Formation of NOx precursors during the pyrolysis of coal and biomass. Part VI. Effects of gas atmosphere on the formation of NH3 and HCN☆

Our results indicate that the gas atmosphere surrounding coal/char particles can greatly affect the formation of NH3 and HCN through its influence on the availability of H radicals. Based on our results, it is believed that the chemisorption of CO2 on the nascent char surface can consume H radicals or block the access of N-sites by H radicals for the formation of NH3 and HCN. For the chars whose thermal cracking generates little H radicals, the gasification of char by CO2 can also generate additional H radicals, enhancing the formation of NH3. However, even gasification of char in CO2 at 950 °C does not lead to the formation of HCN. The oxidation of coal with 4% O2 at low temperatures (400–600 °C) leads to the formation of HCN as well as NH3 due to the enhanced formation of (H) radicals. The gasification of coal with 15% H2O drastically enhances the formation of NH3 due to the greatly enhanced availability of H as an intermediate between the reactions of H2O and char. These results support our reaction mechanisms proposed previously, emphasising the importance of H on the formation of NH3 and HCN during pyrolysis, which can also be extended to the conversion of coal-N during gasification.