Research Doctorate - Doctor of Philosophy (PhD) / An increased demand worldwide for the reduction in pollutants emitted from coal-fired power stations has meant that advanced coal utilisation technologies are being sought as alternatives to pulverised fuel (pf) fired plants. The leading systems use coal gasification to produce a fuel gas which is cleaned and used in a combined-cycle gas turbine system. This produces electricity at high efficiencies and with significant reductions in the emissions of CO2, N- and S- gases and particulates. These systems offer the emission levels approaching those of natural gas combined-cycle plants, with the low fuel cost of coal. Modern coal gasification technologies operate with high temperatures and at pressures many times that of pf boilers: the reliability and efficiency of gasification-based systems are strongly influenced by the performance of the coal used under these conditions. The high-intensity nature of these processes means that generating experimental coal performance data for coal assessment and reactor design is time consuming, expensive or even impossible due to the lack of suitable facilities. Using process models based on fundamental gasification phenomena, coal performance can be predicted over a range of conditions. This is beneficial for both the development of new gasification technologies and in the assessment of Australian coals for use in the evolving international market. The slowest stage of the coal gasification process, i.e. the conversion of the char, has been identified as an important parameter for the design and implementation of such models. In particular, the intrinsic reactivity—characterised by data measured under conditions where chemical processes alone control the reaction rates—is extremely important, as intrinsic data can be readily combined with char physical properties to predict the high-temperature reaction rates of coal chars. The lack of intrinsic data generated at pressures relevant to modern gasification systems has meant that kinetic input into process models has been somewhat unreliable. In particular, there are no high-pressure reactivity data—intrinsic or otherwise—available for Australian black coals. To address this need, work in this thesis has used a pressurised thermogravimetric analyser to measure the effects of pressure (up to 30 atm) on the intrinsic reactivities to O2, CO2 and H2O of several Australian black coal chars, at emperatures between 350 and 900°C. These chars were made under a range of pressures and heating rates, and were in the size range 100 μm to 1.0 mm. In particular, the experiments were performed under conditions where chemical processes alone controlled the reaction rates, and where inhibition of the respective reactions by the products was negligible. It was found (using chars made in bulk at atmospheric pressure with slow heating rates) that whilst the reaction rate increased with reactant pressure in all gases, at pressures above approximately 15–20 atm the rates of reaction with CO2 and H2O ceased to increase with pressure. There was no such observation for the char–O2 reaction up to 16 atm. Activation energies of the reactions were unaffected by pressure. Samples made at high pressures and with high heating rates were found to be orders of magnitude more reactive than the chars made at atmospheric pressure under slow heating rates. These differences were found to be largely due to an increased microporous surface area, such that the intrinsic reactivities (calculated using the CO2 adsorption surface area) were similar. The effects of variations in pyrolysis pressure and parent coal petrography (and consequently char morphology) were also largely accounted for by char surface area, such that intrinsic rates were not greatly affected by these variables. These intrinsic reaction rate data were examined to produce a modified version of the nth order rate equation. This incorporated a pressure order that decreased as the reactant pressure increased, based on the physical process of available surface saturation. This model was compared with measured data and it was shown that the predictive capability of the nth order rate equation over a range of pressures was improved. There is scope for further refinement of this model by investigating the effects of reactant pressure on the development of the surface area of the char during conversion, since it was shown in this work that pressure has a strong effect on the such development. Moreover, this effect of pressure was not consistent between reactant and coal char type. This kinetic model was combined with measured char properties such as surface area, pore size, etc. to crudely predict the high temperature reactivity of a sample. This demonstrated the usefulness of reliable intrinsic data in the development of high temperature gasification models, and highlighted the need for experimental data obtained under process conditions of high temperature and pressure that can be used to validate such models.
| Identifer | oai:union.ndltd.org:ADTP/222121 |
| Date | January 2000 |
| Creators | Roberts, Daniel Geoffrey |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright 2000 Daniel Geoffrey Roberts |
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