The soaring, grimy chimneys of the coal-fired power station have belched the last of their choking fumes into Beijing's air, authorities say -- but experts doubt the plan will ease the capital's smog.

The 50-year-old Gaojing facility is one of four enormous generating plants authorities promised to close after the city was repeatedly blanketed by acrid haze.

Levels of PM2.5 particulates, the smallest and most dangerous, rose to 16 times World Health Organization recommended limits across Beijing over the past week, the toxic smog forcing hundreds of athletes to don masks for Sunday's annual marathon.

World leaders including presidents Barack Obama and Vladimir Putin are due to descend on the city for an Asia-Pacific Economic Co-operation summit next month, when temporary industrial closures and a public sector holiday have been declared.

Around a quarter of the capital's electricity is produced from coal, and in the long term municipal authorities aim to switch to cleaner energy sources.

A pristine gas-fired plant that became operational earlier this year, the Beijing Northwest Thermoelectric Centre, stands near the Gaojing facility, its shiny steel structure glistening in the sun.

But at the same time China is opening a new coal-fired power plant every week, according to environmental campaign group Greenpeace, with 159 under construction and another 337 in the pipeline.

The Gaojing plant's shutdown was announced earlier this year, but a month later clouds of steam still emerged from its cooling towers and staff were unconcerned that they might lose their livelihoods.

"We are going to work at other power stations," said Cheng Mengxiong, one of scores of workers at the site. "There will be a newly built coal power station in Hebei," the province neighbouring Beijing, he added.

PM2.5 levels soared to almost 40 times the maximum recommended limit during a heavy bout of pollution last year, sparking widespread anger among the capital's 21 million population.

In response Beijing authorities announced plans to slash coal consumption by 9.2 million tonnes before 2016.

- 'War on pollution' -

Premier Li Keqiang declared a nationwide "war on pollution" in March, officials talk of "green cities", and the central government has repeatedly pledged to cut the proportion of electricity generated from coal.

But it shies away from promising to reduce the total amount consumed, and observers say the plant closures' overall effect on the Beijing environment will be limited.

"Smog comes into the city, as well as being produced by the city itself, so it may actually have zero net effect and we may not notice anything different than before," said Richard Brubaker, professor in sustainability at the China Europe International Business School in Shanghai.

"We are talking about four power plants from about 2,200-2,300 across China," he added. "There are many people out there saying that by cutting these four power plants Beijing's smog is going to go away, which is anything but."

China will need as much as 400 percent more energy by 2030 to power its developing economy, currently the world's second-largest, Brubaker said.

Increasing urbanisation will see cities' energy footprint "absolutely explode", he added.

Writing in the Lancet last year, former Chinese health minister Chen Zhu cited studies showing air pollution caused up to 500,000 premature deaths a year in the country.

"It is very clear that China needs to start economic reform to not have only a better environment but also a more healthier economy," said Huang Wei, Greenpeace climate and energy campaigner in Beijing.

"The public is paying a lot of health 'burden' to live in such kind of environment and the economy is facing a lot of problems because of the huge over-capacity of steel, cement and glass industries."

China itself has attempted to put the brakes on plans for scores of coal-to-oil and coal-to-gas projects, originally touted as a ground-breaking shift towards cleaner energy use.

For Brubaker, energy efficiency is key, but he was not optimistic.

"The smog is only really going to get worse than it has over the last few years," he said.

"They are burning two units of energy compared to one unit in the US for the same level of GDP. It is just a very inefficient economy."