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15 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. 5. ANNOTATED BIBLIOGRAPHY This section presents a short description of the scope and contents of the references that reported the most essential findings regarding emissions from SAJF. It includes key findings from emission tests project reports (APEX, AAFEX, etc.), ASTM research reports/annexes, and emissions analysis reports by researchers for FAA, CLEEN, DoD, NASA, ACRP, PARTNER, and ASCENT. Altaher, Mohamed A., Andrews, Gordon E., and Li, Hu, PM Characteristics of Low NOx Combustor Burning Biodiesel and its Blends with Kerosene, Proceedings of ASME Turbo Expo 2013: Turbine Technical Conference and Exposition GT2013, June 3-7, 2013, San Antonio, Texas, USA. This work investigated the particulate number concentrations and size distributions of exhaust gases emitted from a radial swirler based low NOx gas turbine combustor. The tests were conducted under atmospheric pressure and 600K at reference Mach number of 0.017 and 0.023. A baseline of natural gas combustion was compared with a waste rapeseed cooking oil methyl ester biodiesel (WME), its blend with kerosene B20, B50 and pure kerosene. â¢ The most common blending ratio is 20% of biodiesel, termed B20. â¢ The combustion test facility consisted of an air supply fan, venturi flow metering, electrical preheaters, 250mm diameter air plenum chamber, 76mm outlet diameter double passage radial swirler, 76mm diameter throat 40mm long wall fuel injector, 330mm long 140mm diameter uncooled combustor, followed by a bend in the water-cooled exhaust pipe with an observation window on the combustor center line. â¢ Lean combustion low NOx radial swirler flame stabilizers were operated close to 0.5 equivalence ratio at 600K and one atmosphere pressure and were shown to have extremely low PM mass emissions of about 1mg/kg for gas and liquid fuel. B100 could be burnt without any major increase in particle number emissions. Anderson, B.E., et al., Alternative Aviation Fuel Experiment (AAFEX), NASA Project Report NSAS/TM-2011-217059, February 2011. This is a report on the AAFEX project, probably the most extensively studied test of alternative jet fuel use in aircraft engines. All of the testing in this project was on-wing engines tested on the ground and evaluation of neat jet fuel (JP-8), a blend of 50% JP-8/50% Fischer-Tropsch fuel produced from natural gas, a blend of 50% JP-8/50% Fischer-Tropsch fuel produced from coal, neat Fischer-Tropsch fuel from natural gas, and neat Fischer-Tropsch fuel from coal. The test bed was a DC-8 with CFM56 engines owned by NASA. Emissions from the aircraftâs APU were also tested. In addition to NASA Langley and Glenn Research Centers, participating research organizations included: U.S. Air Force Arnold and Wright Patterson Air Force Bases, Aerodyne Research, Inc., General Electric, Harvard University, Missouri University of Science and Technology, Montana State University, Pennsylvania State University, Pratt and Whitney, U.S. EPA, and United Technologies. â¢ âRelative to JP-8, burning alternative fuels generally reduced engine CO, THC, and NOx emissions.â â¢ âHAPs emissions were significantly lower for FT fuels.â 4.7. HAP Only a few of the emissions testing programs had specific evaluations of the composition of the UHC emissions so very limited data on HAP emissions are available. To evaluate HAP emissions, UHC emissions must be evaluated for their component species. Additional testing of each SAJF fuel to measure HAPs in the UHC component of the engine exhaust is needed. 4.8. FUTURE TESTING Going forward, additional, more systematic testing would be beneficial to confirm the findings reported here and to fill gaps in our current understanding. In particular, being able to relate the molecular composition of SAJF, and in particular the H/C ratio, would be especially useful. State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
14 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. significant. The age of an engine, time since last overhaul, and cleanliness of fuel components, such as nozzles, can affect combustion and consequently emissions. This is even significant for engines of the same type and model number. This presents a challenge in comparing emissions data across multiple tests, hence the need for more systematic testing to refine the results. 4.2. SOX As noted in Section 2, SOx emissions are proportional to the sulfur content of the fuel. However, not all sulfur is emitted as SOx. Some sulfur is retained on the soot particles that make up a large component of nvPM, and there are some aerosol particles that contain sulfur. It would be very useful if researchers could partition sulfur in fuel into SOx, aerosol, and nvPM. While in general, fuel sulfur content is proportional to SOx it is not possible to complete a material balance without knowing where all sulfur emissions go. This is not essential for understanding the air quality impacts of SAJF per se but is important for understanding and tracking the fate of the fuel sulfur. 4.3. PM Significant reductions in PM emissions as a result of using SAJF blends are a positive outcome. For that reason it would be beneficial to more carefully/specifically relate the number based emission index (EIn) (the number of particles generated per kg of fuel burned), and the mass based emissions index (EIm) (the mass of particles generated per kg of fuel burned), to engine thrust to better quantify the overall benefits. Also, since PM emissions are found to vary monotonically with the aromatic content of the fuel, this relationship should be quantified. Additionally, more detailed study of the relationship between PM emissions and the concentration of naphthalene as a share of total aromatics in the fuel is needed. Naphthalene appears to play an outsized role in the production of nvPM emissions from aircraft engines. 4.4. CO Only modest changes are seen in CO emissions between conventional jet fuel and SAJF. They are largely related to engine design and operating conditions. However, the H/C ratio in the fuel influences combustion efficiency. SAJF often have a slightly higher H/C ratio compared to conventional jet fuel as a result of the hydrotreating of the fuel as one of the final steps in most SAJF processing, or due to a higher overall percentage of paraffins and iso-paraffins versus cyclo-paraffins and aromatics. Since the differences are small, the resulting impact on CO emissions is generally small. Specific testing to identify these changes is needed to more carefully relate CO emissions to H/C fuel ratio. Also, since CO emissions are higher at low power due to engines being less efficient at low power, testing should focus on lower power operations, although testing at multiple thrust levels (7%, 30%, 85%, and 100%) is recommended. 4.5. UHC Similar to CO, UHC emissions are related to engine efficiency and are higher at low power since engines are designed to maximize efficiency at high power when engines consume the most fuel. H/C ratio in the fuel influences combustion efficiency. SAJF often have a slightly higher H/C ratio compared to conventional jet fuel as a result of the hydrotreating of the fuel as one of the final steps in most SAJF processing, or due to a higher overall percentage of paraffins and iso-paraffins versus cyclo-paraffins and aromatics. Since the differences are small, the resulting impact on UHC emissions are generally small. Specific testing to identify these changes is needed to more carefully relate UHC emissions to H/C fuel ratio. Also, UHC emissions are higher at low power since engines are less efficient at low power so this testing should focus on lower power testing although testing at multiple thrust levels (7%, 30%, 85%, and 100%) is recommended. 4.6. NOX As noted in Section 3, NOx emissions reflect flame temperature, flame residence time, fuel air ratio, combustor inlet conditions, engine power settings, humidity, ambient temperature, and fuel hydrogen content. Thus, compared to conventional jet fuel, only minor changes in NOx emissions have been detected in SAJF testing and most commonly there has been no change that resulted from increasing the SAJF blend percentage. Because the differences in emissions are small, thorough testing is needed to evaluate NOx emissions, being careful to repeat the same engine conditions (e.g., fuel air ratio, humidity, and ambient temperature) with neat conventional jet fuel and different blend percentages of SAJF for different fuel composition. State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
13 State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels Copyright National Academy of Sciences. All rights reserved. Conventional Jet Fuel Sustainable Alternative Jet Fuel SAJF Annex A1 FT-SPK Annex A2 HEFA-SPK Annex A3 Annex A4 Annex A5 Engine JP-8 Jet A Jet A-1 FT CTL FT GTL Beef Tallow Camelina Fats & Grease SIP SPK/A ATJ- SPK 131-9 APU B37, A320* â â â â GTCP85 APU B737 â â AE3007 ERJ145 â â CFM56 DC8 â â â â â CFM56-2 DC8 â â â â CFM56-5C4 A340 CFM56-7 B737 â â â â CFM56-7B B737 â â F117 C-17 â â â F117- PW-100 C-17 â â â â PW308C DF 2000 â â PW615F Citation Mustang â â SaM146 RRJ75 â â T63 Bell OH-58 â â â â â T63-A-700 Bell OH-58 â â â â â â TF33 â â B-52 TF34 A-10 â â TPE331-10 J 31 â â TPE331- 19YGD J 41 â â Table 2: Fuel Testing by Engine Type for Reported Emissions Data State of the Industry Report on Air Quality Emissions from Sustainable Alternative Jet Fuels opyright ational cade y of ciences. ll rights reserved.
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.