I dug a little while to try to find this. I couldn't , but I don't have any Spanish sources. Maybe someone who does will stop by, but I can at least do some informed speculation.

As gunpowder is a mixture, a fast and complete reaction of the ingredients depends upon them being in very close contact. Quite early, in the 15th c., though they didn't know anything really about chemistry, gunpowder makers learned that the ingredients had to be ground together for a very long time ( as a damp paste, for safety) . That mixing created that closer contact. But there was also experimentation with getting the right kind of charcoal to start with. The Du Pont family, in their early gunpowder mills in Delaware, liked charcoal made from willow twigs, and this preference for willow twigs had for centuries been shared by other makers- but other things that were "twiggy" like alder, osier, and hazel were also used.

Reducing wood to charcoal was, generally, done by poorly-paid men dressed in rags living in hovels out in the middle of a forest. They'd cut and stack wood, then bury it, set it alight, and control the amount of air getting to it by scraping away or adding dirt. Some of the wood would burn to provide the heat to reduce the rest to carbon. It was a pretty long, miserable process, as the slow fires needed to be tended for a very long time, around the clock. But this resulted in not only charcoal but wood ash, and of course the wood ash was not useful as a fuel or gunpowder ingredient. So, gunpowder makers would also sometimes bake twigs in iron kettles or retorts to reduce them- but this of course was more expensive and a later process, done more in the 19th c.

There is one 1915 reference in the cited article for the Spanish piling hemp into pits for making charcoal ( perhaps old rope? ). But in your online searching you've likely found that citrus peels are a very good material for making activated charcoal, charcoal that is extremely porous. That high porosity means that it can react with or trap other molecules. But that high porosity also means it has very big surface area, and that would therefore make it an excellent ingredient for gunpowder. Again, I can't find any reference for this being done. And, it seems like it would have been hard to burn orange peels in dirt-covered heaps- it seems they would have had to be reduced in pots. But it seems it would have been, at least, a good ingredient.

Buchanan, Brenda J. “CHARCOAL: 'THE LARGEST SINGLE VARIABLE IN THE PERFORMANCE OF BLACK POWDER'.” Icon, vol. 14, 2008, pp. 3–29. JSTOR, www.jstor.org/stable/23787159.