Energy production from pruning waste
The comparison between the combustion of vine and olive pruning residues and natural gas in small boilers shows conflicting results regarding environmental impacts. For woody material, additional emissions from the presence of residues of pesticide treatments have to be carefully considered
“Ecological transition' and 'sustainable development' have become part of everyday vocabulary for some time now, even if we are not always fully aware of the path to their implementation. In the field of energy, an interesting opportunity that is increasingly being considered is that of generating heat and/or electricity from renewable sources: indeed, for some time now, crops have been cultivated for expressly energy-related purposes, but sometimes they falter in terms of sustainability. The case of some first-generation biofuels, such as bioethanol from maize, which not infrequently presented disastrous efficiency indicators, with an EROEI (Energy Return On Energy Invested, an index measuring the energy return per unit of energy invested) below unity, is a sensational one. In other words, excluding the contribution of solar energy, more energy was consumed in the production of that fuel than could be obtained!
Apart from solar and wind power, the most interesting renewable energy sources at the moment are supply chains based on the explotation of waste and scrap, which should no longer be considered as such, but as by-products. It is therefore not surprising that among the various types of ligno-cellulosic biomass of agricultural origin, pruning residues are of considerable interest.
Along with positive aspects and undoubted opportunities, such as the virtuous management of "secondary" matrices whose traditional management represents a cost, the diversification of income and the reduction of dependence on fossil fuels, the development of agro-energy chains based on the exploitation of this type of biomass still shows some criticalities. Often the implementation of plants is hindered by complicated regulatory aspects and/or the lack of specific expertise, while from a logistic point of view it is necessary to consider that pruning residues tend to be widely dispersed on the territory and their availability is in any case characterised by a marked seasonality. In order to exploit this matrix in a sustainable way, it is therefore necessary to organise not only the collection but also the storage in a rational way.
From a technical point of view, since it is a small material in which the quantity of bark compared to wood is not negligible and, moreover, treated with plant protection products, the resulting combustion may give polluting emissions (see box) and to the melting of ashes in the combustion chamber.
Pruning residues (which have a generally medium-low quality, mainly due to the non-negligible content of inerts and/or ash) are mainly burnt in devices with a thermal output of less than 150 kW, if they are intended to produce heat for domestic users, or in gasifiers or larger plants, for the cogeneration of electricity and heat.
Moreover, in recent years many studies have focused on the energy exploitation of fruit pruning residues through direct combustion or gasification starting from the product packaged in small cylindrical bales, or from wood chips or pellets, with particular attention to the technical and economic results, neglecting however the evaluation of the environmental impact, an aspect that is (and remains) however essential for an overall assessment of the sustainability of the supply chain. In previous issues of Mondo Macchina, specific experiences have been analysed and described (e.g. vine shoots for "trigeneration" in wineries, development of harvesting sites for olives and citrus fruits, etc.). In this article we want to analyse the environmental aspect related to the generation of thermal energy, comparing the results with those of heat production from fossil fuels.
Environmental impact
The use of pruning residues in a small power boiler (<50 kW) with a thermal efficiency of 85% was investigated; In detail, vine shoots chipped by means of a chipper were considered, with a Lower Calorific Value (IHC) of 18 MJ/kg and an average combustion humidity of 25%, and olive pruning residues packaged and subsequently stored for natural drying and finally chipped, with an IHC of MJ/kg 18,7 and an average humidity of 15-20%.
The data on heat generation from fossil sources were taken from the Ecoinvent® database, considering natural gas, used in small-medium power devices (< 100 kW). The environmental impact assessment was carried out according to the Life Cycle Assessment (LCA) method. Global warming, particulate matter formation, soil acidification, freshwater eutrophication and consumption of mineral and fossil resources were analysed. As different environmental effects can be quantified in different units, comparisons are usually made by setting the highest environmental impact or benefit equal to 100, with the one(s) being compared calculated proportionally.
The comparison between the different supply chains shows significant differences between the impact of heat generated with pruning residues and the impact of using natural gas. For global warming and ozone depletion, wood residues show a lower impact of more than 80% compared to gas; for other impacts, the use of gas is more sustainable, due to lower emissions during combustion, especially of particulate matter, nitrogen oxides and sulphur dioxide.
The differences between the two solutions involving the use of vine and olive pruning residues depend mainly on the amount of biomass harvested, and secondly on the harvesting method.
The exploitation of olive residues "pays the price" for the smaller amount of material available compared to that of the vineyard.
In conclusion, as highlighted, the energetic exploitation of pruning residues has lights and shadows.
From an economic point of view, it certainly represents an opportunity for farmers who, once sustainability has been verified, can diversify their sources of income by selling the biofuel or exploiting it directly from an environmental point of view.
However, it must be considered that their combustion in small devices can give rise to pollutant emissions and an increase in certain environmental effects. Researchers are working to develop technologies and techniques which can overcome such issues
Emissions
Emissions are undoubtedly a critical point in the energy exploitation of pruning residues by combustion, especially if small or medium-sized devices are used. In fact, burning ligno-cellulosic biomass is much less simple than exploiting methane.
The problem of emissions is common whether heat or electricity is generated, although in the latter case filters and abatement systems are generally used, which however entail a considerable increase in investment costs for the construction of the plant. This increase is in fact admissible only for medium-large plants, while it would undoubtedly be more critical in smaller ones, due to the incidence of emission abatement costs, which would even be preponderant compared to the rest of the plant.
Another issue is the accumulation of residues of pesticides in wood from pruning: an interesting study conducted by researchers from the Mediterranea University of Reggio Calabria and CREA, published in the journal "Fuel", showed that citrus and vine pruning residues show the highest NOx emissions also due to their higher nitrogen content, while olive wood residues produce emissions within the limits of the regulations.
