Pyrolysis Combined with the Dry Reforming of Waste Plastics as a Potential Method for Resource Recovery—A Review of Process Parameters and Catalysts

Round 1

Reviewer 1 Report

"This manuscript presents the review of pyrolysis of different plastics via dry reforming. This review article is well-written and interesting. This reviewer cannot find any major concerns. Only a few typos must be corrected before accepting for publication.

1. “DR it is commercially utilized in industrial processes,” – delete “it”.
2. “thus don’t affect the catalyst” – replace don’t with “do not”.

Author Response

Reviewer 1

This manuscript presents the review of pyrolysis of different plastics via dry reforming. This review article is well-written and interesting. This reviewer cannot find any major concerns. Only a few typos must be corrected before accepting for publication.

1. “DR it is commercially utilized in industrial processes,” – delete “it”.
2. “thus don’t affect the catalyst” – replace don’t with “do not”.

 

thank you for your comments. We checked carefully our manuscript again and corrected typos as it was suggested.

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript is well reviewed on dry-reforming of waste plastic materials. Readers will be able to understand the area with this manuscript. Only one thing, the carbon disposal issue should be disccussed, because the issue is very important to handle in the plant scale sometimes. Hence, would you please input "the strategies or challenge on carbon disposal" after reforming process.

Author Response

Reviewer 2

This manuscript is well reviewed on dry-reforming of waste plastic materials. Readers will be able to understand the area with this manuscript. Only one thing, the carbon disposal issue should be disccussed, because the issue is very important to handle in the plant scale sometimes. Hence, would you please input "the strategies or challenge on carbon disposal" after reforming process.

thank you for your valuable review. You have added a new subsection 4.2. into our manuscript regarding ,,The strategies or challenge on carbon disposal’’. We believe that this section exhaustively covers the topic of your interest. Please, find the added description below.

,,Scaling up the PCDR process is always brings with it additional challenges, such as the need to manage waste. One of the main challenges in the plant scale is the post-process carbon disposal. The main product of PCDR processes is synthesis gas, however, depending on the process parameters, by-products such as liquid oil or char can be formed. These products can also be valuable, so in the industrial scale their separation and further use should be considered.

Liquid oil generated in the pyrolysis of waste plastics, which is rich in liquid hydrocarbons (aromatic, olefin, naphthalene), can be used as recovered resource in energy-related applications such as heating purposes, electricity generation, transportation fuel or feedstock for generation of chemicals. Miandad et al. [71] studied conversion of different types of plastic waste (PS, PE, PP and PET) into valuable liquid oil via pyrolysis process. The obtained liquid oil had higher heating values (HHV) in the range of 41.7 – 44.2 MJ/kg, which is close to conventional diesel. Therefore, the liquid oil generated in the PCDR process could be used as an alternative energy source and as transportation fuel after refining/blending with conventional fuels. Nileshkumar et al. [120] used plastic pyrolysis oil generated from waste HDPE, LDPE and PP as transportation fuel after mixing it with diesel fuel. They concluded that blend containing 20 vol. % of pyrolytic oil exhibited equivalent engine performance as conventional diesel fuel. Rehan et al. [121] proposed the application of liquid oil generated in pyrolysis of municipal plastic waste as fuel in an diesel engine to produce electrical energy, due to its similar characteristic to conventional diesel (density 0.8 kg/m³, viscosity up to 2.96 mm²/s, flash point 30.5°C and energy content 40 MJ/kg). Saptoadi et al. [122] demonstrated that plastic waste (PE, PP, PS, PET) derived oil can be used as partial substitute for kerosene in pressurized cookstoves. They indicated that thermal efficiency of pyrolytic oils blended with kerosene in a volumetric ratio of 1:3 do not differ significantly (~3%) from pure kerosene. Furthermore, liquid oil can be also processed to recover chemicals like styrene, benzene, toluene or polymeric monomers which can be transferred to an established market [123].

Solid by-product that remains after the pyrolysis stage is char. Char also has a wide list of its potential applications: adsorbent in the water treatment [124], activated carbon production [125], reducing agent [126], fuel briquettes [127], raw materials for fabrication of graphene [123], supercapacitors [128], nanocatalysts and nanofillers for composite application [129].

An important aspect that should be taken into account is also the composition of the synthesis gas obtained in the PCDR processes. According to the literature reports on PCDR processes summarized in Table 3, obtained synthesis gas was rich in carbon monoxide (H₂/CO ratio around 1). Therefore, as obtained syngas is suitable for processes such as synthesis of dimethyl ether or oxo-synthesis process in aldehyde and alcohol production [50, 118]. If the synthesis gas produced were to be used in processes requiring a higher content of hydrogen, such as methanol production or the Fisher-Tropsch syntheses, the strategy discussed in Section 4.1.5 involving the addition of steam to the input stream could be followed, resulting in higher H₂/CO ratio. In the industrial scale, it is also worth implementing stream recirculation, which would allow to maximize the use of carbon dioxide feedstock and enhance the efficiency of synthesis gas production.”

 

References:

50. Song, X.; Guo, Z. Technologies for direct production of flexible H2/CO synthesis gas. Energy Convers. Manag. 2006, 47, 560–569, doi:10.1016/j.enconman.2005.05.012.

71. Miandad, R.; Barakat, M.A.; Rehan, M.; Aburiazaiza, A.S.; Ismail, I.M.I.; Nizami, A.S. Plastic waste to liquid oil through catalytic pyrolysis using natural and synthetic zeolite catalysts. Waste Manag. 2017, 69, 66–78, doi:10.1016/j.wasman.2017.08.032.

118. Azizi, Z.; Rezaeimanesh, M.; Tohidian, T.; Rahimpour, M.R. Dimethyl ether: A review of technologies and production challenges. Chem. Eng. Process. Process Intensif. 2014, 82, 150–172, doi:10.1016/j.cep.2014.06.007.

120. Nileshkumar, K.D.; Patel, T.M.; Rathod, G.P. Effect of Blend Ratio of Plastic Pyrolysis Oil and Diesel Fuel on the Performance of Single Cylinder CI Engine. Int. J. Sci. Technol. Eng. 2015, 1, 195–203.

121. Rehan, M.; Nizami, A.S.; Shahzad, K.; Ouda, O.K.M.; Ismail, I.M.I.; Almeelbi, T.; Iqbal, T.; Demirbas, A. Pyrolytic liquid fuel: A source of renewable electricity generation in Makkah. Energy Sources, Part A Recover. Util. Environ. Eff. 2016, 38, 2598–2603, doi:10.1080/15567036.2016.1153753.

122. Saptoadi, H.; Pratama, N.N. Utilization of Plastics Waste Oil as Partial Substitute for Kerosene in Pressurized Cookstoves. Int. J. Environ. Sci. Dev. 2015, 6, 363–368, doi:10.7763/ijesd.2015.v6.619.

123. Miandad, R.; Rehan, M.; Barakat, M.A.; Aburiazaiza, A.S.; Khan, H.; Ismail, I.M.I.; Dhavamani, J.; Gardy, J.; Hassanpour, A.; Nizami, A.S. Catalytic pyrolysis of plastic waste: Moving toward pyrolysis based biorefineries. Front. Energy Res. 2019, 7, doi:10.3389/fenrg.2019.00027.

124. Ravenni, G.; Cafaggi, G.; Sárossy, Z.; Rohde Nielsen, K.T.; Ahrenfeldt, J.; Henriksen, U.B. Waste chars from wood gasification and wastewater sludge pyrolysis compared to commercial activated carbon for the removal of cationic and anionic dyes from aqueous solution. Bioresour. Technol. Reports 2020, 10, 100421, doi:10.1016/j.biteb.2020.100421.

125. Doumer, M.E.; Arízaga, G.G.C.; Da Silva, D.A.; Yamamoto, C.I.; Novotny, E.H.; Santos, J.M.; Dos Santos, L.O.; Wisniewski, A.; De Andrade, J.B.; Mangrich, A.S. Slow pyrolysis of different Brazilian waste biomasses as sources of soil conditioners and energy, and for environmental protection. J. Anal. Appl. Pyrolysis 2015, 113, 434–443, doi:10.1016/j.jaap.2015.03.006.

126. Suopajärvi, H.; Pongrácz, E.; Fabritius, T. The potential of using biomass-based reducing agents in the blast furnace: A review of thermochemical conversion technologies and assessments related to sustainability. Renew. Sustain. Energy Rev. 2013, 25, 511–528, doi:10.1016/j.rser.2013.05.005.

127. Harussani, M.M.; Sapuan, S.M.; Rashid, U.; Khalina, A.; Ilyas, R.A. Pyrolysis of polypropylene plastic waste into carbonaceous char: Priority of plastic waste management amidst COVID-19 pandemic. Sci. Total Environ. 2022, 803, 149911, doi:10.1016/j.scitotenv.2021.149911.

128. Garg, K.K.; Pandey, S.; Kumar, A.; Rana, A.; Sahoo, N.G.; Singh, R.K. Graphene nanosheets derived from waste plastic for cost-effective thermoelectric applications. Results Mater. 2022, 13, 100260, doi:10.1016/j.rinma.2022.100260.

129. Sogancioglu, M.; Yel, E.; Ahmetli, G. Behaviour of waste polypropylene pyrolysis char-based epoxy composite materials. Environ. Sci. Pollut. Res. 2020, 27, 3871–3884, doi:10.1007/s11356-019-07028-3.

Author Response File: Author Response.pdf

Reviewer 3 Report

This manuscript comprises a compilation of previously published information on the transformation of plastic materials into synthesis gas/hydrogen by PCDR and analyzes the catalytic reactions of pyrolysis and dry & steam reforming, paying special attention to the process variables.

In general, the references included are significant and resume the current state of the art.

However, this is still an open field and the manuscript requires a reordering and systematization of the information. In the process there are two reaction stages in which the first can be (or not) catalytic and the second is, but not all investigations use the same procedure and there is no clear differentiation. There are many proposals for processes with different reactors, feeds, temperatures, contact times or catalysts. I believe that the information should be better organized in order to provide the reader with a clear global vision.

Some comments:

• With regard to the reactors (Fig 2), it is recommended to indicate whether they are configurations that have been tested on a laboratory scale. There are other configurations Since the raw materials in the pyrolysis stage are plastics, some research teams have used other mixing reactors, moving beds and specific designs that are not cited.
• The research group of Prof. Bilbao, J. has published kinetic studies of the reforming stage that relate the process variables & yields, including deactivation. Please, consider whether it would be appropriate to include a reference in the context of this review.
• It would be useful to better focus section 4.1.4 in the context of work. The reaction systems presented in Figure 2 are continuous, with fixed bed catalysts and a catalyst inlet is not considered. However, this section discusses the catalyst/feedstock ratio as a continuous catalyst feed, which would be a significantly different process involving subsequent ¿recovery? ¿regeneration? ¿recycle? of the catalyst. It should be better clarified in order to give value to the review, since the cost of the catalyst becomes very relevant. In systems based on fixed bed catalytic reactors, GHSV should be used.
• Although the results of previous works are analyzed, the joint discussion of the results of the different works is scarce. Despite the variability of the studies, a joint analysis of the results would be convenient to reach conclusions about what is already established in the state of the art in relation to the process variables studied.
• Please, do a detailed review to removev some minor typing errors. In special, correct reference to Laura et al. using her last name (see on ref listing). Also, check line 147, there is a "liquid phase" and must be probably "gas phase".

Author Response

Reviewer 3

Thank you very much for your valuable comments. As you requested, we made corrections to systematize the information in our review. Please find our answers below.

This manuscript comprises a compilation of previously published information on the transformation of plastic materials into synthesis gas/hydrogen by PCDR and analyzes the catalytic reactions of pyrolysis and dry & steam reforming, paying special attention to the process variables.

In general, the references included are significant and resume the current state of the art.

However, this is still an open field and the manuscript requires a reordering and systematization of the information. In the process there are two reaction stages in which the first can be (or not) catalytic and the second is, but not all investigations use the same procedure and there is no clear differentiation. There are many proposals for processes with different reactors, feeds, temperatures, contact times or catalysts. I believe that the information should be better organized in order to provide the reader with a clear global vision.

To our best knowledge in literature reports regarding PCDR processes carried out to date, only reforming stage is catalytic, while pyrolysis stage is performed in the absence of the catalyst. The pyrolysis step may be conducted catalytically, however there is no reports reporting combination of catalytic pyrolysis and dry reforming. Therefore, we added the clarification in lines 342 and 343.

,,To our best knowledge there is no reports regarding the use of a catalyst in the pyrolysis stage of PCDR process, only the reforming stage is catalytic.’’

Some comments:

• With regard to the reactors (Fig 2), it is recommended to indicate whether they are configurations that have been tested on a laboratory scale. There are other configurations. Since the raw materials in the pyrolysis stage are plastics, some research teams have used other mixing reactors, moving beds and specific designs that are not cited.

We indicated that they are configurations that have been tested on a laboratory scale (lines 285 and 321). You rightly mentioned that some research teams have used other reactors for pyrolysis process only. As it was suggested the information regarding other reactor’s configuration was added to the manuscript. In Figure 3 are presented various pyrolysis reactors which could be possibly implemented in first stage of PCDR process and we added lines 297-319 to clarify current state of art.

,,For PCDR processes, the reactors used so far have been fixed bed reactors, however there are many types of pyrolysis reactors that could potentially be implemented for the first stage of the PCDR process. The different types of pyrolysis reactors are shown in Figure 3 [65]. Reactors are classified depending on how the plastic feedstock is forced to move inside the reactor: pneumatically (fluidized bed reactor and fixed bed reactor), mechanically (auger reactor, rotary reactor, ablative reactor and reactor with stirrer) and gravitationally (column type reactor). Fixed bed reactors are the main type of reactors for large-scale chemical synthesis, used also in laboratory scale configurations for PCDR processes. It is tube filled with plastic feedstock with gaseous stream flowing through the bed and being converted into pyrolytic products. In fluidized bed reactor gaseous stream is passed through a plastic feedstock at high enough speeds to suspend the solid and cause it to behave as though it were a fluid [66]. Auger reactors are mechanical reactors where screw is used to convey feedstock down the length of a tube [67]. Rotary reactor is a tube, inclined slightly from the horizontal, which is rotated slowly about its longitudinal axis. The feedstock is fed into the upper end of the cylinder and gradually moves down toward the lower end and undergoes mixing [66]. The ablative reactor consist of a chamber, inside of with is a spinning bowl where the feedstock is placed, and a hot plate at the top that moves vertically and applies pressure against the feedstock. The heat is conducted to plastic feedstock by direct contact with a hot surface, and pyrolysis takes place within a thin layer in contact with the hot surface rather than the entire feed. This provides an opportunity to use large pieces of plastics [68]. The mechanical forces associated with mechanical reactors enhance particle mixing and heat transfer, which are key to successful pyrolysis.’’

• The research group of Prof. Bilbao, J. has published kinetic studies of the reforming stage that relate the process variables & yields, including deactivation. Please, consider whether it would be appropriate to include a reference in the context of this review.

The articles Prof. Bilbao research group is related mainly to steam reforming processes and catalyst deactivation-regeneration issues. We added discussion in lines 731-743 with reference to their work regarding catalysts deactivation and regeneration, which is common and important challenge that should be addressed.

“Deactivation and possibility of regeneration of deactivated catalyst is an important aspect of PCDR processes. Catalysts deactivation can occur by series of physicochemical phenomena, including metal sintering, metallic phase oxidation, thermal degradation of the support and coke deposition [105]. The type of deactivation depends on catalyst composition, structure, feedstock and operating conditions. The catalyst regeneration strategy involves reaction/regeneration cycles. The common way to regenerate a catalyst is to use an oxidizing medium at high temperatures, which causes the coke to burn out. According to the literature [106,107], Ni-based catalysts do not fully recover the activity after first reaction/regeneration cycles, but after several cycles Ni particles sintering no longer occures, hence catalysts gain more stability. Moreover, a more stable behavior during reaction/regeneration cycles of Ni-based catalysts can be achieved by using spinel like NiAl₂O₄, alloys of Ni-Fe or by incorporating compound for support basicity regulation like MgO, CeO₂, MnO to the Ni supported catalyst [108-110].”

 

• It would be useful to better focus section 4.1.4 in the context of work. The reaction systems presented in Figure 2 are continuous, with fixed bed catalysts and a catalyst inlet is not considered. However, this section discusses the catalyst/feedstock ratio as a continuous catalyst feed, which would be a significantly different process involving subsequent ¿recovery? ¿regeneration? ¿recycle? of the catalyst. It should be better clarified in order to give value to the review, since the cost of the catalyst becomes very relevant. In systems based on fixed bed catalytic reactors, GHSV should be used.

In the section 4.1.4. it was mentioned that there is insufficient amount of works on PCDR that concern catalyst to plastic ratio, thus we cited related to this topic, regarding catalyst:plastic ratio in processes such as pyrolysis-gasification, pyrolysis-steam reforming, etc. To clarify, the reactors shown in Figure 2 are operated batchwise, only gaseous streams (CO₂ as feed and inert gas such as argon or nitrogen to ensure inert reaction conditions) are continuously fed to the reactor. We agree ,that the researchers should consider the possibility of replacing only the plastic feedstock portion, while the catalyst could be used multiply times. Moreover, regeneration of the catalyst would further extend its service life. These factors have a significant impact on the cost of the catalyst, so further research is necessary in this area, including multiple uses of the catalyst, hence stability studies, as well as studies on catalyst regeneration strategies. Therefore, we added lines 773-778 to highlight this area for further research.

,,It should be noted that in the above-mentioned works on PCDR processes, the catalyst/plastic ratio is considered as a continuous catalyst feed. Taking into account the fact that a given amount of catalyst can be used multiply times while only the plastic feed is replaced by next portion, process costs can vary significantly. Therefore, additional studies on the reuse and stability of the catalysts as well as possibility of their regeneration are necessary.’’

 

• Although the results of previous works are analyzed, the joint discussion of the results of the different works is scarce. Despite the variability of the studies, a joint analysis of the results would be convenient to reach conclusions about what is already established in the state of the art in relation to the process variables studied.

In Section 5 we added joint discussion and conclusions about what is already established regarding the results of the various works on PCDR processes.

 

• Please, do a detailed review to remove some minor typing errors. In special, correct reference to Laura et al. using her last name (see on ref listing). Also, check line 147, there is a "liquid phase" and must be probably "gas phase".

We checked carefully the manuscript again and corrected typing and reference errors, as it was requested. 

 

 

References:

65. Lewandowski, W.M.; Januszewicz, K.; Kosakowski, W. Efficiency and proportions of waste tyre pyrolysis products depending on the reactor type—A review. J. Anal. Appl. Pyrolysis 2019, 140, 25–53, doi:10.1016/j.jaap.2019.03.018.

66. Hita, I.; Arabiourrutia, M.; Olazar, M.; Bilbao, J.; Arandes, J.M.; Castaño Sánchez, P. Opportunities and barriers for producing high quality fuels from the pyrolysis of scrap tires. Renew. Sustain. Energy Rev. 2016, 56, 745–759, doi:10.1016/j.rser.2015.11.081.

67. Campuzano, F.; Brown, R.C.; Martínez, J.D. Auger reactors for pyrolysis of biomass and wastes. Renew. Sustain. Energy Rev. 2019, 102, 372–409, doi:10.1016/j.rser.2018.12.014.

68. Luo, G.; Chandler, D.S.; Anjos, L.C.A.; Eng, R.J.; Jia, P.; Resende, F.L.P. Pyrolysis of whole wood chips and rods in a novel ablative reactor. Fuel 2017, 194, 229–238, doi:10.1016/j.fuel.2017.01.010.

105. Ochoa, A.; Bilbao, J.; Gayubo, A.G.; Castaño, P. Coke formation and deactivation during catalytic reforming of biomass and waste pyrolysis products: A review. Renew. Sustain. Energy Rev. 2020, 119, doi:10.1016/j.rser.2019.109600.

106. Ferella, F.; Stoehr, J.; Michelis, I. De; Hornung, A. Zirconia and alumina based catalysts for steam reforming of naphthalene. Fuel 2013, 105, 614–629, doi:10.1016/j.fuel.2012.09.052.

107. Barbarias, I.; Artetxe, M.; Lopez, G.; Arregi, A.; Santamaria, L.; Bilbao, J.; Olazar, M. Catalyst performance in the HDPE pyrolysis-reforming under reaction-regeneration cycles. Catalysts 2019, 9, doi:10.3390/catal9050414.

108. Arandia, A.; Remiro, A.; García, V.; Castaño, P.; Bilbao, J.; Gayubo, A.G. Oxidative steam reforming of raw bio-oil over supported and bulk Ni catalysts for hydrogen production. Catalysts 2018, 8, doi:10.3390/catal8080322.

109. Li, D.; Koike, M.; Wang, L.; Nakagawa, Y.; Xu, Y.; Tomishige, K. Regenerability of hydrotalcite-derived nickel-iron alloy nanoparticles for syngas production from biomass tar. ChemSusChem 2014, 7, 510–522, doi:10.1002/cssc.201300855.

110. Li, D.; Nakagawa, Y.; Tomishige, K. Development of Ni-based catalysts for steam reforming of tar derived from biomass pyrolysis. Cuihua Xuebao/Chinese J. Catal. 2012, 33, 583–594, doi:10.1016/s1872-2067(11)60359-8.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

New version takes into account all previous comments. Thanks.