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This book explores the use of recent advanced multiple stage conversion technologies. These applications combine conventional fluidised bed systems with new plasma technologies to efficiently generate different energy outputs from waste materials with minimum cleaning effort. Using a mix of modelling and experimental approaches, the author provides fundamental insights into how the key operating variables of the two-stage process may impact the final quality of syngas. This thesis serves as a useful reference guide on the modelling and design of single and multiple-stage systems for thermal waste treatment. Its extended section on plant configuration and operation of waste gasification plants identifies the main technical challenges, and is of use to researchers entering the field.



Chapter 1. Introduction

Growing global population and increasing regulatory, social and environmental antipathy towards high carbon intensity fossil fuels and disposal of waste to landfills, has created a demand for alternative methods of energy production and waste disposal. Although thermochemical processes, like pyrolysis and gasification, are not new concepts, it is only in recent years that they have been used to treat municipal solid waste or refuse derived fuels, for they have the potential of adding value to low- or negative-value feed stocks by converting them into marketable biofuels and products. The research presented in this thesis is concerned principally with the technical analysis and modus operandi of new generation systems based on a two-stage configuration. Given the relatively limited track record of operation of gasification systems on waste, this depth of information is also helpful in understanding and appraising the expected performance of an industrial waste to energy plant. This introductory chapter aims to present the motivations and objectives of this research, and to give a guide post through the different chapters.
Massimiliano Materazzi

Chapter 2. Gasification of Waste Derived Fuels in Fluidized Beds: Fundamental Aspects and Industrial Challenges

Fluidised beds are the most popular technologies for gasification as they are considered to be more robust and versatile than other more conventional reactors. Most of the commercial fluid bed gasifiers (FBG) were originally developed for operation coal and/or pure biomass. The translation of the systems to operation on waste is the issue which represents the most significant technical difficulty, and from this cascade particular concerns regarding the ability to achieve long term stable operation, as well as wider confidence in commercial viability. The purpose of this Chapter is to appraise the fluidized bed gasification systems in light of the exotic characteristics of alternative fuels derived from waste materials. Given the technology’s heritage there is extensive technical information available regarding the modus operandi of fluidized bed reactors. However, given the relatively limited track record of operation on waste, this depth of information is helpful in appraising the expected performance of a waste-fed fluid bed system and the technical issues associated therein.
Massimiliano Materazzi

Chapter 3. Plasma as an Alternative Way to Gas Reforming and Ash Disposal

Plasma has long fascinated scientists and engineers, due to its unique ability to vaporize and destroy any chemical bond in the organic and inorganic forms. The use of electricity as input is also very interesting in waste treatment applications, as it decouples the heat generation from the organic fraction, thus allowing for higher flexibility and better control of the processing unit. However the use of electricity is also a main drawback since it is the most expensive form of energy. Furthermore, there is a lack of data on the reliability of plasma treatment that could prevent its development at large scales. These considerations led to the development of multiple-stage processes, where the plasma is used as a post-processing stage for conditioning the product streams generated from the primary gasification unit (e.g. FBG or rotary kiln). In this way, the majority of the energy input to the process is derived from the controlled oxidation reactions of the solid fuel at the gasifiers, which greatly limits the plasma arc electrical power demand which constitutes only a minor fraction of the total energy content of the fuel to the process.
Massimiliano Materazzi

Chapter 4. Apparatus and Investigative Approach

An extensive literature overview has been presented in previous chapters, and the information summarized therein offers valuable sources for performing experiments and modelling in order to fill the “gaps” regarding waste components behaviour, tar evolution, inorganic partition and overall performance parameters of a two-stage fluid bed-plasma gasification process.
Massimiliano Materazzi

Chapter 5. Modelling of a Two-Stage Process: Comparison of Different Approaches and Performance Analysis

Gasification of solid waste in multiple stage systems has significant potential given the high flexibility and strong policy and technological drivers. Nonetheless, significant ambiguities in the knowledge base of multiple stages processes and their performance are apparent. This Chapter presents a comprehensive comparison of different modelling approaches to describe the behaviour of a two-stage fluid bed gasification—plasma process. The final stage of the assessment is done focusing on the thermodynamic assets of using a two-stage thermal process over the conventional single-stage approach, in order to determine optimum conditions for the gasification of RDF and to understand the limitations and influence of the second stage on the process performance (gas heating value, cold gas efficiency, carbon conversion efficiency), along with other parameters. To this end, a flexible model capable of providing reliable quantitative predictions of product yield and composition after the two-stage process has been developed. Comparison with a different thermal refining stage, i.e. thermal cracking (via partial oxidation) is also performed. The model also helps to predict the behaviour of different waste types and is a useful tool for optimizing the design and operation of the process. The analysis is supported by experimental data from the pilot plant, and provides an overall picture of the process, as well as the main points of interest for the next stages of this research.
Massimiliano Materazzi

Chapter 6. Tar Evolution in the Two Stage Fluid Bed-Plasma Gasification Process

Waste gasification is considered a valuable solution to the production of clean energy and bio-fuels provided that the syngas produced in the gasifier is free of condensable tars and organic sulphur contaminants that cause equipment fouling and deactivation of catalytic stages downstream.
Massimiliano Materazzi

Chapter 7. Fate and Behaviour of Inorganic Constituents

To accelerate progress in the industrial use of RDF as an alternative fuel in gasification plants, the problems associated with hazardous solid residues and ash deposition must also be resolved. The practical approach to reduce these problems must be aimed at preventing the accumulation of fly ash/condensable vapours on heat transfer surface areas while minimising the amount of residual materials that have to be treated before the disposal. This Chapter presents a comprehensive examination of ashes sampled under different operational conditions and in different locations of the fluid bed-plasma demonstration plant. A demonstration test miming the normal commercial operation was conducted over 44 h of operation with RDF from a standard UK municipal solid waste. The results are presented according to solid samples composition, gas composition, and further specific data (e.g., enrichment factor, XRD analysis, leaching test, etc.). An investigation on pollutant removal from the hot syngas, focusing on the partitioning and chemistry of sulphur and chlorine along with other relevant components, is also carried out. Experimental trials revealed a reduced extent of alkali and metals availability in the gas phase, i.e. a minor deposit forming potential into downstream equipment. From 85 to 91 % of the fly ash was captured and vitrified within the plasma converter and made non-leachable with respect to non-volatile heavy metals, allowing for near complete landfill diversion.
Massimiliano Materazzi

Chapter 8. Conclusions and Future Work

Over the past thirty years, the major factor that has prevented the widespread uptake of advanced gasification technologies for treating municipal solid waste has been the presence of tars and char contaminants in the syngas product, which makes the gas unsuitable for power production or bio-fuel synthesis.
Massimiliano Materazzi


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