the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Oct 2023
Opinion: Eliminating aircraft soot emissions
Abstract. Soot from aircraft engines deteriorates air quality around airports and can contribute to climate change primarily by influencing cloud processes and contrail formation. Simultaneously, aircraft engines emit CO2, nitrogen oxides (NOx) and other pollutants which also negatively affect human health and the environment. While urgent action is needed to reduce all pollutants, strategies to reduce one pollutant may increase another, calling for a need to decrease, for example, the uncertainty associated with soot’s contribution to net Radiative Forcing (RF) in order to design targeted policies that minimize the formation and release of all pollutants. Aircraft soot is characterized by rather small median mobility diameters, dm = 8 – 60 nm, and at high thrust, low (< 25 %) organic carbon to total carbon (OC/TC) ratios while at low thrust the OC/TC can be quite high. Computational models could aid in the design of new aircraft combustors to reduce emissions, but current models struggle to capture the soot dm, and volume fraction, fv measured experimentally. This may be in part due to oversimplification of soot’s irregular morphology in models and a still poor understanding of soot inception. Nonetheless, combustor design can significantly reduce soot emissions through extensive oxidation or near-premixed, lean combustion. For example, lean premixed prevaporized combustors significantly reduce emissions at high thrust by allowing injected fuel to fully vaporize before ignition while low temperatures from very lean jet fuel combustion limit the formation of NOx. Alternative fuels can be used alongside improved combustor technologies to reduce soot emissions. However, current policies and low supply promote the blending of alternative fuels at low ratios (~1 %) for all flights, rather than using high ratios (> 30 %) in a few flights which could meaningfully reduce soot emissions. Here, existing technologies for reducing such emissions through combustor and fuel design will be reviewed to identify strategies that eliminate them.
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RC1: 'Comment on ar-2023-15', Anonymous Referee #1, 21 Nov 2023
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General review
The authors present an opinion paper on “eliminating aircraft soot emissions”
According to the abstract the authors aims to give a review of existing technologies to identify strategies to eliminate soot emissions. From my point of view, they have not succeeded to accomplish this aim. There are some important technologies missing from the article, and the actual technologies leading the soot emission reduction are only mentioned briefly and for some reason in the description seems that these technologies are somehow still on a development phase.
Overall the structure is correct but, in some points there are some repetitions or text that seems to be out of place. For example, in introduction, the last paragraph seems quite out of place.
Detalied review:
Introduction
Lines 22-25, you state that aviation has a significant impact on health and climate due mainly to soot, but then you give two citations that deal with soot in general. Indeed, the review from Niranjan and Thakur mainly compile works on synthetic soot like printex and some on diesel soot, but not aeronautic soot. There is not too many works dealing with health effects of aircraft emissions, but still there are some that are much more relevant for the context than the review paper cited (for example: Bendtsen et al. 2019 https://doi.org/10.1186/s12989-019-0305-5, Delaval et al. 2022 https://doi.org/10.1016/j.envpol.2022.119521)
Lines 27-29 you are mistaking jet aircraft smoke visibility and smoke number. Both can be linked (Slusher 1971 FAA report FAA-RD-71-23) but the smoke number is defined as the loss of reflectance of a filter used to trap smoke particles from a prescribed mass of exhaust per unit area of filter (SAE ARP 1179).
Lines 64-65, why do you give a citation about light-duty diesel engine to illustrate soot NOx trade off in aircraft engines?
Lines 67-69, I would like to see a reference about the uncertainties of soot contribution to climate in global models being due to the simplification of soot morphology
Lines 92-99, you may want to have a look to this paper in ACPD https://doi.org/10.5194/egusphere-2023-2441
Lines 107-110 the definition of nvPM should have ben given at the beginning of the introduction
Line 111, the discussion about >26.7 KN engines could be moved to line 32
Lines 116-118, what Avgas have to do with soot? Lead is a contaminant strictly linked to the fuel, I do not see the link with soot reduction or combustor technologies.
Section 3.1
Frist of all, be careful with the term alternative fuel, not all alternative fuels are SAF. I am surprised by not seeing any references to CORSIA criteria when speaking about SAF.
Lines 232-233 No, alternative fuels can not be used as drop-in fuels. They can only be used after blending with jet fuel up to the mixing ratios defined by ASTM for each fuel. There are two alternative fuels which have started the procedure to be certified for their use as 100% drop-in fuel, CHJ and FT-SPK-A but they are not certified yet.
238-239 No, LCAF are defined as: A fossil-based aviation fuel that meets the CORSIA Sustainability Criteria under this Volume (ICAO Annex 16 Vol IV), so indeed LCAF are fossil fuels that manage to reduce their lifecycle emissions by 10% compared to the baseline of 89 gCO2e/MJ
Line 258 that is the actual blend limit for HEFA not for SAF, other SAF are certified for much lower blend ratio, for example SIP is certified just up to 10% blend ratio.
Line 290, I am missing references to on-going project like ECLIF, EcoDemonstrator or VOLCAN, where in flight measurements has been done even with 100% SAF
Lines 304-307, Fit for 55 and ReFuelEU aviation target for a share of SAF in the annual fuel used, not in the blends. Indeed, how things work actually is just as the author describes, the blends used in airports are normally blends of around 30% SAF. It also has to be noted that even if the EU fix a minimum amount of SAF share by year, industry have proposed higher SAF shares, for example Airbus have set as an objective to use 10% SAF share for 2023 for internal movements and 5% for external movements, and again this is done by using blends between 30 and 49% of SAF for some flights, and not blends 1% of SAF for all flights. So last sentence of the paragraph (306-307) are incorrect and must be removed.
After reading SAF section I have the feeling that the authors are not familiar with aeronautic SAF. There are too many wrong or misleading information.
Section 3.2
Overall all this section is too naïf to be a review of existing technology, only speak of general combustor concepts, but there is no mention of different technologies, just as an example, there are different RQL implementations (LEC, TALON …) and each of this have their own specificities. I am missing also TAPS. Regarding technologies in development, they only mention LEAF, while there are other technologies that are much more advance in development, for example WET engines that will be tested on flight in the near future. Also developments like open-rotor engines might be included, thought it is not directly linked to combustor optimization, but again this technologies will be tested on flight soon while LEAF is at laboratory level demonstration.
Lines 327-329 results shown in Kelesidis et al. 2023 has been obtained with a laboratory combustor, that has nothing to do with a real aircraft combustor, despite they might obtain soot with similar properties that soot obtained in some engine regimes, results about oxygen injection cannot be transferred to a real engine.
Line 408 Emission index is defined as emissions by kg of burned fuel.
Lines 412-414, I wondering how the authors have assigned different engines to different technologies. I do not see where they have obtained LDI engine data, or LPP, are they assuming that TAPS is equivalent to LPP? according with line 416 you include in your graph 27 LPP engines, in the data base there is data for 26 TAPS and TAPS II engines, so seems so (though I don’t know what is the extra engine you consider LPP to sum up the 27 points you use), Furthermore, what is the point of comparing old SAC engines with new SAC-TI engines? The authors claim that RQL combustors have a large variation, and that this is most likely due there is more data entries for RQL… no, this is due to the fact that you are comparing technologies based on RQL concept but that are differently implemented and also that you are probably including out of production engines (ICAO data base contain data for 215 engines, 43 of those are out of production, your graph includes 208 points).
Line 440-441 As this is written I interpret that you say that engine manufacturers manipulate the data included in ICAO data base, what is unacceptable , this is just a plain offense to ICAO, SAE-E31 and EASA.
Line 446-448 again authors are forgetting a large number of on-going projects linked to these engines. The end of this sentence “ … if they will be adopted in the future” just illustrate the lack of understanding of the authors in the aeronautic field. LEAP and GEnx engine has been in the market for several years, actually there are almost 10k on service, and if you check the orders of engines only for LEAP engines, this indicates an annual production for the next year of over 2000 engine per year.
Line 450-452 Again an other sentence that indicates that the authors are not familiar with the engine development. CFD combustion models are used in the development of engines, but indeed they represent only a small part on the development, there are several test of different injector configurations, combustion chamber geometries etc, so I do not see the point of the authors here.
Conclusion
I have the feeling that authors are neglecting the potential of engines based on TAPS combustor to reduce both NOx and soot emissions. Indeed, above 30% of engine thrust soot emissions of these engines are close to be under the detection limit of the instruments. The main conclusion seem to be that the solution for eliminate soot from aircraft emissions is the development of CFD models able to predict soot production. This will be of course useful, but I do not think is something realistic. Further more actual models used in engine development are able to give at least order of magnitude values for soot production, and in any case all engine development goes through an experimental optimization of injectors and combustion chamber.
I do not see that this article bring something new to the field, even as a review there are many things missing. For me the question right now in the field is not so much to reduce further soot emission from actual engines ( at least for thrust > 30%) but, what could be other sources of particulate matter once the soot emissions are reduced at ambient background levels and how contrail formation can be affected. Other interesting topic can be if is worthy to produce Lean burn engines or SAF + rich burn engines can achive similar reductions on nvPM emissions.
Citation: https://doi.org/10.5194/ar-2023-15-RC1 -
CC1: 'Comment on ar-2023-15', Marc Stettler, 08 Dec 2023
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This opinion paper aggregates literature in a useful way, however there are several gaps that need addressing, detailed below. Furthermore, it is not entirely clear what the opinion is - is this paper supposed to put forward a view that soot emissions should be minimised? It currently appears that this is more of a review article.
The comments below represent significant omissions and I do not recommend publication of this article in it's current form.
Major comments:
1. The authors have used the ICAO emissions databank to show data on nvPM emissions indices for different combustor types. It would have been useful to show this as a trend in time in addition to with respect to engine rated thrust. Furthermore, a discussion of the nvPM mass could also be shown. There is no mention on whether this data shown has been corrected for line-losses. Discussion on suggestions on improving or adding to the regulatory measurement procedure would be a welcome addition. Discussion on how ground-level measurements scale to cruise conditions would also be welcome, e.g. https://egusphere.copernicus.org/preprints/2023/egusphere-2023-724/.
2. There is extremely limited discussion on the role of other aerosol particles in contrail formation. This is literature going back more than a couple of decades looking at the effect of sulphur, and there is emerging evidence that lubrication oil particles might play a role in the case of low soot conditions (https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1264/). Consideration should be given to the potential contrail impacts under low soot conditions (https://www.nature.com/articles/s41467-018-04068-0).
3. There is no mention of aerosol cloud interactions. These is the most uncertain contribution of aviation to climate change and might be the largest contribution to RF, however both the sign and magnitude the RF is extremely uncertain (https://www.sciencedirect.com/science/article/pii/S1352231020305689). Emerging evidence suggests that the role of soot particles might be less important than ambient particles (https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2441/; https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022JD037881). It is critical that this is covered in the article.
Citation: https://doi.org/10.5194/ar-2023-15-CC1 -
AC1: 'Reply on CC1', Una Trivanovic, 15 Jan 2024
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Please find the authors' reply in the supplement.
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AC1: 'Reply on CC1', Una Trivanovic, 15 Jan 2024
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