Question

Why does a grenish yellow flame appear when ammonia burns in oxygen?

Answer 1

in the presence of oxygen  usually  ammonia does burn .But at high temperature and in presence of catalyst ammonia burns to give greenish yellow flame . 
The colour of the flame is due to the formation of nitrogen oxide as shown in following reaction :


                            catalyst
4NH3   +5O2  _____________  4NO +  6H20