I don't know how to set the fuel for boilers making steam to solid fuel. Every time I try it seems to think I'm adding a new factory with the recipe of creating solid fuel instead of using it as a fuel. Not sure if maybe I have a broken build or some mod is messing with the recipes Helmod understands?

Click on the icon of the machine to edit the recipe line. In the popup, you can choose which tier of machine to use, which fuel, which modules etc.

Hope that makes sense! I'm not able to take screenshots right now sorry.