How to emulate a NFC card with an Android phone? I have installed several "NFC tools" apps, and I am able to read a NFC door card. It seems to be able to duplicate it (when I'll have a blank NFC card) or even write new NFC tags.

But I'd like to do something else:

How to make my Android device act as a NFC tag?

Example: since I can read and get the hexadecimal codes from my NFC door card, I'd like to make my smartphone act as a NFC card, so that, if I forget my door card, my smartphone could serve as a NFC tag. 

How to do this?

PS: it is surely possible, because when paying with a smartphone ("contactless payment with phone"), the smartphone acts as a NFC device indeed for another reader (typically the shop credit card reader).
It is possible physically for the phone hardware but there are software problems, payment works but it's provided by google. I read about using phone as nfc tag recently and even if you can make it work card reader expects like logical address from you when you connect and card has it static while android sets new for itself with every connection so you need to somehow set it also to static.