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-   -   Coal gasification-problem with convergence (http://www.cfd-online.com/Forums/fluent/38403-coal-gasification-problem-convergence.html)

Claudia October 27, 2005 08:20

Coal gasification-problem with convergence
 
Hello FLUENT users, i am working on a coal gasification problem. In principle, the mesh seems to be working as long as I am only using the coal as a gasequivalent (I use the coal composition calculated from the prePDF). As soon as I inject only a small mass flow of real coal, I get problems in getting a converged solution. When I increase the mass flow of the real coal even more, the residuals also increase and don't seem to drop down somewhen. Can anybody help me with this problem? Another problem is that i can't get rid of the turbulence viscosity ratio problem (about 2500 out of 30000 cells) Thanx a lot for your advise! Claudia

RoM October 27, 2005 08:43

Re: Coal gasification-problem with convergence
 
For the turbulent viscosity problem

http://www.cfd-online.com/Forum/flue...cgi?read=29389

What model do you use for gasification? From my experience prePDF is not suitable for gasification of solids since you can specify only one oxidizing species.

RoM

Claudia October 27, 2005 09:08

Re: Coal gasification-problem with convergence
 
Hi RoM, Thanx for the quick response. I will try to solve the turbulence viscosity problem now. I am using the nonpremixed combustion model, realizable ke-turbulence model as well as the p1-model for the particle radiation. I do have 3 different fuels. In the center I have a pilot burner (using methane and oxygen mixed with some steam) and the coal burner (using the coal and oxygen mixed with some steam). So i have a fuel, a secondary fuel and an oxydizer. Why do you think is the prePDF not the appropriate tool for simulating a coal gasification? And what would be the alternative to it?. Claudia

RoM October 27, 2005 09:41

Re: Coal gasification-problem with convergence
 
The main problem about prePDF is, you will have to specify the oxidizing specie for the coal (solid) in the injection panel. You could use O2 as oxydizer with an burnout ratio of 1.33 but somewhere in your gasfier you will run out of oxygen and then CO2 and H2O will become important as they will continue to convert you carbon according to C+CO2->2CO and C+H2O->CO+H2O (both endothermal reactions). Since you can specify only one oxidizer O2 OR CO2 OR H2O you will neglect 2 of the 3 possible oxidizing reactions. This will lead to wrong gas qualities and temperature profiles. If you actually manage to produce reasonable results with prePDF i would be glad to hear about. Best results from my experience came from speciec transport model with multiple surface reactions. This way you can specify as many gas-solid reactions as you want. I am currently writing a new model based upon species transport with finite rate chemistry for gas-solid reactions and equilibrium chemistry for the gas phase. A good reference about gasification modeling can be found here C.Chen, M.Horio, T.Kojima :"Numerical simulation of entrained flow coal gasifiers", Chemical Enginieering Science, 2000, p3861-3883.

RoM

Claudia October 28, 2005 02:05

Re: Coal gasification-problem with convergence
 
But then i am wondering why does Fluent use the nonpremixed combustion (with prePDF) for coal combustion in their tutorials. But could that lead to too long flames in the reactor? The temperatures in the flame seem to be reasonalbe with about 3120 K at the most. Thank your for your advise also regarding the literature.

Claudia

RoM October 28, 2005 02:46

Re: Coal gasification-problem with convergence
 
Nonpremixed combuston with preDF is suitable because you will have oxygen in abundance. You dont dont have to deal with oxygen lean/oxygen rich conditions so the assumption of only one oxydizing reaction is good. In gasification things are unfortunatly not that easy for the reasons mentioned in my last post If you want to optimize your gasifier in terms of carbon conversion then prePDF wont do the job. The flame length is more related to your reaction kinetics and the turbulent mixing while the flame temperature is affected by intermediate (dissociated) species (like OH).

RoM

Dimitris November 7, 2005 15:14

Re: Coal gasification-problem with convergence
 
Halo to everyone, I am new to CFD modelling, but I am starting my phD with the subject "Modelling of Integrated biomass gasification with IC engine" . Since you have a great experience in combustion modelling could you advise me if Fluent is the appropriate software for the above application? I will split the procedure in 3 components (1) biomass gasification (2) gas cleaning (3) ic engine integration can Fluent treat all 3 procedures together?

RoM November 8, 2005 03:08

Re: Coal gasification-problem with convergence
 
I think fluent can handle the gasification and the IC engine. I am not shure about the gas cleaning.

RoM


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