A Numerical Model of Biomass Combustion Physical and Chemical Processes
Round 1
Reviewer 1 Report
The authors approached a very important research topic which has a high applicability potential in an attempt to optimize and streamline the industrial combustion processes of lignocellulose biomass.
Paper’s abstract clearly identified the need and relevance for the numerical model proposed by the authors. I suggest all the abbreviation in the abstract be written also in full (e.g. “computational fluid dynamics”), to help those interested to find the paper easier by using keywords.
Introduction is concise and provides a clear definition of the problem being investigated, also giving a clear overview of the investigation strategy. However, in order to be able to better demonstrate the high novelty and originality of the research, I would recommend the authors to complete the Introduction section with relevant information on the current state-of-the-art in numerical results specific for this field of research. In this regard, I noticed that the bibliographic references [5-15] are cited all at once in Conclusions (see line 447), without any brief mention in Introduction or anywhere in the text regarding related results achieved by other researchers.
The research methodology includes sufficient technical details and theoretical information to comprehensively evaluate the validity of the research. However, elements regarding the research methodology (description of the model, functions and calculation procedures, etc.) are found both in the Introduction and in Chapters 2.1, 2.2, 2.3. I recommend a better delimitation of the introductory elements (by adding some results obtained by other researchers) from those regarding the description of the methods and the model, possibly by creating a chapter of Materials and methods.
The results are expressed very professionally and coherently allowing an easy understanding of phenomena and effects. They are discussed logically and in depth, being accompanied by suggestive images and graphics to compare simulated and experimental data.
The conclusions of the paper analyze the issues addressed and emphasize the relevance of using three-dimensional numerical simulation methods. However, I would recommend to the authors that this chapter be more concise, and focus on the results but less on summarizing the paper's structure.
The authors have proved a high quality level in the correct expression of ideas, using English proficiently without grammar or expression errors. I have identified a minor error that I suggest to correct: Raw 430: replace <all this was> with the plural <all these were>. Or, better the entire phrase be reformulated.
My overall conclusion is that the paper is of a high scientific level, is designed and written professionally and is particularly useful for the scientific community in the field of modeling and simulation of thermal processes for biomass conversion.
Author Response
GENERAL COMMENTS
A revision has been carried out following carefully all the indications, suggestions and observations formulated by the reviewers.
The main operated changes are outlined next:
1) As suggested, an overall revision of the text was carried out in order to correct some small mistakes all along the work. A spell check and editing of the English language and style was also carefully performed. Moreover, several longer phrases and have been modified for a clearer and easier reading.
2) The abstract was modified to some extent, slightly reformulating some words, or word groups, and inserting a new phrase highlighting the most important achievements of this paper. Also, the reference to the first part of the study (ref. [1]) was moved from the abstract’s body to the Introduction.
3) The Introduction was extensively modified. Five new paragraphs were inserted: the first three new paragraphs were added at the beginning, in order to make a connection to the first article from this series of two, and for a more logical transition into the discussion of the paper at hand; the last two paragraphs aim to present a very short presentation of related research – however, please take note that an exhaustive literature review of the paper’s topic is already available in the first article (ref. [1]).
4) The Conclusions were almost entirely re-written. Following the suggestions of the reviewers, greater emphasis was placed on summarizing the achievements, and the presented information is now significantly condensed.
At this point, it has to be also underlined that the authors tried in a very honest way to follow all the suggestions and observations formulated by the reviewers and to operate (as much as it was possible) all the corrections indicated by them. Furthermore, in order to follow the corrections operated in the paper, a version of the manuscript having all the changes operated tracked (with the option track changes) has been also uploaded together with the last form of the manuscript (without the changes tracked).
Finally, the authors would like to sincerely express their gratitude to the reviewers for their valuable comments that obviously helped in improving the quality of the present work.
The specific corrections operated according to the suggestions of the reviewers are given next together with detailed explanations.
Reviewer 1
Comment 1:
I suggest all the abbreviation in the abstract be written also in full (e.g. “computational fluid dynamics”), to help those interested to find the paper easier by using keywords.
Thank you for this observation. We followed your suggestion accordingly.
The revised abstract now reads:
Abstract: Identifying a modeling procedure of biomass thermal decomposition not only simple enough to implement and use, and computationally efficient, but also sufficiently accurate for engineering design activities, and with a spectrum of applications as broad as possible is a very difficult task. The authors propose a procedure which consists of two main stages: a) the static modeling phase with the purpose of generating the algorithm (macro functions) that supplies a Computational Fluid Dynamics (CFD) model with specific input data (source/sink terms and local material properties) and b) the dynamic modeling phase, where the CFD model is bi-directionally coupled to the external biomass decomposition model in the form of a User-Defined Function (UDF). The modeling approach was successfully validated against data obtained from single particle decomposition experiments, demonstrating its applicability even to large biomass particles, under high heating rates and combusting conditions.
Comment 2:
Introduction is concise and provides a clear definition of the problem being investigated, also giving a clear overview of the investigation strategy. However, in order to be able to better demonstrate the high novelty and originality of the research, I would recommend the authors to complete the Introduction section with relevant information on the current state-of-the-art in numerical results specific for this field of research.
Thank you for bringing this to our attention. We have made extensive modifications to the Introduction, for a better understanding of the context and link the paper with the first article of the series (ref. [1]). However, we do respectfully ask the Reviewer to accept that we did not elaborate too much in the direction of the current state-of-the-art, since that was done exhaustively in the first article. The two papers are clearly meant to be studied together, and the paper being reviewed does make complete sense only in conjunction with the previous one; therefore, we feel that re-discussing this aspect would have been redundant and possibly annoying for the reader.
The revised text now reads:
The work presented herein is entirely founded on the mathematical model of biomass combustion physical and chemical processes explained in great detail in [1]. For a more complete understanding and easier assessment of this effort, we strongly encourage the reader to take it into consideration.
The main goal of this study (the previous and the current paper, together) is to present a possible solution for modeling and simulating biomass thermal decomposition regardless of particle size and shape, heating rate, and neutral or oxidizing environment conditions. Moreover, it is formulated such that any biomass source can be used, provided it can be described using its main constituents (fractions of hemicellulose, cellulose, lignin, minerals and moisture).
The first paper detailed the complete path starting from the mathematical formulation of the kinetic-chemical model for biomass transformation, and the physical modeling of biomass properties. It concluded by showing the outcome of the decomposition model, in the form of transfer functions to be implemented in the three-dimensional Computational Fluid Dynamics (CFD) model – the static modeling phase.
This second paper presents the subsequent dynamic modeling phase, involving the actual numerical simulation of biomass thermal decomposition and volatiles & char combustion, as well as its verification and validation.
…
The experimental data published by Lu [4] was chosen for assessing the overall modeling performance and accuracy. Other researchers [5-15] have attempted to model and numerically simulate the decomposition of biomass particles, either in isolation or in the form of particle bed, using various approaches. Generally though, the conclusion was that heat & mass transfer dominated processes – large particles, high heating rates – proved significantly more difficult to simulate. Dimensionally-reduced models also have difficulties in coping with more complex situations, particularly due to the importance of three-dimensional effects and asymmetries.
Many research efforts (including Lu’s) are limited to mass loss and temperature measurements at particular points. Consequently, no validation of volatiles composition could be performed yet.
Comment 3:
…elements regarding the research methodology (description of the model, functions and calculation procedures, etc.) are found both in the Introduction and in Chapters 2.1, 2.2, 2.3. I recommend a better delimitation of the introductory elements (by adding some results obtained by other researchers) from those regarding the description of the methods and the model, possibly by creating a chapter of Materials and methods.
Thank you for your suggestion. We did take it into consideration, as demonstrated by the significant modifications of the Introduction, specifically the reduction of modeling related wording and the additional paragraphs (see also response to previous comment).
Comment 4:
The conclusions of the paper analyze the issues addressed and emphasize the relevance of using three-dimensional numerical simulation methods. However, I would recommend to the authors that this chapter be more concise, and focus on the results but less on summarizing the paper's structure.
Thank you for your valuable observation. The Conclusions were almost completely re-written, as follows:
The complexity of physical and chemical transformations taking place within biomass particles undergoing thermal decomposition is undeniably very high. Reproducing these processes with sufficient accuracy using numerical modelling and simulation requires a comparably detailed approach. Achieving the right balance between model sophistication and the associated computational effort is of great concern, also. Nevertheless, the modelling procedure presented herein is quite successful at reproducing the observable behaviour of relatively large, heavily thermally loaded, biomass particles; and it does that with reasonable costs.
The implementation of the mathematical model for biomass decomposition presented in the first part of the study is performed taking advantage of the extensive capabilities of a commercial CFD software platform. The User-Defined Function is formulated such that the communication between the chemical-kinetic model and the CFD numerical model is bi-directional. Using dedicated subroutines, the program performs calculations for (1) the mass and energy sources for all mass transfer processes associated to the thermal decomposition of biomass and (2) the material properties and those of transport in solid volume, extracting all necessary numerical data from the CFD solver. The calculation of heat & mass transfer, volatiles transport, and combustion, if conditions allow, char burn, etc., are all perfomed within the solver. Of all numerical tasks, the integration of chemical reaction mechanisms for gaseous species combustion seems to be the limiting factor in terms of computational performance.
The comparison of numerical results and experimental data used for validation is very good. The models qualitatively and quantitatively reproduce the experiments conducted by Lu [4] for a series of 9.5 mm cylindrical biomass samples (poplar), thermally decomposed in an oven specially designed and manufactured for this purpose. Both neutral and oxidizing conditions are successfully validated, at low and high initial moisture content, too.
The global performance of the biomass thermal decomposition model developed in this research is remarkable, especially considering the fact that it relies on a relatively simple chemical-kinetic scheme. The numerical model can be successfully used not only for accurate estimation of conversion process duration, either pyrolysis or combustion, or for estimating residual mass (i.e. char), but it can be applied as well for determining combustion temperatures in both key phases (volatiles burning, followed by char burning). Given the achieved level of accuracy, we can assume that the chemical composition of burnt gases might be properly predicted too, but this remains to be confirmed in future research.
The most important conclusion drawn from the analysis of all numerical data and their correspondence with experimental measurements is the following: in some cases it may be possible to apply one-dimensional or two-dimensional simplified models to obtain acceptable results in general engineering design; but an accurate and thorough research of many apparently simple cases can be done only by using three-dimensional, unsteady numerical simulation methods, that must be properly formulated physically and chemically as detailed and close to real phenomena as possible. Achieving high accuracy in numerical modelling cannot be done just by concentrating on aspects of physical and chemical transformations of biomass during thermal decomposition; the complete modelling of heat transport and, especially, mass transport, are found to be equally important.
Comment 5
I have identified a minor error that I suggest to correct: Raw 430: replace <all this was> with the plural <all these were>. Or, better the entire phrase be reformulated.
Thank you for bringing this to our attention. The entire phrase was removed following the re-writing of Conclusions.
Reviewer 2 Report
A Numerical Model of Biomass Combustion Physical and Chemical Processes
In this work, a procedure that consists of two main stages: a static modeling phase to generate the algorithm and dynamic modeling was proposed. The manuscript has been writing with very good language the paper can be a very significant addition to the library of the field. Also, the used references are relevant and up to date. The only minor issue with this work that there is no clear discussion section
Author Response
GENERAL COMMENTS
A revision has been carried out following carefully all the indications, suggestions and observations formulated by the reviewers.
The main operated changes are outlined next:
1) As suggested, an overall revision of the text was carried out in order to correct some small mistakes all along the work. A spell check and editing of the English language and style was also carefully performed. Moreover, several longer phrases and have been modified for a clearer and easier reading.
2) The abstract was modified to some extent, slightly reformulating some words, or word groups, and inserting a new phrase highlighting the most important achievements of this paper. Also, the reference to the first part of the study (ref. [1]) was moved from the abstract’s body to the Introduction.
3) The Introduction was extensively modified. Five new paragraphs were inserted: the first three new paragraphs were added at the beginning, in order to make a connection to the first article from this series of two, and for a more logical transition into the discussion of the paper at hand; the last two paragraphs aim to present a very short presentation of related research – however, please take note that an exhaustive literature review of the paper’s topic is already available in the first article (ref. [1]).
4) The Conclusions were almost entirely re-written. Following the suggestions of the reviewers, greater emphasis was placed on summarizing the achievements, and the presented information is now significantly condensed.
At this point, it has to be also underlined that the authors tried in a very honest way to follow all the suggestions and observations formulated by the reviewers and to operate (as much as it was possible) all the corrections indicated by them. Furthermore, in order to follow the corrections operated in the paper, a version of the manuscript having all the changes operated tracked (with the option track changes) has been also uploaded together with the last form of the manuscript (without the changes tracked).
Finally, the authors would like to sincerely express their gratitude to the reviewers for their valuable comments that obviously helped in improving the quality of the present work.
The specific corrections operated according to the suggestions of the reviewers are given next together with detailed explanations.
Reviewer 2
Comment 1:
The only minor issue with this work [] there is no clear discussion section.
Thank you for this observation. We did not create a dedicated section, but we have reformulated the Conclusions extensively, taking also your comment into account.
The revised text now reads:
The complexity of physical and chemical transformations taking place within biomass particles undergoing thermal decomposition is undeniably very high. Reproducing these processes with sufficient accuracy using numerical modelling and simulation requires a comparably detailed approach. Achieving the right balance between model sophistication and the associated computational effort is of great concern, also. Nevertheless, the modelling procedure presented herein is quite successful at reproducing the observable behaviour of relatively large, heavily thermally loaded, biomass particles; and it does that with reasonable costs.
The implementation of the mathematical model for biomass decomposition presented in the first part of the study is performed taking advantage of the extensive capabilities of a commercial CFD software platform. The User-Defined Function is formulated such that the communication between the chemical-kinetic model and the CFD numerical model is bi-directional. Using dedicated subroutines, the program performs calculations for (1) the mass and energy sources for all mass transfer processes associated to the thermal decomposition of biomass and (2) the material properties and those of transport in solid volume, extracting all necessary numerical data from the CFD solver. The calculation of heat & mass transfer, volatiles transport, and combustion, if conditions allow, char burn, etc., are all perfomed within the solver. Of all numerical tasks, the integration of chemical reaction mechanisms for gaseous species combustion seems to be the limiting factor in terms of computational performance.
The comparison of numerical results and experimental data used for validation is very good. The models qualitatively and quantitatively reproduce the experiments conducted by Lu [4] for a series of 9.5 mm cylindrical biomass samples (poplar), thermally decomposed in an oven specially designed and manufactured for this purpose. Both neutral and oxidizing conditions are successfully validated, at low and high initial moisture content, too.
The global performance of the biomass thermal decomposition model developed in this research is remarkable, especially considering the fact that it relies on a relatively simple chemical-kinetic scheme. The numerical model can be successfully used not only for accurate estimation of conversion process duration, either pyrolysis or combustion, or for estimating residual mass (i.e. char), but it can be applied as well for determining combustion temperatures in both key phases (volatiles burning, followed by char burning). Given the achieved level of accuracy, we can assume that the chemical composition of burnt gases might be properly predicted too, but this remains to be confirmed in future research.
The most important conclusion drawn from the analysis of all numerical data and their correspondence with experimental measurements is the following: in some cases it may be possible to apply one-dimensional or two-dimensional simplified models to obtain acceptable results in general engineering design; but an accurate and thorough research of many apparently simple cases can be done only by using three-dimensional, unsteady numerical simulation methods, that must be properly formulated physically and chemically as detailed and close to real phenomena as possible. Achieving high accuracy in numerical modelling cannot be done just by concentrating on aspects of physical and chemical transformations of biomass during thermal decomposition; the complete modelling of heat transport and, especially, mass transport, are found to be equally important.
Author Response File: 
Author Response.docx
Reviewer 3 Report
The manuscript is about A Numerical Model of Biomass Combustion Physical and Chemical Processes
As for the characteristics of biomass, the literature review was limited, the authors should take into account similar studies that have been carried out in the discussion. The authors described the subject of biomass in a limited way. I recommend that you refer to biomass in more chemical aspects, I recommend reading the article:
Roman, K.; Barwicki, J.; Hryniewicz, M.; Szadkowska, D.; Szadkowski, J. Production of Electricity and Heat from Biomass Wastes Using a Converted Aircraft Turbine AI-20. Processes 2021, 9, 364. https://doi.org/10.3390/pr9020364
For example in above article the PY-GC-MS of straw was shown.
Author Response
GENERAL COMMENTS
A revision has been carried out following carefully all the indications, suggestions and observations formulated by the reviewers.
The main operated changes are outlined next:
1) As suggested, an overall revision of the text was carried out in order to correct some small mistakes all along the work. A spell check and editing of the English language and style was also carefully performed. Moreover, several longer phrases and have been modified for a clearer and easier reading.
2) The abstract was modified to some extent, slightly reformulating some words, or word groups, and inserting a new phrase highlighting the most important achievements of this paper. Also, the reference to the first part of the study (ref. [1]) was moved from the abstract’s body to the Introduction.
3) The Introduction was extensively modified. Five new paragraphs were inserted: the first three new paragraphs were added at the beginning, in order to make a connection to the first article from this series of two, and for a more logical transition into the discussion of the paper at hand; the last two paragraphs aim to present a very short presentation of related research – however, please take note that an exhaustive literature review of the paper’s topic is already available in the first article (ref. [1]).
4) The Conclusions were almost entirely re-written. Following the suggestions of the reviewers, greater emphasis was placed on summarizing the achievements, and the presented information is now significantly condensed.
At this point, it has to be also underlined that the authors tried in a very honest way to follow all the suggestions and observations formulated by the reviewers and to operate (as much as it was possible) all the corrections indicated by them. Furthermore, in order to follow the corrections operated in the paper, a version of the manuscript having all the changes operated tracked (with the option track changes) has been also uploaded together with the last form of the manuscript (without the changes tracked).
Finally, the authors would like to sincerely express their gratitude to the reviewers for their valuable comments that obviously helped in improving the quality of the present work.
The specific corrections operated according to the suggestions of the reviewers are given next together with detailed explanations.
Reviewer 3
Comment 1:
As for the characteristics of biomass, the literature review was limited, the authors should take into account similar studies that have been carried out in the discussion. The authors described the subject of biomass in a limited way.
Thank you for bringing this to our attention. We have made extensive modifications to the Introduction, for a better understanding of the context and link the paper with the first article of the series (ref. [1]). However, we do respectfully ask the Reviewer to accept that we did not elaborate too much in the direction of the current state-of-the-art, since that was done exhaustively in the first article. The two papers are clearly meant to be studied together, and the paper being reviewed does make complete sense only in conjunction with the previous one; therefore, we feel that re-discussing this aspect would have been redundant and possibly annoying for the reader.
At the same time, we also we do respectfully ask the Reviewer to understand that the scope of the paper under review is strongly focused on presenting the numerical implementation and validation of the mathematical model of biomass thermal decomposition described in the previous article (ref. [1]), specifically targeted at combustion simulation applications. This is a rather particular aspect of the biomass subject.
The revised text now reads:
The work presented herein is entirely founded on the mathematical model of biomass combustion physical and chemical processes explained in great detail in [1]. For a more complete understanding and easier assessment of this effort, we strongly encourage the reader to take it into consideration.
The main goal of this study (the previous and the current paper, together) is to present a possible solution for modeling and simulating biomass thermal decomposition regardless of particle size and shape, heating rate, and neutral or oxidizing environment conditions. Moreover, it is formulated such that any biomass source can be used, provided it can be described using its main constituents (fractions of hemicellulose, cellulose, lignin, minerals and moisture).
The first paper detailed the complete path starting from the mathematical formulation of the kinetic-chemical model for biomass transformation, and the physical modeling of biomass properties. It concluded by showing the outcome of the decomposition model, in the form of transfer functions to be implemented in the three-dimensional Computational Fluid Dynamics (CFD) model – the static modeling phase.
This second paper presents the subsequent dynamic modeling phase, involving the actual numerical simulation of biomass thermal decomposition and volatiles & char combustion, as well as its verification and validation.
…
The experimental data published by Lu [4] was chosen for assessing the overall modeling performance and accuracy. Other researchers [5-15] have attempted to model and numerically simulate the decomposition of biomass particles, either in isolation or in the form of particle bed, using various approaches. Generally though, the conclusion was that heat & mass transfer dominated processes – large particles, high heating rates – proved significantly more difficult to simulate. Dimensionally-reduced models also have difficulties in coping with more complex situations, particularly due to the importance of three-dimensional effects and asymmetries.
Comment 2:
The authors described the subject of biomass in a limited way. I recommend that you refer to biomass in more chemical aspects, I recommend reading the article:
Roman, K.; Barwicki, J.; Hryniewicz, M.; Szadkowska, D.; Szadkowski, J. Production of Electricity and Heat from Biomass Wastes Using a Converted Aircraft Turbine AI-20. Processes 2021, 9, 364.
Thank you for this observation. We followed your suggestion and read the recommended article. We also included it in the References (ref. [17]), and in the paper’s body.
The included text reads (Introduction):
Many research efforts (including Lu’s) are limited to mass loss and temperature measurements at particular points. Consequently, no validation of volatiles composition could be performed yet. Nevertheless, an interesting and well-detailed example of the effects of using micronized biomass particles on the quality of the volatiles under pyrolitic conditions is given in [17].
As for the biomass chemistry, we do agree it is a fundamental aspect. Elements of biomass chemistry, relevant to the processes being modeled and simulated – again, targeted at combustion simulation – were included in the first article (ref. [1]). However, a detailed and complete description of biomass from a chemical perspective is outside the scope of both papers.
Author Response File: Author Response.docx
Reviewer 4 Report
Do not mention citation in the abstract [1]. Provide it in the introduction part. (Linguistic ) User-defined function (L21), Coupled with (L21), check the spelling of modelling and behaviour. (L395) occurs
How did you measure the Flame temperature? There is no information about it. How did the authors measure L/D experimentally? What would be mutual effect of devolatisation on the moisture removal?
Conclusion is lengthy. Kindly shorten it.
Author Response
GENERAL COMMENTS
A revision has been carried out following carefully all the indications, suggestions and observations formulated by the reviewers.
The main operated changes are outlined next:
1) As suggested, an overall revision of the text was carried out in order to correct some small mistakes all along the work. A spell check and editing of the English language and style was also carefully performed. Moreover, several longer phrases and have been modified for a clearer and easier reading.
2) The abstract was modified to some extent, slightly reformulating some words, or word groups, and inserting a new phrase highlighting the most important achievements of this paper. Also, the reference to the first part of the study (ref. [1]) was moved from the abstract’s body to the Introduction.
3) The Introduction was extensively modified. Five new paragraphs were inserted: the first three new paragraphs were added at the beginning, in order to make a connection to the first article from this series of two, and for a more logical transition into the discussion of the paper at hand; the last two paragraphs aim to present a very short presentation of related research – however, please take note that an exhaustive literature review of the paper’s topic is already available in the first article (ref. [1]).
4) The Conclusions were almost entirely re-written. Following the suggestions of the reviewers, greater emphasis was placed on summarizing the achievements, and the presented information is now significantly condensed.
At this point, it has to be also underlined that the authors tried in a very honest way to follow all the suggestions and observations formulated by the reviewers and to operate (as much as it was possible) all the corrections indicated by them. Furthermore, in order to follow the corrections operated in the paper, a version of the manuscript having all the changes operated tracked (with the option track changes) has been also uploaded together with the last form of the manuscript (without the changes tracked).
Finally, the authors would like to sincerely express their gratitude to the reviewers for their valuable comments that obviously helped in improving the quality of the present work.
The specific corrections operated according to the suggestions of the reviewers are given next together with detailed explanations.
Reviewer 4
Comment 1:
Do not mention citation in the abstract [1]. Provide it in the introduction part. (Linguistic) User-defined function (L21), Coupled with (L21), check the spelling of modelling and behaviour. (L395) occurs.
Thank you for bringing all this to our attention. The citation was removed from the abstract and included in the Introduction instead. All spelling errors were corrected, including “modeling” and “behavior” – too many to list here.
Comment 2:
How did you measure the Flame temperature? There is no information about it.
If your question addresses the experimental data, we had no contribution to that. The author of the doctoral thesis (Lu [4]) used both thermocouples and thermal imaging.
If your question addresses the numerical data, it is obvious we extracted the values directly from the numerical results, using specific post-processing techniques.
Comment 3:
How did the authors measure L/D experimentally?
We assume that Lu [4] has done that directly, given the fact that the particles were actually large enough to accurately perform the measurements by hand (diameter = 9.5 mm).
Comment 4:
What would be mutual effect of devolatization on the moisture removal?
As described in ref. [1], the mathematical derivation of the model is based on the superposition principle, i.e. it is assumed that there is no direct influence of a biomass component evolution on another. However, this is somewhat disregarded in the actual implementation of the numerical model, because all processes (heat & mass transfer, flow, reactions – both inside and outside the particle) are tightly coupled in the CFD model and solved simultaneously. Therefore, some mutual influence would take place; for instance, inside the particle, the vaporization of moisture would heavily keep local temperature under control, delaying the onset of CL/HCL/LG decomposition. Other than that, there isn’t much else though, since moisture and volatiles do evolve within significantly different temperature intervals.
Comment 4:
Conclusion is lengthy. Kindly shorten it.
Thank you for your suggestion. We have reformulated the Conclusions almost entirely, significantly reducing its length in the process.
The revised text now reads:
The complexity of physical and chemical transformations taking place within biomass particles undergoing thermal decomposition is undeniably very high. Reproducing these processes with sufficient accuracy using numerical modelling and simulation requires a comparably detailed approach. Achieving the right balance between model sophistication and the associated computational effort is of great concern, also. Nevertheless, the modelling procedure presented herein is quite successful at reproducing the observable behaviour of relatively large, heavily thermally loaded, biomass particles; and it does that with reasonable costs.
The implementation of the mathematical model for biomass decomposition presented in the first part of the study is performed taking advantage of the extensive capabilities of a commercial CFD software platform. The User-Defined Function is formulated such that the communication between the chemical-kinetic model and the CFD numerical model is bi-directional. Using dedicated subroutines, the program performs calculations for (1) the mass and energy sources for all mass transfer processes associated to the thermal decomposition of biomass and (2) the material properties and those of transport in solid volume, extracting all necessary numerical data from the CFD solver. The calculation of heat & mass transfer, volatiles transport, and combustion, if conditions allow, char burn, etc., are all perfomed within the solver. Of all numerical tasks, the integration of chemical reaction mechanisms for gaseous species combustion seems to be the limiting factor in terms of computational performance.
The comparison of numerical results and experimental data used for validation is very good. The models qualitatively and quantitatively reproduce the experiments conducted by Lu [4] for a series of 9.5 mm cylindrical biomass samples (poplar), thermally decomposed in an oven specially designed and manufactured for this purpose. Both neutral and oxidizing conditions are successfully validated, at low and high initial moisture content, too.
The global performance of the biomass thermal decomposition model developed in this research is remarkable, especially considering the fact that it relies on a relatively simple chemical-kinetic scheme. The numerical model can be successfully used not only for accurate estimation of conversion process duration, either pyrolysis or combustion, or for estimating residual mass (i.e. char), but it can be applied as well for determining combustion temperatures in both key phases (volatiles burning, followed by char burning). Given the achieved level of accuracy, we can assume that the chemical composition of burnt gases might be properly predicted too, but this remains to be confirmed in future research.
The most important conclusion drawn from the analysis of all numerical data and their correspondence with experimental measurements is the following: in some cases it may be possible to apply one-dimensional or two-dimensional simplified models to obtain acceptable results in general engineering design; but an accurate and thorough research of many apparently simple cases can be done only by using three-dimensional, unsteady numerical simulation methods, that must be properly formulated physically and chemically as detailed and close to real phenomena as possible. Achieving high accuracy in numerical modelling cannot be done just by concentrating on aspects of physical and chemical transformations of biomass during thermal decomposition; the complete modelling of heat transport and, especially, mass transport, are found to be equally important.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The manuscript is properly prepared and I have no comments. The content of the manuscript is very interesting and contains a lot of information about FEM. I recommend publishing the manuscript.
Reviewer 4 Report
It can be considered.
