System Hydrodynamics of a 1 MW_th Dual Circulating Fluidized Bed Chemical Looping Gasifier

Round 1

Reviewer 1 Report

Review: System Hydrodynamics of a 1 MWth Dual Circulating Fluidized 2 Bed Chemical Looping Gasifier

The paper concept is worthy of publication; however, a major revision is warranted.  The paper is the length of two publications and contains 12 to 13 to many figures in its 25 figures.

Technical comments:

1.      Lines 79-81.  The scale-up of bubbling and circulating fluidized beds is not so well defined. Breault and Breault et al. have developed a new approach based upon maintaining the microstructure in the large and small units by ensuring that the behavior of each as defined by maintaining higher order statistics and chaotic parameters.  See

Breault, Ronald. Maintaining Microstructure – The Path to Successful Technology Maturation in Fluidized Systems. United States: N. p., 2023. Web. PowerPoint Presentation (doe.gov)

 

2.  Line 93-94. See comment and references above

3.  Bed materials: lines 192-204.  Particle sizes and particle densities differ than those presented in Table 4.  Scaling and applying Glicksman laws across Geldart classifications will lead to significant errors.  The hot unit is a Geldart Group B material whereas the cold material is on the Geldart Group A/B transition – with behavior of A and B materials.

4.  Table 5: row for density ratio – what is “1.55 104” and “3.64 104” is the author claiming 5 significant figures? What is the space between the “5 and 1” and “4 and 1”?

5.  Figure 7: color of arrows not consistent with key a right.

6.  Line 303.  The term Gs* is normally referred to as the saturation carrying capacity not the “saturation entrainment”. Please correct so readers understand.   Also, see the Breault references above and the following two for newer correlations for Geldart group A and B relationships for the saturation carrying capacity.

Breault and Weber, “Saturation Carrying Capacity for Group A Particles in a Circulating Fluidized Bed”, Energies 2021, 14(10), 2809; https://doi.org/10.3390/en14102809

 

The overall reducing the paper length by about 50% (possibly making two publications) and the scaling law applications are the biggest issues with the scaling law application likely inducing errors and is outdated as compared to recent publications by Breault as noted above.

Author Response

Dear Sir or Madam,

thank you for your valuable feedback and recommendations.

Below you find a list relating to all your comments in detail. The attached rebuttal letter contains a holistic overview over all changes made in the revised manuscript, following the suggestions by all reviewers.

Kind regards,

Paul Dieringer

________

Reviewer 1:

1. Lines 79-81. The scale-up of bubbling and circulating fluidized beds is not so well defined. Breault and Breault et al. have developed a new approach based upon maintaining the microstructure in the large and small units by ensuring that the behavior of each as defined by maintaining higher order statistics and chaotic parameters.  See

Breault, Ronald. Maintaining Microstructure – The Path to Successful Technology Maturation in Fluidized Systems. United States: N. p., 2023. Web. PowerPoint Presentation (doe.gov)

• The stated reference was not available at the time of drafting this manuscript. To accommodate the latest research, its main finding was incorporated into the introduction. 

2. Line 93-94. See comment and references above

• See 1.

3. Bed materials: lines 192-204. Particle sizes and particle densities differ than those presented in Table 4.  Scaling and applying Glicksman laws across Geldart classifications will lead to significant errors.  The hot unit is a Geldart Group B material whereas the cold material is on the Geldart Group A/B transition – with behavior of A and B materials.

• In Section 2.3.1, properties of the fresh materials are provided. In the cold setup, bed material properties do not change significantly. Therefore, the data provided for bronze powder in Chapter 2.3.1 and Table 4 is identical (a rounding error was corrected in Chapter 2.3.1). On the other hand, it is well known that particle properties change inside a hot system during chemical looping [1], [2]. Hence, the properties of the fresh material (given in Chapter 2.3.1) are not used for scaling. Rather, the properties of the used OC material were used for scaling. This information is highlighted in Footnote 3.
• In terms of the Geldart classification, it is true that bronze powder falls onto the threshold between Geldart A and B particles. However, this is inevitable, as a material of higher density is not usable in the CFM due to economic and/or safety reasons. Pröll et al. [3] previously successfully used Bronze powder with a similar particle diameter for scaling of a chemical looping system. Moreover, our observations in the CFM show that the Bronze powder behaves as a Geldart B powder. Nonetheless, we agree with your point that this discrepancy in Geldart classification can impair the accuracy of the scaling approach. Hence, an additional explanatory sentence was inserted on page 13, to disclose this deviation in Geldart classifications and its possible impacts on scaling accuracy.

4. Table 5: row for density ratio – what is “1.55 104” and “3.64 104” is the author claiming 5 significant figures? What is the space between the “5 and 1” and “4 and 1”?

• Thank you for your close attention. During transferal of Table 5 into the MDPI Energies layout, the superscripts were lost. This was corrected. (The density ratios are in the magnitude of 10^4)

5. Figure 7: color of arrows not consistent with key a right.

• The arrows in Figure 7 relate to the colors of the solid streams as given in Figure 6. A short explanation was added to the figure description.

6. Line 303. The term Gs* is normally referred to as the saturation carrying capacity not the “saturation entrainment”. Please correct so readers understand.  Also, see the Breault references above and the following two for newer correlations for Geldart group A and B relationships for the saturation carrying capacity.

Breault and Weber, “Saturation Carrying Capacity for Group A Particles in a Circulating Fluidized Bed”, Energies 2021, 14(10), 2809; https://doi.org/10.3390/en14102809

• The denomination of Gs* was adjusted to saturation carrying capacity
• More recent semi-empirical approaches by Breault et al. [4] and Breault and Weber [5] were referenced in the introduction.
• Moreover, the approach by Breault et al. [4] was also added as a reference approach in addition to that from that by Geldart et al. [6], [7] in Section 2.5 and in Figures 13 and 19. The Figure description and the relating text was adapted accordingly.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

I commend the authors on synthesized a large body of experimental measurements to develop scaling laws for pressure drop and predict solids entrainment in dual CFB risers. The results are a valuable addition to the literature. I would recommend a few minor modifications if deemed feasible:

1. Please replace the word "chapter" throughout the manuscript with "section"

2. The manuscript is too long (and reads almost like a review paper). Since this work appears to build upon several previous endeavors (Eg: Ref's 18, 24 and 44), perhaps a concise tabular summary of those previous works in a tabular form in the Introduction section might assist towards highlighting the novel contributions in a more succinct manner.

3. In a similar vein, reporting previous efforts on scaling CFB risers in a tabular form and highlighting their shortcomings would be beneficial i.e., where there other hot flow studies where the Glicksman scaling laws failed?

4. Given the important role played by PSD, where PSDs before and after the hot tests measured to assess attrition and agglomeration?

Author Response

Dear Sir or Madam,

thank you for your valuable feedback  and recommendations.

Below you find a list relating to all your comments in detail. The attached rebuttal letter contains a holistic overview over all changes made in the revised manuscript, following the suggestions by all reviewers.

Kind regards,

Paul Dieringer

_______

Reviewer 2:

1. Please replace the word "chapter" throughout the manuscript with "section"

• The recommended change was incorporated accordingly.

2. The manuscript is too long (and reads almost like a review paper). Since this work appears to build upon several previous endeavors (Eg: Ref's 18, 24 and 44), perhaps a concise tabular summary of those previous works in a tabular form in the Introduction section might assist towards highlighting the novel contributions in a more succinct manner.

• It is true that the present research paper builds on previous findings made during the advancement of the CLG technology in 1 MW_th scale [8]–[11]. Apart from their relevance for the CLG technology, the findings from the given research paper, allow for the description of the hydrodynamics of any (coupled) CFB system. As the focus of the paper leans more towards this aspect than the CLG technology specifically, an overview over other scaling endeavors is in our opinion conducive (see point 3. below), while providing an overview over previous work on autothermal CLG would “water down” the focus of this research paper. Therefore, a paper detailing previous work on autothermal CLG was not added to the revised manuscript, to streamline the introduction towards the focal point of this research paper.

3. In a similar vein, reporting previous efforts on scaling CFB risers in a tabular form and highlighting their shortcomings would be beneficial i.e., where there other hot flow studies where the Glicksman scaling laws failed?

• An overview over research paper utilizing the scaling approach of Glicksman was added to the introduction. Please note that these endeavors did not fail. The approach is well established and hence widely used. The difference of the references papers to the current manuscript is that they were used in different setups. Further, this research paper shows that while scaling via the scaling approach of Glicksman is viable, it exhibits certain pitfalls.

4. Given the important role played by PSD, where PSDs before and after the hot tests measured to assess attrition and agglomeration?

• Yes, the PSD was measured for the majority of selected solid samples (see e.g. Figure 17). All data is given in the supplementary material. A small clarifying sentence was added in Section 2.6.1.

Author Response File: Author Response.pdf

Reviewer 3 Report

The subject studied by the authors is pertinent. The paper structure is logic, the content is discussed in great detail and it is very well written. It is clear that the authors have put a lot of effort in this investigation. I would even dare say that in my opinion the paper is a bit too lengthy and the reader can get sometimes confused. Overall, however, I recommend acceptance of this paper.

Author Response

Dear Sir or Madam,

thank you for your valuable feedback.

The attached rebuttal letter contains a holistic overview over all changes made in the revised manuscript, following the suggestions by all reviewers.

Kind regards,

Paul Dieringer

Author Response File: Author Response.pdf

Reviewer 4 Report

This paper is well written, and the prediction work on the hydrodynamics of interconnected fluidized bed is very interesting and significant for the industrialization of chemical looping technologies. This paper can be published in the current form.

Author Response

Dear Sir or Madam,

thank you for your valuable feedback.

The attached rebuttal letter contains a holistic overview over all changes made in the revised manuscript, following the suggestions by all reviewers.

Kind regards,

Paul Dieringer

Author Response File: Author Response.pdf

Reviewer 5 Report

This work investigates the hydrodynamics of Chemical Looping Gasification (CLG) dual-fluidized bed technology, which intend to provide a holistic insight into the hydrodynamic behavior of the dual-fluidized bed reactor systems. The study focus on the operating conditions and the solids circulations between the air and fuel reactors and the entire system’s components. The authors utilized two operating setups a cold flow model and 1 MW_th pilot plant in the conducted study, which aided them to propose a model to estimate the solid entrainment of circulating fluidized beds. The model was developed based on using the dimensionless approach of different parameters that were reported as common designs characteristics. The introduction is well written and very informative. The paper is good and can be published after minor revision, suggestions:

1.     Previous efforts for scaling up CFB need to be highlighted and referenced in the manuscript such as:

a.      https://doi.org/10.1016/j.powtec.2018.11.036

b.     https://pubs.acs.org/doi/10.1021/acs.iecr.0c04611

c.      https://doi.org/10.1016/j.powtec.2016.04.037.

2.     It would be very useful to present how long it takes for the system to reach steady state after initial operation?

3.     A table or a sentence on the minimum and maximum holding capacity for each system and reactor is also important to be provided.

4.     A comparison of the presented model with other model available on the literature can be considered.

5.     The author should also elaborates on the limitations of the proposed model.

Good.

Author Response

Dear Sir or Madam,

thank you for your valuable feedback and recommendations.

Below you find a list relating to all your comments in detail. The attached rebuttal letter contains a holistic overview over all changes made in the revised manuscript, following the suggestions by all reviewers.

Kind regards,

Paul Dieringer

______

Reviewer 5:

1. Previous efforts for scaling up CFB need to be highlighted and referenced in the manuscript such as:
2. https://doi.org/10.1016/j.powtec.2018.11.036
3. https://pubs.acs.org/doi/10.1021/acs.iecr.0c04611
4. https://doi.org/10.1016/j.powtec.2016.04.037.

• Thank you for the recommendations. Reference b. was already referenced in the original manuscript. References a. and c. were added to the novel Table 1 in the introduction. Moreover, findings from reference c. were added in Section 4.3.2.

2. It would be very useful to present how long it takes for the system to reach steady state after initial operation?

• A number for the CFM is provided in Section 2.4.2 (2-3 minutes). A similar explanation was added for the 1 MW_th plant in Section 2.4.1.

3. A table or a sentence on the minimum and maximum holding capacity for each system and reactor is also important to be provided.

• A sentence specifying the total system inventory for the 1 MW_th plant and the CFM were added in Section 2.4.1 and Section 2.4.2, respectively.

4. A comparison of the presented model with other model available on the literature can be considered.

• In their work, Marx et al. [10] provide a detailed overview over approaches to determine the solid circulation between two CFBs systems. For the sake of brevity, this exercise is not repeated here. As shown in the referenced paper, the method developed by Marx et al. is the most suitable one for application in a coupled CFB system. Hence, this method is used as a reference in this study.
  Methods to derive the saturation carrying capacity Gs* are presented in this paper and compared to the experimental data (following the suggestion by Reviewer 1, a novel method for the calculation of Gs* has also been included). Yet, as shown in the manuscript, these approaches fail to adequately predict solid entrainment. Other modelling approaches typically require extensive computational efforts (CFD modelling) or fitting endeavors (e.g. freeboard entrainment model detailed by Kunii & Levenspiel [12, p. 201]), which is mentioned in the introduction. However, these approaches are not covered in this paper to prevent a further content-wise extension.

5. The author should also elaborates on the limitations of the proposed model.

• A paragraph elaborating on this aspect was added to Section 4.3.2.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Accept as is