Catalytic Combustion of Hydrogen--i I . an Experimental Investigation of F U N D a M E N T a L Conditions for B U R N E R Design

-The performance of catalysts in different forms was investigated for the design of a catalytic combustor with hydrogen fuel. The catalysts tested had dimensions of 150 x 150 mm, and consisted of a ceramic honeycomb impregnated with Pt, two Ni metal foams coated with Pd powder which differed from each other in pore size, and a ceramic foam coated with Co-Mn-Ag oxide powder. In the diffusive mode of operation, the Pd-coated Ni foam with larger pores exhibited the highest combustion efficiency. The ceramic foam with the oxide coating also provided smooth hydrogen combustion in the range 0.2-1.0 kcal cm -2 h -t. Combustion efficiency was improved by increasing the amount of premixed air and totally supplied air. Spot measurements of surface temperature and gas composition were carried out over the catalyst surface and the characteristic features of each catalyst were compared and discussed. INTRODUCTION Previous work in this series has shown the results of catalyst screening for low-temperature catalytic combustion of hydrogen [1]. It has been found that several inexpensive oxides of 3d transition metals, as well as platinum-group metals and their oxides, are active at intermediate temperatures around 150°C and that palladium metal supported on silica gel can initiate hydrogen combustion even at temperatures below 0°C. The present investigation was undertaken to see what catalyst structure is suitable for a catalytic burner with hydrogen fuel. Although the use of catalytic combustion in heating appliances has invoked much interest [2-7], little attention has been paid to the effect of the catalyst shape on the performance of a catalytic heater. Development of practical catalytic heaters requires the determination of optimum construction and the knowledge of operating properties. The shape of a catalyst body is assumed to be one of the major factors affecting the performance of a catalytic heater, because distribution of fuel over the catalyst body and diffusion of air to its surface against the bulk flow of fuel controls the operating properties in the diffusive mode of operation. In most commercial and prototype catalytic space heaters [2-5, 8], asbestos wool, ceramic wool, fibreglass or an aluminium sheet is used to support the catalyst. Unfortunately, information on the effect of catalyst structure is incomplete and no type of catalyst body other than a fibrous pad and a metal plate impregnated with platinum-group metals seems to have been used for a diffusive catalytic heater. Therefore, four types of catalysts (all non-fibrous) were tested. Their operating properties are described and discussed in the present investigation. EXPERIMENTAL The performance tests of four catalysts (dimensions 150 x 150mm) were carried out using 99.99% pure hydrogen and the apparatus shown in Fig. 1. A ceramic wool mat adapted to facilitate uniform dispersion of hydrogen was disposed opposite the front side of the catalyst. Combustion air dried by silica gel was fed to a premixing chamber concurrently through two tubes and to a gas outlet chamber tangentially through four tubes. Spot temperature and spot composition of gas mixtures on the front surface of the catalysts were measured with nine chromel-alumel thermocouples connected to a digital multi-temperature recorder (Yokogawa Electrics Works Ltd., Type 3874) and with nine microprobes for gas sampling connected to a gas chromatograph (Yanaco, Model G1800). The local spots for both temperature and gas composition measurements were numbered as shown in Fig. 1. Secondary air $ ~ c o s t t t Primary Secondary air air Fig. 1. Schematic diagram of catalytic combustion apparatus. A, ceramic wool mat; B, catalyst body; C, thermocouple; D, gas sampling probe. 729 730 M. HARUTA, Y. SOUMA AND H. SANO Table 1. Catalysts used in performance tests Catalyst Catalyst Catalyst Preparative Dimensions material support method (l x w x d) (mm) H2 comb. eff. at 1.2 kcal cm -2 h -1