Experimental Studies on Co-Combustion of Sludge and Wheat Straw

Round 1

Reviewer 1 Report

The proposed manuscript fit the scope of the journal and is very interesting as is related to the significant problems occurring during the combustion/co-combustion of various solid fuels (biomass, wastes, sludges etc). Slagging or fouling problems are of special attention as they lead to a decrease in the efficiency, heat exchange or even unexpected shut-downs of the boiler and financial losses. These issues appear more frequently when the biomass blends are burnt. It was observed, that in some cases the synergy processes are observed that limit negative consequences of difficult fuels combustion. However, there are still many unsolved issues related to the mechanism and conditions of interactions between blended fuels and their products. Therefore, this manuscript can increase the knowledge in that area.

However, the manuscript requires some clarifications and improvements prior acceptance for publication. 

General remarks:

- The order of manuscript layout is not in line with authors guidelines. The materials and methods must be placed before the experimental results/discussion.

- there are no data related to SEM analysis (apparatus, magnification, etc)

- In the introduction, more attention should be paid on synergy issues and their consequences in co-firing. It will underline the importance of the research proposed by the authors.

- the value of the melting temperature of the fuels and the blend is missing.

- What was the reason for 30/70 blend share selection? Why not 40/60 or 50/50 etc..It limits the discussion point and correlation issue.

Detailed remarks:

Line 59: Cytowanie nie jest precyzyjne. Powinien to być Yu i in. ponieważ w cytowanym artykule jest więcej autorów. Podobnie, Zhang i in. należy użyć. Zaleca się sprawdzenie manuskryptu w tym aspekcie.

Line 78-79: the last sentence should be moved to the material and methods.

Line 79: low heat value? - it is a lower heating value (LHV) not "low".

line 86: the abbreviation WS must be defined prior further use i.e. "wheat straw (WS) is a ......."

line 89, 215 etc.: avoid the use of word "obvious" - in science it is a tricky word.

line 104-105: what does "practical reaction temperature" mean? in combustion technique, such term does not exist. Must be clarified. 

Line 112: Figure 2 - magnification data is missing or not readable.

Line 114: Figure 2(a) - should be Figure 2a

Line 128: It is not allowed to start the section or point with a table, picture or graph etc. Some introduction is required.

- How was the blend 70\30 insured ? Especially for TGA and other experiments, where a sample mass is very small 

- The combustion atmosphere and conditions: in the air? in oxygen?

- what about the evaporation of chlorine?

- Did the authors consider the influence of Cl/S ratio?

- it the materials and methods the residence time of the blended fuels in the vertical tube is missing.

Line 309: what is the unit for 100-150 (size of the particles), microns ?? It is not clear.

- units i.e mg/kg are not recorded according to the guidelines; should be mg*kg-1.

Line 320: is 420mm - should be 420 mm etc.

Line 335-336: try rewrite it: Rins and Rs is ............., accordingly.

Line 348-349: results indicated that co-combustion could lower the ignition temperature and increase the burnout temperature of sludge. It was not proved as the ignition temperature was not investigated/measured. The conclusion is not justified. Next, "co-combustion could ..... increase the burnout temperature of sludge". This conclusion is not clear. Does it mean that the burnout temperature must be higher? If yes, It is not an advantage then. If the authors thought that the burnout rate is higher then the sentence must be re-written another way. 

Comments for author File: Comments.pdf

Author Response

1. The order of manuscript layout is not in line with authors guidelines. The materials and methods must be placed before the experimental results/discussion.

Response:

Thank you very much for your kind comment. I have adjusted the layout of the article to meet the preferences of the journal. Please check on it!

2. There are no data related to SEM analysis (apparatus, magnification, etc)

Response:

Thank you very much for your kind comment. The apparatus was scanning electron microscope (SU8010, HITACHI, Japan), and the magnification was x50000. The details have been filled in the revised paper. Please check on it!

3. In the introduction, more attention should be paid on synergy issues and their consequences in co-firing. It will underline the importance of the research proposed by the authors.

Response:

Thank you very much for your kind comment. Previous studies mainly focused on the slagging control during co-combustion while the release characteristics of heavy metals and the interactions occurred need further study. These have been mentioned in the revised paper. Please check on it!

4. The value of the melting temperature of the fuels and the blend is missing.

Response:

Thank you very much for your kind comment. To quantify the slagging trend of the ashes, deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flowing temperature (FT) are listed in Table 2 in the revised paper in Section 3.2. Please check on it!

5. What was the reason for 30/70 blend share selection? Why not 40/60 or 50/50 etc. It limits the discussion point and correlation issue.

Response:

Thank you very much for your kind comment. Considering the lower heat value and higher heavy metal content in sludge, ratio of sludge in the blends was selected as 30%. This has also been discussed in previous articles and supplemented in the revised manuscript, please check on it!

6. Line 78-79: the last sentence should be moved to the material and methods.

Response:

Thank you very much for your kind comment. I have adjusted the layout of the manuscript, please check on it!

7. Line 79: low heat value? - it is a lower heating value (LHV) not "low".

Response:

Thank you very much for your kind comment. I intended to express that the sludge has a low calorific value but my poor expression caused the misunderstanding. I have modified this in the revised manuscript, please check on it!

8. Line 86: the abbreviation WS must be defined prior further use i.e. "wheat straw (WS) is a ......."

Response:

Thank you very much for your kind comment. I have modified this in section 2.2 in the revised manuscript, please check on it!

9. Line 89, 215 etc.: avoid the use of word "obvious" - in science it is a tricky word.

Response:

Thank you very much for your kind comment. I have replaced it into “distinct” in the revised manuscript, please check on it!

10. Line 104-105: what does "practical reaction temperature" mean? in combustion technique, such term does not exist. Must be clarified.

Response:

Thank you very much for your kind comment. “Practical” here means the value was gained from experiment other than theory. I have clarified in the revised manuscript, please check on it!

11. Line 112: Figure 2 - magnification data is missing or not readable.

Response:

Thank you very much for your kind comment. I have added the data in Figure 2 in the revised manuscript, please check on it!

12. Line 114: Figure 2(a) - should be Figure 2a

Response:

Thank you very much for your kind comment. I have modified these in the revised manuscript, please check on it!

13. Line 128: It is not allowed to start the section or point with a table, picture or graph etc. Some introduction is required.

Response:

Thank you very much for your kind comment. I have moved the tables and figures to the correct place in the revised manuscript, please check on it!

14. How was the blend 70\30 insured? Especially for TGA and other experiments, where a sample mass is very small.

Response:

Thank you very much for your kind comment. Firstly, I weighted 3 g of sludge and 7 g of wheat straw particles in a beaker and then stirred evenly. Lastly, about 6 mg of the blended fuel was weighted to conduct TG experiment.

15. The combustion atmosphere and conditions: in the air? in oxygen?

Response:

Thank you very much for your kind comment. The combustion was in the air supplied by an air compressor. The description has been added in section 2.2 in the revised manuscript, please check on it!

16. What about the evaporation of chlorine?

Response:

Thank you very much for your kind comment. Cl has a great impact on the release of alkali and heavy metals in the combustion. However, due to the limit of the current experiment condition, we lacked the data of Cl. We will improve our facility and pay more attention to Cl in our further research.

17. Did the authors consider the influence of Cl/S ratio?

Response:

Thank you very much for your kind comment. Cl/S ratio is usually discussed in the researches in which the pure substances were used. Sludge and wheat straw are two kinds of complicated compounds and the initial existing forms of the elements remain unknown. As the result, the influence of Cl/S ratio was not taken into consideration in this research.

18. In the materials and methods the residence time of the blended fuels in the vertical tube is missing.

Response:

Thank you very much for your kind comment. Actually the residence time was 45 min and this has already mentioned in the manuscript, please check on it!

19. Line 309: what is the unit for 100-150 (size of the particles), microns ?? It is not clear.

Response:

Thank you very much for your kind comment. I have modified the statement into “0.10~0.15 mm” in the revised manuscript, please check on it! 

20. Units i.e mg/kg are not recorded according to the guidelines; should be mg*kg-1.

Response:

Thank you very much for your kind comment. I have modified the statement in the revised manuscript, please check on it! 

21. Line 320: is 420mm - should be 420 mm etc.

Response:

Thank you very much for your kind comment. I have modified the statement in the revised manuscript, please check on it! 

22. Line 335-336: try rewrite it: Rins and Rs is ............., accordingly.

Response:

Thank you very much for your kind comment. I have modified the statement in the revised manuscript, please check on it!

23. Line 348-349: results indicated that co-combustion could lower the ignition temperature and increase the burnout temperature of sludge. It was not proved as the ignition temperature was not investigated/measured. The conclusion is not justified. Next, "co-combustion could ..... increase the burnout temperature of sludge". This conclusion is not clear. Does it mean that the burnout temperature must be higher? If yes, It is not an advantage then. If the authors thought that the burnout rate is higher then the sentence must be re-written another way.

Response:

Thank you very much for your kind comment. I intended to express that wheat straw could increase the combustion quality and decrease the burnout temperature of sludge however I made the mistake in the expression. I have re-written this sentence in the revised manuscript, please check on it!

Reviewer 2 Report

 Main Remarks:

-        The structure of the article is weird, incorrect.

-        Chapter 2 is the section "result and discussion, and where is the description of the research method and materials?

-        The description of the method is behind the results. This is illogical

-        In introduction, please review the previous studies. What is the novelty for this article compared with existing studies? The literature review is not complete.

Author Response

1.The structure of the article is weird, incorrect.

Response: Thanks very much for your kind comment. I have adjusted the layout of the article in the revised manuscript, please check on it!

2.Chapter 2 is the section "result and discussion, and where is the description of the research method and materials?

Response: Thanks very much for your kind comment. I have moved the part of method and materials to Chapter 2 in the revised manuscript please check on it!

3.The description of the method is behind the results. This is illogical.

Response: Thanks very much for your kind comment. I have adjusted the layout of the article in the revised manuscript, please check on it!

4.In introduction, please review the previous studies. What is the novelty for this article compared with existing studies? The literature review is not complete.

Response: Thanks very much for your kind comment. Previous studies mainly focused on the slagging control during co-combustion while the release characteristics of heavy metals and the interactions occurred need further study. These have been mentioned in the introduction in the revised manuscript. Please check on it! 

Round 2

Reviewer 2 Report

The authors have made the necessary corrections.