Coupling dechlorination and catalytic pyrolysis to produce carbon nanotubes from mixed polyvinyl chloride and polyethylene.

The presence of chlorine in polyvinyl chloride (PVC) presents a major challenge for realizing the high-value utilization of real waste plastics. The objective of this research was to develop a chlorine-resistant process for the preparation of carbon nanotubes (CNTs) from mixed plastics containing PVC. This study investigates the influence of PVC content and various dechlorinating agents (CaO, Na2CO3, red mud (RM), ZSM-5, Fe-Al2O3, Fe(OH)3) on CNTs formation. The results showed that PVC content exceeding 5 % significantly inhibits CNTs formation. Employing dechlorinating agents in the pyrolysis process results in a substantial yield of CNTs from mixed plastics containing 10 % PVC. Among the dechlorinating agents, RM proves to be the most effective, leading to the highest carbon yield (at 30 wt%) and superior CNTs quality. Other dechlorinating agents, except for ZSM-5, yield comparable results, although there were some obvious variations of volatiles. Further investigation of the role of dechlorinating agents from the perspective of volatiles evolution was conducted via Py-GC/MS, and found that the dechlorination agent efficiently absorbs the HCl from mixed plastics pyrolysis, while also exhibiting catalytic and regulatory influence on volatile components. These findings offer valuable insights for the development of a chlorine-resistant process in the preparation of CNTs from mixed plastics that contain PVC.

High-value products from ex-situ catalytic pyrolysis of polypropylene waste using iron-based catalysts: the influence of support materials.

Catalytic pyrolysis is considered a promising strategy for the utilisation of plastic waste from the economic and environmental perspectives. As such, the supporting materials play a critical role in the properties of the catalyst. This study clarified this influence on the dispersion of the iron (Fe) within an experimental context. Four different types of typical supports with different physical structures were introduced and explored in a two-stage fixed-bed reactor; these included metallic oxides (Al2O3, TiO2), a non-metallic oxide (SiO2), and molecular sieves (ZSM-5). The results show that the liquid products were converted into carbon deposits and lighter gaseous products, such as hydrogen. The Al2O3-supported catalyst with a relatively moderate specific surface areas and average pore diameter exhibited improved metal distribution with higher catalytic activity. In comparison, the relatively low specific surface areas of TiO2 and small average pore diameters of ZSM-5 had a negative impact on metal distribution and the subsequent catalytic reformation process; this was because of the inadequate reaction during the catalytic process. The Fe/Al2O3 catalyst produced a higher yield of carbon deposits (30.2 wt%), including over 65% high-value carbon nanotubes (CNTs) and hydrogen content (58.7 vol%). Additionally, more dispersive and uniform CNTs were obtained from the Fe/SiO2 catalyst. The Fe/TiO2 catalyst promoted the formation of carbon fibre twisted like fried dough twist. Notably, there was interesting correspondence between the size of the reduced Fe nanoparticles and the product distribution. Within certain limits, the smaller Fe particle size facilitates the catalytic activity. The smaller and better dispersed Fe particles over the support materials were observed to be essential for hydrocarbon cracking and the subsequent formation of carbon deposits. The findings from this study may provide specific guidance for the preparation of different forms of carbon materials.

Machine learning prediction of pyrolytic gas yield and compositions with feature reduction methods: Effects of pyrolysis conditions and biomass characteristics.

This study aimed to utilize machine learning algorithems combined with feature reduction for predicting pyrolytic gas yield and compositions based on pyrolysis conditions and biomass characteristics. To this end, random forest (RF) and support vector machine (SVM) was introduced and compared. The results suggested that six features were adequate to accurately forecast (R2 > 0.85, RMSE < 5.7%) the yield while the compositions only required three. Moreover, the profound information behind the models was extracted. The relative contribution of pyrolysis conditions was higher than that of biomass characteristics for yield (55%), CO2 (73%), and H2 (81%), which was inverse for CO (12%) and CH4 (38%). Furthermore, partial dependence analysis quantified the effects of both reduced features and their interactions exerted on pyrolysis process. This study provided references for pyrolytic gas production and upgrading in a more convenient manner with fewer features and extended the knowledge into the biomass pyrolysis process.

Pyrolysis of Chinese chestnut shells: Effects of temperature and Fe presence on product composition.

To understand the role of Fe on biomass pyrolysis, Fe-catalyzed biomass pyrolysis in a fixed-bed reactor was investigated. It was found that the introduction of Fe increased the yields of gases and solid char while decreasing the yield of liquid oil. With increasing temperature, Hydrogen content in gaseous products obtained in the presence of Fe increased, while that of CH4 decreased. In the case of liquid oil, the introduction of Fe promoted the formation of ketones and acids at 400-600 °C, and these species became dominant (67.51%) at 700-800 °C. Finally, solid char obtained in the presence of Fe at 700-800 °C featured a larger pore volume, specific surface area, and graphitization degree, and was characterized by a mesoporous structure with narrow pores size distribution (∼5.3 nm).

Pyrolysis behavior and economics analysis of the biomass pyrolytic polygeneration of forest farming waste.

The pyrolysis behavior of Chinese chestnut and Jatropha curcas shells (CNS and JCS, respectively) were investigated to determine the optimum operating temperature for biomass pyrolytic polygeneration systems. At low temperatures (250-450 °C), CO2 was the main component of the pyrolytic gas, and high acidity oil was obtained. When the temperature increased to 550-650 °C, phenol-enriched oil and high LHV biochar (∼26 MJ/kg) were obtained; H2 and CO yields increased. At high temperatures (750-950 °C), heavy-oil and high LHV pyrolytic gas (∼15 MJ/m3) were obtained. Meanwhile, the biochar showed a highly condensed aromatic ring system and low H/C (∼0.1) and O/C (∼0.05) ratios. CNS and JCS biochars showed different tendencies with regard to their structure evolution. An economic analysis was performed, which suggested that the optimum operating temperatures were 450 °C for CNS and 350 °C for JCS.