10th International Symposium on Process Systems Engineering - PSE2009 Rita Maria de Brito Alves, Claudio Augusto Oller do Nascimento and Evaristo Chalbaud Biscaia Jr. (Editors) © 2009 Elsevier B.V. All rights reserved.

Numerical Simulation of Coal Boiler at Electric Thermal Plants Using Computational Fluid Dynamics Jairo Z. Souzaa, Leonardo P. Rangela, Henrique C. Monteiroa, Marcelo Bzuneckb, Luiz Felippeb, Artur R. F. Ellwangerb aESSS – Engineering Simulation and Scientific Software LTDA. Rod. SC 401, km 1, Parq. Tec. Alfa, Ed. CELTA, Sl. 4.01, CEP 88030-000, Florianópolis - SC, Brazil bTractebel Energia S.A – Usina Termelétrica Jorge Lacerda C (UTLC) Av. Paulo Santos Mello, s/n, CEP 88745-000, Capivari de Baixo – SC, Brazil

Abstract One of great challenges found in electric thermal plants boiler operation is to avoid the erosion problem on water wall ducts. This problem is generally caused by three different sources: flame misalignment, thermal attack and erosion due to the contact with chemicals. This work focus on investigate the effect of flame misalignment. This problem can be caused by fluid dynamics factors due the burner geometry, which can be very complex. The key part of the burner the swirler, whist working generates a spiral flow starting in the burner and following towards the rear of the boiler. The numerical simulation of this system has been developed using the commercial software of Computational Fluid Dynamics (ANSYS CFX). It has been chosen based on its robustness in mesh generation and to solver conservation equations by finite volume method. The mathematical model has been developed by two approaches: homogeneous (only air has been simulated) and heterogeneous (the coal particle trajectories inside the boiler has been calculated in the Euler-Lagrange approach). The results presented velocity profiles, pressure profiles, streamlines and other data that is helpful to understand the fluid flow phenomena inside the equipment. The coal particles trajectories simulated allow identifying where particles collide with the boiler wall. All results are in very good concordance with field observations according to engineers at UTLC from Tractebel Energia S.A. Keywords: CFD, Boiler Simulation, Multiphase Flow

1. Introduction Keep high efficiency of boilers in steam generation process is a great challenge for thermal power plants. The efficiency of boilers can be analyzed by reducing maintenance stops due to the expensive coasts in this equipment. Tractebel asked ESSS to guide engineers to understand the fluid flow within the boiler using a numerical simulation. The main objective of this study is to analyze the possible causes of excessive erosion in a specific boiler wall region: the region situated near of the right burner in the third row of burners, as showed in the Figure 1.

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Figure 1 – Boiler region affected by excessive erosion

It has been previously identified that a possible origin for the excessive erosion could be due to the non-burned particles flow as a result of a misaligned flame. The swirler promotes a spiral flow inside the equipment and is developed to stabilize the flame and the air-coal mixture flow within the burner. The strategy to visualize the flow has been to develop a detailed air and coal mixture within the region allowing the detection of recirculation zones. This can be observed with velocity profiles and particle coal pathlines. It has been assumed that the erosion problem is associated to the particles path lines driven to the collision against the boiler wall. This work has been developed with an adiabatic flow model. Thermal effects have not been considered since the hot air entered and no combustion model has been applied. For an additional study, these factors must be considerer to quantify the wall erosion rates, but the computational effort increases significantly. According to what has been presented, these results are qualitative, but enough to understand the boiler internal flow and wall erosion causes.

2. Methodology The proposed boiler-burner numerical simulation has been developed using Computational Fluid Dynamics (CFD) with the commercial CFD software ANSYS CFX®. A CFD work is made of five stages: geometry, mesh, pre-processing, solver and pos-processing. These stages are detailed below. 2.1. Geometry and Computational Mesh The geometry is developed using a CAD model that must specify all important details that are necessary to the correct equipment description. For this work, the initial data available has been only 2D plants and photographs. The geometry has been build using ANSYS® Design Modeler. The final burner CAD model is showed in Figure 2(a), while the Figure 2(b) shows the computational domain. The computational mesh is the representation of finite volumetric elements which are applied the conservation equations. In this stage, has been used the software ANSYS® ICEM CFD. To keep the correct geometry in the discretized domain, the elements sizes are established in accordance to the local length of each equipment part.

Boiler Numerical Simulation in the Eletric Thermal Plants Using CFD 3

(a) (b) Figure 2 – Burner (a) and boiler (b) geometries

Equipment computational mesh examples are shown in Figures 3, where there are two burner parts: deflector (a) and swirler (b). The final computational mesh has 2,2 millions of nodes. This size characterized a high detailed mesh which requires high computational effort.

(a) (b)

Figure 3 – Computational mesh in deflector (a) and swirler (b) regions 2.2. Mathematical Model The main assumptions in the boiler numerical simulation are (i) steady state, (ii) isothermal, (iii) incompressible, (iv) turbulent, (v) lower solid volumetric fraction within the equipment (Euler-Lagrange). With assumptions above, the mathematical model consists by mass continuity equation

( ). . 0Vtρ ρ∂+∇ =

∂

ur (1)

and the Navier-Stokes momentum equations

2DV g p VDt

ρ ρ μ= −∇ + ∇ur

ur (2)

For the turbulence model, it has been assumed the k-ɛ model, the most utilized in industrial equipments with rotation. For the multiphase model, the Eulerian-Lagrangian approach calculates the particle individual track, through the integration of Ordinary Differential Equation (ODE) for particle velocity and position. The particle position can

4 J.Z. Souza et al

be represented as in the Figure 4, which integrated, results the particle position within the equipment.

PP U

dtrd rr

=

(3)

Looking Figure 4, a particle with mass Pm moves with velocity PUr

and it is

influenced by Fr

force generated by continuous phase, which has velocity FUr

.

Figure 4 – Particle schematic representation

The particle velocity is obtained through the particle momentum balance, as shown equation 4.

FdtUdm P

P

rr

=

(4)

The Fr

force acting over the particle has its origin in different phenomena, which depends of particle physical properties, continuous phase, relative velocity, gravity, turbulence, etc. In this model, it has been considered the drag force, the weight and the turbulence influence on particles tracks, through the turbulent dispersion force. To enclosure the mathematical model, the boundary conditions have been obtained through operation data at thermal unity, where has been measured the burner air flow values. The wall has considered as “no-slip” condition. 2.3. Solver ANSYS® CFX-Solver has been applied to solve all conservation equations. It implies the method of finite volume based on finite elements. The convergence criteria has been the RMS (Root Mean Square), which has been monitored up to the residual reaches the acceptable criteria. Several monitor points throughout the equipment have been indicating velocity and pressure values aiming to guarantee the convergence stability. The third convergence criterion is the closure of mass imbalance.

3. Discussion of Results The CFD results allow detailed flow visualization, as recirculation zones, high velocities, etc. These results are generally presented in profiles planes, streamlines or vectors, showing the flow directions and preferential paths. In this work, it has focused the velocity field and particle track. The results allowed identify the equipment erosion tendencies. The numerical solution pos-processing has been realized with ANSYS® CFX-POST. 3.1. Burner and Boiler Flow The burner is the main boiler component and focused in this work. In it that is generated the air flow in a spiral format. Figure 5 shows the streamlines in this part of equipment. It was possible capture this phenomenon through the turbulence model.

Boiler Numerical Simulation in the Eletric Thermal Plants Using CFD 5

(a) (b)

Figure 5 – Burner Geometry effects throughout the primary and secondary inlets In the Figure 5 (a), the spiral flow starts on the blades of the swirler, promoting a spin. As mentioned before, this swirl stabilizes the flame and homogenizes the air-coal flow. The mass flow in this secondary air pipe is higher than the primary air-flow; therefore this spiral flow is predominant. Figure 5 (b) shows the streamlines of primary air pipe in contact with deflector. The radial opening homogenizes this flow with secondary air. In this duct that coal is transported. Deflection and swirlers angles are geometric parameters which can be tested with CFD, supplying tendencies of optimal configuration. The flame destabilization inside the boiler may be caused by two internal pipes in the middle of burner. These are used for flame ignition. Figures 6 and 7 show that these pipes promote a non-symmetric region.

Figure 6 – Velocity Profile in the Interface

between boiler and burner Figure 7 – Streamlines Inside the

Boiler The external annular region present an axial symmetry, however, the central region is non-symmetric. This shows that the internal pipes have significant influence in the flow. Looking towards the air flow within the boiler in a superior view in the Figure 7, it can be observed the spiral flow tendency, follow throughout the boiler. However, there are some streamlines that disengages of principal flow, going to the wall. The streamlines overtopped by the red circle can carry coal particles not burned into the wall, resulting in the excessive erosion. The causes of this disengages streamlines are unknown and it is considered that internal ignition pipes have influence on it. 3.2. Multiphase Analysis The multiphase approach used in this work has been the Eulerian-Lagrangian, with one way coupling between the phases. With this approach only the air flow influences in the

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