Analysis of the Produced Pellets
Considering the direct emissions for CO, NOx and TSP (total suspended particles), the results of measurements are 4.8 × 10−5 kg/MJ, 1.2 × 10−4 kg/MJ and 7.2 × 10−5 kg/MJ, respectively.
shows the results for Eco-indicator 99 for PS1 and PS2. The first option (mobile pelletizer system) shows a higher impact (around 4.3%) than PS2; this is due to the impact generated by transportation of the pellet mobile system, and to the impact relative to the wood ash mixture. The pellet production indicates the lower impact of the pellet production plant than the mobile system, but with very limited differences.
The actual impact evaluation considers ashes as a residue with no specified value, and a potential use as fertilizer or raw material for fertilizers production. Unfortunately, due to the absence of essential information, it would be incorrect to define a standard allocation procedure for input/output of ashes. In this paper two different approaches have been considered and evaluated in order to underline the differences between waste management and the avoided impact procedure for ashes. In the first case (baseline) a process for ash disposal has been considered, applying a negative value in order to exclude from the total impact the one derived from the ash disposal. In the second case (avoided product), following the standard approach, ashes have been considered as a substitute for potassium fertilizer. In Table 5
the results of the analysis are reported.
Employing a midpoint approach, the contribution analysis of PS1 shows the high impact of the wood ash mixture on carcinogens and ecotoxicity; the pellet production instead has more impact on respiratory (organics and inorganics) and acidification/eutrophication. The infrastructures (mobile pellet plant and boiler) show a relevant impact on climate change, radiation, ozone layer, minerals and fossil fuels. For land use, the pellet production shows a negative impact (credit) of about 25%. Electricity has a relevant impact only on climate change, radiation, ozone layer and fossil fuels (Figure 3
Ecoindicator 99 for PS2 (Figure 4
) shows the relevant effects of direct emissions on Respiratory organics, Respiratory inorganics and Acidification/Eutrophication. On land use, the impact of the pellet is negative, as in the previous case, but it makes a relevant contribution in all the other categories except Carcinogens, Respiratory inorganics, Ecotoxicity and Acidification/Eutrophication.
The last consideration is about transport impact: a simulation has been conducted in order to evaluate how much the transport of a pellet from the mobile pelletizer system (PS1) should be reduced in order to reach the impact of the pelletization plant scenario (PS2). As shown in Table 6
, considering a final transport of 10 km instead of 100 km (distance set in the baseline scenario), the impact of PS1 (with reduced transport distance) is almost the same as that of PS2.
Considering ReCiPe Midpoint (H) V1.10, total impact and contribution are reported in Table 7
for PS1 and Table 8
The direct emissions have an impact mainly on TA, ME, POF, PMF and TET. Pellet boiler has a relevant impact on ULO, NLT and MD. Wood ash management is the major contributor to FE, HT, TET and MET, while for all the other impact categories the contribution is negligible. Pellet production makes a relevant contribution to CC, OD, IR, ALO, ULO, NLT, WD, MD and FD, while less impact is reported for all the other categories. Electricity has an impact on all the categories, but this is higher for CC, OD, IR, ALO, WD and FD. PS1 pellet transportation only has a relevant impact on ULO, and its impact is less evident on all the other categories. The negative impact of wood ash on NLT should be mentioned. This negative impact is due to the approach used in baseline ash management.
From the comparison of PS1 with AS1, AS2 and the heat production of the short rotation coppice (SRC) pellet [25
], reported in Table 9
, it is clear that PS1 and AS1 have the same total impact, but considering the single damage categories, PS1 shows a higher impact on Human Health, while AS1 has a higher impact on Resources. Regarding AS2, the Ecoinvent process selected considers a 50 kW boiler (instead the 150 kW one of PS1), and the impact calculated is higher than PS1 and AS1. Regarding heat from SRC, the impact distribution between the damage categories is quite similar to that for PS1, except for Resources. This is due to the use of agricultural inputs for SRC that are completely allocated to the wood produced. On the contrary, for vineyard pruning, no input or impact allocations have been considered.
The results of this study show that, in comparison to a big pelletizing plant, the production of a vineyard pruning pellet with a mobile system is slightly more impactful. Considering different approaches to ash management, such as negative values or avoided product, the relative impact is significantly different and higher for PS1 and PS2 baseline scenarios. This means that the avoided product (avoided production of potassium fertilizer) choice is relevant, and must be carefully considered so as to obtain the most accurate evaluation of the process under analysis. As regards the pellet transportation (PS1), it has an impact of low relevance, but is one of the unit process that can be subject to variations in order to simulate the effects on the final impact, and to define scenarios which, although only slightly improving sustainability, can then be really applied. In detail, by reducing the distance from 100 to 10 km, the relative impact is reduced by around 2%, approaching the impact of PS2. Compared to the reference supply chain (methane), both the considered pellet production scenarios are slightly different from, and relatively lower than, methane. It should be noted that pelletization is likely to be a substitute for more impactful fossil fuels like LPG, and the impact saving should be significantly higher in that case.
Considering the ReCiPe method with the midpoint approach, the impact of PS1 is higher than PS2 for all the considered categories. The contribution analysis shows that the most impactful unit processes are pellet production, direct emissions from pellet combustion and ash management. The less impactful ones are boiler, electricity and transportation (for PS1).
The choice of the ash management system, which represents the third contributor to the total impact, makes a substantial difference to the total impact. In the baseline scenario, the ashes are considered as a normal input, to which a negative value is applied (equivalent to the ash value per functional unit). In the alternative scenario, however, as suggested by many authors, the ashes are considered through an avoided product approach; specifically, the ashes are considered as a substitute for a common potassium fertilizer. The resulting impact is appreciably different, and in favor of the avoided product scenario that shows a lower impact. The reference standard considers the avoided product as one of the best procedures for estimating the impact of co-products or by-products with a potential use. In this specific case, to consider ashes as a potassium fertilizer may not be the best choice due to the low standardization of the ashes and the possible regulatory limits which make ashes unsuitable to use as a fertilizer, and therefore make the replacement procedure difficult to apply. Another noteworthy issue relates to the negative impact of pellet production on the category of Land Use for the Eco-indicator 99 with midpoint approach, due to the use of residual biomass produced by the vine production chain. For ReCiPe, the negative impact of ash on natural land transformation has a similar justification.