Once the UK’s largest coal-fired power station, Drax has recently undergone a green revolution, converting three of its six generators to run on biomass. Reaching this stage has been difficult, prompting innovative technological and supply chain solutions for handling the wood pellet fuel.
Just outside Selby in North Yorkshire, the 750ha Drax site provides around 8% of the country’s electricity supply. Until recently, it was the country’s biggest carbon emitter – making Drax a beacon for environmental protesters. “It was unpleasant at the time,” says Dorothy Thompson, chief executive of Drax. “But it was hard to find a way to reduce our carbon emissions.”
However, the biomass fuel switch has proven what can be achieved in carbon mitigation through innovative technology, she says. When the power station first announced its intentions in 2008, many people believed it simply wasn’t possible to burn 100% biomass in a unit of any size that had previously used coal, she says.
Even Drax’s own analysis concluded the switch couldn’t be done efficiently, nor would it be able to run at full capacity, and it would be hard to do safely, owing to the incendiary nature of the wood pellets.
Proving everyone wrong, the power station is now running three units – two on 100% wood pellets and one at around 85% – safely and reliably. The biomass burnt delivers 86% carbon savings compared with coal, says Thompson.
This progress has been achieved in increments. The power station team started by successfully co-firing a 50:50 mix of coal and wood pellets in one of its furnaces. “Co-firing was our first commercialised attempt at firing biomass,” says Jason Shipstone, research and development manager at Drax. “This was designed to give us 10% by heat – around 400MW.”
Initially, it was thought all six furnaces would operate in this way. However the government, which has supplied the project with renewable subsidies, later asked Drax to explore running whole units on 100% biomass, instead.
When talking about the conversion, the key members behind the project stress that it was by no means ‘just a fuel-switch’. The project demanded logistical and supply chain changes, innovative technical solutions and shifts in mindset as to how they think of and handle fuel.
Coal can be stored safely outside in the elements and is relatively inert and difficult to ignite. In contrast, wood pellets become unusable if damp, give off highly flammable dust, and can compost and combust when stored in large piles.
Gunpowder exercise
“During the trials, we did a mindset-changing exercise with our workforce,” says Shipstone. “We got them to think of the wood pellet dust as gunpowder. We were challenged with a material that would act like 8% gunpowder.”
The Drax team looked to handling and chemical industries for inspiration, but it was the petrochemical industry that delivered the right solution for transporting the pellets around the site. “We have pneumatic seals that move ash, but fuel in power stations is moved on conveyors,” says Brian Greensmith, project lead for the biomass conversion. “We realised that by pneumatically moving the pellets and dust around we had solved not only how you get to multiple mills, but also the safety aspects.”
The biomass is delivered into a vessel at atmospheric pressure, where it’s weighed and measured and then emptied into the pipes below. The pellets are blown at around 50 tonnes an hour down a pipe, propelled by air at 1 bar (100kPa) pressure – which causes them to behave as a liquid. “That was part of the mindset change, to treat biomass fuel more like a petrochemical,” says Greensmith.
The next challenge was how to safely store the massive amounts of wood pellets needed to fuel the three units, which come in at double the volume of coal required to create the same energy.
The novel solution was to build four giant storage domes, each 50m high and capable of storing 112,000m3 of fuel. The structures were delivered by US firm Dome Technology. First, a concrete ring is built in the ground. The dome structure, made from a 1mm-thick PVC, is then fastened to the ring and inflated with air. The inside of the dome is insulated with polyurethane foam, and then sprayed with layers of concrete on top of metal barring. The floor of the dome is made up of 300 metal plates that sit on top of motors that can vibrate the floor, to shake pellets and dust onto channels to transport them elsewhere on site.
To ensure the pellets don’t spontaneously combust while in storage, the atmosphere is transformed to almost 100% nitrogen. Alternate metal floor plates have a gas line and sensors in the dome can detect a temperature rise, so nitrogen can be released in a quadrant to stabilise an area if required.
“Everybody told us we couldn’t store biomass 50m high,” says Shipstone. “And yet, inside the dome, with all that weight pressing down on it, the biomass can maintain a stable temperature.”
Originally, Drax operated with 70 days’ worth of coal stored on-site, but for biomass it must operate with just 14 days’ stock. This requirement has prompted the power station to work hard to ensure its supply chain runs smoothly, with shipments of biomass from North America running continuously, and seamlessly lining up with its supply chain in the UK.
“It’s easy to look at what our transformation has been on site, but we’ve had to put in a whole supply chain across the country,” says Greensmith. “Everything we’ve learned about this fuel, we’ve had to pass on to the ports, as our resilience depends on them.”
Once the biomass has been delivered to one of Drax’s chosen ports, it’s transported on Network Rail lines to the site via a specially designed fleet of freight trains.
“We wanted to redesign our rail freight so we could get the maximum volume the track was capable of,” says Greensmith. “We worked with W H Davis in Mansfield and Lloyd’s Register Rail. The trains look more like passenger trains. The wagons are 30% larger than other freight wagons, and can carry 71.6 tonnes of biomass.”
The transition to working with such a limited amount of fuel stock for three units wasn’t easy, says Shipstone. “We had ‘mind adjustment’ to do to get used to just-in-time delivery.” This ‘mind adjustment’ had to spread across the supply chain, he adds.
Speedy unloading
The added time pressure meant the power station had to design facilities at the site to unload the biomass as quickly as possible. Drax has laid down new rail tracks and signal equipment on site so trains can run at 15mph rather than the previous speed limit, and can now turn a delivery around in just 90 minutes.
The sheer scale of what Drax has achieved over the past decade is impressive. However, far from being content, the team at the power station are raring to begin the conversion of its three remaining coal-firing units.
Frustratingly for Drax, the ability to move forward with the final conversions depends on key decisions from government. So far, the outlook is far from rosy, with carbon-capture storage projects cancelled and subsidies for biomass cut. “Aspects of government policy have been challenging,” says Thompson. “The startling one for us was when, in the July budget, the Chancellor announced that the climate change levy would be applied to renewable power. That was a shock to us and had a material impact on our profitability.”
In another blow to Drax, the European Commission has raised concerns that a £1.7 billion government subsidy contract signed in April 2014 for the third of its six units may have been too generous. The contract would see the power station paid a fixed price of £105 for every megawatt-hour of biomass-fired power that the unit generated until 2027 – more than double the current market price.
Despite these challenges, Drax will work hard to achieve a positive outcome that will result in half of the power station running fully on biomass, says Thompson.
“We don’t think our work here is done – we’re just on part of the way of the journey,” she adds. “We believe that there is a great benefit to the UK for us to convert more units.”
Drax turns to universities for combustion help
While biomass may look similar to coal, its energy is far more easily released, says Brian Greensmith, project lead for the biomass conversion. The ash released can also be problematic in a boiler, causing slagging, fouling and corrosion.
To prevent these problems, the power station worked with several UK universities to come up with chemistry specifications for the supply chain, and for its own mitigant that Drax puts in the boiler.
”The inital question was: ‘can you get the flame to burn?’” says Greensmith. “This is a huge firebox. If you try and model it, there are so many variables. It only gives you a guideline – it can’t tell you the answer. So you have to iterate between what you see and what you discover in practice. You’re not doing this on a Bunsen burner – the burners on our system are the size of Ford Transits and there are 48 of them. It’s a big thing to play with.”
Where does the biomass come from?
The majority of biomass that Drax uses comes from working forests in North America already used by other industries.
Biomass has around two-thirds of the energy density of coal, so double the volume is needed to provide the same amount of energy. To ship such large quantities safely and affordably, the biomass is dried and compressed into pellets.
While the wood chip source isn’t infinite, the US Department of Agriculture has estimated that every year the country wastes 93 million tonnes of wood fibre. In comparison, Drax uses 7 million tonnes of biomass a year.
Since 2008, PwC has audited the power station’s sustainability compliance, says Drax chief executive Dorothy Thompson.