title: Intensification of catalytic process in premixed lean hydrogen/air combustion
Creators: name: Mondal, Md Nur Alam
Publisher: name: University of Glasgow
Year: 2025

description: The global energy use is moving towards hydrogen energy to make the energy sector clean, efficient and sustainable. Therefore, ambitious policies globally on hydrogen energy are made and aimed at transitioning away from fossil fuels to meet the goals of climate change through the Paris Climate Agreement. For this, moving to hydrogen fuel use is a feasible and promising solution, especially, to replacing the fossil fuel use in combustion systems. As NOx emissions pose a major challenge, further innovation and research are crucial for developing low/ultra-low NOx hydrogen combustion systems. In this regard, catalytic aided combustion widely investigated in the literature, is useful for efficient hydrogen combustion with reducing significant NOx emission at low temperatures. However, given the limitations of noble metal use and its high cost, fundamental research is required to optimise the catalytic reactor design with minimal use of expensive catalysts.

The present study begins by numerically investigating the premixed combustion of H2/air over a platinum catalyst in a planar monolithic reactor (a block with parallel channels resembling a honeycomb structure), with the goal of stabilising the flame under lean to ultra-lean conditions. A steady laminar species transport model is initially used, incorporating elementary heterogeneous and homogeneous chemical reaction schemes, and the results are validated against experimental data. A stability map for the equivalence ratios (φ) of 0.15 to 0.20 is obtained from a non-catalytic burner, forming the basis for the catalytic flame analysis. In the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at a Reynolds number (Re) below 571 for φ = 0.18 and below 381 for φ = 0.20. Additionally, a significant amount of unburned hydrogen exits the reactor in all cases. However, with a Pt catalyst coated on the walls, complete H2 combustion is achieved for 0.10 ≤ φ ≤ 0.20, with gas-phase (homogeneous) reactions becoming more prominent at higher Re. In addition, the superadiabatic temperatures are observed close to reactor walls in all studies cases. Moreover, wall radiation and inlet conditions influence combustion kinetics and flame temperature. Under the same conditions, NOx emissions increase with equivalence ratio but are negligibly affected by the inflow Reynolds number.

Given the high cost and scarcity of noble metal catalysts, this study also focuses on a numerical investigation to determine the best way of coating a platinum catalyst inside a catalytic hydrogen reactor. Various planar and non-planar reactor configurations are examined, and the results show that a reactor combining half and full cylinders is the most effective in achieving better H2 conversion. Compared to an equivalent planar catalytic reactor, this non-planar configuration improves H2 conversion by 30.7%. The findings suggest that enhancing mass and heat convection significantly boosts H2 conversion. Moreover, the contours of flow parameters and temperatures reveal that cylinders inside the reactor significantly affect the flow near the catalytic surfaces and have benefits in reducing the intensity of super adiabatic temperatures. Additionally, non-planar reactors, with surfaces of improved mass and heat transfer, can achieve up to 50% catalyst savings while maintaining a conversion rate of 2 kg/s per unit of catalytically-coated surface area.

To simulate a realistic catalytic process, Large Eddy Simulations (LES) have been conducted. This represents the first attempt at LES modelling for catalytic monolith reactors to predict catalytic reacting flows. A premixed hydrogen/air mixture at a fuel-lean equivalence ratio of 0.15 and an incoming Reynolds number of 3500 is used for analysis. Both planar and non-planar reactors are studied and compared under the same conditions and with the same platinum-coated surface area. The simulations employed a turbulent kinetic energy sub-grid model and the eddy dissipation concept to model the turbulent catalytic reacting flow. A discrete ordinate model is used to account for radiation heat transfer. The LES results, validated against experimental data, demonstrate that placing cylinders along the reactor enhances convective mass transfer, intensifies catalytic combustion, and enables efficient combustion with less catalytic surface area. Compared to planar models, non-planar reactors exhibited significantly higher H2 conversion efficiency along the reactor length, allowing for a catalyst savings of nearly 62.5%.

Finally, an experiment was conducted in a catalyst-packed bed tubular reactor to investigate the effect of varying catalyst content (0.3%, 0.5%, or 1.0% Pt in Al₂O₃ pellets and 0.5%, or 5.0% Pd in Al₂O₃ pellets) and catalyst loading (1.0 g, 2.5 g, and 5.0 g). The choice of experimental approach in a packed bed reactor is due to limitations of addressing the effect of catalyst material and amount of catalyst loading in numerical modelling. Measurements were taken in the packed bed reactor across the flow rates ranging from 1 LPM to 5 LPM. The results show that the packed bed with higher Pt/Pd content generates elevated combustion temperatures and demonstrates an effective catalytic performance. Moreover, the pellets with Pt/Pd content, even with a loading of 1 g at low flow rates, exhibit catalytic performance comparable to higher catalyst loadings at different flow conditions.