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20.2.8 Using the SOx Model

When the sulfur content in the fuel is low, SOx concentrations that are generated in combustion generally have minimal influence on the predicted flow field, temperature, and major combustion product concentrations. The most efficient way to use the SOx model is as a postprocessor to the main combustion calculation. However, if the sulfur content is high, then SOx formation should be coupled with the gas phase combustion process rather than treating it as a postprocessing step.

The procedure for activating and setting up the model is as follows:

1.   Calculate your combustion problem using FLUENT.

2.   Enable the SOx model by clicking the SOx Formation button and set the appropriate parameters, as described in this section.

Define $\rightarrow$ Models $\rightarrow$ Species $\rightarrow$ SOx...

Figure 20.2.1: The SOx Model Panel
figure

3.   Define the boundary conditions for $SO_2$ and $H{_2}S$ (and $SO_3$, SH, or SO if necessary) at flow inlets.

Define $\rightarrow$ Boundary Conditions...

4.   In the Solution Controls panel, turn off the solution of all variables except species $SO_2$ and $H{_2}S$ (and $SO_3$, SH, or SO, based on your selections).

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

5.   Perform calculations until convergence (i.e., until the $SO_2$ and $H{_2}S$ (and $SO_3$, SH, or SO, if solved) species residuals are below $10^{-6}$) to ensure that the $SO_2$ and $H{_2}S$ concentration fields are no longer evolving.

Solve $\rightarrow$ Iterate...

6.   Review the mass fractions of $SO_2$ and $H{_2}S$ (and $SO_3$, SH, or SO) with alphanumerics and/or graphics tools in the usual way.

7.   Save a new set of case and data files, if desired.



Activating the SOx Model


To activate the SOx models and set related parameters, you will use the SOx Model panel (e.g., Figure  20.2.1).

Define $\rightarrow$ Models $\rightarrow$ Species $\rightarrow$ SOx...



Setting Fuel SOx Parameters


For fuel SOx models, you will first need to specify the Fuel Type under Model Parameters:

Figure 20.2.2: The SOx Model Panel Displaying Liquid Fuel Parameters
figure

Note that you can use only one of the fuel models at a time. The Gas option is available only when the Species Transport model is enabled (see Section  14.1.3).

Setting Gaseous and Liquid Fuel SOx Parameters

If you have selected Gas or Liquid as the Fuel Type, you will also need to specify the following:

Setting Solid Fuel SOx Parameters

For solid fuel, several inputs are required for SOx model.

Figure 20.2.3: The SOx Model Panel Displaying Solid Fuel Parameters
figure

The following equations are used to determine the mass fraction of sulfur in the volatiles and char


 \dot{m}_{S_{v/c}} = \dot{m}_{v/c} \cdot mf_{S_{v/c}} (20.2-12)


where    
  $\dot{m}_{S_{v/c}}$ = rate of release of fuel sulfur in kg/s
  $\dot{m}_{v/c}$ = rate of release of volatiles (v) or char (c) in kg/s
  $mf_{S_{v/c}}$ = mass fraction of sulfur in volatiles or char


Let    
  $P_{S_{v}}$ = percentage by mass of sulfur in volatiles
  $P_{S_{c}}$ = percentage by mass of sulfur in char
  $TS_{fuel}$ = percentage by mass of sulfur in fuel (daf)
  $S_{split}$ = split of sulfur between volatiles and char
  $F_{vol}$ = mass fraction of volatiles in coal (daf)
  $F_{char}$ = mass fraction of char in coal (daf)

Then the following should hold:


 F_{vol} + F_{char} = 1.0 (20.2-13)


 \frac{P_{S_{v}}}{P_{S_{c}}} = S_{split} (20.2-14)


 (F_{vol} \cdot P_{S_{v}}) + (F_{char} \cdot P_{S_{c}}) = TS_{fuel} (20.2-15)


 P_{S_{c}} = \frac{TS_{fuel}}{(F_{vol} \cdot S_{split}) + F_{char}} (20.2-16)

figure   

Note that if water is assumed to release at the same rate as volatiles, the above calculation has to be slightly modified.



Setting Turbulence Parameters


If you want to take into account turbulent fluctuations when you compute the specified SO $_2$ formation, select one of the options in the PDF Mode drop-down list under Turbulence Interaction.

Figure 20.2.4: The SOx Model Panel for a Gas Fuel Type with Turbulence
figure

As explained in Section  20.1.10, the mixture fraction option is available only if you are using the nonpremixed combustion model to model the reacting system.

Number of Beta Points

You can, optionally, adjust the number of Beta PDF Points. The default value is 10, indicating that the beta function in Equation  20.1-104 or Equation  20.1-105 will be integrated at 10 points on a histogram basis, will yield an accurate solution with reasonable computation time. Increasing this value may improve accuracy, but will also increase the computation time. This option is not available when you select mixture fraction in the PDF Mode drop-down list under Turbulence Interaction. In this case, the mixture fraction points defined in the PDF table will be used.



Specifying a User-Defined Function for the SOx Rate


You can, optionally, choose to specify a user-defined function for the rate of SOx production. The rate returned from the UDF is added to the rate returned from the standard SOx production options, if any are selected. However, if you would like to replace any or all of FLUENT's SOx rate calculations with your own user-defined SOx rate, you can turn on the Replace with UDF Rate option for the SOx formation after loading the UDF file.

Select the desired function in the SOx Rate drop-down list under User-Defined Functions. See the separate UDF Manual. for details about user-defined functions.



Defining Boundary Conditions for the SOx Model


At flow inlet boundaries, you will need to specify the Pollutant SO Mass Fraction, and if necessary, the Pollutant SH Mass Fraction, Pollutant H2S Mass Fraction, Pollutant SO3 Mass Fraction,and Pollutant SO2 Mass Fraction in the Species tab, as demonstrated in Figure  20.2.5.

Define $\rightarrow$ Boundary Conditions...

You can retain the default inlet values of zero for these quantities or you can input nonzero numbers as appropriate for your combustion system.

Figure 20.2.5: The Mass-Flow Inlet Panel and the Species Tab
figure


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