Numerical simulation method for pulverized coal combustion of rotary cement kiln under cooperation of flue gas recirculation and oxygen-enriched combustion

Abstract

The invention discloses a numerical simulation method for pulverized coal combustion of a rotary cement kiln under the synergy of flue gas recirculation and oxygen-enriched combustion, which obtains the following conclusion by performing numerical simulation on pulverized coal combustion characteristics and NOx generation characteristics in the rotary cement kiln under the synergy of flue gas recirculation and oxygen-enriched combustion, the highest temperature in the rotary kiln and the temperature around the combustor can be reduced by increasing the flue gas recirculation rate, and therefore the NOx emission concentration of the rotary kiln can be greatly reduced. The calcination interval of the cement clinker can be shortened by increasing the flue gas recirculation rate, the calcination requirement can be met, and the effect of increasing the yield caused by oxygen-enriched combustion is not affected. In conclusion, the rotary cement kiln pulverized coal oxygen-enriched combustion and flue gas recirculation technology can effectively reduce NOx emission while improving the temperature and the cement calcination quality, and theoretical support is provided for optimizing the rotary cement kiln pulverized coal combustion process, improving the production efficiency and quality of the rotary cement kiln and reducing pollutant emission.

Description

Technical Field

[0001] The invention belongs to the field of cement production control, and in particular relates to a numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and coordinated oxygen-enriched combustion. Background Art

[0002] The temperature of the flame in the cement rotary kiln is crucial to cement calcination. In recent years, scholars have conducted a lot of research on oxygen-enriched combustion technology on rotary kilns to address this issue. Oxygen-enriched combustion technology is to increase the oxygen concentration of the primary and secondary air, reduce the ignition temperature of coal powder, reduce the amount of nitrogen and the heat carried away by the flue gas, thereby increasing the temperature in the kiln. Oxygen-enriched combustion technology has the advantages of increasing the temperature in the kiln, increasing the clinker output, reducing coal consumption and improving the adaptability of coal types.

[0003] Wang Hui et al. studied the application of oxygen-enriched combustion technology in lime rotary kilns and concluded that when coke oven gas and converter gas are selected as fuel without changing the existing equipment, it is more appropriate to control the oxygen concentration at 21.4%; if the existing combustion heat is modified, it is recommended to control the volume concentration of oxygen at 28% to improve the energy utilization efficiency and the thermal efficiency of the rotary kiln. Zong Dian designed an oxygen-enriched burner for the combustion of inferior coal in cement rotary kilns and conducted numerical simulations. The combustion characteristics of Yanzhou coal and Dongluo coal under oxygen-enriched conditions in cement rotary kilns were studied. The research results proved the feasibility of the specific burner designed as an oxygen-enriched burner and showed the feasibility of applying oxygen-enriched combustion technology in cement rotary kilns. Yang Zhifang et al. conducted an oxygen-enriched test on a rotary kiln with a daily output of 4,000 tons in a cement plant in China. In the test, the oxygen enrichment rate of the primary air reached a maximum of 30%. The test results show that when the pulverized coal flow rate remains unchanged, oxygen-enriched combustion can increase the temperature in the rotary kiln by 100-300°C; in the test, every time the volume fraction of oxygen increases by 4%-5%, the temperature in the kiln will increase by 200-300°C.

[0004] Oxygen-enriched combustion technology reduces the content of free CaO in clinker and improves the burning quality of cement clinker. Although oxygen-enriched combustion technology has a great effect on increasing the temperature in cement rotary kilns and has been applied to cement rotary kilns to a certain extent, the NO generated x The flue gas recirculation technology is very effective in suppressing the emission of nitrogen oxides. It can effectively reduce NO by reducing the combustion temperature in the kiln and increasing the specific heat of flue gas. x At present, flue gas recirculation technology has a great impact on the combustion of pulverized coal and NO in cement rotary kilns. x There is a lack of research on the impact of NO emissions. Therefore, it is necessary to conduct in-depth research on pulverized coal oxygen-enriched combustion and NO emissions under flue gas recirculation conditions. xThe generation characteristics of pulverized coal in cement rotary kiln provide theoretical support for optimizing the combustion process of pulverized coal in cement rotary kiln, improving the production efficiency and quality of cement rotary kiln, and reducing pollutant emissions. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxy-enriched combustion.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The numerical simulation method of coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion includes the following steps:

[0008] S1. Use a combination of structured and unstructured methods to partition and grid the cement rotary kiln, so that the grid direction is consistent with the air flow velocity direction, and the grid quality is above 0.8;

[0009] S2. According to the parameters of the kiln under actual working conditions, the basic energy conservation equation, turbulent flow model, turbulent gas-solid two-phase flow model, gas-phase turbulent combustion model, coal powder particle combustion model, radiation heat transfer model and NO x Generate models for numerical simulations;

[0010] S3. The oxygen-enriched combustion process in the rotary kiln is realized by adjusting the primary air volume, oxygen content and secondary air volume. The flue gas recirculation process is realized by calculating the gas generated when the pulverized coal is completely burned in the cement rotary kiln through the basic working conditions, and then the recycled flue gas is sent into the cement rotary kiln according to different flue gas recirculation rates;

[0011] S4. Calculate the air velocity distribution, combustion temperature distribution, O2, CO2, CO and NO in the cement rotary kiln under different conditions according to the model. x The concentration distribution of NO at the kiln outlet x concentration.

[0012] Preferably, in step S1, grid division software is used to divide the cement rotary kiln into zones and grids.

[0013] Preferably, in step S1, unstructured grids are used in parts of the burner and cement rotary kiln.

[0014] Preferably, in step S2, CFD software is used to perform numerical simulation.

[0015] Preferably, in step S2, the turbulent flow model adopts the Standard k-ε model, the turbulent gas-solid two-phase flow model adopts the Lagrangian random particle trajectory model, the gas-phase turbulent combustion model adopts the component transport model, the coal powder particle combustion model adopts a combination of the chemical permeation precipitation model and the diffusion-dynamic controlled combustion model for coke combustion, and the radiation heat transfer model adopts the P-1 model.

[0016] Preferably, in step S3, the gas generated when the coal powder is completely burned in the cement rotary kiln includes CO2, H2O, N2 and O2.

[0017] Preferably, in step S3, the primary air volume, oxygen content and secondary air volume are adjusted to realize the oxygen-enriched combustion link in the rotary kiln, and the specific steps are: increasing the primary air oxygen concentration by injecting pure oxygen into the primary air, and reducing the secondary air flow rate to keep the excess air coefficient in the entire cement rotary kiln unchanged when the primary air volume remains unchanged. The oxygen concentrations of the four channels of the primary air can be equal or unequal, so all channels can be oxygen-enriched (or pure oxygen), or some channels can be oxygen-enriched (or pure oxygen).

[0018] Preferably, in step S3, the recycled flue gas is fed into the cement rotary kiln by being mixed with primary air and sprayed, mixed with secondary air and sprayed, or by setting a separate recycled flue gas nozzle for spraying, or a combination of two or more thereof; further preferably, the recycled flue gas is extracted only from the tail of the rotary kiln and then mixed with the secondary air and fed into the rotary kiln.

[0019] The present invention has the following beneficial effects:

[0020] The present invention studies the combustion characteristics of pulverized coal and NO in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion. x The following conclusions were obtained by numerical simulation of the generated characteristics:

[0021] Under the conditions of flue gas recirculation and oxygen-enriched combustion, the increase in flue gas recirculation rate can reduce the maximum temperature in the rotary kiln and the temperature around the burner, thereby significantly reducing the NOx in the rotary kiln. x The increase in flue gas recirculation rate will shorten the calcination range of cement clinker, but it can meet the calcination requirements and will not affect the effect of increasing production brought by oxygen-enriched combustion. In summary, it can be concluded that the oxy-combustion and flue gas recirculation technology of cement rotary kiln pulverized coal can effectively reduce NO while improving the temperature and cement calcination quality. x This provides theoretical support for optimizing the pulverized coal combustion process in cement rotary kilns, improving the production efficiency and quality of cement rotary kilns, and reducing pollutant emissions, and also provides a certain reference for subsequent thermal engineering research in the cement industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 :(a) is the process flow chart of coal powder oxygen-enriched combustion and flue gas recirculation combustion technology in cement rotary kiln; (b) is the schematic diagram of four-channel burner; (c) is the structural cross-sectional diagram of four-channel burner; (d) is the structural schematic diagram of cement rotary kiln model;

[0024] Figure 2 The air flow velocity distribution cloud diagram in the rotary kiln at different flue gas recirculation rates (z = 0, );

[0025] Figure 3 The air flow velocity distribution cloud diagram in the rotary kiln at different flue gas recirculation rates (z = 0, );

[0026] Figure 4 It is the distribution curve of air flow velocity on the center line of the kiln under different flue gas recirculation rates;

[0027] Figure 5 The distribution cloud diagram of the flue gas temperature in the kiln under different flue gas recirculation rates (z = 0, );

[0028] Figure 6 The distribution cloud diagram of the flue gas temperature in the kiln under different flue gas recirculation rates (z = 0, );

[0029] Figure 7 The flue gas temperature distribution diagram on the center line of the kiln under different flue gas recirculation rates;

[0030] Figure 8 This is the maximum temperature variation curve in the kiln under different flue gas recirculation rates;

[0031] Fig. 9 The O2 concentration distribution diagram in the kiln under different flue gas recirculation rates (z = 0, );

[0032] Fig.10 The O2 concentration distribution in the kiln under different flue gas recirculation rates (z = 0, );

[0033] Fig.11 CO2 concentration distribution in the kiln under different flue gas recirculation rates (z = 0, );

[0034] Fig.12 CO2 concentration distribution in the kiln under different flue gas recirculation rates (z = 0, );

[0035] Fig.13 The CO concentration distribution in the kiln under different flue gas recirculation rates (z = 0, );

[0036] Fig.14 The CO concentration distribution in the kiln under different flue gas recirculation rates (z = 0, );

[0037] Fig.15 is the NO in the kiln under different flue gas recirculation rates x Concentration distribution (z = 0, );

[0038] Fig.16 is the NO in the kiln under different flue gas recirculation rates x Concentration distribution (z = 0, );

[0039] Fig.17 The O2 concentration distribution diagram on the center line of the kiln under different flue gas recirculation rates;

[0040] Fig.18 The CO2 concentration distribution diagram on the center line of the kiln under different flue gas recirculation rates;

[0041] Fig.19 This is the CO concentration distribution diagram on the center line of the kiln under different flue gas recirculation rates;

[0042] Fig. 20 is the NO on the center line of the kiln at different flue gas recirculation rates x Concentration distribution diagram;

[0043] Fig.21 NO at kiln outlet under different flue gas recirculation rates x Concentration distribution diagram. DETAILED DESCRIPTION

[0044] In the following description, specific details such as specific system structures, technologies, etc. are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present invention. However, it should be clear to those skilled in the art that the present invention may be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present invention.

[0045] Reference Figure 1The present invention provides a process for coal powder oxygen-enriched combustion and flue gas recycling in a cement rotary kiln, comprising the following steps: (1) primary air and coal powder enter the rotary kiln through a burner for combustion, and pure oxygen is added to the primary air to make the oxygen content of the primary air higher than 21%, thereby achieving oxygen-enriched (or pure oxygen) combustion. In this way, all primary air ducts can be enriched with oxygen (or pure oxygen), and some of the four primary air ducts can also be selected for oxygen-enriched (or pure oxygen); (2) the flue gas after combustion is extracted from the tail of the cement rotary kiln, and a part of the flue gas is mixed with the secondary air and then returned to the kiln, forming a flue gas recycling combustion mode. This part of the flue gas is called recycled flue gas. It should be emphasized here that the flue gas after combustion is not limited to being extracted from the tail of the cement rotary kiln, but can also be extracted from other positions after the rotary kiln, such as the kiln tail, after the preheater, after the dust collector, or different combinations of these positions, and there are also many ways to send it into the cement rotary kiln, such as mixing with the primary air, mixing with the secondary air, and setting a separate recirculation flue gas nozzle to spray, or different combinations of these methods. The flue gas components, pollutant concentrations and temperatures extracted from different positions are different, and the effects of spraying the recirculated flue gas in different ways are also different. This technical configuration can greatly enhance the regulation effect and flexibility of the flue gas recirculation and oxygen-enriched combustion technology, so that oxygen can be flexibly distributed in different areas according to different needs. For simplicity, the implementation case shows that it is only extracted from the tail of the rotary kiln and mixed with the secondary air and sent to the rotary kiln. In this way, oxygen-enriched combustion and flue gas recirculation technology are closely combined to form a new combustion technology for cement rotary kiln, namely, cement rotary kiln coal pulverized oxygen-enriched combustion and flue gas recirculation technology (OEC-FGR), which has a very broad application prospect.

[0046] In the present invention, air is heated by a grate cooler to form hot air, a portion of which is injected into the kiln from the front end of the rotary kiln as secondary air, and a portion of which is injected into the decomposition kiln as tertiary air to assist combustion (see Figure 1 (a)). Take the cement rotary kiln four-channel burner (see Figure 1 Taking (b) as an example, the burner is divided into four channels: axial flow duct, swirl duct, coal air duct and central duct. The functions of each duct are as follows: the central duct is used to prevent the backflow of coal powder and kiln dust, making the flame more stable and extending the service life of the burner; the coal air duct is used to transport coal powder; the swirl duct is used to make the air have a certain swirl speed, increase disturbance, make the coal powder and air better mixed, and at the same time produce a speed difference with the external wind to form a central recirculation zone; the axial flow duct, because the speed of the axial flow duct is very fast, it can control the length and stiffness of the flame, and at the same time suck in high-temperature secondary air to form a strong external recirculation zone.

[0047] This embodiment specifically uses a cement rotary kiln with a production capacity of 5000t / d in a certain factory, with an actual size of 4.8×72m. Under actual operating conditions, the rear half of the rotary kiln tail tends to be stable, the combustion temperature in the furnace changes very little, and does not affect the combustion process in the front section. Therefore, the embodiment of the present invention starts from the rotary kiln head and intercepts the 4.4×60m area as the simulation calculation object (see Figure 1 (d)). The three-dimensional model of the four-channel burner is shown in Figure 1 As shown in (c). The composition of the recycled flue gas is determined according to the complete combustion composition when the excess air coefficient of the pulverized coal is 1.2. The recycled flue gas components are mainly N2, CO2 and O2, accounting for 76%, 20.6% and 3.4% respectively. Then the amount of each component is determined according to the different flue gas recirculation rates, and introduced into the rotary kiln together with the secondary components. The flue gas recirculation rate r refers to the ratio of the recycled flue gas volume to the flue gas volume at the outlet of the cement rotary kiln. r is calculated according to the following formula:

[0048]

[0049] Where: q v1 ——Flue gas recirculation volume; q v2 ——Total amount of smoke exhaust.

[0050] The primary air oxygen concentrations studied in this embodiment are 27% and 30%, respectively, and the flue gas recirculation rates are 0, 10%, 15%, 20% and 27%, respectively. The coal powder combustion and NOx generation characteristics in the cement rotary kiln, the simulated operating conditions and parameters are shown in Tables 1 and 2.

[0051] Table 1

[0052]

[0053] Table 2

[0054]

[0055] This embodiment also provides a numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion based on the above combustion system, comprising the following steps:

[0056] S1. Use a combination of structured and unstructured methods to partition and grid the cement rotary kiln so that the grid direction is consistent with the air flow velocity direction and the quality is above 0.8;

[0057] In step S1, grid division software is used to divide the cement rotary kiln into zones and grids.

[0058] S2. According to the parameters of the cement rotary kiln under actual working conditions, the basic energy conservation equation, turbulent flow model, turbulent gas-solid two-phase flow model, gas-phase turbulent combustion model, coal powder particle combustion model, radiation heat transfer model and NO x The generated model was numerically simulated using CFD software;

[0059] Among them, since the rotary kiln studied in the present invention has strong swirl characteristics, the swirl of the fluid has a great influence on the working conditions in the entire rotary kiln, so the turbulent flow model here adopts the Realizable k-ε model. The turbulent gas-solid two-phase flow model adopts the Lagrangian random particle orbit model, and the gas phase turbulent combustion model adopts the component transport model;

[0060] In the cement rotary kiln, the combustion of coal powder particles is divided into: water evaporation, volatile analysis and combustion, and coke combustion. The coal powder particle combustion model adopts the chemical permeation precipitation model and the diffusion-dynamic controlled combustion model for coke combustion; radiation heat exchange is a very important heat exchange method in the cement rotary kiln coal powder combustion kiln. The temperature of the coal powder in the kiln is very high when it is completely burned, and the heat exchange between the temperature in the kiln and the kiln wall is mainly carried out through radiation heat exchange. Here, the radiation heat exchange model adopts the P-1 radiation model.

[0061] In the numerical simulation calculation of cement rotary kiln, the quality of the grid plays a vital role in the simulation calculation. It is necessary to partition the cement rotary kiln model and the burner area into grids. Most of them will choose a combination of structured and unstructured grid division. The structure of the burner area in this embodiment is relatively complex. The use of a single grid division method is not conducive to the simulation calculation, and it will also increase the difficulty of grid division and the workload of calculation. Therefore, unstructured grids are used in some areas of the burner and cement rotary kiln, and the structure of the remaining parts is simple and symmetrical, which is suitable for the use of structured grids.

[0062] S3. The oxygen-enriched combustion link in the cement rotary kiln is realized by adjusting the primary air volume, oxygen content and secondary air volume. Specifically, the oxygen content in the entire cement rotary kiln is maintained to meet the requirement of unchanged excess air coefficient by increasing the primary air oxygen concentration and reducing the secondary air flow rate; the flue gas recirculation link is realized by calculating the gas produced when the coal powder is completely burned in the cement rotary kiln through basic working conditions. The gas includes CO2, H2O, N2 and O2; and then, according to different flue gas recirculation rates, the circulating flue gas is introduced into the secondary air inlet and mixed with the secondary air and sprayed into the cement rotary kiln;

[0063] S4. Calculate the air velocity distribution, combustion temperature distribution, O2, CO2, CO and NO in the cement rotary kiln under different conditions according to the model. x The concentration distribution of NO at the kiln outlet x concentration.

[0064] Simulation results:

[0065] (1) Air velocity distribution

[0066] Reference Figure 2 and Figure 3 , respectively showing the distribution cloud diagram of air flow velocity in the rotary kiln at different flue gas recirculation rates when the primary air oxygen concentration is 27% and 30% at the z=0 section; Figure 2-3 The results show that the air velocity distribution in the kiln is basically the same under the two primary air oxygen concentrations. With the introduction of flue gas recirculation, the air flow rate increases, resulting in an increase in the average air velocity and a shortened residence time of coal powder particles in the kiln.

[0067] Reference Figure 4 , is the air flow velocity distribution curve on the center line of the rotary kiln when the primary air oxygen concentration is 27% and 30% and the flue gas recirculation rate is different. Figure 4 (a) and Figure 4 From the results in (b), we can see that the velocity on the center line of the rotary kiln increases sharply and then suddenly decreases, and finally tends to a stable state. This is because the high-speed airflow increases sharply near the burner nozzle area after it comes out of the burner nozzle, but the secondary air in the kiln is sucked in by the axial wind, causing the airflow velocity to decay sharply and finally stabilize. At the same flue gas recirculation rate, the airflow velocity on the center line of the kiln shows a large difference about 11m away from the kiln head. When the primary air oxygen concentration is 27% and 20%, the airflow velocity in the kiln increases with the increase of the flue gas recirculation rate, but the airflow velocity in the kiln when the primary air oxygen concentration is 27% is higher than that when the primary air oxygen concentration is 30%.

[0068] (2) Temperature distribution

[0069] (Ⅰ)Reference Figure 5 and Figure 6 , respectively showing the temperature distribution cloud diagram in the rotary kiln at different flue gas recirculation rates with primary air oxygen concentrations of 27% and 30% at section z=0.

[0070] Depend on Figure 5 and Figure 6The results show that there is no significant change in the flame shape under the two primary air oxygen concentrations. With the increase of the flue gas recirculation rate, the flame length in the rotary kiln is constantly increasing, but the addition of flue gas recirculation can significantly change the temperature distribution near the burner in the rotary kiln. Compared with the addition of flue gas recirculation, there is an obvious high-temperature area near the burner when there is no flue gas recirculation. The addition of flue gas recirculation reduces the oxygen content of the secondary air. Since the main function of the axial flow wind of the burner is to entrain the secondary air in the kiln, the oxygen content around the coal powder particles is reduced, the combustion of the volatile matter of the coal powder is slowed down, and the temperature around the burner is reduced. With the increase of the flue gas recirculation rate, the combustion temperature in the rotary kiln gradually decreases, especially the maximum temperature in the kiln decreases the most. This is because the increase in the flue gas recirculation rate accelerates the air flow velocity in the kiln, thereby increasing the combustion distance of the coal powder. Moreover, the CO2 concentration in the circulating flue gas is higher than that in the case of no flue gas recirculation, which increases the specific heat of the flue gas and requires more heat. The addition of circulating flue gas also reduces the oxygen concentration in the kiln, making the combustion performance of volatile matter and coke in the pulverized coal worse and reducing the combustion effect, thereby reducing the combustion temperature in the kiln.

[0071] (Ⅱ)Reference Figure 7 and Figure 8 , respectively showing the temperature on the center line of the rotary kiln and the maximum temperature in the kiln when the primary air oxygen concentration is 27% and 30% and the flue gas recirculation rate is different. For different flue gas recirculation rates, the overall trend of the temperature distribution in the kiln is consistent, which first increases and then decreases along the kiln axis, but the flue gas temperature is still very different in different areas.

[0072] Depend on Figure 7 Middle (a), Figure 7 (b) and Figure 8 The results show that in the area near the burner, the temperature near the burner is higher when the flue gas recirculation rate is 0 than when the flue gas recirculation rate is added. With the increase of the flue gas recirculation rate, the maximum temperature in the kiln is constantly decreasing, because the combustion of pulverized coal in the kiln is in the coke combustion stage, and the combustion rate is lower than the combustion without flue gas recirculation, resulting in a decrease in the flame temperature. When the flue gas recirculation rate is 0, 10%, 15%, 20% and 27%, the corresponding maximum temperatures in the kiln under the primary air oxygen concentration of 27% are 2044K, 1967K, 1921K, 1897K and 1861K, respectively, and the corresponding maximum temperatures in the kiln under the primary air oxygen concentration of 30% are 2065K, 1989K, 1951K, 1907K and 1885K, respectively.

[0073] For the rotary kiln in the embodiment of the present invention, the temperature interval length for calcining cement clinker is 13.2 to 22 m. Figure 7From (a), it can be concluded that when the primary air oxygen concentration is 27%, the cement calcination length decreases from 16.94m to 12.83m; Figure 7 From (b), it can be concluded that when the primary air oxygen concentration is 30%, the calcination length of cement is reduced from 18.57m to 14.53m; it can also be concluded that when the primary air oxygen concentration is 27% and the flue gas recirculation rate is 27%, the length of the calcination temperature above 1800K is 12.83m<13.2m, and the calcination length of cement clinker does not meet the conditions.

[0074] (3) Concentration distribution of each component

[0075] Figures 9 to 16 The O2, CO2, CO and NO in the rotary kiln at different flue gas recirculation rates when the primary air oxygen concentration is 27% and 30% on the z=0 section. x Concentration distribution cloud map.

[0076] Depend on Fig. 9 and 10 The results show that the O2 concentration in the area near the burner is higher than that in other areas. Due to the addition of flue gas recirculation, the O2 concentration at the kiln head continues to decrease, which inhibits the combustion of coal powder near the burner. From the distribution cloud map of O2 concentration, it can also be seen that the interval of large O2 concentration consumption has been lengthened. This is mainly because the average air flow velocity in the kiln increases after the addition of flue gas recirculation, and the oxygen content around the coal powder particles decreases, which slows down the combustion speed of the coal powder and makes it take longer for the coal powder to burn out. At the same time, due to the incomplete combustion of coal powder, the CO concentration will increase, which is opposite to the distribution of O2 concentration.

[0077] Depend on Fig.11 and 12 It can be concluded that the distribution of CO2 concentration in the kiln is opposite to that of O2 concentration. This is mainly because the combustion of pulverized coal consumes a large amount of O2 and generates a large amount of CO2 gas. With the increase of flue gas recirculation rate, the CO2 concentration at the kiln head is also increasing. In addition, the CO2 generated by the combustion of pulverized coal increases the CO2 concentration at the kiln tail.

[0078] Depend on Fig.13 and 14 It can be concluded that when there is no flue gas recirculation, a large amount of CO gas is generated near the burner. This is mainly because the pulverized coal is sent into the rotary kiln through the pulverized coal air duct, and a large amount of pulverized coal gathers in this area. At the same time, the oxygen content is low, and the incomplete combustion of the pulverized coal leads to a large amount of CO generation. With the addition of flue gas recirculation, the air flow velocity increases, and the oxygen concentration decreases, more CO gas is generated, and this area is getting farther and farther away from the burner.

[0079] Depend on Fig.15 and 16 It can be concluded that flue gas recirculation has an effect on NO during coal pulverization in the kiln.x The generation of NO has a good inhibitory effect. x and fuel-type NO x The generation of NO x The amount of NO generated is related to the temperature in the rotary kiln and the nitrogen content of the pulverized coal. At the same time, the combustion of pulverized coal consumes a large amount of O2, forming a high-temperature and low-oxygen reduction zone. Part of the NO in this area x It will be reduced to N2 by HCN and other substances. When the combustion temperature reaches above 1773K, more thermal NO x Therefore, in the area where coal powder is burned in large quantities in the rotary kiln, NO x The concentration is high, which is consistent with the temperature distribution. When the cement rotary kiln pulverized coal is burned with oxygen with flue gas recirculation, NO x emissions.

[0080] Reference Fig.17 and Fig.18 , respectively showing the distribution of O2 concentration and CO2 concentration on the center line of the rotary kiln at different flue gas recirculation rates with primary air oxygen concentrations of 27% and 30%.

[0081] Depend on Fig.17 and Fig.18 The results show that the O2 concentration in the kiln gradually decreases and the CO2 concentration gradually increases, indicating that the pulverized coal begins to burn in large quantities when it enters the rotary kiln, and there is also a low-oxygen reduction zone. As the flue gas recirculation rate increases, the low-oxygen reduction zone becomes longer and longer, indicating that the addition of flue gas recirculation is beneficial to inhibit NO x Generation of.

[0082] from Fig.18 It can be seen that the CO2 concentration in the kiln is delayed, and then the CO2 concentration rises rapidly as the reaction proceeds. This is because the flue gas recirculation delays the combustion of pulverized coal, so that the CO2 concentration curve moves backward. Since the recycled flue gas contains CO2, the CO2 concentration in the final product increases with the increase of the flue gas circulation ratio. When the primary air oxygen concentration is 27%, the CO2 concentration is 14.07% when there is no flue gas recirculation, and the CO2 concentration is 15.8% when the flue gas recirculation rate is 27%, an increase of 12.3%; when the primary air oxygen concentration is 30%, the CO2 concentration is 14.25% when there is no flue gas recirculation, and the CO2 concentration is 15.91% when the flue gas recirculation rate is 27%, an increase of 11.65%. Compared with the two different oxygen concentrations, although the outlet CO2 concentration is higher when the oxygen concentration is 30%, the increase gradually decreases.

[0083] Reference Fig.19 and Fig. 20, respectively showing the CO concentration and NO concentration on the center line of the rotary kiln at different flue gas recirculation rates when the primary air oxygen concentration is 27% and 30%. x Concentration distribution.

[0084] from Fig.19 It can be seen that with the addition of flue gas recirculation, the oxygen concentration decreases, the degree of incomplete combustion of pulverized coal in the kiln increases, and the CO concentration increases, which can effectively inhibit the conversion of HCN and NH3 to NO x The figure also shows that when the flue gas recirculation rate is 10%, the local CO concentration reaches the maximum; as the flue gas recirculation rate further increases, the CO concentration decreases.

[0085] Through Fig. 20 By comparison, it was found that when no flue gas recirculation was added, the CO concentration in the combustion area of ​​the kiln was lower and the NO x The concentration is higher, and the opposite is true when flue gas recirculation is added. Under the same flue gas recirculation rate, the NO concentration shows a bimodal distribution along the axial direction, and the latter peak is significantly higher than the former peak and has a larger peak surface. The first peak is due to the NO generated by the combustion of volatile matter in the pulverized coal. x The second peak position basically coincides with the temperature peak, which is the fuel-type NO generated by the combustion of coal char at high temperature. x and thermal NO x As the flue gas recirculation rate increases, the first peak value changes from first decreasing to then increasing, while the second peak value becomes lower and lower, but higher than the first peak value. As can be seen from the figure, the addition of flue gas recirculation greatly reduces the NO x When the flue gas recirculation rate increases from 0 to 27% and the primary air oxygen concentration is 27%, the NO x The maximum concentration decreased from 726ppm to 334ppm, a decrease of 392ppm; the primary air oxygen concentration was 30%, and the NO x The maximum concentration decreased from 787ppm to 361ppm, a decrease of 426ppm. This shows that the introduction of flue gas recirculation can effectively reduce NO in the rotary kiln. x By comparison, it was found that when the primary air oxygen concentration was 30%, adding flue gas recirculation had a significant effect on reducing NO in the kiln. x The effect of concentration is more significant.

[0086] (4) Export NO x Concentration comparison

[0087] Reference Fig.21 , shows the NO at the rotary kiln outlet at different flue gas recirculation rates with primary air oxygen concentrations of 27% and 30%. x Average concentration distribution curve. It can be seen from the figure that the flue gas recirculation rate has an effect on the NO xWhen the primary air oxygen concentration is 27%, when FGRP2 = 0%, the outlet NO x The highest concentration is 584ppm; when FGRP2 = 10%, the outlet NO x The concentration is 357ppm; when FGRP2 = 15%, the outlet NO x The concentration is 324ppm; when FGRP2 = 20%, the outlet NO x The concentration is 270ppm; when FGRP2 = 27%, the outlet NO x The concentration is 241ppm. When the primary air oxygen concentration is 30%, FGRP2 = 0% and 27%, the outlet NO x The concentrations are 608ppm and 262ppm respectively. It can be seen that the addition of flue gas recirculation can significantly reduce the outlet NO x Concentration, when the flue gas recirculation rate is in the range of 0-10%, the reduction is the largest; when the flue gas recirculation rate is in the range of 15%-27%, the reduction is smaller.

[0088] Therefore, considering the combustion performance of coal powder in the rotary kiln and the temperature of cement clinker calcination not less than 1800K, the interval is 13.2~22m. When the primary air oxygen concentration is 27% and the flue gas recirculation rate is 20%, the length of cement clinker calcination interval is 14.88m, and the outlet NO x The concentration is 270ppm. Under this condition, the oxygen concentration is 27% and the flue gas recirculation rate is 0-15%. x The outlet concentration is lower, and when the primary air oxygen concentration is 27% and the flue gas recirculation rate is 20%, compared with the primary air oxygen concentration of 30% and the flue gas recirculation rate of 27%, the former has better maximum temperature and temperature range of calcined cement clinker than the latter. x The concentration is 8ppm less than the former. Therefore, the primary air oxygen concentration of 27% and the flue gas recirculation rate of 20% are the optimal working conditions for calcining cement.

[0089] In summary, the embodiment of the present invention numerically simulates a 5000t / d cement rotary kiln and studies the effects of secondary air mixed with recycled flue gas on the combustion characteristics of pulverized coal and NO in the rotary kiln on the basis of primary air oxygen concentrations of 27% and 30%. x The influence of the generated characteristics leads to the following conclusions:

[0090] (1) The increase in the flue gas recirculation rate reduces the maximum temperature in the rotary kiln and the temperature around the burner. When the primary air oxygen concentration is 27% and the flue gas recirculation rate increases from 0 to 27%, the maximum temperature in the rotary kiln decreases from 2044K to 1861K, and the location where it appears is delayed from 21.05m to 29.48m; when the primary air oxygen concentration is 30% and the flue gas recirculation rate increases from 0 to 27%, the maximum temperature in the rotary kiln decreases from 2065K to 1885K, and the location where it appears is delayed from 20.43m to 27.38m.

[0091] (2) The increase in flue gas recirculation rate shortens the calcination interval of cement clinker. When the primary air oxygen concentration is 27% and the flue gas recirculation rates are 0, 10%, 15% and 20% respectively, the lengths of the calcination intervals of cement clinker are 16.94m, 15.43m, 15.24m and 14.88m respectively. When the primary air oxygen concentration is 27% and the flue gas recirculation rate is 27%, the length of the interval where the maximum temperature in the rotary kiln is above 1800K is 12.83m, which does not meet the required temperature interval for calcination of cement clinker: that is, the length above 1800K is required to be >13.2m. When the primary air oxygen concentration is 30% and the flue gas recirculation rates are 0, 10%, 15%, 20% and 27% respectively, the lengths of the calcination intervals of cement clinker are 18.57m, 15.57m, 15.23m, 14.57m and 14.53m respectively.

[0092] (3) Increasing the flue gas recirculation rate can reduce the NO at the rotary kiln outlet x When the primary air oxygen concentration is 27% and the flue gas recirculation rate is 20%, the outlet NO x The concentration can be reduced from 584ppm without smoke gas recirculation to 270ppm, a reduction of 53.77%. When the smoke gas recirculation rate exceeds 20%, NO x However, it is not conducive to the calcination of cement clinker; when the primary air oxygen concentration is 30%, the flue gas recirculation rate is 27%, and the outlet NO x The concentration can be reduced from 608ppm without flue gas recirculation to 262ppm, a reduction of 56.91%. Therefore, when the primary air oxygen concentration is 27% and the flue gas recirculation rate is 20%, it is the best working condition for calcining cement clinker.

[0093] In summary, the present invention studies the combustion characteristics of pulverized coal and NO in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion. xThe following conclusions were drawn from the numerical simulation of the generation characteristics: (1) When only oxygen-enriched combustion is used, as the oxygen concentration of the primary air increases, the maximum temperature of the flame in the rotary kiln increases, the cement clinker calcination range increases, and the cement clinker production can be increased. (2) When the primary air is oxygen-enriched and flue gas recirculation is used, as the flue gas recirculation rate increases, the maximum temperature in the rotary kiln and the temperature around the burner decrease, and the cement clinker calcination range is slightly shortened, but it does not affect the effect of oxygen-enriched combustion on increasing production. The NO at the rotary kiln outlet is x (3) When the primary air oxygen concentration is 27% and the flue gas recirculation rate is 20%, the cement clinker production can be effectively increased while reducing the generation of NOx. In summary, the cement rotary kiln coal powder oxygen-enriched combustion synergistic flue gas recirculation technology can effectively reduce NO while increasing cement calcination production. x This provides theoretical support for optimizing the pulverized coal combustion process in cement rotary kilns, improving the production efficiency and quality of cement rotary kilns, and reducing pollutant emissions, and also provides a certain reference for subsequent thermal engineering research in the cement industry.

[0094] The present invention is not limited to the above-mentioned specific implementation modes. Various changes made by ordinary technicians in this field based on the above-mentioned concepts without creative work are all within the protection scope of the present invention.

Claims

1. A numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion, characterized in that: The following steps are involved: S1. Use a combination of structured and unstructured methods to partition and grid the cement rotary kiln so that the grid direction is consistent with the air flow velocity direction and the quality is above 0.8; S2. According to the parameters in the actual working conditions of the kiln, numerical simulation is carried out through the basic energy conservation equation, turbulent flow model, turbulent gas-solid two-phase flow model, gas-phase turbulent combustion model, pulverized coal particle combustion model, radiation heat transfer model and NOx generation model; S3. The oxygen-enriched combustion process in the rotary kiln is realized by adjusting the primary air volume, oxygen content and secondary air volume. The flue gas recirculation process is realized by calculating the gas generated when the pulverized coal is completely burned in the cement rotary kiln through the basic working conditions, and then the recirculated flue gas is sent into the cement rotary kiln according to different flue gas recirculation rates; S4. Calculate the air velocity distribution, combustion temperature distribution, O2, CO2, CO and NO in the cement rotary kiln under different conditions according to the model. x The concentration distribution of NO at the kiln outlet x concentration.

2. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In the step S1, grid division software is used to divide the cement rotary kiln into zones and grids.

3. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In the step S1, unstructured grids are used in the burner and partial areas of the cement rotary kiln.

4. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In the step S2, CFD software is used to perform numerical simulation.

5. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In step S2, the turbulent flow model adopts the Standard k-ε model, the turbulent gas-solid two-phase flow model adopts the Lagrangian random particle trajectory model, the gas phase turbulent combustion model adopts the component transport model, the coal powder particle combustion model adopts a combination of the chemical permeation precipitation model and the diffusion-dynamic controlled combustion model for coke combustion, and the radiation heat transfer model adopts the P-1 model.

6. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In step S3, the gases generated when the coal powder is completely burned in the cement rotary kiln include CO2, H2O, N2 and O2.

7. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: In step S3, the primary air volume, oxygen content and secondary air volume are adjusted to realize the oxygen-enriched combustion link in the rotary kiln. The specific steps are: increasing the primary air oxygen concentration by injecting pure oxygen into the primary air, and reducing the secondary air flow rate to maintain the excess air coefficient in the entire cement rotary kiln unchanged while keeping the primary air volume unchanged.

8. The numerical simulation method for coal powder combustion in a cement rotary kiln under flue gas recirculation and oxygen-enriched combustion according to claim 1 is characterized in that: The recirculated flue gas is fed into the cement rotary kiln by mixing with primary air and spraying, mixing with secondary air and spraying, or setting a separate recirculated flue gas nozzle to spray, or a combination of two or more.

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