Chemical Engineering Journal 167 (2011) 622–633 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Facing the catalytic combustion of CH 4 /H 2 mixtures into monoliths Stefania Specchia ∗ , Stefano Tacchino, Vito Specchia Department of Materials Science and Chemical Engineering, Politecnico di Torino – Corso Duca degli Abruzzi 24, 10129 Torino, Italy article info Article history: Received 15 April 2010 Received in revised form 22 September 2010 Accepted 5 October 2010 Keywords: Catalytic combustion Methane–hydrogen mixtures Monolith Microchannel Cordierite Silicon carbide Pd/LaMnO3ZrO2 catalyst abstract Catalytic combustion of methane mixed with hydrogen in micro-scale channels is becoming a key research topic, for various portable power generation devices that could replace current batteries to meet the increasing demand for more efficient, longer lasting and more environmentally friendly energy- consuming utilizations. The present work deals with the investigation on the performance of catalyst 2% Pd over 5% LaMnO 3 ·ZrO 2 (PLZ), lined on silicon carbide (SC) or cordierite (CD) monoliths, for the CH 4 /H 2 /air lean mixtures oxidation. The bare and coated monoliths were tested into a lab-microreactor designed to provide a favourable environment for microscale combustion of various CH 4 /H 2 /air lean mixtures to reach high power density (7.6 MW th m -3 ; GHSV 16,000 h -1 ). The main goal of the catalytic combustion tests was to select the best settings to achieve stable combustion conditions at the lowest possible tempera- ture, i.e., full CH 4 conversion with the minimum H 2 concentration in the reactive mixture, accompanied by the lowest possible CO concentration. Depending on the thermal conductivity of the tested monoliths, the existence of the steady-state mul- tiplicity was verified, mainly when the H 2 concentration was quite low. Basically, CD monoliths exhibited shorter ignition times compared to SC ones, due to the formation of spatially localized hot spots that pro- moted catalytic ignition. At the same time, the CD monoliths required shorter times to reach steady-state. But SC materials assured longer time on stream operations. The presence of the catalyst lined on both monoliths allowed reaching lower CO emissions. The best results belonged to the coated SiC monolith, with very low H 2 concentration in the mixtures. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In the recent years, there are remarkable efforts to progress in the field of microscale combustion, i.e., when employing a com- bustor characterized by a diameter smaller than approx. 1 mm [1,2]. The main driving force behind these efforts is the much larger energy density obtainable by using commercial fuels when compared with batteries [2,3]. Several works demonstrated the capability of combustion in microscale: some research groups started to build devices that use the microscale combustors or microburners in various applications, as microcomputers or microburners integrated with thermoelectric [4–7] or thermopho- tovoltaic [8,9] for combined power and energy applications, or micropropulsion [10,11]. During heterogeneous catalytic combustion, the residence time in the burner is a key parameter in determining the rate of species to the solid surface and the adsorption/desorption phe- nomena on the latter. Since in general the residence time will be small in microcombustors, it is important to have small chem- ical time scale to ensure completion of the combustion process ∗ Corresponding author. Tel.: +39 011 0904608; fax: +39 011 0904699. E-mail address: stefania.specchia@polito.it (S. Specchia). within the combustor [1]. In general, small chemical time scale are attained by high combustion temperatures; the latter can be achieved by using stoichiometric mixtures, by reducing the heat losses from the combustion chamber, by preventing radical deple- tion at the wall and using highly energetic fuels. In microscale catalytic assisted combustion, as the combustor size decreases, the attainment of the highest as possible internal surface available for catalyst deposition is a must. The surface increase can be obtained, e.g., by creating small size internal channels. As consequence, the surface-to-volume ratio increases, resulting in increased potential destruction of radical species at the walls and internal heat conduc- tion through the channel walls. These mechanisms will increase the chemical time scale and possibly prevent the onset of the gas-phase combustion reaction or lead to quenching of an ongoing reaction. Consequently, surface effects (interfacial phenomena [1] and time scaling [1,12]) turn out to be more important, and flame quenching (thermal quenching and radical quenching) becomes a problem [1]. The consequence could be incomplete combustion of fuels. In thermal quenching, flame extinction occurs when the heat of combustion cannot compensate the heat loss to the surround- ing environment through the wall; therefore, the wall acts as an enthalpy sink. In addition to the thermal quenching mechanism based on the heat losses, there is another extinction mechanism due to blowout [13,14]. Blowout occurs when a flame gets swept 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.10.051