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Modelling Biomass Combustion via Species Transport

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Old   July 18, 2007, 10:14
Default Re: Modelling Biomass Combustion via Species Trans
  #21
Kino
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Dear Allan,

Now I generally understand how you construct a matrix. For the case of consdering for example CH4, CO2, H2O, H2, CO, NH3, H2S, two additional equations will be required besides 5 elemental balance equations. One of them is based on the data of heating value. Now, my questions are:

(1)for simulation of biomass or coal, how do you determine the ratio of H2O/CO2? Why do you prefer to choosing the relation between H2O and CO2 insead of others? Is it easy to get from experimental findings?

(2) In your calculation, I suppose that the MW of volatiles is assinged directly by you? Am I right?

(3) If I add one product for the volatiles,say Tar, I need an additional equation for balancing. In this case, could you suggest an approach to select one closure relationship?

(4) In addition, I noticed that the volumetric basis (rather than a mass fraction!) is used for moisture fraction in wet combustion. But I aslo noticed that the mass fracton is mentioned t be used in WET combustion (see eq 23.3-27, UG6.2). Now, I am really confusing. What is your opinion on this?

Thank you very much. Kino.

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Old   July 19, 2007, 14:42
Default Re: Modelling Biomass Combustion via Species Trans
  #22
Allan Walsh
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(1) In my general case, if you set the fractions of CH4 and CO2, the concentrations of the other species will be determined. The assumed fractions of CH4 and CO2 will influence the calculated heating value, so if the heating value is independently known, it provides a guide in the assumptions.

(2) The MW is determined in my calculation, independent of Fluent, and then specified in as a fluid property.

(3) Your tar would have some composition (say C7H8O like cresol) which would have to be accounted for in the mass and energy balance. I may not have made it clear, but this approach is to generate a volatile gas that is consistent with the biomass fuel composition. Since there are more unknowns than equations, there is more than one possible combination.

Also, the approach (which I have used) in determining the volatiles composition ahead of time can't account for differences in the products - say as a function of local temperatures. You may want to change the tar fraction if the heat flux is low, for example. To do this takes extra steps.

(4) My opinion on the moisture fraction is that it has to be verified in each case. If you are modeling 10 kg/s of fuel and it is 25% moisture by mass, then there better be 2.5 kg/s of water showing up. The cases where the definition (mass or volume) becomes a problem is when the particle density is not constant. You can try this out for yourself.
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Old   July 19, 2007, 19:44
Default Re: Modelling Biomass Combustion via Species Trans
  #23
Kino
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Dear Allan Walsh,

I still do not exactly understand your answer (1). Maybe, our calculation steps are different. My calculation steps are as follows1)By means of the data of HHV,proximate and ultimate analysis of biomass, the elemental compositions and formation enthalpy for the volatile is calculated firstly; (2) then, I would like to decompose the volatile into CH4,CO,CO2,H2,H2O,tar. Because of step 1, the information of HHV ( or formation enthalpy,alternatively)of the volatile has already been used in my case. Therefore, the problem in this step now turns: How to use just three elemental blance equation (C,H,O) to determine 6 coefficients for CH4,CO,CO2,H2,H2O,tar? Even not considering tar, two additional equations are needed.

At present,I am actually in trouble of choosing the effective relationship or correlations to close all the equations. Especially for biomass prolysis/devolatilzation,how to select out these relations?could you please help me out? Thanks in advance!
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Old   July 20, 2007, 00:40
Default Re: Modelling Biomass Combustion via Species Trans
  #24
Racheal
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Can someone tell me if I have done the above calculations correctly? Hoping for replies, thank you very much!
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Old   July 26, 2007, 12:38
Default Re: Modelling Biomass Combustion via Species Trans
  #25
majid
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i cant define tow or more udf .
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Old   October 20, 2009, 21:48
Question Issues with Steam gasification
  #26
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Elango
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Fluent inherently has a mixture type for woody-volatiles ,that cud be combusted with air.But i would like to do the modeling by gasifying the wood chips with high temperature steam instead of air.

I am having problems with defining the wood chips in the global equation and determining the number of reactions required for modeling since, the product gas composition includes H2,CO,CO2,H2O,CH4
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Old   March 4, 2014, 23:22
Default modelling species-porous zone-
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Hassan Khodaei
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Dear Allan,
Many thanks for good information regarding to modelling species in biomass combustion.
I am trying to create a fixed bed model combustion in Ansys-Fluent 14.5 using porous zone part.I want to apply 12 kw heat flux at the top of the bed (porous media) and study moisture evaporation and volatile released from densified wood pellets which are used in our experimental investigation ( A 20 cm cylendrical fixed bed combustor). Do you think, is it possible to model species by porous zone in fluent or not? If Not is there any other suggestion for modelling fixed bed in Fluent? Do I need writing UDF for modelling moisture evaporation and devolatilisation or the available models in fluent could be responsible to my requirements?
Kind Regards,
Hassan Khodaei.
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Old   April 28, 2016, 06:19
Default CFD modeling of wood pellet combustion
  #28
iam
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Hi all,

This is a post from a few years, but I am experiencing similar doubts at the moment, and I was wondering if someone could help.

I am trying to simulate with Fluent the combustion of wood pellets, At the moment I am using the wood-air model provided by Fluent. Could someone help me to calculate a good molecular weight and heat of formation of the volatiles? I have the elementary analysis of the wood pellets and the burning rate and I would like to estimate the molecular weight and heat of formation from those values.

Once I am done with this calculation, I would like to move on an use a more detailed reaction mechanism for my calculations. I already have a reduced mechanism, but I am not sure how I could estimate the composition of the initial volatile gases from the elementary composition of the pellets (C,H,O,N,S).

Thanks a lot for your help!!!!
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