The Petroleum Institute, UAE
Title: CFD and Process Simulations of air gasification of plastic wastes in a conical spouted bed gasifier
Abdallah S Berrouk is from The Petroleum Institute, UAE.
Apart from coaland biomass gasification, the gasification of plastic wastewhichis commonly deemed asone category of municipal solid waste(MSW), isnot only a promising techniqueto produce syngas because of high content of hydrogen and carbon element but low content of oxygen in plastics which should give rise to high-quality syngas with high heat value , but also an alternate and economicalway for effective treatment of non-decomposable solid waste.The main objective of thisinvestigation is to study numerically air gasification of a plastics feedstock that consists of 100% polyethylene (PE) in a newly designed conical spouted bed reactor(see figure 1). The investigation was carried out using a CFD-based EquivalentReactor Network(ERN) model developed by the authors in Aspen Plus simulator . Numerical Methodology Theconstruction procedure of CFD-based ERN model consists of three steps: (1) A computational fluid dynamics (CFD) model is run that accounts only for the hydrodynamics of the conical spouted bed gasifier.Herein, an Euler-Euler two fluid model, closed using the kinetic theory of granular flow, was used to model the hydrodynamics of the gas-solid flow in the gasifier. (2) An auto-zoning algorithm  is applied to the CFD-generated flow field to create an ensemble of connected zones or compartments. (3) Each zone or compartment is considered as an ideal chemical reactor according to the predominant flow pattern in it. Herein, the spouted bed gasifier was zoned into five connected zones and represented by corresponding chemical reactor chosen from Aspen Plus unit database (see figure 2). Detailed homogenous and heterogeneous kineticsof gasification reactions were nested into Aspen Plus in the form of FORTRAN modules. With the establishedCFD-based ERNmodelfor the spouted bed gasifier, the effects of gasification temperature and equivalence ratio (ER) on air gasification of polyethylene in the conical spouted bed gasifierwerequantified after model validation through compared with available experimental data . Results and Discussion By using the developed CFD-based ERN model, it was found that increased gasification temperature improves the gasifier performance through enhancing the production of CO and H2 and decreasing C2H4 content in syngas. This resulted in a slight decline in Lower Heat Value (LHV) but a considerable increase in CarbonConversion Efficiency (CCE)for the entire range of tested temperature. The highest Cold Gas Efficiency (CGE) of 72.14% was recorded at 700°C. Also, the study revealed the strong influence equivalence ratio (ER) had on the syngas production through its direct influence on carbon conversion and oxidation of syngas. The optimum value of ER for air gasification of PE at 700°C was found to be 0.4 with a value of LHV of 6.2MJ/Nm3 and a value of CCE of 97.3%. Conclusions The present work demonstrated the capabilities of the developed CFD-based ERN model in simulating polyethylene gasification process as well as the appropriateness of the newly-designed conical spouted bed gasifier to carry out such a process.