Well-to-wheel life cycle assessment of Eruca Sativa-based biorefinery

Highlights

• Three biorefiney scenarios based on Eruca Sativa (ES) as feedstock were investigated.

• Life cycle assessment approach from energy balance and environmental point of views was employed.

• ES-based biorefinery could result in a total net energy gain (NEG) of 4.94E+08 MJ.

• Compared with neat diesel ES-based biodiesel could save GHG emissions by 150%.

Abstract

Renewable energy generation through biorefineries is increasingly considered as more sustainable in comparison with fossil-based fuels as well as single-product renewable energy systems. However, biorefineries have many system variations, and therefore, the evaluation of their environmental performance and comparison with conventional systems before large-scale deployment is essential. In this paper, the sustainability of three different biorefiney scenarios (Sc-1, Sc-2, and Sc-3) based on Eruca sativa (ES) as feedstock were investigated using a life cycle assessment approach from energy balance and environmental point of views. Biodiesel, electricity, ethanol, heat, glycerol, and/or biomethane were the marketable products taken into account under the conditions of these scenarios. According to the results obtained, we argue that although biorefineries offer unique features as most effective alternatives for mitigating climate change and reducing dependence on fossil fuels, the selection of biomass processing options and management decisions can widely affect the final evaluation results. Overall, providing transportation fuel through Sc-2 in which biodiesel, electricity, ethanol, heat, and glycerol were produced could decrease GHG emissions by approximately 140% compared with the combustion of neat diesel while also offering a total net energy gain (NEG) of 4.94E+08 MJ/yr. Nevertheless, if biorefineries are to be used for future transportation fuel production, a great deal of efforts should still be made to achieve better environmental performance in the Human Health and Ecosystem quality damage categories.

Introduction

Rapid depletion of fossil resources and the intensifying environmental problems caused by the intensive use of these energy carriers seem to serve as strong drive to further increase the share of renewable energy carriers on the market [1], [2]. On the other hand, while less concern is raised about the future supply of renewable electricity and heat due to the availability of a variety of renewable alternatives (i.e., wind, solar, hydro, biomass, and others), major concerns still exist regarding the transportation sector and the role renewable energies could play to transform this sector remarkably. This is specifically true about the next few decades when the sector has been anticipated to highly rely on biomass-derived energy carriers, also known as biofuels [3].

Historically, initial attempts led to the production of first generation biofuels mainly produced from various feedstocks such as soybean [4], [5], [6], corn [7], [8], [9], and rapeseed [10], [11], [12]. However, in spite of technological availability and economic viability, they suffer from serious drawbacks including food vs. fuel competition over water/land, land use change (LUC), biodiversity loss, and feedstock limitation [13], [14], [15], [16]. Hence, second generation biofuels mainly produced from a variety of biomass resources including non-food crops and residues were introduced and evaluated. In spite of their many advantages, the expensive and sophisticated technologies used for producing these biofuels have imposed serious limitations on their profitability and economic viability [17].

To overcome the changes faced to enhance the economic viability features of the second generation biofuel as future transportation fuel, the biorefinery concept has been frequently investigated [18], [19], [20]. In fact, the main objective of a biorefinery as presented by the International Energy Agency Bioenergy Task 42, is to produce a spectrum of marketable products in order to maximize the profitability of a given system [3], [21]. As for biofuels, the biorefinery concept is translated by integrating different biomass conversion processes in order to produce multiple products of high economic value, e.g., bioethanol, methane, and heat from sugarcane bagasse [21]. It should be mentioned that although economic aspects are of critical importance in achieving a viable energy system, they only constitute one vertex of the sustainability triangle. This means that environmental and energy analyses should also be taken into account to correctly define the behavior of a given biofuel system under consideration [22].

Life Cycle Assessment (LCA), a method by which the potential environmental impacts of a product system could be evaluated through its life cycle [23], is a valuable approach to point out the environmental aspects of biofuel biorefineries and to compare these renewable systems with petroleum refineries. This could assist with finding a trade-off among different energy generation scenarios leading to the employment of the more economic and sustainable ones [24]. In fact, this approach has been widely used to build the eco-profile of various biofuels such as bioethanol [25], [26], [27], [28], biogas [14], [29], [30], and biodiesel [31], [32], [33], [34]. The use of LCA methodology for investigating the biorefinery concept for sustainable production of biofuels and biomaterials has proved to be promising and has been attracted a great deal of interest [3], [35], [36], [37].

Plants producing non-edible oils have been introduced as appropriate feedstocks for developing future biorefineries because they can address the controversies caused by using edible oils as energy feedstocks, i.e., high cost and food security conflicts [19], [38]. Moreover, they can be grown on degraded agricultural soils, infertile wastelands as well as marginal and low grade lands while leading to increased soil carbon pool due to LUC [18]. However, in large scale energy production systems, they may require fertile soil and good farm practices in order to produce economically-viable yields which could consequently result in higher environmental burdens [39]. On the other hand, the process options by which a given feedstock is treated in a biorefinery could also widely affect the amount of energy produced, and consequently the environmental burdens released to the ecosphere.

Among different non-edible oils used so far for second generation biofuel production, Eruca Sativa (ES) has recently attracted a great deal of attention as a valuable biofuel feedstock within the biorefinery framework [39], [40], [41], [42]. This is ascribed to the unique characteristics of this plant, i.e., excellent resistance to drought, poor soil conditions, and diseases, as well as its potential to be used for multiple biofuels production [19], [38]. ES could be cultivated on weak and dry lands while waste water could be used for irrigation [38], [42]. To the best of our knowledge, only one study exists in which biodiesel production from ES with a biorefinery approach was taken into account and the other reports available were only focused on biodiesel production from ES seeds [19], ignoring the high potential of ES leaves, stems, and seed cake, remaining after oil extraction, for biomethane and ethanol production. Moreover, the biorefinery-based study conducted previously with focus on ES as the feedstock failed to include the environmental considerations in the analyses. Moreover, it did not consider the different process management options by which ES feedstocks could be treated in a biorefinery, while these process management strategies could have deeply influenced the system's economy and pollution issues.

Therefore, the present study was set to investigate three distinctive scenarios for production of ES biodiesel and relevant co-products based on a biorefinery approach while a thorough LCA was also carried out taking into account ES cultivation, harvesting, transport, processing, conversion, as well as the final use of the produced ES biodiesel in diesel engines. To show the potential benefits and/or drawbacks of ES biodiesel production on a biorefinery scale, the results were also compared with a fossil fuel reference, i.e., mineral diesel, when ES biodiesel used to replace fossil fuels.