By Louise Flanagan
Wet coal doesn't burn. It clogs up crushing plants, breaks conveyor belts and turns to mud-like sludge.
"Wet coal doesn't work, it's not good for us," said Tutuka power station manager Ryno Lacock.
Tutuka will swallow 10,6-million tons of coal this year and produce about 3-million tons of ash. Half of this coal comes from the New Denmark Colliery run by Anglo Coal, next door to the power station.
This coal is loaded into a massive silo which holds 12 000 tons of coal. It's crushed, then moved by conveyor belt about 7km to the power station.
The other half comes from numerous small mines and is trucked in, already crushed.
Once at the plant, the coal is ground into fine powder, then blown into the furnaces to burn, as this is more efficient than burning lumps of coal.
On average every six minutes, a huge truck arrives with a 30-ton load of coal and dumps it on the piles in the stock yard. Enormous stacker-reclaimer machines pick up the coal with rotating buckets at the end of a long arm and feed it onto conveyor belts to Tutuka.
The stock yard supplies could run Tutuka for about four days and the bunkers at Tutuka add nearly two more days to this. It's a tight schedule, but Lacock said it works.
"We have not lost a single megawatt of transmission because of coal not being available."
Poor-quality coal and wet coal is a bigger problem.
The power station needs crushed coal - ideally particles about 40mm in diameter.
Crushed coal is more susceptible to water damage.
Lacock said the maximum moisture content the plant could handle was 5 percent to 9 percent.
There are two points where coal gets wet - at the mines and in the power station's stock yard. Many coal mines are opencast, which means coal operations get soaked in the rain.
Even in an underground mine, the coal seam may get waterlogged in rain as the groundwater level gets deeper.
The trucks are covered, but loading and offloading facilities get flooded.
Lacock said supplier contracts specified the quality of coal required, and this was checked. "There's a fairly narrow band of specification for coal you can use," he said.
Eskom MD for corporate services Steve Lennon said: "Generally speaking, the average quality is dropping to the lower end of the specifications."
Higher-quality coal is not only hugely expensive - R740 a ton for export-quality coal compared to R100 a ton for low-grade coal used by Tutuka - it's not appropriate for South African power station designs.