Valorization of Rice Husk for the Production of Porous Biochar Materials

Round 1

Reviewer 1 Report

This article is interesting, but it has the following shortcomings:

1. Introduction

Line 34….converting carbon dioxide (CO2) in the atmosphere...  Remark: I recommend editing this sentence. More precisely write …”converting atmospheric carbon dioxide (CO2)…

Lines 40-42. Due to the bulk form, low energy density and high water content of biomass, the conversion can be achieved through by the biochemical and thermochemical routes for producing useful products …  Remark: Not all biomass types have low energy density and high water content, therefore this sentence should be changed, e.g. as follows: “Furthermore, biomass can be converted by biochemical and thermochemical methods into useful products”...

Lines 43-45. Pyrolysis, one of the commonly used thermochemical conversion processes, involves rapid heating of biomass in the absence of air or oxygen at a maximal temperature…  Remark: According to definition “Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere” (Pyrolysis. Compendium of Chemical Terminology. International Union of Pure and Applied Chemistry. 2009. p. 1824). Therefore, terms “rapid heating” and “maximal temperature” do not fit this definition. I recommend editing the sentence as follows: “Pyrolysis, one of the commonly used thermochemical conversion processes, involves decomposition of biomass in the absence of air or oxygen at an elevated temperature”…

Lines 46, 91, 103, 134: … biochar product (s)…  Remark: The word “product(s)” is unnecessary here and should be removed.

Line 51. Therefore, about 150 million metric tons of rice husk will be produced… Remark: the word “Therefore” does not fit here and should be removed. I recommend starting this sentence as follows: “About 150 million metric tons of rice husk will be produced”…

Line 64: …the carbon release as GHG forms…  Remark: It is error that “the carbon” release... Use “the carbon dioxide” instead of “the carbon”.

Line 79. …120, 25 and 25 min.  Remark: Time 25 min repeated twice, and it must be corrected.

Line 84. …huge sorption potential of urea… Remark: It is error. Write here …huge sorption potential of biochar, not urea

2. Materials and methods

Line 112. WCH.  Remark:  What is WCH? Seems, that it is a misprint that should be replaced with RH.

Lines 112, 113. WCH? (RH) contained the high contents of carbon (45.28 wt%), hydrogen (5.51 wt%), oxygen (~ 44 wt%, estimated) and silicon (3.90 wt%), thus showing a relatively low calorific value (16.4 MJ/kg).  Remark: There is a contradiction between "high content of carbon" and low calorific value of the biomass. In addition, the calorific value of 16.4 MJ/kg should not be called low, since a similar value is observed for most lignocellulosic biomasses, including some types of wood, as well as biopolymers - proteins, hemicelluloses and some other polysaccharides. In addition, there is not relation between “high content of carbon” and

Therefore, the last part of this sentence should be corrected, as e.g. … silicon (3.90 wt%), and this biomass has a moderate calorific value (16.4 MJ/kg).

Line 127: …temperature must operate above 400℃… Remark: This phrase should be edited, as e.g. …”temperature must be above 400℃”…

Lines 127-129. …the pyrolysis temperature must above 400℃ because the peak temperature of DTG occurred at about 380℃, indicating the complete devolatilization of hemicellulose and cellulose at around 400℃… Remark: This fragment should be edited, as e.g.…”the pyrolysis temperature must be above 400℃ because the devolatilization temperature of cellulose is about 400℃, while hemicelluloses are devolatilized at about 380℃”…

Lines 138-139. In order to describe the RH-based (i.e., BRH) products easily, the resulting…  Remark: This fragment should be edited, as e.g. “In order to describe the RH-based biochar (i.e., BRH), the resulting”…  

Line 141. … temperature of 400℃ for 30 holding time of 30 min. Remark: Remove 30 before “holding”, as follows … temperature of 400℃ for holding time of 30 min.

3. Results and Discussion

Lines 185-190. As shown, there are three apparent peaks as pyrolysis temperature increased from room temperature to 400°C. … It implied that the second TGA peak (“shoulder”) appears at a temperature (about 300°C)… Remark: This fragment should be edited, as e.g. … “As shown, there are two apparent peaks and one shoulder as the pyrolysis temperature increased from room temperature to 400°C. … It implied that the TGA shoulder appears at a temperature about 300°C…

Lines 190-191. … temperature (about 300°C) lower than that for lignin. Remark: This fragment should be edited, as e.g. …“temperature (about 300°C) that is lower than for lignin”.

Lines 212-213. ..biochar products products… Remark: Misprint since the word “products” was repeated twice.

Fig. 2. Remark: The curve needs to be smoothed.

Fig. 3 and 4.  Remark: Since the pore volume does not change when the pore size were above 15 nm, it is recommended to limit these graphs to a pore size range from 0 to 15 nm.

Fig. 6. EDS spectrum.  Remark: Since the EDS peaks are absent when X <3, it is recommended to limit these graphs to X range from 0 to 3.

4. Conclusions

 Lines 332, 336, 339 …biochar products… The word “product(s)” is unnecessary here and should be removed; write “biochar(s)” only.

Decision: Due to many grammatical errors and misprints, this article cannot be recommended for publication in the presented form, since it needs serious revision and editing.

Author Response

Q1. Line 34….converting carbon dioxide (CO2) in the atmosphere...  Remark: I recommend editing this sentence. More precisely write …”converting atmospheric carbon dioxide (CO2)…

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q2. Lines 40-42. Due to the bulk form, low energy density and high water content of biomass, the conversion can be achieved through by the biochemical and thermochemical routes for producing useful products …  Remark: Not all biomass types have low energy density and high water content, therefore this sentence should be changed, e.g. as follows: “Furthermore, biomass can be converted by biochemical and thermochemical methods into useful products”...

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q3. Lines 43-45. Pyrolysis, one of the commonly used thermochemical conversion processes, involves rapid heating of biomass in the absence of air or oxygen at a maximal temperature…  Remark: According to definition “Pyrolysis is the thermal decomposition of materials at elevated temperatures in an inert atmosphere” (Pyrolysis. Compendium of Chemical Terminology. International Union of Pure and Applied Chemistry. 2009. p. 1824). Therefore, terms “rapid heating” and “maximal temperature” do not fit this definition. I recommend editing the sentence as follows: “Pyrolysis, one of the commonly used thermochemical conversion processes, involves decomposition of biomass in the absence of air or oxygen at an elevated temperature”… 

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q4. Lines 46, 91, 103, 134: … biochar product (s)…  Remark: The word “product(s)” is unnecessary here and should be removed.

Reply: As suggested by the reviewer, the word “product(s)” in the manuscript has been deleted to make it relevant.

Q5. Line 51. Therefore, about 150 million metric tons of rice husk will be produced… Remark: the word “Therefore” does not fit here and should be removed. I recommend starting this sentence as follows: “About 150 million metric tons of rice husk will be produced”…

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q6. Line 64: …the carbon release as GHG forms…  Remark: It is error that “the carbon” release... Use “the carbon dioxide” instead of “the carbon”.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q7. Line 79. …120, 25 and 25 min.  Remark: Time 25 min repeated twice, and it must be corrected.

Reply: The sentence has been changed as follows: 

“Dissanayake et al. conducted the pyrolysis experiments of rice husk at 350, 500 and 650°C by a heating rate of 10°C/min and subsequently maintained for 120, 25 and 25 min, respectively [18].  Herein, the experiment at 350 °C took about 2 hours to complete pyrolysis, while the experiments at 500 °C and 650 °C completed process in about 25 min after reaching the pyrolysis temperature.”

Q8. Line 84. …huge sorption potential of urea… Remark: It is error. Write here …huge sorption potential of biochar, not urea

Reply: The sentence has been changed to make it clear.

 “….. as adsorbents of nutrient nitrogen (i.e., urea), showing a huge sorption potential of biochar due to high functionality and porosity [25].”

Q9. Line 112. WCH.  Remark:  What is WCH? Seems, that it is a misprint that should be replaced with RH.

Reply: Due to the typographical error, the word has been revised. 

Q10. Lines 112, 113. WCH? (RH) contained the high contents of carbon (45.28 wt%), hydrogen (5.51 wt%), oxygen (~ 44 wt%, estimated) and silicon (3.90 wt%), thus showing a relatively low calorific value (16.4 MJ/kg).  Remark: There is a contradiction between "high content of carbon" and low calorific value of the biomass. In addition, the calorific value of 16.4 MJ/kg should not be called low, since a similar value is observed for most lignocellulosic biomasses, including some types of wood, as well as biopolymers - proteins, hemicelluloses and some other polysaccharides. In addition, there is not relation between “high content of carbon” and Therefore, the last part of this sentence should be corrected, as e.g. … silicon (3.90 wt%), and this biomass has a moderate calorific value (16.4 MJ/kg).

Reply: As pointed out by the reviewer, the sentence has been changed to make it precise. 

Q11. Line 127: …temperature must operate above 400℃… Remark: This phrase should be edited, as e.g. …”temperature must be above 400℃”…

Reply: As suggested by the reviewer, the sentence has been changed to make it precise. 

Q12. Lines 127-129. …the pyrolysis temperature must above 400℃ because the peak temperature of DTG occurred at about 380℃, indicating the complete devolatilization of hemicellulose and cellulose at around 400℃… Remark: This fragment should be edited, as e.g.…”the pyrolysis temperature must be above 400℃ because the devolatilization temperature of cellulose is about 400℃, while hemicelluloses are devolatilized at about 380℃”…

Reply: The sentence has been changed to make it clear.   

“This result was consistent with those reported by Johar et al. [31], indicating the complete devolatilization of hemicellulose and cellulose at around 400℃.”

Q13. Lines 138-139. In order to describe the RH-based (i.e., BRH) products easily, the resulting…  Remark: This fragment should be edited, as e.g. “In order to describe the RH-based biochar (i.e., BRH), the resulting”…

Reply: As suggested by the reviewer, the sentence has been changed to make it precise. 

Q14. Line 141. … temperature of 400℃ for 30 holding time of 30 min. Remark: Remove 30 before “holding”, as follows … temperature of 400℃ for holding time of 30 min.

Reply: As suggested by the reviewer, the “30” has been deleted to make it right. 

Q15. Lines 185-190. As shown, there are three apparent peaks as pyrolysis temperature increased from room temperature to 400°C. … It implied that the second TGA peak (“shoulder”) appears at a temperature (about 300°C)… Remark: This fragment should be edited, as e.g. … “As shown, there are two apparent peaks and one shoulder as the pyrolysis temperature increased from room temperature to 400°C. … It implied that the TGA shoulder appears at a temperature about 300°C…

Reply: As suggested by the reviewer, the sentence has been changed to make it precise. 

Q16. Lines 190-191. … temperature (about 300°C) lower than that for lignin. Remark: This fragment should be edited, as e.g. …“temperature (about 300°C) that is lower than for lignin”.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise. 

Q17. Lines 212-213. ..biochar products products… Remark: Misprint since the word “products” was repeated twice.

Reply: As pointed out by the reviewer, the word “products” has been deleted to make it consistent.

Q18. Fig. 2. Remark: The curve needs to be smoothed.

Reply: As pointed out by the reviewer, the curve has been revised to make it smooth.

Q19. Fig. 3 and 4.  Remark: Since the pore volume does not change when the pore size were above 15 nm, it is recommended to limit these graphs to a pore size range from 0 to 15 nm.

Reply: As pointed out by the reviewer, the maximal scales of these graphs have been limited to 15 nm. 

Q20. Fig. 6. EDS spectrum.  Remark: Since the EDS peaks are absent when X <3, it is recommended to limit these graphs to X range from 0 to 3.

Reply: As pointed out by the reviewer, the maximal scale of the EDS graph has been limited to 3. 

Q21. Lines 332, 336, 339 …biochar products… The word “product(s)” is unnecessary here and should be removed; write “biochar(s)” only.

Reply: As the Q3, the word “product(s)” in the manuscript has been deleted to make it relevant. 

Author Response File: Author Response.doc

Reviewer 2 Report

This manuscript presents a study on the pyrolysis of rice husk, with the aim of producing biochar. The study appears to be well-conducted and the results are clear, but the quality of the presentation should be enhanced. Please find a list of comments below.

1. This article was submitted to the journal "Fermentation". However, it has nothing to do with fermentation, since it deals with a thermochemical process. Why did the authors not choose a more suitable journal?
2. The main problem of this article is that the quality of the English is too low, so that many sentences are hardly understandable. There are many typos and grammatical mistakes, such as incorrect uses of words, inconsistent uses of verbal tenses and singular/plurals. A deep revision should be performed. Some representative examples are provided below.
3. Line 54-58: please provide references.
4. Line 58: what is the meaning of "based on its original form"? I also do not understand the meaning of the following sentence (lines 59-62).
5. Page 3: Please explain "WCH".
6. Page 5: Why is the Time-Yield plot not included?
7. Page 6: I do not understand the first equation, please clarify.
8. How was a null holding time achieved?
9. General point: The surface area appears indeed to increase with temperature and residence time. However, aside for the case at 400 °C and 30 min, the increase is quite marginal: if the surface area in the 500-30 case is about 211 m2/g and in the 900-90 case is about 279 m2/g, it is probably not worthy to use such a high amount of energy for such a moderate increase. This should be commented.

English points:

1. Line 13: "value-added valorization"?
2. Line 13-16: the verb is missing.
3. Line 33-36: This sentence makes little sense.
4. Line 45: "at a maximal temperature for holding a specified time"?
5. Line 50-51: this does not explain why rice husk is an important crop residue in Asia.
6. Line 51: will -> are
7. Line 133: available -> suitable?
8. Line 184 and others: Figure 1 indicates
9. Line 295: "The presence of C and O should be original from the biochar phase". Is not this quite obvious?

Author Response

Q1. This article was submitted to the journal "Fermentation". However, it has nothing to do with fermentation, since it deals with a thermochemical process. Why did the authors not choose a more suitable journal? 

Reply: As pointed out by the reviewer, this paper focused on the thermochemical conversion of rice husk into porous biochar.  However, this manuscript was submitted to the Fermentation’s special issue (“Biomass and Waste Valorization”), which covered the recent green pretreatment and fractionation approaches (including biochemical and thermochemical technologies) for biomass reduction and valorization.

Q2. The main problem of this article is that the quality of the English is too low, so that many sentences are hardly understandable. There are many typos and grammatical mistakes, such as incorrect uses of words, inconsistent uses of verbal tenses and singular/plurals. A deep revision should be performed. Some representative examples are provided below.

Reply: As pointed out by the reviewer (and other reviewers), the typos and grammatical mistakes has been carefully checked and revised.

Q3. Line 54-58: please provide references. 

Reply: As suggested by the reviewer, the relevant (three) references about the energy and material uses of rice husk have been added to the paragraph.

Q4. Line 58: what is the meaning of "based on its original form"? I also do not understand the meaning of the following sentence (lines 59-62).

Reply: As suggested by the reviewer, the sentence has been revised to make it readable.

“Obviously, rice husk was directly reused without converting it into useful materials by thermochemical or biochemical processes.  Furthermore, these direct reuse approaches not only generate GHG emissions in a short period of time [2], but also deplete biomass resources without adding valuable applications like porous carbon materials.”

Q5. Page 3: Please explain "WCH".

Reply: Due to the typographical error, the word has been revised.

Q6. Page 5: Why is the Time-Yield plot not included?

Reply: The yields of resulting RH-based biochar prepared at 900℃ for four holding times (0, 30, 60 and 90 min) also indicated a decreasing trend, but had a narrow yield range.  As compared with the significant variation as a function of temperature (Figure 1), the Time-Yield plot was not shown in the manuscript.

Q7. Page 6: I do not understand the first equation, please clarify.

Reply: The particle density (ρ_p) was estimated by the Eq. (1), which was based on its definition (i.e., the mass of a particle divided by it total volume, including intra-particle pore volume).  In contrast, true density (ρ_s) has the same definition, but excludes intra-particle pores (i.e., the mass of a particle divided by it particle volume.  On the other hand, total pore volume (V_t) has the unit with cm³/g (i.e., intra-particle pore volume per one gram of particle mass).  Therefore, this equation can be verified by checking the above-mentioned definitions assuming 1 g of particle mass.

     ρ_s/(V_t x ρ_s + 1) = (1/particle volume)/[pore volume x (1/particle volume) + 1]

                 = (1/particle volume)/[( pore volume + particle volume)/particle volume]

                 = (1/particle volume)/( total volume/particle volume)

                 = 1/total volume = ρ_p

Q8. How was a null holding time achieved?

Reply: In the pyrolysis experiment, the null holding time means that the heating was stopped (powered off) when the specified temperature has reached.

Q9. General point: The surface area appears indeed to increase with temperature and residence time. However, aside for the case at 400 °C and 30 min, the increase is quite marginal: if the surface area in the 500-30 case is about 211 m²/g and in the 900-90 case is about 279 m²/g, it is probably not worthy to use such a high amount of energy for such a moderate increase. This should be commented.

Reply: Based on the efficient energy use, the authors fully agreed to the reviewer’s comment.  As suggested by the reviewer, this comment has been incorporated into the revised version in the Sec. 3.2.

“From the viewpoint of efficient energy use, the pyrolysis conditions at 500℃for 30 min may be suitable for the production of RH-based biochar with high BET surface area of 211 m²/g when comparing with the resulting biochar (BET surface area of 279 m²/g) prepared at 900℃for 90 min.”

Q10. Other minor remarks on English points.

Reply: As pointed out by the reviewer, these English-editing points have been revised or incorporated into the revised manuscript with changes marked in red color. 

Author Response File: Author Response.doc

Reviewer 3 Report

Review on the manuscript “Valorization of rice husk for the production of porous biochar materials”, by Wen-Tien Tsai, Yu-Quan Lin.

This manuscript underlines the importance of using further the rice husk (biomass) for the production of biochar. They also presented how the pyrolysis process (temperature, time, heating rate) influences the properties of the so-formed rice husk biochar.

The manuscript needs to be read and corrected by a native English speaker. I can’t understand parts of the manuscript due to the poor English, especially in the introduction part.

L 281 In addition, the resulting biochar product also exhibited a rigid, irregular and rough surface due to its rigid silica composition and the rigorous carbonization at a higher temperature – How do you know the content of silica? Have you performed an analysis to evidence it’s content in the biochar?

L 284 In this regard, the RH-based biochar may be a good medium for possible applications in the water conservation, wastewater treatment and soil amendment. – Please explain.

L 296 The most significant peak is assigned to the presence of Si due to the high content of silica (SiO2) in the RH. – XRD analysis is required in order to confirm the silica content.

Figure 6 – please assign all the peaks

L 311 In general, the chemical structure of biochar is mainly comprised of aromatic C and inorganic minerals, which are dependent on the pyrolysis conditions applied. – Please explain what do you mean by “chemical structure”, because the chemical composition in regard of the inorganic minerals is not influenced by the pyrolysis conditions.

L 312 The presence of functional groups on the surface of biochar plays a vital role in the soil and

water environments. – Please detail and explain.

L 323 Si-0 framework bands at around 1,115 cm-1 and below 800 cm-1 were evident. – Please correct and write Si-O(oxygen) not Si-0(zero) and add bibliography. As far as I know, at around 1100 cm-1 is the peak characteristic to the Si-O-Si asymmetric stretching vibration.

For the results to be clear it must be performed a XRD analysis that would clarify the phase composition of the biochar and to establish how the temperature influences the composition.

The discussion part need the comparison with the existing literature data, thus confirming the novelty/improvement brought by this study.

Author Response

Q1. The manuscript needs to be read and corrected by a native English speaker. I can’t understand parts of the manuscript due to the poor English, especially in the introduction part.

Reply: As pointed out by the reviewer, these English-editing points have been revised or incorporated into the revised manuscript with changes marked in red color.

Q2. L 281. In addition, the resulting biochar product also exhibited a rigid, irregular and rough surface due to its rigid silica composition and the rigorous carbonization at a higher temperature – How do you know the content of silica? Have you performed an analysis to evidence it’s content in the biochar?

Reply: It is well known that the RH-based biochar contains high content of silica (SiO₂) due to its starting feedstock (i.e., RH) as previously studied by the author (Ref. 30).

Q3. L 284. In this regard, the RH-based biochar may be a good medium for possible applications in the water conservation, wastewater treatment and soil amendment. – Please explain.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

“In this regard, the RH-based biochar may be a good medium for possible applications in the water retention and wastewater treatment in the soil environment due to its highly porous structure.”

Q4. L 296. The most significant peak is assigned to the presence of Si due to the high content of silica (SiO₂) in the RH. – XRD analysis is required in order to confirm the silica content.

Q5. Figure 6 – please assign all the peaks

Reply: In Figure 6, the peaks not assigned by relevant elements mean the coated gold (Au).

Q6. L 311. In general, the chemical structure of biochar is mainly comprised of aromatic C and inorganic minerals, which are dependent on the pyrolysis conditions applied. – Please explain what do you mean by “chemical structure”, because the chemical composition in regard of the inorganic minerals is not influenced by the pyrolysis conditions.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

“In general, the chemical characteristics of biochar are mainly comprised of aromatic C and inorganic minerals, which are dependent on the starting feedstock and pyrolysis conditions applied.”

Q7. L 312. The presence of functional groups on the surface of biochar plays a vital role in the soil and water environments. – Please detail and explain.

Reply: The description about the role of functional groups on the surface of biochar has been revised to make it clear. 

“The presence of functional groups on the surface of biochar plays a vital role in the soil and water environments.  For example, the addition of biochar to soil has been proven to enhance the sorption capacities for heavy metal ions by the electrostatic attraction [42], which can be attributed to the high contents of oxygen-containing functional groups on the surface due to the negatively charged surface.”

Q8. L 323. Si-0 framework bands at around 1,115 cm-1 and below 800 cm-1 were evident. – Please correct and write Si-O(oxygen) not Si-0(zero) and add bibliography. As far as I know, at around 1100 cm-1 is the peak characteristic to the Si-O-Si asymmetric stretching vibration.

Reply: The sentence has been changed to make it clear.

“Si-O framework bands at around 1,115 cm^-1 and below 800 cm^-1 were evident [17].”

Q9. For the results to be clear it must be performed a XRD analysis that would clarify the phase composition of the biochar and to establish how the temperature influences the composition.

Reply: Indeed, XRD was commonly used to determine the presence of crystalline compounds or minerals (e.g., silica) in the biochar.  However, this work focused on the pore properties of RH-based biochars produced by different pyrolysis conditions.  It would be helpful to use the XRD in the analysis of mineral phase.

Q10. The discussion part needs the comparison with the existing literature data, thus confirming the novelty/improvement brought by this study.

Reply: As pointed out by the reviewer, more discussions with red color have been incorporated into the revised version for highlighting the novelty/improvement brought by this study. 

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

The authors made corrections and revised manuscript. However, some shortcomings still remain, namely:

 Abstract

Lines 13-15. …production of RH-based biochars at higher pyrolysis temperatures (i.e., 400, 500, 600, 700, 800 and 900℃) for different holding times (i.e., 0, 30, 60 and 90 min).  Remark:  I recommend to use the temperature and time range, namely: ….production of RH-based biochars at different pyrolysis temperatures from 400 to 900℃ and holding times from 0 to 90 min.

Lines 24-25. … Obviously, the RH-based biochar could be reused… Remark: (1). Word “Obviously” does not suitable and should be deleted; (2). Word “reused” should be replaced with “used”.

Results

Lines 224-225. Based on the definition of porosity (ε_p), the data on true density (ρs), particle density (ρ_p) can be further calculated below [40]: Remark: This sentence is in the wrong place. It should be moved above, namely, place it after reference [39] and before eq.

 ρ_p = ρ_s/(V_t x ρ_s + 1)

Table 1.  Last column – porosity (g/cm³). Remark: The used dimension of porosity, g/cm³, is wrong, because according to definition and eq.  ε_p = 1 – (ρ_p/ρ_s), the porosity is dimensionless parameter. Therefore, the dimension (g/cm³) must be removed from last column of Table 1.

 Line 236, 242. Biochar products. Remark: Word “product” is unnecessary and it should be removed.

Line 279. …the larger pore properties… Remark: I recommend write …the better pore properties…

Decision: The noted shortcomings need to be corrected. After that, this article can be recommended for publication.

Author Response

Q1. Lines 13-15. …production of RH-based biochars at higher pyrolysis temperatures (i.e., 400, 500, 600, 700, 800 and 900℃) for different holding times (i.e., 0, 30, 60 and 90 min).  Remark:  I recommend to use the temperature and time range, namely: …. production of RH-based biochars at different pyrolysis temperatures from 400 to 900℃ and holding times from 0 to 90 min.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q2. Lines 24-25. … Obviously, the RH-based biochar could be reused… Remark: (1). Word “Obviously” does not suitable and should be deleted; (2). Word “reused” should be replaced with “used”.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q3. Lines 224-225. Based on the definition of porosity (ε_p), the data on true density (ρs), particle density (ρ_p) can be further calculated below [40]: Remark: This sentence is in the wrong place. It should be moved above, namely, place it after reference [39] and before eq.

Reply: As suggested by the reviewer, the sentence has been changed to make it precise.

Q4. Table 1.  Last column – porosity (g/cm³). Remark: The used dimension of porosity, g/cm³, is wrong, because according to definition and eq.  ε_p = 1 – (ρ_p/ρ_s), the porosity is dimensionless parameter. Therefore, the dimension (g/cm³) must be removed from last column of Table 1.

Reply: The dimension (g/cm³) has been deleted to make it relevant.

Q5. Line 236, 242. Biochar products. Remark: Word “product” is unnecessary and it should be removed.

Reply: As suggested by the reviewer, the word “product(s)” in the manuscript has been deleted to make it relevant.

Q6. Line 279. …the larger pore properties… Remark: I recommend write …the better pore properties…

Reply: As suggested by the reviewer, the word has been changed to make it precise.

Author Response File: Author Response.doc

Reviewer 2 Report

The authors have provided adequate replies to my queries. However, there are still a few points that need further consideration.

1. The quality of English may still be enhanced. For example, just at the beginning of the abstract, there is a repetition of "therefore". I recommend another check.
2. “Obviously, rice husk was directly reused without converting it into useful materials by thermochemical or biochemical processes.  Furthermore, these direct reuse approaches not only generate GHG emissions in a short period of time [2], but also deplete biomass resources without adding valuable applications like porous carbon materials.” This is not a problem by itself. If rice husk is converted into energy through controlled pathways (such as in doi.org/10.3390/en13215750), it is still a valuable and sustainable choice, regardless of whether other materials are produced or not. Moreover, pyrolysis also generates GHG emissions. I believe the comparison should rather be made with other practices (such as uncontrolled burning on field) that do not valorise the energy content of the material and may generate toxic emissions.
3. The clarification regarding the null holding time should be included in the manuscript as well.

If the authors are able to make these changes, I will pose no further objection to the manuscript's publication.

Author Response

Q1. The quality of English may still be enhanced. For example, just at the beginning of the abstract, there is a repetition of "therefore". I recommend another check.

Reply: As suggested by the reviewer, the rhetorical writings and English grammar in the revised manuscript have been carefully checked and revised with marking in red color.

Q2. “Obviously, rice husk was directly reused without converting it into useful materials by thermochemical or biochemical processes.  Furthermore, these direct reuse approaches not only generate GHG emissions in a short period of time [2], but also deplete biomass resources without adding valuable applications like porous carbon materials.” This is not a problem by itself. If rice husk is converted into energy through controlled pathways (such as in doi.org/10.3390/en13215750), it is still a valuable and sustainable choice, regardless of whether other materials are produced or not. Moreover, pyrolysis also generates GHG emissions. I believe the comparison should rather be made with other practices (such as uncontrolled burning on field) that do not valorise the energy content of the material and may generate toxic emissions.

Reply: As pointed out by the reviewer, the sentence has been revised to make it precise.

“As compared to uncontrolled burning on field, these direct reuse approaches do not valorize the energy content of the material, and may generate toxic emissions without adding valuable applications like porous carbon materials.”

Q3. The clarification regarding the null holding time should be included in the manuscript as well.

Reply: In the pyrolysis experiment, the null holding time means that the heating was stopped (powered off) when the specified temperature has reached.  This clarification has been included in the Sec. 2.3.

Author Response File: Author Response.doc

Reviewer 3 Report

Comparing the first and the actual version of the manuscript, I remarked that the improved version still needs some work.

As I recommended before, English must be improved.

The queries that I recommended before were partially addressed.…

I also recommended the author to elaborate the discussion part but I do not see an improvement in the discussion area. Therefore, in order to help the authors improve their discussion part I searched some articles of interest and I attached the links.  I urge the authors to consider at least half of the articles (what they think is more close to their research) for the discussion part.

https://0-doi-org.brum.beds.ac.uk/10.3390/polym13081256 

https://0-doi-org.brum.beds.ac.uk/10.3390/ma14071776 

https://0-doi-org.brum.beds.ac.uk/10.3390/su13041817 

https://0-doi-org.brum.beds.ac.uk/10.3390/app11010108

https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10070990 

https://0-doi-org.brum.beds.ac.uk/10.3390/su12062458

Author Response

Q1. As I recommended before, English must be improved.

Reply: As suggested by the reviewer, the rhetorical writings and English grammar in the revised manuscript have been carefully checked and revised with marking in red color.

Q2. I also recommended the author to elaborate the discussion part but I do not see an improvement in the discussion area.  Therefore, in order to help the authors improve their discussion part I searched some articles of interest and I attached the links.  I urge the authors to consider at least half of the articles (what they think is more close to their research) for the discussion part.

https://0-doi-org.brum.beds.ac.uk/10.3390/polym13081256 

https://0-doi-org.brum.beds.ac.uk/10.3390/ma14071776 

https://0-doi-org.brum.beds.ac.uk/10.3390/su13041817 

https://0-doi-org.brum.beds.ac.uk/10.3390/app11010108

https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10070990 

https://0-doi-org.brum.beds.ac.uk/10.3390/su12062458

Reply: In order to aid in enhancing manuscript quality, the applications of rice husk biochar extracted from the four references (new Ref. No. 28-31) have been incorporated into the discussion.  By the way, the new Ref. No. 30 (https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy10070990) has been listed in the submitted version (the last paragraph of the Introduction).

Author Response File: Author Response.doc