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Characterization of pellets from mixing olive pomace and olive tree pruning
Renewable Energy 88 (2016) 185e191
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Characterization of pellets from mixing olive pomace and olive tree pruning M. Barbanera a, *, E. Lascaro a, V. Stanzione b, A. Esposito b, R. Altieri b, M. Bufacchi b a b
CRB e Biomass Research Centre, Via G.Duranti, 63, 06125 Perugia, Italy Italian National Research Council, Institute for Agriculture and Forest Systems in the Mediterranean, CNR-ISAFOM, Via Madonna Alta, 06128 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2015 Received in revised form 1 October 2015 Accepted 11 November 2015 Available online 5 December 2015
Olive pomace is an interesting agro-industrial byproduct that can be a potential raw material for densified biomass products. At first, 2-phase (2 PH) and 3-phase (3 PH) olive pomace pellets were analyzed in order to evaluate their quality in terms of the main parameters required by the European Standard EN 17225-6. The characterization of the pure pellets has shown important problems because of out of limits values of nitrogen, durability and copper in the two olive pomace. To improve the properties of olive pomace pellets, the possibility of manufacturing pellets by mixing olive pomace and olive tree pruning (PR) was investigated. Several blends at different weight ratios were analyzed in order to verify the effect of mixing on the pellet properties. It can be concluded that the physical properties of all mixtures are in compliance with the requirements of the standard. In particular, two best blends in terms of physical, chemical and mechanical characteristics were identified as becoming potential fuel for combustion and gasification applications: 75PR252 PH (75% pruning and 25% 2-phase pomace) and 50PR503 PH (50% pruning and 50% 3-phase pomace). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Olive pomace Pellet Durability Blends Olive tree pruning
1. Introduction The energy dependency derived from the use of fossil fuels and the increasing environmental concerns, have prompted the need to develop an energy system with a more renewable energy percentage and a reduction of greenhouse gas emissions [1]. Among renewable energy sources, biomass is particularly interesting because it can be used for heat, electricity, and transportation [2] and it can also be stored, unlike other renewable energy sources [3]. In particular a rapid growth of the wood pellet, production and consumption for power and heating has been registered in the last years. According to the European Bioenergy Outlook 2013 [4], the world wood pellet production in 2013 is equal about to 24.5 Mton, of which about 50% is produced in the European countries. In terms of wood pellet consumption, European countries are the biggest consumers with about 80% of the total world consumption. However, since only wood pellet from forestry residues have
* Corresponding author. E-mail address: [email protected] (M. Barbanera). http://dx.doi.org/10.1016/j.renene.2015.11.037 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
already successfully established technologies and markets, it seems to be interesting to focus on evaluating the pelletization of agroindustrial biomass, which is periodically collected and represents an interesting option for energy recovery, being an alternative to their disposal. In particular, most byproducts from agriculture or agroindustrial processes are characterized by low bulk density and, therefore, cannot be efficiently transported over long distances to areas where they can be effectively employed. There are also other important factors which make the energy use of agricultural residues difficult, as the local availability, since they are widespread over a relatively large area, and the costs of the treatments needed for their proper removal [1]. The olive oil industry is one of the agro-industrial activity that produces a significant amount of by-products. There are almost 900 million olive trees occupying over 10 million hectares worldwide, 98% of those are located in the Mediterranean Countries [5]. It can be assumed that 1 ha of olive tree produces about 2500 kg of olives and about 35 kg of olive pomace is obtained for 100 kg of treated olives [6]. Also olive, tree pruning are about 5% of the olive weight [7]. As regards the olive pomace, global annual production was
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estimated to approach 400 million tons of dry matter [8]. As regards the olive pomace, today two kinds of process are mainly used to separate oil from olive pastes: the three-phase centrifugation system, which produces a relatively dry solid waste named three-phase pomace and a large volume of olive mill waste waters, and the two-phase system in which the extraction water injection is carried out only in the final vertical centrifugation step, reducing by one-third on average the volume of liquid effluent [9]. The management of these residues is widely considered as a big concern, due to their potential environmental impact on soil and water [10]. Actually several options were tested for olive mill solid waste management. The main uses reported in Literature [11] are: animal feed, biogas production, extraction of useful materials, fertilizers. Densification by pelletizing can be an interesting option for increasing the bulk density of olive oil industry residues and their energetic employment. However, the high oil content of the pomace reduces significantly the quality of the pellet and makes difficult the densification during the pelletizing process. According to Kalyan and Morey [12] the higher content of fat or oil in the feed before densification should not exceed 6.5%, to not affect the pellet durability. Therefore, it could be necessary to blend the olive pomace with other wood biomass to obtain suitable characteristics for an ideal pelletization. Many authors investigated the use of different biomass blends as fuel for energy production: Liu et al. [13] studied the quality of pellets resulting from the mixing of bamboo and rice straw while Mediavilla et al. analyzed the pelletizing properties of blends including vine shoots and cork [14]. The behavior of domestic boilers fed with different types of agro-pellets was studied by lez et al. [16] carried out a comparative Verma et al. [15] and Gonza study on the performance of tomato, olive stone, and cardoon residues. However, to the best of our knowledge, the effect of mixing olive tree pruning and olive pomace on the pellet properties was not investigated in the Literature before. Therefore, the aim of the paper is to analyze the mechanical, chemical and physical properties of the pellets produced from olive tree pruning (PR), olive pomace, from two (2 PH) and three phase (3 PH) centrifugation system, and their blends at different mixture ratios. Furthermore, the quality parameters are discussed and compared with the technical standard EN 17225-6 [17] to check if the samples meet the requirements established for graded non-woody pellets. 2. Materials and methods 2.1. Raw materials Olive mill husk from two and three phase decanter, collected from local manufactures from Central Italy regions and olive tree pruning, collected from orchards located in central Italy (Toscana and Umbria) were used in this study. Exhausted olive mill husk was not considered because actually in Italy the production of olive husk oil is decreasing due to the very low quality of the oil, if compared to the more valuable extra virgin olive oil, derived from mechanical extraction. The initial moisture content of the raw materials was about 47.4%, 57.2% and 72.9% for PR, 2 PH and 3 PH, respectively. 2.2. Materials preparation Due to the high moisture content, raw materials were dried before grinding. Drying of 2 PH and 3 PH was performed in wintertime using a greenhouse provided by a heating system and
putting the raw materials in thin layers over heated beds, waiting for natural drying. To speed up the drying process 2 PH and 3 PH were periodically turned by hand (at least once per day). OP was previously mechanically chipped using an 8hp XL MTD wood chipper before drying that was performed in the same environment. When both raw materials reached moisture content below 20%, they were ground using an ultra centrifugal mill (mod. ZM200, Retsch) and sieved in order to obtain a particle size lower than 4 mm (diameter). The fraction size distribution of three representative 50 g samples for each material was analyzed by manual sieving on a stack of sieves arranged from the largest to the smallest opening. The sieving procedure terminated when no further notable passing took place. The sieve sizes 3150, 2800, 2000, 1400, 1000, 500 and 250 mm were used. The fraction size distribution of the milled material is reported in Table 1. Subsequently, the samples were subjected to an air drying process during the time necessary to attain an equilibrium moisture content (approximately 17%). In order to observe the behavior in densification of residues, as well as its characteristics, a classification of the samples was established, based on the percentage in weight of the components of each one of the mixtures. Therefore, the following blends were considered: 1002 PH: composed only by 2 PH; 1003 PH: composed only by 3 PH; 100 PR: composed only by PR; 75PR252 PH: 75% olive tree pruning and 25% two-phase pomace; 50PR502 PH: 50% olive tree pruning and 50% two-phase pomace; 25PR752 PH: 25% olive tree pruning and 75% two-phase pomace; 75PR253 PH: 75% olive tree pruning and 25% three-phase pomace; 50PR503 PH: 50% olive tree pruning and 50% three-phase pomace; 25PR753 PH: 25% olive tree pruning and 75% three-phase pomace.
olive olive olive olive olive olive
2.3. Densification of the raw materials The pellets were then manufactured using laboratory pellet mill (Betasystem srl), was used to perform pelletizing tests. This pelletizer has a nominal power of 1.7 kW. The feed material is pressed through open-ended cylindrical holes die made in the periphery of a series of rings. Two small rotating rolls push the feed material into the die holes from inside of the ring towards the
Table 1 Fraction size distribution of the milled materials. Fraction (mm)
>3.15 2.80÷3.15 2.00÷2.80 1.40÷2.00 1.00÷1.40 0.50÷1.00 0.25÷0.50 <0.25
Weight (%) of the sample Pr
2 PH
3 PH
0.00 0.12 1.45 11.15 17.56 48.54 14.16 7.02
0.00 0.05 1.98 8.27 24.95 47.94 9.97 6.84
0.00 0.08 1.78 7.58 22.56 50.17 12.34 5.50
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outside of the ring. The skin friction between the feed particles and the wall of the die resists the free flow of feed and thus the particles are compressed against each other inside the die to form pellets. The pellet mill parameters were set to press channel of 20.0 mm, and pellet diameter of 6.0 mm. The compression pressure for all the experimental runs was fixed at 100 kg/cm2. The temperature of biomass pellets exiting the die was between 75 C and 85 C. The increase in biomass temperature was due to frictional heating of the die during pelleting. Several runs of pellets were obtained to supply the pellet press continuously with the milled and sieved biomass. Once the process stabilizes, the temperature of the material increases due to the friction between the stationary flat die, the driven roller, and the residue. Once pellets came out from the pelletizer, they were air cooled at room temperature and representative samples were taken for the analyses described below. 2.4. Characterization of the pellets Moisture content was determined according to UNI EN 14774-2 [18] introducing the sample in a forced ventilation oven at 105 C for 24 h. The moisture content was then calculated by weighing the difference. The determination of the volatile matter and ash content was carried out according to UNI EN 15148 [19] and UNI EN 14775 [20] using a Thermogravimetric Analyzer TGA-701 LECO. The volatile loss and ash content were obtained by weigh and the fixed carbon percentage was determined by difference. The ultimate analysis (C, H, N, O) was performed according to UNI EN 15104 [21], using an Elemental Analyser (Vario Macro Cube, Elementar), whose operation is based on a thermal conductivity method. The net calorific value analysis was carried out according to UNI EN 14918 [22] employing an isoperibolic calorimeter (mod. Parr 6200). The device gives a gross calorific value of the sample; then the net value is obtained by calculation, from moisture and hydrogen content of the sample. Sulfur and chlorine contents were analyzed following UNI EN 15289 [23], using the isoperibolic calorimeter and Dionex ICS-90 IC system. Major (Na, Ca, Mg, Fe, K) and minor elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) determination was carried out according to respectively UNI EN 15290 [24] and UNI EN 15297 [25] with a previous digestion of the biomass sample in a closed container, using a mixture of acids (HNO3, SigmaeAldrich; H2O2, Panreac) and a microwave oven (mod. Ethos One, Milestone). The mineralization product is then analyzed by inductively coupled plasma mass spectrometry (ICPMS mod. 7500ce, Agilent). Acid-insoluble lignin was determined after a two-stage sulfuric acid hydrolysis as recommended NREL/TP-510-42618 method [26]. Oil content was determined using Soxhlet extraction with nhexane, according to the ISO 659 standard [27]. The mechanical durability of the sample was determined according to UNI EN 15210-1 [28] using a mechanical durability tester (mod. Holmen Lignotester). The durability is expressed as the percent ratio of the pellets retained on a sieve after tumbling in reference to the mass before tumbling. The test was conducted on 500 g samples of pellets tumbled at 50 rpm for 500 rotations. A 3.15 mm mesh sieve was used to collect crushed pellets after tumbling. The bulk density was calculated according to UNI EN 15103 [29] by the mass material contained in a standard container of 2 L volume for the milled material. The pellet length and diameter of a representative sample of
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pellets were measured according to ISO 17829 [30] by using an electronic caliper. To get significant mean values, all analyses were performed in triplicate; data reported in the table represents the mean values with difference among replicates below 5%. 3. Results and discussion After the pelletization process, the samples made of pure raw materials (1002 PH, 1003 PH, 100 PR) were analyzed in order to evaluate the potential advantages arising from the pellet blends. First of all the results obtained were compared with the guidelines established for graded non-woody pellets, summarized in Table 2 according to the European Standard EN 17225-6 [17]. Both the samples 1002 PH and 1003 PH do not satisfy the requirements established by B class due to a higher copper content than 20 mg/kg and a lower mechanical durability than 96% for the first one and a higher nitrogen content than 2% for the other. Considering the results obtained for the sample 100 PR, it is clear that mixing the olive tree pruning with olive pomace could improve the value of these parameters, meeting the requirements of the Technical Standard. Two important parameters, affecting strength and durability of the pellets, not required by EN 17225-6 are lignin and oil content. In particular, an oil content in biomass higher than 6.5% results in lower pellet durability [12], because oil has a low viscosity and this allows the fibers to easier slip if they are connected through mechanical interlocking [31]. From the other hand, the lignin in biomass acts as a binder and, during the pelletization process, it softens and helps the binding process [12]. Averaged oil content (dry basis) was respectively 10.3% for 1002 PH and 4.4% for 1003 PH, while the averaged lignin content (dry basis) was 34.1% for 100 PR, 53.0% for 1002 PH and 51.9% for 1003 PH. Furthermore, it seemed to be interesting to analyzed the minerals composition of the pure pellets (Table 3) both because they are the main ash forming elements of biomass materials [32] and because but they can cause serious effect such as slagging and fouling to the thermal system during combustion or gasification process [33,34]. In particular, the difference in the ash content of the pellets was mainly due to these element variations. However, high calcium content can have a positive effect during combustion of biomass because it can sustain the thermal operation and thus may prevent agglomeration phenomenon [35]. At this regard, the highest value for 100 PR (17148.4 mg/kg on dry basis) allows to state that mixing olive tree pruning with olive pomace could optimize the combustion properties. In the following paragraphs, the influence of the blending ratio on the main chemical and physical parameters was investigated. 3.1. Dimension The geometric dimensions of pellet are important properties to assess the solid fuel efficiency during the combustion; diameter and length are a specification in the quality Technical Standard [17]. It has been showed [36] that size particles affects burning rate and ignition speed during combustion, in particular, smallest particles have the best ignition speeds and burning performance. In order to determine the variability of length and diameter in the different pellet blends, measurements of these two variables were carried out and the mean values of ten replicates for each kind of pellet were reported in Fig. 1 and Fig. 2. The mean diameter ranges more for blends with 3 PH olive pomace than 2 PH ones; all the obtained values are in compliance to European Standard EN 17225-6 [17].
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Table 2 Physical and chemical parameters of the pure pellets. Parameter
EN 17225-6 classification A
Size (diameter and length) (mm) a
Moisture content (%, wb) Ash content (%, db)b Mechanical durability (%, wb) Net calorific value (MJ/kg, wb) Bulk density (kg/m3, wb) Nitrogen (%, db) Sulphur (%, db) Chlorine (%, db) Arsenic (mg/kg, db) Cadmium (mg/kg, db) Chromium (mg/kg, db) Copper (mg/kg, db) Lead (mg/kg, db) Mercury (mg/kg, db) Nickel (mg/kg, db) Zinc (mg/kg, db) a b
Pellet B
D06 to D25, D ± 1; 3.15 < L < 40 (from D06 to D10) 3.15 < L < 50 (from D12 to D25) M12 12 M15 15 A5.0 5 A10.0 10 DU97.5 97.5 DU96.0 96.0 Q14.1 14.1 Q13.2 13.2 BD600 600 N1.5 1.5 N2.0 2.0 S0.20 0.20 S0.20 0.20 Cl0.20 0.20 Cl0.30 0.30 1 0.5 50 20 10 0.1 10 100
100 PR
1002 PH
1003 PH
D6.1 e L23
D6.1 e L21
D6.1 e L22.5
2.9 96.8 17.8 644.1 1.0 0.15 0.02 <0.1 <0.05 2.1 4.7 0.1 <0.05 0.7 15
4.3 92.6 20.4 653.6 2.0 0.11 0.07 <0.1 <0.05 0.7 21.3 2.2 <0.05 2.5 8
2.4 97.5 19.6 622.5 2.3 0.13 0.05 <0.1 <0.05 1.2 14.2 0.2 <0.05 1.6 10
wb: wet basis. db: dry basis.
Table 3 Minerals content of the pure pellets.
Na (mg/kg, db) Ca (mg/kg, db) Mg (mg/kg, db) K (mg/kg, db) Fe (mg/kg, db)
100 PR
1002 PH
1003 PH
92.1 17148.4 1189.7 11366.2 54.0
214.3 1693.0 808.1 28433.9 302.3
103.8 3218.7 511.1 16020.2 87.4
Fig. 2. Pellet diameter and length in the blends of pruning and three-phase pomace.
Fig. 1. Pellet diameter and length in the blends of pruning and two-phase pomace.
3.2. Bulk density The bulk density is an important parameter to assess pellet quality [31], and also the Technical Standard [17] regulates its range of variability to establish the quality class. It is defined as the mass of particles of the material divided by the total volume they occupy that includes particle volume, interparticle volume, and internal pore volume. The storage and transport issues are related to pellet bulk density because of the economic costs rise if the bulk density is too low; higher this physical parameter allows to enhance the transport efficiency and facilities and reduce the costs. The mean bulk density trend of the two types of pellet blends is shown in Fig. 3. The black line indicates the minimum threshold
according to the requirements for pellet quality [17], and all the blends result in compliance with this regulation. It can be noted that for blends of pruning and two-phases pomace (black bars), the bulk density decreases with increasing of percentage of pruning in the blend from 656.3 kg/m3 to 644.1 kg/ m3 of 100 PR pellet; the opposite trend is typical of the 3-phase pomace and pruning blends (grey bars), the bulk density ranges from 622.5 kg/m3 of 100 PH to 644.1 kg/m3 of 100 PR pellet. This difference is related to the initial moisture of two different pomace and their storage before pelletization: low moisture content and long time storage of 2 PH pomace involve higher bulk density values [37]. 3.3. Mechanical durability Durability is the prevalent form of measurement and expression of pellet quality; this property is influenced by different factors, in particular, raw material composition and dimension. In Fig. 4 the two different trends of pellet blends durability are reported, and the black line indicated the minimum threshold value according to EN 17225-6 [17]. The mechanical durability is lower in 2 PH pomace than 3 PH due to the higher content of oil and lignin in the first one; in fact, several studies [12] have tested the negative influence of fat/oil and
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Fig. 3. Bulk density trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
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Fig. 5. Ash content trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
In this work, the net heating value was used to express the energetic feature of different pellet blends because of the technical standard referring to this parameter in its regulation. In Fig. 6 the net calorific values of different pellet blends are shown; as we can see, thanks to high heating values of pomace, all the pellet blends are in compliance to EN 17225-6 [17], that recommends at least 14.4 MJ/kg to consider a good quality of pellet. Net heating values are strongly affected by raw material composition; in fact the higher oil content in 2 PH pomace reflects an important energetic value of the 2 PH pellet blends. Nevertheless, a little difference from 2 PH and 3 PH pellet blends is clear because of 3 PH blends are characterized by a lower ash content that, as described above, influences the heating value. Fig. 4. Durability trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
lignin on pellet durability. Therefore, the increase of the percentage of pruning in the 2 PH pellet blends allows to obtain a maximum value of durability of 96.2% for 75PR25 PH blend that which it is the only one that exceeds the value required by the International Standard of pellet class quality B. The 3 PH pellet blends, instead, hold the highest values of durability never less 96.8% because of 3 PH pomace have a little content of oil (4.4%) and also a lower lignin percentage than 2 PH. Also the particle distribution in the raw material influences the mechanical durability of the pellet blends; as we have seen, 3 PH pomace presents a larger concentration of fine particles than 2 PH and pruning and this fact increases the corresponding pellet durability.
3.5. Relationship between the main physical and chemical properties To give some indications about the best pellet blends characteristics, the relationship between bulk density, durability and energetic density is reported in Fig. 7 and Fig. 8 for the two series of pellet blends. The energetic density is an important factor to consider, mostly in relation to the sizing of the storage place of the pellets. This parameter is the ratio between the heating value and the bulk density of the pellet. In the figures is represented only a black dashed line corresponding to the durability minimum threshold (96%) according to EN 17225-6 [17] because the requirements of the bulk density are observed for all the blends. For 2 PH pellet blends (Fig. 7) only 75PR252 PH sample presents
3.4. Ash content and calorific value The ash content influences the energetic quality of the pellet; therefore, the energetic value is greatly related to this parameter. In Fig. 5 the variability of ash content in the different pellet blends is shown; the ash percentage in the 2 PH pomace pellet is nearly double (4.3%) than 3 PH one (2.4%), due to the extremely high values of ash-forming elements as Fe, Mg, K. Nevertheless, all the pellets blends are in compliance with the Technical Standard [17] since the ash content is less than 5% on a dry basis, the maximum acceptable value. The main parameter to define the energetic quality as the fuel of the pellets is the heating value, as the maximum amount of energy that can be gained by converting completely a unit mass of fuel in standard conditions.
Fig. 6. Net calorific value for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
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Fig. 7. Relationship between durability, bulk density and energetic density for pellet blends of pruning and two-phase pomace.
Fig. 8. Relationship between durability, bulk density and energetic density for pellet blends of pruning and three-phase pomace.
a durability value >96.0% and then is in compliance with the Technical Standard; instead, 3 PH pellet blends (Fig. 8) are characterized by higher values of durability and so the 50PR503 PH seems to have the most appropriate behavior in terms of physical and energetic parameters due to its high energetic density and durability and a medium value of bulk density better than 25PR753 PH. 4. Conclusions The analysis of pellet blends composed by olive pomace and olive tree pruning was carried out to investigate the improvements of the physical and chemical properties respect to pure olive pomace pellets. The pure 2-phase and 3-phase olive pomace pellets are not in compliance with European Standard of pellet quality and then several mixtures with olive tree pruning were produced and
analyzed in the most important physical, chemical and mechanical features. The results show that, adding olive tree pruning, the chemical composition of pellet blends respects the standard requirements in terms of mechanical durability and nitrogen and copper content; also the bulk density is enhanced allowing a reduction of transport and storage economic cost. Considering all the quality pellet parameters, two pellet blends have the best behavior as 75PR252 PH among the 2-phase mixtures and 50PR503 PH between the 3-phase blends, and can be used for combustion and gasification processes. Acknowledgments This research received support by the European Commission,Seventh Framework Programme for European Research and Technological Development (2012e2015) within the project “FFW”
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“Liquid and gas Fischer-Tropsch fuel production from olive industry waste: fuel from waste”. Grant Agreement n 308733. References [1] A. Garcia-Maraver, M.L. Rodríguez, F. Serrano-Bernardo, F. Diaz, M. Zamorano, Factors affecting the quality of pellets made from residual biomass of olive trees, Fuel Process Technol. 129 (2015) 1e7. [2] J. Zuwała, Life cycle approach for energy and environmental analysis of biomass and coal co-firing in CHP plant with backpressure turbine, J. Clean. Prod. 35 (2012) 164e175. [3] C. Gokcol, B. Dursun, B. Alboyaci, E. Sunan, Importance of biomass energy as alternative to other sources in Turkey, Energy Policy 37 (2009) 424e431. [4] AEBIOM, European Bioenergy Outlook, 2013. [5] M. Sesli, E.D. Yegenoglu, RAPD-PCR analysis of cultured type olives in Turkey, Afr. J. Biotechnol. 8 (2009) 3418e3423. [6] FAOSTAT. 2014. Available at:; April 23, 2015. [7] J.M. Romero-García, L. Nino, C. Martínez-Patino, C. Alvarez, E. Castro, M.J. Negro, Biorefinery based on olive biomass. State of the art and future trends, Bioresour. Technol. 159 (2014) 421e432. [8] A. El Asli, A. Qatibi, Ethanol production from olive cake biomass substrate, Biotechnol. Bioprocess Eng. 14 (2009) 118e122. [9] N. Kalogeropoulos, A.C. Kaliora, A. Artemiou, I. Giogios, Composition, volatile profiles and functional properties of virgin olive oils produced by two-phase vs three-phase centrifugal decanters, LWT e Food Sci. Technol. 58 (1) (2014) 272e279. [10] S. Dermeche, M. Nadour, C. Larroche, F. Moulti-Mati, P. Michaud, Olive mill wastes: biochemical characterizations and valorization strategies, Process Biochem. 48 (2013) 1532e1552. [11] H.A. Qdais, H. Alshraideh, Selection of management option for solid waste from olive oil industry using the analytical hierarchy process, J. Mater Cycles Waste Manag. (2014) 1e9. [12] N. Kaliyan, R.V. Morey, Factors affecting strength and durability of densified biomass products, Biomass Bioenergy 33 (2009) 337e359. [13] Z. Liu, X. Liu, B. Fei, Z. Jiang, Z. Cai, Y. Yu, The properties of pellets from mixing bamboo and rice straw, Renew. Energy 55 (2013) 1e5. ndez, L.S. Esteban, Optimization of pelletisation and [14] I. Medialvilla, M.J. Ferna combustion in a boiler of 17.5 kWth for vine shoots and industrial cork residue, Fuel Process Technol. 90 (2009) 621e628. [15] V.K. Verma, S. Bram, F. Delattin, P. Laha, I. Vandendael, A. Hubin, et al., Agropellets for domestic heating boilers: standard laboratory and real life performance, Appl. Energy 90 (1) (2011) 17e23. lez-García, A. Ramiro, J. Gonza lez, E. Sabio, J. Gana n, [16] J.F. Gonz alez, C.M. Gonza M.A. Rodriguez, Combustion optimisation of biomass residue pellets for domestic heating with a mural boiler, Biomass Bioenergy 27 (2004) 145e154. [17] EN ISO 17225-6:2014, Solid Biofuels Fuel Specifications and Classes. Part 6: Graded Non-woody Pellets, National Standards Authority of Ireland, Ireland, UK, 2014.
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[18] UNI EN 14774e2, Solid Biofuels. Determination of Moisture Content e Oven Dry Method e Part 2: Total Moisture e Simplified Method, 2010. [19] UNI EN 15148, Solid Biofuels. Determination of the Content of Volatile Matter, 2010. [20] UNI EN 14775, Solid Biofuels. Determination of Ash Content, 2010. [21] UNI EN 15104, Solid Biofuels. Determination of Total Content of Carbon, Hydrogen and Nitrogen - Instrumental Methods, 2011. [22] UNI EN 14918, Solid Biofuels. Method for the Determination of Calorific Value, 2010. [23] UNI EN 15289, Solid Biofuels. Determination of Total Content of Sulfur and Chlorine, 2011. [24] UNI EN 15290, Solid Biofuels. Determination of Major Elements, 2011. [25] UNI EN 15297, Solid Biofuels. Determination of Minor Elements e as, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, V and Zn, 2011. [26] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter, D. Templeton, D. Crocker, Determination of Structural Carbohydrates and Lignin in Biomass, Technical report NREL/TP-510e42618, National Renewable Energy Laboratory, Golden, 2012. [27] International Organization for Standardization, ISO 659:1998. Oilseed e Determination of Oil Content (Reference Method), ISO, Geneva, 1998. [28] UNI EN 15210e1, Solid Fuels. Determination of Mechanical Durability of Pellets and Briquettes e Part 1-Pellets, 2010. [29] UNI EN 15103, Solid Biofuels. Determination of Bulk Density, 2010. [30] ISO/DIS 17829, Solid Biofuels. Determination of Length and Diameter of Pellets. 2014, 2014. [31] M. Arshadi, R. Cref, P. Geladi, S. Dahlqvist, T. Lestander, The influence of raw material characteristics on the industrial pelletizing process and pellet quality, Fuel Process Technol. 89 (2008) 1442e1447. [32] J. Werkelin, D. Lindberg, D. Bostrom, B.-J. Skrifvars, M. Hupa, Ash-forming elements in four Scandinavian wood species. Part. 3. Combustion of five spruce samples, Biomass Bioenerg. 35 (2011) 725e733. [33] N.Y. Harun, M.T. Afzal, Chemical and Mechanical Properties of Pellets Made from Agricultural and Woody Biomass Blends, ASABE Paper No. 141908650, St. Joseph, Michigan: American Society of Agricultural and Biological Engineers, 2014. [34] M. Filipponi, G. Cavalaglio, B. Castellani, L. Brinchi, Second Generation Biodiesel via Fischer-tropsch Synthesis: BioDie2 Research Project and Preliminary Tests, 20th European Biomass Conference and Exhibition, June 18e22, Milan, Italy, 2012. [35] S.V. Vassilev, D. Baxter, C.G. Vassileva, An overview of the behaviour of biomass during combustion: part I. Phase-mineral transformations of organic and inorganic matter, Fuel 112 (2013) 391e449. [36] C. Ryu, Y.B. Yang, A. Khor, N.E. Yates, V.N. Sharifi, J. Swithenbank, Effect of fuel properties on biomass combustion: part I. Experiments e fuel type, equivalence ratio and particle size, Fuel 85 (2002) 1039e1046. € stro € m, T.A. Lestander, Effect of biomaterial [37] R. Samuelsson, M. Thyrel, M. Sjo characteristics on pelletizing properties and biofuel pellet quality, Fuel Process Technol. 90 (2009) 1129e1134.
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Characterization of pellets from mixing olive pomace and olive tree pruning M. Barbanera a, *, E. Lascaro a, V. Stanzione b, A. Esposito b, R. Altieri b, M. Bufacchi b a b
CRB e Biomass Research Centre, Via G.Duranti, 63, 06125 Perugia, Italy Italian National Research Council, Institute for Agriculture and Forest Systems in the Mediterranean, CNR-ISAFOM, Via Madonna Alta, 06128 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 June 2015 Received in revised form 1 October 2015 Accepted 11 November 2015 Available online 5 December 2015
Olive pomace is an interesting agro-industrial byproduct that can be a potential raw material for densified biomass products. At first, 2-phase (2 PH) and 3-phase (3 PH) olive pomace pellets were analyzed in order to evaluate their quality in terms of the main parameters required by the European Standard EN 17225-6. The characterization of the pure pellets has shown important problems because of out of limits values of nitrogen, durability and copper in the two olive pomace. To improve the properties of olive pomace pellets, the possibility of manufacturing pellets by mixing olive pomace and olive tree pruning (PR) was investigated. Several blends at different weight ratios were analyzed in order to verify the effect of mixing on the pellet properties. It can be concluded that the physical properties of all mixtures are in compliance with the requirements of the standard. In particular, two best blends in terms of physical, chemical and mechanical characteristics were identified as becoming potential fuel for combustion and gasification applications: 75PR252 PH (75% pruning and 25% 2-phase pomace) and 50PR503 PH (50% pruning and 50% 3-phase pomace). © 2015 Elsevier Ltd. All rights reserved.
Keywords: Olive pomace Pellet Durability Blends Olive tree pruning
1. Introduction The energy dependency derived from the use of fossil fuels and the increasing environmental concerns, have prompted the need to develop an energy system with a more renewable energy percentage and a reduction of greenhouse gas emissions [1]. Among renewable energy sources, biomass is particularly interesting because it can be used for heat, electricity, and transportation [2] and it can also be stored, unlike other renewable energy sources [3]. In particular a rapid growth of the wood pellet, production and consumption for power and heating has been registered in the last years. According to the European Bioenergy Outlook 2013 [4], the world wood pellet production in 2013 is equal about to 24.5 Mton, of which about 50% is produced in the European countries. In terms of wood pellet consumption, European countries are the biggest consumers with about 80% of the total world consumption. However, since only wood pellet from forestry residues have
* Corresponding author. E-mail address: [email protected] (M. Barbanera). http://dx.doi.org/10.1016/j.renene.2015.11.037 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
already successfully established technologies and markets, it seems to be interesting to focus on evaluating the pelletization of agroindustrial biomass, which is periodically collected and represents an interesting option for energy recovery, being an alternative to their disposal. In particular, most byproducts from agriculture or agroindustrial processes are characterized by low bulk density and, therefore, cannot be efficiently transported over long distances to areas where they can be effectively employed. There are also other important factors which make the energy use of agricultural residues difficult, as the local availability, since they are widespread over a relatively large area, and the costs of the treatments needed for their proper removal [1]. The olive oil industry is one of the agro-industrial activity that produces a significant amount of by-products. There are almost 900 million olive trees occupying over 10 million hectares worldwide, 98% of those are located in the Mediterranean Countries [5]. It can be assumed that 1 ha of olive tree produces about 2500 kg of olives and about 35 kg of olive pomace is obtained for 100 kg of treated olives [6]. Also olive, tree pruning are about 5% of the olive weight [7]. As regards the olive pomace, global annual production was
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estimated to approach 400 million tons of dry matter [8]. As regards the olive pomace, today two kinds of process are mainly used to separate oil from olive pastes: the three-phase centrifugation system, which produces a relatively dry solid waste named three-phase pomace and a large volume of olive mill waste waters, and the two-phase system in which the extraction water injection is carried out only in the final vertical centrifugation step, reducing by one-third on average the volume of liquid effluent [9]. The management of these residues is widely considered as a big concern, due to their potential environmental impact on soil and water [10]. Actually several options were tested for olive mill solid waste management. The main uses reported in Literature [11] are: animal feed, biogas production, extraction of useful materials, fertilizers. Densification by pelletizing can be an interesting option for increasing the bulk density of olive oil industry residues and their energetic employment. However, the high oil content of the pomace reduces significantly the quality of the pellet and makes difficult the densification during the pelletizing process. According to Kalyan and Morey [12] the higher content of fat or oil in the feed before densification should not exceed 6.5%, to not affect the pellet durability. Therefore, it could be necessary to blend the olive pomace with other wood biomass to obtain suitable characteristics for an ideal pelletization. Many authors investigated the use of different biomass blends as fuel for energy production: Liu et al. [13] studied the quality of pellets resulting from the mixing of bamboo and rice straw while Mediavilla et al. analyzed the pelletizing properties of blends including vine shoots and cork [14]. The behavior of domestic boilers fed with different types of agro-pellets was studied by lez et al. [16] carried out a comparative Verma et al. [15] and Gonza study on the performance of tomato, olive stone, and cardoon residues. However, to the best of our knowledge, the effect of mixing olive tree pruning and olive pomace on the pellet properties was not investigated in the Literature before. Therefore, the aim of the paper is to analyze the mechanical, chemical and physical properties of the pellets produced from olive tree pruning (PR), olive pomace, from two (2 PH) and three phase (3 PH) centrifugation system, and their blends at different mixture ratios. Furthermore, the quality parameters are discussed and compared with the technical standard EN 17225-6 [17] to check if the samples meet the requirements established for graded non-woody pellets. 2. Materials and methods 2.1. Raw materials Olive mill husk from two and three phase decanter, collected from local manufactures from Central Italy regions and olive tree pruning, collected from orchards located in central Italy (Toscana and Umbria) were used in this study. Exhausted olive mill husk was not considered because actually in Italy the production of olive husk oil is decreasing due to the very low quality of the oil, if compared to the more valuable extra virgin olive oil, derived from mechanical extraction. The initial moisture content of the raw materials was about 47.4%, 57.2% and 72.9% for PR, 2 PH and 3 PH, respectively. 2.2. Materials preparation Due to the high moisture content, raw materials were dried before grinding. Drying of 2 PH and 3 PH was performed in wintertime using a greenhouse provided by a heating system and
putting the raw materials in thin layers over heated beds, waiting for natural drying. To speed up the drying process 2 PH and 3 PH were periodically turned by hand (at least once per day). OP was previously mechanically chipped using an 8hp XL MTD wood chipper before drying that was performed in the same environment. When both raw materials reached moisture content below 20%, they were ground using an ultra centrifugal mill (mod. ZM200, Retsch) and sieved in order to obtain a particle size lower than 4 mm (diameter). The fraction size distribution of three representative 50 g samples for each material was analyzed by manual sieving on a stack of sieves arranged from the largest to the smallest opening. The sieving procedure terminated when no further notable passing took place. The sieve sizes 3150, 2800, 2000, 1400, 1000, 500 and 250 mm were used. The fraction size distribution of the milled material is reported in Table 1. Subsequently, the samples were subjected to an air drying process during the time necessary to attain an equilibrium moisture content (approximately 17%). In order to observe the behavior in densification of residues, as well as its characteristics, a classification of the samples was established, based on the percentage in weight of the components of each one of the mixtures. Therefore, the following blends were considered: 1002 PH: composed only by 2 PH; 1003 PH: composed only by 3 PH; 100 PR: composed only by PR; 75PR252 PH: 75% olive tree pruning and 25% two-phase pomace; 50PR502 PH: 50% olive tree pruning and 50% two-phase pomace; 25PR752 PH: 25% olive tree pruning and 75% two-phase pomace; 75PR253 PH: 75% olive tree pruning and 25% three-phase pomace; 50PR503 PH: 50% olive tree pruning and 50% three-phase pomace; 25PR753 PH: 25% olive tree pruning and 75% three-phase pomace.
olive olive olive olive olive olive
2.3. Densification of the raw materials The pellets were then manufactured using laboratory pellet mill (Betasystem srl), was used to perform pelletizing tests. This pelletizer has a nominal power of 1.7 kW. The feed material is pressed through open-ended cylindrical holes die made in the periphery of a series of rings. Two small rotating rolls push the feed material into the die holes from inside of the ring towards the
Table 1 Fraction size distribution of the milled materials. Fraction (mm)
>3.15 2.80÷3.15 2.00÷2.80 1.40÷2.00 1.00÷1.40 0.50÷1.00 0.25÷0.50 <0.25
Weight (%) of the sample Pr
2 PH
3 PH
0.00 0.12 1.45 11.15 17.56 48.54 14.16 7.02
0.00 0.05 1.98 8.27 24.95 47.94 9.97 6.84
0.00 0.08 1.78 7.58 22.56 50.17 12.34 5.50
M. Barbanera et al. / Renewable Energy 88 (2016) 185e191
outside of the ring. The skin friction between the feed particles and the wall of the die resists the free flow of feed and thus the particles are compressed against each other inside the die to form pellets. The pellet mill parameters were set to press channel of 20.0 mm, and pellet diameter of 6.0 mm. The compression pressure for all the experimental runs was fixed at 100 kg/cm2. The temperature of biomass pellets exiting the die was between 75 C and 85 C. The increase in biomass temperature was due to frictional heating of the die during pelleting. Several runs of pellets were obtained to supply the pellet press continuously with the milled and sieved biomass. Once the process stabilizes, the temperature of the material increases due to the friction between the stationary flat die, the driven roller, and the residue. Once pellets came out from the pelletizer, they were air cooled at room temperature and representative samples were taken for the analyses described below. 2.4. Characterization of the pellets Moisture content was determined according to UNI EN 14774-2 [18] introducing the sample in a forced ventilation oven at 105 C for 24 h. The moisture content was then calculated by weighing the difference. The determination of the volatile matter and ash content was carried out according to UNI EN 15148 [19] and UNI EN 14775 [20] using a Thermogravimetric Analyzer TGA-701 LECO. The volatile loss and ash content were obtained by weigh and the fixed carbon percentage was determined by difference. The ultimate analysis (C, H, N, O) was performed according to UNI EN 15104 [21], using an Elemental Analyser (Vario Macro Cube, Elementar), whose operation is based on a thermal conductivity method. The net calorific value analysis was carried out according to UNI EN 14918 [22] employing an isoperibolic calorimeter (mod. Parr 6200). The device gives a gross calorific value of the sample; then the net value is obtained by calculation, from moisture and hydrogen content of the sample. Sulfur and chlorine contents were analyzed following UNI EN 15289 [23], using the isoperibolic calorimeter and Dionex ICS-90 IC system. Major (Na, Ca, Mg, Fe, K) and minor elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) determination was carried out according to respectively UNI EN 15290 [24] and UNI EN 15297 [25] with a previous digestion of the biomass sample in a closed container, using a mixture of acids (HNO3, SigmaeAldrich; H2O2, Panreac) and a microwave oven (mod. Ethos One, Milestone). The mineralization product is then analyzed by inductively coupled plasma mass spectrometry (ICPMS mod. 7500ce, Agilent). Acid-insoluble lignin was determined after a two-stage sulfuric acid hydrolysis as recommended NREL/TP-510-42618 method [26]. Oil content was determined using Soxhlet extraction with nhexane, according to the ISO 659 standard [27]. The mechanical durability of the sample was determined according to UNI EN 15210-1 [28] using a mechanical durability tester (mod. Holmen Lignotester). The durability is expressed as the percent ratio of the pellets retained on a sieve after tumbling in reference to the mass before tumbling. The test was conducted on 500 g samples of pellets tumbled at 50 rpm for 500 rotations. A 3.15 mm mesh sieve was used to collect crushed pellets after tumbling. The bulk density was calculated according to UNI EN 15103 [29] by the mass material contained in a standard container of 2 L volume for the milled material. The pellet length and diameter of a representative sample of
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pellets were measured according to ISO 17829 [30] by using an electronic caliper. To get significant mean values, all analyses were performed in triplicate; data reported in the table represents the mean values with difference among replicates below 5%. 3. Results and discussion After the pelletization process, the samples made of pure raw materials (1002 PH, 1003 PH, 100 PR) were analyzed in order to evaluate the potential advantages arising from the pellet blends. First of all the results obtained were compared with the guidelines established for graded non-woody pellets, summarized in Table 2 according to the European Standard EN 17225-6 [17]. Both the samples 1002 PH and 1003 PH do not satisfy the requirements established by B class due to a higher copper content than 20 mg/kg and a lower mechanical durability than 96% for the first one and a higher nitrogen content than 2% for the other. Considering the results obtained for the sample 100 PR, it is clear that mixing the olive tree pruning with olive pomace could improve the value of these parameters, meeting the requirements of the Technical Standard. Two important parameters, affecting strength and durability of the pellets, not required by EN 17225-6 are lignin and oil content. In particular, an oil content in biomass higher than 6.5% results in lower pellet durability [12], because oil has a low viscosity and this allows the fibers to easier slip if they are connected through mechanical interlocking [31]. From the other hand, the lignin in biomass acts as a binder and, during the pelletization process, it softens and helps the binding process [12]. Averaged oil content (dry basis) was respectively 10.3% for 1002 PH and 4.4% for 1003 PH, while the averaged lignin content (dry basis) was 34.1% for 100 PR, 53.0% for 1002 PH and 51.9% for 1003 PH. Furthermore, it seemed to be interesting to analyzed the minerals composition of the pure pellets (Table 3) both because they are the main ash forming elements of biomass materials [32] and because but they can cause serious effect such as slagging and fouling to the thermal system during combustion or gasification process [33,34]. In particular, the difference in the ash content of the pellets was mainly due to these element variations. However, high calcium content can have a positive effect during combustion of biomass because it can sustain the thermal operation and thus may prevent agglomeration phenomenon [35]. At this regard, the highest value for 100 PR (17148.4 mg/kg on dry basis) allows to state that mixing olive tree pruning with olive pomace could optimize the combustion properties. In the following paragraphs, the influence of the blending ratio on the main chemical and physical parameters was investigated. 3.1. Dimension The geometric dimensions of pellet are important properties to assess the solid fuel efficiency during the combustion; diameter and length are a specification in the quality Technical Standard [17]. It has been showed [36] that size particles affects burning rate and ignition speed during combustion, in particular, smallest particles have the best ignition speeds and burning performance. In order to determine the variability of length and diameter in the different pellet blends, measurements of these two variables were carried out and the mean values of ten replicates for each kind of pellet were reported in Fig. 1 and Fig. 2. The mean diameter ranges more for blends with 3 PH olive pomace than 2 PH ones; all the obtained values are in compliance to European Standard EN 17225-6 [17].
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Table 2 Physical and chemical parameters of the pure pellets. Parameter
EN 17225-6 classification A
Size (diameter and length) (mm) a
Moisture content (%, wb) Ash content (%, db)b Mechanical durability (%, wb) Net calorific value (MJ/kg, wb) Bulk density (kg/m3, wb) Nitrogen (%, db) Sulphur (%, db) Chlorine (%, db) Arsenic (mg/kg, db) Cadmium (mg/kg, db) Chromium (mg/kg, db) Copper (mg/kg, db) Lead (mg/kg, db) Mercury (mg/kg, db) Nickel (mg/kg, db) Zinc (mg/kg, db) a b
Pellet B
D06 to D25, D ± 1; 3.15 < L < 40 (from D06 to D10) 3.15 < L < 50 (from D12 to D25) M12 12 M15 15 A5.0 5 A10.0 10 DU97.5 97.5 DU96.0 96.0 Q14.1 14.1 Q13.2 13.2 BD600 600 N1.5 1.5 N2.0 2.0 S0.20 0.20 S0.20 0.20 Cl0.20 0.20 Cl0.30 0.30 1 0.5 50 20 10 0.1 10 100
100 PR
1002 PH
1003 PH
D6.1 e L23
D6.1 e L21
D6.1 e L22.5
2.9 96.8 17.8 644.1 1.0 0.15 0.02 <0.1 <0.05 2.1 4.7 0.1 <0.05 0.7 15
4.3 92.6 20.4 653.6 2.0 0.11 0.07 <0.1 <0.05 0.7 21.3 2.2 <0.05 2.5 8
2.4 97.5 19.6 622.5 2.3 0.13 0.05 <0.1 <0.05 1.2 14.2 0.2 <0.05 1.6 10
wb: wet basis. db: dry basis.
Table 3 Minerals content of the pure pellets.
Na (mg/kg, db) Ca (mg/kg, db) Mg (mg/kg, db) K (mg/kg, db) Fe (mg/kg, db)
100 PR
1002 PH
1003 PH
92.1 17148.4 1189.7 11366.2 54.0
214.3 1693.0 808.1 28433.9 302.3
103.8 3218.7 511.1 16020.2 87.4
Fig. 2. Pellet diameter and length in the blends of pruning and three-phase pomace.
Fig. 1. Pellet diameter and length in the blends of pruning and two-phase pomace.
3.2. Bulk density The bulk density is an important parameter to assess pellet quality [31], and also the Technical Standard [17] regulates its range of variability to establish the quality class. It is defined as the mass of particles of the material divided by the total volume they occupy that includes particle volume, interparticle volume, and internal pore volume. The storage and transport issues are related to pellet bulk density because of the economic costs rise if the bulk density is too low; higher this physical parameter allows to enhance the transport efficiency and facilities and reduce the costs. The mean bulk density trend of the two types of pellet blends is shown in Fig. 3. The black line indicates the minimum threshold
according to the requirements for pellet quality [17], and all the blends result in compliance with this regulation. It can be noted that for blends of pruning and two-phases pomace (black bars), the bulk density decreases with increasing of percentage of pruning in the blend from 656.3 kg/m3 to 644.1 kg/ m3 of 100 PR pellet; the opposite trend is typical of the 3-phase pomace and pruning blends (grey bars), the bulk density ranges from 622.5 kg/m3 of 100 PH to 644.1 kg/m3 of 100 PR pellet. This difference is related to the initial moisture of two different pomace and their storage before pelletization: low moisture content and long time storage of 2 PH pomace involve higher bulk density values [37]. 3.3. Mechanical durability Durability is the prevalent form of measurement and expression of pellet quality; this property is influenced by different factors, in particular, raw material composition and dimension. In Fig. 4 the two different trends of pellet blends durability are reported, and the black line indicated the minimum threshold value according to EN 17225-6 [17]. The mechanical durability is lower in 2 PH pomace than 3 PH due to the higher content of oil and lignin in the first one; in fact, several studies [12] have tested the negative influence of fat/oil and
M. Barbanera et al. / Renewable Energy 88 (2016) 185e191
Fig. 3. Bulk density trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
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Fig. 5. Ash content trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
In this work, the net heating value was used to express the energetic feature of different pellet blends because of the technical standard referring to this parameter in its regulation. In Fig. 6 the net calorific values of different pellet blends are shown; as we can see, thanks to high heating values of pomace, all the pellet blends are in compliance to EN 17225-6 [17], that recommends at least 14.4 MJ/kg to consider a good quality of pellet. Net heating values are strongly affected by raw material composition; in fact the higher oil content in 2 PH pomace reflects an important energetic value of the 2 PH pellet blends. Nevertheless, a little difference from 2 PH and 3 PH pellet blends is clear because of 3 PH blends are characterized by a lower ash content that, as described above, influences the heating value. Fig. 4. Durability trend for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
lignin on pellet durability. Therefore, the increase of the percentage of pruning in the 2 PH pellet blends allows to obtain a maximum value of durability of 96.2% for 75PR25 PH blend that which it is the only one that exceeds the value required by the International Standard of pellet class quality B. The 3 PH pellet blends, instead, hold the highest values of durability never less 96.8% because of 3 PH pomace have a little content of oil (4.4%) and also a lower lignin percentage than 2 PH. Also the particle distribution in the raw material influences the mechanical durability of the pellet blends; as we have seen, 3 PH pomace presents a larger concentration of fine particles than 2 PH and pruning and this fact increases the corresponding pellet durability.
3.5. Relationship between the main physical and chemical properties To give some indications about the best pellet blends characteristics, the relationship between bulk density, durability and energetic density is reported in Fig. 7 and Fig. 8 for the two series of pellet blends. The energetic density is an important factor to consider, mostly in relation to the sizing of the storage place of the pellets. This parameter is the ratio between the heating value and the bulk density of the pellet. In the figures is represented only a black dashed line corresponding to the durability minimum threshold (96%) according to EN 17225-6 [17] because the requirements of the bulk density are observed for all the blends. For 2 PH pellet blends (Fig. 7) only 75PR252 PH sample presents
3.4. Ash content and calorific value The ash content influences the energetic quality of the pellet; therefore, the energetic value is greatly related to this parameter. In Fig. 5 the variability of ash content in the different pellet blends is shown; the ash percentage in the 2 PH pomace pellet is nearly double (4.3%) than 3 PH one (2.4%), due to the extremely high values of ash-forming elements as Fe, Mg, K. Nevertheless, all the pellets blends are in compliance with the Technical Standard [17] since the ash content is less than 5% on a dry basis, the maximum acceptable value. The main parameter to define the energetic quality as the fuel of the pellets is the heating value, as the maximum amount of energy that can be gained by converting completely a unit mass of fuel in standard conditions.
Fig. 6. Net calorific value for pellet blends of pruning and two-phase pomace (black bars) or three-phase pomace (grey bars).
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Fig. 7. Relationship between durability, bulk density and energetic density for pellet blends of pruning and two-phase pomace.
Fig. 8. Relationship between durability, bulk density and energetic density for pellet blends of pruning and three-phase pomace.
a durability value >96.0% and then is in compliance with the Technical Standard; instead, 3 PH pellet blends (Fig. 8) are characterized by higher values of durability and so the 50PR503 PH seems to have the most appropriate behavior in terms of physical and energetic parameters due to its high energetic density and durability and a medium value of bulk density better than 25PR753 PH. 4. Conclusions The analysis of pellet blends composed by olive pomace and olive tree pruning was carried out to investigate the improvements of the physical and chemical properties respect to pure olive pomace pellets. The pure 2-phase and 3-phase olive pomace pellets are not in compliance with European Standard of pellet quality and then several mixtures with olive tree pruning were produced and
analyzed in the most important physical, chemical and mechanical features. The results show that, adding olive tree pruning, the chemical composition of pellet blends respects the standard requirements in terms of mechanical durability and nitrogen and copper content; also the bulk density is enhanced allowing a reduction of transport and storage economic cost. Considering all the quality pellet parameters, two pellet blends have the best behavior as 75PR252 PH among the 2-phase mixtures and 50PR503 PH between the 3-phase blends, and can be used for combustion and gasification processes. Acknowledgments This research received support by the European Commission,Seventh Framework Programme for European Research and Technological Development (2012e2015) within the project “FFW”
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“Liquid and gas Fischer-Tropsch fuel production from olive industry waste: fuel from waste”. Grant Agreement n 308733. References [1] A. Garcia-Maraver, M.L. Rodríguez, F. Serrano-Bernardo, F. Diaz, M. Zamorano, Factors affecting the quality of pellets made from residual biomass of olive trees, Fuel Process Technol. 129 (2015) 1e7. [2] J. Zuwała, Life cycle approach for energy and environmental analysis of biomass and coal co-firing in CHP plant with backpressure turbine, J. Clean. Prod. 35 (2012) 164e175. [3] C. Gokcol, B. Dursun, B. Alboyaci, E. Sunan, Importance of biomass energy as alternative to other sources in Turkey, Energy Policy 37 (2009) 424e431. [4] AEBIOM, European Bioenergy Outlook, 2013. [5] M. Sesli, E.D. Yegenoglu, RAPD-PCR analysis of cultured type olives in Turkey, Afr. J. Biotechnol. 8 (2009) 3418e3423. [6] FAOSTAT. 2014. Available at:
191
[18] UNI EN 14774e2, Solid Biofuels. Determination of Moisture Content e Oven Dry Method e Part 2: Total Moisture e Simplified Method, 2010. [19] UNI EN 15148, Solid Biofuels. Determination of the Content of Volatile Matter, 2010. [20] UNI EN 14775, Solid Biofuels. Determination of Ash Content, 2010. [21] UNI EN 15104, Solid Biofuels. Determination of Total Content of Carbon, Hydrogen and Nitrogen - Instrumental Methods, 2011. [22] UNI EN 14918, Solid Biofuels. Method for the Determination of Calorific Value, 2010. [23] UNI EN 15289, Solid Biofuels. Determination of Total Content of Sulfur and Chlorine, 2011. [24] UNI EN 15290, Solid Biofuels. Determination of Major Elements, 2011. [25] UNI EN 15297, Solid Biofuels. Determination of Minor Elements e as, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, V and Zn, 2011. [26] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter, D. Templeton, D. Crocker, Determination of Structural Carbohydrates and Lignin in Biomass, Technical report NREL/TP-510e42618, National Renewable Energy Laboratory, Golden, 2012. [27] International Organization for Standardization, ISO 659:1998. Oilseed e Determination of Oil Content (Reference Method), ISO, Geneva, 1998. [28] UNI EN 15210e1, Solid Fuels. Determination of Mechanical Durability of Pellets and Briquettes e Part 1-Pellets, 2010. [29] UNI EN 15103, Solid Biofuels. Determination of Bulk Density, 2010. [30] ISO/DIS 17829, Solid Biofuels. Determination of Length and Diameter of Pellets. 2014, 2014. [31] M. Arshadi, R. Cref, P. Geladi, S. Dahlqvist, T. Lestander, The influence of raw material characteristics on the industrial pelletizing process and pellet quality, Fuel Process Technol. 89 (2008) 1442e1447. [32] J. Werkelin, D. Lindberg, D. Bostrom, B.-J. Skrifvars, M. Hupa, Ash-forming elements in four Scandinavian wood species. Part. 3. Combustion of five spruce samples, Biomass Bioenerg. 35 (2011) 725e733. [33] N.Y. Harun, M.T. Afzal, Chemical and Mechanical Properties of Pellets Made from Agricultural and Woody Biomass Blends, ASABE Paper No. 141908650, St. Joseph, Michigan: American Society of Agricultural and Biological Engineers, 2014. [34] M. Filipponi, G. Cavalaglio, B. Castellani, L. Brinchi, Second Generation Biodiesel via Fischer-tropsch Synthesis: BioDie2 Research Project and Preliminary Tests, 20th European Biomass Conference and Exhibition, June 18e22, Milan, Italy, 2012. [35] S.V. Vassilev, D. Baxter, C.G. Vassileva, An overview of the behaviour of biomass during combustion: part I. Phase-mineral transformations of organic and inorganic matter, Fuel 112 (2013) 391e449. [36] C. Ryu, Y.B. Yang, A. Khor, N.E. Yates, V.N. Sharifi, J. Swithenbank, Effect of fuel properties on biomass combustion: part I. Experiments e fuel type, equivalence ratio and particle size, Fuel 85 (2002) 1039e1046. € stro € m, T.A. Lestander, Effect of biomaterial [37] R. Samuelsson, M. Thyrel, M. Sjo characteristics on pelletizing properties and biofuel pellet quality, Fuel Process Technol. 90 (2009) 1129e1134.