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  • Рік видавництва: 2007


    Журнал: Проблеми регіональної енергетики


    Наукова стаття на тему 'Advanced Computational Methods for Complex Simulation of Thermal Processes in Power Engineering'

    Текст наукової роботи на тему «Advanced Computational Methods for Complex Simulation of Thermal Processes in Power Engineering»

    ?Advanced Computational Methods for Complex Simulation of Thermal Processes in Power Engineering

    Risto V. Filkoski, Ilija J. Petrovski

    Faculty of Mechanical Engineering, University "Sts. Cyril & Metodius "

    P. O. Box 464 1000 Skopje, Republic of Macedonia rfilko @ mf. edu. mk

    Abstract

    The overall frame and principal steps of complex numerical modelling of thermal processes in power boiler furnaces on pulverised coal with tangential disposition of the burners are presented in the paper. Computational fluid dynamics (CFD) technique is used as a tool to perform comprehensive thermal analysis in two test cases. The methodology for creation of three-dimensional models of boiler furnaces is briefly described. Standard steady k-s model is employed for description of the turbulent flow. The coupling of continuity and momentum is achieved by the SIMPLEC method. Coal combustion is modelled by the mixture fraction / probability density function approach for the reaction chemistry, with equilibrium assumption applied for description of the system chemistry. Thermal radiation is computed by means of the simplified P-N model, based on expansion of the radiation intensity into an orthogonal series of spherical harmonics.

    Comparison between the simulation predictions and available site measurements leads to a conclusion that the model produces realistic insight into the furnace processes. Qualitative agreement of the results indicates reasonability of the calculations and validates the employed sub-models. The described test cases and other experiences with CFD modelling stress the advantages over a purely field data study, such as the ability to quickly and cheaply analyse a variety of design options without actually modifying the object and the availability of significantly more data to interpret the results.

    Key words: pulverised coal-fired boiler, CFD modelling, combustion, thermal radiation, heat transfer

    Rezumat. In lucrarea sunt prezentate fazele modelarii numerice a proceselor in focarele cazanelor care functioneaza cu pulbere de carbune. In calitate de procedeu principal la modelarea sunt utilizate metodele numerice de calcul a dinamicii fluidelor. Compararea rezultatelor modelarilor §i a rezultatele investigatiilor experimentale confirma corectituidenea modelului obiectului real.

    Cuvinte cheie. Cazan, care functioneaza cu pulbere de carbune, ardere, modelarea cu utilizarea metodelor de calcul al dinamicii lichidului, radiatia termica, transfer de caldura.

    Резюме. В роботі наводяться основні етапи цифрового моделювання теплових процесів в топках котлів, що працюють на пилоподібному вугіллі. В якості основного засобу для моделювання використані методи обчислювальної рідинної динаміки. Порівняння між результатами моделювання та результатами випробувань підтверджує відповідність моделі об'єкту.

    Ключові слова. Котел, що працює на пилоподібному паливі. Моделювання з використанням методів обчислювальної рідинної динаміки, горіння, теплова радіація, теплопередача.

    1. Introduction

    Numerical simulation techniques through the last decades have grown from being promising, mainly scientific tool, to a basic technology, unavoidable in engineering practice. With the development of the methods, the use of numerical simulation tools is changing from the

    traditional physics-based approach towards the application-based view. Numerical simulations performed on basis of computational fluid dynamics / computational thermal analysis (CFD / CTA) provide great potential in analysing, design, retrofitting and optimisation of performances of fossil fuel power systems.

    Efficient use of low quality coals is crucial to the power industry in the most South and East European countries and utility boiler with tangential burners disposition is a basic model that serves most of the power plants, which was the main motivation for undertaking this investigation. The combustion process of pulverised coal in boiler furnace is an example of very complex 3-D turbulent flow, accompanied by strong coupling of mass, momentum and energy in two phases. Comprehensive modelling of furnace processes enables simulation of operational state and it can be applied in diagnostics and foresight of behaviour, operational conditions and situations of boiler plants in efforts to improve their combustion efficiency, fuel economy and to reduce pollutants emission. Thus, it is relatively easy to analyse how changes of the fuel supply system, fuel type or milling quality affect combustion, heat transfer, temperature distribution, heat flux, pollutants emission, erosion of heat exchanging surfaces etc. This paper presents two test cases of CFD simulations: 1) OB-380 120 MWe utility boiler in the Thermal Power Plant "Oslomej", Kicevo, Republic of Macedonia and 2) TENT A2 210 MWe utility boiler in the Thermal Power Plant "Nikola Tesla "Obrenovac, Serbia.

    2. Description of the Mathematical Model

    Differential models, based on solving equations for fluid flow, heat and mass transfer, thermal radiation and chemical reactions, including combustion, offer local values ​​of relevant variables and detailed insight into the complex processes and phenomena in the computational domain, regarding the actual geometry, fuel characteristics and other operating conditions. They enable widespread and fast analysis of the impact of huge number of parameters and operational modes, compared to measurements or common conventional engineering calculations, which offer limited reliability when applied to changing exploitation conditions.

    Three-dimensional models of industrial and utility scale furnaces, including models of tangentially fired furnaces, have been developed and successfully applied for years now [113]. However, there is still an area for further improvements, having as a subject a detailed mathematical description of physical and chemical processes in certain specific conditions. The models of combustion systems are often similar to each other in many ways and the majority use variations of the SIMPLE algorithm for coupling of velocity and pressure and the ks gas turbulence model, or some derivatives, like RNG ks model [2], or ks -kp two-phase turbulence model [10]. Gas phase conservation equations are mostly time-averaged and two-phase flow, as the one occurring in boilers fired with pulverised coal, is usually described by Eulerian-Lagrangian approach and PSI-CELL method for taking into account the influence between phases, with some exceptions using Eulerian-Eulerian approach, or two-fluid trajectory model [10]. Most of the combustion submodels given in [2,7,8,9-11,13] separately treat particle devolatilisation, char oxidation and additional gas phase reactions. Thermal radiation in the furnace is modelled by means of various approaches, like discrete transfer method [7], discrete ordinates method [8,10,13], six-fluxes method [9], Monte Carlo method [2], or so called PN model [14], as in this paper. Commercial CFD codes are applied successfully [11,12,13], but also research efforts are given worldwide to the comprehensive models specially developed for simulation of the furnaces [7-10]. In general, it should be noted that a comprehensive model of the furnace processes must balance sub-model sophistication with computational practicality.

    prePDF preprocessor

    - combustion,

    "Look-up" tables

    PDF

    files

    GAMBIT

    - geometry creation

    - generation of numerical mesh

    FLUENT

    - import of geometry and numerical mesh

    - physical models

    - boundary conditions

    - material properties

    - calculation procedures

    - postprocessing

    Object geometry + 2D / 3D mesh

    I

    In the both cases described in this paper the furnace geometry is described in details, with particular emphasize on burners [15,16]. General structure of the case set-up and solution with CFD / CTA technique in this research is presented in Fig. 1. Fluent CFD software is employed for description of turbulent fluid flow, devolatilisation, coal combustion, gas phase chemical reactions, species transport and heat transfer, with Gambit preprocessor used as a graphic tool for geometry creation and mesh generation [17]. The simulations are performed in 3-D domains for boilers 'steady state operating

    Fig. 1. Structure of the case set-up and solution with the conditions.

    CFD technique

    Turbulent mixing is quantified by the standard k-s model. Common values ​​of the

    constants are used in the transport equations: crk = 1.0, crs = 1.3, C1s = 1.44 and C2s = 1.92. Coupling of velocity and pressure is achieved by the SIMPLEC algorithm. Numerical simulation of the pulverised coal combustion involves modelling of continuous gas phase flow field and its interaction with discrete phase - coal and ash particles. Stochastic tracking model is used in the calculations to take into account the effect of turbulence on the particles trajectories. The polydisperse coal particle size distribution is assumed to fit the Rosin-Rammler equation. Mass flow rate, temperature and mixture fraction is assigned at coal and air inlets, while outflow is prescribed at the recirculating holes and at the furnace exit, which, in this test case is located after the platen superheater. Soot formation and emission of pollutants, such as NOx, are also taken into consideration in the research.

    The coal particles, travelling through the air-gas mixture, devolatilise, creating a source of fuel for reaction in the gas phase and undergo char combustion. Energy balance to the particles is used to calculate the particle temperature and to describe the coal evolution. In both cases, two-competiting-kinetic-rates model is selected as a devolatilisation model. The combustion is modelled as non-premixed kinetics / diffusion-limited process with the mixturefraction / probability density function (PDF) approach for the reaction chemistry [17,18]. Full equilibrium chemistry is selected as chemistry model and the turbulence-chemistry interaction is modelled with P probability density function. It is assumed that PDF mixture consists of 16 species: C (S), C, H, O, N, O2, N2, CO2, H2O, H2O (L), CH4, CO, OH, NO and HCN.

    One of the important issues in the case of coal combustion modeling is inclusion of the effect of discrete phase, coal and ash particles, on the radiation absorption coefficient. The basic radiative transfer equation for an absorbing, emitting and scattering medium with contribution of the particulate phase, at position r in direction s is

    where I is total radiation intensity, which depends on position r and direction s; 5 is path length; ap is the equivalent absorption coefficient due to the presence of particulates; ap is equivalent particle scattering factor; Ep is the equivalent particle emission; a is absorption coefficient; n is refractive index; a is Stefan-Boltzmann constant, o = 5,672-10'8 W / m2K4); T is local absolute temperature; s 'is scattering direction vector; O is phase function and Q 'is solid angle. The product (a + s ^ s is optical thickness or opacity of the medium.

    dI (r, s) XT,. 2 aT4

    ---- + (a + ap + sp) I (r, s) = an -----------------

    ds n

    4n

    + Ep + - fl (r, s ') O (s • s') dQ '

    (1)

    Thermal radiation in this work is taken into account in the heat transfer simulations through the so-called P-1 model [14,17,19,20]. It is based on expansion of the radiation intensity I into an orthogonal series of spherical harmonics. If only four terms in the series are used, the following equation is obtained for the radiation flux qr:

    q- =-,<| + \ R- VG (2)

    3 \ a + ° s j-C ° s

    where G is incident radiation, <js is scattering coefficient and C is linear-anisotropic phase function coefficient. Variable absorption coefficient a is computed by the weighted-sum-of-gray-gases model [17, 19, 21].

    Besides the relative simplicity, the P-1 model has several advantages over other radiation models, treating the radiative transfer equation (1) as an easy-to-solve diffusion equation. It can easily be applied to complicated geometries and for combustion applications where the optical thickness is large it works reasonably well. Also, the particle emissivity, reflectivity and scattering can effectively be included in the calculation of the radiation heat transfer.

    The transport equation for G is

    V (rVG) + 4tc a- + Ev - (a + ap) G = 0 (3)

    I n J

    in which the parameter Y is defined through the equivalent absorption coefficient ap and the equivalent particle scattering factor <jp:

    r = 7 ------ 1 ------ 1 (4)

    3 \ a + ap + ° p j

    With substitution qr = -rVG in eq. (3) the following expression is obtained for -Vq /.

    ( <jT ^ ^

    -Vqr = -4rc a - + Ep + (a + ap) G (5)

    which can be directly included into the energy equation to account for heat sources due to radiation.

    The flux of the incident radiation at wall qrw is determined with the expression * |-4 -G-) (6)

    where sw is wall emissivity, Tw is wall temperature and Gw is incident wall radiation.

    3. Case 1: Utility Boiler OB-380 120 MWe

    The tangential coal fired steam generator OB-380 is designed and manufactured by RAFAKO S.A., Raciborz, Poland. Its simplified configuration, with disposition of the heat exchanging surfaces, is displayed in Fig. 2 and the main technical characteristics are listed in Tab. 1. The boiler shape is conventional, with two gas passes and with natural water-steam circulation. Membrane walls form the furnace, crossover pass and a part of the convective pass. The furnace is 12.055 m wide, 9.615 m long and 40.0 m high. Six pulverised coal burners are arranged in such manner, shown in Fig. 3, to form a swirling flow of gas-solid mixture. OB-380 boiler is fired with low-grade lignite, with huge content of ballast materials and with calorific value varying between 6500 and 8800 kJ / kg. The average proximate and ultimate

    Fig. 3. Direction of the burners in the furnace and burner vertical cross-section

    analyses of the coal are given in Tab. 2. Approximate fuel consumption of the boiler operated at full load is 45-52 kg / s, while flue gases outflow is 160-200 m3 / s. The boiler has already expanded its design operational lifetime, working often at maximum capacity.

    Table 1. Main characteristics of the boiler OB-380

    Property Value

    - Steam output - Parameters of superheated steam - Parameters of reheated steam - Parameters of feed water - Pressure in the boiler drum - Temperature of preheated air - Flue gases outlet temperature - Boiler efficiency 105.6 kg / s 138 bar / 540oC 27.7 bar / 540oC 165 bar / 230oC 154 bar 260oC 150oC 85-88%

    Fig. 2. Scheme of the boiler OB-380, TPP "Oslomej", Kicevo, Macedonia

    Table 2. Average proximate and ultimate analysis of the Oslomej lignite

    Proximate analysis,% Ultimate analysis,%

    Char 29.15 C 23.45

    Volatiles 21.35 H 2.11

    Cfix 13.38 O 7.50

    Ash 15.77 N 1.10

    Moisture 49.50 S 0.57

    Several basic cases of boiler operating conditions are investigated and three of them are subject of consideration in this article: mode R1 corresponding to 83% boiler load (100 MW electrical output) with five burners in service and modes R2 and R3 conducted on the basis of almost full load (115 MW electrical output), with values ​​of some of the boiler parameters and operating conditions given in Table 3 [15,22].

    The furnace computational domain and mesh as they are generated for the purpose of this research are presented in Fig. 4. Numerical mesh of 124839 finite volume cells, 375573 faces and 125880 nodes is employed. The superheater is modelled with parametric heat exchanger model to account for the heat absorption and pressure loss [17]. For that purpose, a separate fluid zone is defined to represent the superheater core, Fig. 4c, which is subdivided into macroscopic cells along the coolant path [15]. The coolant inlet temperature to each macro cell is computed and then subsequently used to compute the heat rejection from each macro cell. This approach provides realistic heat rejection distribution over the heat exchanger core.

    The coal particles size distribution is represented with the Rosin-Rammler equation with a mean diameter dpm = 90-120 ^ m and a spread parameter of 3.5. Particle trajectory data, coal devolatilisation and combustion parameters used in the model are given in Tables 4 and 5.

    Recirculation of the flue gases through holes in the upper part of the furnace is included in the computations with a coefficient rg = 0.25-0.31, depending on the working mode.

    The wall emissivity in this test case is specified in the range 0.65-0.8 at the furnace walls and 1.0 at the furnace bottom and exit.

    Table 5. Coal combustion parameters

    Table 4. Coal particle trajectory data

    Number of particle stream start locations 18

    Maximum number of steps in each

    trajectory 700

    Length scale 0.1 m

    Number of particle diameters 8

    a) Coal devolatilisation data b) Combusting particles properties

    Devolatilisation model - two competing rates Density 1250 kg / m3

    1) First rate Specific heat capacity - picewise-linear profile

    - pre-exponential factor 2.0-105 s-1 Thermal conductivity 0.05 W / mK

    - activation energy 7.50-107 J / kmol Mechanism factor 2

    - weighting factor 0.3 Binary diffusivity 4-10-5 m2 / s

    2) Second rate Particle emissivity 0.8

    - pre-exponential factor -s r- '0 co Particle scattering factor 0.5

    - activation energy 1.45-108 J / kmol Swelling coefficient 1.0

    - weighting factor 1.0 Mass diffusion limited rate constant 5.0-10-12 Kinetic rate pre-exponential factor 0.002 Activation energy 9.5 -107 J / kmol

    "A)

    W

    .45 56 1.69 /8.68 06 15 No. 4

    No. 3 I No. 3

    ------------cT----------------

    Fig. 4. Boiler furnace: a) feature, b) finite-volume mesh and c) superheater zone

    Sidewall temperature is calculated on a basis of the near-wall heat transfer conditions.

    As results of the simulations regarding the OB-380 boiler, informations are obtained on flow fields, velocity vectors, particles path lines, temperature contours, heat flux profiles to the furnace walls, contours of O2, CO2 and other species concentrations, as well as many other variables [15]. Some typical results are displayed in the following figures. Figure 5a shows gas phase velocity vectors in vertical furnace intersection. Predictions of coal particles path lines initiated from the fuel inlets of the burner No. 1 are shown in Fig. 5b. Knowing probable path lines of the fuel particles can be very important information for prediction of position where the most intensive combustion occurs, but also it can help in gaining closer insight into the reasons for eventual appearance of incomplete combustion. Traces of coal particles released from the burner No. 4 are displayed in Fig. 5c, showing general swirling flow field in the furnace.

    a) b) c)

    Fig. 5. a) Gas phase velocity vectors in central vertical cross-section; b) Path lines of coal particles streams released from the burner No. 1, coloured by DPM burnout; c) Traces of particles released from the burner No. 4, coloured by temperature, view from the furnace top

    Simulation results of typical temperature distribution in certain intersections of the computational domain at boiler full load are presented in Fig. 6. The plots highlight flame shape and furnace hot spots outside the burner flame boundaries. The highest temperatures,

    somewhat above 1300 ° C according to the CFD predi ctions, are detected in the furnace core. The tangential movement of the flue gases-particles mixture in the horizontal intersection at the burners 'level is clearly visible, appearing as a consequence of the burners' position. Central position of the flame suggests that the temperature and heat loads of the furnace are well balanced.

    In tangential coal fired boilers, the gas temperature deviation at the furnace exit could occur as the scale of the boiler becomes larger. It is commonly considered that it results from the after twirl in the furnace exit, which depends mainly on the dimensions and shape of the platen superheater, the way of the

    Fig. 6. Temperature fields in central vertical intersection at full load and at different horizontal levels

    N

    secondary air introduction and the shape of the furnace. This phenomenon can result in damage of superheaters 'and reheaters' pipes. Although the investigated boiler unit could not be treated as a large capacity boiler, according to the present simulations, temperature deviation appears in some extent in the upper part of the furnace, Fig. 6. Further investigations in this direction are necessary. It can be noticed that the presented numerical method slightly overestimates the expected temperature at the furnace core, Still, the average furnace outlet temperature, which, according to the long-term experience with the boiler operation, should be 950-980 ° C, is asserted with the model, with insignificant deviations.

    Present simulations include analysis of NOx formation and reduction during the combustion process. An example of the results concerning this issue is presented in Fig. 7.

    Since the fuel is low calorific lignite and, consequently, furnace temperatures are moderate, appearance of thermal NOx is irrelevant and the total NOx emission is not very high.

    Temperature and heat flux to the walls in the furnace are measured through 31 measurement points at four levels: 13.9 m, 20.4 m, 23.0 m and 26.4 m (the bottom of the furnace funnel is located approximately at elevation 6.5 m), with aspiration pyrometer , non-cooled temperature probe and digital optical pyrometer [15].

    Typical profiles of measured and computed temperatures at elevation 26.4 m are shown in Fig. 8 [15]. Relatively well conformity between the CFD predictions and available field data can be noticed at the right side, but the discrepancy is considerable on the left side of the central furnace cross-section. Also, profiles of measured and average area-weighted temperature along the furnace height at modes: R1 (83% of full load), R2 and R3 (both full load), are displayed in Fig. 9 [15]. Appearance of temperature peaks at approximate height of 18 m can not be verified, neither denied with the available measurements. Figure 10 depicts area-weighted average heat flux to the walls along the furnace height, predicted with CFD and confronted with measurements [15]. According to the simulations, maximum local values ​​in the zone of intensive combustion do not exceed 140-150 kW / m2, which is in agreement with recommendations for this type of boiler furnace.

    The change of the average thermal efficiency of the furnace walls along the boiler height

    1.50 * -03

    1.44c-03

    1.3Sc-03

    1.32c-03

    1.36c-03

    t20c-03

    lHc-03

    1.08c-03

    1.024-03

    9.60c-04

    9.00 «-04

    8.40o-04

    7.81 * -04

    7.21 -04

    6.61c-04

    6.01c-04

    5.41t-04

    4.S1C-04

    4.2k-04

    3.61c-04

    3.01c-04

    2.41c-04

    1.81c-04

    1.2k-04

    6.11c-05

    Fig. 7. Contours of NO mass fraction at the furnace central cross-section and at the furnace exit

    o

    o

    <D

    : 5

    e

    12 3 4

    Distance from the front w all,

    Eevation [m]

    Fig. 8. Temperature contours at elevation 26.4 m (approx. 20 m above the furnace bottom), mode R1: CFD-P15, CFD-P22 - model, 1.075 m from the left and right sidewall, respectively; M-P15, M-P22 - measurements, 1.075 m from the left and right sidewall

    Fig. 9. Area-weighted average temperature along the furnace height: R1 - measured (83% load), R2 and R3 - measured (full load); CFD-R1 - model (83% load), CFD-R2 and CFD-R3 - model (full load)

    0

    5

    is given in Fig. 11 [15]. In this diagram, the results obtained by the CFD simulations are confronted to the values ​​calculated indirectly on the basis of the heat flux and temperature measurements. The change of the walls thermal efficiency, according to the Normative

    Elevation [m]

    Fig. 10. Heat flux distribution along the furnace height: R1, R2 and R3 - measurements; CFD-R1, CFD-R2 and CFD-R3 - model results

    0.55

    0.5

    0.45

    0.4

    0.35

    0.3

    0.25

    0.2

    0.15

    0.1

    I

    if

    V | M er. FD - RR1 _

    0 5 10 15 20 25 30 35 40

    Elevation [m]

    Fig. 11. Average coefficient of thermal efficiency of the furnace walls

    o

    c

    0)

    o

    it =

    <D

    c

    c

    o

    1 />

    .Q

    E

    o

    O

    100

    99.5

    99

    98.5

    98

    97.5

    97

    | R1-mer,% | R2-mer,% A R3-mer,% 0 R1-CFD,% - - R2-CFD,% - -? T - - R3-CFD,%

    * n

    9

    15

    17.5

    20

    25

    27.5

    30

    22.5 Level, m

    Fig. 12. Combustion efficiency along the furnace in the modes R1, R2 and R3

    Method of the CKTI (according to [23]), is presented in the same figure for comparison.

    Combustion efficiency in the modes R1-R3, according to the measurements and CFD simulations, is illustrated with Fig. 12. Estimations show high fuel conversion in the cases of 83% and full boiler loading, with predicted unburned fuel loss below 1.5%, suggesting that the coal combustion in the boiler runs successfully and is completed before the upper furnace zones.

    4. Case 2: Utility Boiler TENT A2 210 MWe

    The steam generator of the 210 MWe TPP 'Nikola Tesla "A2 in Obrenovac, Serbia is characterised with conventional" n "silhouette gas tract and with natural circulation, Fig. 13. The furnace is designed for thermal load of 3400-3500 kW / m2 of the intersection and for relatively long flame. Six coal mills and corresponding burners are disposed around the

    furnace in such manner to create tangential movement of gas-solid particles stream and to

    provide efficient fulfilment of the volume with hot gases, Fig. 14. Gas-fuel dust mixture with temperature of 165oC is transported from the mill into an inertial separator and through a vertical channel and eight rectangular channels is led to the jets. The furnace is 15.5 m wide, 13.5 m long and about 34 m high. To achieve the design temperature at the furnace outlet, which is 1000oC, it is necessary to maintain an average heat flux to the furnace membrane walls of ~ 340 kW / m2. The slag is lead away from the furnace bottom in solid state. The most important characteristics are given below:

    - steam production 650 t / h

    - steam pressure / temperature 138 bar / 540oC

    Fig. 13. Disposition of the boiler and burner intersection

    Fig. 14. Horizontal arrangement of the burners a - pulverized coal-air mixture ducts, b - secondary air ducts

    - feed water temperature 240oC

    - drum pressure 152 bar

    - boiler efficiency 0.85

    - fuel consumption 73.6 kg / s

    The boiler is designed to operate with lignite with lower calorific value of 5500-6700 kJ / kg, humidity (as received) 50-56% and ash content 14-22%. More detailed description of the boiler design, water-steam circulation scheme, operating modes and data on fuel properties are given in [16,24]. The fuel composition corresponding to the basic test case presented in this work (Kolubara lignite-field D), is given in Tab. 6. Pulverised coal sieve analysis is presented in Tab. 7 and the laboratory determined kinetic parameters of the fuel are given in Tab. 8 [16].

    Table 6. Kolubara lignite composition (field D)

    Proximate analysis (after the mills, at W = 14.00%),% Ultimate analysis,%

    Combustibles 65.60% Compo- nent As received After the mills

    Char 47.00% C 22.70 41.00

    Cfix 26.60% H 2.13 3.80

    Volatiles 39.00% S 0.61 1.11

    Moisture 14.00% O 10.40 19.70

    Ash 20.40% N 0.49

    <N O o 0.14% W 52.44 14.00

    S (total) 1.11% A 11.23 20.40

    Table 7. Coal fractions: Kolubara lignite, field D

    In this case, also, the Fluent CFD package, including Gambit graphical processor and prePDF pre-processor, was employed for creation of the furnace numerical model. The geometry is created in a 3-D domain, which represents the whole boiler furnace, including the superheaters installed in its upper zone, Fig. 15. The generated numerical mesh consists of 315555 volume cells, 796804 faces and 189939 nodes.

    Fraction 4 (0-50) | im,% (50-90) | im,% (90-200) l ^ m,% (200-500) | im,% >500 | im,% Mean particle diameter, | im

    Milling

    Fine, R90 = 48.4% 40.68 10.92 20.74 17.84 9.82 194

    Med., R90 = 60.15% 24.83 13.95 28.90 21.47 10.85 225

    Raw, R90 = 73.85% 7.55 18.60 31.43 25.10 17.32 295

    dpi, | im 25 70 145 350 850

    Table 8. Kinetic parameters of the Kolubara lignite, field D

    The numerical simulations of the TENT A2 210 MWe steam generator with the described model are performed at steady state operating conditions. The main results of this case study consist of flow fields, particles path lines, temperature contours, heat flux profiles to the furnace walls, contours of O2, CO2 and other species concentrations, as well as other variables. Figure 16a shows gas phase velocity vectors in horizontal furnace intersections.

    Kinetic parameters Combustion rate at тисячі двісті сімдесят три K, kr, m / s

    Pre-exponential coefficient A, m / s Activation energy E, kJ / kmol

    8.90-103 9.54-104 1.08

    a)

    b)

    Fig. 15. Furnace geometry and numerical grid

    Fig. 16. a) Velocity vectors at several horizontal intersections; b) Traces of coal particles

    Traces of particles thrown into the furnace from a single burner are presented in Fig. 16b. Simulation results of typical temperature distribution in vertical and horizontal intersections of the computational domain at boiler full load are presented in Fig. 17. The plots highlight the flame shape and high temperature spots outside the near-burner-flame boundaries.

    Fig. 17. Temperature contours in various vertical and horizontal intersections

    Figure 18a presents the temperature profile along the furnace height. When compared to the profile given in [16], it can be noticed that there is a fairly good qualitative agreement of the profiles shape and even the temperature maximum has been obtained by both approaches at the same vertical level, around or somewhat above 25.0 m . Horizontal temperature profiles

    Temperature, K Distance from the wall, m

    a) b)

    Fig. 18. a) Average temperature profile along the furnace height; b) Horizontal temperature profiles close to

    the right-hand wall at level 26.3 m

    from the right-hand wall to the furnace core at level 26.3 m, obtained by the model simulations and measurements, are presented in Fig. 18b. The diagram demonstrates good accordance between the modelling results and the measured ones [16].

    5. Conclusion

    The paper presents methodology used to model and simulate processes of tangential pulverised coal-fired boilers furnaces, based on CFD technology. The described method gives a possibility to investigate the operation of boilers in various modes and situations, with different load, as well as with redistribution of coal and air mass flow at the inlets, which

    would lead to certain changes of the flame position and other parameters. On a basis of comparisons with available site records a conclusion can be drawn that the model produces realistic insight into the furnace processes. Values ​​of temperature, heat flux and combustion efficiency are in expected limits, typical for the boiler types and for the coals used and, in general, they follow the trend line of measurements.

    The presented cases and other experiences with CFD modelling show that computational thermal modelling and analysis can successfully be applied to practical combustion systems. The procedure discussed in the paper and applied to utility boiler furnaces has wide band assertion applicability. The justification for this resides in the variety of processes and phenomena, which the CFD has already been shown to be able to handle.

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    Ключові слова: pulverised coal-fired boiler /cfd modelling /combustion /thermal radiation /heat transfer

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