8.11.4. Hydrocarbon Fuel Model Setup

The hydrocarbon fuel model offers a simple method of creating a solid particulate or liquid droplet combustion model starting from the ultimate and proximate analysis of the fuel. It uses the Lagrangian particle transport model to track representative fuel particles and uses the Eddy Dissipation model and/or the Finite Rate Chemistry model for the combustion of the volatile gases in the gas phase. Devolatilization (or pyrolysis) of the hydrocarbon fuel particles and oxidation of the resultant char are modeled by semi-empirical kinetic models with parameters that depend on the fuel composition and a standard fuel analysis. The model was mainly designed for the modeling of pulverized coal combustion but can also be applied to combustion of other solid hydrocarbon fuels in particulate form or liquid sprays.

This section describes the setup for the hydrocarbon fuel model. Three variants are covered here:

  • Proximate/ultimate fuel analysis starting from the standard library template

  • Proximate/ultimate fuel analysis when setting up manually

  • Using generic multiphase reactions set-up

The first method, using the fuel analysis directly, is recommended. The third method is mainly included for compatibility with earlier releases of the software.

 

8.11.4.1. Setup using Library Template (Recommended)

The recommended way of setting up the hydrocarbon fuel model is to start by loading from the library provided. This approach uses the full functionality of the model, but avoids unnecessarily repeating the definition of materials and reactions. All fuel dependent data, like material composition and heat release, is gathered in the hydrocarbon fuel material definition. Therefore, after loading the library, most objects do not need to be changed by you. In most cases, it will be sufficient to only edit the hydrocarbon fuel material object.

The pre-defined multiphase reactions assume certain names for the hydrocarbon fuel material and for the gas mixture. The name of the hydrocarbon fuel is HC Fuel. The gas mixture material name depends on the fuel-nitrogen model: Gas Mixture HCN NO with the fuel-nitrogen model; and Gas Mixture without.

The hydrocarbon fuel model without the fuel-nitrogen sub-model can be set up using the provided library as follows:

  1. From either the Materials or the Reactions object, select Import Library Data from the context menu to import all objects from the hydrocarbon fuel library Hydrocarbon_Fuel.ccl located in the reactions-extra/ directory. Additional materials and reactions will be included automatically from the standard libraries. Note that when loading the Hydrocarbon_Fuel.ccl file using the general File > Import CCL... route, then some additional materials and reactions will not be loaded automatically, therefore causing corresponding physics error messages. In this case the missing materials and reactions need to be loaded manually in order to resolve the error messages.

  2. Edit the imported material HC Fuel. The parameters on the Proximate/Ultimate Analysis tab need to be adjusted in order to match the actual fuel specification:

    1. Proximate analysis

      • As Received (mass fractions sum to 1)

      • Dry Ash Free (fixed carbon and volatiles sum to 1)

    2. Ultimate analysis

      • As Received (mass fractions sum to 1 – ash – moisture)

      • Dry Ash Free (mass fractions sum to 1)

    3. Heating value

    4. Volatiles yield enhancement (ratio of actual volatile yield to proximate yield) - Defaults for remaining parameters

  3. Create the domain:

    1. Fluid material: Gas Mixture

    2. Particle material HC Fuel

    3. Multiphase reactions:

      • HC Fuel Devolat

      • HC Fuel Char Field or HC Fuel Char Gibb

    4. Heat transfer model fluid dependent:

      • Fluid: Thermal or Total Energy

      • Particle: Particle Temperature

    5. Combustion model fluid dependent:

      • Fluid: Eddy Dissipation

    6. Radiation model fluid dependent:

      • Fluid: P1, Discrete Transfer, or Monte Carlo

    7. Fluid component details:

      • N2: Constraint

      • Others: Automatic

    8. Particle diameter change during pyrolysis (Fluid: HC Fuel):

      • Option = Swelling Model

      • Reference Material = Raw Combustible

      • Swelling Factor = <real> # 0.0 for constant diameter

    9. Thermal radiation transfer (Fluid Pair: HC Fuel | Gas Mixture HCN NO):

      • Option = Blended Particle Emissivity

      • Reference Material = Raw Combustible

      • Base Emissivity = <real> # Emissivity of char and ash

      • Blend Emissivity = <real> # Emissivity of raw combustible

  4. Set particle ignition under Solver Control / Particle Control:

    • Particle Ignition / Ignition Temperature = 1000 [K]

No boundary conditions for particle components will be defined. The remaining setup is the same as for pure-material particles and a variable composition fluid.

The fuel-nitrogen model can be enabled by the following changes to the above procedure:

  1. Edit the HC Fuel material. On the Mixture Materials tab change the option in the Gas Mixture section:

    1. Gas Mixture/Option = Mixture with HCN NO

    2. Mixture Material = Gas Mixture HCN NO

    3. Component materials:

      • HCN Material = HCN

      • HCO Material = HCO

      • NO Material = NO

    4. Remaining parameters unchanged

  2. Change the fluid and the multiphase reactions on the Edit Domain form:

    1. Fluid material Gas Mixture HCN NO

    2. Multiphase reactions:

      • HC Fuel Devolat HCN

      • HC Fuel Char Field HCN or HC Fuel Char Gibb HCN

 

8.11.4.2. Set Up Manually (Experts)

The manual set up is usually not required and is intended for expert users only. For this procedure, the definition of material and reaction objects is completely left to you. When doing so, keep in mind that the hydrocarbon fuel model can detect only certain types of reactions for computation of mass coefficients and stoichiometric coefficients. The auto-computed coefficients are reported to the CFX-Solver Output file and should be checked for consistency.

In order to set up the coal model from scratch, the following additional steps are required to create the required materials and reactions:

  1. Create component materials:

    1. Gas phase components: Fuel Gas, O2, CO2, H2O, N2, …, NO, HCN, HCO

    2. Hydrocarbon fuel components: Ash, Char, Raw Combustible

  2. Create gas phase reactions:

    1. Fuel gas oxidation

    2. Pollutants reactions (for example, for NO)

  3. Create mixture materials:

    1. Homogeneous binary mixture: Fuel Gas, Raw Combustible

    2. Gas phase reacting mixture: components and reactions previously created

    3. Hydrocarbon fuel: Proximate/ultimate analysis data; homogeneous binary mixture material; gas mixture material

  4. Create multiphase reactions:

    1. Devolatilization (with or without fuel nitrogen)

    2. Char oxidation (with or without fuel nitrogen)

From this point on the setup can be continued as described in section Setup using Library Template (Recommended), with the materials and reactions replaced with those created locally.

 

8.11.4.3. Using Generic Multiphase Reactions Setup

A third way of specifying combustion of a hydrocarbon fuel is to use the generic multiphase multi-component and reaction machinery. In this case, it is up to you to set the appropriate particle component boundary values, stoichiometric coefficients for reactions, and heat release for the gas phase reactions (by means of volatiles reference enthalpy).