Tag: wood pellets

Inside the dome

6 October 2016

Sustainable bioenergy

There are four storage domes at Drax Power Station and each of them can hold 80,000 tonnes of compressed wood pellets. It’s these biomass pellets, a sustainable fuel, that Drax is being upgraded to run on and produce renewable electricity.

Wood pellets are an incredible fuel that can match coal for efficiency – the challenge is you just need more of them as the density and calorific value of coal is greater. However, storing such large quantities in a confined space presents risks that have to be managed, 24/7.

Atmospheric control

The crucial difficulty with storing the pellets is their chemical volatility. Wood, which the pellets are made from, emit carbon monoxide (CO). In a confined space such as the storage dome, this CO can build up and – due to CO’s extreme flammability – require the entire internal atmosphere to be regulated by a set of highly sophisticated engineering solutions.

As long as materials are emitting more heat into the atmosphere than they are storing in themselves, there is no risk of combustion. A single wood pellet in a fuel store poses no fire risk. Nor does a small pile. But when thousands upon thousands are piled together, the pressure builds up and causes the pellets to heat up.

Gradually, the rate of temperature increase speeds up, and before you know the flashpoint threshold has been crossed and there’s potential for danger.

However, remove or limit the oxygen supply in the silo and purge the CO that’s emitted from the pellets, and the risk of a thermal event is substantially reduced. The challenge for the engineers at Drax constructing the domes was finding a way to manage temperatures within the dome.

Neutral nitrogen

To do this they created a system to automatically inject nitrogen into the storage dome. While nitrogen isn’t a truly inert gas, it is much less reactive than CO and oxygen.  With this pumped into the dome’s atmosphere it is a much safer environment.

To get a steady supply of nitrogen, regular air from our atmosphere – which is 78% nitrogen – is passed through a molecular filter, which removes the larger oxygen molecules. The gas collected at the other end is 96% nitrogen.

This nitrogen-rich air is then injected from underneath the dome and continually distributed around it. Not only is this a fire prevention method, but also a firefighting one that can be pumped in larger quantities in the event of combustion. Separate to the above measures which are there to manage fuel temperatures, the dome is also fitted with a carbon dioxide (CO2) injection system and water deluge system which are there as fire extinguishing precautions.

The big ear inside the dome

The next problem facing the designers was how to accurately monitor the quantity of compressed wood pellets inside the dome. To achieve this, each dome is fitted with a sonar system – which sounds a bit like a chirping bird – that provides continuous feedback on how full the dome is.

The sonar monitoring system provides level, profile and volume information which is translated into a 3D image of the stored biomass. This method of volumunetric measurement allows the operators to view and monitor in ‘real time’ the effects of their actions when filling and unloading domes, so they can target specific areas particularly when unloading and for fuel accounting purposes.

Other tools and tricks

Five thermocouple arrays measure the pile temperature and provide feedback in real time to the operators to allow them to assess the status of the dome and effectively plan material filling and reclaim. Gas monitors measure the levels of CO and CO2 as well as O2 depletion within the head space of the dome.

A dome breather vent (a two way acting valve, which as its name suggests, allows the dome to breathe) is fitted to the top of the dome and acts as a vacuum breaker maintaining a relatively even pressure allowing air in during unloading and releasing head space gasses during nitrogen inserting.

The final piece of the atmospheric control puzzle is regulating pressure. At the top of each dome is a controllable aperture called a slide gate which is closed unless the dome is being filled to allow material to enter. A dome aspiration system is installed here to filter and remove displaced air from within the head space during filling, but also allow a route for CO and other offgassing products to escape.

All the hidden systems within these four huge white domes allow the operator to effectively control their atmospheric conditions and crucially to store massive amounts of potentially volatile biomass safely on site.

Find out more about these giant storage domes – read the story about how they were constructed. 

The single biggest transformation of our century

Sustainable bioenergy

At the turn of the millennium, Drax was facing a serious issue. Demand for electricity was high and increasing, but so was the desire for sources of power that were less harmful to the environment than coal, at that time Drax’s fuel.

To continue to meet demand in a cleaner and more sustainable way, an alternative approach was needed. Drax had a legacy in this field – in 1988, it was the first coal-fired power station to install flue-gas desulphurisation technology, which removes 90% of coal’s harmful sulphur dioxide (SO2) emissions.

In the two decades that followed, however, the sustainability conversation moved beyond how to make coal cleaner. Instead, the focus was finding a truly viable alternative fuel.

Finding a new fuel

In those early days, the idea of converting a fully coal-fired station to another fuel seemed outlandish to say the least.

“We made a lot of people’s heads hurt with this project,” says Drax Strategic Projects Engineering Manager Jason Shipstone. “No one had the answers. It was a bit like going for a walk but not knowing where you’re going.” Back then it was all about experimentation.

Jim Price, Alternative Fuel manager at the time, explains: “Initially, we found a few distressed cargos of wood pellets and sunflower husks that someone had ordered but didn’t want. We mixed that with coal at very low concentration.”

Price and his team found they could use the plant-based fuel alongside coal at low percentages without it detrimentally affecting the boilers. It was a long way from being a new business model, but it was a start. They spent the next year working with willow wood, a subsidized energy crop that proved difficult to turn into a fuel that could be used efficiently to power a boiler.

Then in 2005, after building a prototype plant and finding a way to pulverise the willow into a fine powder – called wood flour – and combine it with coal dust, the team hit its first key milestone. It was able to power a Drax boiler.

“That was the Eureka moment,” says Price.

“No one had the answers. It was a bit like going for a walk but not knowing where you’re going.”

A change in attitude

The response to the success was immediate. Senior management support for the project had been in place from the beginning, but now there was a change across the whole company. “People started to think maybe it can be done,” says Price.

Work continued on the project and – after more experiments – Drax eventually settled on compressed wood pellets. This form of biomass ultimately required investment in four vast storage domes that between them store 80,000 tonnes of pellets.

Then there was the issue of supply and delivery. Materials were sourced from the US, shipped to the UK, then freighted to the plant in specially designed covered train wagons, each carrying up to 7,600 tonnes.

“Everything else had to carry on as normal. This had to be seamless. We had to work the same as Drax has always worked – reliable and available,” says Shipstone.

Jason Shipstone, Drax Strategic Projects Manager, played an instrumental role in upgrading Drax.

Jason Shipstone, Drax Strategic Projects Manager, played an instrumental role in upgrading Drax.

The final hurdle

In 2009 the team overcame one of the final challenges, and successfully adapted the boilers to combust the new fuel, proving that co-firing (the process of using two fuels powering one boiler – in this case wood pellets and coal) could work. It was enough to show there was a future in wood pellets and it could work at scale.

Although nothing was fully built yet, but Dorothy Thompson, CEO of Drax, was convinced. Shipstone remembers the conversation after Thompson signed the contract to begin the transition in earnest. “’So we can do 10%. What does it take to get to 50%?’ she asked,” recalls Shipstone. His response? No problem. “It was the right answer,” he says.

Toward a coal-free future

Fast forward to 2016, and Drax is Europe’s largest decarbonisation project – reducing emissions by at least 80% of the 12 million tonnes of carbon dioxide that the three, now converted, former coal generation units would have released per year. Although only half of Drax’s six units have been upgraded from coal to use compressed wood pellets, 65% of the electricity generated at the power station is the result of a renewable, rather than a fossil fuel. Its three biomass units produce enough electricity to power the equivalent of four million homes – or more than half of all residential properties in northern England.

Given the challenges the world faces regarding the future of energy production, decisive action is required if we’re to meet carbon reduction targets. In the UK the government has voiced ambitions of phasing out coal by 2025. Drax has aims of doing it quicker. Thompson has spoken of plans that see all coal units taken off the Drax system by 2020, if not before.

The story of energy since the dawn of the Industrial Revolution has been one of fossil fuels. This simply has to change. By finding a way to ease the transition away from coal, Drax is helping to write the next chapter.

The biggest balls of electricity generation

Power generation

When making a cup of tea, it’s unlikely you consider the industrial equipment kicked into action the moment you switch on your kettle. And of all of the activity going on behind the scenes, it’s even more unlikely you think about a 1.2-tonne steel ball.

But without a number of 1.2-tonne balls and the electricity they help generate, your kettle would be nothing more than a fancy jug.

How do giant balls help to generate power?

The answer lies in the way fuels like coal and compressed wood pellets are used to power boilers and generate electricity. Drax started its life as a coal power station, but today it is in the process of upgrading to run on biomass. Progress has already been made – three of the station’s six units already run on compressed wood pellets, Drax’s biomass fuel, generating around 20% of the UK’s renewable electricity.

To generate enough power to supply 8% of the UK’s demand – as Drax does – a lot of fuel is needed. Hundreds of thousands of wood pellets are delivered to Drax every day, arriving on custom-built trains travelling from the Ports of Tyne, Hull, Immingham and Liverpool.

The pellets pass through a system of conveyor belts until they arrive at one of four massive conical storage domes, located on site in Yorkshire. Before the wood pellets can be converted into fuel, they need to be crushed: this is where the balls come into play.

The pulveriser

The wood pellets used at Drax are compressed and dried wood that is formed into small capsules the size of a child’s crayon. But, like with coal, to get the best results in the power station’s huge boilers, the material needs to be turned into a very fine powder in pulverising mills. When very fine, the fuel burns as efficiently and as quickly as a gas.

Inside each mill are 10 giant steel balls that grind down either the wood pellets or coal. Each ball is three quarters of a metre in size, made of hollow cast steel alloy and weighs roughly 1.2 tonnes – equivalent in weight to British-made Jaguar XE mid-sized saloon car or an entire football team.

And to make sure that each one is up to the task of extreme pulverisation, they need to be hard. Each one is heat treated during manufacture to make sure they’re up robust enough to consistently crush raw fuel.

The benefit of this durability is that they can readily pulverise fuel to feed Drax boilers, to power kettles across the country – a big responsibility for a big ball.