About 100 protesters marched at Wales' largest coal-fired power station calling for its closure on Saturday.
The protest at Aberthaw, Vale of Glamorgan also called for more renewable energy jobs.
In September, the European Court of Justice found it had been emitting illegal levels of air pollution.
Plant operators RWE said they were investing £5m in technology that would "significantly lower" the station's toxic nitrogen oxides emissions.
A spokesman said the station would also see a reduction in generation levels this year to ensure it can continue to provide electricity competitively while operating within its Environmental Permit.
But Alyson Austin, from the United Valleys Action Group, said this was "a sticking plaster for a severed arm, it's just not good enough".
She said: "We need to put something positive in place of Aberthaw. It is inevitable Aberthaw is going to close.
"Renewable energy could provide thousands of jobs."
She called for Aberthaw workers to be "re-skilled" now, to allow this to happen.
Her action group was joined at the protest by Reclaim the Power, Bristol Rising Tide and the Coal Action Network.
An RWE spokesman said: "The combination of fewer operating hours and a lowering of emissions will ensure that Aberthaw Power Station can provide security of electricity supply for Wales and beyond into the 2020s.
"UK generation margins have fallen to record low levels a number of times so far this winter, with associated warnings from National Grid.
"Whilst Aberthaw provides an important bridge to the UK's vision of a renewable future, in the short to medium term it is an important partner to balance renewable technologies and help to keep the nation's lights on."
He added: "For the more than 200 men and women who work at the site, the impact of the changed operations means that staff structures are being updated and it is, regrettably, a challenging time for them."