Sustainable Aviation Fuels Research Articles

The pathway for achieving sustainable aviation fuel production in the Middle East and North Africa region

AbstractThe Middle East and North Africa (MENA) region faces significant challenges in the production of sustainable aviation fuel (SAF) due to high costs, limited feedstock availability, and underdeveloped infrastructure. However, the region has diverse feedstock options and can leverage existing refining capabilities and advanced conversion technologies for SAF production. Addressing these challenges requires research and development (R&D) collaboration, supportive policies, and international knowledge exchange. To promote the production of SAF in the MENA region, a comprehensive strategy is required that includes collaboration, policy alignment, research and development, infrastructure development, and training. Market incentives such as regulations and financial support can help drive demand for SAF. Collaborating with international organizations can improve knowledge sharing and promote sustainable practices. Research and development collaboration is crucial for innovation and establishing a strong SAF industry. Policy harmonization can create a supportive framework for investment and innovation. Monitoring and reporting mechanisms can ensure transparency and verify sustainability. By addressing these aspects, the region can become a leading hub for SAF production and contribute to a sustainable aviation industry.

Critical interplay between ruthenium oxide and water for the catalytic conversion of lignin to sustainable aviation fuel

The production of renewable drop-in fuels from lignocellulosic biomass is desirable for achieving net-zero carbon emissions in the transportation sector. However, the high irregularity of lignin and its resistance to complete ring saturation and deoxygenation hinder its conversion to sustainable aviation fuels (SAFs) such as C7–C16 isoparaffins and alkylated cycloalkanes. In this study, we achieve the direct production of SAFs from lignin-derived oil over a nonacidic Ru/C catalyst in water. Hydrogen atoms generated by H2 dissociation combine with neighboring water molecules to produce hydronium ions, which are then transferred via H-shuttling to attack the double bonds and cyclic –OH groups of adsorbed reactants and intermediates. Density functional theory calculations suggest that hydrodeoxygenation in water decreases the activation energy for the direct hydrogenolysis of cyclic –OH groups over oxygen-vacant ruthenium oxide sites. The critical interplay between oxygen-vacant ruthenium oxide and water results in the almost-complete conversion of lignin-derived monomers, dimers, and bio-oils to desirable SAF components.

A convolutional neural network prediction model for aviation nitrogen oxides emissions throughout all flight phases

In recent years, there has been an increasing amount of research on nitrogen oxides (NOx) emissions, and the environmental impact of aviation NOx emissions at cruising altitudes has received widespread attention. NOx may play a crucial role in altering the composition of the atmosphere, particularly regarding ozone formation in the upper troposphere. At present, the ground emission database based on the landing and takeoff (LTO) cycle is more comprehensive, while high-altitude emission data is scarce due to the prohibitively high cost and the inevitable measurement uncertainty associated with in-flight sampling. Therefore, it is necessary to establish a comprehensive NOx emission database for the entire flight envelope, encompassing both ground and cruise phases. This will enable a thorough assessment of the impact of aviation NOx emissions on climate and air quality. In this study, a prediction model has been developed via convolutional neural network (CNN) technology. This model can predict the ground and cruise NOx emission index for turbofan engines and mixed turbofan engines fueled by either conventional aviation kerosene or sustainable aviation fuels (SAFs). The model utilizes data from the engine emission database (EEDB) released by the International Civil Aviation Organization (ICAO) and results obtained from several in-situ emission measurements conducted during ground and cruise phases. The model has been validated by comparing measured and predicted data, and the results demonstrate its high prediction accuracy for both the ground (R2 > 0.95) and cruise phases (R2 > 0.9). This surpasses traditional prediction models that rely on fuel flow rate, such as the Boeing Fuel Flow Method 2 (BFFM2). Furthermore, the model can predict NOx emissions from aircrafts burning SAFs with satisfactory accuracy, facilitating the development of a more complete and accurate aviation NOx emission inventory, which can serve as a basis for aviation environmental and climatic research. SynopsisThe utilization of the ANOEPM-CNN offers a foundation for establishing more precise emission inventories, thereby reducing inaccuracies in assessing the impact of aviation NOx emissions on climate and air quality.

Rapid assessment of jet engine-like soot from combustion of conventional and sustainable aviation fuels using flame spray pyrolysis

Rapid assessment of jet engine-like soot from combustion of conventional and sustainable aviation fuels using flame spray pyrolysis

Geospatially explicit technoeconomic assessment of sustainable aviation fuel production: A regional case study in Virginia

There is strong interest in sustainable aviation fuels (SAF) to decarbonize aviation; however, local decision-makers will need to consider what additional incentives could stimulate SAF commercialization within their own jurisdictions. This study analyzed SAF production in Virginia, evaluating two biomass-to-energy platforms (gasification Fischer Tropsch [GFT] and pyrolysis) and two regionally abundant feedstocks (woody wastes and municipal solid wastes). A suite of open-access modeling tools were applied to possible SAF supply chains encompassing feedstock collection and transportation, conversion, and fuel upgrading and transport. Key modeling outputs were minimum product selling price (MPSP) ($/gallon) and life-cycle global warming potential (GWP) (g CO2eq/MJ). Results suggest that early SAF production via GFT will require local incentives of approximately $3.61 per gallon compared to $0.75 per gallon for pilot-scale pyrolysis. Location of production facility (by county) influences economic and environmental metrics but is not nearly as important as facility size (tonnes/year). Different formats of financial incentives (i.e., tax credits, loan forgiveness, etc.) offer markedly different reductions in SAF MPSP. Finally, under current federal incentives in the US, it is still more economically efficient to use pyrolysis (with higher GWP) than GFT (with lower GWP). Therefore, regional stakeholders will need to navigate the tradeoff between economic and environmental performances of these platforms. Though Virginia was used as a case study, the methodology is replicable for other jurisdictions, insofar it can be adapted for use in other locations without decision-makers having to completely build their own TEA models.

Experimental and data-driven chemical kinetic modeling study of alcohol-to-jet (ATJ) synthetic biofuel for sustainable aviation fuels

This work assesses the chemical ignition delay behavior of an isoparaffinic alcohol-to-jet (ATJ) fuel, one of the certified sustainable aviation fuels (SAFs). Chemical ignition delay measurements of ATJ were performed at a compressed pressure of PC = 2 MPa and three equivalence ratios (phi = 0.5, 1.0, & 1.3) in synthetic dry air, between 667 K and 1250 K. The unique chemical structures of isoalkanes result in high thermal decomposition rate, but low chemical oxidation reactivity. The new lumped mechanism (ARLMech-HC-ATJ) is developed using a genetic algorithm based on empirical ignition delays. The mechanism shows good performance over the temperature range from 667 K to 1250 K and varied equivalence ratios. Using the suggested mechanism, this study presents sensitivity analysis for the temperature range to understand chemical kinetics of ATJ fuel. The results show the impact of using ATJ modeling to design and optimize propulsion systems operating in the negative temperature coefficient (NTC) or low temperature chemistry (LTC) regime in the future and a methodology that can be adapted to other novel fuels.

An adaptive energy management strategy for airports to achieve carbon neutrality by 2050 via waste, wind, and solar power

After the border lockdown, global traffic has returned, making green airports a government goal to reach carbon neutrality by 2050. Sustainable aviation fuel (SAF) use for commercial aircraft alone won't help achieve net-zero emissions (NEZ). An independent renewable energy supply system at airports is urgently needed to implement green airports worldwide. This study develops a renewable energy power supply system that integrates wind, photovoltaic (PV), and waste-to-energy (WTE) sources to investigate a new adaptive model predictive control (MPC) energy management strategy based on airport operational characteristics. Simulations of Copenhagen Airport’s energy programme confirmed this energy management strategy’s feasibility. Energy management scenarios for two typical summer and winter load days were simulated for case studies. Field data from Copenhagen was used for simulation analysis, and experimental modelling of the airport’s energy scheme proved the energy management program’s practicality. The simulations show that Copenhagen airport renewable energy generates half of its electricity in winter and 81.0% in summer. A total operating cost of USD 36,087.83/day in summer and USD 102,061.20/day in winter is needed to reduce carbon emissions by 41.19 Mt/day in summer and 43.96 Mt/day in winter. This approach should help deploy green airports worldwide and reach carbon neutrality by 2050.

Abating CO2 and non-CO2 emissions with hydrogen propulsion

Abstract This contribution focuses on the abatement with hydrogen of CO2 and non-CO2 emissions. It is agenda-setting in two respects. Firstly, it challenges the globally accepted hydrocarbon sustainable aviation fuel (SAF) pathway to sustainability and recommends that our industry accelerates along the hydrogen pathway to ‘green’ aviation. Secondly, it reports a philosophical and analytical investigation of appropriate accuracy on abatement strategies for nitrogen oxides and contrails of large hydrogen airliners. For the second contribution, a comparison is made of nitrogen oxide emissions and contrail avoidance options of two hydrogen airliners and a conventional airliner of similar passenger capacity. The hydrogen aircraft are representative of the first and second innovation waves where the main difference is the weight of the hydrogen tanks. Flights of 1000, 2000, 4000 and 8000 nautical miles are explored. Cranfield’s state of the art simulators for propulsion system integration and gas turbine performance (Orion and Turbomatch) were used for this. There are two primary contributions to knowledge. The first is a new set of questions to be asked of SAF and hydrogen decarbonising features. The second is the quantification of the benefits from hydrogen on non-CO2 emissions. For the second generation of long-range hydrogen-fuelled aircraft having gas turbine propulsion, lighter tanks (needing less thrust and lower gas temperatures) are anticipated to reduce NOx emissions by over 20%; in the case of contrails, the preliminary findings indicate that regardless of the fuel, contrails could largely be avoided with fuel-burn penalties of a few per cent. Mitigating action is only needed for a small fraction of flights. For conventional aircraft this penalty results in more CO2, while for hydrogen aircraft the additional emission is water vapour. The conclusion is that our research community should continue to consider hydrogen as the key ‘greening’ option for aviation, notwithstanding the very significant costs of transition.

Does carinata, an oilseed crop for sustainable aviation fuel, improve the eco-efficiency of crop rotations in South Georgia, United States?

In 2019, the aviation sector emitted about five percent of the total energy-related carbon dioxide equivalent (CO2e) emissions in the United States. The replacement of conventional aviation fuel with drop-in sustainable aviation fuel (SAF) derived from various biomass-based feedstocks is vital to reduce the overall CO2e emissions of the aviation sector. The SAF derived from Brassica carinata (carinata), an oilseed winter crop, could help the aviation sector mitigate CO2e emissions. Therefore, it is vital to develop a framework for assessing the eco-effectiveness of carinata in crop rotations by combining economic and environmental benefits. We identified 292 crop rotations (over a four-year rotation period with and without carinata) in South Georgia, a major agricultural region of the United States. We conducted a comprehensive life cycle assessment to determine the total carbon emissions of selected crop rotations. We combined the carbon emissions and profitability information using data envelopment analysis (DEA) to estimate the eco-efficiency score of each crop rotation. The results indicate that crop rotations with carinata emit less CO2e than crop rotations without it, especially when carinata replaces winter wheat. Carinata's profitability (net present value of $2996/acre) is highest in a “corn-corn-soybean” rotation at a contract price of $441 per metric ton. Finally, “cotton-carinata-cotton-fallow-soybean-fallow-cotton-carinata” is the most eco-efficient rotation at the same contract price. Overall, carinata increases eco-efficiency at the farm level and, thereby, has the potential to promote the bioeconomy in South Georgia and beyond.

Techno-Economic Evaluation of Jet Fuel Production via an Alternative Gasification-Driven Biomass-to-Liquid Pathway and Benchmarking with the State-of-the-Art Fischer–Tropsch and Alcohol-to-Jet Concepts

Around 65% of the mitigation needed for the targeted net-zero carbon aviation emissions in 2050 is expected to come from Sustainable Aviation Fuels (SAFs). In this study, an alternative gasification-driven Biomass-to-Liquid (BtL) concept for the production of SAFs is introduced and evaluated. In particular, a fuel synthesis scheme based on the double-stage fermentation of the produced syngas (syngas → acetic acid → TAGs) is assessed instead of the conventional Fischer-Tropsch (FT) or Alcohol-to-Jet (AtJ) synthesis. The objective of the present work is the techno-economic evaluation of a large-scale (200 MWth) replication of the mentioned BtL concept, whose performance has been simulated in Aspen PlusTM (V.11) with reasonable upscaling considerations and models validated at a pilot scale. The estimated baseline Total Capital Investment (TCI) of €577 million lies in the typical range of €500–700 million that many recent techno-economic studies adopt for gasification-driven BtL plants of similar capacity, while the estimated annual operating costs of €50 million correspond to a 15–40% OpEx reduction compared to such plants. A discounted cash flow analysis was carried out, and a baseline Minimum Jet Selling Price (MJSP) equal to 1.83 €/L was calculated, while a range of 1.38–2.27 €/L emerged from the sensitivity analysis. This study sets the biological conversion of gasification-derived syngas into triglycerides (TAGs) as a promising alternative route for the production of SAFs. In general, gasification-driven BtL pathways, led by the relatively mature FT and AtJ technologies, are capable of thriving in the coming years based on their capability of advanced feedstock flexibility.

Assessment of Sustainable Aviation Fuel Production Methods: A Promethee II Approach

Sustainable aviation fuels (SAF) present a feasible solution to decarbonize modern aviation. Unlike traditional jet fuels, SAFs are produced in a variety of ways, thereby choosing one of these processes as a complicated Multi-Criteria Decision challenge that involves conflicting priorities. This study evaluates SAF production processes using a multicriteria methodology, PROMETHEE-2. With SAF technology in its nascent stage and limited data, several stakeholders in the aviation sector were enlisted to assist in the collection of data and preferences. The suggested framework’s strength lies in its adaptability to suit the subjective opinions of diverse stakeholders, selection of ranking system, and robustness of outcomes. This research engaged stakeholders in a participative manner to rank 11 (A1 to A11) SAF production paths based on 24 parameters categorized into social, environmental, economic, and technological evaluation criteria. Industry professionals were given a form to rate SAF production methods according to a performance criterion. Data is validated using fuzzy TOPSIS and fuzzy VIKOR and PROMETHEE-II to reduce professionals’ judgmental personal prejudice. Results indicate the optimal feedstock for SAF production is the direct transition process of CO2 to SAF (A11) in the gasification or Fischer-T synthesis group.

Powering aircraft with 100 % sustainable aviation fuel reduces ice crystals in contrails

Abstract. Powering aircraft by sustainable aviation fuels (SAFs) is a pathway to reduce the climate impact of aviation by lowering aviation lifecycle CO2 emissions and by reducing ice crystal numbers and radiative forcing from contrails. While the effect of SAF blends on contrails has been measured previously, here we present novel measurements on particle emission and contrails from 100 % SAF combustion. During the ECLIF3 (Emission and CLimate Impact of alternative Fuels) campaign, a collaboration between the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Airbus, Rolls-Royce, and Neste, the DLR Falcon 20 research aircraft performed in situ measurements following an Airbus A350-941 source aircraft powered by Rolls-Royce Trent XWB-84 engines in 1 to 2 min old contrails at cruise altitudes. Apparent ice emission indices of 100 % HEFA-SPK (hydro-processed esters and fatty acids–synthetic paraffinic kerosene) were measured and compared to Jet A-1 fuel contrails at similar engine and ambient ice-supersaturated conditions within a single flight. A 56 % reduction in ice particle numbers per mass of burned fuel was measured for 100 % HEFA-SPK compared to Jet A-1 under engine cruise conditions. The measured 35 % reduction in soot particle numbers suggests reduced ice activation by the low-sulfur HEFA fuel. Contrail properties are consistently modeled with a contrail plume model. Global climate model simulations for the 2018 fleet conservatively estimate a 26 % decrease in contrail radiative forcing and stronger decreases for larger particle reductions. Our results indicate that higher hydrogen content fuels as well as clean engines with low particle emissions may lead to reduced climate forcing from contrails.

High-Resolution Lipidomics Reveals Influence of Biomass and Pretreatment Process on the Composition of Extracted Algae Oils As Feedstock for Sustainable Aviation Fuels.

The increasing demand for sustainable aviation fuel (SAF) creates a need for innovative biomass and lipid sources with compositions that are compatible with refineries. Algae-derived oils present an opportunity to supply a process-compatible lipid feedstock at yields higher than those of conventional oilseed crops. With few documented reports on chemical composition, the process readiness remains elusive. We present data on extraction efficiency, yield, and purity of lipids from algae with and without the application of a low-concentration sulfuric acid pretreatment of the biomass. The pretreatment process increased the oil yield and positively impacted the quality of the extracted oils. Results from fatty acid and lipidomics analysis revealed that the low-lipid biomass sources extracted 70-80% of the available lipids, and the non-fatty acid co-extractants exceeded 40% of the extracted oils. For a high-lipid algae sample, derived from a genetically engineered strain, we show >90% extraction yield with >85% FAME purity. This work provides insights into the composition of algae-derived oils and quality metrics that are essential to determining the viability of lipid hydroprocessing to SAF.

Pathway to net zero: Reviewing sustainable aviation fuels, environmental impacts and pricing

Supporting the pathway to net zero carbon emissions in the aviation industry, sustainable aviation fuels (SAF) will contribute to a large extent to reduce net CO2 emissions in the next decades to deliver on climate targets. For this reason, the large-scale uptake of SAFs in aviation is on top of the agenda of regulatory policy. However, challenges remain for building the respective infrastructure and supply chains at the appropriate scale to produce and distribute SAF at a reasonable cost. Currently, only one percent of fuels consumed by airlines in Europe are blended with sustainable aviation fuels. The paper examines current SAF production pathways, environmental benefits, and estimated fuel prices from the state-of-the art literature. Further, regulatory policies established by governments, regulatory agencies and industry associations are examined, regarding blending mandates in different regional markets. The authors conclude with the statistical analysis of minimum selling prices for different types of SAF, and discuss their respective availability.

Life Cycle Assessment of Aviation Fuel Production from Steel-Industry Off-Gas

The steel industry in China, the world’s largest, contributes to about 15% of the nation’s total carbon emissions. Instead of direct combustion, the technology of converting off-gas from the steel industry into liquid fuels not only enhances the added value of this byproduct but also helps alleviate carbon emissions. This study, for the first time, integrates the specific circumstances of China to evaluate the carbon emissions of Ethanol to Jet (ETJ) and Fischer–Tropsch to Jet (FTJ) fuel technologies utilizing Basic Oxygen Furnace Gas (BOFG) and Coke Oven Gas (COG) as feedstocks. Six cases were examined using Aspen Plus (V11) for mass and energy balance: Case 1: BOFG/ETJ, Case 2: BOFG/FTJ, Case 3: COG/ETJ, Case 4: COG/FTJ, Case 5: (COG + BOFG)/ETJ, and Case 6: (COG + BOFG)/FTJ. The analysis underscores that the FTJ pathway exhibits superior carbon reduction efficiency relative to ETJ. Compared to traditional petroleum-based aviation fuels (86.65 g CO2eq/MJ), the FTJ pathways utilizing COG or COG + BOFG as feedstocks exhibit significant advantages in greenhouse gas (GHG) emission reductions, with carbon emissions of 23.60 g CO2eq/MJ and 41.48 g CO2eq/MJ, respectively, representing reductions of 72.76% and 52.13%. Furthermore, employing uncertainty analysis based on the Monte Carlo method establishes the credibility of the findings. Finally, sensitivity analysis for parameter optimization and process improvements demonstrates the significant impact of the life cycle assessment (LCA) allocation method on computational results for exhaust gas feedstocks. Given the limited coverage of lifecycle assessments for Ethanol to Jet and Fischer–Tropsch to Jet pathways in China, this study could assist policymakers in determining the development trajectory of sustainable aviation fuel (SAF) in China.

Sustainable Aviation Fuels for Clean Skies: Exploring the Potential and Perspectives of Strained Hydrocarbons

Sustainable Aviation Fuels for Clean Skies: Exploring the Potential and Perspectives of Strained Hydrocarbons

Assessing the Impact of Preprocessing and Conversion Technologies on the Sustainable Aviation Fuel Supply from Forest Residues in the Southeast U.S.A.

Forest residues have been suggested as potential feedstock for sustainable aviation fuel (SAF) production through different pathways. However, the bulky density and scatter distribution require efficient size-reduction processes to enhance the feedstock supply. Thus, this study analyzes the impact of alternative preprocessing and conversion technologies on the SAF supply chains from logging residues (the predominant share of forest residues) using a two-stage mixed-integer linear programming model. The model determines the optimal location of biomass preprocessing depots, conversion facilities, and airports receiving SAF by minimizing the net present value of the total supply chain cost over 10 years. Using high-resolution spatial data in the Southeast U.S.A. as a case study, the results show that the technology and scale of the facilities heavily influenced the SAF’s breakeven cost and maximum supply quantity. The most economically efficient system that adopts a rotary shear milling system and pyrolysis conversion process could generate up to 650 million gallons of SAF from logging residues in the region, with nearly 62% of the SAF being produced from hardwood residues. Also, the most efficient system has an estimated breakeven cost of US$5.51 per gallon of SAF.

The properties of sustainable aviation fuel II: Laminar flame speed

This work reports on the study of flame characteristics and laminar flame speeds (LFS) of sustainable aviation fuel (SAF) and conventional fuels (Jet-A1 and JP-5) premixed with air at elevated pressures. The experimental facilities included the constant volume combustion chamber (CVCC) and used a shadowgraph system to quantify the LFS. The effects of equivalence ratio and ambient pressure on the flame speeds were examined by varying the initial pressure 1, 2, and 3 bar (in addition to 0.1 and 0.5 bar for flame morphology) and the equivalence ratio from 0.8 to 1.8 at 423 K of temperature. The results indicated that JP-5 had the highest LFS value (90.1 cm/s) compared to the other two fuels. Jet-A1 and SAF have the same LFS result, which is less than a 15% difference, excluding the high equivalence ratio and initial pressure. The higher the initial pressure, the lower the LFS for all conditions. The equivalence ratio of 1.2 resulted in the highest LFSs. In addition, in order to understand the laminar flame speed of SAF and its combustion chemical kinetics. Therefore, the sensitivity analysis of SAF was performed. Results indicated that the chain branching reaction (O2 + HOH + O) positively enhanced flame speed as equivalence ratios (φ) increased, while chain propagation reactions, notably O2 + HHO2 (+M), had a negative effect on flame speed, particularly diminishing their influence at higher φ due to limited oxygen availability.

An upcycling bioprocess for sustainable aviation fuel production from food waste-derived greenhouse gases: Life cycle assessment and techno-economic analysis

Biogas generated from food waste through anaerobic digestion is typically utilized for heat and electricity. However, recent developments in synthetic biology have enabled the conversion of carbon dioxide (CO2) and methane (CH4) in biogas into desired products by microorganisms. This study introduced a novel bioroute for the production of sustainable aviation fuel (SAF) using food waste in a carbon–neutral and cost-effective manner. Both CO2 and CH4 derived from food waste are sequentially bioconverted into lipids by photosynthetic and methanotrophic microorganisms, which are then upgraded into SAF. A comparative life cycle assessment was conducted, revealing that this proposed bioroute has a lower environmental impact than a thermo-route involving hydrothermal liquefaction. Furthermore, a techno-economic analysis of the bioroute was also performed to evaluate both the capital and operating costs, showcasing the feasibility of this concept with current cutting-edge technologies. With potential food waste disposal subsidies taken into consideration, the bioroute-based SAF could yield an internal rate of return of up to 38%, demonstrating its economic viability in the market. By leveraging food waste and innovative biotechnology, this bioroute has the potential to contribute to a more eco-friendly and economically sustainable aviation industry.

Technical, economic and ecological assessment of European sustainable aviation fuels (SAF) production

AbstractMultiple options for sustainable alternatives to fossil kerosene have been assessed. The route using biomass gasification, hotgas cleaning, hydrogen addition (derived from water electrolysis using renewable power), Fischer-Tropsch synthesis and product upgrading towards ASTM D7566 certified SAF has technical, economic and environmental advantages to be discussed in detail. The technical assessment shows carbon resp. energy efficiency of fuel production at 61% resp. 55%. The economic assessment of brown-field installations in Finland predicts costs in the range of 1.00 €2019/l. For the evaluation of 200 MWth biomass plus 160.2 MWe grid power plant size, an electricity price and biomass price of 42 €2019/t and 51 €2019/MWh were assumed, respectively. The environmental assessment shows a global warming potential reduction of 91% (corresponds to 8.8 gCO2-eq./MJFuel,LHV) compared to fossil fuel using Finnish wind energy and biomass residues, far better than the required 65% GWP reduction of the European RED II regulation. Based on the techno-economic and environmental assessment of the preferred FT route, a European deployment potential between 10 and 60 Mt/a SAF seems achievable in early 2030 years. The authors propose to implement appropriate regulation with high urgency.