Experimental Study of a Rotating Electrode Plasma Reactor for Hydrogen Production from Liquid Petroleum Gas Conversion

Round 1

Reviewer 1 Report

This is a comprehensive study with many precious data. This is definitely a good manuscript to be published in Applied Sciences. However, some minor revision is required prior to publication. The number of figure are to much. I suggest to combine some figures together and move some to the supplementary data. Figures 1 and 2 can be combined. Figures 7-9 can be combined and present in a figure with multiple sub-figures. Figure 5 can be inserted in the supplementary data. 

Author Response

Thank you by your comments that have helped to improve the manuscript. Please, see the attached document for my responses.

Author Response File: Author Response.pdf

Reviewer 2 Report

I would recommend careful proof reading and editing prior to publication.

Author Response

Thank you by your comments that have helped to improve the manuscript. Please, see the attached document for my responses.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors studied the LPG decomposition by using a rotating electrode plasma reactor for hydrogen production. The manuscript is well written, and the discussion is suitable to publish in Applied Sciences. However, some critical issues have to be addressed before acceptance:

1. Line 143-144 is not scientific statement. Cost should be discussed with other systems.

2. Explanation for Figure 3 is not there. Explanation should be added, or Figure 3 should be deleted.

3. Authors just explained the result of Table 1, and the discussion is not there. The suitable reason of the rotation velocity and gas flow rate dependence should be added.

4. If the flow rate is increased, the decomposed gases also should be increased even if conversion yield is lowered. This means the tendency of conversion yield should not be related directly to peak intensity and the consumed power. Please explain the reason.

5. Energy efficiency should be estimated by “(Enthalpy change by decomposition of C4H10 and C3H8 × moles of each gas converted/s)/P”.

Author Response

Thank you by your comments that have helped to improve the manuscript. Please, see the attached document for my responses.

Author Response File: Author Response.pdf

Reviewer 4 Report

Title: Experimental Study of a Rotating Electrode Plasma Reactor for Hydrogen Production from Liquid Petroleum Gas Conversion

Manuscript Nr.: applsci-1659037-peer-review-v1

The authors present an experimental study on the production of H2 from LPG fuels using a novel DBD-like reactor, which features a rotating electrode. The authors combine the results obtained using gas chromatography with the electrical and optical characterization of the plasma. The authors explained the attained performances based on the characteristics of the plasma. The work is of good quality but there are generals points and scientific questions that must be addressed before considering it for publication. They are listed below.

English usage

There are several instances where the English must be revised. A non-exhaustive list is given below. A thorough revision is advised.

• Line 41: “The support type joints the specific surface area…”. What do the authors mean?
• Line 42: “performs” to be replaced with “exhibited the”
• Line 54: “are” to be replaced with “were”
• Line 63: “residence” instead of “resident”
• Line 83: “lack” instead of “shortage”
• Line 116: one should read “The main plasma parameters of the discharges produced…”
• Line 118: one should read “These parameters…”
• Line 143: “… the designing and fabricating cost…” to be replaced with “… the design and fabrication costs…”
• Line 303: “its selectivity … when velocity grows up.” to be replaced with “… when the rotation velocity increases or is raised.”
• Line 350: one should read at the end “by the following reactions”
• Line 355: “leads” instead of “lead”
• Line 358: “s” instead of “sec”

Units, Plots

• It would be of improved value if the authors would avoid using Torr for pressure. The SI unit Pa should be used instead.
• Is lpm standard liter per minute? If so, it would be better to use sLm instead, since it is a more commonly used unit.
• Figure 4.b) what does “constant” mean? Is this supposed to be static?
• Speed, rotational frequency are more accurate terms than velocity (which is formally used in the context of the vector velocity)
• “Flowrate” should be replaced with “Flow rate”
• Figure 9: “The hydrogen selectivity…” should be a mistake.

Scientific comments and questions

• Why did the authors use polyamide as the dielectric? This is not a usual material. And this is a bit troublesome because, later in the results, the authors argue that there is air in the chamber (lines 364 – 365) – which is already worrying by itself – because they observe nitrogen and oxygen. But I would assume this could also come from the dielectric itself. Could the authors discuss this? Did the authors perform other experiments with for example quartz or alumina?
• Line 132: the authors say that the glass tube was evacuated to a base pressure of 760 Torr, which is atmospheric. This seems to be a contradiction. And did the authors characterize the background gas with GC? What is effectively the lowest pressure achieved? What is the percentage of air in all discharges?
• Line 139: “the ionization rate … is significantly high”. What do the authors compare it to? The authors should give here a number that convinces the reader.
• Why did the authors study the voltage dependency in static configuration? This complicates the statements given by the authors. It would be desirable to have all voltage dependency plots replaced with data obtained at reference conditions of e.g. 18000 rpm and 1 lpm.
• Lines 226-227: the authors here provide the hypothesis, that the conversion trend is correlated with the number of current shocks in the plasmas. But later in the manuscript the authors (lines 313/314) give it as a fact. Do the authors think that the impact of the voltage, velocity and flow rate is solely explained by the increase in the number of shocks? This should be clearly explained.
• The energy efficiency is defined as units of mmol/kJ. Usually, one would expect the efficiency to be given in %, relative to suitable quantities. Could the authors comment on this? Nevertheless, even using mmol/kJ, there is no comparison given between the results and what the literature provides. It is entirely missing a discussion on how these values compare with those given in literature.
• The authors show that both H2 and C2 increase with voltage and decrease with the flow. For the voltage dependency, selectivities beyond 90% are simultaneously achieved for both H2 and C2. The question that follows is what is the concentration of H2 at the outlet of the reactor? Presumably this is a mixture with an important presence of C2. A discussion on the concentration of H2 – which is highly important for the applicability of this technology for H2 generation – should be given.
• The dissociation of CH4 in (5) is repeated twice and that of C3H8 and C2H4 are repeated once. Is the dissociation of C4H10 correctly written?
• The authors state that there are “some air impurities” in the discharge tube. From the OES spectrum given, it is difficult to accept this as “impurities”. What do the authors mean by impurities? What is the concentration of background in air in the measurements?
• Lines 324-325: the authors state that the OES measurements represent a time-average of the discharges taking place at much shorter times. Could the authors provide a number on the typical number of discharges that occur during the OES acquisition time?
• Line 352/353: Could the authors explain the sentence on the H2 production being twice that of carbon?
• Figure 12c). The authors state in the text (390-391) that the concentrations of all species decrease with the gas flow rate. This seems to be true at different degrees for different species (some of them being constant). Can the authors comment on this?
• At the end of the conclusions, the authors state that there is soot accumulation but in the core of the manuscript this is not discussed, although the authors state in lines 304-305 that accumulation of carbon deposits can explain the decrease of C2 when the flow rate increases. This calls for the need to discuss the presence of soot/deposits after the experiments. Did the authors characterize the amount of soot created? How is it impacted by the range of conditions studied?

Other comments

• Please introduce acronyms in the Introduction.
• Line 42: “performs” to be replaced with “exhibited the”
• Line 54: “are” to be replaced with “were”
• Line 63: “residence” instead of “resident”
• Line 83: “lack” instead of “shortage”
• Figure 3 is not introduced in the text

Author Response

Thank you by your comments that have helped to improve the manuscript. Please, see the attached document for my responses.

Author Response File: Author Response.pdf

Reviewer 5 Report

The use of atmospheric plasma for gas reforming has been investigated in the last years for several kinds of gases. The paper of Barkhordari et al. reports on the development of a rotating plasma reactor for treatment of C4H10/C3H8 gas mixture aiming hydrogen molecule production. Several electrical, optical and neutral gas analysis techniques were used in this study and the results evidenced that the rotation velocity of the power electrode can influence considerably in generated plasma chemistry and consequently in C4H10/C3H8 conversion. In general the paper is well-written and the subject is of interest for the readers of the Applied Science MPDI journal. Before paper acceptance some improvements are required:

(1) Please remove the dot in the end of the paper title.

(2) Abstract: Please define "butane / propane (C4H10/C3H8)" in abstract;

(3) Materials and Methods:

3.1 Please inform the thickness of the disk electrode. The authors inform that this electrode act as anode, please verify if this information is correct?;

3.2 Figure 2: Insert the ground in the HV power supply + electrodes;

3.3 Please inform the manufacturer of the AC power supply.

(4) Figure 13: The experimental data is not possible to see, please correct this figure;

(5) Figure 16: It is more conventional to present the electron temperature in eV.

Author Response

Thank you by your comments that have helped to improve the manuscript. Please, see the attached document for my responses.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The manuscript was carefully revised. I recommend to publish the manuscript by Applied Sciences.

Reviewer 4 Report

The authors carefully tackled each point raised in the review. I recommend the manuscript for acceptance.