“hard coal”造句，怎麼用hard coal造句

hard coal; anthracite

Finally great depth of burial, long time and higher temperature may metamorphose the lignite into soft coal or bituminous coal and in extreme cases, to hard coal or anthracite.

anthracite; hard coal

The first caving roof sections in hard coal layers can effectively reduce the pressure during the course of caving and guarantee safe production and efficiency.