Life-cycle assessment of electricity from biomass: Case studies of two biocrops in Spain

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Life-cycle assessment of electricity from biomass: Case studies of two biocrops in Spain Isabela Butnar a,*, Julio Rodrigo b, Carles M. Gasol c, Francesc Castells a a

Departament d’Enginyeria Quı´mica, Universitat Rovira i Virgili, Campus Sescelades, 43007 Tarragona, Spain SIMPPLE S.L., 43007 Tarragona, Spain c SosteniPrA-Institut de Cie`ncia i Tecnologia Ambientals (ICTA), Universitat Auto`noma de Barcelona, Campus de la UAB, 08193 Barcelona, Spain b

article info

abstract

Article history:

Through the Renewable Energies Plan 2000e2010, Spain has fixed the objective of covering

Received 4 September 2009

12% of the primary energy demand from renewable sources. The achievement of this

Accepted 9 July 2010

objective implies an annual increase of 22.4% of the energy produced from renewable

Available online 4 August 2010

sources. In this context, the objective of this study is to determine if the electricity from biomass produced in Spain would be environmentally competitive with electricity from

Keywords:

natural gas or from the Spanish electricity mix. For that, the environmental impacts

LCA

associated to the whole life cycle of two energetic crops in Spain, Poplar and Ethiopian

Biofuels

mustard, used for power generation were evaluated. The overall assessment includes the

Electricity

cultivation and collection of biomass, its transport and the processes of its energetic

Spain

transformation. We calculated different scenarios of electricity production from biomass in different capacity power plants (10, 25 or 50 MW), different transport scenarios and different productivities for biomass production. Our results show that, given the assumptions of this study, Ethiopian mustard is more impacting than Poplar when used for electricity production. Also, the transportation of biomass from the field to the power plant is an important stage that has to be carefully planned in order to get the maximum amount of electricity with a minimum environmental impact. Compared to electricity from natural gas or the Spanish electricity mix, the electricity obtained from biomass is more impacting in three from six impact categories we present here. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Bioenergy (energy from biomass and waste) is recognised as the most important amongst the renewable energy sources and its potential contribution to several global policies is considered very high [1]. The Spanish National Plan 2000e2010 gives an important role to the promotion of renewable energies and emerging technologies. In the field of biomass, it promotes the R&D on new herbaceous and woody species

with a good productivity, as well as on the machinery needed for their recollection [2]. An important weight is accorded to the regional studies, in order to prevent the loss of biodiversity, land-use conflicts, water resources depletion, or other type of unwanted local effects. Some of the energy crops analysed in experimental and demonstration parcels for their implementation in Mediterranean areas are annual species such as Cardoon (Cynara cardunculus) [3e5], Ethiopian Mustard (Brassica carinata)

* Corresponding author. Tel.: þ34 977 55 8556; fax: þ34 977 55 9621. E-mail addresses: [email protected] (I. Butnar), [email protected] (J. Rodrigo), [email protected] (C.M. Gasol), [email protected] (F. Castells). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.07.013

b i o m a s s a n d b i o e n e r g y 3 4 ( 2 0 1 0 ) 1 7 8 0 e1 7 8 8

[6e8], Winter rape (Brassica napus) [6], and also short rotation coppices (SRC) as Poplar (Populus spp.) [8e10] or Eucalyptus (Eucalyptus globulus) [11]. The Cardoon has a wide spectrum of potential applications in Spain which have been investigated in the last years (see [4] for a review of the industrial application of Cardoon). Winter rape is the most common rapeseed cultivated in continental Europe, but not so suitable for clay- and sand-type soils and in semiarid temperate climate such as of some parts of Spain. A better productivity in these areas was proved by the Ethiopian mustard which demonstrated to be suitable for producing biodiesel by transesterification of the oil extracted from its seeds but not as a lignocellulosic biomass crop used to generate heat and power [6,12,13]. In this study we focus on two species: Poplar and Ethiopian mustard for their good yields in Spain and also because of the implemented experimental and demonstration parcels in Soria, Spain, which can provide us with field data on their cultivation. Poplar crop has been selected in this study due to its friendlier overall environmental performance and its highest biomass production yields per hectare in Mediterranean areas compared with annual herbaceous crops [14]. An environmental disadvantage of Poplar is the high consumption of water for its cultivation, e.g. in Soria, Spain 28,000 m3/ ha for a 16 years cultivation [10]. As water is a scarce resource in Spain and other Mediterranean countries [15], the implementation of Poplar as energy crop competes with areas with water availability. Currently in Spain, these areas are extensively occupied by woody crops aimed to produce wood for the paper and packaging industry. In order to reach the national renewable plans targets, the surface needed for the cultivation of Poplar would be approximately 10% of the total current arable land surface, or 36% of the 5 million hectares dedicated to ligneous crops [16]. The objective of this study is to assess the environmental impacts associated to the production of electricity from Poplar and Ethiopian Mustard. We include into the analysis the whole life cycle of these two energetic crops: the cultivation and collection of biomass, its transport and the processes of its energetic transformation. For the cultivation stage we assumed two different productivities for each energetic crop: 4.72 and 8.07 t/ha for Ethiopian mustard and

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9 and 13.5 t/ha for Poplar. For the transport of biomass from the field to the power plant we also considered two scenarios: when the biomass cultivated area (CA) does not exceed 15% from the regional irrigated arable land (RIAL), we considered a distance of 25 km from the field to the power plant; for the other scenarios the distance was set to 50 km. To calculate these distances, we simulated the possible localization of the plant in the Catalan region of Tarragone`s that occupies approximately 25% from the surface of Catalonia. The environmental assessment was done for the production of electricity in different capacity power plants (10, 25 or 50 MW). We analysed the electricity from Ethiopian Mustard vs. electricity from Poplar and compared them to the Spanish electricity mix and electricity from Natural Gas.

2.

Methodology

In order to determine the effects of the production and use of biomass for electricity production, the methodology of LifeCycle Assessment (LCA) was chosen. LCA is an analytical methodology used to provide information on a product’s or service’s energy, material, wastes and emissions from a lifecycle perspective. The technique examines every stage of the life cycle of the product, from raw materials acquisition through manufacture, distribution, use, possible use/reuse and recycling, and then final disposal [17,18]. If real environmental improvements are to be made, it is important to use LCA so that any system changes do not cause greater environmental deteriorations at another time or location in the life cycle. In the case of biomass for electricity production, the assessment was done from “cradle to grave” including all relevant impacts from crop cultivation and harvesting, its transport, to its energy conversion (see Fig. 1). We considered similar life cycles for both biomasses analysed here: they begin with the biomass cultivation, followed by its harvesting, natural drying in the field, transport to warehouses and power plant, combustion, and finally transport of ashes from the combustion of biomass. Biomass is recollected in chips (Poplar) or in bales (Ethiopian mustard)

Fig. 1 e LCA boundaries for power generation from biomass.

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with a water content (w.c.) of 50% at the time of harvesting. It is left in the field to dry naturally until reaching a water content of 25%. The transport of biomass within the field and from the field to temporal warehouses is done with tractors. From there, chips/bales are transported by trucks to the power plant where they are stored until their use. The life-cycle ends with the transport of ashes resulted from the combustion of biomass. The disposal of ashes is not considered within the boundaries of this study. All necessary inventory data for the two biomasses analysed in this work were collected in the project CTM2004-05800-C03-02 sponsored by the Spanish Ministry of Education and Science and complemented with additional data from the Swiss life-cycle inventory database: ecoinvent Database v.1.1 [19]. In Section 2.1 we describe some particularities of Poplar and Ethiopian Mustard cultivation and harvesting in Spain. Section 2.2 points out the assumptions we made for the transportation of biomass. Section 2.3 describes the scenarios thought for power generation and SiAGROSOST tool used for the LCA assessment in this paper.

2.1.

Cultivation of biomass for power generation

2.1.1.

Poplar (Populus spp)

2.1.2.

Ethiopian mustard (Brassica carinata)

Ethiopian Mustard is an annual allotetraploid herbaceous specie derived by crossing Brassica oleracea and Brassica nigra, which is considered to have major potential as an energetic crop in southern Europe due to its adaptation and productivity in the Mediterranean areas as Spain, Italy and France [6]. An important advantage of Ethiopian Mustard’s cultivation is its good adaptability to soils with lower nutritional levels and dry climate regions [12]. The boundaries of Ethiopian Mustard’s cultivation system considered in this work are shown schematically in Fig. 3.

2.2.

Transport (stage) of biomass

2.2.1.

Agricultural inputs transport

To make possible the comparison between the two biomass systems, we assume for both energetic crops the same transport distances and vehicle characteristics. We assumed 16 t regional distribution lorries and distances of 25 km for the transport of seeds to the field and of residues from field works. For the transport of agrochemicals (fertilizers, pesticides) we assumed distances of 500 km and 32 t distribution lorries. Within the field, there are used tractors with trailers for average distances of 0.5 km. We assumed the transport of workers to be done in passenger cars for distances of 25 km.

Poplar is grown as a perennial crop with multiple harvest cycles occurring between successive plantings. In the experimental parcels used for this study in Soria, Spain, Poplar cultivation stages cover a period of 16 years, with 3 five-yearsrotations and one year of field preparation. After the first year of growth, the plants are cut and removed from the field. Best period considered for harvesting is during autumn and winter, when leaves are fallen. At the end of the 16 years, Poplar stools are removed from the site [10]. For the environmental assessment of Poplar’s cultivation system we considered the processes represented in Fig. 2. Besides the field works, they include the transport of herbicides, fertilizers, packaging materials, etc., symbolized by T in Fig. 2, and the use of tractors or other farm machines within the cultivation area (symbolized by M).

2.2.2.

Biomass transport

We assume Poplar is harvested in form of chips, meanwhile Ethiopian Mustard in form of bales. The recollection of biomass within the field and its transport from the field to temporal warehouses is done with tractors. The transport of chips/bales from the field to the power plant is assumed to be made by truck, applying the load limits authorized in Spain: 16 t legal useful load for a regional transport truck [20]. For a biomass density superior to 340 kg/m3, the load weight becomes a limiting factor of transport. For smaller densities the volume of biomass limits the transport load. For the bales of Ethiopian Mustard, we considered a density of 150 kg/m3 (25% w.c). The assumed Poplar’s chips density is 280 kg/m3 (25% w.c) and therefore, in order to fulfill the volume limits,

Multinutrient fertilizer production

Herbicide

(NPK 8/24/8)

production T

M

Herbicide application

M

T

Multinutrient Pesticide

fertilizer production

Top fertilization

production

(NPK 8/24/8) M Field prepa ration

T

Planting

M First cut

T

T

M

Top fertilization

M

Pesticide application

Ha rvest

3 5-years rotations

Base fertilization

KEY T = transport in 16 or 32t trucks M = farm machinery (tractors)

M

T Simpple nutrient fertilizer production (ammonium nitrate)

Fig. 2 e Schematic view of Poplar’s cultivation system.

Poplar stool elimination

Poplar biomass

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Simpple pp nutrient

Multinutrient

Herbicide

fertilizer production

fertilizer production

production

(ammonium nitrate)

T

M

Herbicide application

T

(NPK 8/24/8) M

Base fertilization

M

T

Planting

M

T

Disk

T

M

Top fertilization

Harvest

Rake , Ballin gg& Balles removal

B. Carinata biomass

KEY T = transport in 16 or 32t trucks M = farm machinery (tractors)

Fig. 3 e Schematic view of Ethiopian Mustard’s cultivation system.

we applied a correction factor of 1.214, getting the real trucks capacity of 13 t [8]. For the transport of biomass from the field to the power plant, simulated as located in the Catalan region of Tarragone`s, we considered two different cases depending on the biomass availability for different power plant capacities. For the scenarios where the biomass cultivated area (CA) does not exceed 15% from the regional irrigated arable land (RIAL), we considered a distance of 25 km from the field to the power plant; for the other scenarios the distance was of 50 km.

2.3.

Combustion of biomass

The Spanish legislation on SO2 emissions resulted from combustion processes fixes a limit of 50 mg SO2/Nm3 of combustion gases from biomass [21]. In order to meet his limit, in the simulation of the combustion process we assumed for both biomass types an emission of 50 mg SO2/ Nm3. Other assumption we made in this study is that biogenic CO2 emissions resulted from the combustion of biomass are compensated by the CO2 absorbed during the growth of biomass. For the simulation of biomass fuelled power plants, we have considered European power plants of different capacities, 10, 25, and respectively 50 MW. The considered efficiencies for electricity production are of 25% for 10 MW power plants, 28% and 30% for 25 and respectively 50 MW power plants. We have considered the power plants produce only electricity. We did not consider the transport and distribution of electricity.

2.4.

CA ¼ BF=Pr

(2)

Ethiopian Mustard’s production yields vary between 4 and 10 t/ha [22], and Poplar’s between 9 and 20 t/ha (d.b.) [23]. The scenarios we propose include the lower limits of these productivities intervals and also two average values: 13.5 t/ha for Poplar and 8.07 t/ha for Ethiopian Mustard (see Table 1). All the LCA-calculations in this study were performed with SiAGROSOST, a tool created within our research group. It was designed with two main functions. Firstly, it is a database which allows organizing and managing all the data on the cultivation of Poplar and Ethiopian Mustard, gathered within the project CTM2004-07420-C03-02. All the missing data of the processes along the life cycle of the electricity production from biomass for which we couldn’t obtain local data were imported from the ecoinvent Database v.1.1 [19], reorganized in the form required by our software. Secondly, SiAGROSOST is an assessment tool that allows performing all the LCAcalculations and identifying the life-cycle components that cause the greatest environmental impacts. In this way, the tool is able to provide support for decision-making on impact reduction strategies or for choosing products or services with a better environmental behaviour. An overview of the structure of SiAGROSOST is given in Fig. 4. The modular structure of SiAGROSOST allowed us to update systematically the database with new local data, at the

Scenarios definition and calculation (SiAGROSOST)

Considering different productivities of the selected crops (in dry base), different transport scenarios and different power plant capacities for the production of electricity, we defined 24 scenarios for the environmental assessment of electricity from Poplar and Ethiopian Mustard (see Table 1). The annual biomass requirement (BF ) for biomass power plants is calculated in dry base tonnes (t d.b.) following Eq. (1). BF ¼

an LHV of 18.2 MJ/kg in dry base (d.b.) and for the Ethiopian Mustard 17.70 MJ/kg (d.b.). The cropping area needed to supply power plants CA (ha) in each scenario is calculated through the annual cropping productivity Pr (t/ha), see Eq. (2):

P 3600  H  e=100 LHV  ð1  h=100Þ  103

(1)

where P stands for power plant capacity (MW), H for the number of annual operation hours (here 8000 h), e is the energetic efficiency of the plant, h is the water content (%) and LHV the low heating value of biomass. For Poplar we assumed

Table 1 e Scenarios for the production of electricity from Poplar and Ethiopian Mustard. Scenarios

1 2 3 4 5 6 7 8 9 10 11 12

Ethiopian Mustard 4,72 8,07 4,72 productivity (t/ha) Poplar productivity 9 13,5 9 (t/ha) Transport to the 25 50 power plant (km) Capacity of the power 10 25 50 10 25 50 10 25 50 10 plant (MW) Efficiency of the 25 28 30 25 28 30 25 28 30 25 power plant (%)

8,07 13,5

25 50 28 30

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heat and power generation in combined-cycle power plants besides the generation of electricity from biomass in high capacity power plants (10, 25 or 50 MW), only the last make the object of this paper.

3.

Results and discussion

Using SiAGROSOST we evaluated the scenarios presented above in order to determine the optimum combination of biomass production, transport and power plant capacity. Section 3.1 presents the evaluation of these scenarios and comments the differences we found between Poplar’s and Ethiopian mustard’s behaviours. In Section 3.2 we extend the comparison to the Spanish electricity mix and the electricity obtained from natural gas. Fig. 4 e SiAGROSOST: structure and functions.

same time with the definition, characterization and assessment of various scenarios of electricity generation from biomass. Although during the execution of the project CTM2004-07420-C03-02 we assessed also scenarios of heat generation by burning of biomass in domestic boilers, and

3.1.

Poplar vs. Ethiopian mustard

Fig. 5 presents the global results of environmental impact assessment of Ethiopian mustard and respectively Poplar production chains including biomass cultivation, transport and use for electricity production. For each type of biomass, ten impact indicators were calculated in twelve scenarios (combination of two different productivities, two transport

Fig. 5 e Environmental impacts of Ethiopian Mustard (BC :) vs. Poplar (PS C) production chains. The discontinued lines represent the 25 km transport assumption, the continued lines 50 km assumption.

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Fig. 6 e Distribution of acid and GHG emissions over the production chains of Ethiopian Mustard (productivity of 8.07 t/ha) and Poplar (productivity of 13.5 t/ha).

scenarios and three power plant capacities). For clarity sake, we give here only the results obtained for six impact categories: global warming, acidification, human toxicity, ozone layer depletion, abiotic depletion and photochemical oxidation. The results in Fig. 5 are given in total impacts per kWh produced electricity from biomass. As expected, for all the selected indicators, the impacts decrease with the increase of biomass productivity. Also, an increase of the distance from the field to the power plant implies higher environmental impacts, which reflects the importance of the power plant location for the system environmental behaviour. The calculated impacts caused by the production chain of Ethiopian Mustard are 20e60% bigger than for Poplar, depending on the analysed impact category. This is explained by the small productivity of Ethiopian Mustard compared to Poplar (it is about twice minor) and also the smaller heating value of Ethiopian Mustard: 17.70 MJ/kg vs. 18.20 MJ/kg of Poplar. Concerning the power plant capacity, our results show that the “greenest” electricity is obtained in 10 or 25 MW power plants. Although for bigger capacity power plants the impacts are smaller per kWh of produced electricity, the availability of biomass to feed big power plants becomes a problem, e.g. to feed a 50 MW power plant with Poplar during 1 year of functioning, there are necessary 98,901 t of Poplar, 25% w.c. Considering a productivity of 13.5 t/ha, this amount translates to 7326 ha used for Poplar’s cultivation, which for Catalonia it would suppose 3% from its irrigated land and it would imply

transport of biomass from more than 50 km from the plant. If the Poplar is to be cultivated in the Catalan region of Tarragone`s, this would suppose occupying 11.5% of its irrigable arable land and the transport would be less than 50 km from the plant, as recommended by Ref. [24] in order to maintain the profitability of transport. It’s interesting to observe the relation biomass productivity e transport e environmental impacts per kWh for each biomass type (Fig. 5), as it seems that the two selected crops behave differently. For the Ethiopian Mustard, the productivity has a significant impact on the overall environmental impacts of obtained electricity: for an increase of productivity from 4.72 to 8.07 t/ha the impacts decrease from 21 to 57% for the same power plant capacity. Varying the transport from field to the power station from 25 to 50 km, the increase of the associated environmental impacts is smaller: up to 24%. However, for the Poplar, an increase of productivity from 9 to 13.5 t/ha implies up to 23% smaller environmental impacts per kWh of electricity produced in the same power plant capacity, but a variation of transport from 25 to 50 km implies up to 50% increase of the environmental impacts. These behaviours are important when planning the production of electricity from different biomass types. The results presented in Fig. 5 are aggregated overall the production chain and therefore more difficulty to see where the impacts originate. If these results are disaggregated over the main parts of the production chain (i.e. cultivation,

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Fig. 7 e Contribution of individual field works to the total impact of cultivation.

transport and transformation in power plants), we can obtain more detailed information on how the emissions are distributed along the production chains of the two biomass types investigated here. In Fig. 6 we present detailed results for acidification and global warming impact categories, for three power plant capacities, two transport scenarios and two productivities: 8.07 t/ha for Ethiopian Mustard and 13.5 t/ha for Poplar. The production of electricity from biomass has an important contribution to the generation of acid gases, even if we assumed lower SO2 emissions (50 mg SO2/Nm3) in order to meet the Spanish legislation [21] on SO2 emissions in

Table 2 e Environmental assessment of electricity from natural gas and electricity from the Spanish mix, low voltage. Functional unit: 1 kW h produced electricity.

Acidification (kg SO2 eq.) Global warming (kg CO2 eq.) Human toxicity (kg 1,4-DCB eq.) Photochemical oxidation (kg ethylene eq.) Abiotic depletion (kg antimony eq.) Ozone layer depletion (kg CFC-11 eq.)

Natural Gas

Electricity ES

3,33E-04 4,23E-01 4,83E-02 3,13E-05

8,32E-03 5,90E-01 2,23E-01 3,11E-04

3,61E-03

4,12E-03

5,18E-08

1,81E-08

combustion gases from biomass. In the case of global warming, the power plant has a small contribution to the overall release of GHG because it is assumed that the CO2 emissions are compensated by the CO2 absorbed during the growth of biomass. The observed GHG emissions for electricity production in Fig. 6 are due to the infrastructure of the power plant. In both acidification and global warming, the cultivation of biomass has an important contribution, mainly due to the use of fertilizers in the field works (see Fig. 7). A possible measure for the reduction of these impacts would be the replacement of the mineral fertilizers with alternative natural fertilizers from agriculture, livestock wastes, etc. as suggested in Ref. [7]. The transportation of biomass within the field and from the field to the power plant is also an important element in the production chain of electricity from biomass. As the productivity of Ethiopian Mustard is smaller that Poplar’s, more transport is required in the case of using Ethiopian Mustard as biofuel in a power plant. Also, an increase in the distance from the field to the power plant can lead to an unsustainable process of electricity production from biomass as the costs and emissions from the transport phase exceed the environmental and cost savings of producing electricity from renewable sources.

3.2.

Biomass vs. natural gas and Spanish electricity mix

To compare the environmental profile of electricity from biomass with that of electricity from natural gas and with

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electricity supplied to the Spanish electrical network, we calculated with SiAGROSOST the environmental impacts of low voltage electricity, at grid production in Spain, and of electricity produced from natural gas in a 400 MW combinedcycle plant, see Table 2. All the data are taken from ecoinvent Database v.1.1, where the year of production is 2002. Observe that the LCA of electricity from natural gas and the Spanish mix include the transport and distribution of electricity, meanwhile the LCA of electricity production from biomass does not include it. If we compare the results in Table 2 with those presented in Fig. 5, we can observe that the electricity from biomass, under the assumptions we made in this study, has a worst environmental profile (more associated environmental impacts) than the electricity from natural gas for the categories Acidification, Human toxicity and Photochemical oxidation. For the left categories it has a better environmental profile. Comparing electricity from biomass with electricity supplied at grid in the Spanish network, we observe that electricity from biomass is “cleaner” for all the selected impact categories. This is due to the high dependence of the Spanish electricity mix at grid on fossil fuels.

4.

Conclusions

Our results show that, given the assumptions of this study, Poplar is less impacting than Ethiopian Mustard when used for energetic purposes. Compared to electricity from natural gas or the Spanish electricity mix, the electricity obtained from biomass is more impacting in three from six impact categories we analysed (Acidification, Human toxicity and Photochemical oxidation). Also, better environmental profiles are obtained for 10 or 25 MW power plants, as for bigger power plant capacities the higher efficiency of electricity production is overtaken by higher distances between the field and the power plant and by problems of land availability for the cultivation of biomass. In order to ensure a good environmental profile of the electricity obtained from biomass, a good productivity of biomass cultures has to be maintained and the transport distance from the field to the power plant has to be kept as small as possible. To get good productivities the most important factors are the soil characteristics and the weather, but also the fertilization of soil. As the most impacting step in the cultivation of biomass is the use of fertilizers (they account for up to 82% from the total impacts of cultivation) a possible measure to reduce these impacts is to replace the mineral fertilizers with alternative natural fertilizers from agriculture, livestock wastes, etc. However, the implementation of these small power plants has to go integrated in good biomass management plans that consider forestall residues or other type of recycled residues (e.g. coming from the cleaning of gardens, public parks, etc.). This available biomass can be used for space heating in residential and commercial buildings, or co-firing of biomass in existing coal-fired power plants in order to improve the environmental profile of the national electricity mix. The results presented here have to be interpreted and used carefully, as we analysed only few LCA indicators. To have a global view on the issue of biomass for energy production,

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additional aspects as land-use, the use of water for some biomass types, e.g. Poplar analysed here, costs of production, etc. would have to be taken into account.

Acknowledgements The authors are grateful to the Spanish Ministry of Science and Technology for financing the “Life-cycle assessment of biomass for energy production: application to biofuels production (CTM2004-05800-C03-02)” project within which this study was carried out.

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