* 4.8 mln households converted in 2018, up from 4 mln in 2017
* War on pollution getting harder, minister tells reporters (Adds detail, quote, background)
BEIJING, March 11 (Reuters) - China expanded its coal-to-gas and coal-to-electricity projects to 35 cities in 2018 from 12 cities the previous year, China's environment minister, Li Ganjie, said on Monday, as the world's second-largest economy stepped up its fight against smog.
China's winter heating programme used to burn an estimated 400 million tonnes of coal a year, and switching it to cleaner types of fuel was identified as a major part of the country's war on pollution, now in its sixth year.
The programme to convert households to low-emission heating ran into difficulties last winter amid widespread natural gas shortages, but 4.8 million households still managed to make the switch from coal to natural gas and electricity last year, up from 4 million households switched in 2017, Li said.
China has also installed ultra-low emissions technology at more than 80 percent of its total coal-fired power generation capacity, Li added.
But, speaking to journalists at a press conference on the sidelines of China's annual parliamentary meeting, Li warned that despite the strides made in 2018, China's war on smog was getting harder.
"In reality the pressures are huge and it isn't easy to be optimistic about the trends," he said. "The things that could easily be done have already mostly been done, and the things that need to be done afterwards are much harder."
While China cut the share of coal in its total energy mix to 59 percent last year, down from 68.5 percent in 2012, it remained too high, he said. There were also still regional disparities in the way environmental protection was being enforced, he said.
China said last week that it would implement special emissions restrictions for a third consecutive winter, after Premier Li Keqiang told parliament that the state would continue to strengthen pollution controls in 2019.
China's economy expanded at its slowest rate since 1990 last year, and the environment ministry has been at pains to state that it would not relent in its efforts to curb pollution.
However, though concentrations of hazardous small particles known as PM2.5 fell overall last year, they rebounded over winter, with averages in 39 smog-prone northern cities up 13 percent over the October-February period.
(Reporting by Meng Meng in Beijing; Writing by Chen Aizhu in Singapore and David Stanway in Shanghai; Editing by Tom Hogue and Christian Schmollinger)
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