Indoor heating was restored yesterday to more than 10,000 households in Urumqi, capital of Northwest China's Xinjiang Uygur Autonomous Region, ending three days of anguish for the freezing residents.

The sudden breakdown of a 450-ton boiler at a local power plant on Sunday cut central heating to an area covering 4 million square metres, mainly in the Shuimogou and Tianshan districts of Urumqi, according to reports by the Xinhua News Agency.

Thousands of people had to cope with the low indoor temperatures by wrapping themselves with thick quilts. Local media urged older people and children to dress warmly to avoid catching colds.

Du Wei, the Party secretary of Xinjiang Weihuliang Power Plant, was quoted by Xinhua as saying that a batch of substandard coal had melted and then stuck to the interior surface of the boiler.

The melted coal formed a layer of residue more than 30 centimetres thick inside the boiler, which led to the breakdown.

Du said strong demand for coal had strained supplies. The plant's supplier, the Shenhua Energy (Xinjiang) Co Ltd, could only meet 80 per cent of its demand.

"We had to buy coal from some small coal mines," Du was quoted as saying.

Cui Dong, an engineer at the plant, told China Daily that clean-up work had only concluded at 8 am yesterday, and that the plant was gradually resuming its normal heat supply.

Cui said the substandard coal had met the requirements for producing heat, though it contained high levels of calcium oxide, which has a comparatively low melting point.

Cui said that another 450-ton boiler had been damaged by the low-quality coal, though not enough to prevent it from providing heat to local hospitals during the three-day break-down.

To prevent future break-downs, Urumqi's government has transported coal from another local mine to ensure an adequate supply at the plant, Cui said.

Electric heaters sold out at local shopping malls, local media reported.

Those heaters came at a right time the local observatory reported that the temperature dropped to a low of -14 C yesterday.