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-   -   Bubbling Fluidized Bed reactor burning biomass (http://www.cfd-online.com/Forums/fluent/33876-bubbling-fluidized-bed-reactor-burning-biomass.html)

Mattias June 3, 2004 08:24

Bubbling Fluidized Bed reactor burning biomass
 
Hi everyone!

Im simulating a Bubbling Fluidized Bed (BFB) reactor burning biomass. The geometry is 12 m high and 3*4 m wide (800 000 cells). I have set my boundary at the bed surface to avoid modelling the dense bed. The splash-zone and freeboard is modelled as a porous zone with a momentum resistance. The fuel is modelled as gas (CH4 and CO) and the combustion as a 2 reaction mechanism. Im using the eddy disipation model to calculate the reaction rate and realizable k-e turbulence model.

My problem is that the reaction are going to fast. I have a complete burnout of fuel before it get to the overfire distributors and therefore the temperature in the lower part of the furnace is to high while in the upper part, the overfire air cools the gases down to much.

Any sugestions? Is the EBU-model better or should I use a more complex reaction mechanism? Or is a 2 phase model better?

Thanks! /Mattias

Ryan Zarnitz June 3, 2004 14:19

Re: Bubbling Fluidized Bed reactor burning biomass
 
If your fuel is completely burning out before the overfire air inlets, then it's possible that you have too much undergrate air for your amount of fuel. I suggest checking on that, provided that you are confident about your kinetic model.

Mattias June 4, 2004 02:18

Re: Bubbling Fluidized Bed reactor burning biomass
 
The boundary conditions are pre-set from measuredments on a real boiler. The mass balance over the furnace i correct. It is the kinetic model that im am unsure of. I have modeled the solid fuel as gas and the combustion of gas is much faster than the solid fuel. Is a two-phase model required?

/Mattias

Ryan Zarnitz June 4, 2004 11:32

Re: Bubbling Fluidized Bed reactor burning biomass
 
I can't comment on the need for a two-phase model wihtout knowing detailed information about your system, but you could try using the non-premixed combustion model and see if that works better. That works by equilibrium, so you won't need detailed kinetics. Also, keep in mind that air-flow measurement in real boilers is usually not very good, and the burnout stoiciometry for methane is probably different than your biomass, so you may want to take a second look at that.

Ryan

ap June 6, 2004 14:32

Re: Bubbling Fluidized Bed reactor burning biomass
 
Well, a bubbling fluidized bed is a multiphase system, so a multiphase model is needed, but reaction and mass transfer aren't available yet in the Eulerian Model.

Hi :)

ap

Mattias June 7, 2004 07:02

Re: Bubbling Fluidized Bed reactor burning biomass
 
The problem is that I have limited CPU power and simulatin a BFB with all that sand in 3D would take enourmus resources. I have developed a corse model to include the heat transfer effect of the sand particle motion through a UDF. Still, the problem is that the fuel is reacting to fast since im using the eddy disipation model. I have ahd thought of using the EBU to include Ahrrenius expressions for the reaction.

/Mattias

Karthick June 8, 2004 02:53

Re: Bubbling Fluidized Bed reactor burning biomass
 
Hello Mattias

What do u mean by " I have developed a corse model to include the heat transfer effect of the sand particle motion through a UDF". Are u patching the sand temperauture/velocity in your eddy dissipation model? What are u coming to say by "reacting fast". The rate of the reactions all are direcly proportional to the preexponential factor and activation energy which you are giving as input. These values are to be adjusted to change the rate of the reaction.

Regards Karthick



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