When it comes to dirty to cleam energy sources the stack rank goes like this, from dirtiest to cleanest:

Coal
shale, tar, and tar oil
oil
gas
geothermal (there are types of geothermal that vent a lot of particulates that can fit between coal and tar oil)
nuclear
Hydro
ocean thermal
wind
solar

Disciples of particular sources might argue with me over the placement of some of these.