- Title
- Molten carbonate recycle and recovery in direct carbon fuel cell
- Creator
- Moradmand, Simin
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The Direct Carbon Fuel Cell (DCFC) is an electrochemical device used to generate electricity at high temperature using different types of solid carbon in the presence of a molten carbonate electrolyte. DCFC’s show significant electrical efficiency and fuel utilisation compared to a traditional power plant, so could be a potential low emission replacement for conventional coal-fired power plants. Coal, as a cheap and abundant fuel, shows very promising efficiency to convert carbon to electricity in a DCFC arrangement. However, before employing this technology on an industrial scale, it needs to address some of the shortcomings associated with the application of coal as the fuel. Although there are many studies that investigate different properties of coal to find out their correlation to DCFC performance, studies focused on the influence of coal mineral matter on DCFC output is scarce. The impact of the coal mineral content on the DCFC performance has not been studied comprehensively. For instance, how the performance of the cell depends on the mineral concentration or how different types of minerals can affect the thermal properties of molten electrolyte is not known. This study considers all above literature gaps in DCFC application. Moreover, there is no understanding of the recovery of contaminated DCFC systems with coal minerals to control the level of mineral concentration in the cell with the lowest impact on the cell electrochemical performance. Alternatively, if the cell reaches the highest level of contamination and cannot be recovered in-situ, methods which can be applied to recover the system have not been investigated. Common coal minerals (kaolin, silica, alumina, sodium oxide) and coal derived minerals have been considered here. Their impact has been assessed on the electrochemical oxidation of a model carbon when they are added to the molten eutectic ternary carbonate (Li2CO3/Na2CO3/ K2CO3) electrolyte at different levels. The addition of dopants to the electrolyte can bring valuable information about practical DCFC operation if the slurry type of DCFC is used on an industrial scale. A slurry of carbon and electrolyte in DCFC configuration has previously revealed the highest power output compared to other setups and acts to mimic a DCFC operating with a continuous feeding system. Coal char is expected to be added as the fuel and its unused mineral concentration will accumulate in the cell. Finding the correlation between the performance of the cell with increasing mineral in electrolyte content is a focus of this work. Graphite has been used in this work as a model carbon source as it shows very good stability and performance reproducibility in molten carbonate media up to high temperatures. A three-electrode cell has been employed for electrochemical evaluation (OCP (Open Circuit Potential), LSV (Linear Sweep Voltammetry), and CA (Chronoampemetry) used extensively) to study coal mineral influence precisely without the interference of any unwanted reactions. Each dopant has been added to the molten carbonate at different levels to find out the highest allowable concentration of that mineral in the cell without a significant decrease in the cell current density. For instance,15 wt.% of silica dopant improved the current density about seven times compared to undoped electrolyte while increasing its concentration to 20 wt.% reduced further than an undoped electrolyte. The comparison of electrochemical performance of different minerals and coalderived minerals contaminated electrolytes indicated that silica impact on the cell performance is very similar to coal-derived minerals as expected due to the highest concentration of silica in the ash composition compared to other minerals. Since most of the minerals showed catalytic behaviour in electrochemical performance, it is proposed that oxide concentration in the electrolyte increases after the addition of minerals in the electrolyte due to dissolution in the electrolyte. This oxide increase improves the cell performance and this hypothesis was tested with YSZ (yttria-stabilized zirconia) concentration cell. YSZ concentration cell was designed to compare the oxide concentration in pure electrolyte and mineral doped electrolytes at different levels. The concentration cell confirmed that oxide released after the dissolution of minerals in the electrolyte at high temperature was one reason for the catalytic behaviour of dopant toward carbon electrooxidation. As well as it was found that the change of CO2 partial pressure can be another reason for impacting carbon oxidation in the electrochemical cell. It was suggested that the interaction between new compounds such as K2SiO3 or Na2SiO3 and CO2 was the main reason for the change in CO2 partial pressure as it was confirmed with the mass gain in the thermal analysis studies in presence of CO2 gas environment and lost weight in absence of CO2. The impact of kaolin as one of the coal minerals on the thermal properties of the melt has been examined using DTA/TGA (Differential Thermal Analysis/Thermogravimetric Analysis) tests. The results showed that kaolin after reaction with the melt, decreased the melting point of electrolyte at the concentration of 10 wt.% in the electrolyte and, after this point, the melting point increased due to its saturation in the electrolyte. The electrochemical improvement of kaolin resulted in a current density of 162 mA.cm-2 at 15 wt.% kaolin compared to 18 mA.cm-2 for undoped electrolyte. Kaolin resulted in catalytic activity towards carbon oxidation due to increasing oxide concentration in the melt after the dissolution of kaolin in the melt confirmed by YSZ concentration cell results. However, the silica doped electrolyte showed more intensive influence on thermal properties of the electrolyte, as observed by DTA/TGA studies, in terms of formation of more reactive compounds toward CO2 absorption as well as increasing the melting point of electrolyte by more than 17°C at 20 wt.% which led to irreversible change in electrolyte composition. The formation of different alkali silicates has been examined by different characteristics tests such as XRD, SEM, and FTIR. The electrochemical impact of different doped electrolytes such as silica, sodium oxide, alumina, and coal-derived minerals on carbon oxidation have been investigated in half-cell setup and YSZ cell concentration. The results showed that all dopants enhanced carbon oxidation depending on the type of dopant and its ratio in the electrolyte. Long-term (18 hr) of operation for 10 wt.% silica doped electrolyte reported the reduction of the current density after the first hour to less than one-third of the beginning which suggested the inhibitory behaviour of silica in the long-term operation which is not observed in short term studies. The removal of silica from the electrolyte is therefore necessary to maintain performance outputs. Another reason for selecting silica to remove from the electrolyte was its high concentration in the coal mineral composition, so its impact would be the highest. To remove silica from the electrolyte, the electrodeposition method at high temperature was chosen which is similar to DCFC operation temperature and doesn’t need to cool down the electrolyte to remove the impurities. Electrodeposition also generates value-added process outputs that can be used in different energy storage devices including traditional lithium-ion batteries. A silica doped electrolyte has been used in a three-electrode cell to undergo the electrochemical reduction experiments. Under the constant potential of -2 V vs C/CO2/CO3 2-, a mixture of products composed of Si/SiC/SiO2 has been generated from the electrolysis at 700°C under CO2 atmosphere as different characterisation tests confirmed. This successfully demonstrated that the electrodeposition method can be a good technique to remove silica from the electrolyte. The results from this work clearly demonstrates that different coal minerals have various impacts on both thermal and electrochemical behaviour of DCFC depending on the type of mineral, their concentration in the melt, and the extent of their interaction with the electrolyte. Although all of the minerals doped in the electrolyte enhanced carbon electrooxidation up to a certain level, the increasing mineral concentration beyond that level would decrease the current density and inhibit carbon oxidation which is likely occurring on the industrial scale of DCFC in long term operation. Therefore, finding an effective way to remove minerals from the system is vital to allow more reliable DCFC functions without disruption. It was shown here that removal of mineral impurity from electrolyte can result in the production of added-value materials depending on the selection method to remove minerals from the electrolyte.
- Subject
- direct carbon fuel cell; molten carbonate; coal; electrolyte
- Identifier
- http://hdl.handle.net/1959.13/1482393
- Identifier
- uon:50922
- Rights
- Copyright 2022 Simin Moradmand
- Language
- eng
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