Anode Recycle

The steam needed for reforming processes is delivered by looping in part of the anode gas. Usually, the amount of recycled gas is varied to achieve a constant steam-to-carbon ratio, and the recycle ratio is defined by the following equation  

Mainly, the re-circulation is forced by the ejector, in which the working medium is a fuel (methane) supplied to the system (Fig. 5.35). The presence of the ejector substantially restricts the range of possible operating conditions. 

The design and off-design performance evaluation of an anodic recirculation system based on ejector technology for solid oxide fuel cell hybrid system 

Gas velocities at anode side are relatively low, which means low pressure drops across anode channels. Pressure drop values of 1-1.5% have been found [18]. 

High pressure inside SOFC-M inereases fuel cell elficiency as well as the installation cost of air compressor and gas turbine devices. A technical-cost analysis provides the adequate pressure ratio for the SOFC-GT hybrid system. Most advanced SOFC-Ms work at pressures of around 9 bar, giving pressure drops across anode channels of between 0.09 and 0.135 bar. In those cases, the range of the pressure ratio of the ejector is from 1.01 to 1.015. These values are relatively low, meaning that theoretically it is possible to use a compressor/fan instead of an ejector. The temperature of the recycle fluid is high (800°C), which will probably present additional difficulties. 

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Figure 1-14 shows a simplified layout for an SOFC-based APU. The air for reformer operation and cathode requirements is compressed in a single compressor and then split between the unit operations. The external water supply shown in figure 1-14 will most likely not be needed the anode recycle stream provides water. Unreacted anode tail gas is recuperated in a tail gas burner. Additional energy is available in a SOFC system from enthalpy recovery from tail gas effluent streams that are typically 400-600°C. Current thinking is that reformers for transportation fuel based SOFC APUs will be of the exothermic type (i.e. partial oxidation or autothermal reforming), as no viable steam reformers are available for such fuels. 

This eliminates the need for steam to be supplied from another source. The anode-anode recycle also heats the fresh fuel feed to the designated anode inlet temperature. 

For recycling to improve the performance of an MCFC network, it must provide benefits that outweigh its inherent disadvantages. If carbon dioxide is not separated from the anode-anode recycle, the concentration of carbon dioxide in the anode is increased. This reduces the Nemst potential. The Nemst potential is similarly reduced by the anode-cathode recycle if steam is not condensed out, since recycled steam dilutes reactant concentrations in the oxidant. In addition, part of the power generated by the network is consumed by the equipment necessary to circulate the recycle streams. Such circulation equipment, along with the additional ducting required by recycling, also increases the capital cost of the MCFC network. 

Among three predominant modes of reforming, POX is more prone to coking, but SR is more difficult to operate and requires more equipment. ATR is a good compromise because of anode-recycle potential. A portion of... 

A large pressure drop along the gas channels can prevent local starvation conditions by dragging out liquid water from the gas pathways. This can be achieved by increasing the total volume flux, for example, by use of an anode recycle loop. Furthermore, sensitive control of the temperature profile can avoid condensation and therefore liquid water blockage within the cell [6]. 

Fuel, oxidant, and water delivery Natural gas compressor, anode recycle blower/compressor, cathode recycle blower/compressor, startup water pump, valves... 

The above results were for SOFC buttrui cells, and more work is needed to demonstrate direct-methane utilization in stacks. One step towards this goal is simulation of stack operation. Such results indicate that in a tubular stack with a barrier layer over the first 30 % of the fuel-flow field, the anode-gas composition is maintained in the thermodynamically non-coking regime. Furthermore, the direct-methane feed case yielded overall stack performance nearly as good as the anode recycle case [33]. 

Another novel system concept from the ECN project is the so-called SMARTER stack, shown schematically in Figure 7.20. In this stack configuration, two types of cells of slightly different geometries are embodied in the one stack. The two cells differ in that anode recycle is applied to one and single-pass operation is applied to the other. Again,... 

The advantage of the anodic recycling at a graphite electrode of Pd(II) is that it proceeds efficiently under atmospheric pressure of carbon monoxide, avoids the use of copper or halide ions and high pressure of O2 gas which also implies the formation of water and causes undesired side reactions. 

One of the first applications of this electrochemical methods, palladium catalyzed, was been the carbonylation of terminal alkynes. Terminal acetylenes were carbonylated imder very mild conditions to yield acetylenecarboxylates under atmospheric pressme of carbon monoxide at room temperature using palladium(II) catalyst in combination with its anodic recycling at graphite electrodes. [4]... 

An other application of the electrochemical method for the reoxidation of Pd(0), was been the efficient synthesis of N,N -disubstituted ureas. In this case, aromatic and aliphatic primary amines imdergo oxidative carbonylation under atmospheric pressure of carbon monoxide, using Pd(II) catalyst in combination with its anodic recycling at a graphite electrode. [12]... 

Electrolyze Desulfurized fuel lank Sam de box Anode recycle blower Boiler feed pump Water Irealment pump... 

Simple addition of a gas turbine subsystem to the SOFC-M raises efficiency to 57%. The efficiency increase is mostiy caused by the anode recycle process, which allows the cell to be worked at a low fuel utilization factor (21%) while simultaneously utilizing 75% of the fuel delivered to the system. The power generated by the gas turbine subsystem represents about 30% of the total system power. 

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