Review Reports

Round 1

Reviewer 1 Report

This manuscript provides an overview of materials for biogas desulfurization, where authors applied Methodi Ordinatio and summarized the advantages and disadvantages of the state-of-the-art H2S sorbents. The reviewed literature is mostly recent and represents the current directions.

Authors summarized iron-based materials in a separate paragraph, which is not very well justified, because there are many other active metal oxides species that have higher sulfur capacities than iron oxide, such as CuO, ZnO. And zeolites section should be a separate topic, and not to be mixed with other oxides.

Therefore, some reorganization of the paragraphs and some deeper discussions are required.

Author Response

REVIEWER #1: This manuscript provides an overview of materials for biogas desulfurization, where authors applied Methodi Ordinatio and summarized the advantages and disadvantages of the state-of-the-art H₂S sorbents. The reviewed literature is mostly recent and represents the current directions.

Authors summarized iron-based materials in a separate paragraph, which is not very well justified, because there are many other active metal oxides species that have higher sulfur capacities than iron oxide, such as CuO, ZnO. And zeolites section should be a separate topic, and not to be mixed with other oxides.

Therefore, some reorganization of the paragraphs and some deeper discussions are required.

The authors are thankful your comments and corrections.

Changes have been made and the text was improved.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments to the manuscript energies-2166912: Reactive Processes for H2S Removal

The present manuscript deals with the abatement of H2S by different technologies in biogas. The topic is within the scope of energies. Hence, this manuscript might be published after some major revisions:

·         Line 12+13 (and Table 1): There is also a quite large share of H2, which should be addressed.

·         Line 15: What do you mean with “and wear”?

·         Line 45: It should be addressed that these contaminants will cause a crude mixture of waste gas contaminants during incineration. The following reference (especially supplementaries) might be very helpful to point out this aspect.

https://doi.org/10.1016/j.jclepro.2019.04.258

·         Line 53: …as well as support food industry.

·         Figure 1 seems to be a collection of potential technologies, but a couple of them are technically inadequate to be used for biogas treatment or should be described clearly i.e. membranes are not selective for H2S and may pass CH4 as both molecules are not lipophilic (skipping thin film membranes) and nearly of same size (skipping porous membranes). Cryogenic separation has dramatically high costs as H2S as permanent gas cannot easily be freezed out. Please give more details about biopercolators? Finally, please substitute biowashers by bioscrubbers.

·         Figure 1: please add technics as biotrickling filter and bioprocesses under microaerophilic conditions.

·         Line 139: Please combine Results and Discussion

·         Line 164: … was used…

·         Line 167: In general concentration of H2S should be listed to get a better idea about contact times and concentrations.

·         Line 169: please explain ‘hollow absorbers’. Commonly, scrubbers are ‘hollow’

·         Line 171: how should alternating processes be established in large-scale system. Please give some ideas.

·         Line 177; Please add the concentrations of NaOH, Ca(OH)2 and MEA to calculate the molar concentrations of additives to adsorb H2S. Most probably the Ca(OH)2-concentration was the lowest due to lower solubility.

·         Table 2 Reference No. 23: Please list the concentration of H2S – or do you mean adsorption of H2S equivalents pointing out a unit of L/100g (seems non realistic)

·         Line 238: Literature reference should be checked whether H2S adsorption capacity of untreated charcoal is really a slow as 0.5 mg/g, equivalent to 0.05 w%. Even though AC shows poor adsoprtion behavior against H2S this value seems to be a factor of 10 in minimum too low.

·         Line 268: it should be pointed out that contact times of 71 s are quite inadequate for industrial application.

·         Line 376: It should be pointed out – due to reference 15 and common experimental experiences that goethite (FeOOH) shows very very poor efficiency as catalyst. Reference no. 15 shows that low performance life time of less than a few minutes.

·         Lin 387: Formation of biofilms was not adressed before. Furthermore, highly relevant techniques like biotrickling filters are not adressed.

Possible references are:

o   doi: 10.1631/jzus.B0920064

o   HAL Id: hal-01945143
https://hal.archives-ouvertes.fr/hal-01945143
Eric Dumont. H2S removal from biogas using bioreactors: a review. International Journal of Energy and Environnement, 2015, 6 (5), pp.479-498. hal-01945143 

o   https://doi.org/10.1080/10962247.2019.1645761

Please be aware oft he oxygen management as BTF systems can be subdivided into systems at ambient oxygen levels to abate H2S in waste gas and microaerophilic conditions of less than 2 vol % O2 to pre-treat biogas.

·         Chapter Conclusion: A final suggestion about the best fitting technologies must be presented. Otherwise you get the idea about reading the paper: ‚Fine paper, and now what about the solution for my problem….?‘

·         References: Within the paper the number of 111 papers of basis for this review was mentioned. However, only 37 are citated. There is a gap, which should be closed. Furthermore it would be very very helpful to present a table showing the key results of these 111 papers, i.e. concentration of H2S, technique, contact time, temperature, efficiency, volume specific loading and elimination…

Figure 1 shows 12 different technologies – however only some of these techniques are addressed in the review. Where are the other ones or why aren´t they presented? Furthermore, as mentioned in the comments to the conclusion a critical comparison and evaluation of the most valuable techniques is missing

Author Response

The authors are thankful for the comments and corrections of the reviewers.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Introduction:

Dear Authors,

Thank you for this comprehensive paper on reactive processes for the removal of H2S. This is indeed a relevant topic on which attention is required for the coupling of anaerobic biological processes between them or when it comes to the purification of gaseous products such as methane from biogas. In general, I would strongly recommend to work on English editing in order to enhance readability and comprehensiveness (see for more details subsequent reviewing sections). The paper contains meaningful and comprehensive figures, which help the reader obtaining a deep and clear overview of H2S removal techniques and processes depending on the application target (could be of value to enhance the references towards local regulations for injecting biomethane which varies from place to place). Finally, I would also add few examples on recently developed process solutions such as in-situ or ex-situ biomethanation with methanogenic archaea growing on CO2 and H2S. This allows to transform the purification process into a bioprocess adding value by allowing to increase the production from a biogas digester (convert the CO2 fraction from biogas in natural gas), see major comment section for more details.

 

Major comments:

Revise English formulations in all the manuscript starting with the introduction section: e.g. line 29 to 32: “The generation of renewable energy challenges companies to involve a chain process, in which all the links are co-responsible for the generation of new products, processes, an  for responsible consumption and production, protecting the environment and improving the lives of the people they work with. maintain interactions.”

And ending with the reformulation of the conclusion section, see the following line 431 to 433: Biogas purification, especially with regard toH2S removal, is a very relevant topic, both for the enrichment of biogas energetic potential and mitigation of harmful and polluting effects H2S, due to its corrosive and dehydrating power. à This is not a comprehensive sentence despite one could grasp the meaning indirectly e.g.: Biogas and more specifically H2S removal is a relevant topic to support applications with biogas and/or purified biomethane. Furthermore, H2S in combination with water contributes to enhance most material corrosion and must be removed (to avoid compromising the life of equipment mechanical components) //// and to allow subsequent applications based on biogas and/or purified biomethane.

 

It would be great to integrate some references to the published works on biological methanation with methanogenic archaea made by companies such as Electrochaea, Krajete GmbH, Microbenergy and Researchers such as Seifert et al. Rittmann et al. Bernacchi et al. Thema et al. and many more around topic of biogas upgrading and power to gas using biological methanation (both in situ and ex-situ). In such bioprocess, which is already commercially available, raw biogas is used to supply CO2 and H2S to axenic or mixed culture of methanogenic archaea in order to upgrade biogas to biomethane in a single step at high volumetric rates (Conversion rate of CO2  >20 [Nm3 * Nm-3 * h-1]) with conversion > 98% for CO2 and 90% for H2S).

 

Minor comments:

A reformulation of the abstract is suggested for easing the subject comprehension <<The development of renewable and sustainable energy, the application and utilization of biogas has attracted significant attention to produce high quality biomethane >> à Could be reformulated in the following direction: Growing demand for renewables and sustainable energy production contributes to a growing interest in producing high quality biomethane from biogas.

 

The rest of the abstract is also “hard to read”: However, there is a complexity of the anaerobic digestion process due to the variation of the process parameters. Biogas, which is mainly composed of methane (CH4), carbon dioxide (CO2), nitrogen (N2) and hydrogen sulfide (H2S). à Biogas is a dynamic multivariate process in which the composition of biogas varies over time. For producing biomethane from biogas, other compounds such as CO2, N2, H2S, NH3, H2O, need to be removed in variable amounts depending on …… (maybe speak about local target variabilities from different gas grid norms etc..)

 

Instead of speaking of biogas decontamination you could use more often terms such as biogas purification, biogas requires selective removal of H2S and so forth. (see line 67 to 70)

 

Author Response

The authors are thankful for the comments and corrections of the reviewers.

Author Response File: Author Response.pdf