Feature

Biomass: When could torrefaction be commercially viable?


Andrew Mourant

Part 1. The pros and cons of the roasting process – torrefaction – for biomass crops is now under scrutiny as never before by university research teams around the UK, as Andrew Mourant reports...

WHAT DOES the next decade hold for exploiting biomass as a commercially viable energy crop? And is there a business future for pre-roasting crops such as willow and poplar so these can be used effectively alongside coal in power stations? The idea has been around for almost 40 years but has yet to take off commercially. Many unanswered questions still surround the science and economics, but the energy world is striving to close that knowledge gap.

The roasting process – torrefaction – is now under scrutiny as never before by university research teams around the UK, and is a significant part of the Supergen Bioenergy hub project. This, a five-year programme, has just started gathering momentum having attracted a £3.5mn grant from the Energy and Physical Science Research Council (EPSRC).

Supergen, based at the University of Manchester, intends uniting the best brains of industry and academia. It's looking far beyond the process of torrefaction, adopting a “whole systems” approach – from crop production to its use in power stations. It's also examining social and economic impacts besides the science.

The problems created by untreated biomass at power stations are well-documented. The material is burned alongside coal which is crushed to powder in huge mills before being blown into the burners. But while coal is easily ground, most biomass is springy and fibrous. This limits the amount that can be processed and used in co-firing.

Some power stations have invested heavily in separate mills for cutting biomass so they can use more of it. Torrefaction, however, has the potential to make a huge difference to their operations. After biomass is roasted in an airless environment at about 280°C, moisture, along with some gases and volatile substances, is lost. What's left is transformed into a harder fuel that's easier to crush, move and store. It also has a longer shelf life.

Shrouded in secrecy

Much of the new research is a work in progress, one in which industry collaborators are reluctant to discuss their involvement. Drax, for example, working alongside the University of Leeds, was reluctant to answer detailed questions from Renewable Energy Focus, with a spokeswoman saying only: ‘a lot of what we do in this area is confidential’.

Drax provides ‘some guidance on the direction which research takes… through sharing feedback on our findings… we provide a link to the real world of business and engineering,’ a spokeswoman said. But she declined to say if there had been significant discoveries about which crops work best for torrefaction; or if Drax has learned from commercially advanced operations such as Topell in Holland – the Netherlands leads the world in the business application of this type of torrefaction technology.

Eon, another partner with Leeds University, was also unforthcoming about its role. What is known is that the university's work is wide-ranging. It's considering, for instance, whether or not torrefied biomass could be at risk of spontaneous combustion; also its potential explosiveness when roasted and ready for use.

Environmental impact

Meanwhile environmental impacts are being examined at Bath University by researcher Dr Paul Adams. “We've done a lot of work on the use of resources and energy,” he said. “You often find there's a lot of ‘embodied’ energy in creating biomass, for instance using inorganic fertilisers on the crop feedstock. Producing these is quite energy-intensive.

“There are hundreds of crops that can be used for torrefaction. But another approach is to use industrial bi-product from forestry sawmills or furniture manufacturers that would otherwise go into landfill, decompose aerobically and release methane. Its global warming potential is 25 times higher than that of CO2.

“Our assessment looks at the whole system of where biomass feedstock is coming from – from cradle to grave. We're not just interested in carbon, but for instance, water, which is used not only in crop cultivation but the industrial process. Another aspect to consider is the use of metal in creating a torrefaction plant.”

Other scientists are examining what torrefaction means for CO2 emissions, among them Dr William Hall at Coventry University. “We've been looking at the burning temperature and time the process takes,” he says. “It's been theoretical: we've used hardwood and softwood in applying the model. We found that the CO2 emissions are linked to the conditions – there's quite a narrow window of temperature. If you stray outside that window, you can increase emissions rather than reducing them.”

Dr Hall has published detailed findings in the Journal of the Energy Institute. His work took two approaches. The first was to look at use the latent heat of hot syngas for torrefaction (syngas is a mix of carbon monoxide, carbon dioxide and hydrogen produced from gasifying a carbon-containing fuel).

“But that's only possible when the gasification plant and torrefaction plant are on the same site,” says Dr Hall. “So we also considered what happens when you use the heat from volatile torrefaction products that have been combusted.”

In terms of reducing CO2, Dr Hall concludes that the ideal torrefaction temperature is 280°C for hardwood and 300°C for softwood where syngas heat was used to heat pre-dried woodchips. When torrefaction volatiles provided the heat source, the optimum temperature dropped to 240°C and 260°C for hardwood and softwood respectively.

“The downside to torrefaction is that mass is lost and therefore the energy yield is never 100%,” says Dr Hall. “There are many uncertainties remaining about the process.” So, although torrefied products have a greater energy density than other biomass and less energy is needed to grind it up in power stations, the power industry must decide whether that overall energy loss is worth the effort and expense.

Explosive risks?

The focus at Leeds is about building up a knowledge base of how fuels behave; the dusts; the explosive risks, says research team leader Professor Jenny Jones. “We're looking to see if torrefied fuels can be stored outside – whether they're at risk of self-heating spontaneous combustion,” she says. “At the moment power stations have to invest in underground storage for biomass. It's a big capital investment. As torrefied biomass has been heat-treated, you've removed a lot of the material that moulds will attack and cause it to spontaneously ignite.”

Dr Daniel Nowakowski, who's based at Aston University and has studied torrefaction for over two years, agrees. “Removing moisture and some volatiles makes it less sensitive to degradation and also hydrophobic (water repellent),” he said. “It's good for storage and handling – there are major cost-savings.”

Dr Nowakowski and his team used a small experimental reactor to torrefy various crops including beech, willow and switch grass. Trials were conducted at temperatures ranging from 225–300°C, with torrefying times ranging from 30 minutes to two hours.

Meantime, Professor Gordon Adams is leading the Leeds University study into explosiveness. One problem, he says, is finding sufficient torrefied material with which to experiment.

“While Jenny Jones's group is making small quantities, one test we do is in a metre cube explosion vessel and you need kilos for that,” he said. “We'd been working 18 months before we got one of the manufacturers to deliver 20kgs. Several companies are making it – it will be big business one day.”

However, such is the commercial sensitivity, and with negotiations between suppliers and power companies underway, Professor Adams was unwilling to name the producer.

The explosiveness test for torrefied biomass uses a pulverised material whisked up with air in an upright pyrex Hartmann tube. “In terms of how little you need to react with air, the indication is that biomass is very reactive compared with coal,” says Professor Adams. “There have been a number of biomass plant explosions. Burning torrefied biomass is do-able but it will release volatility sooner.”

The dust clouds created by biomass tipped into storage silos presents a hazard against which the industry has always to be on guard. The dangers were underlined by a fire at a Npower biomass plant in Tilbury, Essex, in February that prevented it exporting power to the grid for four months.

While Npower claimed the fire was caused by a “number of relatively minor events that, taken in isolation would not have escalated”, smouldering dust from wood pellets ignited by drafts of air were widely speculated to have been the cause. Npower says it has since improved safety measures at the site.

Still some way to go

Developing a biomass supply chain and infrastructure for industrial scale torrefaction ‘could probably take a decade to get into place’, says Professor Adams. It raises big logistical questions – transporting the bulky crop with its significant water content is cumbersome and costly. However pellet plants are now being built; and the pellets, which are dried during the process, are easier and cheaper to ship than raw biomass.

In business terms, working out just where to build a torrefaction plant will be crucial. That's a question Professor Adams, for one, still struggles to answer. “Do you place it at source (near the feedstock crop) or near the power station?” he wonders. “The size of a big-coal fired station is such that you would need a torrefaction plant of its own. No one knows what the economics will be.”

Part 2. We look at the world's largest torrefaction plant...

About: Andrew Mourant is a freelance journalist whose specialisms include renewable energy, education and the rail industry

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