Conversion of Biomass-Derived Tars in a Fluidized Catalytic Post-Gasification Process
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis study analyzes the conversion of biomass tar by steam and steam/H2/CO2 reforming using different Ni/CeO2-Al2O3 catalysts. The topic is really interesting and it would attract the interest of the potential readers of Catalysts. Moreover, the authors have conducted a great effort in the characterization of the catalysts by a wide range of techniques, and they have analyzed the catalyst performance at different temperatures and reaction times. However, some parts of the manuscript must be further discussed and the results must be compared with similar literature results.
Accordingly, prior to the manuscript publication, the following points must be addressed:
- Why was 2-methoxy-4-methylphenol selected as tar model compound instead of more predominant ones such as naphthalene? Please, explain it.
- The authors synthesized the catalysts by incipient wetness impregnation, and they calcined the catalyst at 500 ºC. Moreover, the in-situ reduction temperature was set at 615ºC. How were these temperatures selected? Could the higher temperature used in the reduction step (and also in some reaction temperatures, 525 and 550ºC) affect the textural properties of the catalyst?
- In the experimental section, the authors are suggested to explain in more detail how metal and crystallite size are determined. Why this characterization was conducted at 615 ºC? Please, explain it.
- The numbering of the manuscript contains some errors (two 5.1 sections appear). Please, revise it.
- In section 2.1, the results of the SBET of the gamma-Al2O3 reported on Table 1 and in the text are not the same. Please, revise it.
- In Page 3, lines 109-111, the authors stated that “However, the 5 wt% and 10 wt% CeO2 mildly decreased the surface area, pore volume, and pore diameter. This was attributed to the blockage of some of the micropores of the γ-Al2O3”. If micropore blockage had occurred, the average pore diameter would have increased. Please revise the discussion of these results.
- The main crystalline phases ascribed to the different diffraction angles should be included in Figures 3 and 4 (not only in the text). Moreover, why the authors did not include de XRD patterns of the reduced catalysts, which are the ones used in the experimental runs?
- The number of Figures and Tables is very high. I would suggest moving some of them to the Supplementary Material.
- In the results section (lines 334-335), the authors explained that: “When these results are compared to those without a catalyst loaded in the CREC Riser Simulator, one can see that runs with and without the catalysts display similar 2M4MP overall conversions” Why? Please, include additional explanations. Moreover, why the authors did not conduct the reforming experiments at higher temperatures, where the reforming reaction would have been enhanced?
- The authors include CO, CO2, CH4 and C1+ selectivities. However, the H2 selectivity, which is normally the desired product to be obtained in reforming reactions, is not included. The authors are suggested to include it and provide a proper discussion of the results.
- The authors analyze the influence of reaction time on tar conversion at 5, 7 and 10s. However, this brief period of time is not enough to analyze catalyst activity decay or coke formation in reforming reactions, and the differences observed may be attributed to experimental error. The authors must justify the selection of these conditions and proper discuss the results obtained.
- Did the authors repeat the experiments in order to ensure the reproducibility of the results? If so, bar errors must be included.
- In Figure 14, Cat15 shows the worst performance in terms of tar conversion. The authors must relate the performance of each catalyst to the extensive characterization conducted.
- The results must be compared with similar literature results in which the steam or steam-CO2 reforming of biomass tar is analyzed.
Author Response
Please refer to the attached file.
Hugo de Lasa
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is about one step in biomass conversion (tar post gasification) in which a Ni/Ce/Al2O3 catalyst is needed. Because of that, I would have appreciated a schematic or flow diagram to more easily understand wher in the process sequence this step would take place (So I advise the authors to use such an illustration).
When I see a paper using a Nickel Catalyst in a Carbon monoxide environment, I always fear an exposure of the personel with Nickel Carbonyl. There are serious industrial accidents which have been reported (and probably also lab accidents). I was myself surprised to see the case in Oil refineries, using Nickel based catalysts for hydrotreatment processes, where I do not expect to see a lot of oxygen. So in this paper, I would appreciate to see a section related to that risk analysis. Formation on nickel carbonyle depends on partial pressure and temperatures, so that should have been part of the HAZOP study done before experimental work. I would also appreciate to see a section on safety measures before opening the reactor after test. NOTE that for the reason of the toxicity of NiCarbonyle, I might have selected the option that "I have an ethical concern with this publication". It is also our duty to raise the safety risks associated with this research and educate the readers.
The tests which have been done have probably been too short to be abale to measure a Nickel loss from the catalyts, but that would certainly be monitored it it reaches pilot or commercial scale;
I have been struggling to find the pressure at which the tests have been carried out. That's clear only at the end of the paper. But because the pressure is extrelemy low, and the CO+H2 partial pressure is also very low, it would be difficult to use the gas in a methanol synthesis process (which require a high pressure). Most of the gases are hydrocarbons. So I do not understand the last conclusion on the CO/H2 ratio.
The model molecule used is a Methyl-Methoxy-Phenol, I write it like that on purpose, as I would easily guess that the molecule would crack/hydrocrack/hydrolyses as Methane, Methanol, CO2/CO/H2 and benzene/hydrocarbon. There would most probably be a gap in the hydrocarbons (no C2-C5). I couldn't find easily a description of the other hydrocarbons. This model molecule my also give the illusion of an easy separation of the products, while a real tar would generate much more polyaromatics and coke. I would be relevant to justify in more details the choice of the model molecule. A dimeric molecule or other longer chain alkyl-methoxy-phenols or unsaturated ones could have give different results. So what make you chose this one?
Should I have to do this reaction industrially, would I chose a Riser or a Downer? I would appreciate if the authors are giving their opinion. I don't think the reaction will continuously generate a large volume of gas. Which is going to dominate: heat or mass transfer? On which basis should we make the selection?
Author Response
Please refer to the attched file.
Hugo de Lasa
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsComments:
1. The sentence in line 110 showed that the blockage of some of the micropores leads to the changes in physical parameters. Please provide the SEM or TEM images.
2. Please provide the characterization data to explain the sentence “this can be attributed to the good dispersion of the nickel crystallites” in line 162.
3. The figures need to be improved. For example, the smooth process needs to be applied to the curves of XRD in figure 3.
4. The word “Figure 5A, 6B and 6C” should be “Figure 5A, 5B and 5C”in line 205. And the same mistake also appears in line 254.
5. The H2-TPR curve of 5wt% CeO2/γ-Al2O3 mentioned in line 284 is not found.
6. Please provide the SEM or TEM images of catalysts after reactivity to analyze the coke formation issue.
Author Response
Please refer to the attached file.
Hugo de Lasa
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsTaking into account the advice of the reviewers, the authors have thoroughly revised the paper and soundly responded and clarified their comments and doubts. Undoubtedly, the authors have done a great effort and the paper has been greatly improved and clarified, and in the reviewer humble opinion it is now publishable.
