Why Kusile power station’s chimney broke

[Nat Quinn]

Faulty emission control systems and human errors led to a cement-like deposit forming in the flue-gas duct of the Kusile power station, causing a part of the chimney-like structure to collapse on 23 October 2022, insiders have told Rapport.

The power utility first publicly confirmed the failure at the power station near Emalahleni in Mpumalanga on Wednesday, 2 November 2022 — 10 days after it occurred.

Photos of the wrecked chimney began circulating on social media, after which Eskom quickly issued a media statement.

It explained a section of the duct exiting the sulphur dioxide absorber failed on the horizontal rubber expansion joint and compensator.

“[The compensator] is a bend to direct flue gas up the [duct] and allow for thermal expansion of the chimney,” Eskom explained.

The failure occurred while Kusile unit 1 was on forced shutdown for flue-gas desulphuration recirculating pump repairs.

“Investigations and assessments are in progress to establish the cause of failure and to ascertain the extent of the damage, as well as the recovery scope of work,” Eskom stated.

The collapse will result in Eskom likely being unable to generate power from the unit for several months, while unit 2’s return to service from maintenance was also put on hold as a precaution.

As a result, a combined 1,600MW of generation has been cut from the grid, further exacerbating capacity shortages that will necessitate additional load-shedding.

Unit 3, the only other unit connected to the same chimney, has continued to run at stable load, Eskom said.

The section of one of the flue-gas duct’s compensators that collapsed on 23 October 2022

Two sources with close knowledge of the matter reportedly provided Rapport with an explanation of the series of factors which led to the collapse.

To understand these, it is important to note that Kusile has been fitted with a wet flue-gas desulphurisation (WFGD) system intended to remove harmful sulphur dioxide from its coal-fired emissions.

One source explained that the problem started with the first desulphurisation stage, where baghouse filters were supposed to capture fly ash from the emitted gasses.

But the source said these filters were not functioning as intended.

As a result, some of the flu ash — which contains high amounts of silica — got carried along to the desulphurisation facility.

At this stage, hydrated lime is sprayed through the emitted gasses to chemically bind with the sulphur dioxide and create calcium sulfite.

However, due to bad management of the process, some lime water spray gets into the chimney.

When combined with heat and fly ash, a hard crust forms along the sides of the chimney.

Over time, the weight of this deposit becomes too much for the structure to support, and it collapses.

One of the sources told Rapport the problem with the leaking fly ash was so severe that much of the environment in the chimney’s immediate vicinity was covered in it, including the parking bays for plant workers’ cars.

Kusile is badly designed, years late, and way over budget

Alongside the Medupi power station near Lephalale in Limpopo, construction on Kusile began in 2007 and was supposed to be completed by 2014.

But years of corruption and poor design have pushed back its completion date by more than a decade.

According to Eskom’s latest timeline, which has been adjusted repeatedly due to various setbacks, the power station will only reach final completion by May 2026.

Four of the six 800MW units have been completed, but with the chimney damaged, only two — unit 3 and unit 4 — are operational.

Unit 5 suffered a fire in September 2022, which has pushed back its completion date by a year — from December 2023 to December 2024.

Eskom COO Jan Oberholzer recently told Parliament that incident would cost Eskom around R150 million per month.

Unit 6 is planned to come online around the middle of 2024.

According to Eskom’s figures, the power station’s construction cost R147.4 billion by August 2022, and a further R14 billion would have to be spent to complete it.

That comes to a total cost of R161.4 billion, compared with an original budget of R81 billion.

It is unclear what the cost of repairing the damage to the chimney would be, but given that it is expected to have a months-long impact, it is likely not going to come cheap.

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