Sub-standard coal and poor quality control — understanding Eskom’s high failure rate

For the time being at least, coal-fired boilers remain the backbone of Eskom’s electricity generation fleet. Coal-fired boilers work by boiling water to produce high-pressure steam which is then used to spin a turbine. To do this, water is pumped at high pressure through boiler tubes which are heated by a coal-fired furnace to produce high-pressure steam.

This combination of extreme pressure and temperature places severe stress on these tubes and over time, they corrode and fail, sometimes accelerated by the presence of corrosive chemicals in the hot furnace gases. This corrosion inevitably results in a rupture and requires that the whole unit be shut down for maintenance. These occasional failures are a normal part of a power station’s operation and are factored into power generation planning. When they happen more often than expected, however, they result in under-delivery of power and, in South Africa’s case, rolling blackouts.

Eskom has stated that boiler-tube failures are the leading cause of unavailability of power generation or, in simpler terms, they are the main culprits for rolling blackouts. The rate at which these failures occur has been rising. Eskom set themselves a target of one tube failure per year per unit but, as of 2021, were averaging 2.3 failures a year. One can only assume that this number has been rising since then, with 2022 by far our worst year of rolling blackouts ever.

On 8 December it was reported that Eskom’s coal fleet was operating with just 40% of its installed capacity available, woefully short of its stated target, a more normal 75% availability. Part of the problem lies in maintenance. Eskom has reported that funding delays, among other issues, often force it to delay planned maintenance on units, heightening the risk of failure, particularly unexpected failure. Perhaps more alarming, when maintenance is carried out, it doesn’t always meet standards and the units fail anyway.

Researchers at Wits, however, have identified underlying causes that are more fundamental to the coal fleet’s chronic boiler-tube failures.

Professor Josias Van Der Merwe, the Head of School at the Wits School of Chemical and Metallurgical Engineering, and KG Moloko, a postgraduate researcher in that school, have conducted a range of chemical analyses on boiler tubes from Eskom power stations to determine the mechanisms and causes behind their corrosion.

They found that the main culprit is sulphidation, a chemical process that degrades steel through the formation of brittle compounds of iron and sulphur. Two chemical conditions must be in place for this to occur — the presence of sulphur, and a low concentration of oxygen, which allows the sulphur to react with iron rather than being oxidised.

The Wits analyses indicate that these conditions are both present in the Eskom boilers that they examined, creating chemical pre-conditions that lead to heightened rates of corrosion and frequent failures.

There is no doubt that Eskom is beset by a multitude of crises ranging from shortages in funding and personnel, along with an ageing fleet that has been overworked to keep the lights on, and even sabotage and threats of violence.

Nevertheless, as long as the chemical pre-conditions for high failure rates are present, energy availability will remain low even if these issues were resolved and, perhaps more concerning, availability could deteriorate further still as more units fail.