Pyrolysis characteristics of low-rank coal in a low-nitrogen pyrolysis atmosphere and properties of the prepared chars

A low-nitrogen pyrolysis atmosphere plays a very important role in the clean and efficient application of low rank coal. The characteristics of low-rank coal pyrolyzed in a low-nitrogen atmosphere were studied using the thermogravimetry-mass spectrometry method. The chemical properties and physical structure of char samples produced by low-rank coal pyrolysis under N2, H2-containing, CH4-containing, CO-containing and a low-nitrogen atmosphere were investigated by Raman spectroscopy, X-ray diffraction, and N2 adsorption. The results showed that low-rank coal pyrolysis under low-nitrogen atmosphere promoted weight loss (16.63% increase) compared with N2 atmosphere, and the release of gas products H2, CH4 and CO were increased. Physicochemical microstructure analysis showed that low-nitrogen pyrolysis atmosphere effectively promoted the coal molecules into dense, aromatic structures and forms regular and orderly carbon structures at a higher temperature (>450 °C). The ordered carbon structure proportion (AG/AAll) of char prepared in low-nitrogen atmosphere (char-LN) was about three times that of char prepared in N2 (char-N2). Overall, low-nitrogen pyrolysis atmosphere reduced the combustion and CO2 gasification reactivity of char. There was a good linear correlation between the volatile content and the combustion reactivity (R2 = 0.98), and a good correlation between AG/AAll and CO2 gasification reactivity (R2 = 0.97) of char-LN.