PetroChina Co, the nation's biggest oil company, has won state approval for its plan to double ethylene capacity at its Daqing petrochemical plant in northeastern China, the nation's top economic planner said.

PetroChina will increase the unit's ethylene capacity to 1.2 million metric tons a year, the National Development and Reform Commission said in a statement on its Website, without giving an investment figure for the project.

China's petrochemicals makers are raising capacity and output as a faster pace of economic expansion stokes demand for chemicals used to make yarn and plastics, Bloomberg News reported. China, whose economy grew 11.1 percent in the first quarter, imports half its ethylene needs.

PetroChina will increase its ethylene capacity to seven million tons a year by 2010 from 2.63 million tons last year, Liu Jie, deputy chief economic adviser at PetroChina's chemicals and market unit, said earlier.

It will build six ethylene projects in Daqing, Jilin, Fushun in the northeast, Lanzhou, Dushanzi in the northwest and Sichuan in the southwest, by 2010, Liu said.