Mixed Heterogeneous-Homogeneous Combustion of Dodecane in a Parallel Plate Microreactor

摘要

Conversion of chemical energy to electrical energy using commercial thermoelectric (TE) devices typically yields efficiencies of ～1%. An alternative and largely overlooked approach is to use thermophotovoltaics (TPV) to convert IR radiation to electrical output. TPV devices offer potential benefits in size, efficiency and steady state output compared to TEs. Both platforms benefit from low pressure drop, heat recirculation and mild temperature gradients in the power harvesting section, however, they operate under very different conditions. TEs have shown promise to convert heat at relatively low temperatures (～500 °C) and achieve operating efficiency approaching 10%. To achieve a comparable operating efficiency in a TPV device, the combination reactor/emitter walls must be very hot (～1000°C) so that there is a sufficient flux of in-band (>0.5eV) photons to be harvested. For a low-power TPV device with 100W chemical input, reaching these conditions limits emitter size to ～10 cm~2. Confining reactions to catalytic surfaces can provide a stable reaction zone with a mild temperature gradient under conditions of high flow velocity and considerable heat loss. In the limit of highly confined heterogeneous dominant operation, heat is efficiently transferred to the reactor walls, however, small residence times (several ms) results in fuel breakthrough. On the other hand, purely homogeneous operation is a less effective means to transfer heat to the reactor walls and is susceptible to blowout. Increasing the limiting dimension increases the rate of homogenous reactions and numerical results have shown that combined heterogeneous-homogeneous (HH) reactions offers greater stability to blowout and extinction than purely homogeneous or heterogeneous reactions. Moreover, peak temperatures in HH combustion are lower than those encountered in purely homogeneous combustion, an important factor in increasing reactor and catalyst lifetimes while limiting the production of pollutants. It follows that HH combustion may provide a long, stable and complete reaction zone at temperatures ideal for the clean and efficient operation of TPV devices over a long lifetime. In this work, the HH combustion of lean dodecane/air mixtures in a parallel plate microreactor is examined. Control over the relative rates of homogeneous and heterogeneous reactions is achieved through varying the gap distance between the platinum-coated reactor walls. Conditions that provided high wall temperatures with mild gradients - suitable for TPV reactors - are indentified and examined on the basis of relative homogeneous and heterogeneous reaction rates and heat transfer rates.