Density functional theory study on N element migration during the secondary cracking of rice husk tar

The existence of tar is the most severe problem hindering the development of biomass gasification technology. The high-temperature cracking method is an effective method to remove tar, but the nitrogen-containing compounds in the tar could be decomposed and converted into NOx precursors such as HCN and NH3, which will pollute the environment. This study aims to reveal the transition process, combining the methods of experiment and quantum chemical calculation to investigate the migration path of nitrogen in the secondary tar cracking. The cracking products of rice husk tar at 900–1200 °C are prepared and collected by a biomass tar cracking experimental platform, and GC-MS analyzes the main compounds and their relative contents at different cracking temperatures. Through density functional theory (DFT), a comprehensive theoretical study of the high-temperature cracking process of main nitrogen-containing compounds is carried out, while the kinetic for reactions paths are calculated. The results show that the main nitrogen-containing compound is FMOC-l-Valine in the tar. The FMOC-l-Valine could continue to crack to generate HCN and NH3 at high temperatures, and the optimal reaction paths have been determined, respectively. All nitrogen elements have been transferred to gaseous products at 900–1000 °C. Compared to HCN, NH3 is more accessible to generate in the process.