(186d) CFD and Process Simulations of Air Gasification of Plastic Wastes in a Conical Spouted Bed Gasifier

Gasification of plastic waste which is commonly deemed as one category of municipal solid waste (MSW), is a promising technique to produce high-quality syngas with a high heat value because of the high content of hydrogen and carbon element and low content of oxygen in plastics. It also an alternate and economical way for effective treatment of non-decomposable solid waste.

The main objective of this investigation is to study numerically air gasification of a plastics feedstock that consists of 100% polyethylene (PE) in a newly designed conical spouted bed reactor. The investigation was carried out using a CFD-based Equivalent Reactor Network (ERN) model developed by the authors in a process simulator.

The construction procedure of the CFD-based ERN model consists of three steps:

(1)   A computational fluid dynamics (CFD) model is built to account  for the hydrodynamics of the conical spouted bed gasifier. An Euler-Euler two fluid model, closed using the kinetic theory of granular flow, is 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. The spouted bed gasifier was zoned into five connected zones and represented by corresponding chemical reactor chosen from the process simulator database. Detailed homogenous and heterogeneous kinetics of gasification reactions are nested into the simulator in the form of FORTRAN modules.

The developed ERN model is validated against available experimental data. With the established CFD-based ERN model for the spouted bed gasifier, the effects of gasification temperature and equivalence ratio (ER) on air gasification of polyethylene in the conical spouted bed gasifier were quantified.  

By using the developed CFD-based ERN model, it is found that increased gasification temperature improves the gasifier performance through enhancing the production of CO and H₂ and decreasing C₂H₄ content in syngas. This results in a slight decline in Lower Heat Value (LHV) but a considerable increase in Carbon Conversion Efficiency (CCE) for the entire range of tested temperature. The highest Cold Gas Efficiency (CGE) of 72.14% is recorded at 700°C. Also, the study reveals 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%.

As conclusion, the present work demonstrates 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.