## 15.9.5 Modeling Coal Combustion Using the Non-Premixed Model

If your model involves coal combustion, the fuel and secondary stream compositions can be input in one of several ways. You can use a single mixture fraction (fuel stream) to represent the coal, defining the fuel composition as a mixture of volatiles and char (solid carbon). Alternatively, you can use two mixture fractions (fuel and secondary streams), defining the volatiles and char separately. In two-mixture-fraction models for coal combustion, the fuel stream represents the char and the secondary stream represents volatiles. This section describes the modeling options and special input procedures for coal combustion models using the non-premixed approach.

There are three options for coal combustion:

• When coal is the only fuel in the system, you can model the coal using two mixture fractions. When this approach is used, one stream is used to represent the char and the other stream is used to represent volatiles. Generally, the char stream composition is represented as 100% C(s). The volatile stream composition is defined by selecting appropriate species and setting their mole or mass fractions. Alternatively, you can use the empirical method (input of atom fractions) for defining these compositions.

 Using two mixture fractions to model coal combustion is more accurate than using one mixture fraction as the volatile and char streams are modeled separately. However, the two-mixture-fraction model incurs significant additional computational expense since the multi-dimensional PDF integrations are performed at run-time.

• When coal is the only fuel in the system, you can choose to model the coal using a single mixture fraction (the fuel stream). When this approach is adopted, the fuel composition you define includes both volatile species and char. Char is typically represented by including C(s) in the species list. You can define the fuel stream composition by selecting appropriate species and setting their mole fractions, or by using the empirical method (input of atom fractions). Definition of the composition is described in detail below.

 Using a single mixture fraction for coal combustion is less accurate than using two mixture fractions. However, convergence in FLUENT should be substantially faster than the two-mixture-fraction model.

• When coal is used with another (gaseous or liquid) fuel of different composition, you must model the coal with one mixture fraction and use a second mixture fraction to represent the second (gaseous or liquid) fuel. The stream associated with the coal composition is defined as detailed below for single-mixture-fraction models.

Defining the Coal Composition: Single-Mixture-Fraction Models

When coal is modeled using a single mixture fraction (the fuel stream), the fuel stream composition can be input using the conventional approach or the empirical fuel approach.

• Conventional approach:

To use the conventional approach, you will need to define the mixture of species in the coal and their mole or mass fractions in the fuel stream. Use the Boundary tab in the Species Model panel to input the list of species (e.g., C H , CH , CO, CO , C(s)) that approximate the coal composition, and their mole or mass fractions.

Note that C(s) is used to represent the char content of the coal. For example, consider a coal that has a molar composition of 40% volatiles and 60% char on a dry ash free (DAF) basis. Assuming the volatiles can be represented by an equimolar mixture of C H and CO, the fuel stream composition defined in the Boundary tab would be C H =0.2, CO = 0.2, and C(s)=0.60. Note that the coal composition should always be defined on an ash-free basis, even if ash will be considered in the FLUENT calculation.

To define ash properties, go to the Materials panel and select combusting-particle as the Material Type.

The following table illustrates the conversion from a typical mass-based proximate analysis to the species fraction inputs required by FLUENT. Note that the conversion requires that you make an assumption regarding the species representing the volatiles. Here, the volatiles are assumed to exist as an equimolar mix of propane and carbon monoxide.

 Proximate Analysis Weight % Mass Fraction (DAF) Moles (DAF) Mole Fraction (DAF) Volatiles - C H - CO Fixed Carbon (C(s)) Ash 30 60 10 0.2035 0.1295 0.667 - 0.004625 0.004625 0.05558 - 0.07134 0.07134 0.85732 - (Total) 0.06483 1.0

Moisture in the coal can be considered by adding it in the fuel composition as liquid water, H O(l). The moisture can also be defined as water vapor, H O, provided that the corresponding latent heat is included in the discrete phase material inputs in FLUENT. If the liquid water is used as a boundary species, it should be removed from the list of excluded species (see Section  15.10.1).

 Note that if water is included in the coal, the water release is not modeled as evaporation, which is typically the case in the wet combustion model, described in Section  22.12.2.

• Empirical fuel approach:

To use the empirical approach, enable the Empirical Fuel Stream option in the Chemistry tab. This method is ideal if you have an elemental analysis of the coal.

In the Chemistry tab, input the lower heating value and mean specific heat of the coal. FLUENT will use these inputs to determine the mole fractions of the chemical species you have included in the system. Then, in the Boundary tab, define the atom fractions of C, H, N, S, and O in the fuel stream.

Note that for both of these composition input methods, you should take care to distinguish atomic carbon, C, from solid carbon, C(s). Atomic carbon should only be selected if you are using the empirical fuel input method.

See Section  15.15 for details about further inputs for modeling coal combustion.

Defining the Coal Composition: Two-Mixture-Fraction Models

If your FLUENT model will represent the coal components using both the fuel stream and the secondary stream, one stream is used to represent char and the other stream is used to represent volatiles. (In the procedures below, it is assumed that the fuel stream represents the char and the secondary stream represents the volatiles.)

As in single-mixture-fraction cases, the fuel stream and secondary stream compositions in a two-mixture-fraction case can be input using either the conventional approach or the empirical fuel approach.

• Conventional approach:

To use the conventional approach, you will need to define the mixture of species in the coal and their mole or mass fractions in the fuel and secondary streams.

Use the Boundary tab of Species Model panel to define the mole or mass fractions of volatile species in the secondary stream (e.g., C H , CH , CO, CO , C(s)). Next, define the mole or mass fractions of species used to represent the char. Generally, you will input 100% C(s) for the fuel stream.

• Empirical fuel approach:

To use the empirical fuel approach, enable the Empirical Secondary Stream option in the Chemistry tab for the volatile (in this case, secondary) stream. This method is ideal if you have an elemental analysis of the coal.

In the Chemistry tab, input the lower heating value and mean specific heat of the coal. Then, in the Boundary tab, define the mole or mass fractions of species used to represent the char. Generally, you will input 100% C(s) for the fuel stream. Finally, define the atom fractions of C, H, N, S, and O in the volatiles. FLUENT will use these inputs to determine the mole fractions of the chemical species you have included in the system. For example, consider coal with the following DAF (dry ash free) data and elemental analysis:

 Proximate Analysis Wt % (dry) Wt % (DAF) Volatiles 28 30.4 Char (C(s)) 64 69.6 Ash 8 -

 Element Wt % (DAF) Wt % (DAF) C 89.3 89.3 H 5.0 5.0 O 3.4 3.4 N 1.5 2.3 S 0.8 -

(Note that in the final column, for modeling simplicity, the sulfur content of the coal has been combined into the nitrogen mass fraction.)

You can combine the proximate and ultimate analysis data to yield the following elemental composition of the volatile stream:

 Element Mass Mass Fraction Moles Mole Fraction C (89.3 - 69.6) 0.65 5.4 0.24 H 5.0 0.16 16 0.70 O 3.4 0.11 0.7 0.03 N 2.3 0.08 0.6 0.03 Total 30.4 22.7

This adjusted composition is used to define the secondary stream (volatile) composition.

The lower heating value of the volatiles can be computed from the known heating value of the coal and the char (DAF):

• LCV = 35.3 MJ/kg

• LCV = 32.9 MJ/kg

You can compute the heating value of the volatiles as

or

Note that for both of these composition input methods, you should take care to distinguish atomic carbon, C, from solid carbon, C(s). Atomic carbon should only be selected if you are using the empirical fuel input method.

See Section  15.15 for details about further inputs for modeling coal combustion.

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