Characterization of coal-based humic acids in relation to their preparation methods

Abstract

ABSTRACT Lignite resource is abundant in China. Its high moisture and high self-ignition tendency have limited its utilization as a fuel. Preparing humic acids is considered to be an economic choice for the utilization of lignite. Herein regenerated humic acids (RHAs) have been obtained by means of hydrolysis of Chinese-aged lignite with NaOH as the catalyst. It has been verified that the hydrothermal process has enabled 67% increase in the extraction yield of humic acids (HAs). Moreover, RHAs, original humic acids (OHAs) and commercial humic acids (CHAs) were characterized by means of UV-Vis, acid functional groups analysis, FT-IR spectroscopy, XPS and thermogravimetric analysis in addition to basic elemental analyses in relation to their preparation methods. Comparison of the studied HAs shows that they are apparently different from each other in structural features regarding the degree of aromaticity, the oxygen content and the distribution of functional groups. CHAs contain the most aromatic structure and largest oxygen content than RHAs and OHAs, and the carboxyl groups are the main oxygen-containing functional groups. These structural characteristics are probably responsible for the lowest pyrolysis temperature and maximum mass loss of CHAs during the pyrolysis process. Although there are some distinct differences between the structure OHAs and RHAs, but the thermogravimetric curves of them are almost the same in the temperature range from ambient temperature to 1100°C.