Engineering360: "Ammonia combustion systems planned for thermal power plant boilers"

Where does the ammonia come from? Does the manufacture of the ammonia leave a carbon footprint?

Yes.

"...the global average is 2.867 tons CO2 emitted per ton NH3 produced (451/157.3). You may be interested to know that North American producers emit, on average, 2.129 tons CO2 per ton NH3 produced." _Google search

Further, use of NH3 as fuel is net energy consuming: "...Each metric ton of ammonia packs about 5 MW h of energy. “The best, most efficient Haber-Bosch plants work at around 10 MW h per metric ton of ammonia,”..." __ https://cen.acs.org/environment/green-chemistry/Industrial-ammonia-production-emits-CO2/97/i24