Plastic Mesoscale Swissroll Reactor
and Applications to Electric Power Generation
Jeongmin Ahn[1], Paul Ronney1, Zongping Shao[2], and Sossina Haile2
Recent experimental and theoretical studies of heat-recirculating reactors have demonstrated the importance of thermal conduction through the structure of the reactor on its performance. In particular, this solid-phase heat conduction inevitably degrades performance via transfer of heat out of the reaction zone to the surrounding structure, which is then lost to ambient. This in turn leads to a reduction of reaction temperature and thus sustainable reaction rates. By use of platinum-based catalysts in spiral counterflow "Swiss roll" heat-recirculating reactors, we have been able to sustain nearly complete combustion of propane-air mixtures at temperatures less than 150 ˚C using reactors built with titanium (thermal conductivity (k) of 7.1 W/m˚C). Such low temperatures suggest that high-temperature polymers (e.g. polyimides, k ≈ 0.2 W/m˚C) may be employed as a reactor material. With this motivation, a polyimide Swiss roll reactor was built using CNC milling and tested over a range of Reynolds numbers with propane fuel and Pt catalyst. Due to the limitation of CNC milling technique, this is not suitable fabrication technique for small-scale reactor. The primary limitation of this technique is that very thin-walled structures preferred for maximum performance are difficult to mill. A second, even simpler fabrication strategy developed was to fold very thin sheets of polyimides into free-standing spirals and glue them to a polymer base. For both the CNC-milled and the much smaller, folded reactors, continuous, sustained operation of these reactors at temperatures up to 450˚C were demonstrated. These temperatures are high enough to provide excellent single-chamber solid oxide fuel cells (SOFC) performance when using our ruthenium and rhodium anode catalysts. Moreover, the external temperatures of reactor were below 50˚C in practically all cases, which leads to minimal thermal signature and touch-temperature hazards. Reynolds numbers as low as 2-supported combustion, with thermal power as low as 3 watts and temperatures as low as 72 ˚C. These initial results suggest that polymer reactors may prove more practical for meso- or microscale thermochemical devices due to their lower thermal conductivity and ease of manufacturing. Applications to electric power generation via single-chamber solid oxide fuel cells were developed. With the single-chamber design, fuel/oxygen crossover due to cracking of seals via thermal cycling is irrelevant. Appropriate SOFC operating temperatures were maintained even at low Reynolds numbers (Re) via combustion of the fuel cell effluent at the center of the Swiss roll. Extinction limits and thermal behavior of the integrated system were determined in equivalence ratio - Re parameter space and an optimal regime for SOFC operation was identified. SOFC power densities up to 420 mW/cm2 were observed at low Re. These results suggest that single-chamber SOFC's integrated with heat-recirculating reactors may be a viable approach for small-scale power generation devices.