what happens to a Necrogen Censer when all of the charge counters on it are removed?
what happens to a Necrogen Censer when all of the charge counters on it are removed?
Nothing. There's no rule that destroys or sends an artifact to the graveyard just for having 0 charge counters. If a particular artifact would be affected this way, it would say that on the card itself.
Once you've used the charge counters, Necrogen Censer will remain on the battlefield. This allows you to either place charge counters on it later, or to use it as a sacrifice (ie, Throne of Geth).