"The problem for a closed system is finding a cheap source of sterile carbon dioxide (CO2). Several experimenters have found the CO2 from a smokestack works well for growing algae.[22][23] To be economical, some experts think that algae farming for biofuels will have to be done next to power plants, where they can also help soak up the pollution.[18]"
http://en.wikipedia.org/wiki/Algae_fuel

So, if algae farming was viable, they would actual pay some money for waste CO2 from power station.
This would mean that making electrical power from coal would be even cheaper than today, because not only are they being paid for the electrical power, but if they only got somewhere around say 5 cent per ton of CO2 [far below current market prices]] they could make as much from CO2 as the electrical power.