DEVELOPMENT AND VALIDATION OF A COAL COMBUSTION MODEL FOR PULVERISED COAL COMBUSTION

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1 Proceedings of the 14th International Heat Transfer Conference IHTC14 August 8-13, 010, Washington, DC, USA IHTC14- DEVELOPMENT AND VALIDATION OF A COAL COMBUSTION MODEL FOR PULVERISED COAL COMBUSTION M.J. Chernetsky Kutateladze Institute of Thermophysics, Laboratory of Power Plant Aerodynamics, Russian Academy of Sciences, Siberian Branch, , 1, Acad. Lavrent ev Avenue, Novosibirsk, Russia A.A. Dekterev Kutateladze Institute of Thermophysics, Laboratory of Power Plant Aerodynamics, Russian Academy of Sciences, Siberian Branch, , 1, Acad. Lavrent ev Avenue, Novosibirsk, Russia Siberian Federal University, Krasnoyarsk, Russia ABSTRACT To fully understand the processes of heat-and-mass transfer on the laboratory-scale and full-scale coal boilers, computer models are needed to develop, which can predict flow fields, heat transfer and the combustion of the coal particles with reasonable accuracy. In the work reported here, a comprehensive model for pulverized coal combustion has been presented. Attention has been given to the char burnout submodel, x formation sub-model and accurate calculation of the temperature of the particles. The model predictions have been compared with the experimental measurements of the laboratory-scale pulverizedcoal combustion burner. INTRODUCTION Efficient and environment combustion of coal is of great concern and will remain so for decades to come. For some time now, strict environmental legislation and increasing competition has forced power generators to look for the most efficient conditions of combustion. This often implies expensive laboratory-scale testing of the coal combustion. More recently, as an alternative, the focus has been on investigating the modelling of pulverised fuel coal combustion in the hope that this method may be quicker and cheaper way of assessing coal combustion performance and optimising burner and furnace conditions. The use of computational fluid dynamics (CFD) models to describe the combustion of coal in utility combustion chambers has become an important aid in the design process. The use of CFD codes with advanced submodels for the more detailed treatment of devolatilisation, volatile combustion, and char burnout stages could make this possible, but very often we don t have knowledge of the detailed combustion processes for specific sort of coal. Because, some time sufficiently simplified model of coal combustion is necessity. This work presents model for single coal particle and comprehensive model for pulverized coal combustion in fired systems. This model was implemented in the in-house CFD code «σflow» [Dekterev et al., 003]. MATHEMATICAL MODEL In this paper, flow, combustion and heat transfer equations are solved to predict the boiler performances. All transport processes are represented by a general convective transport equation which could be written as ρφ + + Γ = t φ = {1, uvwh,,,, f, k, ε} ( ρ φ) ( φ) S v φ φ where φ is a dependent variable, Γ φ is the effective viscosity coefficient, and S ϕ is a source term. A SIMPLE-C method is employed to determine velocities and pressures of flow field. The gas composition consists of N, O, CO, CO, H O and complex of volatiles. The modified high-reynolds k-ε model of turbulence (Chen k-ε model) is used to describe the turbulent characteristics of flow. The Eddy Dissipation concept was proposed by Magnussen and Hjertager [1981] and it is used to calculate the gas phase reactions. The motion of coal particles in fluids is described in the Lagrangian approach by solving a set of ordinary differential equations along the trajectory. i 1 Copyright 010 by ASME

2 Stochastic separated flow (SSF) model is used to describe the particle dispersion. When the coal particle advances in the furnace, the reaction processes of vaporization, coal pyrolysis and char combustion are considered. The coal particle consists of four components: water, volatiles, carbon and ash. Vaporization of moisture from the coal particle is described by the diffusion-limited model. Coal pyrolysis is modelled by a simple, one-step mechanism and the volatile composition is assumed to be constant. The reaction rate of coal pyrolysis is taken from experimental data. Char combustion is controlled by the chemical surface reaction and the oxygen diffusion to the particle. This model includes the factor η which describes the transition between the char combustion regime limited by the rate of oxygen diffusion and the regime is sufficiently limited by the chemical reaction rate. Char particles are considered to burn at constant density and variable size. The diameter change of a particle follows: dδ = * Kc dτ ρ s к KC = βc (73/ T )* K S O g α 1 αk = if α < ηα 1 1 kkin. kdiff. + α α kkin. kdiff. α K = α if α > ηα k. diff k. kin k. diff α k. diff NuDD = δ Nu 0.Pe0.66 D = + E / RT α = K e к kkin.. к ρ k is the density of the char particle (kg m 3 ); K S C is the char combustion rate (kg m - *s -1 ); Nu D is the diffusion Nusselt number; D is the bulk molecular diffusion coefficient (m /s); α k,kin is the reaction-rate coefficient for a chemical reaction (ms -1 ), α k,diff is the reaction-rate coefficient for diffusion (ms -1 ) The instantaneous burning rate of an individual particle is determined from temperature, velocity, and size information by solving the energy balance for the particle, assuming a spherical, homogeneous, reacting particle surrounded by a chemically frozen boundary layer (i.e., single-film model). Heat losses from convection and radiation are considered, as well as the effects Stefan flow: mc dt QH = εσ( T T ) + α ( T T ) + 4π 4 p p p 4 4 rad p conv p rp πrp α conv is the convection heat transfer coefficient: Nuλ α conv = r The correction to the heat-transfer equation due to Stefan flow is provided modification of Nusselt number : Pe 37 PePe Nu = + Pe Pe where Pe is a Peclet number, Pe is a modified version of the Peclet number (i.e., the ratio of the convective velocity of the net mass leaving the particle surface to the diffusive velocity of heat leaving the surface). When char burn, heat losses from convection is much larger than that is predicted for not burning particles. In the model is used correlation coefficient K comb [Babiy V.I., Kuvaev Y.F., 1986]: comb α conv comb αconv = αconvк comb, αconv p Kcomb 5000 T e g = are convective heat transfer coefficients for burning and not burning coal particle respectively. Three mechanisms are identified for the production of x in combustion systems: termal, fast and fuel. For the reaction kinetics of thermal formation, the Zeldovich mechanism is used. For the fast formation, the Fenimore s model [Fenimore, 1979] is used. For the reaction kinetics of fuel formation, the modified DeSoete mechanism [Magel et al., 1996] is used. Additional equations for and intermediate hydrogen cyanide HCN transfer is given by: ρ f t ρ f t HCN ( ρv ) ( ) + f = D f + S ( ρv ) ( ) + f = D f + S HCN HCN HCN The source term in the equation of transfer describing thermal mechanism [Zeldovich, 1946] is expressed as: S =M thermal-x [ ] Copyright 010 by ASME

3 where d[]/ is given by: ( ) [ ] [ O] k1k [ O ][ N ]-k-1k- [ ] = k [ O ] + k [ ] -1 Assuming that the oxygen atom density [O] is at partial equilibrium, [O] defined as: 1/ [ O] = 36.64T [ O ] 1/ exp( -713/ T) the reaction rate expressed in Arrhenius form as - ( T) ( ) 8 k1 = exp / 7 k-1 = exp -45/ T 4 k = T exp( / T) 3 k = T exp -080 / T ( ) Prompt - hydrocarbon radicals produced when the fuels being burned react with N to produce HCN or CN which subsequently be oxidized to. Under most practical combustion conditions, the contribution of the prompt to total formation is small. For the fast formation, Fenimore s model is used [Fenimore, 1979]. The source term in the equation of transfer can be written as S = M [ ] prompt- x, Where d[]/ is given by: where [ ] a Ea = kpr [ O] [ N][ VOL] exp -, RT kpr Ea a+ ( RT ) 1 7 = / p = 60 kcal mol parameter a depends on the flame conditions [de Soete, 1975]. The fuel x is a result of the reaction between oxygen and fuel nitrogen. In the process of coal fuel gasification and char burn there takes place the transformation of nitrogen containing compounds to NH 3 (ammonia) and HCN (hydrogen cyanide). Depending on the scheme of chemical reactions between these compounds and combustion gases, formation of or N takes place. The modified de Soete model [Magel et al., 1996], consisting of three global reactions, is realized to calculate the fuel x : 1 dx dx dx RESULTS HCN HCN exp( 3370 / T) xhcn = exp( 3000 / T) xhcn = a O exp( 9466 / T) xxcnh m = Applicability of the coal combustion model was validated by comparing its predictions with experimental data of single coal particle combustion [Babiy V.I. and Kuvaev Y.F.,1986]. The particles of the anthracite and the Nazarovo brown coal were considered. The simulation was carried out for different diameters of particles (0-800 μm), different temperatures ( К) of gas and various oxygen concentrations (5-1%). In all cases, results of single particle combustion model were in good agreement with the experimental data. Applicability of the comprehensive model was validated by comparing its predictions with the experimental data of a laboratory-scale pulverized-coal combustion burner of Siberian Thermal Engineering Institute (SibVTI) [ SibVTI, 1996]. The furnace has a length of 6.0 m and an internal diameter of 0.4 m. The primary air with pulverized coal is injected in the burner centre. The secondary air surrounds the primary air. The Irsha- Borodin coal is used. The operating parameters are presented in Table 1. The analysis of the coal used is shown in Table. The calculation results are shown for aerodynamics and heat exchange in a combustion chamber. These results were calculated for excess air coefficient equal Maximum gas temperatures within the furnace are 155 C by the calculations and 150 C by the measurements. Graphic 1 shows the temperature of gas along the furnace. As it can be seen, the comprehensive model provides reasonable agreement with the experimental data. The concentrations of O, CO along the furnace are illustrated in Graphic. The computed concentrations of O, CO is in agreement with the measurements.. The calculation results for excess air coefficient equal 1. are shown in Graphic. 3. Graphic 4 shows concentration x (O 6%) along the furnace. Table 1 Operating Conditions Of The Laboratory-scale Pulverized-coal Combustion Burner Fuel rate (kg/h) 60 Total air flow (Nm 3 /h) 465 Temperature secondary air (ºС) 05 Temperature primary air (ºС) 15 Excess air coefficient 1.34 x x 3 Copyright 010 by ASME

4 Table Proximate And Ultimate Analysis Of The Irsha-Borodin Coal Proximate and ultimate analysis (%) MJ/kg W r A d V daf C daf H daf S d N daf Q r a) b) Figure Calculation Results For Excess Air Coefficient Equal 1.34: a) Velocity Field Of Boiler In Vertical Section b) Temperature Field In Vertical Section Figure 1 The Laboratory-scale Pulverized-coal Combustion Burner 100 Experiment Temperature, 0 C Calculation results(along center of furnace) T, 0 C Calculation results(average value along furnace) T, 0 C Experiment CO Experiment O Calculation results (along center of furnace) CO Calculation results (average value along furnace) CO Calculation results (along center of furnace) O Calculation results (average volue along furnace) O Temperature, 0C Concentration, m3/m Graphic 1 Temperature ( C) Along The Furnace (alfa=1.34) Graphic Concentration CO, O Along The Furnace (alfa=1.34) 4 Copyright 010 by ASME

5 Temperature, 0 C Experiment Calculation results(along center of furnace) T, 0 C Calculation results(average value along furnace) T, 0 C 1100 Concentration x, mg/m 3 (O 6%) Calculation results (alfa=1.) Calculation results (alfa=1.34) Experiment (alfa=1.) Experiment (alfa=1.34) Temperature, 0C x, mg/m 3 (O 6%) Graphic 3 Temperature ( C) Along The Furnace (alfa=1.) Graphic 4 Concentration x, (mg 3 /m 3 ) along the furnace (O 6%) 6 CONCLUSIONS 1. A mathematical model of coal combustion in the pulverized coal-fired boiler has been developed.. Applicability of the coal combustion model has been validated by comparing its predictions with experimental data of single coal particle combustion. Applicability of the comprehensive model has been validated by comparing its predictions with the experimental data of a laboratory-scale pulverized-coal combustion burner. 3. The present model has been integrated in the in-house CFD code «σflow» and this integration is increased the performance capabilities optimization for a design and an operations of pulverized coal-fired boilers. MENCLATURE u, v, w : Gas velocity m/s h : Specific enthalpy J/kg T : Temperature K ρ : Gas density kg/m 3 k : Turbulence energy m /s ε : Turbulent dissipation rate m /s 3 Subscripts j : Gas species p : Particle REFERENCES 1. Dekterev A.A., Gavrilov A.A., Harlamov E.B., Litvintcev K.Y., (003), Application of CFD code σflow for numerical investigation of technological facilities, J.Computation Technologies, Vol. 8, Part 1, pp (in Russian). Chen, Y.S., and Kim, S.W., (1987), Computation of turbulent flows using an extended k-ε turbulence closure model, NASA CR Magnussen, B.F., and Hjertager, B.W., (1981), On the structure of turbulence and a generalised eddy dissipation concept for chemical reaction in turbulent flow, 19th AIAA Aerospace Meeting, St. Louis, USA. 4. Babiy V.I., Kuvaev Y.F., 1986, Pulverized coal particles combustion and calculation of coal-dust flame, Energoatomizdat, Moscow (in Russian) 5. Zeldovich Y.B., (1946), The oxidation of nitrogen in combustion and explosions, Acta Physicochim. USSR 1:577. (in Russian) 6. Fenimore C.P., (1979), Studies of fuel-nitrogen in rich flame gases, 17th Symp. (Intl.) Comb., The Combustion Institute, Pittsburgh, P Magel H. C., Greul U., Schnell U., Spliethoff H., Hein K.R.G., 1996, x- reduction with staged combustion - comparison of experimental and modeling results, In Proc. 5 Copyright 010 by ASME

6 Joint Meeting of the Portuguese, British, Spanish and Swedish 9. Report Development of technical innovation for Section of the Combustion Institute, Madeira. the 500 t/h boiler design, Krasnoyarsk: 1996, SibVTI (in 8. G. G. De Soete, (1975), Overall Reaction Rates of and Russian) N Formation from Fuel Nitrogen, In 15th Symp. (Int'l.) on Combustion, page The Combustion Institute. 6 Copyright 010 by ASME