High-sulphur coal, poor quality control led to Eskom’s woes

Fundamental chemical problems in the boiler tubes in Eskom’s coal-fired power stations are the cause of the current chronic failures.

Even if the company solves the issues of sabotage, lack of funds and experienced personnel, the chemical problems mean energy availability will remain low and may even get worse as generating units continue to fail.

Eskom said boiler-tube failures were the leading cause of unavailability of power generation and load shedding. The rate at which these failures occur has been rising.

Eskom coal fleet operation

Yesterday, it was reported that Eskom’s coal fleet was operating with just 40% of its installed capacity, woefully short of its target.

Researchers at the University of the Witwatersrand identified underlying causes that were more fundamental to the coal fleet’s chronic boiler-tube failures.

Professor Josias van der Merwe, head of the Wits School of Chemical and Metallurgical Engineering, and KG Moloko, a postgraduate researcher, 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 the main culprit is sulphidation, a chemical process which degrades steel through the formation of brittle compounds of iron and sulphur.

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

Eliminating or mitigating the chemical pre-conditions is therefore a crucial first step. Coal reserves vary in sulphur content and, generally, lower sulphur coal comes with a higher price tag.

Sulphur also results in emissions that are damaging to the environment and to human health. As with most mining activities, the most appealing reserves tend to be mined first, so the standards of remaining reserves decline over time.

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This is one factor in the declining performance of Eskom’s coal fleet, but several scientific articles have found South Africa’s coal reserves are generally low in sulphur. Eskom should be able to avoid sulphur-driven corrosion if motivated to do so. The issue, then, is not what coal South Africa has, but what coal Eskom chooses to use.

Eskom coal inspection

Eskom has a proven history of purchasing sub-standard coal, having spent hundreds of millions of rands buying coal containing two percent sulphur from Gupta-owned Tegeta mining – well above the specified limit of 1.3%.

Investigators also found evidence that coal inspection processes had been interfered with, with samples from one mine allegedly swapped out to obscure the sulphur content.

There have been reports that some of the coal supplied to Eskom is even mixed with sand and rocks to increase the weight. The fact that such obviously sub-standard product manages to make it into Eskom’s boilers reveals shocking deficiencies in inspection and quality control.

The low oxygen levels inside the boiler tubes is a complex operational issue. When asked whether feeding higher ratios of air to coal could reduce the rate at which this type of corrosion occurs, Van der Merwe said while it may help, it would result in increased emissions of NOx compounds which have extremely high global warming potential, as well as a range of detrimental health effects.

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The main reason that boilers are operated in conditions of restricted oxygen is to prevent the formation of these compounds. That may be a trade-off South Africa has to accept as a stopgap, while Eskom finds a way to switch to coal of an adequate standard.

Neil Thomas Stacey lectures in biomedical engineering and on waste-water management at Wits.

This article originally appeared on GroundUp, read original here