Learning Forum

[sunyi24]

I am using the EDC model in Fluent to simulate the co-firing of ammonia and coal. I employed a simplified Okafor mechanism to describe the gas-phase reactions. However, when analyzing the results, I noticed the presence of NH2 radicals in the pipe carrying pure ammonia, and a relatively high concentration of O atoms in the OFA pipe supplying pure air. The reaction mechanism I used does not include the direct decomposition reaction of ammonia (NH3 = NH2 + H). In theory, there should be no NH2 in the ammonia pipe and no O in the OFA pipe. Although the concentrations are not high, this significantly affects the credibility of my results. Is this due to numerical false diffusion or computational error?

[Essence]

Hello,

Which reaction mechanism you have used?

[sunyi24]

Thanks for your reply！

I used Okafor mechanism reduced by myself to simulate the gas reaction.This mechanism comprises 102 homogeneous (gas-phase) reactions and 5 gas-solid reactions.  I have compared the reduced mechanism with original Okafor mechanism using Chemkin, in my opinion, it's usable.

Although determining the cause remains a goal, the presence of NH₂ radicals within the tube is now less concerning. More intriguingly, significant NO formation is observed near the wall upstream of the outlet following ammonia injection, despite the absence of a recirculation zone in the flow field.

[Essence]

I see. Can you please check if you have any NO and/or the NH2 in the species list in the mechanism? Generally, it is quite difficult to diagnose the issue if you have modified the things like - mechanism itself. Can you try out in the other unmodified mechanism just in case?

[sunyi24]

Yes，NO , NH2, NH, N are all in the mixture.Unmodified mechanism usually needs much time to solve out. That's why I need to do some reduction.Is the noticeable NO generation reaction phenomenon upstream of the ammonia pipe outlet caused by the grid, the algorithm, or something else?

[Essence]

Please check if the mass/mole fraction of the NO, NH2, NH and N are defined as zero. If not, then you can expect these species in your domain. Also please re-check if there is really no reaction involving the before mentioned species.