Characteristic Study of Briquette Cyanobacteria as Fuel in Chemical Looping Combustion with Hematite as Oxygen Carrier

Round 1

Reviewer 1 Report

Manuscript has been deeply revised and improved. Therefore I recommend to publish this paper.

Author Response

Issue: Manuscript has been deeply revised and improved. Therefore I recommend to publish this paper.

Discussion:

Thanks for your advice and encouragement. We will work harder about CLC of briquette cyanobacteria.

Reviewer 2 Report

Reviewer’s comment on manuscript applsci 1177381

 

Manuscript topic is of potential interest for Applied Sciences journal. It deals with cyanobacteria processing by chemical looping thermochemical conversion. Apart from interesting topic and experimental results presented in the manuscript, several drawbacks can be documented: Most important, the aspect of nitrogen and phosphorus content receives little attention in the experimental part and the related discussion, on the contrary to Introduction part. Another major drawback is the absence of critical experimental data comparison with available literature, with the prospect of pointing out possible differences due to unique feedstock. Apart from that, manuscript requires language and style improvement. Abstract and Introduction could be condensed. A few formal and formatting issues were detected as well.

 

As a result and based on the list of queries listed below, I recommend a deep and thorough manuscript revision. Without seriously dealing with the nitrogen and phosphorus content in the feedstock and without including a proper comparative discussion, I do not see the manuscript to be ever fit for publishing in Applied Sciences journal.

 

Abstract: Is quite lengthy and should be rewritten to a more compact form. Moreover, stylistic improvement is desirable. Just as an example: “cyanobacteria” repeats almost in each sentence.

 

English polishing (both language and style) of the whole manuscript is highly recommendable.

 

Introduction should be shortened, omitting well known facts, highlighting the advantages of CLC especially regarding cyanobacteria processing. Study objectives could be better formulated in a separate paragraph or in bullet points.

 

Part 2: Statistics of the data provided and test methods described should be stated. How many repetitions of each measurement type were performed?

 

Figure 4 is of poor quality. Equations (1) and (2) are blurry.

 

Lines 341-343: reformulate this please. The two sentences are very much alike.

 

Figure 16,19: Check y-axis description. Concentration is usually given in other units than %.

 

Chemical reactions in part 3.3.2 should be renumbered as to match with previous equations.

 

Experimental section and the related discussion are incomplete. As the authors rightly point out in Introduction, nitrogen and phosphorus content in cyanobacteria is of great importance. So, as a minimum, experimental results should deal with NOx formation during the process as well as with phosphorus emissions / deposition on catalyst particles and how it could be recovered. This is very important information.

 

Second major drawback of this study is the absence of any comparison of obtained results with those in biomass CLC experiments documented in the available literature. The authors state in Introduction that major differences of the process should be observable due to different chemical nature of the material. This is a perfect opportunity to prove this assumption.

 

The last two comments are crucial for a good scientific paper. I hope the authors are prepared to deal with them thoroughly; otherwise I am not prepared to recommend this manuscript for publishing.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The article is undoubtedly of interest, since the problem of purifying water from cyanobacteria is urgent.True, it is not entirely clear how the authors intend to utilize methane and carbon dioxide released during the processing of cyanobacterial briquettes.

I believe that this manuscript can be published and will be of interest to the target audience of the journal.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The authors undertook interesting research work. The whole presents itself at a high level. I just have a few comments:

1. What is the estimated availability potential of the analyzed biomass? The management of cyanobacteria is extremely important, but is it possible to do it on a larger scale from the energy point of view?
2. How can the analyzed biomass be collected for further energy processing? Is it profitable?
3. What is the energy potential of the analyzed biomass, eg what is the calorific value of the briquettes created?
4. What is the overall energy balance of the entire sample preparation and gasification process in relation to the obtained final raw materials?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Dear authors,

I am glad to see you dealt with my questions and suggestions seriously and implemented the needed changes in the revised manuscript. From the contentual point of view I consider the revised submission fit for publishing.

However, a minor revision aimed at formal and formatting aspects is necessary. Please take the following points as inspiration and perform a thorough manuscript proofreading before resubmission:

• the newly added or modified text needs to be checked for grammar and spell errors
• check for correct numbering of references in the References section
• check and use correct symbol "°C" instead of "^oC" or the like

Author Response

 "Please see the attachment."

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Most of my comments (listed below) were not taken into account, which means that I have wasted my time reviewing this paper. Because of such ignorance I recommend to reject this paper.

 

 

Detailed comments:

 

Section 2.1.1: The shape and size of pellets obtained from pelleting unit should be mentioned in this section.

 

Section 2.1.3: Heating rate(s) and gas(es) flux(es) should be provided in this section.

 

General: In that kind of experiment term: “drying” seems to be better than “dewatering”. For consideration by the Authors.

 

2.1.4: The range of operating temperature and type of fuel feeding (batch/continuous) should be mentioned as well as the gas velocity in the column. What about the voidage of the bed or height of the bed under operation?

 

The values in the Table 1 do not correspond to the values in the text (line 175).

 

Section 2.2: Mass of OC in the FB unit should be provided in this section.

 

2.3.2: Try to avoid using active voice “we think” (line 227)

 

2.3.4: “… different experimental conditions including temperature, bed materials and so on.” Is there other bed material than hematite? Please describe it clearly in this section.

“… and so on.” – please be more specific.

 

3.1.1.: The maximum is …% when moisture content is 10%. (not 5%) (lines 289-290)

 

I do not think that increase of 28% is a “little change” (line 294)

 

3.2.1: And again, decrease in relax density of about 30% is not “a slight decrease” (lines 343-344)

 

Line 405: Figure14 (not 12)

 

Section 3.3.1/ first paragraph: The size of quartz sand should be provided.

 

“The peaks of CO, CO2, and H2 appear at the first minute and the peak of CH4 is at the second minute.” - Please check the compounds; correct the legend in Figures 16 or rewrite the sentence (lines 455-456) 

 

Please compare the values in Figures 16a and 16d, which increase of about 2 times. Is it really “slight increase”? (457-458)

 

Carbon conversion is very low and the process is not completed even in the temperature of 900oC. What is the reason of that? (lines 461-470)

 

“… dewatering (..) process”??? You are describing curves of carbon conversion rate. (lines 477-479)

 

“… initial min and …” (line 496) – please use full words not abbreviations in the text (apply to the whole paper)

 

Line 532: 800oC (not 750oC)

 

The differences described in paragraph – lines 527-541 seems to be rather accidental.

 

English should be generally improved.

Reviewer 2 Report

V2 cyanobacteria combustion

 

Compared with the precedent submission, some points have been modified, but the main problems are still present:

1/ English quality is not acceptable and a number of misused words or expressions are present.

2/ Excepted for one tiny modification, all Figures are equal to Figures in precedent submission.

3/ The authors discussed to much CLC, whereas their experiments forgot totally the second part of a chemical looping process, i.e. the reoxidation of the oxygen carrier, in their case impure hematite.

During combustion of cyanobacteria, some ash is produced and a fraction of this ash may contaminate the surface of “partially reduced hematite”, modifying its oxygen donor capacity after some reduction oxidation cycles.

What the authors have studied is the pelleting of cyanobacteria and some mechanical properties of the pellets, together with the use of “hematite” as a catalyst for cyanobacteria pellet combustion, nothing else. Even the project title is confusing.

4/ Many discussions are difficult to understand and many experiments impossible to repeat due to a lack of basic informations.

 As a first example, Table 1 in section 2.1 can be observed:

The title of the Table contains …..”chemical components of cyanobacteria”, whereas only elements are presented, C, H, O, N and S. The values given in Table 1 for these elements are not the values presented in the text preceding the Table. In Table 1, some parameters are expressed as Vd, FCd, Ad, but are not explained in the experimental part or in the legend of the Table. The granulometry of “hematite” is given, but not that of quartz sand used for blank experiments.

As a second example, Figure 5 presents the scheme of the fluidized bed reactor used during the work. During the first step of a CLC, i.e. oxidation of biomass by labil oxygen from “hematite” or better during the reduction of “hematite” by reduced species issued from the degradation of biomass, the gases produced are deviated through a “cooler” (what is its temperature? what is its size?) then through a dryer (what are its temperature, composition and size) and a filter (also undefined) before some permanent gases are analysed.

In the same Figure, at the top of the reactor is placed a piece named “fuel chute” suggesting that cyanobacteria pellets (or ground pellets) are introduced by this entry. But before the cyanobacteria is added to the reactor, what occurs in the reactor is not described (presence of “hematite”, thermal heating of “hematite” (how long the treatment), control of the fluidized bed quality, …….

The reader does not know if the “hematite” is changed after one combustion or if the same “hematite” is permanently present and is used all over the described experiments.

As a third example, in the section 2.4.3., the Carbon balance is based on CO, CO2 and CH4. As a non- oxidized carbon molecule (CH4) is present, it is also useful to verify that other molecules like HCN, C2H4, are not formed. No information on this experimental caution is mentioned.

5/ The authors used an excellent TGA equipment, but do not present any mass variation of their “hematite” or cyanobacteria along their publication project, whereas TG and DTG are, theoretically, fully able to define the main mass loss intervals allowing to precisely separate dewatering, devolatilization and combustion temperatures ranges.

6/ The presence of an unexpected sentence is observed on lines 232-235, sentence that seems one of previous reviewer.

7/ On lines 265-267, the authors wrote “The carbon conversion  and ash properties are investigated under different experimental conditions including temperature and bed materials(quartz sand and hematite OC)”. But the reader was unable to find data on “ash properties”.

8/ On line 461 “The fluidizing agent of N2 is fixed at 1.5L/min and steam flow is at 0.8g/min”.

The referee is quite surprised by the presence of steam in the reactor. On one hand, the paper comments a lot on the importance of dewatering after pelleting and define an optimum water content (some 10%) for the stability of the cyanobacteria pellets. On the other hand, an addition of water during the first step of CLC is susceptible to change the optimized water content of biomass pellet. Rather difficult to understand.

Then, during the first step of CLC, what is the effect of water pressure on the stability of the biomass mean particle size. It appears for the referee that:

• either all the discussion on dewatering needs simplification and summarizing
• or a control of steam addition (0.8 g/min) on the mechanical resistance of the cyanobacteria pellets is necessary.

9/ The gas phase composition partially studied in the present work needs new insights: In the presence of CO2, CO, H2, CH4 among others, and the presence of “catalyst like FeOx”, reactions such as Water Gas Shift, Reverse Water Gas Shift, Methane Steam Reforming, Methane Dry Reforming, and many others are susceptible to take place to different extent and possibly explain the differences observed when comparing the first CLC step in presence of sand and in presence of “hematite”. The present paper must also discuss such possible situation.

To conclude, the actual presentation content seems quite far from an international standard publication. Important points are missing and secondary details are occupying to much space.