By Adam Rose and David Stanway

BEIJING (Reuters) - A chronic shortage of natural gas is hurting China's plan to move away from burning coal to heat homes and offices, raising the prospect of more choking air pollution this winter and beyond.

The problem is worst in northern China, where air pollution mainly caused by decades of reliance on coal has lowered life expectancy by an estimated 5.5 years compared to the south, Chinese and international researchers said in July.

The frigid northeastern city of Harbin, home to 11 million people, virtually ground to a halt last week when airborne contaminants were around 50 times the level recommended by the World Health Organization. Beijing had its own emergency in January when air pollution was 45 times the level.

"I suspect we will have severe incidents of air pollution in Beijing again this winter," said Alvin Lin, China Climate and Energy Policy Director for the U.S.-based Natural Resources Defense Council.

China sees natural gas as the way to cleaner air. Authorities have said Beijing's urban core should use only gas for heating. But domestic output cannot keep up with demand.

"With the current natural gas situation, maybe you can guarantee (supply to) big cities like Beijing, but if you want to expand to the cities of northern China that need it ... I think that's still quite difficult," said Tao Guangyuan, a renewable energy expert and columnist based in Beijing.

The shortage has forced the government to ration gas supplies, even banning construction of new natural gas-fired power stations. The government also said last week it would control the increase in new gas users, prioritizing supplies to residential users and public transport during the winter.

Nevertheless, the gas shortage may still be 10 percent higher this winter than last year because more users have switched over, official media said last week, citing an unidentified executive from PetroChina, China's largest gas producer and importer.

The government has said it would raise natural gas use to 230 billion cubic meters by 2015, more than double the 2010 rate, but disappointing domestic production growth coupled with insufficient pipeline and storage capacity has left it increasingly reliant on imports and prone to shortages.

Importers also risk losses because the government keeps the price of gas low to curb inflation and ease the impact on consumers, although recent incremental price hikes have helped.

"If there's not enough gas, many people will just go back to burning coal, secretly," said Ming Sung, East Asia chief representative of the Clean Air Task Force, a U.S.-based environmental advocacy group.

SMOG ALERT SYSTEM FOR "GREYJING"

China wants to tackle air pollution to stem potential unrest as its increasingly affluent urban populace turns against a growth-at-all-costs economic model that has spoiled much of the country's air, water and soil.

Its cities are among the world's most polluted. Air pollution in Beijing, derided as "Greyjing" or "Beige-jing" by English-speaking residents, exceeded national standards 62 percent of the time during the third quarter, the Ministry of Environmental Protection said.

Beijing authorities have unveiled a new smog alert system that will impose curbs on driving and halt manufacturing and construction if three consecutive days of hazardous pollution are forecast for the capital, located in northern China.

"I think there's now more attention being paid to what are the immediate, short-term things that could be done when the air pollution gets that bad," said Lin.

Images of the acrid smog over Harbin on October 21 were beamed around the world after authorities literally turned on the heat by switching on a network of mostly coal-fired heating plants that warm large swaths of the city. Officials blamed thick fog on the same day for contributing to the pollution.

The same sort of plants are used across northern China, providing so-called centralized heating. Some also supply electricity while others burn coal just to generate steam which is piped into city grids for heat.

Centralized heating dates back to the 1950s in Beijing. It had spread across the north by the mid-1980s. At the time, China was still impoverished, which meant the government could only afford to give its colder north centralized heating. Residents in cities south of the Yangtze River such as Shanghai tend to rely on electric space heaters.

Most of the north's heating plants are turned on around mid-November. Many institutions and residential complexes have private heating, but also use coal-fired boilers.

The coal-fired heating plants use coal-scrubbing filters which reduce soot emissions, said Tao, the renewable energy expert. The real pollution culprits are small coal boilers, as well as traditional "kang" - stove-heated beds that people sleep on - and other forms of "rustic heating" that residents depend on to survive chilly winters. Such stoves produce tar and hazardous smoke, experts say.

Air pollution in Harbin, which gets 65 percent of its warmth from public and private centralized heating, was within WHO recommended levels on Tuesday after being 7.5 times acceptable levels on Monday.

COAL IS STILL KING

Despite China's rush to gas, coal still supplies the bulk of the country's total electricity needs.

Under a new plan announced last month to tackle air pollution, China would cut consumption of the fossil fuel to below 65 percent of primary energy use by 2017, down from 66.8 percent last year.

The plan also aims to raise the share of non-fossil fuel energy to 13 percent by 2017, up from 11.4 percent in 2012.

PetroChina Chairman Zhou Jiping said earlier this year it would take at least four to five years to build up new natural gas supply capacity, which would still not be enough to meet demand.

"For the whole country to move to natural gas in the way that you kind of see in the U.S., ... it's going to be a few decades," said Lin of the NRDC.

Natural gas demand rose 13.5 percent in the first nine months of 2013 for example, 4.3 percentage points faster than production, the government said last week.

With demand set to jump further, the government has put Beijing at the top of the list for supplies.

Beijing, Shanghai and Guangzhou in southern China are also expected to increasingly get energy from neighboring regions under last month's anti-pollution plan.

The capital, for example, is supposed to get 70 percent of its power by 2017 from neighboring regions, such as Shanxi and Inner Mongolia, where coal is still in widespread use.

"You can think of it as a redistribution of pollution," added Lin.

(Additional reporting by Beijing Newsroom.; Editing by Ben Blanchard and Dean Yates)