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Jeberti April 7, 2010 09:23

Solid Fuel Modelling

I am working on solid fuel combustion( both in biomass and coal). i was encountering problem in defining the mixture property of the solid HC fuels. In the manual of Ansys_CFX,for the volatile fuel material option, it gives the user an option to change among pure substance, fixed composition mixture and variable composition mixture.
once i define the fixed composition mixture with some mass fraction the
simulation stops with error message telling " only pure substance can be
put as volatile fuel material". i would like to know how to solve this

thanks in advance,

neilduffy1024 July 20, 2010 12:03

Hi, I'm only at a similar stage myself but have you checked that all the volatiles defined in your fixed composition mixture, i.e. CO, CH4, C2H2 etc, have actually been defined individually as pure substances in your model, along with the corresponding reactions?


Jeberti July 20, 2010 16:29

Hi Neil,

Thanks for replying to my long time request!!!

the thing is i have got a solution to my problem. its not possible to define variable composition mixture or fixed compostion mixture in the HC fuel set up. As a result you can do it in the following step;

1- in HC fuel material set the fuel gas as a a compound CxHyOz( you can also add up S, Cl and N, depending on your problem)
2- set the molecular wieght of the volatile to be automatically calculated,( CFX assumes the compund to be a mixture of CH4, CO and H2,check the manual)
3- go in to your devolatalization reaction and set the devolatalization reaction out put to CH4, Co and H2. butu dont forget to balance the toatal amount of energy should be balanced ( E of CxHyOz = E of Ch4+ E of Co + E of H2). if the energy is balanced you can also balance the mass coeffiecents as well.

finally instead using CxHyOz in the devolatalization output you can use the mixture of volatile outputs.

I hope this will helpfull to advance in your work, if not i will have also another method to propose to you which depends on objective of your work :)

Enjoy working with CFD


neilduffy1024 July 21, 2010 17:43

Solid fuel model and user routine for reaction rate
Hi Taha,

Thanks for the info. The HC Fuel analysis seems to be quite limited alright. I am looking at biomass in particular and was hoping to include a range of additional species (in addition to CO, H2 and CH4) such as C2H2, C3H3 etc, so the HC Fuel model does not seem to be suitable for this. I would be interested in the other method you used.

On a slightly different topic, I am trying to set up sub routines to describe reaction/conversion rates for devolatilisation and char oxidation. For the char oxidation case I am currently trying to link a fortran routine (to describe the reaction rate - hopefully conversion rate later) to a multiphase reaction in CFX-Pre. However, despite following the manual it will not let me select "particle user routine", the only options available are mass arrhenius, gibb and field. This was even the case when I tried to load the example ccl given. If anyone knows why this is the case, it would be very helpful.

All the best,


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