Rogelio Sotelo-Boyás*, Yolibelt Guerrero-Téllez, Malinalli Pérez-Vigueras,
Instituto Politécnico Nacional. ESIQIE, Mexico City. 07738, Mexico
* Corresponding author
Abstract
The use of a more environmentally friendly jet fuel produced from renewable feedstocks, i.e., SAF (sustainable aviation fuel), is a key factor in meeting the aviation industry's climate goals. A life cycle assessment (LCA) was conducted to evaluate the environmental impact and energy consumption of the production of hydroprocessed renewable jet fuel from Mexican jatropha curcas L. oil, and the results were compared against those of conventional fuel. The LCA was performed with SimaPro software V9.5. The functional unit was 1 MJ of SAF. The boundaries of this study were analyzed from cradle to grave to evaluate CO2e emissions along with 10 other environmental impact categories during cultivation, harvesting, biomass transport, oil extraction, industrial hydroconversion and final use under the conditions of Nuevo Leon state in Mexico. The methodology used for this analysis is based on the ISO 14040–ISO 14044 standards and considers the guidelines established by the International Aviation Carbon Offsetting and Reduction Scheme (CORSIA). The results revealed that the major environmental impact is due to the agricultural stage. Compared with greenhouse gas (GHG) emissions from fossil jet fuel, GHG emissions from jatropha oil-based SAF were 62.05% lower. This study shows that the use of clean fuels is favorable for the environment and is a viable option to start implementing in Mexico.
Keywords: green diesel, SAF, jet fuel, LCA, HEFA, jatropha.
The rapid growth of the aviation sector in recent years has generated growing concern about its impact on the environment [1]. This concern has motivated aviation stakeholders to adopt a common goal: net-zero emissions by 2050 [2]. Current actions include the use of new and optimized aircraft designs to increase fuel efficiency. However, this action is not enough to achieve such a decarbonization goal [3]. As an alternative, the use of drop-in alternative jet fuel, known as sustainable aviation fuel (SAF), has been identified as the foremost option to reduce the emissions of CO2e in the international aviation sector. Dichter et al. [4] claim that the use of SAF could reduce the aviation carbon emissions by 70 to nearly 100%. In addition, the SAF can be used as a partial substitute for fossil jet fuel, up to 50% volume, without any engine modifications in the aircraft and without requiring any additional refueling infrastructure [5,6].
In 2018, as a means to contribute to a cleaner aviation sector, the United Nation’s International Civil Aviation Organization (ICAO) established eligibility criteria for the SAF and proposed an approach to calculate its life cycle GHG emissions under the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) [7].
Several countries have already implemented or considered SAF policy options [8]. In Mexico, however, infrastructure does not exist, and the biofuel-based aviation status involves the search for sustainable feedstocks and technically viable processing technologies. In this work, we propose the use of jatropha curcas L. as a potential feedstock as well as the implementation of hydroprocessed ester and fatty acid (HEFA) technology inside or close to Mexican petroleum refineries.
Jatropha curcas L. is a native plant from Mexico and Central America [9]. The oil of Jatropha Curcas L. has gained importance in the last two decades from an environmental and energy point of view, since it has several advantages over other oils used in the production of fuel, such as its ability to grow on marginal soils for up to fifty years, its ability to grow on marginal soils, and its oil content ranges from 30% to 40% by weight, and it grows relatively fast.
On the other hand, the HEFA process is based on hydrotreating and hydrocracking processes that are commonly carried out in petroleum refineries for treating vacuum gas oil. Mexican refiners have been subjected to hydrotreating processes for more than 50 years. Thus, infrastructure and technical know-how are already in place. Hydroprocessing Jatropha Curcas L. oil therefore seems to be promising for the production of SAFs in Mexico.
In addition to the SAF process, the HEFA process results in a significant yield of green diesel and a lower amount of green gasoline and LP gas. To investigate the environmental impact of producing SAFs and green diesel, in this work, we perform an LCA study following the CORSIA guidelines.
The software SIMAPRO V9.5 was used to perform the LCA following the ISO 14040 and 14044 methods [10]. The goal of this study was to evaluate the environmental impact of green diesel and SAF. The scope of this study encompasses the cradle-to-crave life cycle from jatropha cultivation to transportation, oil extraction, green diesel and SAF industrial production, and combustion.
The functional unit is 1 MJ of lower heating value from fuel combustion for both green diesel and SAF. This unit is designed according to the function of aviation fuel, i.e., to power aircraft engines.
The CML-IA baseline V3.03/EU25 methodology developed by the Institute of Environmental Sciences of the University of Leiden in the Netherlands [11] was used for impact assessment. The impact categories determined in this study are abiotic depletion, abiotic depletion (fossil fuels), global warming (GWP100a), ozone layer depletion (ODP), human toxicity, freshwater aquatic ecotoxicity, marine aquatic ecotoxicity, terrestrial ecotoxicity, photochemical oxidation, acidification and eutrophication.
The electricity input was modified based on average utilization for the northern region of Mexico. The capacity of the plant is 2000 barrels per day. Other materials and energy inputs were selected mostly from the Ecoinvent database in SimaPro. Mass and economic allocations were considered to account for the environmental impact of byproducts. Finally, the hot spots of the life cycle were determined, and a sensitivity analysis of the global carbon dioxide equivalent (CO2e) emissions was performed.
In this stage, 24.6 g CO2e/MJ were estimated to be generated. The main contributions to environmental impact are the use of herbicides, followed by the use of fossil fuel for transport and cultivation activities and the use of fertilizers.
The impact of abiotic depletion is that glyphosate and fertilizer (triple superphosphate and urea) are more prevalent because, for the production of these two elements, large amounts of materials derived from nature are extracted. The impact of transport refers to the use of fuel for the use of conventional diesel for tractors and for the transportation of the harvested Jatropha Curcas L. seeds to the biorefinery (approximately 40 km).
Urea is considered to be main component in fertilizer used, whose main impacts are in the categories of global warming and ozone depletion, mainly because to the emissions of ammonia (NH3), molecular nitrogen (N2) and nitrous oxide (NO2), which are generated during its use and contribute equally to the emission of GHGs.
In this stage, the jatropha oil is considered to be extracted via a solvent extraction method, which is one of the most viable methods for recovering a relatively high percentage of oil from the seed. N-hexane was used as the solvent, and a Soxhlet-type extractor was used for this method. At this stage, transportation is not considered, since it is assumed that the extraction plant is located within the biorefinery.
Electricity has a greater percentage of impact in most categories because the electricity used comes from Mexico's energy mix, which is generated from fossil sources.
The amount of emissions generated at this stage was 1.17 g CO2e/MJ of SAF.
During this stage, the SAF is produced by hydroprocessing. Thus, this stage is one of the most important. The main utilities input to the HEFA production process are similar to those of a typical refining system and include steam, natural gas, cooling water, and electrical power.
Natural gas and electricity are the main utilities that contribute a large percentage of the impact in most categories of environmental impact. In the case of electricity, its impact is relevant because its generation comes from fossil fuels.
In the case of natural gas, its impact is because during its production process, GHG emissions are generated. Thus, in this stage, 4.09 g CO2e/MJ of SAF produced were estimated to be generated.
The emissions generated during final transport were considered from the biorefinery to the Monterrey International Airport, which is approximately 30 km long. The transport of the SAF is carried out by pipelines. The annual demand for the SAF at Monterrey International Airport alone is considered to be 1,000,000 L/year. However, according to the capacity of the production plant, 317,840 L of SAF (2000 barrels) are produced per day. With the emission factor of the national electricity system 0.444 kg CO2e/kWh and considering a density of 0.775 kg/L and a heat value of 42 Mj/kg, the emissions generated in this transport stage are 3.53 g CO2e/MJ.
The emissions of CO2e in the combustion of SAF in the aircraft are considered to be of abiotic origin; therefore, they are not taken into account in the estimation of the global warming potential.
The baseline established for carbon intensity for conventional jet fuel is 88 g/MJ [12. In this study, the CO2e emissions from convention SAF from jatropha curcas L. oil amounted to 33.39 CO2e/MJ. Thus, there is reduction of 62.05 % of CO2e emissions.
This work fulfills the main objective of examining the environmental impacts and emissions of obtaining SAFs from Jatropha Curcas L. oil. After comparing the GHG emissions generated during the production process of conventional jet fuel with those of SAF, a 62.05% reduction in emissions is obtained, so replacing fossil fuels with green fuels is an environmentally feasible option.
Importantly, all the inventory data have been scaled only to obtain 1 MJ of SAF; however, the demand for jet fuel in Mexico per year is 19,757,780 L/year, and the number of hectares that are needed to supply a crop that covers the demand is 28,401.81 ha; for this, the state of Nuevo León was selected since it is the state that meets the available hectares of crop. An important consideration is to designate more hectares of cultivation for biofuels to meet the demand for national fuel. In addition, this would provide business opportunities in Mexico, both for large companies and for small agricultural entrepreneurs.
In addition, to carry out the production process, it is necessary to implement biofuel processing plants in the country or biorefineries that are relatively close to the crop fields to reduce the transport emissions generated from the cultivation field to the biorefinery.
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