Originally developed in the 1980s in France, torrefaction of biomass has been derived from a process traditionally used to dry and roast coffee beans, which makes them brittle and gives them their distinctive flavour. The process involves gently roasting coffee beans up to temperatures of around 280°C. Now, Jenny Jones and Toby Bridgeman, from the University of Leeds, have shown that torrefaction could have huge benefits if used on energy fuels.
Jones and Bridgeman examined the effect of torrefaction on four different energy crops: willow, reed canary grass, switchgrass and wheat straw from agricultural residue. In each case the crop was heated in a nitrogen gas atmosphere, to a maximum temperature of 300°C. Throughout the process they carried out chemical analysis to examine the way in which the crops decomposed. Afterwards they measured the mass and energy content of the new fuel.
Writing in the May issue of the journal Fuel, Jones and Bridgeman describe how the end product resembled low-grade coal. The results showed that torrefaction was beneficial to all of the energy crops examined, driving off moisture and creating a higher energy fuel. Mass was reduced by between 10 to 30%, making the fuels more economic to transport.
As well as increasing the energy content, torrefaction also helps to dry out the fuel and alter its chemical composition. "This means that it absorbs less moisture when it is stored and reduces the likelihood of fungal attack," Jones told environmentalresearchweb.
Perhaps most exciting of all, the torrefaction process made the fuel more friable, making it easier to grind. This might mean that milled biomass could be used in ordinary coal-fired power stations, burned alongside the pulverised coal. "In its normal state biomass is very hard to mill. After torrefaction it becomes much easier and raises the possibility of co-firing energy crops with coal using existing milling facilities," says Jones.
Willow in particular responded well to torrefaction. "Ordinary willow pellets produced from fresh biomass have an energy density of around 10 Gigajoules per cubic metre, but after torrefaction this increased to as much as 18 Gigajoules per cubic metre," explained Bridgeman.
And although energy is required to carry out the torrefaction process, the energy gains can outweigh the cost. What's more, in some cases the by-products can be re-used. "The gases and liquids that are driven off during torrefaction can be recycled and used to drive the process," said Bridgeman.
As far as they are aware, Jones and Bridgeman believe that none of the power stations in the UK have torrefaction facilities, but say it would be possible to introduce. "On a small-scale, the equipment is quite simple, but there would be challenges associated with large scale production, and our continuing research is addressing some of these," said Jones.