Sustainable Aviation Research Articles

Impacts of a sustainable aviation fuel mandate on airline competition — Full-service carrier vs. low-cost carrier

The aviation industry has proposed the collective goal of achieving net-zero carbon emissions by 2050. In addition to the existing market-based methods (MBMs), the use of renewable energies, in particular, sustainable aviation fuel (SAF), could be one of the most promising means for achieving this long-term target. This paper investigates how an SAF mandate from the government can lead to heterogeneous impacts on two types of airlines, namely, full-service carriers (FSCs) and low-cost carriers (LCCs), that compete in the same markets. A game-theoretic economic model is constructed that accounts for the different service quality levels and cost structures (i.e., fuel efficiency) between FSCs and LCCs. Our analytical results suggest that the SAF mandate favors LCCs over FSCs. The share of SAF blended into fuel raises the airfares of both types of airlines while reducing the traffic and profit of FSCs. The impacts on the LCC traffic and profit depend on its fuel efficiency relative to that of FSC. When LCCs are sufficiently fuel efficient, the SAF mandate can increase their traffic and profit. The government subsidy on airline's SAF use is also analyzed, deriving the optimal subsidy levels for FSC and LCC respectively to achieve pareto-improvement in social welfare and consumer surplus. We also examine how air passengers’ environmental awareness affects the outcomes of SAF mandate implementation. Our economic model also accounts for passengers’ heterogeneous preferences in terms of service quality and price sensitivity.

Caproic acid production from short-chained carboxylic acids produced by arrested anaerobic digestion of green waste: Development and optimization of a mixed caproic acid producing culture

Caproic acid is an important compound for producing a variety of chemicals and a potential precursor for Sustainable Aviation Fuels (SAF). This study aimed at developing a stable mixed culture producing caproic acid using short-chain carboxylic acids (SCCAs) derived from arrested anaerobic digestion (AAD) of green waste and optimizing chain elongation (CE) conditions. The results showed that the mixed culture was dominated by Clostridium sensu stricto 12 (53.09 %), optimally producing caproic acid at 37 °C, pH 7.20, with a 15 % inoculum concentration. Under these conditions, 75.59 % of the consumed electrons contributed to caproic acid formation. Gradual ethanol feeding in bioreactor experiments resulted in caproic acid titer of 13.36 g/L, with carbon conversion efficiency (CCE) of 81.91 % and electron transfer efficiency (ETE) of 76.39 %, surpassing batch experiments without gradual ethanol addition. These findings demonstrate that the effectiveness of a mixed culture can be improved, resulting in increased caproic acid production.

An Examination of the Sustainability Activities of Global Airline Collaborations within the Scope of IATA's Sustainability Goals

This study aims to evaluate the sustainability activities of the global airline alliances Star Alliance, SkyTeam, and OneWorld within the framework of the International Air Transport Association's (IATA) sustainability goals. Specifically, it focuses on key areas such as achieving net-zero carbon emissions by 2050, promoting new aircraft technologies, enabling greener travel (in terms of noise and air quality), enhancing energy, resource, and operational efficiency, managing cabin waste and recycling, adopting sustainable aviation fuels (SAF), and protecting biodiversity. Using a content analysis methodology, data were gathered from publicly available sources, including airline sustainability reports, annual reports, and information from the IATA website. This systematic analysis provides an objective evaluation of how well these airline alliances are aligning their strategies with IATA’s sustainability goals. The findings reveal significant disparities in the alliances’ commitments. For instance, Star Alliance and OneWorld members exhibit deficiencies in biodiversity conservation, while SkyTeam members show slower progress towards achieving the net-zero carbon emissions target. Additionally, all three alliances demonstrate varying degrees of success in adopting SAF and implementing other sustainable practices, such as waste management and the promotion of new aircraft technologies. By identifying the strengths and weaknesses across the alliances, this research offers critical insights into how the aviation sector can more effectively contribute to global sustainability efforts.

Optical engine investigation of two-injection strategies for energy-assisted compression-ignition combustion of low cetane number sustainable aviation fuels

Optical engine investigation of two-injection strategies for energy-assisted compression-ignition combustion of low cetane number sustainable aviation fuels

Measurements of particle emissions of an A350-941 burning 100 % sustainable aviation fuels in cruise

Measurements of particle emissions of an A350-941 burning 100 % sustainable aviation fuels in cruise

The Global South Air transport Belt: A catalyst for sustainable tourism and economic growth

This paper explores specific technical dimensions of a Global South Air Transport Belt concept, emphasizing international air transport's pivotal role in fostering global trade, stimulating tourism, promoting people-to-people exchange, and enhancing supply chain efficiency. A notable disparity exists between the air transport infrastructures of the Global North and the Global South. Furthermore, the Carbon Offset Scheme for International Aviation poses an obstacle to the air transport advancement in developing nations. This market-based initiative, designed to cap aviation emissions from 2020, has the potential to impede the actual international network connections expansion in developing countries. As an alternative, this paper advocates for the adoption of operational measures, the utilization of sustainable aviation fuels, and the implementation of technological advancements. This strategy aligns with the United Nations' Sustainable Development Goals. The paper highlights the potential for sustainable development through the air transport infrastructure expansion in developing countries, focusing particularly on the BRICS nations. Creating a cohesive Global South Air Transport Belt will facilitate the growth of tourism and foster economic development across the participating nations. Such an initiative seeks a sustainable pathway toward inclusive economic growth and social progress for humanity.

Investigating the oxidation characteristic of a hydro-processed bio-jet fuel: Experimental and modeling study

A rapid transition from conventional jet fuels to sustainable aviation fuels (SAFs) is imperative in order to reduce carbon emissions. Hydro-processed esters and fatty acids synthetic paraffinic kerosene (HEFA-SPK), as a type of SAF, exhibits broad applications. In this study, a new HEFA-SPK named ZH-HEFA was investigated. The fuel comprises 14% n-alkanes, 85% iso-alkanes and only 1% cycloalkanes by weight, with the majority of alkanes ranging from C9 to C17. Oxidation experiments of the fuel were conducted using an atmospheric pressure flow reactor at temperatures ranging from 550 K to 1075 K under three equivalence ratios (0.5, 1.0 and 1.5). Species mole fraction profiles were measured by an on-line gas chromatographic (GC). For comparison purposes, an experiment was also performed on RP-3, a conventional jet fuel commonly used in China, under the equivalence of 0.5. Compared to RP-3, ZH-HEFA exhibited significantly stronger low temperature reactivity and higher combustion conversion rates while demonstrating considerably lower yields of aromatics at high temperatures. The kinetic simulation of ZH-HEFA was achieved by proposing two surrogates and their corresponding kinetic models. Surrogate S-1 consisted solely of n-dodecane, while S-2 comprised 35% n-dodecane and 65% 2,6,10-trimethyl dodecane by weight. Both surrogate models were validated by the experimental data. S-1 exhibited a closer resemblance to the global oxidation characteristics of ZH-HEFA, whereas S-2 demonstrated improved accuracy in predicting the formation of small hydrocarbon intermediates during the fuel oxidation. Rate of production analysis revealed that the branched alkane component in S-2 possessed more pathways and greater capability than S-1 in generating C3 intermediates, which are important for the generation of aromatics. Furthermore, both models displayed good predictive performance for the auto-ignition properties of HEFA-SPK fuels.-

The ‘hangar of the future’ for sustainable aviation

AbstractSustainability is becoming a major strategic driver within the aviation industry, which has moved from providing primarily economic benefits to delivering the ‘triple bottom line’, including social and environmental impact as well as financial performance. Sustainable aviation is also being tracked by the International Civil Aviation Organisation (ICAO) Global Collation for Sustainable Aviation. Operations and Infrastructure is an important near-term opportunity to deliver sustainability benefits. Digital Technologies, Integrated Vehicle Health Management (IVHM) and Maintenance Repair and Overhaul (MRO) play a prominent role in implementing these benefits, with a particular focus on operational efficiencies. As part of this, the sustainable smart hangar of the future is a concept that is becoming more and more important in forming the future of the aviation industry. The Hangar of the Future is an excellent opportunity for innovation, combining the progress in manufacturing, materials, robotics and artificial intelligence technologies. Succeeding in developing a hangar with these characteristics will provide us with potential benefits ranging from reduced downtime and costs to improved safety and environmental impact. This work explores some of the key features related to the sustainable smart hangar of the future by discussing research that takes place in DARTeC’s (Digital Aviation Research and Technology Centre) hangar led by the IVHM Centre in Cranfield. Additionally, the paper touches on some longer-term aspirations.

DESIGN and be SMART: Eleven engineering challenges to achieve sustainable air transportation under safety assurance in the year 2050

The aviation industry faces various challenges in meeting long-term sustainability goals amidst surging demand for air travel and growing environmental concerns of the general public. The year 2050 is set as an ambitious goal for net zero emissions, a substantial reduction in carbon dioxide emissions per passenger kilometer flown, major improvements in aircraft energy efficiency, and a development towards autonomous, intelligent operations. This review explores the pivotal role of advancements in engineering for achieving sustainability in aviation. Through a comprehensive review of existing literature and case studies, our work highlights how innovations in all aspects of aircraft engineering coupled with operations-related technologies, offer promising solutions to mitigate environmental impact, enhance efficiency, and ensure long-term sustainability in aviation operations. To discuss the necessary advances, we promote the so-called ‘DESIGN and be SMART’ framework, consisting of eleven complementary engineering challenges towards reaching sustainability. To address the high safety levels reached in air transportation, our DESIGN and be SMART framework also addresses the safety assurance challenge that is overarching each of the eleven engineering challenges. We believe that through an orchestrated integration of hardware advancements with innovative software solutions, and novel safety assurance methods, the aviation industry can realize synergistic benefits that drive sustainable growth of air transportation. Our review contributes to such an orchestration by describing the status quo and research challenges ahead.

Deoxygenation Pathways for Sustainable Aviation Fuel from Used Cooking Oil: A Review on Catalyst and Operating Parameters

Deoxygenation Pathways for Sustainable Aviation Fuel from Used Cooking Oil: A Review on Catalyst and Operating Parameters

Aircraft performance of a novel SAF: Lower costs, lower environmental impact, and higher aircraft performance

Investing in Sustainable Aviation Fuel (SAF) is crucial for reducing the aviation industry’s carbon footprint and mitigating climate change. As global air travel demand increases, SAF offers a viable solution to significantly lower greenhouse gas emissions and enhance energy security, ensuring a more sustainable future for aviation. Additionally, converting biomass, particularly waste materials, into SAF adds value by turning potential environmental liabilities into valuable energy resources, promoting a circular economy and reducing overall waste. This study evaluates the aircraft performance of a novel sustainable aviation fuel (SAF) derived from multiple feedstocks in a hybrid biorefinery. SAF performance is compared to two conventional jet fuels, specifically a blend of 30% kerosene and 70% gasoline and JET-A1. The results demonstrated that the optimal SAF outperformed conventional fuels in terms of both thrust and range. Specifically, SAF exhibited a 17% increase in thrust and a 10% increase in range compared to conventional Jet A1 fuel. This novel fuel did not only mitigate CO2 emissions and achieve a cost reduction of 0.13 to 8.08%, but also exhibited superior aircraft performance. In addition, this fuel also meets the criteria of a “drop-in fuel” as it does not necessitate significant alterations to the currently existing CFM56-7B turbofan engine. This is due to its similar key thermodynamic indicators, such as heat capacities and combustion temperature, which are comparable to those of conventional jet fuels. In addition, this paper identifies the sensitivity of the CFM56–7B turbofan engine fuelled by the novel fuel.

Machine learning-enabled techno-economic uncertainty analysis of sustainable aviation fuel production pathways

Stochastic techno-economic analysis (TEA) is pivotal in assessing the financial viability and risks inherent in biofuel production processes. In this method, the Monte Carlo approach entails the random sampling of input variables and multiple runs of the TEA model to create probability distributions of economic metrics. However, traditional Monte Carlo TEA, reliant on iterative calls to process simulation, is resource-intensive and time-consuming, hindering widespread adoption. To address these challenges, we present an accessible framework that harnesses machine learning methods to estimate techno-economic uncertainty in biofuel production pathways. Our approach streamlines the conventional simulation process by automating dataset generation and machine learning model training. These trained models enable rapid predictions of minimum fuel selling prices at any scale, accommodating randomized input variables based on their defined distributions. We illustrate the efficacy of our framework through examples from sustainable aviation fuel production pathways. Our research entails identifying the primary factors influencing uncertainties in minimum selling prices, exploring the synergistic effects of pathway inputs, and assessing how price variability is impacted by financial, technical, and supply chain factors. These examples underscore the framework's effectiveness in addressing breakeven price uncertainties in biofuel production across diverse input scenarios.

Combustion enhancement of sustainable aviation fuel containing ethanol in aero jet engine combustors by regulating the intrinsic characteristics of the fuel

Combustion enhancement of sustainable aviation fuel containing ethanol in aero jet engine combustors by regulating the intrinsic characteristics of the fuel

Life cycle inventories for aviation: Background data, shortcomings, and improvements

In the context of growing environmental awareness and a drive towards sustainable aviation, Life Cycle Assessment (LCA) emerges as a pivotal tool for evaluating the environmental impacts of current and novel technologies. This paper focuses on Life Cycle Assessment within the aviation sector, with a specific emphasis on Life Cycle Inventories (LCIs) and databases. Recognizing a relevant data gap in existing databases regarding aircraft maintenance, our study seeks to address this limitation. A maintenance, repair and overhaul use-case is proposed as an illustrative example to enrich underrepresented data in LCIs. Our methodology considers the entire service life of aircraft, building a cumulative life cycle inventory in a cradle-to-gate approach. Geographical representativeness is ensured for maintenance activities conducted in Germany, with extrapolation applied across Europe where necessary. Our findings underscore the need to differentiate maintenance activities between aircraft components and engines, as well as the importance of considering various flight scenarios, ranging from short to long haul. This paper contributes to the advancement of LCA in aviation by providing insights into improving data accuracy and completeness. It also delves into how and why data generation is possible and what are the necessary data improvements within the topic. This paper is aimed at LCA practitioners in both research and industry, thus fostering sustainable practices in aviation.

On the relationship between air connectivity and economic development: A comparative analysis of inequality evolution for 2000–2019

Throughout the past decades, aviation has experienced a tremendous growth of passenger and cargo demand. This growth may come with problems however, when considering the Sustainable Development Goals set by the United Nations in 2015, particularly those related to the sustainable and fair use of resources and reduction of inequalities among countries. This study discusses the air connectivity inequalities of countries from various perspectives, covering topology-based and schedule-based connectivity indicators. We focus on the connectivity changes of countries between the years 2000 and 2019, i.e., before the impact of the COVID-19 pandemic. Results indicate that our aviation system is dominated by a few highly-developed countries in Western Europe and Northern America. While this dominance is gradually weakened, mainly due to the emergence of aviation hubs in the Middle East and the role of China as a hub in Asia, there need to be more efforts towards a homogeneous aviation infrastructure. Moreover, the flight connectivity between countries appears to be correlated to the Human Development Index growth value, but not significantly correlated with the potential travel demand between countries. Finally, detailed analysis on network-induced inequalities of connectivity reveal that countries are most unequally distributed concerning betweenness centrality but most equally according to closeness centrality. We believe that our study contributes towards a better understanding of sustainable aviation planning and raises important policy issues concerning the achievement of Sustainable Development Goals.

Revisiting Contrail Ice Formation: Impact of Primary Soot Particle Sizes and Contribution of Volatile Particles.

Aircraft contrails, formed largely on soot particles in current flights, are important for aviation's non-CO2 climate impact. Here we show that the activation of nonvolatile soot particles during contrail formation is likely determined by the sizes of primary soot particles rather than the effective sizes of soot aggregates as assumed in previous studies, which can explain less-than-unity fractions of soot particles forming contrail ice particles as recently observed during ECLIF (Emission and CLimate Impact of alternative Fuels) campaigns. The smaller soot primary sizes compared to aggregate sizes delay the onset of contrail ice formation, increase the maximum plume supersaturation reached in the contrail plume, and thus increase the probability of small volatile particles contributing to the total contrail ice particle number. This study suggests that the range of conditions for volatile plume particles to contribute significantly to the contrail ice number budget is wider than previously thought. As the aviation industry is moving toward sustainable aviation fuel and/or lean-burning engine technology, which is expected to reduce not only the emission index of nonvolatile soot particles but also the sizes of primary soot particles, this study highlights the need to better understand how the combined changes may affect contrail formation, contribution of volatile particles, and climate impacts.

Tuning ZSM-22 zeolite through NH4F etching enables enhanced performance and tailored product distribution in n-alkane isomerization for sustainable aviation fuels

Isomerization of n-alkane remains a significant strategy for producing high-quality sustainable aviation fuels (SAF). Improving zeolite catalyst performance and optimizing iso-product distribution to meet the SAF specification are still challenging. As the limitation mainly originated from the strict shape selectivity of the microporous zeolite structure, feasible methods were highly demanded. In this work, the NH4F etching approach was utilized to controllably regulate the framework structure of ZSM-22 zeolite, leading to the rational variation in the crystallinity, porosity, morphology, and acidity, which were closely correlated with the etching severity. Compared to the elliptical 10-membered ring micropore mouths of the parent zeolite, the micropore mouths of the etched ZSM-22 zeolite tended to be circular, as observed by HRTEM and strongly supported by Raman, realizing the regulation of zeolite structure. For n-decane isomerization, the etched catalysts exhibited a considerable increase in catalytic performance, surpassing the parent Pt/ZSM-22 catalyst and the highest isomer yield of 82.1% at the conversion of 91.4% was achieved. The optimized iso-product distribution with approximatively equal amounts of several mono-branched isomers and much higher contributions of the originally disfavored isomers was also observed due to the tuned shape selectivity and the balanced bifunctionality. This work presented an efficient method to design zeolite catalysts for linear paraffin isomerization beyond the limit set by the zeolite structure and shape selectivity that meets the strict standard specification for the freezing point of the SAF.

Towards hydrogen gas turbine engines aviation: A review of production, infrastructure, storage, aircraft design and combustion technologies

This study explores the potential of hydrogen gas turbine engines for sustainable aviation, with a particular focus on their development for low subsonic to transonic flight regimes. Hydrogen offers notable advantages, including zero CO2 emissions and high energy density. However, its widespread adoption is prevented by significant challenges in production, infrastructure, storage, aircraft design, and combustion technology. The limited availability of green hydrogen – the global production totaled only 127 kt in 2023, with 76% produced in China - raises concerns about the feasibility of a rapid shift to hydrogen-powered general aviation. Despite numerous pledges for hydrogen adoption in the Western world, major aircraft manufacturers have primarily engaged in marketing activities rather than substantial product innovation. To date, the only hydrogen aircraft demonstrator from this century is the 2012 Boeing Phantom Eye, which utilized modified hydrogen internal combustion engines rather than a gas turbine engine. The only hydrogen aircraft demonstrator with a gas turbine engine was the 1988 hybrid Tupolev Tu-155 which coupled one hydrogen gas turbine engine to other two jet fuel engines. This paper evaluates the current state of hydrogen gas turbine technology, comparing its combustion characteristics with traditional jet fuels and examining various NOx emission control techniques. It also investigates the potential of micro-mix lean combustion and staged combustion to achieve efficient and low-emission hydrogen combustion in gas turbines. Given the more extensive experience with terrestrial gas turbines compared to their aeronautical counterparts, the review includes insights from terrestrial power generation applications. Additionally, it considers hythane - a blend of hydrogen and natural gas - as a transitional solution for significantly reducing CO2 emissions while progressing towards net-zero targets. The review underscores the urgent need for advancements in green hydrogen production, technology, and infrastructure to overcome existing barriers and enable a more sustainable aviation future powered by hydrogen. Importantly, the review emphasizes the need to move beyond mere virtue signaling and make tangible progress toward a hydrogen economy.

Aerodynamic and aeroacoustic sensitivities of contra-rotating open rotors to operational parameters

Abstract Open rotors can play a critical role towards transitioning to a more sustainable aviation by providing a fuel-efficient alternative. This paper considers the sensitivity of an open-rotor engine to variations of three operational parameters during take-off, focusing on both aerodynamics and aeroacoustics. Via a sensitivity analysis, insights to the complex interactions of aerodynamics and aeroacoustics can be gained. For both the aerodynamics and aeroacoustics of the engine, numerical methods have been implemented. Namely, the flowfield has been solved using unsteady Reynolds Averaged Navier Stokes and the acoustic footprint of the engine has been quantified through the Ffowcs Williams-Hawking equations. The analysis has concluded that the aerodynamic performance of the open rotor can decisively be impacted by small variations of the operational parameters. Specifically, blade loading increased by 9.8% for a 5% decrease in inlet total temperature with the uncertainty being amplified through the engine. In comparison, the aeroacoustic footprint of the engine had more moderate variations, with the overall sound pressure level increasing by up to 2.4dB for a microphone lying on the engine axis and aft of the inlet. The results signify that there is considerable sensitivity in the model and shall be systematically examined during the design or optimisation process.

Diels–Alder Cycloaddition of Cyclopentadiene with α-Olefins for the Synthesis of High-Performance Sustainable Aviation Fuels

Diels–Alder Cycloaddition of Cyclopentadiene with α-Olefins for the Synthesis of High-Performance Sustainable Aviation Fuels