Can I use charcoal briquettes from the supermarket to make black powder?

carbon is carbon, why not....

now all you need is a little sulphur and salt peter...

No, carbon is not carbon in this case, because charcoal is not carbon! Charcoal is a mixture of some carbon but also complex hydrocarbons and who knows what else, depending on how it was made.

Referring to the question... yes, you can grind supermarket charcoal for blackpowder use, but it will be lower quality than if it were made from higher grade willow or softwood charcoal designed for black powder use. Supermarket charcoal contains flame-retardant dopants to slow burnrates.

However, you can't just mix sulfur KNO3 and charcoal and expect to have gunpowder...it will burn, but very slowly. To make proper blackpowder, component integration is needed with a ball mill and proper grinding media. If you're serious about making good black powder in quantity, get one, they're great. Instructions are here...

http://www.pyrocreations.com/making_black_powder

Use the "ball mill method."

specific i think you could. Black powder-gunpowder- is produced from charcoal, potassium nitrate (saltpeter), and a few sulfur compound. Charcoal could be produced from any style of timber. this is made by utilising heating timber interior the absence of oxygen. all of the different "stuff" is going away and you're left with carbon. Your appropriate wager is merely to purchase charcoal.

If you can grind them up to the required fineness.