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10 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. be evaluated in more detail in a subsequent phase of this project. This report describes why these findings are expected based on analysis of the mechanisms of pollutant production when burning jet fuel in aircraft engines and how this is repeatedly confirmed by the data collected from numerous tests and measurement campaigns. 3. POLLUTANT EMISSIONS This section summarizes the findings of the impact of SAJF use on the pollutants of interest. It is an evaluation of the body of literature on SAJF emissions testing. Since we are in the early stages of SAJF production, fuels from only a few different production pathways have been tested. However, the fuels tested meet the D7566 specification so their emissions performance is expected to be an excellent indicator of the emissions performance of predominantly paraffinic fuels produced by future production pathways. 3.1. CO2 AND H2O CO2 and H2O are the primary products of hydrocarbon- based fuel combustion (>99%) and the relative proportion of each species is defined by the H/C ratio of a given fuel. SAJFs typically are found to have higher H/C ratios than conventional fuels (~1%), largely due to the additional hydroprocessing, which is the final step in most fuel production processes. 3.2. SOX SOx emissions are produced by the oxidation of sulfur present in the fuel, and emissions levels are directly proportional to the fuel sulfur content. Typically, the sulfur content in SAJFs is very low (< 0.003%wt) and in the case of blends of SAJFs with conventional fuels the sulfur content is dominated by the level of sulfur in the conventional jet fuel component of the blend (Corporan 2010, Stratton, Corporan 2012, Moses 2008). For conventional jet fuels, typical sulfur levels of 0.3%wt are reported. In Beyersdorf, the authors report the use of pure FT fuels resulted in EISO2 reductions of greater than 90%, and intermediate reductions for blends. Table A.1: Alternative Fuel Impact on SOx Emissions in the Appendix summarizes the SOx emission impacts reported in the literature reviewed for this project. 3.3. PM2.5 PM2.5 as defined in the U.S. National Ambient Air Quality Standards (NAAQS), is a regulatory standard for the criteria pollutant described as fine particulate matter and is based on measuring the mass of the particles with diameters <2.5 micrometers. Particulate matter directly emitted from jet engines and detected at the engine exit plane falls into this category, however, these particles typically have diameters that range in the 10 to 100 nanometers (a nanometer is one thousand times smaller than a micrometer). These particles are the products of incomplete combustion within the engineâs combustor and are largely carbonaceous. The non-carbonaceous particles, referred to as volatile particles, are typically heavy hydrocarbons. The carbonaceous particles are often referred to as non-volatile particulate matter (nvPM) and sometime referred to as soot. They are found to vary monotonically with the aromatic content of the fuel. Suitable metrics for nvPM emissions are (1) the number-based emission index (EIn), which is the number of particles generated per kg of fuel burned, and (2) the mass-based emissions index (EIm), which is the mass of particulate matter generated per kg of fuel burned. The NAAQS is a mass-based regulation. The U.S. does not regulate for particle number emissions, however particle number may be more important for evaluating the health effects of particle emissions, therefore, Eln has attracted more interest recently. Changes in EIn ranged from -22 to -99% (Christy 2015, Timko, Andersen 2011, Dally, Moore, Christy 2017, Chen, Shila, Chan, Colker, Byersdorf, Li 2013, Cain, Huang, Moore, Lobo 2011, Corporan 2010). Changes in EIm ranged from -20 to -95% (Christy 2015, Timko, Andersen 2011, Dally, Moore, Christy 2017, Shila, Chan, Colker, Beyersdorf, Li 2013, Cain, Lobo 2011). And changes in GMD (geometric mean diameter) ranged from -2 to +16% for FAME (Lobo 2011, Timko) and from -12 to +1% for 100% FT and 50% blend (Lobo 2011). Similar reductions were observed but not quantified for SPK (Cain), FT coal (Vander Wal, Timko), and biofuels (Chen). Chen analyzed for sulfate ions present in the nvPM component of the exhaust and found them to be the dominant particle-bound anion. Chen also found them to be reduced in concentration for the range of alternative fuels studied compared to conventional jet fuel. Table A.2: Alternative Fuel Impact on PM2.5 Emissions in the Appendix summarizes the PM emission impacts reported in the literature reviewed for this project. State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
9 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. terms but in this report the differences were ignored. Two of the reports use the chemical class âaldehydesâ as a surrogate for HAPs and one report measures formaldehyde (HCHO) as a representative of all aldehydes. This has proven to be reasonable in prior research as indicative of that component of the emissions. Formaldehyde is commonly the most prevalent aldehyde in aircraft engine emissions and the proportion of formaldehyde to the other hydrocarbons in the emissions remains consistent at different emission rates. PM (particulate matter), often referred to as soot, means nvPM (non-volatile PM) when used in this report. Some studies measured PM2.5, which is non-volatile PM smaller than 2.5 micrometers in diameter and is the regulated size classification of PM. Aircraft engine emissions of nvPM are even smaller, in the PM1.0 and smaller range. For the purpose of this report, these terms are assumed to be equivalent. 2.7. REPORT IDENTIFICATION The research team team collected, reviewed, and compiled data from research reports from all aircraft engine emission tests of SAJF and related research projects. This includes testing sponsored by DoD, NASA, FAA, aircraft and engine Original Equipment Manufacturers (OEMs), fuel producers, and other organizations. Missouri University of Science & Technology (MS&T) technical library database, the open Internet, and frequently cited reports were searched to identify the reports in this literature survey. The collected information includes university and government publications, briefings, and other technical reports. Table 1 shows how the universe of reports was reduced to just the reports pertinent for this study. An analysis of these reports and data shows that SAJF, when blended with conventional jet fuel, significantly reduces SOx and PM, generally reduces CO and UHC emissions, and minimally reduces or has no effect on NOx emissions. The variability in emissions data will molecular weight hydrocarbon compounds into straight chain compounds (normal paraffins or n-paraffins). Finally, distillation separates the primary fuel components from lighter and heavier molecules to leave a fuel stream comprised mostly of molecules containing 8-18 carbon atoms. Some branched molecules (iso-paraffins) are present. One significant result of this process is that the synthetic fuels have a higher hydrogen content and higher energy mass density compared to conventional jet fuel. As the volumetric energy density goes up with increased hydrogen content, the mass of fuel used decreases. An indication of the changes to the fuel properties was that frequently during test campaigns, fuel flow and shaft speeds decreased with increasing SAJF blend percentage. These changes are consistent with higher energy mass density of the alternative fuels, which result in a constant energy input with lower fuel mass flow. Higher heating value (BTU/lbm) leads to lower fuel burn and reduced emissions on a mass basis. This improves fuel efficiency and reduces emissions. 2.6. POLLUTANT SPECIES Specific emissions and pollutant species addressed in this report include: â¢ Sulfur oxides expressed as SOx â¢ Non-volatile particulate matter (nvPM also referred to as PM) â¢ Carbon monoxide (CO) â¢ Unburned hydrocarbons (UHC) â¢ Nitrogen oxides expressed as NOx â¢ Hazardous air pollutants (HAP) For some pollutant species, slightly different terms are used in different reports. For example, UHC (unburned hydrocarbons) as used in this report. Other research reports use HC (hydrocarbons), VOC (volatile organic carbon), and THC (total hydrocarbons). There are slight differences in the chemical compounds that makeup these Document Hits Search Criteria 35,136 Alternative jet fuel emissions 9,369 Alternative jet fuel emissions + criteria pollutants 73 Alternative jet fuel emissions + criteria pollutants + emission measurements 51 Reports with quantitative emissions analysis (used in this literature review) Table 1: Identifying Reports for Literature Review State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
8 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. materials, which can include various sources of renewable biomass such as municipal solid waste, agricultural wastes, forest wastes, wood, and energy crops. Synthesis gas (CO and H2) is then converted employing catalytic processes in a Fischer-Tropsch (FT) reactor into liquid hydrocarbons such as diesel or jet fuel. FT-SPK must be blended with conventional jet fuel at levels up to 50%. 2. Synthesized Paraffinic Kerosene from Hydroprocessed Esters and Fatty Acids (HEFA-SPK) is produced by reacting an oil or fat-based feedstock, such as fats and oils derived from vegetables, animals, or waste oil, with hydrogen. HEFA-SPK can be blended with conventional jet fuel up to 50%. 3. Synthesized Iso-Paraffins produced from Hydroprocessed Fermented Sugars (HFS-SIP), are produced by hydroprocessing and fractionation of synthetic hydrocarbons derived from the fermentation of plant-based sugars such as sugar, corn, or forest wastes, to produce farnesene, a 15-carbon hydrocarbon molecule. HFS-SIP must be blended with conventional jet fuel, at levels up to 10%. 4. Synthesized Kerosene with Aromatics Derived by Alkylation of Light Aromatics from Non-Petroleum Sources (SPK/A), includes aromatics that can possibly reach higher blend rates than other synthetic fuels. A minimum of 8% aromatics is required in SAJF blends to ensure sufficient seal swell as a way to prevent fuel system leaks. According to the ASTM D7566 specification, the SPK/A synthetic blending component shall be comprised of FT-SPK combined with synthesized aromatics from the alkylation of non- petroleum derived light aromatics, primarily benzene. These fuels must be blended with conventional jet fuel, at levels up to 50%. 5. Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) is produced from alcohols that have been produced from fermentable sugars which in turn have been produced from renewable feedstocks such as sugar, corn, or industrial or forest wastes. ATJ-SPK to be blended with conventional jet fuel, at levels up to 30% (although as of this writing, an effort is underway to increase the maximum blending level to a higher 50%). Several other fuel production pathways are currently being evaluated under ASTM D7566 for approval as blending components. At the time of this report, the following fuels are in the approval process: 1. HDO-SAK â A bioforming process that converts aqueous carbohydrate solutions into a mixture of aromatic hydrocarbons via a process described as HydroDeOxygenation. 2. Catalytic Hydrothermolysis Jet (CHJ) â A two- step process of catalytic hydrothermolysis and hydroprocessing where bio-oils are converted to hydrocarbons in the jet fuel range using water (under high temperature and pressure) as a catalyst. Alkylation produces aromatics so it is possible the fuel could be used without the need for blending. 3. FHP-HEFA â High Freeze Point HEFA (aka Green Diesel, or HEFA+), is using hydrogenation-derived renewable diesel as a blending agent with jet fuel. 2.5. SUSTAINABLE ALTERNATIVE JET FUEL PRODUCTION The five SAJF approved by the industry in ASTM D7566 each follow a different production process beginning with somewhat different feedstocks, however, the resulting fuel when blended with conventional jet fuel meets the specification for conventional jet fuel. Fuels produced from non-sustainable feedstocks such as coal, petroleum, or natural gas can also meet the ASTM specifications under Annex 1 and perform identically to conventional jet fuel. Some of the emissions testing evaluated for this report were conducted on GTL (gas-to-liquid) fuels. Because they meet the D7566 specification, their emissions performance is equivalent to SAJF and a valid source of data for this analysis. Hydroprocessing is an important step in producing jet fuels, both conventional and SAJF. In refineries producing conventional jet fuel, one of the final steps is treating the fuel with hydrogen in the presence of a catalyst. The hydrogen reacts with sulfur, nitrogen, and other unwanted elements (heteroatoms) present in the fuel to reduce impurities required by internationally accepted fuel specifications. Hydroprocessing at the refinery reduces, rather than eliminates, these components to ensure they are below the allowable limits. Hydroprocessing is also the final step in the five SAJF production pathways approved to date. It is an important step in producing HEFA-SPK fuels to remove oxygen from feedstocks (i.e., deoxygenate). This is typical for feedstocks containing triglycerides such as animal fats and oils from soybeans, palm, algae, jatropha, and other oily plants. In all five SAJF production processes, hydroprocessing essentially removes all sulfur and heteroatoms that may be present. It also converts the fuel stream into a more uniform composition, converting aromatics and high State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
7 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. are produced at high temperature, are higher at high thrust and lower at low thrust. Similarly, with an APU, CO and UHC emissions are higher at the âno loadâ setting and lower at âmaximum loadâ while the reverse is true for NOx. 2.3. CONVENTIONAL JET FUEL Various emissions tests of jet fuel, SAJF, and blends of the two are evaluated in this literature review. Different research groups used different jet fuels in their experiments, which usually reflected the most readily available conventional jet fuel. The different fuels, based on different fuel specifications, however, are very similar. The Jet A specification is used commercially throughout the U.S. and generally not available outside of the country. Jet A-1 specification fuel, which is used outside the U.S., is very similar to Jet A with the primary difference being a slightly lower freeze point for Jet A-1. The U.S. military formerly used JP-8 jet fuel, which is very similar to Jet A-1 but includes a static dissipater, corrosion inhibitor, and anti-icing additives. They have recently changed to Jet A-1 with those additives. The combustion of these three fuels is very similar from an emissions standpoint and for the purpose of this report they are assumed to be equivalent. 2.4. SYNTHETIC FUELS ASTM International, an international standard setting organization, maintains a standard specification D1655 for aviation turbine fuels, referred to as conventional jet fuel. The organization also is responsible for establishing specifications for synthetic blending components, designated D7566. While most of the reports reviewed for this investigation used alternative fuels that would meet D7566, some of the fuels would not meet this specification (even though these studies were conducted prior to publication of the D7566 specification). These include fatty acid methyl ethers (FAME) and ethanol, which are both oxygenated compounds. These emission studies provided useful data illustrating the relationship of fuel composition and emissions. Also, the D7566 specification does not address the sustainability of the fuel per se. Generally, the sustainability of any alternative jet fuel production depends not only on the source of the hydrocarbons used to produce the fuel, but many other measures of societal acceptability associated with the fuelâs production, including economic, environmental, and social factors. Fuel produced from coal, petroleum, or natural gas feedstocks is not considered sustainable, as the feedstocks are fundamentally not renewable. Fuel produced from renewable biological feedstocks (e.g., plant-derived oils, agricultural wastes, forestry residues) or recycled carbonaceous sources (e.g., municipal solid waste, industrial off-gases, atmospheric CO2)), might be considered as meeting the minimum threshold for consideration of sustainability. However, being renewable or recyclable is not a sufficient criterion, and other aspects of societal impact must be evaluated to entitle such fuels as âsustainable.â Determining the sustainability of a given fuel type or production lot is often complex and beyond the scope of this report, but information exists that shows that there are examples of fuels being able to be sustainably produced per the definitions in ASTM D7566 annexes. To date, the aviation industry has added five annexes to D7566 for producing different synthetic blending components. Fuels produced according to these annexes are referred to in this report as sustainable alternative jet fuels (SAJF). 1. Fischer-Tropsch Hydroprocessed Synthetic Paraffinic Kerosene (FT-SPK), uses a synthesis gas feedstock, produced by thermally converting hydrocarbon State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
6 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. 2. BACKGROUND This State of the Industry report is a âreference documentâ that captures the current status of knowledge regarding emissions from the use of sustainable alternative jet fuels (SAJF). The research team conducted a review of available research, literature, and measurement campaigns to enhance our current understanding of the local air quality emissions benefits and impacts of SAJF as well as SAJF blends relative to conventional jet fuel. We drilled down into the data and extracted and analyzed the essential emissions testing data to quantify typical emissions impacts of SAJF use. The review results are focused specifically on air quality emissions of criteria pollutants from SAJF. The report also includes an analysis of gaps in our current understanding of the production of pollutants from SAJF. 2.1. TEST CAMPAIGNS Department of Defense (DoD) and National Aeronautics and Space Administration (NASA) have been primarily responsible for most of the SAJF testing to date. DoD conducted tests on many of the different aircraft they operate to qualify their use of SAJF. NASA has been conducting research on aircraft engine emissions to evaluate their environmental impact for several years. Many of these research programs included DoD research labs, Federal Aviation Administration (FAA) experts, aircraft and engine manufacturers, universities, and scientific experts. Some of the more significant test programs were: â¢ APEX, September 2006 â¢ AAFEX-I, January 2009 â¢ AAFEX-II, March 2011 â¢ ACCESS-I, February-April, 2013 â¢ ACCESS-II, May, 2014 Much of the literature reviewed in this report comes from reports on these projects or analyses of the data produced during these projects. 2.2. AIRCRAFT ENGINES Todayâs commercial and military aircraft rely on modern, high-efficiency, sophisticated turbine engines to deliver the safety, operability, and efficiency demanded from the sector. Additionally, auxiliary power units (APU) are smaller turbine engines utilizing similar design principles which also perform consistently and reliably. Aircraft main engines and APUs were the primary test beds for SAJF emissions testing. The various testing campaigns showed that emissions testing on a given engine could produce repeatable and consistent results for any given fuel. However, engine to engine differences are significant. The age/pedigree of the engine, time since last overhaul, and cleanliness of fuel components, such as nozzles, can affect combustion and consequently emissions. For a series of tests performed on a given engine, the changes in emissions will be a reflection of the thrust setting (i.e., fuel flow) and fuel composition. In general, fuel chemistry has a much greater impact on emissions than the difference among engines. The relationship between fuel composition and emissions is discussed below. The emissions tests conducted in the reports included in this literature review were performed at various engine thrust settings intended to reflect an aircraftâs main engine performance. The basis for selecting specific thrust settings correlates with the landing-and-takeoff (LTO) cycle as defined by the International Civil Aviation Organization (ICAO) and used for engine certification testing. As a general representation, taxiing aircraft use low thrust (4%). Full thrust (100%) is used to represent takeoff as the aircraft comes up to speed quickly to get off the ground. Once in the air, the thrust is reduced somewhat (85%) as the aircraft climbs to cruise altitude. A thrust setting of 30% is representative of the thrust an aircraft uses on approach to landing. These thrust settings are commonly used for emissions testing although in actual operation, thrust settings vary and typically are lower than these values. In a similar way, tests conducted on APUs use three power settings â âno loadâ or âready-to-load,â environmental control system, and âmaximum loadâ or âmain engine start,â which reflect in-use APU thrust settings. The no load setting is equivalent to idle on aircraft main engines. The environmental control system setting is an intermediate power setting used when the APU is providing secondary electric power and ventilation to an aircraft parked at the gate. The main engine start setting is a high-power setting used when starting the aircraft main engines. Aircraft main engines are designed to operate most efficiently at cruise power since the majority of fuel use is during cruise. Lower power operation, such as for idling and taxiing, is less efficient from a fuel combustion standpoint. As a result, emission species that reflect engine efficiency, notably CO and UHC, are higher per unit of fuel consumed at low power operation and lower at high power operation. Conversely, NOx emissions, which State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
5 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. While the primary purpose for airlines to use SAJF is to reduce CO2 emissions, emissions of other pollutants may also be reduced, which could be significantly beneficial to airports. However, these reductions are not yet well defined, leaving airports unable to realize what may be substantial benefits. The research team team analyzed the published technical literature to validate that SAJF use does reduce air pollutant emissions (i.e., PM2.5, SOx, CO, UHC, NOx) and does not cause any of them to increase. Table ES-2 summarizes the body of literature that was screened to identify essential reports that include quantitative data on results from emissions testing of SAJF. The data in these reports was analyzed in detail to define the impact of using SAJF on air pollutants of interest to airports. The summarization of these reports and their emissions data shows that SAJF, when blended with conventional jet fuel as defined in D7566, significantly reduces SOx and PM, generally reduces CO and UHC emissions, and minimally reduces or has no effect on NOx emissions. Figure ES-2 summarizes the impact of SAJF on aircraft emissions. This report describes why these findings are expected based on an understanding of the mechanisms of pollutant production when burning jet fuel in aircraft engines and how this is repeatedly confirmed by the data collected from numerous tests and measurement campaigns. Following this Executive Summary, Section 2 provides a discussion of the scope of this report including emission testing campaigns, SAJF production and approved fuels, and pollutant species. Section 3 provides information on the source of the different pollutant species that result from fuel combustion. Section 4 describes the knowledge gaps in the current literature and testing to date. Section 5 is an annotated bibliography that highlights key findings from several reports, which influenced the findings of this literature survey. Section 6 includes a complete reference list, and Section 7 is an appendix, which summarizes the impacts of alternative fuels on the emissions of SOx, PM2.5, CO, UHC, NOx, and HAP from individual reports. Figure ES-1: Fuel Production Pathways Currently Undergoing Review Document Hits Search Criteria 35,136 Alternative jet fuel emissions 9,369 Alternative jet fuel emissions + criteria pollutants 73 Alternative jet fuel emissions + criteria pollutants + emission measurements 51 Reports with quantitative emissions analysis (used in this literature review) Table ES-2: Identifying Reports for Literature Review SOx PM2.5 CO UHC NOx HAP 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Neat 50% Blend R ed uc ti o ns (% ) Figure ES-2: Representative Air Pollutant Emission Reductions from the Use of SAJF FUEL PRODUCTION PATHWAY ATJ-SPK; Expansion of Annex A5 (ATJ-SPK) to include the use of ethanol as a feedstock HDO-SAK; Synthesized Aromatic Kerosene via the catalytic conversion of sugars CHJ; Catalytic Hydrothermolysis Jet via Isoconversion of lipids, fats, oils or greases HFP-HEFA; High Freeze Point HEFA, using HDRD (aka Green Diesel) as a blending agent State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.