Investigations on Victorian brown coal as a fuel for power generation in direct carbon fuel cells
2017-02-27T05:19:23Z (GMT) by
Coal accounts for almost forty-percent of global power generation. Globally, coal is likely to retain a central role in power generation given its abundance and economic advantage over other fuels for the foreseeable future. However, current coal-fired power stations are inefficient (25-35% efficiency) and contribute to significant CO2 emissions. Therefore, there is a concerted effort to improve the efficiency of coal use, potentially resulting in reduced CO2 emissions. Amongst alternative coal-based technologies, research into the direct carbon fuel cell (DCFC) has gathered momentum over the last decade. This is largely due to the high efficiency and carbon capture and storage compatibility of this promising and novel technology. Current research efforts in the DCFC field include the trialling of various types of carbons, predominantly from coal and biomass derived fuels, and assessing the influence of fuel properties on fuel cell performance and operability. In addition, understanding carbon reaction and conversion mechanisms, long-term fuel cell operation, as well as the compatibility of critical fuel cell components with cell operating environments are all pressing issues for advancing this technology. The present work addresses some of these key areas of interest in this field, in the context of Victorian brown coal char as a fuel in physical-contact solid oxide electrolyte DCFC. The studies were mostly focussed on lanthanum strontium cobalt ferrite (LSCF), a mixed ion-electronic conducting (MIEC) anode for the cell with yttria-stabilised zirconia (YSZ) as the electrolyte and LSCF as the cathode. However, due to long term instability of the LSCF in fuel environments, an alternative anode was also investigated. Following a review of the desirable properties of solid fuels for use in a DCFC, the first investigation in this body of work directly addresses the influence of fuel-based properties on the performance of a DCFC. The results from DCFCs operated on Victorian brown coal are encouraging. A thorough characterisation and analysis of chars produced from the coal used has highlighted the contribution of inorganic species, inherent in the coal, to fuel reactivity and ultimately to fuel cell performance. These results were benchmarked against pure carbon in the form of carbon black. Subsequent investigation into extended cell operation revealed insights into potential sources of cell performance degradation. Through careful monitoring of cell performance via electrochemical impedance spectroscopy, a mechanism of carbon consumption contributing to loss of electrical conducting pathways was proposed. The state of the MIEC anode before and after cell operation was also investigated and showed that there were changes occurring to the anode phase relating to the coarsening of LSCF particles as well as minor phase instabilities. In addition, a phenomenon of power overshoots, not reported in any detail in the literature, during voltage-current density scans was observed and the influence of the fuel chamber atmosphere on this was evaluated. Noting the structural changes to the LSCF electrode over extended periods of operation in strongly reducing atmospheres, a new anode was fabricated and trialled in the DCFC. The anode, composed of nickel, Gadolinia-Doped Ceria (GDC), and YSZ, delivered promising stability and operability when using a demineralised coal char as the fuel. Ash accumulation at the anode has been proposed as a likely contributor to degradation in the cell performance with time in the case of raw coal char. In order to gain further insight into the role of coal impurities within the DCFC, carbon black was impregnated with various catalytic species (compounds of Ca, Mg and Fe) for a targeted investigation into the effect of these inorganic species on DCFC performance. Thermogravimetric analysis revealed effects of these catalytic elements on the reactivity (Ca > Fe > Mg) of the catalysed carbon fuels towards Boudouard gasification under a carbon dioxide atmosphere. The catalysed carbon reactivity translated into enhanced fuel cell performance in a similar order, supporting the relationship between carbon reactivity and cell performance identified in the earlier study. The research outcomes from this project have generated substantial knowledge in the field of fuel (Victorian brown coal) preparation and characterisation, DCFC operation, carbon oxidation mechanism, cell performance, and lifetime of critical cell components with Victorian brown coal as a fuel for power generation. The findings presented in this study are expected to contribute to the development of this technology for the operation of direct-contact solid electrolyte based DCFCs using solid carbonaceous fuels.