- Title
- Coal oxidation at low temperatures: oxidation products, reaction mechanism and chemical kinetics
- Creator
- Wang, Haihui
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2002
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Low temperature oxidation of coal is a complicated phenomenon, involving oxygen transport in coal pores, chemical reactions and heat release. This phenomenon is not only the major hear source responsible for self-heating and spontaneous combustion of coal, but also a significant contributor to greenhouse gas emissions. The current work focuses on the development of a fast method for studying coal oxidation at low temperatures, examination of the characteristics of oxidation products, analysis of the reaction mechanism of coal oxidation and modelling of chemical reactions occurring in this process. An isothermal flow reactor facility was designed and constructed to investigate the characteristics of coal oxidation under various conditions. Equipped with a sensitive oxygen analyser and a micro gas chromatograph, this facility provides simultaneous measurement of oxygen consumption and production of carbon oxides, Experimental data show that oxidation products are present in both gas and solid phases. Gaseous oxidation products consist of CO₂ CO and H₂O. Products formed in the solid phase include single or double bond oxygenated complexes. The molar ration among H₂O, CO₂ and CO production is about 21:3:1 for a coal undergoing oxidation. During oxidation experiments, the production rate of CO₂ decreases sharply at the initial stage, followed by a more progressive decay, which the rate of CO production decreases gently with time. The ratio of CO₂ to CO production rates is initially high, but decreases with time to reach a steady state value. Analysis of the ratio of CO₂ to CO production rates at steady state suggests that, for oxidation of weathered coal at temperatures above 70°C, the direct burn-off reaction plays a major role in the production of carbon oxides. A method is subsequently developed to determine the activation energy for the direct burn-off reaction. The decomposition behaviour of solid oxygenated complexes formed at coal surfaces is examined under pure nitrogen. The independence of the formation rates of carbon oxides of the sample particle sizes confirms that the thermal decomposition process is dominated by chemical kinetics rather than diffusion. It is found that an Elovich-type equation provides an excellent description for the production rates of carbon oxides, and the activation energies for the production of CO₂ and CO are 52.1 ± 6.0 kJ/mol and 72.0 ± 6.0 kJ/mol, respectively. This indicates two separate reaction pathways for the decomposition of the solid oxygenated complexes. It is also found that even unoxidised coal incorporates oxygenated complexes in its structure. These species commence decomposition at temperatures between 50 and 70°C. Comparison between the liberated carbon oxides from coal oxidation and from thermal decomposition of oxidised coal at the same temperature suggests two parallel reaction sequences contributing to the emission of carbon oxides during coal oxidation. The differences in the rates of CO₂ and CO production during oxidation of coal samples at two levels of water content indicate that water participates in the chemical reactions in the coal oxidation process. Detailed reaction steps occurring in coal oxidation are proposed to explain the observed behaviour. Based on the current understanding of reaction mechanism and the active sites theory, a kinetic model is developed for the prediction of the rates of oxygen consumption and formation of carbon oxides. The model serves to gain additional insight into the oxidation process. The rate expression for oxygen consumption developed in the thesis provides more accurate description of the oxidation kinetics than is presently included in the Frank-Kamenetskii analysis of self-ignition of solids.
- Subject
- coal oxidation; reaction mechanism; chemical kinetics; low temperatures
- Identifier
- http://hdl.handle.net/1959.13/1321558
- Identifier
- uon:24393
- Rights
- Copyright 2002 Haihui Wang
- Language
- eng
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