Density and porosity of the cut and ground material of energy plants

Annals of Warsaw University of Life Sciences SGGW Agriculture No 58 (Agricultural and Forest Engineering) 2011: 21 28 (Ann. Warsaw Univ. Life Sci. SGGW, Agricult. 58, 2011) Density and porosity of the cut and ground material of energy plants ALEKSANDER LISOWSKI 1, ADAM ŚWIĘTOCHOWSKI 1, KATARZYNA SZULC 2, ANDRZEJ LENART 2 1 Department of Agricultural and Forest Machinery, Warsaw University of Life Science SGGW 2 Department of Food Engineering and Process Management, Warsaw University of Life Science SGGW Abstract: Density and porosity of the cut and ground material of energy plants. The work aimed at determination of bulk and specific density and porosity for the three energy plant species: rose (Rosa multifl ora), giant miscanthus (Miscanthus sinensis gigantean) and Virginia mallow (Sida hermaphrodita). The differences in bulk and specific density and porosity are the species features of energy plants and result from the method for material fragmentation. The bulk density of cut material was by 17% higher than that of the ground material, and for the rose (240 kg/m 3 ) by over 3.5 times higher than for the miscanthus and the Virginia mallow; these values for the latter created a homogeneous group (68 kg/m 3 ). The specific plant material density of Virginia mallow, rose and miscanthus, designed for briquette production, amounted to 1207, 1162 and 640 kg/m 3, respectively. The cut material porosity depended on the energy plant species and method for its fragmentation, as well as on dimensions and shape of the particles. Key words: energy plants, density, porosity. INTRODUCTION The biomass demand increases constantly in Poland. The solid biomass has been currently obtained from the wastes of: forests, agriculture, wood industry, urban greens; small amounts from the sorted organic municipal wastes. The supplementary supply of biomass on energy market is obtained from perennial plantations of energy plants [Stolarski et al. 2005]. From the viewpoint of ecology, the biomass is more attractive fuel than the mineral, but it creates many technical problems. One of them is to small density that makes difficult its transport, storage and metering to boilers; it negatively affects the combustion process and its stability. This problem can be solve by appropriate processing of plant raw materials. Most often the biomass is processed into fuel briquettes and pellets, that should be characterized by high density of 650 900 kg/m 3. The briquettes and pellets are obtained as a result of pressure agglomeration process of plant materials, influenced by a series of factors; they can be divided into the following groups [Hejft and Obiedziński 2007]: Chemical and biological factors (chemical composition of material, biological structure of particles), Material factors connected with material preparation for densification process (moisture content, temperature, size distribution of the mixture, density),

22 A. Lisowski et al. Design factors (die diameter, diameter and number of compacting rollers, diameter, length and state of hole surface in the die, die-roller clearance, etc.), Process factors connected with the course of densification process (compaction pressures, flow rate of material subjected to densification, densification velocity, process temperature, conditioning). According to Sokhansanj and Lang [2003] one of the main aims of densification process investigating is determination of the effect of material features on quality of concentrated product. Therefore, knowledge of material features is essential to ensure the minimal energy consumption for densification process. The investigations aimed at determination of specific and bulk density and porosity of the fragmented material of energy plants. MATERIAL AND METHODS In investigations there were used the energy plants cultivated on plots of Experimental Station in Skierniewice, belonging to Agriculture and Biology Faculty WULS in Warsaw; the plants were harvested in April 2009. The three plant species were selected: rose (Rosa multifl ora), giant miscanthus (Miscanthus sinensis gigantean) and Virginia mallow (Sida hermaphrodita). The plants were chopped with forage harvester Z374 equipped with the chopping shredding unit. During operation the chopping unit was equipped 5 knives, while the knife disk rotational speed amounted to 1000 rpm. At peripheral speed of pulling- -compacting rolls of 0.82 m/s and cutting frequency of 83 Hz, the theoretical length of particles amounted to 9.9 mm. The cut plant material was dried under natural conditions and ground in the beater grinder equipped with a perforated screen with holes of diameter 15 mm. Moisture content of the cut and ground plant material was determined by the drying-and-weighing method according to PN-EN 13183-1 Standard with accuracy 1% (Tab. 1). The samples were weighed on an electronic scale RADWAG WPS 600/C with accuracy 0.01 g. Degree of fragmentation was determined with the use of sieve separator according to ASAE S424.1 Standard [1993]. The analysis of particle dimension distribution after cutting and fragmentation was given in the works of Lisowski et al. [2009, 2010]; the values of geometric mean of particle dimensions TABLE 1. Characteristic of plant material Plant material parameter Virginia mallow Rose Giant miscanthus Material moisture content at harvest, % 10.8 30.6 20.9 Cut material moisture content, % 9.7 29.5 20.0 Ground material moisture content, % 12.1 9.5 12.7 Mean geometric dimension of cut material particles, mm 9.69 9.35 10.40 Geometric standard deviation for cut material 1.92 2.13 1.83 Mean geometric dimension of ground material particles, mm 4.32 2.79 5.19 Geometric standard deviation for ground material 2.36 2.51 2.42

Density and porosity of the cut and ground material... 23 and geometric standard deviation are listed in Table 1. Methodology of the investigations included determination of bulk and specific density and porosity of the cut and ground material. Bulk density of the cut plant material was determined by twice repeated weighing of empty container (of volume 10 dm 3 ) and container with sample, on electronic scale with accuracy of 0.1 g. m mn ρl (1) V where: ρ L bulk density of fragmented plant material, kg/m 3, m mass of container with material, kg, m n mass of container, kg, V volume of container, m 3. In determination of plant material specific density the stereopycnometer of Quantachrome Instruments was used. The measurements are automatic and based on Archimedes principle (the volume of a body immersed in liquid or gas is proportional to liquid or gas volume displaced by this body). The gas is supposed to fill all material slits and inter-particle spaces. For small particles, especially after material grinding, and in order to maintain high accuracy, the helium was used. The specific density measurement involved compression of gas in a measuring chamber of volume 100 dcm 3 with material of known mass. The pressure in measuring chamber was recorded for this state, then the valve was opened and gas was directed to the datum chamber, where the gas pressure was measured. The material volume was calculated from expression: V P V C VA p 1 1 p2 (2) where: V p volume of investigated material, m 3, V C volume of measuring chamber, m 3, V A volume of datum chamber, m 3, p 1 pressure in measuring chamber, MPa, p 2 pressure in datum chamber, MPa. The plant material specific density was determined with dependence: m ρ V P (3) Porosity of investigated material was calculated on the basis of bulk and specific density: 100 1 ρ L L (4) ρ where: ε L porosity of material, %, ρ L bulk density, kg/m 3, ρ specific density, kg/m 3. The maximal relative errors in determination of material bulk and specific density and porosity were calculated on the basis of total differential method; for cut material they amounted to: 5, 1.7 and 1.5%, respectively, while for ground material to 4.3, 2.3 and 1.8%, respectively. Data analysis was carried out with the use of Statgraphics v.4.1 computer program, with application of variance analysis procedure and Duncan test. The bulk density measurements were carried out in Department of Agricultural and Forest Machinery WULS in Warsaw, while specific density measurements were executed in Department of Food

24 A. Lisowski et al. Engineering and Production Management WULS in Warsaw. RESULTS AND DISCUSSION The analysis of variance showed that specific and bulk densities as well as material porosity were significantly influenced by both the energy plant species and the material fragmentation method (Fisher-Snedecor statistics values amounted to: bulk density 4204.7 and 120.8, respectively, specific density 2449.3 and 509.8, porosity 1899.9 and 13.7, at significance level not higher than 0.05). Mean values of parameters for the cut and ground material and plant species are presented in Tables 2, 3 and 4, while their interactions graphical interpretation in Figures 1, 2 and 3. The highest differences in parameter values between fragmentation methods were found for bulk density (17%), while the least differences for porosity (1%). Density of the ground material was higher than that of the cut material (Fig. 1). The highest material bulk density was found for rose, both after chopping and after grinding (the mean values amounted to 237 and 241 kg/m 3, respectively. The bulk density values for giant miscanthus and Virginia mallow were similar and created a homogeneous group (Tab. 2); for the cut and ground material it amounted to 55 and 61 kg/m 3 and 53 and 63 kg/m 3, respectively. It is then evident that material bulk density of rose was over 3.5 times higher than for miscanthus and Virginia mallow. Frączek et al. [2003] found that bulk density depended on the moisture con- 300 250 237.0 240.9 Bulk density [kg/m 3 ] 200 150 100 50 55.4 61.4 52.7 63.0 cięcie cutting mielenie grinding 0 Miscanthus Rose Virginia mallow FIGURE 1. Bulk density of cut and ground material of energy plants TABLE 2. Homogeneous groups of bulk density Fragmentation method Plant species Mean Homogeneous Mean, Homogeneous Kind of material Plant kg/m 3 group kg/m 3 group Cut 115.7 X Virginia mallow 67.9 X Rose 239.9 X Ground 135.1 X Giant miscanthus 68.4 X

Density and porosity of the cut and ground material... 25 tent, pressure, degree of contamination, rate and method of deposit forming and the height of seed falling. This parameter varies widely depending on the species and variety, moisture content, filling method, height of deposit, degree of contamination and other factors. As it is evident from our investigations, the bulk density values well correspond with dimensions of material particles after chopping and grinding (Tab. 1). Susceptibility of plant material to fragmentation was determined by the skeletal structure of tissues and hardness. The rose material was lignified, and the fragmented material mixture, especially obtained by grinding, breaking-up and crushing in the beater shredder, had the least geometric mean of particle dimensions (2.79 mm). It allowed for better packing of fragmented material; this was proved by its least porosity amounted to 78 and 79% for the cut and ground material, respectively (Fig. 3). The higher deposit graining, the higher porosity that depends also on the shape of particles; therefore, the more fibrous miscanthus material had slightly lower porosity than the Virginia mallow material; the respective values amounted to 91 and 94% (Tab. 4). Specific density of the ground material of Virginia mallow and rose was higher than that of the cut material and amounted to 1207 and 1094 kg/m 3 and 1162 and 1072 kg/m 3, respectively; these values for miscanthus were opposite and equal to 640 and 720 kg/m 3, respectively (Fig. 2). Although the bulk density values for miscanthus and Virginia mallow were comparable, the specific density differed greatly (by 47%). This difference partially resulted from different particle dimensions of the fragmented material in both plants. Andrejko [2005] reported that particle dimensions and their variability can cause the increased or decreased specific density, depending on kind of material. The moisture content of TABLE 3. Homogeneous groups of specific density Fragmentation method Kind of material Mean, kg/m 3 Homogeneous group Plant Plant species Mean, kg/m 3 Homogeneous group Cut 961.8 X Virginia mallow 1150.5 X Rose 1117.1 X Ground 1069.9 X Giant miscanthus 779.9 X TABLE 4. Homogeneous groups of porosity Fragmentation method Kind of material Mean, kg/m 3 Homogeneous group Plant Plant species Mean kg/m 3 Homogeneous group Cut 88.4 X Virginia mallow 94.1 X Rose 78.5 X Ground 87.6 X Giant miscanthus 91.3 X

26 A. Lisowski et al. 1250 1207.4 1162.3 1100 1072.0 1093.7 Specyfi c density [kg/m 3 ] 950 800 719.9 cutting grinding 650 639.7 500 Miscanthus Rose Virginia mallow FIGURE 2. Specific density of cut and ground material of energy plants 100 95 90 92.3 95.2 93.1 90.3 85 Porosity [%] 80 75 70 77.7 79.3 cutting grinding 65 60 Miscanthus Rose Virginia mallow FIGURE 3. Porosity of cut and ground material of energy plants cut miscanthus was almost twice higher than that of Virginia mallow, however, for the ground material it was comparable. Thus, no significant differences in specific density can be connected with material moisture content; it was reported by other researchers. It was found that an increase in moisture content in the range of 10.0 30.0% caused a decrease in the specific, bulk and shake density of loose plant material [Deshpande et al. 1993; Mieszkalski 1999; Sokhansanj and Lang 1996; Szot and Stępniewski 2001]. The material specific density values for rose and Virginia mallow were higher than that for oak (600 900 kg/m 3 ), therefore,

Density and porosity of the cut and ground material... 27 the fuel formed into pellets or biquettes should meet the density requirements for such kind of products (650 900 kg/m 3 ). In the case of miscanthus material of specific density 640 kg/m 3, one can not produce pellets or biquettes that meet the Standard s requirements. CONCLUSIONS 1. Differences in the bulk and specific density and porosity are the speciesrelated features of energy plants and result from the material fragmentation method. 2. The cut material bulk density was higher by 17% than that of the ground material, while for the rose (240 kg/ m 3 ) was over 3.5 times higher than that of giant miscanthus and Virginia mallow; its values created a homogeneous group (68 kg/m 3 ). 3. The material specific density for Virginia mallow, rose and giant miscanthus, designed for production of briquettes, amounted to 1207, 1162 and 640 kg/ /m 3, respectively. 4. Porosity of the ground material of energy plants depended on the plant species, its fragmentation method and dimensions and shape of the particles. REFERENCES ANDREJKO D. 2005: Wpływ wilgotności i wymiarów cząstek na gęstość sypkich surowców roślinnych. Inżynieria Rolnicza. 11(71), 9 17. ASAE S424.1 MAR98: Method of determining and expressing participle size of chopped forage materials by screening. DESHPANDE S. D., BAL S., OJHA T. P. 1993: Physical properties of soybean. J. Agric. Engng Res. 56(2), 89 98. FRĄCZEK J., KACZOROWSKI J., ŚLIPEK Z., HORABIK J., MOLENDA M. 2003: Standaryzacja metod pomiaru właściwości fizyczno- -mechanicznych roślinnych materiałów ziarnistych. Rozprawy i Monografie 92, Lublin, p. 160. HEJFT R., OBIDZIŃSKI S. 2007: Wpływ parametrów aparaturowo-procesowych na wartości nacisków zagęszczających w procesie granulowania pasz. Inżynieria Rolnicza. 5(93), p. 313 319. LISOWSKI A., ÇAÐLAYAN N., ŚWIĘTOCHOW- SKI A. 2010: Rozkłady wymiarów cząstek roślin energetycznych zmielonych w rozdrabniaczu bijakowym. Annals of Warsaw University of Life Sciences SGGW, Agriculture (Agricultural and Forest Engineering) (w druku). LISOWSKI A., NOWAKOWSKI T., SYPUŁA M., CHOŁUJ D., WIŚNIEWSKI G., URBANOVIČOVÁ O. 2009: Suppleness of energetic plants to chopping. Annals of Warsaw University of Life Sciences SGGW, Agriculture (Agricultural and Forest Engineering). 53, 33 40. MIESZKALSKI L. 1999: Badania podstawowych właściwości fizycznych nasion łubinów. Probl. Inż. Roln., 1, 51 58. PN-EN 13183-1. Wilgotność sztuki tarcicy. Część 1: Oznaczenie wilgotności metodą suszarkowo-wagową. SOKHANSANJ S., LANG W. 1996: Prediction of kernel and bulk volume of wheat and canola during adsorption and desorption. J. Agric. Engng. Res., 63(2), 129 136. SOKHANSANJ S., MANI S., BI X., ZAINI P., TABIL L. 2005: Binderless pelletization of biomass. ASAE Paper No 056061. ASAE, 2950 Niles Road, St. Joseph, MI 49085-9659 USA. STOLARSKI M., SZCZUKOWSKI S., TWOR- KOWSKI J., KWIATKOWSKI J., GRZEL- CZYK M. 2005: Charakterystyka zrębków oraz peletów (granulatów) z biomasy wierzby i ślazowca jako paliwa. Problemy Inżynierii Rolniczej. 1(47), p. 13 22. SZOT B., STĘPNIEWSKI A. 2001: Niektóre właściwości fizyczne nasion polskich odmian soczewicy. Acta Agrophysica. 46, 187 196. Research work financed by Ministry of Science and Higher Education in 2007 2010 as the project PBZ-MNiSW-1/3/2006

28 A. Lisowski et al. Streszczenie: Gęstość i porowatość pociętego i zmielonego materiału z roślin energetycznych. Do badań użyto materiał ślazowca pensylwańskiego, miskanta olbrzymiego i róży wielokwiatowej po cięciu sieczkarnia polową przy prędkości zasilania 0,82 m/s i częstotliwości cięcia 83 Hz. Pocięty materiał zmielono w rozdrabniaczu bijakowym, wyposażonym w ekran perforowany o średnicy otworów 15 mm. Do wyznaczenia gęstości usypowej zastosowano pojemnik o objętości 10 dm 3, a do gęstości właściwej piknometr helowy. Stwierdzono, że różnice w gęstości usypowej, właściwej oraz porowatości są cechą gatunkową roślin energetycznych i wynikają ze sposobu rozdrobnienia materiału. Gęstość usypowa materiału pociętego była o 17% większa niż zmielonego oraz dla róży wielokwiatowej (240 kg/m 3 ) była ponad 3,5-krotnie większa niż miskanta olbrzymiego i ślazowca pensylwańskiego, którego wartości utworzyły grupę homogeniczną (68 kg/m 3 ). Gęstość właściwa materiału z roślin ślazowca pensylwańskiego, róży wielokwiatowej i miskanta olbrzymiego przeznaczonego do produkcji brykietów wynosiła odpowiednio 1207, 1162 i 640 kg/m 3. Porowatość rozdrobnionego materiału z roślin energetycznych zależała od gatunku rośliny i sposobu jego rozdrobnienia oraz wymiarów i kształtu cząstek. MS. received November 2009 Authors address: Aleksander Lisowski, Adam Świętochowski Wydział Inżynierii Produkcji Katedra Maszyn Rolniczych i Leśnych Szkoła Główna Gospodarstwa Wiejskiego ul. Nowoursynowska 164 02-787 Warszawa Andrzej Lenart, Katarzyna Szulc Wydział Nauk o Żywności Katedra Inżynierii Żywności i Organizacji Produkcji Szkoła Główna Gospodarstwa Wiejskiego ul. Nowoursynowska 164 02-787 Warszawa Poland e-mail: aleksander_lisowski@sggw.pl