Review Reports - The Role of Catalysts in Biomass Hydrothermal Liquefaction and Biocrude Upgrading

Round 1

Reviewer 1 Report

Dear Authors:

I am glad to review this "review" article and learn about the role of catalyst in biomass hydrothermal liquefaction process.

In the abstract, please mention the novelty your literature review than others. For instance, please add your opinions on the new research directions.

In line 40-42, you mentioned combustion, gasification, pyrolysis are using pre-drying process. But, some research also used raw materials. Please add few more studies to explain the end product of combustion, gasification, pyrolysis using the following papers [1, 2, 3]. For instance, poultry litter was used in the combustion process directly to produce heat and electricity [2].

[1] Kirubakaran, V., Sivaramakrishnan, V., Nalini, R., Sekar, T., Premalatha, M., & Subramanian, P. (2009). A review on gasification of biomass. Renewable and Sustainable Energy Reviews, 13(1), 179-186.

[2] Qian, X., Lee, S., Chandrasekaran, R., Yang, Y., Caballes, M., Alamu, O., & Chen, G. (2019). Electricity evaluation and emission characteristics of poultry litter co-combustion process. Applied Sciences, 9(19), 4116.

[3] Shafizadeh, F. (1982). Introduction to pyrolysis of biomass. Journal of analytical and applied pyrolysis, 3(4), 283-305.

In line 46, please add the references for the range of temperature and pressure for the HTL.

In lines 54-58, there are existing references on this topic. Why your study is unique? Please add the need of this study in the following paragraph.

Please add the description of each factor in the equation 1.

Please explain the HHV in the line 135. HHV, also known as the gross calorific value or gross energy, refers to the heat released by the complete combustion of fuel, assuming that the water originally present in the fuel and any generated water are present in a condensed state [4, 5]

[4] Sheng, C.; Azevedo, J.L.T. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenergy 2005, 28, 499–507.

[5]Qian, X., Lee, S., Soto, A. M., & Chen, G. (2018). Regression model to predict the higher heating value of poultry waste from proximate analysis. Resources, 7(3), 39.

Equation [2] is not labeled correctly.

Figure 2 in not clear. Please update it.

Please add note to explain the symbols in the Tables. For instance, WHSV/LHSV in Table 6.

In the Section 6, authors explained catalysis types. What if the type of biomass are different?

I hope these comments will be addressed in the revised manuscript.

Reviewer

Author Response

In the abstract, please mention the novelty your literature review than others. For instance, please add your opinions on the new research directions.

We thank the reviewer for his/her comment. The novelty of our literature review was already mentioned in the abstract, but we added a sentence to make it even more explicit. The revised text will read as:

“This review has the ambition to summarize the different available information to draw an updated overall picture of catalysis applied to HTL. The different catalysis strategies are reviewed, high-lighting the specific effect of each kind of catalyst on the yields and properties of the HTL products, by comparing them with the non-catalyzed case. This allows drawing quantitative conclusions on the actual effectiveness of each catalyst, in relation to the different biomass processed”.

Our opinions on the new research directions are expressed in the last section of the manuscript. We prefer not to include them in the abstract for sake of conciseness.

[1] Kirubakaran, V., Sivaramakrishnan, V., Nalini, R., Sekar, T., Premalatha, M., & Subramanian, P. (2009). A review on gasification of biomass. Renewable and Sustainable Energy Reviews, 13(1), 179-186.

[3] Shafizadeh, F. (1982). Introduction to pyrolysis of biomass. Journal of analytical and applied pyrolysis, 3(4), 283-305.

The focus of this study is to investigate the role of homogeneous and heterogeneous catalysts during the HTL and biocrude upgrading so explanation regarding the end-product of combustion, gasification, and pyrolysis is out of the scope.

In line 46, please add the references for the range of temperature and pressure for the HTL.

It is already mentioned in the paragraph, i.e Ref#10

In lines 54-58, there are existing references on this topic. Why your study is unique? Please add the need of this study in the following paragraph.

HTL has undergone in recent years a sudden increase in the number of publications, however the knowledge generated regarding role of homogeneous and heterogeneous catalysts is rather fragmentary, and for this reason the purpose of this review is to critically analyze the current research developments on the use of alkali salts, organic acids, transition metals, metal oxides, and activated carbon during HTL and upgrading process, to enhance simultaneously the biocrude yield and quality. The common catalytic effects corresponding at each classes of materials were highlighted too.

Please add the description of each factor in the equation 1.

Following the reviewer’s remark, we have added a short description of each term in equation 1.

[4] Sheng, C.; Azevedo, J.L.T. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenergy 2005, 28, 499–507.

[5]Qian, X., Lee, S., Soto, A. M., & Chen, G. (2018). Regression model to predict the higher heating value of poultry waste from proximate analysis. Resources, 7(3), 39.

The definition of higher heating value (HHV) is generally known and it is part of the basic knowledge in engineering. Therefore, we think it is not necessary to explain it.

Equation [2] is not labeled correctly.

Following the reviewer’s remark, we corrected the labeling of eq. 2 by using two round brackets, i.e. "(2)".

Figure 2 in not clear. Please update it.

Following the reviewer’s remark, we provided a better version of Figure 2 in the revised manuscript.

Please add note to explain the symbols in the Tables. For instance, WHSV/LHSV in Table 6.

In revised manuscript, we addressed the reviewer comment and added a note under Table 6 which explains the symbols such as WHSV/LHSV.

In the Section 6, authors explained catalysis types. What if the type of biomass are different?

From this comment, we imagine that reviewer is referring to Section 5 (Catalysis for HTL bio-crude upgrading) where the effect of different catalyst types (i.e., sulfided and non-sulfided catalysts) on HTL biocrude upgrading (i.e., hydrotreating) are comprehensively explained. Here, reviewer asked an interesting question (What if the type of biomass are different?). However, the focus of this original work is not to explain the different types of HTL biocrudes and the practical challenges that arise due to the complex molecular structures which are directly related to the biomass used. It could be indeed an interesting point of discussion for a much more detailed future work.

However, in present scope we do not see it worthy to further prolong the concluded discussion with different possible topics. On the other hand, we have stated right from the start that this work will focus on the role of different catalysts in biomass hydrothermal liquefaction and bio-crude upgrading. In Tables 5-7, reviewer can clearly observe the data related to different catalysts and their effect on different biomasses. In all cases, catalysts managed it upgraded the oil irrespective of the biomass used, with a clear better performance of sulfided catalysts over non-sulfided ones.

Reviewer 2 Report

This is a comprehensive review highlighting the results of catalytic hydrothermal liquefaction (HTL) of different feeddstocks using both homogeneous and heterogeneous catalysts. The authors have analyzed the results from the literature based on a common metric – energy recovery of bio-crude from both non-catalytic and catalytic HTL, which is valuable. Change in HHV of the bio-crude due to catalytic treatment is also employed to analyze the positive/negative effect of use of catalysts on energy content. In the later part of the paper, the authors have considered catalytic hydrotreatment of bio-crude using different sulfide and non-sulfided catalysts, and have analyzed the results based on H/C, N/C and O/C ratios. This manuscript can be published in Processes after the following minor comments are addressed.

It will be valuable to also include a dedicated discussion/section on the effect of solvents on bio-crude yield, quality and energy recovery. While a few literature studies on solvent-assisted catalytic HTL are included in this paper, it is not comprehensive. The authors may consider referring (but not limited to) the following works: J. Cleaner Production 2019, 227, 292-301; Energies 2020, 13, 2618; Bioresour. Technol. 2021, 339, 125537.

Many more recent studies have been published on this topic of catalytic HTL of macroalgae, and the authors may consider citing and discussing some of them: Bioresour. Technol. 2022, 346, 126515; Bioresour. Technol. Rep. 2021, 15, 100796

The authors discuss an important point in the conclusion section on the fate of metal ions as they are usually carried away in the aqueous phase. While this is true, and only a few studies report the concentration of ions (cations and anions) in the aqueous phase, the ions basically get distributed to bio-char, aqueous and bio-crude phases, and there is no study currently in the literature that is focused on this aspect, i.e. thorough analysis of ions in all three phases after the reaction. This may be a valuable point for future work.

Another important scope for future work is to understand the fundamental reaction mechanism of catalytic action during HTL of biomass feedstocks, which is relatively less explored. While this review is focused more on applied aspects, it is worthwhile to include the need for fundamental mechanistic studies, so that rational catalyst design may become an opportunity.

Author Response

This is a comprehensive review highlighting the results of catalytic hydrothermal liquefaction (HTL) of different feedstocks using both homogeneous and heterogeneous catalysts. The authors have analyzed the results from the literature based on a common metric – energy recovery of bio-crude from both non-catalytic and catalytic HTL, which is valuable. Change in HHV of the bio-crude due to catalytic treatment is also employed to analyze the positive/negative effect of use of catalysts on energy content. In the later part of the paper, the authors have considered catalytic hydrotreatment of bio-crude using different sulfide and non-sulfided catalysts, and have analyzed the results based on H/C, N/C and O/C ratios. This manuscript can be published in Processes after the following minor comments are addressed.

We thank the reviewer for his/her comments and appreciation of our work. Although we also think that the topic of the solvent effect would be very interesting, we preferred not to include this in our study, as we wanted to cover only, or mostly, “pure” hydrothermal liquefaction, where water is the only solvent.

The reviewer is right. Indeed, in the revised manuscript we have included a number of further more recent studies on catalytic HTL in Tables 2 and 4.

We agree with the reviewer with the importance of tracing the ions in the different phases, which was a concept that was already described in the text. However, we decided to make it more extensive and to take into account the “ions” instead of only metals.

We appreciate the reviewer’s suggestion. We have included a statement in the conclusions: “Moreover, in order to come to a rational design of the catalyst, it is necessary to perform fundamental mechanistic studies, to obtain a better understanding of the different reaction pathways”.

Reviewer 3 Report

Journal: Processes
Manuscript ID: processes-1529264
Type of manuscript: Review
Title: The role of catalysts in biomass hydrothermal liquefaction and bio-crude upgrading
Authors: Ayaz Ali Shah, Kamaldeep Sharma, Muhammad Salman Haider, Saqib Sohail Toor, Lasse Aistrup Rosendahl, Thomas Helmer Pedersen, Daniele Castello

The manuscripts review the literature on the use of homogeneous and heterogeneous catalyst in hydrothermal liquefaction of biomass and waste feedstocks. In addition, the catalytic upgrading of HTL biocrudes in reviewed.

This is an adequate review of the topic but not comprehensive. The main drawback is that it is significantly out of date on recent literature particularly on catalytic hydrothermal liquefaction. This review could be updated to include important recent papers in the last 18 months which has been cited on other publications, including:

ACS Sustainable Chemistry & Engineering 2020 8 (17), 6877-6886

Sustainable Energy Fuels, 2021,5, 941-955

The effect of ash on catalytic HTL is not explained in detail
The catalyst stability under htl conditions is a very important which is not addressed in detail. This should be addressed or at least work in this area should be referred to in the text.
Table 1 and 3 could include ER for comparison
Figure 4 The cellulose molecule is not clear.
I addition, there are many minor spelling and grammar improvements are required as indicated as follows. The authors should have the manuscript proofed carefully:

Pg1 l38 “…most developed…”

Pg1 l45 “…of the biomass.”

Pg2 l56 “.. the HTL process…”

Pg2 l73 “.. role of a catalyst…”

Pg3 l119 “… affect several outputs…”

Pg3 l139 “…reported…”

Pg3 l148 “…very effective…”

Pg4 l119 “…not sufficient…”

P6 l221 “…reported the …”

Pg 1227 “… derived from…”

Pg9 1302 “…were determined…”

P12 l377 “…in the presence…”

P14 l461 “…was carried out…”

P17 l536 “…very important…”

P17 l550 “… por stability.”

P18 l587 “…40% catalyst…”

P19 l635 hardwood biooil?

P21 l697 “…handful of…”

P24 l768 “.. with attention…”

In conclusion, I recommend this manuscript for publication after major revision.

Comments for author File: Comments.pdf

Author Response

This is an adequate review of the topic but not comprehensive. The main drawback is that it is significantly out of date on recent literature particularly on catalytic hydrothermal liquefaction. This review could be updated to include important recent papers in the last 18 months which has been cited on other publications, including:

ACS Sustainable Chemistry & Engineering 2020 8 (17), 6877-6886

Sustainable Energy Fuels, 2021,5, 941-955

We thank the reviewer for his/her comment, which we took carefully into consideration. Now several studies from the latest literature have been included dated from 2020 and 2021, among which both above articles mentioned above. These studies are included in the new Tables 2 and 4.

The effect of ash on catalytic HTL is not explained in detail

The effect of ashes on catalytic HTL was treated in Section 3.1. However, following the reviewer’s remark, we reformulated our text to include a clearer explanation of the phenomena involved and, especially, the link between the catalytic activity and the high content of carbohydrates in the feedstock.

The catalyst stability under htl conditions is a very important which is not addressed in detail. This should be addressed or at least work in this area should be referred to in the text.

According to the reviewer comment, the catalysts stability under hydrothermal conditions has now been addressed in the revised manuscript. Please see section 4.4 in the revised manuscript.

Table 1 and 3 could include ER for comparison

ER for both homogeneous and heterogeneous catalysts has already been provided in the manuscript in a graphical form, i.e. in Figures 1 and 5 in the revised manuscript. Therefore we preferred not to duplicate this piece of information also in the tables, as it would be redundant.

Figure 4 The cellulose molecule is not clear.

Figure 4 was reproduced from the work by Hirano et al. (2020), published on the Journal of Analytical and Applied Pyrolysis. Following the reviewer’s remark, a better-quality version of it was provided.

I addition, there are many minor spelling and grammar improvements are required as indicated as follows. The authors should have the manuscript proofed carefully:

Pg1 l38 “…most developed…”

Pg1 l45 “…of the biomass.”

Pg2 l56 “.. the HTL process…”

Pg2 l73 “.. role of a catalyst…”

Pg3 l119 “… affect several outputs…”

Pg3 l139 “…reported…”

Pg3 l148 “…very effective…”

Pg4 l119 “…not sufficient…”

P6 l221 “…reported the …”

Pg 1227 “… derived from…”

Pg9 1302 “…were determined…”

P12 l377 “…in the presence…”

P14 l461 “…was carried out…”

P17 l536 “…very important…”

P17 l550 “… por stability.”

P18 l587 “…40% catalyst…”

P19 l635 hardwood biooil?

P21 l697 “…handful of…”

P24 l768 “.. with attention…”

We thank the reviewer for all his/her remarks on language and style. We corrected all the occurrences that were listed and we carefully proof-read the whole document. We hope that now the language reached to a sufficient quality level.

Round 2

Reviewer 1 Report

Dear Authors:

I am glad to review your revised manuscript.

As you mentioned in the line 34, low value feedstocks, e.g., biomass fuels are very important to generate energy. In order to give general introduction and provide more comprehensive solutions, I suggest to add following contents in the introduction in line 36. Biomass fuels are widely used an alternative fuels instead of fossil fuels using the several conversion technologies, combustion, gasification, pyrolysis [1, 2, 3]. For instance, poultry litter as one of agricultural biomass was used in the combustion process directly to produce heat and electricity [2].

[3] Shafizadeh, F. (1982). Introduction to pyrolysis of biomass. Journal of analytical and applied pyrolysis, 3(4), 283-305.

Figure 2 is not very and need more clear one.

Add future study in the conclusion.

Check one more time on the grammar and spelling.

Best regards,

Reviewer

Author Response

[3] Shafizadeh, F. (1982). Introduction to pyrolysis of biomass. Journal of analytical and applied pyrolysis, 3(4), 283-305.

The reviewer is suggesting to include three references that are not related to our work. Indeed, the three suggested sources are, respectively, on gasification, combustion and pyrolysis: none of the three is actually related to the work we did, which is on the usage of catalysts during hydrothermal liquefaction. We think that adding these references would not bring any advantage to the reader, therefore we decided once again to kindly refuse the reviewer's suggestion.

Figure 2 is not very and need more clear one.

We think the reviewer wanted to write that the figure is not very clear. However, we have some doubts on what he/she means with "clear", i.e. is he/she referring to the concept represented in the figure or to the graphical quality of the image itself? In case of the former, Figure 2 was reproduced from a literature work by Yin and Tan (2012) in Applied Energy. All content in it appears quite clear and all acronyms are well explained within the picture. Instead, if he/she is referring to the resolution of the picture, probably the one provided in the reviewers' copy is at low resolution. In any case, we have a high resolution source that will be provided to the publisher during the article production procedure.

Add future study in the conclusion.

Following the reviewer's remark, we added the following sentence to the conclusions (L 868): "However, the effect of biocrude origin, i.e. of the type of biomass used for its production, on the effectiveness of the upgrading catalyst needs to be analyzed and discussed more in deep".

Check one more time on the grammar and spelling.

Following the reviewer's remark, we had the document checked again.

Reviewer 3 Report

corrections to the manuscript are sufficient

Author Response

We thank the reviewer for his/her appreciation and for the interesting comments provided.