Processing real-world waste plastics by pyrolysis-reforming for hydrogen and high-value carbon nanotubes.

Producing both hydrogen and high-value carbon nanotubes (CNTs) derived from waste plastics is reported here using a pyrolysis-reforming technology comprising a two-stage reaction system, in the presence of steam and a Ni-Mn-Al catalyst. The waste plastics consisted of plastics from a motor oil container (MOC), commercial waste high density polyethylene (HDPE) and regranulated HDPE waste containing polyvinyl chloride (PVC). The results show that hydrogen can be produced from the pyrolysis-reforming process, but also carbon nanotubes are formed on the catalyst. However, the content of 0.3 wt.% polyvinyl chloride in the waste HDPE (HDPE/PVC) has been shown to poison the catalyst and significantly reduce the quantity and purity of CNTs. The presence of sulfur has shown less influence on the production of CNTs in terms of quantity and CNT morphologies. Around 94.4 mmol H2 g(-1) plastic was obtained for the pyrolysis-reforming of HDPE waste in the presence of the Ni-Mn-Al catalyst and steam at a reforming temperature of 800 °C. The addition of steam in the process results in an increase of hydrogen production and reduction of carbon yield; in addition, the defects of CNTs, for example, edge dislocations were found to be increased with the introduction of steam (from Raman analysis).

Dechlorination of waste polyvinyl chloride (PVC) through non-thermal plasma.

Dechlorination is essential for the chemical recycling of waste polyvinyl chloride (PVC) plastics. This study investigated the use of non-thermal plasma (NTP) for chlorine removal, with a focus on the effects of treatment time and discharge power on dechlorination efficiency. The results showed that longer treatment times and higher discharge powers led to better dechlorination performance. The maximum efficiency (98.25%) and HCl recovery yield (55.72%) were achieved at 180 W power after 40 min of treatment where 96.44% of Cl existed in the form of HCl gas, 1.44% in the liquid product, and 2.12% in the solid residue product. NTP at a discharge power of 150 W showed better dechlorination performance compared to traditional thermal pyrolysis treatment in temperatures ranging from 200 to 400 °C. The activation energy analysis of the chlorine removal showed that compared to pyrolysis-based dechlorination (137.09 kJ/mol), NTP-based dechlorination (23.62 kJ/mol) was more easily achievable. This work presents a practical method for the dechlorination of waste PVC plastic using a novel technology without requiring additional thermal and pressure input.

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.

Bimetallic carbon nanotube encapsulated Fe-Ni catalysts from fast pyrolysis of waste plastics and their oxygen reduction properties.

Carbon-based bimetallic electrocatalysts were obtained by catalytic pyrolysis of waste plastics with Fe-Ni-based catalysts and were used as efficient oxygen reduction reaction (ORR) catalysts in this study. The prepared iron-nickel alloy nanoparticles encapsulated in oxidized carbon nanotubes (FeNi-OCNTs) are solid products with a unique structure. Moreover, the chemical composition and structural features of FeNi-OCNTs were determined. The iron-nickel alloy nanoparticles were wrapped in carbon layers, and the carbon nanotubes had an outer diameter of 20-50 nm and micron-scale lengths. FeNi-OCNT with a Fe/Ni ratio of 1:2 (FeNi-OCNT12) exhibited remarkable electrochemical performance as an ORR catalyst with a positive onset potential of 1.01 V (vs. RHE) and a half-wave potential of 0.87 V (vs. RHE), which were comparable to those of a commercial 20% Pt/C catalyst. Furthermore, FeNi-OCNT12 exhibited promising long-term stability and higher tolerance to methanol than the commercial 20% Pt/C catalyst in an alkaline medium. These properties were attributable to the protective OCNT coating of the iron-nickel alloy nanoparticles.

De-novo formation of dioxins and furans and the memory effect in waste incineration flue gases.

The formation of dioxin (polychlorinated-rho-dibenzodioxins - PCDD) and furan (polychlorinated dibenzofurans - PCDF) in waste incineration flyash in relation to temperature has been investigated in a bench scale reactor. The PCDD/PCDF released into the exhaust gases of the reactor and trapped in a condensation and vapour phase adsorption system were also determined. The concentrations of PCDD/PCDF in the flyash were analysed in detail for all congeners and isomers including the mono-, di- and tri-PCDD/PCDF. De-novo synthesis of PCDD/PCDF was clearly seen in the flyash with large changes in concentrations of PCDD/PCDF with only small changes in temperature. Exhaust gas PCDD/PCDF were detected at all temperatures, with the maximum emission occurring at a flyash temperature of 350 degrees C, where the equivalent of over 42 times the total dioxin/furan concentration of the original flyash was lost to the vapour phase per unit mass of initial flyash. Although the desorbed species were mainly the lower mono- to tri-congener group of dioxins and furans, the desorbed dioxin/furans had a considerable toxic equivalent value.

Pyrolysis of latex gloves in the presence of Y-zeolite.

In this study we have investigated the possibility of processing waste rubber gloves using pyrolysis. Y-zeolite catalyst was employed to upgrade the pyrolysis products to give higher yields of valuable aromatic compounds such as toluene and xylenes. The composition of the pyrolysis products was determined using gas chromatography with linked mass spectrometry (GC-MS), gas chromatograph equipped with a flame ionization detector (GC-FID), gas chromatograph fitted with dual thermal conductivity detectors (GC-TCD), and Fourier Transform Infra-Red Spectrometry (FT-IR). It was found that when rubber gloves were pyrolysed in the absence of a catalyst, the pyrolysis oil consisted mainly of limonene and oligomers of polyisoprene. When Y-zeolite was added to the reaction system, the yields of toluene, xylene, methylbenzenes, ethylbenzenes, and naphthalenes increased dramatically. The Y-zeolite also catalysed the decomposition of limonene, which was absent from the catalytic pyrolysis products. The presence of the Y-zeolite catalyst also increased the yield of hydrocarbon gases. The tests were carried out at both 380 degrees C and 480 degrees C and it was found that the higher reaction temperature led to increased yields of all the major compounds, both in the presence and absence of the Y-zeolite catalyst.

Effect of decabromodiphenyl ether and antimony trioxide on controlled pyrolysis of high-impact polystyrene mixed with polyolefins.

The controlled pyrolysis of polyethylene/polypropylene/polystyrene mixed with brominated high-impact polystyrene containing decabromodiphenyl ether as a brominated flame-retardant with antimony trioxide as a synergist was performed. The effect of decabromodiphenyl ether and antimony trioxide on the formation of its congeners and their effect on distribution of pyrolysis products were investigated. The controlled pyrolysis significantly affected the decomposition behavior and the formation of products. Analysis with gas chromatograph with electron capture detector confirmed that the bromine content was rich in step 1 (oil 1) liquid products leaving less bromine content in the step 2 (oil 2) liquid products. In the presence of antimony containing samples, the major portion of bromine was observed in the form of antimony bromide and no flame-retardant species were found in oil 1. In the presence of synergist, the step 1 and step 2 oils contain both light and heavy compounds. In the absence of synergist, the heavy compounds in step 1 oil and light compounds in step 2 oils were observed. The presence of antimony bromide was confirmed in the step 1 oils but not in step 2 oils.

Pyrolysis-catalysis of waste plastic using a nickel-stainless-steel mesh catalyst for high-value carbon products.

A stainless-steel mesh loaded with nickel catalyst was produced and used for the pyrolysis-catalysis of waste high-density polyethylene with the aim of producing high-value carbon products, including carbon nanotubes (CNTs). The catalysis temperature and plastic-to-catalyst ratio were investigated to determine the influence on the formation of different types of carbon deposited on the nickel-stainless-steel mesh catalyst. Increasing temperature from 700 to 900°C resulted in an increase in the carbon deposited on the nickel-loaded stainless-steel mesh catalyst from 32.5 to 38.0 wt%. The increase in sample-to-catalyst ratio reduced the amount of carbon deposited on the mesh catalyst in terms of g carbon g-1 plastic. The carbons were found to be largely composed of filamentous carbons, with negligible disordered (amorphous) carbons. Transmission electron microscopy analysis of the filamentous carbons revealed them to be composed of a large proportion (estimated at ∼40%) multi-walled carbon nanotubes (MWCNTs). The optimum process conditions for CNT production, in terms of yield and graphitic nature, determined by Raman spectroscopy, was catalysis temperature of 800°C and plastic-to-catalyst ratio of 1:2, where a mass of 334 mg of filamentous/MWCNTs g-1 plastic was produced.

Production of activated carbons from waste tyres for low temperature NOx control.

Waste tyres were pyrolysed in a bench scale reactor and the product chars were chemically activated with alkali chemical agents, KOH, K2CO3, NaOH and Na2CO3 to produce waste tyre derived activated carbons. The activated carbon products were then examined in terms of their ability to adsorb NOx (NO) at low temperature (25°C) from a simulated industrial process flue gas. This study investigates the influence of surface area and porosity of the carbons produced with the different alkali chemical activating agents on NO capture from the simulated flue gas. The influence of varying the chemical activation conditions on the porous texture and corresponding NO removal from the flue gas was studied. The activated carbon sorbents were characterized in relation to BET surface area, micropore and mesopore volumes and chemical composition. The highest NO removal efficiency for the waste tyre derived activated carbons was ∼75% which was obtained with the adsorbent treated with KOH which correlated with both the highest BET surface area and largest micropore volume. In contrast, the waste tyre derived activated carbons prepared using K2CO3, NaOH and Na2CO3 alkali activating agents appeared to have little influence on NO removal from the flue gases. The results suggest problematic waste tyres, have the potential to be converted to activated carbons with NOx removal efficiency comparable with conventionally produced carbons.

Alkaline reforming of brominated fire-retardant plastics: fate of bromine and antimony.

High-impact polystyrene (HIPS) flame retarded with decabromodiphenyl ether (DDE), has been reacted in supercritical water from 380 to 450 degrees C and 21.5 to 31.0 MPa pressure in a batch reactor. Different concentrations of sodium hydroxide additive were used in situ to neutralize the corrosive inorganic bromine species released during the reactions. It appeared that supercritical water conditions lowered the decomposition temperature of both the fire-retardant DDE and HIPS. The reaction products included oils (up to 76 wt%), char (up to 18 wt%) and gas (up to 2.4 wt%) which was mainly methane. The presence of the alkaline water led to up to 97 wt% debromination of the product oil, producing virtually bromine-free oil feedstock. The removal of antimony from the oil product during processing was of the order of 98 wt%. The oil consisted of many single- and multiple-ringed aromatic compounds, many of which had alkyl substituents and/or aliphatic C(n)-bridges (n=1-4). The major single-ringed compounds included toluene, xylenes, ethylbenzene, propylbenzene and alpha-methylstyrene. Bibenzyl (diphenylethane), stilbene, diphenylmethane, diphenylpropane, diphenylcyclopropane, diphenylpropene, diphenylbutane, diphenylbutene and diphenylbuta-1,3-diene were the major C(n)-bridged compounds. Diphenyl ether and acetophenone were the major oxygenated compounds found. The process thus has the potential to produce bromine-free and antimony-free oils from fire-retardant plastics.

Fluidised bed catalytic pyrolysis of scrap tyres: influence of catalyst:tyre ratio and catalyst temperature.

Pyrolysis with on-line Zeolite catalysis of scrap tyres was undertaken in a fluidised bed reactor with the aim of maximising the production of higher value single ring aromatic hydrocarbons in the derived oil. Experiments were carried out in relation to the ratio of the catalyst to tyre feedstock and the temperature of the catalyst bed. Two Zeolite catalysts were examined, a Y-type Zeolite catalyst and Zeolite ZSM-5 catalyst of differing pore size and surface activity. The composition of the oils derived from the uncatalysed fluidised bed pyrolysis of tyres showed that benzene concentration was 0.2 wt%, toluene concentration was 0.8 wt%, o-xylene was 0.3 wt%, m/p-xylenes were 1.8 wt% and limonene was 4.3 wt%. Benzene, toluene and xylenes present in the oils showed a significant increase in the presence of both of the catalysts. The maximum concentrations of these chemicals for the Y-Zeolite (CBV-400) catalyst was 1 wt% for benzene, 8wt% for toluene, 3 wt% for o-xylene and 8.5 wt% for m/p-xylenes, produced at a catalyst:tyre ratio of 1.5. There was less influence of catalyst temperature on the yield of benzene, toluene and xylenes, however, increasing the temperature of the catalyst resulted in a marked decrease in limonene concentration. The Y-type Zeolite catalyst produced significantly higher concentrations of benzene, toluene and xylenes which was attributed to the larger pore size and higher surface acidity of the Y-Zeolite catalyst compared to the Zeolite ZSM-5 catalyst.