Biofuel can be produced from wood, too. The key to this is the suitable pretreatment of the plant parts in order to break up the wood structure. Researchers at the Transport Processes and Reactions Laboratory, ETH Zurich, have found and patented a method of this kind.
Thomas Pielhop fills spruce sawdust into a reactor vessel into which he then injects hot steam at a temperature of 220 degrees to break up the wood structure. (Image: F. Gribi / NRP66)
Wood, straw or grass – already today these are the components for ethanol production from renewable resources. They are seen as a viable alternative to maize and sugar cane from which fuel can, in some cases, be produced on a large scale. However, this is a controversial ethical issue. Producing ethanol from wood-containing plant biomass can have economic, ecological and energy advantages
But there is a snag. Wood mainly consists of usable sugars which are bound in the form of the structural element cellulose and the energy store hemicellulose. But it also contains up to 30 percent lignin which cross-links the structural materials in a stable manner. Lignins are large, multi-branched molecules which give the wood its characteristic stiffness and protect it from degradation (through microorganisms).
Boiling wood with steam
To use wood-containing plants for fuel generation, the lignin braces must initially be separated by means of appropriate pretreatment. Thomas Pielhop, a doctoral student of Philipp Rudolf von Rohr, Professor for Process Engineering, is working on wood pretreatment of this kind.
“The preferred method for the treatment of woody biomass is steam”, he says. “That is the easiest and most environmentally-friendly procedure.” To break up the structure and lignin linkages of spruce wood, the researcher literally gets things steaming. He inserts fine shavings into a pressure chamber and then introduces hot steam at a temperature of 220 degrees. Then the wood is boiled for up to 20 minutes which partially breaks the lignin bonds. Although cellulose and lignin cannot be completely separated from each other, the enzymes added afterwards can succeed in breaking up the cellulose.
Blasting the wood structure
To give the enzymes a larger area of attack, Pielhop abruptly reduces the pressure in the chamber by opening a flap. The water, which had previously condensed on and above all in the wood shavings, evaporates explosively. When the water evaporates, it needs more than 1,000 times more space. The wood parts are really blasted and further broken up for enzymatic degradation.
The method does, however, have one downside. The fragments of lignin produced tend to immediately hook another lignin fragment as a new bonding partner. The molecular fragments can link up so strongly that the resulting biomass can scarcely harness sugar any more.
To prevent the unwanted lignin crossing reactions, Pielhop admixes a substance to the steam which “captures” the reactive lignin fragments. The chemists call substances of this kind scavengers. In concrete terms, the ETH scientist used 2-naphthol, a low-priced compound to be found in coke ash from coal power plants. It meets the requirements of a scavenger. It only reacts once and has an affinity to the lignin fragments.
The initial experiments confirmed to Pielhop that he was on the right track. Pretreatment increased the efficacy of the enzymes and the sugar yield increased twofold compared to steam pretreatment with no naphthol admixture. “Thanks to this we are able to make almost 100 percent of the sugar contained in the spruce wood available for ethanol production. That’s unusual”, stresses the chemical engineer.
Pielhop initially developed and tested his method on spruce wood in a small laboratory reactor and in a larger pilot facility, a steam gun. Michael Studer, a former colleague from the Rudolf von Rohr group, was involved in designing facility. Today the steam gun is located at Bern University of Applied Sciences. The tests are promising. Via the technology transfer office of ETH Zurich, ETH Transfer, the researchers then also had the method patented.
The researcher is not completely happy about the fact that he uses a fossil raw material, 2-naphthol as the radical scavenger. Pielhop said that he was currently on the lookout for a scavenger that came from renewable resources. However, the precondition for commercial success is that it is readily available at a favourable price.
Roll-out in the USA and Scandinavia?
Pielhop’s pretreatment method is likely to meet with interest above all in the USA and in Scandinavia. Extensive research is already underway there on how to produce ethanol from wood. There is also a pilot plant that processes lignocellulosic biomass close to Milan. Besides wheat straw and rice, it also processes giant cane (Arundo donax). This is specifically cultivated for ethanol production on areas which agriculture cannot/can no longer use. However, giant cane has turned out to be an invasive plant in the USA that crowds out indigenous vegetation.
In this pioneer facility at Bern University of Applied Sciences the structure of wood can be broken up with steam so that the cellulose can be digested by enzymes. (Photo: M. Studer FH Bern)