From the journal Environmental Science: Atmospheres Peer review history

28-May-2022

Dear Miss Wright:

Manuscript ID: EA-CRV-03-2022-000029
TITLE: Decarbonisation of heavy-duty diesel engines using hydrogen fuel: a review of the potential impact on NOx emissions

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Reviewer 1

This paper reviews the potential impact of heavy duty diesel engine operated with hydrogen-diesel fuel on NOx emissions.
The main finding of this study that H2D would be best used in lower average load applications is interesting. However, I still doubt the reliability of the conclusion because the number of references is not enough.
As the author mentioned in the paper, the formation of NOx emissions is highly dependent on the variety of factors such as engine operating conditions (engine load, engine speed, H2 ratio, and injection timing, etc.), EGR ratio, and aftertreatment devices.
My questions are as follows:
1) What do you think is the most dominant factor to affect the formation of the NOx emissions? I suppose that the engine load was selected as the most dominant factor because you divided the change in NOx emissions resulting from a hydrogen-blending policy for heavy-duty diesel engines by engine load.
2) What is the exact reason for the change of NOx emissions according to the load range?
3) What is the mitigation strategy for the NOx emissions? Are the aftertreatment devices, such as LNT and SCR, developed for pure diesel engine still valid to reduce the NOx emissions of H2D engine?
If these devices works well in H2D engine, can we apply the H2D stategy in heavy-duty diesel engine regardless of the engine load?

Reviewer 2

Alter the abstract section with more research findings
Compare and elaborate on the recent literature review publications related to NOx, hydrogen, and diesel engines (commercial, heavy, agriculture and other application).
Authors need to compare the present review with other forms of gases like natural, gas from waste and others.
Discuss and include more emissions.
Include NOx emissions mechanism
More literature review must be addressed in the form of a table
Include more results and reviews based on the recent literature.
This critical review is not up to the mark.
Various forms of emissions must be included and expand
Lag in the introduction section and literature so the authors must be addressed the below potent articles
Hunt, J. D., Nascimento, A., Nascimento, N., Vieira, L. W., & Romero, O. J. (2022). Possible pathways for oil and gas companies in a sustainable future: From the perspective of a hydrogen economy. Renewable and Sustainable Energy Reviews, 160, 112291.
Spatolisano, E., de Angelis, A. R., & Pellegrini, L. A. (2022). Middle Scale Hydrogen Sulphide Conversion and Valorisation Technologies: A Review. ChemBioEng Reviews.
Yoon, H. J., Seo, S. K., & Lee, C. J. (2022). Multi-period optimization of hydrogen supply chain utilizing natural gas pipelines and byproduct hydrogen. Renewable and Sustainable Energy Reviews, 157, 112083.
Wu, X., Zhang, H., Yang, M., Jia, W., Qiu, Y., & Lan, L. (2022). From the perspective of new technology of blending hydrogen into natural gas pipelines transmission: Mechanism, experimental study, and suggestions for further work of hydrogen embrittlement in high-strength pipeline steels. International Journal of Hydrogen Energy.
• https://doi.org/10.1080/15567036.2019.1675810
• https://doi.org/10.1016/j.energy.2020.117821
• https://doi.org/10.1016/j.energy.2020.117821
• https://doi.org/10.1080/15567036.2020.1856973.
• https://doi.org/10.1002/9781119793038.ch17
• https://doi.org/10.1016/j.energy.2022.123806
What is the main result of this review and include further scope? It should be given in the end of the conclusion section.
Include scope, research gap, novelty and research methodology of recent literatures
Include the combustion and other emission analysis to enhance the quality of the manuscript.
Overall English, typo errors and nomenclature must be reviewed and corrected.

Response to reviewer comments on the manuscript ‘Decarbonisation of heavy-duty diesel engines using hydrogen fuel: a review of the potential impact on NOx emissions’

The authors would like to thank the reviewers for providing constructive feedback on our work. Below are our responses to each question/suggestion. Changes and new references within the revised manuscript are highlighted in red and referenced in responses below. We have also picked up and improved the text for readability in a number of places, but do not list these exhaustively here.

REVIEWER 1:

1. Doubt the reliability of the conclusion because the number of studies is not enough.

RESPONSE: We agree that the literature database is limited, but these ~100 papers and report represent all the available information that is relevant to the problem being addressed. Policy decisions cannot always wait until perfect evidence has been assembled, hence we see considerable value in synthesis of existing information, suitably caveated for uncertainties. Taking onboard the reviewer’s wider point however, aspects of the manuscript (abstract/conclusion/paragraph 4 of results and discussion) have now been reworded to address this concern. Now in the conclusion, rather than making a firm statements, we have only highlighted that NOx emissions may be lower for low load applications, based on our interpretation of data. See answer to 2b for why we consider this is still valid.

2. As the author mentioned in the paper, the formation of NOx emissions is highly dependent on the variety of factors such as engine operating conditions (engine load, engine speed, H2 ratio, and injection timing, etc.), EGR ratio, and aftertreatment devices. My questions are as follows:

a. What do you think is the most dominant factor to affect the formation of the NOx emissions? I suppose that the engine load was selected as the most dominant factor because you divided the change in NOx emissions resulting from a hydrogen-blending policy for heavy-duty diesel engines by engine load.

RESPONSE: Using the available literature and relevant datasets, all linear regressions were plotted on the same graph, then split according to different factors e.g. with vs without EGR; low and high load; different engine speeds; different hydrogen injection parameters. Although there were wide ranges of NOx outcomes for each case, engine load was the only parameter where there was a clear difference in the bounds for NOx emissions between the low/high meta-analyses. Thus, engine load was deemed to be the most important factor (excluding e.% of hydrogen in the dual fuel).

b. What is the exact reason for the change of NOx emissions according to the load range?

RESPONSE: Although we cannot be certain, based on evidence contained within around half of the studies in Table 1, we suggest that the effect of load on increasing combustion temperature becomes outweighs any NOx reducing effects (such as hydrogen as thermal sink). The effect on ignition delay seems to be key here. Explanation of this has been added to paragraph 6 of ‘Hydrogen fuelling in large diesel engines’. Confirming this has also been added into possible extensions of this work in the conclusion section.

c. What is the mitigation strategy for the NOx emissions? Are the aftertreatment devices, such as LNT and SCR, developed for pure diesel engine still valid to reduce the NOx emissions of H2D engines?

The short answer to this is yes, all existing abatement options for diesel are likely to be effective to a degree, and we include some additional literature examples within the text where we discuss abatement in the conclusions.

d. If these devices work well in H2D engine, can we apply the H2D strategy in heavy-duty diesel engine regardless of the engine load?

Potentially yes, although the requirement to add further aftertreatment technologies changes the economics of the power unit and the fuel. One potential advantage in a well optimised H2 only fuelled system is that NOx emissions might be sufficiently low that aftertreatment could be avoided, and this would be a considerable cost saving. We have added some more references (103-105) to performance of aftertreatment when H2 is co-fuelled.


REVIEWER 2:

1. Alter the abstract section with more research findings

RESPONSE: Thank you for your suggestion. The abstract now contains both quantitative and qualitative results.

2. Compare and elaborate on the recent literature review publications related to NOx, hydrogen, and diesel engines (commercial, heavy, agriculture and other application).

RESPONSE: We have included all relevant literature up to end of 2021 and aim to have discussed that in our review and included data in our meta analysis where we can. This is however a constantly evolving field, and we have looked to see if any other newer reviews exist that either duplicate or complement the work here. We do not find any other work of that kind, although have included some very recent literature, including a 2022 Editorial commentary from the International Journal of Engine Research on hydrogen combustion that including NOx as an issue for policy consideration.

3. Authors need to compare the present review with other forms of gases like natural, gas from waste and others.

RESPONSE: We recognise that there are alternative fuels for heavy-duty diesel engines. These have now been mentioned in paragraph 5 of the introduction. However, we have focussed our review on hydrogen dual fuel combustion because the UK Hydrogen Strategy suggests this is very likely to become an important part of decarbonisation of difficult-to-electrify sectors in the short-term, at least in the UK. We are not necessarily proposing hydrogen as the optimal solution, but are addressing the evidence gaps on NOx emissions assuming that hydrogen will be a major energy source for these sectors in the UK. In this manuscript, we therefore make this assumption of hydrogen uptake, then look at the aspects of this action for which relatively little is known (point-of-use air quality- especially NOx), and try to suggest the range of possible outcomes. We have tried to make this clear by adding in some text in paragraph 3 of the introduction. The scope of the review does not extend to assessment of the emissions impacts of all alternative/replacement diesel fuel sources since that would be impractically complex and lengthy for a journal article.

4. Discuss and include more emissions.

RESPONSE: Other emission results have been added to Table 1 and discussed further on pages 8 and 9, and also in other places including the introduction, discussion and conclusions. Whilst the effect of these should be kept in mind when assessing overall benefits/disbenefits, our primary concern in the review is NOx emissions.

5. Include NOx emissions mechanism

RESPONSE: we have now added a short section on mechanisms in the introduction, and provided a signpost of other recent literature that covers the chemical mechanisms in more detail. The start of Section 3 also provides considerable detail on mechanisms of formation, which we consider more than adequate for the purposes of this review.

6. More literature review must be addressed in the form of a table

RESPONSE: more emissions have been added in Table 1 and are now discussed. As a result, the ‘data location’ column has been removed since this only referred to the NOx emissions results. However, our wider discussion remains primarily with NOx since this is a key issue for UK, as described in paragraphs 4 and 6 of the introduction.

7. Include more results and reviews based on the recent literature.

RESPONSE: See our response to Point 2. We have looked to include all literature up to end 2021, but have now added further a small number of further papers including from 2022. We note that the reviewer doesn’t point us to any papers in particular, making this difficult to respond to. For a review of this kind there has at some stage to be a cut-off date for literature, otherwise the review can never be completed. On searching we do not find any new literature, including reviews, that have been published in 2022 that would change our conclusions.

8. This critical review is not up to the mark.

RESPONSE: Thank you for your feedback, although this is quite a difficult comment to address since it doesn’t particularly guide us to where the reviewer sees gaps. The main sub-section of the review, titled ‘Hydrogen Fuelling in Large Engines’, has however been substantially expanded. We did originally remove this added text in an attempt to make the paper more concise, but realise now that it removed some important information. The conclusions and recommendations section is also expanded to give more space of review.

9. Various forms of emissions must be included and expanded

RESPONSE: Results for Hydrocarbon, CO and PM emissions have been included in Table 1 and are now included in the literature review section. These are undoubtedly important to consider and we discuss these in the paper in several new places. However, our key focus remains with NOx emissions; with electrification of the passenger vehicle fleet, based on current data from the UK National Atmospheric Emissions Inventory, non-electrified diesel engines could become the major source of NOx some areas of the UK in a business as usual scenario. This is now explained in paragraph 6 of the introduction.

10. Addition of suggested articles to the manuscript’s introduction and literature review sections

RESPONSE: Thank you for pointing out these articles, which focus largely on issues associated with hydrogen production and distribution. Whilst these are incredibly important aspects of a hydrogen economy, we focus our research on point-of-use/downstream emissions because this has received much less attention. This is especially true for the UK, where a lot of research, planning and investment has gone into development of hydrogen supply. Over 2/3 of gas pipelines have been converted to polyethylene in the Iron Mains Risk Reduction Program, which can distribute hydrogen safely. At the risk of making this review too long, we have decided not to include detailed explanation of the main distribution and production impacts from these articles, but we have now signposted some aspects from the UK’s Hydrogen Strategy which hopefully highlight this, as well emphasising our motivations for studying point-of-use air quality impacts. See paragraphs 3 and 5 of the introduction section.

11. What is the main result of this review and include further scope? It should be given in the end of the conclusion section.

RESPONSE: The main results of this review are 1) engine load and EGR are factors that significantly influence the NOx emissions from hydrogen-diesel fuel blends; 2) current knowledge and available data is still not enough to accurately determine the effect of hydrogen blending for HDEs on a large-scale e.g. city or national emissions. Further scope includes 1) investigating the effect of engine speed on NOx from these set-ups; 2) looking at the distribution of types of heavy-duty engines used in the UK to determine a most likely scenario for NOx as a result of a hydrogen blending policy; 3) confirming that engine load is a dominating factor influencing NOx from H2D combustion in heavy-duty engines, and in the way described in this review.
This has all been added to the conclusion section.

12. Include scope, research gap, novelty and research methodology of recent literatures

RESPONSE: As already stated in the response to 8, more detail on the methodology has been added into the literature review. We have not included recent, novel decarbonisation methods because this is not the focus of our review- we are looking at fast, technologically straightforward decarbonisation options that can be scaled up easily. We look at this because of the recommendations provided to the government, for example from the Committee on Climate Change, who highlight the speed at which deep decarbonisation needs to occur if the legally binding 2050 net zero target is to be achieved. Novel techniques often have lower technological readiness levels (TRLs) or suffer from scalability issues, hence we focus only on H2D. The datasets in Table 1 of the original manuscript are all the accessible relevant sets of data, which provide sufficient methodology details (engine type/load parameters etc.) to provide reliable and relevant emissions data.

13. Include the combustion and other emission analysis to enhance the quality of the manuscript.

RESPONSE: Results for other emissions such as CO and PM have been included in Table 1 and now discussed in the literature review section. These are undoubtedly important to consider. However, our key focus remains with NOx emissions because with electrification of the passenger vehicle fleet, based on current data from the UK National Atmospheric Emissions Inventory, non-electrified diesel engines could become the major source of NOx some areas of the UK.

14. Overall English, typo errors and nomenclature must be reviewed and corrected.

RESPONSE: Any errors spotted have been corrected and are highlighted in red on the revised manuscript

04-Jul-2022

Dear Professor Lewis:

Manuscript ID: EA-CRV-03-2022-000029.R1
TITLE: Decarbonisation of heavy-duty diesel engines using hydrogen fuel: a review of the potential impact on NOx emissions

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Reviewer 1

My concerns are resolved.

Reviewer 2

All the review comments were addressed properly.