The only unit which requires Coal is the Ironclad. While Coal Plants may be built without access to Coal, they will not provide any unless Coal is present in the city. Coal, along with Oil , allows Workers to build railroads. Besides increasing transportation speed, railroads give +1 with Lumbermills, Mines, and Quarries. Coal provides an additional +50% with the Ironworks.
The Production bonuses of Coal come with significant Unhealthiness penalties. Coal produces +2 with Factories and +1 with Industrial Parks. When producing Power , Coal Plants (and Shale Plants) produce +4 compared to the +2 with clean power sources. When operational, clean power sources supersede Coal Plants (and Shale Plants), thereby reducing by 2.
While Power may be provided by alternative means, and railroads may be built with Oil , the +50% bonus with the Ironworks absolutely requires access to Coal. If is a severe issue, it may be desirable to cut off access to Coal by trading away all Coal trade resources, or by destroying Coal Mines; however, this strategy will result in the loss of the +50% Ironworks bonus.
Coal is also relevant to Corporations. When present in a city with Aluminum Co., Coal provides +3 per Coal resource, and also provides access to within that city. When present in a city with Mining Inc., each Coal resources provides +1 .
More than 300 million years ago, plant life growing in swamps began to decompose. Layers of sand and mud covered the decomposing plants, and the decaying plants were compressed by the combined weight of water and sediment. Over time, the plant matter hardened and became coal. Coal deposits are mined all over the world, and coal was one of the major fuel sources in the world through the 1970s. In many countries, concern over the environmental effects of the burning of coal has led to a decline in its use. The smoke produced by burning coal has a high acid content, and creates an environmental condition known as "acid rain," which is harmful to plant and animal life. Despite the environmental concerns, some of the largest coal mines in the U.S. each still produce more than 450,000 metric tons annually, making coal mining a very profitable industry.