Why are moisture and ash important when evaluating coal as a fuel?

Evaluating moisture and ash content is essential when assessing coal as a fuel because neither component contributes to the coal's heating value.1

Moisture
The presence of moisture is a critical factor in both the storage and utilization of coal. It reduces the overall calorific value of the fuel and adds unnecessary weight, which increases costs during transportation. Additionally, high moisture levels can create significant handling difficulties.2

Ash
Ash consists of the mineral matter remaining after coal is burned. It is generally considered an undesirable residue and a source of pollution. The amount and nature of ash are vital for plant design because, at high temperatures, ash can sinter (become sticky) or melt into slag, which may foul or corrode heat-transfer surfaces. Consequently, engineers must account for ash-fusion temperatures to manage clinkering potential within combustion systems. While ash is typically a liability, it is occasionally useful in specific industrial processes, such as coal liquefaction.314

Because these components negatively impact energy efficiency and operational maintenance, they are standard metrics in proximate analysis used to determine coal quality.1