PROCESS FLOW DESIGN OF THE THERMOCHEMICAL CONVERSION OF FOOD WASTE IN GASIFIER
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CHAPTER ONE
1.0 INTRODUCTION
1.1 BACKGROUND OF STUDY
As development countinues to grow, every country try to look for the best way to dispose refuse, especially the food waste. There have being so much discussion concerning the research topic especially in the developing countries. most of the waste found in the soil are mainly plastic waste, the growing need for the conversion of food waste to other form of enery is the major problem facing most of the developed countries.
Many cities are confronted with the problem of how to discard large quantities of municipal solid waste (MSW). Currently, landfills are the primary means of MSW disposal taking in approximately 60% of the residential garbage generated in the US (SCS Engineers, 1992). However, rising landfill tipping fees and their proven negative environmental impacts (Denison, 1996; Miranda and Hale 1999), have led to the search for cleaner and less costly alternatives for municipal waste disposal. High temperature energy recovery from MSW, known as waste-to-energy (WTE), is one such alternative. Waste-to-Energy reduces the amount of materials sent to landfills, can prevent air/water contamination, improves recycling rates and lessens the dependence on fossil fuels for power generation. The two most commercially viable forms of large scale WTE are combustion and gasification. Combustion is a well-established practice, while gasification is still in its early stages as a large-scale commercial industry. The purpose of this study was to assess MSW gasification technology as an alternative to combustion and also to examine its potential role in a zero-emission waste-to-energy (ZEWTE) process.
1.2 STATEMENT OF PROBLEM
Most developed countries are still Faced with the costly problem of waste disposal and the need for more energy, a growing number of countries are turning to gasification, a time-tested and environmentally-sound way of converting the energy in MSW into useful products such as electricity, fertilizers, transportation fuels and chemicals. On average, conventional waste-to-energy plants that use mass-burn incineration can convert one ton of MSW to about 550 kilowatt-hours of electricity. With gasification technology, one ton of MSW can be used to produce up to 1,000 kilowatt-hours of electricity, a much more efficient and cleaner way to utilize this source of energy. Gasification can help the world both manage its waste and produce the energy and products needed to fuel economic growth.
1.3 OBJECTIVS OF STUDY
The aim of the research work is to:
- To use the gassification plant to manage waste and produce other forms of energy from it.
- Produce a true RDF cost-effectively remains one of the most difficult tasks in thermochemical conversion of solid waste.
- Remove tar from the gas product
- convert MSW that would typically be incinerated into a clean, useful syngas.
- reduce the need for landfill space, decreasing methane emissions from the decomposition of organic materials in the landfill.
- reduce the risk of surface water and groundwater contamination from landfills.
1.4 SIGNIFICANCE OF STUDY
The research work is a very important one as it will discuss in details the process involved in the conversion of food waste to other forms of energy, it will also discuss the use of gassification plant to manage waste and produce other forms of energy from it. It will also discuss the process of removal of tar from the gaseous product during the process of concersion of food waste to other forms of energy.
1.5 SCOPE OF STUDY
The research work is only limited to the process involved in the design of the thermochemical conversion of food waste in gassifier assembly plant.
1.6 METHODOLOGY
Gasification is a thermochemical process that generates a gaseous, fuel rich product. Regardless of how the gasifier is designed, two processes must take place in order to produce a useable fuel gas. In the first stage, pyrolysis releases the volatile components of the fuel at temperatures below 600°C (1112°F). The by-product of pyrolysis that is not vaporized is called char and consists mainly of fixed carbon and ash. In the second gasification stage, the carbon remaining after pyrolysis is either reacted with steam or hydrogen or combusted with air or pure oxygen. Gasification with air results in a nitrogen-rich, low BTU fuel gas. Gasification with pure oxygen results in a higher quality mixture of carbon monoxide and hydrogen and virtually no nitrogen. Gasification with steam is more commonly called “reforming” and results in a hydrogen and carbon dioxide rich “synthetic” gas (syngas). Typically, the exothermic reaction between carbon and oxygen provides the heat energy required to drive the pyrolysis and char gasification reactions. (EREN, 2002) The basic gasification reactions that must be considered are:
1) C + O2 CO2 -393 kJ/mol (exothermic)
2) C + H2O —– CO + H2 +131 kJ /mol (endothermic)
3) C + CO2—– 2CO +172 kJ/mol (endothermic)
4) C + 2H2—– CH4 -74 kJ/mol (exothermic)
5) CO + H20 ——CO2 + H2 -41 kJ/mol (exothermic)
6) CO + 3H2—— CH4 + H20 -205 kJ/mol (exothermic) (Krigmont, 1999)
All of these reactions are reversible and their rates depend on the temperature, pressure and concentration of oxygen in the reactor
1.7 LIMITATION OF STUDY
FINANCIAL CONSTRAINTS: The researcher was unable to get all the materials needed for the purpose of the research work due to insufficient finance but he or she was able to get the meaningful information required for the completion of the research work.
TIME CONSTRAINTS: The research being a students will be involved in other departmental activities like seminar presentation and other departmental assignment which made the researcher to be left with limited time but he or she was able to meet up with the time allocated for the completion of the research work.
1.8 DEFINITION OF RELATED TERMS
THERMOCHEMISTRY: is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling.
GASIFICATION: is a process that converts organic or fossil fuel based carbonaceous materials into carbon monoxide, hydrogen and carbon dioxide. This is achieved by reacting the material at high temperatures (>700 °C), without combustion, with a controlled amount of oxygen and/or steam.
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