Before entering the catalyst chamber the ammonia, or other reductant (such as urea), is injected and mixed with the gases.

In general, the SCR unit is located between the furnace economizer and the air heater, and the ammonia is injected into the catalyst chamber through an ammonia injection grid.

Reactions 1 and 2 are extremely exothermic (the catalyst chamber can reach 800 C in a matter of milliseconds,) and they produce large volumes of hot gas from a small volume of liquid, making hydrazine a fairly efficient thruster propellant with a vacuum specific impulse of about 220 seconds.