Experimental Study of Entrainment and Mixing of Renewable Active Particles in Fluidized Beds

Round 1

Reviewer 1 Report

Please find the attached Review file. 

Comments for author File: Comments.pdf

Author Response

 

1-a) The reviewer wondered about the reasons for not discussing the results with correlations available in the literature.

Discussion:

Comparison and validation of the results with data and correlations from the literature increase the scientific value of the work. In our present study, we investigated industrial-based active particles and not representative spherical or cylindrical particles, as discussed in the literature proposed by the reviewer. To our best knowledge, there is no fluid dynamic correlation for such heterogeneous particles. However, we made comparisons with literature entrainment correlations (Fig. 7), and segregation profile data (Fig. 10 “revised manuscript”) which are close to our case study.

1-b) The reviewer claimed that the literature review is superficial and suggested citation of relevant references.

Discussion:

The references suggested by the reviewer were cited in the relevant parts in the introduction. Two sentences were added to the literature review paragraph “Earlier, Win et al. [21] investigated the transport velocity of coarse particles in a multi-solid fluidized bed system. They reported that the transport velocity of coarse particles decreases with increasing the solids circulation rate.”.

2) The reviewer requested a clear explanation of the novelty of the paper.

Discussion:

Two sentences were added to the introduction to clarify the novelty of the present paper “Concluding over the literature survey, we can say that the study of the fluid dynamic characteristics of coarse active particles was not covered with sufficient details. For example, the majority of the literature measurements were performed on representative particles (spherical tracer or spherical beads). However, the real heterogeneous properties of the active particles are more complex and are expected to have some differences in behavior compared to the representative spherical particles.”.

3-a) The reviewer requested justification of the proposed particle size analysis method.

Discussion:

As we mentioned in the manuscript, we digitized the images by using the MATLAB Image Processing Toolbox. Then, the projection areas of the investigated fuel particles were distinguished. The shape properties of the empty spaces were calculated using a user defined code. The background page dimensions were used in the caliberation of the code. The same method was applied in studied in the literature as we showed in the revised sentence “The particle size distribution (PSD) analysis was performed by using a MATLAB code that is based on a sieveless segmentation technique [22-24].”.

3-b) The reviewer asked about the validation of the applied image size analysis method.

Discussion:

First, as mentioned before, we calibrated the code using the background A4-size paper. We could not do further validation using other methods due to the large-size and heterogeneous nature of the investigated fuel particles. However, the photographic images in Fig. 1 can provide a rough (approximate) validation of the present particle size analysis method.

4) The reviewer claimed missing error analysis information.

Discussion:

The experimental error data were added to the revised entrainment figure (Fig. 6) because another reviewer requested this issue. Also, a brief description was included in the caption of Fig. 6.

5) The reviewer requested citing references for the bed-frozen method which was applied in the mixing measurements.

Discussion:

The bed-frozen method is the most commonly used method in the literature for mixing and segregation studies. And it is known that it is the most accurate method at low superficial velocities. Among many studies, we cited four top-cited references for the bed frozen-method as in the revised sentence “Mixing of the fuel particles in binary-mixture fluidized beds was measured by the commonly used bed-frozen method [25-28]”.

 

6) The reviewer requested marking each fuel with the same symbol in all figures.

Discussion:

We changed the symbols in the particle size distribution figure (Fig. 5) to match with uniform marking of the investigated fuels. Also, the symbols coloring in Fig. 7 was revised for the same reason.

 

7) The reviewer asked to mention the operating conditions in the “Materials and methods” section.

Discussion:

The operating conditions were mentioned in the materials and methods section e.g., the following sentence was revised as follows “Measuring the entrained fuel amount in the filter at five fluidization velocities within the range of 13 m/s.”.

Reviewer 2 Report

In my opinion, the quality of the paper must be improved. The paper shows an experimental research but I have not seen its novelty. Please, clarify it and explain the improvements compared with other papers.

Regarding the English language, there are several mistakes throughout the paper. Please, revise the English of the paper.

About the results:

• Particle size distribution (table 4) —> the R2 values are good. Is it normal in these types of particles or are typical values?
• In Figure 7, the data of wheat shell does not distinguish among the others. Please, modify it.

In order to look for the application of this research, I have a lot of questions, such as, ould these types of renewable active particles improve the combustion process? are these particles better than the biomass? how much does the energy performance or efficiency increase with these particles? should these particles be used in the fluidized beds?...

Author Response

1/ The reviewer requested showing the novelty of the paper over the other studies.

Discussion:

We added two sentences to the introduction to clarify the novelty of the present study “Concluding over the literature survey, we can say that the study of the fluid dynamic characteristics of coarse active particles was not covered with sufficient details. For example, the majority of the literature measurements were performed on representative particles (spherical tracer or spherical beads). However, the real heterogeneous properties of the active particles are more complex and are expected to have some differences in behavior compared to the representative spherical particles.”.

 

2/ The reviewer claimed English language mistakes in the paper.

Discussion:

The English language was revised after checking by a higher English proficiency academic person “In the present study, entrainment and mixing measurements are performed on SRF, bark, sunflower shell, and wheat shell. The results of these nonspherical and heterogenous are presented and analyzed. The entrainment results of the investigated fuel particles were plotted on the Haider-Levelspiel chart to obtain the representative sphericity values. Also, the vertical arrangements of these fuels in binary-mixture fluidized beds are discussed. Finally, the axial concentration profiles of these nonspherical active particles are compared to the probability function of the presence of a spherical tracer measured by others.”

 

3/ The reviewer wondered if the goodness of the R² values are normal for these types or are typical values.

Discussion:

The MATLAB curve fitting toolbox calculated the regression (R²) parameter mentioned in Table 3. The goodness of the fitting does not depend on the particle type, but rather on the internal iterative code of the curve fitting toolbox and the proper selection of the fitting relation. In the present study, we selected the widely used equation for the particle-size distribution, which is the Rosin-Rammeler equation.

 

4/ The reviewer requested distinguishing the data of wheat shell in Fig. 7.

Discussion:

The symbols representing the investigated fuels were set similar in all figures. The representative symbol of the wheat shell in Fig. 7 was distinguished by changing its color.

 

5/ The reviewer asked about the impact of the investigated renewable active particles on the combustion process.

Discussion:

The investigated renewable fuels have considerable energy potential as represented by the heating values in Tables 1 and 2. Also, we mentioned their chemical characteristics in Table 2. The combustion characteristics of these fuels are out-of-scope of the present study. However, the combustion characteristic will be an exciting research topic due to the importance of these renewable fuels which already used as coal-alternatives in power plants nowadays [Szűcs, T., Szentannai, P., Szilágyi, I.M. et al. Comparing different reaction models for combustion kinetics of solid recovered fuel. J Therm Anal Calorim 139, 555–565 (2020). https://doi.org/10.1007/s10973-019-08438-8].

 

6/ The reviewer wondered if the investigated particles are better than the biomass.

Discussion:

The term biomass can have two meanings from in engineering:

- The first biomass meaning is the material that contains hydrocarbons and can be converted to energy through chemical or thermal processes such as bark, sunflower shell, and wheat shell.

- The second definition of biomass is solid wastes (mostly from humans and animals) used in the biological conversion of the hydrocarbonaceous content into biogas by the action of anaerobic bacteria.

The first definition is the one that we referred to in the present study. Therefore, according to the fluid dynamic perspective of the present study, we can say that the SRF has relatively better entrainment characteristics than biomass fuels. However, biomass fuels have better mixing characteristics, especially at elevated superficial velocities.

 

7/ The reviewer asked about the influence of the investigated fuels on efficiency.

Discussion:

From the Engineering experiences, the SRF powered energy plant has low efficiency, but it is still worthy because of the environmental aspects. The other biomass fuels such as bark, sunflower shell, and wheat shell are used in co-combustion with coal in relatively higher efficiency energy plants. The exact scientific details will be interesting subjects for future studies.

 

8/ The reviewer wondered about the suitability of the investigated fuels for fluidized beds.

Discussion:

From the mixing perspective, all fuels are suitable for fluidized bed applications. However, the utilization of sunflower shell and wheat shell is somewhat more challenging and require a special design. We added one sentence to the conclusions from a suggestion of another reviewer to clarify this issue “Finally, we can conclude that the sunflower shell and wheat shell are not suitable for fluidized bed combustion because they entrain from the bed rapidly.”.

Reviewer 3 Report

The properties of a cold fluid medium are completely different from the properties (especially viscosity) in the warm working conditions. Therefore, the significance of the performed evaluation is only theoretical. Cold tests do not show a fundamental change in the properties of the fuel in the environment of the operating temperatures of the fluidized bed. Sunflower shell and wheat shell are the fuels completely not proper for the fluidized bed combustion. The results of the tests performed
and the conclusions of the article prove it.
It would be appropriate to extend the conclusions with
generalizations for the combustion of the investigated fuels.

Author Response

1) The reviewer argued the applicability and relevance of the cold flow measurements for the prototype hot basis situation.

Discussion:

We included additional text and new figure to subsection 3.2. in the revised manuscript to illustrate this issue “In fluidized bed energy applications, the operating temperature is high and it depends on the conversion process e.g., incineration, gasification, pyrolysis, and carbonization. The present measurements were performed on a cold basis to avoid conversion of the active particles. …”. Also, this new information can make the results more practical by linking the superficial velocity on the hot basis to the investigated cold basis superficial air velocity.

2) The reviewer suggested an extension of the conclusion to generalize the combustion of the investigated fuels.

Discussion:

A new sentence was added to the conclusions for generalization of combustion of the investigated fuels “Finally, we can conclude that the sunflower shell and wheat shell are not suitable for fluidized bed combustion because they entrain from the bed rapidly.”.

Reviewer 4 Report

This work presents the experimental study on particle entrainment and mixing of some biomass. The different particle size distribution and entrainment were observed, and discussed. Overall, this work might be interesting to the fluidized bed application. However, I do have some comments for this work as follow:

• Since the application of this work is fluidized bed, the major concern I have is this experimental work was completed in room temperature without considering the combustion. This will be hard to estimate how does the temperature and combustion impact on the particle entrainment in real case.
• In figure 6, the authors demonstrated the experimental data of the accumulated entrained mass fraction over the superficial air velocity. For a reliable experimental work, the error bars are strongly suggested. Because it will clarify the uncertainty of this measurement. As a result, these data can be better referred in future. 

Author Response

1/ The reviewer wondered about the impact of the temperature on the particle entrainment in real-world combustion applications.

Discussion:

We agree with the reviewer's opinion about the significance of the effect of the operating temperature. There are published experimental studies of the same field in the literature that were performed on cold flow basis. However, we believe that the hot basis must be connected to the modeling study somehow to provide a clear indication of the relevance of the study. In the meantime, the investigated fuels can be converted/processed under different operating temperatures which depend on the application/output, e.g., carbonization (300 °C), pyrolysis (600 °C), gasification (800 °C), or incineration (950 °C). For this reason, we added a new figure with explanatory text at the end of subsection 3.2 to explain this important relationship with the real-world application.

 

2/ The reviewer suggested plotting of the error bars on Fig. 6.

Discussion:

We agree with the reviewer's opinion about the importance of plotting error bars in Fig. 6. Accordingly, the figure was revised to show the error bars of the measured data. However, the data in the figure was slightly changed due to the averaging with the new measured data for showing the experimental repeatability error.

Round 2

Reviewer 1 Report

The paper “Experimental study of entrainment and mixing of renewable active particles in fluidized beds” has been substantially revised and improved.

In general, it seems that the authors have managed to successfully address my comments. Therefore I suggest accepting the paper.

Reviewer 2 Report

Thank you for your answers.