The fate of char-nitrogen in low-temperature oxidation

Abstract

Nitrogen release during the oxidation of coal char has been studied in a thermogravimetric analyzer (TGA) at 873 K with 2% O2 in He. Species profiles are determined using Fourier transform infrared (FTIR) spectroscopy (NO, NO2, CO, CO2, N2O, and HCN) and high-speed gas chromatography (N2), which allows all of the char-N to be accounted for in the gas phase. Char-N is released predominantly as N2 with smaller amounts of NO and HCN also present. The proportion of char-N released to the gas phase as N2, NO, and HCN is relatively constant throughout burnout. At the low-temperature conditions of this study, N2O is not produced and the results suggest that HCN is a primary product formed via heterogeneous oxidation and not by slow or secondary devolatilization. The gaseous product species profiles suggest that nitrogen is preferentially retained in the char as the carbon is oxidized and this is confirmed directly by X-ray photoelectron spectroscopy (XPS) and elemental analysis of partially oxidized char samples. Analysis of the XPS results also shows that enrichment of the char in nitrogen is a surface effect that occurs with an increase in the pyridinic N content of the char relative to pyrrolic N. At low temperatures, the enrichment of the char in nitrogen as burnout proceeds is the major contributing factor to the increase in the [NO]/([CO2]+[CO]) ratio.