Acta Polytechnica DOI:10.14311/AP.2019.59.0543 Acta Polytechnica 59(6):543–553, 2019 © Czech Technical University in Prague, 2019 available online at https://ojs.cvut.cz/ojs/index.php/ap 3-D MODELING OF HEAT AND MASS TRANSFER PROCESS DURING THE COMBUSTION OF SOLID FUEL IN A SWIRL FURNACE Aliya Askarovaa,b, Saltanat Bolegenovaa,b, Valeriy Maximovb, Symbat Bolegenovab, Aizhan Nugymanovaa,∗, Pavel Šafaříkc a al-Farabi Kazakh National University, Physics and Technology Faculty, al-Farabi av. 71, 050040, Almaty, Kazakhstan b al-Farabi Kazakh National University, Scientific Research Institute of Experimental and Theoretical Physics, Tole bi, 94a, 480012, Almaty, Kazakhstan c Czech Technical University in Prague, Faculty of Mechanical Engineering, 166 07 Praha 6, Prague, Czech Republic ∗ corresponding author: nugymanova.aizhana@gmail.com Abstract. In this work, a comprehensive study of thermal processes and aerodynamic and concentra- tion characteristics of the combustion chamber of the boiler BKZ-75 of the Shakhtinskaya thermal power plant (Kazakhstan) are presented. A comparison of the characteristics of the combustion processes for two cases is given for the direct-flow method of supplying the mixture - the burners are located on opposite-side walls and the swirl-air mixture supplying method - burners with a swirl angle of the air mixture flow and their inclination to the centre of symmetry of the boiler by 30 degrees. The research results allow us to determine the optimal technological parameters of the studied object, to improve the methodology for the numerical study of heat and mass transfer processes in high-temperature and chemically reacting flows in the presence of turbulence, and also develop appropriate technological solutions for installing burner devices (direct-flow or swirl) in the studied combustion chamber. Keywords: Modeling, thermal power plants, direct-flow and swirl burners, carbon monoxide, nitrogen dioxide. 1. Introduction The fuel and energy complex in the economy of any state is the most important component of ensuring the functioning and development of productive forces and, improving the living standards of the population [1]. In this regard, the creation of new, highly efficient energy- and resource-saving technologies of energy processes becomes an urgent issue of the choice of operation. Therefore, it is necessary to implement a whole range of measures, the most important of which is the application of the most accurate methods for calculating heat and power processes. It is possible to increase the efficiency and envi- ronmental safety of boiler units of thermal coal-fired power plants with the swirl method of burning fuel, which uses the aerodynamic features of a swirling flow having a complex structure, separation ability and a high degree of turbulence. The Iinternal aerodynamics of the combustion device ensures the achievement of specified technical and environmental indicators when burning fuel in a swirl flow. The optimal organization of the flow aerodynamics can affect the uniform dis- tribution of temperature and heat fluxes, increase the residence time of the fuel particle in the combustion chamber, and consequently, the completeness of fuel burn out, increase slag collection, reduce the emission of toxic ingredients and increase the energy efficiency of the steam generator [2]. At the moment, in the implementation of a compre- hensive study of the processes of burning pulverized coal in the combustion chambers of boilers of indus- trial facilities (thermal power plants, etc.), the most optimal way is to conduct computational experiments using 3D modelling methods and with the use of a modern computer equipment, computer technology and application software packages [3–9]. When conducting a computational experiment to build physicotechnical models, model ideas about the mechanism of really occurring processes in the com- bustion chambers of energy objects are used. The methodological principles for creating such models are based on an understanding of the flow of technological processes (the stage of combustion pulverized coal, the formation of harmful dust and gas emissions, ash, etc.). Such processes are based on the knowledge of the laws, the so-called “elementary” processes of aero- dynamics, mass and heat transfer, chemical kinetics, phase transition processes, nonlinear effects of convec- tive and radiation heat transfer, diffusion processes, etc. In this paper, using modern technologies of numeri- cal methods [10–18], a comprehensive study of thermal processes and aerodynamic characteristics of the com- 543 https://doi.org/10.14311/AP.2019.59.0543 https://ojs.cvut.cz/ojs/index.php/ap A. Askarova, S. Bolegenova, V. Maximov et al. Acta Polytechnica Figure 1. Geometry and finite-difference grid of the combustion chamber of the boiler BKZ-75 of the Shakhtinskaya TPP. bustion chamber of an existing Kazakhstan energy facility (Shakhtinskaya thermal power plant (TPP), Kazakhstan) is carried out. Based on the numerical solution of the system of convective heat and mass transfer equations [19], taking into account the kinet- ics of chemical reactions, two-phase flow, nonlinear effects of convective and radiation heat transfer and 3D modelling methods, aerodynamic, thermal, and concentration characteristics are obtained over the entire volume of the combustion chamber, in its main sections and on an output from itits output [20–26]. The results of the studies allow us to determine the optimal technological parameters of the inves- tigated object, to improve the methodology for the numerical study of heat and mass transfer processes in high-temperature and chemically reacting flows in the presence of turbulence, and also develop appropriate technological solutions for installing burner devices (direct-flow or swirl) in the combustion chamber under study. 2. Description of the combustion chamber for conducting computational experiments For carrying out computational experiments, the com- bustion chamber of the BKZ-75 boiler installed at the Shakhtinskaya TPP (Shakhtinsk, Kazakhstan) was selected. The steam boiler of the factory mark BKZ- 75–vertically water-tube, with a productivity of 75 t/h (51.45Gcal/h) [27]. The boiler is equipped with four pulverized coal burners which are installed in two on the front and back walls in one tier. During the oper- ation of this combustion chamber, direct blowing of dust from individual dust preparation systems is used, which is provided by two hammer mills. Fig. 1 shows the geometry and the finite-difference grid of the com- bustion chamber of the BKZ-75 boiler for conducting computational experiments. In the computational experiment for the steam boiler BKZ-75, the dust of Karaganda ordinary coal grade KR-200 was burned, the characteristics of which are indicated in the Table 1. The designs of various burner devices of the combus- tion chamber of the BKZ-75 boiler are shown in Fig. 2. To conduct computational experiments in the com- bustion chamber of the BKZ-75 boiler, two cases were investigated: 1) the direct-flow method of supplying an air mixture - burners are located on opposite-side walls; 2) swirl air mixture supplying method - burners with a swirl angle of the air mixture flow and their inclination to the centre of symmetry of the boiler by 30 degrees. 3. Results of Numerical Modeling Figures 3-13 show the results of computational exper- iments, the aerodynamic flow pattern (distribution of the overall velocity), temperature and concentration fields of carbon monoxide CO and nitrogen dioxide NO2 for two cases of fuel supply to the combustion chamber of the BKZ-75 boiler, direct-flow and swirl. The distribution of the overall velocity vector field in various cross-sections of the combustion chamber of the BKZ-75 boiler is shown in Figures 3 - 4. We can see that with the direct-flow method of supplying the air mixture, the flows, colliding in the center centre at a right angle (Fig. 3a), are dissected, and combined into two main streams are and sent to the exit from the combustion chamber (Fig. 4a). With the swirl method of supplying the air mixture, four swirling flows directed from the swirl burners collide with each other in the central part at an angle of 30 degrees (Fig. 3b), and then, combined, are sent to the exit from the combustion chamber, with the formation of a swirl flow of higher intensity (Fig. 4b). Figures 5 - 6 illustrate the temperature fields in various cross-sections of the combustion chamber of the BKZ-75 boiler for the two studied modes of the supply of air mixture (direct-flow and swirl). We see that the temperature has maximum values in the region close to the location of the burner. With a direct-flow method of supplying the air mix- ture, the temperature value area of the burner section (z = 4.0m) of the combustion chamber of the BKZ-75 boiler is 620.56 °C, and the region of maximum temper- ature values for the direct-flow burner is located near the walls of the combustion chamber, which causes their additional heat load (Fig. 5a, Fig. 6a). The average temperature in the case of swirl burn- ers with a swirl angle of 30 degrees is distributed differently. Here, the temperature in the burner area increases and amounts to 834.74 °C, and the core of a torch is observed in the centre of the combustion cham- ber (Fig. 5b, Fig. 6b), thereby reducing the thermal load on its walls. This is due to the swirl character of the flow, providing a maximum convective transport and an increase in the residence time of coal particles in the combustion chamber of the boiler. 544 vol. 59 no. 6/2019 3-D modeling of heat and mass transfer process. . . Name Symbol Unit Value Type of coal KR-200 - - Grinding fineness R90 % 20 Coal density ρ kg/m3 1350 Calorific value of coal Qc kJ/kg 3.4162 · 104 Dry ash content Ad % 35.10 Volatile release to combustible mass V % 22.00 Humidity (coal as received) War % 10.60 Carbon (coal as received) C % 79.57 Hydrogen (coal as received) H2 % 6.63 Oxygen (coal as received) O2 % 9.65 Sulfur (coal as received) S2 % 1.92 Nitrogen (coal as received) N2 % 2.23 The chemical composition of the ash (macro components) SiO2 % 60.2 Al2O3 % 25.5 Fe2O3 % 5.85 CaO % 3.65 MgO % 1.05 TiO2 % 0.95 SO3 % 0.8 K2O % 1.65 Na2O % 1.06 Table 1. Characteristics of the combustion chamber of the boiler BKZ-75 and fuel. Figure 2. Designs of burner devices of the combustion chamber of the boiler BKZ-75: left) direct-flow burners; right) swirl burners. 545 A. Askarova, S. Bolegenova, V. Maximov et al. Acta Polytechnica Figure 3. Distribution of the overall velocity in the area of the burner section (z = 4.0m) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. Figure 4. Distribution of the overall velocity in the central section (y = 3.3) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. 546 vol. 59 no. 6/2019 3-D modeling of heat and mass transfer process. . . Figure 5. Temperature distribution in the area of the burner section (z = 4.0m) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. Figure 6. Temperature distribution in the central sections (y = 3.3) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. 547 A. Askarova, S. Bolegenova, V. Maximov et al. Acta Polytechnica Figure 7. Temperature distribution along the height of the combustion chamber of the BKZ-75 boiler with: 1– direct-flow method of supplying an air mixture; 2 – swirl method of supplying an air mixture; N – are theoretical values obtained by the method of thermal calculation (CBTI - Central Boiler and Turbine Institute) [28]. Figure 7 shows a comparative analysis of the distri- bution of the average temperature in the cross section over the height of the combustion chamber for the two studied modes of the air mixture supply (direct-flow and swirl). In the case of a swirl method of supplying an air mixture, an increase in the extent of the zone of maximum temperatures is observed. The minimal in the curves are associated with the low temperature of the air mixture entering the combustion chamber through the burners. The temperature at the outlet of the combustion chamber is confirmed by its theoret- ical value calculated by the method of CBTI (Central Boiler-and-Turbine Institute) for direct-flow supplying of air mixture [28]. An increase in the temperature in the core of the flame and a decrease in its output has a significant effect on the chemical processes of the formation of combustion products. Figures 8 - 10 below show the distribution of carbon monoxide concentrations CO in various sections of the combustion chamber for the studied modes of the supply of air mixture (direct-flow and swirl). An analysis of the figures shows that with a direct- flow method of supplying an air mixture, the average values of the concentration of carbon monoxide CO in the central sections (for y = 3.3) are 6 · 10−3 kg/kg (Fig. 8a), while with a swirl method of supplying an air mixture, the minimum average values of the concentration of carbon monoxide are observed CO – 4 · 10−3 kg/kg (Fig. 8b). As the combustion chamber moves upwards, the concentrations of carbon monox- ide CO decrease and, at the outlet (x = 7.1), make up 7 · 10−4 kg/kg for the direct-flow method of supplying the air mixture (Fig. 9a), and 6 · 10−4 kg/kg for the swirl method of supplying the air mixture (Fig. 9b). This is confirmed by Fig. 10, which shows the curves of the distribution of carbon monoxide concentrations over the height of the combustion chamber of the BKZ-75 boiler for the two studied cases. The concentration distributions of nitrogen diox- ide NO2 in various cross-sections of the combustion chamber are shown in Figures 11 - 13. As can be seen from the figures, the zone of the maximum formation of nitrogen dioxide NO2 is the region of high temper- atures and intense swirl flow. Intensive mixing of fuel and oxidizing agent created by turbulent flows of in- jected aerosol mixtures near the burners, as well as the high temperature in the torch core, create favourable conditions for the formation of nitrogen dioxides. In this area, the concentration of nitrogen dioxide NO2 reaches its maximum values equal to 1067.63mg/m3 n (Fig. 11a) with a direct-flow method of supplying an air mixture, and 1045.91mg/m3 n (Fig. 11b) with a swirl method of supplying an air mixture. However, towards the exit from the combustion chamber (Figures 12 - 13), a uniform decrease in the NO2 concentration is observed, since this region con- tains less oxygen and a fuel component. In addition, in the case of the use of swirl burner devices, the tem- perature along the height of the combustion chamber monotonously decreases, as a result of which the rate of formation of nitrogen dioxide NO2 decreases. At the outlet of the combustion chamber, the average value of the concentration of nitrogen dioxide NO2, when using a direct-flow method of supplying an air mixture, is 564.34mg/m3 n (Fig. 12a and Fig. 13 curve 1), and with swirl burner devices – 439.35mg/m3 n (Fig. 12b and Fig. 13 curve 2), which is 125mg/m3 n less. The obtained results point to the advantages of the selecting swirl burners to optimize the combus- tion of high-ash coal in the furnaces of power plants and reduce harmful dust and gas emissions into the environment. 548 vol. 59 no. 6/2019 3-D modeling of heat and mass transfer process. . . Figure 8. Distribution of carbon monoxide CO in the central sections (y = 3.3) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. Figure 9. Distribution of carbon monoxide CO at the outlet of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. 549 A. Askarova, S. Bolegenova, V. Maximov et al. Acta Polytechnica Figure 10. Distribution of the concentration of carbon monoxide CO over the height of the combustion chamber of the boiler BKZ-75 with: 1 - direct-flow method of supplying an air mixture; 2 - swirl method of supplying an air mixture. Figure 11. Distribution of nitrogen dioxide NO2 in the central sections (y = 3.3) of the combustion chamber of the BKZ-75 boiler with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. 550 vol. 59 no. 6/2019 3-D modeling of heat and mass transfer process. . . Figure 12. Distribution of nitrogen dioxide NO2 at the outlet of the combustion chamber of the boiler BKZ-75 with: a) direct-flow method of supplying an air mixture; b) swirl method of supplying an air mixture. Figure 13. Distribution of the concentration of nitrogen dioxide NO2 along the height of the combustion chamber the boiler BKZ-75 with: 1 – direct-flow method of supplying an air mixture; 2 – swirl method of supplying an air mixture. 4. Conclusion Based on the research results, the following conclu- sions can be formulated: • A comparison of the characteristics of combustion processes for two cases is given for a direct-flow method of supplying the air mixture – burners are located on opposite-side walls and a swirl method of supplying the air mixture – burners with a swirl angle of the air mixture and their inclination to the boiler symmetry centre by 30 degrees. • The results of computational experiments on the distribution of the overall velocity, temperature fields, concentration fields of carbon oxides CO and nitrogen dioxides NO2 over the entire volume of the combustion chamber and their comparative analysis for the two studied modes of the supply of air mixture (direct-flow and swirl) are presented. • Two cases of the method of supplying the air mix- ture with the direct-flow and swirl methods of sup- plying the mixture were investigated, and it was found that the aerodynamics of the flow in both cases is different. In the direct-flow method of sup- plying air mixtures, the flows, colliding in the centre at a right angle, are cut in the region of the cold funnel and towards the exit from the combustion chamber, with the formation of a swirl flow of lower intensity. In the swirl method of supplying the air mixture, four swirling flows area of the burner sec- tion guided from the swirl burners collide with each other in the central part of the combustion chamber at an angle of 30 degrees. Then, having united in two main streams, they interact, forming swirl flows in the horizontal region of the combustion chamber. High stability of the swirl flow position increases the residence time of the coal dust in the combustion zone and will significantly reduce high concentrations of harmful substances. • During a swirl feed of an air mixture, an increase in the extent of the zone of maximum temperatures is observed. The temperature values greatly differ depending on the area of the burner section. An 551 A. Askarova, S. Bolegenova, V. Maximov et al. Acta Polytechnica increase in the temperature in the flame core and a decrease in its outlet have a significant effect on the chemical processes of the formation of combus- tion products, since the temperature is the main factor in determining the rate of combustion of the components of the fuel mixture. • The swirl method of supplying the mixture is prefer- able for the studied combustion chamber, since, at the outlet from the combustion chamber, the av- erage concentration of carbon monoxide CO and nitrogen dioxide NO2 decreases when using swirl burners. The value of the concentration of harmful substances (NO2, CO) at the exit from the com- bustion chamber corresponds to the maximum per- missible concentrations standards adopted in the power system of the Republic of Kazakhstan. 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