Unpacking the Carbon Balance: Biochar Production from Forest Residues and Its Impact on Sustainability

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor and Authors,

The research is on relevant subjects as it relates to the study of the effect of the pyrolysis rate of woody biomass on the sustainability of biochar production. The stated assessment was considered using Life Cycle Assessment (LCA), which highlights the complex approach to the objective. However, the manuscript has several key points that should be discussed before it is considered for publication.

1) How do the authors relate pyrolysis and bioenergy? A clearer accent should be shown in the abstract. Pyrolysis as a process is used to produce target products (biochar, gas, resin), not energy. Moreover, pyrolysis does not generate gasification (line 44). Authors should be careful in terms.

2) The Introduction section of the manuscript requires information on the classification of pyrolysis rates. Which pyrolysis rate corresponds to FAST pyrolysis and which to the SLOW?

3) Figure 1 should be zoomed in for better readability.

4) To extend the impact of the publication, authors can specify wider applications of biochar such as metallurgy, chemical industry, etc. For example, https://doi.org/10.3390/agronomy11040615; http://dx.doi.org/10.3390/en16124673, etc.

5) What software was used to calculate the LCA?

6) What input characteristics of woody biomass are considered? These (size, moisture content, ash, volatile yield) determine the efficiency of the pyrolysis process and the yield of products.

7) Table 1. Biochar yield is definitely underestimated. It is commonly more than 30% but depends on the pyrolysis temperature. Why the sum of biochar, syngas and oil is 70%. What is the other 30%?

8) Do not use ‘our’ and ‘we’ in the manuscript text. Scientific articles are written in the third person.

9) The manuscript needs proofreading for technical and grammatical errors.

Comments on the Quality of English Language

The manuscript needs proofreading for technical and grammatical errors.

Author Response

Dear reviewer,

before addressing your specific comments, we would like to express our gratitude for your insightful feedback. Your comments have significantly helped us improve the quality of our work. We appreciate your thorough review and valuable suggestions

Comment 1: How do the authors relate pyrolysis and bioenergy? A clearer accent should be shown in the abstract. Pyrolysis as a process is used to produce target products (biochar, gas, resin), not energy. Moreover, pyrolysis does not generate gasification (line 44). Authors should be careful in terms.

Response 1: Thank you for your valuable comments and suggestions. We have considered your feedback and made the necessary revisions. Specifically, the abstract and the entire manuscript have been modified to address your requests. The suggested details have been clarified in the introduction section with comprehensive changes from line 36 to 66. We believe these modifications enhance the clarity and quality of our manuscript.

Comment 2: The Introduction section of the manuscript requires information on the classification of pyrolysis rates. Which pyrolysis rate corresponds to FAST pyrolysis and which to the SLOW?

Response 2: Modifications have been performed to fulfil the request (see lines 44-49)

Comment 3: Figure 1 should be zoomed in for better readability.

Response 3: As requested, figure 1 has been modified for better readability

Comment 4: To extend the impact of the publication, authors can specify wider applications of biochar such as metallurgy, chemical industry, etc. For example, https://doi.org/10.3390/agronomy11040615; http://dx.doi.org/10.3390/en16124673, etc.

Response 4: In the results and discussion section we have exluded other effect on soil rather than carbon sequestration. Moreover, we also provided a wider range of possibile applications of biochar that also were not taken into consideration (see lines 236 to 238).

Comment 5: What software was used to calculate the LCA?

Response 5: This information is already provided at line 136.

Comment 6: What input characteristics of woody biomass are considered? These (size, moisture content, ash, volatile yield) determine the efficiency of the pyrolysis process and the yield of products.

Response 6: Of all suggested biomass characteristics, moisture is already mentioned in table 1; density and size have been added in the material and methods section at lines 155, 163 and 164); while ash and volatile yield were not considered.

Comment 7: Table 1. Biochar yield is definitely underestimated. It is commonly more than 30% but depends on the pyrolysis temperature. Why the sum of biochar, syngas and oil is 70%. What is the other 30%?

Response 7: The remaining 30% is water. The process giving 3% of biomass is pyrolysis followed by a gasification, teh amount of biochar accounting is coherent with the literature.

Comment 8: Do not use ‘our’ and ‘we’ in the manuscript text. Scientific articles are written in the third person.

Response 8: All pronouns suggested have been modified as requested throughout the entire manuscript.

Comment 9: The manuscript needs proofreading for technical and grammatical errors.

Response 9: The entire manuscript has been subjected to a technical and grammatical review to eliminate errors.

Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the environmental benefits of utilizing forest residues for biochar production through slow and fast pyrolysis using Life Cycle Assessment (LCA). The content of the paper appears to be low and does not meet the requirements of this journal.

 

Author Response

Thanks to the suggestions from both reviewers, we have significantly improved the quality of the paper. 

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript entitled “Unpacking the Carbon Balance: Biochar Production from Forest Residues and Its Impact on Sustainability” is dedicated to estimating the environmental benefits of utilizing forest residues for biochar production through slow and fast pyrolysis using Life Cycle Assessment. No doubt, global industrialization accelerates the use of fossil fuels and, consequently, the excessive emissions of greenhouse gases, especially carbon dioxide, which poses a serious threat to the ecological environment. Therefore, it is essential to seek ways to reduce CO₂ emissions and utilize alternative energy sources. The manuscript provides valuable information on evaluating the economic viability of using forest residues as precursors for syngas, bio-oil, and biochar, which in turn can be used for CO₂ capture. For this reason, the article is worthy of publication. Nevertheless, there are some comments provided below that the authors should address.

 

1) Why did the authors choose to evaluate the efficiency of capturing atmospheric CO₂ by incorporating biochar into the soil? It is well-known that pristine biochar can be used directly for CO₂ capture. Perhaps synthesizing biochar-based carbon adsorbents would be more economically feasible and more effective for absorbing atmospheric CO₂?

2) Line 180-184: I recommend rewriting the sentence and dividing it into several sentences for better understanding.

 

3) Figure 3: There is a typos: it should be "Avoided heat," not "eat."

Author Response

Dear reviewer, Before addressing your specific comments, we would like to express our gratitude for your insightful feedback. Your comments have significantly helped us improve the quality of our work. We appreciate your thorough review and valuable suggestions

Comment 1: Why did the authors choose to evaluate the efficiency of capturing atmospheric CO₂ by incorporating biochar into the soil? It is well-known that pristine biochar can be used directly for CO₂ Perhaps synthesizing biochar-based carbon adsorbents would be more economically feasible and more effective for absorbing atmospheric CO₂?

Response 1: To better address the remark pointed out in these questions, a better explanation about carbon sequestration through biochar as an amendant has been integrated into the manuscript. A reference has also been added (see lines 64-66 and reference 21).

Comment 2: Line 180-184: I recommend rewriting the sentence and dividing it into several sentences for better understanding.

Response 2: As suggested, the paragraph has been divided (see lines 216-222).

Comment 3: Figure 3: There is a typos: it should be "Avoided heat," no

Response 3: Figure 3 has been revised to eliminate the typos, as kindly suggested.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor and Authors,

The authors have carefully addressed the comments of the first report and improved the manuscript. However, minor revisions are still needed to progress the manuscript.

1) It is indicated (line 43) that fast pyrolysis follows a heating rate of 10-200 °C/s, and slow pyrolysis responds to 0.1-1 °C/s. Do you mean exactly 1 °C? Then how should the rate from 1 to 10 °C/s be classified?

2) In Figure 1, the pathways of syngas and oil are only for heat and energy. Which heat and energy are meant can be clarified. Additionally, the use of oil is broader than just combustion. Generally, gas is burned, but not oil.

Moreover, the arrow from Forest Management is directed to Wood. Although wood is a raw material and therefore the start point of the scheme, if I understand correctly.

3) Regarding the 16% biochar yield after slow pyrolysis. This value is underestimated and is usually 30%wt. Or was there also a subsequent gasification? Although then what is the point of obtaining biochar and then pyrolyzing it?

These points should be explained in the manuscript.

Author Response

Comment 1: It is indicated (line 43) that fast pyrolysis follows a heating rate of 10-200 °C/s, and slow pyrolysis responds to 0.1-1 °C/s. Do you mean exactly 1 °C? Then how should the rate from 1 to 10 °C/s be classified?

Response 1: Pahnila et al. provide a review of pyrolysis technologies and discuss the impact of process parameters on biocarbon properties in their article "A Review of Pyrolysis Technologies and the Effect of Process Parameters on Biocarbon Properties" published in Energies and cited in our manuscript (Ref. 12). In Table 1 of their article, they present the parameters that we indicated in our paper, along with the references from which they obtained them. At the same time, the authors state that the boundary between slow and fast pyrolysis is somewhat questionable. Generally, sources draw the line between slow and fast pyrolysis at a heating rate of 10 °C/s, where heating rates lower than 10 °C/s are referred to as slow pyrolysis and those faster than 10 °C/s as fast pyrolysis. We modified accordingly (see lines 42-46).

Comment 2: In Figure 1, the pathways of syngas and oil are only for heat and energy. Which heat and energy are meant can be clarified. Additionally, the use of oil is broader than just combustion. Generally, gas is burned, but not oil. Moreover, the arrow from Forest Management is directed to Wood. Although wood is a raw material and therefore the start point of the scheme, if I understand correctly.

Response 2: Thank you for the suggestion. We have updated Figure 1 and clarified that thermal energy can be obtained from the thermochemical process, and subsequently, electricity can be generated from it, as you kindly requested. Despite the various applications of bio-oil, we are analyzing two systems in which tar production is minimized and the amount produced is used for energy purposes. Several works have demonstrated in the literature that it has the potential to be used in burners, boilers, and furnaces, as well as in gas turbines and compression-ignition engines. (please check ref. 40 now added to the manuscript.) https://doi.org/10.1016/j.pecs.2020.100834

Comment 3: Regarding the 16% biochar yield after slow pyrolysis. This value is underestimated and is usually 30%wt. Or was there also a subsequent gasification? Although then what is the point of obtaining biochar and then pyrolyzing it? These points should be explained in the manuscript

Response 3: Thank you for the suggestion, as we modified in the text (see lines 199-202), biochar represents 16% of the input mass. Typically, biochar accounts for 30% of the products obtained. In our pyrolysis process, when calculating the percentage based on the products, biochar constitutes 23%. These are primary data, and various factors can influence the yield, including temperature and residence time. However, comparing these results with the literature and with other facilities is not always possible.

Reviewer 2 Report

Comments and Suggestions for Authors

This study investigates the environmental benefits of utilizing forest residues for biochar production through slow and fast pyrolysis using Life Cycle Assessment (LCA). The content of the paper appears to be low and does not meet the requirements of this journal.

Author Response

Comment 1: This study investigates the environmental benefits of utilizing forest residues for biochar production through slow and fast pyrolysis using Life Cycle Assessment (LCA). The content of the paper appears to be low and does not meet the requirements of this journal.

Response 1: Thank you for your review. We appreciate the time and effort you have dedicated to evaluating our paper. However, we are surprised that, despite our extensive revisions and improvements based on the suggestions from other reviewers, the feedback remains the same and appears to be quite generic.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The present study investigated the environmental benefits of utilizing forest residues for biochar production through slow and fast pyrolysis using Life Cycle Assessment (LCA). The environmental benefits of thermochemical processes utilising forest residues to produce valuable energy-dense products, such as syngas, bio-oil, and biochar, providing a carbon sink were investigated. The paper generally meets the requirements of this journal now, but there are still a few questions that need to be answered or revised before it can be published. The comments are as follows.

 

1、line#40-51：In the introductory section, the authors emphasize the importance of biomass feedstock utilization and describe different biomass pyrolysis pathways that can lead to diversified utilization. Among them, biochar as a porous material is widely used in air purification, wastewater treatment and soil remediation, which is very helpful for the improvement of ecological environment. Thus, the following newer relevant works on biomass activated carbon should be cited to improve the manuscript,

e.g., https://doi.org/10.1016/j.indcrop.2024.118991 （bamboo-based activated carbon）

https://doi.org/10.1016/j.renene.2024.120209（wood-based activated carbon）

https://doi.org/10.1016/j.scp.2024.101634（shell-based activated carbon）

 

2、Can the authors provide the specific tree species of the biochar precursors described in the paper? Interspecific differences in tree species have a direct impact on the structural properties of biochar and its ecological impact.

 

3、The molecular formula for carbon dioxide in Figure 2 and Figure 3 is written in a poor format.

Comments on the Quality of English Language

 Quality of English language is fine.

Author Response

Comment 1: line#40-51：In the introductory section, the authors emphasize the importance of biomass feedstock utilization and describe different biomass pyrolysis pathways that can lead to diversified utilization. Among them, biochar as a porous material is widely used in air purification, wastewater treatment and soil remediation, which is very helpful for the improvement of ecological environment. Thus, the following newer relevant works on biomass activated carbon should be cited to improve the manuscript,

e.g., https://doi.org/10.1016/j.indcrop.2024.118991 （bamboo-based activated carbon）https://doi.org/10.1016/j.renene.2024.120209（wood-based activated carbon）https://doi.org/10.1016/j.scp.2024.101634（shell-based activated carbon）

Response 1: Thank you for the suggestion. Now in the introduction section we added your insights about potential different applications for biochar other than carbon stockage in soil for agricultural purposes. (please see lines 64-69).

 

Comment 2: Can the authors provide the specific tree species of the biochar precursors described in the paper? Interspecific differences in tree species have a direct impact on the structural properties of biochar and its ecological impact.

Response 2: Biochar in this study has been obtained from a mixture of wood residues, made mainly of Turkey oak and beech chipped wood. We addressed this observation in the manuscript (please see line 146-147)

 

Comment 3: The molecular formula for carbon dioxide in Figure 2 and Figure 3 is written in a poor format.

Response 3: The molecular formula for carbon dioxide in Figure 2 and Figure 3 has been improved as kindly suggested.