High resolution TEM image analysis of the aggregate structural characteristics under heat treatment：A case study of Yimin coal

The aggregate structural characteristics of Yimin lignite under heat treatment at 384,456,528 and 600 ℃ were examined by HighResolution Transmission Electron Microscopy (HRTEM) technique in this paper.Thermogravimetry coupled with mass spectrometry (TG-MS) was employed to measure the gaseous evolution characteristics from 30 ℃ to 900 ℃.The results indicated that the length,crooked degree,orientation distribution,layer spacing and stacking height of the aromatic fringes varied accordingly with increasing temperature,and were closely related to the gaseous release characteristics during pyrolysis.From 384 ℃ to 456 ℃,1×1 aromatic layers increased while the larger ones decreased,the number of aromatic layer with a crooked degree of 1.0-1.2 increased while the one with more crooked decreased,and the aromatic layer became more oriented.All of those were related to the release of CO2,which evolved by the decomposition of carboxyl or oxygen containing heterocycles linked to the aromatic rings,meanwhile,the spacing distance decreased while the stacking height increased.From 456 ℃ to 528 ℃,the main reaction of coal pyrolysis was the breaking of arylmethyl or alkyl ether bond and the polycondensation of the small aromatic layer which formed in the above stage.Therefore,it caused the reduction of 1×1 aromatic layers and the increase of larger ones,the reduction of aromatic layers with a crooked degree of 1.0-1.2 and the increase of others,the decrease of the aromatic layer spacing distance,the increase of stacking height,and better orientation distribution.From 528 ℃ to 600 ℃,a large amount of H2 was generated from the polycondensation of small aromatic layers.Therefore,1×1 aromatic layers decreased and the larger ones increased,the number of aromatic layer with a crooked degree of 1.0-1.2 decreased while the number of more crooked one increased,the stacking height increased significantly.