LANZHOU, August 14 -- Once one of China's most polluted cities, the northwest city of Lanzhou has rid itself of heavy air pollution and become a model for pollution control over the past two years.

The Ministry of Environmental Protection organized a group of environmental officials from other heavily polluted areas to visit and learn from Lanzhou City, capital of northwest China's Gansu Province, last Thursday.

Lanzhou was once one of the top 10 most polluted cities in China. In 2009, it ranked as the most polluted among the country's provincial capital cities.

As a heavy chemical industry base, Lanzhou has suffered from serious air pollution over the past decade. The city is situated in the Yellow River valley and is surrounded by mountains, which hinders the dispersal of pollutants.

Yan Zijiang, the director of the Lanzhou Municipal Environmental Protection Bureau, said that the city's pollution is especially heavy in winter because of coal-fired heating systems.

There were more than 1,000 coal-fired heating boilers, three large thermal power plants and about 200,000 civil small boilers in the city's urban area, Yan said.

To remove the source of pollution, Lanzhou has spent two years encouraging gas use rather than coal for heating.

All 1,000 heating boilers in Lanzhou have been replaced by natural gas-fired heating systems. The three thermal power plants have been given emission limitations, and residents have been ordered to use anthracite as an alternative to burning coal, according to Yan.

So far, a total of 916 projects, including reducing industrial and coal-generated emissions and automobile exhaust control, have been implemented to curb the severe pollution.

Thirteen polluting enterprises with excessive capacity have been closed, more than 200 highly polluting enterprises were suspended in winter, and 78 industrial enterprises have moved to an industrial park outside the city.

Lanzhou's citizens have been mobilized to help with the pollution control campaign.

The city has been divided into 1,482 districts, with six staff in charge of pollution supervision in each area, according to Chen Yaolong, an official with the community office of Dunhuang Road in Lanzhou's Qilihe District.

Lanzhou had 299 days of excellent or good air quality last year, 57 days more than in 2001. So far this year, the city has seen more than 180 days of good air quality, according to statistics from the city's environmental protection bureau.

Zhang Yucun, 62, a resident in Lanzhou, said that in the past he could rarely see the mountains clearly due to the heavy pollution, but now people can do morning exercises in a clean environment.