The postcombustion technology involved the application of the deep air-staged combustion technology to circulating fluidized bed (CFB) combustion. To further develop its de-NOx potential, three kinds of fuel were adopted to investigate the conversion of carbon and nitrogen along the postcombustion chamber (PCC) in a 0.15 MWth CFB test platform. By ultimate analysis of fly ash and gas components analysis, it was found that the char content in fly ash and CO in the gas phase gradually decreased along the PCC and the flue gas pass. The majority of CO and char particles in fly ash would burn out in the PCC. Similar to char and CO, the nitrogen content in fly ash also gradually decreased along the PCC and the flue gas pass. Although the fuel-N was gradually released into the gas phase, the NOx concentration first sharply increased and then slightly decreased instead of increasing monotonically. In addition, the relationship between fuel-N conversion and fuel characteristics was established. The results indicated that the conversion rate of fuel-N in the furnace and in the PCC increased with volatile content and fixed carbon content, respectively. This phenomenon proved that the volatile nitrogen (volatile-N) and char nitrogen (char-N) had different transformation tendencies in the postcombustion technology. Moreover, the most likely conversion paths of fuel-N in the postcombustion technology were proposed. Based on the above findings, it was inferred that the NOx from the combustion of coal with low volatile content and high fixed carbon content would be higher in the postcombustion technology. The NOx emission experiment results agreed with the above reference well.