In a study published by the Open Renewable Energy Journal 2015/05/06, the effect on combustion and flue temperatures was studied by the University of Eastern Finland.
"According to the European standard on wood pellets, the minimum pellet length is 3.15mm and the maximum is 40mm, up to 1% 45mm (EN 14961-2). The length standard was made primarily to ensure the fluent working of conveyors. The initiative to this study was the finding of several pellet users that pellet length also has an effect on combustion. This practical observation has also led pellet stove manufacturers to create different operating programs for short and long pellets. The purpose of this study was to assess if pellet length has any effect on combustion characteristics in addition to calorific heat, and more precisely to identify the reasons for the possible effects. Current pellet production technologies do not carefully control pellet length. Cutting knives are used to cut pellets before they get too long but other control systems do not exist. Wood pellets have been used on a large scale for about ten years. Knowing important features from the usability point of view gives milestones for further development of production and raw materials. If pellet length really does have a significant effect on combustion, the future development of pellet production should pay attention to the improved control of pellet length. "
They found in their conclusions that "Pellet length has a significant effect on the combustion temperature and momentary energy released in a pellet stove. Increasing the average length of pellets from 5.8mm to 13.1mm decreased the average burning temperature by 31% and flue gas temperature by 25%. A major contributing factor deemed to be the better functioning of the feeding screw of the pellet stove. Shorter pellets work more fluently in the screw and the pellet flow is steadier. Another contributing factor is more intense combustion conditions caused by a greater surface area subject to combustion and a denser fuel bed with the shorter pellet. Long and heavy pellets are sometimes suppressing the flame in the burning cup and the combustion process goes down because the ignition of long pellets takes time. Based on observations in this pilot study, the authors recommend that whenever pellet combustion is studied, the pellet length distribution should be stated and direct comparison should be made only between pellets of similar length. For further development of pellet production, improved control of pellet length might increase the efficiency of small-scale combustion."
Full Copy of the Study
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