This is NOT a duplicate of the issue for steam engines. While a fuel type that uses both a solid fuel as well as something like water would certainly be welcome, I'm making a separate issue to specifically request a fuel type that uses only solid fuel.

The main application that I'm thinking of for this is hot air balloons (while modern ones use gas, earlier ones used charcoal), although I'm sure that others could come up with their own applications.

This should not just accept all furnace fuels. Instead, like existing fuel types, pack authors would be able to create their own solid fuel types with lists of items and ore dictionary entries that are allowed inputs. Additionally, there should be separate crates for fuel and cargo (handled by the slot with a boolean value), to prevent it from using fuel that is being transported as cargo.

This seems like a really big edge case. Something that is in theory desirable, but maybe not. I do see folks wanting to make a engine that just burns burnables, so you might get this even if it's not exactly what you want.

As to crates, that's already possible via the feedsVehicles setting on crates and linkedParts on part slots.