PEi visits the giant Drax coal fired power station in North Yorkshire, UK. The second largest coal plant in Europe is the site of a co-firing system that will allow for the displacement of ten per cent of its coal throughput in favour of biomass, thus reducing its sizeable carbon footprint by around two million tonnes a year.

Tim Probert, Deputy Editor

Drax has its work cut out. The owner of the UK’s largest power plant of the same name, which is second in size only to Poland’s Belchatow in Europe, burns between 9-10 million tonnes of coal a year, including approximately 250 000 tonnes a week in winter. This 4 GW plant, therefore, has a rather large carbon footprint – 22.3 million tonnes per year – not to mention that it is liable for an increasingly large bill for emissions permits.

Bulk silos at the Drax 4 GW power plant, the largest power station in the UK
Click here to enlarge image

Drax’s sheer size – it accounts for eight per cent of the UK’s output alone at full load – has meant that it has attracted some unfavourable and largely undeserved brickbats from environmental groups. In fact, Drax claims that it is the most efficient coal fired plant in the UK in terms of carbon emissions – it produces 809 kg per MWh versus an industry average of 950 kg/MWh.

However, faced with the rising costs of burning coal, Drax has been forced to explore its options to reduce its carbon emissions. A £100 million ($160 million) steam turbine modernization programme to raise the operational efficiency of the plant from 38 per cent to 40 per cent units is underway, but the scope for carbon mitigation from this method is ultimately limited. Carbon capture and storage technology is arguably at least ten years away at best, while combined-cycle gas turbines are neither Drax’s area of expertise nor its competitive advantage.

Yet there is one alternative source of fuel that Drax is taking very seriously, namely biomass. This FTSE 100-listed firm sees biomass as not only a way to reduce its carbon footprint, but also as a stand alone industry that will allow it to diversify from coal in a significant way. Drax has already built a straw pelletization facility in Goole in the UK’s Humberside and has ambitions to build three, 300 MW dedicated power plants, which will exclusively burn biomass; one at the Drax site itself, the second at the Port of Immingham, one of the Britain’s largest ports by tonnage, and the third at an as yet undisclosed location in the UK.

Co-firing facility

These plants are still in planning, but from June 2010 the 4 GW thermal power plant at Drax will have the capability of displacing up to ten per cent of its coal throughput with biomass – equivalent to 400 MWe – via a £60 million ($98.4 million) co-firing system currently under development with Alstom and Doosan Babcock. When operational, this will be the largest biomass co-firing facility in the world.

The cost is relatively low because Alstom is not installing dedicated biomass burners but rather using the existing coal burners. Thus, the biomass will be mixed with pulverized coal and fed into the burners using existing fuel lines.

Much of the cost is being spent on unloading, conveying and filtering the highly combustible biomass for ferrous matter, as well as storage in four giant silos. In addition, Doosan Babcock is supplying the ‘plumbing’ that will ‘tie in’ the direct injection biomass co-firing system for an additional £10 million. Drax is also spending £20 million on upgrading its rail system so that trains, as well as trucks, can transport and unload biomass to the facility.

Due to the lack of available space, much of the unloading and storage of the biomass takes place away from the boiler buildings. Indeed, part of the plant’s 2.5 million tonnes coal stockpile had to be moved to make way for the unloading area.

Until the rail upgrade, biomass will be delivered to the site by truck, up to 250 tonnes per hour, and tipped into a purpose-built underground reception area. From the underground storage area, the biomass is transported via an enclosed conveyor system, which utilizes an air cushion to reduce fuel agitation and contain dust emissions.

The highly combustible biomass then enters into a purpose-built processing facility where the material undergoes a three-phase process to remove ferrous, non-ferrous and oversized objects in the raw material. The first phase uses a vibratory screen to remove the over-sized objects, while the second and third phases use magnets to remove metal, which not only causes damage to plant equipment, but also increases the risk of sparks and, ultimately, explosions.

A sample of biomass from each delivery is then taken for testing using a proprietary sampling system. This is a condition, imposed by the UK’s electricity market regulator, Ofgem, which must be met in order for Drax to be eligible for the Renewable Obligation Certificates scheme.

The biomass is then transported by conveyor to four 20 metre diameter concrete bulk storage silos, each with a capacity of 4000 m3. From the bulk silos it is transported across the station site, just outside the boiler building, into three day silos, of which each holds enough fuel for two of the plant’s six units.

From here the biomass is fed through a series of hammer drills that grind the fuel into a fine powder. Finally, before the fuel arrives at the mills, an air knife system is harnessed as a final check to remove any further unwanted particulates and contaminates.

These processes are designed to ensure that the biomass will burn efficiently and that any materials that might reduce the working performance and efficiency of the boilers are removed. Using a dedicated milling system, says Alstom, ensures that the biomass does not affect the existing coal delivery system at the plant.

After the milling process, and beyond Alstom’s scope of supply, the biomass is transported through Doosan Babcock-installed pipes, which inject the biomass into the main pulverized coal fuel network downstream of the existing coal mills. The coal and biomass is then pneumatically transported to the existing burners for combustion in the boiler.

Challenges of co-firing

Alstom’s earlier biomass co-firing installation at Fiddlers Ferry, in Warrington, UK, involved the design and installation of a dedicated system, up to 20 per cent thermal heat input, at two of the plant’s four 500 MW units. Drax’s capability to use the existing burners for the plant’s wall-fired boilers, however, has significantly reduced the required capital investment for the project.

Sam Saidi, head of business development for Alstom UK’s boiler retrofit division and is leading the Drax project for the French firm, said: “If you have a tangentially fired boiler, like those at Fiddlers Ferry, then you can remove one of the cold air nozzles, which induce fuel into the boiler and replace it with a dedicated burner. A dedicated burner for a wall-fired boiler is difficult, it can be done, but it would cost more than just using the existing burners and feeding into the existing fuel lines.”

Schematic overview of the biomass screen house. Drax will be able to burn up to 60 different types of biomass
Click here to enlarge image

Furthermore, no further modifications to the burners are needed at the coal fired plant. Peter Emery, Drax’s production director, said: “We built a pilot plant in 2005 that was designed to check that our burners could burn coal and biomass correctly to controlled stochiometry. The good news is the pilot worked and worked well. It also meant that the existing combustion controls didn’t need changing. The panel operator couldn’t believe that the biomass was almost invisible to him and that he could still control it!”

Drax anticipates that co-firing will reduce the amount of ash produced and, it is estimated, the impact of biomass on nitrogen oxides emissions will be neutral. However, some products, in particular straw, have relatively high levels of corrosive constituents, including chlorine. Emery said: “Although we could have gone up to 15 per cent, we have designed the project to burn ten per cent biomass and at those levels we do not anticipate any problems with corrosion.

“We have been cautious because Drax is an efficient, profitable plant. Biomass has a lower calorific value than coal and our calculations show that there is a break-point between 10 per cent and 20 per cent where steam turbine capacity is reduced.

“In the future it may be more economic to eat into our generating capacity but increase our biomass burning capability. That’s one of the attractions of this system: because we’re using the same burners, we could install more injection facilities, as only 20 per cent of the plant’s burners are being used for biomass co-firing.”

Fuel flexibility

For now, Drax expects to burn up to 1.5 million tonnes of biomass a year. The remit of the project was for Drax to be fuel flexible to quickly adapt to market conditions of various biomass products. The only constraint is that the moisture content of the fuel must be less than 15 per cent, says Saidi, other than that the system can handle and fire a wide variety of biomass in pellet or meal form.

The types of biomass which are likely to be co-fired include wood-based materials, such as forestry residues, agricultural by-products like straw, sunflower husk pellets and olive cake, as well as purposely grown energy crops, such as short rotation coppice willow and miscanthus.

Tests on the pilot plant showed that Drax can successfully burn between 50-60 different types of biomass. But coming up with a ‘recipe’ of fuel incorporating at times low quality coals with differing biomass products of varying standard will be challenging, Emery concedes.

“Different biomass products have varying calorific values and burn in different ways – it won’t be simply mixed up together. In reality we have to handle the batching of it. Units 1 and 2, say, may run on a different fuel on different days than will units 5 and 6. We’ve been asking our biomass suppliers to supply fuel at a certain quantity to a certain quality. There have been some interesting negotiations about how big that window of quality is, but we are confident we can do this.”

What will be burned at any particular time will be almost purely down to economics. Emery said: “The price of these biomass products goes up and down so we will burn them according to market signals. Therefore with this project we have put a lot of effort into maintaining fuel optionality. If, for example, a dry summer produces more straw then we would burn more straw and vice versa.”

The biomass will be sourced from a variety of locations, as the UK alone is incapable at present of supplying Drax’s requirement of 1.5 million tonnes a year. Drax’s Goole wheat/barley/rapeseed/straw processing facility, which shreds and pressurizes bales into high density, low moisture pellets, produces a healthy 200-300 tonnes a day, but it is expected that the majority of fuel will be imported in large cargoes – most likely wood from Scandinavia, the Baltics and North America.

Drax says that the carbon emissions generated from, for example, the transatlantic shipping of 50 000 tonnes-plus biomass cargoes are relatively small. Emery said: “This is a red herring. To transport anything in large quantities in ships does not cost very much in terms of carbon dioxide per tonne. It’s probably far more carbon intensive to transport coal mined 914 metres underground 32 km from here than from shipping opencast mined coal from Colombia.”

Drax’s ‘Sustainability Policy’ dictates that it will not burn any biomass that does not reduce carbon dioxide versus the coal alternative. “Even though we’re shipping biomass transatlantic, we’re still getting 85-90 per cent carbon reductions versus coal,” claims Emery.

Eventually, Drax hopes that the UK will be able to supply the majority of its biomass but, with the market still in short trousers, this may take some time. Emery said: “We are trying to generate a new industry. It’s a chicken-and-egg situation: the farmer won’t commit to growing crops until he’s seen the plant and I won’t build the plant until I’ve seen the fuel! But once people realize that this is a credible going concern linked to a baseload power plant like Drax then the supply chain will start to snowball.”

Subsidies and carbon allowances

Drax could expect a return from its co-firing investment reasonably quickly. For co-firing coal with biomass, Drax would be eligible for one-half of a Renewable Obligation Certificate, with each worth £37.19 per MWh for 2009-2010. With an estimated annual sold output of 24 million MWh – ten per cent of which will now be biomass – that equates to at least £45 million a year for a guaranteed 20 years.

Biomass in the day silos is discharged to the mills before being transported into the main fuel network and injected into Drax’s existing burners
Click here to enlarge image

Furthermore, Drax expects biomass co-firing to reduce its carbon dioxide emissions by around 2 million tonnes per year. Once operational, co-firing could save the company £30 million a year in emissions allowances at the current price of £15/tonne. Of course, biomass is more expensive and has a lower calorific content than coal, so it will have higher fuel costs.

According to the Biomass Energy Centre, large shipments of wood pellets could cost between £80-100/tonne depending on the quality of wood. As a major consumer of biomass, Drax would be able to negotiate some very favourable deals for large-scale contracts gor 50 000 tonnes shipments.

Even if Drax’s fuel bill rises by as much as £50 million a year, co-firing still provides a handsome return for going green(ish). Small wonder, then, that Drax is planning those three 300 MW dedicated biomass units.

More Powe Engineerimg International Issue Articles

Powe Engineerimg International Archives