Coal-Scenedesmus Microalgae Co-Firing in a Fixed Bed Combustion Reactor: A Study on CO₂, SO₂ and NOx Emissions and Ash

Round 1

Reviewer 1 Report

Coal-Scenedesmus microalgae co-firing in a fixed bed combustion reactor: A study on CO₂, SO₂ and NO_x emissions and ash

 

Authors: Nokuthula E. Magida, Gary Dugmore and Adeniyi S. Ogunlaja

 

 

 

Recommendation:

The manuscript concerns research focused on the reduction of CO₂ and NO_x emissions by co-combustion of algae with coal. The subject is important and much effort has gone into this investigation. A wide range of research methods was used: from exhaust gas analysis to XRF and TEM analyzes. Unfortunately, the values obtained from exhaust gas analyzes (especially CO₂ mass) raise serious concerns, because in my opinion they significantly exceed the results that can be obtained. If the authors are unable to explain the obtained results, the manuscript should be rejected due to the very low precision of the measurements.

 

COMMENTS FOR THE AUTHOR

General comments

ü  Importance and interest: Interesting.

ü  Scientific soundness: Sufficient.

ü  Originality: It contains original investigations but few new information's.

ü  Degree to which conclusions are supported: Improvement required.

Specific comments

ü  Title: OK.

ü  Abstract: Avoid using the names "Coalgae® 10% and Coalgae® 15%" which have not been previously explained. This material may be unknown to readers.

ü  Key words: OK

ü  1. Introduction: The section should clearly distinguish any gap that may be addressed in comparison to previously published research deal combustion and co-combustion biomass with coal.

ü  2.3. Combustion system description: Figure 1 is missing from the manuscript.

ü  2.4. Combustion Method: Avoid Repetitions: "Gas emissions were measured at the flue gas outlet every minute." and next "Gas concentration readings are then recorded every minute."

ü  2.5. The O₂ referencing: "O₂ reference (3% or 6% are often used)" - this entry is not very fortunate. Indicate clearly that the 3% oxygen content in the exhaust gas applies only to gaseous fuels, while the 6% oxygen content in the exhaust gas relates to solid fuels such as coal.

ü  3.3. Effect of coal…: “Figure 4 shows the effect of peak temperatures (°C) of coal and Coalgae® blends”. Please indicate where these temperatures were measured in the combustion chamber.

ü  The analysis of the combustion process and co-combustion of coal with biomass requires the measurement of carbon monoxide (CO). By evaluating the CO / CO₂ ratio, we can determine the combustion efficiency. The authors should measure the concentration of CO in the exhaust gas.

ü  “3.4. Effect of coal - Scenedesmus microalgae co-firing and air flow rate on CO₂ emissions”. Complete this section with the information for which air flow rate the results are presented in Fig. 5.

ü  How was air flow rate determined ? From a scientific point of view, it is worth defining the excess air ratio.

ü  I have serious reservations about this section: How was the mass in grams of CO₂ determined? From the calculations based on the proportion: 12 gC - 44 gCO₂, this means that 130.8 gC (65% of the sample mass) can obtain a maximum of 479.6 gCO₂ (assuming that all the carbon has been burned), which is not possible because CO and hydrocarbons are always produced. All values in Fig. 7 for coal combustion alone are well above 479 gCO₂. How can this be explained? Please also consider SO₂ above 2g for coal combustion (Figure 9).

ü  Tables: OK.

ü  Conclusion: Improvement required.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The papers showed Coal-Scenedesmus microalgae co-firing in a fixed bed combustion reactor. In the introduction, the authors presented several articles devoted to the use of coal and the co-combustion of biomass in energy systems. In “Materials and Methods” section, the authors presented materials with their chemical analyses as well as the combustion system description. Next, the results with discussion were shown. The effect of co-firing coal and Scenedesmus microalgae on mass loss, temperature, and emissions were presented. The ash analysis with XRF as well as SEM and TEM were also presented. The conclusions are supported by research results. I have a few comments:

1.         There is a reference to Figure 1 in the text, but there is no figure.

2.         No diagram or photo of the test stand.

3.         What was the temperature of the air pumped into the reactor?

4.         With what device was the flue gas temperature measured?

5.         With what device was the O2, NOx, and SO2, as well as the mass reduction measured?

6.         No information about the error analysis of the study method. Could you write some information about uncertainty in your study? Do you consider the impact of the uncertainty on the results of your work?

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Despite extensive explanation from the Authors, I am sad to say that the current version of the manuscript has not improved compared to the previous one. It is highly incomprehensible to me why the Authors did not mention anything in the response that they change all values ​​in Fig. 3. In the previous version, the maximum temperature was 500oC, and the current one was 1000oC (analogically in Fig. 4). This raises serious doubts. Probably, the SO2 (g) values in Fig. 9 (value for coal at 17.2l / min) have been recalculated and corrected. Although the authors report no changes. There was an error in the title of the X-axis in Fig. 9 that was not there before.

I also regret to admit that I disagree with the explanation regarding the calculation of the weight in grams (g) for CO2, etc. No values and units have been given that would confirm the correctness of the calculations. I believe that it contains errors:

a) CO2 was measured in% and not in ppm,

b) the density of CO2 is not 1.84

c) the unit for all conversion equations (ppm -> g) will not be in grams. If we multiply mg / m3 by the air stream w in l/min.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

I have no new comments.

Author Response

There are no comments reviewer 2.

Thank you for the invaluable reviewal.

Round 3

Reviewer 1 Report

Dear Authors,
At the outset, I would like to point out that my goal is not to be malicious towards the authors, but only to care for the quality of the research results published in the journal.
As a reviewer, I cannot agree that the unit of emission in the quoted equation "CO2 (mg) = 50721350.4 mg / m3 * 0.0172 m3 / min = 872407.2 mg" is mg. In this case, it would be mg / min. Unfortunately, it is not a matter of adding only this minute in units. The problem is that the sample combustion process did not last one minute (only several dozen or several hundred) - as shown in Figures 2 and 3. This suggests that the adopted calculation procedure is incorrect.

Author Response

Please see the attachment.

Author Response File: Author Response.docx