Coal gasification
 

I am modeling coal gasification in Fluent.I am using FINITE RATE chemistry with multiple surface reaction model for combustion and gasification.I have defined 7 reactions for gasification and combustion.But its seen that the surface reactions are not taking palce.

Can anybody suggest any modification ar alteration required for this?

Dyana

Re: Coal gasification
 

I work on a similar subject. I use a non-premixed combustion model with DPM, and it seems to work. Maybe you could send me a more detailed explanation of your models/problems?

Re: Coal gasification
 

Thanks Andy for ur suggestion.Is it possible to do coal gasification using non-premixed combustion model? Are u defining any pdf for this? In non-premixed combustion model how will u define more than 2 mixture fractions?I tried using non premixed but couldnt move forward.In this model how will u define all the gasification reactions?

Presently I am trying with finite rate chemistry with multiple surface reaction model.But it is also not working properly.

Can u please help me?

Dyana

Re: Coal gasification
 

non-premixed combustion needs only two mixture fractions, the other micture fractions are computed from these fractions. Non-premixed combustion uses "mixed-is-burnt", as far as I know, so that they can reduce it to two mixture fractions. Disadvantag is, that you can't see the slow reactions (e.g. a CO shift or something), but I think, those slow reactions might not be so important. So, actually you don't have to define the gasification reactions, you just create a pdf for your streams. Of course you need to know the composition of your coal and its heating value. What kind of coal gasification do you simulate?

Re: Coal gasification
 

Thanks for ur kind reply Andy. Ya, non-premixed combustion uses mixed is burnt, I know it.But how will u define the gasification reactions there?? There are reactions of coal with O2,CO2, H2O etc which are the main gasification reactions u can not neglect.After all, if u want to do steam gasification, how will u define it in pdf?? If u go thru any one of the published literatures u can see a number of reactions defined for coal gasification.Obviously one can do coal combustion in fluent but gasification is really a problem.

In ur case, r u getting good results?? What are the molefracions of CO and H2 u r getting??

Are u getting a well converged solution of DPM model?

pl reply

Dyana

Re: Coal gasification
 

Molefractions of CO and H2 are okay so far and agree relatively good with measurements. Of course, it's all in equivalence, so you get good results similar to simulations with ASPEN. Convergence actually is a problem. For steam gasification, I usually use coal as primary fuel, oxygen as oxidizer and steam as secondary fuel. It has the disadvantage, that you have to use primary fuel for volatiles and char, but if you can live with that, it's ok. The reactions of the coal (or the primary fuel) with O2, CO2 and H2O are included in the pdf. Those pdfs are usually created for combustion, but the gasification reactions are included (since even at combustion there are areas, where lambda<0.6). Another possibility for the simulation is a complete kinetic model (e.g. the GRI 3.0), but I don't know exactly how to use that. But that one really slows your simulation down, because it adds a lot of equations to your system. So, as far as I understand that system, you get some gas from the volatiles at first, which reacts with the other gases as soon as it's mixed, and you get a char (with the composition of the fuel) that reacts with the gas, too, limited by diffusion. It was possible to obtain acceptable results with this method, although there still is a lot to make better.

Re: Coal gasification
 

Thanks for ur valuable instantaneous reply Andy. What is GRI 3.0, I dint hear about it.

I use finite rate chemistry by defining different reactions.I am using multiple surface reaction model for char gasification.As u said in this model volatiles are released first.Then gasification reactions taking place according to the reactions defined.But i am getting the molefraction of CO as 30% which is higher I think.Is it acceptable? Again I have a doubt that whether the surface reactions are taking place. The reaction rate for the surface reaction in DPM showing zero value. Can u help me in this regard?

Are u doing any research in coal gasification?

Dyana

Re: Coal gasification
 

Of course the value of molefraction of CO depends on the composition of your fuel and the amount of oxygen. Maybe you should make a molecular bilance for every atom (C,O,H,N,S) to check wether the results at your outlet are OK or not. You can also have a look at trajectory samples and particle histories (it's described in chapter 23.12.6. and 23.12.7 in the fluent users guide, just try a little bit :) ), so you can see, how the mass of different particles evolves during the gasification process. You should see one step, where volatiles are released and another period, where the surface reactions take place. In finite rate chemistry, how do you define the composition of your fuel? Maybe you should have a look at this - maybe it's not correctly identified as reaction partner? When using pdf and dpm as reaction model, you can see devolatilization and burnout in your reactor under contours-->dpm-->devolatilization / burnout. GRI 3.0 is the description of the complete kinetic reaction scheme of burning methane, which can be used for gasification too (although it might not be as exact as for combustion). And yes, I do some research in gasification of coal and oil, but I'm not in that topic for all too long.

Re: Coal gasification
 

Ya u r right, the CO value depends on the composition and O2.I know to check trajectory and particle history and checked the conversion of volatiles and char, that is ok.But in report panel in discrte phase the summary is not coming and another problem with my model is the reaction rates for all the surface reactions showing zero.D o u know why it is showing zero??

In finite rate chemistry i have many reactions.Based on the proximate analysis and ultimate anlysis i will find out the volatiles composition on ash basis , then the volatiles combustion is defined for which the kinetic data are available in fluent database for diff coals.Then the char combustion and gasification reactions are defined.

In ur pdf how will u define the steam percentage ?if u want to inject different amount of steam , how will u do in pdf? may i know ur location?

Re: Coal gasification
 

Please send an email to l i t t l e a n d y 2 0 0 3 ( a t ) w e b ( d o t) d e , so it's easier to talk (remove spaces).

Re: Coal gasification
 

i have send u a mail, but no reply

dyana

Re: Coal gasification
 

Sorry, didn't receive it. Did you remove spaces and put in @ for (at) and . for (dot)?

Re: Coal gasification
 

ya, urs id is littleandy2003@web.de?

Re: Coal gasification
 

Yes, indeed.

Re: Coal gasification
 

I have send 2 mails, but no reply from u. Here is my id dyanadas_1982@yahoo.com

pl send me a mail

Sorry, I have a work that similar as this case, which multiphase model did you use ?

You should use species transport (Non-Premixed). also you should enter fuel as solid combusting particle as discrete phase.

I have the same simulation. Do you mean the secondary fuel is (as the same as) secondary stream? Because, I have read that the secondary stream is volatile component of coal. If no, can you explain more? Thanks.

have you changed the combustion model to multiple surface reactions and specified the mass fraction of the solid species?

Thanks for your attention.

I have simulated dry coal gasification using Non-Premixed Combustion. I didn't use multiple surface reactions because I could not know surely what are the reactions and I had difficulty when I created PDF table, there was error. So, I used one stream-fuel and one stream-oxidizer. I used kinetic/diffusion-limited surface reaction rate and single rate model. It worked. The result was good enough.