Mixed Plastic Waste Gasification in a Large Pilot-Scale Fluidized Bed Reactor Operated with Oxygen-Enriched Air and Steam

Worldwide, management of plastic waste by means of mechanical recycling often produces just a downcycling, particularly when applied to mixed plastic waste (MPW), consisting of multilayer packaging and mixtures of heterogeneous polymers. This poor management is the main cause of the too low percentage of plastic recycled in the world (about 9%), with the consequent uncontrolled dispersion of contaminants (mainly microplastics and accompanying additives) and the related and dramatic increase of impacts on the environment and human health. In this alarming framework, advanced autothermal steam-oxygen gasification technologies, aimed at producing hydrogen and building block chemicals, represent a promising solution. This study describes a series of gasification tests carried out with MPWs coming from separate collection of municipal solid waste, in a large pilot-scale fluidized bed, using steam and O2-enriched air with increasing fractions of oxygen (up to 0.69). Obtained data allow investigating how the main operating parameters (equivalence ratio, ER; and steam-to-carbon molar ratio, StC) and the state variables (bed temperature and axial temperature profile) affect crucial process performance indicators (such as yield and heating value of syngas, grams of hydrogen per kilogram of MPW, and profiles of syngas composition along the reactor height). An increased molar fraction of oxygen in the enriched air seems to improve the yield of hydrogen (up to 12 gH2/kgMPW) and the H2/CO ratio (up to 1.6). Steam injection, when larger than that necessary for the stoichiometry of the water-gas reaction, slightly affects syngas composition but remains crucial to control the temperature along the reactor. ER and StC jointly determine the reactor temperatures and, in turn, the syngas composition and overall process performance. The results are useful to define the design and operating criteria of large-scale steam-oxygen fluidized bed gasifiers and to support the optimization of descriptive or predictive numerical models.