Reformer feed evaporation system

Delsman et al. analysed a 100-W methanol fuel processor/fuel cell system [4], It was composed of a microstmctured reformer/bumer heat-exchanger reactor for methanol steam reforming and anode off-gas combustion, several heat-exchangers, a cooled microstmctured heat-exchanger reactor for preferential oxidation of carbon monoxide, a heat-exchanger for feed evaporation and a low temperature fuel cell. 

At this point in time, the temperature of the reformer feed amounted to about 110 ° C and the reformate left the reactor at a temperature of220 °C. The full reformate flow was achieved after 30 s and the temperature of the reformate was then 500 °C. After 60 s the steam left the evaporator at a temperature of400 °C and the reformate outlet temperature was already 700 °C. The total mass of a future 50 kW fuel processor was estimated to be 55 kg, which corresponded to a total energy demand of 7 MJ for startup heating. From this value, the power demand of an air blower, which had to provide some 22 m min air to the system during start-up, was calculated to about 1 kW. As this power would have been required only during the rapid system start-up (60 s) a normal battery would have been sufficient as the power supply. An efficiency of 78% was calculated for the entire future fuel processor. 

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. 

The concept of the system was to vaporize/preheat a methanol/air mixture, combust it in a separate combustor and feed the methanol steam reforming reaction with the energy of the hot combustion gases (see Figure 2.75). light-off of the combustion gases occurred at 70 °C [115]. The combustion gases were further used subsequently to supply the fuel pre-heater/evaporator of the combustor and finally the fuel pre-heater/evaporator of the reformer. Unconverted methanol and water were removed in a separator from the reformate prior to GC analysis. 

A block diagram of the FP integrated with FC is presented in Figure 15.i. in this system the fuel is methanol, which is stored in a storage tank as an aqueous solution. Excess water in the feed is needed to reduce CO formation in the reformer and avoid dehydration of the FC. Methanol is evaporated in the vaporizer as all reactions in the FP-FC system take place in the gas phase. In the reformer methanol is converted into a hydrogen-rich gas at 250 °C over a Cu-based catalyst according to the endothermic steam reforming reaction. 

An obvious advantage of partial oxidation is that only an air feed is required, apart from the fuel. This makes the system simpler because evaporation processes, as required for steam reforming, are avoided. On the other hand, the amount of carbon monoxide formed is considerably higher compared with steam reforming. This puts an additional load onto the subsequent clean-up equipment, but only where CO-sensitive fuel cells are cormected to the fuel processor. When fuels are converted by partial oxidation, some total oxidation usually takes place as an undesired side reaction [46]. In practical applications, an excess of air is fed to the system and consequently even more fuel is subject to total oxidation. The water formed by the combustion process in turn gives rise to some water-gas shift. Another typical byproduct of partial oxidation is methane, which is formed according to reaction (3.5). Coke formation is a critical issue (see Sections 4.1.1 and 4.2.11). Coke may be formed by reaction of carbon monoxide with hydrogen ... 

The development of a methanol fuel processor prototype was described by Hdhlein et al. [556]. The methanol burner dedicated to this system has been described in Section 7.5. Later, a complete methanol reformer was developed by Wiese et al. [154]. It was operated at a S/C ratio of 1.5 and a pressure of 3.8 bar. The feed was evaporated and superheated to 280 °C. The reformer itself consisted of four pairs of concentric stainless steel tubes. In the annular gap between the tubes, steam was condensed at 65 bar and 280 °C for the heat supply, while the inner tube carried the copper/zinc oxide catalyst for steam reforming. The reformer response time to a load change from 40 to 100% was about 25 s, which was mainly attributed to the slow dynamics of the dosing pump. Because the dynamic behaviour of the reformer was too slow for an automotive drive system, which had been the target appUcation of the work, an additional gas storage system was considered. To improve the system dynamics, Peters et al. considered the application of microreactor technology for a subsequent improved fuel processor [569]. 

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