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Top-class biofuel from the depths of the forest

May 22, 2013 — Tops and branches from tree-felling sites are reborn in the laboratory as compact pellets. However, the energy industry will not act until the price is right.

We have all seen it when we walk in the forest after the lumberjacks have been there. When the logging machinery moves on, what it usually leaves behind are piles of branches and tops.

Norway possesses major unexploited energy resources in the form of these branches and tops -- known in their Norwegian acronym as GROT (see Fact-box). Samples of this logging waste regularly arrive at SINTEF to be transformed into fuel.

In the raw form in which the biomass arrives at the laboratory, it is regarded as a problematic and therefore low-value fuel. But when the scientists and technicians have finished processing it, they are left with a valuable source of heat -- ready for use in industrial heating furnaces that are currently fuelled with wood pellets or chips, and for domestic pellet stoves.

The transformation is effected via a process called torrefaction, a sort of extreme sauna for timber and vegetation.

Problematic raw material

Senior scientist Øyvind Skreiberg shows Gemini a fistful of fragments of wood with an admixture of spruce needles. It looks just like the sort of debris that litters the pavement around the Christmas tree stands on Christmas Eve. This is chopped-up GROT from spruce logging.

"Cheap fuel, and Norwegian logging sites are full of it. But it is a poor-quality fuel, because it is so variable in composition," says Skreiberg.

If the GROT is tipped into the furnace, the woody component of the mixture may burn in one instant, bark and needles in the next.

"This mixture means that the efficiency and characteristics of the combustion process are extremely variable. In the worst case, they can destroy a combustion chamber," says Skreiberg. Then he opens his other fist and shows us the evidence that the problem is capable of being solved.

Fibre broken down

On its way through the highly insulated steel cylinders of the laboratory rig, the raw material has been heated to 275 oC. The heat treatment has broken down the fibrous structure of the biomass. According to Skreiberg, this has two benefits.

"In the first place, the torrefied material can easily be crushed into the powder that you can see here, which can be stirred around to form a homogeneous, and therefore combustion-friendly, mass."

"Secondly, this powder can be pressed into pellets with a high energy content per unit of weight and volume; in other words, it is also a transport- and storage-friendly fuel. Pellets of torrefied biomass can withstand getting wet, just like coal, and are very stable under storage."

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