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12 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. 3.6. NOX NOx emissions arise from the oxidation of nitrogen in the combustor. The primary source of nitrogen in the combustor is that which is present in the combustion air flow. Compared to this atmospheric source, nitrogen chemically bound in the fuel is not considered a significant source of engine NOx emissions. Factors affecting NOx emissions include flame temperature, flame residence time, fuel air ratio, combustor inlet conditions, engine power settings, humidity, ambient temperature, and fuel hydrogen content. Since thermal NOx is not specifically related to fuel composition, it is instead very similar between conventional jet fuel and SAJF. The increased H/C ratio in SAJF noted previously can produce small reductions in NOx emissions due to the lower mass of fuel burned and may also increase the rate of NOx creation due to higher combustion temperature. The evidence that fuel composition indirectly affects NOx emissions is mostly reported to be small, less than 10% (Wey, Li 2013, Cain, Corporan 2010, Bhagwan, Colker, Chan, Chi, Rahmes, Corpran 2010, Andersen 2011, Corpran 2011, TImko, Boeing, Carter 2011, Stratton, Corpran 2012, Del Rosario, Roland, Andersen 2015, Christy 2015, Edwards). The exception was from a study of biofuels by Chen where reductions in NOx of up to 70% were observed. However this study evaluated several oxygenated fuels including ethanol, which resulted in lower combustion temperature for these blends. Among all of the tests specifically reporting NOx emissions, 25 of 45 showed no change in NOx emissions, 9 showed NOx reductions of less than 10%, 7 showed NOx emissions either slightly up or slightly down depending on engine power setting, 2 showed NOx increases of 5% or less, and, as mentioned, 1 showed a 70% reduction. Table A.5: Alternative Fuel Impact on NOx Emissions in the Appendix summarizes the NOx emission impacts reported in the literature reviewed for this project. 3.7. HAP Hazardous air pollutants (HAP) are volatile organic compounds (VOC) found in the UHC component of aircraft exhaust emissions. ICAO reported some examples of HAPs that have been identified as representative pollutants from airport sources including formaldehyde, acetaldehyde, acrolein, 1,3-butadiene, benzene, naphthalene, toluene, xylene and propionaldehyde (ICAO). These compounds play an important role in atmospheric chemistry and urban air quality (ICAO, Leikauf, Koenig) and have major health concerns (Li 2014). Studies concerning the impact of alternate fuels on HAP emissions are extremely limited. Four alternate fuels and their blends with Jet A-1 were studied by Li et al. using an APU as the emissions source. Overall, all four alternate fuels/blends showed equivalent (two HEFA blends) or lower aldehyde emissions (FAE blend) compared to Jet A-1. Formaldehyde appeared to be the dominant aldehyde species (Li 2014). In similar studies by Corporan (Corporan 2012) and Timko discussing the limit of uncertainty for HAPs measurements, no significant differences in HAPs production are seen for the alternate fuels studied. Table A.6: Alternative Fuel Impact on HAP Emissions in the Appendix summarizes the HAP emission impacts reported in the literature reviewed for this project. 4. KNOWLEDGE GAPS As noted in earlier sections of this report, there have been several emissions tests conducted to evaluate the performance of SAJF as well as to measure their emissions. The emissions testing was primarily focused on evaluating emissions of nvPM, however, many of the testing programs also recorded emissions of other criteria pollutants. In light of the evolution of SAJF development, the emissions testing was not systematic or extensive. However, the SAJF tested met the D7566 specification and when blended, the tested fuels met the commercial jet fuel specification D1655. As a result, the emissions performance should be representative of SAJF more broadly. Tests were conducted on a variety of engines ranging from auxiliary power units to commercial aircraft main engines as well as several military aircraft engines. Also, as noted earlier, different fuels produced from different feedstocks were used for blending which may have some (probably minor) effect on emissions. The result is a limited data set to be used for evaluating emissions performance across a range of engines, power settings, pollutant species, and blend percentages. A limited data set results in larger error bars and more limited confidence in the relationship between SAJF composition engine emissions. 4.1. SCOPE OF TESTING Table 2 shows the range of engines tested on different fuels. As noted earlier, emissions testing on a given engine could produce repeatable and consistent results for any given fuel. Engine to engine differences, however, are State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
11 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. 3.4. CO CO emissions are the product of incomplete combustion. They are quantitatively dependent on engine type, engine combustor technology, and engine combustion efficiency. Factors that influence the production of CO include fuel air ratio, fuel injection/atomization/mixing, combustor inlet conditions, and engine power settings. The net result being that, independent of fuel type burned, for a given engine type, CO emissions are found to decrease with increasing engine power setting since at low power engines operate less efficiently (Boeing). Fuels with higher hydrogen/carbon (H/C) ratios yield greater combustion efficiency and lower CO emissions and therefore modest reductions in CO emissions are observed when SAJFs with higher H/C ratios (10-25% Corpran 2011, Boeing) are compared with conventional fuels (Corpran 2010, Timko, Carter 2011, Corpran 2012, Christy 2015, Andersen 2011). In contrast to the generally observed reductions in CO emissions, for the case of SIP fuels no significant changes in CO were observed when compared to conventional fuels (Roland), and in the case of AATJ-SPK fuels CO was observed to increase compared to conventional fuels for low engine power settings (Edwards). Table A.3: Alternative Fuel Impact on CO Emissions in the Appendix summarizes the CO emission impacts reported in the literature reviewed for this project. 3.5. UHC UHC emissions are the products of incomplete combustion. Factors governing UHC emissions include engine type, combustion efficiency and associated parameters such as combustor temperature and pressure, engine power setting, and the H/C ratio of the fuel. Incomplete combustion can result in both cracking and partial combustion of the fuel. Both processes result in the formation of species not present in the original fuel composition. In seven studies using four engines and three combustor rigs and a range of alternative fuels, changes in UHC emissions appeared to be both engine/ combustor rig and fuel specific, sometimes decreasing (Cain, Beyersdorf, Chen, Li 2013), and in some cases no change was observed (Corporan 2010, Altaher, Chi). Table A.4: Alternative Fuel Impact on UHC Emissions in the Appendix summarizes the UHC 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.
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.