Calcium-Bearing Minerals Transformation during Underground Coal Gasification
Round 1
Reviewer 1 Report
Authors adequately answered the reviewer's questions and comments and made respective changes to the paper.
Author Response
Shuqin Liu
China University of Mining and Technology (Beijing)
D11, Xueyuan Road
Haidian District,
Beijing, 100083
P.R. China
E-mail: 13910526026@163.com
November 11, 2019
Re: minerals-633678Title: "Calcium-Bearing Minerals Transformation during Underground Coal Gasification"
Dear Reviewer,
Thank you very much for your comments for our manuscript (minerals-633678). We appreciate these careful reviews and detailed comments, which are very useful and have been incorporated into the revised manuscript. I and my coauthors hereby submit our revised manuscript.
The following is a list of the changes we have made based on the suggestions.
Reviewer 1
Comments and Suggestions for Authors
Authors adequately answered the reviewer's questions and comments and made respective changes to the paper.
Response: Thanks.
We hope the revised version of the manuscript is acceptable for publication. However, please let me know if you have any further questions or comments.
With best wishes,
Shuqin Liu
On behalf of all co-authors
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript describes the transformation of calcium minerals during UCG process, providing better understanding about the mechanism of ash transformation during UCG which is relatively rare. I recommended to accept this manuscript but with minor revision.
My comments are as follows.
1. I think author should mention in introduction that UCG process is able to address the ash-related problems (fouling, slagging, agglomeration) of Zhundong coal because it doesn't need a boiler or gasifier.
2. In introduction, author mentioned that the study of mineral characteristics of ash is important to explore the actual reaction condition of UCG process. I wondered how and when ash minerals are collected during a real UCG process.
3. Author should mention equipment used for SEM-EDS analysis in section of sample characterization
4. In line 222-223, author speculated that the mineral is oldhemite because the ratio of S to Ca is close to 1:1. However, the contents of S and Ca are very small (about 1%) and close to that of Mg (0.54%). Therefore, it is not oldhemite, but probably the reduction form of ankerite.
5. It would better to exclude the value of C when determining elemental components of ash by SEM-EDS analysis, particularly for fig 5-6.
6. Figure 9 should be revised by adding arrows of another components such as SiO2, Al2O3 or Fe2O3 to describe the conversions of CaO to Gahlenite and Gahlenite to Anorthite, and finally Anorthite to iron-bearing minerals.
minor mistakes
1. In line 195, "formed from" is more appropriate than "converted from"
2. the mark/typo of celsius should be revised in line 162,174,186,194,203, 204, 284, 286, 297, 314, 315, 342, 349, 353, 354.
Author Response
Shuqin Liu
China University of Mining and Technology (Beijing)
D11, Xueyuan Road
Haidian District,
Beijing, 100083
P.R. China
E-mail: 13910526026@163.com
November 12, 2019
Re: minerals-633678Title: "Calcium-Bearing Minerals Transformation during Underground Coal Gasification"
Dear Reviewer,
Thank you very much for your comments for our manuscript (minerals-633678). We appreciate these careful reviews and detailed comments, which are very useful and have been incorporated into the revised manuscript. I and my coauthors hereby submit our revised manuscript.
The following is a list of the changes we have made based on the suggestions.
Reviewer 2
Comments and Suggestions for Authors
The manuscript describes the transformation of calcium minerals during UCG process, providing better understanding about the mechanism of ash transformation during UCG which is relatively rare. I recommended to accept this manuscript but with minor revision.
My comments are as follows.
I think author should mention in introduction that UCG process is able to address the ash-related problems (fouling, slagging, agglomeration) of Zhundong coal because it doesn't need a boiler or gasifier.
Response: Thanks. We have added the corresponding information in the “introduction” as follows: “Due to characteristics of high volatile substances, Zhundong coal is easily ignited. However, the severe scaling and slagging problems during the pyrolysis or combustion process limit its utilization as cleaning fuel. The main reason is due to the high contents of alkali and alkaline earth metallic species (AAEMs) in the coal. Underground coal gasification can solve the ash-related problems (fouling, slagging, agglomeration) of Zhundong coal because it doesn’t need a boiler or gasifier facility. This paper focuses on the transformation of calcium-bearing minerals during the UCG process”.
In introduction, author mentioned that the study of mineral characteristics of ash is important to explore the actual reaction condition of UCG process. I wondered how and when ash minerals are collected during a real UCG process.
Response: After the completion of underground coal gasification, the sample of ash and slag left underground are sampled by drilling. These samples are suffered from different heating conditions, calcium bearing minerals may reflect the reaction temperature during the UCG process. This is always used in the stage of field test before commercialization.
|
Raw coal |
Gypsum (CaSO4.2H2O) |
|
Semi-coke (800 °C) |
Anhydrite (CaSO4) |
|
Reduction ash (900-1300 °C) |
Oldhamite (CaS) |
|
Oxidation ash (1100-1500 °C) |
1100 °C (Gehlenite (Ca2Al2SiO7)) |
|
1200 °C (Gehlenite (Ca2Al2SiO7)) |
|
|
1300 °C (Gehlenite (Ca2Al2SiO7) (Mullite (Al6Si2O13)) |
|
|
1400 °C (Anorthite (CaAl2Si2O8)) |
|
|
1500 °C (Cordierite (Fe2Al4Si5O18)) |
Author should mention equipment used for SEM-EDS analysis in section of sample characterization.
Response: Thanks. We have added the corresponding information in the “Sample Characterization” as follows:
“A field emission scanning electron microscope (FE-SEM, MERLIN Compact, Zeiss, Jena, Germany) and an energy dispersive X-ray spectrometer (EDS, INCA, Oxford, UK) were used to study the morphology and compositions of raw coal, ash and slag, as well as the distribution of main elements.”
In line 222-223, author speculated that the mineral is oldhamite because the ratio of S to Ca is close to 1:1. However, the contents of S and Ca are very small (about 1%) and close to that of Mg (0.57%). Therefore, it is not oldhamite, but probably the reduction form of ankerite.
Response: Thanks for your suggestions. We have revised in the manuscript and added the corresponding information as follows:
“After the analysis of the ash surface elements, the results (Figure 5B) indicate that the contained elements in the mineral are complex, including C, O, Na, Mg, Al, Si, S, and Ca, and the ratio of S to Ca is close to 1:1. However, the content of S and Ca are very small (about 1%) and close to that of Mg (0.57%). Thus, we speculate that the mineral is the reduction form of ankerite.”
It would better to exclude the value of C when determining elemental components of ash by SEM-EDS analysis, particularly for fig 5-6.
Response: Thanks. Your suggestion is very useful for our research, but this time it is too late to make an appointment for SEM-EDS experiment. We will consider this suggestion in the following researches.
Figure 9 should be revised by adding arrows of another components such as SiO2, Al2O3 or Fe2O3 to describe the conversions of CaO to Gehlenite and Gehlenite to Anorthite, and finally Anorthite to iron-bearing minerals.
Response: Thanks. We have revised in the manuscript and the Figure 9 has been redrawn as follows:
Figure 9. Sequential transformation of calcium-bearing minerals in UCG process.
minor mistakes
In line 195, "formed from" is more appropriate than "converted from" the mark/typo of Celsius should be revised in line 162,174,186,194,203, 204, 284, 286, 297, 314, 315, 342, 349, 353, 354.
Response: Thanks. We have revised in the manuscript.
We hope the revised version of the manuscript is acceptable for publication. However, please let me know if you have any further questions or comments.
With best wishes,
Shuqin Liu
On behalf of all co-authors
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
This article is well organized but the introduction and abstract part can be improved.
For example: Although the coal is easily ignited due to the high volatile substances, but the severe scaling and slagging problems during the pyrolysis or combustion process limit its utilization as cleaning fuel. What is the relationship between this sentence and your research aims?
Besides, please improve the abstract by state your research novelty and impact. Please check the logic and sequence of your statement carefully.
Author Response
Shuqin Liu
China University of Mining and Technology (Beijing)
D11, Xueyuan Road
Haidian District,
Beijing, 100083
P.R. China
E-mail: 13910526026@163.com
September 27, 2019
Re: minerals-594286Title: "Calcium-Bearing Minerals Transformation during Underground Coal Gasification"
Dear Reviewer,
Thank you very much for your comments for our manuscript (minerals-594286). We appreciate these careful reviews and detailed comments, which are very useful and have been incorporated into the revised manuscript. I and my coauthors hereby submit our revised manuscript.
The following is a list of the changes we have made based on the suggestions.
Reviewer 1
Comments and Suggestions for Authors
This article is well organized but the introduction and abstract part can be improved.
For example: Although the coal is easily ignited due to the high volatile substances, but the severe scaling and slagging problems during the pyrolysis or combustion process limit its utilization as cleaning fuel. What is the relationship between this sentence and your research aims?
Besides, please improve the abstract by state your research novelty and impact. Please check the logic and sequence of your statement carefully.
Response: The purpose of this sentence is to explain the characteristics of Zhundong coal and the existing problems in its utilization. For surface gasification process, the severe scaling and slagging problems during the pyrolysis or combustion process limit its utilization as cleaning fuel. But for UCG process, the coal seam is fixed while the fire face (or gasification face) moves, thus the slagging problems will not block the diffusion of air/oxygen into the fresh coal surface. In addition. the molten ash/slag can also form an underground support body, which is beneficial to the penetration of gas in the coal seam.
We have rewritten the abstract and introduction in the revised manuscript. The abstract has been rewritten as follows:
Abstract: Calcium-bearing minerals are one of the main typical minerals in coal and coal ash. In the process of coal thermal conversion, calcium-bearing minerals undergo different morphological transformation in which the reaction temperature, pressure, and atmosphere are important factors affecting their transformation. The reaction process of underground coal gasification (UCG) could be clearly divided into pyrolysis, reduction, and oxidation and the typical calcium-bearing minerals are expected to indicate the actual reaction conditions of UCG. A high-calcium coal, Zhundong coal, was used in this research. The products of UCG were prepared and the minerals were identified by X-ray diffraction (XRD) and a scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The thermodynamic calculation was used to assist in understanding the transformation behaviors of calcium-bearing minerals. The experimental results show that the calcium-bearing mineral is gradually converted from gypsum (CaSO4·2H2O) in the raw coal into anhydrite (CaSO4) during the pyrolysis process. In the reduction stage, anhydrite reacts with the reducing gas (CO) to produce oldhamite (CaS), and the oldhamite is stably present in the reduction ash. During the oxidation process, oldhamite is first transformed into CaSO4, and then CaSO4 is converted into CaO. Finally, CaO reacts with Al2O3 and SiO2 to produce gehlenite (Ca2Al2SiO7) at 1100 ℃. As the oxidation temperature rises to 1400 ℃, gehlenite is transformed into the thermodynamically-stable anorthite (CaAl2Si2O8). With the further progress of the reaction, anorthite will co-melt with iron-bearing minerals above 1500 ℃. The ternary phase diagram of SiO2-Al2O3-CaO system proves that anorthite and gehlenite are the typical high-temperature calcium-bearing minerals when the mole fraction of SiO2 is higher than 0.6. Moreover, the gehlenite is converted to anorthite with the temperature rises, which is consistent with the experimental results. This study provides a scientific basis for understanding the UCG reaction conditions.
We hope the revised version of the manuscript is acceptable for publication. However, please let me know if you have any further questions or comments.
With best wishes,
Shuqin Liu
On behalf of all co-authors
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper outlines results of a mainly experimental laboratory study on the transformation of calcium containing minerals in one particular type of coal. Since the experiments shall simulate the behavior under underground coal gasification, the experimental sequence covers pyrolysis, reduction and oxidation of the samples. Samples are characterized by XRD and SEM. The experimental investigations are accompanied by thermodynamic calculations. In principle, the obtained results confirm the transformation behavior found for other Ca-rich fuels. The following questions and comments should be considered before this paper is suitable for publication.
Chapter 2. The main experimental parameters, e.g. gas compositions, flow rate, temperatures etc., need to be to be given in this section, not only in a reference and also not in the introduction.
Chapter 2.4. Which database was used? Which solution phases were selected?
167, MgCO3. According to XRD ankerite is present as Mg-containing carbonate.Fig. 5, related discussion. What is the fate of Fe, Si, Al etc. when the respective minerals disappear? What about the amorphous part? If FT=1280°C there should be slag at 1300°C under reducing conditions. This should be visible in XRD and SEM images of 1300°C samples. There should be somehow a closed mass balance for all elements.
216 and 218. Stoichiometry is wrong.Fig. 9. Similar to Fig. 5., show all detected minerals including amorphous content if possible or label as “selected” minerals.
Chapter 3.5. It is a pity that authors use FactSage only for calculation of phase diagrams, here. At least the phase diagram of the system Al2O3-SiO2-CaO can easily be taken from literature. For example, more complex calculations using the equilib module could be done to calculate the equilibrium for the entire ash system or parts of the system under the used experimental conditions. A comparison of such results with the experimental results would help to understand which observed mineral transformations reach equilibrium and which are kinetically constrained. This information would be of great interest.
264. CaO is an alkaline earth metal oxide.Author Response
Shuqin Liu
China University of Mining and Technology (Beijing)
D11, Xueyuan Road
Haidian District,
Beijing, 100083
P.R. China
E-mail: 13910526026@163.com
September 27, 2019
Re: minerals-594286Title: "Calcium-Bearing Minerals Transformation during Underground Coal Gasification"
Dear Reviewer,
Thank you very much for your comments for our manuscript (minerals-594286). We appreciate these careful reviews and detailed comments, which are very useful and have been incorporated into the revised manuscript. I and my coauthors hereby submit our revised manuscript.
The following is a list of the changes we have made based on the suggestions.
Reviewer 2
Comments and Suggestions for Authors
The paper outlines result of a mainly experimental laboratory study on the transformation of calcium containing minerals in one particular type of coal. Since the experiments shall simulate the behavior under underground coal gasification, the experimental sequence covers pyrolysis, reduction and oxidation of the samples. Samples are characterized by XRD and SEM. The experimental investigations are accompanied by thermodynamic calculations. In principle, the obtained results confirm the transformation behavior found for other Ca-rich fuels. The following questions and comments should be considered before this paper is suitable for publication.
Chapter 2. The main experimental parameters, e.g. gas compositions, flow rate, temperatures etc., need to be to be given in this section, not only in a reference and also not in the introduction.
Response: Thanks. We have added the corresponding experimental parameters in the manuscript as follows: “During the pyrolysis process, coal samples were placed in the experimental system with N2 (2L/min) protective atmosphere. The temperature rises from room temperature to 800 ℃, the heating rate is 5 ℃/min, and the constant temperature is 30 min; During the reduction stage, H2O (g) (5g/min) and CO2 (2L/min) were injected into the experimental system, the completion times of the reduction process from 900 ℃ to 1300 ℃ were 55 min, 40 min, 25 min, 15 min and 20 min, respectively; During the oxidation stage, the completion time of the oxidation process from 1100 ℃ to 1500 ℃ was about 60 min. ”
Chapter 2.4. Which database was used? Which solution phases were selected?
Response: For the thermodynamic calculations, the databases Ftoxid and FactPs were used, and the solution phases of Ftoxid-SLAG and Ftoxid-oPyr were selected.
167, MgCO3. According to XRD ankerite is present as Mg-containing carbonate.
Response: Yes. The formula of ankerite is (Fe, Ca, Mg) CO3. We made revision in the manuscript.
Fig. 5, related discussion. What is the fate of Fe, Si, Al etc. when the respective minerals disappear? What about the amorphous part? If FT=1280 °C there should be slag at 1300 °C under reducing conditions. This should be visible in XRD and SEM images of 1300 °C samples. There should be somehow a closed mass balance for all elements.
Response: These questions could be clarified in the following Figure 1 and Table 1, which gives the quantitative analysis of XRD, in which we use tridymite instead of amorphous. Slag is formed in the reduction ash at 1300 °C, as shown in the following Figure 1. Since the manuscript focus on the calcium-bearing minerals, these date for other minerals are not included.
Figure 1. 1300 °C reduction slag
|
Reduction ash |
Magnetite |
Hematite |
Oldhamite |
Lime |
Periclase |
Goethite |
Vaterite |
Calcite |
Nepheline |
Magnesite |
Andradite |
|
900 °C |
14% |
7% |
29% |
7% |
10% |
10% |
19% |
26% |
13% |
- |
- |
|
1000 °C |
8% |
- |
54% |
- |
9% |
6% |
13% |
17% |
12% |
- |
- |
|
1100 °C |
7% |
- |
74% |
- |
8% |
- |
- |
- |
10% |
- |
- |
|
1200 °C |
3% |
- |
80% |
- |
7% |
- |
- |
- |
9% |
- |
- |
|
1300 °C |
2% |
- |
87% |
- |
- |
- |
- |
- |
- |
5% |
7% |
Table 1. Quantitative analysis of minerals in the reduction ash and slag
216 and 218. Stoichiometry is wrong.
Response: Thanks for marking this mistake. We have made modifications in the manuscript.
2/3Fe3O4+1/6O2→Fe2O3
2CaFeSi2O6 + 2Al2O3 + SiO2 → Fe2Al4Si5O18 + 2CaO
Fig. 9. Similar to Fig. 5, show all detected minerals including amorphous content if possible or label as “selected” minerals.
Response: The results of quantitative analysis of minerals in the oxidation ash and slag are shown in Table 2. Figure 9 in the manuscript shows the quantitative analysis of calcium-bearing minerals.
Table 2. Quantitative analysis of minerals in the oxidation ash and slag
|
Oxidation ash |
Magnetite |
Hematite |
Nepheline |
Magnesite |
Anhydrite |
Gehlenite |
Quartz |
Mullite |
Cristobalite |
Anorthite |
Microcline |
|
1100 °C |
- |
13% |
14% |
9% |
74% |
- |
- |
- |
- |
- |
- |
|
1200 °C |
7% |
10% |
10% |
4% |
34% |
27% |
- |
- |
- |
- |
- |
|
1300 °C |
7% |
8% |
- |
- |
- |
11% |
21% |
33% |
6% |
19% |
5% |
|
1400 °C |
- |
- |
- |
- |
- |
- |
19% |
40% |
9% |
31% |
- |
|
1500 °C |
- |
- |
- |
- |
- |
- |
9% |
80% |
2% |
- |
- |
Chapter 3.5. It is a pity that authors use FactSage only for calculation of phase diagrams, here. At least the phase diagram of the system Al2O3-SiO2-CaO can easily be taken from literature. For example, more complex calculations using the equilib module could be done to calculate the equilibrium for the entire ash system or parts of the system under the used experimental conditions. A comparison of such results with the experimental results would help to understand which observed mineral transformations reach equilibrium and which are kinetically constrained. This information would be of great interest.
Response: Thanks. Your suggestion is very useful for our research. Since calcium-bearing compounds are easy to react with silicon and aluminum compounds, we use “Phase Diagram” to study the system of Al2O3-SiO2-CaO and understand the calcium-bearing minerals transformation with temperature. According to your valuable suggestion, we will use the “Equilib” module to calculate the equilibrium for the entire ash system or parts of the system under the used experimental conditions in following research work.
CaO is an alkaline earth metal oxide.Response: Thanks for marking this mistake. We have revised in the manuscript.
We hope the revised version of the manuscript is acceptable for publication. However, please let me know if you have any further questions or comments.
With best wishes,
Shuqin Liu
On behalf of all co-authors
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Thank you for considering my comments and making respective changes to the paper. However, I am unfortunately not completely satisfied with the answers.
XRD analysis and respective figures and discussion. On the one hand, authors say that they focus on calcium-bearing minerals, and on the other hand, they list, show, and discuss also other minerals, which I think is important to understand the whole system. The formation of a melt / amorphous phase and indication of mineral phases disappearing to for the melt are important information which should be at least included in the discussion. Furthermore, I still suggest putting all quantified phases incl. amorphous phase into Figs. 5 and 9. They need to be consistent with Figs. 4 and 8. Unfortunately, these figures and tables 1 and 2 of the response letter are not consistent for several temperatures. Please check.
The comment on FactSage calculations was not only meant as suggestion for the future.
