Simcenter STAR-CCM+ How is the coal volatile and char fraction contribution considered in coal combustion simulation
Summary
we will understand where to enter the coal volatile and char fraction terms in the simulation and see how it affects the results.
Details
To simulate coal combustion in Simcenter STAR-CCM+ you require the following information
- Proximate analysis of the Coal sample
- Calorific value of the Coal sample (LCV)
- Specific Heat of Coal
- Density of Coal
As a solid fuel, raw coal goes through a 4-step breakdown and combustion process. Simcenter STAR-CCM+ has models for each of these steps.
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Step 1: Evaporation of the moisture content in the coal via the Coal Moisture Evaporation model
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Step 2: Coal volatile formation and release via the Coal Devolatilization model. (The coal volatile gases will then burn through your chosen gas phase combustion model)
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Step3: Oxidation of the remaining carbon via the Char Oxidation model. (Once the volatile gases are released from the coal in Step 2, most of what is left is solid carbon known as char.)
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Step 4: Production of NOx from the nitrogen content of the coal via the Coal Fuel NOx model.
In this article you will see how the to account for the volatile fraction and char fraction in the simulation setup. Two examples of how to specify the split between the volatile fraction and remaining char are provided.
Scenario 1: You will enter the volatile fraction directly into the simulation based on the measured proximate and ultimate analyses of the fuel. The char fraction is then calculated automatically by the software.
Scenario 2: Alternatively you specify the char fraction directly at the injector. This implies there is no char calculated from the proximate analysis which means RawCoal has only volatile matter.
Scenario1
Let’s take the following proximate analysis. This information is used to specify the RawCoal mass fraction at the injector, and the volatile yield in the Coal Devolatilization model.
Proximate analysis:
|
Volatile Matter (VM) |
37.4 |
|
Fixed Carbon (Char) |
54.3 |
|
Ash |
8.3 |
At the Injector the RawCoal mass fraction is determined from the proximate analysis: RawCoal = VM + Char = 0.374+0.543 = 0.917. (Note the Char is 0 at the injector)
The volatile yield, YY, is just the volatile matter fraction in the raw coal. In our example 0.374/0.917 = 0.40785 which we enter below in the simulation.
The remaining fraction in Raw Coal which is nothing but char. This is equal to 1-VM fraction = 1-0.40785 = 0.59215 which will be used automatically by the char oxidation reaction in the background (you need not enter any value)
Scenario 2
To account for char oxidation independent of the proximate analysis, you would enter the value of the char mass fraction directly at the injector, and set YY=1 under the devolatilization model. The char value at the injector is then used for the char oxidation reaction directly, and YY=1 implies there is no char calculated from the proximate analysis which means RawCoal has only volatile matter.
Regardless of scenario used, there are two useful reports which will help you to quantify the results:
- Coal Volatile Matter (VM) Yield report = Volatile Matter Produced/Initial Mass of Raw Coal
- Char Burn Out report = Char Oxidized/(Char Produced + Initial Mass of Char)
See also : Simulating Physics > Combustion and Other Reacting Flows > Interphase