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Title: Clean energy from waste : fundamental investigations on ashes and tar behavior in a two stage fluid bed-plasma process for waste gasification
Author: Materazzi, M.
ISNI:       0000 0004 7659 8782
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2015
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Over the past thirty years, the major factor that has prevented the widespread uptake of advanced gasification technologies for treating municipal solid waste (MSW) and biomass fuels has been the presence of tars and char contaminants in the syngas product, which makes the gas unsuitable for power production using energy efficient gas engines or turbines. Furthermore, the large quantities of ashes and volatile material in waste materials produce a large amount of residues downstream, as well as significant corrosive inorganic vapours and ash deposition issues. Advanced Plasma Power (APP) have developed a 2-stage thermal process where the raw syngas generated in a conventional bubbling fluid bed gasifier (FBG) is further treated in a plasma converter (PC) unit to crack and reform these tar and char species to provide a refined syngas suitable for use in a power island. At the same time, inorganic particulate and ash-type components are converted into a stable vitrified product that can be recycled as ceramic glass or road paving material. The fate of the potential deposit-forming elements arising from waste materials in a two-stage process will clearly influence the conversion efficiency, as well as the nature and extent of any harmful deposits along the thermal plant. Therefore, how the main constituents differentiate into gas phase and solid products can be monitored and controlled in the FBG first, and in the PC after, becomes a very important question. The purpose of this PhD project was to gain a fundamental understanding as to how the key process operating variables may impact the final quality of the syngas exiting the thermal plant, especially with regard to the fate of the ash forming components (i.e. agglomeration, slugging, fouling and vitrification) and the behaviour of the volatile matter (i.e. mixing, segregation and gas phase reaction mechanism) in the two stages. A systematic study was conducted to evaluate the effect of the key operating variables on the quality and quantity of the syngas exiting each unit, with specific attention to the behavior of tar components, and other key contaminants (chlorine, sulphur, heavy metals, etc.). On this side, the FBG reactor seems to play a crucial role on the two stage process efficiency evaluation. Within this context, a large part of the study herein was aimed at developing a fundamental understanding on the fluid dynamic behaviour (fluid-particle and particle-particle interaction) of a bubbling fluidized bed operated at high temperature, up to 800˚C. This included process analysis based on operation of a pilot plant using a municipal waste feedstock (40-100 kg/hr). In addition to fluidization tests, laboratory analyses, such as X-ray diffraction (XRD), X-ray fluorescence (XRF), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), were carried out to investigate the characterization and speciation of bottom and fly-ashes. The results obtained from these physical tests could be used to explain the phenomena observed for some of the materials tested in demonstration runs at APP, which showed changes in the fluidization behaviour for different ash compositions. In parallel, the potential of thermal plasma for the reforming of fluid bed tars and ash vitrification was investigated. Evaluation of plasma chemistry was performed by comparing experimental data from the pilot plant with thermodynamic and thermal kinetic predictions. Oxygen atoms initially formed from CO2 were identified as the major active species involved in the oxidative decomposition of hydrocarbon intermediates and soot precursors. The same mechanism was used to describe the reforming of organosulphur compounds, produced from gasification of sulphur-rich wastes (e.g. automotive shredded residues, demolition wood, etc.). This provides a clearer understanding of the mechanism as to how potentially hazardous elements evolve and provides guidance in the implementation of two-stage processes utilising solid wastes as alternative fuels.
Supervisor: Lettieri, P. L. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available