The location of the injection point is dependent on the flue gas temperature. The amonia to NOx reaction happens in a certain temperature range. Amonia injections systems I have seen typically have the injection point in the superheater section.
The dirty little secret about SNCR is that much of the urea is converted and emmitted as N2O, nitrous oxide ( laughing gas), which is not currently regulated by EPA, but has a much larer ozone destructive potential than CO2 or Methane.