Influences of Hydrothermal Modification on Nitrogen Thermal Conversion of Low-Rank Coals

The influence of hydrothermal modification on thermal conversion of coal-N in low-rank coals was investigated in this paper. Hydrothermal modification has been performed using three typical Chinese low-rank coals at final modification temperatures of 200, 250, and 300 °C. An analytical pyro-probe coupled to a pyrolysis–gas chromatography mass spectrometry (Py-GC/MS) system was used to study the release characteristics of nitrogen-containing compounds from raw and modified coal. In addition, a tube furnace apparatus was used to study the release characteristics of nitrogen pollutants during pyrolysis and combustion of Zhaotong raw and modified coal. Results showed that hydrothermal modification affected the content and type of nitrogen-containing compounds. Some triple bonds present in the coal structure are broken to form amides. The presence of azoles decreases sharply after modification, but it increases as the hydrothermal modification temperature increases. During tube furnace pyrolysis, the HCN yield decreases after modification; similarly, with the increase of modification temperature, NH₃ is released in greater quantity after modification, but the release decreases as the modification temperature increases. NH₃ may be converted from HCN through secondary reactions. Hydrothermal modification is conducive to NO emission reduction in coal combustion process; however, increasing the modification temperature restrains the NO emission to some extent.