1. 煤
    Coal is a hard black substance that is extracted from the ground and burned as fuel.

    e.g. Gas-fired electricity is cheaper than coal...
           天然气发电比煤便宜。
    e.g. Today, oil and natural gas have replaced coal and wood in most areas.
           如今在大部分地区，燃油和天然气已经取代了煤炭和木头。

2. (燃烧着的)煤块
    Coals are burning pieces of coal.

    e.g. The iron teakettle was hissing splendidly over live coals...
           煤火烧得正旺，上面的铁茶壶嘶嘶地响个不停。
    e.g. It is important to get the coals white-hot before you start cooking.
           很重要的一点是要把煤块烧到白热状再开始做饭。

3. 申斥;严厉责备
    If a person in authority hauls or drags someone over the coals, they speak to them severely about something foolish or wrong that they have done.

    e.g. I heard later that Uncle Jim had been hauled over the coals for not letting anyone know where we were...
           后来，我听说吉姆叔叔因为不告诉别人我们在哪儿而受到了严厉斥责。
    e.g. The museum's P.R. man was going to be dragged over the coals for sure.
           博物馆的公关人员肯定会受到斥责。

4. 多此一举;徒劳无功
    If someone is taking coals to Newcastle, they are trying to give or sell someone something that they already have a lot of.

    e.g. Taking a gun to the United States would be like taking coals to Newcastle.
           带把枪去美国简直是多此一举。