1. Introduction
Bioenergy is expected to have an important role in global scenarios for achieving global low temperature stabilization targets. Using renewable energy sources to displace fossil fuels, enhancing terrestrial carbon sinks, and capturing and storing carbon are among the key mitigation options to achieve climate targets [
1]. For example, the different Shared Socio-economic Pathways (SSPs) indicate that the demand for dedicated crops for bioenergy can range from less than 5000, up to about 20,000 million tonnes per year by 2100, corresponding to about 200–1500 million ha of land for dedicated energy crops [
2,
3]. In these scenarios, bioenergy deployment is often considered, in combination with technologies for capture and storage of carbon emissions (BECCS) [
3]. BECCS allows achieving negative CO
2 emissions, after energy and transportation services are used to replace fossil fuels. However, many studies alert to the fact that feasibility of large-scale deployment of BECCS has not yet been demonstrated, nor have its potential and risks been fully examined [
1,
4,
5,
6].
The international market of forest biomass in the form of wood pellets have increased dramatically in the recent years, with Europe being the main importer and North America the main exporter [
7,
8,
9,
10,
11], mostly due the Renewable Energy Directive (RED) [
12]. In this policy, the European countries aim to increase the share of renewable energy in their gross final energy consumption to 20% by 2020. Much has been written about the environmental issues of this increasing use of woody biomass for bioenergy, e.g., [
1,
7,
13,
14,
15], in terms of net climate change mitigation benefits, competition for land resources, and potential adverse side-effects on biodiversity and other ecosystem services. A large body of work has also quantified the life cycle greenhouse gas (GHG) emissions of wood pellets from short-rotation coppice (SRC) systems [
16,
17,
18,
19,
20,
21,
22,
23,
24,
25], roundwood [
1,
26,
27,
28,
29], logging residues [
8,
25,
30,
31], and wood industry residues [
32,
33], as well as the energy and transportation services from the international market of wood pellets, e.g., [
28,
30,
34,
35,
36,
37,
38]. Results, including the international trade of bioenergy products, usually suggest that the highest avoided emissions occur when wood pellets are used to replace energy systems from high-emission fossil fuel, such as coal e.g., [
6,
7,
8]. For example, climate impacts savings are reported ranging between 50% and 85% for electricity derived from imported wood pellets from the United States, in comparison to electricity derived from fossil fuels in Europe [
30,
37]. Climate impacts related to wood pellet production and transatlantic shipment (e.g., from North America to Europe) are found to contribute as of approximately 50% and 30% of total climate impacts, respectively [
37]. Other environmental impacts are reported, varying mostly due to technical aspects of the chosen bioenergy system (e.g., biomass production system, transportation, plant efficiency and technology), modelling assumptions, and methodological issues (e.g., methods to deal with the coproducts) [
17,
18,
19,
28].
High-density energy wood plantations managed on short-rotation coppice (SRC) are seen as a promising source of biomass for different final uses, since they could help to mitigate some of the environmental issues arising from using forest wood resources for bioenergy [
17,
18,
39,
40]. It is mostly because SRC production systems present relatively higher yields than conventional forestry systems [
39], and the possibility to be gown on marginal land to reverse grassland degradation, thereby being beneficial under both a climate change mitigation and land restoration perspective [
16,
41].
To the best of our knowledge, no previous studies examined the environmental implications of alternative energy and transportation services in Europe form novel technologies and pioneering eucalyptus short-rotation coppice (SRC) systems in Brazil. This research gap is particularly important because many global temperature stabilization scenarios aligning with a 2 or 1.5 °C target predict a large increase in the biomass supply to the international markets, especially from land-rich regions, such as Latin America and Africa, to high energy-demanding regions, such as Europe and East Asia [
2,
3,
42,
43]. However, most of the existing literature focuses on the climate impacts of SRC systems in North America, e.g., [
17,
44], or Europe, e.g., [
16,
18,
19,
41], while a few focus on other regions, e.g., [
27,
40]. The environmental conditions and locations in which biomass resources grow considerably affect yields, management systems, and the product characteristics [
40]. Wood pellets from eucalyptus SRC produced in Brazil are a promising option because eucalyptus is the most important forest species for wood supply in the country [
39], and plantation trials have demonstrated that it is possible to double productivity when eucalyptus is grown under SRC management [
45].
In this study, we apply the life cycle assessment (LCA) methodology to quantify relevant environmental impacts of different bioenergy systems delivering energy and transportation services in Europe from imported wood pellets from pioneering eucalyptus SRC systems produced in Brazil, including production of heat, electricity, advanced liquid biofuels, and BECCS. Using this approach, we aim to determine the best bioenergy systems to convert biomass resources in energy and transportation services in Europe, identify the supply chain stages with higher environmental impacts, and compare the environmental benefits and adverse side effects of bioenergy displacing fossil fuel products. Finally, we carry out a comprehensive sensitivity analysis to address the influence of changes in key technical aspects, modelling assumptions, and methodological issues in the environmental impacts of bioenergy systems.
3. Results
The breakdown of environmental impacts over the life cycle stages is presented in
Figure 2. In these results, unallocated environmental impacts are presented as having 1 tonne
(db) of eucalyptus SRC pellets processed in the four bioenergy systems as the reference flow. The shipping of pellets from Brazil to Europe has remarkably high impacts in most of the environmental impact categories. Eucalyptus SRC establishment has high impacts on eutrophication (EP) and acidification (AP), notably due to use of nitrogen fertilizers. The pellet production stage presents a significant contribution to climate change (GWP100) and photochemical oxidant formation (POFP), mostly because of its high power and thermal energy demand. Harvesting and chipping of eucalyptus SRCs shows relatively moderate contribution to the selected environmental impacts. The BECCS bioenergy system undoubtedly promotes a significant reduction in the climate impacts, while additional inputs needed to capture, compress, transport, and store the carbon account for only 3% of its total climate impacts.
The environmental impacts of different energy and transportation services provided by the four bioenergy systems are presented in
Table 2. In this table, environmental impacts are allocated to the different outputs based on the exergy content of outputs, as described in the methods section. Fossil fuels impacts are also presented in the table as reference systems. All the energy and transportation services from eucalyptus SRC pellets presented lower climate impacts in comparison with fossil counterparts. The second lower climate impacts for heat and electricity production is obtained with the CHP system, logically after BECCS, which yields negative CO
2 emissions.
Considering the other environmental impact categories, HP and FTD options presented higher impacts than to fossil references. These aspects are better visualized in
Table 3, where environmental impacts per tonne
(db) of eucalyptus SRC processed are shown before and after substitution of their fossil reference systems. Most of the bioenergy systems presented slightly higher impacts on EP and AP in comparison to fossil reference systems; however, these categories have a local scope and, therefore, especially important for consideration in site-specific development projects. The CHP and BECCS options reduce environmental impacts in POFP and AP categories when substituting its fossil references. This is mostly due to the electricity from wood pellets substituting electricity from natural gas. In all the bioenergy systems, EP impacts are higher for the bioenergy systems in comparison to the fossil references, due to the use of fertilizers in eucalyptus SRC production systems.
4. Discussion
Climate impacts in
Table 2 are generally in accordance with reported ranges for heat, electricity, and transportation services from wood [
84] and short-rotation woody crops [
6]. Also, climate impacts of heat production in HP and CHP bioenergy systems are similar to heat production from SRC systems using willow in Canada [
17], poplar in Spain [
19], and eucalyptus in France [
18]. Electricity impacts are also similar to electricity from forest residues produced in the United States and transported to United Kingdom [
30]. However, it should be noted that the climate impacts of CHP systems, based on wood feedstock, present high variability in the literature [
84]. Similarly, emission savings ranging from 61 to 115% for FT diesel, compared to fossil diesel, are reported for biomass-to-liquid fuels [
85], in accordance with our results, indicating a 66% reduction in climate impacts in comparison to using fossil diesel.
These results from the other environmental impact categories confirm previous analyses, indicating higher EP impacts for bioenergy systems from wood pellets in comparison to fossil fuels [
17,
18,
19]. The bioenergy system considering Fischer-Tropsch diesel production is the option with the lowest climate change mitigation potential after substitution, meaning the lower climate benefit per tonne(db) of processed biomass. It occurs mainly because heat and electricity are considered to substitute relatively “dirty” natural gas power plants in the present study [
49]. Some studies call the attention that there are more alternatives for decarbonize electricity and heat (e.g., solar, wind, hydro, etc.) than liquid fuels for transportation demand [
86]. Therefore, the potential deployment of liquid transportation fuels from Fischer-Tropsch option remains, especially in regions of increasing demand for liquid fuels and lower capacity of investments for major changes in transportation infrastructure (e.g., massive introduction of electric mobility systems), such as Southeast Asia and Latin America.
4.1. Sensitivity Analysis
A sensitivity analysis was performed to address how changes in key technical aspects, modelling assumptions, and methodological issues, would affect the environmental impacts of the different bioenergy systems. See
Table S7 for details on the life cycle modelling changes implemented in the sensitivity analysis. The sensitivity results in
Table 4 are shown as a percent change in the unallocated environmental impacts and total energy output per tonne
(db) of processed biomass, in relation to the indicated reference system (selected from one of the evaluated bioenergy systems in this study).
Our analysis indicated that alternative modelling choices in the bioenergy systems might significantly alter the results. For example, considering higher moisture content in wood pellets (from 5% to 10%) increased impacts to all categories, except climate change, due to lower energy output (−6.2%). Using wood chips instead of pellets as energy carrier significantly promote higher impacts, due to lower thermal efficiency of wood chips in comparison to wood pellets. In addition, higher degradation of biomass is expected for wood chips, being translated into the lower energy yields (−10.3%). Torrefaction of pellets is seen an interesting improvement to reduce climate impacts in this value chain [
28]. In this alternative, biomass goes through a torrefaction process before the densification stage. The use of torrefied biomass, both as lose material or pellets, can promote lower environmental impacts in comparison to wood pellets, mainly due to more efficient intercontinental transportation system. Torrefaction is identified as a promising alternative from the environmental impact perspective, but at the cost of significant reduction in the energy output. In addition, it is important to highlight that lost torrefied biomass is an uncommon energy carrier, therefore, unlikely to be traded in the international market without any densification step [
87].
Different background energy mixes were investigated for pellet production and FTD processes, since they present comparatively high energy demands. Using additional wood chips as energy source for the pellet production process promoted a reduction on environmental impacts in relation to the reference bioenergy system. However, this would introduce an additional demand on external biomass (and, consequently, more resources, land, etc.). If these impacts are included in the product system cannibalizing some of the wood chip feedstock for providing the energy demand for the pelletizing process, the reduction in energy output is expressive (−8.2%). The emissions in Brazilian electricity mix were found to be particularly high in comparison to similar processes in other countries, mainly due to emissions of CO
2 and CH
4 from flooded areas for hydropower production (the highest share in the Brazilian electricity mix, of about 70%). These emissions are highly uncertain, and obviously should be properly depreciated during the life span of the hydropower plant [
88]. Excluding CO
2 and CH
4 emissions in the hydropower process led to a decrease in the climate impacts of about 11%, and a minor decrease in POFP in comparison to the reference system. Considering, the FTD process located in Brazil causes a decrease in all categories, except for climate change. This is mainly due to the differences in the energy mixes between Brazil and the reference energy mix adopted for the reference country used in Europe (i.e., Norway).
The sensitivity to higher fertilizer inputs in eucalyptus SRC systems assumes figures that are about four times higher than the reference case, reflecting the faster growing rates reported in ref. [
39]. Our results indicated that higher impacts due the higher fertilization rates are not compensated by the higher energy outputs. The use of a smaller ship size, for transoceanic transportation of pellets from Brazil to Europe, is translated to an increase in about 5% to 30% on the various environmental impact categories, highlighting the importance of transportation systems in the results. In particular, it draws attention to more efficient long-haul marine transportation systems. Higher impacts are also observed when using a tractor for forwarding wood instead of a large lorry, once more, highlighting the importance of efficient transportation systems in the bioenergy value chains. Transporting wet biomass to the pellet factory will increase road transportation emissions. However, these impacts are relatively small in comparison to the other supply chain alternatives’ sensitivities presented here. Finally, the alternative of performing carbon capture and storage by using the post-combustion absorption [
48] increases environmental impacts and decreases energy output, mainly because this technology requires additional energy inputs in comparison to the reference case for carbon capture and storage facility.
4.2. Large-Scale Bioenergy Deployment
The quantification of energy outputs and environmental impacts from an idealized large-scale bioenergy deployment, with the introduction of 25 million hectares of SRC eucalyptus in Brazil being used for bioenergy production in Europe, is presented in
Table 5 and
Table 6. In this exercise, we assume that SRC eucalyptus plantations are established on marginal areas previously occupied with pastureland. This premise avoids concerns about the additional pressure of bioenergy systems on productive agricultural land, while minimizing competition with other land uses. This ideal analysis is performed to estimate a benchmark for the possible environmental profile of a large-scale bioenergy production as envisioned by many future energy scenarios.
The considered large-scale bioenergy deployment would be able to deliver significant shares of current energy and transportation demands in Europe (
Table 5). Highest energy services are obtained with the HP and CHP. With the idealized large-scale bioenergy deployment, the heat demand in Europe can be outreached by approximately 2 to 3 times, while the demand of liquid fuel for transportation is attained by 34%, and electricity by 18% to 22%, depending on the bioenergy system. BECCS is a very promising bioenergy alternative regardless of presenting the smallest energy output. The magnitude of the environmental impacts due to large-scale bioenergy deployment in
Table 6 results from the upscaling of figures shown in
Table 2. The energy and transportation services produced in the different bioenergy systems achieve a reduction between 0.9% and 2.4% of global CO
2 emissions in 2015. When compared to emissions from Europe only, large-scale bioenergy deployment is able to provide climate change mitigation between 5.7% and 16%. For the other environmental impact categories and their effects on European total impacts, results largely vary for the different bioenergy systems. POFP is increased with HP (1.7%) and FTD (0.7%), but decreased with CHP (1.5%) and BECCS (1.0%). AP shows the same trend, although with larger relative increases (6.4% for HP and 5.8% for FTD). EP impacts uniformly increase between 4.6 and 5.1%. BECCS delivers the smallest fraction of energy service (
Table 5), but it achieves the highest emission savings, while presenting moderate co-benefits in POFP and AP, and the smallest trade-offs in EP in comparison to the other bioenergy systems considered here.
5. Conclusions and Future Work
All the energy and transportation services from eucalyptus SRC pellets presented lower climate impacts in comparison with fossil counterparts. However, most of them presented slightly higher impacts on EP and AP. Results indicate that the combined heat and power plant with carbon capture and storage is the best option to convert imported biomass from SRCs of eucalyptus in Brazil, in terms of maximizing climate change mitigation. However, this can raise the environmental impacts in acidification, photochemical oxidant formation, and eutrophication, in relation to the reference fossil system. The most impacting activities in the life cycle of a bioenergy chain are primarily attributable to the biomass transport stages (including transoceanic shipping), followed by eucalyptus SRC stand establishment and, finally, the pellet production process. Our sensitivity analysis indicated that bioenergy systems with best environmental performance include on-site biomass storage, transportation of wood chips with trucks, use of pellets as an energy carrier, and large ship sizes for transoceanic transportation. This study supports that the international market of densified biomass from eucalyptus SRC systems is an interesting alternative to decarbonize the transport and energy sectors in Europe. Bioenergy options addressed here are appreciated as a sizeable climate change mitigation measure, without major burden shifting in other relevant environmental impact categories, although there can be some adverse side-effects that should be managed and mitigated. Future refining of the life cycle inventory modelling of energy and transportation services from eucalyptus SRC, during its initial deployment, will be instrumental to identify, manage, and prevent potential conflicting implications of biofuel systems in several environmental areas of concern. Our analysis considered only climate impacts from the life cycle of well-mixed greenhouse gases emissions, such as CO
2, CH
4 N
2O, and some fluorinated species. Further refinement in this analysis can entail the contribution of short-lived climate forcers (such as NOx, SOx, organic carbon, and black carbon) [
89,
90,
91], biophysical aspects such as changes in albedo and evapotranspiration following establishment of eucalyptus plantation [
92,
93], potential changes in biogenic carbon dynamics [
94,
95], as well as applying complementary climate metrics tacking into consideration heterogeneities in the climate system response [
89,
90,
96]. In addition, the inclusion of other relevant environmental impact categories that are normally included in the bioenergy debate, such as water depletion and biodiversity, would provide a better understanding of the co-benefits and tradeoffs between the multiple environmental sustainability implications of bioenergy systems. The inclusion of these aspects in a consistent life cycle assessment framework will allow a better representation of the various environmental challenges our society is facing, as recognized by the Sustainable Development Goals (SDG) agenda.