A Critical Review of Recent Progress and Perspective in Practical Denitration Application

Round 1

Reviewer 1 Report

This review deals with current state of art of the denitration application. The topic of the manuscript is interesting, and it reports some interesting information on the subject, in my opinion there are only some points that should be clarified before the publications.

In particular, the authors should take into consideration the following considerations:

 

- Please specify the acronyms the first time that compare in the manuscript to facilitate the reading. See for example pag. 4 line 137 (SCR) or pag. 10 line 317 (SCO) or pag. 15 line 464 (“have poor S (sulfur) and P (phosphorus) resistance”..)

 

-In my opinion it is more correct use the world “researchers” instead of “scholars” (see for example pag. 6 line 168).

 

- Please correct the misprinting at pag. 12 line 376 in the world “proceses”

 

- Please correct the misprinting on the table 3 in the world “temperature”

 

-Please correct the error in the title of the paragraph at pag. 18 it should be H₂-SCR

 

-Please revise the title of each paragraph for example the title of 5.1 paragraph is in italic font

 

-Paragraph 3.3. I think that would be useful discuss also to the new oxidation processes for the NO_x removal (even as hints) as photocatalysis or photoFenton processes.

 

-In my opinion at the end of paragraph 5 the authors should discuss the economical point of view of the plasma processes in comparison with the conventional denitration treatments. In fact, although the plasma process is very promising it could be more expansive.

 

For my point of view the manuscript can be published in Catalysts after a minor revision according to the above reported comments.

 

 

 

Author Response

Dear Editors and Reviewers,

We greatly appreciate the time and effort you’ve spent in reviewing our manuscript titled

“A Critical Review of Recent Progress and Perspective in Practical Denitration Application” (Manuscript number: catalysts-586977), and your constructive suggestions are valuable to our manuscript and future research work. According to the reviewers’ comments, we have made further analysis and revised the manuscript carefully. We hope the Editor and Reviewers will be satisfied with the revisions for the original manuscript. The followings are the reviewers’ comments and the corresponding point-by-point responses to the concerns.

Reviewer 1

Comments and Suggestions for Authors

This review deals with current state of art of the denitration application. The topic of the manuscript is interesting, and it reports some interesting information on the subject, in my opinion there are only some points that should be clarified before the publications.

In particular, the authors should take into consideration the following considerations:

Please specify the acronyms the first time that compare in the manuscript to facilitate the reading. See for example pag. 4 line 137 (SCR) or page. 10 line 317 (SCO) or pag. 15 line 464 (“have poor S (sulfur) and P (phosphorus) resistance”)

Thanks for your valuable advice. I have revised the above matters. Page. 4 line 137 (SCR to Selective catalytic reduction ) or page. 12 line 362 (SCO to Selective catalytic oxidation) or page. 18 line 528 and other modifications.

It was found that 3% O₂ was beneficial to increase the conversion of NO, which was attributed to NO oxidation as the first step of selective catalytic reduction Selective catalytic oxidation method has the advantages of relatively low cost and high removal efficiency of NO_x. At the same time, they have poor sulfur and phosphorus resistance, so it is particularly important to develop non-precious metal catalysts. In my opinion it is more correct use the world “researchers” instead of “scholars” (see for example page. 6 line 168).

I quite agree with you, so I made a change. (Page 6 line 183).

Since the development of a low temperature oxidation technology called LoTOx by BOC Company in the United States, more and more researchers have been studying the denitrification performance of O₃.

3 Please correct the misprinting at pag. 12 line 376 in the world “proceses”

I'm terribly sorry .It was my carelessness that caused the spelling mistakes. I've changed it on page 14 line 421.

NH₃-SCR is one of most efficient processes for denitrification.

Please correct the misprinting on the table 3 in the world “temperature”

I've adjusted the table layout.

Catalyst	Synthetic method	GHSV（hr-1）	Temperature (oC)	Conversion (%)	Ref
Ce-Mo/TiO2	Co-precipitation	19000	200-400	90	[178]
MnAl/ LDO	FNP	60000	150-250	100	[179]
MnOx-CeO2-Al2O3	self-propagating synthesis	15384	50-400	100	[180]
Mn-Ce-Ti	Hydrothermal	64000	150-400	98	[181]
MnOx-CeO2-TiO2	Sol-ge	10000	100-300	90	[182]
Co/Ni-CeO2	Co-precipitation	48000	75-200	93	[183]
Fe-Mn-Ce/γ-Al2O3	Sol-ge	10000	100-450	95	[184]
MnOx/CeO2-ZrO2-Al2O3	Impregnation	10000	50-300	90	[185]

Please correct the error in the title of the paragraph at pag. 18 it should be H₂-SCR

I apologized to you once again. (I've changed it on page 20 line 579.

4.2.4. H₂-SCR

Please revise the title of each paragraph for example the title of 5.1 paragraph is in italic font

I have changed the italic font for each secondary heading.

For example, 5.1. Direct decomposition of NO_x by plasma

Paragraph 3.3. I think that would be useful discuss also to the new oxidation processes for the NO_x removal (even as hints) as photocatalysis or photoFenton processes.

I have carefully considered this valuable suggestion, and then I added a detailed section, introducing photocatalytic oxidation in detail and in depth. Paragraph 3.3.3. (Page 10 line 301)

3.3.3. Photocatalytic oxidation

Photocatalytic oxidation is a new process of NO_x treatment which has been developed gradually in recent ten years. The principle of photocatalytic oxidation is to irradiate semiconductor catalysts with light of specific wavelength, to stimulate valence band electrons on semiconductor materials to transit into conduction band, and to generate holes in valence band. Conduction band electrons and valence band holes have strong reductive and oxidative properties, respectively. When they are in contact with flue gas, H₂O, O₂ and NO_x adsorbed on the catalyst surface will generate active free radicals under the action of light, and then catalytic oxidation reaction will take place to oxidize NO_x to NO₃^-. Because it does not need to inject additional reductant, and is simple to operate, low cost, no secondary pollution, it is the focus of photocatalytic technology research at present [109-110].

The catalysts for photocatalytic oxidation are mainly metal oxide semiconductor materials. Among them, TiO₂ has the merits of high catalytic activity, photochemical stability and low price. It is the most commonly used catalytic material in photocatalytic reaction. Duan et al. [111] found that a small amount of Ag nanoclusters supported on TiO₂ was beneficial to improve photocatalytic activity. He et al. [112] synthesized carbon-encapsulated nanocrystalline TiO₂ catalyst, which achieved 71% NO conversion under visible light. Yuan et al. [113] prepared TiO₂ aluminosilicate fiber nanocatalyst for photocatalytic removal of NO_x. Jin et al. [114] prepared SrTiO₃ catalyst with SrCO₃ successfully as auxiliaryr by one-step in-situ pyrolysis for removal of NO_x. Huy et al. [115] reported for the first time the removal of NO_x by visible light on SnO₂/TiO₂ nanotube heterojunction catalysts. The reaction mechanism was shown in Figure 6a. The removal efficiency of NO_x under visible light was represented in Figure 6b. Figure 6c exhibited that SnO₂/TiO₂ nanotube heterojunctions had been successfully synthesized. Figure 6d was proved that superoxide anion radical played an important role in visible-light photocatalytic oxidation by using active oxygen detection materials.

Figure 6. (a) Reaction mechanism of visible light removal of NO_x by SnO₂/TiO₂ nanotube heterojunction, (b) removal efficiency of NO_x under different conditions, (c) transmission electron microscopy of SnO₂/TiO₂ nanomaterials, (d) detection process of free radicals under different conditions (reproduced with the permission from refs. [115]).

In my opinion at the end of paragraph 5 the authors should discuss the economical point of view of the plasma processes in comparison with the conventional denitration treatments. In fact, although the plasma process is very promising it could be more expansive.

I quite agree with you, then at the end of paragraph 5, I not only emphasize the advantages of plasma method, but analyze its economic deficiencies. (Page 26 line 735)

In recent years, the form of plasma has been innovated rapidly. Dielectric barrier discharge, radio frequency discharge and microwave discharge are all new non-thermal equilibrium plasma discharge forms [266-267]. The effect of plasma denitrification is remarkable, which can improve the NO conversion rapidly on short notice. However, compared with the traditional denitrification technology, the plasma process has made some progress, but there are still some problems: high energy consumption, short power supply life and performance to be improved; moreover, the expensive price of plasma equipment, high cost of system operation and maintenance, and complex equipment structure make the technology not widely used and restrict its development.

What’s more, we have added some new contents in order to briefly introduce the application of ‘monolith catalyst' in NH₃-SCR, as below.

It is worth mentioning that the powder catalyst in the laboratory stage has been quite mature, which can efficiently remove nitrogen oxides from simulated flue gas. However, how to apply these excellent catalysts to practice is a question worthy of consideration. Obviously, powder catalysts must be prepared as monolith catalysts to meet the needs of stationary industrial installations. Monolith catalysts have many dominant positions, such as strong mechanical stability, thermal conductivity, mass transfer capacity, small pressure drop and recycling, which are conducive to the catalytic process and practical application.

Yu Feng research group of Shihezi University has studied the monolith catalyst deeply. Tian et al. [176] prepared nanoporous microspheres Mn–Ce–Fe–Ti mixed oxide catalysts for NH₃-SCR by spray drying (Figure. 11a). The samples were processed by Focused Ion beam (FIB) and observed by scanning electron microscopy (Figure. 11b,c). After application in flue gas treatment, it was found that the monolith catalyst still maintained good catalytic activity. Wang et al. [159] first designed and prepared spherical MnO_x-CeO₂-Al₂O₃ powder catalysts by spray drying, then applied it to monolith honeycomb catalysts(MHC). Compared with the same metal oxides catalysts prepared by co-precipitation (CP-MHC), spray drying (SD-MHC) method exhibited excellent SCR denitrification performance at 50-150 ^oC (Figure. 11d,e).

Figure 11. (a) The SEM image of nanoporous microspheres Mn–Ce–Fe–Ti mixed oxide catalysts, (b) FIB images and (c) SEM image of nanoporous microspheres Mn–Ce–Fe–Ti [176], (d) SEM images of MnO_x-CeO₂-Al₂O₃ (CP-MHC) and (e) MnO_x-CeO₂-Al₂O₃ (SD-MHC) [159].

Author Response File: Author Response.pdf

Reviewer 2 Report

The comments are listed below:

Authors should enter more references for NOX adsorption by various kinds of adsorbents (maybe a new table). For Oxidation part, authors should enter a new section for the applications of photocatalysts (especially TiO2) for NOX removal.  Conclusion and prospects need more description. 

Author Response

Dear Editors and Reviewers,

We greatly appreciate the time and effort you’ve spent in reviewing our manuscript titled

“A Critical Review of Recent Progress and Perspective in Practical Denitration Application”  (Manuscript number: catalysts-586977), and your constructive suggestions are valuable to our manuscript and future research work. According to the reviewers’ comments, we have made further analysis and revised the manuscript carefully. We hope the Editor and Reviewers will be satisfied with the revisions for the original manuscript. The followings are the reviewers’ comments and the corresponding point-by-point responses to the concerns.

Reviewer 2

The comments are listed below:

Authors should enter more references for NO_x adsorption by various kinds of adsorbents (maybe a new table). For Oxidation part, authors should enter a new section for the applications of photocatalysts (especially TiO2) for NO_x removal. Conclusion and prospects need more description.

Authors should enter more references for NO_x adsorption by various kinds of adsorbents (maybe a new table):

Thanks for your valuable advice, and then I added a new section with a new table. (Page 5 line 147.

2.3. Other crucial NO_x adsorbents

There are numerous kinds of NO_x adsorbents, low-priced and efficient adsorbents are required for denitrification. Li et al. [63] prepared fly ash derivative Cu/SAPO-34 by acid-alkali combined hydrothermal method to absorb NO_x. Mire et al. [64] studied the removal of NO_x using H-ZSM-5 as adsorbents and found that Fe-modified Fe/H-ZSM-5 had the strongest adsorption capacity. Chen et al. [65] synthesized Fe-ZSM-5@CeO₂ adsorbent for adsorbing NO_x. Baran et al. [66] prepared Cu-BEA molecular sieve to study the adsorption characteristics of NO_x. Ma et al. [67] developed a new Ag nano-g-C₃N₄/WS₂ material to adsorb NO_x. Imai et al. [68] developed Niobium phosphate adsorbent remove NO_x. Xiao et al. [69] studied the adsorption of NO_x on mesoporous molecular sieve doped with platinum (Pt/SBA-15). Wang et al. [70] found that when iron and cobalt were loaded on the active semi-coke material, it was beneficial to NO_x adsorption. Chen et al. [71] studied NO removal performance based on sinter adsorption materials. The synthetic methods and reaction conditions of different adsorption materials are shown in Table 2.

Table 2. Effect of different adsorbents on removal of NO_x

Adsorbent material	Synthetic method	Reaction conditions	NO Conversion %	Ref
Cu/SAPO-34	Acid–alkali hydrothermal	300 ppm NO, 3% O2 GHSV 12000 h−1, N2	90	[63]
Fe/H-ZSM-5	Hydrothermal	5000  ppm NO, 5% O2, GHSV = 35000 h−1, He	63.4	[64]
Fe-ZSM-5 @CeO2	Dopamine polymerization	1000 ppm NO, 5% O2 , GHSV   33600 h−1, N2	90	[65]
Cu-BEA	Precipitation	1000 ppm NO, 3.5 vol.% O2 1000 mL/min, N2	100	[66]
g-C3N4/WS2	Solvent evaporation	500 ppm NO, 20vol% O2, GHSV 400 mL/min, N2	72.5	[67]
Niobium phosphate	Impregnation	1000 ppm NO, 1125 ppm O2, GHSV 100 mL/min, He	100	[68]
Pt/SBA-15	Thermal hydrolysis	4000 ppm NO, 10 vol.% O2 , GHSV 50 mL/min, Ar,	100	[69]
Semi-coke	Hydrothermal	1000 ppm NO, GHSV 6000 h-1,N2	100	[70]
Sintered ore	Impregnation	400 mg/m3 NO, 15% O2, GHSV=1000 h-1, Ar	61.6	[71]

For Oxidation part, authors should enter a new section for the applications of photocatalysts (especially TiO₂) for NO_x removal.

I quite agree with you, and then I added a detailed section, introducing photocatalytic oxidation in detail and in depth. Paragraph 3.3.3. (Page 10 line 301)

3.3.3. Photocatalytic oxidation

Photocatalytic oxidation is a new process of NO_x treatment which has been developed gradually in recent ten years. The principle of photocatalytic oxidation is to irradiate semiconductor catalysts with light of specific wavelength, to stimulate valence band electrons on semiconductor materials to transit into conduction band, and to generate holes in valence band. Conduction band electrons and valence band holes have strong reductive and oxidative properties, respectively. When they are in contact with flue gas, H₂O, O₂ and NO_x adsorbed on the catalyst surface will generate active free radicals under the action of light, and then catalytic oxidation reaction will take place to oxidize NO_x to NO₃^-. Because it does not need to inject additional reductant, and is simple to operate, low cost, no secondary pollution, it is the focus of photocatalytic technology research at present [109-110].

The catalysts for photocatalytic oxidation are mainly metal oxide semiconductor materials. Among them, TiO₂ has the merits of high catalytic activity, photochemical stability and low price. It is the most commonly used catalytic material in photocatalytic reaction. Duan et al. [111] found that a small amount of Ag nanoclusters supported on TiO₂ was beneficial to improve photocatalytic activity. He et al. [112] synthesized carbon-encapsulated nanocrystalline TiO₂ catalyst, which achieved 71% NO conversion under visible light. Yuan et al. [113] prepared TiO₂ aluminosilicate fiber nanocatalyst for photocatalytic removal of NO_x. Jin et al. [114] prepared SrTiO₃ catalyst with SrCO₃ successfully as auxiliaryr by one-step in-situ pyrolysis for removal of NO_x. Huy et al. [115] reported for the first time the removal of NO_x by visible light on SnO₂/TiO₂ nanotube heterojunction catalysts. The reaction mechanism was shown in Figure 6a. The removal efficiency of NO_x under visible light was represented in Figure 6b. Figure 6c exhibited that SnO₂/TiO₂ nanotube heterojunctions had been successfully synthesized. Figure 6d was proved that superoxide anion radical played an important role in visible-light photocatalytic oxidation by using active oxygen detection materials.

Figure 6. (a) Reaction mechanism of visible light removal of NO_x by SnO₂/TiO₂ nanotube heterojunction, (b) removal efficiency of NO_x under different conditions, (c) transmission electron microscopy of SnO₂/TiO₂ nanomaterials, (d) detection process of free radicals under different conditions (reproduced with the permission from refs. [115]).

Conclusion and prospects need more description.

Thanks for your valuable suggestion again. I have made a new statement of my Conclusion and prospects, which is more comprehensive and convincing than before. (Page 26 line 744)

Along with the global emission of NO_x in the large number influence continues to deteriorate planet earth environment and human health in the Anthropocene, it becomes progressively significant to collect and summarize the efficient approaches of governance and treatment of NO_x. Optimizing the above four distinct denitrification methods and controlling the emission of NO_x are the main challenges for future atmospheric environmental treatment. Finally, in order to solve the problem of air pollution that plagues all countries in the world, the feasibility of four main technologies in the field of flue gas denitrification and industrial application of flue gas tail purify is evaluated.

Adsorption treatment of NO_x has the feature s of environmental friendliness, energy saving and high efficiency. The process is simple to operate, controllable, for low concentration poisonous gases, efficient capture can be achieved. However, due to its limited adsorption capacity, frequent regeneration of adsorbents, huge size of adsorbents and large -scale investment, the application of adsorbents is limited. Oxidation process is a relatively mature method in the field of wet denitrification technology because of its simple route, easy operation and outstanding denitrification effect. Some of them have been industrialized. However, the oxidation tower has the problems of expensive raw materials and equipment corrosion, which limits its development due to its cost and safety. Reduction is the most efficient and mature denitrification technology with the most extensive application as well as the most exceptional means to control NO_x pollution. But at the same time, it has some shortcomings, high investment, high cost of catalyst regeneration, NH₃ escaping from secondary pollution, and poor sulfur and water resistance of catalyst and so on. The removal of NO_x by plasma technology is remarkable, which can rapidly improve the denitrification performance in a short time, and promote the stability and low temperature activity of the catalyst. However, the required equipment covers a large area, consumes high energy, and costs high investment, operation and maintenance, which is limited in practical application.

Future, the general requirement of denitrification technology is low cost, high efficiency and green. The overall development trend of technology is to realize the coordinated removal of multiple pollutants by coupling multiple technologies. Different regions and industries are suitable for different denitrification means. In order to decrease the emission level of NOx, cut down the expense of treatment and recovery, and improve the economic performance, different regions and industries should choose appropriate denitrification technology according from the resource situation to product usage. Therefore, it is of great practical significance to develop denitrification methods with high efficiency, low energy consumption, low secondary pollution and low investment.

What’s more, we have added some new contents in order to briefly introduce the application of ‘monolith catalyst' in NH₃-SCR, as below.

It is worth mentioning that the powder catalyst in the laboratory stage has been quite mature, which can efficiently remove nitrogen oxides from simulated flue gas. However, how to apply these excellent catalysts to practice is a question worthy of consideration. Obviously, powder catalysts must be prepared as monolith catalysts to meet the needs of stationary industrial installations. Monolith catalysts have many dominant positions, such as strong mechanical stability, thermal conductivity, mass transfer capacity, small pressure drop and recycling, which are conducive to the catalytic process and practical application.

Yu Feng research group of Shihezi University has studied the monolith catalyst deeply. Tian et al. [176] prepared nanoporous microspheres Mn–Ce–Fe–Ti mixed oxide catalysts for NH₃-SCR by spray drying (Figure. 11a). The samples were processed by Focused Ion beam (FIB) and observed by scanning electron microscopy (Figure. 11b,c). After application in flue gas treatment, it was found that the monolith catalyst still maintained good catalytic activity. Wang et al. [159] first designed and prepared spherical MnO_x-CeO₂-Al₂O₃ powder catalysts by spray drying, then applied it to monolith honeycomb catalysts(MHC). Compared with the same metal oxides catalysts prepared by co-precipitation (CP-MHC), spray drying (SD-MHC) method exhibited excellent SCR denitrification performance at 50-150 ^oC (Figure. 11d,e).

Figure 11. (a) The SEM image of nanoporous microspheres Mn–Ce–Fe–Ti mixed oxide catalysts, (b) FIB images and (c) SEM image of nanoporous microspheres Mn–Ce–Fe–Ti [176], (d) SEM images of MnO_x-CeO₂-Al₂O₃ (CP-MHC) and (e) MnO_x-CeO₂-Al₂O₃ (SD-MHC) [159].

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

All of my comments are amended in the manuscript. This manuscript can be accepted and published in this journal