Coaltek, a Tucker, Ga., company that tries to lower the greenhouse gas emissions created by the burning of coal, has raised $33.4 million in a third round of financing, according to VentureWire (sub required).

Lightspeed Venture Partners led the round, and was joined by existing investors Braemar Energy Ventures, Draper Fisher Jurvetson, Element Venture Partners, Technology Partners, and Warburg Pincus — brining the company’s total funding to more than $50 million in three rounds of funding.

The VentureWire piece has a good summary of the debate about coal-processing, between critics who say it the process is too costly to really make coal clean enough, and supporters who say coal’s predominance as an energy source necessitates moving ahead with the technology.

From the piece:

While venture backed start-ups, like Cambridge, Mass.-based GreatPoint Energy Inc., are also looking at coal-gasification technologies, Coaltek has developed a process that converts low grade coal into higher quality, cleaner burning coal for use in traditional electric power plants.