Coal is fossilised plant matter. After millions of years of compression, it forms various stages of a hard brown or black substance known as coal. As it ages the water content decreases and the carbon content increases. Coal at all stages also contains moisture, ash, sulphur volatiles and fixed carbon.
Water is contained in pores within the coal.
It diminishes the heating value and can make it more difficult to ignite. In the processing stage it can affect the handling and crushing properties. Particular problems can be encountered in cold climates where the water can freeze. A low moisture content is desirable, but is not critical. Ash is non-combustable material which lowers the heating value and also presents problems for its disposal after burning.