Query regarding choice of pyrolysis model

[Devananda Vijayananda Vivek]

Dear friends,

I am trying to simulate the pyrolysis of a polymer (such as HDPE or wood). I read the FDS user guide and some parts of the validation guide. I am confused about the various property values I see from different sources. Below is the screenshot of the polyethylene properties that I set up.

I am now considering using wood as the fuel. I do not have any preference for the fuel. 

Kindly guide me. Thank you so much.

Sincerely,

D Vijayananda Vivek 

[Devananda Vijayananda Vivek]

[fde]

Question is not clear to me.

[Devananda Vijayananda Vivek]

Hello,

Thank you for your reply. I am not sure about how to set up a pyrolysis model. I am looking at various fuels such as polyethylene and wood. Different sources show different thermo-physical properties of the fuels. 

[dr_jfloyd]

That is to be expected. No two pieces of wood are identical. Different species of trees will have different properties, and there is even variance within pieces of wood from the same species. Even the same type of plastic will have some variation from manufactuer to manufacturer depending on what additives (plasticizers, UV protection, etc.) are included in the plastic. Even for the same manufacturer there may be some variance between batches if additives were sourced from different manufacturers. There are also different types of test apparatus used to determine thermo-physical properties. Even for the same test apparatus there may be different models used to convert the data the tests collects into a property. This is one of the reasons we recommend in the guide that before you do a full scale simulatiokn with a set of properties that you model a TGA/DSC and/or cone type test with your properties and make sure that you are getting sensible results.

[Jonathan Hodges]

The FDS user guide does a good job of explaining what each parameter in the pyrolysis model does. Accurately resolving material pyrolysis is a focus of on-going research and may not be the best approach for your application. Can you provide a bit more context on the application you are trying to model? That will help us provide better guidance.

[Devananda Vijayananda Vivek]

Thank you so much for your reply Mr. Hodges,

I am trying to simulate a backdraft situation. I want to use the fuel that we encounter in our daily life. So I chose wood or plastic. So I want the polymer to decompose due to a concentrated heating source like a cigarette bud. The heating continues even after the oxygen in the compartment has depleted thus making it fuel rich. 

[Devananda Vijayananda Vivek]

Dear Friends,

I have a query regarding the example cases of FDS. In the case of birch_tga_1step_20.fds, the reaction selected is methane. It assumes the birch wood produces methane gas, water vapor, and char as the only by-products. There is no soot yield.

In the example of couch.fds, the reaction is directly selected as polyurethane. The chemical formula of polyurethane is given and an ignitor particle is kept in the couch. 

When I tried to define a user-defined fuel called wood using the cellulose formula C6H10O5, FDS warns me as below

So can I define a fuel called Wood using cellulose as the base material just like in the couch case or should I consider the products of pyrolysis of wood such as methane, carbon monoxide, and hydrogen, and define these reactions separately? Please guide me. Thank you so much.

Sincerely,

Vijay 

[Jonathan Hodges]

Both backdraft and pyrolysis are complex phenomena, and be aware that your intended application is in the research space. Take a look at the BST/FRS Wood Crib models from the validation manual which should provide a good starting point for the wood pyrolysis and gas phase reaction. Note the reaction in the example does not reduce the decomposition rate based on local oxygen concentration which may be important in your application.

[Kevin]

The WARNING just means that your gas species is not listed. FDS will assume it has the properties of nitrogen except for whatever you specify as the molecular weight. As for the example cases, we sometimes list METHANE as a pyrolysis product because we are only interested in testing the solid phase algorithm. It does not matter, in these cases, what the fuel gas is.