Why ‘wet coal’ causes problems for power generation

Eskom has 13 coal-fired power stations around the country, which all have various requirements when it comes to coal.

The one thing they all have in common is they can use wet coal – when it is appropriately sized.

“Producing electricity from coal starts when the coal is pulverised in huge mills into a fine powder before it is blown into huge kettles, called boilers,” the Eskom website states.

“Due to the heat in the boiler, the coal particles combust and burn to generate heat to turn water into steam.”

The problem is when coal arrives at the mill as mud, it cannot be moved by the blowers, much like a blocked fuel injector on a modern-day car engine which has to atomise the fuel before it can combust and create power.

“There was wet coal at 10 units causing loss of generation,” energy expert Ted Blom said.

“Normally when you buy coal, you would stipulate no more than 10% fine. I’m guessing people are buying up to 50% fine which means there is a lot of money on the table going into someone’s pocket.”

Eskom noted the temperature of the coal entering a boiler furnace was ± 90ºC.

“The reason for having the coal at such a high temperature is to ensure that combustion is taking place in the shortest possible time and within the confinement of the boiler furnace. When the coal is removed from the mills through a blast of hot air, if the coal is too wet, the drying out is ineffective and has a negative effect on the combustion of coal.

“In other words, if the total moisture content becomes too high, the amount of heat energy required to evaporate the moisture is greater than the boiler design allows. This limits the amount of coal that can be dried for the milling process and the amount of pulverised coal that can be fired into the boiler, in turn limiting the amount of electricity that can be generated,” Eskom said.

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