Operating micro-tubular SOFCs with wood-gas components

Solid Oxide Fuel Cells are known to be able to handle a large variety of different fuels. Because of the greenhouse effect the use of carbon dioxide neutral gases or liquids are of special interest. Wood-gas is considered to be an inexpensive renewable fuel. The gas, which is of major interest for our group, is generated by a fluidized bed steam gasifier and consists of various components such as 25 Vol-% carbon monoxide, 20 Vol-% carbon dioxide, 10 Vol-% methane, 2.5 Vol-% ethylene, 0.5 Vol-% propylene, 2 Vol-% nitrogen and the rest hydrogen (values in dry state). The water concentration of the original pyrolysis gas is about 35 Vol-%. Besides these main ingredients there are of course many impurities like dust, tars, ammonia, hydrogen sulphide and hydrogen chloride present in the product gas. Especially the last two ones may lead to degeneration of the fuel cell anode and must therefore be almost totally removed before feeding the gas into the cell. In order to reduce energy losses, hot gas cleaning systems are favoured. This, however, limits the possibility to reduce the impurity concentrations to very low levels. Therefore the aim of this work was to define the maximum acceptable output concentrations for the hydrogen chloride absorber also in combination with hydrogen sulphide, since for a micro-tubular SOFC there are as yet hardly any data available. To determine the influence of the hydrogen chloride on the performance of the fuel cell, different concentrations of this impurity were fed to the cell. Here, also the flow rate was changed while the electrochemical output was determined. In addition it was analysed if there was any effect when changing from pure hydrogen to the HCl containing fuel. This was investigated at 1123 K and 1173 K, which are the preferred working temperatures for our cells. Cooling down as well as heating up procedures were tested with cells between 1173 and 573 K. In a second series of experiments combinations of hydrogen chloride and hydrogen sulphide of variable concentrations were tested. As before, changing between pure hydrogen and the acid containing fuel at above given temperatures was analysed by determining the cell performance. In parallel to the above experiments, synthetic wood-gas was used for operating the micro-tubular fuel cell while monitoring the electrochemical output with time.