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Numerical calculations of the WR-40 boiler with a furnace jet boiler system
Energy xxx (2015) 1e13
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Numerical calculations of the WR-40 boiler with a furnace jet boiler system Bartłomiej Hernik Institute of Power Engineering and Turbomachinery, Silesian University of Technology in Gliwice, ul. Konarskiego 18, 44-100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 12 May 2015 Accepted 26 May 2015 Available online xxx
The aim of this work was to develop a numerical model of combustion in the furnace of a WR-40 boiler equipped with a FJBS (furnace jet boiler system). Based on previous calculations [1] and measurements [2], appropriate competent sub-numerical models were developed to match a temperature value and composition of the flue gas obtained with measurements taken at the real working boiler. The FJBS System has eight in-furnace jet-pump ventilators that uses gas propulsion (compressed air). Its task is to mix the flue-gas and any substrates of combustion at vortex, fire it up, and alignment align the temperature field. Numerical modelling results for the combustion chamber of the WR-40 boiler was shown. The concentration of O2, CO, CO2 in the flue gas, the temperature of flue gas for the whole combustion chamber, the change of the mass fraction of H2O in the coal, and the change in the mass of coal particles along the grate were measured to demonstrate that the processes taking place during combustion of fuel on the grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). Numerical calculations confirm measurement results qualitatively rather than quantitatively. Generally, the CFD (Computational Fluid Dynamics) modelling confirmed the conclusions from measurements. Obtaining reliable temperatures at the furnace chamber outlet using numerical modelling is possible based on the 0-dimensional model of the boiler. In order to get reliable results from numerical modelling, it is necessary to validate the model with real measurement data. The results obtained by numerical simulation show that the FJBS system aligns the field temperature and concentration and decreases CO in flue gas. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Numerical modelling Boiler Grate Combustion Jet
1. Introduction The values obtained by means of measurements [2] of the WR40 boiler were compared with the results of numerical calculations performed using Ansys.Fluent [3] to verify the numerical model and to select the sub-models. Authors experience with previous calculation of this boiler [1] was also helpful in selecting appropriate competent sub-numerical models. The average temperature of flue gases at the furnace outlet toutlet was obtained using the classical 0-dimensional balance model of a boiler [4,5]. The 0dimensional model of the boiler as inputs need for the calculations (considered as correct) the mass flows of water, steam and injection, the pressure of agents, the temperature of agents at the boiler inlet obtained by measurements. The 0-dimensional model is fully balanced in comparison with measurements which are always encumbered with unavoidable errors. The average temperature of
the flue gases at the furnace outlet achieved using it may be considered as highly reliable. The methodology used in the 0dimensional model was showed in Refs. [6e8]. The combustion chamber is calculated employs the CKTI (Centralny KotÅ,owo-Turbinowy Instytut (in Polish), in English means Central Institute of Boilers and Turbine) method [6,7]. Russian studies was the base to develop that method in Poland generally used. The calculations of the convection part of the boiler used a method based on studies conducted at the Silesian University of Technology [8]. The WR-40 boiler (Fig. 1) is a two-pass, radiant, stoker-fired boiler fed with hard coal. Primary air is supplied from below the stoker, and secondary air I and II is fed by a set of nozzles over the furnace ignition arch. The furnace has a height of 9096 mm, and the dimensions of the grate is 6440 6770 mm. The FJBS (furnace jet boiler system) has eight in-furnace jet-pump ventilators with gas propulsion (compressed air). Its task is to mix the flue-gas and any substrates of combustion at the vortex and to fire up it and align the temperature field.
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2015.05.137 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
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B. Hernik / Energy xxx (2015) 1e13
Fig. 1. Contour of the WR-40 boiler.
2. Modelling description and boundary conditions The numerical model's calculation of the contour of the furnace chamber of the WR-40 boiler with FJBS is shown in Fig. 2. The FJBS composed of eight jet-pump ventilators is shown in Figs. 2 and 3.
The geometrical model and the numerical mesh composed of 1,431,936 numerical cells are presented in Fig. 4. Measurements of the concentrations of O2, CO, CO2, and temperature of flue gas was done only at the measured planes [2] e see Fig. 2. However, the classical 0-dimensional balance model of a
Fig. 2. Drawing of the WR-40 boiler furnace chamber with FJBS.
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3
Fig. 3. Drawing of the one jet-pumps ventilators.
Fig. 4. Numerical mesh of the WR-40 boiler furnace chamber.
Table 2 The boiler operating parameters.
Table 1 Coal analysis (as-received state). Qri
Ar
Wrt
Cr
Hr
Sr
Or
Nr
kJ/kg
%
%
%
%
%
%
%
22,114
21.5
11.2
57
3.9
1.1
4.8
0.6
Data
Unit
Value
Coal mass flow Primary air mass flow Secondary air I mass flow Secondary air II mass flow FJBS air mass flow Primary air temperature Secondary air temperature FJBS air temperature
kg/s kg/s kg/s kg/s kg/s C C C
2.22 20.5 1.04 1.04 0.016 150 80.00 30
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Table 3 Assumptions of the numerical model. Two-phase model Turbulence model Combustion model
For a coal particle e devolatilization
e combustion
Radiation model
Eulere Lagrange [14,15,18] k-ε real [9e12,17,18] Finite-Rate/Eddy-Dissipation Model [9,11e13,17e19] Reaction k1 Reaction k2 A ¼ 2.119e þ 11 A ¼ 2.239e þ 12 E ¼ 2.027e þ 8 J/kmol E ¼ 1.7e þ 8 J/kmol Flue gas absorptivity: wsggm-domain-based model, calculates absorptivity based on the concentration of CO2 and H2O [12e14,19] Single-rate model A ¼ 382,000 E ¼ 7.4 e þ 7 J/kmol Kinetic-diffusion model [14,16] C1 ¼ 5e þ 12 C2 ¼ 0.002 EA ¼ 7.9 e þ 7 J/kmol DO [9,13,17e19] Flow Iterations per radiation iteration: 5 Theta divisions: 4 Theta pixels: 3 Phi divisions: 4 Phi pixels: 3
boiler [4,5] was used to obtain average temperature of flue gases at the furnace outlet e toutlet.The concentration values of O2 and CO at the measured planes was achieved using an Ultramat 23 analyser. The modelling input data implemented into Ansys.Fluentc are presented below in Tables 1 and 2. The steady-state governing equations were solved using the SIMPLE algorithm [10,18,19]. As defined in the model, the first process on the grate is the drying of wet coal, the second process is the devolatilization of coal, and then combustion of coal follows. Combustion of the volatiles depends on their volumetric reactions. Table 3 presents the numerical model assumptions. Below, the two overall reactions of the volatile fraction combustion is presented [9] (m, n, l, k e based on coal composition):
Cm Hn Ol Nk þ
ml n n k k1 þ O !mCO þ H2 O þ N2 2 4 2 2 2
k2
CO þ 0:5O2 !CO2
(1)
(2)
Coal as injection's in DPM (Discrete Phase Model) is fed onto the grate. Injection is provided as surface at front of boiler below Secondary air I nozzles. The temperature of wall of ignition arch was set to 970 K. Coals particle is dried, devolatilized, burned according to appropriate models e see Table 3. Porous zone was set above the grate to extend resistance time of coal particle in devolatilized and burned zone. 3. Results of numerical modelling The results of numerical calculations of combustion chamber of WRe40 boiler is shown. The first stage included building the numerical model with no working FJBS and CFD (Computational Fluid Dynamics) calculations. However, the second stage concerns numerical modelling of furnace with working FJBS. The O2, CO, and CO2 concentration in the flue gas, the temperature of the flue gas for the whole combustion chamber, the change of mass fraction of H2O in the coal, and the change in the mass of coal particles along the grate was shown. This demonstrates that the processes taking place during combustion of fuel on the grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). What proves the selected numerical assumptions (sub models and constants adopted in the model) was taken correctly. 3.1. The first stage e model with no working FJBS Fig. 5 shows the mass fraction of oxygen in the planes of the central boiler. Its contents were in a layer of coal on the grate and
Fig. 5. The mass fraction of O2 in the central planes on x and z axis, without FJBS.
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Fig. 6. The mass fraction of CO in the central planes on x and z axis, without FJBS.
decreased as a result of its participation in the process of burning coal particles (i.e., from the bottom, under the grate where the primary air is fed). Over the coal layer the oxygen content is decreased in the exhaust gas due to the oxidation of CO to CO2. Close to the right boiler wall is too much oxygen because of smaller utilization of O2 in combustion process of CO. The devolatilization process occurred more intensive near opposite wall (see Fig. 9)
leading to generated higher concentration of CO in flue-gas (see Fig. 6). Fig. 6 shows the mass fraction of CO in the planes of the central boiler. You can see the highest concentration in a region of the degassing process of the carbon particles. Decreases in the CO content of the flue gas results from the oxidation of CO to CO2. High concentrations of CO over a carbon layer on the grate is the
Fig. 7. The mass fraction of CO2 in the central planes on x and z axis, without FJBS.
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Fig. 8. The temperature of flue gas [K] in the central planes on x and z axis, without FJBS.
effect of the combustion reaction of volatile matter (see Equation (1)). Fig. 7 shows the mass fraction of carbon dioxide in the central planes of the boiler. The mass fraction of CO2 in the flue gas in the model is determined from the CO oxidation reaction. The temperature field in the planes of the central boiler is shown in Fig. 8. The temperature of combustion of the carbon particles on the grate does not exceed 1000 C; this is lower than the softening temperature of the ash for the Polish coal. The highest temperature occurs in the combustion of volatiles.
Fig. 9 shows the coal devolatilization [kg/s] in the central planes of the boiler. The devolatilization process occurred before combustion zone. 3.2. The second stage e model with working FJBS The mass fraction of oxygen in the planes of the central boiler was showed on Fig. 10. The results of numerical calculations is very similar as no working FJBS case. Its contents were in a layer of coal on the grate and decreased as a result of its participation in the process
Fig. 9. The coal devolatilization [kg/s] in the central planes on x and z axis, without FJBS.
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Fig. 10. The mass fraction of O2 in the central planes on x and z axis, with FJBS.
of burning coal particles. Over the coal layer the oxygen content is decreased in the exhaust gas due to the oxidation of CO to CO2. Fig. 11 shows the mass fraction of CO in the planes of the central boiler. Such as on Fig. 6, the highest concentration in a region of the degassing process of the carbon particles. Decreases in the CO content of the flue gas results from the oxidation of CO to CO2. High concentrations of CO over a carbon
layer on the grate is the effect of the combustion reaction of volatile matter (see Equation (1)). As you can see combustion process on the grate and in the region over the grate but below FJBS level is very similar for two consider cases. Fig. 12 shows the mass fraction of carbon dioxide in the central planes of the boiler. The mass fraction of CO2 in the flue gas in the model is determined from the CO oxidation reaction.
Fig. 11. The mass fraction of CO in the central planes on x and z axis, with FJBS.
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Fig. 12. The mass fraction of CO2 in the central planes on x and z axis, with FJBS.
The temperature field in the planes of the central boiler is shown in Fig. 13. The temperature of combustion of the carbon particles on the grate does not exceed 1000 C; this is lower than the softening temperature of the ash for the Polish coal. The highest temperature occurs in the combustion of volatiles. For case with FJBS the change
of mass fraction of H2O in the coal (Fig. 14), and the change in the mass of coal particles along the grate (Fig. 15) was shown. However, what was pointed before combustion process is very similar for cases with and without FJBS, because of FJBS has no influence on process occurring below its level. This demonstrates that the processes taking place during combustion of fuel on the
Fig. 13. The temperature of flue gas [K] in the central planes on x and z axis, with FJBS.
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Fig. 14. The change of mass fraction of H2O in the coal on the grate, with FJBS.
grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). The velocity on the outlet of jet-pumps ventilators was showed on Fig. 16 and the temperature of flue gas [K] in the outlet window plane, without FJBS was presented on Fig. 17. 4. Comparison of CFD modelling with measurements The measurements [2] showed that FJBS homogenizes the temperature profile of the flue gas and the concentration of CO and
O2, and it also decreases the concentration of CO in boiler e see Figs. 18, 20 and 22. The numerical calculations confirm the measurements qualitatively rather than quantitatively. Generally, CFD modelling confirmed the conclusions from measurements. To make it easier for comparison, the results of the CFD calculations (Figs. 19, 21 and 23) and measurement results are presented below. Measurement ranges were conditioned by the length of the probe so that numerical results are presented in the same range. The temperature at the outlet of the combustion chamber (see Fig. 2) obtained from the 0-dimensional model of the boiler (based
Fig. 15. The change of coal's particle mass in [kg] on the grate, with FJBS.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
Fig. 16. The velocity [m/s] on the outlet of jet-pumps ventilators, with FJBS.
Fig. 17. The temperature of flue gas [K] in the outlet window plane, without FJBS.
Fig. 18. The temperature of flue gas [K] in the measured planes obtained from measurements, left with FJBS, right without FJBS.
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Fig. 19. The temperature of flue gas [K] in the measured planes obtained from CFD, left with FJBS, right without FJBS.
Fig. 20. The mass fraction of O2 in the measured planes obtained from measurements, left with FJBS, right without FJBS.
Fig. 21. The mass fraction of O2 in the measured planes obtained from CFD, left with FJBS, right without FJBS.
on the data obtained from the measurement of the boiler) and obtained CFD modelling (see Fig. 17) was shown in Table 4. 5. Conclusions 1. Obtaining a reliable temperature at the furnace chamber outlet by means of numerical modelling is possible using a 0-
dimensional boiler model based on the data obtained from the boiler measurement. 2. This paper showed that development of a CFD model should be used with measurements for validation (i.e., to select proper submodels), then the numerical model will generate real values. 3. The CFD modelling in this paper showed that FJBS homogenizes the temperature profile of the flue gas and concentration
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Fig. 22. The mass fraction of CO in the measured planes obtained from measurements, left with FJBS, right without FJBS.
Fig. 23. The mass fraction of CO in the highest measured planes obtained from CFD, left with FJBS, right without FJBS.
Table 4 The temperature of the flue gas in the outlet of the furnace. Data
Unit
0-dimensional model
CFD model
toutlet
982
982
C
of CO and O2 and also decreases the concentration of CO in the boiler.
Acknowledgements Investigations presented in this work were partially financed within the KIC InnoEnergy innovation project on Efficient Coal Fired Stoker Boiler (EcoStoker).
References [1] Hernik B. Numerical calculations of the WR-40 boiler based on the boiler zerodimensional model. Chem Process Eng 2014;35(2):173e80. http://dx.doi.org/ 10.2478/cpe-2014-0013. wnanie [2] Kalisz S, Wejkowski R, Ostrowski P, Gramatyka F, Pronobis M. Wyro l temperatury i ste˛ ze _ n składniko w spalin w komorze paleniskowej kotła po rusztowego za pomoca˛ nape˛ du strumienicowego FJBS. In: III Konferencja łczesne Technologie i Urza˛ dzenia Energetyczne”, Naukowo-Techniczna “Wspo w, 10; 2013. p. 15e8. Krako [3] Ansys Fluent. Computational fluid dynamics, Ansys Inc., England. [4] Hernik B, Pronobis M. A zero-dimensional model used as a basis for numerical modelling of OP-650 boiler. Archives Energetics 2012;42:17e26.
[5] Hernik B, Pronobis M. Verification of the boiler numerical model with a zerodimensional model. In: Stepan G, editor. Gepeszet, proceedings of the eight international conference on mechanical engineering, Budapest, Hungary, may 24-25, 2012. Budapest: University of Technology and Economics. Faculty of Mechanical Economics; 2012. p. 612e23. CD-ROM. ski W, Szwarc E. Kotły parowe [Steam boilers]. WNT [6] Orłowski P, Dobrzan Warsaw. 1979. [7] Kuznetsov NW, Mitor WW, Dubovski IE, Karasina ES. Standards for thermal calculations of power boilers [Teplovoi raschet kotelnyh agregatov]. Moskva: Energia; 1973 [inRussian]. [8] Pronobis M. Wymiana ciepła w zanieczyszczonych powierzchniach konwekw [Heat transfer in boiler convection surfaces with fouling]. Book cyjnych kotło ˛ skiej, series: Energetyka; 1992. 115. Gliwice: Zeszyty Naukowe Politechniki Sla [9] Yin Ch, Rosendahl L, Clausen S, Hvid SL. Characterizing and modeling of an 88MWgrate-fired boiler burning wheat straw: experience and lessons. Energy 2012;41:473e82. http://dx.doi.org/10.1016/j.energy.2012.02.050. [10] Zhang X, Chen Q, Bradford R, Sharifi V, Swithenbank J. Experimental investigation and mathematical modelling of wood combustion in a moving grate boiler. Fuel Process Technol 2010;91:1491e9. [11] Dong W, Blasiak W. CFD modeling of ecotube system in coal and biomass grate combustion. In: The 2nd international symposium on advanced energy conversion systems and related technologies, Dec.1e3; 1998. [12] Blasiak W, Yang WH, Dong W. Combustion performance improvement of grate fired furnaces using ecotube system. J Energy Inst 2006;79(2):67e74. [13] Hajek J, Jurena T. Modelling of 1 MW solid biomass combustor: simplified balance-based bed model coupled with freeboard CFD simulation. Chem Eng Trans 2012;29:745e50. http://dx.doi.org/10.3303/CET1229125. [14] Eaton AM, Smoot LD, Hill SC, Eatough CN. Components, formulations, solutions, evaluation, and application of comprehensive combustion models. Prog Energy Combust Sci 1999;25:387e436. http://dx.doi.org/10.1016/S03601285(99)00008-8. [15] He B, Zhu L, Wang J, Liu S, Liu B, Cui Y, et al. Computational fluid dynamics based retrofits to reheater panel overheating of no. 3 boiler of dagang power plant. Comput Fluids 2007;36:435e44. http://dx.doi.org/10.1016/ j.compfluid.2005.09.005.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
B. Hernik / Energy xxx (2015) 1e13 [16] Williams A, Backreedy R, Habib R, Jones JM, Pourkashanian M. Modelling coal combustion:the current position. Fuel 2002;81:605e18. http://dx.doi.org/ 10.1016/S0016-2361(01)00158-2. [17] Yin Ch, Rosendahl L, Kær SK, Clausen S, Hvid SL, Hille T. Mathematical modeling and experimental study of biomass combustion in a thermal 108 MW grate-fired boiler. Energy Fuels 2008;22:1380e90. http://dx.doi.org/10.1021/ef700689r.
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[18] Scharler R, Obernberger I. Numerical modelling of biomass grate furnaces. In: Proceedings of the fifth European conference on industrials furnaces and boilers, Rio Tinto, Portugal, ISBN 972-8034-04-0. INFUB. [19] Kær SK, Rosendahl L, Baxter LL. Towards a CFD-based mechanistic deposit formation model for straw-fired boilers. Fuel 2006;85:833e48. http:// dx.doi.org/10.1016/j.fuel.2005.08.016.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Numerical calculations of the WR-40 boiler with a furnace jet boiler system Bartłomiej Hernik Institute of Power Engineering and Turbomachinery, Silesian University of Technology in Gliwice, ul. Konarskiego 18, 44-100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 12 May 2015 Accepted 26 May 2015 Available online xxx
The aim of this work was to develop a numerical model of combustion in the furnace of a WR-40 boiler equipped with a FJBS (furnace jet boiler system). Based on previous calculations [1] and measurements [2], appropriate competent sub-numerical models were developed to match a temperature value and composition of the flue gas obtained with measurements taken at the real working boiler. The FJBS System has eight in-furnace jet-pump ventilators that uses gas propulsion (compressed air). Its task is to mix the flue-gas and any substrates of combustion at vortex, fire it up, and alignment align the temperature field. Numerical modelling results for the combustion chamber of the WR-40 boiler was shown. The concentration of O2, CO, CO2 in the flue gas, the temperature of flue gas for the whole combustion chamber, the change of the mass fraction of H2O in the coal, and the change in the mass of coal particles along the grate were measured to demonstrate that the processes taking place during combustion of fuel on the grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). Numerical calculations confirm measurement results qualitatively rather than quantitatively. Generally, the CFD (Computational Fluid Dynamics) modelling confirmed the conclusions from measurements. Obtaining reliable temperatures at the furnace chamber outlet using numerical modelling is possible based on the 0-dimensional model of the boiler. In order to get reliable results from numerical modelling, it is necessary to validate the model with real measurement data. The results obtained by numerical simulation show that the FJBS system aligns the field temperature and concentration and decreases CO in flue gas. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Numerical modelling Boiler Grate Combustion Jet
1. Introduction The values obtained by means of measurements [2] of the WR40 boiler were compared with the results of numerical calculations performed using Ansys.Fluent [3] to verify the numerical model and to select the sub-models. Authors experience with previous calculation of this boiler [1] was also helpful in selecting appropriate competent sub-numerical models. The average temperature of flue gases at the furnace outlet toutlet was obtained using the classical 0-dimensional balance model of a boiler [4,5]. The 0dimensional model of the boiler as inputs need for the calculations (considered as correct) the mass flows of water, steam and injection, the pressure of agents, the temperature of agents at the boiler inlet obtained by measurements. The 0-dimensional model is fully balanced in comparison with measurements which are always encumbered with unavoidable errors. The average temperature of
the flue gases at the furnace outlet achieved using it may be considered as highly reliable. The methodology used in the 0dimensional model was showed in Refs. [6e8]. The combustion chamber is calculated employs the CKTI (Centralny KotÅ,owo-Turbinowy Instytut (in Polish), in English means Central Institute of Boilers and Turbine) method [6,7]. Russian studies was the base to develop that method in Poland generally used. The calculations of the convection part of the boiler used a method based on studies conducted at the Silesian University of Technology [8]. The WR-40 boiler (Fig. 1) is a two-pass, radiant, stoker-fired boiler fed with hard coal. Primary air is supplied from below the stoker, and secondary air I and II is fed by a set of nozzles over the furnace ignition arch. The furnace has a height of 9096 mm, and the dimensions of the grate is 6440 6770 mm. The FJBS (furnace jet boiler system) has eight in-furnace jet-pump ventilators with gas propulsion (compressed air). Its task is to mix the flue-gas and any substrates of combustion at the vortex and to fire up it and align the temperature field.
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2015.05.137 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
2
B. Hernik / Energy xxx (2015) 1e13
Fig. 1. Contour of the WR-40 boiler.
2. Modelling description and boundary conditions The numerical model's calculation of the contour of the furnace chamber of the WR-40 boiler with FJBS is shown in Fig. 2. The FJBS composed of eight jet-pump ventilators is shown in Figs. 2 and 3.
The geometrical model and the numerical mesh composed of 1,431,936 numerical cells are presented in Fig. 4. Measurements of the concentrations of O2, CO, CO2, and temperature of flue gas was done only at the measured planes [2] e see Fig. 2. However, the classical 0-dimensional balance model of a
Fig. 2. Drawing of the WR-40 boiler furnace chamber with FJBS.
Please cite this article in press as: Hernik B, Numerical calculations of the WR-40 boiler with a furnace jet boiler system, Energy (2015), http:// dx.doi.org/10.1016/j.energy.2015.05.137
B. Hernik / Energy xxx (2015) 1e13
3
Fig. 3. Drawing of the one jet-pumps ventilators.
Fig. 4. Numerical mesh of the WR-40 boiler furnace chamber.
Table 2 The boiler operating parameters.
Table 1 Coal analysis (as-received state). Qri
Ar
Wrt
Cr
Hr
Sr
Or
Nr
kJ/kg
%
%
%
%
%
%
%
22,114
21.5
11.2
57
3.9
1.1
4.8
0.6
Data
Unit
Value
Coal mass flow Primary air mass flow Secondary air I mass flow Secondary air II mass flow FJBS air mass flow Primary air temperature Secondary air temperature FJBS air temperature
kg/s kg/s kg/s kg/s kg/s C C C
2.22 20.5 1.04 1.04 0.016 150 80.00 30
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Table 3 Assumptions of the numerical model. Two-phase model Turbulence model Combustion model
For a coal particle e devolatilization
e combustion
Radiation model
Eulere Lagrange [14,15,18] k-ε real [9e12,17,18] Finite-Rate/Eddy-Dissipation Model [9,11e13,17e19] Reaction k1 Reaction k2 A ¼ 2.119e þ 11 A ¼ 2.239e þ 12 E ¼ 2.027e þ 8 J/kmol E ¼ 1.7e þ 8 J/kmol Flue gas absorptivity: wsggm-domain-based model, calculates absorptivity based on the concentration of CO2 and H2O [12e14,19] Single-rate model A ¼ 382,000 E ¼ 7.4 e þ 7 J/kmol Kinetic-diffusion model [14,16] C1 ¼ 5e þ 12 C2 ¼ 0.002 EA ¼ 7.9 e þ 7 J/kmol DO [9,13,17e19] Flow Iterations per radiation iteration: 5 Theta divisions: 4 Theta pixels: 3 Phi divisions: 4 Phi pixels: 3
boiler [4,5] was used to obtain average temperature of flue gases at the furnace outlet e toutlet.The concentration values of O2 and CO at the measured planes was achieved using an Ultramat 23 analyser. The modelling input data implemented into Ansys.Fluentc are presented below in Tables 1 and 2. The steady-state governing equations were solved using the SIMPLE algorithm [10,18,19]. As defined in the model, the first process on the grate is the drying of wet coal, the second process is the devolatilization of coal, and then combustion of coal follows. Combustion of the volatiles depends on their volumetric reactions. Table 3 presents the numerical model assumptions. Below, the two overall reactions of the volatile fraction combustion is presented [9] (m, n, l, k e based on coal composition):
Cm Hn Ol Nk þ
ml n n k k1 þ O !mCO þ H2 O þ N2 2 4 2 2 2
k2
CO þ 0:5O2 !CO2
(1)
(2)
Coal as injection's in DPM (Discrete Phase Model) is fed onto the grate. Injection is provided as surface at front of boiler below Secondary air I nozzles. The temperature of wall of ignition arch was set to 970 K. Coals particle is dried, devolatilized, burned according to appropriate models e see Table 3. Porous zone was set above the grate to extend resistance time of coal particle in devolatilized and burned zone. 3. Results of numerical modelling The results of numerical calculations of combustion chamber of WRe40 boiler is shown. The first stage included building the numerical model with no working FJBS and CFD (Computational Fluid Dynamics) calculations. However, the second stage concerns numerical modelling of furnace with working FJBS. The O2, CO, and CO2 concentration in the flue gas, the temperature of the flue gas for the whole combustion chamber, the change of mass fraction of H2O in the coal, and the change in the mass of coal particles along the grate was shown. This demonstrates that the processes taking place during combustion of fuel on the grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). What proves the selected numerical assumptions (sub models and constants adopted in the model) was taken correctly. 3.1. The first stage e model with no working FJBS Fig. 5 shows the mass fraction of oxygen in the planes of the central boiler. Its contents were in a layer of coal on the grate and
Fig. 5. The mass fraction of O2 in the central planes on x and z axis, without FJBS.
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Fig. 6. The mass fraction of CO in the central planes on x and z axis, without FJBS.
decreased as a result of its participation in the process of burning coal particles (i.e., from the bottom, under the grate where the primary air is fed). Over the coal layer the oxygen content is decreased in the exhaust gas due to the oxidation of CO to CO2. Close to the right boiler wall is too much oxygen because of smaller utilization of O2 in combustion process of CO. The devolatilization process occurred more intensive near opposite wall (see Fig. 9)
leading to generated higher concentration of CO in flue-gas (see Fig. 6). Fig. 6 shows the mass fraction of CO in the planes of the central boiler. You can see the highest concentration in a region of the degassing process of the carbon particles. Decreases in the CO content of the flue gas results from the oxidation of CO to CO2. High concentrations of CO over a carbon layer on the grate is the
Fig. 7. The mass fraction of CO2 in the central planes on x and z axis, without FJBS.
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Fig. 8. The temperature of flue gas [K] in the central planes on x and z axis, without FJBS.
effect of the combustion reaction of volatile matter (see Equation (1)). Fig. 7 shows the mass fraction of carbon dioxide in the central planes of the boiler. The mass fraction of CO2 in the flue gas in the model is determined from the CO oxidation reaction. The temperature field in the planes of the central boiler is shown in Fig. 8. The temperature of combustion of the carbon particles on the grate does not exceed 1000 C; this is lower than the softening temperature of the ash for the Polish coal. The highest temperature occurs in the combustion of volatiles.
Fig. 9 shows the coal devolatilization [kg/s] in the central planes of the boiler. The devolatilization process occurred before combustion zone. 3.2. The second stage e model with working FJBS The mass fraction of oxygen in the planes of the central boiler was showed on Fig. 10. The results of numerical calculations is very similar as no working FJBS case. Its contents were in a layer of coal on the grate and decreased as a result of its participation in the process
Fig. 9. The coal devolatilization [kg/s] in the central planes on x and z axis, without FJBS.
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Fig. 10. The mass fraction of O2 in the central planes on x and z axis, with FJBS.
of burning coal particles. Over the coal layer the oxygen content is decreased in the exhaust gas due to the oxidation of CO to CO2. Fig. 11 shows the mass fraction of CO in the planes of the central boiler. Such as on Fig. 6, the highest concentration in a region of the degassing process of the carbon particles. Decreases in the CO content of the flue gas results from the oxidation of CO to CO2. High concentrations of CO over a carbon
layer on the grate is the effect of the combustion reaction of volatile matter (see Equation (1)). As you can see combustion process on the grate and in the region over the grate but below FJBS level is very similar for two consider cases. Fig. 12 shows the mass fraction of carbon dioxide in the central planes of the boiler. The mass fraction of CO2 in the flue gas in the model is determined from the CO oxidation reaction.
Fig. 11. The mass fraction of CO in the central planes on x and z axis, with FJBS.
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Fig. 12. The mass fraction of CO2 in the central planes on x and z axis, with FJBS.
The temperature field in the planes of the central boiler is shown in Fig. 13. The temperature of combustion of the carbon particles on the grate does not exceed 1000 C; this is lower than the softening temperature of the ash for the Polish coal. The highest temperature occurs in the combustion of volatiles. For case with FJBS the change
of mass fraction of H2O in the coal (Fig. 14), and the change in the mass of coal particles along the grate (Fig. 15) was shown. However, what was pointed before combustion process is very similar for cases with and without FJBS, because of FJBS has no influence on process occurring below its level. This demonstrates that the processes taking place during combustion of fuel on the
Fig. 13. The temperature of flue gas [K] in the central planes on x and z axis, with FJBS.
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Fig. 14. The change of mass fraction of H2O in the coal on the grate, with FJBS.
grate was included in the numerical model (i.e., drying, degassing, the burning of carbon particles, and the burning of volatiles). The velocity on the outlet of jet-pumps ventilators was showed on Fig. 16 and the temperature of flue gas [K] in the outlet window plane, without FJBS was presented on Fig. 17. 4. Comparison of CFD modelling with measurements The measurements [2] showed that FJBS homogenizes the temperature profile of the flue gas and the concentration of CO and
O2, and it also decreases the concentration of CO in boiler e see Figs. 18, 20 and 22. The numerical calculations confirm the measurements qualitatively rather than quantitatively. Generally, CFD modelling confirmed the conclusions from measurements. To make it easier for comparison, the results of the CFD calculations (Figs. 19, 21 and 23) and measurement results are presented below. Measurement ranges were conditioned by the length of the probe so that numerical results are presented in the same range. The temperature at the outlet of the combustion chamber (see Fig. 2) obtained from the 0-dimensional model of the boiler (based
Fig. 15. The change of coal's particle mass in [kg] on the grate, with FJBS.
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Fig. 16. The velocity [m/s] on the outlet of jet-pumps ventilators, with FJBS.
Fig. 17. The temperature of flue gas [K] in the outlet window plane, without FJBS.
Fig. 18. The temperature of flue gas [K] in the measured planes obtained from measurements, left with FJBS, right without FJBS.
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Fig. 19. The temperature of flue gas [K] in the measured planes obtained from CFD, left with FJBS, right without FJBS.
Fig. 20. The mass fraction of O2 in the measured planes obtained from measurements, left with FJBS, right without FJBS.
Fig. 21. The mass fraction of O2 in the measured planes obtained from CFD, left with FJBS, right without FJBS.
on the data obtained from the measurement of the boiler) and obtained CFD modelling (see Fig. 17) was shown in Table 4. 5. Conclusions 1. Obtaining a reliable temperature at the furnace chamber outlet by means of numerical modelling is possible using a 0-
dimensional boiler model based on the data obtained from the boiler measurement. 2. This paper showed that development of a CFD model should be used with measurements for validation (i.e., to select proper submodels), then the numerical model will generate real values. 3. The CFD modelling in this paper showed that FJBS homogenizes the temperature profile of the flue gas and concentration
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Fig. 22. The mass fraction of CO in the measured planes obtained from measurements, left with FJBS, right without FJBS.
Fig. 23. The mass fraction of CO in the highest measured planes obtained from CFD, left with FJBS, right without FJBS.
Table 4 The temperature of the flue gas in the outlet of the furnace. Data
Unit
0-dimensional model
CFD model
toutlet
982
982
C
of CO and O2 and also decreases the concentration of CO in the boiler.
Acknowledgements Investigations presented in this work were partially financed within the KIC InnoEnergy innovation project on Efficient Coal Fired Stoker Boiler (EcoStoker).
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