AN EXPERIMENTAL STUDY ON THE PERFORMANCE OF A REFORMER UTILIZING THE CATALYTIC COMBUSTION IN HIGH TEMPERATURE FUEL CELLS

Abstract

Stationary fuel cell power generation systems are composed of many subunits. A reformer, which is one of the parts of the systems, is a device that supplies hydrogen giving it, a great amount of influence on the overall efficiency of the fuel cell power generation systems. This study focuses on the ways to utilize the off-gas discharged from the fuel cell to increase the efficiency of the high temperature fuel cell power generation systems. Many combustible components in the off-gas exist. Therefore, the off-gas can be used directly as a heat source for the reformer. Meanwhile, due to containing inactive ingredients in the off-gas, the catalytic combustion is desirable to maintain a stable combustion in the lean fuel conditions. Through burning the off-gas assisted a catalytic combustor, the production of hydrogen occurs by directly transferring the generated thermal energy to the reforming reaction at that time. As the hydrogen being generated in this way is supplied to the fuel cell, the overall efficiency of the fuel cell power generation systems could be improved. A series of experimental studies were conducted in order to achieve the higher hydrogen yield of the reformer. First, reaction characteristics for each of off-gases were identified through catalytic combustion experiments. The reactivity of hydrogen was fast and strong, and the inhibition effect to catalytic combustion by water vapor were also confirmed. A mixing of pre-heated air and the off-gas was found to be an important problem. A concentric cylindrical-type reactor, which is coupled with catalytic combustor and reformer, was made for the experiments. Inside of the coupled reactor, fins were installed to promote the heat transfer rate from the catalytic combustor to the reformer. The installation of the fins increased the temperatures distribution in the reformer and brought about higher hydrogen production. The experiments were conducted in various operating conditions. The actual operating conditions were set as the fuel utilization in the fuel cell, the steam to carbon ratio within the reformer, the inlet temperature in a catalytic combustor, amount of air for combustion, a change to the amount of catalysts, etc. The temperature distribution in the coupled reactor was obtained, and the thermal behavior and the reaction characteristics of the reactor had been identified. The local heat transfer coefficients inside the coupled reactor were determined theoretically at the reaction temperatures. These values were used in the numerical simulation. By the semi-empirical analysis method which takes the temperatures derived from experiments as input values, the temperature distributions in the reformer could be predicted, these were in solid agreement with the experimental data. From the results of the sensitivity analysis to the operating conditions, the variation of the fuel utilization rate in fuel cells appeared as the factor that gave the most impact to the coupled reactor. The next important parameter was the inlet temperature of the catalytic combustor. All the experimental results showed that the hydrogen production corresponding to the temperature shifted about 30℃ from the thermodynamic equilibrium state. The results of this study suggests a number of possibilities for the utilization as an external reformer in the high-temperature fuel cell power generation systems. Overall, it is meaningful that the results of the experiments are utilized for the basic data for the design of and further improvements to the coupled reactor.