- Title
- The influence of common coal contaminants on the electrochemical oxidation of carbon in a molten carbonate electrolyte
- Creator
- Tulloch, John
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2013
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Fundamental and applied investigations of the electrochemical oxidation of carbon under direct carbon fuel cell conditions have been performed, with a focus on contamination of the carbon fuel and the molten carbonate electrolyte with common coal based impurities. This is of key importance for further development and understanding with respect to the implementation of a coal fuelled direct carbon fuel cell (DCFC). The DCFC is a high temperature fuel cell, with strong correlations to molten carbonate fuel cell (MCFC) and solid oxide fuel cells (SOFC). The DCFC shows great potential as an alternative energy generation technology to current practices, which would be strengthened were the technology able to operate on an economical carbon fuel source, as in coal. Historically and despite its inherent advantages, the DCFC has had comparatively little research focus when equated to its fuel cell siblings. However, in an age where energy production, environmental concerns and long term forecasts for economically viable fossil fuel reserves appear bleak, interest in energy technologies such as the DCFC are being rekindled. Theoretically, the DCFC can operate on unclean carbon sources such as coal, and furthermore, a review of DCFC focused literature has confirmed cases where small-scale cells have functioned on coal as a fuel feedstock. However, for such a fuel cell technology to operate competitively, a comprehensive understanding of the electrochemical processes occurring on the coal fuel type needs to be established so that cell power output can be maximised. Understanding the behaviour of the coal material under DCFC conditions has been identified as a key issue in this work, as this factor ultimately dictates the type of carbon material and its incorporated contaminants that are present at the site of the carbon oxidation reaction. Quartz, kaolin, pyrite and anatase have been identified as common coal contaminants that will retain most of their primary structure within an operational DCFC and as such were selected for further investigation. Furthermore, the surface functionality (which, according to literature plays a role in the electrochemical oxidation process) of a selection of coal materials has been shown to undergo significant variation on exposure to the thermal conditions of the DCFC. The surface, structure and composition of the selected coal materials was investigated through several analytical techniques and whilst a substantial difference was observed between the heat treated coal sub samples and their original material, there was no obvious trend between coal composition and the behaviours seen on the samples surface and internal structure pre and post the heat treatment protocol. The electrochemical oxidation reaction was studied on a selection of carbon materials through the use of a specifically designed electrochemical test cell. By confining the electrochemical carbon oxidation to a single well-defined carbon surface working electrode, the electrochemical results from the oxidation processes reflected those seen on the carbon surface, rather than those seen from the bulk solution from a electrolyte-carbon slurry. In order to confine the electrochemical oxidation reaction to a designated carbon surface the carbon materials were pelletised with specific additives to improve the conductivity and handling properties of the carbon pellets and then mounted into a specific working electrode design. Varying degrees of success were observed from this process, with materials such as graphite showing excellent pelletability with no additives, whereas biochar and activated carbon required substantial concentrations of additives to achieve a semi-stable carbon pellet. Furthermore, it was found that the carbon materials with a higher degree of amorphic like structure tended to give less reproducibility in the peletisation process. From these findings a graphite material was discovered to be preferable as a model carbon fuel source to study the electrochemical effects of contaminants on the carbon oxidation mechanism. Through the electrochemical test cell design, electrochemical effects of the common coal contaminants (identified to survive the coal baking protocol) were shown to have a range of effects on the electrochemical oxidation of the model carbon fuel used in the contamination studies. From the systematic introduction of the common coal contaminants to the model carbon fuel it was discovered that clay materials, such as kaolin and montmorillonite were identified to give the largest increase in the normalised current from the electrochemical test cell. Metal oxides and sulphides such as anatase, alumina and pyrite gave a limited increase in the normalised current, whereas quartz gave a significant decrease in the normalised current. The proposed oxidation mechanism for carbon was modelled by taking an electrochemical kinetic approach, and through the classical equation for electrode kinetics a series of expressions was derived for each step of the proposed carbon oxidation mechanism. Through the derived expressions the rate determining step(s) of the oxidation mechanism were determined on many of the contaminated series of electrodes. Furthermore, it was established that the rate determining step of the electrochemical oxidation process within the electrochemical test cell was not the same as that proposed by most literature; and moreso, the rate determining step of the mechanism depended significantly on both the applied potential to the electrochemical cell and the type and concentration of the contaminant within the carbon fuel.
- Subject
- coal contaminants; direct carbon fuel cell; DCFC; carbon
- Identifier
- http://hdl.handle.net/1959.13/1036046
- Identifier
- uon:13202
- Rights
- Copyright 2013 John Tulloch
- Language
- eng
- Full Text
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