This study reports on the application of chemical looping combustion (CLC) in pressurized packed bed reactors using syngas as a fuel. High pressure operation of CLC in packed bed has a different set of challenges in terms of material properties, cycle and reactor design compared to fluidized bed operation. However, high pressure operation allows the use of inherently more efficient power cycles than low pressure fluidized bed solutions. This paper quantifies the challenges in high pressure operation and introduces a novel reactor concept with which those challenges can be addressed. Continuous cyclic operation of a packed bed CLC system is simulated in a 1D numerical reactor model. Importantly, it is demonstrated that the temperature profiles that can occur in a packed bed reactor as a result of the different process steps do not accumulate, and have a negligible effect on the overall performance of the system. Moreover, it has been shown that an even higher energy efficiency can be achieved by feeding the syngas from the opposite direction during the reduction step (i.e. countercurrent operation). Unfortunately, in this configuration mode, more severe temperature fluctuations occur in the reactor exhaust, which is disadvantageous for the operation of a downstream gas turbine. Finally, a novel reactor configuration is introduced in which the desired temperature rise for obtained hot pressured air suitable for a gas turbine is obtained by carrying out the process with two packed bed reactor in series (twostage CLC). This is shown to be a good alternative to the single bed configuration, and has the added advantage of decreasing the demands on both the oxygen carrier and the reactor materials and design specification.

in Wiley InterScience (www.interscience.wiley.com).The process of flooding has been examined with a single-channel fuel cell that permits direct observation of liquid water motion and local current density. As product water flows through the largest pores in the hydrophobic GDL, drops detach from the surface, aggregate, and form slugs. Flooding in polymer electrolyte membrane (PEM) fuel cells occurs when liquid water slugs accumulate in the gas flow channel, inhibiting reactant transport. Because of the importance of gravity, we observe different characteristics with different orientations of the flow channels. Liquid water may fall away from the GDL and be pushed out with minimal effect on the local current density, accumulate on the GDL surface and cause local fluctuations, or become a pulsating flow of liquid slugs and cause periodic oscillations. We show that flooding in PEM fuel cells is gravity-dependent and the local current densities depend on dynamics of liquid slugs moving through the flow channels. 2008 American Institute of Chemical Engineers AIChE J, 54: [1313][1314][1315][1316][1317][1318][1319][1320][1321][1322][1323][1324][1325][1326][1327][1328][1329][1330][1331][1332] 2008 Keywords: multi-phase flow, fuel cells, porous media, transport IntroductionPerhaps the greatest challenge facing fuel cells is the difficulty in maintaining stable operation and control due to flooding by liquid water. The build up of water produced at the membrane/cathode interface is known to limit the current output from PEM fuel cells. To describe the effects of flooding, several models have been proposed. [1][2][3][4][5][6] Most of these hypothesize that liquid water condenses in the pores of the gas diffusion layer (GDL) creating a mass transfer resistance for oxygen to get to the membrane/electrode interface as illustrated in Figure 1.Our group recently examined water permeation through the GDL and obtained results that contradicted the previous hypotheses about liquid flooding. 7 The GDL is a woven cloth or paper of carbon fibers that is usually treated with Teflon 1 to increase its hydrophobicity. We showed that water does not enter the GDL until a sufficient hydraulic pressure is applied to overcome the repulsive surface energy. The largest pores in the GDL are the first to permit water penetration, and once water penetrates the pores it can freely drain. Our results suggested a two-highway system for liquid and gas transport through the GDL, as illustrated in Figure 2. Liquid is driven by a hydraulic pressure from the membrane/cathode interface through the largest pores while gas moves from the gas flow channel to the membrane/cathode interface through smaller, but more plentiful, pores. Results that support these conclusions have also been reported using florescence microscopy to view the ex-situ transport of water through carbon paper. 8 Recently, water intrusion has been used to determine the capillary pressure vs. liquid saturation curves for different GDL materials 9,10 , providing pore volume distributions...

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A model fuel cell with a single transparent straight flow channel and segmented anode was constructed to measure the direct correlation of liquid water movement with the local currents along the flow channel. Water drops emerge through the largest pores of the GDL with the size of the droplets that emerge on the surface determined by the size of the pore and its location under the gas flow channel or under the land. Gravity, surface tension, and the shearing force from the gas flow control the movement of liquid in the gas flow channel. By creating a single large diameter pore in the GDL, liquid water flow emergent from the GDL was forced to be in specific locations along the length of the channel and either under the land or under the channel. The effects of gravity were amplified when the large pore was under the channel, but diminished with the large pore under the land. Current fluctuations were minimised when the dominant water transport from the GDL pore was near the cathode outlet. The results show that it is possible to engineer the water distribution in PEM fuel cells by modifying the pore sizes in the GDL.

in Wiley Online Library (wileyonlinelibrary.com).Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O 2 /N 2 ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O 2 /N 2 mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell.