The Jacob Marley column: Supercritical

August 4th, 2014, Published in Articles: Energize



This topic invites a strange introduction. In October 1901, several daring Boer commandos attacked the rearguard of British Colonel Benson’s column at Bakenlaagte in Mpumalanga. During the ensuing melee, great bravery was demonstrated by both sides. Colonel Benson, a Magersfontein veteran, died the next morning from wounds received on the field of battle.

Why mention this? Well, Bakenlaagte is just 12 km down the hill from where Kriel Power Station looms over the countryside today. This is where the first Benson boilers were installed by Eskom seventy years later. The choice of boiler was not related to the intrepid soldier, of course, but it is a curious point of nominative determinism that these Benson boilers are the precursors and preferred type of boilers to be used for supercritical operation at Medupi and Kusile.

Imagine a substance so outlandish that it almost defies description. It consists of nothing but hydrogen and oxygen, and exists only in a world where the temperature is always more than 374°C and the pressure exceeds 22 Mpa. Something like the atmosphere of the planet Venus, perhaps. However, in behaviour this strange substance is more Mars than Venus, if I may employ that dreadful modern comparison.

Because distinct liquid and gas phases no longer exist, a supercritical “fluid” can effuse through solids like a gas and dissolve materials like a liquid. Even more to the point, small changes in temperature or pressure result in large changes in density. This it highly unstable and difficult to control. A bit spooky, I am sure Colonel Benson may have thought.

A supercritical fluid need not consist of hydrogen-oxygen molecules, of course, but that happens to be the fluid of choice when it comes to driving turbines coupled to electric generators. High temperatures are desirable from a point of efficiency, so engineers are forever chasing that Holy Grail by driving Messers Carnot and Rankine’s cycles to ever-greater heights.

It is comparatively easy to boil water and separate the steam from it in a boiler drum, but natural circulation ceases at high pressures and temperatures. Besides, boiler drums are heavy, expensive and prone to explode at the drop of a hat. So in 1922 Mark Benson (a Bohemian engineer not related to our Colonel) invented a boiler without a drum which uses a simple once-through concept. One pumps water in at the bottom and gets steam out at the top. This is what happens at Kriel and its successors, although still at subcritical conditions (about 17 Mpa and as much as 516 to 540°C).

With a Benson boiler, however, it is relatively easy to increase both pressure and temperature above the critical point and achieve cycle efficiencies which are not possible at subcritical conditions. In the search for ever higher efficiencies, a Benson boiler (or “steam generator”, since supercritical fluid does not “boil”) is the logical starting point. Both Medupi and Kusile will be equipped with these beasts, typically operating at 25 to 26 Mpa and 560 to 570°C. The gain is several percentage points improvement in efficiency, with a commensurate reduction in coal consumption and greenhouse gas emissions. Sounds too good to be true?

Well, there is the point of unstable behaviour of a supercritical fluid. It was only 30 or so years ago when sophisticated control systems made it possible to operate supercritical steam generators of any practical size with some reliability. Although the technology has improved considerably over the years, it is a bit unusual to throw out the control system in midstream and start all over again, as happened at Medupi. So the risk of a malfunction remains ever-present and just like a large airliner needs a well-trained pilot to take over in some critical situations, so does a supercritical steam generator rely on experienced operators to keep it safe and reliable.

Having heard about “unfortunate accidents” which have beset Eskom’s power plants recently, one cannot help wondering how prepared the utility is to keep riding the supercritical roller coaster once they get it going. It will be a bit like riding a wild mustang with an uncertain pedigree at a rodeo, and however much the powers that be might not like it, it might a jolly good idea to bring in some experienced grey-beards with years of training and a steady hand at the tiller rather than some old-hand cadres.

Secondly, supercriticality requires the most sophisticated purified water treatment possible. Just as demineralised water contains many times fewer impurities than evaporated water, so sophisticated condensate polishing and oxygenated water chemistry needs to keep the water supply for a supercritical once-through plant much purer compared to a subcritical installation. That not only requires chemists of the highest calibre but also a vigilance and a backup plan which will react instantly.

With all the horror stories doing the rounds about air and water pollution issues at Eskom’s power stations in the recent past, it is no wonder that one might be concerned not so much about the necessary pieces of paper as about the fact that there is only one chance to prevent a glitch, because that will be the last.

And last but not least, there is the matter of supercritical fluid behaving like a liquid which can dissolve solid substances like water or effuse through steel like gas through a permeable membrane. The special steels used in supercritical boilers are usually able to withstand this aberrant behaviour, but the welds, which are a mixture of various substances, sometimes brought together in less that exact and pristine circumstances, remains an area of weakness.

And who can forget the furore over the past few years around the suspect welds at Medupi? And all the delays and unrest? Who knows how many welds are still left open to attack, not by the intrepid Boers of so many years ago, but by that spooky supercritical fluid in Col. Benson’s namesake steam generators?

So no matter how supercritical the smooth operation of Medupi may be to the ailing economy, these glossed-over aspects of supercriticality, despite all the fulsome promises, may well trip up any optimistic forecasts, perhaps not so much about the commisioning dates as about the firm commercial operation dates of these first units. After all, winning the battle counts.

Without thus meaning to be critical, Sir, I remain your humble and obedient servant

Jacob Marley

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