Th US has a trading system in place for sulphur dioxide emission. This trading system has a tendency to favor existing installations over new build installations. So 'repairing' or 'refitting' an existing intsllation is chaeper than building a new one if one wants more capacity. And before you ask, refineries are major sulphur emitters.

Interesting tie to this - On the watt podcast #75 they discuss how the coal power plant reduction happened.

Lower heat content, lower sulfur coal was shipped in VS using 'local' higher heat, higher sulfur coal.

Thus lower sulfur emitted, higher CO2 emissions.

WHEEE!

Now I'll bet they wish they installed scrubbers (or whatever it is that captures the sulfur), and had "waste" sulfur to sell. Last I heard it was something like $900/ton.