Effect of mineral extraction on the evolution of nitrogen functionalities during coal pyrolysis

Abstract

The mineral composition of coal has a significant effect on the pyrolysis of coal and the evolution of the nitrogen-containing functional groups. Thermogravimetry-differential thermal analysis-Fourier transform infrared-mass spectrometry (TG-DTA-FTIR-MS), X-ray photoelectron spectroscopy (XPS), and pyrolysis–gas chromatography-mass spectrometry (Py-GC-MS) were used to study the effects of extraction pretreatment (water-washing, NH4Ac-washing, and HCl-washing) on the pyrolysis characteristics and the three-phase nitrogen transformation mechanism in Xinjiang Hongshaquan coal. The results show that the minerals in each extraction stage can inhibit the transformation of char-N to tar-N/gas-N. The presence of H2O-soluble and HCl-soluble minerals makes the tar-N content of the raw coal (R-coal) 25.5% lower than that of the water-washed coal (W-coal), and the tar-N content of the NH4Ac-washed coal (N-coal) 18.7% lower than that of the HCl-washed coal (H-coal). By contrast, NH4Ac-soluble minerals make the tar-N content of the W-coal 86.3% higher than that of the N-coal. The gas-N was increased by the extraction. The HCl-soluble minerals were the most important factor affecting the gas-N yield, which results in a large amount of char-N being converted into gas-N, while the NH4Ac-soluble minerals inhibit the process. The results of this study will provide data support and theoretical basis for clean coal combustion and improving the quality of coal tar.