CO Clean-up Preferential Oxidation

The main unit is the catalytic primaiy process reactor for gross production, based on the ATR of biodiesel. After the primary step, secondary units for both the CO clean-up process and the simultaneous increase of the concentration are employed the content from the reformated gas can be increased through the water-gas shift (WGS) reaction by converting the CO with steam to CO and H. The high thermal shift (HTS) reactor is operating at 575-625 K followed by a low thermal shift (LTS) reactor operating at 475-535 K (Ruettinger et al., 2003). A preferential oxidation (PROX) step is required to completely remove the CO by oxidation to COj on a noble metal catalyst. The PROX reaction is assumed to take place in an isothermal bed reactor at 425 K after the last shift step (Rosso et al., 2004). 

However, most fuel cell systems can tolerate methane concentrations up to at least 1% in the reformate, no special purification reactions are required. In contrast, hence, removing small residual amounts of carbon monoxide from pre-purifled reformate applying the methanation reaction may be considered as an alternative to the preferential oxidation of carbon monoxide, provided that the CO concentration is low enough to have no significant impact on the hydrogen yield. However, no applications of methanation for CO clean-up in micro structured devices appear to have been reported, hence the issue is not discussed in depth. Finally, during hydrocarbon reforming all hydrocarbon species (saturated and unsaturated) smaller than the feed molecule may be formed. 

Reduce the CO concentration to a level amenable to clean-up by preferential oxidation catalysts (fuel cell applications). 

The output of the reformer is typically fed to PEM fuel cells with operation temperatures of about 80 °C, hence the PEM feed needs special attention. The carbon monoxide content must be kept below 50 ppm so as not to adsorb on the PEM catalyst and lower the fuel cell efficiency. Therefore, a gas clean-up is needed at the exit of the fuel processor. Depending on the conversion route and the fuel - or in most cases the conversion temperature, which determines the water gas shift equilibrium between CO2 and CO - a complete fuel cell system needs a gas clean-up like a water gas shift reactor (reaction Equation 23.5) or a preferential oxidation reactor, i.e. selective oxidation of CO in the presence of H2 (reaction Equation 23.6) [21, 22]. 

Pan and Wang switched four adiabatic preferential oxidation reactors in series downstream of the reformer/evaporator described in Section 7.1.3 [546]. Heat-exchangers were installed after each reactor. The four reactors were operated at the same inlet temperature of 150 °C, and the O/CO ratio in the feed increased from 1.6 to 3, to minimise the heat formation in the first reactors. Despite these measures, the temperatures rise in the first reactor was as high as 121 K, in the second reactor it was still at 82 K and decreased to 28 K in the third and 8 K in the last reactor. While only 50% carbon monoxide conversion could be achieved in the first reactor, conversion was complete after the last stage. The combined steam reformer/clean-up system was operated for 24 h. The carbon monoxide content ofthe reformate could be maintained below 40 ppm. 

A methane or natural gas fuel processor with 2.5-kW thermal energy output was described by Heinzel et al. [17]. It consisted of a pre-reformer, which made future multi-fuel operation possible, the reformer itself, which carried a nickel catalyst [433], it was operated between 750 and 800 ° C, and had catalytic carbon monoxide clean-up. The preferential oxidation reactor was operated at an O/CO ratio of 3.5 [433]. A carbon monoxide content of between 20 and 50 ppm could be achieved during steady state operation. An external burner suppUed the steam reforming reaction with energy. The natural gas was desulfiirised by a fixed bed of impregnated charcoal. Figure 9.21... 

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