Numerical Simulation of Coal Boiler at Electric Thermal Plants Using Computational Fluid Dynamics

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1 10th Intenational Symposium on ocess Systems Engineeing - SE2009 Rita Maia de Bito Alves, Claudio Augusto Olle do Nascimento and Evaisto Chalbaud Biscaia J. (Editos) 2009 Elsevie B.V. All ights eseved. Numeical Simulation of Coal Boile at Electic Themal lants Using Computational Fluid Dynamics Jaio Z. Souza a, Leonado. Rangel a, Henique C. Monteio a, Macelo Bzuneck b, Luiz Felippe b, Atu R. F. Ellwange b a ESSS Engineeing Simulation and Scientific Softwae LTDA. Rod. SC 401, km 1, aq. Tec. Alfa, Ed. CELTA, Sl. 4.01, CE , Floianópolis - SC, Bazil b Tactebel Enegia S.A Usina Temelética Joge Laceda C (UTLC) Av. aulo Santos Mello, s/n, CE , Capivai de Baixo SC, Bazil Abstact One of geat challenges found in electic themal plants boile opeation is to avoid the eosion poblem on wate wall ducts. This poblem is geneally caused by thee diffeent souces: flame misalignment, themal attack and eosion due to the contact with chemicals. This wok focus on investigate the effect of flame misalignment. This poblem can be caused by fluid dynamics factos due the bune geomety, which can be vey complex. The key pat of the bune the swile, whist woking geneates a spial flow stating in the bune and following towads the ea of the boile. The numeical simulation of this system has been developed using the commecial softwae of Computational Fluid Dynamics (ANSYS CFX). It has been chosen based on its obustness in mesh geneation and to solve consevation equations by finite volume method. The mathematical model has been developed by two appoaches: homogeneous (only ai has been simulated) and heteogeneous (the coal paticle tajectoies inside the boile has been calculated in the Eule-Lagange appoach). The esults pesented velocity pofiles, pessue pofiles, steamlines and othe data that is helpful to undestand the fluid flow phenomena inside the equipment. The coal paticles tajectoies simulated allow identifying whee paticles collide with the boile wall. All esults ae in vey good concodance with field obsevations accoding to enginees at UTLC fom Tactebel Enegia S.A. Keywods: CFD, Boile Simulation, Multiphase Flow 1. Intoduction Keep high efficiency of boiles in steam geneation pocess is a geat challenge fo themal powe plants. The efficiency of boiles can be analyzed by educing maintenance stops due to the expensive coasts in this equipment. Tactebel asked ESSS to guide enginees to undestand the fluid flow within the boile using a numeical simulation. The main objective of this study is to analyze the possible causes of excessive eosion in a specific boile wall egion: the egion situated nea of the ight bune in the thid ow of bunes, as showed in the Figue 1.

2 2 J.Z. Souza et al Figue 1 Boile egion affected by excessive eosion It has been peviously identified that a possible oigin fo the excessive eosion could be due to the non-buned paticles flow as a esult of a misaligned flame. The swile pomotes a spial flow inside the equipment and is developed to stabilize the flame and the ai-coal mixtue flow within the bune. The stategy to visualize the flow has been to develop a detailed ai and coal mixtue within the egion allowing the detection of eciculation zones. This can be obseved with velocity pofiles and paticle coal pathlines. It has been assumed that the eosion poblem is associated to the paticles path lines diven to the collision against the boile wall. This wok has been developed with an adiabatic flow model. Themal effects have not been consideed since the hot ai enteed and no combustion model has been applied. Fo an additional study, these factos must be considee to quantify the wall eosion ates, but the computational effot inceases significantly. Accoding to what has been pesented, these esults ae qualitative, but enough to undestand the boile intenal flow and wall eosion causes. 2. Methodology The poposed boile-bune numeical simulation has been developed using Computational Fluid Dynamics (CFD) with the commecial CFD softwae ANSYS CFX. A CFD wok is made of five stages: geomety, mesh, pe-pocessing, solve and pos-pocessing. These stages ae detailed below Geomety and Computational Mesh The geomety is developed using a CAD model that must specify all impotant details that ae necessay to the coect equipment desciption. Fo this wok, the initial data available has been only 2D plants and photogaphs. The geomety has been build using ANSYS Design Modele. The final bune CAD model is showed in Figue 2(a), while the Figue 2(b) shows the computational domain. The computational mesh is the epesentation of finite volumetic elements which ae applied the consevation equations. In this stage, has been used the softwae ANSYS ICEM CFD. To keep the coect geomety in the discetized domain, the elements sizes ae established in accodance to the local length of each equipment pat.

3 Boile Numeical Simulation in the Eletic Themal lants Using CFD 3 (a) (b) Figue 2 Bune (a) and boile (b) geometies Equipment computational mesh examples ae shown in Figues 3, whee thee ae two bune pats: deflecto (a) and swile (b). The final computational mesh has 2,2 millions of nodes. This size chaacteized a high detailed mesh which equies high computational effot. (a) (b) Figue 3 Computational mesh in deflecto (a) and swile (b) egions 2.2. Mathematical Model The main assumptions in the boile numeical simulation ae (i) steady state, (ii) isothemal, (iii) incompessible, (iv) tubulent, (v) lowe solid volumetic faction within the equipment (Eule-Lagange). With assumptions above, the mathematical model consists by mass continuity equation ρ u +.( ρ. V ) = 0 t and the Navie-Stokes momentum equations u DV u 2 ρ = ρg p+ μ V (2) Dt Fo the tubulence model, it has been assumed the k-ɛ model, the most utilized in industial equipments with otation. Fo the multiphase model, the Euleian-Lagangian appoach calculates the paticle individual tack, though the integation of Odinay Diffeential Equation (ODE) fo paticle velocity and position. The paticle position can (1)

4 4 J.Z. Souza et al be epesented as in the Figue 4, which integated, esults the paticle position within the equipment. d = U (3) dt m Looking Figue 4, a paticle with mass moves with velocity U and it is influenced by F foce geneated by continuous phase, which has velocity U F. Figue 4 aticle schematic epesentation The paticle velocity is obtained though the paticle momentum balance, as shown equation 4. du = (4) m dt F The F foce acting ove the paticle has its oigin in diffeent phenomena, which depends of paticle physical popeties, continuous phase, elative velocity, gavity, tubulence, etc. In this model, it has been consideed the dag foce, the weight and the tubulence influence on paticles tacks, though the tubulent dispesion foce. To enclosue the mathematical model, the bounday conditions have been obtained though opeation data at themal unity, whee has been measued the bune ai flow values. The wall has consideed as no-slip condition Solve ANSYS CFX-Solve has been applied to solve all consevation equations. It implies the method of finite volume based on finite elements. The convegence citeia has been the RMS (Root Mean Squae), which has been monitoed up to the esidual eaches the acceptable citeia. Seveal monito points thoughout the equipment have been indicating velocity and pessue values aiming to guaantee the convegence stability. The thid convegence citeion is the closue of mass imbalance. 3. Discussion of Results The CFD esults allow detailed flow visualization, as eciculation zones, high velocities, etc. These esults ae geneally pesented in pofiles planes, steamlines o vectos, showing the flow diections and pefeential paths. In this wok, it has focused the velocity field and paticle tack. The esults allowed identify the equipment eosion tendencies. The numeical solution pos-pocessing has been ealized with ANSYS CFX-OST Bune and Boile Flow The bune is the main boile component and focused in this wok. In it that is geneated the ai flow in a spial fomat. Figue 5 shows the steamlines in this pat of equipment. It was possible captue this phenomenon though the tubulence model.

5 Boile Numeical Simulation in the Eletic Themal lants Using CFD 5 (a) (b) Figue 5 Bune Geomety effects thoughout the pimay and seconday inlets In the Figue 5 (a), the spial flow stats on the blades of the swile, pomoting a spin. As mentioned befoe, this swil stabilizes the flame and homogenizes the ai-coal flow. The mass flow in this seconday ai pipe is highe than the pimay ai-flow; theefoe this spial flow is pedominant. Figue 5 (b) shows the steamlines of pimay ai pipe in contact with deflecto. The adial opening homogenizes this flow with seconday ai. In this duct that coal is tanspoted. Deflection and swiles angles ae geometic paametes which can be tested with CFD, supplying tendencies of optimal configuation. The flame destabilization inside the boile may be caused by two intenal pipes in the middle of bune. These ae used fo flame ignition. Figues 6 and 7 show that these pipes pomote a non-symmetic egion. Figue 6 Velocity ofile in the Inteface between boile and bune Figue 7 Steamlines Inside the Boile The extenal annula egion pesent an axial symmety, howeve, the cental egion is non-symmetic. This shows that the intenal pipes have significant influence in the flow. Looking towads the ai flow within the boile in a supeio view in the Figue 7, it can be obseved the spial flow tendency, follow thoughout the boile. Howeve, thee ae some steamlines that disengages of pincipal flow, going to the wall. The steamlines ovetopped by the ed cicle can cay coal paticles not buned into the wall, esulting in the excessive eosion. The causes of this disengages steamlines ae unknown and it is consideed that intenal ignition pipes have influence on it Multiphase Analysis The multiphase appoach used in this wok has been the Euleian-Lagangian, with one way coupling between the phases. With this appoach only the ai flow influences in the

6 6 J.Z. Souza et al l volumetiic solid conceentation, as solid flow. This simpliffication is posssible due the low matical modeel section. Fig gue 8 shows coal paticle tajectoies explained in the mathem and collisiion against thhe boile wall. These ae feew tajectoies (the most ppat follows towads thhe back of the boile), but thhese paticles can attack thee wall with eoosion. Figuue 8 Coal aaticles Tackks Figue 9 Boile B Wall Eosion Figue 9 shows wall contact c of paaticles. The ed ellipse inn left side is the bune position. The T white linees ae only foo efeence. The T eosion shhowed by colooed points within the ed cicle aee calculated thhough the maateial popetties (coal steaal), velocity and collisiion angle betw ween the coaal paticles and wall. The eds points ae the moe intense colllision points. The egion whee w the coall paticles colllide against thhe wall is in vey goodd concodancee with the fieeld obsevatio ons fom engiinees of Tacctebel. The showed egion is about 3.5 m of bunne offset. 4. Conclu usions Eosion poblem on wate w wall duccts of coal bo oiles can be analyzed byy numeical simulationn using CFD. In this wok, it has been sh hown how bune swil andd deflection affect the paticles p and ai a path lines by calculating g the tubulennt multiphase fflow with a commeciaal CFD tool. Velocity poofile and steamlines show wed the flow tendencies within the boile with paticles p collission at 3.5 m of bune offsset, the exact position of the highesst detection of eosion. This is caused because b thee is no symmeety of two intenal iggnition pipes within w the bunne. This fact geneates a misalignment m in the spial flow, whaat may pom mote the flamee destabilizattion as well. A quantitatiive eosion analysis has not been pefomed p at this t stage, butt it can be devveloped with addition of themal annd chemical effects e like coombustion. Th he qualitative analysis povvided good esults is eal ageemennt with the pooblem at Tacctebel Enegiaa owe lantt validating the popossed model of this t wok. Refeencces Bid, R. B. ; Stewat, W. E. E ; Linghtfoot, E. N. (1960) Tanspot henomena. NY: Johhn Wiley. 2 ed. Rio de Janneio: Guanabaa Dois. Foust, A. S. (1982) incíppios das opeações unitáias Edição. Maliska, C.. R. (2004) Tannsfeência de Calo C e Mecânicca dos Fluidos Computacional. C Editoa LTC. Mille, A. Gidaspow G D. (1992) Dense, veetical gas-solid d flow in a pipe.. AIChE Jounaal, v.38, n.11, p