A Role of Mineral Oxides on Trace Elements Behavior during Pulverized Coal Combustion
Round 1
Reviewer 1 Report
The article refers to the important problem of reducing the emission of trace elements during the coal combustion process.
The article is written correctly. However, I have a few comments:
1. The article requires linguistic verification.
2. In the introduction, I suggest to organize the types of used sorbents (found in the literature) and summarized in the table, which will include: sorbent (first column), findings (second column; for example: Kaolinite is the best absorber for ....) and Reserchers (third column; name and year).
Among the presented sorbents to capture trace elements, the use of vanadium pentoxide and titanium dioxide (titania) is missing. Please also include them in the table.
3. The authors describe CFA (in Materials and Methods) showing the chemical composition of fly ash obtained from a power plant in Japan. I would like to know what temperature was in the pulverized coal combustion boiler (range can be given). As you know, temperature has a significant influence on trace elements behavior.
4. Did the authors measure ash fusion temperature? Are they in possession of such data? If not, it would be good to write something about it based on the literature data.
Author Response
Comments and Suggestions for Authors
The article refers to the important problem of reducing the emission of trace elements during the coal combustion process.
The article is written correctly. However, I have a few comments:
- The article requires linguistic verification.
Response 1: Thank you for your recommendation. We will do a linguistic editing service for our manuscript.
- In the introduction, I suggest to organize the types of used sorbents (found in the literature) and summarized in the table, which will include: sorbent (first column), findings (second column; for example: Kaolinite is the best absorber for ....) and Reserchers (third column; name and year).
Among the presented sorbents to capture trace elements, the use of vanadium pentoxide and titanium dioxide (titania) is missing. Please also include them in the table.
Response 2: Thank you for your recommendation. We summarized into Table 1. Please take a look at the manuscript.
- The authors describe CFA (in Materials and Methods) showing the chemical composition of fly ash obtained from a power plant in Japan. I would like to know what temperature was in the pulverized coal combustion boiler (range can be given). As you know, temperature has a significant influence on trace elements behavior.
Response 3: The temperature was max. 1500 °C.
- Did the authors measure ash fusion temperature? Are they in possession of such data? If not, it would be good to write something about it based on the literature data.
Response 4: No, we do not measure the ash fusion temperature and we mention the literature data about fusion temperature in section 4.2.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The review of the manuscript is provided in the appendix.
Comments for author File: Comments.pdf
Author Response
Comments and Suggestions for Authors
The manuscript deals an important topic from the point of view of the increasing current demand for coal. The method of equilibrium modelling of trace elements during coal combustion was correctly selected since the kinetics of the trace element reaction has not been developed. In the last paragraph of the introduction, the Authors motivate the aim of the study, pointing to the lack of thermodynamic calculations reflecting the interaction of trace elements with metal oxides, e.g., CaO, MgO, Al2O3, Fe2O3 and K2O. I recommend that you read the articles where this issue has been studied. e.g. 10.1021/acs.energyfuels.6b02814, 10.1080/15567036.2015.1110643 Moreover, the manuscript lacks a description of the knowledge, what has already been developed in the modelling of trace elements during the combustion process, and what is still lacking e.g., the effect of: gas recirculation, pressure, the combustion atmosphere, excess air combustion, the combustion temperature etc. Detailed comments:
- It is necessary to add an explanation before Table 1, which method was used to determine the ash composition: XRF?, ICP-OES?.
Response 1: Thank you for your recommendation. We added the method to determine the ash composition. The method is XRF. Please take a look at the manuscript.
- Coal fly ash does not contain unburned carbon - can the Authors explain this?
Response 2: In my understanding, if the unburned carbon contain in fly ash can be an indicator of inefficiencies in the combustion process, and an excessive proportion of unburned carbon in the fly ash means a significant loss of energy in such ash power production. So, the power thermal do the separation of unburned carbon from fly ash to achieve a higher efficiency in the utilization of waste fly ash and greater economic and environmental benefits.
- I propose to modify Table 1: Add the notation: main oxides (%) Below, leave trace elements and add a unit.
Response 3: Thank you for your suggestion, we modified it.
- The description of the calculation procedure should also include system pressure and input reactants temperature. It is a reference point for calculating Gibbs free energy in the systems under consideration.
Response 4: Thank you for your suggestion, we rewrite the calculation procedure and the information of system pressure and reactants temperature. Please take a look at the manuscript.
- There are no liquid compounds in the reaction products, e.g. KBO2_liquid or K2B4O7 Has it been assumed that the liquids will not be formed in the considered systems?
Response 5: Yes, during the studying interaction of trace elements-minerals oxide or trace elements-all coal fly ash components, no liquid compounds in the reaction products were detected. So we assumed that the liquids not be formed in this considered systems, but when we input all trace elements-all coal fly ash component interaction, liquid compounds was formed as we mention in the figure respond no. 7.
- The oxide composition of ash was adopted for thermodynamic modelling. This is a considerable simplification, and it is worth mentioning it in the calculation procedure. Coal fly ash will never consist of only oxides, which can be confirmed by XRD analysis (crystalline and amorphous phases).
Response 6: Thank you for your suggestion. Yes, coal fly ash will never consist of only oxides, also consist crystalline and amourphous phases, such as (CaCO3) and ettringite (Ca6Al2(SO4)3(OH)12·26H2O) as reported by Yasumasa Ogawa et al. (2018). In case to our study, the chemical properties of the coal fly ash sample was determined based on X-ray fluorescence (XRF) as shown in Table 1. We do not do yet the XRD analysis. For further study, we will analysis it.
- The manuscript adopted a calculation model: one trace element - one metal oxide. In this way, the possibility of interaction of trace elements (e.g. CrAsO4) was eliminated. How could it influence the presented calculation results?
Response 7: Yes, in our manuscript, we study one trace elements – one metal oxide, then one trace element – all minerals in coal fly ash. Based on the literature studying by M.L Contreras et al. (2009), they reported that despite an interaction between trace elements is possible, it must be noticed that the low concentration of these elements in a real combustion system, make decrease the potential reactions. So that, we neglected the interaction of trace elements. We also study the possibility of trace elements interaction as shown in figure below, but we do not add the results to our manuscript.
- Line 121: Calculations were made from 100 °C, but in this line it is written that from 0°C.
Response 8: We revised and rewrite it to “from 100 °C”.
- Table 2 lists the main speciation forms of the elements studied. In the "B - O2 - K" system, KBO2 (s) was indicated. But when T < 900 °C, the dominant form of the boron compound is KBO2 (g).
In addition, I would like to ask you to explain why there are TEs-air-metal oxides systems in the description of the figures, while in Table 2 there are TEs-O2-metal
Response 9: Yes, in Table 2 we listed the main speciation of the elements studied during combustion process (Temperatures in the range 100-1600 °C). In case in Table 2, we listed only for the main speciation in solid species that form during combustion process. As mentioned in Figure 2.d, interaction B-O2-K2O, when T < 900 °C, boron mostly form in the gaseous specie KBO2(g), but when temperature reaches higher, the KBO2(s) is the dominant specie, that why we listed this species in Table 2. Related to TEs-air-metal oxides and TEs-O2-metal oxides, the correct one is TEs-O2-metal oxides as same as our calculation procedure using pure oxygen, so we change the description of the figure from TEs-air-metal oxides TO TEs-O2-metal oxides. Please take a look at the manuscript.
- Line 216: “slag” – please use identical nomenclature throughout the manuscript, in this case it will be (s) from the word solid.
Response 10: Thank you for your suggestion, we rewrite it.
- Due to the fact that global equilibrium system is analysed, there is no justification for presenting specific reactions (i.e. equation 1-5). If the authors had determined the Gibbs free energy for each possible reaction as a function of temperature, this is an argument. However, I did not see such a thermodynamic prediction in this manuscript.
Response 11: In our study, the determination of specific reactions equation 1-5 based on the FactSage analysis. After the predicted species ware form, so we predict the possible reaction based on the input reactants and the species formed. We do not determined the possible reaction based the Gibbs free energy as a function of temperature.
- Table 3. Ionic forms of the elements were the products of combustion? E.g. “B+” or “B-“?
Response 12: In our results study using FactSage 7.2, the ionic B+ and B- were formed and detected, so we added this ionic forms of the elemental to our results.
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear Editor,
The MS entitled with “A Role of Mineral Oxides on Trace Elements Behavior during Pulverized Coal Combustion” reports theoretical prediction of selected hazardous trace elements retention by major oxides during coal combustion. The organization and theoretical background of the study are quite well; however, the authors should be careful with typing element names and citations in entire MS. Although using software like FactSage 7.2 could be useful, the theoretical predication of trace element retention during coal combustion should also be supported by the direct methods. Since concerned elements like B, F and Se are very volatile during coal combustion and not easy to detect by the SEM-EDX, the authors could perform SEM-EDX analyses for As and Cr in order to find out main As- and Cr-bearing minerals or phases in the studied FA sample. Thus, the prediction about As and Cr could be more robust. I added several corrections and suggestions in the revised MS. The authors should check them carefully. Overall, I would like to reconsider the MS after suggested corrections will have done.
Comments for author File: Comments.pdf
Author Response
The MS entitled with “A Role of Mineral Oxides on Trace Elements Behavior during Pulverized Coal Combustion” reports theoretical prediction of selected hazardous trace elements retention by major oxides during coal combustion. The organization and theoretical background of the study are quite well; however, the authors should be careful with typing element names and citations in entire MS. Although using software like FactSage 7.2 could be useful, the theoretical predication of trace element retention during coal combustion should also be supported by the direct methods. Since concerned elements like B, F and Se are very volatile during coal combustion and not easy to detect by the SEM-EDX, the authors could perform SEM-EDX analyses for As and Cr in order to find out main As- and Cr-bearing minerals or phases in the studied FA sample. Thus, the prediction about As and Cr could be more robust. I added several corrections and suggestions in the revised MS. The authors should check them carefully. Overall, I would like to reconsider the MS after suggested corrections will have done.
Response: Thank you for your suggestion and recommendation of several literature on support our manuscript, we rewrite and revise it based on your suggestion. For more detail please take a look at manuscript.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The revised version of the manuscript is suitable for publication in the Minerals journal.
Author Response
Dear Reviewer
Thank you for your advise and suggestion to our manuscript.
Author Response File: 
Author Response.docx
Reviewer 3 Report
The authors did most of the suggested corrections, especially about the typo errors. Please write “arsenic” as “As” in the entire text, expect the beginning of a paragraph (over than 100 times “As” was written as “Arsenic”). Furthermore, the authors should cross-check entire text for elements’ symbols. One point, I did not understand that why the authors provide a table about literature overview instead of writing a brief summary paragraph about literature in the introduction chapter. Secondly, the authors did not provide a basic data about feeding coal of analyzed fly ash. Because the source matter is also important for understanding elemental compositions of fly ashes and chemistry of the boiler atmosphere during combustion. As I noted earlier, in order to predict behaviors of trace elements during combustion, we need to have microanalytical analyses (e.g., SEM-EDX). With this way we could find out affiliation of elements in FAs more correctly, and software do not always show to use certain behaviors of trace elements during combustion. Please, take in consider this point in your future studies. Overall, I suggest a moderate minor correction, and I would like to see the revised MS before sending to production service.
Author Response
Dear Reviewer
Thank you for your advise and suggestion to our manuscript. We make a revision related your suggestion and studying more based on your advise.
Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
The authors did almost suggested corrections in the second round. I do not have any further comments.
