There are many fuels, coal, charcoal, and redstone being the most common. I am unsure about covalence dust. If the SWRG consumes a bit of fuel that's worth much more EMC, it won't consume more fuel until it's exhausted that. An Aeternalis Fuel will last a lot longer than coal, and it won't show up in inventory either way, so you may have thought you weren't using any fuel.