Swelling and Char Structures from Density Fractions of Pulverized Coal

摘要

Char samples are prepared in a drop tube furnace (DTF) in a nitrogen atmosphere using density-separated coal samples of two Australian coals. The structure and morphology of the char samples are analyzed under a scanning electron microscope (SEM). The porosities of individual chars are determined through image processing of the SEM images. The char particle size distribution (PSD) is analyzed using a Malvern Laser Sizer and through image analysis. The results demonstrate the heterogeneity of the char structures and morphology among the density fractions of the same coal. For both coals, porous chars with a cenospheric structure (Group I chars) are generated in the coal samples of low density (F1.25 and S1.25-F1.30). Solid char structures (Group III chars) are formed from the high density coal samples (S1.35-F1.50 and S1.50). The medium density samples contain different types of chars with a moderate porosity and wall thickness. The weight loss, swelling ratio, and the porosity decrease with increasing parent coal density. Results of size fraction samples suggest that a larger swelling occurs in DTF when the particle size decreases. Mechanisms for the formation of different char structures from different density fractions are discussed, and influences of the presence of ash on char structure formation, in addition to the dominant role of maceral compositions reported in previous works, are identified.