Review Reports

Round 1

Reviewer 1 Report

Dear author, kindly perform the revision as follows before potential acceptance:

1.      Kindly add a space between number and degree Celsius, please be consistent throughout the manuscript.

 

2.      “Temperature, residence time, and feedstock to water weight ratio are the key parameters affecting the products yield and properties.” Catalytic HTC also been reported recently, kindly improve the content in the introduction: 10.14710/ijred.2022.42595.

 

3.      Please include the hypothesis of this study in the introduction.

 

4.      “The moisture and ash content of feedstock was determined 70 to be 62.91 ± 0.87 wt.% and 1.57 ± 0.16 wt.%, respectively” How these data were obtained?

 

5.      Please improve the resolution quality of Figure 2.

 

6.      Some of the similar graphs can be combined into one (For example Figure 9 and 10).

Author Response

[1] Kindly add a space between number and degree Celsius, please be consistent throughout the manuscript.

Response: Thank you for this comment. This has been addressed in the manuscript.

[2] “Temperature, residence time, and feedstock to water weight ratio are the key parameters affecting the products yield and properties.” Catalytic HTC also been reported recently, kindly improve the content in the introduction: 10.14710/ijred.2022.42595.

Response: As kindly suggested by the review, the relevant sentences have been re-written, “Based on recent studies, temperature, residence time, feedstock to water weight ratio, and catalyst type/dosage are the key parameters affecting the products yield and properties [10,11].”

[3] Please include the hypothesis of this study in the introduction.

Response: Additional sentences have been added in the revised manuscript, “As earlier described, HTC could be an energy-saving technology for converting high water containing feedstocks like SCG when compared with conventional approaches like torrefaction and pyrolysis. Additionally, in consideration of the composition of SCG, it belongs to lignocellulosic biomass primarily containing cellulose, hemicellulose, lignin, fatty acids, and other polysaccharides, thereby ensuring SCGs excellent raw materials for producing fuels and chemicals [7]. Among them, the preparation of solid fuel could be a potential application of SCGs.”

[4] “The moisture and ash content of feedstock was determined 70 to be 62.91 ± 0.87 wt.% and 1.57 ± 0.16 wt.%, respectively” How these data were obtained?

Response: Thank you for this comment. You can find the experimental procedure to determine moisture and ash contents in Page 5, Line 111-113, “The moisture content of feedstock was determined by drying the samples in an oven at 105 ℃ for 24 hours. The ash content of feedstock was measured by combusting the dried samples in a muffle furnace at 575 ℃ for 3 hours.”

[5] Please improve the resolution quality of Figure 2.

Response: A higher resolution Figure 2 has been added in the revised manuscript.

[6] Some of the similar graphs can be combined into one (For example Figure 9 and 10).

Response: This comment has been addressed in the revised manuscript, accordingly.

Reviewer 2 Report

In the manuscript entitled "Hydrothermal carbonization of spent coffee grounds for producing solid fuel" the authors evaluated the application of hydrothermal carbonization as an attractive approach to valorize SCG to valuable bioproducts, such as hydrochar. Besides, the surface morphology, fuctionality, and combustion behavior of hydrochar were also investigated. From the research standpoint this is a standard research with a great deal of experiments and huge amount of data. Overall, this manuscript is demonstrating an important and promising research direction, however part of this manuscript seems to be enhanced in much clearer discussion. Herein, it is suggested that the manuscript could be accepted for publication in Sustainability unless major corrections has been conducted according to the recommended points.

 

Recommended points:

i) Abstract section: It is suggested that the authors should emphasize and point out the importance and significance of this study, especially the environmental influence of SCG that from the coffee brewing process. Please balance this situation. Furthermore, the vital experiment data of HTC process and hydrochar produced against others should be added.

 

ii) Key words section: Line 29, spent coffee grounds should be changed into spent coffee grounds (SCG), which is the same as hydrothermal carbonization (HTC). Moreover, hydrochar should be also added.

 

iii) Introduction section: The authors have demonstrated the consumption and environment concern of SCG as the feedstock to produce useful bioproducts in this section. However, the authors should also give a brief comparison and summary of SCG against other types of biomass, such as microalgae that recognized as the third bioenergy feedstock. It is suggested that some recent literature related to the advantages of microalgae biomass thermal conversion and by means of thermal analysis could be referred to, such as (a) and (b): (a) Journal of Cleaner Production, 2022, 355, 131768; (b) Biochemical Engineering Journal, 2022, 179, 108330; Besides, the current status of SCG treatment technology should be also supplied compared with the HTC.

 

iv) All the format of figures should be improved and uniformed for clearly demonstration, such as Figure 1 and Figure 2..., please combined Figure 9 a and b, Figure 10 a and b for better illustration. Besides, many tables in this manuscript can be combined into one table, such as Table 4 and 5, 7 and 8…

 

v) In the Methods section, please mention that if triplicate sets of measurements had been carried out. This is very critical for an analytical manuscript, in which the statistical analysis will be the fundamental of the conclusions. Besides, please also indicate what is the software of data analysis in this study.

 

vi) It is suggested that the font size of the contents in each equation, should be in a uniform format.

 

vii) The font color of the contents below Fig. 7 X-axis , should be in Black.

 

viii) It is suggested that explain the necessity of including DTG curves, since it is very important to demonstrate the effectiveness of such method during data analysis.

 

ix) It is suggested that the resolution of Fig. 10 should be improved. The current form also did not show some important data points or temperature values. Please try to include or mark the important points with some values.

 

x) For the Acknowledgement section, if it has any grant numbers, please also included them.

Author Response

[1] Abstract section: It is suggested that the authors should emphasize and point out the importance and significance of this study, especially the environmental influence of SCG that from the coffee brewing process. Please balance this situation. Furthermore, the vital experiment data of HTC process and hydrochar produced against others should be added.

Response: As duly suggested by the reviewer, the abstract has been revised accordingly, “Spent coffee grounds (SCG) are industrial biowaste resulting from the coffee brewing process, and they are often underutilized and ended in landfills, thereby leading to the emissions of toxic gases and environmental damage. Hydrothermal carbonization (HTC) is an attractive approach to valorize wet biomass like SCG to valuable bioproducts (i.e., hydrochar). Thus, in this work, HTC of SCG was carried out in a 500 L stainless steel vessel at 150, 170, 190, 210, and 230 ℃ for 30 min, 60 min, 90 min, and 120 min and a feedstock to water weight ratio of 1:5, 1:10, and 1:15, and the use of the resulting hydrochar as the solid fuel was evaluated. The results showed that a high energy recovery (83.93%) and HHV (23.54 MJ/kg) of hydrochar was obtained at moderate conditions (150 ℃, 30 min, and feedstock to water weight ratio of 1:5) when compared with conventional approaches like torrefaction. Following this, the surface morphology, functionality, and combustion behavior of this hydrochar were characterized by SEM, FTIR, and TGA, respectively. In short, it can be concluded that HTC is an effective approach for producing solid fuel from SCG and the resulting hydrochar has a potential to be applied either in domestic heating or large-scale co-firing plant.” 

[2] Key words section: Line 29, spent coffee grounds should be changed into spent coffee grounds (SCG), which is the same as hydrothermal carbonization (HTC). Moreover, hydrochar should be also added.

Response: This comment has been addressed in the revised manuscript.

 

[3] Introduction section: The authors have demonstrated the consumption and environment concern of SCG as the feedstock to produce useful bioproducts in this section. However, the authors should also give a brief comparison and summary of SCG against other types of biomass, such as microalgae that recognized as the third bioenergy feedstock. It is suggested that some recent literature related to the advantages of microalgae biomass thermal conversion and by means of thermal analysis could be referred to, such as (a) and (b): (a) Journal of Cleaner Production, 2022, 355, 131768; (b) Biochemical Engineering Journal, 2022, 179, 108330; Besides, the current status of SCG treatment technology should be also supplied compared with the HTC.

Response: Thank you for this comment. The additional sentences have been added in the revised manuscript, “Undoubtedly, it is significant to search for renewable and sustainable resources to produce fuels and materials with aims for lowering the use of fossil fuels and reducing CO₂ emissions. To date, a range of biomass and organic waste including first-, second-, and third-generation biomass have been extensively investigated as the raw materials for the generation of biofuels and biomaterials. Biofuels production obtained from first-generation biomass like corn is a well-established process; however, it would lead to fuel vs. food competition. Second-generation biomass like agricultural waste could be an alternative resource to first-generation biomass for producing biofuels. Microalgae belongs to third-generation biomass, and it often is utilized as the feedstocks for biodiesel production owing to its high concentration of lipid [1,2].”

[4] All the format of figures should be improved and uniformed for clearly demonstration, such as Figure 1 and Figure 2..., please combined Figure 9 a and b, Figure 10 a and b for better illustration. Besides, many tables in this manuscript can be combined into one table, such as Table 4 and 5, 7 and 8…

Response: Figure 1 and Figure 2 have been re-drawn. Figure 9 and Figure 10 have been combined. Tables have been combined as well.

[5] In the Methods section, please mention that if triplicate sets of measurements had been carried out. This is very critical for an analytical manuscript, in which the statistical analysis will be the fundamental of the conclusions. Besides, please also indicate what is the software of data analysis in this study.

Response: Thank you for this comment. Additional sentences have been added in the revised manuscript, “All experiments were conducted at least two times to ensure the reproducibility and reliability of the results, and Excel software was used for data processing”.

[6] It is suggested that the font size of the contents in each equation, should be in a uniform format.

Response: The font size of the contents in each equation has been adjusted to be uniform throughout the manuscript.

 

[7] The font color of the contents below Fig. 7 X-axis , should be in Black.

Response: This comment has been addressed in the revised manuscript.

[8] It is suggested that explain the necessity of including DTG curves, since it is very important to demonstrate the effectiveness of such method during data analysis.

Response: As kindly suggested by the reviewer, the additional sentences have been added in the revised manuscript, “Based on the TG curves, DTG curves were depicted and could be helpful for understanding the rate of biomass decomposition with respect to temperature, the major biomass degradation region, and the degradation temperature.”

[9] It is suggested that the resolution of Fig. 10 should be improved. The current form also did not show some important data points or temperature values. Please try to include or mark the important points with some values.

Response: Figure 10 has been combined with Figure 9 to show more valuable information.

[10] For the Acknowledgement section, if it has any grant numbers, please also included them.

Response: The grant number has been added in the revised manuscript.

Reviewer 3 Report

Journal: Sustainability

Comments on the manuscript entitled “Title: Hydrothermal carbonization of spent coffee grounds for producing solid fuel” (Manuscript No. sustainability-1808434)

 

The article is not suitable for publication in its present form. It needs a minor revision. Below are my comments:

Some specific comments:

1.      Abstract:  It is suggested to add some background with a few objectives and possible applications of this study and highlight the novelty of this work. The abstract only contains some parameters without any process conditions or key values from results, which is insufficient to delineate the whole picture of contribution and possible application of this study.

2.      Revise keywords add more specific and novel keywords with broader meanings (5-7 words).

3.      Add some references in the Introduction section to strengthen the literature review related to the fuel

·         Yaqoob H, Teoh YH, Jamil MA, Sher F. Energy, exergy, thermoeconomic and sustainability assessment of tire pyrolysis oil in common rail direct injection diesel engine. Fuel 2022;311:122622. https://doi.org/https://doi.org/10.1016/j.fuel.2021.122622.

·         Murugesan, A.; Umarani, C.; Subramanian, R.; Nedunchezhian, N. Bio-diesel as an alternative fuel for diesel engines—A review. Renew. Sustain. Energy Rev. 2009, 13, 653–662.

4.      The introduction lacks sufficient background information, which is unable to give the reader detailed background knowledge and possible wide application of this study. Research gaps should be highlighted more clearly and future applications of this study should be added.

5.      When explaining engine thermal efficiency, authors should refer to the combustion analysis.

6.      Most important: Authors should prove their scientific originality by defining what are their main scientific findings, which have not yet been presented in other studies.

7.      The tables/figures inserted are not explained or discussed well in the text please discuss critically and explain all tables/figures in the text.

8.      Against which standard was the different equipment used?

9.      What are the tolerances for the different types of equipment?

10.  What was the sampling rate of the solid fuel?

11.  There are lot of experimental details missing from this paper. i.e., uncertainties with the instruments, methodology for running the experiment.

12.  Conclusions of this manuscript lacks clear findings and future aspects. The authors are advised to write the conclusion comprehensively.

13.  There are some grammatical issues, particularly in the introduction, and results & discussion section.

14.  Validate the results section with the help of physicochemical properties not just with the previously published results.

 

15.  Between the lack of methodology and limited novelty, the authors have a significant amount of work to do to bring this paper up to a publishable standard.

Author Response

[1] Abstract:  It is suggested to add some background with a few objectives and possible applications of this study and highlight the novelty of this work. The abstract only contains some parameters without any process conditions or key values from results, which is insufficient to delineate the whole picture of contribution and possible application of this study.

Response: Thank you for this comment. The abstract was re-written in the revised manuscript, “Spent coffee grounds (SCG) are industrial biowaste resulting from the coffee brewing process, and they are often underutilized and ended in landfills, thereby leading to the emissions of toxic gases and environmental damage. Hydrothermal carbonization (HTC) is an attractive approach to valorize wet biomass like SCG to valuable bioproducts (i.e., hydrochar). Thus, in this work, HTC of SCG was carried out in a 500 L stainless steel vessel at 150, 170, 190, 210, and 230 ℃ for 30 min, 60 min, 90 min, and 120 min and a feedstock to water weight ratio of 1:5, 1:10, and 1:15, and the use of the resulting hydrochar as the solid fuel was evaluated. The results showed that a high energy recovery (83.93%) and HHV (23.54 MJ/kg) of hydrochar was obtained at moderate conditions (150 ℃, 30 min, and feedstock to water weight ratio of 1:5) when compared with conventional approaches like torrefaction. Following this, the surface morphology, functionality, and combustion behavior of this hydrochar were characterized by SEM, FTIR, and TGA, respectively. In short, it can be concluded that HTC is an effective approach for producing solid fuel from SCG and the resulting hydrochar has a potential to be applied either in domestic heating or large-scale co-firing plant.”

[2] Revise keywords add more specific and novel keywords with broader meanings (5-7 words).

Response: As kindly suggested by the reviewer, the keywords were revised in the revised manuscript, “Spent coffee grounds (SCG); Hydrothermal carbonization (HTC); Hydrochar; Biofuel; Solid fuel; Valorization”.

[3] Add some references in the Introduction section to strengthen the literature review related to the fuel

Yaqoob H, Teoh YH, Jamil MA, Sher F. Energy, exergy, thermoeconomic and sustainability assessment of tire pyrolysis oil in common rail direct injection diesel engine. Fuel 2022;311:122622. https://doi.org/https://doi.org/10.1016/j.fuel.2021.122622.

Murugesan, A.; Umarani, C.; Subramanian, R.; Nedunchezhian, N. Bio-diesel as an alternative fuel for diesel engines—A review. Renew. Sustain. Energy Rev. 2009, 13, 653–662.

Response: Thank you for this comment. Additional references were added in the revised manuscript.

[4] The introduction lacks sufficient background information, which is unable to give the reader detailed background knowledge and possible wide application of this study. Research gaps should be highlighted more clearly and future applications of this study should be added.

Response: As duly suggested by the reviewer, the introduction was re-written in the revised manuscript, “Undoubtedly, it is significant to search for renewable and sustainable resources to produce fuels and materials with aims for lowering the use of fossil fuels and reducing CO₂ emissions. To date, a range of biomass and organic waste including first-, second-, and third-generation biomass have been extensively investigated as the raw materials for the generation of biofuels and biomaterials [1–4]. Biofuels production obtained from first-generation biomass like corn is a well-established process; however, it would lead to fuel vs. food competition. Second-generation biomass like agricultural waste could be an alternative resource to first-generation biomass for producing biofuels. Microalgae belongs to third-generation biomass, and it often is utilized as the feedstocks for biodiesel production owing to its high concentration of lipid [5,6].

The consumption of the spent coffee grounds (SCG) as a result of the waste originally comes from coffee brewing process has gradually increased. In 2020 – 2021, the global coffee consumption is expected to grow until 167.1 million bags of 60 kg, which leads to the annual generation of around 6 million tons of SCG per year [7]. This massive amount of SCG poses a serious environmental concern owing to the current waste disposal practices, i.e., landfills [8]. Alternatively, SCG is a carbonaceous material and can be used as the feedstock to produce useful bioproducts. In particular, converting SCG to solid fuel by improving its fuel properties has been investigated by torrefaction of dried SCG at 250 – 350 ℃ for 0 – 30 min by Nepal et al. [9]. Barbanera and Muguerza [10] performed torrefaction of SCG for the production of solid biofuels at 210 – 260 ℃ for 90 min. However, an energy-intensive pre-drying step is needed prior to solid fuel preparation via torrefaction and thus it is not a suitable conversion process for wet biomass like SCG.

Hydrothermal carbonization (HTC) is an alternative approach for preparing solid fuel, especially for high water containing biowaste. This is due to the fact that water can be used as a reaction medium in the HTC process and thus the energy intensive pre-drying stage can be eliminated and reduces the overall energy consumption. At subcritical conditions, the physical properties of water are dramatically different from those at ambient conditions. For instance, the dielectric constant of water reduced from 78 F/m at 25 ℃ and 0.1 MPa to 27.1 F/m at 250 ℃ and 5 MPa, along with an increase in the ionic product (pK_w) from 14 at 25 ℃ and 0.1 MPa to 11.2 at 250 ℃ and 5 MPa [11]. Thus, HTC is selected as the conversion technology in this study to convert wet SCG into solid fuel. Based on recent studies, temperature, residence time, feedstock to water weight ratio, and catalyst type/dosage are the key parameters affecting the products yield and properties [3,12].

As earlier described, HTC could be an energy-saving technology for converting high water containing feedstocks like SCG when compared with conventional approaches like torrefaction and pyrolysis. Additionally, in consideration of the composition of SCG, it belongs to lignocellulosic biomass primarily containing cellulose, hemicellulose, lignin, fatty acids, and other polysaccharides, thereby ensuring SCGs excellent raw materials for producing fuels and chemicals [9]. Among them, the preparation of solid fuel could be a potential application of SCGs. Till now, most recent studies relating SCG valorization are primarily limited to soil conditioner [13], energy storage [14], adsorbent [15], biodiesel [16], and bio-crude oil [17]. Aside from these applications, SCG could be an excellent source for producing solid fuel. In addition, although few studies have investigated the use of SCG for preparing solid fuel, most studies have used traditional torrefaction as the conversion approach and lacks investigations on the alternative method – HTC [9,10]. As a result, in this study, non-catalytic HTC of SCG was carried out at temperatures of 150 – 210 ℃, residence time of 30 – 120 min, and feedstock to water weight ratio of 1:5 – 1:15. A series of analytical instruments including CHNS elemental analyzer, bomb calorimeter, scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were applied to comprehensively characterize SCG-derived hydrochar in terms of elemental composition, higher heating value (HHV), surface morphology, functionality, and combustion behavior, respectively. Overall, this present study can offer a theoretical basis for expanding the application fields of SCG and improving the utilization efficiency of SCG through the use of alternative conversion technology.”

[5] When explaining engine thermal efficiency, authors should refer to the combustion analysis.

Response: The authors agree with the reviewer that the combustion analysis should be conducted when discussing the engine thermal efficiency. The engine thermal efficiency was not discussed in the revised manuscript.

[6] Most important: Authors should prove their scientific originality by defining what are their main scientific findings, which have not yet been presented in other studies.

Response: As kindly suggested by the reviewer, the knowledge gap of this study was redefined and re-elaborated in the revised manuscript.

“However, an energy-intensive pre-drying step is needed prior to solid fuel preparation via torrefaction and thus it is not a suitable conversion process for wet biomass like SCG.”

“As earlier described, HTC could be an energy-saving technology for converting high water containing feedstocks like SCG when compared with conventional approaches like torrefaction and pyrolysis. Additionally, in consideration of the composition of SCG, it belongs to lignocellulosic biomass primarily containing cellulose, hemicellulose, lignin, fatty acids, and other polysaccharides, thereby ensuring SCGs excellent raw materials for producing fuels and chemicals [9]. Among them, the preparation of solid fuel could be a potential application of SCGs. Till now, most recent studies relating SCG valorization are primarily limited to soil conditioner [13], energy storage [14], adsorbent [15], biodiesel [16], and bio-crude oil [17]. Aside from these applications, SCG could be an excellent source for producing solid fuel. In addition, although few studies have investigated the use of SCG for preparing solid fuel, most studies have used traditional torrefaction as the conversion approach and lacks investigations on the alternative method – HTC [9,10]. As a result, in this study, non-catalytic HTC of SCG was carried out at temperatures of 150 – 210 ℃, residence time of 30 – 120 min, and feedstock to water weight ratio of 1:5 – 1:15.”

[7] The tables/figures inserted are not explained or discussed well in the text please discuss critically and explain all tables/figures in the text.

Response: Thank you for this comment. Some figures were combined into one figure and re-drawn. Some tables were recombined into one table as well. All the tables and figures were discussed in the text in the revised manuscript. All the data obtained from this study were compared with previous studies to provide an in-depth discussion.

[8] Against which standard was the different equipment used?

Response: The relevant sentences were re-written in the revised manuscript, “The moisture content of feedstock was determined by drying the samples in an oven at 105 ℃ for 24 hours. The ash content of feedstock was measured by combusting the dried samples in a muffle furnace at 575 ℃ for 3 hours. The methods adopted in this study for measuring moisture and ash content followed our previously published papers [20,21]. The elemental composition of feedstock and hydrochar was determined using the Organic Elemental Analysis Equipment (Flash 2000 CHNS-O, Thermo Fisher Scientific, Waltham, USA). The tolerance for this elemental analyzer is total carbon and nitrogen ranges between 100 ppm to 100% level, and 2,5-bis(5-tert-2-benzo-oxazol-2-yl) thiophene (BBOT) is used as the standard to build up the calibration curve. The functionality of feedstock and hydrochar was determined using FT-IR analysis (PerkinElmer, Massachusetts, USA) in the range of 400-4000 cm^-1 with a resolution of 4 cm^-1. The morphology of feedstock and hydrochar was characterized using SEM (TM3000, Hitachi, Ibaraki, Japan), and the sample was coated with platinum before the analysis. The imaging was conducted at an acceleration voltage of 15 kV. HHV of feedstock and hydrochar was determined using a bomb calorimeter (Parr 6100 Calorimeter, Moline, USA). The precision classification of this Parr 6100 Calorimeter given by the manufacture is 0.1 – 0.2% class. The combustion performance of hydrochar was determined using a TGA analyzer (TGA Q500), and the parameters used in the equipment set-up and analysis were followed by He et al. [22].”

[9] What are the tolerances for the different types of equipment?

Response: The response to this comment can be found in the Comment [8].

[10] What was the sampling rate of the solid fuel?

Response: You could find this response in the Page 6 of the revised manuscript, “All experiments were conducted at least two times to ensure the reproducibility and reliability of the results, and Excel software was used for data processing.”

[11] There are lot of experimental details missing from this paper. i.e., uncertainties with the instruments, methodology for running the experiment.

Response: As kindly suggested by the reviewer, additional sentences about the tolerance of instruments was added in the revised manuscript, and this response can be found in the Comment [8]. In addition, the methodology for running the experiment can be found in the Page 3-6 of the revised manuscript.  

[12] Conclusions of this manuscript lacks clear findings and future aspects. The authors are advised to write the conclusion comprehensively.

Response: According to the reviewer’s comment, the conclusions were re-written in the revised manuscript, “The inappropriate disposal of SCG from coffee brewing industry by landfill leads to serious environmental problems. SCG is an excellent source for producing value-added bioproducts like solid fuel; however traditional conversion process (i.e., torrefaction) is an ideal approach for high water-containing feedstocks like SCG. Thus, in this study, SCG was converted to a solid fuel called hydrochar via HTC at temperatures of 150 – 210 ℃, residence times of 30 – 120 min, and feedstock to water weight ratio of 1:5 – 1:15. HTC is an alternative and promising technology to convert wet biomass into hydrochar, and one of its applications is to be used as a solid fuel for domestic heating or co-firing power plant. After HTC experiments, the elemental composition, HHV, functionality, surface morphology, and combustion behavior of hydrochar was determined using CHNS elemental analyzer, bomb calorimeter, FTIR, SEM, and TGA, respectively. The results showed that hydrochar obtained at 150 ℃ for 30 min and feedstock to water weight ratio of 1:5 had the highest energy recovery of 84%, along with a yield of 81.93 ± 2.04 wt.% and HHV of 23.54 MJ/kg. No significant differences were observed in the functionality, surface morphology, and combustion performance of feedstock and hydrochar. In conclusion, SCG might be a promising source to prepare solid fuel via HTC in terms of its relatively high calorific value and excellent combustion performance. Further research is needed to explore the other fuel properties like combustion emission characteristics.”

[13] There are some grammatical issues, particularly in the introduction, and results & discussion section.

Response: The manuscript was revised to correct the language.

[14] Validate the results section with the help of physicochemical properties not just with the previously published results.

Response: Additional sentences were added in the revised manuscript, “As suggested by the ash content of SCG, the total content of volatile matter and fixed carbon (dried basis) is ~98%. When considering the chemical composition of SCG, previous study has reported that cellulose (~12 wt.%), hemicellulose (~39 wt.%), and lignin (~24 wt.%) were main composition of SCG and lipid (~ 2 wt.%) and protein (~ 17 wt.%) were also observed in the SCG [36]. Based on the physiochemical analysis, it can be speculated that some fraction of volatile matter was decomposed during HTC reaction, which implies by a dark color of the water phase. As confirmed by TGA analysis, some small molecular weight chemicals could be released from the biomass, especially the protein and hemicellulose due to their relatively lower degradation temperature compared to other chemical composition like lignin.”

[15] Between the lack of methodology and limited novelty, the authors have a significant amount of work to do to bring this paper up to a publishable standard.

Response: Thank you for this comment. The methodology used in this study was revised to add more details, as suggested by the reviewer. Besides, the knowledge gaps were re-elaborated in the revised manuscript, “As earlier described, HTC could be an energy-saving technology for converting high water containing feedstocks like SCG when compared with conventional approaches like torrefaction and pyrolysis. Additionally, in consideration of the composition of SCG, it belongs to lignocellulosic biomass primarily containing cellulose, hemicellulose, lignin, fatty acids, and other polysaccharides, thereby ensuring SCGs excellent raw materials for producing fuels and chemicals [9]. Among them, the preparation of solid fuel could be a potential application of SCGs. Till now, most recent studies relating SCG valorization are primarily limited to soil conditioner [13], energy storage [14], adsorbent [15], biodiesel [16], and bio-crude oil [17]. Aside from these applications, SCG could be an excellent source for producing solid fuel. In addition, although few studies have investigated the use of SCG for preparing solid fuel, most studies have used traditional torrefaction as the conversion approach and lacks investigations on the alternative method – HTC [9,10]. As a result, in this study, non-catalytic HTC of SCG was carried out at temperatures of 150 – 210 ℃, residence time of 30 – 120 min, and feedstock to water weight ratio of 1:5 – 1:15.”

Round 2

Reviewer 2 Report

The improved version had made some changes according to the comments. However, the current version still needs some revisions prior to the acceptance. The current quality of formatting and expression should be checked in very serious manner.

 

(1) In the main text, some numbers of figures or tables were formatted in Italic font, please check if it is necessary.

 

(2) Line 32, in the list, it only shows TGA. However, there are a lot of TG and DTG in the main text, but not explained at the abbreviation words list.

 

(3) It is suggested that the numbers with standard deviations in Table 1 and Table 3 should be placed within one line so that some misunderstandings could be avoided.

 

(4) In the S%column of Table 1, Table 3 and Table 5, is it “zero” or “not detected”?

 

(5) In the HHV column of Table 1, Table 2 and Table 4, how could 22.83±0 happen? In Table 4, how could 22.85±0 happen?

 

(6) Please replace Fig. 4 with a higher resolution version. The current one looks in the fog, especially the numbers nearby the axis.

 

(7) Line 226 and Line 270, it should be 150°C but not the current symbol.

 

(8) Line 234, Line 266, Line 341, what does the table and figure refer to?

 

(9) Line 356, please check if it is a typo or not.

 

Author Response

[1] In the main text, some numbers of figures or tables were formatted in Italic font, please check if it is necessary.

Response: Thank you for this comment. The numbers of figures or tables were formatted consistently in the revised manuscript.

[2] Line 32, in the list, it only shows TGA. However, there are a lot of TG and DTG in the main text, but not explained at the abbreviation words list.

Response: This comment was addressed in the revised manuscript.

[3] It is suggested that the numbers with standard deviations in Table 1 and Table 3 should be placed within one line so that some misunderstandings could be avoided.

Response: This comment was addressed in the revised manuscript.

[4] In the S%column of Table 1, Table 3 and Table 5, is it “zero” or “not detected”?

Response: The S% cannot be detected due to the limitation of the CHNS analyzer. So, the S% column of Table 1-5 was changed to n.d. in the revised manuscript.

[5] In the HHV column of Table 1, Table 2 and Table 4, how could 22.83±0 happen? In Table 4, how could 22.85±0 happen?

Response: Thank you for this comment. The HHV of all hydrochar samples and feedstock was measured by bomb calorimeter twice and the average value was reported. The raw data for feedstock was the same.

[6] Please replace Fig. 4 with a higher resolution version. The current one looks in the fog, especially the numbers nearby the axis.

Response: As kindly suggested by the reviewer, Figure 4 was replaced with the figure with a higher resolution.

[7] Line 226 and Line 270, it should be 150°C but not the current symbol.

Response: Thank you for pointing this mistake out. This was addressed in the revised manuscript.

[8] Line 234, Line 266, Line 341, what does the table and figure refer to?

Response: This comment was addressed in the revised manuscript.

[9] Line 356, please check if it is a typo or not.

Response: Thank you for this comment. This was addressed in the revised manuscript.