Pore and Thermochemical Properties of Biochar Materials Produced from Moso Bamboo Under Different Carbonization Conditions
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsArticle “Pore and Thermochemical Properties of Biochar Materials Produced from Moso Bamboo Under Different Carbonization Conditions˝ explains precisely moso bamboo (Phyllostachys edulis, PE) pyrolyze proces in a high-temperature carbonization furnace to produce porous biochar materials with high carbon contents un-der different carbonization temperatures (500, 600, 700, 800, and 900 °C) and heating rates (10 and 20 °C/min). The results obtained are presented very nicely and in detail and the techniques are well described.
My recommendations and questions for improving the representativeness of this paper are as follows:
- Nowhere in the paper is it stated which device was used for SEM analysis?
- If a comparison of morphologies is to be made, the micrographs must be shown at the same magnification for the initial and final samples. Also, the bars representing lengths are different in the images and are not very clearly shown (I assume they are the dots at the bottom of the image which are not very pleasing).
- It would be nice to see micrographs of the pores at a higher magnification (at least 3000x for both samples) for the moso bamboo and optimal biochar.
- Is it better to have a larger amount of product after combustion or is the advantage of a higher porosity of the material?
- What will the resulting product of bamboo combustion be used for? The work would be much better if some application of the resulting product was also shown.
Author Response
Q1. Nowhere in the paper is it stated which device was used for SEM analysis?
Reply: Thank you for your comment. The device used for SEM analysis was mentioned in the manuscript (S-3000N, Hitachi Co., Tokyo, Japan), which was written in section 2.4.
Q2. If a comparison of morphologies is to be made, the micrographs must be shown at the same magnification for the initial and final samples. Also, the bars representing lengths are different in the images and are not very clearly shown (I assume they are the dots at the bottom of the image which are not very pleasing).
Reply: Thank you for your suggestion. As recommended, the micrographs at the suggested magnifications were included into the manuscript for better comparisons. The dots that are shown at the bottom of the micrographs are not scale bars that represent the lengths but are a part of the formatting from the instrument printout. These dots cannot be removed or modified because it is embedded into the image
Q3. It would be nice to see micrographs of the pores at a higher magnification (at least 3000x for both samples) for the moso bamboo and optimal biochar.
Reply: As per your recommendation in the previous comment, we included micrographs at the desired magnifications (seen in Figure 6). Thank you for the suggestion, it indeed improved the quality of our manuscript.
Q4. Is it better to have a larger amount of product after combustion or is the advantage of a higher porosity of the material?
Reply: That is a very good question, thanks. Well it depends on what is the intended use of the final biochar product. In most cases, porosity plays an important role if the biochar is to be used for remediation or as a scaffolding material for secondary products through modification. On the other hand, if the intended use is for more general purpose and nothing too specific, then the yield plays an important role so that you can maximize the product output from the inputted biomass material. It must be noted however that product output and porosity are related to the pyrolysis conditions.
Q5. What will the resulting product of bamboo combustion be used for? The work would be much better if some application of the resulting product was also shown.
Reply: Thank you for your comment, however the purpose of the research was not to delve into the uses of the product but to characterize the pore and thermochemical properties of the biomass and biochar products. As we mentioned in the conclusions that the insights were to provide guidance on future pyrolysis conditions that required high surface area applications. It was not necessary to discuss these uses in detail in the current paper because it would distract from the main objectives of the research.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe topic is relevant; however, the manuscript lacks a clear scientific positioning and a mechanistic discussion linking TGA results, biochar yield, pore development, and elemental composition. Most interpretations remain descriptive and insufficiently supported by the literature; therefore, major revisions are recommended
The authors do not clearly state how their work provides genuine novelty with respect to the overall existing literature, and not only in comparison with a previous study conducted by the same research group.
The interpretation of the TGA curve is not adequately supported by appropriate literature references.
The discussion of the TGA results remains superficial and largely descriptive, without in-depth exploitation of the DTG data.
The evolution of biochar yield as a function of temperature is presented only in tabular form, which reduces clarity and interpretability; a graphical representation is necessary.
The decrease in yield with increasing temperature is mentioned but insufficiently explained from the standpoint of devolatilization mechanisms and carbon matrix rearrangement.
The influence of temperature on porosity development is observed but not discussed in terms of pore formation, pore opening, or pore collapse. The underlying physical and chemical mechanisms (volatile release, reorganization of the carbon structure, aromatization processes) should be explicitly analyzed.
The effect of heating rate on porosity is poorly explained and lacks a clear mechanistic interpretation.
The evolution of elemental composition with temperature is described but not analyzed in depth.
The phenomena of dehydrogenation, deoxygenation, and aromatization are mentioned but not discussed in a detailed and structured manner.
The absence of atomic H/C and O/C ratios severely limits the structural and chemical interpretation of the obtained biochars.
Overall, the discussion lacks an integrated mechanistic approach linking TGA results, yield, porosity, and chemical composition.
The conclusions merely summarize the results without providing critical insight or clear scientific perspectives.
Finally, several factors that are well documented in the literature are neglected, including the influence of residence time, surface mass loading, and other operational parameters on the final properties of biochars
Author Response
Q1. The authors do not clearly state how their work provides genuine novelty with respect to the overall existing literature, and not only in comparison with a previous study conducted by the same research group.
Reply: Thank you for your suggestion. A brief statement was added to clarify the novelty of the present work with respect to the broader literature, emphasizing the systematic investigation of low heating-rate carbonization across a wide temperature range and the combined analysis of pore development and elemental compositions. The changes were highlighted in the last paragraph of Introduction.
“In the previous study [30], an induction-heating pyrolysis system was employed to produce porous bamboo-derived biochars at high heating rates (110-170 °C/min). The results indicated that a heating rate of approximately 150 °C/min was optimal for generating microporous biochars with BET surface areas exceeding 250 m2/g and pore volumes greater than 0.12 cm3/g. In contrast to the predominantly high heating-rate conditions reported in the literature, the present study systematically examines the effects of low heating-rate carbonization across a wide temperature range, with simultaneous evaluation of pore structure evolution and elemental composition, thereby providing complementary mechanistic insight into bamboo biochar formation. …….”
Q2. The interpretation of the TGA curve is not adequately supported by appropriate literature references.
Reply: Thank you for your insight. As per your recommendation, the interpretations of the thermal degradation curves were improved in the manuscript to give the readers a better understanding of the mechanisms at hand. These changes were similar to the recommendation you gave that follows. In addition, two appropriate references (Refs. 37 and 38) have been added to support the TGA results of this work.
Q3. The discussion of the TGA results remains superficial and largely descriptive, without in-depth exploitation of the DTG data.
Reply: Thank you for your suggestion. As per your recommendation along with the recommendation of other reviewers, this section of the manuscript was revised. A more in-depth explanation of the DTG data was included in the second paragraph of the Sec. 3.1. These changes are presented in a red color.
“………. DTG analysis indicates that Stage II proceeds through a complex, multi-step mechanism characterized by overlapping degradation reactions of hemicellulose and cellulose. The thermal decomposition of hemicellulose is generally initiated at lower temperatures (≈ 200 - 300°C) and is associated with the cleavage of relatively weak acetyl, ether, and glycosidic bonds, resulting in the rapid evolution of CO2, CO, acetic acid, and other oxygenated volatile species. With further temperature increase (≈ 300 - 400°C), cellulose is subjected to depolymerization via the breakdown of β-1,4-glycosidic linkages [39], leading to the formation of levoglucosan, light hydrocarbons, and condensable tar compounds. The overlap of these degradation processes gives rise to the broadened and asymmetric DTG peak observed in Figure 2b, reflecting the concurrent occurrence of intense devolatilization, secondary cracking of volatile products, and the initial formation of char. The pronounced mass loss and elevated DTG peak intensity during this stage highlighted its dominant role in governing char yield, pore development, and surface chemical characteristics. …………”
Q4. The evolution of biochar yield as a function of temperature is presented only in tabular form, which reduces clarity and interpretability; a graphical representation is necessary.
Reply: Thank you for your suggestion. As recommended, the graphical representation of the biochar’s yield as a function of temperature was added to the manuscript (seen in Figure 3). Your suggestion improved the quality of our work.
Q5. The decrease in yield with increasing temperature is mentioned but insufficiently explained from the standpoint of devolatilization mechanisms and carbon matrix rearrangement.
Reply: Thank you for this valuable comment. The manuscript was revised to provide a more detailed mechanistic explanation for the decrease in biochar yield with increasing carbonization temperature (seen in the Sec. 3.1). The additions clarify the physicochemical processes governing yield reduction and strengthen the discussion of the experimental observations. The corresponding revisions were highlighted in the manuscript.
“……….. At higher carbonization temperatures, primary devolatilization reactions were intensified, leading to the extensive release of non-condensable gaseous products such as CO2, CO or CH4, and condensable tar species that were generated from the breakdown of residual lignocellulosic structures. Furthermore, secondary cracking reactions of tar intermediates were promoted at elevated temperatures, whereby condensable products were further converted into lighter gaseous compounds, thereby reducing the retained solid fraction. Concurrently, the carbon matrix rearrangement occurred through aromatization and ring condensation in the carbon framework. These processes may have been accompanied by the continuous elimination of heteroatoms in the form of volatile species which contributed to additional mass loss despite the formation of a more thermally stable carbon structure. As shown in Table 2, the reduction in yield between 500 and 900 °C was relatively moderate, which was consistent with the mass loss behavior observed in the TGA curves (Figure 2). This observation suggested that the majority of the readily volatilizable components were removed during the primary devolatilization stage, while subsequent yield reduction at higher temperatures was mainly governed by slower secondary devolatilization processes and carbon matrix restructuring. The visual representation of the relationship biochar yield had with pyrolysis temperature is presented in Figure 3.”
Q6. The influence of temperature on porosity development is observed but not discussed in terms of pore formation, pore opening, or pore collapse. The underlying physical and chemical mechanisms (volatile release, reorganization of the carbon structure, aromatization processes) should be explicitly analyzed.
Reply: Thank you for your insight. We revised the manuscript to provide a more detailed mechanistic discussion of porosity development as a function of carbonization temperature and heating rate (seen in the Sec. 3.1). The revised section now addresses pore formation and pore opening induced by volatile release, as well as pore collapse and burn-off effects at higher temperatures. These additions clarified the physicochemical mechanisms observed by the BET surface area trends and strengthen the interpretation of the experimental results. We highlighted the revised text in the manuscript.
“Second, the BET surface area of the biochars produced at a heating rate of 10 °C/min increased sharply from 5.04 m2/g at 500 °C to 496.03 m2/g at 800 °C [40], followed by a slight decrease to 450.02 m2/g at 900 °C. The pronounced increase in BET surface area observed between 500 and 800 °C may be attributed to progressive pore formation and pore opening processes driven by intensified devolatilization reactions. As the carbonization temperature increased, the release of volatile compounds and tar intermediates from the lignocellulosic matrix generated internal voids, thereby promoting the development of micropores and small mesopores. Simultaneously, thermal reorganization of the carbon framework occurred through aromatization and structural condensation, which contributed to the stabilization and widening of newly formed pores. At 900 °C, the slight decline in surface area suggests that pore collapse and excessive burn-off became increasingly significant. Under such severe thermal conditions, continued carbon matrix rearrangement and ring condensation may have caused pore wall shrinkage, coalescence, or partial structural collapse. In addition, the accelerated removal of carbon atoms through gasification reactions likely reduced the integrity of thin pore walls, resulting in a net loss of accessible porosity despite increased carbon ordering [41].”
Q7. The effect of heating rate on porosity is poorly explained and lacks a clear mechanistic interpretation.
Reply: As per your previous comment, we improved the explanation for the relationship temperature and heating rate has on porosity and pore formation (seen in the Sec. 3.1). The incorporated content improved the mechanistic interpretation of the experimental results. All new content was highlighted.
“In contrast, biochars produced at a heating rate of 20 °C/min exhibited a pronounced increase in BET surface area only at elevated temperatures, rising from 1.96 m2/g at 800 °C to 240.04 m2/g at 900 °C, with minimal pore development observed in the temperature range of 500-800 °C. This behavior may be explained by kinetic limitations associated with rapid heating, whereby insufficient residence time was available for gradual volatile release and controlled pore development at lower temperatures. As a result, pore formation was suppressed until higher temperatures were reached, at which point intensified devolatilization and structural rearrangement processes facilitated delayed pore opening. Overall, the biochar with the most developed pore structure characterized by a BET surface area of 496.03 m2/g and a total pore volume of 0.1771 cm3/g was obtained at 800 °C using a heating rate of 10 °C/min with a holding time of 30 min. This condition appears to represent an optimal balance between volatile-driven pore generation, carbon matrix reorganization, and structural stabilization, prior to the onset of significant pore collapse or burn-off at higher temperatures.”
Q8. The evolution of elemental composition with temperature is described but not analyzed in depth.
Reply: Thank you for the comment. This was answered together with the one that follows being that they are related to each other and pertains to the same section of the manuscript.
Q9. The phenomena of dehydrogenation, deoxygenation, and aromatization are mentioned but not discussed in a detailed and structured manner.
Reply: Thank you for these constructive comments. In response, the third paragraph of the Section 3.3 has been substantially revised to provide a more detailed and structured analysis of the evolution of elemental composition as a function of carbonization temperature and heating rate. The revised discussion now includes the roles of dehydrogenation, deoxygenation and aromatization reactions. In addition, the slight reversal in elemental trends observed at the highest temperature has been further analyzed in terms of carbon burn-off and surface re-oxidation effects. Thank you again for the suggestions, they have enhanced the quality of our overall discussion. All the changes have been highlighted in the manuscript.
“The progressive increase in carbon content accompanied by the reduction in hydrogen and oxygen contents may be attributed to temperature-driven dehydrogenation and deoxygenation reactions that occurred during pyrolysis. As carbonization temperature increased, hydrogen was preferentially removed in the form of H2 and H2O through dehydration, decarboxylation, and cracking reactions, while oxygen was eliminated primarily as CO and CO2 via decarboxylation and decarbonylation pathways. These reactions led to a continuous decrease in the H/C and O/C atomic ratios, indicating a gradual transition from aliphatic structures toward more condensed carbon frameworks. Concurrently, aromatization processes were promoted at elevated temperatures through structural rearrangement, ring condensation, and the fusion of polyaromatic domains. The removal of heteroatoms facilitated closer packing of carbon layers and increased structural ordering, resulting in the formation of thermally stable, aromatic-rich biochar matrices. This evolution is consistent with the observed enrichment of carbon content up to 800 °C and reflects the dominance of carbon condensation reactions over volatilization processes within this temperature range. The slight decrease in carbon content and the concurrent increase in oxygen content observed at 900 °C for both heating rates may be attributed to enhanced carbon burn-off and surface re-oxidation effects. Under severe carbonization conditions, partial gasification of the carbon matrix may occur, leading to the loss of carbon as gaseous species. In addition, the formation of thermodynamically stable oxygen-containing surface functional groups or post-carbonization exposure to residual oxygen may contribute to the apparent increase in oxygen content at the highest temperature.”
Q10. The absence of atomic H/C and O/C ratios severely limits the structural and chemical interpretation of the obtained biochars.
Reply: Thank you for your suggestion. We included the H/C and O/C ratios in the manuscript to improve the interpretation of structural and chemical changes. The changes were included in the corresponding table (seen in Table 3) and highlighted.
Q11. Overall, the discussion lacks an integrated mechanistic approach linking TGA results, yield, porosity, and chemical composition.
Reply: Thank you for your recommendation. The discussion section of the manuscript was revised thoroughly. The explanations linking the TGA results, yield, porosity and chemical compositions were improved as suggested. All changes were highlighted in the revised manuscript.
Q12. The conclusions merely summarize the results without providing critical insight or clear scientific perspectives.
Reply: Thank you for your input. The Conclusions section was revised as suggested. These revisions provided clearer scientific insight and strengthened the contribution of the study. The revisions were highlighted in the manuscript.
“…… These trends suggest the existence of an optimal carbonization temperature window in which volatile release promotes effective pore opening and structural development, while excessive carbon matrix shrinkage and rearrangement at higher temperatures may compromise pore stability. However, an inverse relationship between biochar yield and carbonization severity was observed. Specifically, the biochar yield decreased from 31.19 % at 500 °C to 25.87 % at 900 °C at a heating rate of 10 °C/min, and from 28.62 % to 23.38 % over the same temperature range at 20 °C/min. This behavior may be attributed to the more gradual devolatilization and extended thermal exposure associated with lower heating rates, which facilitate carbon matrix reorganization and the stabilization of newly formed pores. Elemental analysis revealed an overall increase in carbon content and a corresponding decrease in hydrogen and oxygen contents with increasing temperature, reflecting progressive dehydrogenation, deoxygenation, and aromatization during carbonization. The evolution of elemental composition provides mechanistic evidence of the transformation from aliphatic biomass components to condensed aromatic carbon structures, which is closely associated with improved thermal stability and altered surface chemistry of the biochars. Overall, the results demonstrate that careful control of carbonization temperature and heating rate is critical for tailoring the structural and chemical properties of bamboo-derived biochars. The insights gained from this study contribute to a deeper understanding of structure–process relationships under slow heating-rate conditions and provide guidance for optimizing biochar production for adsorption and other environmental applications.”
Q13. Finally, several factors that are well documented in the literature are neglected, including the influence of residence time, surface mass loading, and other operational parameters on the final properties of biochars.
Reply: Thank you for your observation. In response, parts of the introductory discussion on biochar production has been revised to acknowledge the influence of additional operational parameters, including residence time, surface mass loading, and heat and mass transfer conditions, on biochar yield, porosity, and surface chemistry. While the present study focused mainly on the effects of carbonization temperature and heating rate, the revised text clarified the broader process variables reported in the literature and delineates the scope of the current work. Your suggestions provided a more comprehensive interpretation of the work. The relative additions were highlighted in the manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsBiochar is one of the modern eco-friendly materials for use in a variety of applications. The development of methods for its production and detailed pyrolys processes of natural precursors make it possible to create carbon materials with controlled morphology. From this point of view, the presented study is relevant and practically significant. The authors position the presented manuscript as a continuation and development of their own systematic research, which is undoubtedly assessed positively.
Questions and comments
- In the authors' previous study, a pyrolysis system at high heating rates (110-170°C/min) was used to produce porous biochars from bamboo. What accounts for the sudden transition to heating rates of 10 and 20°C/min? Why weren't intermediate values used? If the authors expected an explosive increase in surface area upon transition to low heating rates, similar to [26-29], then it was necessary to provide comparative information in the conclusions.
- When discussing the mechanism of pyrolysis, it is necessary to present a chemical diagram of the process, even in a simplified form.
- The three stages of moso bamboo pyrolysis were described using experimental TGA data as shown in Figure 2a. However, DTG analysis (Figure 2b) indicates that Stage 2 proceeds via a complex mechanism. Can the authors detail the main pyrolysis process at Stage II?
Author Response
Q1. In the authors' previous study, a pyrolysis system at high heating rates (110-170°C/min) was used to produce porous biochars from bamboo. What accounts for the sudden transition to heating rates of 10 and 20°C/min? Why weren't intermediate values used? If the authors expected an explosive increase in surface area upon transition to low heating rates, similar to [26-29], then it was necessary to provide comparative information in the conclusions.
Reply: Thank you for the insight. In my previous study [30], the faster heating rates by using an induction-heating pyrolysis system were employed compared to our current research. If you observed the pyrolysis system was also different, it was capable of extremely fast heating rates compared to the pyrolysis system used in this investigation. In addition, slower heating rates of 10 and 20°C/min were used in this experiment because of corresponding literature and the limitations of the pyrolysis apparatus.
Q2. When discussing the mechanism of pyrolysis, it is necessary to present a chemical diagram of the process, even in a simplified form.
Reply: Thank you for your comment. While we agree that the presentation of the mechanism in visual form is beneficial, there are numerous images of such in the literature and its addition would increase the contents and put a strain on the readers. In addition, we took your suggestion and expanded the discussion and interpretation of the mechanisms involved so that the readers can have a more in depth understanding of the processes that are occurring during the stages of pyrolysis.
Q3. The three stages of moso bamboo pyrolysis were described using experimental TGA data as shown in Figure 2a. However, DTG analysis (Figure 2b) indicates that Stage 2 proceeds via a complex mechanism. Can the authors detail the main pyrolysis process at Stage II?
Reply: Thank you for this insightful comment. We agree that Stage II involves a complex thermal decomposition mechanism, as evidenced by the DTG profile. In response, we expanded the manuscript to provide a more detailed description of the dominant reactions occurring in Stage II, including the overlapping decomposition of hemicellulose and cellulose, bond cleavage mechanisms, volatile evolution, and the char precursor formation. As seen in the Sec. 3.1, these additions clarified the multi-step nature of Stage II pyrolysis and strengthened the interpretation of the DTG peak behavior. The revised text was added to the manuscript and was highlighted accordingly.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe revised manuscript has significantly improved in clarity and scientific quality. I am satisfied with the revisions and recommend acceptance.
