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Collecting and baling prunings to produce energy
Mr. Carlo Bisaglia - CRA-ING -Research laboratory of Treviglio (BG)

The pruning of the main tree cultivations in Italy (vines, olives and orchards) produces a quantity of residues (shoots, prunings, etc.) that varies from 1 to 3 t/hectare each year, depending on the conditions.

Tab. 1 Amount of pruning residues estimated for various species of trees

 
 

Species

Pruning residuies (t/ha)

Vine

2,9

Olive tree

1,7

Apple tree

2,4

Pear tree

2,0

Peach tree

2,9

Citrus plantation

1,8

Almond tree

1,7

Hazelnut-tree

2,8

 
 

Therefore, considering the entire surface area covered with these cultivations in Italy, a total quantity of prunings can be estimated that varies from 2 to 6 million tons/year.

Traditionally, these residues are left on the ground after they have been shredded or are collected and destroyed in the case of parasite problems that need to be dealt with.  These operations represent a real cost for the farmer (this can be estimated from 60 to 140 €/hectare) who has to dedicate time and specific mechanical equipment to the operations.  

An alternative use can be made by exploiting the energy content in the prunings; for example, if we compare this value with the energy contained in fuel oil we can estimate an energy potential in the residues that is comparable to the energy potential of almost 1.3 billion litres of fuel oil, sufficient to heat almost 600,000 flats.

Therefore, it is easy to appreciate the opportunity offered by the use of pruning residues as a source of energy.  This possibility is even more tangible, today, by observing the increasing availability on the market of heating systems that use wood residues to produce heat or even electric power in large centralised systems.

Therefore, there is a new opportunity for the commercial farm that, however, needs to be managed carefully, since it involves recovering sub-products, which if, on the one hand, they have an attractive energy potential, on the other hand, they generally have mediocre qualitative characteristics (low lignification, the presence of bark and other impurities, high level of humidity, variable composition, etc.).  One of the keys to success to recover prunings is to have available mechanical sites able to produce good technical results at limited costs.  The principal aspects to be considered concern the possibility of moving between rows which are frequently narrow or beneath overhead trellises or pergolas, having to manoeuvre in limited headers or on sloping ground or even on terracing.

In addition, there is the need to reduce the volume effectively to facilitate transportation and storage of the prunings until they are ready to be used.
Lastly, it is necessary to identify the conditions in which it is easy to reduce the humidity to favour the product's preservation, limit fermentation and improve the energy yield.

One of the technical solutions offered by the market today concerns the in-field baling of residues.  This technique is very well-suited to collecting thin residues, up to a maximum diameter of approximately 4 cm, generally difficult and arduous to handle.  Lightweight balers are also available on the market, today, among the various solutions; these machines are able to move easily between the narrowest rows and are operated by small tractors (generally between 30 and 40 kW).

In particular, the compact round-balers (with a bale diameter of 40 cm and weight varying from 25 to 30 kg) represent an interesting option, since they have a good working performance (0.5-0.6 hectare/h, corresponding to approximately 45-50 bales produced per hour), limited power requirements, thanks to the mechanical efficiency and the "in line" working method with the tractor that ensures very limited transverse overall dimensions, in particular, if working with specialised narrow track tractors (less than 1.15 m).  The working capacity of these machines can virtually double (1.0-1.2 hectares/h) when they are equipped with accessories to transport up to 8 round bales.  This enables a collecting autonomy of up to 240 m to be achieved without having to unload the round bales along the rows, but the bales are concentrated at the headers where the loading operations on the trailers can be performed more easily and rapidly.  

Lastly, the round bales permit a significant quantity of humidity to be released from the prunings into the atmosphere favouring the drying process and avoiding the presence of fermentation or moulds.  
In this way the baled prunings can be used 'as is' inside specific boilers, which are available on the market today, or can be cut or even shredded to be used in boilers which can be fed with chipped biomasses, since they can exploit automatic feed systems.

The new environmental awareness with the need to limit CO2 emissions and the uncertainties generated by the decreasing availability of traditional energy sources and the respective prices, undoubtedly represent an important reason to consider with interest every possible exploitation of renewable energy sources.

 
 
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