Emission Factors for Biochar Production from Various Biomass Types in Flame Curtain Kilns

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript in reference addresses an important topic related to biochar production, specifically the emission factors for various non-woody feedstocks when using "Kon-Tiki" flame curtain kilns. The manuscript has interesting elements as biochar production is increasingly considered a strategy for carbon sequestration and soil amendment, especially in resource-poor regions. The paper's focus on smallholders in the tropics using low-cost, accessible technology is particularly pertinent to current global challenges related to sustainable agriculture and climate change mitigation.  However, several issues were detected that limit its further consideration.

1. Title: the title is somewhat long and could be improved for clarity and conciseness. A good title should be succinct while still conveying the key aspects of the study.

2. The manuscript appears to be a preliminary study rather than a complete one. The study lacks detailed results, a comprehensive methodology, and an in-depth discussion of findings, including the implications of different feedstocks on emissions during biochar production in "Kon-Tiki" flame curtain kilns. A preliminary study typically provides initial findings with less extensive data collection and analysis. The reduced information, absence of a conclusion, and potentially shorter or more general discussion sections all point to the possibility that this is an early-stage investigation rather than a comprehensive, full-scale study. This fact might compromise the scientific merit of this manuscript.

3.   The study does not seem to adequately control for or explore the potential effects of other variables, such as the specific microclimate conditions during pyrolysis, feedstock preparation (e.g., particle size), or the exact timing of quenching. These factors could significantly affect the emission factors reported.

4. The paper mentions the use of t-tests to compare emission factors between feedstocks, but it does not provide enough detail on the statistical rigor, including p-values or confidence intervals, which would strengthen the conclusions. Additionally, the paper would benefit from a more detailed presentation of the data variability and more robust statistical methods for multiple comparisons.

5. While the study covers some non-woody feedstocks, the choice of feedstocks could be expanded to include a broader range of agricultural residues commonly found in tropical regions. This would make the findings more broadly applicable.

6. The discussion of why finely-grained feedstocks like coffee husks result in higher emissions is not sufficiently detailed. The study could benefit from a deeper exploration of the underlying mechanisms, perhaps through additional experimental data or a review of relevant literature on the pyrolysis behavior of different feedstock types.

7. The results from specific field conditions in Uganda might not be readily generalizable to other regions or production scales. The paper should include a deeper discussion on the potential limitations of extrapolating these findings to different contexts, considering factors such as regional differences in biomass composition and kiln operation practices.

8. Although it is not mandatory, the manuscript does appear to be missing a conclusion section. A conclusion is essential as it summarizes the key conceptual findings, discusses the implications of the study, and may suggest future research directions. Including this section would enhance the manuscript's completeness and provide readers with a clear takeaway from the research.

Comments on the Quality of English Language

A deep scrutiny must be performed to revise various grammar and stylistic issues.

Author Response

Reviewer 1

The manuscript in reference addresses an important topic related to biochar production, specifically the emission factors for various non-woody feedstocks when using "Kon-Tiki" flame curtain kilns. The manuscript has interesting elements as biochar production is increasingly considered a strategy for carbon sequestration and soil amendment, especially in resource-poor regions. The paper's focus on smallholders in the tropics using low-cost, accessible technology is particularly pertinent to current global challenges related to sustainable agriculture and climate change mitigation.  However, several issues were detected that limit its further consideration.

Thank you for your kind words and extensive, well-worded review.

1. Title: the title is somewhat long and could be improved for clarity and conciseness. A good title should be succinct while still conveying the key aspects of the study.

We agree and reworded the title into “Emission Factors for Biochar Production from Various Biomass Types in Flame Curtain Kilns”.

2. The manuscript appears to be a preliminary study rather than a complete one. The study lacks detailed results, a comprehensive methodology, and an in-depth discussion of findings, including the implications of different feedstocks on emissions during biochar production in "Kon-Tiki" flame curtain kilns. A preliminary study typically provides initial findings with less extensive data collection and analysis. The reduced information, absence of a conclusion, and potentially shorter or more general discussion sections all point to the possibility that this is an early-stage investigation rather than a comprehensive, full-scale study. This fact might compromise the scientific merit of this manuscript.

We agree and have addressed this constructive comment in the following ways:

5. Detailed results were added in the Supplementary Information: all raw data are now presented in Tables S1-5.
6. A conclusion section was added: “The biomass types investigated in the present study, excluding the fine-grained ones (< 2-3 cm), represent a large opportunity for carbon sequestration upon the identification of a viable mechanism to incentivize biochar production by residues from maize, cacao, cassava and similar crops. Flame curtain kilns are not suited for fine-grained feedstocks because of the resulting flame quenching and subsequent aerosol and methane emissions. The low emissions from maize cob, grass and sesame stem pyrolysis in flame curtain kilns documented here are important due to the relevance of these biomass residues in cropping systems in sub-Saharan Africa. In 2018, maize was the second-most abundant crop in sub-Saharan Africa (93 million metric tons (Mbanjo et al., 2021)), after cassava (169 million metric tons (Mbanjo et al., 2021)), from which the stems also likely provide the same high quality feedstock biomass  as sesame stems and twigs, due to their similarity. Grass is abundant and accessible to most farmers in the region. The 33 million smallholder farmers in sub-Saharan Africa could potentially obtain at least 50 million tons of biochar feedstock from their maize cobs, cassava stems, and grass. Annually, this could potentially sequester 70-80 million tons of CO2-eq and improve 12.5 million ha of farmland (application rate 4 t ha^-1 (Martinsen et al., 2014).”

• Discussion was added on how the use of various feedstocks would influence emissions during biochar production:
  1. “Properties for the combustion analysis of biomass are usually classified into physical, chemical and thermal properties (Demirbas, 2004a). Physical properties include density, porosity, and internal surface area. Twigs and sesame stems create a low density and large porosity at the flame curtain front, allowing for the full combustion of pyrolysis gases. Grass needs to be added with care and lifted at intervals to provide sufficient porosity. Its advantage is its low density. Dense materials that form non-porous clusters such as coffee husk, grains, and rice husk are thus not ideal for maintaining the flame curtain, which is important to effectively combust the pyrolysis gases. Important chemical properties for combustion are composition of the biomass and heating value of the volatiles (Demirbas, 2004a). Higher lignin content of olive husk (50%) was found to result in a higher biochar yield in comparison with corncob (15% lignin) (Demirbas, 2004b). Lignin, generally more stable than cellulose or hemicellulose (Börcsök and Pásztory, 2021), is completely decomposed at above 500 °C (Demirbas and Kucuk, 1994). Lignin contents of grasses are typically 22-25% (8 grasses (Oliveira et al., 2020)), of sesame stems 22% (Su et al., 2020) and of woody materials typical for the Miombo forest biome 15-16% (Mtambanengwe and Kirchmann, 1995). Coffee husk contains up to 42% lignin, similar to olive husk (de Almeida et al., 2024). This indicates that not only particle size and biomass moisture content but also lignin content probably plays a role in the pyrolysis process, with denser, smaller, wetter and high-lignin biomasses being the most challenging to pyrolyze with low emissions and high efficiency.

Thermal properties include specific heat, heat of pyrolysis, and thermal conductivity. Caloric contents of all feedstocks are in the same range of 15 to 20 MJ kg^-1 (Constantin, 2012). Thus, the caloric content is probably not an important factor causing substantial differences in emissions between feedstocks. The effect of temperature and biomass grain size on biochar yield was also studied in a simple batch pyrolysis reactor (Demirbas, 2004b). A high temperature (> 700 °C, similar to the temperature of a flame curtain kiln (Schmidt and Taylor, 2014)) and smaller particles increased the heating rate resulting in a decreased biochar yield.”

19. “Earlier work showed higher ratios of organic to black carbon in green fuels (19.2 ± 4.2) compared to dry, wooden logs (7.3 ± 1.9), indicating that more moisture in green biomass resulted in more smouldering-phase combustion and higher aerosol emissions (Zhang et al., 2013). Further, higher combustion temperatures in wood stove burns led to lower emissions (Zhang et al., 2013). This corroborates our findings that pyrolysis of the currently studied green wastes resulted in higher aerosol emissions than woody twigs (Cornelissen et al., 2023). Our study thus confirms that fire intensity plays a role in aerosol emission patterns, with higher aerosol emissions for greener fuel types burning with lower-intensity (the non-woody feedstocks here and field underburns, green leaves and branches in Zhang et al. (2013)).
20.  The study does not seem to adequately control for or explore the potential effects of other variables, such as the specific microclimate conditions during pyrolysis, feedstock preparation (e.g., particle size), or the exact timing of quenching. These factors could significantly affect the emission factors reported.

This is a good point, and important details have now been added to the methods.

• Microclimate: we reduced the effect of microclimate by doing the measurements under comparable weather conditions, and in the presence of large wind shields protecting the kilns from sudden gusts of wind. Added: “We reduced the effect of variations in microclimate by doing the measurements under comparable weather conditions, and in the presence of large wind shields (Fig. S1)”
• Particle size: inherent for each feedstock. In practice farmers will not cut feedstocks to uniform size either. Homogeneity of feedstocks was ensured though: “Care was taken to use feedstock that was as homogeneous and uniformly sized as possible”
• Timing of quenching: “Quenching was done right after the last flame curtain disappeared, and with the same amount of water and at the same rapid speed for all measurements.”

4. The paper mentions the use of t-tests to compare emission factors between feedstocks, but it does not provide enough detail on the statistical rigor, including p-values or confidence intervals, which would strengthen the conclusions. Additionally, the paper would benefit from a more detailed presentation of the data variability and more robust statistical methods for multiple comparisons.

We agree, and added p-values wherever comparisons were made:

• “CO emissions were higher for maize cobs (70-90 g kg^-1) than for grass (p=0.02), sesame stems (p=0.0007), and grass/twigs (p=0.04; all 30-40 g kg^-1)”
• “Aerosol emissions (TSP; PM₁₀) were in the range of 16-45 g kg^-1 biochar (Table 2) for all biomass types except coffee husk (331 g kg^-1 biochar), and significantly lower for grass and sesame stems (< 25 g kg^-1 biochar) than for grass/twigs and maize cobs (> 30 g kg^-1 biochar; sesame vs. grass/twigs, p=0.003; sesame vs. maize, p=0.04; grass/twigs vs. grass, p=0.002; grass vs. maize, p=0.02).”

Variation has been included in the form of interquartile ranges in Table 1. We feel it is a bit redundant to also add confidence intervals – and due to the large variation in the data because of the nature of the pyrolysis process, we considered interquartile ranges more relevant than confidence intervals:

• “Median values and interquartile ranges were reported to avoid data bias due to the high inherent variability of the measurements.”

Otherwise, our statistical analyses were similar to those in our earlier similar papers in 2016 and 2023 and found sufficient for reputable journals such as Science of the Total Environment (Cornelissen et al 2023) and PLOS One (Cornelissen et al 2016).

5. While the study covers some non-woody feedstocks, the choice of feedstocks could be expanded to include a broader range of agricultural residues commonly found in tropical regions. This would make the findings more broadly applicable.

We could not agree more but unfortunately, we are restricted by the logistics of getting all instruments to Uganda, as well as limited project budgets. Ideally of course the dataset should be expanded. However, we do think that the current data are so useful that they merit publication, since this is the first data on non-woody biomasses in flame curtain kilns. The current data are as such important for life-cycle analyses (LCA) of converting various ubiquitous biomasses into biochar using the simple and directly accessible flame curtain technology. Emission measurements are mandatory for a correct LCA of biomass conversion processes (Osman et al., 2021).

Added to the manuscript: “The current data are the first concerning non-woody biomasses in flame curtain kilns. They are important for life-cycle analyses (LCA) of converting various ubiquitous biomasses into biochar using the simple and directly accessible flame curtain technology. Emission measurements are mandatory for a correct LCA of biomass conversion processes (Osman et al., 2021).”

6. The discussion of why finely-grained feedstocks like coffee husks result in higher emissions is not sufficiently detailed. The study could benefit from a deeper exploration of the underlying mechanisms, perhaps through additional experimental data or a review of relevant literature on the pyrolysis behavior of different feedstock types.

We agree and added the following discussion, where we considered various factors that could influence emissions patterns from different feedstocks in a more systematic way (lignin content, caloric value, particle size, porosity):

“Earlier work showed higher ratios of organic to black carbon in green fuels (19.2 ± 4.2) compared to dry, wooden logs (7.3 ± 1.9), indicating that more moisture in green biomass resulted in more smouldering-phase combustion and higher aerosol emissions (Zhang et al., 2013). Further, higher combustion temperatures in wood stove burns led to lower emissions (Zhang et al., 2013). This corroborates our findings that pyrolysis of the currently studied green wastes resulted in higher aerosol emissions than woody twigs (Cornelissen et al., 2023). Our study thus confirms that fire intensity plays a role in aerosol emission patterns, with higher aerosol emissions for greener fuel types burning with lower-intensity (the non-woody feedstocks here and field underburns, green leaves and branches in Zhang et al. (2013)).

“Properties for the combustion analysis of biomass are usually classified into physical, chemical and thermal properties (Demirbas, 2004a). Physical properties include density, porosity, and internal surface area. Twigs and sesame stems create a low density and large porosity at the flame curtain front, allowing for the full combustion of pyrolysis gases. Grass needs to be added with care and lifted at intervals to provide sufficient porosity. Its advantage is its low density. Dense materials that form non-porous clusters such as coffee husk, grains, and rice husk are thus not ideal for maintaining the flame curtain, which is important to effectively combust the pyrolysis gases. Important chemical properties for combustion are composition of the biomass and heating value of the volatiles (Demirbas, 2004a). Higher lignin content of olive husk (50%) was found to result in a higher biochar yield in comparison with corncob (15% lignin) (Demirbas, 2004b). Lignin, generally more stable than cellulose or hemicellulose (Börcsök and Pásztory, 2021), is completely decomposed at above 500 °C (Demirbas and Kucuk, 1994). Lignin contents of grasses are typically 22-25% (8 grasses (Oliveira et al., 2020)), of sesame stems 22% (Su et al., 2020) and of woody materials typical for the Miombo forest biome 15-16% (Mtambanengwe and Kirchmann, 1995). Coffee husk contains up to 42% lignin, similar to olive husk (de Almeida et al., 2024). This indicates that not only particle size and biomass moisture content but also lignin content probably plays a role in the pyrolysis process, with denser, smaller, wetter and high-lignin biomasses being the most challenging to pyrolyze with low emissions and high efficiency.

Thermal properties include specific heat, heat of pyrolysis, and thermal conductivity. Caloric contents of all feedstocks are in the same range of 15 to 20 MJ kg^-1 (Constantin, 2012). Thus, the caloric content is probably not an important factor causing substantial differences in emissions between feedstocks. The effect of temperature and biomass grain size on biochar yield was also studied in a simple batch pyrolysis reactor (Demirbas, 2004b). A high temperature (> 700 °C, similar to the temperature of a flame curtain kiln (Schmidt and Taylor, 2014)) and smaller particles increased the heating rate resulting in a decreased biochar yield.”

 

7. The results from specific field conditions in Uganda might not be readily generalizable to other regions or production scales. The paper should include a deeper discussion on the potential limitations of extrapolating these findings to different contexts, considering factors such as regional differences in biomass composition and kiln operation practices.

This is of course true, but we do argue that the conditions during the dry season in Uganda are representative for those in much of sub-Saharan Africa and elsewhere in similar climatic conditions. For example, the twigs/grass data in the current work were highly similar to those obtained in dry coastal Peru in our 2023 paper.

We expect the main challenge rather to be the diligence of the skilled operators vs. what will happen in practice upon large-scale implementation where not all farmers will be equally careful while adding a more challenging feedstock such as grass.

Of course, the data are not necessarily representative of those in temperate and wet climates (e.g. rainforest).

These useful points we now make in the paper:

“A challenge for feedstocks such as grass and maize cobs is that care and diligence are required to keep emissions as low as those reported in this work. If the grass is not carefully added and lifted at intervals,  as described here, emissions will inevitably spike. Careful instructions need to be given and adhered to. Another limitation is the applicability of the work to more temperate and humid tropical regions (e.g., rainforests), where other feedstocks and weather conditions will dominate. The current study should be considered most relevant for sub-Saharan Africa and regions with similar climatic conditions, and where similar crops are cultivated. Another remaining challenge is finding pyrolysis technologies for finely grained feedstocks such as coffee husk, rice husk, cacao husk, cashew, and other nut shells. Gasification with efficient syn-gas combustion in a heat engine is a way forward, but emission factors from the simplest and most economical of such systems, e.g. those manufactured by Husk Power Systems ® (Patna, India; Fort Collins, CO, USA), have not yet been published.”

8. Although it is not mandatory, the manuscript does appear to be missing a conclusion section. A conclusion is essential as it summarizes the key conceptual findings, discusses the implications of the study, and may suggest future research directions. Including this section would enhance the manuscript's completeness and provide readers with a clear takeaway from the research.

We agree and added a conclusion section (see response under point 2).

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The aim of the manuscript entitled “Emission Factors for Biochar Production from Maize Cob, Sesame Stem, Grass, and Coffee Husk in “Kon-Tiki” Flame Curtain Kilns” is to assess the emission factors associated with the production of biochar using simple, low-cost "Kon-Tiki" flame curtain kilns from various feedstocks, including maize cobs, sesame stems, grass, and coffee husk. The study seeks to evaluate the suitability of these kilns for smallholder use in the tropics by quantifying the emissions of gases and aerosols, such as methane, carbon monoxide, total suspended particles, and nitrogen oxides, during the pyrolysis of different feedstocks. The research aims to identify the conditions under which these kilns can produce biochar with minimal harmful emissions, particularly focusing on whether finely-grained feedstocks like coffee husk require more advanced pyrolysis systems to meet environmental standards.

The manuscript is interesting and fits well with the scope of the Journal. The manuscript is generally well-prepared, but some issues should be resolved. My specific comments are given below.

The abstract is packed with data and numbers, making it somewhat dense and challenging to read. Besides, it does not clearly state the research objective or hypothesis. It would benefit from a more explicit statement of what the study aimed to achieve or explore beyond simply documenting emissions. Also, there is no mention of the methodology used to measure the emissions, which is a critical aspect of understanding the results.

The introduction is informative.

“Carbon, hydrogen and nitrogen contents in the biochars were measured by element analysis according to previously described methods (Cornelissen et al., 2016).” - All methods should be described in detail, even if they are published before, for the sake of completeness. Also, the authors should include photographs of the experimental setup.

“For the first, quenching probably needs to be delayed to after the flames have disappeared after the ultimate feedstock addition as a sign of completed pyrolysis.” - The word "probably" introduces uncertainty and suggests a lack of confidence in the conclusion. It is better to use more definitive language or to specify the conditions under which the conclusion holds.

Table 2 is not visible well.

In the section “Implications,” there is some repetition, particularly regarding the importance of dry biomass and the challenges in humid tropics. These points could be consolidated to avoid redundancy. Also, this section does a good job of highlighting the advantages and potential of Kon-Tiki kilns, but a more balanced discussion that also addresses the limitations and challenges would provide a fuller picture. It should be included.

The conclusion is missing.

The font size is not the same throughout the manuscript.

Comments on the Quality of English Language

Minor changes are required. 

Author Response

Reviewer 2

The aim of the manuscript entitled “Emission Factors for Biochar Production from Maize Cob, Sesame Stem, Grass, and Coffee Husk in “Kon-Tiki” Flame Curtain Kilns” is to assess the emission factors associated with the production of biochar using simple, low-cost "Kon-Tiki" flame curtain kilns from various feedstocks, including maize cobs, sesame stems, grass, and coffee husk. The study seeks to evaluate the suitability of these kilns for smallholder use in the tropics by quantifying the emissions of gases and aerosols, such as methane, carbon monoxide, total suspended particles, and nitrogen oxides, during the pyrolysis of different feedstocks. The research aims to identify the conditions under which these kilns can produce biochar with minimal harmful emissions, particularly focusing on whether finely-grained feedstocks like coffee husk require more advanced pyrolysis systems to meet environmental standards.

The manuscript is interesting and fits well with the scope of the Journal. The manuscript is generally well-prepared, but some issues should be resolved. My specific comments are given below.

Thank you for this positive feedback.

The abstract is packed with data and numbers, making it somewhat dense and challenging to read. Besides, it does not clearly state the research objective or hypothesis. It would benefit from a more explicit statement of what the study aimed to achieve or explore beyond simply documenting emissions. Also, there is no mention of the methodology used to measure the emissions, which is a critical aspect of understanding the results.

We agree and restructured the abstract to contain:

1. Less numbers: removed all numbers from the abstract.
2. Explicit aims: “The main aims of the current work were to document emissions for ubiquitous non-woody feedstocks, contrast emissions for finely grained, high-lignin feedstock (coffee husk) to those for coarser low-lignin ones (maize cobs, grass, sesame stems).”

• Method description: “All carbon-containing gases and NO_x were monitored during full pyrolysis runs using hand-held sensitive instruments equipped with internal pumps, and carbon balances were utilized to establish emission factors in gram per kg biochar.”

1. More overarching interpretation: “The reported data should aid overarching life-cycle analyses of the integration of biochar practice in climate-smart agriculture and facilitate carbon credit procurement for tropical smallholders.”

 

The introduction is informative.

Thank you.

“Carbon, hydrogen and nitrogen contents in the biochars were measured by element analysis according to previously described methods (Cornelissen et al., 2016).” - All methods should be described in detail, even if they are published before, for the sake of completeness. Also, the authors should include photographs of the experimental setup.

We thank the reviewer for these useful suggestions, and added the required information:

1. “Carbon, hydrogen and nitrogen contents in the biochars were measured by element analysis after combustion at 1050 °C and chromatographic detection in a Leco 836 element analyzer (Cornelissen et al., 2016).”
2. “Moisture in the feedstock was measured with a Protimeter Timbermaster BLD5609 (1% accuracy) for maize cobs.”

We also added pictures of the setup to the SI:

1. The flame curtain kiln with steel shields to protect from wind and create uniform conditions.
2. The measurement devices.

• Using rakes to lift the newly added grass.

“For the first, quenching probably needs to be delayed to after the flames have disappeared after the ultimate feedstock addition as a sign of completed pyrolysis.” - The word "probably" introduces uncertainty and suggests a lack of confidence in the conclusion. It is better to use more definitive language or to specify the conditions under which the conclusion holds.

We agree and removed the word “probably” here. This was a valuable comment: we checked for “probably” throughout and removed the word in at least half the occasions.

Table 2 is not visible well.

We suspect this may have to do with the pdf file the reviewer has looked at? We suggest to leave the ultimate layout of Table 2 to the journal’s editorial staff, if this is acceptable to the reviewer and editor.

In the section “Implications,” there is some repetition, particularly regarding the importance of dry biomass and the challenges in humid tropics. These points could be consolidated to avoid redundancy. Also, this section does a good job of highlighting the advantages and potential of Kon-Tiki kilns, but a more balanced discussion that also addresses the limitations and challenges would provide a fuller picture. It should be included.

We corrected for redundancy and included more discussion on challenges and limitations, as also suggested by reviewer 1:

“A challenge for feedstocks such as grass and maize cobs is that care and diligence are required to keep emissions as low as those reported in this work. If the grass is not carefully added and lifted at intervals,  as described here, emissions will inevitably spike. Careful instructions need to be given and adhered to. Another limitation is the applicability of the work to more temperate and humid tropical regions (e.g., rainforests), where other feedstocks and weather conditions will dominate. The current study should be considered most relevant for sub-Saharan Africa and regions with similar climatic conditions, and where similar crops are cultivated. Another remaining challenge is finding pyrolysis technologies for finely grained feedstocks such as coffee husk, rice husk, cacao husk, cashew, and other nut shells. Gasification with efficient syn-gas combustion in a heat engine is a way forward, but emission factors from the simplest and most economical of such systems, e.g. those manufactured by Husk Power Systems ® (Patna, India; Fort Collins, CO, USA), have not yet been published.”

The conclusion is missing.

Thank you for pointing this out. We now added a conclusion: “The biomass types investigated in the present study, excluding the fine-grained ones (< 2-3 cm),  represent a large opportunity for carbon sequestration upon the identification of a viable mechanism to incentivize biochar production by residues from maize, cacao, cassava and similar crops. Flame curtain kilns are not suited for fine-grained feedstocks because of the resulting flame quenching and subsequent aerosol and methane emissions. The low emissions from maize cob, grass and sesame stem pyrolysis in flame curtain kilns documented here are important due to the relevance of these biomass residues in cropping systems in sub-Saharan Africa. In 2018, maize was the second-most abundant crop in sub-Saharan Africa (93 million metric tons (Mbanjo et al., 2021)), after cassava (169 million metric tons (Mbanjo et al., 2021)), from which the stems also likely provide the same high quality feedstock biomass  as sesame stems and twigs, due to their similarity. Grass is abundant and accessible to most farmers in the region. The 33 million smallholder farmers in sub-Saharan Africa could potentially obtain at least 50 million tons of biochar feedstock from their maize cobs, cassava stems, and grass. Annually, this could potentially sequester 70-80 million tons of CO2-eq and improve 12.5 million ha of farmland (application rate 4 t ha^-1 (Martinsen et al., 2014).”

The font size is not the same throughout the manuscript.

Corrected.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

In this manuscript, the author reported the emission factor from biochar production with different woody feedstock. However, the investigation is not sufficient to support a scientific paper.

1. There are only two tables, value data for the results. No spectrum, no images, no pictures, and no VARIABLE was investigated, which can not to be a scientific paper.

2. The section of “HIGHLIGHTS” should delete.

3. The format of References should revised.

4. What is the size of the feedstocks?

5. What is the parameters of different feedstocks?

6. What is the unit of kg dw?

7. Table 1, how can you determined the C, H, N in the biochar?

8. As show in Table 1, the “Duration” are different, how can you compared the results of different feedstocks?

9. Table 2 was not fully displayed.

10. The format of some sentences in page 9 is not consistent.

11. What is “Implications” means?

12. Where is the Conclusion?

13. The parameters of biochar production procedure were not investigated.

14. The gas phase was not revealed with chromatography.

15. The composition of the obtained biochar was not characterized with FTIR, XRD or XPS.

Comments on the Quality of English Language

Extensive editing of English language required.

Author Response

In this manuscript, the author reported the emission factor from biochar production with different woody feedstock. However, the investigation is not sufficient to support a scientific paper.

1. There are only two tables, value data for the results. No spectrum, no images, no pictures, and no VARIABLE was investigated, which can not to be a scientific paper.

In principle we agree with the comment, and testing biomass characteristics against emission data would have been the ultimate goal. However, limited funding and logistical challenges precluded such an extensive study. The current 4 feedstocks did not really allow for a systematic study between biomass characteristics and emission patterns.

However, we addressed the comment by adding literature data on “variables” for the biomasses – especially data on lignin content and caloric content. While the caloric content values were too similar between feedstocks to provide any additional mechanistic insights, the lignin contents were actually very different between coffee husk (and olive husk) and the other feedstocks. Thus, this comment helped in adding the following discussion:

“Properties for the combustion analysis of biomass are usually classified into physical, chemical and thermal properties (Demirbas, 2004a). Physical properties include density, porosity, and internal surface area. Twigs and sesame stems create a low density and large porosity at the flame curtain front, allowing for the full combustion of pyrolysis gases. Grass needs to be added with care and lifted at intervals to provide sufficient porosity. Its advantage is its low density. Dense materials that form non-porous clusters such as coffee husk, grains, and rice husk are thus not ideal for maintaining the flame curtain, which is important to effectively combust the pyrolysis gases. Important chemical properties for combustion are composition of the biomass and heating value of the volatiles (Demirbas, 2004a). Higher lignin content of olive husk (50%) was found to result in a higher biochar yield in comparison with corncob (15% lignin) (Demirbas, 2004b). Lignin, generally more stable than cellulose or hemicellulose (Börcsök and Pásztory, 2021), is completely decomposed at above 500 °C (Demirbas and Kucuk, 1994). Lignin contents of grasses are typically 22-25% (8 grasses (Oliveira et al., 2020)), of sesame stems 22% (Su et al., 2020) and of woody materials typical for the Miombo forest biome 15-16% (Mtambanengwe and Kirchmann, 1995). Coffee husk contains up to 42% lignin, similar to olive husk (de Almeida et al., 2024). This indicates that not only particle size and biomass moisture content but also lignin content probably plays a role in the pyrolysis process, with denser, smaller, wetter and high-lignin biomasses being the most challenging to pyrolyze with low emissions and high efficiency.

Thermal properties include specific heat, heat of pyrolysis, and thermal conductivity. Caloric contents of all feedstocks are in the same range of 15 to 20 MJ kg^-1 (Constantin, 2012). Thus, the caloric content is probably not an important factor causing substantial differences in emissions between feedstocks. The effect of temperature and biomass grain size on biochar yield was also studied in a simple batch pyrolysis reactor (Demirbas, 2004b). A high temperature (> 700 °C, similar to the temperature of a flame curtain kiln (Schmidt and Taylor, 2014)) and smaller particles increased the heating rate resulting in a decreased biochar yield.”

2. The section of “HIGHLIGHTS” should delete.

Thank you – deleted.

3. The format of References should revised.

Thank you. We suggest to collaborate with editorial staff in the next round to get the references in accordance to journal policy.

4. What is the size of the feedstocks?

Thank you for this good question. Now added to the method section: “Size of the feedstocks were: maize cobs 15-20 cm; sesame stems 30-50 cm length and 5 mm thick; grass 80-100 cm length and 2-3 mm thick, twigs 80-100 cm length and 10-20 mm thick; coffee husk 10 mm diameter.”

5. What is the parameters of different feedstocks?

We included the following characteristics in the manuscript:

• C, H, N
• Lignin content (literature values)
• Caloric content (literature values)
• Particle size (this reviewer’s suggestion)
• Biochar pH
•  

6. What is the unit of kg dw?

Kg dry weight. Specified now (Table 1 heading).

7. Table 1, how can you determined the C, H, N in the biochar?

Added: “Carbon, hydrogen and nitrogen contents in the biochars were measured by element analysis  after combustion at 1050 °C and chromatographic detection in a Leco 836 element analyzer (Cornelissen et al., 2016).”

8. As show in Table 1, the “Duration” are different, how can you compared the results of different feedstocks?

For all feedstocks, the median values over the whole run were presented. For smaller-size materials forming denser layers such as maize cobs and coffee husk, pyrolysis was slower and thus duration longer. We tried to have a rather similar number or data points for each feedstock.

9. Table 2 was not fully displayed.

We suspect this may have to do with the pdf file the reviewer has looked at? We suggest to leave the ultimate layout of Table 2 to the journal’s editorial staff, if this is acceptable to the reviewer and editor.

10. The format of some sentences in page 9 is not consistent.

We do not really understand which sentences this comment is referring to. The editorial staff will help harmonize formats though.

11. What is “Implications” means?

Generally, Implications mean consequences or effects, the likely results or outcomes of an action or decision. Replaced by “Consequences”.

12. Where is the Conclusion?

Thank you for pointing this out. We now added a conclusion:

“The biomass types investigated in the present study, excluding the fine-grained ones (< 2-3 cm),  represent a large opportunity for carbon sequestration upon the identification of a viable mechanism to incentivize biochar production by residues from maize, cacao, cassava and similar crops. Flame curtain kilns are not suited for fine-grained feedstocks because of the resulting flame quenching and subsequent aerosol and methane emissions. The low emissions from maize cob, grass and sesame stem pyrolysis in flame curtain kilns documented here are important due to the relevance of these biomass residues in cropping systems in sub-Saharan Africa. In 2018, maize was the second-most abundant crop in sub-Saharan Africa (93 million metric tons (Mbanjo et al., 2021)), after cassava (169 million metric tons (Mbanjo et al., 2021)), from which the stems also likely provide the same high quality feedstock biomass  as sesame stems and twigs, due to their similarity. Grass is abundant and accessible to most farmers in the region. The 33 million smallholder farmers in sub-Saharan Africa could potentially obtain at least 50 million tons of biochar feedstock from their maize cobs, cassava stems, and grass. Annually, this could potentially sequester 70-80 million tons of CO2-eq and improve 12.5 million ha of farmland (application rate 4 t ha^-1 (Martinsen et al., 2014).”

13. The parameters of biochar production procedure were not investigated.

We agree, but temperature, the main parameter, was investigated during our previous study in 2023. Added: “With dry feedstock, temperatures have earlier been reported to range from 600 °C to 810 °C for Kon Tiki kilns (Cornelissen et al., 2023).”

14. The gas phase was not revealed with chromatography.

Unfortunately, we did not have the opportunity to take gas samples in rural Africa. However, we used advanced hand-held sensors from the top market segment, as specified in the Method section. This is a field study with its limitations (no advanced characterizations) and merits (relevance).

15. The composition of the obtained biochar was not characterized with FTIR, XRD or XPS.

We agree such analyses would have been interesting, but unfortunately, we did not have easy access to such devices, and no budget to potentially cover external analyses. We do think spectroscopic data would not have been easy to connect to the emission data and considered it not really worth the effort required.

Extensive editing of English language required.

Native speakers have gone through the manuscript and edited the English.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors adequately addressed the raised comments, satisfying the process for improving quality and content. Accordingly, I recommend acceptance for publication.

Author Response

Thank you for re-reviewing our manuscript!

Reviewer 2 Report

Comments and Suggestions for Authors

The authors addressed all my comments. 

Comments on the Quality of English Language

Minor changes are required. 

Author Response

Thank you for re-reviewing our manuscript!

We revised the English, and actually discovered a few typographic errors. We also rewrote the sentences marked in yellow.

Reviewer 3 Report

Comments and Suggestions for Authors

1. The format of References should revised.

 

1. Table 1 was not fully displayed.

Comments on the Quality of English Language

Moderate editing of English language required.

Author Response

Thank you for re-reviewing our manuscript!

We revised the English, and actually discovered a few typographic errors. We also rewrote the sentences marked in yellow.

We trust the editorial staff in revising the reference list and Table 2 into the perfect format for the journal.