Warning: This is highly simplified and thus not exactly correct.
The temperature at which a substance freezes, melts, condenses, or boils is not fixed, but varies with pressure.
When a liquid crosses the "critical temperature" (at atmospheric pressure), it boils and becomes a gas with an obvious transition.
Less well known, when a liquid is subjected to pressure above the "critical pressure" (at a fixed temperature), it can actually become a compressible liquid.
The location on the graph where "critical temperature" and "critical pressure" meet is the "critical point" and that's where you can become supercritical. In that region, there is no difference between liquid and gas; there is no "boiling" or "condensing", because the substance effectively behaves like both at the same time. It has no surface tension, yet can dissolve things like a liquid solvent, but it can also diffuse through tiny holes like a gas.
Tiny variations in temperature or pressure can radically alter the density, allowing you to adjust for the exact behavior you want. Above the critical temperature, no amount of pressure can force the substance into liquid form, you can only solidify it. Some substances refuse to be made liquid or solid due to crazy critical temperature or pressure values required. Mixtures are another matter entirely, for example high pressure may force one component of a mixture to solidify and drop out of solution.
We think of matter as having "normal" properties because that's how it behaves at the earth's atmospheric pressure and temperature ranges but in reality the way we experience matter is just one of many different and just as crazy states.
To bring it back down to earth, the point of mentioning "supercritical steam" is that it means they can heat the water well above boiling by keeping it under high pressure. If they didn't, additional heat added to the water would just create more steam, not heat the water any further (and steam is vastly less efficient at absorbing heat than liquid water).
The temperature at which a substance freezes, melts, condenses, or boils is not fixed, but varies with pressure.
When a liquid crosses the "critical temperature" (at atmospheric pressure), it boils and becomes a gas with an obvious transition.
Less well known, when a liquid is subjected to pressure above the "critical pressure" (at a fixed temperature), it can actually become a compressible liquid.
The location on the graph where "critical temperature" and "critical pressure" meet is the "critical point" and that's where you can become supercritical. In that region, there is no difference between liquid and gas; there is no "boiling" or "condensing", because the substance effectively behaves like both at the same time. It has no surface tension, yet can dissolve things like a liquid solvent, but it can also diffuse through tiny holes like a gas.
Tiny variations in temperature or pressure can radically alter the density, allowing you to adjust for the exact behavior you want. Above the critical temperature, no amount of pressure can force the substance into liquid form, you can only solidify it. Some substances refuse to be made liquid or solid due to crazy critical temperature or pressure values required. Mixtures are another matter entirely, for example high pressure may force one component of a mixture to solidify and drop out of solution.
We think of matter as having "normal" properties because that's how it behaves at the earth's atmospheric pressure and temperature ranges but in reality the way we experience matter is just one of many different and just as crazy states.
To bring it back down to earth, the point of mentioning "supercritical steam" is that it means they can heat the water well above boiling by keeping it under high pressure. If they didn't, additional heat added to the water would just create more steam, not heat the water any further (and steam is vastly less efficient at absorbing heat than liquid water).