15.15 Coal Modeling Inputs in FLUENT

Within FLUENT, the DPM coal combustion simulation is defined as usual when the non-premixed combustion model is selected. The air (oxidizer) inlets are defined as having a mixture fraction value of zero. No gas phase fuel inlets will be included and the sole source of fuel will come from the coal devolatilization and char burnout. The coal particles are defined as injections using the Set Injection Properties panel in the usual way, and physical properties for the coal material are specified as described in Section  22.14. You should keep in mind the following issues when defining injections and discrete-phase material properties for coal materials:

• In the Set Injection Properties panel, you will specify for the Oxidizing Species one of the components of the oxidizer stream. This species concentration field will be used to calculate the diffusion-controlled driving force in the char burnout law (if applicable).

The specification of the char and volatile streams differs depending on the type of model you are defining:

• If the coal is modeled using a single mixture fraction, the gas phase species representing the volatiles and the char combustion are represented by the mixture fraction used by the non-premixed combustion model.

• If the coal is modeled using two mixture fractions, rather than specifying a destination species for the volatiles and char, you will instead specify the Devolatilizing Stream and Char Stream.

• If the coal is modeled using one mixture fraction, and another fuel is modeled using a second mixture fraction, you should specify the stream representing the coal as both the Devolatilizing Stream and the Char Stream.

• In the Materials panel, Vaporization Temperature should be set equal to the fuel inlet temperature. This temperature controls the onset of the devolatilization process. The fuel inlet temperature that you define in the Boundary tab of the Species Model panel should be set to the temperature at which you want to initiate devolatilization. This way, the look-up tables will include the appropriate temperature range for your process.

• In the Materials panel, Volatile Component Fraction and Combustible Fraction should be set to values that are consistent with the coal composition used to define the fuel (and secondary) stream composition.

• Also in the Materials panel, you will be prompted for the Burnout Stoichiometric Ratio and for the Latent Heat. The Burnout Stoichiometric Ratio is used in the calculation of the diffusion controlled burnout rate but has no other impact on the system chemistry when the non-premixed combustion model is used. The Burnout Stoichiometric Ratio is the mass of oxidant required per mass of char. The default value of 1.33 assumes that C(s) is oxidized by O to yield CO. The Latent Heat input determines the heat required to generate the vapor phase volatiles defined in the non-premixed system chemistry. You can usually set this value to zero when the non-premixed model is used, since your definition of volatile species will have been based on the overall heating value of the coal. However, if the coal composition includes the water content, the latent heat should be set as follows:
• Set latent heat to zero if the water content of the coal has been defined as H2O(L). In this case, the system chemistry will include the latent heat required to vaporize the liquid water.

• Set latent heat to the value for water (2.25 10 J/kg), adjusted by the mass loading of water in the volatiles, if the water content of the coal has been defined using water vapor, H2O. In this case, the water content you defined will be evolved along with the other species in the coal but the system chemistry does not include the latent heat effect.

• The Density you define for the coal in the Materials panel should be the apparent density, including ash content.

• You will not be asked to define the Heat of Reaction for Burnout for the char combustion.

Postprocessing Non-Premixed Models of Coal Combustion

FLUENT reports the rate of volatile release from the coal using the DPM Evaporation/Devolatilization postprocessing variable. The rate of char burnout is reported in the DPM Burnout variable.

Previous: 15.14 Defining Non-Premixed Physical
Up: 15. Modeling Non-Premixed Combustion
Next: 15.16 Solution Strategies for