very low grade coal

例句：

1. 1. The mine produces very lowgrade coal, which is difficult to refine.
   这个矿出产非常低等级的煤炭，难以提炼。
2. 2. Due to the low grade ofthe coal, the production costs are high.
   由于煤炭等级低，生产成本高。
3. 3. The companyhas been exploring for very low grade coalin remote areas.
   该公司一直在偏远地区寻找非常低等级的煤炭。