[pelletburner]

Dear CFD Online users!

First of all I would like to thank the community for many insightful threads that I have read so far and fund them very useful!

I am turning to this forum with a few questions about a simulation I am currently working on! Any kind of help would be very useful and kindly appreciated!

The simulation I am working on is a small pellet boiler with an power output rating of 35 kW. The fuel used is standard wood pellets. I am using CFX as I was more use to this environment at the start.

The domain is the whole gas side of the boiler together with the burner. For combustion I am using an empirical bed model, that provides the temperature and velocity profile at the burner inlet and the species mass fractions of the generated gas mixture. The species involved are volatiles and carbon monoxide, where volatiles are a custom compound obtained from subtracting the fixed carbon content from the ultimate analysis composition. The model used for the gas phase combustion is EDM.

The problems and questions follow:

1. I have problem with the outlet temperature. The boiler operates with an inlet water of 50°C and an output of 70°C. For the wall boundary of the heat exchanger I used a temperature boundary condition with a value of 100°C. The simulated outlet temperature is too high so I think this setup is not right. What kind of boundary condition would be more appropriate?

2. The most difficult problem is that of getting a normal value of the CO mass fraction at the outlet. I get a very very low mass fraction of CO at the outlet. When testing the boiler we got about 100 - 200 ppm of CO in dry flue gas. The simulation predicts only a CO mass fraction of 2.10e-7 which is far to low. After looking at the molar reaction rate of CO i found that due to the complex geometry and the EDM used there are zones where CO reacts even at low temperatures far from the burner, for example when gas enters the tube heat exchanger there is a zone of high molar reaction rate due to narrowings. This results with an almost zero mass fraction of CO at the outlet. I looked in the solver theory guide and found the there are two coefficients (A and B) that work as product and reactant limiters. In some articles and publications I found a few variations of those coefficients for example A=0.6 and B=0.5, or even A=4 and B=0.5. As the results are very very dependent on those two numbers I ask myself what is the right setup. I tried a lot of combinations but cannot lift the CO mass fraction at the outlet, i only manage to decrees it to 0. Also I cant understand how the chemical reaction kinetics and has any influence when using EDM. If the reaction rate is controlled with mixing what is the influence of the chemical kinetics term that had to be supplied for the two reactions? I am kindly asking for an explanation for how to use the two coefficient and what their "job" in the simulation is. What would be the correct setup?

Thank you!