Waste to Carbon: Biocoal from Elephant Dung as New Cooking Fuel

Round 1

Reviewer 1 Report

As a general comment, the work is important once biomass is a promising alternative to fossil fuels in some cases, and a sustainable way of producing energy from it should be endeavoured. Nevertheless, the novelty of the work, apart from being pointed out as the first to valorise elephant dung, should be clarified and enforced from the circular economy point-of-view.

Also, more dedicated suggestions follow:

1- There is a good command of the English language, but some typos and grammar flaws must be revised, example: lines 20-21 (lack of verb to be), line 112 (folled), line 485 (endodermal), line 488 (double requires). Maybe the word “dung” could be sometimes replaced by “manure” or other synonym, in order to comply with the related literature.

2- the specific heat model referred in Figure 2 should be added at least as supplementary information or annex.

3- the results need a deeper discussion as the exposed arguments are a bit weak. Only comparing with other literature values is not enough, scientific justifications are required.

4- phrases about the statistical differences (or lack of them) are consecutively repeated.

5- the HHV of the torrefied dung is not much higher than the raw sample. This should be better analysed and explained. Maybe a different set of experimental conditions should be tested.

Author Response

Dear Reviewer:

Thanks for your comments concerning our manuscript entitled “Waste to Carbon: biochar from elephant dung as new cooking fuel”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guidance to our on-going and future research. We have studied comments carefully and have made corrections. Revised text is highlighted using "Track Changes" function in the manuscript. The point-to-point response to your comments is listed in the table below. We appreciate the opportunity to respond to your constructive feedback. Our responses to your comments are in attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

All the experiments are performed and explained well with proper citation of previous works. I have main concern about the feasibility and scale up of this process as there is only countable number of elephants through out the world and number of elephants per unit area is very low. Other concern about the process, is it make any economic benefit as dung torrefaction process also require energy. Production of one fuel at the expense of other fuel don't seems an economic approach. Need to justify these in introduction.

Author Response

Dear Reviewer:

Thanks for your comments concerning our manuscript entitled “Waste to Carbon: biochar from elephant dung as new cooking fuel”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guidance to our on-going and future research. We have studied comments carefully and have made corrections. Revised text is highlighted using "Track Changes" function in the manuscript. The point-to-point response to your comments is listed in the table below. We appreciate the opportunity to respond to your constructive feedback. Our responses to your comments are in attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper presents an interesting study of the characteristics of a sample of elephant dung after torrefaction. The mass yield, the energy densification ratio and the energy yield of the biomass, also with the temperature and time used in the process were used as the main parameters to compare the different characteristics of the sample before and after the thermal degradation. In addition, the kinetics parameters were obtained by thermogravimetric analysis (TGA) and the exothermicity of the overall process was assessed by Differential Scanning Calorimetry (DSC). Although this paper contributes some novel knowledge to the related field, it needs some deep improvements before acceptance for publication. The major comments are as follows:

(1) English editing must be extensive improved. In some parts of the manuscript, it is difficult to follow what the authors want exactly to say. Some corrections are proposed by this reviewer, but there are much more that need revision.

(2) Line 20: "dung could processed" must be "dung could be processed".

(3) Line 27: "three durations of the..." must be expressed in a different way.

(4) Line 30: "kinetics" must be "kinetics assessment".

(5) Line 31: replace "use" by "it could be used".

(6) Line 32: specify some key fuel properties.

(7) Line 46: replace "continent" by "continent they can be found".

(8) Lines 49-52: it seems that you are proposing to valorize all elephant dung in the world. Be concise here. Can you add a reference to support the dung production?

(9) Lines 70-76: explain better. It is difficult to understand the idea that the authors want to describe.

(10) Line 80: Why do you know that 30km is a critical distance? Please add a reference here.

(11) Line 82: "disposal)" must be "disposal".

(12) Lines 92-94: you are describing some results of your study. In the introduction section this must be avoided.

(13) Line 95: It is not clear.

(14) Line 101: "method allowing" must be "method".

(15) Line 108: write the hypothesis but not in a question form (see previous lines).

(16) Line 112: folled??

(17) Line 113: Why 0.425 mm? Add a reference here.

(17) Line 121: Don´t repeat the torrefaction time again.

(18) Line 122: "for torrefaction" must be "from torrefaction".

(19) Line 123: replace "Scheme" by "A scheme". This sentence must be placed at the beginning of the methods description.

(20) Line 131: In Fig. 2 it seems that proximate analysis were performed only for torrefacted biomass.

(21) Line 144: replace "kinetics" by "the kinetics".

(22) Line 145: a brief explain about the tests and calculations is needed.

(23) Lines 146, 148 and 157: replace "was" by "were".

(24) Line 149: "was made" is not a correct expression here.

(25) Line 156: replace "Thermal analysis" by "DSC thermal analysis" (there are more thermal processes in this study).

(26) Line 187: replace "dry ash-free bases" by "dry and ash-free base".

(27) Line 213: replace "is" by "are". In addition, this sentence is not clear.

(28) Lines 215-217: the redaction is not clear.

(29) Line 223: 80ºC, at which pressure?

(30) Line 243: the redaction is not clear.

(31) Line 246: "determine" is duplicated.

(32) Line 279: the changes with temperature were significant?

(33) Line 299: if the lowest OM value was obtained for 280ºC, it is not true that the content of OM decreased as the temperature increase because you reached 300ºC as the highest temperature.

(34) Line 309: replace "% raw" by "% of raw".

(35) Line 331: if the decrease in the HHV was observed also with the increase of temperature and time, why the highest HHV was obtained for 60 min?

(36) Lines 332-339: many "the" definite articles are missing.

(37) Line 335: it is interesting to compare the biomass in both bases, but, do you think that it could be profitable to supply this biomass as d.a.f.?

(38) Line 337: R2=0.21 cannot be acceptable as a linear trend validation.

(39) Line 345: Figure 10 must be presented with the same orientation that Figure 9 for a better analysis.

(40) Line 356: some explanation is needed to understand the consequences of a no statistical difference between 280 and 300ºC for k values (also for 200-260ºC range). Can you add some references of similar studies with these results?

(41) Line 359: I can´t see that the mass loss is stabilised after 500ºC!

(42) Line 370: please, add a reference.

(43) Line 372: Figure 11 must be drawn with thiner lines than actual ones (as Figure 12). 

(44) Line 378: "at the process start" must be replaced by "at the beginning of the experiment".

(45) Line 379: replace "the first observed transformation" by, for example, "the first observation that it was observed". Also, replace "a" by "the". 

(46) Line 385: "It is likely...". This sentence must be moved to the discussion section, and a deep explanation is necessary. Add some references.

(47) Lines 387-388: the sentence is not clear.

(48) Line 413: replace "a high ash" by "a higher ash" and replace "and was higher compared to" by "than".

(49) Line 414: can you provide some explanation about why these values are so different if the feeding is similar?

(50) Line 419: replace "trend" by "relation".

(51) Lines 419-423: do you think that moisture adsorption could be relevant here? Have you found some references where it is?

(52) Lines 437-439: please, clarify the sentence.

(53) Line 443: can you make the same comparison with the HHV (daf) of that pellets? I think it could be relevant, and your conclusion can be different.

(54) Line 452: "here". Where? Do you refer to the previous data or to your experiment results?

(55) Line 454: replace "and temperature range" by "and a temperature range".

(56) Line 463: Ea values were estimated or they were obtained by experimental analysis?

(57) Line 468: can you add some references where OM was also found to be a critical parameter?

(58) Line 475: please, add a reference here.

(59) Line 479: replace "faster" by "at a lower temperature range".

(60) Lines 482-483: these sentences are not clear.

(61) Line 488: the word "requires" is duplicated.

(62) Line 497: "were" is wrong here.

(63) Line 502: be careful with the scale-up effects. You cannot conclude this from your experiments. The first sentence must be deleted.

(64) Line 509: do you think that 485 and 484.81 kJ/kg are different?

(65) Line 514: this sentence has no sense in its actual redaction.

(66) Line 515: "household" must be in plural here.

(67) Line 534: Can you explain why you obtained a lower moisture content at 40 min than that observed at 60 min for 200 and 260ºC?

(68) Line 537: Can you explain why you obtained a different mass yield trend at 200ºC than that observed at 260 and 280ºC?

Author Response

Dear Reviewer:

Thanks for your comments concerning our manuscript entitled “Waste to Carbon: biochar from elephant dung as new cooking fuel”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guidance to our on-going and future research. We have studied comments carefully and have made corrections. Revised text is highlighted using "Track Changes" function in the manuscript. The point-to-point response to your comments is listed in the table below. We appreciate the opportunity to respond to your constructive feedback. Our responses to your comments are in attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

The manuscript describes torrefaction of elephant dung. Experiments are well performed and clearly described, but the statisticla models used are questionable. Specific comments:

The term biochar is used to denote the torrefied biomass. I suggest to change this to biocoal. “Biochar” is primarily used to describe thermochemically treated biomass intended for soil amendment. If treated at above 350C, biochar is chemically stable, recalcitrant to microbial decomposition. As this paper describes materials treated at a lower temperature, intended for uses as fuel, the term “biochar” is inappropriate.

Animal dung is first and foremost a source of nitrogen fertilizer for soils. In this case, the dung is used as fuel instead. However, the nitrogen issue should be mentioned, at least in the discussion. What is the nitrogen content of the raw elephant dung? How much of this will be vaporized during torrefaction, and how much will remain in the fuel? I understand that this has not been measured, but it may be possible to say something about it based on other literature.

In the introduction, previous research on treatment of elephant dung is described. However, there should be more to gain from also giving a background on manure from other animals, for comparison. In particular, the introduction should cover torrefaction of other materials, including other manures.

L 114. Refrigeration refers to cool storage. At -15C, the material is frozen

L 133-140. Please describe key properties of the methods used, such as temperatures and times

L227. The choice of polynomial models is not obvious in this case and should be reconsidered, and if you decide to keep it, its relevance should be well motivated. To use models with 7-9 terms does not seem appropriate for such a limited amount of data, in many cases just 14-15 data points.

Your data on moisture, ash and OM are presented with to decimal points. I doubt that this precision reflects the precision of your analysis. Please adjust.

Line 438: “ways to reduce the ash content in elephant dung” is mentioned. It would be helpful if you could refer to any such method that could be applicable in practice.

Author Response

Dear Reviewer:

Thanks for your comments concerning our manuscript entitled “Waste to Carbon: biochar from elephant dung as new cooking fuel”. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guidance to our on-going and future research. We have studied comments carefully and have made corrections. Revised text is highlighted using "Track Changes" function in the manuscript. The point-to-point response to your comments is listed in the table below.  We appreciate the opportunity to respond to your constructive feedback. Our responses to your comments are in attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors successfully revised their contribution.

Author Response

Thank you for your service and feedback.