The current resources to make gunpowder is for black powder, however black powder was outmodded a long time ago;
https://en.wikipedia.org/wiki/Smokeless_powder I don't think modern smokeless powder uses coal. I think the recipe should be updated.
Or simply have both. Not two different gunpowder types, just two different recipes to make gunpowder.
Have each require their own patent, and one require the other. That way it is an upgraded more advanced recipe unlocked by researching a second patent.
Special perquisites can be added to the advanced recipe patent, such as electricity or plastic tools.
Off-topic: I have wanted large missiles that use grav-couples forever too.
The uses for coal are already fairly limited...
I'd suggest keeping the 'Black Powder' Recipe if for no other reason than to avoid oversimplifying the core resource requirements in game. Coal may only be used for gunpowder and power generation but it still offers it's quality to be used to improve or limit the quality of explosives and other munitions.
I know it's an opinion with room to be opposed but between the designers and overall grand scale of Hazeron it is far from a simple game. We don't need another resource, like Magmium, with no use.