The deportation is only possible if you have the country of origin that is willing to receive the person. If you can't determine his nationality, you're not able to deport him.
In German law, which I think is the same according to the return directive, detention for deportation is not allowed just to bring pressure on people to tell us who they are or where they came from. Detention is only allowed to facilitate the deportation procedure, in order to prevent that person from—