Multi-Scale Modeling of Plastic Waste Gasification: Opportunities and Challenges

Abstract

Among the different thermo-chemical recycling routes for plastic waste valorization, gasification is one of the most promising, converting plastic waste into syngas (H2+CO) and energy in the presence of an oxygen-rich gas. Plastic waste gasification is associated with many different complexities due to the multi-scale nature of the process, the feedstock complexity (mixed polyolefins with different contaminations), intricate reaction mechanisms, plastic properties (melting behavior and molecular weight distribution), and complex transport phenomena in a multi-phase flow system. Hence, creating a reliable model calls for an extensive understanding of the phenomena at all scales, and more advanced modeling approaches than those applied today are required. Indeed, modeling of plastic waste gasification (PWG) is still in its infancy today. Our review paper shows that the thermophysical properties are rarely properly defined. Challenges in this regard together with possible methodologies to decently define these properties have been elaborated. The complexities regarding the kinetic modeling of gasification are numerous, compared to, e.g., plastic waste pyrolysis, or coal and biomass gasification, which are elaborated in this work along with the possible solutions to overcome them. Moreover, transport limitations and phase transformations, which affect the apparent kinetics of the process, are not usually considered, while it is demonstrated in this review that they are crucial in the robust prediction of the outcome. Hence, possible approaches in implementing available models to consider these limitations are suggested. Finally, the reactor-scale phenomena of PWG, which are more intricate than the similar processes—due to the presence of molten plastic—are usually simplified to the gas-solid systems, which can result in unreliable modeling frameworks. In this regard, an opportunity lies in the increased computational power that helps improve the model’s precision and allows us to include those complexities within the multi-scale PWG modeling. Using the more accurate modeling methodologies in combination with multi-scale modeling approaches will, in a decade, allow us to perform a rigorous optimization of the PWG process, improve existing and develop new gasifiers, and avoid fouling issues caused by tar.

Combustion, Chemistry, and Carbon Neutrality

“…Biomass and waste streams offer potential contributions for such demands. Recent reviews are a rich source of information on the progress in pyrolysis and gasification in view of the transformation toward sustainable and circular processes. − Introductory, overview, and perspective articles address general procedures, ,, experimental techniques, − , and modeling strategies ,,, regarding the conversion of biomass, ,− specifically of biopolymeric building blocks such as lignocellulose and lignin, , and of plastics, ,− aiming not only at energy recovery but, e.g., at producing chemicals and fuels. , …”

“…Similar to combustion, gas-phase pyrolysis and gasification processes proceed via sequences of multiple radical reactions with hundreds of species, and only models that encompass these radical chain reactions will enable predictions for a broad range of conditions. , Typical reaction families, including initiation, propagation, and termination reactions (H-abstraction from different sites, β-scission, addition, recombination, or cyclization reactions) are differently represented in current detailed polymer degradation mechanisms . Further work is needed for a consistent description of polymer conversion processes regarding reaction parameters, fluid dynamics, heat and mass transfer, and phase changes, including gas and solid phases but also the presence of molten polymer if applicable. , Model development could feasibly build on detailed investigations for monodisperse single polymers of limited chain length and related validation experiments. Multiscale mathematical models based on fundamental principles are much needed to predict optimal reactor design and efficient process conditions with favorable selectivity for desired products, but they are not yet available for real mixed-feedstock, industrial-scale processes. , The feasibility and overall sustainability of thermochemical processes must also be examined against other chemical recycling strategies, e.g., using depolymerization to monomers or other molecular building blocks to access valuable products from fine chemicals to materials .…”

“…Further work is needed for a consistent description of polymer conversion processes regarding reaction parameters, fluid dynamics, heat and mass transfer, and phase changes, including gas and solid phases but also the presence of molten polymer if applicable. , Model development could feasibly build on detailed investigations for monodisperse single polymers of limited chain length and related validation experiments. Multiscale mathematical models based on fundamental principles are much needed to predict optimal reactor design and efficient process conditions with favorable selectivity for desired products, but they are not yet available for real mixed-feedstock, industrial-scale processes. , The feasibility and overall sustainability of thermochemical processes must also be examined against other chemical recycling strategies, e.g., using depolymerization to monomers or other molecular building blocks to access valuable products from fine chemicals to materials . Techno-economic assessments are helpful for the evaluation of potential process routes, as, e.g., for converting PP and PP/PET laminate waste predominantly to pyrolysis gas as a natural gas substitute …”

“…Recent reviews are a rich source of information on the progress in pyrolysis and gasification in view of the transformation toward sustainable and circular processes. 634−650 Introductory, overview, and perspective articles address general procedures, 634,645,646 experimental techniques, [636][637][638][639]641 and modeling strategies 642,644,649,640 regarding the conversion of biomass, 636,641−644 specifically of biopolymeric building blocks such as lignocellulose and lignin, 636,644 and of plastics, 634,645−649 aiming not only at energy recovery but, e.g., at producing chemicals and fuels. 646,647 Different thermochemical conversion technologies for these purposes have several features in common, including their robustness and good economics.…”

“…634 Further work is needed for a consistent description of polymer conversion processes regarding reaction parameters, fluid dynamics, heat and mass transfer, and phase changes, including gas and solid phases but also the presence of molten polymer if applicable. 634,649 Model development could feasibly build on detailed investigations for monodisperse single polymers of limited chain length and related validation experiments. Multiscale mathematical models based on fundamental principles are much needed to predict optimal reactor design and efficient process conditions with favorable selectivity for desired products, but they are not yet available for real mixed-feedstock, industrial-scale processes.…”