Conclusiones

Conclusions

As an economical way to produce chemicals and fuels from sustainable low-cost feedstock, fast co-pyrolysis of biomass and plastic was studied in this PhD work in order to elucidate the underlying chemicals and physical phenomena occurring during pyrolysis.

Overall, co-pyrolysis with plastics reduced the formation of oxygenated compounds from biomass, increasing HHV of bio-oil. The catalytic co-pyrolysis of biomass and plastics promotes the production of high-quality liquid products catalyzed by zeolite catalyst. The co-pyrolysis process can be further optimized through conducting feedstock pretreatment, changing reactant-catalyst contact mode and carrier gases during pyrolysis.

First, a continuous co-pyrolysis of HDPE and red oak was successfully implemented in a bench-scale fluidized bed reactor from 525 ºC to 675 ºC. The yield of pyrolysis oil was optimized at 625 ºC. The presence of 20% HDPE in the feedstock promoted the formation of furans and acids from holocellulose and alkylated phenols from lignin. Water was found to increase during co-pyrolysis, possibly due to the enhanced hydrodeoxygenation reaction of red oak derived oxygenates by hydrogen transfer from HDPE. The yield of pyrolysis char from red oak decreased. The difference of char SEM pictures between red oak-only pyrolysis and co-pyrolysis further suggested the interaction among red oak and melted HDPE.

Second, this study focused on unravelling the thermal synergy and catalytic synergy during the co-pyrolysis of biomass and plastic. The thermal synergy manifested in terms of increased production of light oxygenates from cellulose and phenolic monomers from lignin.

production constituted the catalytic synergy. Increase of temperature was found to promote the yield of aromatic hydrocarbons. The catalytic synergy was favored at moderate catalyst temperature and became insignificant at higher catalyst temperature.

This study further investigated the possibility of enhancing the synergistic effects between biomass and plastic. Pretreatment of corn stover by sulfuric acid infusion and leaching processes can significant enhancing the cross-reaction between corn stover and polyethylene. The yield of levoglucosan was found to increase with the co-pyrolysis of acid infused corn stover and polyethylene during non-catalytic co-pyrolysis. The neutralized potassium sulfate as well as lignin components in corn stover catalyzed the cracking of polyethylene chain. The catalytic co-pyrolysis of raw/acid leached/acid infused corn stover and polyethylene further confirmed the contribution of Diels-Alder reaction between furans (dehydration products of levoglucosan) and olefins (depolymerization production of PE) to the synergistic effects.

Finally, four main waste plastics were catalytic pyrolyzed to evaluate their potentials for hydrocarbon production. The product species and distribution heavily depended on the feedstock-catalyst contact mode and plastic types. The difference of product outcomes between in-situ and ex-situ catalytic pyrolysis indicated different reaction mechanisms existing in these two scenarios. By changing the carrier gas of pyrolysis, hydrogen was found to improve the conversion of hydrogen deficient plastics including PS and PET, while played a much less important role in the hydropyrolysis of PE and PP. Further co-pyrolysis of PE and PS/PET confirmed the hydrogen transfer from PE to these hydrogen deficient plastics, thus improving the overall process performance in terms of enhancing hydrocarbon

production and inhibition of catalytic coke formation.

Overall, cross reactions between biomass and plastic were proven in both non-catalytic and non-catalytic fast co-pyrolysis. The co-pyrolysis with plastics can be considered to improve the performance of biomass conversion technologies, such as increasing stabilized products and valuable chemicals. These finds and improvements further prove the concept and feasibility of processing and recycling Municipal Solid Waste through thermochemical technologies to produce advanced products.

Future work

Work in this dissertation has helped the understanding of interaction during fast co-pyrolysis of biomass and plastic and improve the methods for producing fuels and chemicals from co-pyrolysis process. Such a work is believed to be useful in using waste plastics as additives in biomass conversion to improve bio-oil yield and quality.

However, there are still some controversies about the role of pyrolysis heating-rate in promoting the interaction between plastic and biomass (fast pyrolysis VS. slow pyrolysis).

Thus, part of the future work can be done to unravel these controversies by both experimental study and kinetic modeling.

Besides, future research will be focused on realizing the successful pyrolysis-oriented conversion of Municipal Solid Waste based on the current findings. Different from the co-pyrolysis of well-defined plastics and biomass performed in this dissertation work, several issues need to be addressed before successful bench-scale and pilot-scale pyrolysis of MSW.

To be able to apply the knowledge gained in this work to real world MSW, pyrolysis and catalytic pyrolysis of other waste streams (waste paper, grass clippings, food waste etc.) should also be evaluated.

Although the major pyrolyzable components of MSW are similar, including organic waste (paper, food waste, yard waste) and plastic waste, the seasonal and regional variations of these components require a more robust reactor design which can achieve stable heat and mass transfer as well as operating conditions regardless of the composition difference.

Furthermore, the inorganic elements in MSW are far more complex than AAEMs mentioned in Chapter 4. These elements can include sulfur, nitrogen, chloride and heavy metals like iron, copper, cobalt and mercury. Understanding the catalytic effects and

evolution of these elements during pyrolysis is important for predicting the product outcomes as well as preventing the pollutants emission. Evaluation is also needed for the possible catalyst poison by these inorganic elements during catalytic pyrolysis process.