Thanks for your reply!

To answer your questions:

Are you coupling with Fluent? yes as the combustion module is only applicable by coupling with CFD!

how high is the solids fraction: so it's a fluidized bed with a height of almost 0.1 out of 0.5 m. 

The problem is not related to any physical concept. For the starter, let's forget about any physical concept like radiation heat transfer. In the combustion module, I just defined the rate of heat transfer to be a fixed value of 1 W and I'd expect to see this value in the UI of the software as well no matter what happens. But it is showing a totally different fixed value. I tried to attach the screenshot again. Hopefully, you could see it!