Liquid Organic Hydrogen Carriers Applied on Methane–Hydrogen-Fueled Internal Combustion Engines: A Preliminary Analysis of Process Heat Balance
Round 1
Reviewer 1 Report
The authors presents an heat analysis of the dehydrogenation of LOHC and the combustion of methane/hydrogen system. Firstly, the calculation is the basic thermodynamics calculations with little novelty. Secondly, why would methane exist in such system is not well explained, i.e. how to store the methane in a vehicle? Any reason not using the LOHC- as the fuel? Thirdly, the introduction of the article is unnecessary long and messy. Lastly, the unit in the figures is missing and the results are not presented clearly. Since the manuscript is poorly prepared, I suggest that this article should be rejected.
1. Line 43, “as it is produced almost entirely through fossil sources”, is this 95%? It is not clearly presented.
2. Line 46. “Due to the lower molecular weight, the density of hydrogen is decidedly low and 46 significantly lower than diesel fuel and petrol. “ The reasoning does not make sense, for example, water has a lower molecular weight than diesel fuel and petrol, but higher density.
3. The introduction part has to be re-written, it is much longer than needed to introduce the topic of interest.
4. The paper does not explain why the CH4 will be there and include in the system.
5. The figure does not have any units in the axis, and the quality of the figure is low.
6. The unit of temperature is not unified, for example, °C and K are co-existing.
7. There should be a table summarize all the parameters. For examples, it is confused whether ???ℎ is calculated from the combustion or calculated from the minimum temperature needed for dehydrogenation.
8. Line 263, what is the reasoning for this assumption?
9. Could the author plot or list the “minimum exhaust temperatures” and “the literature and experimental exhaust temperatures” in the same graph, and compare?
Author Response
Comment 1: Line 43, “as it is produced almost entirely through fossil sources”, is this 95%? It is not clearly presented.
Response: Thanks for your suggestion. We modify the content as you suggested.
Original content: …as it is produced almost entirely through fossil sources...
Modified content:…as it is produced almost 95% through fossil sources….
Comment 2: Line 46. “Due to the lower molecular weight, the density of hydrogen is decidedly low and 46 significantly lower than diesel fuel and petrol. “ The reasoning does not make sense, for example, water has a lower molecular weight than diesel fuel and petrol, but higher density.
Response: We have revised the word through your suggestion.
Original content: Due to the lower molecular weight, the density of hydrogen is decidedly low and 46 significantly lower than diesel fuel and petrol
Modified content: The density of hydrogen is decidedly low and significantly lower than diesel fuel and petrol.
Comment 3: The introduction part has to be re-written, it is much longer than needed to introduce the topic of interest.
Response: Thanks for your suggestion. We shorted the introduction, delating the text from line 110 to line 154, giving more importance on hydrogen storage options.
Comment 4: The paper does not explain why the CH4 will be there and include in the system.
Response: According to your suggestion, we added a specific motivation regarding the use of methane-hydrogen mixture and how methane can be store in a vehicle.
Original content: This study considers a compressed natural gas (CNG) internal combustion engine (1.4L, eight valves) vehicle with variable composition.
Modified content: This study considers a FireTM 1.4 litres, 8 valves, 4 stroke spark ignition engine manufactured by FiatTM. The choice of this engine was made because FIAT already converted this engine to methane as fuel for some commercial vehicle (i.e. Fiat PandaTM and Fiat Grande PuntoTM) where methane is stored in compressed tanks at 350 bar maximum pressure. The brake power curve of the engine fuelled with Compressed Natural Gas (CNG) is given by the manufacturer and used in a previous work where methane was mixed syngas from biomass gasification [13].
Comment 5: The figure does not have any units in the axis, and the quality of the figure is low.
Comment 6: The unit of temperature is not unified, for example, °C and K are co-existing.
Response: According to your suggestion, we increased figures resolutions, we added units in the y-axis of the Figures 2,3,4 and we unified temperature unit to K.
Comment 7: There should be a table summarize all the parameters. For examples, it is confused whether ???ℎ is calculated from the combustion or calculated from the minimum temperature needed for dehydrogenation.
Response: Thank you for the suggestion, we decided to add an “Abbreviations and Symbols” section at the end of the paper where all the parameters of the models are reported and defined.
Comment 8: Line 263, what is the reasoning for this assumption?
Response: We back calculate the values of (, , and ) using Eq. 26. starting from the volumetric mixtures in case A (100% H2 + 0% CH4); case B (75% H2 + 25% CH4); case C (50% H2 +50% CH4) and case D (25% H2 + 75% CH4). The choice of these volumetric mixture was made to equally cover the mixture range.
Comment 9: Could the author plot or list the “minimum exhaust temperatures” and “the literature and experimental exhaust temperatures” in the same graph, and compare?
Response: In Figure 2, the observed values of some studies from the literature have been added, and we believe that your suggestion will add value to our article. Thank you for your valuable comments.
Modified content: Figure 2 shows the calculated values for H18-DBT and H12-NEC with changes in the hydrogen ratio. Data from some experimental studies in the literature are also shown in the same figure. It should be noted that while the values obtained from our study show the minimum exhaust temperatures required to self-sustain the process, the exhaust gas temperature values observed in experimental studies are independent of the dehydrogenation process. As shown in Figure 2, exhaust gas temperature values observed with low amounts of hydrogen in the fuel may be sufficient to initiate dehydrogenation. It should be noted that the values for different references indicated in the image are calculated using different fuel compositions.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
A brief summary
Simone PEDRAZZI, etc. investigate the feasibility of using exhaust gas heat from an internal combustion engine to drive the dehydrogenation of Liquid Organic Hydrogen Carriers (LOHCs) for hydrogen storage. A 0D model was used to analyse a mixture of hydrogen and methane in varying ratios, considering Perhydro-dibenzyltoluene and Perhydro-N-Ethylcarbazole as LOHC compounds. The study aimed to determine the minimum exhaust gas temperatures required to self-sustain the dehydrogenation process. The results demonstrated that the minimum exhaust temperatures for different running regimes and hydrogen-to-methane ratios are lower than those reported in the literature and experimental data, suggesting that LOHC dehydrogenation using exhaust gas heat is a viable option. I would like to recommend this work be published after a minor revision.
General concept comments
1. In Equation 13, which calculates the engine power using syngas, should the term Kgas be replaced with Ksyngas to clarify the distinction and avoid ambiguity?
2. In line 251, is ? = -3.595×10-9 a typo?
3. In Figure 3 and Figure 4, it is recommended to add the unit of Y-axis.
4. N-Ethyl carbazole might be not a good candidate for powering a vehicle as it has high melting point, which make the storage difficult.
Specific comments
1. I would suggest author to add 0% H2, 100% CH4 for better comparation in this system.
2. Are there any significant factors or limitations that could affect the assumption of ???? = ??????? = 20°C?
3. What are the potential solutions for increasing exhaust gas temperatures if they are lower than the minimum required for the dehydrogenation process to occur?
Author Response
Comment 1: In Equation 13, which calculates the engine power using syngas, should the term Kgas be replaced with Ksyngas to clarify the distinction and avoid ambiguity?
Response: In Equation 13, Kgas was replaced with Ksyngas. As you mentioned, this is better to clarify the paper.
Comment 2: In line 251, is ? = -3.595×10-9 a typo?
Response: In line 251, “?” should be “d”, we corrected this error.
Comment 3: In Figure 3 and Figure 4, it is recommended to add the unit of Y-axis
Response: For the Figure 3 and 4, units of Y-axis were added.
Comment 4: N-Ethyl carbazole might be not a good candidate for powering a vehicle as it has high melting point, which make the storage difficult.
Response: Good comment. The high melting point of N-Ethyl carbazole can be overcome by designing specialized methods for storage and transportation that can maintain the necessary temperature or by combining N-Ethyl carbazole with other chemicals to lower its melting point. However, these methods can add complexity and cost to the refueling process, which can make refueling less practical when used on vehicles. We added this consideration in the results and discussion section of the paper.
Modified content: H12-NEC needs a lower thermal power compared to the H18-DBT and then exhaust temperatures requirement are lower for the H12-NEC. However this LOHC the has an high melting point that can be overcome by designing specialized methods for storage and transportation that can maintain the necessary temperature or by combining H12-NEC with other chemicals to lower its melting point. However, these methods can add complexity and cost to the refueling process, which can make refueling less practical when used on vehicles.
Comment 5: I would suggest author to add 0% H2, 100% CH4 for better comparation in this system.
Response: the case that you indicates is already discussed in a previous paper [6], we decide to not discuss here this case to avoid unless repetition.
Comment 5: Are there any significant factors or limitations that could affect the assumption of ???? = ??????? = 20°C?
Response: The presumption that the air temperature is identical to the syngas temperature may be affected by a number of variables and restrictions. The local climate, the hour of the day, and altitude are only a few examples of the variables that might affect the temperature of the air and syngas. The temperature of the combination may also be impacted by the syngas and air constituents. If there is a significant heat transfer between the two elements prior to mixing, the presumption that air and syngas are both at the same temperature might not hold true. Therefore, even though the simplistic assumption of Tair = Tsyngas = 20°C may be appropriate in some circumstances, it is vital to take into account the considerations and constraints.
Comment 6: What are the potential solutions for increasing exhaust gas temperatures if they are lower than the minimum required for the dehydrogenation process to occur?
Response: Some potential solutions are described from line 325 to line 333 of the paper.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The author addressed all the comments. I suggest publish in the present form.
