Characterization of different aggregate structures in coal based on HRTEM

The HRTEM image of Caocun No. 2 coal shows that the lattice fringes with an ordered-disordered distribu- tion. Dividing the different regions of the original image,three different fringe region types of aggregated structural re- gions were defined: good orientation, similar to a ring-shaped, and disordered. Based on HRTEM image processing method,the authors carried out a series of quantitative characterizations on the aggregated structures in different re- gions,including the length and orientation of lattice fringes,and curvature characteristics ( segment angle,cumulative angle and tortuosity). Results show that the single lattice fringe exhibits large curvature,which is characterized by a large segmentation angle and tortuosity,in a fringe region with good orientation. For the fringe region with similar to a ring-shaped,the rose pattern exhibits a “cross-shaped” distribution in terms of orientation. In the disordered fringe re- gion,the linearity of the single fringe is better,which is characterized by smaller segmentation angle and tortuosity. The lattice fringes in three regions exhibited a relatively uniform length distribution. The distribution of cumulative angles shows that the fringes in the good orientation regions have large curvature,and the short fringes contribute most of the tortuosity. In addition,the pore size distribution of three regions was analyzed,and in all three regions developed micro- pores. The pore size were concentrated at 0. 4-0. 5 nm in a fringe region with good orientation,while it was slightly lar- ger in the disordered fringe region. Combining the molecular structure parameters of FTIR and Raman,as well as the lattice fringe characteristics and pore size distribution of different regions in coal,the formation mechanism of aggregate structure in coal was discussed. The aggregate state in coal is the result of the interaction of different aromatic struc- tures,macromolecular group formed between aliphatic structure and other molecules,defects or pores inside the molec- ular group.