Tag: wood pellets

7 principles of a sustainable forest biomass policy

28 November 2017

Sustainable bioenergy

Biomass is playing an important role in moving the UK away from coal. At Drax Power Station, in the form of compressed wood pellets, biomass is already supplying roughly 17% of Great Britain’s renewable power.

But more than just being a low carbon replacement for fossil fuel generation, it is also crucial in maintaining the stability of the power network. Among renewable sources of power, biomass is unique in being able to provide the same range of ancillary services that can be provided by coal power stations – such as frequency control and inertia. This inherrent flexibility is vital in maintaining stability on Britain’s high voltage transmission system. Wood pellets can also reliably generate power, helping to fill in the gaps left by intermittent renewables when the wind doesn’t blow and the sun doesn’t shine and avoiding reliance on diesel, coal and gas.

However, for the UK and the wider global environment to reap the maximum benefits from biomass, it must be produced sustainably. More than this, its supply chain must be low in emissions so that clear savings can be made versus power generation with fossil fuels.

To ensure this, the use of biomass is regulated in the UK under EU Timber Regulations and the Renewables Obligation (RO). But further guidelines are set to be introduced as part of the European Parliament’s update to the Renewable Energy Directive (RED), which will specify criteria for all biomass.

There is a clear need for this, but for these to be truly successful they need to be based on a set of robust key principles. A new report by Drax outlines seven of these which can ensure sustainable biomass usage in the future.

1. Forest biomass for bioenergy should be sourced from sustainable forests

The sustainability of the forests from which biomass is sourced is key to ensuring its usage has a positive impact on the environmental, social and economic health of that supply region.

For example, a properly managed forest can boost carbon stock as the younger, faster growing trees that are replanted after felling absorb more CO2 than older, over-mature trees.  Thinning operations also increase the growth of the biggest and best trees, ensuring more carbon is stored in longer term solid wood products.

Generators should be able to demonstrate they are avoiding biomass sourced from higher-risk areas where extracting biomass could cause long-term carbon stock decreases in soils or ecosystems, as well as other factors such as biodiversity loss, soil erosion or depletion of water sources.

2. Bioenergy from forest biomass should not be produced from high-risk feedstocks

Feedstocks, the raw materials turned into biomass pellets, must come from sustainable sources and avoid protected and sensitive sites that could be considered a risk.

In 2016 around 40% of all feedstock supplied to Drax originated as a sawmill residue. Processes such as thinning also serve as a source of biomass feedstock, while also benefitting the overall health and quality of the forest. Thinning a semi mature stand of trees allows the forest owner to maximise the production of higher value saw-timber trees, storing more carbon and generating more stable revenue streams. Having a variety of wood products markets from saw logs through to biomass incentivises land owners to maintain healthy forests and reduces the risk of conversion of forest to agriculture or urban development.

3. Carbon savings and emissions should be properly accounted

To understand the effectiveness of biomass sustainability policy, carbon savings need to be measured.

Factors such as fossil fuel substitution and the emissions associated with harvesting, processing and transporting biomass are relatively straightforward to measure.

4. Bioenergy should be limited to what can be sustainably supplied

Unlike coal or oil, which will eventually run out, more trees can be planted, grown and harvested.

That said, there is a natural limit to the amount of biomass available on the planet, and so it should not be considered an infinite resource. This is why it’s crucial biomass is sourced from sustainable forests managed following set guidelines. In short, to ensure biomass truly is sustainable, it is essential that working forests are actively managed and maintain or increase productivity.

5. Support should be given to all technologies that achieve significant carbon savings

One of the major advantages of biomass over other renewable sources is its potential to help the UK rapidly adapt to meet the EU target of achieving 27% of final energy consumption from renewables.

The fastest way for biomass to make an impact to the UK’s carbon emissions is through converting coal power stations to biomass, as is the case at Drax Power Station.

This repurposing of existing facilities not only offers rapid adoption of renewable energy, but also the ability to provide vital ancillary services other renewable sources can’t.

Quickly deploying biomass solutions in this manner will serve to help it become an established part of the energy system as it continues to decarbonise.

6. The efficient use of raw materials is supported by encouraging buoyant forest biomass markets

Globally, there are substantial amounts of forest residue and forestry industry by-products that currently go unused.

Biomass should be sourced from regions where the largest surpluses exist and the forest carbon balance can be maintained. To enable this to function effectively on a global scale, trade restrictions should be avoided.

Pelletisation offers one of the most efficient ways for this raw material to be used by making it safe, cost-efficient and low-carbon to transport around the world.

These principals are tried and tested by Drax and known to protect forests and ecosystems, as well as optimise supply chains to ensure carbon emissions are kept to a minimum. Ultimately, Drax’s experience in sustainably using biomass serve as a guide for other producers and governments to quickly decarbonise energy systems.

7. The sustainability of forest biomass should be independently verified

One of the best ways to guarantee biomass is sourced sustainably is by introducing third-parties and official guidelines that generators and suppliers can work with.

In Europe, forest level management certification schemes can act as an effective indicator that forests are managed in accordance with the guidelines laid out by Forest Europe. Outside of Europe, where Drax sources most of its biomass, independent, third part auditors can ensure the UK’s stringent criteria are being met on the ground.

Read the full report: The 7 Principles of a Sustainable Forest Biomass Policy – Proven to Work

Keeping the options open

19 October 2017

Sustainable bioenergy

Roughly 750 million acres of the US is covered in forestland – an area nearly 12 times the size of the UK. Approximately two-thirds of that land is working timberland, producing wood used for construction and furniture. In short, US forestry is a massive industry.

Enviva is the world’s largest wood pellet producer and biggest biomass supplier to Drax Power Station, but in the context of the US forestry industry in which it operates, Enviva does things differently.

“We’re leading the industry in sustainability and transparency in our sourcing practices,” says Jennifer Jenkins, Vice President and Chief Sustainability Officer at Enviva. “We’ve created unique tracking systems and we conduct science-based sourcing, both of which encourage sound forest stewardship.”

Specifically, Enviva draws on best practices to make decisions about which areas it sources from and how it protects the areas it doesn’t.

Protecting bottomland forests

A bottomland forest is an area of low-lying marshy area near rivers or streams that can be home to unique tree and wildlife species. These forests are flooded periodically and they can be ecologically important. However, they’re also a part of south-eastern America’s working forest landscape.

In fact, Enviva sources 3-4% of its wood from these areas, but only where harvesting improves the life of the forest. For example, in some cases, harvesting mimics naturally occurring storms, clearing the canopy so young seedlings and forest floor species thrive. More than that, harvesting can also help keep forests as forests.

“In the areas where we work, one of the biggest threats to forests is being converted to another use – specifically to developed or agricultural land,” explains Dr. Jenkins. “Our goal is to keep forests as forests. We want to preserve those with the highest risk of being converted for another use.” If landowners can gain a steady income from regular harvests, they’re likely to keep their land as working forests.

However, this is only true for carefully assessed forests where harvesting is deemed safe. Any land that doesn’t meet Enviva’s set of strict criteria means Enviva won’t source from it – it can simply walk away. The landowners, on the other hand, don’t have that luxury.

“Isn’t it our responsibility to provide another option for a landowner who might not want to facilitate a harvest?” asks Dr. Jenkins. “Maybe they recognize its value. Maybe they would prefer to conserve it instead. In recognition of our responsibility, we made a commitment.”

A fund that keeps forests as forests

Enviva’s commitment was to partner with the US Endowment for Forestry & Communities to set up the Enviva Forest Conservation Fund, a $5 million, 10-year programme designed to protect tens of thousands of acres of sensitive bottomland forests in the Virginia-North Carolina coastal plain.

It works by inviting submissions from projects looking to protect areas of high conservation value. Last year it awarded its first round of funding to four projects. More recently, in June 2017, the Enviva Forest Conservation Fund announced a total of $500,000 to go toward a second round of projects with partners such as Ducks Unlimited, an organization which – with the grant – plans to acquire more than 6,000 acres of wetlands to operate as a public Wildlife Management Area.

The Fund follows a history of proactive sustainability programmes, including a strict supplier assessment process and the company’s Track & Trace tool, a one-of-a-kind publicly-accessible system that tracks every ton of primary wood Enviva purchases back to the forest from which it was sourced. It is entirely transparent and is a testament to Enviva’s commitment to sustainability and doing things differently.

As Dr. Jenkins explains, this approach stems back to the origins of the company in 2004: “As a company that makes wood pellets, Enviva’s reason for being is to help lower greenhouse gas emissions. An emphasis on sustainability has always been a part of Enviva’s DNA.”

Active management of forests increases growth and carbon storage

28 July 2017

Sustainable bioenergy

A study on the historical trends in the forest industry of the US South, carried out by supply chain consultancy Forest2Market, and commissioned by Drax Group, the National Alliance of Forest Owners and the U.S. Endowment for Forestry and Communities, found that over the last 60 years, as demand for forestry products has increased, the productivity of the area’s forests has increased too. In short, the more we’ve come to use them, the more forests have grown.

A forest is not like a mine – there is not a finite amount of wood in the ground that disappears when it is extracted, never to return. Forests are a renewable resource that can be replanted, improved, and harvested for as long as the land is managed responsibly.

What’s more, landowners have a strong financial incentive to not only maintain their holdings but to improve their productivity – after all, the more of something you have, the more of it you have to sell. It is an economic incentive that works.

Six decades of growth

The Forest2Market report found that increased demand for wood is statistically correlated with more annual tree growth, more wood volume available in the forest, and more timberland.

For example, between 1953 and 2015, tree harvests increased by 57%, largely driven by US economic growth and increased construction. Over the same period, annual wood growth increased by 112%, and inventory increased by 108%. In total, annual growth exceeded annual removals by 38% on average.

Annual forest growth in the US South increased from 193 million cubic metres in 1953 to 408 million cubic metres in 2015. Inventory increased from 4 billion to 8.4 billion cubic metres.

The forest products industry funded private-public research projects to enhance the quality and performance of seedlings and forest management practices to ensure a stable supply of wood. Because of these efforts, landowners saw the value of active management techniques, changing their approach to site preparation, fertilization, weed control, and thinning, and the use of improved planting stock. Healthy demand made it easy for landowners to take a long-term view, investing in more expensive management practices up front for greater returns in the future. And the results were extraordinary: seedlings established in the last 20 years have helped plantations to become nearly four times as productive as they were 50 years ago.

A changing market

Markets for wood have changed over the last two decades, precipitated by decreased demand for writing paper and newsprint, increased demand for absorbent hygiene products and containerboard and increased demand for biomass. These changes in demand have not impacted the way that landowners manage timberland, however. As sawtimber (the largest trees in the forest) remains the most valuable timber crop, it also remains the crop the landowners want to grow. Pulpwood is harvested from the forest only when the forest is being thinned to create optimal conditions for growing sawtimber or when sawtimber is harvested and the forest is being cleared for replanting.

Pellet production has expanded rapidly in the US South, though its overall footprint is still small in comparison to more traditional forest product industries.

In some local wood basins, however, like the one surrounding Bastrop, Louisiana (the location of one of Drax’s US production facilities), the use of biomass to create pellets has filled a gap left by the closure of an 80-year old paper mill. Drax’s decision to locate in this area is integral to supporting forest industry jobs and landowners who carefully consider local demand when deciding whether to continue growing trees.

According to Tracy Leslie, Director of Forest2Market’s biomaterials and sustainability practice, “As history has shown, forests in the US South have benefitted from increased demand for all types of wood. The rise of the pulp and paper industry did not detract from landowner objectives to maximize production of higher value sawtimber nor did it result in a shift in focus to growing only pulpwood. This new demand did, however, provide landowners with important interim and supplemental sources of income. Forest2Market’s research shows that an increase in demand for pellets will have the same effect, incentivizing landowners to grow and re-grow forests, increasing both forest inventory and carbon storage.”

The real threat to forests

Not only does demand for forest products increase the productivity and carbon storage of forests and provide an incentive for landowners to continue growing trees, but it also helps counter factors that irrevocably destroy this natural resource. The real threat to forests in the US is not demand for wood, but urbanisation. Nearly half of all US forestland that converted to another use between 1982 and 2012 was cleared for urban development.

Commercial forestry protects forested lands from development; between 1989 and 1999, just 1% of managed pine plantations in the US South were cleared for non-forest development compared to 3% of naturally-regenerated forest types.

Urbanisation, not the forest products industry, places the most pressure on forests in the US South. Forest2Market’s findings demonstrate that demand associated with healthy timber markets promotes the productivity of forests and mitigates forest loss by encouraging landowners to continue to grow, harvest, and regenerate trees.

Read the full report: Historical Perspective on the Relationship between Demand and Forest Productivity in the US South. An At A Glance version plus an Executive Summary are also available as a separate documents.

 

How lasers reduce emissions

25 July 2017

Power generation
Drax laser

Of the air that makes up our atmosphere, the most abundant elements are nitrogen and oxygen. In isolation, these elements are harmless. But when exposed to extremely high temperatures, such as in a power station boiler or in nature such as in lightning strikes, they cling together to form NOx.

NOx is a collective term for waste nitrogen oxide products – specifically nitric oxide (NO) and nitrogen dioxide (NO2) – and when released into the atmosphere, they can cause problems like smog and acid rain.

At a power station, where fuel is combusted to generate electricity, some NOx is inevitable as air is used in boilers to generate heat. But it is possible to reduce how much is formed and emitted. At Drax Power Station, a system installed by Siemens is doing just that.

It begins with a look into swirling clouds of fire.

Not your average fireplace

“Getting rid of NOx is, at heart, a problem of getting combustion temperatures to a point where they are hot enough to burn fuel effectively. Too hot and the combustion will form excess amounts of NOx gases. Too cool and it won’t combust efficiently,” says Julian Groganz, a Process Control Engineer who helped install the SPPA-P3000 combustion optimisation system at Drax. “Combustion temperatures are the result of the given ratio of fuel and air in each spot of the furnace. This is our starting point for optimisation.”

An industrial boiler works in a very different way to your average fireplace. In Drax’s boilers, the fuel, be it compressed wood pellets or coal, is ground up into a fine powder before it enters the furnace. This powder has the properties of a gas and is combusted in the boilers.

“The space inside the boiler is filled with swirling clouds of burning fuel dust,” says Groganz. Ensuring uniform combustion at appropriate temperatures within this burning chamber – a necessary step for limiting NOx emissions – becomes rather difficult.

Heat up the cold spots, cool down the hotspots

If you’re looking to balance the heat inside a boiler you need to understand where to intervene.

The SPPA-P3000 system does this by beaming an array of lasers across the inside of the boiler. “Lasers are used because different gases absorb light at different wavelengths,” explains Groganz. By collecting and analysing the data from either end of the lasers – specifically, which wavelengths have been absorbed during each beam’s journey across the boiler – it’s possible to identify areas within it burning fuel at different rates and potentially producing NOx emissions.

For example, some areas may be full of lots of unburnt particles, meaning there is a lack of air causing cold spots in the furnace. Other areas may be burning too hot, forcing together nitrogen and oxygen molecules into NOx molecules. The lasers detect these imbalances and give the system a clear understanding of what’s happening inside. But knowing this is only half the battle.

A breath of fresher air

“The next job is optimising the rate of burning within the boiler so fuel can be burnt more efficiently,” explains Groganz. This is achieved by selectively pumping air into the combustion process to areas where the combustion is too poor, or limiting air in areas which is too rich.

“If you limit the air being fed into air-rich, overheated areas, temperatures come down, which reduces the production of NOx gases,” says Groganz. “If you add air into air-poor, cooler areas, temperatures go up, burning the remaining particles of fuel more efficiently.”

Drax Laser 2

It’s a two-for-one deal: not only does balancing temperatures inside the boiler limit the production of NOx gases, but also improves the overall efficiency of the boiler, bringing costs down across the board. It even helps limit damage to the materials on the inside the boiler itself.

Thanks to this system, and thanks to its increased use of sustainable biomass (which naturally produces less NOx than coal), Drax has cut NOx emissions by 53% since the solution was installed. More than that, it is the first biomass power station to install a system of this sophistication at such scale. This means it is not just a feat of technical and engineering innovation, but one paving the way to a cleaner, more efficient future.

This is how you unload a wood chip truck

30 June 2017

Sustainable bioenergy
Truck raising and lowering

A truck arrives at an industrial facility deep in the expanding forestland of the south-eastern USA. It passes through a set of gates, over a massive scale, then onto a metal platform.

The driver steps out and pushes a button on a nearby console. Slowly, the platform beneath the truck tilts and rises. As it does, the truck’s cargo empties into a large container behind it. Two minutes later it’s empty.

This is how you unload a wood fuel truck at Drax Biomass’ compressed wood pellet plants in Louisiana and Mississippi.

What is a tipper?

“Some people call them truck dumpers, but it depends on who you talk to,” says Jim Stemple, Senior Director of Procurement at Drax Biomass. “We just call it the tipper.” Regardless of what it’s called, what the tipper does is easy to explain: it lifts trucks and uses the power of gravity to empty them quickly and efficiently.

The sight of a truck being lifted into the air might be a rare one across the Atlantic, however at industrial facilities in the United States it’s more common. “Tippers are used to unload trucks carrying cargo such as corn, grain, and gravel,” Stemple explains. “Basically anything that can be unloaded just by tipping.”

Both of Drax Biomass’ two operational pellet facilities (a third is currently idle while being upgraded) use tippers to unload the daily deliveries of bark – known in the forestry industry as hog fuel, which is used to heat the plants’ wood chip dryers – sawdust and raw wood chips, which are used to make the compressed wood pellets.

close-up of truck raising and lowering

How does it work?

The tipper uses hydraulic pistons to lift the truck platform at one end while the truck itself rests against a reinforced barrier at the other. To ensure safety, each vehicle must be reinforced at the very end (where the load is emptying from) so they can hold the weight of the truck above it as it tips.

Each tipper can lift up to 60 tonnes and can accommodate vehicles over 50 feet long. Once tipped far enough (each platform tips to a roughly 60-degree angle), the renewable fuel begins to unload and a diverter guides it to one of two places depending on what it will be used for.

“One way takes it to the chip and sawdust piles – which then goes through the pelleting process of the hammer mills, the dryer and the pellet mill,” says Stemple. “The other way takes it to the fuel pile, which goes to the furnace.”

The furnace heats the dryer which ensures wood chips have a moisture level between 11.5% and 12% before they go through the pelleting process.

“If everything goes right you can tip four to five trucks an hour,” says Stemple. From full and tipping to empty and exiting takes only a few minutes before the trucks are on the road to pick up another load.

Efficiency benefits

Using the power of gravity to unload a truck might seem a rudimentary approach, but it’s also an efficient one. Firstly, there’s the speed it allows. Multiple trucks can arrive and unload every hour. And because cargo is delivered straight into the system, there’s no time lost between unloading the wood from truck to container to system.

Secondly, for the truck owners, the benefits are they don’t need to carry out costly hydraulic maintenance on their trucks. Instead, it’s just the tipper – one piece of equipment – which is maintained to keep operations on track.

However, there is one thing drivers need to be wary of: what they leave in their driver cabins. Open coffee cups, food containers – anything not firmly secured – all quickly become potential hazards once the tipper comes into play.

“I guess leaving something like that in the cab only happens once,” Stemple says. “The first time a trucker has to clean out a mess from his cab is probably the last time.”

Forests, sustainability and biomass – the expert’s view

Matthew Rivers, Director of Corporate Affairs, Drax, Group Special Advisor

6 October 2016

Opinion

It was a forestry catastrophe that first inspired Matthew Rivers’ interest in forests.

Dutch Elm trees, an iconic part of the UK landscape for over 250 years were becoming infected with a fatal and fast-spreading disease. The race was on to save them.

A schoolboy in North London at the time, Rivers joined the after curricular school team tasked with saving its trees – first by injecting them with insecticide, and when that didn’t work, by felling and replanting them. It was an early foundation in how forests work and the challenges of keeping them healthy.

Decades later, Rivers is Director of Corporate Affairs at Drax. It’s a role he finds himself in following a career as a forester, helping to manage forestry businesses, and supporting the setting up of wood product manufacturing plants.

His own estimation of his working life is a humble one, however. “I think I’m probably a failed farmer,” he says.

“A forester always plants in hope.”

Rivers studied forestry at university in Scotland before taking up jobs in the forestry industry across the UK, Uruguay and Finland. Working in this industry, he says, is one that requires patience.

“In the UK we’re talking about 30- or 40-year growth cycles. The trees I planted at the start of my career are only just coming to maturity now,” he explains.

But more than the long investment of time, being a forester relies on faith. “A forester always plants in hope,” he says. When a forester plants a tree, he or she most commonly does not know who the end customer will be.

So when the call came from Drax for a forestry expert to help guide the company through an important transformation – upgrading the power station from coal to biomass – the challenge was one he was ready to take.

“Drax already had ambitions of converting three boilers to run on biomass. That would mean consuming tonnes of compressed wood pellets,” he says. The business needed a supply, and Rivers was drafted in to set this up.

As part of the supply solution, and Chaired by Rivers, Drax set up Drax Biomass, a pellet manufacturing business in the USA that makes and supplies compressed wood pellets to Drax Power Station.

Setting up its own manufacturing plant not only means Drax needs to rely on fewer external suppliers, but also that it can use the learnings about the technologies, the economics and the sourcing of the process to continually hone its supply chain.

To operate responsibly and receive governmental support, Drax has to be sustainable, and this is particularly important when it comes to where and how it sources its fuel. This comes with its own challenges.

No universal definition of sustainability

“To my understanding, there is no universal definition of sustainability,” says Rivers. So how do you proof your business for an unclear entity?

“At its heart, sustainability is about not doing anything today that would prejudice doing the same thing for the next generation or generations to come.”

A responsibly managed forest is one that is as healthy, productive, diverse and useful in 100 or 500 years’ time as it is today. They key to this, is to think of forests as a whole.

Rivers explains: “Think about a single tree – you fell it and use it to heat your home over one winter. But it’s going to take perhaps 30 years for that tree to grow back,” he says. “What do you do for the next 30 years?”

“In a sustainably managed forest you have all different ages of tree represented – one thirtieth devoted to each age- and, when you use an older tree to warm you in winter, you plant a replacement. That way, for every year you’ll have trees reaching maturity ready to provide your power.” It’s a cycle that, if managed responsibly, keeps delivering a useful resource as well as maintaining the health of the forest.

Rivers continues: “Sustainability is the very nature of what a forester does; because if we don’t take care of our forests, and ensure we have a crop to harvest year after year, we lose our livelihood.”

forests_trees_growing_for_winter_heating_smh4nj

Becoming a private forester

Two decades ago, Rivers completed a loop he started decades ago amidst the Dutch Elm crisis and became a forest owner himself. In Scotland, he bought, and now manages, his own private forest.

“We’ve had kids’ birthday parties, we’ve dug out a pond, we harvest chanterelles in the autumn – there’s a millennium capsule buried somewhere,” he says.

It’s not only a family heirloom. It’s a place for him to exercise a passion – maintaining and managing a responsible and healthy forest.

 

This is how you make a biomass wood pellet

Sustainable bioenergy
Compressed wood pellets

Wood has been used as fuel for tens of thousands of years, but this wood – a compressed wood pellet – is different. It’s the size of a child’s crayon and weighs next to nothing, but when combined with many more it is a smart solution to generating cleaner electricity compared to coal.

Wood pellets like these are being used at Drax Power Station to generate electricity and power cities. Not only are they renewable and sustainable, but because they are compressed, dried and made from incredibly fine wood fibres, they’re also a very efficient fuel for power stations.

This is how a compressed wood pellet is made at the Drax Biomass Amite BioEnergy Pellet Plant in Mississippi.

The wood arrives to the yard

Wood arrives at the plant via truck and is sent to one of four places: the wood storage yard, the wood circle (where wood is primed for processing), the piles of sawdust and woodchip, or straight into processing.

Bark is removed and kept for fuel

Logs are fed into a debarker machine, which beats the logs together inside a large drum to remove the bark. The bark is put aside and used to fuel the woodchip dryer, used later in the process.

Thinned wood stems become small chips

The logs – low-value fibre from sustainably managed working forests – need to be cut down into even smaller pieces so they can then be shredded into the fine material needed for creating pellets. Inside the wood chipper multiple blades spin and cut the logs into chips roughly 10mm long and 3mm thick. The resulting chips are fed into the woodchip pile, ready for screening.

Chips are screened for quality and waste is removed

Chipped down wood can include waste elements like sand, remaining bark or stones that can affect pellet production. The chips are passed through a screener that removes the waste, leaving only ideal sized wood chips.

The biggest hairdryer you’ve ever seen

The wood chips need to have a moisture level of between 11.5% and 12% before they go into the pelleting process. Anything other than this and the quality of the resulting pellets could be compromised. The chips enter a large drum, which is blasted with hot air generated in a heater powered by bark collected from the debarker. The chips are moved through the drum by a large fan, ready for the hammer mill.

Wood pellet Hammer Mill

Small woodchips become even smaller woodchips

Inside the hammer mill there’s a spinning shaft mounted with a series of hammers. The wood chips are fed into the top of the drum and the spinning hammers chip and shred them down into a fine powdery substance that is used to create the pellets.

Putting the chips under pressure – a lot of pressure

The shredded woodchip powder is fed into the pellet mill. Inside, a rotating arm presses the powdered wood fibre through a grate featuring a number of small holes. The intense pressure heats up the wood fibre and helps it bind together as it passes through the holes in a metal ring dye, forming the compressed wood pellets.

Resting and cooling down

Fresh pellets from the mill are damp and hot, and need to rest and cool before transporting off site. They’re moved to large storage silos kept at low temperatures so the pellets can cool and harden, ready for shipping.

One of the biggest domes you’ve ever seen

This is the final stage before shipping. Specially designed and constructed storage domes are used to store the wood pellets after they are transported to the Mississippi River, Louisiana and before they make their way across the Atlantic to the UK.

Inside the dome

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.