The influences of different temperatures and sewage treatment processes on the pyrolysis of sludge obtained from three municipal wastewater plants in Shenyang, China, were studied in a fixed-bed reactor. To clarify nitrogen transformation mechanisms, the functional forms of sewage sludge nitrogen (SS-N) were evaluated through X-ray photoelectron spectroscopy; the NOx precursor was identified with a spectrophotometric method. The results show that nitrogen present in sludge in forms of protein N (P-N), pyridine N (N-6), pyrrole N (N-5), quaternary N, and nitrogen oxides (N-X); P-N and N-6 account for approximately 80% of the total nitrogen in raw sludge samples (SS-Raw). NH3 is the main product of P-N conversion during sludge pyrolysis; the majority of N-6 tends to be converted into HCN at 400-600 degrees C. The N-6 removal rate is closely linked to the type and origin of municipal sludge that originates from different sewage treatment processes as well as the various stages of the microbial growth curve. The anaerobic process facilitates the removal of N-6 from chars and its conversion to HCN. SS-Raw does not contain N-X; an increase in the temperature increases the amount of bound N-X in the SS-N fraction. This N-X mainly originates from the conversion of heterocyclic N (N-6 and/or N-5). The transformation routes of fuel nitrogen during sludge pyrolysis are discussed as well.