Modeling and Analysis of Coal-Based Lurgi Gasification for LNG and Methanol Coproduction Process
Round 1
Reviewer 1 Report
The language needs improvement. Please carefully revise and eliminate all errors. Attached, please find the detailed review comments. Authors need to supply a list of changes.
Comments for author File: 
Comments.pdf
Author Response
In this revision, the manuscript has been undergoing an extensive English editing to improve its readability. The author is apologizing for all reading inconvenience caused by the last format.
Here is a brief introduce for out new process just for your information:
A coal-based coproduction process of LNG and Methanol (CTLNG-M) is developed and key units are simulated. It is studied to find improvements to the low earing Coal to Synthesis Natural Gas (CTSNG) process using the same raw material but producing a low margin, single SNG product. In the CTLNG-M process, there are two innovative aspects. Firstly, the syngas is separated to LNG product by cryogenic separation unit and the remaining lean-methane syngas then for methanol synthesis, which outputs consisting of high-valued form of methane product and methanol with profit. Secondly, CO2 as carbon emission from acid gas removal unit is partially reused to supply on synthesis reaction, which further increases the element utilization.
And there are several responses to the reviewers' comments as below.
Firstly, for the comment about “carbon emission decreased while energy consumption increased, which is really confusing.” A detailed description about carbon track and break-down analysis of energy cost structure have been new added in the revision.
Lower carbon emission: That is mainly because CO2 emission has been partially converted into product. the reason why CO2 can be reused as gas feed for synthesis is reaction requirement for H/C ratio in the new CTLNG-M scenario. In the conventional process, all syngas has to be converted to only synthesized natural gas (SNG) which requires the H/C ratio of 3.1 by element balance equation. This is higher than the ratio in the syngas output originally from the Lurgi gasification as 2.7. thereby, that process needs more hydrogen which is provided by the gas shift reaction unit, in where CO is consumed and additional CO2 is emitted. However, In the new coproduction CTLNG-M process, methanol is presented as a suitable product from chemical synthesis with products methane is separated and cryogenic cooled to directly produce the LNG product. The remaining syngas is only used for methanol synthesis which requires of a lower H/C ratio of 2.1. In this case, the syngas has excessive hydrogen. We then introduce CO2 into the syngas to adjust the ratio. In this study case, a reduction of CO2 emission by 130,000 tons/a can be realized compared to the CTSNG process.
Higher energy consumption: the CTLNG-M process has higher energy efficiency and carbon element utilization ratio, which are improvements to the former CTSNG process. Moreover, an energy consumption analysis is detailed developed now. That is because when considering the CTLNG-M process is under a coproduction design with a new added cryogenic separation unit, a unit that is specially need low temperature environment and more electricity, quantitative analysis for energy use is a necessary.
The energy consumption is defined as utilities consisting of steam cost and electricity cost in this paper. For a more convenient comparison, both steam cost and electricity consumption are converted to the same units as MJ/a. As far as we calculated, the CTLNG-M process consumes 4.3×109 MJ of steam and 9.5×109 MJ of electricity for a year, and the CTSNG process consumes 5.9×109 MJ of steam and 6.7×109 MJ of electricity. It shows that the coproduction system has a lower steam cost about 1.6×109 MJ for one year, which mainly because of a flexible way to integrate heat exchange when there is no only one route for product processing. However, there are some space for an improvement on electricity use in CTLNG-M process. It is because its nitrogen circulation refrigeration process needs more power assistant as modelling data indicates. Since there is no power can be generated in the inside system, it takes more capital investment that should be under a further analyze.
thereby, indirect carbon emission is not within the border of our system. More energy consumption is not caused in a chemical processing way, but for more cooling machine’s work. There is no power can be generated in the CTLNG-M process but it does in need. We attribute those electricity cost as one part of variable cost in the capital investment, which are studied in the following paper.
Secondly, for the comment about “not enough unit modelling and simulation in the whole CTLNG-M process” A explanation for why we only stimulated all new units and a model of nitrogen circulation refrigeration process have been new added in this revision.
Why there is not all units’ simulation but only Cryogenic separation unit and Methanol synthesis unit? This paper is studied to find improvements to the low earing Coal to Synthesis Natural Gas (CTSNG) process using the same raw material but producing a low margin, single SNG product. Thereby, the Lurgi Coal gasification unit and Acid gas removal unit are remaining the same but other two unit are new added when the new process is raised. The detailed simulation can be found the previous work in our group, there are many as we listed in the reference. This paper mainly gives description of the newly added units’ modelling, as the cryogenic separation unit and the Methanol synthesis unit.
Thirdly, we specially revised and added new data as fig.1 and fig.3 to show China's SNG price recording (2014-2018) and LNG market price recording (2017.1-2019.1). That is for a better understanding why output consisting of LNG is a higher valued product.
For SNG product: the market price of synthetic natural gas (SNG) products is not set base on its cost structure, neither according to the guidance from a market mechanism. Departments of China offer a rather than low price to the public, especially give the priority to civil use, transportation field, etc. Thus, low economic returns are commonly to all CTSNG projects. The price of natural gas has fallen sharply since Nov. 2015 in the country after the National Development and Reform Commission issued a report about price adjustment, which is shown as fig.1. It indicates that if SNG price keeps fluctuating at the low-price level, those CTSNG projects are likely to face severe losses. Moreover, it is mainly supplying to civil use like urban heating in the winter, thus, there is a peak-valley difference of natural gas demand between winter and summer. Since natural gas cannot be stored for a long time, coal-based gas projects are facing production cuts during the non-heating period, which brings huge economic losses.
For LNG product: The syngas from a Lurgi gasifier contains 12% to 18% methane. Because of this high composition of methane, Lurgi gasification technology is usually used in CTSNG projects. However, from another point of view, LNG products can also be obtained if by separating methane from the syngas by through a new added Cryogenic Separation technology. LNG is a relatively high value-added product form of coal based natural gas, which price can reflect the supply and demand mechanism as shown in fig. [03], In addition, LNG can be transported and stored in a more flexible way. It indicates that coproduction is a possible way coal chemical processes based on Lugri gasification technology, with more economic benefit.
Again, thanks for all the working and comments on out manuscript.
Best regards.
Author Response File: Author Response.pdf
Reviewer 2 Report
The authors propose in this manuscript a concept of coproduction of chemical products in coal-to-chemical processes, to avoid seasonal fluctuations and enhance overall profitability. The concept is interesting and could of great value to the coal-to-chemical industry. Validation of this concept presented in the manuscript is also well designed, conducted, described, and discussed. The only problem with the manuscript is that its readability can be further improved. Especially use of too many abbreviations greatly reduces joy of reading the manuscript, which the authors may want to avoid during revision.Author Response
In this revision, the manuscript has been undergoing an extensive English editing to improve its readability. The author is apologizing for all reading inconvenience caused by the last format.
Here is a brief introduce for out new process just for your information:
A coal-based coproduction process of LNG and Methanol (CTLNG-M) is developed and key units are simulated. It is studied to find improvements to the low earing Coal to Synthesis Natural Gas (CTSNG) process using the same raw material but producing a low margin, single SNG product. In the CTLNG-M process, there are two innovative aspects. Firstly, the syngas is separated to LNG product by cryogenic separation unit and the remaining lean-methane syngas then for methanol synthesis, which outputs consisting of high-valued form of methane product and methanol with profit. Secondly, CO2 as carbon emission from acid gas removal unit is partially reused to supply on synthesis reaction, which further increases the element utilization.
And there are several responses to the reviewers' comments as below.
Firstly, for the comment about “carbon emission decreased while energy consumption increased, which is really confusing.” A detailed description about carbon track and break-down analysis of energy cost structure have been new added in the revision.
Lower carbon emission: That is mainly because CO2 emission has been partially converted into product. the reason why CO2 can be reused as gas feed for synthesis is reaction requirement for H/C ratio in the new CTLNG-M scenario. In the conventional process, all syngas has to be converted to only synthesized natural gas (SNG) which requires the H/C ratio of 3.1 by element balance equation. This is higher than the ratio in the syngas output originally from the Lurgi gasification as 2.7. thereby, that process needs more hydrogen which is provided by the gas shift reaction unit, in where CO is consumed and additional CO2 is emitted. However, In the new coproduction CTLNG-M process, methanol is presented as a suitable product from chemical synthesis with products methane is separated and cryogenic cooled to directly produce the LNG product. The remaining syngas is only used for methanol synthesis which requires of a lower H/C ratio of 2.1. In this case, the syngas has excessive hydrogen. We then introduce CO2 into the syngas to adjust the ratio. In this study case, a reduction of CO2 emission by 130,000 tons/a can be realized compared to the CTSNG process.
Higher energy consumption: the CTLNG-M process has higher energy efficiency and carbon element utilization ratio, which are improvements to the former CTSNG process. Moreover, an energy consumption analysis is detailed developed now. That is because when considering the CTLNG-M process is under a coproduction design with a new added cryogenic separation unit, a unit that is specially need low temperature environment and more electricity, quantitative analysis for energy use is a necessary.
The energy consumption is defined as utilities consisting of steam cost and electricity cost in this paper. For a more convenient comparison, both steam cost and electricity consumption are converted to the same units as MJ/a. As far as we calculated, the CTLNG-M process consumes 4.3×109 MJ of steam and 9.5×109 MJ of electricity for a year, and the CTSNG process consumes 5.9×109 MJ of steam and 6.7×109 MJ of electricity. It shows that the coproduction system has a lower steam cost about 1.6×109 MJ for one year, which mainly because of a flexible way to integrate heat exchange when there is no only one route for product processing. However, there are some space for an improvement on electricity use in CTLNG-M process. It is because its nitrogen circulation refrigeration process needs more power assistant as modelling data indicates. Since there is no power can be generated in the inside system, it takes more capital investment that should be under a further analyze.
thereby, indirect carbon emission is not within the border of our system. More energy consumption is not caused in a chemical processing way, but for more cooling machine’s work. There is no power can be generated in the CTLNG-M process but it does in need. We attribute those electricity cost as one part of variable cost in the capital investment, which are studied in the following paper.
Secondly, for the comment about “not enough unit modelling and simulation in the whole CTLNG-M process” A explanation for why we only stimulated all new units and a model of nitrogen circulation refrigeration process have been new added in this revision.
Why there is not all units’ simulation but only Cryogenic separation unit and Methanol synthesis unit? This paper is studied to find improvements to the low earing Coal to Synthesis Natural Gas (CTSNG) process using the same raw material but producing a low margin, single SNG product. Thereby, the Lurgi Coal gasification unit and Acid gas removal unit are remaining the same but other two unit are new added when the new process is raised. The detailed simulation can be found the previous work in our group, there are many as we listed in the reference. This paper mainly gives description of the newly added units’ modelling, as the cryogenic separation unit and the Methanol synthesis unit.
Thirdly, we specially revised and added new data as fig.1 and fig.3 to show China's SNG price recording (2014-2018) and LNG market price recording (2017.1-2019.1). That is for a better understanding why output consisting of LNG is a higher valued product.
For SNG product: the market price of synthetic natural gas (SNG) products is not set base on its cost structure, neither according to the guidance from a market mechanism. Departments of China offer a rather than low price to the public, especially give the priority to civil use, transportation field, etc. Thus, low economic returns are commonly to all CTSNG projects. The price of natural gas has fallen sharply since Nov. 2015 in the country after the National Development and Reform Commission issued a report about price adjustment, which is shown as fig.1. It indicates that if SNG price keeps fluctuating at the low-price level, those CTSNG projects are likely to face severe losses. Moreover, it is mainly supplying to civil use like urban heating in the winter, thus, there is a peak-valley difference of natural gas demand between winter and summer. Since natural gas cannot be stored for a long time, coal-based gas projects are facing production cuts during the non-heating period, which brings huge economic losses.
For LNG product: The syngas from a Lurgi gasifier contains 12% to 18% methane. Because of this high composition of methane, Lurgi gasification technology is usually used in CTSNG projects. However, from another point of view, LNG products can also be obtained if by separating methane from the syngas by through a new added Cryogenic Separation technology. LNG is a relatively high value-added product form of coal based natural gas, which price can reflect the supply and demand mechanism as shown in fig. [03], In addition, LNG can be transported and stored in a more flexible way. It indicates that coproduction is a possible way coal chemical processes based on Lugri gasification technology, with more economic benefit.
Again, thanks for all the working and comments on out manuscript.
Best regards.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The revised manuscript is full of errors (language related, problems with figure numbers, wrong figure caption, etc.). There are many typographical errors which the authors should have identified and corrected before submission. Also, several of the errors are because carelessness. I suggest the authors consider carefully all the marked changes and comments indicated in their revised manuscript. Attached, please find the detailed review comments.
The standard of the quality of writing in the revised manuscript does not satisfy the requirements for publication in /Processes/. Hopefully, the authors will do a better job in their next version.
Please provide a marked revised version so the changes in the next revised manuscript can be evaluated rapidly. Therefore, please submit a clean version and a marked revised version.
Comments for author File: Comments.pdf
Author Response
Dear professor,
Thanks for your kindly review work.
In this revision, the manuscript has been undergoing an extensive English editing to improve its readability. We are apologizing for all misunderstanding descriptions and errors in the last format.
As you've asked, a marked revised version and a clean version are provided as below in 1 file* (It seems that there is only 1 file can be uploaded, so we would like to suggest you start with the clean revision as shown in the page 21-40.)
Again, thank you for all the helpful work, including those detailed comments and useful advice on this manuscript.
Best regards.
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
While the revised manuscript has improved, unfortunately, it still contains too many language related errors to satisfy the publication requirements.
Here is a list of statements where there are errors (authors should carefully read what they have written, identify the error and correct the error):
line 177
lines 190-191
Lines 200-208 (the text has too many errors)
lines 212- 213
line 231
line 245
line 270
Line 276
line 292-293
line 368-369
line 375-376
line 390-391
line 407-409
line 453-455
The authors should use a language expert or other help to address the language related errors in different sentences in the lines listed above.
Author Response
Dear professor,
Thanks for your kindly review.
As you've asked, a marked revised version and point-by-point response are provided. Again, thank you for all the helpful work, including those detailed comments and useful advice on this manuscript.
Best regards.
*point-by-point response:
line 177
(The stream at the bottom of the column consists of a large amount of methane and carbon monoxide.)
The stream extracted from the bottom of washing tower mainly consists of methane and carbon monoxide.
lines 190-191 3. Modeling and Simulation
(As each module of CTLNG-M process has described in Section 2, including coal gasification, acid gas removal, cryogenic separation and methanol synthesis unit.)
As have been mentioned above, there are four main units involved in CTLNG-M process, namely coal gasification, acid gas removal, cryogenic separation and methanol synthesis unit. The detailed simulation of coal gasification and acid gas removal unit can be found in our group’s previous work [28-30]. Consequently, this paper gives modelling and simulation for two added units, as the cryogenic separation and methanol synthesis unit.
Lines 200-208 (the text has too many errors)
3.1 Cryogenic separation unit
(To set apart CH4 component for obtaining LNG product, cryogenic separation unit is added [31]. The flow-diagram of that unit is shown as figure 6. Clean syngas, as SYNGAS-C marked in figure 6, enters a molecular sieve tower to absorb H2O and methanol remained in the gas. In this unit, T2-W and T3-D are mainly set for CH4 gathering [32]. After heat exchange, syngas as steam S1 drops its temperature and enters T2-W. Outlet steam S2 from the top of T2-W is sent to next unit to produce methanol. Steam S4 form the bottom of T2-W is mainly consisted of CO and CH4. The distillation tower T3-D is set for splitting the mixture syngas into CO-rich gas, Stream S5 and LNG product as stream S8.)
The role of cryogenic separation unit played in the new process is to obtain purified LNG products [31]. As illustrated in figure 6, the clean syngas is firstly introduced to an absorption tower for H2O and methane removal. After that, the resulted syngas (S1) is cooled into liquid phase. Most of the remained carbon monoxide component in S2 is sent to methanol synthesis unit. Stream S4 from the bottom of T2-W is mainly consisted of CO and CH4, which needs further distillation in T3-D [32]. Stream S5 is then separated in to two parts, one is recycled to the top of T2-W, and another for methanol synthesis. Stream S8 obtained from the downside of the T3-D is the LNG product.
lines 212- 213
(In this paper, SRK property is used to predict the steam status.)
In this paper, SRK method is used to predict the physical properties of streams.
line 231
Simulation results are shown in table 3.
line 245
(A small part of unreacted gas is purged for mass balance.)
A small part of unreacted gas is discharged as purge gas.
line 270
(These two parameters are highly corelated with the composition of syngas and methanol production. The impact of these parameters is analyzed in detail.)
These two parameters have significant impact on the composition of syngas and methanol production, which are analyzed in detail as follows.
Line 276
Figure 9(a, b). Effect of temperature and pressure on the H/C ratio and inert gas concentration.
line 292-293
Figure 10(a, b). Effect of recycling ratio on methanol productivity and compressor duty.
(The yield of methanol, indicating that the conversion grows as the recycling ratio increases. This finding is the same as that of Man et al. (2016).)
The productivity of methanol shows an upward tendency with the recycling ratio increasing, which indicates that a higher carbon utilization efficiency can be achieved by adjusting that ratio. The result demonstrates good accordance with the previous study of Man et al. (2016) [39].
line 368-369
(Thereby that process needs more hydrogen which is provided by the gas shift reaction unit, in which CO is consumed and additional CO2 is emitted.)
It is necessary for CTSNG process to use water gas shift unit to increase that ratio to 3.1 for methanation reaction. In this course, CO2 emission is increased.
line 375-376
4.3 Energy consumption analysis
(In the discussion above stated, it is shown that the CTLNG-M process has improvements to the former process with higher energy efficiency and carbon element utilization ratio.)
As has shown in the above discussion, the CTLNG-M process has a higher energy and carbon utilization ratio than CTSNG process.
line 390-391
(However, there are some space for an improvement on electricity use in CTLNG-M.)
However, more electricity is consumed in the new process.
line 407-409
(The capital investment in these projects mainly includes fixed assets investment and working capital. The former one is the funds needed to provide production equipment and production-related facilities, including equipment, installation pipelines, electrical appliances, plant, land, engineering design and construction costs, etc.)
The total capital investment (TCI) for a given construction project mainly includes fixed capital investment and variable cost. The investment for manufacturing and plant facilities are defined as the fixed capital investment, while those for the plant operation are the working capital.
line 453-455
(Figure 14 shows a comparison of internal rates of return between these two processes. It can be seen that IRR of both CTLNG-M and CTSNG processes is excel the industrial criterion as 12%, so both projects are feasible from the economic point of view.)
The IRRs of CTLNG-M and CTSNG process are compared in figure 14, which are higher than the industrial criterion as 12%.
Author Response File: Author Response.pdf
Round 4
Reviewer 1 Report
Please kindly address the following changes:
Line 88: Change "Some work are also been proven ...." to " Some works are also being proven ...."
Line 194: Change "As have been ..." to "As has been ...."
Line 194: Change "... involved in ..." to "...involved in the ..."
Line 195: Change to "Namely, ...."
Line 197: Change "...modelling and simulation for ..." to "...modelling and simulation results for ..."
Line 218: Change "remained" to "remaining"
Line 219: Change "is mainly consisted" to "mainly consists"
Line 221: Change "downside" to "bottom"
Line 225: Change to "the SRK method ...."
Line 259: Change to "Simulation results ....."
Line 261: Change "shown" to "given"
Line 287: Change to "Simulation results ...."
Line 331: Change "The result demonstrates good accordance ..." to "The results confirm a good match ..."
Line 370: Change "System performances ....." to "System performance parameters ....."
Line 415: Change "... that ratio ..." to "...the ratio...."
Line 417: Change "by which" to "through which"
Line 423: Change to "As has been stated in the above discussion, ...."
Line 518: Change to "criterion of"
Author Response
Dear professor,
Thanks for your kindly review.
As you've asked, a marked revised version and point-by-point change/response are provided. Again, thank you for all the helpful work, including those detailed comments and useful advice on this manuscript.
Best regards.
*Point-by-point change/response:
Line 88: Change "Some work are also been proven ...." to " Some works are also being proven ...."
Some work are also being proven by demonstration projects.
Line 194: Change "As have been ..." to "As has been ...."
Line 194: Change "... involved in ..." to "...involved in the ..."
Line 195: Change to "Namely, ...."
As has been mentioned above, there are four main units involved in the CTLNG-M process. Namely coal gasification, acid gas removal, cryogenic separation and methanol synthesis unit.
Line 197: Change "...modelling and simulation for ..." to "...modelling and simulation results for ..."
Consequently, this paper gives modelling and simulation results for two added units.
Line 218: Change "remained" to "remaining"
Most of the remaining carbon monoxide component in S2 is sent to methanol synthesis unit.
Line 219: Change "is mainly consisted" to "mainly consists"
Stream S4 from the bottom of T2-W is mainly consists of CO and CH4.
Line 221: Change "downside" to "bottom"
Stream S8 obtained from the bottom of the T3-D is the LNG product.
Line 225: Change to "the SRK method ...."
In this paper, the SRK method is used to predict the physical properties of streams.
Line 259: Change to "Simulation results ....."
Table 3. Simulation results of the cryogenic separation unit.
Line 261: Change "shown" to "given"
Simulation results are given in Table 3.
Line 287: Change to "Simulation results ...."
Table 4. Simulation results of the methanol synthesis unit.
Line 331: Change "The result demonstrates good accordance ..." to "The results confirm a good match ..."
The results confirm a good match with the previous study of Man et al. (2016) [39].
Line 370: Change "System performances ....." to "System performance parameters ....."
Table 7. System performances parameters of CTLNG-M and CTSNG.
Line 415: Change "... that ratio ..." to "...the ratio...."
It is necessary for CTSNG process to use water gas shift unit to increase the ratio to 3.1 for methanation reaction.
Line 417: Change "by which" to "through which"
Methanol is present as a suitable product from chemical synthesis through which product methane is separated and cryogenically cooled to directly produce the LNG product.
Line 423: Change to "As has been stated in the above discussion, ...."
As has been stated in the above discussion
Line 518: Change to "criterion of"
The IRRs of CTLNG-M and CTSNG process are compared in figure 14, which are higher than the industrial criterion of 12%.
Author Response File: 
Author Response.docx
