ABSTRACT
Coal combustion is responsible for the majority of electricity production in the United States. It is however, also the primary cause for carbon dioxide emissions, which contribute to global warming. With oil reaching its peak production in the near future, alternative fuel sources will be needed to meet the worlds growing energy demands. Coal is an abundant resource that has the potential to meet those demands. In contrast to coal combustion, coal gasification only partially oxidizes the coal to produce a syngas containing of hydrogen and carbon monoxide, which means less carbon dioxide emissions. Utilizing coal in gasification technologies is the key to using coal in a more environmentally friendly way. Coal utilized gasification technologies have a variety of different applications. These applications include production of synthetic natural gas, production of methanol, to converting the syngas to gasoline, or chemicals like ammonia or a more efficient method to produce electricity for power generation. There are some challenges associated with coal when trying to extract its energy. These challenges exist due to the impurities that are inherent in coal. These impurities get released upon combustion and gasification systems and cause corrosion and erosion which can lead to damaging of expensive equipment used in chemical processing plants. Therefore research is needed to address these challenges, in order to improve the gasification systems so they can become more efficient. One area of gasification technology that can utilize coal to generate useful products is fluidized bed gasification. Fluidized bed gasification is not as widely used as other gasification technologies in industry. This is because these systems have their own unique set of challenges associated with them. This research is focused on fluidized bed gasification with lignite as the design fuel. In this work a fluidized bed gasifier was designed, constructed, commissioned and optimized for hydrogen production. The design was based off of the literature and centered on the minimum fluidization velocity. Shakedown experiments were performed as part of commissioning the system. Experiments were run under combustion conditions, air blown gasification, oxygen blown gasification, oxygen combustion, and a hydrogen retort. A hydrogen rich syngas was produced, containing 58% hydrogen for the retort experiments and as high as 55% for oxygen blown gasification. This hydrogen rich stream was largely because of the water gas shift reaction that took place downstream of the gasifier. Along with these experiments, deposits from the impurities were formed under realistic conditions. The deposits were prepared and analyzed using scanning electron microscopy. The two methods which were used to characterize the deposits were morphology, which uses EDS to identify the atoms present in the sample, and point count (SEMPC) which uses a computer program to compare and classify the mineral phases present in the sample. Based on the results of the SEMPC analysis the mechanism from which the deposits formed was through viscous flow sintering. The atomic species most responsible for the sintering was found to be organically associated sodium and calcium in the lignite.
CHAPTER I
INTRODUCTION
1.1 Objective
The objective of this research is to learn about gasification systems by designing and constructing a fluidized bed gasifier with Lignite coal as the design fuel.
1.2 Scope
The scope of this project is design a fluidized bed for the gasification of lignite based on the literature. Once that design is determined to be valid, the system will be constructed and commissioned. The system will consist of a fluidized bed gasifier, along with post gasification cleanup systems. Gasification experiments will be performed and behavior of the system will analyzed including syngas composition, carbon conversion, cold gas efficiency, tar formation, and impurity generation.
1.3 Motivation
With Oil prices being higher than ever, there is a new interest in developing green/alternative energy technologies. Historically, compared to other types of alternative energy, petroleum derived fuels have dominated the energy market, and realistically will continue to dominate the energy market for the foreseeable future. Coal, like oil, is a fossil fuel and is the main source for the majority of the world’s electricity. Coal is an abundant energy resource that will play an important role in future energy requirements(Annual Energy Outlook 2011, 2011). The U.S has a large amount of coal reserves that will last for a couple hundred years. In fact, in 2010 the United States used coal to generate 46% of its total power, and reflects a 4.5% increase in coal usage from the previous year(World Energy Council, 2013). This makes the utilization of coal as an energy resource attractive due to its high abundance worldwide, and low cost. However, a lot of the coal in the U.S is lignite, which is a low ranked coal that has not been used as much for energy applications as its high rank counterparts. This is partly due to the inherent problems that are associated with lignite that prevent technology from being further developed for its use. Specifically lignite has a high moisture content, as well as large amounts of alkali metals namely sodium, that can cause a lot of fouling of equipment and other problems.(Matsuoka, Suzuki, Eylands, Benson, & Tomita, 2006). Coal presents some environmental challenges; it can create pollution and contribute to global warming by releasing carbon dioxide into the atmosphere. The utilization of coal is very versatile; coal can be used to produce chemicals, in electricity production, and the production of hydrogen. One of the technologies that can utilize coal in a more productive was is a process known as gasification. Gasification is a process in which turns any material with a high carbon content, into a gaseous fuel with a heating value. It is a process which is similar to combustion; however, gasification limits the amount of oxygen that is available to react with fuel source, which creates a reducing environment. Ultimately, the product gas in gasification is known as a syngas, and typically contains hydrogen and carbon monoxide. There are different types of coals, each one with its own unique properties. The unique properties of each coal present with it different problems and challenges that need to be overcome. Depending on the application that is desired, for example, the production of chemicals, hydrogen or electricity, as well as the availability, will determine the type of coal to be used. Once the type of coal is selected, this will dictate the type reactor that will be implemented to carry out the gasification process. Although the type of reactor does depend on the type of coal being used there is an underlying common problem that affects each type of gasification process in its own unique way that is the impurities in the coal. This work will show how gasification, fluidized bed technology, along with coal can provide cleaner and more efficient ways to utilize energy in the future. It will also provide a brief history on gasification, what gasification is, and what its applications are and address some of the problems that researchers are trying to overcome. As well go through actually designing, constructing and commissioning of the fluidized bed system, present and interpret the data and offer recommendations for future work.
