What is a supercritical boiler, and why are their deployments growing?

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Dear Control Engineering: Looking at the article about new ultra-supercritical power plants, I have to ask what supercritical means, and why it matters.

If you think of what water looks like in a boiling pot, steam forms in bubbles in the water and rise to the surface. If the pressure in the boiler is higher than 3,200 psi, bubbles cannot form in the water, so the water converts directly to steam without conventional boiling. Ultra-supercritical boilers use pressures in the 4,500 psi range. Typically these are used in coal-fired power plants where the whole idea of using such high pressures is to gain efficiency. The higher the pressure, the higher the efficiency.

However, there are limits. Given the temperatures and pressures involved, the stress on the boiler is enormous. Practical deployments of supercritical plants capable of base loading have been relatively recent since boilers require materials and construction methods that are still being developed. The process of forcing feedwater into a boiler is also a challenge and requires proportionally larger turbines. Regulating steam production requires more sophisticated control systems which are also improving along with the construction methods. (Read an article about controlling such plants.)

Nonetheless, the desire for greater efficiency, here measured as kilowatt hours per ton of coal, is a worthwhile goal.

--Peter Welander, pwelander(at)cfemedia.com