Gasification of Refuse Derived Fuels in Bubbling Bed Reactors: Bed-Freeboard Coupled Models, Results and Validation

Gasification is the thermochemical conversion of solid feedstock into syngas, which can be accomplished through several reactor designs. Feedstocks containing high ash and volatile fractions, such as refuse-derived fuel (RDF) produced from solid wastes are commonly treated in fluidized bed reactors, which are particularly suitable due to their flexibility with respect to feedstock and operating conditions. Differently from conventional fuels, waste is generated from a number of different sources and influenced by a wide range of factors that create a complex mixture of materials, resulting into an extensive variability in RDF composition and, therefore, process operating conditions.

The complexity of simultaneous reaction phenomena occurring during RDF gasification and the corresponding lack of real plant data to validate models pose a significant challenge for accurately describing and predicting process performance. Thus, experimentally validated models that can correlate the large variations in process operating conditions and feedstock compositions to products quality (including an accurate prediction of tars and VOCs contents) are highly beneficial and of great industrial interest.

In this study, two models were developed to better describe the kinetics and hydrodynamics of different reactor sections of an RDF gasification process. The fluidised bed zone was modelled on Matlab to include feeding, fuel devolatilization and gas-particles interactions. The freeboard zone was modelled using Ansys Chemkin as a non-isothermal plug flow reactor to describe composition and temperature profiles of the syngas product, including over 300 components. The two models were then combined and validated with experimental data from laboratory and pilot scale reactors. The resulting system models allow for an accurate prediction of the effects of the operating conditions, such as pressure, equivalent ratio and use of novel gasifiying agents (e.g. pure oxygen, steam or CO₂ and their combination), on yield and quality of product syngas over a wide range of feedstock compositions.