Molten Carbonate Electrolyte Based Thermocell for High Temperature Waste Heat Recovery

Abstract

Industrial processes for the production of metals and alloys by metallurgical and electrochemical methods generate a lot of waste heat due to irreversible losses. This waste heat could be used as a power source to produce electricity. An inexpensive molten carbonate electrolyte based thermocell demonstrated the possibility to use this waste heat as a power source. A thermocell is an electrochemical cell with two symmetrical electrodes placed in an electrolyte solution, but a difference in temperature is established between the electrodes. The temperature difference creates a potential difference between the electrodes by ion migration in the electrolyte, and electrical energy may be generated. The ion-conducting molten carbonate electrolyte with two symmetrical gas (CO2|O2) electrodes in the thermocell delivers a high Seebeck coefficient (~ -1 mV/K). It shows the advantage to harvest industrial waste heat at high temperatures and utilize the available CO­­2 rich off-gases from metal producing industries. In this thesis, the molten carbonate thermocell components were further optimized to enhance the Seebeck coefficient and power conversion efficiency, which was not studied systematically before.

First, the flow rate of the gas supply to the electrodes and content of solid oxide in the molten electrolyte mixture were optimized. The dispersion of solid oxide in the molten carbonate was found to reduce the heat flux and enhance the conditions for thermoelectric conversion. The change in Seebeck coefficient was reported for various ratios of eutectic (Li,Na)2CO3 and dispersed solid oxide MgO and for varying gas (CO2|O2) flow rates to the electrode/electrolyte interfaces.

The surface charge of the dispersed solid oxide plays a crucial role in thermocell behavior. So, the change in electrical and thermal conductivities of the electrolyte mixtures dispersed with a different surface area of MgO were measured. AC impedance spectroscopy technique was used to measure the electrical conductivity (σ) of the electrolyte mixture at 550 °C by constructing a conductivity cell with capillary electrodes. A simple heat flux DSC was used to measure the thermal conductivity (λ) of molten electrolyte with solid oxide mixture for the first time. The determined thermal and electrical conductivities and thermocell Seebeck coefficient (-1.7 mV/K) were used to estimate the figure of merit. The electrolyte mixture dispersion with a larger surface area of solid MgO provides a better (σ/λ) ratio and enhanced figure of merit (ZT) of 1.1. The ZT was comparable to the semiconductor thermoelectric materials.

The thermocell performance was also investigated with various selected solid oxides dispersed in the electrolyte mixture. The thermal and chemical stability of the dispersed solid oxides and the electrolyte mixtures were systematically analyzed. The solid oxides of Al2O3 and LiAlO2 showed a significant chemical reactivity to the carbonate melt compared to MgO and CeO2, and subsequent changes on the thermocell Seebeck coefficient was experienced. The electrolyte containing the MgO and LiAlO2 gave a Seebeck coefficient of -1.8 mV/K at Soret equilibrium (after 100 h), which are -1.6 (MgO) and -0.9 (LiAlO2) mV/K at initial time. Thus, the solid MgO with larger surface area offered the better conditions for the thermoelectric conversion and high chemical stability.

The thermo-physical and physicochemical properties of the electrolyte mixture may be tuned to reduce the liquidus temperature to ~ 400 °C in order to operate the molten carbonate thermocells below 500 °C (liquidus temperature of (Li,Na)2CO3). The multi-component (ternary and quaternary) carbonates mixtures were studied to achieve a low liquidus temperature, by mixing the molten (K and Ca) carbonate and LiF additives into binary (Li,Na)2CO3. Still, the Seebeck coefficient of the thermocells remains larger (-1.5 mV/K) for the multi-component carbonates electrolyte mixture.

In the above-mentioned preliminary experiments, a metallic gold was used as the current collector for the gas (CO2|O2) electrodes to avoid the formation of interference oxide layers during operation. Finally, for further reduction in energy generation cost, an inexpensive and stable alternative metal current collector was identified to replace the gold. The present compositions of the electrolyte mixture and electrode gas of the thermocell were analogous to the cathode side half-cell of the molten carbonate fuel cell (MCFC). So, in this study the suitability of the MCFC’s nickel-based cathodes to operate the molten carbonate thermocell was investigated. Thus, in this thesis a thermocell with non-critical and inexpensive molten carbonate-based electrolyte mixtures with reversible (CO2|O2) gas electrodes was demonstrated to recover the high temperature (> 400 °C) waste heat to produce electricity.