U.S. power generator NRG Energy Inc. and Japanese energy producer JX Nippon Oil & Gas Exploration Corp. have finished the world's largest system to capture carbon dioxide produced from burning coal at a power plant.

The $1 billion Petra Nova project, which collects carbon emissions from an existing coal-fired power plant southwest of Houston, passed testing in late December and was turned over for operations, the companies said in a statement Tuesday. The carbon dioxide, a heat-trapping gas that contributes to global warming, is sent by pipeline to an oil field jointly owned by NRG, JX Nippon and Hilcorp Energy Co. where it can be used to draw crude out of the ground.

The achievement comes at a precarious time for so-called clean coal projects that have for years benefited from federal policies encouraging lower emissions from power plants to battle climate change. While the Petra Nova project was finished on schedule, another clean coal plant being built by Southern Co. in Mississippi has been delayed by years and is billions over budget. U.S. President-elect Donald Trump has, meanwhile, vowed to roll back rules that require power generators to curb emissions.