Coal-ebu

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Tutorial: Coal Combustion with Eddy Break Up (EBU) Model

Introduction The purpose of this tutorial is to provide guidelines and recommendations for setting up and solving a coal combustion case with the Eddy Break Up (EBU) model. This tutorial demonstrates how to do the following: • Set up and solve a coal combustion case. • Use the Eddy Break Up (EBU) model. • Solve the case using appropriate solver settings. • Postprocess the resulting data.

Prerequisites You should be familiar with the FLUENT interface and have a good understanding of basic setup and solution procedures. In this tutorial, you will use turbulence and combustion models, so you should have some experience with them. This tutorial will not cover the mechanics of using these models. Instead, it will focus on the application of these models to coal combustion. If you have not used these models before, it would be helpful to first refer to the FLUENT 6.3 User’s Guide and the FLUENT 6.3 Tutorial Guide.

Problem Description A 3D cutaway of the furnace is shown in Figure 1. Two annular inlets on the left-hand side and a circular outlet on the right-hand side are visible. Only one quarter of this geometry is modeled due to symmetry. The inner annular inlet has inner and outer radii of 0.055 m and 0.067 m respectively. The outer annular inlet has inner and outer radii of 0.07 m and 0.117 m respectively. The outlet radius is 0.425 m. Coal and carrier air enter the combustion chamber through the inner annular region. Hot, swirling, secondary air enters through the outer annular region. Combustion takes place and the products exit at the pressure outlet.

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Coal Combustion with Eddy Break Up (EBU) Model

Preparation 1. Copy the files coal-ebu.msh.gz and coal-ebu.c to the working folder. 2. Start the 3D (3d) version of FLUENT.

Figure 1: Problem Figure

Setup and Solution Step 1: Grid 1. Read the mesh file (coal-ebu.msh.gz). 2. Check the grid. Grid −→Check 3. Display the grid. Display −→Grid... (a) Select all the surfaces from the Surfaces selection list. (b) Click Display and close the Grid Display panel.

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 2: Grid Display

Step 2: Models 1. Select the standard k-epsilon (2 eqn) turbulence model. Define −→ Models −→Viscous... 2. Enable the Energy Equation. Define −→ Models −→Energy... 3. Select the Species Transport model. Define −→ Models −→ Species −→Transport & Reaction... (a) Select Species Transport from the Model list. (b) Enable Volumetric from the Reactions list. (c) Select coal-hv-volatiles-air from the Mixture Material drop-down list. (d) Select Eddy Dissipation from the Turbulence-Chemistry Interaction list. (e) Click OK to close the Species Model panel. 4. Select the Discrete Ordinates model. Define −→ Models −→Radiation... (a) Select Discrete Ordinates (DO) from the Model list. The Radiation Model panel expands to show the related inputs. (b) Set Flow Iterations per Radiation Iteration to 1. (c) Set Theta Divisions and Phi Divisions to 4 in the Angular Discretization group box.

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Coal Combustion with Eddy Break Up (EBU) Model

(d) Set Theta Pixels and Phi Pixels to 3 in the Angular Discretization group box. (e) Click OK to close the Radiation Model panel. 5. Enable the Discrete Phase model. Define −→ Models −→Discrete Phase... (a) Enter 40000 for Max. Number of Steps. (b) Enable Specify Length Scale and enter 0.0025 m for Length Scale. (c) Click OK to close the Discrete Phase Model panel. Step 3: Injections 1. Define 9 injections from surface v-1. Define −→Injections... (a) Click the Create button to open the Set Injection Properties panel. (b) The common properties for the 9 injections are shown in the table: (c) Click the Turbulent Dispersion tab and enable Discrete Random Walk Model. Parameter Particle Type Material Devolatilizing Species Product Species Oxidizing Species Point Properties Turbulent Dispersion

Value Combusting coal-hv hv vol co2 o2 Temperature = 343, Z-Velocity = 23.11 Stochastic Model with values for Number of Tries and Time Scale Constant as 10 and 0.15 respectively.

Table 1: Common Injection Properties (d) The properties specific to each injection are shown in the table: Injection Name injection-0 injection-1 injection-2 injection-3 injection-4 injection-5 injection-6 injection-7 injection-8

Diameter (m) 1e-6 5e-6 1e-5 2.5e-5 5e-5 7.5e-5 0.0001 0.0002 0.0003

Flow Rate 0.00018264 0.00073056 0.00127848 0.00438336 0.00584448 0.00347016 0.00146112 0.00073056 0.00018264

Table 2: Specific Injection Properties

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Coal Combustion with Eddy Break Up (EBU) Model

(e) Retain the default values for the other parameters. (f) Close the Injections panel. Step 4: Materials 1. Modify the properties for the coal-hv-volatiles-air mixture. Define −→Materials... (a) Add carbon-monoxide (co) from the FLUENT materials database. i. Click the Fluent Database... button to open the Fluent Database Materials panel. ii. Select fluid from the Material Type drop-down list. iii. Select carbon-monoxide (co) from Fluent Fluid Materials panel. iv. Click Copy and close the Fluent Database Materials panel. (b) Click the Edit... button to the right of the Mixture Species drop-down list to open the Species panel. i. Add carbon-monoxide (co) to the list of Mixture Species. Note: Make sure nitrogen is the last species in the list. If not, remove nitrogen and add it again. ii. Click OK to close the Species panel. (c) Click the Edit... button to the right of the Reaction drop-down list to open the Reactions panel. i. Edit the Eddy-Dissipation reaction model as follows: Reactants Number of Reactants = 2 Stoich. Coefficients hv vol =1 o2 = 2.46

Products Number of Products = 4 Stoich. Coefficients co = 2.17 co2 = 0.633 h2o = 2.118 n2 = 0.071

Table 3: Reaction 1 Reactants Number of Reactants = 2 Stoich. Coefficients co = 1 o2 = 0.5

Products Number of Products = 1 Stoich. Coefficient co2 = 1

Table 4: Reaction 2

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Coal Combustion with Eddy Break Up (EBU) Model

ii. Retain the default values for the other parameters. iii. Click OK to close the Reactions panel. (d) Set the physical properties for other parameters: Parameter Density Cp Thermal Conductivity

Viscosity

Absorption Coefficient Scattering Coefficient Scattering Phase Function

Value incompressible-ideal-gas mixing-law polynomial The first and second temperature coefficients are 0.01006 and 5.413e-5 respectively. polynomial The first and second temperature coefficients are 9.18e-6 and 3.161e-8 respectively. wsggm-domain-based constant with a value of 0.5. isotropic

Table 5: Properties for other Parameters 2. Set the properties for the combusting particle coal-hv. Property Density Cp Latent Heat Vaporization Temperature Volatile Component Fraction Binary Diffusivity Swelling Coefficient Burnout Stoichiometric Ratio Combustible Fraction Heat of Reaction for Burnout React. Heat Fraction Absorbed by Solid

Combustion Model

Value 1000 1100 0 343 55 3e-5 2 2.67 36.7 3.29e7 0 kinetics/diffusion-limited Mass Diffusion-Limited Rate Constant = 5e-12 Kinetics-Limited Rate Pre-Exponential Factor = 6.7 Kinetics-Limited Rate Activation Energy = 1.138e8

Table 6: Combusting Particle Material Properties

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Coal Combustion with Eddy Break Up (EBU) Model

Vaporization temperature of coal is 773 K. But, to start the reactions, we will lower it to 343 K and once the flame shape is obtained, it will be changed to the original value. 3. Set properties for o2, co2, h2o, co, and n2. (a) Select piecewise-polynomial from the Cp drop-down list for o2, co2, h2o, co, and n2 species and accept the default values. 4. Set properties for coal volatiles coal hv volatiles. (a) Enter 50 for Molecular Weight and -1.8474e7 for Standard State Enthalpy. 5. Click Change/Create and close the Materials panel. Step 5: Operating Conditions Define −→Operating Conditions... 1. Retain the default operating conditions. Step 6: Compiling the Interpreted User Defined Functions (UDFs) These functions will be used later to set the boundary conditions. For more information on interpreted UDFs, refer to the FLUENT 6.3 UDF Manual. Define −→ User-Defined −→ Functions −→Interpreted... 1. Enter the name of the C function (coal-ebu.c) for Source File Name. 2. Specify the C preprocessor to be used in the CPP Command Name. Keep the default Stack Size setting of 10000, unless the number of local variables in your function will cause the stack to overflow. In this case, set the Stack Size to a number that is greater than the number of local variables used. 3. Select the Use Contributed CPP option if you want to use the preprocessor supplied by Fluent Inc., instead of using your own. 4. Click Interpret and close the Interpreted UDFs panel. In case there are errors while interpreting, you can keep the panel open and continue debugging and interpreting simultaneously until no errors are reported.

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Coal Combustion with Eddy Break Up (EBU) Model

Step 7: Boundary Conditions Define −→Boundary Conditions... 1. Set the boundary conditions for v-1 as specified in Table 7. Parameter Velocity Magnitude Temperature Turbulence Intensity Hydraulic Diameter Species Mass Fractions Internal Emissivity Discrete Phase BC Type

Value 23.11 m/s 343 K 10% 0.013 m o2 = 0.2315 1 escape

Table 7: Boundary Conditions for v-1 2. Set the boundary conditions for v-2 as specified in Table 8. Parameter Velocity Specification Method Coordinate System Radial-Velocity Tangential-Velocity Axial-Velocity Temperature Turbulence Intensity Hydraulic Diameter Species Mass Fractions

Value Components Cylindrical (Radial, Tangential, Axial) 0 udf vinlet2wvel udf vinlet2uvel 573 K 12 % 0.047 m o2 = 0.2315

Table 8: Boundary Conditions for v-2 3. Set the boundary conditions for p-1 as specified in Table 9. Parameter Gauge Pressure Backflow Total Temperature Backflow Turbulence Intensity Backflow Hydraulic Diameter Species Mass Fractions

Value 0 1000 K 10% 1m o2 = 0.2315

Table 9: Boundary Conditions for p-1 4. Set the boundary conditions for the wall zones. The Temperature and Internal Emissivity are specified in Table 10.

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Coal Combustion with Eddy Break Up (EBU) Model

Zone w-1 w-2 w-3 w-4 w-5 w-6 w-7 w-8 w-9

Temperature 343 573 873 1273 udf wall5temp udf wall6temp udf wall7temp 1323 1073

Internal Emissivity 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5

Table 10: Wall Boundary Conditions 5. Set the boundary condition for periodic as Rotational. 6. Close the Boundary Conditions panel. Step 8: Non-Reacting Flow Solution 1. Disable Volumetric reactions. Define −→ Models −→ Species −→Transport & Reaction... 2. Deselect the Discrete Ordinates radiation model. Solve −→ Controls −→Solution... (a) Deselect Discrete Ordinates from the Equations selection list. (b) Click OK to close the Solution Controls panel. 3. Disable Interaction with Continuous Phase. Define −→ Models −→Discrete Phase... 4. Initialize the flow field from all-zones. Solve −→ Initialize −→Initialize... 5. Enable the plotting of residuals during the calculation. Solve −→ Monitors −→Residual... 6. Start the calculation by requesting 100 iterations. Solve −→Iterate... 7. Change the solution control parameters. Solve −→ Controls −→Solution... (a) Select PRESTO! for Pressure. (b) Set the Under-Relaxation Factors for Pressure, Momentum, and Turbulence to 0.5, 0.2, and 0.7 respectively.

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Coal Combustion with Eddy Break Up (EBU) Model

(c) Click OK to close the Solution Controls panel. 8. Request 100 more iterations. Step 9: Initiate Reacting Flow Solution 1. Enable Interaction with Continuous Phase and set the Number of Continuous Phase Iterations per DPM Iteration to 1. 2. Enable Volumetric reactions. 3. Patch high temperature and product species mass fractions in reaction zone. Adapt −→Region...

(a) Select Cylinder from the Shapes list. (b) Enter the Input Coordinates as shown in the panel. (c) Click Mark and close the Region Adaption panel. 4. Patch the following values in the reaction zone: Solve −→ Initialize −→Patch... (a) Select cylinder-r0 from the Registers to Patch selection list and patch the following values: • Temperature = 2000 K • h2o mass fraction = 0.01 • co2 mass fraction = 0.01 (b) Close the Patch panel.

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Coal Combustion with Eddy Break Up (EBU) Model

5. Set the Under-Relaxation Factors as follows: Energy Species Discrete Phase Sources

0.95 0.95 1

6. Request 1 iteration. 7. Save the case and data files (coal-ebu-react-start.cas.gz and coal-ebu-react-start.dat.gz). 8. Change the Number of Continuous Phase Iterations per DPM Iteration to 50. Define −→ Models −→Discrete Phase... 9. Set the Under-Relaxation Factor for Discrete Phase Sources to 0.1. 10. Request 300 more iterations. Step 10: Obtain Converged Solution 1. Change the solution control parameters. Solve −→ Controls −→Solution... (a) Select Discrete Ordinates from the Equations selection list. (b) Set the Under-Relaxation Factor for Density to 0.7. 2. Request 500 additional iterations. 3. Save the case and data files (coal-ebu-1.cas.gz and coal-ebu-1.dat.gz). 4. Select Second Order Upwind from the Momentum, Turbulent Kinetic Energy, Turbulent Dissipation Rate, hv vol, o2, co2, h2o, co and Energy drop-down lists in the Discretization group box. 5. Request an additional 300 iterations. 6. Save the case and data files (coal-ebu-2.cas.gz and coal-ebu-2.dat.gz). 7. Enable Particle Radiation Interaction. Define −→ Models −→Discrete Phase... 8. Modify the properties for combusting-particle. Define −→ Materials... −→ (a) Enter 773 K for Vaporization Temperature. (b) Set Particle Scattering Factor to 0.15. 9. Set the Under-Relaxation Factors for all species, Energy and turbulence to 1, 0.98, and 0.6 respectively.

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Coal Combustion with Eddy Break Up (EBU) Model

10. Request another 2000 iterations. Solve −→Iterate... 11. Save the case and data files (coal-ebu-final.cas.gz and coal-ebu-final.dat.gz). Step 11: Postprocessing 1. Check the mass balance for convergence. Report −→Fluxes... (a) Select Mass Flow Rate from the Options list. (b) Select all the zones from the Boundaries selection list and click Compute. This is net gas phase mass flux. The negative number indicates net gas mass flux leaving the domain. (c) Close the Flux Reports panel. Report −→Volume Integrals... (d) Select Sum from the Report Type list. (e) Select Discrete Phase Model... and DPM Mass Source from the Field Variable drop-down lists. (f) Select fluid from the Cell Zones selection list and click Compute. This is net mass transfer from the discrete phase coal particles to the gas phase. (g) Close the Volume Integrals panel. Note: The two mass balances should add up to a very small number as compared to the total mass flow rate from the inlets, v-1 and v-2. 2. Check the net heat transfer. Report −→Fluxes... (a) Select Total Heat Transfer Rate from the Options list. (b) Select all the zones from the Boundaries selection list and click Compute. This is net gas phase heat transfer. (c) Close the Flux Reports panel. Report −→Volume Integrals... (d) Select Sum from the Report Type list. (e) Select Discrete Phase Model... and DPM Enthalpy Source from the Field Variable drop-down lists. (f) Select fluid from the Cell Zones selection list and Compute. This is net discrete phase heat transfer. (g) Close the Volume Integrals panel.

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Coal Combustion with Eddy Break Up (EBU) Model

Note: The sum of net gas phase and discrete phase heat transfer should be much smaller than a representative heat flux, such as heat flux from the outlet boundary, p-1. 3. Display filled contours of velocity magnitude (Figure 3). 4. Display filled contours of static temperature (Figure 4). 5. Display filled contours of mass fraction of hv vol (Figure 5), o2 (Figure 6), co2 (Figure 7), co (Figure 8), and h2o (Figure 9) on x=0m plane. 6. Display particle tracks for injection-5 (Figure 10).

Figure 3: Contours of Velocity Magnitude

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 4: Contours of Static Temperature

Figure 5: Mass Fraction of Volatiles

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 6: Mass Fraction of o2

Figure 7: Mass Fraction of co2

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 8: Mass Fraction of co

Figure 9: Mass Fraction of h2o

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 10: Particle Tracks

Step 12: NOx Modeling 1. Select the NOx model. Define −→ Models −→ Species −→NOx... (a) Enable Thermal NO, Prompt NO, and Fuel NO from the Pathways list. (b) Select partial-equilibrium from the [O] Model and [OH] Model drop-down lists. (c) Click the Prompt tab and select hv vol from the Fuel Species selection list. (d) Enter 2.8 and 0.685 for Fuel Carbon Number and Equivalence Ratio respectively. (e) Click the Fuel tab and select Solid from the Fuel Type group box. (f) Set the following parameters: Fuel Type N Intermediate Volatile N Mass Fraction Char N Mass Fraction Partition Fractions HCN NH3 Char N Conversion

Solid hcn/nh3/no 0.0398 0.0596 0.9 0.1 no

(g) Click the Turbulence Interaction tab and select temperature from the PDF Mode drop-down list in the Turbulence Interaction Mode drop-down list. (h) Enter 20 for Beta PDF Points.

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Coal Combustion with Eddy Break Up (EBU) Model

(i) Click Apply and close the NOx Model panel. 2. Change the solution control parameters. Solve −→ Control −→Soltuion... (a) Deselect all equations except Pollutant no, Pollutant hcn, and Pollutant nh3 from the Equations selection list. (b) Increase the Under-Relaxation Factors for these equations to 1. 3. Change the convergence criteria for all the three equations to 1e-06. Solve −→ Monitors −→Residual... 4. Request for 100 iterations. Solve −→Iterate... 5. Select Second Order Upwind from the Pollutant no, Pollutant hcn, and Pollutant nh3 drop-down lists in the Discretization group box. Solve −→ Control −→Solution... 6. Request another 100 iterations. Solve −→Iterate... 7. Save the case and data flies (coal-ebu-final-no.cas.gz and coal-ebu-final-no.dat.gz). 8. Display contours of mass fraction of Pollutant no on x=0m plane (Figure 11). Display −→Contours... (a) Select NOx... and Mass Fraction of Pollutant no from the Contours of drop-down lists. (b) Select x=0m plane from the Surfaces selection list. (c) Click Display and close the Contours panel.

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Coal Combustion with Eddy Break Up (EBU) Model

Figure 11: Mass Fraction of Pollutant no

Results In this tutorial, nine injections are introduced at the inlet. The coal-hv particles travel a short distance before they start releasing volatiles. At this point, reactions start and the temperature increases. This high temperature zone can be seen inside the furnace slightly away from the inlet. In Eddy Break Up (EBU) coal combustion, coal particles release volatiles that react with oxygen and produce combustion products. Similar trends can be seen for NOx. The stoichiometric coefficients can be calculated once chemical composition of coal volatiles is known. For more information on determining coal volatile composition, refer to the FLUENT 6.3 Tutorial Guide.

Summary Application of the EBU model in a coal combustion case has been demonstrated.

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