3D CFD Simulation of Combustion in a 150 MWe Circulating Fluidized Bed Boiler

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1 Engneerng Conferences Internatonal ECI Dgtal Archves 10th Internatonal Conference on Crculatng Fludzed Beds and Fludzaton Technology - CFB-10 Refereed Proceedngs Sprng D CFD Smulaton of Combuston n a 150 MWe Crculatng Fludzed Bed Boler Nan Zhang State Key Laboratory of Multphase Complex Systems, Insttute of Process Engneerng, Chnese Academy of Scences Bona Lu State Key Laboratory of Multphase Complex Systems, Insttute of Process Engneerng, Chnese Academy of Scences We Wang State Key Laboratory of Multphase Complex Systems, Insttute of Process Engneerng, Chnese Academy of Scences Jngha L State Key Laboratory of Multphase Complex Systems, Insttute of Process Engneerng, Chnese Academy of Scences Follow ths and addtonal works at: Part of the Chemcal Engneerng Commons Recommended Ctaton Nan Zhang, Bona Lu, We Wang, and Jngha L, "3D CFD Smulaton of Combuston n a 150 MWe Crculatng Fludzed Bed Boler" n "10th Internatonal Conference on Crculatng Fludzed Beds and Fludzaton Technology - CFB-10", T. Knowlton, PSRI Eds, ECI Symposum Seres, (2013). Ths Conference Proceedng s brought to you for free and open access by the Refereed Proceedngs at ECI Dgtal Archves. It has been accepted for ncluson n 10th Internatonal Conference on Crculatng Fludzed Beds and Fludzaton Technology - CFB-10 by an authorzed admnstrator of ECI Dgtal Archves. For more nformaton, please contact franco@bepress.com.

2 3D CFD SIMULATION OF COMBUSTION IN A 150 MW e CIRCULATING FLUIDIZED BED BOILER Nan Zhang, Bona Lu, We Wang, Jngha L State Key Laboratory of Multphase Complex Systems, Insttute of Process Engneerng, Chnese Academy of Scences, Bejng , Chna ABSTRACT Euleran granular multphase model wth meso-scale modelng of drag coeffcent and mass transfer coeffcent, based on the energy mnmzaton mult-scale (EMMS) model, was presented to smulate a 150 MW e CFB boler. The three-dmensonal (3D), full-loop, tme-dependent smulaton results were presented n terms of the profles of pressure, solds volume fracton and solds vertcal velocty, the dstrbutons of carbon and oxygen, as well as the temperature. The EMMS-based sub-grd modelng allows usng coarse grd wth proven accuracy, and hence t s sutable for smulaton of such large-scale ndustral reactors. INTRODUCTION The gas-sold flow, heat /mass transfer and chemcal reactons n Crculatng Fludzed Bed (CFB) reactors are nherently coupled wth spato-temporal mult-scale structures, and featured wth non-lnear non-equlbrum behavor (1). For such a mult-phase complex system, t s nadequate to reproduce the real physcal process by refnng the grd n the conventonal Two-Flud Model (TFM), thus callng for establshng meso-scale models (2). The present study frstly presents a 3D full-loop smulaton for a 150 MW e CFB boler, showng the powerful capablty of the sub-grd EMMS model n predctng the hydrodynamcs n a CFB. Then, a reduced EMMS/mass model s ncorporated nto the mass conservaton equatons for combuston speces, and a 3D combuston smulaton s realzed for the furnace part.

3 MODEL DESCRIPTION AND SIMULATION SETTING Governng equatons The Euleran granular model n Fluent was used to study the flow behavor n the full-loop of the boler, n whch the stress of the sold phase was descrbed wth the knetc theory of granular flow; the drag coeffcent correlaton was corrected wth consderaton of partcle clusters, whch was detaled n (3) and would not be stressed here. By neglectng of energy due to vscous dsspaton, compresson or expanson and nterfacal flow, the nternal energy balances and the conservaton equatons of chemcal speces n gas and sold phase were used to study the combuston n the furnace. As a frst step to combuston smulaton, only the man reacton of carbon react wth oxygen to carbon doxde was consdered here. The volatle combuston and mosture release was consdered n the heat balance through a UDF whch flled the gap between the low heatng value of the coal and the reacton heat as carbon was combusted completely, and the value was added averagely on the solds phase n the lower secton (h <= 6m) of the furnace. The volatle and mosture was neglected n the mass balance equaton, as they are less than 2% compared wth the total ar nput, detaled n (4). Geometry and mesh The 150 MW e CFB boler was desgned by Harbn Boler Co. Ltd. and nstalled n Guangdong, Chna. The man cross secton of the furnace s a rectangle of m 2 ; the chamber heght s 36.5 m; detaled nformaton can be found n (3). The geometry and surface mesh of the whole loop of the solds materal was shown n Fg. 1. As can be seen from the fgure, most of the boler was meshed wth hexahedron, others meshed wth polyhedron, all wth sze scale of 0.1 m. For convenence, the prmary ar and the loop-seal aeraton were assumed n plug flow from ther bottoms. To balance the sold nventory n the boler, the solds extng from the cyclone outlets, whch was about 6%(mass fracton) of the crculatng solds durng the smulaton, were returned va the coal-feed nlets. The orgn pont s set at the center of the prmary nlet at the bottom of the furnace; the x-axs s along the front-to-back wall drecton (wdth drecton); the y-axs s along the sde-to-sde wall drecton (depth drecton), and the z-axs s aganst the gravty drecton. Smulaton settngs

4 For full-loop hydrodynamc smulaton, the boler was consdered operated at the desgn temperature of 917 and atmospherc pressure, thus the physcal propertes of the gas was consdered as a const, whle for combuston smulaton n the furnace, the mxng laws of the gas mxture were summarzed n Table 1. The boundary and ntal condtons of the hydrodynamc smulaton and the combuston smulaton are lsted n Table 2 and Table 3 separately. Fg. 1. Geometry and surface mesh of the whole loop of the 150 MW e CFB boler. Table 1. Mxng laws of the gas mxture Property Unt Mxng law Equaton Densty (kg/m 3 ) Incompressble deal gas law Specfc Heat (J/kg k) Mxng law Capacty Thermal Conductvty (w/m k) mass weghted mxng law Vscosty (kg/m s) mass weghted mxng law Mass Dffuson (m 2 /s) Dlute Approxmaton ρ = p RT ( Y MW ) c op = Yc p p, k = Yk μ = Yμ D m,

5 Table 2. Boundary and ntal condtons of hydrodynamc smulaton n full-loop of the boler Boundary and Intal Gas Phase Sold Phase Condtons Flow rate Area Velocty (kg/s) (m 2 ) (m/s) Prmary ar nlet Secondary ar Inlet Slag-cooler nlet Loop-seal nlet Coal-feed nlet UDF Intal sold packng heght 2.5 m Cyclone Outlet Pa Wall No-slp Partal slp Table 3. Boundary and ntal condtons of combuston smulaton n the furnace Boundary and Intal Gas Phase Sold Phase Condtons Temperature Velocty Temperature Velocty (K) (m/s) (K) (m/s) Prmary ar nlet Secondary ar Inlet Slag-cooler nlet Loop-seal nlet Coal feed nlet returned Intal sold packng heght 1.5 m Cyclone Outlet Pa Wall No-slp Partal slp HYDRODYNAMIC FLOW IN THE FULL-LOOP Fg. 2(a) shows the smulated pressure balance n the boler: pressure gradent s large at the bottom and comparatvely small at the top n the furnace, whle the largest gradent occurs at the return legs. Fg. 2(b) shows that the general trends of the smulated data were comparable wth expermental data. Fg. 3 and Fg. 4 show profles of solds volume fracton and solds vertcal velocty at dfferent heghts, respectvely. Fg. 3(a) shows the so-called core-annulus structure,.e. a denser solds concentraton near the wall than n the center. Fg. 3(b) shows that solds concentraton profles along the depth drecton are flatter than those along the wdth drecton. Fg. 4(a) shows fallng clusters near the walls whle rsng partcles n the center. Fg. 4(b) shows two maxmum rsng veloctes not far from the walls, whch may be affected by the secondary ar nlets at the sde walls.

6 (a) (b) Fg. 2. Pressure dstrbuton: (a): n the whole loop (b): compared wth expermental data n the furnace (smulaton data were taken from the center lne of the furnace). Fg. 3. Solds volume fracton at dfferent heghts n the boler. Fg. 4. Solds vertcal velocty at dfferent heghts n the boler. COMBUSTION IN THE FURNACE PART Fg. 5 shows the smulated temperature n the furnace, whch s low at the bottom because of the njected cold ar and s hgh at the top because of combuston energy released. Fg. 6 and Fg. 7 show snapshots of smulated carbon concentraton and oxygen concentraton wth several slces n dfferent drectons, respectvely. Fg. 6(a) and Fg. 6 (b) Show that carbon concentraton s large at the bottom and small at the top, Fg. 6(b) and Fg. 6 (c) show that carbon concentraton reaches local maxmum

7 near the solds return ports and causes non-unform n the bottom of the furnace, and ths local non-unform caused by local ports wll decrease along the heght because of combuston and dsperson. Fg. 7(a) shows that oxygen concentraton s large at the bottom and s small at the top because of combuston. Fg. 7(b) shows that the njected secondary ar causes local maxmum of oxygen concentraton, whle the maxmum oxygen s not at the center lne of the furnace but there are two maxmum near the center lne, whch s caused by the fact that the njecton does not reach the center of the furnace, ths phenomenon was also reported by Myöhänen et al. (5). Fg. 7(c) shows that the njected secondary ar leads to non-unform oxygen concentraton at the bottom and ths non-unformty decreases along the heght but stll exsts at the top of the furnace. Fg. 5. Temperature profle n the furnace. (a) (b) (c) Fg. 6. Smulated carbon concentraton dstrbutons wth vertcal slces (a): sde-to-sde slce,(b): front-to-back slces and (c): horzontal slces.

8 (a) (b) (c) Fg. 7. Smulated oxygen concentraton dstrbutons wth vertcal slces (a): sde-to-sde slce,(b): front-to-back slces and (c): horzontal slces. CONCLUSIONS Smulaton results show the capablty of the current model, wth emphass on the EMMS-corrected drag coeffcent, n predctng the two-phase flow behavor. The reduced mult-scale mass transfer model and the drag coeffcent correcton based on the EMMS model were coupled to realze combuston smulaton n the furnace. It s an extenson to our experence on vrtual expermentaton to nvestgate the combuston wthn an ndustral reactor. ACKNOWLEDGMENT The authors are grateful to Professor Junfu Lu of Tsnghua Unversty for provdng the blueprnt of the boler. The fnancal supports from NSFC under Grant No , MOST under Nos. 2007AA and 2008BAF33B01, and CAS under No. KGCX2-YW-222 are also gratefully acknowledged. NOTATION c p specfc heat capacty, J/kg k D mass dffuson coeffcent, m 2 /s k thermal conductvty, w/m k p pressure, Pa

9 R gas constant, J / mol K MW molecular weght, kg/mol T temperature, K or V velocty, m/s x coordnate, m y coordnate, m Y mass fracton, dmensonless z coordnate, m Greek letters ε volume fracton, dmensonless μ vscosty, kg/(m s) ρ densty, kg/m 3 Subscrpts s sold phase speces REFERENCES 1. J. L, M. Kwauk, Partcle-flud two-phase flow: The energy-mnmzaton mult-scale method. Bejng: Metallurgcal Industry Press, B. Lu, W. Wang, J. L, Searchng for a mesh-ndependent sub-grd model for CFD smulaton of gas-sold rser flows. Chemcal Engneerng Scence 2009, 64 (15), N. Zhang, B. Lu, W. Wang, J. L, 3D CFD smulaton of hydrodynamcs of a 150 MW e crculatng fludzed bed boler. Chemcal Engneerng Journal 2010, 162 (2), N. Zhang, B. Lu, W. Wang, J. L, 3D CFD smulaton of combuston n a 150 MW e crculatng fludzed bed boler. To be submtted. 5. K. Myöhänen, T. Hyppänen, M. Loschkn, Convertng Measurement Data to Process Knowledge by Usng Three-Dmensonal CFB Furnace Model, n: K. Cen (Eds.), Crculatng Fludzed Bed Technology VIII - Proceedngs of the Eghth Internatonal Conference on Crculatng Fludzed Beds, Internatonal Academc Publshers, World Publshng Corp., Hangzhou, 2005, pp