Green and Drax

The village of Drax in North Yorkshire gets its name from the Old English words Dreora and Ac, which loosely translate into timbered area haunted by the wild - a derivation echoed recently when a band of ecowarriors set up camp outside the Drax Power coal-fired power station, nearby.

To ’green’ activists, Drax is a by-word for pollution; a 240-metre chimney pumping out over 20 million tonnes of carbon dioxide (CO2) emissions amongst 12 massive cooling towers that dominate the local landscape. The 4,000 megawatt (MW) facility, which can supply almost 7% of the UK’s electricity demand, has no future in a world threatened by climate change, they argue.

Amid the fossil-fuel phobia, Drax Power is working to reinforce the plant’s position as a key contributor to the UK’s increasingly diversified energy mix. The ambitious strategy centres on reducing the environmental impact of what, it says, is already the UK’s largest, cleanest and most efficient coal-fired power station.

“There is no question in people’s minds here today that there is a long-term future for Drax, provided we continue with the kind of upgrades we are doing, and carving out an environmental leadership position,” said Melanie Wedgbury, Drax Power’s head of external affairs, in an interview at the power plant.

By 2011, Drax plans to cut its annual CO2 emissions by 15% through improving efficiency, emissions reduction and co-firing biomass materials in its fuel mix. The plan will tap into the skills and experience of the 580 employees at the plant, which has undergone a series of major upgrades since it began generating electricity in 1974.

The biggest current scheme is a £100-million upgrade to replace the low-pressure and high-pressure turbines on the six 660MW generators at the power station. Earlier this year, Siemens Power Generation won the four-year contract for this work. which will also support the introduction of co-firing technology at Drax.

Following completion of the turbine upgrade, Drax expects to improve its overall baseload efficiency — the ratio of energy out to energy in when operating at full capacity — by 5%, to around 43%, and so reduce CO2 emissions by one million tonnes/year.

According to Les Lemmon, head of the engineering and mechanical section at Drax, the efficiency gains will be principally down to the design of the turbine blade path. The existing turbines, he noted, were designed and built about 40 years ago — well before software tools such as CNC machining or finite element analysis for optimising the steam path and modelling where losses occur.

“Small, almost undetectable changes to the blade shape will increase the efficiency of the plant’s high-pressure cylinder to 92% compared to 87-88% at present,” the engineer said.

Drax is, meanwhile, progressing technology to enable it to burn renewable biomass materials alongside coal. The aim is to generate 10% of its electricity from renewable sources and reduce CO2 emissions by two million t/yr by the end of 2009.

A large area of the site is now given over to the storage of vast quantities of locally-grown elephant grass, willow billets and waste timber from the forestry industry. These renewable resources are the feedstock for a series of trials for burning solid biomass on one boiler, with another boiler scheduled to start co-firing by the end of the year.

The biomass has raised a number of unexpected challenges for Drax, such as selecting the best wood chipping equipment, according to a Drax manager. Willow proved particularly difficult to process due to its fibrous nature. Within the boilers, meanwhile, the biofuel/coal mix must burn within two seconds to ensure that the ash by-product meets the quality requirements of the building industry.

“It is not like-for-like substitution as the calorific values are different for a start,” said Wedgbury. “The engineers think it is technically feasible to go to 20% [biomass content]. We are targeting 10% and probably between 10% and 15% there is a window that needs to be explored.”

Another feature of the Drax facility is the flue gas desulphurisation (FGD) equipment, which is installed on all boilers. The installation, which was completed in 1996 at a cost of £680 million, is the biggest of its kind in the world.

The FGD equipment, in which boiler flue gas is passed through a series of equipment spraying limestone slurry, removes 90% of sulphur dioxide from the gasses, in compliance with current regulatory requirements. The process also produces 750,000 t/yr of gypsum — a by-product of the reaction between SO2 and limestone, which is sold to British Gypsum to make plasterboard. Drax started a programme in 2004 to retrofit all six boilers by installing Boosted Over-Fire Air systems. “Four out of the six units have been retrofitted, the fifth will be retrofitted next quarter and the final unit next year,” said Wedgbury, noting that this NOx emission reduction technology also increases fuel efficiency and reduces plant disruptions.

Another focus for Drax Power’s engineers concerns plant availability, which last year hit 90% — compared with an average of 71% for the remainder of the coal-fired sector. This reflects a continuing development of the company’s maintenance strategy, particularly for the turbines and boilers, said Lemmon.

For instance, the Siemens package includes the installation of remote monitoring equipment that will allow access via the Internet to monitor how the upgraded plant is performing and the condition of the plant equipment. “The idea,” said Lemmon, “is to datalog all the information. Siemens engineers will then be able to access that from a remote centre to check if the plant is performing at its optimum.”

Much of Drax investment’s strategy is driven by the Emissions Trading Scheme (ETS) — the European Union mechanism for putting a price on carbon emissions. The problem is that the ETS has yet to establish a clear enough framework to allow operators to make solid investment decisions beyond the current phase, which runs to 2012.

“We know there is a price on carbon up to 2012— ETS phase II emission allowances are currently trading at Euro22 — but beyond that there is no certainty,” said Wedgbury. “Within that five-year window we can make investments on the turbine upgrade to get to our target of 10% by co-firing. Beyond that there is no signal in the market today.”

One logical step for Drax would be to evaluate installing super critical boiler technology, which uses steam at high temperature and pressure to boost plant cycle efficiency, making more electricity from less coal.

“But,” said Wedgbury, “with six large boilers here, it would be incredibly expensive, with payback periods of 10-15 years. That’s why we need certainty and predictability.”

With regard to future environmental initiatives, Drax is working with other industry players and funding research in areas such as carbon capture.

“Carbon capture and storage is a technology that’s still on the horizon. We are not ignoring that as a possibility, but it is something we need to see demonstrated on a large commercial scale before a plant the size of Drax can actually take that step,” Wedgbury commented

The Drax spokeswoman concluded: “When you talk to government, they think we need to know a price to set this up and for them to tell us how everything is going to be. We don’t’. The market sets the prices, the government sets the parameters within which the markets work. It is just the commitment that is needed.”


Generating on a huge scale

The Drax power plant can handle 3,800 tonnes/hour of coal and uses 160 million litres of water a day from the nearby River Ouse in its cooling towers.

Coal is delivered by trains; releasing automatically to underground hoppers without actually stopping at the site. A conveyor system transports the coal for immediate use in the six boiler units, or for storage.

In the plant, the coal is ground to fine powder in pulveriser mills and passed via a powerful flow of hot air to the boiler burners to convert high purity feedwater to high pressure steam at 568°C and 166 bar.

Drax produces up to 1.9 million tonnes/year of ash in its boilers. Around 60% of the ash is sold to the building industry, with the remainder sent to a managed disposal site to create a landscaped nature reserve on the 1,854-acre site.

Each boiler weighs 4,000 tonnes, is the height of a 15-storey building and contains enough steel tubing to stretch 480km.

Steam is discharged from the boiler system at high pressure through a series of nozzles to rotate the blades on a series of high-, intermediate- and low-pressure turbines. These are connected to a huge cylindrical electromagnet, comprising a hydrogen-cooled generator rotor enclosed within the water-cooled stator.

Electricity is produced in the stator copper bars by the rotation of the electromagnetic field at 3,000 rpm. The electricity voltage is increased from 23,500V to 400,000V in a generator transformer.