"You can't dictate future technologies like in a planned economy," he told an energy conference in Berlin last week.

Germany's E.ON and RWE are among major power generators that want to cut the pollution caused by burning coal for electricity in a world worried about climate change.

They have launched studies on capturing the greenhouse gas carbon dioxide (CO2), which is produced in vast amounts by burning coal, and burying it under the ground or the seabed.

The success of tests at pilot plants will depend on whether storage will be available and at what expense, Bernotat said.

E.ON is involved in CO2-free projects in Germany, Britain and the United States, which Bernotat said might be concluded by 2020, but he added that there was no guarantee.

Experts cite vast additional energy and financial requirements, as well as technical and legal problems, as obstacles to the success of such schemes.

It is still uncertain how the economics of carbon capture stack up -- for example the European Commission has not yet decided how to reward the technology under its carbon trading scheme.

EU Commissioner Andris Piebalgs said on Jan. 10 that all new coal-fired power stations from 2020 should offer CO2 capture and storage.

In response, German Economy Minister Michael Glos said in Berlin: "We want the CO2-free coal and gas plant but we will only be able to say in a few years when we can achieve this."

PIPE DREAM

Energy analyst Manuel Frondel of the RWI Insitute in Essen said CO2-free technology was "still a pipe dream to combat coal's negative image, but a lot of homework needs to be done".

RWE Power, RWE's generation arm, is speeding plans for a 1 billion euro (US$1.3-billion) clean coal power station meant to start up in 2014 in Germany, and it is involved in a UK project.

Like E.ON, it is also installing more efficient technology at newly built conventional coal plants.

"The two trends work hand in hand. If we can boost the efficiency of conventional plants, we can afford to spend some of the energy surplus for the capture and storage process, which necessarily absorbs extra energy again," said an RWE spokesman.

E.ON and RWE aim to eventually raise coal plant efficiency rates, which measure energy output as a proportion of raw material input, to above 50 percent from the 43 percent that is now the best achieved in Germany.

In many other countries, the norm is just 30 percent.

Boosting efficiency involves coal drying techniques, heat recycling and use of steels that can resist higher temperatures and pressure. Germany hopes to export such technology.

Demand for coal generation is booming in an increasing number of countries, despite the difficulties posed by the carbon emissions.

"There is no way around coal usage. It is the least subsidised, cheapest and most widely available source of power," the chief executive of RWE Power, Jan Zilius, said in Berlin.