Learning Forum

[scholar]

Hello everyone,nIam simulating the coal gasification process in Eulerian framework. Mixture-air and mixture-coal are two phases. For Mixture-coal, the particulate stresses are solved using KTGF model. I have the following queries.na. Coal is composed of char (solid carbon), volatiles (hv_vol), ash (SiC), moisture (h2o). In this volatiles and moisture are not solid phases in reality. But still, should they be considered in the mixture-coal phase as species?nb. For coal devolatilization reaction, Kobayashi two-equation method is most widely used in literature (Eq. (1)). In this, it has two rate constants which lead to two-reaction. How to include two reaction rates for one reaction in UDF? nc. Eq. (1)-(2) are mentioned as heterogeneous in the literature. So the volatiles, tar, and moisture should be considered in mixture-coal? If that is the case, then in the KTGF model an averaged density is required in which the densities of volatile, moisture will also be used. But in reality, these are not solid phases. nd. How to obtain the chemical formula of volatile that is required for the reaction rate?Eq. (1): volatile-----> 0.07CO+0.01CO2+0.03CH4+0.03H2+0.02H20 n A1 = 2*10^5 s-1, E1 =104600 J/mol, nA2 = 1.3*10^7 s-1, E2 = 167400 J/molnK1 = A1 exp (-E1/RT), K2 = A2 exp (-E2/RT)nEq. (2): mositure---- >H2OnnAny clarification will be greatly helpful.nThank you in advancen

[Rob]

From your other attempt at posting - it's forum related so not a problem.Hello everyone,nI want to include coal devolatilization reaction in the Eulerian framework. I have two mixture phases, one mixture-air and another mixture-coal.nUsually, coal is composed of char (solid carbon), volatiles, moisture, and ash. I took default coal-hv-volatiles-air as mixture-coal and added carbon solid, hv_vol, SiC (ash), h2o (moisture) species in it.nI am using KTGF model to solve particulate stresses in the mixture-coal phase. I have the following doubts on solving the KTGF model and inclusion of coal devolatilization.nSince I am using KTGF model for mixture-coal, should I include moisture and volatiles in mixture-air as they are do not come into solid phase-in reality?nTo include devolatilization reaction the most used model is Kobayashi two-equation method (see Eq. (1) in 'devolatilization-rxn.jpg' in the attachment). For this, we need the chemical formula volatile. How to obtain it?nEq. (1) leads to two kinetic rates (two equations). How to include them as one in UDF? Since there should be only one devolatilization reaction. Or am I missing something here?nSince Eq. (1) is mentioned as a heterogeneous reaction (taken from literature), should we consider volatiles as solid phases and include it as species in mixture-coal?nEq. (3) in 'devolatilization-rxn.jpg'. Moisture converted to water vapor. Since it is mentioned as a heterogeneous reaction, should h20 (moisture) included in the mixture-coalnAny clarification on the above queries would be greatly helpful.n

[scholar]

Hi Rob, I did not understand your comment. n

[Rob]

You posted what looked like the same question twice. Normally we'd delete one, but as the text wasn't identical I copied it into this thread. There were some issues overnight so we have more duplicate posts than normal: we usually don't like people posting the same thing more than once. nLooking at the questions, yes, you need to add the moisture etc to the solid phase. However, you can then use a mass transfer to move the moisture from the solid to gas phase. nVolatiles will need to be included as a species of some sort, that's for you to define, but again as it's a computer model there's nothing stopping you making up a material to balance with the evolved gases. You can also have multiple evolved gases if you include the cracking energy in the reaction/phase transition.

[scholar]

Hi Rob,nI did ask few questions related to the heterogenous reactions in the previous posts in which it was char (solid carbon) reacting with the gas phase.nThis post is related to the coal devolatilization reaction in which the kinetics is different than other heterogenous reactions. Therefore, I created a new discussion. nThanks for your reply and for clarification of few doubts. nCan you look into question numbered (3) once and kindly let me know your thoughts on this.

[Rob]

OK, they looked similar enough to be a result of some glitches we seem to have had. nYou need to work out which mechanism is correct, I understand combustion chemistry but not the modelling. If you do have two mechanisms how would you determine which is active in the solver? n

[scholar]

The mechanism I have chosen is from the literature. It is most widely used. n

[ai0013]

a) You can take a look at the manner in which the E-L framework handles this. If you have wet combustion on, a droplet material is included in your mixture, which is also defined by the moisture content of your PA. There's mass transfer from liquid to gaseous phase through evaporation. However I haven't used the EE framework, somehow you'll need to indicate a mass transfer mechanism to do the same as in EL. (Maybe define a liquid phase to represent moisture and then use mass transfer,)nCoal-volatiles is a gaseous species, and hence is just transfer from gas to gas phase.nnb) Maybe try using single Kinetic model first? Kobayashi Is useful when you have both fast/slow devolatilization. Look at your system and check which mode dominates, otherwise maybe look at the C3M software (free access) and review the devolatilization reaction there. It automatically creates UDF ready to be implemented in Fluent, but I'm not sure if the kobayashi model is employed there.nnc) Can you specify more about this? What is the input for KTGF? volatiles, tar and moisture are part of the full gasifiction reaction, so you need to define tar as well, and tar cracking reaction. d) I have the same question. I just know that some models represent Volatiles as a CHONS and do mass balance from here. Stoichiometric models based on equilibrium are useful to balance the devolatilization. For tar cracking and tar oxidation, look at C3M.nn