New Delhi, May 20: New thermal power plants will have to generate electricity with 25 per cent less soot particles under modified rules of operation announced by the government that will also impose fresh limits on other emissions.

The Union environment ministry has released a draft notification specifying the revised norms from 2017 to slash emissions of particulate matter, nitrogen oxides, sulphur dioxide and mercury.

Thermal plants are a major source of air pollution. A survey of 47 coal-powered plants released by the NGO Centre for Science and Environment (CSE) earlier this year found that over half the units violated limits on various emissions.

The CSE has estimated the new rules will cut particulate emissions by 25 per cent, sulphur dioxide by 90 per cent, nitrogen oxides by 70 per cent and mercury by 75 per cent.

"The proposed changes may go a long way in safeguarding public health," the CSE said in a release yesterday.

Under the rules, thermal plants established after 2003 need to meet slightly lower standards, while emissions will be even more relaxed for plants constructed before 2003.

The CSE said coal-based power plants account for 60 per cent of particulate matter, 45 per cent of sulphur dioxide, 30 per cent nitrogen oxides and 80 per cent mercury emissions spewed by the power sector.

Two years ago, Greenpeace India had released a document that dubbed coal power as "India's dirtiest energy source", and estimated that between 80,000 and 100,000 people died prematurely from the health consequences of coal-related emissions.

The report had also estimated that over 20 million asthma attacks in 2011-12 could be attributed to thermal plant emissions.

The ministry's revised standards also require existing cooling tower-based units to restrict water consumption to 3.5 cubic metres per watt hour. Plants established after January 2017 will have to work with 2.5 cubic metres per watt hour.

The proposed changes will help reduce the amount of freshwater use by thermal power plants. The CSE estimates that, cumulatively, their freshwater use will decrease from about 22 billion cubic metres in 2011-12 to about 4.5 billion cubic metres in 2016-17.

"We're happy that less water will be used by power plants and more will be available for people," said CSE deputy director-general Chandra Bhushan.