Constructing a grenade (for a novel I'm writing.) Ok, before any eyebrows get raised, this is for a novel that I'm writing and inspired by this answer from worldbuilding and this reference answer. I just want to make sure the device I'm trying to use is as close to accurate as can be.

So, in the novel, I've devised a compartmentalized glass urn. The urn contains three compartments, one will house a mixture of a finely powdered combination of charcoal, sulfur, and saltpeter. The second compartment would contain slightly acidic water and the third one would contain limestone. What I'm expecting is that when the urn is dropped, the shattering of the glass would cause the water to react with lime resulting in an exothermic reaction which would help to ignite the gunpowder mixture. Is this idea liable to work or is too quixotic and fanciful to consider ? If so, what adjustments can I make to it so that it works? Do keep in mind that the sources have to be natural as it is an apocalyptic world.

Also, can I just call it a mixture of a finely powdered combination of charcoal, sulfur, and saltpeter or does it need to be prepared in a special way or maybe there is a special term for it?

Apologies if this is not the right place to ask the question but I could not think of any other apt place.
In short-no your technique will not work.  Reacting limestone with acid does not release enough energy to reach an appropriate temperature to ignite gunpowder and the acid would hinder combustion.

Fear not though, spontaneous ignition may be achieved by mixing glycerol and potassium permanganate which works well and is a classic example.  If you would like to be more out of the ordinary, zinc powder and ammonium nitrate with a minor amount of copper or ferric chloride and water will also create a spontaneous ignition for your gunpowder.