The Climate Change Potential (excluding biogenic carbon of the PtE scheme is approximately 12 times smaller than of the PtS scheme. This is mainly caused by a substantial difference in CO2
emissions within the power generation processes. For the PtE and PtS value chains (Figure 4
), the Climate Change Potential of the combustion process for power generation is 26 kg CO2
and 2340 kg CO2
, respectively. The incineration of pruned peach branches in the power plant does not cause direct, additional carbon dioxides emissions, as the carbon dioxide here released is assumed to be biogenic, and for this reason climate neutral. However, in the PtS scheme, the applied electricity substitution stemming from the thermal conversion of hard coal causes significant amounts of fossil carbon dioxide to be released directly.
When only the cradle-to-gate processes are considered (without the power generation processes), the results show a more varied picture. The agricultural processes the peach orchard cause similar CO2
emissions for the chipping and collection process (PtE) and the mulching process (PtS). In both scenarios the total consumption of diesel is comparable. The large differences in total Climate Change Potential between the scenarios show that a combination of efficient harvesting of the PPR together with low fuel consumption leads to both an advantageous energy balance [11
] as well as a lower CO2
In the PtE scenario, there is an additional transport stage, which is lacking in the Pruning-to-Soil scenario. This transport of the PPR to the end user could lead to additional environmental effects by fuel combustion in trucks. In the considered scenario, however, the PPR transport (as loose chips) has a small Climate Change Potential effect (17 CO2
), whereas the covered distance of 50 km (25 km to storage + 25 km to end-user) is relatively large for local biomass-to-energy schemes. Even with larger distances, the impact of the transport to the overall results stays limited, as it was shown by Dyjakon et al. [44
], where the transport distance (by tractor) was increased from 6 to 600 km.
The result of the PPR incineration for power generation (PtE scenario) might be the application additional fertilizers to make up for the deficit of soil nutrients or some additional measures to prevent erosion in the orchard. The exporting of the PPR causes a loss in nitrogen, phosphorous, and potassium. The additional fertilizing (production, spreading, emissions stemming from biochemical soil processes) leads to combined extra emissions of 89 kg CO2
. A risk of erosion caused by not leaving the branches in the orchards is likely to be expected in less humid regions (e.g., Mediterranean climates) or in orchards with lots of rocks. Grass-covered soils (or by other plants) do not show substantial differences in the balance of organic carbon when the pruning residues are removed [31
]. Before planning a PtE scheme, it should be assessed, whether an erosion risk can be expected in the given circumstances [12
In the PtS scenario, on the other hand, the PPR remain in the orchard. In this way both, heavy metals (that are contained in the peach branches), with negative environmental consequences and nutrients and organic carbon, with positive environmental consequences, are introduced to the soil in small amounts. The introduction in particular of nitrogen, contained in the peach pruning residues, to the soil leads to Climate Change related emissions (NH3
, and consequently, of N2
O and NOx
). These are similar to the PtE scenario (with nitrogen contained in artificial fertilizer). In the PtS scheme, the additional amount of organic carbon remaining in the field leads to a limited sequestration effect, as a small share of the organic carbon remains in the soil for a long time. This results in a negative Climate Change Potential of the PtS scenario amounting to −31 CO2
. This reduction of GHG emissions is therefore positive to the environment. These results of energy production from biomass leading to positive GHG emissions for collecting and combustion the biomass and negative GHG emissions for remaining the pruned branches are also reported by Nieto et al. [45
] and Morlat et al. [42
]. The soil effects methodology applied is described in more detail by Den Boer et al. [40
] and Den Boer and Den Boer [41
]. Even though in the PtE scheme considerably higher soil-related Climate Change Potential effects were found, the overall results still show an over ten times, lower Climate Change Potential than the PtS scenario. The limited contribution to the combined results of the soil-related processes, in comparison to the traditional fuel incineration for energy production, is also shown by Ruiz et al. [47
], Boschiero et al. [48
], and Cowie et al. [49
The PPR incineration for power generation in the PtE scheme still leads to limited emissions of GHG because of the use of chemicals for the flue gas cleaning, the construction of infrastructure, and the emission of small amounts of non-CO2 flue gas components affecting Climate Change. The prevented generation of electricity from hard coal incineration within the PtS scheme is the dominant cause of the joint Climate Change Potential as a result of the avoided fossil CO2 emissions.
In the PtE scheme, the Photochemical Ozone Formation category is dominated by the PPR combustion for power generation (9.1 × 10−2 IE·ha−1), mainly caused by the emission of nitrogen oxides, and less so by harvesting machinery production and use (2.6 × 10−2 IE·ha−1). Ethylene, sulphur dioxide, carbon monoxide, and nitrogen oxides emissions caused by hard coal burning (3.6 × 10−1 IE·ha−1) are the predominant drivers in the PtS scheme.
Particulate matter/Respiratory inorganics effects are mainly caused by fine dust (PM2.5) emissions in power generation processes in both of the considered scenarios. The significant difference between the overall scenarios (6.0 × 10−2 IE·ha−1 for PtE and 5.9 × 10−1 IE·ha−1 for PtS) is can be lead back to the incineration process and properties of the applied fuels. In the PtS scheme, sulphur dioxide emissions stemming from the incineration of hard coal have a dominating impact (compared to the PPR combustion, as the PPR is very low in sulfur). Also, the higher the ash content in hard coal (approximately 20%) impacts the overall results. The lower content in the pruned peach branches (3.7% only), notwithstanding the higher amounts of biomass to be combusted in the PtE (accompanied with a lower LHV), leads to a lower emission of fly ash than in the PtS scheme. Ammonia emission resulting from artificial fertilizer application is a small contributor in the PtE scenario. The total Eutrophication Terrestrial potential (2.8 × 10−1 IE·ha−1) is dominated by NH3 emissions (1.4 × 10−1 IE·ha−1) from the use of artificial fertilizer and emission of nitrogen oxides (1.1 × 10−1 IE·ha−1) from the PPR incineration in the PtE scheme. In the alternative scenario, the impact is predominantly caused by the nitrogen oxides emitted during the combustion of substituted hard coal (2.4 × 10−1 IE·ha−1).
Overall, the production of energy based on agricultural wood residues has better environmental results than the alternative scenario system, even though in some cases trade-offs may occur. As an example, with bioenergy systems saving up to 92% of Climate Change Potential, it generally is accompanied by a higher potential in toxicity impacts [48
]. In the specific case of the use of peach pruning residues for electricity generation however, also the toxicity impacts are limited.
Heavy metal (Zn and Cu) to soil emissions caused by the landspreading of the PPR combustion ashes (2.9 × 10−1 IE·ha−1) and Zn emissions stemming from the PPR burning for electricity generation in the PtE scheme (2.3 × 10−2 IE·ha−1) are the main reasons for Ecotoxicity Freshwater impacts. The impact of the PtS scheme (which is larger) is mainly caused by Cr (VI), V, Cu, Ni, and Zn emissions from hard coal incineration (1.5 IE·ha−1). A lesser effect has the Cu and Zn in the PPR stay behind in the peach orchard (2.4 × 10−1 IE·ha−1). A subsequent lesser contributor constitutes the emissions of heavy metal caused by the mulching machinery (1.9 × 10−2 IE·ha−1).
In general, the impact category Acidification is related to a limited number of emissions only. In the PtE scheme, the foremost contributors are NH3 emissions caused by additional artificial fertilizers application (1.2 × 10−1 IE·ha−1) and the emission of nitrogen oxides caused by the incineration of the PPR (7.9 × 10−2 IE·ha−1). In the PtS scheme, the impacts are dominated by mainly Sulphur dioxide and, as a smaller contributor, by the emission of nitrogen oxides from the hard coal incineration for power generation (6.2 × 10−1 IE·ha−1).
In practice, the two major significant parameters affecting the environmental impacts for the PtE scenario are the overall distance (orchard-final user) and the type of substituted energy. As mentioned above, it was shown by Dyjakon et al. [43
], that even with larger distances, the impact of the transport to the overall results stays limited, even where the transport distance is increased by a factor of up to 100. Therefore, the sensitivity of the considered scenarios was determined towards a change in energy source for the modeling of the substituted power generation. In the basic scenario, electricity from coal was used, which was changed to the Spanish electricity mix.
4.2. Sensitivity Analysis—Change in Substituted Energy Source for Power Generation
In Figure 6
, the effects in the considered ILCD LCIA categories are given for the two investigated scenarios: energetic use of the PPR for electricity production (PtE) and leaving the PPR in the orchard with consequent additional power generation according to the Spanish electricity mix.
The results showed that the change in energy source drastically changes the environmental impacts. The PtE scenario remains the same, as no changes were introduced there (see also Figure 5
, which scale was kept in Figure 6
for the sake of comparison). For the PtS scenario, however, the impact in all impact categories is strongly reduced. Now, the PtS scenario shows smaller impacts than the PtE scenario in all considered impact categories except for the Climate Change Potential. The Climate Change Potential for the PtS scenario also significantly drops, but it stays larger than for the PtE scenario.
The strong decrease in impacts observed in the PtS scenario due to change from the substitution of coal power to the Spanish electricity mix is not surprising, considering the high share of low emission energy carries in that mix.
It can be noted (Table 1
) that almost 60% of the energy sources in Spain cause no emissions in the use phase (wind, hydro, photovoltaic, thermal solar, and nuclear). Also, the emissions from natural gas and biogas combustion are significantly lower, whereas the emission limits for waste incineration are more stringent than those for other energy sources. Thus the strong decrease in environmental impacts caused by the change in energy source for the substituted electricity generation can be explained.
Notwithstanding the above, the choice for the substitution of electricity generation based on hard coal is justified. In the first place, in practice, only a hard coal power plant allows for a change in fuel from hard coal to pruning residues. As a result, without any significant changes, in the existing power plant, the PPR can be burnt. In other electricity generation technologies, this is not possible. Secondly, hard coal power plants are the most polluting ones, with the highest GHG emissions, and are therefore the most likely to be replaced by a renewable energy source, as the pruning residues are. Thirdly, coal power plants tend to be older than the more recent renewable plants and, therefore, the first in line to be closed down.