Review Reports - Gasification of Waste Machine Oil by the Ultra-Superheated Mixture of Steam and Carbon Dioxide

Round 1

Reviewer 1 Report

Gasification of waste machine oil by the ultra-superheated mixture of steam and carbon dioxide, is an interesting work and fall within the scope of the journal. My few comments are below;

1. The schematics of gasifier are given, it would be more informative to add real figure too.

2. The fluctuation or a jump in gas outcome in fig.3a, does it correspond to something?

3. There are some repetitive sentences, those can be avoided.

4. The syngas parameters can be discussed in terms of their energy flow of the feed stock

5. The given references and arguments are too little, other works can be added, to relate the syngas outcome from different feedstock types and gasification benefits over other pyrolysis methods.

6. The authors should refine the conclusion.

Very nice work, best wishes.

Author Response

We are grateful to the reviewer for valuable comments. We have made our best to follow all the comments. All changes in the revised manuscript are marked in yellow.

Gasification of waste machine oil by the ultra-superheated mixture of steam and carbon dioxide, is an interesting work and fall within the scope of the journal. My few comments are below;

1.The schematics of gasifier are given, it would be more informative to add real figure too.

To follow this comment, we have added the image of the experimental facility to Figure 1.

The fluctuation or a jump in gas outcome in fig.3a, does it correspond to something?

To follow this comment, we have slightly changed the text related to Figure 3. First, we have noted that VARIO gas analyzer does not register nitrogen and hydrocarbons other than CH4. These compounds are presented in Figure 3 as N2+CxHy. Second, we replaced CxHy in Figure 3 by N2+CxHy. Third, we have added the following sentence to the description of Figure 3:

“The sharp increase in the contents of N2+CxHy and O2 after the shutdown of the PDGs corresponds to the displacement of the product syngas by atmospheric air in the gas analyzer.”

There are some repetitive sentences, those can be avoided.

We have made our best to avoid repetitive sentences.

The syngas parameters can be discussed in terms of their energy flow of the feed stock.

To follow this comment, we have added the values of HHV to Tables 3 and 4, which would help to construct the energy flow of feedstock.

The given references and arguments are too little, other works can be added, to relate the syngas outcome from different feedstock types and gasification benefits over other pyrolysis methods.

To follow this comment, we have added a paragraph to the Discussion section, where we compared our results with those available in the literature for engine oil and biomass. In view of it, we have added some more new references.

The authors should refine the conclusion.

We have refined the conclusions.

Very nice work, best wishes.

We appreciate your comments very much. Thank you!

Author Response File: Author Response.pdf

Reviewer 2 Report

Have you considered the energy consumption to run the process?
The gasification temperature up to 2000oC seems to be too high.
The material construction of gasifier will give some challenges and may not be economically feasible.

Author Response

We are grateful to the reviewer for valuable comments. We have made our best to follow all the comments. All changes in the revised manuscript are marked in green.

Have you considered the energy consumption to run the process?
This issue was addressed earlier in [28]. To follow this comment, we have highlighted the sentence in Section 2.2 of the original manuscript:

“Fourthly, the energy consumption for the cyclic operation of the PDG is negligible.”

The gasification temperature up to 2000oC seems to be too high.
To follow this comment, we have extended one sentence in Section 2.2 by adding

“…with the temperature reduced to about 2000 °C [21,34].”

In Refs. [21,34], we discuss in detail the issue addressed by the reviewer.

The material construction of gasifier will give some challenges and may not be economically feasible.

We have addressed this issue in our previous publications (see, e.g., [28]). Actually, the gasifier can be made of conventional construction materials because the PDG is effectively cooled down from interior during the fill process, and can be water cooled from the exterior. We have highlighted this statement in the original manuscript at page 5.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article " Gasification of waste machine oil by the ultra-superheated mixture of steam and carbon dioxide", Reported in the article is further progress in the development of the novel Pulsed Detonation Gun (PDG) technology, by optimizing design and operating parameters to minimize the content of carbon dioxide in gasification products. The research purpose of the manuscript is clear, the structure is reasonable, and only part of the content is unclear.

Detailed comments are as follows:

1. Line 147, "The PDG is a tube with one end open and the other closed." This indicates that one end is open and the other is closed. Please mark in the figure which port can be opened, or does each port have the characteristics of opening and closing , please specify the content. In addition, it is recommended to supplement the key information in figure 1, such as spark, etc.

2. Line 181-182, the content indicates that it has "mendous destructive power". Will this destructive power cause damage to the flow reactor and is the degree of damage predictable? What is the pressure value that the flow reactor can withstand?

3. In the experimental conditional part of 2.3, it is recommended that the author add a table to the content of lines 205-214, which may compare the experimental conditions more intuitively.

4. Line 244, "overpressure in the flow reactor, ??, and temperature of the wall, ??", please give the specific pressure and temperature measuring point location information, which should also be marked in Figure 1. In addition, whether the position of the measuring point will affect the analysis of the results should be judged.

Author Response

We are grateful to the reviewer for valuable comments. We have made our best to follow all the comments. All changes in the revised manuscript are marked in blue.

Detailed comments are as follows:

1.Line 147, "The PDG is a tube with one end open and the other closed." This indicates that one end is open and the other is closed. Please mark in the figure which port can be opened, or does each port have the characteristics of opening and closing , please specify the content. In addition, it is recommended to supplement the key information in figure 1, such as spark, etc.

To follow this comment, we have replotted Figures 1a and 1b where we added showed the locations of the spark plug and the supply ports of natural gas and oxygen.

Line 181-182, the content indicates that it has "mendous destructive power". Will this destructive power cause damage to the flow reactor and is the degree of damage predictable? What is the pressure value that the flow reactor can withstand?

This issue was discussed in our earlier publications (see, e.g., [28]). In case of misfire, the fresh fuel-oxygen mixture can fill the gasifier and can be exploded by the subsequent detonation pulse. The flow reactor must withstand the constant-volume explosion of natural gas – oxygen mixture diluted with the products of the previous detonation pulse. So far, we used cylindrical and spherical vessels made of conventional steel with the wall thickness less than 5 mm and did not detect any deformation or damage. This is because the volume of PDGs is an order of magnitude less than the volume of gasifier. As for the feedstock, it is fragmented effectively by the shock waves.

To follow this comment, we have added the following sentence to Section 2.2:

“The issues of operation safety are discussed in detail in [28].”

In the experimental conditional part of 2.3, it is recommended that the author add a table to the content of lines 205-214, which may compare the experimental conditions more intuitively.

To follow this comment, we have added a new table (Table 5) and renumbered all subsequent tables.

Line 244, "overpressure in the flow reactor, ??, and temperature of the wall, ??", please give the specific pressure and temperature measuring point location information, which should also be marked in Figure 1. In addition, whether the position of the measuring point will affect the analysis of the results should be judged.

To follow this comment, we have added to Figures 1a and 1b the locations of the pressure sensor and thermocouple.In addition, we have added two sentences in Section 2.3:

“The locations of the thermocouple and pressure sensor are shown in Figures 1a and 1b. The changes of these locations on the surface of the flow reactor did not affect essentially the measured temperature and pressure.”

Author Response File: Author Response.pdf

Reviewer 4 Report

This paper examines a novel reactor configuration for the gasification of organic liquid wastes. The reactor is original and it is worthy to further progress on its development. In general the manuscript is well written and the results are interesting. However, I cannot be accepted for publication in the present form. Literature comparison is very poor. 33 references were cited in the introduction whereas only 2 in the results section. Moreover, one third of the references correspond to the authors. In order to accept it, the following should be supplemented.

In section 2.1 how was the moisture content measured?

The H2O and CO2 used as gasifying agents are coming from the combustion of natural gas?

At which point of the operation was the liquid waste supplied? How was it fed? Please specify it on the manuscript

A detailed scheme of the PDG is encouraged.

Feed location should appear in the experimental conditions, where four schemes are detailed.

Which are the dimensions of the reactor?

Which is the flow rate of the GA? What is the ratio between H2O and CO2?

How was the temperature on the reactor controlled? Was it measured using the thermocouple in the reactor wall? Which temperature was selected for the gasification experiments?

What does G_f mean? Is it refered to the natural gas o to the syngas?

Why were different feedstock rates used to study the effect of feedstock supply location? It seems more logic to carry out the experiments under similar conditions.

How much does the residence time increased when operating with the valve?

The use of the reaction indices usually employed to assess the gasification performance such as gas yield (Nm3 gas per kg of feedstock) and H2 production (H2 g per 100 g of feedstock) are highly encouraged.

In Table 4, results corresponding to 2 and 4 schemes cannot be compared as there is so much difference (almost 100 ºC) in the reaction temperature

In section 3.2, if the residence time increases due to longer PDGs and the use of the valve, why H2 and CO concentrations decrease and that of CH4 and other light hydrocarbons increase? Explain that.

In section 3.3 why does the CH4 concentration increase when changing from minimum to intermediate residence time? Explain it in the manuscript.

In section 3.4, if the residence time of the feedstock in the PDG is much lower than that in the reactor, why are the H2 and CO contents higher? Please explain that.

In Table 5, are the test no 1 and 4 comparable?

Literature comparison should be improved. The authors must compare their results with those in the bibliography.

Author Response

We are grateful to the reviewer for valuable comments. We have made our best to follow all the comments. All changes in the revised manuscript are marked in gray.

1.Literature comparison is very poor. 33 references were cited in the introduction whereas only 2 in the results section. Moreover, one third of the references correspond to the authors. In order to accept it, the following should be supplemented.

To follow this comment, we have extended the literature review by adding a paragraph in the Discussion section.

2.In section 2.1 how was the moisture content measured?

The moisture content of the feedstock was measured by gravitational phase separation. This is now written in the footnote to Table 1.

3.The H2O and CO2 used as gasifying agents are coming from the combustion of natural gas?

The H2O and CO2 used as gasifying agents are coming from DETONATION of slightly fuel rich natural gas – oxygen mixture. To follow this comment, we have added a new table (Table 4) with the measured composition of detonation products. We have also added the following sentence with the reference to the table:

“Table 4 shows the composition of the GA – the mixture of ultra-superheated steam and carbon dioxide – measured in the experiments without feedstock supply.”

4.At which point of the operation was the liquid waste supplied? How was it fed? Please specify it on the manuscript

The procedure of feedstock supply is described in Section 2.3. To clarify this issue, we have added the following sentences to the captions of Figures 2 and 3:

Figure 2: “Zero time approximately corresponds to the conditions when the second transient stage of WG operation comes to an end.”

Figure 3: “Zero time approximately corresponds to the start of the second transient stage of WG operation.”

The first and second transient stages of WG operation are described in the last paragraph of Section 2.2. We have highlighted this text.

A detailed scheme of the PDG is encouraged.

The detailed scheme of PDG and its operation principle have been considered in our previous publications (see, e.g., [28, 34]). In this manuscript, we have just explained the PDG operation principle in the second paragraph of Section 2.2.

Feed location should appear in the experimental conditions, where four schemes are detailed.

To follow this comment, we have added the information on the feedstock location ports to Table 5.

Which are the dimensions of the reactor?

To address this comment, we have added the following sentence to Section 2.2:

“The flow reactor is based on the standard 40-Liter gas cylinder (241 mm in diameter and 997 mm long).”

Which is the flow rate of the GA? What is the ratio between H2O and CO2?

To address this comment, we have added one more column to Tables 6-9 with the values of mass flow rates of oxygen G_ox in the natural gas – oxygen combustible mixtures. The flow rate of GA is then the sum G_f + G_ox. The ratio between H2O and CO2 is seen from the new Table 4.

How was the temperature on the reactor controlled? Was it measured using the thermocouple in the reactor wall? Which temperature was selected for the gasification experiments?

The steady-state temperature of the reactor wall mainly depends on the PDG volume, PDG operation frequency, reactor volume, and PDG-reactor cooling conditions. In this study, we did not change any of these parameters and conditions and just registered the time history of wall temperature to reach a steady-state condition. The reactor wall temperature was measured by a thermocouple (its position is shown in Figure 1). Note that the measured wall temperature is not relevant to the process temperature, as the gasification reactions proceed in the reactor volume at a considerably higher temperature of the gasifying agent. The analogy is the operation process in an internal combustion engine: its wall temperature has nothing to do with the processes inside the engine cylinder.

What does G_f mean? Is it refered to the natural gas o to the syngas?

It is related to the natural gas. To clarify it, we have refined the definition of G_f.

Why were different feedstock rates used to study the effect of feedstock supply location? It seems more logic to carry out the experiments under similar conditions.

The objective was to achieve the maximum conversion of feedstock into syngas. Therefore, we present here only the results meeting this objective. Note that the supply of liquid oil to the PDG can affect the detonation process: at larger mass flow rates of oil, we could not obtain a DDT and detonation failed.

To address this comment, we have added the following text to Section 2.2:

“Note that waste supply to the PDG can affect both DDT and developed detonation. Depending on the location and the mass flow rate of waste supply, they can fail or decay, respectively.”

How much does the residence time increased when operating with the valve?

To answer to this question, one must conduct a 3D calculation and construct the probability density function of residence time. This is planned to do in the nearest future, as we already have such experience. Nevertheless, we believe that the valved scheme provides the increase in the mean residence time by a approximately factor of 3 as compared to the valveless scheme.

To address this comment, we have added the following note in Section 3.1:

“(the mean residence time is expected to increases by a factor of about 3)”

In this study, we did not collect the product syngas: we just burned it in a burner. Nevertheless, one could readily estimate the flow rate of the product syngas as the sum G_f+G_ox+G_w. To address this comment, we have added a paragraph to Section 3.5:

“The value of sum G_f+G_ox+G_w and the process time (~600 s) can be used to estimate the amount of the produced wet syngas in the WG of valveless schemes 1 and 3. For the cases with the absence of unreacted feedstock (W_w) in Tables 8 and 9, this amount ranges from 2.5 to 2.6 kg. Taking into account the residual water, W_sr, the amount of the produced dry syngas ranges from 1.8 to 2.2 kg. This means that the yield of dry syngas per unit mass of feedstock ranges from 1.8 to 2.5 kg syngas/kg feedstock. Knowing the syngas composition, this gives the volumetric yield of syngas ranging from 2.1 to 3 Nm3/kg feedstock. The gasification process provides the volumetric hydrogen production ranging from 0.63 to 0.90 Nm3 H2/kg feedstock or mass-based hydrogen production ranging from 50 to 72 g H2/kg feedstock.”

In Table 4, results corresponding to 2 and 4 schemes cannot be compared as there is so much difference (almost 100 ºC) in the reaction temperature.

As we mentioned earlier, the reaction wall temperature IS NOT the process temperature. The process temperature is determined by the temperature of gasifying agent, which is initially above 2000 C.

In section 3.2, if the residence time increases due to longer PDGs and the use of the valve, why H2 and CO concentrations decrease and that of CH4 and other light hydrocarbons increase? Explain that.

To address this comment, we have added the following sentence to Section 3.2:

“The latter could be explained by the larger relative loading of the reactor with the feedstock ( to ratio 0.56 vs. 1.21) and possibly incomplete gasification of feedstock.”

In section 3.3 why does the CH4 concentration increase when changing from minimum to intermediate residence time? Explain it in the manuscript.

We have analyzed this comment and found that the case with the intermediate residence time corresponds to test #3 in Table 9. In this test, a considerable amount (400 g) of unreacted or partly reacted feedstock was found in the exhaust system of the flow reactor due to insufficient residence time. Therefore, the increase in the yield of CH4 when changing from minimum to intermediate residence time could be explained by incomplete gasification of feedstock.

To address this comment, we have added the following sentence to Section 3.3:

“The increase in the yield of CH4 when changing from minimum to intermediate residence time could be explained by incomplete gasification of feedstock (see below).”

In section 3.4, if the residence time of the feedstock in the PDG is much lower than that in the reactor, why are the H2 and CO contents higher? Please explain that.

The reviewer possibly means the irregularity between the contents of CO for cases 5 and 6 in Figure 7. To address this comment, we have added the following sentence to Section 3.4:

“Nevertheless, some inconsistencies must be noted, e.g., the irregularity between the contents of CO for cases 5 and 6 in Figure 7. Case 5 corresponds to the short and valved PDG with feedstock supply at a distance of 295 mm from the closed end (see Table 7, scheme #2). Case 6 corresponds to the long and valved PDG with the feedstock supply at the same distance from the closed end (see Table 7, scheme #4). The expected residence time in Case 6 must be longer, and the yields of H2 and CO must be higher. However, Table 7 and Figure 7 show that despite the yield of H2 is higher indeed (30.6% vs. 33.2%), the yield of CO is somewhat smaller (43% vs. 40.6%). The possible reason for this is the incomplete gasification of feedstock in Case 6 due to a larger relative loading of the reactor with the feedstock as compared with Case 5 ( to ratio is 1.21 vs. 0.75).”

In Table 5, are the test no 1 and 4 comparable?

Test #1 can be omitted as it differs in the mass flow rate of natural gas. To address this comment, we have deleted the line corresponding to test #1 and renumbered the tests in the table.

Literature comparison should be improved. The authors must compare their results with those in the bibliography.

To address this comment, we have included some comparisons with the literature data. The corresponding changes are highlighted in the text at the end of Discussion section:

“It is worth to compare the obtained results with the available literature data. Despite there are many articles on the gasification of such organic wastes as plastic, biomass, tires, etc. (see, e.g., [35–37]), there is unfortunately little data on the gasification of waste machine oil [38,39]. Reported in [38] are the results of gasification of fresh synthetic engine oil with steam and supercritical water at a temperature ranging from 500 to 800 °C and a high pressure ranging from 50 to 500 bar. At the optimal conditions (750 °C, 250 bar, reaction time 1.9 min) found by the authors, over 85% of the feedstock was gasified and produced 1.6 kg syngas/kg feedstock and 60 g H2/kg·feedstock. The obtained results look also comparable with the literature data on the gasification of organic feedstock. Experiments on H2O, CO2, and mixed H2O/CO2 gasification of rice straw in an electrically heated reactor at temperatures 750–950 ◦C and pressure 1 bar were reported in [40]. In general, experiments showed that substitution of H2O with 30 and 60%vol. CO2 in the GA lowered the H2 yield and enhanced the CO yield. Thus, in gasification tests at 950 °C the yield of H2 decreased with the addition of CO2 from 82 to 1 g H2/kg feedstock, and the yield of CO increased from 560 to 900 g CO/kg feedstock. The thermal efficiency of gasifier was higher at a higher CO2 blending ratio. The authors of [41] reported the results of thermodynamic calculations on the gasification of rice straw with steam and CO2 as gasifying agents at 900 °C and 1 bar. Calculations were made for the composition of syngas at various CO2-to-feedstock ratios (from 0 to 0.87) when temperature and steam-to-feedstock ratio were kept constant at 900 °C and 0.3, respectively. The yield of H2 decreased from 54 to 34%vol. while that of CO increased from 37 to 40%vol. The conclusion was made that the selection of a proper CO2-to-feedstock ratio could result in attaining the highest gasification efficiency. These findings are well comparable with our results presented in Section 3.5.”