Integrated microdevices are increasingly being explored for application in distributed and portable power generation (1-4) due to the high energy density of fuels. Aside from meeting safety and stability requirements, these devices should also be fast starting, i.e., the time required for device warm-up from cold-start conditions to the operating temperature should be short. This is also desirable from an environmental standpoint to minimize emissions during startup. Two commonly employed startup strategies are preheating of the feed stream to a temperature higher than the ignition point and resistive heating. The aim of this work is to compare these two startup strategies for microscale devices at various operating parameters using a hierarchy of models, including pseudo-homogeneous 2D and full 2D computational fluid dynamics (CFD) simulations. Furthermore, the role of reactor solid structure in transient light-off is investigated.

Our results show a remarkably different behavior of ignition of microdevices from that of large-scale counterparts. When inlet preheating is applied, microburners with insulating walls ignite earlier, i.e., with a lower inlet feed temperature. The reason for the earlier ignition of insulating walls can be traced to their lower heat losses. When electric resistive preheating is employed, highly conducting walls require lower electric power for ignition. This dependence of steady state ignition characteristics on wall conductivity is rather weak. However, it has a strong effect on the transient time required to attain steady state. Highly conducting walls have a shorter response time than insulating walls. One reason for this behavior is their higher axial heat recirculation through the conducting walls. The heat capacity of the solid structure also affects the startup time. Higher heat capacity requires, as expected, longer time to reach steady state. The higher the inlet velocity, the greater is the amount of electric power required to preheat the gas to a certain (ignition) temperature. A good start-up strategy is to preheat the reactor to a desired temperature using resistive heating, and then start the flow. Finally, it was found that faster steady state is achieved with only part of the reactor is preheated. In this strategy, the effect of preheating location on the power required for ignition is rather small, but is significant on the time to steady state. Preheating of the front-end of the reactor is preferable.

References

1. J.D. Holladay, E.O. Jones, M. Phelps, J. Hu, J. Power Sources 108 (2002) 21-27.

2. S.R. Deshmukh, D.G. Vlachos, Chem. Engng. Sci. 60 (2005) 5718-5728.

3. J.C. Ganley, E.G. Seebauer, R.I. Masel, AIChE J. 50 (2004) 829-834.

4. A.C. Fernandez-Pello, Proc. Combust. Inst. 29 (2003) 883-889.