Nitrogen release during high temperature pyrolysis of coals and catalytic role of calcium in N2 formation

Pyrolysis of 10 coals with carbon contents of less than 80 wt%(daf) has been studied with a fixed bed quartz reactor to examine mainly nitrogen release from char-N without volatile matters. When temperature is raised from 1000 to 1350 °C, N₂ yield increases but char-N decreases for all the coals used. There is a strong reverse correlation between N₂ and char-N, which points out that most of N₂ arises from char-N via solid phase reactions. NH₃ is also formed from char-N at high temperatures of ≥1000 °C. In the pyrolysis of low rank coals, demineralization by HCl washing increases yields of tar-N, HCN and char-N, but decreases NH₃ and N₂. The addition of 3 wt% Ca to the demineralized coals shows almost the reverse effect. The XRD measurements after pyrolysis at 1000–1350 °C reveal that the Ca exists predominantly as CaO with the average crystallite size of 25–65 nm and promotes carbon crystallization. As the extent of crystallized carbon increases, N₂ yield increases remarkably. It is likely that the highly dispersed CaO catalyzes efficiently conversion reactions of char-N to N₂ in the process of carbon crystallization. The reaction mechanism is discussed in term of interactions between CaO particles and char-N.