In order to investigate the effect of limestone on NO emission during petroleum coke combustion, experiments were conducted in a horizontal fixed-bed reactor connected with an on-line FTIR gas analyzer. Calcium based materials (limestone, CaO and CaSO4) were employed as solid additives, while CO2 and SO2 were mixed with the carrier gas to research their effects on NO emission. The results showed that char-N was the most important source of NO. Char-NO, which was produced by the combustion of char-N was significantly promoted with the addition of limestone. This was mostly caused by the catalytic effect of CaO, which was produced by the decomposition of limestone. To clarify the mechanism, pyrolysis experiments were carried out, and the transformation of fuel-N to gaseous-N and char-N was investigated. The conversion ratio of fuel-N to gaseous-N was promoted to a higher value by the additives, whereas the proportion of char-N slightly decreased. However, most of fuel-N still remained in char regardless of additives. Therefore, it could be concluded that the conversion ratio of char-N to NO obviously increased due to the addition of additives and then resulted in an increase in the overall emission of NO during combustion. The nitrogen species in raw material and chars were analyzed using X-ray photoelectron spectroscopy (XPS). The results showed that the conversion route of fuel-N to char-N during pyrolysis was not obviously affected by limestone, however, it was greatly affected by CaO. More nitrogen was restrained in the form of N-X in chars produced by the co-pyrolysis of petroleum coke and CaO, due to which, the positive effect of CaO on NO emission was weaker than that of the limestone.