Environmental profile of ethanol from poplar biomass as transport fuel in Southern Europe
Introduction
The progressive depletion of non-renewable fossil fuels and mankind's growing concern regarding climate change and atmospheric pollution, has led to an interest in the use of renewable fuels. Nowadays, 40% of the total energy consumption worldwide is in the form of liquid fuels such as gasoline and diesel [1]. In fact, transport is almost fully dependent on these kinds of liquid fuels. Special attention has been paid to the potential use of biomass for production of alternative and renewable fuels to be used in vehicles. Liquid biofuels, especially ethanol, provide one of the few options for fossil fuels substitution in the short- to medium-term and are strongly promoted by the European Union [2]: they have the potential to offer both greenhouse gas (GHG) savings and a secure supply of energy [3].
Ethanol derived from biomass has the potential of being an environmental friendly transportation fuel as well as an alternative to gasoline [4]. In fact, it is used in the light duty vehicle fleet in a large number of countries [5]. Traditionally, ethanol has been produced from starch and sugar crops such as cassava, rice, wheat, barley, corn grain or sugarcane [6]. However, a variety of biomass feedstocks can be used for ethanol production (second-generation ethanol), e.g. mill wastes, municipal solid wastes and/or lignocellulosic biomass [7]. Corn based ethanol processes have been commercialized, but an economical and energy efficient process for the conversion of cellulosic material to ethanol has not been achieved yet [8], [9]. However, the land use requirement can cause the competition with food and nature, which is becoming the main driving force of the development of advanced process technologies to produce ethanol from lignocellulosic materials (low value agricultural co-products or wastes like corn stover, wheat straw, sugarcane bagasse, wood or grass).
Several studies were conducted on the environmental impact of ethanol, focusing mainly on two main aims related to the use of biofuels: reduction of primary fossil fuel extraction and reduction of GHG [10], [11], [12], [13], [14], [15]. According to these studies, the production and use of cellulosic ethanol have the potential to result in significant abatement of GHG emissions since the carbon released as CO2 from combustion of the fuel would be incorporated into the regrowth of the plant. In addition, reduction of up to 50% of fossil fuel consumption could be achieved in comparison with the use of gasoline [16].
This study is focused on one woody potential feedstock to produce ethanol. Poplar (Populus sp.) is an example of the implementation of energy crops in Europe for biomass or biofuel in energy forestry systems. It is a fast growing and short rotation energy crop, which is widely used for the manufacture of paper pulp, pallets and cheap plywoods [17]. The advantages of the agricultural production of poplar include the fact that it is a crop with a known tradition in Europe specifically in Spain where 135,720 ha were cultivated in 2008 [18], high yields, high ecological interest and low biomass production costs [19], [20]. Fig. 1 shows the distribution of poplar dedicated areas in Spain based on statistics data from the Spanish Ministry of the Environment and Rural and Marine Affairs [15], which were mapped using the ArcView GIS 3.2 program [21]. Another social and economic benefits of using poplar as feedstock to ethanol production are that this crop does not compete with food and feed crops such as cereals [8], [11]. However, the poplar cultivation presents also disadvantages such as high water requirements which limit the natural distribution of poplar [19], [20]. Therefore, establishment of poplars as an energy crop could compete with other crops in areas having sufficient water and land availability [22]. Strategic planning should be a key factor in the environmental and energetic performance of this crop taking into account the location and local climatic conditions.
Poplar is an ideal candidate for cellulosic ethanol production [23], [24], [25] and an alternative to other energy crops since its biomass presents a chemical composition richer in cellulose and hemicellulose [26] which are the main raw materials for sugar conversion. In addition, poplar could also be interesting in comparison with the use of forest waste with this aim because of forest wastes are commonly used as raw materials in power plants (heat and electricity production) [27].
This study focuses on poplar biomass as a second-generation biofuel, involving the use of cellulosic technology. The objective is to perform an assessment of energy and environmental performance of ethanol using life cycle assessment (LCA) approach. In particular, the analysis compares the environmental performance of i) ethanol in a 10% blend with gasoline (E10), ii) ethanol in an 85% blend with gasoline (E85) and iii) pure ethanol (E100) with conventional gasoline (CG). The full life cycles of ethanol and gasoline are analysed, including the production and transport of the raw materials and fuels, the production of electricity and the use of fuels in a middle size passenger car. So far, although previous LCA studies have been conducted to assess the environmental impacts of cellulosic ethanol from cellulosic feedstocks, no studies were found on ethanol produced from poplar biomass.
Section snippets
Methodology
Life Cycle Assessment approach (LCA) is defined as a methodology for the comprehensive assessment of the impact that a product has on the environment throughout its life cycle (from extraction of raw materials through manufacturing, logistics and use to scrapping and recycling, if any), which is known as a “from cradle to grave” analysis [28]. LCA is an objective process to evaluate the environmental burdens associated with a product by identifying natural resources consumption and emissions to
Life cycle energy and environmental performance
Life Cycle Impact Assessment was conducted using characterization factors from CML methodology [50]. The following potential impact categories have been considered in the analysis: abiotic resources depletion (AD), global warming (GW), ozone layer depletion (OD), human toxicity (HT), fresh water aquatic ecotoxicity (FE), marine aquatic ecotoxicity (ME), terrestrial ecotoxicity (TE), photochemical-oxidants formation (PO), acidification (AC) and eutrophication (EP). In addition, the analysis was
Discussion
Although the amount of biofuels produced in the European Union is growing, the quantities remain small compared to the total volume of fossil transport fuels sold (approximately 0.3% of all EU petrol and diesel fuel in 2003) [51].
There are a few obstacles and constraints that need to be overcome if second-generation ethanol is to be regarded as a sustainable and cost-effective source of energy. Nowadays, biofuels are commercially uncompetitive with fossil fuels (petrol and diesel) in Europe
Conclusions
The present study shows the results of an LCA performed upon poplar biomass based fuel ethanol and its use in FFV whether blended or not with gasoline. The poplar biomass composition, rich in cellulose and hemicellulose, makes it an interesting alternative to making ethanol to compete with conventional gasoline. Poplar cultivation, biomass processing and transport to refinery gate, ethanol conversion and transport to blending refinery, blending of ethanol with gasoline in two ethanol fuel
Recommendations
The production and use of ethanol as transport fuel allow an improvement of the environmental impacts in terms of greenhouse gas emissions and fossil fuel extraction in comparison to gasoline. However, impacts are still considerable in terms of other impact categories such as acidification and nutrient enrichment, where improvements should be made to reduce the contributions (specifically focused on agricultural practices). Further research could be focused on developing a strategic planning to
Acknowledgements
This study was developed within the framework of the AQUATERRE Integrated Project (Project Reference: 212654). Dr. S. González-García and Dr. C. Martinez Gasol would like to express their gratitude to the Spanish Ministry of Education for financial support (Grant references AP2005-2374 and AP2005-2518).
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