Review Reports - Fouling and Slagging Investigation on Ash Derived from Sasol Coal Using ICP and XRF Analytical Techniques

Round 1

Reviewer 1 Report (Previous Reviewer 1)

This article purports compositions a fly ash and a bottom ash and their precursor coal sample from the boiler furnaces at the Sasol synfuel site. The authors have made minor improvements from the first submission. The biggest problem with this article is that a major critique leveled has not been addressed by the authors - the error on the measurements. Without errors the significance of the claims made (summarized in Table 4) can not be verified. Further, it is not clear what the motivation or insight gained from this study is. The introduction loosely focuses on efficiency loss of the system resulting from fouling and slagging of the fuels used. A fly and a bottom ash are analyzed for their chemistry and then evaluated using established metrics for slagging/fouling and found to be low for all, except the Na2O content. Therefore, there seems to be no problem. If the motivation of the study is exploratory, "We do not know what the ash is?" then this needs to be stated. But if the motivation is that there is a problem that may be improved, then the intro does not clearly establish the issue at hand. Finally, it must be questioned whether 2 samples alone are actually representative of the slagging/fouling potential of all output from the furnace. For these reasons alone the study can not be published in its current form.

Major issues:
(1) Errors on the chemistry must be reported. Also, could the authors please add detection limits for trace element data?

(2) The authors claim (in abstract and line 248) to have developed slagging/fouling indices (Table 5), however, this table reports well established indices that many other authors have already developed; see Garcia-Maraver et al., 2017 and references therein. This is a clear over-statement by the authors.

(3) Could the authors add some discussion regarding the physical basis for the "predicting factors"? For example, the Sr metric, is based on SiO2 being known to increase the liquidus temperature and viscosity of the melt/ash. The samples have quite a high Sr (77 & 81), indicating SiO2-rich samples that should be quite viscous and have increased slagging potential whereas the indices used show that the higher the SiO2 the lower the slag viscosity potential -- the opposite of the expected behavior. Could the authors explain the rationale here please?

The manuscript would benefit greatly from a brief explanation of the rationale for each of the 'predicting factors' evaluated.

(4) The authors entire thesis is that the chemistry of these 2 samples alone are sufficient to understanding slagging/fouling at Sasol. Could the authors link their compositional analysis to actual observations from the site so that the reader may better understand the significance of the results? For example, whether significant slagging or fouling is observed? or if the furnace efficiency has changed or decreased during the study period?

(5) The conclusions are not conclusions - it is more of an outline of the article. The first of each of the intitial sentences in the conclusions start: "The article starts...", "The methods...", "The results...", "A summarised conclusion and references..." Please improve.

(6) In general this manuscript is difficult to follow with the text mainly repeating what is in the table in long run-on sentences. For example, the sentence on starting on line 229 reads, "Ash deposition on the boiler furnace wall and heat transfer surfaces during the boiler operations is mainly dictated by three different routes i.e., initial slagging on the furnace wall induced by pyrites (Fe and S) particles resulting from its larger density and lower melting temperature in a reducing atmospheric conditions, initial induced fouling on the superheaters and economizers tubes as a result of the condensation alkaline vapours and thermophoretic aerosols/fume particles deposition, and the slagging/sintered passage formed by the molten basic components reacting with clay and quartz [38, 40, 41]." Therefore, the manuscript requires significant re-writting.

Other issues:
- Table 1 and table 2 are redundant.
- line 146 is incomplete...

There are many typos, missing punctuation and run-on sentences.

Author Response

Response to reviewer 1 comments

Comment 1

Errors on the chemistry must be reported. Also, could the authors please add detection limits for trace element data?

Response

The ash samples were prepared and analysed using a precision form of instrument repeatability for the ICP-OES and ICP-MS. The common instrumental error estimation for the ICP-OES and ICP-MS analysis is less than 5% in the equipment’s analytical region and a 10 % error at the detection limit.

The estimated lower limit of detection for the ICP-OES analysed data is 0.05 mg/kg. The estimated assumed safe limit of detection for the ICP-MS analysis is 0.1ug/L (0.0001 mg/Kg) for most elements and 0.01ug/L (0.00001mg/Kg) for Hg and Cd.

For the XRF chemical composition analysis, the standard error estimation for the concentrated analysed major elemental composition (i.e., Al₂O₃ and SiO₂) is 2%. The error/uncertainty tends to be higher for minor elements concentration (i.e., CaO, TiO₂, Fe₂O₃, MgO etc).

The limit of detection for the analysed XRF results is approximately 0.05 mass (%) (500mg/Kg).

Comment 2

The authors claim (in abstract and line 248) to have developed slagging/fouling indices (Table 5), however, this table reports well established indices that many other authors have already developed; see Garcia-Maraver et al., 2017 and references therein. This is a clear over-statement by the authors.

Response 2

The correction has already been made in the manuscript, the prediction indices used were previously published and established indices and they were used as a guide to evaluate the fouling and slagging potential during the operation of Sasol boiler using the analysed ash chemical composition obtained in this study.

Comment 3

Could the authors add some discussion regarding the physical basis for the "predicting factors"? For example, the Sr metric, is based on SiO2 being known to increase the liquidus temperature and viscosity of the melt/ash. The samples have quite a high Sr (77 & 81), indicating SiO2-rich samples that should be quite viscous and have increased slagging potential whereas the indices used show that the higher the SiO2 the lower the slag viscosity potential -- the opposite of the expected behavior. Could the authors explain the rationale here please?

Response 3.

This has already been addressed in the revised manuscript, The slag viscosity index (Sr) is used to predict the slagging tendency in the boiler furnace wall since the furnace wall deposits are similar to the coal ash constituents that are made up of mostly silicate materials. A higher slag viscosity index represents a high viscosity of the silica constituents inside the boiler furnace with a corresponding low slagging potential i.e., the higher the viscosity on the flowing hot flue gases the lower the slagging potential in the boiler furnace vice versa.

Comment 4

The authors entire thesis is that the chemistry of these 2 samples alone are sufficient to understanding slagging/fouling at Sasol. Could the authors link their compositional analysis to actual observations from the site so that the reader may better understand the significance of the results? For example, whether significant slagging or fouling is observed? or if the furnace efficiency has changed or decreased during the study period?

Response 4

These two samples were collected during the boiler operation. However, since the same coal is used as the primary feedstock for firing all the boilers and the boilers are of the same designed. It is presumed that during coal combustion inside the boiler furnace, the coal ash produced in all the boilers will be the same. Hence there is every possibility that the chemical composition, the fouling and slagging prediction will be similar for all the boilers.

The researcher is currently collecting data during the boiler operation to evaluate the fouling and slagging potential. Attached is the questionnaire of the ongoing data collection during the boiler operation.

Date

Time

Furnace Temperature (θ₁) ^oC

Super Heater Temperature

(θ₂) ^oC

Economizer Temperature

(θ₃) ^oC

Output Steam Pressure

(Tons)

Exit flue gas Temperature

(θ₄) ^oC

8 AM

4 PM

The table above is the designed questionnaire for collection data during the boiler operation to assess the fouling and slagging potential. Data on the furnace temperature, superheaters, economizers, steam produced, and the exit flue gas temperature was collected from the operational site main control room for 60 days. An increase in the furnace temperature leads to a corresponding increase in the steam produced. Rapid increase in the furnace temperature and the exit flue gas temperature results to a drop in the steam produce which indicates an increase in ash accumulation in the furnace wall and convective heat transfer surfaces. This data will be used to develop a fouling and slagging mathematical prediction model which is not in the scope of this article.

Comment 5

The conclusions are not conclusions - it is more of an outline of the article. The first of each of the intitial sentences in the conclusions start: "The article starts...", "The methods...", "The results...", "A summarised conclusion and references..." Please improve

Response 5

This article is an investigation into predicting fouling and slagging occurrences during the operation of the installed boilers operated by Sasol synfuel limited. The investigation involved analysing the coal fly ash and bottom ash residue collected from Sasol operational site in Secunda using ICP-MS, ICP-OES and XRF laboratory analytical techniques. Previously published fouling and slagging indices based on the coal ash chemical composition were used as a guide to evaluate the fouling and slagging potential of the installed boilers operated by Sasol synfuel limited. The prediction results obtained using the analysed chemical composition of the coal ash samples in this study indicates or predict a low to medium fouling potential in the boiler convective heat transfer surfaces, and a low slagging potential in the boiler furnace wall. Hence the research objective of this study had been achieved.

Comment 6

In general this manuscript is difficult to follow with the text mainly repeating what is in the table in long run-on sentences. For example, the sentence on starting on line 229 reads, "Ash deposition on the boiler furnace wall and heat transfer surfaces during the boiler operations is mainly dictated by three different routes i.e., initial slagging on the furnace wall induced by pyrites (Fe and S) particles resulting from its larger density and lower melting temperature in a reducing atmospheric conditions, initial induced fouling on the superheaters and economizers tubes as a result of the condensation alkaline vapours and thermophoretic aerosols/fume particles deposition, and the slagging/sintered passage formed by the molten basic components reacting with clay and quartz [38, 40, 41]." Therefore, the manuscript requires significant re-writting.

Response 6

Significant changes have been made to the manuscript. The tables have been corrected as well.

Reviewer 2 Report (Previous Reviewer 2)

now article is fine.

one remark: tables 1 and 2 are much better now, but still I can not understand why in table 1 there is Al at 32.66 % and in table 2 it is Al2O3 at 32.66 % (and the same for other elements). The oxygen adds weight, this is not consistent!

Author Response

Response to reviewer 2 comments

Comment 1

Response 1

This has been addressed in the revised manuscript, the data set from the various analytical methods used in this study have been displayed separately, for more clarity on the results obtained during the analysis.

Author Response File: Author Response.docx

Reviewer 3 Report (New Reviewer)

The manuscript presents fouling and slagging investigation on ash derived from an industrial power plant using various analytical techniques to investigate the boiler’s deposition problems based on the ash chemical composition. The subject is within the scope of the journal and of common interest all around in the arena of coal fired power. Authors have presented a number of datasets based on the analysis. However, the presentation is not clear in many senses and the novelty of the study is not properly outlined. As a decision I would like to recommend a major revision of the paper before any further decision can be made. Please find the comments.

1. The writing structure of the paper is not up to the standard of scientific writing. A major revision is needed in terms of meaningful sentence formation, avoiding complex sentence, grammar and syntax errors. Professional editing service can help to improve the overall writing. Some sections must be changed as these are seemed not meaningful or complex sentences.
Line 40-45, 150-153, 247-250 etc.

Introduction is written in a single paragraph which is difficult to read and follow.

2. The abstract of the paper may be revised to clearly present the novelty and outcome of the study. Abstract should have at least one sentence per each: context and background, motivation, hypothesis, methods, results, conclusions.

The first line of the abstract can be omitted as it is too generic.

3. Literature review is not comprehensive. It should summarize relevant research to provide context, and explain what findings of others, if any, are being challenged or extended. The no of references presented is minimum. More recent literature's may be discussed and similar works can be grouped together for general discussion, findings and challenges.

Some important review papers may be discussed for more information:

# Co-firing of biomass and slagging in industrial furnace: A review on modelling approach. Journal of the Energy Institute 90 (6), 838-854

# A review on thermo-chemical characteristics of coal/biomass co-firing in industrial furnace. Journal of the Energy Institute 91 (1), 1-18

4. The novelty of the present study is not clearly outlined. It should be emphasized. There have been numerous studies on Coal Combustion Slagging including most recent numerical works to estimate or predict slagging and fouling (See review paper mentioned above).

So, what research gaps the authors are trying to fill up must be clearly mentioned. Is it a stand-alone problem solution for the Sasol Plant or authors are trying to establish a general relationship among known parameters? If the latter is the case, authors must compare their results to other literatures and establish their findings.

5. The common writing patterns of academic papers have not been followed. Figures and tables should be placed close to their first citation in any text or paragraphs. Authors have mentioned Table 5 (line 81) in front of Table 1 and 2 (line 115-117). Although several tables and figures have been mentioned in the text much prior to their appearance after 2-3 pages. These must be revised.

6. In Methodology section, more details of the investigation methods ICP-OES, ICP-MS, and XRF analytical techniques etc should be presented with appropriate reference.

The experimental methods can be better presented in flow charts, schematic diagrams. Some pictures of experiments, test analysers can be added for reference.

7. Results and discussions section must be improved. The intro of this section (line 194-196) is not meaningful. Please start with general comments and then move in to the details.

The discussion on result found is not comprehensive. There is no comparison with the results obtained. The findings of the study should be more clearly presented and discussed to establish some novelty. More figures can be added for better visual presentation.

8. Table 5, fouling and slagging prediction indices, are these outlined only for the present study or these have been taken from some other studies as guidelines?

9. The conclusion is not justified. It more looks like a summary of the writings not the study in detail. Conclusion must be rewritten. Also, some limitations and future directions must be presented.

Author Response

Response to reviewer 3 comments

Comment 1

The writing structure of the paper is not up to the standard of scientific writing. A major revision is needed in terms of meaningful sentence formation, avoiding complex sentence, grammar and syntax errors. Professional editing service can help to improve the overall writing. Some sections must be changed as these are seemed not meaningful or complex sentences.

Line 40-45, 150-153, 247-250 etc.

Introduction is written in a single paragraph which is difficult to read and follow.

Response 1

This problem has been addressed in the revised manuscript.

Comment 2

The abstract of the paper may be revised to clearly present the novelty and outcome of the study. Abstract should have at least one sentence per each: context and background, motivation, hypothesis, methods, results, conclusions.

Response 2

The abstract has been revised as suggested.

Abstract - During coal combustion in boilers, light fly ash particles are carried away along with the hot flue gases and the heavier bottom ash particles fall to the bottom of the boiler. The fly ash particles stick on the convective heat transfer surfaces and the furnace wall causing fouling and slagging deposition problems during the boiler operation. The fouling and slagging effect reduces the boiler's operational efficiency. This study was motivated by the decline in the operational efficiency of the installed boilers at Sasol synfuel operations in Secunda, Mpumalanga province in the Republic of South Africa. It was assumed that the drop in the boiler efficiency was caused by the coal ash deposition during the boiler operations and hypothesized that the rate of ash deposition and accumulation effect on the convective heat transfer tubes and furnace water walls during the boiler operation depends on the chemical composition of the coal ash produced during combustion in the boiler. Coal fly and bottom ash samples from the operational site were collected for laboratory analysis to determine their chemical composition using induced coupled plasma optical emission spectroscopy, induced coupled plasma mass spectroscopy (ICP-OES, ICP-MS) and X-Ray fluorescence (XRF) analytical methods. The major, minor and trace elements by mass (%) in the ash samples were obtained from the ICP-OES and ICP-MS, whereas the elemental composition in an oxidised atmosphere was obtained from the XRF analysis. The amount of unburnt coal particles within the ash samples was determined from the loss on ignition (LOI) test. The fouling and slagging prediction during Sasol boiler operation was evaluated using previously developed fouling and slagging indices as a guide using the analysed ash chemical composition results obtained in this study. It was concluded from the analysed results using the guided evaluated indices from the analysed coal ash chemistry that during the operations of Sasol boiler(s) there is a low to medium fouling prediction occurrence on the convective heat transfer tubes and a low slagging in the boiler furnace walls.

Comment 3

Literature review is not comprehensive. It should summarize relevant research to provide context, and explain what findings of others, if any, are being challenged or extended. The no of references presented is minimum. More recent literature's may be discussed and similar works can be grouped together for general discussion, findings and challenges.

Response 3

The article has been reviewed stating some outcomes on different boiler fouling and slagging predictions.

Comment 4

The novelty of the present study is not clearly outlined. It should be emphasized. There have been numerous studies on Coal Combustion Slagging including most recent numerical works to estimate or predict slagging and fouling (See review paper mentioned above).

Response 4

This study is a stand-alone problem aimed at predicting the boilers fouling and slagging potential based on the coal ash chemical composition.

The main objective of this study is to investigate the decline in the overall efficiency and steam output pressure of the boiler(s) operated by Sasol synfuel limited, this challenge might be a result of the coal ash accumulation on the boiler heat transfer tubes. In meeting this research objective, an investigation on the coal ash produced from the coal combustion was carried out to determine the coal ash chemical concentration.

Comment 5

The common writing patterns of academic papers have not been followed. Figures and tables should be placed close to their first citation in any text or paragraphs. Authors have mentioned Table 5 (line 81) in front of Table 1 and 2 (line 115-117). Although several tables and figures have been mentioned in the text much prior to their appearance after 2-3 pages. These must be revised.

Response 5

The manuscript has been revised as recommended.

Comment 6

In Methodology section, more details of the investigation methods ICP-OES, ICP-MS, and XRF analytical techniques etc should be presented with appropriate reference.

The experimental methods can be better presented in flow charts, schematic diagrams. Some pictures of experiments, test analysers can be added for reference.

Response 6

The experimental methods have been reviewed and pictures of the equipment added. Though the methodology not presented in flow chart because of the number of texts used in describing the methods and equipment used in preparing and analyzing the coal ash samples.

Comment 7

Results and discussions section must be improved. The intro of this section (line 194-196) is not meaningful. Please start with general comments and then move into the details.

Response 7

The results and discussion section has been reviewed with separate tables for the ICP-OES and ICP-MS added. A section discussing the fouling and slagging indices was also added.

Comment 8

Table 5, fouling and slagging prediction indices, are these outlined only for the present study or these have been taken from some other studies as guidelines?

Response 8

Comment 9

The conclusion is not justified. It more looks like a summary of the writings not the study in detail. Conclusion must be rewritten. Also, some limitations and future directions must be presented.

Response 9

The conclusion has been revised in the manuscript as follows:

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report (Previous Reviewer 1)

The authors have made substantial improvements to this work, which now contains the necessary uncertainties and motivation for the study. As such, it only requires minor revisions before being publishable. I would recommend one major improvement. In the abstract, the authors state, "This study is motivated by the decline in operational efficiency...", however, the results show that the slagging/fouling potential is low for their samples. So, I would request that the authors include some discussion of whether their results can explain, or not, the decline in operational efficiency? If they can not explain this than some brief speculation as to the origin would help the reader. The conclusions should be updated accordingly. For these reasons, I find that this manuscript is acceptable for publication after minor additions.

Other comments:

- Fig 2 is unnecessary.

- The final statement, "Hence, the research objective has been achieved." Should be changed to, "Hence, we can eliminate the ash as the source of the decline in operational efficiency." or some other concluding statement that applies.

Author Response

Response to Reviewer 1 Comment

Comment

I would recommend one major improvement. In the abstract, the authors state, "This study is motivated by the decline in operational efficiency...", however, the results show that the slagging/fouling potential is low for their samples. So, I would request that the authors include some discussion of whether their results can explain, or not, the decline in operational efficiency? If they cannot explain this than some brief speculation as to the origin would help the reader. The conclusions should be updated accordingly.

Response

The results from the analyzed coal ash samples predicted that the fouling and slagging potential during the operations of the installed boilers at Sasol synfuel in low and hence therefore the decline in the boiler’s operational efficiency is not as a result of the coal ash produced during combustion. However, the accumulation of the ash in the boiler furnace wall and convective tube surfaces over long operational period, would result in the formation of thick insulated layer reducing the amount of heat transfer to the moving fluid inside the tubes resulting to a decline in the overall efficiency. The installation of smart soot blowers for ash removal on the boiler tubes is therefore recommended as to achieve an optimal operational efficiency.

The conclusion has been updated accordingly and Figure 2 removed.

Author Response File: Author Response.docx

Reviewer 3 Report (New Reviewer)

The manuscript has been revised and the quality has been improved to some level. However, the result and discussion section is still requiring a revision.

All major results have been presented in Tabular form, some figures could have been included for comparison/presentation of results.

In result and discussion section, the second paragraph stating, "The main objective of this study is to investigate..."should be brought forward and the section may start with this paragraph.

After that the Table 3 and 4 can be presented.

Fouling and slagging prediction indices this section can be a part of the Methodology in section 3.

All tables needs more discussion in the main result section.

Author Response

Response to Reviewer 2 Comment

Comment

All major results have been presented in Tabular form, some figures could have been included for comparison/presentation of results.

In result and discussion section, the second paragraph stating, "The main objective of this study is to investigate..."should be brought forward and the section may start with this paragraph.

After that the Table 3 and 4 can be presented.

Fouling and slagging prediction indices this section can be a part of the Methodology in section 3.

All tables needs more discussion in the main result section.

Response

The data sets were presented in a tabular manner for a better understanding of the interpreted results.

The paragraph had been moved in the reviewed manuscript.

The fouling and slagging prediction index paragraph had been moved to the methodology section as 3.3 in the revised manuscript.

The tables discussion was minimized to avoid repetition as commented by reviewer 1 in the initial submitted manuscript.

Author Response File: Author Response.docx

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

In this work, Kamara et al., study the composition of fly and bottom ash on coal combustion in boilers and its influence on fowling and slagging which causes degradation in the performance of the boilers. The authors do not report errors/uncertainties in their measurements and their figures often lack numerical scales on the axis. They present major element and trace element data together in a way that makes little sense and is inconsistent with most other papers presenting this type of data. With respect to the interpretation, the authors provide clear metrics with respect to fowling and slagging potential but when describing (either in the abstract, intro or conclusions) these results they have 1 or 2 good statements in a paragraph with vague statements about the "influence" or "changes" that are not directly connected to the chemical results. For example, the tables 4-9 provide the different metrics for slagging/fowling but their samples/data are never evaluated directly in a table NOR figure!

This study has potential but requires a complete re-write and revision of most figures and tables before it can be published. For these reasons, I recommend rejection but encourage the authors to re-evaluate how they present their research both graphically and in writing and then re-submit this study.

Author Response

Response to Reviewer 1 Comments

Point 1: In this work, Kamara et al., study the composition of fly and bottom ash on coal combustion in boilers and its influence on fowling and slagging which causes degradation in the performance of the boilers. The authors do not report errors/uncertainties in their measurements and their figures often lack numerical scales on the axis. They present major element and trace element data together in a way that makes little sense and is inconsistent with most other papers presenting this type of data. With respect to the interpretation, the authors provide clear metrics with respect to fowling and slagging potential but when describing (either in the abstract, intro or conclusions) these results they have 1 or 2 good statements in a paragraph with vague statements about the "influence" or "changes" that are not directly connected to the chemical results. For example, the tables 4-9 provide the different metrics for slagging/fowling but their samples/data are never evaluated directly in a table NOR figure

Response 1: the ash analysed in this study is obtained from the combustion of low grade pulverized bituminous coal. The results obtained in this study however agreed with those obtained from previous analysed literature. The manuscript has been fully revised, restructured and checked for grammatical errors. The figures and tables have been corrected and rearranged with defined values on both axis on the figures. A structured table consisting of three developed fouling and slagging prediction indices based on the coal ash chemistry have been developed (Table 5). Table 4 is the evaluated fouling and slagging type prediction occurrence during the boiler operation at Sasol synfuel operations using the chemical composition results obtained from the analysed fly ash (Sample A) and bottom ash (Sample B) samples in this study as displayed below.

Table 4: Fly ash and Bottom ash samples fouling and slagging prediction

Predicting Factor		Evaluating Index	Quantity/ Unit
Predicting Factor		Evaluating Index	Sample A	Sample B	Index/ Type
a	Fouling tendency prediction (Rf)		0.15	0.14	Low
b	Fouling potential prediction (fu)		0.25	0.26	Low
c	Fouling potential prediction on Na₂O presence	Quantity of Na₂O in the ash sample	0.91	0.75	Medium
d	Slagging type prediction (Rs)		0.03	0.04	Low
e	Slag viscosity prediction (Sr)		80.68	77.14	Low
f	Acid Based ratio slagging prediction		0.15	0.19	Low

Author Response File: Author Response.docx

Reviewer 2 Report

some remarks:

-l 163: it is doubtful if just HNO3 is enough to dissolve the ash samples. some silicates and/or quartz very likely remain undissolved
- chapter 3.2: this is basic knowledge and may not be reiterated here - any textbook on the matter will give these information
- table 2 - no page break please
- table 1 and 2: same numbers, different formulas - something must be wrong here
- table 5 - for no reason there is a different formatting than the other table around
- fig 1 and 2 depict the same (and the same problem) as table 1 and 2, this is unnecessary
- discussion: no indication that the authors have ever tried to cross-check their results concerning slagging and fouling in the real Sasol boilers. Does the reality show that it is as you have calculated?
- also, no evaluation has been done if all these indices are any good

Author Response

Response to Reviewer 2 Comments

Point 1: It is doubtful if just HNO3 is enough to dissolve the ash samples. some silicates and/or quartz very likely remain undissolved.

Response 1: The ash samples were not completely dissolved in the HNO3 solution, because the sample was heated to a temperature of 180^oC for 25 minutes which is far below the melting point of the silicates/ quartz. The samples were heated in the HNO3 to obtain a homogenous solution to be fed into the analysers i.e., ICP-OES and ICP-MS analysers. Because the ash is not completely dissolved that is why the researcher used a 0.22 µm acrodisc syringe to filter the sample to separate the undissolved quartz and clay particles.

Point 2: Chapter 3.2: this is basic knowledge and may not be reiterated here - any textbook on the matter will give these information

Response 2: Chapter 3.2 is a detailed explanation of the instruments used and the experimental procedures carried out in obtaining the results in this study. The researcher is not certain if the reviewer was referring to 2.2 which is a reviewed literature on the ash physical and chemical properties.

Point 3: Table 2 - no page break please

Response 3: this has been adjusted in the revised manuscript

Point 4: Table 1 and 2: same numbers, different formulas - something must be wrong here

Response 4: the different test carried out in this study were ICP-OES, ICP-MS and XRF, the samples used were the same i.e., the elemental composition must be the same. Table 1 however is the result obtained from the ICP-OES and ICP-MS analysers used to determine the element presents i.e., major, minor and trace elements and Table 2 were the results obtained from the XRF analyser where a non-destructive x-ray beam is directed through a boron fusion disk in determining the elements present in the ash samples. However, all the elements cannot be discovered using the XRF analyser for eg trace elements such as Cd, Pb, Hg, Se and As were not discovered by the XRF analyser. Also, the unburnt carbon present within the ash samples referred to as the Loss on ignition (LOI) cannot be determined using the ICP-OES and ICP-MS. Because the samples were analysed using different analytical methods their chemical composition remains the same, hence the similar chemical composition in Table 1 and 2.

Point 5: Table 5 - for no reason there is a different formatting than the other table around

Response 6: this has been resolved in the revised manuscript, all developed indices used to predict the fouling and slagging during the boiler have been grouped in one Table as displayed below.

Predicting Factor	Evaluating Index	Low	Medium	High	Severe
Fouling tendency prediction (Rf) [36, 38]		< 0.2	0.2 – 0.5	0.5 – 1.0	> 1.0
Fouling potential prediction (fu) [39, 18]		<0.6		0.7 – 40	Above 40
Fouling potential prediction on Na₂O presence [38]	Quantity of Na₂O in the ash sample	<0.5	0.5 - 1.0	1.0 – 2.5	>2.5
Slagging type prediction (Rs) [36]		<0.6	0.6 - 2.0	2.0 – 2.6	>2.6
Slag Viscosity prediction (Sr) [34]		Above 72	65 – 72	Below 65
Acid Based ratio slagging prediction [36, 37]		< 0.5	0.5 – 0.9	1.0 – 1.74	Above 1.75

Point 6: Fig 1 and 2 depict the same (and the same problem) as table 1 and 2, this is unnecessary

Response 6: This have been addressed in the revised article.

Point 7: discussion: no indication that the authors have ever tried to cross-check their results concerning slagging and fouling in the real Sasol boilers. Does the reality show that it is as you have calculated?

Response 7: The researcher is currently testing these results at Sasol synfuel operations. A questionnaire was designed to collecting data during the boiler operation. The data collection includes furnace temperature, flue exit gas temperature (FEGT) for slagging assessment, and superheaters, economizers temperature for fouling assessment, and it effects on the quantity of steam produced during the boiler operation. The researcher is working on developing a liner regression model for the boilers at Sasol based on the collected data, but the data collection is still in progress and a manuscript will be submitted on that later.

Point 8: also, no evaluation has been done if all these indices are any good

Response 8: These were published developed indices used in coal fired boilers designed operations. As the coal fired boiler efficiency is strongly dependent on the coal fuel used for firing the boiler.

Author Response File: Author Response.docx