The beast has its Virgil: Pauline Butler, who, through earplugs and headphones because it’s so noisy, explains precisely how this terrifying yet magnificent feat of engineering pumps enough electricity each day to fire the homes of 6 million people. Its output is about 3.4 gigawatts, making it the UK’s biggest single power generator, providing 8 per cent of all our electricity.

To create such magic, Drax eats up about 8.5 million tonnes of coal but spews out about 22 million tonnes of dirty CO2 gases each year. Now the beast is changing its diet. For the past year one of the six burners has been fed with wood pellets that have travelled 2,000 miles from the forests of Mississippi by ship and rail to North Yorkshire. Once here on the Selby site, the biomass is stored in brand-new ballooned-shaped domes as big as Istanbul’s majestic Hagia Sofia.

A few hundred metres away from this extraordinary moon-like landscape I wait in offices that have the feel of a glorified Portakabin to meet the woman behind Drax’s newest industrial revolution: its chief executive, Dorothy Carrington Thompson. “Ah, Dorothy,” says the receptionist, with the same sigh of awe that everyone uses when they talk about the boss: “She’ll be here any minute.”

The City has a new name for her: the Iron Lady. This is out of respect for her toughness in persuading the Government to back Drax’s ambitious plan to go green because of new carbon taxes. But also for the way she and her team have led the world in converting such a vast coal-fired station to biomass. By doing so, Mrs Thompson has secured more than 1,250 jobs and created another 800 jobs for British contractors. Analysts have been converted too: Drax’s shares have steamed ahead by nearly a third to 750.5p over the last year, valuing it at £3bn, and the shares are still rated a buy.

Mrs Thompson arrives looking glamorous; incongruous against the mud and vapour of the site where she spends most of her working week, commuting from York. It’s hot in the room and she takes off her jacket; her bare arms look as though she works out. So how did this Iron Lady pull off such a transformation? “No one thought coal-fired stations could be converted on this scale but our engineers have been working by trial and error on this for years, experimenting and designing over and over again.”

Their biggest problem was working out which biomass to use and how to store so much of the stuff in controlled conditions; biomass dust is many times more combustible than coal dust as it ignites spontaneously. Eventually, her engineers came up with the solution: domes made by inflating gigantic polyurethane balloons and then lining them with concrete – a sort of inside-out papier-mâché process.

Two of the four domes are now feeding one of the boilers with biomass fuel which is powering about 1 million homes, she says. The aim is to have half the plant operating with biomass by 2016, thus reducing CO2 emissions by half. But it’s been costly: Drax has spent £700m on the domes, new conveyor belts and the trains bringing the pellets from the ports. Two new upstream supply plants, and new port facilities, are also being built in the US.

 With coal so cheap, is it worth the cost? “But of course,” she says, a little archly. “I confess I am a believer in climate change. We need to do all we can to ensure a low-carbon environment and be as careful as we can about sustainability. CO2 emissions are already much lower as a result of the one burner using wood.”

Biomass is also the most flexible and reliable of all renewables: “It is the only renewable which can deliver low-carbon electricity on demand, at the scale the grid needs and when it’s needed. At a time when margin capacity is so low, the UK needs a good balance of power sources. Other sources such as wind farms are nowhere near as reliable.”

Yet Drax has its critics. Ultra-greens argue that taking wood from the US forests is as ecologically damaging as using coal, while others claim that the taxpayer is paying too much in subsidies: under a new contract-for-difference scheme, the Government has given Drax a guaranteed strike price, which some argue keeps electricity prices higher than they could be.

But she swats down both arguments: “The biomass we use is the by-product of the forests, which are sustainable and renewable. As to subsidies, Drax is a national strategic asset and government needs the security when energy supplies are so fragile.

“Everyone here feels a real responsibility towards the public to ensure supplies are kept going. What we have achieved is totally home-grown and we feel great pride in what’s been done.”

And so should we: pride in the workers such as Mrs Butler, who guides thousands of people around the site each year, and all those who risk their lives daily in the inferno, and those in the control rooms glued to their screens monitoring the fires that burn to keep our lights on.