1 Introduction

In Brazil, Eucalyptus is a promising genus for renewable energy production in short rotation coppice (SRC) (Guerra et al. 2012) given that the surface area covered by this species plantations designed for production was around 5.6 million ha in 2014 (IBA 2015). Conventional commercial plantations (of 3.0 × 2.0 m spacing, harvest at 6–8 years) can reach mean productivities of 45 m3 ha–1 year–1 due to genetic improvement and ideal edaphoclimatic conditions (Gonçalves et al. 2013). The key components of Eucalyptus high growth rate are its nutrients capture and use-efficiency and abundant water supply, mainly (Stape et al. 2004). According to Guerra et al. (2014), eucalypt short-rotation plantations, clearcut at 2–3 years, may present superior biomass production in shorter time when compared with the conventional forestry, reaching 120 m3 ha–1 in just 1 year.

Short rotation coppice (SRC) systems are design to increase the population density resulting in high quantity of low-priced final biomass product (Spinelli et al. 2009). In addition, as a fast growth species, eucalypt cultivated in SRC, system characterized by constant removal of biomass, requires an extra fertilization to maintain the soil fertility and high productivity rates (Mitchell et al. 1999). Therefore, this system demands highly efficient operations to active success, especially within harvesting, whereas its cost can account of 50% of total (Spinelli et al. 2009). In addition, as energy source, the final product woodchips should have uniform size (Spinelli et al. 2011) and low moisture content. This latter quality parameter is directly linked to market price (Nyström and Dahlquist 2004) and calorific power (Ergül and Ayrilmis 2014). It is important for power plants that the delivered woodchip has low moisture content variation to avoid combustion problems (Nyström and Dahlquist 2004).

In some European and American countries, the use of modified forager machines for forest harvesting has been the target of studies since the 1990s. Modified foragers have the characteristic of transforming the entire tree into woodchips in one-single pass, mostly known as cut-and-chip system (CAC). Some researchers like, Spinelli and Magagnotti (2010), Schweier and Becker (2012a, b) and Eisenbies et al. (2014) analysed the operating performance of available forage harvesters in the market from the following brands; Jaguar, Class, John Deere, and New Holland. In these studies, the main trees genus used were poplar (Poplar spp.) and willow (Salix spp.).

Forest mechanized harvesting is normally composed by two different systems in Brazil: cut-to-length (CTL) and whole-tree logging (WTL). In the former case, the machine responsible for cutting the trees, the harvester header cut-base the stems, delimber, cross-cut and bunch in one process, then, the forwarder machine collect the harvested stems and remove from the plot to alongside the road (Nurminen et al. 2006). In the latter case, the operation consists of a feller-buncher, responsible for cutting-base the trees and bunching, a processor, for delimbering, and a grapple skidder, to remove the logs outside the plot (Ghaffariyan et al. 2011). The CTL system is considered more environmental friendly because has higher productivity rate and requires less machines on the field. Both harvesting types need another machine in order to process the logs and cut them into woodchips, known as woodchipper. Chippers can be divided in mobile or stationary (Spinelli and Magagnotti 2010) and be classified for its cutting device, drum or discs (Manzone and Balsari 2015). Drum devices are more productive than discs (Spinelli et al. 2013) but produce higher diversification of medium-size chips (Spinelli et al. 2005).

The introduction of modified forager machines in forest systems is quite recent in Brazil. A few years ago, New Holland brought to the country a forager machine (Fig. 1) with the purpose to harvest SRC Eucalyptus plantation with high quantity of low-priced woodchips.

Fig. 1
figure 1

Woodchip pile, forager and Eucalyptus SRC

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A forager machine and a coppice header (Fig. 2) compose the harvester. The header or cutting head is equipped with two circular saws that cut-base located at the bottom of two feeding towers, one push-bar that push the felled trees towards opposite direction to the machine, one paddle roll that lifts up the trees into two verticals infeed rolls toward inside the forager. The drum chipper with knives is located inside the forager after the forage’s infeed roll and metal detector roll. After the chipper, the woodchips are engaged by the blower and discharged through the outlet pipe.

Fig. 2
figure 2

Main features scheme of adapted forage harvester

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The chip length can be chosen electronically from the cab through increasing or decreasing the feeding speed, giving the option for chip length from 30 to 5 mm (Fig. 3) in 1 mm steps. The largest one is recommended for directly burning and the smaller, can be used for pellet manufacturing.

Fig. 3
figure 3

Woodchips length: 30 mm (top) and 5 mm (bottom)

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The further concern regarding this kind of harvesting is the stump quality after the coppice (Fig. 4). In order to produce high standard cutting, the circular saw must have the following features: be sharpened, ideal rotation speed and ideal forward-motion speed (forager harvesting speed). Low cutting quality decrease the odds for stumps to produce vigorous resprouts. In other hand, “clean” cutting allows both healthy resprouting (Soler et al. 2013) and increase the biomass productivity (Dillen et al. 2013). However, one must have in mind that the regrowth ability varies between species (Fig. 5).

Fig. 4
figure 4

Cut quality: low (top) and high (bottom)

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Fig. 5
figure 5

Eucalypt resprouts on stump few weeks after mechanized harvested

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Hence, there is plenty harvesting machines available on market. However, evaluating the economic feasibility of the introduction of eucalypt as energy crop, the harvesting and chipping cost represents the main cost item (Sgroi et al. 2015). For this reason, a wisely choice is necessary for a profit production. Each harvesting system has pros and cons (Culshaw and Stokes 1995). Figure 6 represents each harvesting system discussed above.

Fig. 6
figure 6

CTL, WTL and CAC harvesting process (Eufrade Junior et al. 2016)

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Thus, a case study was conducted to analyse the cost of this new system in Brazil using modified forager harvester and pulled-tractor silage trailer in short-rotation eucalypt plantation.

2 Material and Methods

The study was conducted at Sao Paulo State (48°24′43″W, 22°58′10″S), in a 1.7 ha Eucalyptus grandis × Eucalyptus urophylla hybrid short-rotation plantation. The plantation spacing was 3.0 × 1.0 m (3333 plants ha–1) and at harvest time, trees had 2.8 years and an average base diameter (at 9 cm of height) of 10 cm. The area maximum land slope was 6%. The climate is mesothermic with dry winter and an average temperature of 20°C. The average annual precipitation is 1500 mm with 58% occurring from January to June. The annual potential evapotranspiration is 945 mm with 33% occurring in summer time (Cunha and Martins 2009).

Integrated the harvest system: a New Holland self-propelled forager machine attached to New Holland coppice header, two New Holland tractors and four TMA 24 m3 silage trailers (Table 1).

Table 1 Equipments description
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The system’s cost analysis methodology was adapted from ASABE (2011a) and the costs obtained were determined in two units: cost per time and quantity harvested in oven-dry ton (odt).

Ownership costs or fixed costs (FC) consist of depreciation, interest on fixed assets and other costs (taxes, housing and insurance), which focus on all the equipment of the mechanized harvesting system (harvester, tractor, and silage trailer) and are detailed in the equation:

$$ FC=\frac{\left\{\left[\frac{V_A+{V}_R}{2}+i\right]+\left[\frac{V_A-{V}_R}{EL}\right]+ THI\right\}}{pmh} $$

Where:

  • FC = fixed cost (€ hour–1)

  • VA = acquisition value of machinery and equipment (€)

  • VR = residual value of machinery and equipment (€)

  • i = interest rate per year (%)

  • EL = economic life (years)

  • THI = taxes, housing and insurance (% of VA in € year–1)

  • pmh = productive machine hours per year

Operating costs or variable costs (VC) consist of fuel, oil and lubricants, repairs and maintenances, and labor focusing on forage machines and tractors. The average fuel consumption is based on the actual power that is required or on the actual consumption measured in the field. The data of fuel consumption were collected using the machine’s on-board computer (Intelliview). To collect the tractor’s hourly consumption we followed the methodology described by Fiorese et al. (2012). To calculate oil lubricants and greases costs, we used the 15% factor in the cost of fuel.

According to ASABE (2011b), the accumulated costs of repairs and maintenance to a typical field velocity can be determined with the following expression using the repair and maintenance factors RF1 and RF2:

$$ CRM=\frac{\left\{{V}_A\times RF1\times {\left(\frac{h+ pmh}{1000}\right)}^{RF2}\right\}-\left\{{V}_A\times RF1\times {\left(\frac{h}{1000}\right)}^{RF2}\right\}}{pmh} $$

Where:

  • CRM = cost of repair and maintenance (€ hour–1)

  • RF1 and RF2 = repair and maintenance factors

  • VA = acquisition value of machinery and equipment (€)

  • h = hours accumulated

  • pmh = productive machine hour per year

Labor cost was calculated based on the monthly wage and work hours, including a correction factor of 25% due to idle time, in other words, time taken for repairs and supply of machines. Cost spend with employees transportation to the workstation were discarded. One workday consisted by two shifts of 4 h each. Wages and labor charges were based on the database provided by the forestry companies’ partners.

Some harvester data were estimated because the product is not considered as commercial machine in Brazil. Prices were acquired in Brazilian currency (R$) and converted to Euros (€) using the average exchange rate for 2015 of R$ 3.16 €–1 according to the official website of the Central Bank of Brazil (www.bcb.gov.br). Only repairs and maintenance cost were included on variable costs for silage trailers (Table 2).

Table 2 Input parameter for cost calculation
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In order to estimate the cost per tonne it was used the productivity expressed in dry tonne per hour, once the woodchip moisture was 52%.

The effective field productivity (EFP) was calculated as shown:

$$ EFP=\frac{d\times s}{1000\times t} $$

Where:

  • EFP = effective field productivity (ha h–1)

  • d = distance (m)

  • s = space between rows (m)

  • t = time (h)

3 Results and Discussion

EFP and productivity were 0.44 ha h–1 and 31.0 odt h–1, respectively. Using the same forage machine in a SRC poplar in Germany, an EFP average of 0.90 ha h–1 and a productivity of 14.6 odt h–1 were obtained (Schweier and Becker 2012a). Tests conducted in the United States, harvesting willow, showed results from 1.8 to 2.3 ha h–1 of EFP and a productivity between 23.9 and 24.9 odt h–1, requiring speeds from 8.0 to 10.0 km h–1 in which is unrealistic regarding a SRC (Eisenbies et al. 2014). These studies were conducted in temperate regions where the most common source of raw material are Salix spp. and Poplar spp. Plantations. These species are generally harvested around 3 and 4 years, and the basic wood density average is 350 and 410 kg m–3, respectively (Tharakan et al. 2003). Eucalyptus spp. has an average basic wood density of 430 kg m–3 (Garcia 2013). This information might justify the result of higher productivity in Brazil, once Eucalyptus’s wood density is 4.6% and 18.6% higher than willow and poplar, respectively.

The harvester EFP is related to the working speed in which could be limited by terrain conditions (i.e. slope and soil type), planting condition (i.e. trees diameter, planting spacing and presence of old stumps between planting lines), operator experience level, and forage harvester power.

The harvest system’s total operational cost was € 258 pmh–1 or € 18.9 odt–1 being the harvester the largest contributor of total cost with fixed total cost of €~87 pmh–1 and € 6.4 odt–1 (Table 3).

Table 3 Operational costs
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Schweier and Becker (2012a) determined an estimated total cost of € 281 h–1 and € 19.70 odt–1. The harvester per unit time individual cost (excluding labor costs) was found by Berhongaray et al. (2013), which was 212.5 € h–1. Despite the high value of labor charges on the operator’s wage and the rise of the exchange rate, operational costs are below those found in the literature. This difference is even greater due to the mean annual increment (MAI) of eucalypt in SRC in Brazil and its impact on the productivity generated by the harvester in this system.

The percentage contributions to total cost of each equipment item are listed in Table 4.

Table 4 Participation in percentage of each element on individual cost
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Depreciation and fuel are the two factors that most contributed to the total cost of the harvester, justified by the high purchase price and high fuel consumption of this equipment. Operator’s experience can be crucial to reduce fuel consumption because it is necessary to adjust the speed according to the forest conditions in order to increase wood chips production while manage operational time and fuel consumption efficiently, without wasting trees and preserving both sets of base cutting disks and cutting knives.

Whereas this forager harvester is non-commercial equipment, lifespan and productive annual hours were estimated, thus, with the adaptations improvement for Brazilian forests conditions both parameters can be even greater reducing the depreciation cost of the harvester.

The greater part of the tractor’s cost is related to the consumed diesel and here, again, the experience of the operator is decisive. Proper engine rotation for each harvest stage, proper adjustment of ballast weights, the type of tires and their internal calibration are factors that significantly influence the tractor’s performance (Lopes et al. 2003; Filho et al. 2010; Monteiro et al. 2011; Berhongaray et al. 2013).

Suitable fleet sizing is essential to reduce idle time in the harvesting process; however, this analysis demands full time studies and measurement of the maximum distance between harvested area and wood chip discharge area in order to optimize the logistics. In this assessment, the required fleet sizing was estimated from authors’ experience.

Once the produced biomass destination is bioenergy production, the calorific value contained in this material becomes relevant. Guerra et al. (2014) conducted a study testing the same clone in order to quantify the calorific value of a SRC under different spacing and fertilization levels. At 2 years old, in 2.8 × 1.0 m spacing (3571 plants ha–1) and applying the conventional fertilization dose, this clone reached an average of 20 GJ t–1 or 761 GJ ha–1. Converting to megawatt-hour, a productive day of harvesting would be able to produce around 2500 GWh. This energy is sufficient to generate electricity during 3 h in a European city with 542,000 households (Kavgic et al. 2013), or 16 h in a large Brazilian city with 2,000,000 inhabitants (Pereira and de Assis 2013). Clearly, this comparison is both informative and illustrative because it does not include generation neither transmission loss of energy, among other factors.

4 Conclusion

The use of modified forage machines for harvesting eucalypt in SRC, despite harvesting a smaller area per time, could reach a greater amount of harvested material per area compared to consolidated harvesting systems for willow and poplar plantations in temperate countries due to the difference of wood basic density.

Even with high labor charges values and high exchange rates, the total estimated cost is cheaper than those from temperate countries, with depreciation and fuel consumption being the biggest influences of total cost. The experience level of the harvester and tractor operators is crucial to this system economy.