Eskom said pollution reduction equipment at its biggest operational plant hasn't been working properly since early last year.
The equipment, which cuts particulate emissions at the 4 116 megawatt coal-fired Kendal power station in South Africa's eastern province of Mpumalanga, began malfunctioning in early 2018. It was then damaged during a strike in July and August of that year. Eskom has 15 coal-fired power plants either operating or under construction.
The state-owned utility's inability to curb pollution from its plants, which experts say kills hundreds of people a year, has prompted a lawsuit against the government from environmental activists because of the high pollution levels from Eskom and Sasol in Mpumalanga. South Africa's emission limits are more lenient than those in China and India.
"Kendal power station is currently operating with very high particulate emissions," Eskom said in an emailed response to questions. "The high emissions began at the beginning of last year."
The delays in ensuring that Kendal complies with pollution standards are "unacceptable," said Robyn Hugo, program head for pollution and climate change at the Cape Town-based Centre for Environmental Rights, a legal organization that represents environmental activists.
"For far too long, Eskom has run rough-shod over the rights of people affected by its pollution - our Constitution demands that Eskom operate in compliance with the law," she said.
While Eskom said it initially began addressing operational issues to cut the dust emission, a strike meant that it had to operate for a time without using its so-called fly-ash removal systems.
"This resulted in significant damage to the dust-handling plant and electrostatic precipitators," Eskom said.
Fly ash is produced from burning powdered coal. Electrostatic producers collect particles on charged plates using an electrostatic charge.
Kendal operates six 686-megawatt units and "the damage on units five and six is significant," Eskom said. Short maintenance outages have allowed some repairs and a long outage scheduled for later this year will allow some repairs to unit five at a cost of more than R50m ($3.6m).
The world's largest dry-cooled coal-fired power station when it opened in 1994, Kendal will be outstripped in size when Eskom fully opens its Medupi and Kusile plants. They each have a capacity of about 4 800 megawatts.