Influence of chipping system on chipper performance and wood chip particle size obtained from peach prunings

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Research paper

Influence of chipping system on chipper performance and wood chip particle size obtained from peach prunings Luigi Pari, Alessandro Suardi∗, Angelo Del Giudice, Antonio Scarfone, Enrico Santangelo Consiglio per la ricerca in agricoltura e l'analisi dell'economia agraria (CREA), Unità di Ricerca per l'Ingegneria Agraria, Monterotondo, Rome, Italy

A R T I C L E I N F O

A B S T R A C T

Keywords: Pruning Comminution Permanent crops Residues

Many obstacles hamper the full exploitation of pruning residues for energy. Among these, harvesting is a crucial point in the development of a sustainable supply, because it greatly affects the quality of the fuel and profitability of production. In normal forestry operations, drum or disk chippers are the tools most frequently used for comminution. A new chipper has been designed which can collect different pruning residues produced under different field conditions and reduce them to a standard chip size (P45) suitable for boilers. Comminution is carried out by a double-auger on which different types of blades can be mounted. This paper compares outcomes in terms of machine performance and particle size distribution as a consequence of the use of a helical (HELK) or hoe-shaped (HSK) blade. The study was conducted on a peach plantation in Spain grown with the “open centre” or "vase" training method. Analysis of the main elements illustrated the difference between the types of blades. The HELK blade performed better in terms of coverage of area (0.52 ha h−1) and material processed (1.06 t h−1). Moreover the main fraction of the particle size (60% of the fraction 3.15 ≤ P ≤ 45 mm) reached 79.2% with the HELK compared with the 67.9% with the HSK. However, from a mechanical standpoint, the HSK blade was found to perform better in terms of ease of maintenance, though the helical blade gave better results for the collection of peach pruning residues.

1. Introduction Despite its huge potential as biomass, pruning residues can be deemed a neglected source, poorly utilized until now for bioenergy purposes [1]. There are many reasons for this, including variability in space, time and typology of pruning; the reluctance of farmers to introduce innovations; the lack of an efficient and cost-effective supply chain. Of course, exploitation will become an effective opportunity only when the biomass can be delivered to the end user at a reasonable price [2,3]. In the framework of a logistics chain, harvesting plays a pivotal role in producing high-grade fuel at an affordable cost. Following this course, a number of machine manufacturers have started to develop dedicated equipment for collecting pruning residue, using either shredding [2,4–7] or baling technology [8–11]. However, some problems remain due to the limited productivity of harvesting–processing machinery [2], and particle size distribution [12] so that use has been restricted mainly to olive and vineyard pruning residues. The size of wood chips must comply with quality standards and will determine whether the fuel can be used in industrial or residential

furnaces, as well as its economic value. As pointed out by several scholars [13,14] particles size distribution of wood chips is one of the most important parameters affecting storage and handling methodologies [15,16,17,18]. Moreover, meeting target particle size standards also means being able to influence machine productivity, diesel fuel consumption and bulk density [17]. Different comminution processes have been studied in considerable depth for wood energy crops [19–22] equipping the commercial machines with disc, drum or cone-screw chipper systems [22–24]. To overcome some of the technical barriers related to harvesting, on-site pre-treatment, quality and transport, the EuroPruning project (www.europruning.eu) studied the development of new improved logistics for pruning residues [25]. One of the most important outcomes of this activity was the design of a chipper to collect and process the largest number of pruning residue types, because harvesting is a key point affecting product quality, the development of a logistics chain and the economic sustainability of the chain [25]. The chipper developed under the aegis of the project could install alternative cutting systems using helical (HELK) or hoe-shaped (HSK) blades). The objective was to achieve a particle size distribution that would comply with the standard

∗

Corresponding author. E-mail addresses: [email protected] (L. Pari), [email protected] (A. Suardi), [email protected] (A. Del Giudice), [email protected] (A. Scarfone), [email protected] (E. Santangelo). https://doi.org/10.1016/j.biombioe.2018.01.002 Received 1 December 2016; Received in revised form 12 January 2018; Accepted 12 January 2018 0961-9534/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Pari, L., Biomass and Bioenergy (2018), https://doi.org/10.1016/j.biombioe.2018.01.002

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product [27]. A gooseneck at 2.5 m height from the ground discharges the product with four alternative options: “big bag”, a breathable plastic bag (volume 1.5 m3) held by the prototype, a machine tilting box (volume 3 m3), a trailer towed by the machine; a tractor-trailer unit moving alongside the machine. The choice of the unloading method to use depends on farm logistics and facilities availability, and each unloading system has pros and cons that several authors have already described in other studies [6,43]. During the test, the big bag configuration was used. The pickup system of the machine collects the prunings from the ground by means of a roller 1.570 mm wide with 18 teeth. The height of the pickup system is adjustable from the control panel on the tractor. Afterwards, the feeding system conveys the prunings into the chipping device. It consists of an ascending central chain and two toothed counter-rotating rollers (CRR) below the chain. The chain has an array of retractable teeth (220 mm long) and dragging teeth (70 mm long), which direct the pruning residues into the chipping system. The speed of the counter-rotating feed rollers (∅ = 250 mm L = 350 mm), as well as that of the windrowing and pick-up rollers can be adjusted by the machine's hydraulic system. The higher the speed of the rotating rollers, larger the size (length) of the chips. During the trial in Fraga, the chipper was powered by a New Holland tractor mod. TF030F with a 56.7 kW engine. The speed of the CRR was set at 1.23 Hz. The key element of the chipping system is an auger (rotating speed 13.3 Hz) powered by the PTO of the tractor with a ratio 1:2, so that each rotation of the PTO shaft causes two rotations of the auger. Two types of blades can be mounted on the auger, helical (HELK) and hoeshaped (HSK) (Fig. 3). The HELK blade set consists of two elements positioned consecutively in the direction of the auger rotation. Once installed, the two elements function like a single blade cutting left to right with a “scissor effect” created by the blade and counter-blade. This cuts the wood cleanly, minimizing the use of the tractor's power. The HSK blade set, instead, includes six separate elements that perform a “guillotine” cut, similar to the drum chipper of a forage harvester. HSK blades have been indicated by the constructor as hoe-shaped blades due to their curved shape. The HSK was designed to produce a cut “one blade at a time” which, considering the cutting angle, would reduce the power necessary to comminute the prunings. This would lead to a theoretical reduction of the power and fuel required. Prototype performance (Table 1) was evaluated by measuring the working time according to ASABE standard methods [28]. Other references include the Commission Internationale de l’Organisation Scientifique du Travail en Agriculture (C.I.O.S.T.A.) and the Italian Society of Agricultural Engineering (A.I.I.A.) 3A R1. A reference to these methods can be found in Ref. [29]. The big bags with the comminuted product were weighed on the field using a dynamometer made by PCE Italia Srl (CS 1000N model range of measurement 1000 kg and sensitivity 0.2 kg) connected to a tractor equipped with a fork. No repetitions were carried out during the biomass harvesting. The yield per each experimental field was calculated considering the comminuted product collected (big bag weights) over the experimental field area. Fuel consumption was measured during the performance test starting with a full tank and refilling the tank at the end of the work. In this way the total fuel consumption (comprehensive of the forward progress, turns, time for maintenance on field with the motor running, and unloading) was assessed. Furthermore, an additional test was performed to estimate fuel consumption for the actual work (productive work) that is, the work done in the chipping stage. The fuel consumption for the actual work separates it from consumption due to unproductive work (e.g. turns, stops, maintenance on the field). This test was carried out in separate rows. For each row the machine started with a full tank which was

Fig. 1. Demonstration field test at Fraga (Spain).

requirements requested by stakeholders interviewed during the project (technical experts, boiler constructors, woodchip producers) [26]. In particular, because the chip particle size P45 was the one most requested by the stakeholders, all the tests focused on obtaining that size. The purpose of this paper is to analyze both the performance of the machine and the compliance of the product with particle size requirements when HELK or the HSK blades were used to harvest and chip peach (Prunus persica (L.) Batsch) pruning residues and to determine which type of blade is more appropriate for the purpose. 2. Materials and methods The tests were conducted in 2015 in an agricultural setting typically devoted to the cultivation of peach orchards. The field, almost flat, was located at Fraga (41° 53′ N, 0° 35′ E, 118 m a.s.l.), in the Aragonese region of Spain (Fig. 1). The shape of the field was irregular, with variable length of the rows. The plantation was 3 years old with plants of the Paraguayo variety grown in a layout 5 × 3 m (666 plant ha−1). The trees were trained in the open center or vase form, which has become the most popular training system for new orchards for its early bearing, easy mechanization and relatively low labour input and establishment costs. Trees trained in this way tend to remain relatively small, up to 3 m high, with an open centre, trained via repeated mechanical summer pruning in the first two years. Accurate winter pruning begins in the second year to control yield and maximize fruit quality. The pruning stage was managed manually by the farmer and co-workers, who stacked the cut biomass in the middle of the inter-row. In accordance with the vegetative growth of the peach tree, the residue showed a fairly irregular structure. Before harvesting, five plots of 10 m2 were randomly selected in the field within the windrows to evaluate the total biomass of the plot. Moreover, for each plot the following data were acquired on twenty observations: windrow height, windrow width, pruning diameter, and pruning length. The prunings were harvested using a chipper made by ONG snc di Naldoni Domenico & C., mod. PC50 [1,27]. Briefly, the chipper is 1.75 m wide, 3.90 m long, and needs to be towed by a tractor with a minimum power of 45 kW (Fig. 2). The chipper is designed for windrowing, picking up, conveying, chipping and collecting the final

Fig. 2. The PC50 chipper with the rear big-bag.

2

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Fig. 3. Chipping system using hoe shaped blades (HSK) and helicoidal blade (HELK).

Table 1 Type of test and size of the test area.

Table 2 Characteristics of pruning and windrows.

Test

Blade type

N. of row

Average row length (m)

Area (ha)

Machine performance and total fuel consumption Fuel consumption of the actual work

Hoe shaped Helicoidal Hoe shaped Helicoidal

10 4 3 2

78 133 120 145

0.39 0.27 0.19 0.15

refilled at the end of the row. It is important to highlight that, even if the refill method to measure the fuel consumption is very common and has the advantage of being very easy to apply in the field, its accuracy has been questioned in many studies, especially when the amounts to be measured are minute, and the measurement error is difficult to assess. The biomass losses were estimated by gathering and weighing the material not harvested during passage of the machine, and any wood chips left on the ground. Ten random plots of 5 m2 area were chosen within the inter-row of each field of the performance test. The biomass present in each plot was collected and weighed using a KERN GmbH dynamometer (CH 50K50 model - range of measurements 50 kg and sensitivity 50 g). The quality standards of the wood chips were analyzed in accordance with the requirements of the European standard [26]. To determine moisture content, immediately after harvesting, five samples of wood chips (app. 500 g each) were taken from the pruning residues chipped with HSK and HELK. The samples were sealed in polyethylene bags to prevent drying, duly tagged and sent to the CREA-IT laboratory in Italy (Monterotondo, RM). The moisture content was calculated as described by the European standard [30]. The bulk density was measured using a normalized cylinder with an internal volume of 26 dm3 [31]. Five samples of wood chips per each blade type were used to fill the cylinder loosely to the top and weighed with a dynamometer (50 kg ± 50 g). The particle size distribution was determined according to the European standard [32]. Approximately 20 kg of product for each blade type were collected and brought to the lab for evaluation. After drying, a sub-sample of 12 L, divided into 4 lots of 3 L each was selected and separated in the mechanical vibrator sieve (Analysette 18, Fritsch). Seven sieves (normalized in accordance with ISO 3310-1) were used in order to separate the following eight wood chip classes: 350-120 mm, 120-100 mm, 100-63 mm, 63-45 mm, 45-16 mm, 16-8 mm, 8–3.15 mm and < 3.15 mm.

Unit

Minimum

Mean

Maximum

Pruning Diameter Length

mm mm

8 170

21 ± 8 1300 ± 570

33 2470

Windrow Height Width

mm mm

110 1400

290 ± 150 1710 ± 200

600 2200

diameter greater than 5 cm and for this reason no problems were observed with the PC50 in chipping the residues, which had an average diameter of 2 cm with a range from 0.8 to 3.3 cm. The yield of both fields harvested in the performance test with the HSK or the HELK was slightly higher than 2 t ha−1 (Table 3). The plants were pruned one week before the test, when the biomass moisture mass fraction content was comparable between the fields, 41.2% for branches harvested with hoe shaped blades and 39.9% for those collected with the helical blades. For reasons of sanitation and plant management, the prunings can only remain in the orchard for brief periods. As a result, the collection or harvest must be accomplished on material having a high moisture content. Also, the availability of prunings from the main permanent crops (olive, vineyard, peach, apricot, plum, pear, apple, kiwifruit) is concentrated between October and April [1,33], when the environmental conditions are not favorable to rapid drying. The dry matter yields obtained during the performance tests with HSK and HELK were 1.43 t ha−1 and 1.23 t ha−1 respectively, (1.33 t ha−1 ± 0.14 in average). The losses observed differed by 4%, being 17% for the HSK and 13% for the HELK (Table 3). The bulk density of the final product is directly affected by the action of the blades. During the test of HSK and HELK the densities reported were respectively 222.4 and 209.3 kg m3. However, the figures were comparable between the two chipping systems and no statistical differences emerged according to Student's t-test (t = 0.416, n = 5). Use of the HELK did have an effect on the performance of the machine, however (Table 4). When the PC50 installed the HELK blade set, the operating speed was 63.5% faster (0.63 vs. 1.03 km h−1). Moreover,

Table 3 Mean productive data of the performance test. The standard deviation is reported for the repeated data. Blade type

3. Results The “open center” or “vase” method ” is a non-intensive training scheme with a plantation layout of 5 × 3 m [1]. The plants grow in volume, producing long branches. In the present study the average length was 130 cm, but branches reached a maximum length of 247 cm (Table 2). The manual pruning system required the formation of a windrow in the middle of the interrow which occupied a width of 171 cm. The machine was developed to process branches with a

Worked area Yield (fresh matter) Moisture content Yield (dry matter) Losses Bulk density

3

ha t ha−1 % t ha−1 tdm ha−1 % m3 kg−1

Hoe shaped

Helicoidal

0.39 2.44 41.2 ± 0.9 1.43 0.52 ± 0.25 17 209.3 ± 19.6

0.27 2.05 39.9 ± 0.5 1.23 0.31 ± 0.16 13 222.4 ± 27.8

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Table 4 Main results on machine performance.

Table 5 Loadings of the principal component analysis on the classes of particle size distribution (gray shadings indicate main constituents of the first two PCs). Unit

Operating speed Field efficiency Theoretical Field Capacity Effective Field capacity Material capacity Total fuel consumption Fuel consumption of actual work

km h−1 % ha h−1 ha h−1 t h−1 l ha−1 l t−1 l ha−1 l t−1

Knife Hoe shaped

Helicoidal

0.63 77 0.33 0.31 0.77 38.5 15.8 37.5 14.2

1.03 71 1.41 0.52 1.06 23.3 11.4 20.5 7.0

120-350 63-100 45-63 16-45 8-16 3,15-8 ≤3,15 Explained variance (%) Cumulative variance (%)

the effective field capacity increased 67.7% when with the HELK blade set installed (0.52 ha h−1) as compared with HSK (0.31 ha h−1), although the field capacity in this case is a poor descriptor of the performance that is influenced by the amount of biomass available in the field. However, a similar result was observed analyzing the material capacities obtained during the two tests that resulted 1.5 time higher for the HELK (1.06 t h−1) compared with the HSK (0.77 t h−1). The fuel consumption of the helical blade matched almost perfectly the value reported by Pari and Sissot (2001) [38] for the collection of the peach prunings using a shredder mounting a rotor with hammers. Use of the HELK blades resulted in a general reduction of the fuel consumption. Taking into account the consumption per product unit (l t−1), the machine mounting the HSK and HELK reported a reduction of fuel consumption between total and actual work of 10.1% (15.8 vs. 14.2 l t−1) and 38.6% (11.4 vs. 7.0 l t−1) respectively. The saving effect of the HELK was evident in the comparison of the consumptions for actual work: the HSK consumed two-fold the fuel required by the HELK per ton (14.2 vs. 7.0 l t−1) (Table 4). The PCA analysis revealed the breakdown of particle size distribution generated using the different blades (Fig. 4). Overall, the first two components accounted for the 89.9% of the variability (Table 5) thus explaining almost the totality. The first principal component (PC1) was the most important, comprising more than two-thirds (72.9%) of the variability. The highest positive load of PC1 resulted for the classes

PC 1

PC 2

PC 3

PC 4

PC 5

PC 6

PC 7

0.24 0.42 0.26 0.42 −0.42 −0.43 −0.40 72.95

0.70 −0.05 −0.63 0.22 0.21 −0.07 0.04 17.00

0.52 −0.49 0.64 −0.02 0.01 −0.07 0.27 5.89

−0.12 0.12 −0.16 0.14 −0.42 −0.26 0.82 3.10

0.37 0.30 −0.02 −0.53 −0.51 0.47 −0.02 0.85

0.01 0.22 0.19 0.63 0.11 0.69 0.17 0.21

0.14 0.66 0.24 −0.26 0.57 −0.19 0.25 0.00

72.95

89.95

95.84

98.94

99.79

100.00

100.00

longer than 16 mm, while the negative ones accounted for the classes below such a value (Table 5). The convex hull of each blade was positioned in the positive (HSK) and negative (HELK) semi-axis of PC1. This distribution should be matched against the different cutting methods achieved with by the two types of blade. The action of the HSK privileged the production of the coarser fraction that shifted towards the finer classes by the HELK. The type of blade affected to a lesser extent the classes 120–350 and 45–63 mm, but these loads showed high values in the second component (PC2), which accounted for 17% of variability. A more detailed breakdown of particle size distribution clarifies better the indications that emerged from PCA. The product of the two chipping systems was different, as the HSK blades produced a greater proportion of long chips compared to the HELK blades. Analyzing particle size distribution in detail, (Fig. 5), use of the HSK blade produced a higher proportion of chips in the 120–350 mm, 63–100 mm. 45–63 mm and 16–45 mm fractions, but the difference was statistically significant only for the 16–45 range (28.4 vs. 19.9%). Probably this significance should also be attributed to the 63–100 mm class (9.9 vs. 1.0%) where the excessive variability among samples masked the statistical extent of the treatments. Correspondingly, the mechanical action of the HELK blade was responsible for the production of smaller chips (from 16 to < 3.15 mm) with particular reference to fractions Fig. 4. Spatial representations of the results of PCA.

4

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Table 6 Compliance of the particle size distribution to the reference standards. Class

EN ISO 17225-4: 2014 Main fraction (%) 3,15 ≤ P ≤ 45 mm Coarse fraction (%) Max length particles

P > 63 mm ≤350 mm

EN ISO 17225–1:2014 Fine fraction (%) P ≤ 3,15 mm Classification

Fig. 5. Particle size distribution observed for the different blades. Significant differences between blade types for each class were determined by Student's t-test. Where reported, *P < .05 and **P < .01 probability level.

Limit

at least 60% ≤10%

Blade type HSK

HELK

67.9 ± 5.8

79.2 ± 1.3

18.1 ± 8.1

6.0 ± 6.0

Yes

Yes

9.9 ± 5.1 F10

13.5 ± 4.4 F15

a reduction in harvesting losses (Table 3) and an improvement of the quality of the particle size (Table 6). When comparing four commercial shredders designed to blow the shredded prunings into a container [6], losses ranging from 3.4% to 61.9% of the total residue on site were reported, with an average value of 20.7%, on pear, apple and vineyard pruning residues. The shredder tested by Ref. [9] showed 21% of losses on the peach orchard, but only 4% when used on olive prunings, although we know that the harvesting losses for olive prunings tend to be lower due to the high pruning yield acting as a divider in the percent calculation [4]. Thus, the PC50 appears to ensure a reduction of losses probably as a consequence of the interaction of the machine design and the efficiency of the pick-up system, which consists of a toothed roller whose height is adjustable from the control panel of the tractor [27]. Both factors (machine design and pick-up system) are crucial for ensuring the most complete recovery of the residues without severe contamination (soil, inorganic material) of the comminuted residue. It will be interesting to analyze the harvesting losses of the PC50 on the olive prunings where a mismatch between windrow width and machine work width frequently occurs [4]. The particle size distribution of the chips, together with the moisture content, is one of the main parameters to take into consideration to define the comminuted biomass quality. The size determines the type of fuel-burning plant, and the way in which the product can be stored. In particular, the presence of oversize chips can create problems to the feeding system of boilers in small and mediumsized systems while, on the other hand, fine material is more difficult to preserve and produces more ash in combustion (Suadicane, Gamborg 1999; Paulrud, Nilsson 2004; Hartmann et al., 2006; Cavalli et al., 2011). The specifications of the solid biofuels to be produced or delivered by the equipment develop within the Europruning Project identified P45 as the particle size most suitable for European consumers [25]. Such a distribution identifies a fuel with three conditional characteristics: (1) 60% of the total mass must consist of particles between 45 and 3.15 mm; (2) the coarse fraction should represent less than 10% of the total mass; (3) the maximum length of the particles must be less than 350 mm (Table 6). Both types of blades were able to produce chips in the P45 category, although the helical blade gave better results. The figures are encouraging and lead us to think that the modulation of chipping conditions might lead to further improvements. The effect of the comminution is linked to the interaction between the mechanical properties of the wood and the chipping parameters [13,21,36,39,40]. In this study the characteristics of the wood were uniform, and, hence, the differences in the particle size were entirely dependent on the type of blade. As reported previously, hoe-shaped blades and helical blades are conceptually different from the cutting systems on drum, disk and conecrew chippers. As described, the particle size distribution of chips produced by the

8–16 mm and 3.15–8 mm where the difference was significant. Use of the HELK blades thus led to an improvement in the characteristics of the wood chips obtained from peach pruning residues, according to the reference standards [26,34]. Both types of blade produced more than 60% of wood chips between 3.15 and 45 mm, but while the HSK exceeded the threshold value of 10% relatively to the chips greater than 63 mm, the reduction achieved by the HELK ensured compliance with the standard requirements. 4. Discussion The P50 chipper was designed to process the widest possible range of pruning residues, and was therefore provided with two alternative chipping systems. The helical blade derived from a fixed-point machine [35] used for cutting trunks with large diameters, as it cuts more efficiently with less engine power. The second system, employing hoeshaped blades arose from its technical handiness. In case of replacement (due to breakage or wear of the blade), removal of the helical blades is laborious and requires extensive technical skill. As shown by the PCA analysis (Fig. 3), the comminution device used greatly affected the characteristics of the final product because the conformation of the blade changed the type and effect of its impact on the wood, in turn altering particle size distribution. In the field of wood biomass it was shown that the type of chipping device (drum, disc or auger) [20,22,36] or alterations to the chipping apparatus [13] or a different parameter settings [17] all played a significant or primary role in producing a feedstock in accordance with specific quality standards. But the choice of different alternatives can also affect such other aspects as machine productivity, energy consumption and power requirements [22]. The data reported in this study confirm the importance of the cutting method for pruning residues as well as its influence on machine performance. Unlike wood biomass, where a remarkable amount of data is available, the performance of chippers for pruning residues is still scattered and scarce. Moreover, the use of innovative chipping systems makes it more difficult to compare results. The material capacity of the PC50 with the helical blade (1.06 t h−1), and in some cases also with the HSK (0.77 t h−1), was comparable to that of shredders mounting hammers or blades tested in the vineyard by Ref. [2] (1.18–1.50 t h−1). In the case of olive tree prunings [37] higher performance (4.88 t h−1) was reported, and also [3] in that of kiwifruit (1.80–3.7 t h−1). Peach prunings were collected [38] using a shredder equipped with a rotor and hammers. In this case the material capacity was 1.82 t h−1. It is critical to note that the tests were performed on a small experimental field (Table 3) and over a very short period of time. This does not invalidate the comparative character of the test but may weaken its ability to correctly represent productivity. In our study, the higher chipping capacity of the helical blade led to 5

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PC50 complied entirely with the current standard specifications when the helical blade was used. This result could not be taken for granted because in the available literature a target particle size was not always obtained [41]. reported that the particle size distribution of vineyard prunings chipped with seven different machines available in the European market was highly variable, and none of the tested machines was able to comply with the P45 specifications. Only 2.4% of the samples fulfilled P45 specifications, as the percentage of oversize and/or undersize particles was too high. However, the reader must be aware that the conditional characteristics of P45 used by the authors [42] were slightly different from those updated and used in this present study, and, consequently compliance may have changed. The application of a refining device led to a higher percentage of “good” particles (not exceeding 45 mm). Also, when harvesting olive prunings using machines with different pick-up devices, or a different number of hammers on the rotor, the particle size distribution may be displaced toward oversize or undersize particles [4,37]. The percentage of oversize and undersize particles must be balanced adequately and their contribution to the total product must be minimized [17]. This is true with particular regard to chips for residential use because the presence of a large proportions of oversize and/or undersize particles makes the fuel more suitable for industrial use [12]. An important outcome of this study concerns this issue because the feedstock produced by means of the helical blade was attuned to the standard specifications. However, it is important to note the high amount of fine fraction in the comminuted material obtained by the two blade types. Furthermore, as observed by Refs. [44,45], fines under 3 mm of length (wood dust) constitute a health hazard because they reduce air circulation during storage, supporting bacterial proliferation, and an increased risk of spontaneous combustion.

[5]

[6]

[7]

[8]

[9] [10] [11]

[12]

[13]

[14]

[15]

[16]

[17]

5. Conclusion

[18]

The study reported on the performance of a chipper designed for harvesting pruning residues equipped with either hoe-shaped or helical blades, a comminution system conceptually different from the drum, disk or cone-crew devices used for the wood energy crop. The presence of the hoe-shaped or helical blades affected the performance of the machine in terms of technical efficiency (effective field capacity, fuel consumption). Although more complicated to manage, the helical blade offered better productivity and a good match to the standard specifications for the collection of peach pruning residues. Since the study was conducted on a peach plantation with specific pruning characteristics, the results must be deemed valid only for the conditions described above. Further tests are needed to investigate a possible blade/pruning type interaction aimed at broadening the range of chipper utility as well as the effect on particle size distribution of blade wear.

[19]

[20]

[21]

[22] [23]

[24]

Acknowledgements [25]

This paper was produced within the framework of the EuroPruning project (Development and implementation of a new and non-existent logistics chain for biomass from pruning). EuroPruning was co-financed by the European Commission's 7th Framework Programme for Research and Technological Innovation, Knowledge-Based Bio-Economy.

[26] [27]

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