Impact of Catalyst Quantity on Premixed Ultralean Hydrogen/Air Combustion in a Packed Bed Reactor

Catalytic combustion of an ultra-lean hydrogen/air mixture is a promising technique for generating heat with extremely low emissions, especially in low-temperature heating applications. A major challenge of this technique is the reliance on noble catalysts, which are both expensive and rare in nature. Therefore, minimizing the use of catalysts is essential for cost-effective catalytic combustion system designs. This work experimentally investigates the effects of varying catalyst contents and loadings in a packed bed tubular catalytic reactor. Active catalyst sites are applied to the surfaces of Al2O3 pellets that make up the packed bed. An ultra-lean premixed mixture of 2% H2 by volume in air is used for the catalytic combustion. The catalyst contents tested include 0.3, 0.5, and 1.0% Pt in Pt/Al2O3 pellets and 0.5 and 5.0% Pd in Pd/Al2O3 pellets. Catalyst loadings for both catalyst pellets were set at 1.0 2.5, and 5.0 g. Measurements were taken in the packed bed reactor across the flow rates ranging from 1 to 5 LPM. The results show that the packed bed with higher Pt or Pd content generates elevated combustion temperatures and demonstrates an effective catalytic performance. Additionally, the occurrence of superadiabatic conditions was observed, and hydrogen conversion rates were significantly influenced by the catalyst contents. Notably, the pellets with high Pt or Pd content exhibited catalytic performance comparable to higher catalyst loadings at different flow conditions, even with a loading of 1 g at low flow rates. However, increasing the catalyst loading affects the reaction mechanism, shifting it from kinetic control to diffusion control.