Twin screw extruder. Figure 2. Open twin screw extruder with material inside (left), whole extruder opened (right). (Source ATB.) As shown in Figure 2, in a twin screw extruder, opposing screws apply mechanical energy to the woodchips. The raw material is crushed, sheared and squeezed. The moist wood chips are split mainly parallel to the fibre bundle structure. During this process the pressure is up to 500 kPa and the temperature reaches approximately 100 °C. A sudden relaxation (steam explosion) results in tearing of the cell structure. The last part of the screw in front of the aperture is designed to force a partial reverse flow. This favours a plug formation (a formation of material being compressed into a plug) and the build-up of an internal overpressure in the ‘comminuting’ space (space inside the extruder where the material is sheared, crushed and mixed) and leads to a particularly intense pressure and shear stress. The high temperature in the extruder also causes the lignin to reach its “glass transition temperature”. This transition is manifested as an abrupt softening of the lignin either at a single temperature (the glass transition “point”) or over a relatively narrow temperature range. (Irvine 1985, Xxxxxxx, 2004)
Twin screw extruder. Figure 5. Open twin screw extruder with material inside (left), whole extruder opened (right) at ATB [Source: ATB.] As shown in Figure 5, in the twin screw extruder at ATB, opposing screws exert mechanical energy upon the wood chips. The raw material is crushed, sheared, squeezed and grated. The moist wood chips are split mainly parallel to the fibre bundle structure. During this process the pressure is up to 500 kPa and the temperature reaches approximately 100 °C. A sudden relaxation (steam explosion) results in tearing of the cell structure. The last part of the screw in front of the outlet opening (aperture) is designed to force a partial reverse flow. This favours a plug formation (the formation of material being compressed into a plug) and the build-up of an internal overpressure in the comminuting space. This is the space inside the extruder where the material is sheared, crushed and mixed and this leads to a particularly intense pressure and shear stress. The high temperature in the extruder also causes the lignin to reach its glass transition temperature. This transition is manifested as an abrupt softening of the lignin either at a single temperature (the glass transition “point”) or over a relatively narrow temperature range. (Irvine 1985); Xxxxxxx 2004) Assessments will need to be made regarding the economic and environmental feasibility of farmers making use of this technology. If a farm is located close to a processing facility, then the energy expended transporting raw materials and finished products will be minimal but for farmers located more remotely, other solutions may be required. It is feasible to fit an extruder of industrial scale, like the one at ATB, into a 40 foot container. As the extruded fibre should not have a moisture content above 20 % before being processed into pellets, a dryer is necessary as well; a small industrial scale dryer can also be fitted into a 40 foot container. Therefore, both extruder and dryer can be mobile. Another economic possibility would be that agricultural drying and pelleting plants buy an extruder since drying and pelletising technology is already available at the location. Only the transportation of the wood chips from the farmer to the agricultural drying and pelleting plant would have to be organised. Transportation lines to these locations are usually already established due to transportation of regular crops to be dryed or processed. Adaption of transportation to wood chips should not be probl...
