Particle Size Distribution and Its Impacts on Ash Deposition and Radiative Transfer during Oxy-Combustion of Rice Husk–Natural Gas

Round 1

Reviewer 1 Report

Very interesting article on the use of leftover biomass with oxy-fuel combustion. My main point of criticism is that the structure of the article feels somewhat odd., making it difficult for a reader who is not exactly active in the same field to follow (it was so at least for me). Much of what is included in your introduction belongs in a separate section describing the state-of-the-art, in my opinion, while the introduction should serve to introduce the reader to the topic at hand. When starting to read the article, it is not exactly clear which application you are talking about, i.e. oxy-fuel combustion as it is used in power generation, i.e. the combustion of a solid fuel with an O2/CO2 blend as a preparation for CCS, or oxy-fuel as it is for example used in the glass or metals industries as a way to enhance process performance. There is important information missing to provide some context for your work. Also, terms such as "2nd generation oxy-fuel" should be explained here, for me at least, this term was quite new.

Another part that would improve your work is a CO2 balance, in my opinion. The main purpose of the technology that you describe is to reduce GHG emissions, so if you look at this technology, the potential for GHG emissions reduction (or even "negative" emissions) ought to be assessed in comparison to a reasonable reference case. 

There are some minor language issues (articles missing, some typos, e.g. "Or instance" instead of "For instance", the layout is sometimes a bit odd, but my main suggestion would be to change the structure of the text somewhat to make it more accessible and allow readers to put it into context.

Author Response

Itemized response to reviewer comments (Ms. No. methane-2306364)

We would like to thank the reviewers for their time and comments. An itemized response to the review questions are provided below. We believe the reviews greatly assisted towards improving the quality of the manuscript without altering our original results and conclusions.

Reviewer #1:

1. Very interesting article on the use of leftover biomass with oxy-fuel combustion. My main point of criticism is that the structure of the article feels somewhat odd making it difficult for a reader who is not exactly active in the same field to follow (it was so at least for me). Much of what is included in your introduction belongs in a separate section describing the state-of-the-art, in my opinion, while the introduction should serve to introduce the reader to the topic at hand.

Response: We appreciate your encouragement and value your comment. Ash deposition and radiative transfer are two inter-dependent processes within a biomass combustor that are strongly governed by the fly-ash PSD. Further, this has not been investigated in second generation, atmospheric pressure oxy-combustion systems. To fill this void, this paper attempts a comprehensive assessment of both these phenomena as a function of fuel and fly-ash PSD. Therefore, the Introduction section is a bit long to enable a succinct summarization of the current state of the art in ash deposition as well as radiative transfer as it relates to oxy-combustion systems.

An additional line (Line 98) in the revised manuscript has been included to highlight this.

In addition, lines 115 – 139 in the revised manuscript raise the questions and the knowledge gap that this manuscript is attempting to fill.

2. When starting to read the article, it is not exactly clear which application you are talking about, i.e. oxy-fuel combustion as it is used in power generation, i.e. the combustion of a solid fuel with an O2/CO2 blend as a preparation for CCS, or oxy-fuel as it is for example used in the glass or metals industries as a way to enhance process performance. There is important information missing to provide some context for your work. Also, terms such as "2nd generation oxy-fuel" should be explained here, for me at least, this term was quite new.

Response: Thank you for your comment! Lines 47 – 51 in the Introduction has been revised to include the definition and benefits of second generation, atmospheric pressure oxy-combustion systems.

Second generation, atmospheric pressure oxy-combustion systems use an oxidizer stream that is highly enriched in oxygen (often > 50% O₂ by volume, OXY50) aimed at minimizing the volumetric flow rate of flue gas flowing through the combustor (as recycle), thereby potentially improving efficiencies.

Oxy-combustion in the context of this study may be employed to both power generation as well as high temperature processes associated with glass melting and metal operations. However, since gaseous fuels are more typically employed in the industrial scenarios, the problem of ash deposition and particle radiation may not arise there.

3. Another part that would improve your work is a CO2 balance, in my opinion. The main purpose of the technology that you describe is to reduce GHG emissions, so if you look at this technology, the potential for GHG emissions reduction (or even "negative" emissions) ought to be assessed in comparison to a reasonable reference case. 

Response: Thank you for your comment! We agree with your comment that the potential for GHG emission reduction should be assessed with respect to a reasonable reference case. In addition, we would like to add that the reduction in GHG emission has to be looked at in conjunction with economics and the cost of electricity production. In the context of atmospheric pressure oxy-combustion there are significant economic costs and overall plant efficiency reductions associated with: air separation, flue gas recycle and CO2 compression. There are several recent papers that address this, for instance:

Gopan, A., Verma, P., Yang, Z. and Axelbaum, R.L., 2020. Quantitative analysis of the impact of flue gas recirculation on the efficiency of oxy-coal power plants. International Journal of Greenhouse Gas Control, 95, p.102936.

The goal of second generation, atmospheric pressure oxy-combustion systems is to use an oxidizer stream that is highly enriched in oxygen (often > 50% O₂ by volume, OXY50) aimed at minimizing the volumetric flow rate of flue gas flowing through the combustor (as recycle), thereby potentially improving efficiencies.

4. There are some minor language issues (articles missing, some typos, e.g. "Or instance" instead of "For instance", the layout is sometimes a bit odd, but my main suggestion would be to change the structure of the text somewhat to make it more accessible and allow readers to put it into context.

Response: Thank you for your comment. The revised manuscript has been carefully checked for grammar and typos and changes have been made where appropriate.

Reviewer 2 Report

The paper is very well written, and the add-on models for ash deposition/shedding and radiative properties were employed in CFD simulations, it revealed that enhanced ash deposition rates conditions with deposition/shedding also dependent on the particle size distribution (PSD) of the parent RH fuel. There are some problems, which must be solved before it is considered for publication.

The suggestions for enriching the work in my opinion are:

1）What is the novelty of the present work? Why it was necessary to discuss current study? What is the applied value of the current study?

2）In my opinion, the diagrams and explanations of Figure 3 do not better express the author's intention, the format of Figure 3. (a) and Figure 3. (b) are not uniform. I suggest that the author check all Figures and tables，and use a uniform format for some tables and Figures to make them more readable.

3）I suggest the authors to improve the text quality and correct some grammar errors.

Author Response

Itemized response to reviewer comments (Ms. No. methane-2306364)

We would like to thank the reviewers for their time and comments. An itemized response to the review questions are provided below. We believe the reviews greatly assisted towards improving the quality of the manuscript without altering our original results and conclusions.

Reviewer #2:

The paper is very well written, and the add-on models for ash deposition/shedding and radiative properties were employed in CFD simulations, it revealed that enhanced ash deposition rates conditions with deposition/shedding also dependent on the particle size distribution (PSD) of the parent RH fuel. There are some problems, which must be solved before it is considered for publication.

 

Response: Thank you! We appreciate your encouragement.

 

1）What is the novelty of the present work? Why it was necessary to discuss current study? What is the applied value of the current study?

Response: Thank you for your comment. Ash deposition and radiative transfer are two inter-dependent processes within a biomass combustor that are strongly governed by the fly-ash PSD. Further, this has not been investigated in second generation, atmospheric pressure oxy-combustion systems. To fill this void, this paper attempts a comprehensive assessment of both these phenomena as a function of fuel and fly-ash PSD. Therefore, the Introduction section is a bit long to enable a succinct summarization of the current state of the art in ash deposition as well as radiative transfer as it relates to oxy-combustion systems.

In addition, lines 115 – 139 in the revised manuscript raise the questions and the knowledge gap that this manuscript is attempting to fill.

 

2）In my opinion, the diagrams and explanations of Figure 3 do not better express the author's intention, the format of Figure 3. (a) and Figure 3. (b) are not uniform. I suggest that the author check all Figures and tables and use a uniform format for some tables and Figures to make them more readable.

Response: Thank you! All tables and figures have been checked carefully. Since Figure 3 highlights the particle kinetic energy – particle viscosity based particle capture framework employed in this study, we could not discern a way of improving these figures. In summary,

Figure 3a shows the accuracy of the particle viscosity model employed in our study by comparing against recent measurements of viscosities of the silica rich RH ash.

Figure 3b, shows the sticking criterion (Eqs. 3 and 4) represented as a function of particle viscosity (µP) and particle kinetic energy (PKE) is represented as a diagonal line demarcating the sticking and rebounding conditions.

 

3）I suggest the authors to improve the text quality and correct some grammar errors.

Response: Thank you for your comment. The revised manuscript has been carefully checked for grammar and typos and changes have been made where appropriate.

 

Reviewer 3 Report

Overall it's a good study but I have following suggestions:

1. The paper is too long, please try to condense it.

2. Show picture of mesh resolution near the deposition probe. 

3. Mention y+ and wall treatment details also because they are important for heat transfer.

4. In figure 2, why measurement and modeled data is put on same plot? It's hard to understand how good/bad the match is.

5. Do we really need table 5? Can it be not covered in introduction? 

6. In Figure 5, measurement and simulation temperatures are quite different for axial distance < 1 m. Any explanations? Also, it's an overstatement to say call it an "excellent agreement" between measurement and predictions because there are clear differences.

Author Response

Itemized response to reviewer comments (Ms. No. methane-2306364)

We would like to thank the reviewers for their time and comments. An itemized response to the review questions are provided below. We believe the reviews greatly assisted towards improving the quality of the manuscript without altering our original results and conclusions.

Reviewer #3:

 

Overall it's a good study but I have following suggestions:

1. The paper is too long, please try to condense it.

Response: We appreciate your encouragement and value your comment. Ash deposition and radiative transfer are two inter-dependent processes within a biomass combustor that are strongly governed by the fly-ash PSD. Further, this has not been investigated in second generation, atmospheric pressure oxy-combustion systems. To fill this void, this paper attempts a comprehensive assessment of both these phenomena as a function of fuel and fly-ash PSD. Therefore, the Introduction section is a bit long to enable a succinct summarization of the current state of the art in ash deposition as well as radiative transfer as it relates to oxy-combustion systems.

In addition, lines 115 – 139 in the revised manuscript raise the questions and the knowledge gap that this manuscript is attempting to fill.

 

2. Show picture of mesh resolution near the deposition probe.

Response: Thank you for your valuable suggestion! Figure 1c in the revised manuscript shows the mesh resolution near the deposition probe.

 

3. Mention y+ and wall treatment details also because they are important for heat transfer.

Response: Thank you for your comment. The following sentences have been included in Lines 174 – 176 of the revised manuscript:

Although, the maximum y+ at the probe resulting from the finely resolved mesh (cf. Figure 1c) was less than 1 across all investigated scenarios, the y+ was greater than 1 in several regions away from the probe. Consequently, the Menter-Lechner (y+ in-sensitive) modeling option was employed for the near-wall treatment.

 

4. In figure 2, why measurement and modeled data is put on same plot? It's hard to understand how good/bad the match is.

Response:  Thank you for your comment. The goal of Figure 2 was to highlight the reasonableness of modeling the PSD employed in the simulations. The measured parent fuel PSD and fly-ash PSD in Figure 7 were fit to the functional form of a Rosin-Rammler distribution function and simulated using 100 discrete bins. Such a fine size resolution is generally more than adequate to get accurate predictions of ash impaction and particle kinetic energy distributions near the deposit surface.

 

5. Do we really need table 5? Can it be not covered in introduction?

Response: We value your comment. While the particle radiation (and its radiative properties) did not influence the results of this study, we felt it was important to highlight that the absorption and scattering efficiencies (Qabs, Qscat) employed in our study (and reported in Table 6) were in line with more sophisticated estimates made from Mie Theory in the literature and did not have any bearing on our conclusion that gas radiation did dominate in these systems. Further, we felt it was best to place Table 5 close to equations 1 and 2 where particle absorption (k_abs) and scattering coefficients (k_scat) of the particles were computed from absorption (Q_abs) and scattering efficiencies (Q_scat).

 

6. In Figure 5, measurement and simulation temperatures are quite different for axial distance < 1 m. Any explanations? Also, it's an overstatement to say call it an "excellent agreement" between measurement and predictions because there are clear differences.

Response: Thank you for this comment. It is generally difficult to predict the temperatures accurately inb the high temperature regions in the vicinity of the burner. Experimental uncertainties in this highly turbulent region in conjunction with refining our devolatilization and char combustion methodologies might improve the agreement. However, as shown in Figure 4, complete char burnout has been accomplished in the ignition zone well upstream of the deposit probe. Since no unburnt carbon was noted in the probe ash deposits in the experiments this provides a preliminary confirmation regarding the adequacy of combustion models employed in the simulations (cf. Table 4). In addition, a slightly earlier onset of burnout prediction (closer to the burner) is noted in the OXY70 scenarios as a result of increased thermal (higher temperatures) and mass diffusion (higher O₂ concentrations). This is aligned with visual observations of OXY flames in this combustor where the flame lift-off region was closer to the burner at enriched oxygen concentrations in the oxidizer stream (Zhang et al., 2011).

Zhang, Jingwei, Kerry E, Kelly, Eric G, Eddings, Jost OL. Wendt Ignition in 40 kW co-axial turbulent diffusion oxy-coal jet flames. Proceed Combust Instit 2011;33(2):3375–82.