Catalytic Combustion of H Y D R O G E N I--its Role in H Y D R O G E N Utilization System and Screening of Catalyst Materials

-The role which low-temperature catalytic combustion should play in the future hydrogen energy system is discussed in comparison with those of conventional flame-type combustion and fuel cells. A hybrid of catalytic and flame combustion is also referred to in the comparison. Screening of catalyst materials proved that several inexpensive oxides of the 3d transition metals were active at intermediate temperatures around 150°C. A volcano-like relation was observed between the catalytic activity and the heat of formation of metal oxides per gram-atom of oxygen. Platinum-group metals supported on a carrier were active enough to initiate hydrogen combustion even at temperatures below 0°C. The effect of hydrogen concentration and space velocity on combustion rate was also investigated. INTRODUCTION HYDROGEN has been proposed as one of the most desirable and clean forms of secondary energy that can supplement and replace fossil fuels in commercial, residential, and industrial utilization. However, our recent work has revealed that the conventional flame-type combustion of hydrogen produces large amounts of nitrogen oxides in a diffusion flame, while it suffers from the danger of backflash in a premixed flame where NOx emission level is very low [1]. Safe and clean combustion of hydrogen requires the solution for the hydrogen flame of this dilemma between NOx emission and flashback. In residential utilization of hydrogen, where high temperatures are not usually necessary, low-temperature flameless combustion over the catalyst surface appears to be the best solution, in consideration of the fact that hydrogen is much more readily oxidizable than any other fuels. The catalytic combustion technique itself has already been utilized, since it has been applied to space heaters with LPG in European countries [2]. Its application to hydrogen fuel was first proposed by Sharer [3], but not much experimental and developmental work has been published except for the fundamental work on platinum-plate catalysts [4,5], and the developmental work on catalytic-combustion appliances by IGT in the U.S.A. [6, 7]. Since 1975, we have been carrying out research and development work in catalytic combustion of hydrogen as one of the research programmes of a national project for hydrogen energy systems in Japan. We have been working on the development of inexpensive oxide catalysts and the fabrication of several prototype catalytic burners [8]. As the first publication of our still-continuing work, the present paper deals with the comparative evaluation of catalytic combustion techniques with flame combustion and fuel cells, and the results of screening of catalyst materials. Succeeding papers will cover the developmental work on composite oxide catalysts and fundamental studies for burner construction. C H A L L E N G I N G ASPECTS OF CATALYTIC COMBUSTORS IN H Y D R O G E N UTILIZATION SYSTEMS Hydrogen can be converted to thermal energy by combustion or to electric energy by fuel cells in industrial, commercial, and residential utilization. Conventional flame-type combustion has to cover the industrial demands for high-temperature heat, despite the problems of high NOx emission and the occurrence of flashback. In residential utilization, low-temperature catalytic combustion is more favorable than flame combustion because safety from fire hazards and elimination of pollutants are required. However, the heat output obtainable from low-temperature catalytic combustion is limited to a certain range and is small in comparison with flame combustion, since combustion takes place only on the surface of a catalyst.