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INDUSTRY NEWS


Production unit The pellets were produced in a production unit located at the Discipline Environmental and Energy System at the Department of Engineering and Chemical Sciences at Karlstad University, Sweden. This has recently been complemented by the installation of a Type MK 200 Mixing Conditioner from Amandus Kahl GmbH & Co KG and a small cooling tower for pellets. The pellet machine was run until stationary conditions were obtained. Before every new test, there was a break-in period of five minutes with the current additive to ensure stationary conditions. Every test run lasted for five minutes. The feed control for the dried sawdust was set at a fixed rpm, e.g., (4.0 Hz) corresponding to approximately 85 kg of sawdust/h (the rapeseed tests were done with various rpm). The additive flow was subsequently increased based on weight of pressing mass dry bases.


Measurements


During the tests, die temperature, screw frequency, electricity used by the pelletising machine, and pressure from the rollers on the die (‘die pressure’) were measured every ten seconds. The die temperature was measured using Pt-100; the error in the measurements is ±0.5 °C. The current load was measured with an accuracy of ±1% (From the measured load current, the specific energy use was calculated according to the method described in a previously published study conducted at Karlstad University). The pressure from the rollers was measured with an accuracy of ±1.25 Bar. The die temperature and the pressure from the rollers indicate stationary and stable conditions. The produced pellets were cooled down to ambient room temperature and sieved, before being analysed. The analysis was performed by testing and comparing the produced pellets with the quality parameter settings in the Swedish Standard, complemented with additional tests. The tested parameters were


moisture content (%, wb) for sawdust, additives and pellets determined according to SS 187170, mechanical durability determined according to SS 187180 measured as the percentage of fine particles < 3 mm (amount of fines).


Results


The results show that the energy consumption decreased using rapeseed cake and all of the starch grades, but was unaffected by the increased use of Kraft lignin. The oxidised cornstarch showed the best result; when 2.8% of cornstarch was added, the average energy consumption was reduced by 14%. Regarding durability, the amount of fines increased with increasing rapeseed cake in the wood-fuel pellets, contrary to what was intended. Pellets produced using 1–4% of Kraft lignin show that an increased amount of Kraft lignin in the pellets increased their mechanical durability. The results also show that dry lignin yields pellets with higher durability as compared to wet lignin. Using various percentages (0.7−3.1% on dry solids (% ds).) of different kinds of starch shows that starch increases the durability of the pellets significantly, already at


0.7% starch. Oxidised starches increase the durability more than native starches and the best results were obtained by adding oxidised cornstarch.


Summary


Energy-efficient solutions reduce the production costs, therefore it is important to investigate how the energy use changes when an increased amount of additive is used in the process. In the research undertaken at Karlstad University, it was found that the energy consumption of the pelletising machine decreased with an increasing amount of rapeseed cake being mixed in with the sawdust pellets. There is also a noticeable reduction in average energy consumption when starch is added for a given press length. The results also show that the uncut pellet length increases when starch is added. This means that the press length can be shorter, which in turn will lead to even more reductions in average energy consumption or, even better, to increased production. Regarding kraft lignin, the specific energy use was unaffected by the increased amount of kraft lignin used, but the specific energy use seems to decrease with higher


PELLETISING


amounts of moisture in the Kraft lignin.


It is clear that the addition of starch and kraft lignin before pelletising Norway spruce (Picea Abies) increases the cohesiveness of the pellets, which is confirmed by the longer uncut pellets and their increased durability. This improved with increased amounts of starch and kraft lignin (for the percentages tested). The improvements in pellet length and durability are greater when adding oxidised starch as compared to native starch, independently of die pressure. To some extent, the water also works as a binder and strengthens the durability even more, since the moisture content increases with increased amounts of, for example, starch. Further studies made in a full-scale pellet plant are recommended to find the economically optimal amount of added additive.


At present, home users represent 50% of the wood- fuel pellet market in Sweden. Accordingly, their demands for a high-grade fuel are now even more important than previously to the pellet industry. Moreover, households demand quality since, for example, they have relatively simple


The pellet production unit consists of: a diagonal mixer (1), a conveyor screw (2), an inlet feeder where conditioning takes place (if needed) (3) an Amandus Kahl C33–390 pelletizing press with a flat die and a maximum output of 300 kg/h (4) and a volumetric feeder for additives and a cooling tower (5).


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