Clean Energy Fuels (NASDQ: CLNE) said Monday it has bought Dallas Clean Energy, which owns a landfill-gas processing plant in Texas, for approximately $19.1 million in cash.

It's the latest in a series of green investments by the distributor of natural gas for transportation, which was co-founded by T. Boone Pickens, a Texas oilman and head of hedge fund BP Capital in Dallas (see Earth2Tech post).

Clean Energy Fuels last week committed $10 million of a $160 million financing for Troy, Mich.-based Vehicle Production Group, which plans to build natural gas-powered taxis and paratransit vehicles.

Natural gas emits fewer pollutants than gasoline, but it is still a fossil fuel, leading some to scoff about it being called "green."

Perseus, a bank and private-equity management firm, led the $160 million financing, and Pickens said he also plans to invest $10 million of his own money into the project (see VentureBeat post).

Pickens is also building a 4-gigawatt wind-power project, complete with transmission lines, and has touted a plan, called the Pickens Plan, to promote wind power and natural gas (see T. Boone Pickens Has a Plan, Summit Aims for Energy Policy Compromises and Wind Power Waiting on Transmission-Line Boom).

Clean Energy Fuels bought Dallas Clean Energy from Camco International, a U.K.-based company that develops projects to reduce greenhouse-gas emissions and that sells carbon credits.

Dallas Clean Energy owns the McCommas Bluff plant, which produces methane gas from a landfill owned by the city of Dallas.