Energy Recovery via Thermal Gasification from Waste Insulation Electrical Cables (WIEC)

Round 1

Reviewer 1 Report

This work is an interesting study about co-gasification of woody biomass together with plastic waste materials coming from electric cables. The topic is very interesting and the experimental section has been carried out fairly. However, I suggest some revisions in order to improve the quality of the manuscript, in particular to clarify some comments regarding the experimental results:

1) Page 3, line 111: "Several controlled gasification tests were carried out, that is, without the engine and the generator running, to identify the effect of the blending ratio." This is the first time in the paper that you mention an engine and generator; please mention in the text if these components are normally connected to the gasifier used for the experiments;

2) Page 4, line 140: "Fuel is supplied from the top as the air moves downward, being preheated through contact with the reactor’s walls" What is the size of the biomass in the hopper? Does it have to be grounded before being inserted in the hopper? Please give some details;

3) Page 4, line 146: Please add some more details about the temperatures of the process and of the downstream components. For example, which is the temperature in the gas filter? The residual water and tar are supposed to condense there, before the ICE?

4) Page 4, line 157: Please report the total duration of the tests;

5) Page 5, line 176: Is this the condensate accumulated after the biomass filter? What is the temperature of the deposit of condensate? Please report some more details on the tar sampling system;

6) Page 5, line 190: "The equivalence ratio is defined as: ER = (A/F)stoic / (A/F)" Actually the Equivalence ratio is defined as the ratio of actual air fuel ratio to the stoichiometric air fuel ratio, according to (Combustion and Gasification in Fluidized Beds, Basu). Please correct the formula or cite a reference that defines it as you reported here, or else define this as a different process parameter;

7) Page 5, line 195: In gasification processes you usually dont operate with an ER>1 (i.e. with an A/F ratio higher than the stoichiometric), because that would cause a combustion process with total oxidation, instead of a gasification process with partial oxidation. But this is probably related to a different definition of the ER parameter;

8) Page 6, Table 1: Cl was not analysed in the elemental analysis of WIEC? It could be present if WIEC is also composed of PVC;

9) Page 7, line 239: "However, a low VM content also reduces fuel HHV and increases the probability of tar formation." Did you mean that a higher amount of VM increases the HHV but is also cause of a higher probability of tar formation? Please revise the sentence;

10) Page 7, line 250: "A very relevant aspect is that the fixed carbon in a gasification reactor is responsible not only for increasing the temperature, but also as a determinant for the thermal cracking of the tar produced during the gasification process." Please add a reference for this sentence;

11) Page 7, line 281: "The oxygen content in the pine was 42.2% db" Please add a definition the first time that you use an acronym in the text (in this case d.b.=dry basis);

12) Page 9, Table 2: Please add the definitions of Trst and Tred. Is Tred the temperature of the reduction (gasification) zone? Is Trst the temperature of the combustion zone?

13) Page 9, Table 2: Is there a reason why you reported the raw syngas composition, comprehensive of N2, instead of the dry N2 free gas composition?

14) Page 9, Table 2: Can you comment about the temperature of the tests? Higher temperatures usually lead to better syngas compositions; why is the temperature in the reduction zone always lower than 600? Moreover, it seems that the temperature of the 1st sample is always very low; does it represent a transitorial phase or is there another explanation for this fact? Please add a comment on this.

15) Page 9, Table 2: Why is the Vair and thus the ER so variable during a test? Are the flows of air and fuel controlled in some way?

16) Page 10, line 293: "It is possible to observe that the temperature increases with the increase in the percentage of WIEC due to large amounts of volatiles, they are more easily synthesized than wood, during the thermochemical process." When you mention that volatiles are easily synthesized, the meaning is that they are burnt and so the temperature rises? Please explain better or rephrase;

17) Page 10, line 303: "With the increase in the percentage of WIEC, and the consequent increase in the reactor temperature and the flow of syngas, it is possible to observe that the Boudouard reaction (C + CO2 ⇔ 2CO) of CO formation and the water-gas shift reaction (CO + H2O⇔CO2 + H2) of H2 formation, are favored." When you comment on the results of the tests do you refer to the average values of samples 1, 2 and 3? If so, the flow of syngas is not the highest in the tests with higher WIEC content, it seems that the highest Vsyngas correspond to the first 2 tests. Moreover, the higher temperature could enhance the boudouard reaction but not the water gas shift reaction, that is exothermic and thus thermodynamically favoured at lower temperatures. Please revise the sentence considering this observations;

18) Page 11, line 308: "When LHV increases, ER tends to decrease, and gasification efficiency also increases." Why do you relate the ER to the LHV? ER is an operating condition, an input parameter, while the LHV is an output. It should be commented that the LHV is influenced by the ER, not the opposite. Please consider revising the sentence or add an explanation;

19) Page 11, line 314: "Although the rate of gas production increases and the CGE increases, the calorific value of syngas decreases, the efficiency of gasification also decreases." It seems that a higher ER leads to higher Volume of syngas but with a lower LHV; the increase of the Volume is higher than the decrease of the LHV, resulting thus in a higher CGE. But it has to be highlighted that higher ER leads to a poorer gas composition. Please extent this discussion;

20) Page 12, Figure 3: Why in the 3rd sample collected the Vsyngas is always the lowest, even if the temperature seems to be the highest for the 3rd sample of each test? For higher temperatures the gas yield should be higher and thus the volume flow of syngas produced should increase. Please try to give an explanation for this;

21) Page 13, line 340: "The blending ratio 80:20 was considered the most viable, despite having a calorific value 10% lower than that obtained in the blending ratio 90:10" The lower calorific value is referred to the gas, not to the fuel mixture. Please specify;

22) Page 13, Table 3: Comparing sample 3 of 5kW and sample 2 of 10 kW, that have approximately the same temperatures (and also the same input fuel) it seems that sample 3, with a higher ER, has a lower content in CO2 and a higher content in H2 compared to sample 2. This brings to a higher LHV. Furthermore the volumes of gas produced are almost the same but the fuel flow rate is almost half in sample 3 compared to sample 2. This results seem unusual, expecially compared to what mentioned before about the influence of the ER in the syngas composition. Please try to give an explanation of this results;

23) Page 13, Table 3: If a higher power of the load brings to higher air suction, why are the air flows here decreasing with the increase of the power load? (from 5 to 10 kW);

24) Page 15, line 355: "During the tests performed, the largest volume fraction is of CO followed by the H2. This is due to the water-gas shift reaction producing more CO than H2 and CH4." Why do you state that CO is produced by WGS? Do you suppose that the reverse WGS takes place, converting CO2 and H2 into CO and H2O? Maybe the higher content of CO could instead be related to the higher extent of the reforming reactions, enhanced indeed at higher temperature. Please explain better this sentence;

25) Page 16, line 395: "The studies explain the effects of ER on the gasification of polymeric residues, with air as an oxidizing agent [45] in which the results obtained showed that the lowest heating value decreased from 13.42 to 7.05 MJ/Nm³ when the ER was increased from 0.21 to 0.41" Please check this sentences with the results reported in the table 3. In some experiments, for equal temperatures the results show that for higher ER the H2 content is higher. What could be the explanation for this phenomena?

26) Page 16, line 411: "The total efficiency of the system increases with the production of energy. Maximum efficiency is 30% for the 15 kW load." Here you refer to the total efficiency but you mention 30% as maximum in the 15 kW case, that seems to be instead the gasifier efficiency (first row of Table 4). Please revise and correct;

27) Page 17, line 424: "Regarding the operational problems that were detected during the tests, it is observed that limiting the incorporation of WIEC by a maximum of 30% is necessary for allowing better gas quality and thus recommended operational parameters." Experiments with WIEC fractions higher than 30% are not described in this study, why the conclusions mention that higher fractions are not feasible? Has there been a preliminary study on the possible WIEC fraction to be incorporated?

28) Page 17, Conclusions: Tests with different electric loads connected to the engine and the relative efficiencies obtained are not mentioned in the conclusions. Please add conclusive sentences about these topics;

29) Please check the page numbers, expecially after page 8;

30) Please check the English language throughout the manuscript.

Author Response

Dear Reviewer,

We appreciate the corrections You made, and we are sure that they are of great value to improving the content and appearance of the manuscript.

Following the questions, we realized that we made a mistake when transposing the data to the excel tables. This error was due to the volume of air in the load tests (Table 3.), which resulted in errors in the ER and other related parameters. We apologize for this error and hope the Reviewer appreciate the corrections and effort that we've made.

Review 1:

1) Page 3, line 111: "Several controlled gasification tests were carried out, that is, without the engine and the generator running, to identify the effect of the blending ratio." This is the first time in the paper that you mention an engine and generator; please mention in the text if these components are normally connected to the gasifier used for the experiments;

Page 4, lines 123 to 129:

“For the tests, pine forest biomass (PFB) and waste insulation for electrical cables (WIEC) were used in different blending ratios PFB/WIEC. The gasification experiments were carried out in a gasifier, which results from a combination of a downdraft reactor, a 3.0 liter 4-cylinder internal combustion engine coupled with a 20 kW electric power generator and an electronic control unit. Several controlled gasification tests were carried out, that is, without the engine and the generator running, to identify the effect of the blending ratio. Four tests were carried out with pine forest biomass and cables in different blending ratios, 100:0, 90:10, 80:20 and 70:30. “

2) Page 4, line 140: "Fuel is supplied from the top as the air moves downward, being preheated through contact with the reactor’s walls" What is the size of the biomass in the hopper? Does it have to be grounded before being inserted in the hopper? Please give some details;

Page 5, lines 156 to 164:

“The gasification tests were performed on an AllPowerLabs PP20 Power Pallets - a gasifier with a power of 15kW, illustrated in Figure 1, a common downdraft reactor that is combined with an electric power generator and an electronic control unit. The equipment consists of a storage silo, where the biomass is simultaneously dried by recirculating the hot gases produced in the reactor. Fuel is supplied from the top as the air moves downward, being preheated through contact with the reactor’s walls. It should be noted that the gasifier admits grinded raw material, with dimensions between 1 to 4 cm. Pine biomass had these characteristics and was grinded before the process. Insulation residues for electrical cables (WIEC) have dimensions between 0.2 and 1 cm, that is, they have been reduced during the metal removal process.”

3) Page 4, line 146: Please add some more details about the temperatures of the process and of the downstream components. For example, which is the temperature in the gas filter? The residual water and tar are supposed to condense there, before the ICE?

Page 5, lines 165 to 174 and page 7:

“At the bottom of the reactor, there is a char collection system, using an endless screw, which pushes the unconverted material into an accessory tank. The synthesis gas leaves the reactor at a temperature between 450 to 550 °C. Downstream of the reactor there is a cyclone filter, which removes the finer particles that follow with the synthesis gas produced. Then the synthesis gas is taken to a heat exchanger, whose function is to reduce the temperature of the synthesis gas to below 100 °C and, at the same time, to heat the biomass found in the hopper. Subsequently, the synthesis gas is cleaned through a filter composed of biomasses of various granulometries. In this filter, there is the retention of tars, which are condensed there. The clean synthesis gas, with a temperature of 50 to 70 °C, can be collected for analysis or injected directly into the engine. Condensed matter is collected at the bottom of the biomass filter.”

4) Page 4, line 157: Please report the total duration of the tests;

Page 6, line 181 to 185:

“During each test, lasting 180 minutes, some parameters were controlled, namely the values of temperature (the temperature sensors are welded outside the reactor, next to the air inlet) and pressure in the upper and lower parts of the reactor (oxidation and reduction zones, respectively), pressure in the biomass particle filter, inlet air flow rate and lastly the amount of biomass consumed during the test.”  

5) Page 5, line 176: Is this the condensate accumulated after the biomass filter? What is the temperature of the deposit of condensate? Please report some more details on the tar sampling system;

Dear Reviewer,

The Reviewer is correct in addressing this issue. In fact, due to the compact configuration of the equipment, it is difficult to measure the tars retained in all parts of the equipment. However, we admit that the tars are retained in elements before the biomass filter. What occurs is that when cleaning and maintaining the gasifier, the amount of tar found is not very relevant, but are counted.

We try to rectify the statement according to the intended.

Page 6 and 7, lines 202 to 208.

“Thermal gasification converts fuel into syngas with chars and tar as by-products formed during the process which restricts practical applications. These by-products are also harmful to the environment and human health, therefore, it is essential to reduce their production [27]. To evaluate the quantity of tar production during the tests, as previously mentioned, the tars condense and are retained in the bottom biomass filter well as in the piping end where it condensates due to temperatures below 300 °C (last stage before the flare), where at the end of the process it was possible to remove and measure the volume produced.”

6) Page 5, line 190: "The equivalence ratio is defined as: ER = (A/F)stoic / (A/F)" Actually the Equivalence ratio is defined as the ratio of actual air fuel ratio to the stoichiometric air fuel ratio, according to (Combustion and Gasification in Fluidized Beds, Basu). Please correct the formula or cite a reference that defines it as you reported here, or else define this as a different process parameter;

Therefore, for fuel rich mixtures, the ER > 1, and for fuel lean, ER < 1

Page 7, lines 225 to 233.

“The equivalence ratio is defined as the ratio between the amount of oxygen added and the stoichiometric oxygen needed for complete combustion of the feedstock:

ER = (A/F) / (A/F)_stoic                                                                                                                                                                      (2)

Where:

ER is the equivalence ratio;

(A/F) is the same mass ratio but under the experimental conditions that were adopted.

(A/F)_stoic is the mass ratio of air/fuel at stoichiometric conditions;

Therefore, for rich mixtures, the ER > 1, and for poor mixtures, ER < 1, and for stoichiometric mixtures, ER = 1.”

7) Page 5, line 195: In gasification processes you usually dont operate with an ER>1 (i.e. with an A/F ratio higher than the stoichiometric), because that would cause a combustion process with total oxidation, instead of a gasification process with partial oxidation. But this is probably related to a different definition of the ER parameter;

Page 7, lines 223 to 226.

“Taking advantage of the fact of the need to calculate the volume of syngas, the equivalence ratio was calculated. The equivalence ratio is commonly used to indicate quantitatively whether a fuel oxidizer mixture is rich, poor or stoichiometric. The equivalence ratio is defined as the actual air–fuel ratio (used in the gasification) to the stoichiometric air–fuel ratio for combustion”

8) Page 6, Table 1: Cl was not analysed in the elemental analysis of WIEC? It could be present if WIEC is also composed of PVC;

Dear Reviewer,

Due to the limitations of the elemental analysis equipment, it was not possible to detect chlorine. However, we calculate by XRF, which was added to table 1.

Page 5, lines 150 to 153.

“XRF - X-Ray Fluorescence

 The amount of chlorine present in WIEC was monitored by X-ray fluorescence analysis, in order to verify the viability of the process. The analysis was determined by X-ray fluorescence analysis using a Thermo Scientific Niton XL 3T GoldD+ analyzer.”

9) Page 7, line 239: "However, a low VM content also reduces fuel HHV and increases the probability of tar formation." Did you mean that a higher amount of VM increases the HHV but is also cause of a higher probability of tar formation? Please revise the sentence;

Page 7, lines 279 to 280.

“However, a higher amount of VM increases the HHV but is also cause of a higher probability of tar formation.”

10) Page 7, line 250: "A very relevant aspect is that the fixed carbon in a gasification reactor is responsible not only for increasing the temperature, but also as a determinant for the thermal cracking of the tar produced during the gasification process." Please add a reference for this sentence;

Dear Reviewer,

The reference was included, as suggested.

Page 9, line 287 to 289.

“A very relevant aspect is that the fixed carbon in a gasification reactor is responsible not only for increasing the temperature, but also as a determinant for the thermal cracking of the tar produced during the gasification process.”

11) Page 7, line 281: "The oxygen content in the pine was 42.2% db" Please add a definition the first time that you use an acronym in the text (in this case d.b.=dry basis);

Page 9, line 320 to 322.

“The oxygen content in the pine was 42.2% dry bases and 45% dry bases for WIEC. This element has a great influence regarding the fuel HHV, hence it is important, sometimes, to pre-treat the fuel and deoxidize it in order to increase the HHV.”

12) Page 9, Table 2: Please add the definitions of Trst and Tred. Is Tred the temperature of the reduction (gasification) zone? Is Trst the temperature of the combustion zone?

Dear Reviewer,

A list of abbreviations (nomenclature table) was made at the beginning of the paper. It is expected to be in accordance with the intended.

13) Page 9, Table 2: Is there a reason why you reported the raw syngas composition, comprehensive of N2, instead of the dry N2 free gas composition?

Dear Reviewer,

N₂ was reported in the Table 2. because all the gases detected were placed according to the gas chromatograph. As the gasification carried out with air as an oxidizing agent, it was decided to set the N₂ value.

14) Page 9, Table 2: Can you comment about the temperature of the tests? Higher temperatures usually lead to better syngas compositions; why is the temperature in the reduction zone always lower than 600? Moreover, it seems that the temperature of the 1st sample is always very low; does it represent a transitorial phase or is there another explanation for this fact? Please add a comment on this.

Page 12, line 331 to 340.

“In the gasification tests, the samples were taken at minute 60, 120 and 180, as a criterion and in order to be able to observe the evolution of the synthesis gas produced, until the process stabilized. It should be noted that the temperature of the 1st sample (60 minutes) is constantly low, it is still a transitory phase of the process, when the temperature is rising. In the second sample, the temperature of the process is still rising, however, the inertia of the equipment is much lower and the stabilization of the process is more noticeable, not extinguishing a large amount of oxidizing agent, as in the 1st sample. The third sample is considered to be the stabilization of the process, where the temperature variation is zero or nearly zero. As previously mentioned, the temperature sensors are welded on the outer wall of the reactor, for this reason, the oxidation and reduction temperatures may appear to be lower compared with a normal gasification process.”

15) Page 9, Table 2: Why is the Vair and thus the ER so variable during a test? Are the flows of air and fuel controlled in some way?

Dear Reviewer,

The air flow is controlled via the vacuum pump. With this control we were able to stabilize the temperature, and the variation of Vair and ER can be derived from this. Regarding the flow of fuel, it is not directly controllable, but it is related to the increase in temperature.

16) Page 10, line 293: "It is possible to observe that the temperature increases with the increase in the percentage of WIEC due to large amounts of volatiles, they are more easily synthesized than wood, during the thermochemical process." When you mention that volatiles are easily synthesized, the meaning is that they are burnt and so the temperature rises? Please explain better or rephrase;

Page 12, line 344 to 347.

“The volatiles are released as the temperature increases, for lignocellulosic biomass the devolatilization occurs at higher temperatures, around 500 °C. WIEC have higher amounts of volatiles that require lower temperatures to devolatilize, which increases temperatures during the thermochemical process, by burning part of these volatiles.”

17) Page 10, line 303: "With the increase in the percentage of WIEC, and the consequent increase in the reactor temperature and the flow of syngas, it is possible to observe that the Boudouard reaction (C + CO2 ⇔ 2CO) of CO formation and the water-gas shift reaction (CO + H2O⇔CO2 + H2) of H2 formation, are favored." When you comment on the results of the tests do you refer to the average values of samples 1, 2 and 3? If so, the flow of syngas is not the highest in the tests with higher WIEC content, it seems that the highest Vsyngas correspond to the first 2 tests. Moreover, the higher temperature could enhance the boudouard reaction but not the water gas shift reaction, that is exothermic and thus thermodynamically favoured at lower temperatures. Please revise the sentence considering this observations;

Dear Reviewer,

The sentence was revised after the observation and it was corrected in the text. Regarding the samples question, they all refer to a same test, that is, each feedstock blending corresponded to a test in which 3 samples of produced gas were collected at different periods of the gasification experiment. For that reason, the comments made refers to the average values of the samples.

Page 12, line 358 to 361.

“With the increase in the percentage of WIEC, and the consequent increase in the reactor temperature, it is possible to observe that the Boudouard reaction (C + CO₂ ⇔ 2CO) of CO is favored and, additionally, light hydrocarbons suffer cracking reactions at high temperatures favoring H₂ formation.”

18) Page 11, line 308: "When LHV increases, ER tends to decrease, and gasification efficiency also increases." Why do you relate the ER to the LHV? ER is an operating condition, an input parameter, while the LHV is an output. It should be commented that the LHV is influenced by the ER, not the opposite. Please consider revising the sentence or add an explanation;

Page 14, lines 366 to 368.

“The LHV is influenced by the ER, when ER decrease, the LHV tends to increase, and gasification efficiency also increases. In order to maintain the ideal operating conditions of a commercial gasifier, they are normally operated with an equivalence ratio of 0.25”

19) Page 11, line 314: "Although the rate of gas production increases and the CGE increases, the calorific value of syngas decreases, the efficiency of gasification also decreases." It seems that a higher ER leads to higher Volume of syngas but with a lower LHV; the increase of the Volume is higher than the decrease of the LHV, resulting thus in a higher CGE. But it has to be highlighted that higher ER leads to a poorer gas composition. Please extent this discussion;

Page 13, lines 328 – 334 and page 14, lines 381 to 385.

“In the gasification tests, the samples were taken at minute 60, 120 and 180, as a criterion and in order to be able to observe the evolution of the synthesis gas produced, until the process stabilized. It should be noted that the temperature of the 1st sample (60 minutes) is constantly low, it is still a transitory phase of the process, when the temperature is rising. In the second sample, the temperature of the process is still rising, however, the inertia of the equipment is much lower and the stabilization of the process is more noticeable, not extinguishing a large amount of oxidizing agent, as in the 1st sample.”

“An increase in ER leads to a poorer syngas composition, not only because of the N₂ dilution effect, but also because the H₂ and CO contents decrease. With high ER more oxidizing agent enters in the process, so higher CO₂ content was observed, which can be concluded that combustion reactions are dominant over gasification reactions.”

20) Page 12, Figure 3: Why in the 3rd sample collected the Vsyngas is always the lowest, even if the temperature seems to be the highest for the 3rd sample of each test? For higher temperatures the gas yield should be higher and thus the volume flow of syngas produced should increase. Please try to give an explanation for this;

Page 13, line 379 to 384.

“Although the rate of gas production increases and the CGE increases, the calorific value of syngas decreases, the efficiency of gasification also decreases (Fig. 3), with steam responsible for contributing to the water-gas reaction and how much the greater the amount of value, the lower the conversion to synthesis gas [37]. For comparison of the different operating temperatures, the yield of the synthesis gas decreased with the increase in temperature, indicating that the high temperature may not have been so beneficial for the production volume but for the quality of the syngas”

21) Page 13, line 340: "The blending ratio 80:20 was considered the most viable, despite having a calorific value 10% lower than that obtained in the blending ratio 90:10" The lower calorific value is referred to the gas, not to the fuel mixture. Please specify;

Page 16, line 423 to 428.

“The 80:20 mixing ratio was considered the most interesting for the use and energetic valuation of WIEC, despite having a calorific value of syngas is 10% lower than that obtained in the 90:10 mixing ratio. Greater incorporation of waste allows better use of polymeric waste for energy recovery. Regarding the 70:30 mixture not being considered, this is due to the fact that there is a drastic increase in temperatures inside the reactor during the gasification tests, since the operation with load tends to increase the temperature a lot and results in problems with agglomeration materials."

22) Page 13, Table 3: Comparing sample 3 of 5kW and sample 2 of 10 kW, that have approximately the same temperatures (and also the same input fuel) it seems that sample 3, with a higher ER, has a lower content in CO2 and a higher content in H2 compared to sample 2. This brings to a higher LHV. Furthermore the volumes of gas produced are almost the same but the fuel flow rate is almost half in sample 3 compared to sample 2. This results seem unusual, expecially compared to what mentioned before about the influence of the ER in the syngas composition. Please try to give an explanation of this results;

Dear Reviewer,

First of all, thanks for the alert that was given regarding the ER values and the consequent synthesis gas composition. Following the call for attention, the excel in which the original data is inserted was checked, and it was possible to observe that there was a mistake in transposing the values that were obtained from air volume to the manuscript, which resulted in strange values of ER, and CGE.

We apologize for the lapse, and the error has already been corrected in Table 3.

23) Page 13, Table 3: If a higher power of the load brings to higher air suction, why are the air flows here decreasing with the increase of the power load? (from 5 to 10 kW);

Dear Reviewer,

Following the previous question, the error has already been corrected.

24) Page 15, line 355: "During the tests performed, the largest volume fraction is of CO followed by the H2. This is due to the water-gas shift reaction producing more CO than H2 and CH4." Why do you state that CO is produced by WGS? Do you suppose that the reverse WGS takes place, converting CO2 and H2 into CO and H2O? Maybe the higher content of CO could instead be related to the higher extent of the reforming reactions, enhanced indeed at higher temperature. Please explain better this sentence;

Page 19, lines 440 to 443.

“During the tests performed, the largest volume fraction is of CO followed by the H₂. CO concentration reaches optimum when the amount of air in the reduction zone decreases Under these conditions, the partial oxidation reaction of char and air in the reduction zone results in an increase in CO and temperature.”

25) Page 16, line 395: "The studies explain the effects of ER on the gasification of polymeric residues, with air as an oxidizing agent [45] in which the results obtained showed that the lowest heating value decreased from 13.42 to 7.05 MJ/Nm³ when the ER was increased from 0.21 to 0.41" Please check this sentences with the results reported in the table 3. In some experiments, for equal temperatures the results show that for higher ER the H2 content is higher. What could be the explanation for this phenomena?

Page 20, lines 484 to 487.

“In the table 3., some experiments, for lowest or equal temperatures the results show that for higher ER the H₂ content is higher, due to the fact of a lower temperature in the oxidation zone favors a greater capitalization of light hydrocarbons, which when entering in the reduction zone, undergo secondary cracking, producing shorter chain hydrocarbons and other products of the gas phase, such as H₂.”

26) Page 16, line 411: "The total efficiency of the system increases with the production of energy. Maximum efficiency is 30% for the 15 kW load." Here you refer to the total efficiency but you mention 30% as maximum in the 15 kW case, that seems to be instead the gasifier efficiency (first row of Table 4). Please revise and correct;

Page 21, lines 497 to 498.

“The results in Table 4. show that the efficiency of the engine remains practically the same, regardless of the electrical load that was used, standing between 16% and 19% throughout the tests. However, the best results are for 10 and 15 kW loads.”

27) Page 17, line 424: "Regarding the operational problems that were detected during the tests, it is observed that limiting the incorporation of WIEC by a maximum of 30% is necessary for allowing better gas quality and thus recommended operational parameters." Experiments with WIEC fractions higher than 30% are not described in this study, why the conclusions mention that higher fractions are not feasible? Has there been a preliminary study on the possible WIEC fraction to be incorporated?

Dear Reviewer,

In tests carried out previously, we have already detected that with greater incorporation of plastics in lignocellulosic biomass, there were a lot of problems observed in the operation of the equipment. We sent a couple of doi from manuscripts that we were made, where the same phenomenon refers:

• 3390/app1013460;
• 3390/su12031036;
• 3390/proceedings2019038009;
• 3390/en12234413;
• 3390/en12050912.

28) Page 17, Conclusions: Tests with different electric loads connected to the engine and the relative efficiencies obtained are not mentioned in the conclusions. Please add conclusive sentences about these topics;

Page 20, Line 532 to 536.

“• In tests with a load of 5 kW the efficiency of the syngas was on average 80%. For loads of 10 and 15 kW the efficiency was slightly lower, around 74%.

• Loads of 5, 10 and 15 kW were used for the 80:20 mixture. The efficiency of the engine increases as energy increases.
• The efficiency of the engine for the different applied loads averaged approximately 18%.”

29) Please check the page numbers, expecially after page 8;

Dear Reviewer,

The page numbers have been revised.

30) Please check the English language throughout the manuscript.

Dear Reviewer,

English was revised throughout the article.

Author Response File: Author Response.docx

Reviewer 2 Report

Review notes: Energy recovery via thermal gasification from WIEC  – Waste Insulation Electrical Cable

The study proposes results from gasification process of blending of WIEC (Waste Isolation Electrical Cable) and lignocellulosic biomass (pine). Effects by WIEC/biomass ratio and system electrical load were evaluated under different operational conditions.

The study contains very interesting hints of research. Furthermore, environmental concerns related to the use of WIEC in co-gasification, need to be studied in deep.

Some revisions are required.  

Introduction

Line 69: for the thermochemical conversion of solid feedstocks, surely gases such as air or oxygen/air (enriched air) and steam mixtures can be useful used as gasifying agents. Supercritical water is a medium used for such particular processes such as supercritical gasification. It is my opinion that such a kind of processes need further developments and actually are far away from market. My suggestion is to well argue the statement of authors about supercritical water. 

Line 71: It isn’t well clear what authors indeed. Gasification occurs under restricted oxygen conditions (equivalence ratio 0.22 – 0.35). It is suggested to better explain the statement.

Line 73: instead ‘coals’, ‘char’ is most appropriate.

Linea 74: ‘gasifying medium’ instead of ‘oxiding medium’.

Line 78 and other: references must be revised: [9] [10] as [9, 10] and so on.

2.2 Gasification test

Line 149: is 400°C the gasification temperature? It’s ok ‘at least’, but it is very low. The gas quality in terms of tar content at such temperature is very low. I think that in these conditions it is very difficult to feed an internal combustion engine that requires a gas tar content below 50-100 mg/Nm3. Which kind of gasifier is? (Imbert, ..)?

2.3.2. Tars

It is my opinion that the procedure used to evaluate tars wasn’t properly correct: greatly underestimate quantities produced during gasification and no information are available about the amount that enter the ICE. Furthermore, it must be highlighted that a great amount of tar, condensate along piping and equipment where the temperature is below 350 – 300°C so that the volume accumulated isn’t indicative of the tar inside the producer gas. On-line methods had to be used so that to obtain gravimetric and chromatographic values (i.e. according to CEN/TS 15439:2006).

2.3.4. Theoretical parameters

The presented form whose equations are written must be revised in the whole document.

By equation (1), I indeed that the volumetric flow rate of produced gas was simply estimated. Does the volumetric flow you calculated, on wet or dry basis? How do you estimated water content in the produced syngas?

Equation (2) must be revised. ER is defined as the actual air–fuel ratio (used in the gasification) to the stoichiometric air–fuel ratio for combustion.

Line 207: It isn’t well clear as you measured mass flow rate of streams in the plant. It is suggested to well explain the experimental plant setup.

3.3 Proximate analysis

Line 260: It isn’t well clear what you mean with lower temperature than updraft and fluidized bed. In the core region of downdraft gasifier, where the oxidation is conducted, very high temperature is reached (about 1100°C) while, lower temperature (about 800°C) is achieved in the reduction area. Gases from primary decomposition reactions (pyrolysis area) are forced to walk through the high temperature zone where tar cracking reactions are promoted. This particular condition allows to obtain in downdraft gasifier a very low tar content (in the order of 1 g/Nm3 of dry syngas) in the producer gas than fluidized bed (tar content in the order of 10-20 g/Nm3) and updraft (in the order of 50-150 g/Nm3) reactors. It is highly suggested to revise your statements.    

4. Results

It is suggested to introduce a section in the paper where symbols used are collected. Some of the symbols used are not commented (i.e. Trst, Tred). Furthermore, style of symbols used must be revised (subscripts, …).

TABLE 2. Gasification results collected in the table, I think, are carried out for system load equal to zero (syngas was simply burnt in the torch: it is correct?). For each set of biomass/WIEC blend, three set of results were showed. I see that the experiments were conducted for each of these at different ER (i.e. ER decreases from sample 1 to sample 3). It is correct? It is noticeable that when ER decrease, Trst (temperature at the core where oxidation occurs, I think. It is correct?) as well as Tred (temperature of reduction area, I think) increases. How you can explain this? When ER increases (i.e. much more air is introduced in the gasifier) oxidation reactions should be promoted so that higher temperature should be reached at the gasifier throat, contrary when ER decreases. Furthermore, the reduction temperature reached is very very low (289°C, 371°C, …). It seems that passing from sample1 to sample 3, steady conditions of the reactor were reached: Tred = 515°C at 100:0 – sample 3, is again low but is most representative of gasification conditions. Of course, the temperature depends on the position of the sensor (in this way a most accurate description of the setup must be done). Tars must be expresses as concentration (i.e g/Nm3). How was fuel flow measured? Experiment time, I think, are in minutes.

4.3 Controlled gasification theoretical parameters

Line 308: LHV is an independent variable of the system so that when ER decreases, LHV increases and consequently the cold efficiency.

4.5 Load Gasification experiments

Table 3: Temperatures reached are now representative of gasification: on the contrary when tests with load equal to zero were carried out, now gasification occurs. Again, tar content, must be expressed as concentration even if it is very difficult because of the method you used to measure them. As it is possible see cold efficiency falls down as load increases even if system efficiency is nearly constant (Table 4): eta_tot = 0.16 – 0.17 . How do you explain this condition (cold efficiency is 0.27 – 0.30 at 15 kW but system efficiency is 0.17)? Surely engine efficiency had a key role at full load but the gap in efficiencies is very high.

Line 405: In Table 4, efficiencies are showed, so that it is most appropriate the statement energy balance instead of mass balance.

Line 412: Maximum efficiency is 30% .. What efficiency you indeed?

5. Conclusion

Line 435: ER is an operational parameter. This mean that it is a variable that you must control to get the desired conditions of gasification. When an internal combustion engine is coupled to gasifier, suction is controlled by the load at the generator, but this doesn’t mean that ER became an independent variable of the system. Gasifier should be designed to get the desired operational parameters in accordance with load at the engine.  It is suggested to revise the statement.

Comments for author File: Comments.docx

Author Response

Dear Reviewer,

We appreciate the corrections you made and we are sure they were of great value in improving the appearance of the manuscript. Please find below our answers and comments regarding your questions.

Introduction

Line 69: for the thermochemical conversion of solid feedstocks, surely gases such as air or oxygen/air (enriched air) and steam mixtures can be useful used as gasifying agents. Supercritical water is a medium used for such particular processes such as supercritical gasification. It is my opinion that such a kind of processes need further developments and actually are far away from market. My suggestion is to well argue the statement of authors about supercritical water. 

Page 3, line 73 to 80:

“For the conversion to occur, a medium is necessary, usually a gas, however, recent studies are using supercritical water as a medium for the gasification process, this process results in higher LHV of produced gas than those obtained by air gasification; however, air gasification is the most extensively studied and applied process because the gasification agent is inexpensive, the reaction process is easy, the reactor structure is simple, nevertheless the supercritical water gasification should be better studied and developed since the reactor plugging is a critical problem when feedstocks with high biomass content are gasified, among other problems.”

Line 71: It isn’t well clear what authors indeed. Gasification occurs under restricted oxygen conditions (equivalence ratio 0.22 – 0.35). It is suggested to better explain the statement.

Page 3, line 71 to 73:

“Gasification is a thermochemical process that transforms carbon-rich materials, such as biomass, into gaseous fuels, throughout sub-stoichiometric levels of oxidant agent for partial oxidation of these carbon-rich materials (char and the high-molecular-weight volatiles)”.

Line 73: instead ‘coals’, ‘char’ is most appropriate.

Page 4, line 83

The word was changed as requested.

Linea 74: ‘gasifying medium’ instead of ‘oxiding medium’.

Page 4, line 84

The word was changed as requested.

Line 78 and other: references must be revised: [9] [10] as [9, 10] and so on.

The references were revised and corrected.

2.2 Gasification test

Line 149: is 400°C the gasification temperature? It’s ok ‘at least’, but it is very low. The gas quality in terms of tar content at such temperature is very low. I think that in these conditions it is very difficult to feed an internal combustion engine that requires a gas tar content below 50-100 mg/Nm3. Which kind of gasifier is? (Imbert, ..)?

Dear Reviewer,

We agree with what was mentioned by the reviewer. The reactor is the Imbert type, which helps the thermal cracking of the tars produced in the pyrolysis zone when it gasifies at low temperatures. However, it should be noted that the temperature sensors are welded to the outer wall of the reactor, it is admitted that the reactions inside the reactor are taking place at slightly higher temperatures. Another problem is that the vacuum pumps in the system have lower performance when compared to the engine, and it is not possible to raise the temperatures as quickly when the controlled tests are carried out. We tried to explain the temperatures in the manuscript a little better.

Page 5, line 158 to 171

“It should be noted that the gasifier admits ground raw material, with dimensions between 1 to 4 cm. Pine biomass had these characteristics and was ground before the process. Insulation residues for electrical cables (WIEC) have dimensions between 0.2 and 1 cm, that is, they have been reduced during the metal removal process. At the bottom of the reactor, there is a char collection system, using an endless screw, which pushes the unconverted material into an accessory tank. The synthesis gas leaves the reactor at a temperature between 450 to 550 °C. Downstream of the reactor there is a cyclone filter, which removes the finer particles that follow with the synthesis gas produced. Then the synthesis gas is taken to a heat exchanger, whose function is to reduce the temperature of the synthesis gas to below 100 °C and, at the same time, to preheat the biomass in the hopper. Subsequently, the synthesis gas is cleaned through a filter composed of biomasses of various granulometries. In this filter, there is the retention of tars, which are condensed there. The clean synthesis gas, with a temperature of 50 to 70 °C, can be collected for analysis or injected directly into the engine. Condensed matter is collected at the bottom of the biomass filter.”

2.3.2. Tars

It is my opinion that the procedure used to evaluate tars wasn’t properly correct: greatly underestimate quantities produced during gasification and no information are available about the amount that enter the ICE. Furthermore, it must be highlighted that a great amount of tar, condensate along piping and equipment where the temperature is below 350 – 300°C so that the volume accumulated isn’t indicative of the tar inside the producer gas. On-line methods had to be used so that to obtain gravimetric and chromatographic values (i.e. according to CEN/TS 15439:2006).

Dear Reviewer,

We agree with the analysis. In fact, the best method of quantifying the tars produced during gasification processes is not being used. However, at the end of each gasification, the equipment was cleaned, and an attempt was made to account for all the tar produced and retained in the equipment. However, the errors associated with the tar accounting method are admitted and the best accounting method is to follow the CEN / TS 15439: 2006 standard, suggested by the Reviewer.

R: Line 201 to 205: “To evaluate the quantity of tar production during the tests, as previously mentioned, the tars condense and are retained in the bottom biomass filter well as in the piping end where it condensates due to temperatures below 300 °C (last stage before the flare), making it possible to remove and measure the volume produced.”

2.3.4. Theoretical parameters

The presented form whose equations are written must be revised in the whole document.

By equation (1), I indeed that the volumetric flow rate of produced gas was simply estimated. Does the volumetric flow you calculated, on wet or dry basis? How do you estimated water content in the produced syngas?

Dear Reviewer,

The gas analysis was performed on a dry basis. The gas is collected in appropriate bags that are analyzed by gas chromatography at the end of each test, that is, the synthesis gas is analyzed at room temperature. The synthesis gas produced is introduced into a gas line, where two filters are located, to remove the moisture present in the synthesis gas.

Equation (2) must be revised. ER is defined as the actual air–fuel ratio (used in the gasification) to the stoichiometric air–fuel ratio for combustion.

Page 7, line 222 to 231

“The equivalence ratio is defined as the ratio between the actual air-fuel ratio (used in the gasification) and the stoichiometric air-fuel ratio for combustion [31]:

ER = (A/F) / (A/F)stoic                                                                                                (2)

Where:

ER is the equivalence ratio;

(A/F) is the same mass ratio but under the experimental conditions that were adopted.

(A/F)stoic is the mass ratio of air/fuel at stoichiometric conditions;

Therefore, for rich mixtures, the ER > 1, and for poor mixtures, ER < 1, and for stoichiometric mixtures, ER = 1.”

Line 207: “It isn’t well clear as you measured mass flow rate of streams in the plant. It is suggested to well explain the experimental plant setup.”

Page 7, line 210 to 213

“To estimate the volumetric air mass entering the reactor, an appropriate flowmeter was used, which is located at the equipment's air intake. As the equipment has no flowmeter to measure the synthesis gas produced, an equation was used in which the amount of volumetric air entering the reactor is combined with the percentage of nitrogen present in the synthesis gas.”

3.3 Proximate analysis

Line 260: It isn’t well clear what you mean with lower temperature than updraft and fluidized bed. In the core region of downdraft gasifier, where the oxidation is conducted, very high temperature is reached (about 1100°C) while, lower temperature (about 800°C) is achieved in the reduction area. Gases from primary decomposition reactions (pyrolysis area) are forced to walk through the high temperature zone where tar cracking reactions are promoted. This particular condition allows to obtain in downdraft gasifier a very low tar content (in the order of 1 g/Nm3 of dry syngas) in the producer gas than fluidized bed (tar content in the order of 10-20 g/Nm3) and updraft (in the order of 50-150 g/Nm3) reactors. It is highly suggested to revise your statements.    

Page 9, line 295 to 298

“The thermal gasification process employing a downdraft reactor is the most appropriate when the fuel has a high ash content, because of the constant removal of unconverted material and ashes at the bottom of the reactor, which prevents the referred problems.”

4. Results

It is suggested to introduce a section in the paper where symbols used are collected. Some of the symbols used are not commented (i.e. Trst, Tred). Furthermore, style of symbols used must be revised (subscripts, …).

Page 2

A nomenclature table was added at the beginning of the paper, where all symbols and acronyms are explained.

TABLE 2. Gasification results collected in the table, I think, are carried out for system load equal to zero (syngas was simply burnt in the torch: it is correct?).

Dear Reviewer,

Correct, the syngas was burned in the flare.

For each set of biomass/WIEC blend, three set of results were showed. I see that the experiments were conducted for each of these at different ER (i.e. ER decreases from sample 1 to sample 3). It is correct?

Dear Reviewer,

It is correct. Each sample was collected at a certain moment during the gasification, that is, sample 1 was collected first (60 min), then sample 2 (120 min) and finally sample 3 (180 min). What happens in controlled tests, is that as the test goes on, the system tends to stabilize and lose inertia, therefore, less exothermic reactions (combustion) are needed to maintain the gasification temperature, so the ERs tend to decrease because the process doesn’t need the same quantity of oxidant agent (air) as in the beginning.

It is noticeable that when ER decrease, Trst (temperature at the core where oxidation occurs, I think. It is correct?) as well as Tred (temperature of reduction area, I think) increases. How you can explain this? When ER increases (i.e. much more air is introduced in the gasifier) oxidation reactions should be promoted so that higher temperature should be reached at the gasifier throat, contrary when ER decreases. Furthermore, the reduction temperature reached is very very low (289°C, 371°C, …). It seems that passing from sample1 to sample 3, steady conditions of the reactor were reached: Tred = 515°C at 100:0 – sample 3, is again low but is most representative of gasification conditions.

Dear Reviewer,

Yes, it is correct. Trst represents the oxidation temperature and Tred is the oxidation temperature. Initially, the ER is elevated because a lot of heat is necessary to start up the gasifier and thus the oxidation reactions must be favored. After some time, the air inlet is reduced to favor reduction reactions, consequently, the calculated ER is also reduced since less air enters the system. The reason that the temperature continues to rise is because the reactor is well isolated, so the heat losses are reduced, this means that, although the ER reduces, the temperature continues to rise but more slowly.

Of course, the temperature depends on the position of the sensor (in this way a most accurate description of the setup must be done).

Dear Reviewer,

It is now stated in the text (section 2.2 – line 182 and section 4.1 – line 339) that the temperature sensors are welded outside the reactor’s refractory wall, next to the air inlet.

Tars must be expresses as concentration (i.e g/Nm3).      

Dear Reviewer,

The tars produced by the volume of produced syngas are now expressed in concentration, as suggested by the reviewer.

How was fuel flow measured?

Dear Reviewer,

The fuel flow was measured by dividing the consumed material over the experiment for the total time, discounting the amount of material not utilized inside the hopper thus not consumed.

Experiment time, I think, are in minutes.

Dear Reviewer,

The time unit was corrected to minutes.

4.3 Controlled gasification theoretical parameters

Line 308: LHV is an independent variable of the system so that when ER decreases, LHV increases and consequently the cold efficiency.

Page 14, Line 367 to 369

“The LHV is influenced by the ER, when the ER decreases, the LHV tends to increase, and gasification efficiency also increases. In order to maintain the ideal operating conditions of a commercial gasifier, they are normally operated with an equivalence ratio  around 0.25.”

4.5 Load Gasification experiments

Table 3: Temperatures reached are now representative of gasification: on the contrary when tests with load equal to zero were carried out, now gasification occurs. Again, tar content, must be expressed as concentration even if it is very difficult because of the method you used to measure them.

Dear Reviewer,

Thank you for the suggestion, the tars content is now expressed as concentration.

As it is possible see cold efficiency falls down as load increases even if system efficiency is nearly constant (Table 4): eta_tot = 0.16 – 0.17 . How do you explain this condition (cold efficiency is 0.27 – 0.30 at 15 kW but system efficiency is 0.17)? Surely engine efficiency had a key role at full load but the gap in efficiencies is very high.

Dear Reviewer,

Please excuse us. There was an exchange of values when transposing the data to excel. The error has already been solved and the tables presented are now correct, and of course, with different efficiency values which we believe are as expected now. 

Line 405: In Table 4, efficiencies are showed, so that it is most appropriate the statement energy balance instead of mass balance.

Dear Reviewer,

Thanks for the suggestion, it was changed as requested.

Line 412: Maximum efficiency is 30% .. What efficiency you indeed?

Page 21, line 502 to 505

“Regarding the efficiency of the gasifier, it is inversely proportional to the efficiency of the engine, this fact is related to the increase in the flow of fuel that enters the gasifier, decreasing the efficiency of syngas. In the case of 5 kW to 15 kW, there was an increase of about 186% in fuel consumption.

5. Conclusion

Line 435: ER is an operational parameter. This mean that it is a variable that you must control to get the desired conditions of gasification. When an internal combustion engine is coupled to gasifier, suction is controlled by the load at the generator, but this doesn’t mean that ER became an independent variable of the system. Gasifier should be designed to get the desired operational parameters in accordance with load at the engine.  It is suggested to revise the statement.

Page 21, line 524 to 534

“It was noted that as the percentage of incorporation increased, the ER tended to suffer an increase. The highest LHV and ER obtained during the tests were 5.2 MJ/Nm³ and 0.35.

• The synthesis gas obtained can be considered for use in internal combustion engines as can be seen in the tests performed.
• In the tests using the engine, it was possible to observe that there was an increase in the gasification temperature. This factor is related to the suction of the engine.
• In tests with a load of 5 kW, the efficiency of the syngas was around 80%. For loads of 10 kW and 15 kW, the efficiency was slightly lower, around 74%.
• Loads of 5, 10, and 15 kW were used for the 80:20 mixture. The efficiency of the engine increases as energy increases.
• The efficiency of the engine for the different applied loads was around 18%, on average.”    

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Form of equations should be revised

Author Response

Thank you very much for your wise comments.
We have proceeded to review the form of the equations.
We take this opportunity to congratulate the Reviewers for their excellent work.

Author Response File: Author Response.docx