Acid Gas and Tar Removal from Syngas of Refuse Gasification by Catalytic Reforming
Round 1
Reviewer 1 Report
Review of Manuscript ID: catalysts-2034302
Title of the article: Acid gas and tar removal from syngas of refuse gasification by catalytic reforming
The authors performed a relevant topic in acid gas and tar removal from syngas. As sustainability and environmental impacts have been the two major challenges of fossil fuel energy in the present time. There is a strong requirement for the development of new technologies and methods for acid gas and tar removal to produce clean energy. Production of synthesis gas via reforming of hydrocarbons has paramount importance. Although the researchers did a huge effort and presented reliable results, however, to comply with the journal standard, a minor revision is rated for the publication of the article. Therefore, the following comments and suggestions are required to be achieved before final acceptance.
Ø The abstract should be enlarged by adding the characterization units used in this work. The main results should be stated explicitly in numbers.
Ø The authors should revise equation 3, the formula for the space velocity.
Ø The authors should discuss deeply the XRF characterization results of Ni-based in Table1.
Ø The nitrogen adsorption-desorption isotherms of the catalysts must be plotted.
Ø The authors should elaborate on the differences between the BET characterization of NaAlO2 before and after the reaction in Table 2.
Ø For accurate determination of elemental analysis an ICP technique is recommended instead of EDX.
Ø References of the article must be updated particularly the reforming section by including the following references.
https://doi.org/10.3390/catal12040361
https://doi.org/10.1002/ese3.1063
https://doi.org/10.1016/j.apcatb.2020.119445
https://doi.org/10.1016/j.jiec.2013.05.013
Comments for author File: 
Comments.pdf
Author Response
Open Review #1
Review of Manuscript ID: catalysts-2034302
Title of the article: Acid gas and tar removal from syngas of refuse gasification by catalytic reforming
The authors performed a relevant topic in acid gas and tar removal from syngas. As sustainability and environmental impacts have been the two major challenges of fossil fuel energy in the present time. There is a strong requirement for the development of new technologies and methods for acid gas and tar removal to produce clean energy. Production of synthesis gas via reforming of hydrocarbons has paramount importance. Although the researchers did a huge effort and presented reliable results, however, to comply with the journal standard, a minor revision is rated for the publication of the article. Therefore, the following comments and suggestions are required to be achieved before final acceptance.
The abstract should be enlarged by adding the characterization units used in this work. The main results should be stated explicitly in numbers.
Response: We have accordingly expanded the abstract to include characterization units of the research. Also, we have detailed the main results with numbers for clarity.
L17-35: “The existence of acid gas and tar in syngas of municipal solid waste gasification limits its downstream utilization as a clean energy source. Here, we investigated the catalytic removal of HCl and tar. The key parameters affecting the catalytic reaction, including space velocity, temperature, the amounts of active metals in the catalyst and the carrier material, were studied, targeting optimized operating conditions for enhance syngas purification. The morphology, mineral phases, surface area and pore size before and after the reaction were investigated to understand the mechanism to dominate the reaction. The results showed that the removal rate of CaO adsorbent and HCl reached 96% at 400 °C. When the space velocity ratio was 1.0 and the temperature was 400 °C, the HCl removal efficiency (97%) by NaAlO2 was even better. Nevertheless, clogging of NaAlO2 pores were observed via the BET test after reaction to jeopardize its durability. 25% Ni doping on Zr1-x(Cex)O2 support provides high stability for tar removal. This is because the Zr1-x(Cex)O2 carrier has a higher carbon deposition resistivity than the Al2O3 carrier. The EDX results confirmed that a large amount of C (79.3%) was accumulated on the commercial catalyst surface supported by Al2O3 (25% Ni-based). As per the temperature, a temperature higher than 800 °C could not enhance the efficiency of tar removal, likely due to the catalyst deactivation. Carbon deposition and coking are the two main causes of catalyst deactivation. At 800 °C, 25% Ni-based synthetic catalyst can convert 48.5±19.4% tar to low molecular weight organic compounds. By contrast, such a conversion rate under the same temperature only accounted for 5.0±6.8% based on the commercial catalyst. These insights point to the important role of catalyst support materials.”
The authors should revise equation 3, the formula for the space velocity.
Response: Equation 3 was revised accordingly (L468-470).
(Eq. 3)
Where (/h) stands for gas hourly space velocity, refers to the volumetric flow rate of naphthalene (L/h), and V refers to the volume of catalyst (L).
The authors should discuss deeply the XRF characterization results of Ni-based in Table1.
Response: Thanks for pointing out the issues. Since the content of the doped CeO2 component is difficult to detect in small amounts, we focused our discussion on the content of the active component Ni and the impurities. The specific modifications are as follows,
L475-482: “The nickel content of the commercial catalysts was close to the corresponding theoretical values, whereas that of the self-made catalysts (17.43% and 27.83%) was slightly higher than the theoretical values (15% and 25%). Noteworthy, cerium could not be detected in the XRF spectrum likely due to their low sensitivity for detection. In addition to active component and support components, several types of trace metal oxides were also detected. Commercial catalysts tend to be mixed with more SO3, CaO, Fe2O3 etc., which may be the admixture of impurities in the preparation process. Self-made catalysts, on the other hand, exhibit doping with trace amounts of noble metal oxides because of the impurities inherent from the support.”
The nitrogen adsorption-desorption isotherms of the catalysts must be plotted.
Response: The nitrogen adsorption-desorption isotherm diagrams of the catalysts (including 25% Ni-based synthetic catalysts and NaAlO2) before and after catalyst reaction were attached with BET results. The specific modifications are as follows (L189-191),
Figure 3. The nitrogen adsorption-desorption isotherms of NaAlO2 before a) and after b) catalytic reforming process.
L178-180: “The type â…¢ isotherm indicates that the material interaction between adsorbent molecules is strong, and it is difficult to adsorb the adsorbent at the initial stage. With the progress of the adsorption process, the adsorption appears self-accelerating phenomenon (Fig. 3).”
Figure 13. The nitrogen adsorption-desorption isotherms of the 25% Ni-based synthetic catalysts a) before and b) aftercatalytic reforming process.
L368-369: “The type IV isotherm with an H1 hysteresis loop at higher relative pressure demonstrate that the synthesis catalyst is a mesoporous material (Fig. 13a).
L373-375: “As a result, the catalysts have become macro porous solid materials and no longer have adsorption characteristics, which is supported by the III type isotherm in Figure 3b.”
The authors should elaborate on the differences between the BET characterization of NaAlO2 before and after the reaction in Table 2.
For accurate determination of elemental analysis an ICP technique is recommended instead of EDX.
Response: Thank you for your suggestions. Here we focused more on the distribution and the content of elements on the catalyst surface, while ICP loses this information despite the more accurate quantification of its overall elemental analysis.
References of the article must be updated particularly the reforming section by including the following references.
https://doi.org/10.3390/catal12040361
https://doi.org/10.1002/ese3.1063
https://doi.org/10.1016/j.apcatb.2020.119445
https://doi.org/10.1016/j.jiec.2013.05.013
Response: The suggested articles has been updated in the revised manuscript.
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript is dealing with cleaning of syngas and improving its quality for end use. Which is of worthy of investigation and work has been well presented. There are some grammatical and language errors and needs to be corrected prior to publication. In addition, please add error bars in Figure 7-9 as well.
Author Response
Open Review #2
Comments and Suggestions for Authors
The manuscript is dealing with cleaning of syngas and improving its quality for end use. Which is of worthy of investigation and work has been well presented. There are some grammatical and language errors and needs to be corrected prior to publication. In addition, please add error bars in Figure 7-9 as well.
Response: Thank you for your suggestions. We have corrected the grammatical and language errors in our revised manuscript wherever is necessary. In addition, we have added error bars in our Figures 1-2 and Figures 7-9.
Author Response File: Author Response.pdf
Reviewer 3 Report
The manuscript is clear, relevant for the field and presented in a well-structured manner. The cited references mostly recent publications (within the last 5 years) but also old manuscripts like a base for research. The manuscript is scientifically sound, experimental part is appropriate to test the hypothesis and clearly described (I believe that results are reproducible based on the details given in the methods section).
Some figures are not clear (too small font, e.g. fig. 1, figures with SEM characterization) - it should be corrected. Tables are clear and good prepared. The conclusions are consistent with the evidence and arguments presented. In terms of editing, unfortunately, the amount of conclusions in relation to the introduction looks very poor. The article contains a lot of research and data, conclusions should be more extensive. However the introduction is clear, comprehensive and of relevance to the field. The Authors approach the topic very meticulously. This article should be interest to the scientific community.
The amount of municipal and industrial waste generated increases every year. Limited possibilities of their storage necessitate the use of more and more efficient methods of disposal. The solution that allows to deal with millions of tons of waste on a large scale are waste incineration plants and thermal municipal waste treatment plants. Effective design of dust extraction and flue gas treatment in such facilities to minimize environmental pollution is a very important issue. Any attempt to improve such methods and technologies should be widely described and discussed. In the article, particular attention is paid to the temperature limit related to the effectiveness of tar removal. Contrary to popular belief, simply increasing its scope does not improve the efficiency of the process, but only harms its economy.
Author Response
Open Review #3
Comments and Suggestions for Authors
The manuscript is clear, relevant for the field and presented in a well-structured manner. The cited references mostly recent publications (within the last 5 years) but also old manuscripts like a base for research. The manuscript is scientifically sound, experimental part is appropriate to test the hypothesis and clearly described (I believe that results are reproducible based on the details given in the methods section).
Some figures are not clear (too small font, e.g., fig. 1, figures with SEM characterization) - it should be corrected. Tables are clear and good prepared. The conclusions are consistent with the evidence and arguments presented. In terms of editing, unfortunately, the amount of conclusions in relation to the introduction looks very poor. The article contains a lot of research and data, conclusions should be more extensive. However, the introduction is clear, comprehensive and of relevance to the field. The Authors approach the topic very meticulously. This article should be interest to the scientific community.
The amount of municipal and industrial waste generated increases every year. Limited possibilities of their storage necessitate the use of more and more efficient methods of disposal. The solution that allows to deal with millions of tons of waste on a large scale are waste incineration plants and thermal municipal waste treatment plants. Effective design of dust extraction and flue gas treatment in such facilities to minimize environmental pollution is a very important issue. Any attempt to improve such methods and technologies should be widely described and discussed. In the article, particular attention is paid to the temperature limit related to the effectiveness of tar removal. Contrary to popular belief, simply increasing its scope does not improve the efficiency of the process, but only harms its economy.
Response: Thank you for your general positive comments to support our manuscript. We also appreciate for the reviewer’s comments to further improve our manuscript. For Figure 1 (now Figure 14 in the revised manuscript, due to Methods moving behind Results and Discussion), we have enlarged the font size in the figure, and the SEM images have also been enlarged. In the conclusion, we also included some additions. We added the optimal temperature and the optimal space velocity ratio for the removal of HCl and summarized the optimal working conditions. At the same time, the optimal condition of catalytic reforming of tar in the syngas of domestic waste is also included, as well as the characteristics before and after the catalyst reaction and the causes of deactivation. Besides, we also supplemented the prospect of future research.
Figure 14. Simulation diagram of synthetic gas purification experiment for municipal solid waste.
Figure 12. SEM characterization of synthetic catalysts before and after catalytic reforming process.
L497-500: “The reaction between NaAlO2 adsorbent and HCl is more intense, but with the increase of the amount of adsorbent, HCl is difficult to enter the interior of the adsorbent and react with it. Therefore, for NaAlO2, the removal efficiency of HCl is the highest when the reaction space velocity ratio is 1.0.”
L508-514: “For tar catalytic reforming experiments, no matter for commercial or self-made catalysts, the oxygen of nickel oxide in the catalyst is consumed in the catalytic reforming process to form CO, CO2, etc., which indicates that a small amount of oxygen left after the reaction may come from the carrier. Naphthalene is difficult to reach the catalyst in the process of high temperature reaction, thus forming an incomplete reaction to yield fixed carbon, covering the reactor and the surface of catalyst particles, thus preventing further catalytic reaction. Carbon deposition and coking are the two main causes of catalyst deactivation.”
L516-519: “However, the larger scale of domestic waste purification technology needs to be discussed in future studies, and more attention should be paid to the possibility of converting tar into small molecular organic compounds for reuse and the treatment of deactivated catalysts.”
Author Response File: Author Response.pdf
