Solids and gas hydrodynamic characteristics in square-based spouted beds for thermal applications

Gasification is considered a favourable method to convert a biomass or other organic materials to a multipurpose gas; the syngas produced can be addresses to the chemical industry as a synthesis reactant or considered a mere fuel. New concepts based on the valorisation of non-conventional resources have been developed in the recent years to save fossil fuels and reduce the generation of civil and industrial waste. According to these ideas, refuses can be transformed into a valuable energy source instead of being disposed. In conformity with these perspectives, updated technologies should be investigated, both to optimize gasification processes and obtain a medium heating value gas. The quality of syngas depends on the properties of the raw material used, the operative conditions and the design of the gasifier. In case the fuel used is a by-product of a certain industrial operation, the chemical composition cannot kept constant, and only an average value can be estimated to predict the composition of the syngas generated by the thermal reaction. According to these considerations, the gasification reaction should be run in a unit adaptable to a variable source of materials. The present work aims at improving the understanding of spouted beds, which display advantages if compared to more common fluidization techniques; a spouted bed can be operated with a better hydrodynamic control, thanks to lower pressure drop and gas flow rate, slightly exceeding the minimum spouting condition, rather than a much higher flow rate required by bubbling or turbulent fluidization. Additionally, the hydrodynamic features of a spouted bed offer optimal mixing of solid phases characterized by very different density or shape; specifically, a light particulate reacting waste, continuously added to a stationary buffer inert, is not affected by segregation. Spouted beds appear to go through a revival, proved by a very recent and comprehensive book on the topic (Epstein and Grace, 2011). This renewed interest arises by implementing new concepts in scaling-up spouting contactors and devising potential applications to high temperature processes, noticeable examples being given by pyrolysis and gasification of biomass, kinetically controlled drying of moist seeds to guarantee the requested qualities and polymer upgrading processes. Moving to our area of interest, a textile district produces waste fibres and fibre fragments at a noticeable mass rate. As a case study, a survey within the Biella district has revealed a daily generation close to 5000 kg. This by-product represent a cost for the sector because they have to be compacted, stored and disposed, while a small-scale thermal process can be a prompted reasonable solution of valorisation. A square-based spouted bed pilot unit was designed and constructed to investigate the gasification of various types of textile waste, fed to the reactor upon pelletization. To guarantee a reducing or low oxidizing atmosphere in the reaction zone at gas flow rate proper for spouting, steam was in-line generated by gas combustion to close the mass and enthalpy balance