The coal accumulation process took place during their certain periods.
聚煤作用发生在特定的演化阶段。
The coalbed and coal - rich zone was an effects which the factor of coal accumulation process systematic action.
煤层和富煤带的形成是聚煤作用因素系统作用的结果。
Owing to complex coal-forming process, the trace elements in coal present unique accumulation and scatter laws.
由于复杂的成煤过程，煤炭中微量元素呈现独特的集散规律。
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