Fuel that has been augmented by enough dust that it's damage value it's higher than 16384 will not be excepted by generators.
I took a peek in the code and found out why, in TileentityIronFurnace.getFuelValueFor:
If the damage value is higher than 16384 then "itemstack.getItemDamage() * 2" will result in a number higher than a short can handle.