NO reduction in a flue gas due to the presence of "CH4N2O" (UREA)

[gaor...@gmail.com]

Hello Cantera users, 

I would like to know if it is possible to use the Urea compound to perform a NO reduction. Urea in contact with heat decomposes into NH3 and HNCO, compounds found in kinetic mechanisms, but urea is not. Could Cantera carry out this decomposition somehow?  

Thank you so much in advance,

Regards.

[leo...@gmail.com]

Hello,

The only manner that I know is to provide a kinetic model  containg the reactions for urea decomposition. If your model does not contain reactions with urea, cantera cannot be used to your finnalities. But you can evaluate the Arrhenius parameters for urea decompostion using quantum chemical methods, and use this parameters to kinetic modelling. Or you can search in the literature to see if anybody studied the desired reaction. The follwoing three article may help you:

https://doi.org/10.1021/ie034052j

https://doi.org/10.1016/j.ces.2013.11.013

DOI: 10.1039/C9CP01529A (Paper) Phys. Chem. Chem. Phys., 2019, 21, 16785-16797

Cordially

Leonardo

[Steven DeCaluwe]

Leonardo is correct, in that Cantera only calculates properties for reactions that you tell it to analyze, via the input file.  While Cantera does ship with a limited number of mechanism files, these are mostly for demonstration purposes.  At present, the software is not in the business of endorsing any particular mechanism over another.

Best,

Steven

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[gaor...@gmail.com]

Thank you so much for your help

Regards.

[bryan.a....@gmail.com]

Worrying about the chemical kinetics of urea decomposition seems like an awkward "middle" level of fidelity.

On the one hand, it might be reasonable to simply skip the urea decomposition step and assume a feed of NH3 and HNCO, or even simply NH3.  This makes sense if, for example, you are simulating SCR over a catalyst above a certain temperature threshold (say ~250 C), where the system is generally designed such that the urea is reasonably decomposed (and, in the NH3 case, hydrolized) before it hits the catalyst and the reactions that are really of interest begin.

On the other hand, if you do need to model decomposition for whatever reason, one must ask how the urea got into the flue gas stream.  If, as is typical, it was injected in aqueous solution, it's reasonable to assume that droplet evaporation is at least as important as chemical kinetics.  Aqueous urea droplets in flue gas are practically their own field of study, involving heat transfer to the droplet, evaporation, and potentially kinetics in both liquid and gas phases, with the properties of the liquid changing as water evaporates (I recall at least one mention of droplets developing a hard shell of urea as the water on the surface evaporates, although I suspect that the conditions to do this would be quite particular).  So far as I am aware, CANTERA does not include a means of modelling any of this except the kinetics.

If you're modelling a different urea delivery approach, I am sure that whatever delivery mechanism you have in mind has its own complexities - potentially less challenging, but also with less prior study.

In short, you might not need to model urea decomposition, but if you do, the kinetics may be the least of your worries.

[gaor...@gmail.com]

Thanks for your reply Bryan and sorry for my late reply.

My idea is to inject liquid urea into a flue gas. Until now I have been using NH3 to simplify the process and the truth is that it obtained very satisfactory results with Cantera. Now I want to go one step further, and use urea instead of NH3.

As you mentioned, not only the thermal decomposition is important but also the evaporation of the drop, the liquid-gas and gas-gas kinetics, ...

Estimating an NH3 and HCNO feed is a good idea, but it is complicated due to the particular conditions of my project. So the ideal would be to find a library that can carry out a urea hydrolysis process and combine it with Cantera using the results obtained. I could also use CFD, but first I would like to have a 1D model with Python. So far I have not found anything to support myself to do it, so I see that it is going to be a complicated task.

Regards.