Modeling municipal solid waste gasification: molecular-level kinetics and software tools

Date
2016
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University of Delaware
Abstract
Municipal Solid Waste (MSW) is a valuable energy resource that is underutilized by today’s society. Waste-to-Energy (WTE) is a low-hanging fruit in a multifaceted energy landscape that incorporates conventional fuels and a plethora of renewable alternatives. From an environmental standpoint, WTE reduces the storage of MSW in landfills which can contaminate groundwater and release methane, a potent greenhouse gas. The most attractive WTE technology is gasification, a process where nonstoichiometric amounts of oxygen or air are fed to a high temperature reactor. The output from gasification is syngas, a ubiquitous product that can be used for a range of purposes, including liquid fuel synthesis and conversion to electricity via combustion. Plasma-arc gasification is an extension of conventional gasification that utilizes a plasma torch to obtain extreme reactor temperatures. The solid byproduct from plasma gasification is an inert vitrified slag, which is usable as a construction material. Plasma-arc gasification successfully utilizes the entire MSW feedstock, thereby removing the need for landfills. However plasma-arc gasification is a relatively new WTE technology, and there is a need to better understand the underlying chemistry in order to optimize process parameters. Molecular-level kinetic modeling has proven valuable in gaining insight on process chemistries ranging from naphtha reforming to biomass pyrolysis. To this end, this dissertation focuses on the development and application of a molecular-level kinetic model for MSW gasification. For model development, the MSW stream was divided into plastics and biomass. Kinetic models were constructed separately for the gasification of each of these streams, using literature data. These models were then combined to construct the MSW gasification model. This model was used to simulate a 1000 metric ton per day plasma-arc gasifier that was divided into three zones for MSW: combustion, gasification, and freeboard. The reactor model was utilized to study the effects of process parameters on syngas quality and tar formation. Increasing the relative oxygen flow to the bed was found to reduce tar formation at the cost of syngas quality. Variations in MSW composition affected the oxygen content in tar molecules but had little impact on syngas quality. Lastly, the localized extreme temperatures in the combustion zone had a potentially negative impact on both syngas quality and tar production due to the oxidation of CO. While studying MSW gasification, modeling approaches were developed for the depolymerization of both linear and cross-linked polymers that could be applied to other complex feedstocks and processes. In particular, this dissertation focuses on heavy oil resid pyrolysis. Resid pyrolysis is an attractive field for modeling due to recent advances in experimental techniques. This study highlighted the ability of molecular-level kinetic models to predict >50,000 molecules from detailed mass spectrometry measurements. Orthogonal to kinetic model development, this thesis focused on the construction of software tools. Software tool development highlighted the interface between molecular-level kinetic models and users. There are three types of users of kinetic models: model developers, research collaborators, and process engineers. Each user has their own goals while using the model. For instance, software tools for model developers or research collaborators might focus on organizing the incredible amount of information contained in a molecular-level kinetic model. To address this aim, one tool focused on the visualization of the reaction network to understand the network structure. In contrast, software tools designed for process engineers target measured inputs and outputs while abstracting the underlying molecular detail. This allows a process engineer, regardless of training in detailed kinetics, to reap the benefits of a molecular model and study the effects of operation parameters such as temperature or feed variation. These examples showcase the ability of software tools to increase the accessibility of detailed kinetics by molding the user-model interface to correspond to the user’s needs. This dissertation focused on gasification of waste, and culminated in a reactor model for the most environmentally friendly of WTE option: plasma-arc gasification. This work was then taken one step further by developing the software necessary to increase the accessibility of the model to a wider audience, ranging from process engineers to model developers. This accessibility makes the model not only fundamental, but also practical for industrial partners. Gasification and other WTE technologies have been, and will continue to be, a future topic of research. Undoubtedly, detailed kinetic models will play a central role in the future conversion of waste to energy.
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