1 - The most common emission and byproducts that are generated by burning coal are carbon dioxide, nitrogen oxides and dioxides and CCPs in genearl (coal combustion products).
2 - Mercury is a heavy liquid metal with some toxical properties. When ingested (most commonly trough water) can be fatal [for further info search Mercury Poisoning]. Something I should also point out is that most of the contaminated water sources are residual waters.
Hope it helped,
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