Abstract: Semi-coke has a low volatile content,so it is difficult to ignite and would produce large amount of NOx during combustion process.An effective improvement method is to super finely pulverized it before burning in the furnace.In order to study the superfine comminution characteristics of low rank coal and its semi coke,Shenhua bituminous coal and Ximeng lignite were selected and their dehydrated samples and 700 ℃ semi-cokes were prepared by tube furnace.The samples were characterized by scanning electron microscope and X-ray diffraction instruments and the comminution experiments with different input energy level were carried out by Hardgrove apparatus and vibration mill.The comminution mechanism was analyzed by laser particle size analyzer combined with fractal theory.Micro-physical and chemical analysis shows that during the pyrolysis process of Shenhua bituminous coal,there was a thermoplastic stage,which produces bubble-like pores.The pores of Ximeng lignite are mainly affected by dehydration process and there was no such stage.Compared with raw coal,both types of semi-coke have increased aromatic structure and increased degree of carbonization.When limited comminution energy is input,compared with raw coal,the grindability of Shenhua dehydrated sample has little change,while the grindability of semi-coke sample is worse;The grindability of both Ximeng dehydrated sample and its semi-coke are much better than that of raw coal.When excessive energy is input,the comminution effect of the two types of semi-coke is better than that of raw coal.The fractal analysis of particle size shows that compared with raw coal,the size range of semi coke is smaller.The results show that the developed and irregular pore structure is conducive to saving the input energy and improving the comminution efficiency in the early stage of comminution;With the increase of temperature,the microcrystalline size in the semi coke becomes larger,and uneven compaction occurs,causing internal stress,more micro cracks are produced.In the late stage of crushing,when the input energy is excessive,the above changes make the semi coke show more brittleness on the macro level,and makes overall shattering easy to happen.