Cellular ceramics in combustion environments

Efficient conversion of chemical energy into heat by combustion processes is very important for modern civilization. The demand for higher radiant heating rates has to be compatible with the tightened environmental regulations, which require minimal pollutants emission. Recently, several investigations have shown the advantages of the porous burner technology. The combustion takes place in a cellular material with a high thermal conductivity which transports the heat from the reaction zone upstream to the incoming gases. The preheating of the fuel/air mixture raises the flame temperature, reduces emissions and stabilizes the flame. Porous burner offer low emissions of CO, NOx and HC, high energy efficiency and high power density. Therefore combustion in porous media is of growing interest. Both metal and ceramic con-cepts are used for several applications. As an alternative to metallic burners cellular ceramics permit higher operating temperatures, more efficient heat extraction through the radiating porous surface and/or volume and potentially better performance concerning flashback safety. As a result of the increase of power density up to some MWm-2 caused by new designs, the requirements on the materials regarding thermal shock and operating temperature have also raised. But in spite of recent scientific and technical success in developing new or improved burner concepts, a lot of problems have remained to be solved for materials science. Most of these problems are connected with the long time stability of ceramics in high temperature combustion environments. This includes the special properties of cellular ceramics, e. g. mi-crostructure, inner surface and mechanical strength. Porous burners made of silicon carbide are currently under development for different applica-tions. Although the oxidation rates of foams are higher than those for bulk materials, SSiC is the most promising material. By lowering the inner porosity a significant decrease of the oxi-dation rate has been achieved. The task for future work is the advancement of microstructure to reduce inner porosity further. This allows the use of SSiC in long term applications such as afterburner in the SOFC system.