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To understand the necessity and the benefits of mass deploying and using sustainable aviation fuels (SAFs), it is vital to understand the nature and the scale of aviations impact on present-day climate forcing. Aviation has a long-term impact on climate from its net carbon dioxide (CO2) emissions and shorter-term impacts from non-CO2 emissions and effects, which include the emissions of water vapor (H2O), nitrogen oxides (NOx), sulphate aerosols, compounds from incomplete combustion (unburnt hydrocarbons, UHC) and non-volatile particulate matter (soot). The emitted molecules are transported in the atmosphere and alter a wide range of atmospheric processes including the formation of contrail-cirrus and ozone and the depletion of methane. Although there are uncertainties, data from measurement campaigns such as ECLIF12 combined with global models show that contrail cirrus contributes the largest to aviations climate impact. Several mitigation options to reduce aviations climate impact have been put forward, including re-routing to avoid contrail prone regions, flying lower altitudes, or using SAF, which are all less radical in their implementation than a complete change in the existing aircraft propulsion system and infrastructure (e.g. H2 aircraft). We focus here on the use of SAF. Alternative aviation fuels are non-conventional or advanced fuels as their feedstock is not crude oil (fossil). Feedstocks for producing sustainable aviation fuels are renewable resources and varied; ranging from cooking oil, plant oils, municipal waste, waste gases, and agricultural residues, or even CO2, water, and renewable energy, which yield fuels that are called power-to-liquid (PTL) or sun-to-liquid (STL). Renewable fuels result in a reduction in carbon dioxide (CO2) emissions (one of the main greenhouse gas emission) across their life cycle. There is more to a sustainable fuel than just the fact that it is produced from a renewable resource. In the particular case of the aviation industry, the focus is on sourcing sustainable aviation fuels that can be mass produced at low cost with minimal socio-economic, environmental, and climate impact. All SAF users including commercial airlines request a certificate of sustainability (CoS) in addition to the technical document (certificate of analysis CoA) when purchasing SAF from a supplier to ensure that fuels are sustainably produced. Concerning the technical feasibility, aviation turbine fuel has to meet the regulatory specification requirements defined in international standards or in similar national adoptions of these specifications at every airport. The two major jet fuel (Jet A-1/Jet A) specifications are the Defense Standard 91-91 and the ASTM D1655. In 2009, based on the pioneering certification process that the Sasol Semi-Synthetic Jet Fuel had been through, the standard was modified to include approval procedures of new aviation turbine fuels: ASTM D4054-09 Standard Practice for Qualification and Approval of New Aviation Turbine Fuels and Fuel Additives. During this qualification process, more than 70 different properties are tested to assess the basic fuel chemical/physical properties and the fuel performance in the aircraft engine, fuels system, and ground handling of the fuels. If a fuel is successfully approved, it will be annexed to ASTM D7566, the standard specification for aviation turbine fuels containing synthesized hydrocarbons. Following the most important step, which is the technical feasibility then regularly clearing SAF candidates with respect to the latest sustainability criteria (e.g. RED II), we see SAF as one rapid solution to address the two components of the problem. SAF synergistically addresses both CO2 emissions and non-CO? impacts from contrail cirrus. Findings and analysis from the DLR ECLIF1 and the DLR/NASA (ECLIF2/ND-MAX) flight and ground measurement campaigns as well as related modeling results are presented. Focus is on the impact of SAFs composition and their physical and chemical properties on non-volatile particulate matter (nvPM or soot) emissions. At cruise altitudes, the reduction in nvPM when burning SAF with respect to conventional Jet A-1 translates directly into a reduction in the number of contrail cirrus ice crystals, thus reduced warming effect. During ground aircraft operations, this translates into improved air quality. |
