Mechanism study of N2 formation from dinitrogen-containing carbonaceous materials during coal pyrolysis

Abstract

N2 is an important nitrogen-containing product in the process of coal pyrolysis, and understanding its formation mechanism is crucial to control NOx emissions in coal combustion. In this paper, the pyrrole-pyridine dinitrogen structure and dipyridine nitrogen structure were selected as the carbonaceous surfaces to investigate effect of different nitrogen forms on N2 production. The density functional theory (DFT) was used to study the formation mechanism of N2 in the process of pyrolysis at the microscopic level. The calculation results show that the form of nitrogen does not change the process of coal pyrolysis to N2. Both of the two dinitrogen char models experienced four processes: nitrogen stripping, nitrogen migration, the second nitrogen stripping and N2 desorption. However, the different nitrogen forms would result in different charge distribution and Mayer bond order in the structure. The presence of pyrrole nitrogen endowed N8 and C9 having relatively strong positive and negative Mulliken atomic charges, respectively, which increased the Mayer bond order between C9 and N8, resulting in a higher activation energy for the stripping of N8 atom from the pyrrole ring. As a result, the char model with dipyridine nitrogen was easier to pyrolysis to N2. The theoretical calculation results also reveal that the energy barriers needed to be overcome at the rate-determining step in the pyrolysis of pyrrole-pyridine dinitrogen structure and dipyridine nitrogen structure to N2 are 552.74 kJ/mol and 492.36 kJ/mol, which are higher than those of HCN and NH3. It well explained the experimental results that higher temperature is required for N2 generation.