Life Cycle Assessment of Hydrogen Transportation Pathways via Pipelines and Truck Trailers: Implications as a Low Carbon Fuel
Round 1
Reviewer 1 Report
The article is interesting.
The main question addressed in the study is the comparison of transportation methods for Hydrogen. Truck or pipeline.
The topic original address a specific gap in the field.
The assessment proposes an interesting method to evaluate the transport of hydrogen in California state.
In the methodology section, the authors say: "The LA production plant was selected after discussions with industry professionals".
What is the meaning of LA? Please clarify.
Please describe the "discussions with industry professionals", methods used, etc.
Besides, the discussions should be improved. Are there other similar studies in the USA or other countries? Please compare.
The conclusions could be improved. The reviewer think that a better discussion should be added, and compare the results with other similar case of studies.
The references are appropriate.
The figures are in the correct way and well described.
In the phase "and consumption of the fuel. (Figure 10)", please move the period to the end.
Author Response
Reviewer Number 1:
- In the methodology section, the authors say: "The LA production plant was selected after discussions with industry professionals".
What is the meaning of LA? Please clarify.
Answer: line (250)
The Los Angeles production plant was selected after discussions with expert in the field.
- Please describe the "discussions with industry professionals", methods used, etc.
Answer: line (251)
The Los Angeles production plant was selected after discussions with expert in the field.
“The expert in the field is refer to Dr. Anil Prabhu of the California Air Resources Board and Dr. Michael Wang, a Senior Scientist in the Energy Systems Division, who was the primary developer of GREET which we acknowledge them in the acknowledgments.”
- Besides, the discussions should be improved. Are there other similar studies in the USA or other countries? Please compare.
Answer: line (207-212)
Sinha et al.[36] analyzed the lifecycle assessment of renewable hydrogen FCEVs in the northern and southern parts of California. Although they consider transportation pathways for short distances in their analysis, it is just limited to the compression factor prior to the loading on tube trailers. California has other resources for producing hydrogen, like SMR and biomass production sites, which are far away from the consumption stations.
- The conclusions could be improved. The reviewer think that a better discussion should be added, and compare the results with other similar case of studies.
Discussion: line(482-498)
Based on the Ecoinvent database, Simons et. al [44] estimated the GHG emissions of SMR hydrogen production 125 gCO2ep per 1 MJ of hydrogen. However, according to our findings 78.93 gCO2eq are produced per 1 MJ of hydrogen. The authors in [35] estimated 111 gCO2ep/1 MJ H2 for SMR-truck and 89 gCO2ep/1 MJ H2 for SMR-pipeline using the GREET database. Even after excluding compression prior to loading, we estimated 125.36 gCO2ep/1 MJ H2 for SMR-truck and 90.323 gCO2ep/1 MJ H2 for SMR-pipeline (70% attribution) using the GREET database. These findings highlight the significance of taking the LCA of pipeline and tube production into account.
The goal of this study is to assess the life cycle impacts of various hydrogen production, delivery to a hydrogen refueling station, and consumption by an FCEV pickup truck; however, water consumption could be a significant limitation, particularly in California. According to this study, all truck pathways use more water than pipeline alternatives. Furthermore, water consumption does not differ significantly between pipeline attrib-ution scenarios. This could be interesting because solar electrolysis pathways, which use water as their main feedstock, use roughly the same amount of water as SMR pathways but much less than biomass pathways (Figures 12 and 13).
Line (506-510)
The inclusion of innovative technologies in the production of onboard hydrogen tanks has the potential to improve the environmental performance of all pathways. Furthermore, taking into account developing new technology in heavy duty fleets and improving fuel economy could enhance the environmental performance of each tube trailer truck-based approach.
Answer: line (207-212)
Based on the Ecoinvent database, Simons et. al [44] estimated the GHG emissions of SMR hydrogen production 125 gCO2ep per 1 MJ of hydrogen. However, according to our findings 78.93 gCO2eq are produced per 1 MJ of hydrogen. The authors in [35] estimated 111 gCO2ep/1 MJ H2 for SMR-truck and 89 gCO2ep/1 MJ H2 for SMR-pipeline using the GREET database. Even after excluding compression prior to loading, we estimated 125.36 gCO2ep/1 MJ H2 for SMR-truck and 90.323 gCO2ep/1 MJ H2 for SMR-pipeline (70% attribution) using the GREET database. These findings highlight the significance of taking the LCA of pipeline and tube production into account.
- In the phase "and consumption of the fuel. (Figure 10)", please move the period to the end.
Answer: Done
Reviewer 2 Report
General comments
This paper analyses the LCA of 6 scenarios for hydrogen production and distribution. Although the paper's topic is interesting, the results are predictable, which compromises the paper's novelty. Furthermore, I consider that the paper is not replicable in its current form. Authors are suggested to take more profound advantage of the results, improve their discussion, and add more material that eases the replication of the paper.
Abstract
It needs to be more quantitative.
Literature review
In lines 66 – 77, please specify if the GHG emissions are based on a lifecycle basis.
Methodology
In lines 225 to 228, please explain in detail why you did not include the compression of the hydrogen as part of your system boundaries. Is that because you are comparing various systems and the emissions from this stage are similar among them?
In lines 241 – 249, please specify why you did not include the construction, maintenance, and operation of both the hydrogen production plant and fuel station. Is that because they are negligible in terms of a lifecycle basis?
It is weird that you include some part of the emissions derived from the infrastructure to distribute the hydrogen (pipelines and trucks) but not those related to the different facilities for hydrogen production (SMR, solar, and biomass). I understand why you decided to include the former emissions, but you must state that you assumed that the latter emissions do not affect the conclusion of your results (are you sure about this?). If possible, authors should include the emissions derived from the infrastructure for hydrogen production as part of their results.
In line 268, why did you allocate 1% of the GHG emissions of the pipeline construction to the functional unit? In this case, the GHG intensity factor should be computed considering the amount of hydrogen that goes throughout the pipelines during their lifespan. Please check this.
Currently, some information in the appendix is related to the LCA results by stage. However, it is hard to visualize the life cycle inventory in the paper and, therefore, ensure its reproducibility. Can the authors include a table in the appendix with more detailed information regarding the mass and energy balances per functional unit? It would allow for a better understanding of the inputs and outputs of the system and make it easier to replicate.
In the biomass-based hydrogen production pathway, what type of biomass did the authors take into account? What is the lifecycle GHG emission intensity factor of this biomass? Please specify.
I think that the goal of the LCA should reflect that the authors are estimating the variations in GHG emissions among the analyzed hydrogen production pathways rather than calculating a GHG emission intensity factor for each of them.
Results
The results shown in the first section are somehow obvious, which compromises the novelty of the paper. Authors are suggested to expand the results and show how the different stages involved in the hydrogen supply chain (depicted in Figure 1) affect the total emissions. What are the activities that contribute the most to the emissions? This could add value to the results.
The authors state that policymakers should consider VOC, NOx, and PM2.5 for the decision-making processes as they affect human health. However, this claim must be seen carefully as these emissions occur in different locations of the supply chain, including areas that might not affect human health.
In the sensitivity analysis, the authors explain better how they decided on the allocation of the emissions of the pipeline construction. However, it seems that it does not follow the expected computation. I would have expected that authors take the GHG intensity factor of the construction of a 1 km pipeline, multiply this by the pipeline length (required to distribute the hydrogen), and then divide the result over the amount of hydrogen transported during the whole lifespan of the pipeline. If other gases are distributed in these pipelines, an allocation should be included considering flow rate or other criteria.
I can tell that the model provides information regarding water consumption on a lifecycle basis. Authors should also include this in the paper given the relevance of this resource in California (and the world). Furthermore, they could even undertake some projections towards the future water consumption in the region if hydrogen were to be produced there under the projected production capacities. Could California afford such a water consumption?
Discussion
The authors stated that they noticed that other LCA tools showed other (more pessimistic) results compared to those of the GREET model. However, this is not reflected in the paper, and so, the reader cannot reach the same conclusion. Moreover, if authors consider that other tools can lead to more accurate results, why do they not use them?
The authors mention that they did not consider the construction of the refueling station and fuel storage tank at the fuel station. If these emissions are the same for all the analyzed case studies, then removing them from the system boundaries is appropriate. However, the goal of the study should be changed to reflect the fact that the authors are not estimating a GHG emission intensity factor but rather the differences in the environmental performance of the assessed system (variation in GHG emission among the analyzed case studies). Activities that are not common for all the scenarios, such as the construction of the hydrogen facility, should be involved unless authors consider (and justify) their emissions are negligible and do not change the conclusions.
I expected to see some proposals to enhance the environmental performance of each hydrogen pathway. Considering the hot spots of emissions, what actions could improve the environmental performance of the supply chain of hydrogen production? Maybe the inclusion of new technologies? Perhaps the inclusion of other types of feedstocks? Is there any possibility of capturing non-carbon emissions (NOx, PM2.5, etc.) by using filters at some points of the supply chain?
Conclusion
Conclusions are kind of vague. They must be more quantitative and highlight the activities that contribute the most to the emissions of hydrogen production.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 3 Report
Abstract:
1. Instead of merely stating that the results are compared across multiple scenarios, the abstract should contain the observed trends and results of comparison of different production and transit processes with certain value or percentage.
2. The abstract does not provide an accessible summary of the paper.
Introduction:
3. In section 1.2 of introduction, the references used for literature review should be of recent years, as it’s impossible to argue about the current understanding by referencing old articles.
4. At the end of this section, it should state the research aims. In my opinion, it should be revised.
Data and methods:
5. What is GREET model mentioned on the line 262? Explain the purpose of using this model for obtaining the reference life cycle inventories (LCIs).
6. Revise the sentences on lines 262 to 268. Explanations on these lines are not clear.
7. Explain the reason of using reference life cycle inventory of hot-rolled steel pipe production in case of both pipelines and tubes.
Results and discussions:
8. Provide reasons for the LCA pipeline attribution assumptions made by the author in section 3.1, line 317.
9. What are the LCI tools mentioned in section 4 such as GaBi or Ecoinvent and why GREET model is preferred over these two? (On line 434-436)
10. It is not mentioned how output data is obtained or recorded using which instrument or software.
11. The figures in section 3, page 7, should be described what the data is showing.
12. The discussion could be enlarged to explain if there are any similarities or discrepancies with the data published and the data obtained.
Conclusion:
13. The author should provide the insights obtained from the graphs involving emission intensity and sensitivity analysis. How do the results in this work compare with those of related studies?
14. There are certain parts of the paper that could be communicated better, such as parts of the discussion and conclusion.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 4 Report
The authors compare the life cycle environmental impacts of hydrogen transportation via pipeline and truck via an illustrative California case study.
General comments:
The life cycle inventory and/or inputs to the GREET model need to be better documented to improve the transparency and reproducibility of the study.
Some discussion is needed on the physical basis for the differences in impacts for carbon footprint, NOx, VOC, PM2.5 between pipeline and trucking. For example, are the impacts happening in the construction phase of the pipeline and minimal during pipeline operation?
Specific comments:
Abstract - add result that solar H2 + pipeline is the lowest GWP scenario, and add the 1% attribution of the pipeline LCA as a key sensitivity
Line 18 - why is the word "despite" used?
Section 1.2 - add reference to another recent California H2 LCA study: https://doi.org/10.1016/j.seta.2021.101188
Line 75 - change "CO2" to "metric tons CO2"
Line 190 - explain here how the current study extends the related work of Frank et al. In discussion section, compare results of the current study to Frank et al.
Line 227 - explain why compressing is not in system boundary
Figure 1 - indicate foreground processes in the figure
Section 3.1 - add some analysis and discussion of potential pipeline leakage
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Accept in present form
