To reduce the energy cost during pulverizing process of lignite

the previous researches were summarized. It pointed out that several factors could influence the grindability of the lignite

such as the moisture content

petrographic composition and geographical conditions. The moisture content is considered as the major reason for poor grandability of the lignite

the higher the moisture content

the poorer the lignite's grindability. So the most effective method to improve the lignite's grindabilty is drying it. However

during the drying process

the pore structure and specific surface area of the lignite will change. The cracks created by moisture precipitation are most likely the main reason in explaining the increasing of the coal grindability as a result of the drying process. On this basis

the grindability of microwaving exposed coals and conventionally heating coals were compared. The results show the microwave heating has the advantages of fast drying rate

short drying time and high energy utilization efficiency. Finally

the evaluation indexes of coal grinding were discussed and the shortage of HGI was figured out. Some new evaluation indexes based on fractal theory and others were also introduced.