(553c) Preliminary Studies on Catalytic Open Cells Foams Methane Combustion Recovered from Fuel Processor Systems

 

Giuliana Ercolino*, Carmen W. Moncada Quintero, Stefania Specchia^*

Department of Applied Science and Technology, Politecnico di Torino, CorsoDucadegli Abruzzi 24, 10129 Torino, Italy;

*giuliana.ercolino@polito.it, *stefania.specchia@polito.it

The vision of “hydrogen economy” and utilization of H₂ as an energy carrier will require increasing H₂ production by more than an order of magnitude of the current production levels. [1]. H₂ can be produced from a variety of sources. Fossil fuels may be used to produce H₂ by first converting their fuel value to gas phase by reaction with steam, oxygen or air (gasification/reforming) followed by H₂ enrichment and separation. Hydrogen is produced through reforming of fossil fuels followed by Water Gas Shift reactors to reduce CO up to 0.5 – 1%. Tolerable level of 10 ppmv CO for PEMFC’s Pt catalyst requires further cleaning step. There are several suitable methods to remove CO from synthesis gas. But for PEM-FC application, CO selective methanation (CO-SMET) is one of the feasible choice because of the limited available spaces and low operating pressures. CO-SMET produces methane that acts as inert in fuel cell and it is recycled back to the after-burner resulting in lower primary fuel consumption [2].

The idea is to burn the methane produced by CO-SMET by catalytic combustion. For this purpose, Pd-based catalysts supported on simple metal oxides, mixed metal oxides (such as spinels or perovskites) have been extensively studied. Nowadays, the attention is focused on ceramic open cells foams. These ceramic foams have some advantages with respect to the packed bed reactors, such as lower pressure drops, which allow low residences times, and high geometric surface area, that improves mass transfer properties increasing the effectiveness factors and enhancing radial convection. Good radial convection within the foam plays especially important role in the exothermic reactions by improving thermal stability through elimination of hot spots [3].

We investigated the catalytic methane oxidation on a series of ceramic open cells foams (OCF) of zirconia (Zir) and silicon carbide (SiC) with 3 wt.% Pd/Co₃O₄ obtained via solution combustion synthesis and wetness impregnation (WI) as catalytic phase. This catalyst was chosen based on our previous studies [4]. All coated structures were characterized by measuring the pressure drops and evaluating the heat transfer coefficients. The heat transfer coefficients across the OCF were estimated at different superficial velocities and temperatures. Each foam-based catalyst was tested towards the combustion of methane in lean conditions to evaluate the influence of the geometrical properties of the structures on the catalytic activity. During the tests, the reactor was fed with 100, 200, and 300 NmL min^−1 (equivalent to a weight hourly space velocity, WHSV, of 30, 60, and 90 NL s^−1 g_cat^−1, respectively) of a gaseous mixture containing 1 vol.% methane in nitrogen, with an excess of oxygen. Our previous studies shows that Zir OCF exhibit a better catalytic activity than SiC OCF. We can explain these results considering that the Zir OCF have lower overall heat exchange coefficients, thus in these systems the convective heat removal via the flue gases is favored. Heat management is a fundamental aspect of adiabatic or quasi-adiabatic systems, like the system analysed in this study. Zir OCF maintain a stable thermal behavior, thanks to the low thermal conductivity value, thus the heat of reaction is easily removed by convection, without influencing the thermodynamics of the exothermic reaction of combustion. Instead, for the SiC OCF the catalytic performance worse compared to those of Zir OCF especially at high temperature. Based on these considerations, in the aim of this work is arrange an optimal catalytic system by placing first a catalytic SiC OCF at the beginning of the reactor (to favor the ignition of the reaction, that is, the kinetics, especially at high WHSV), followed by a catalytic Zir OCF to favor the reaction at higher temperatures (to favor the thermodynamics).

References

[1] K. Liu, American Institute of Chemical Engineers 2010

[2] G. Ercolino et al., Appl. Energy 143 (2015) 138.

[3] G. Ercolino et al., Cat. Today 257 (2015) 67