Abstract. Herbaceous biomass potential is considered as a valuable local renewable energy resource in Latvia. Cereal crop straw (mainly wheat straw), common reeds, rape straw and reed canary grass are the most prospective stalk materials for solid biofuel production. The main biomass conditioning operation in solid biofuel production technology is shredding. Hammer mills are the essential equipment for this operation. Hammers and their articulation are rapidly wearing during hammer mill lifetime. It is recommended to design hammers so that collisions with biomass particles are applied in the hammer centre of percussion. Then the hammer articulation hole is less wearing, unbalance and vibration of the hammer mill rotor are avoided. The main objective of this paper is to determine compliance with this rule for different size rectangular style manufactured hammers. Calculations were made using the Mathcad program for four different size hammers. Only for one size hammer positioning of articulation partly accomplished the mentioned recommended condition of the hammer centre of percussion. The recommended positions and sizes of articulation holes were determined for the mentioned four size hammers. If the hammer design accomplishes the recommended condition of the hammer centre of percussion, it is possible for collisions with particles during milling in different points of the hammer. Then some impact forces in the hammer articulation hole will appear. Therefore, it can be concluded that, if the centre of percussion is designed at the end part of the hammer, the impact forces in hammer articulation would be minimized. The results of calculation in compliance of four sizes of manufactured hammers with the condition of the centre of percussion at the end point of the hammer show that only one hammer size partly satisfies this condition. This shows that the hammer mill hammer design so that interaction between the hammer and material occur at the percussion centre of the hammer is still actual. The sizes of articulation holes have to be also calculated in accordance with the pin shaft strength conditions. Changing the size of the articulation holes the articulation position must be changed.Keywords: hammer mill hammers, centre of percussion. IntroductionHammer mills are the essential equipment for herbaceous biomass shredding before compacting as solid biofuel. Hammers are the main working tools of the hammer mill and are rapidly wearing during the hammer mill lifetime. The percussion interaction between the hammer and material produce additional perturbations generating irregularities [1] during the work. This leads to the consideration of the hammer design so that interaction between the hammer and material occur at the percussion centre of the hammer [2; 3], then the percussions in the articulation are eliminated and so the work of the machine is not disturbed.The main aim of this paper is to determine compliance with this rule for different size rectangular style existing manufactured hammers. The articulation hole...

The 2003 reform o f the EU Common agricultural policy stimulates farmers to grow more energy crops, including short rotation coppice and other perennial crops. Peat can be used as additive for manufacturing o f solid biofuel, because it improves density, durability o f stalk material briquettes (pellets) and avoid corrosion o f boilers. For these reason herbaceous biomass compositions with peat fo r solid biofuel production is recommended. The main conditioning operation before biomass compacting is shredding. It was stated that common reed stalk material particle size reduction during cutting (shredding) process increased energy consumption very significantly. The calculation o f energy consumption fo r common reed cutting to sizes 0.6 and 0.5 mm was giving results 31.3 k J kg'1 and 43.5 k J kg'1. The shredder cutter bar has to be designed with friction energy losses decreased to minimum. This aim can to be realized by reducing o f area o f cutter bar knives moving into stalk biomass and minimizing biomass pressure (Patent LV13447) on cutter bar.

The paper present experimental investigation results of common reeds (Phragmites australis) particle compacting in closed die. Common reeds are important natural biomass resource in Latvia. Compacting of biomass is very complicated process for solid biofuel production and there are many factors influencing to this process. The quality parameters of compacted biomass are described by European countries standards for solid biofuel. Density is the most important quality parameter of biomass compacting. The paper present results of common reeds particles compacting mechanism in closed die. Maximum pressure 212 MPa had been achieved in compacting. For compacting process evaluation has been determined pressing energy and density of briquettes. The minimum of density 0.87 g cm-3 have briquettes with particle size 12 – 13 mm, but maximum density 1.03 – 1.04 g cm-3 two particle sizes < 0.5 mm and 32 – 33 mm briquettes. Maximum value of energy consumption for compacting (~172 kJ kg-1) has been obtained for reed particle size 32 – 33 mm, minimum value (~53 kJ kg-1) for particle size less than 0.5 mm.

The main steps of rural policy to the development of renewable energy resources are energy crop production. Latvia can provide for the use of approximately 0.36 million ha of the unused now agricultural land for herbaceous energy crop growing. More than 230 million tones of peat are available for solid biofuel production. Peat is recomended as additive because it improves density and burning properties of herbaceous material solid biofuel. Herbaceous stalk material can be harvested in winter season with humidity less than 15% and used for compacting without drying. The aim of experimental investigation is to determine conditioning methods for solid biofuel production with minimum energy consumption. Common reed stalk material is used for experiments, because it experimentally stated ultimate tensile strength is 256 ± 27 N mm-2. This value testifies that common reeds are twice strongest material than cereal crop stalk materials and conditioning mechanization equipment of common reeds would be useful for other energy crops.

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