Coal emissions

The hotter you can make steam, the more efficient your power station will run. It's simple thermodynamics - the hotter the steam gets, the more energy it has which can be transferred to your turbines for the same amount of coal burnt.

Unfortunately it's somewhat difficult to find a pipe (to run your steam through) which can accept being put in the flame of the boiler - at around 1400DegC (possibly up to 1700DegC / beyond).

Generally the more nickel you put into your steel (and nickel is expensive) the better things get (although I should say that I'm not a metallurgist).

Comtes 700 is a research project designed to create a power station that can run with a steam cycle at 700DegC - which is plenty hot!
It's operating efficiency would be 50, 55% I believe - approximately equivalent to a combined cycle gas turbine plant.
I believe its gCO2 / KWh emissions will be in the region of 600g / KWh - obviously these can be brought down by adding biomass, but I don't believe that they will be able to be brought below the 500g / KWh mark, although 550g / KWh is possibly feasible.

Adding carbon capture to the plant will obviously decrease it's efficiency, but probably only to around 40%.
Let's remember that adding carbon capture to a combined cycle gas turbine will do something similar to its efficiency.