Evaluation of Fundamental Characteristics of High Functional Direct Carbon Fuel Cell Using Wood Pellet as Fuel

Abstract

After the Great East Japan Earthquake, because the power supply of Japan shifted from nuclear power generation to thermal power generation, the exhaust amount of CO2 as the greenhouse gas increased drastically. On the other hand, Japan has a lot of organic wastes such as a food waste and unused biomass. To solve these problems, we have been developing High Functional Direct Carbon Fuel Cell (HF-DCFC) composed of molten salts gasification system and tubular molten carbonate fuel cell (T-MCFC) since 2012. Molten salts gasification is a technology to produce H2 and CO for T-MCFC from organic wastes. Our previous study obtained the fundamental characteristics of HF-DCFC using soy sauce cake as fuel. Generally, because the operating condition of HD-DCFC depends on a fuel type, we examine the fuel diversity of HF-DCFC. Therefore, this paper aimed to evaluate the fundamental characteristics of HF-DCFC using wood pellet made of unused timber. Moreover, because the performance of T-MCFC depends on the electrolyte loading ratio of both electrodes, this paper also examined the optimum electrolyte loading ratio. In the results of the optimum electrolyte loading ratio, although the performances of the cell to which electrolyte of 45% or more was impregnated were very low under 650℃ of the cell temperature, it has been improved when the cell temperature was raised to 750℃ which was the operating temperature of HF-DCFC. Especially, the diffusion overpotential of each cell was improved greatly by the cell temperature rise. Generally, about three days are required so that the electrolyte is normally distributed to each cell component of a plate type MCFC. Although the distribution of T-MCFC was worse than that of a plate type MCFC because of the difference of the pore structure, it was improved by the decrease in the viscosity of electrolyte by the cell temperature rise. Consequently, we confirmed that the optimum electrolyte loading ratio was in about 25%. On the HF-DCFC experiment, the fundamental characteristic of HF-DCFC was evaluated by the cell voltage change when the wood pellet of 3g was supplied to HF-DCFC generating electricity by a constant current density of 100mA/cm2. The fuel gas utilization was changed from 40% to 80% to evaluate the influence of the gasification gas from the wood pellet on the cell performance. Although the cell power decreases rapidly immediately after the supplying of the wood pellet, thereafter it increases significantly. This tendency becomes large with the increase of the fuel gas utilization. The power descent immediately after the supplying of wood pellet originates to the influence of the air that mixed at the pellet supplying. The cell power descent immediately after the supplying of the pellet originates to the decrease of the partial pressure of hydrogen because of the combustion of air that mixes at the pellet supplying and the T-MCFC fuel gas. Thereafter, the cell power increases rapidly by the increase of fuel gas produced by gasification of the wood pellet. The ratio of the gasification gas in the anode gas increases so that the supply flow rate decreases as the fuel gas utilization increases. Therefore, this tendency increases with the increase of the fuel gas utilization. Consequently, we were able to confirm that HF-DCFC is effective enough as the distributed power source and the waste disposal equipment if wood pellet was supplied to HF-DCFC continuously. Figure 1