer, the tracer concentration collected at the upstream and downstream sampling positions of the injection port, and the three measurement variables all have good traceability, so the final flue gas flow measurement result also has traceability.

[0060] As shown in Figure 1 and Figure 2 The present embodiment provides a traceable flue gas flow measurement system for large-diameter chimneys, which comprises:

[0061] A tracer gas cylinder 1 for storing tracers;

[0062] A flow controller 3 for controlling the output flow of the tracers;

[0063] A tracer injection probe 4 is installed at an upstream position of the chimney duct 5 to inject the tracer gas outputted from the flow controller into the chimney duct to mix with the flue gas;

[0064] A tracer sampling probe 6 is installed at a downstream position of the chimney duct to extract the mixed gas sample by a gas pump 7;

[0065] A gas analyzer 8 is used to analyze the obtained gas sample for calculating the flue gas volume flow in the chimney duct.

[0066] As an optional embodiment, referring to Figure 2 , the measurement system further comprises a data acquisition and control device 9 connected with the gas analyzer 8 and the flow controller 3 through the Ethernet 2.

[0067] The working principle of the above system is as follows: the tracer gas cylinder provides high-purity tracer gas, and the tracer gas is injected into the chimney duct at a constant rate from the tracer injection probe through the flow controller. The flue gas sampling probe is installed at a downstream position, and the mixed gas sample is extracted by a pump, and the flow rate of the gas sample is controlled to be stable into the gas analyzer. The data acquisition and control device can control the devices in the system to realize the above operation through the connected Ethernet, and communicate with other devices to realize data synchronization, centralized monitoring and recording, and calculate the flue gas volume flow in the monitoring duct by the following formula:

[0068]

[0069] In the formula, X T,1 is the concentration of the injected tracer, V T,1 is the volume flow of the injected tracer; X T,D is the concentration of the tracer in the gas sample collected downstream; X T,U is the concentration of the tracer measured upstream of the injection point, which is 0 in this embodiment.

[0070] The measurement accuracy of the tracer gas dilution method depends on the mixing degree of the tracer and the gas in the pipe, so the strong turbulence and complex flow characteristics of the gas flow in the pipe can promote the mixing of the tracer and the flue gas, thereby improving the measurement accuracy of the method. In order to improve the measurement accuracy, the embodiments of the present application also conduct research based on numerical simulation, which will be described in detail below in combination with the drawings and the specific application scene of the large-diameter chimney of the thermal power plant. However, the application scene of the present application is not limited to the large-diameter chimney of the thermal power plant, and can be applied to various pipeline gas flow measurement scenes.

[0071] (1) CFD model establishment and verification

[0072] 1.1) Geometric model and mesh division

[0073] The chimney model of the embodiment is shown in Fig. 1(a). Figure 3 The chimney diameter is 7.8 m, the height is 210 m, there are measuring platforms at the heights of 80 m and 120 m, but due to a 40 m high platform at the bottom of the chimney pipe, the actual length of the chimney pipe is 170 m, for the convenience of analysis, the bottom of the chimney pipe is defined as the 0 m height, and at this time, the heights of the measuring platforms are 40 m and 80 m respectively. In addition, a straight pipe model as shown in Fig. 1(b) is also provided in the embodiment to compare with the above real chimney model to study the influence of different flue gas flow characteristics on the mixing effect of the tracer and the flue gas, and the diameter and length of the straight pipe model are the same as those of the chimney model. Figure 3

[0074] In the embodiment, Fluent meshing is used to divide the model into grids, and Poly-hexcore is used to divide hexahedral grids. In order to make the numerical simulation results more accurate, the grid near the tracer injection port is encrypted, and the specific situation can be seen in the example in Fig. 2. In the embodiment, four grid division schemes are set to verify the grid independence, and the relative standard deviation of the tracer concentration in the chimney cross section is used as the evaluation index to analyze the mixing effect of the tracer and the flue gas. The simulation results are as follows: the simulation results of the grid number of 216w and 240w are almost the same, and the simulation results of the grid number of 156w and 180w have local differences with the simulation results of the grid number of 200w and 223w. Based on the above results, the scheme of the grid number of 216w is selected for subsequent simulation work in the embodiment. Figure 4

[0075] 1.2) Mathematical model and boundary conditions

[0076] The flow of flue gas in the pipe can be assumed to be steady and incompressible, and its continuity equation, momentum equation and energy equation are shown in formula (2):

[0077]

[0078] Wherein: ρ is the density of flue gas, kg / m 3 ; u is the flue gas velocity, m / s; t is time, s; Γ φ is the generalized diffusion coefficient, φ is the general variable; S φ is the generalized source term.

[0079] In the embodiment, the standard k-ε turbulence model is selected. The flow field is solved by using the SIMPLEC algorithm.

[0080] The turbulent kinetic energy equation is:

[0081]

[0082] The dissipation rate ε equation is:

[0083] ​​

[0084] wherein: G k is the turbulent kinetic energy; C 1ε , C 2ε are constants; σ k and σ ε are the turbulent Prandtl numbers for the k-equation and the ε-equation, respectively.

[0085] The present embodiment reflects the diffusion of the tracer and the mixing with the flue gas during the transportation of the flue gas and the tracer by the component transportation model in the FLUENT numerical simulation software. The material transportation equation and the mass diffusion equation are as follows:

[0086]

[0087] wherein: R i represents the net generation rate of the chemical reaction substance; S i represents the discrete term; Sc t represents the turbulent Schmidt number.

[0088] The simulation inlet is a velocity inlet, and the flue gas inlet velocity is set to 6.04 m / s, 6.75 m / s and 7.45 m / s according to the 60%, 80% and 100% load of the power plant operation. The outlet is set as a pressure outlet. The wall surface is a standard no-slip wall condition.

[0089] (2) Selection of the tracer

[0090] According to the Maxwell-Gilliland formula, the diffusion coefficient between two gases can be calculated by formula (7)

[0091]

[0092] wherein, T is the temperature, K; p is the pressure, kPa; M is the molecular mass; v refers to the volume of 1 mol of substance in liquid state at its normal boiling point, cm 3 / mol; A and B represent two gases, respectively.

[0093] In the present embodiment, in order to analyze the influence of the type of tracer on the mixing effect of the tracer and the flue gas, the mixing effect of SF6 and He as tracers with the flue gas is analyzed in the chimney model and the straight pipe model, respectively. It should be noted that the present embodiment only analyzes SF6 and He, but is not limited to these two substances. The simulation is carried out under 80% load, and the tracer is injected at a height of 20 m chimney. Figure 5For the chimney model and the straight pipe model cross-section concentration relative standard deviation with height variation graph, the following conclusions can be drawn: First, whether in the chimney model or the straight pipe model, the relative standard deviation of the cross-sectional concentration of SF6 is always greater than that of He, which is consistent with the above analysis of the diffusion coefficients of the two tracers, that is, He has better diffusion and can further reduce the unevenness of the cross-sectional concentration distribution. Second, in the chimney model, the relative standard deviation of the cross-sectional concentration of the two tracers with height variation curve is almost the same, while in the straight pipe model, it shows a big difference. This is because in the chimney model, the smoke flows through the corner with strong turbulence and complex flow characteristics, which plays a leading role in the mixing of tracers and smoke, while in the straight pipe, the smoke does not have the above-mentioned flow conditions, so the mixing effect of tracers and smoke is mainly affected by the diffusion coefficient. Third, in the chimney model, the relative standard deviation of the cross-sectional concentration of the tracer drops to below 10% at a height of about 9D. According to the description in the standard ASTM E2029-11 (2019), the tracer and smoke have achieved sufficient mixing at this height, meeting the requirements of tracer gas dilution method in pipe flow measurement. In the straight pipe model, due to the lack of strong turbulence and complex flow to promote, the distribution of tracers in the pipe is still greatly uneven, which does not meet the requirements of tracer gas dilution method in pipe flow measurement.

[0094] Based on the above comparison, combined with the criteria for selecting tracers and the properties of common tracers, the embodiment selects SF6 as the tracer for the next research, which has a higher detection level and is more convenient to obtain in power plants.

[0095] (3) Selection of tracer dilution ratio

[0096] The ratio of tracer to smoke will affect the diffusion of tracer in the pipe and the mixing effect with smoke, so in this embodiment, three schemes as shown in Table 1 are set to study the influence of the dilution ratio of tracer in the smoke on the mixing effect of smoke. The simulation in this embodiment is carried out under the condition of 80% load, and the flow rate of smoke in the chimney is 14.34 m / s.

[0097] Table 1 Different tracer dilution ratio schemes (smoke volume flow rate 685.3 m 3 / s)

[0098]

[0099] The simulation results are as follows: the mixing effect of the three tracer dilution ratio schemes varies with the height of the chimney, but scheme 2 is better than the other two schemes in terms of the mixing effect of the tracer and the flue gas. Scheme 1 is less effective than scheme 2 in terms of the disturbance to the flue gas, because the injection speed of the tracer is too small relative to the main flow of the flue gas in the pipe. In addition, the tracer carries less kinetic energy, so the diffusion of the tracer in the flue gas and the efficiency of mixing with the flue gas are low. Scheme 3 is less effective than scheme 2 in terms of the mixing of the tracer and the flue gas, because the injection speed of the tracer is too high, which causes the tracer to directly penetrate the flue gas and reach the far end of the pipe. In addition, a low-pressure area may be formed on the surface of the high-speed tracer jet, which hinders the mixing of the tracer and the flue gas. In comparison with the mixing effect of the tracers and the flue gas in the three schemes and considering the detection limit of the conventional gas analyzer, scheme 2 is selected for subsequent research in this embodiment. In this case, the amount of tracer used is moderate, and the concentration of the diluted tracer can also be easily detected.

[0100] (4) Tracer injection cross-section scheme

[0101] The measurement accuracy of the tracer gas dilution method depends on the degree of mixing of the tracer and the flue gas. Although increasing the length of the pipe (mixing distance) can effectively improve the degree of mixing of the tracer and the flue gas, in actual applications, we hope that the tracer and the flue gas can be fully mixed in as short a distance as possible. To achieve this goal, this embodiment simulates the effects of chimney and flue injection on the mixing of the tracer and the flue gas and the flow measurement error, as shown in Figure 6 , where the chimney injection cross-section is located at the 20m height plane, and the flue injection cross-section is located 2m before the chimney inlet. At the same time, three sampling cross-sections and six sampling point arrangement schemes are set to evaluate the consistency and stability of the measurement results under each injection scheme, as shown in Figure 7 , and the sampling cross-sections are located at distances of 3D, 8D and 12D (for flue injection, the upper plane of the flue before the chimney inlet is defined as 0D) from the injection cross-section. The 3D and 8D cross-sections are located near the 40m and 80m measurement platforms of the chimney, respectively.

[0102] Table 2 shows the flow measurement error of each case in this embodiment under different tracer sampling point arrangement schemes, and the calculation formula is as follows:

[0103]

[0104] where Q 测量值 is calculated by substituting the tracer concentration at the sampling point into formula (1), and Q 真实值 is the volume flow rate of the flue gas in the simulation case.

[0105] Table 2 Relative error of flow measurement under different conditions (%)

[0106]

[0107] Figure 8 The influence of different injection sections on the mixing degree of tracer and flue gas under 80% load is shown. As can be seen from the figure, when injected into the flue, the relative standard deviation of tracer concentration decreases to 10% at about 2D section position, which meets the requirement of standard ASTM E2029-11(2019) for sufficient mixing of tracer. Compared with stack injection, the mixing effect of tracer and flue gas is greatly enhanced, and the mixing distance required to achieve sufficient mixing is reduced by about 7D, and the relative standard deviation of section concentration has decreased to 5% at about 5D height. The results prove that compared with stack injection, the tracer can achieve more sufficient mixing with flue gas in a shorter distance when injected into the flue. This is because when injected into the flue, the tracer flows through the elbow together with the flue gas, the flow direction changes sharply, the flue gas forms a secondary flow, and strong turbulence is generated, which increases the mixing efficiency of the tracer and the flue gas. From this, we can also see that under the action of the elbow, the requirement for the length of the straight pipe section of the tracer gas dilution method is greatly reduced compared with other flow measurement methods based on the velocity area method.

[0108] Figure 9 The flow measurement error of each sampling scheme at different sampling sections under each working condition is shown. The results show that the measurement performance of each scheme between different working conditions has strong similarity, so 80% load is taken as an example for analysis. As can be seen from the figure, under the condition of stack injection, the flow measurement error between each sampling scheme shows a wide range of fluctuations, with a fluctuation range of-32.02%~49.75%, but when three-point sampling, the measurement error at 3D, 8D, 12D section is only-3.59%, -0.69%, -1.05% respectively. The measurement error under the condition of flue injection is smaller than that under the condition of stack injection, and the measurement error fluctuation between different sampling schemes is relatively stable, all within ±10%, which meets the requirement of standard DL / T 2376-2021 that the flow velocity / flow measurement error should not exceed 10%. In addition, when three-point sampling, the measurement error at 3D, 8D, 12D section is only 0.98%, -0.52%, 0.21% respectively, all within ±1%. The measurement performance is greatly improved when injected into the flue compared with the stack injection.

[0109] In summary, the application provides a trace gas dilution method for measuring the flue gas flow of a large diameter chimney, and the best scheme for implementing the method is studied based on numerical simulation. The trace gas dilution method directly calculates the volume flow of flue gas in the chimney by monitoring the concentration change of the tracer during the flow process in the pipe. The measurement accuracy of the method mainly depends on the mixing of the tracer and the flue gas, and the strong turbulence and various complex flow characteristics of the flue gas during the flow process in the chimney can well promote the mixing of the tracer and the flue gas, thereby greatly reducing the required straight pipe length for measurement and increasing the accuracy of the flow measurement.

[0110] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the present specification and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0111] The above embodiments are only for the purpose of illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made according to the essence of the present application should be covered within the protection scope of the present application.

Claims

1. A traceable flue gas flow measurement method for a large diameter chimney, characterized in that: The following steps are involved: Build a physical model of the chimney and the flue in front of the chimney inlet, mesh the physical model, and simulate the model; Determine the type of tracer based on the mixing effect between the tracer and the flue gas; Determine the tracer dilution ratio based on the gas mixing effect; Determine the injection cross section of the tracer based on the gas mixing effect; Build a flue gas flow measurement system based on the information determined by the simulation to measure the flue gas flow; The step of determining the injection cross section of the tracer according to the mixing effect of the gas comprises: Two tracer injection sections, chimney injection and flue injection, were set to simulate the effects of chimney and flue section injection on the mixing of tracer and flue gas and flow measurement error. The final tracer injection cross section is selected based on the simulation results and flow measurement errors; The flue gas flow measurement system is a traceable flue gas flow measurement system for large-diameter chimneys, comprising: Tracer gas cylinder, used to hold tracer; A flow controller, used to control the output flow of the tracer; The tracer injection probe is used to inject the tracer output from the flow controller into the upstream position of the chimney duct to mix with the flue gas; The tracer sampling probe is installed in the downstream position of the chimney duct and extracts the mixed gas sample through the air pump; The gas analyzer is used to analyze the obtained gas sample and obtain the tracer concentration in the flue gas sample for calculating the flue gas volume flow in the chimney duct.

2. The method for measuring the traceable flue gas flow rate of a large diameter chimney according to claim 1, characterized in that: The physical model of the chimney and the flue in front of the chimney inlet is constructed, the physical model is meshed, and the model is simulated, including: Determine the size and layout of the chimney and the flue in front of the chimney entrance, and build a chimney model; Fluent meshing was used to mesh the model, and the mesh around the tracer injection port was encrypted. It is assumed that the flow of flue gas in the chimney duct is steady and incompressible, and the standard k-ε turbulence model is adopted; The component transport model in the FLUENT numerical simulation software is used to reflect the diffusion of the tracer and its mixing with the flue gas during the transportation process. The material transport equation and mass diffusion equation are as follows: Where R i Indicates the net generation rate of substances participating in the chemical reaction; S i Represents a discrete term; Sc t represents the turbulent Schmidt number; ρ represents the density of the mixed fluid, Y i represents the mass fraction of component i, represents the velocity vector of the mixed fluid, D i,m represents the molecular diffusion coefficient of component i in the mixture, μ t represents the turbulent viscosity of the mixture, is a vector differential operator.

3. The method for measuring the traceable flue gas flow rate of a large-diameter chimney according to claim 1, characterized in that: The method of determining the type of tracer according to the mixing effect between the tracer and the flue gas includes: Under preset conditions, the mixing effects of various tracers with flue gas in the chimney are simulated, and the main factors affecting the mixing effect of tracers and flue gas are obtained; The final tracer type is selected based on a combination of simulation results, gas detection level, and cost.

4. The method for measuring traceable flue gas flow in a large diameter chimney according to claim 1, characterized in that: The step of determining the tracer dilution ratio based on the gas mixing effect includes: Set multiple tracer dilution ratios to simulate how the tracer dilution ratio affects the tracer injection rate; According to the simulation results, the influence of the tracer dilution ratio on the mixing effect of the tracer and flue gas is analyzed.

5. The method for measuring the traceable flue gas flow rate of a large diameter chimney according to claim 1, characterized in that: The method for measuring the traceable flue gas flow rate of a large-diameter chimney further comprises the following steps: Three sampling sections were set up, located 3D, 8D, and 12D downstream of the pipeline from the tracer injection position. During flue injection, the upper plane of the flue in front of the chimney inlet was defined as 0D. 3D and 8D were the two existing measurement sections close to the power plant chimney, and D represented the chimney pipe diameter.

6. The method for measuring traceable flue gas flow in a large diameter chimney according to claim 1, characterized in that: The calculation formula of the flue gas volume flow rate is as follows: Where, X T,1 is the concentration of the injected tracer, V T,1 is the volume flow rate of the injected tracer; X T,D is the concentration of the tracer in the gas sample collected downstream; X T,U is the tracer concentration measured upstream of the injection point.

7. The method for measuring traceable flue gas flow in a large diameter chimney according to claim 1, characterized in that: The tracer is sulfur hexafluoride, and the tracer dilution ratio is 1:500,000.

8. The method for measuring traceable flue gas flow in a large diameter chimney according to claim 1, characterized in that: The tracer is injected into the chimney pipe by flue injection or chimney injection; and / or, The tracer sampling probe adopts a three-point sampling method for sampling.

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