BEIJING -- China's capital of Beijing has set a target of completing environmentally friendly renovations on all its coal-fired heating furnaces by 2015.

Inefficient coal burning is considered one of the major causes of lingering smog in the city, which is dependent on fossil fuels for centralized heating during its four-month-long winter.

The municipal commission of city administration and environment said on Monday that Beijing has conducted energy efficiency retrofits on 266 coal-fired furnaces, which supply heating for 72 million square meters of residential areas.

The reconstruction has improved their heating efficiency, reducing the amount of coal they cumulatively consume each year by 500,000 tons, and the amount of sulfur dioxide, nitric oxide and soot they discharge annually by 86.5 tons, 650 tons and 43.5 tons, respectively, said a spokesman with the commission.

He said there are still 30 more coal-fired furnaces located within the city's fifth ring road waiting for the technical transformation. The work is expected to be fully completed before 2015.

Partly due to humidity and a lack of strong wind, Beijing experienced 23 smoggy days from January 1 to 28. This was about 10 more than on average in the same period in the past 10 years and the most since 1954.

The average density of PM2.5 tiny airborne matter in January was 180 micrograms per cubic meter in Beijing, about 30 percent higher than the same period between 2009 and 2011, according to official meteorological data.

Coal burning and car exhaust emissions are the two primary sources of air pollution in the crowded, traffic-clogged city.