I'm surprised the larger ships aren't burning coal again. A small several megawatt power plant could easily fit inside a big ship.
Steel, assuming you actually believe the content of your own post..
I'll give this a shot:
1. First off, although it was certainly cheap, nobody ever in history just "burned off" heavy oil. Amongst other things like oil lamp fuel, it was used to make the roads you drive on.
2. Coal is basically solid carbon. Do we assume you've not heard of Global Warming/Climate Change, etc?
3. What do you think the greenies would have to say about coal-burning ships?
4. Have you any idea of the handling issues surrounding large quantities of coal? Besides the obvious fire/explosion risk in confined spaces ship-board, think about what coal dust does to your lungs!..
5. You won't get either a "several megawatt power plant", or anyone to run coal-fired boilers, inside any big ship these days. Even if the economy was there (and it isn't, when you take space contraints into account), OH&S simply will not allow it.
IIRC, one of the major reasons behind the move away from steam was the enormous amount of space the boilers and the associated ventilation apparatus took up... but there were many, many other reasons - mostly to do with safety of both ship and crew.
Have another think about why they won't come back..