Ammonia is a liquid product primarily generated as byproduct of processing Sour Water in the Sour Water Stripper. Its primary use in the early game is in the production of Fertilizer, though it also serves as an accessible source of elemental Hydrogen for the production of Fuel Gas with the beneficial side-effect of Carbon Dioxide capture.
In the mid-game, Ammonia is a stop-gap source of Hydrogen for the small-scale production of Silicon (Poly). While it is possible to utilize Ammonia in the late game as the Hydrogen source for processing of Heavy Oil in the Cracking Unit, it is unlocked in close proximity to the Hydrogen Reformer and thus generally replaced by Fuel Gas for that purpose.
Below are all the recipes, which contain this item. Note, that some of them may be locked behind Research, and not immediately available in your game.