Application of Thermodynamic Methods to the Study of Plant Biomass and Its Components—A Review
Round 1
Reviewer 1 Report
Comments and Suggestions for Authorsthe review article is very well written, most of the points were addressed which were related to the topic and Thermodynamic Methods. However, i believe few points are required to be highlighted for the improvement of this review article:
Keywords: please rewrite the keywords in alphabetic order
Suggestion 1: please expend the explanation on factors in Thermodynamic Methods
suggestion 2: I believe it is important to mention the factors that can degrade cellulose in Thermodynamic Methods ( Temperature, concentration and duration of the experiment)? please provide some information to it.
Author Response
The answer of the author on comments of reviewer 1
The review article is very well written, most of the points were addressed which were related to the topic and Thermodynamic Methods. However, i believe a few points are required to be highlighted for the improvement of this review article:
Keywords: please rewrite the keywords in alphabetic order. Answer: I did it.
Suggestion 1: please expand the explanation of factors in Thermodynamic Methods. Answer: I have expanded the explanation of various factors that influence the thermodynamic functions. (See Lines 187-197).
Suggestion 2: I believe it is important to mention the factors that can degrade cellulose in Thermodynamic Methods (Temperature, concentration and duration of the experiment)? please provide some information to it. Answer: These factors (sample amount, temperature, oxygen pressure, etc.) were mentioned for example in the description of the sample combustion in bomb calorimeters (see Lines 254-264). Temperature, process durability, and other factors of the anaerobic decomposition of biomass by methanogenic bacteria were described (see Lines 508-539). In additions factors influencing the process of enzymatic hydrolysis of pretreated biomass or cellulose were discussed (see Lines 540, etc.)
Reviewer 2 Report
Comments and Suggestions for AuthorsThe comments were included in an attached file.
Comments for author File: 
Comments.pdf
Author Response
Answer of the author on comments of reviewer 2
After reading the article, I find the title rather enigmatic in two respects: 1. The application of thermodynamic methods. 2. The phrase Abstract In this section, the Author outlines what the article presents. However, one may ask: is this truly a review? The energy potential of various biomass types, as well as biomass-based solid, liquid, and gaseous biofuels, is determined. A method of additive contributions of combustion enthalpies of main components is proposed. It has also been established that the thermal energy potential of the initial biomass is higher than the energy potential of secondary biofuels derived from it. The thermodynamic functions of plant biopolymers are calculated. Moreover, the thermodynamic stability of various crystalline allomorphs of cellulose and amorphous cellulose is studied. The melting enthalpies of crystallites with different crystalline structures are estimated, and a thermochemical method for determining cellulose crystallinity is proposed. In the manuscript, the Author expresses the potential of biomass evaluation through thermochemical equations and transformations of various energy forms during chemical reactions and phase transitions. The Author summarizes his previous works and the methods used to study biomass potential and its changes during processing (e.g., thermochemical reactions). However, it seems this review heavily relies on the Author's previous studies.
Answer: This seems to be an exaggerated judgment. Although this review article contains some results of my and my co-authors' research, their share, according to the journal's editorial board, does not exceed 18%. Thus, although I used references to my research, over 80% of the review article pertains to data from other researchers.
Objective of the Review The purpose of the review is stated in Chapter 2: "This review article will show how thermochemical methods can be applied to the study of plant biomass and its main components, cellulose, hemicelluloses, lignin, etc. Considerable attention will be paid to the thermodynamics of physicochemical and chemical transformations of cellulose and nanocellulose, as well as other components of plant biomass. The thermodynamics of second solid, liquid, and gaseous biofuels released from the initial biomass was also considered." However, is "thermochemical" equivalent to "thermodynamic"?
Answer: At the beginning of the introduction, it was noted that thermochemical" and "thermodynamic” terms are not equivalent. Thermodynamics is the most general concept the main tasks of which are to determine thermodynamic functions, study the transformations of various forms of energy during chemical reactions and phase transitions, predict the feasibility and direction of chemical processes, and study the ability of chemical systems to perform useful work. In contrast to this general concept, the task of thermochemistry is specific, namely the study of the thermal effects of reactions.
Moreover, are methods like X-ray or scanning electron microscopy truly thermodynamic methods? This chapter contains relatively little about the actual methods.
Answer: It is clear that these methods are not thermodynamic. However, at present it is impossible to conduct a full and reliable study using only one method. For example, without using these structural methods it is difficult to understand whether a plant polymer is crystalline, semi-crystalline, or amorphous. And what proportion of crystalline and amorphous material does the biomass itself contain? In addition, without data on structural methods, it is difficult to calculate the thermodynamics of reactions. For example, if according to the data of structural studies, the cellulose sample has an amorphous structure, then to calculate the thermodynamics of its hydrolysis it is necessary to use the thermodynamic functions corresponding to the amorphous state. But if the sample is partially crystalline, then the calculations should take into account the thermodynamic functions both of amorphous and crystalline domains.
A more substantial introduction and justification, as seen at the beginning of Chapter 3, would be helpful. For instance, outlining the purpose of the work, its novelty, and originality. What does this study contribute to scientific knowledge and industrial practices? What does it add to the Author's previous works? How might industrial practice utilize this knowledge?
Answer: I have taken this comment into account and added to the introduction an explanation of why it is so important to use thermodynamic methods to study biomass and its components and what practical applications this may have.
The Author suggests that the total content of thermal energy is highest in raw (intact) biomass but also points out the need for material purification or densification. Of course, the specific expectations significantly influence this assessment. Regarding Figure 5, what is the sense of presenting Energy yield without considering other factors? The Author speculates that it may be better to burn raw material instead of processing it into secondary products. This is like to recommending eating a salad as a whole instead of its individual ingredients. On the one hand, the Author proposes the "rule of additive contributions," while on the other, he claims raw biomass is more exothermic than its derived products. Does this assessment consider the potential energy yield of by-products (e.g., meal from oil extraction or digestate from anaerobic digestion)?
Answer: If compare the energy output, these calculations are accurate. They take into account the energy potential of both individual types of biofuels and the total energy potential of various types of biofuels. I gave the following example in the manuscript section “Thermal Energy Potential of Secondary Biofuels”. Using slow pyrolysis of 100 kg woody biomass, the following yield of secondary biofuel was determined: 30 kg biochar, 18.1 kg bio-oil, and 14 m3 biogas [59]. It can be calculated that the total content of thermal energy (TEC) of such biofuel will be TEC = - 1.24 GJ, which is only 62% of the thermal energy content in 100 kg initial biomass equal to -2 GJ.
Check these calculations. The experimental calorific value (CV) of wood is -20 MJ/kg; thus is -2 GJ. CV of biochar is ca -27 MJ/kg; thus 30 kg has TEC= -0.81 GJ. Bio-oil has a maximum CV ca -16 MJ/kg; thus, CV of 18.1 bio-oil is -0.29 GJ. Biogas has a maximum CV ca -10 MJ/m3; thus, CV of 14 m3 biogas is -0.14 GJ. The total content of thermal energy of all formed biofuels is -(0.81+0.29+014) = -1.24 GJ which is 62% of TEC = -2 GJ of 100 kg biomass.
The reason for the low energy yield of secondary biofuels is, firstly, due to the low mass yield of a particular type of biofuel, of no more than 30-40%, and secondly, that a bio-fuel has non-combustible components that reduce its calorific value. For example, bio-oil contains such non-combustible components as water, acetic and formic acid, and phenols; while biogas contains carbon dioxide and often nitrogen. In addition, just as in physics, the efficiency of various devices is always below 100%, so in thermochemistry, you cannot transfer 100% of the energy of the initial biomass to secondary products.
Regarding “This is like to recommending eating a salad as a whole instead of its individual ingredients. On the one hand, the Author proposes the "rule of additive contributions," while on the other, he claims raw biomass is more exothermic than its derived products”. Answer: This example is incorrect because biomass does not consist of secondary bio-products obtained by pyrolysis, i.e., it is not a mixture of these secondary substances. You cannot mix char, bio-oil, and biogas and get whole biomass back. However, since biomass contains cellulose, lignin, and hemicelluloses its energy potential can be calculated as the sum of the energy contributions of these three main components.
Methodology and Data Presentation As this is a review article without original experimental methodology, tables and figures should relate to data from cited sources. Proper citations should be provided.
Answer: If you look closely at the tables and figures, you will see that they have corresponding references. Some illustrations were prepared by the author specifically for this review. In this case, of course, there are no references.
Discussion The discussion contains several generalizations that the Author highlights but which are later repeated in the conclusions. Additionally the Author states "Values of standard thermodynamic functions (TDFs) for cellulose and other components of plant biomass can also be found in scientific literature. In addition, standard TDFs for most reagents and reaction products are given in handbooks on chemical thermodynamics." In my opinion, this chapter does not effectively address the application of thermodynamic methods as the title suggests. Does the discussion truly align with the article's topic? In my opinion, not entirely.
Answer: The Discussion section has been revised and expanded. Unresolved issues and promising directions that should be pursued in further research have been noted.
Conclusions They can be shortened. The same is comprised in the Discussion section It is difficult to agree with the conclusion that, based solely on energy yield, it is best to burn raw material, possibly in a densified form. Why not produce gas and use by-products for biofuel or gas production in a second stage? Interaction effects are also significant. Regarding Conclusion 3, proposing a new model for evaluating cellulose crystallinity was not a stated objective of this work, making the title enigmatic. Note: Thermonuclear fusion is considered an exothermic reaction, but its efficiency depends on achieving very high temperatures. Final Remarks The paper falls under the scope of the Applied Biosciences. Though the Author took care on many aspects both scientific and editorial some issues need to be respected before publishing. The revised text should ensures coherence and removes ambiguity, while maintaining a critical tone and clear observations.
Answer: The conclusions were revised. The points that “it is best to burn the raw material, possibly in a compacted form” etc., and “new model for evaluating cellulose crystallinity” were removed. The importance of using some types of biofuels was emphasized.
Reviewer 3 Report
Comments and Suggestions for AuthorsComments
The review manuscript titled “Application of Thermodynamic Methods to the Study of Plant Biomass and its Components– A Review” by Michael describes the application of thermodynamics to study the cellulose, hemicellulose and lignin contents of plant biomass. Although, the manuscript has presented an interesting topic, there are some major concerns that deserve thorough attention before publication. The overall language of the manuscript is a kind of weak and has a scope for further improvements. Thus, authors are advised to improve the manuscript’s readability, merits as well as its understanding for the journal readers.
1. The introduction section of the manuscript is very long and deviating from the main topic. Here the authors are advised to highlight the novelty of the manuscript by precisely stating how thermodynamics is important for bioconversion of plant biomass into biofuels or other related products.
2. Some statements are very long and confusing. Revise for clarity. E.g. Lines 190, 214 and so on.
3. The sections 1 and 2 are very long, authors can merge it and present it precisely while giving the problem statement and aims/objectives of the manuscript at the end of this section.
4. Making paragraphs of 2 or 3 sentences is often annoying and distracting the attention of the readers from the main goal of the paper. Hence, authors should reorganize the manuscript in a systematic order to appeal the readers.
5. Many statements are superfluous and vague not relevant to plant biomass. E.g., 225-228.
6. Line 275; What is DTA. Please give full forms for abbreviations on first mention.
7. What methodology was used for the compilation of the review topic?
8. Line 280: Is this manuscript really dealing with nanocellulose? Till now I cannot see any description on nanocellulose.
9. Lines 456-461: please give citations to the statements.
10. Line 673: Numbering needed for the subheading.
11. Line 737: Revise and correct the statement.
12. The challenges of the available methods for thermodynamic of plant biomass is lacking in the manuscript.
13. Authors should also add a section on, Future perspectives” for the thermodynamics of plant biomass.
14. Authors can present the conclusions very briefly with a particular focus on thermodynamics of lignocellulosic biomass into biofuels.
15. Many references are outdated. E.g., 4, 5, 92, 93, and so on.
Comments on the Quality of English LanguageComments
The review manuscript titled “Application of Thermodynamic Methods to the Study of Plant Biomass and its Components– A Review” by Michael describes the application of thermodynamics to study the cellulose, hemicellulose and lignin contents of plant biomass. Although, the manuscript has presented an interesting topic, there are some major concerns that deserve thorough attention before publication. The overall language of the manuscript is a kind of weak and has a scope for further improvements. Thus, authors are advised to improve the manuscript’s readability, merits as well as its understanding for the journal readers.
1. The introduction section of the manuscript is very long and deviating from the main topic. Here the authors are advised to highlight the novelty of the manuscript by precisely stating how thermodynamics is important for bioconversion of plant biomass into biofuels or other related products.
2. Some statements are very long and confusing. Revise for clarity. E.g. Lines 190, 214 and so on.
3. The sections 1 and 2 are very long, authors can merge it and present it precisely while giving the problem statement and aims/objectives of the manuscript at the end of this section.
4. Making paragraphs of 2 or 3 sentences is often annoying and distracting the attention of the readers from the main goal of the paper. Hence, authors should reorganize the manuscript in a systematic order to appeal the readers.
5. Many statements are superfluous and vague not relevant to plant biomass. E.g., 225-228.
6. Line 275; What is DTA. Please give full forms for abbreviations on first mention.
7. What methodology was used for the compilation of the review topic?
8. Line 280: Is this manuscript really dealing with nanocellulose? Till now I cannot see any description on nanocellulose.
9. Lines 456-461: please give citations to the statements.
10. Line 673: Numbering needed for the subheading.
11. Line 737: Revise and correct the statement.
12. The challenges of the available methods for thermodynamic of plant biomass is lacking in the manuscript.
13. Authors should also add a section on, Future perspectives” for the thermodynamics of plant biomass.
14. Authors can present the conclusions very briefly with a particular focus on thermodynamics of lignocellulosic biomass into biofuels.
15. Many references are outdated. E.g., 4, 5, 92, 93, and so on.
Author Response
The answer of the author on comments of Reviewer 3
Reviewer 3.
The review manuscript titled “Application of Thermodynamic Methods to the Study of Plant Biomass and its Components– A Review” by Michael describes the application of thermodynamics to study the cellulose, hemicellulose and lignin contents of plant biomass. Although, the manuscript has presented an interesting topic, there are some major concerns that deserve thorough attention before publication. The overall language of the manuscript is a kind of weak and has a scope for further improvements. Thus, authors are advised to improve the manuscript’s readability, merits as well as its understanding for the journal readers.
- The introduction section of the manuscript is very long and deviating from the main topic. Here the authors are advised to highlight the novelty of the manuscript by precisely stating how thermodynamics is important for bioconversion of plant biomass into biofuels or other related products. Answer: The importance of thermodynamic methods for studying biomass and its bioconversion was added to the introduction.
- Some statements are very long and confusing. Revise for clarity. E.g. Lines 190, 214 and so on. Answer: Lines 190 & 214 are removed
- The sections 1 and 2 are very long, authors can merge it and present it precisely while giving the problem statement and aims/objectives of the manuscript at the end of this section. Answer: I agree. These sections were merged.
- Making paragraphs of 2 or 3 sentences is often annoying and distracting the attention of the readers from the main goal of the paper. Hence, authors should reorganize the manuscript in a systematic order to appeal the readers. Answer: The number of paragraphs has been reduced.
- Many statements are superfluous and vague not relevant to plant biomass. E.g., 225-228. Answer: Lines 225-228 are removed
- Line 275; What is DTA. Please give full forms for abbreviations on first mention. Answer: The full name of this method was introduced
- What methodology was used for the compilation of the review topic?
Answer: The methodology was the following. From the topic, it follows that the objects of this review are biomass and its main components - cellulose, lignin, hemicelluloses, and other polysaccharides, as well as some other components (lipids, resins, etc.). Therefore, at the beginning, information was presented on the thermochemical characteristics of various types of biomasses and their dependence on the chemical composition. Since plant biomass is also the raw material for secondary biofuels, the analysis of the thermal energy potential of various biomass-based biofuels was performed. Further, an overview of the thermodynamic properties of the main components of biomass isolated from it - cellulose, lignin, hemicelluloses, etc. was given. Since cellulose is the most common natural polymer, which also has wide practical applications, a thermodynamic analysis of various cellulose reactions such as enzymatic hydrolysis, production of cellulosic ethanol, etherification, esterification, etc was carried out and discussed.
- Line 280: Is this manuscript really dealing with nanocellulose? Till now I cannot see any description on nanocellulose. Answer: Yes, in section 5.1, the phenomena of aggregation of nano-scale cellulose and recrystallization of small nano-crystallites III into larger crystallites IV and some other processes were discussed.
- Lines 456-461: please give citations to the statements. Answer: The reference was introduced.
- Line 673: Numbering needed for the subheading. Answer: Subheadings were removed.
- Line 737: Revise and correct the statement. Answer: It is taken into account.
- The challenges of the available methods for the thermodynamics of plant biomass is lacking in the manuscript.
Answer: The problems of the thermodynamic methods are noted.
- Authors should also add a section on, Future perspectives” for the thermodynamics of plant biomass. Answer: Future perspectives were added to Discussions
- Authors can present the conclusions very briefly with a particular focus on the thermodynamics of lignocellulosic biomass into biofuels. Answer: The conclusions were improved.
- Many references are outdated. E.g., 4, 5, 92, 93, and so on.
Answer: Although these articles are old, they are reliable and exemplary. Some of them (e.g., the article of Kelley et al.) are considered classics. A typical example is Scherrer's article from the early 20th century on the X-ray method for determining the size of crystallites; most modern publications use this article, although it was published 95 years ago. In most modern publications, especially overviews, it is customary to refer to old but reliable works to avoid being accused of appropriating priority. Therefore, I also referred to these old publications.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsAuthors have made substantial modifications to the revision manuscript and addressed all of the reviewers concerns satisfactorily. Hence i recommend acceptance of the manuscript for possible publication.
