Optimization of Thermodynamic Parameters of the Biological Hydrogen Methanation in a Trickle-Bed Reactor for the Conditioning of Biogas to Biomethane

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1

Shoul add abbreviation index

Thank you for your comment. We added an abbreviation index. Since the MDPI layout did not give a section for abb. Index we suggest the editorial board decides if it fits.

I guess this word should be change, because in this work only modify temperature and pressure

Thank you for your indication. Since temperature and pressure are thermodynamic parameters, we liked this more elegant and compact phrasing. In the article itself we then declare temperature and pressure as the parameters we varied.

Should improve keywords, because there are many exteneive

The key words were changed as follows: Biological methanation; trickle-bed reactor; biogas upgrading; biomethane; optimization of operation parameters

Must indicate in this section, the statistical analysis used, and also software used too. because in figures mentioned Tuke ́s Test

Thank you for your indication. The relevant information is added in the lines 162-165 of the manuscript: The sensor data logged during the experiment and stored in database, as well as results of gas measurement were evaluated and process with the help of MATLAB software package. The statistical analysis of the gas measurement data and lab analysis data was performed with the Tukey’s test (p<0.05).

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Should reduce size of this figure.

Figure 1 is resized

Must improve title. Of table 1, and should not use table word again

Thank you for your comment. The title of the table was changed as follows:  Generated design of experiment by JMP Pro variating temperature and pressure.

Must use the acronym.

In the lines 138-139 the acronym for hours was used.

Must correct, using sub index

Thank you for you indication. The line 177 was corrected.

Should use 2 number, after point

Thank you for your commnt. We are using three decimal places to amplify differences between the different results and high precission of the data. Values for the flow rates were corrected accordingly.

Authors should to use only once reference 9 in this paragraph, improving redaction.

Lines 244 were corrected according to reviewer. 

Must improve the quality of figures.

Thank you for your comment. The quality of the figures was improved and they will be provided to editorial board  in formats jpg, png and pdf.

Delete acronyms in figure captions

We deleted acronyms as reviewer suggested.

Is “reactors´ effluent” correct in Line 295

We had our manuscript checked with language control and it is correct.

Must verify reference, I guess is a web page

Thank you for your comment. The reference in line 354 is a report by IEA Bioenergy. The citation style was automatically updated according to the MDPI format.

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Submitted paper presents a great contribution related to the use of biogas in the field of natural gas grids, in this case more related to the specifications from German market.

In first chapter the authors present in detail the requirements that biogas had to meet to be used in natural gas grids, considering both grids (L-gas and H-gas). Also, in same chapter they mention some studies related to the CO2 methanation. The novelty of the paper is based on the co-methanation of biogas and H2 in a trickling bed reactor.

Chapter 2 presents the experimental setup used to validate the main idea and also presents some parameters achieved in tests. The method is explained well covering also explanations related to lab scale equipment used to register process parameters and also the subchapter 2.3 explains the analysis of experimental data.

In chapter 3 the authors describe the experimental results using well explained figures and graphs.

Only thing to mention is that from chapter 3 the paper jumps to chapter 5, probably this is a typing error while chapter 4 is missing.

In conclusion I consider this paper worth to be accepted for publication after the correction of this minor mistake with chapter numbers.

Thank you for you comments, indications and praise, we are very grateful. We revised the typing error.

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3

The compsotion ranges of the biogas stated in the literature should be revised as the summation doesnt equal 100%.

If the reviewer refers to the following sentence (Lines 36-38): “Biogas consists of 50 - 75 % CH4 and about 25 - 50 % CO2, less than 10 % water vapor, and trace amounts of H2 (< 1 %) and hydrogen sulfide (< 3 %) [1],…”, than the following explanation could help: The availability of each gas component is mentioned in the certain ranges, and exact share of each gas in the gas mixture could vary: for example 50% of CH4 and 50% of CO2 without H2 and H2S, or 75% of CH4, 20% of CO2 and the rest will be water vapor, H2 and H2S. The sum of gases will always be 100%, but the share of each gas will vary in certain ranges.

The literature review is not adequate. Authors have to discuss further facts regarding the hydrogen conversion and the BHM process from literature.

Thank you for your remark. The present article describes an experimental work, and our main focus is the production of biomethane. We added additional literature on the co-methanation divoted to the topic of biomethane injection into the gas grid
(doi:10.1016/j.apenergy.2013.04.080,  doi:10.1016/j.biortech.2021.125135). Our literature review embraces the operational parameters, description of BHM process, and bench mark studies are mentioned. The works [7-10] describe the hydrogen conversion in BHM plants.

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The authors mentionned the usage of converted hydrogen into the biogas upgrading. How can this be efficient compared to other pathways such as conversion to methanol or ammonia or other more feusible routes? refer to this reference "https://doi.org/10.1016/j.rser.2022.112556  "

Thank you for your comment and your interesting reference. The scope of our research is related to the usage of renewable energy resources, and the proposed technical solution of biogas upgrade. By that, the calorific value of biogas will be increased, as well as its quality, and biomethane could be injected into the national gas grid. The conversion of methanol or ammonia are established technologies for transportation of H2, which is not our goal.

We focus on the usage of excess energy to produce green hydrogen which then can be used to upgrade biogas to biomethane. Thereby we can store this excess energy in an already excisting infrastructure.

The contribution of this work have to be given in details. The authors mention the differece compared to CO2 methanation, however, also distinguishing this work in regard of biogas upgrading and biogas to biomethane have to be discussed.

As we mention in line 70, there is a gap in investigation of BHM as an upgrading technology for biogas. That is why we can not provide literature on biogas upgrading and present our experiments on that topic as a novelty in this paper.

Table 1's title is given mistakenly

Thanks for your indication. The title was corrected as indicated by the reviewer.

The scheme of experiment (Fig. 2) should be more detailed

Thank you for your comment. The detailed scheme of the experimental plant was provided in earlier works of T. Ullrich [6]. For the needs of the present article we have shown the simplified scheme describing the construction of the plant which is sufficient for understanding of the experimental procedure.

Fig. 2 illustrates that the biomethane purety is more than 99%, but the abstract claim that it achieved 95 of methane contnet at the optimised conditions. how can the authors explain this?

As it follows from Fig.2, the graph demonstrates the relation between biomethane quality and ratio of incoming gases H2:CO2, and the importance of this is declared in the outlook of the manuscript. The purity above 99% could be achieved at the optimal operation parameters, but it was rather an outlier and therefore is not shown on the box plots in figure 3 and table 2. In the abstract we mentioned 95 % as the median value , which is the threshold of biomethane quality to be fed into the natural gas grid.

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4

In the methodology section:

During the experiments, the main thermodynamic parameters, namely temperature and pressure, were varied in a certain range to find the optimal operating point.

Mention what the range was.

Thank you for your comment. The following section was revised:  During the experiments, the main thermodynamic parameters, namely temperature and pressure, were varied in the range of 50 to 60 °C and 2 to 9 bar to find the optimal operating point. (Lines 93-94)

For the design of experiments established, why were not higher temperatures chosen for this type of system?

Thank you for your remark. In the presented study we sticked to the favorable conditions for thermofilic methanogenic species (40-70°C).

Since our plant was build for thermophilic conditions, the maximum temperature is 60 °C due to operational limits of the sensors.

It is necessary to carry out additional experiments at high temperatures, since the experimental design presents few experiments for biogas production.

Thank you for your indication. This is definitely an interesting topic. Although, the change to higher temperature will cause change in microbial community: the microorganisms active at 50-60 degrees will be suppressed at temperature above 70 degrees, and other type of microorganisms (hyperthermophiles) will be active at elevated temperature. In this case, it will be difficuilt to compare the experimental results, since another type of microorganisms will be responsible for methanantion process (https://doi.org/10.1016/j.biortech.2018.05.105 ). As mentioned above our experimental plant is not designed for higher temperature. As we suggested in the outlook, racing temperature even higher as in our experiments does raise the question of economical feasibility.

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5

The abstract section mainly described the main results of this work and should add more important experimental data.

Thank you for your comment. Results related to the biomethane quality and methane formation rate and educt ratio is presented in the abstract with corresponding operating parameters.

The introduction section should further emphasize the novelty of this draft.

Thank you for your remark. In our opinion the last section of the introduction does emphasize the novelty of our study:  Since previous studies focused mainly on CO2-methanation, which was later referred to as mono-methanation, this study analyzes the co-methanation of biogas and H2 in a TBR. The objective of the study was to verify the possibility of upgrading biogas with 55 % CH4 to biomethane with 95 % CH4 with BHM, and to optimize the thermodynamic pa-rameters of the operating process by applying a design of experiments (DoE).

Conclusion. Please add some key experimental data.

Thank you for your indication. The core results of our work is presented in the conclusion. Methane quality, hydrogen conversion rate and methane formation rate with the corresponding operating parameters are our key experimental data.

Line 133-135: After the reaction mixture had been allowed to stand for 30 min at room temperature, its absorbance at 517 nm was measured immediately using a UV–VIS-NIR spectrophotometer (UV-3600, Shimadzu Co., Kyoto, Japan). This approach is also adopted by other research fields such as remote sensing, to quantify atmospheric water vapor absorption and vegetation chlorophyll concentration, as reported in "Requirement sensitivity studies for a future Landsat sensor", "Potential of Red Edge Spectral Bands in Future Landsat Satellites on Agroecosystem Canopy Chlorophyll Content Retrieval" and "Potential of red edge spectral bands in future landsat satellites on agroecosystem canopy green leaf area index retrieval".

We are not sure that this comment is related to our work, since in our manuscript the lines 133-135 are related to the table with the experimental plan with varied temperature and pressure.