(219e) Improved Thermal Management of H2O2 Decomposition for a Compact SOFC Oxidant Source

            The
catalytic decomposition of hydrogen peroxide (H₂O₂) into
water (H₂O) and oxygen (O₂) [H₂O₂
→ H₂O + ½ O₂] is notoriously susceptible to thermal
runaway (Heat of Reaction: -98 kJ/mol). However, liquid H₂O₂ has
high oxygen density making it a particularly attractive oxidant for
applications where air-independence and space limitations elicit additional
power supply design challenges. Microchemical technologies have intrinsically
higher surface-to-volume ratios which facilitate heat and mass transfer, and
thus a mechanism to harness the benefits of H₂O₂ decomposition
as an oxidant source while reducing the risk of uncontrollable heat rise. This
study investigated two small-scale reactors to illustrate thermal management
and oxygen production feasibility during the multiphase decomposition of H₂O₂
as an oxidant for an unmanned undersea vehicle (UUV) solid oxide fuel cell
(SOFC). A miniature tubular, catalyst-coated (TCC) reactor model with channel
radius of 1.5 mm and a microscale packed bed (MPB) reactor model with channel
radius 0.5 mm were simulated in the finite element modeling program COMSOL. The
properties of the multiphase product stream were described by weighted averages
of the components in the stream. This approach assumed that the stream
maintained a well-mixed, dispersed phase. Heat generated during simulated
decomposition in the convectively cooled reactor channels caused temperature
rises greater than 400 K (neglecting the energy required to evaporate water). Although
small-scale channel geometry alone failed to prevent significant heat rise, the
extension of surface areas around the reactor channel improved passive cooling
resulting in simulated temperature rises less than 24 K (TCC) and 7 K (MPB) and
experimentally observed temperature rises less than 56 K (TCC) and 4 K (MPB).
Pulsating oxygen production, consistent with two-phase slug flow, was observed
in both reactors over a range of flow rates. Although slug flow was not
accounted for in the model, the time-averaged empirical behavior compared well
to the simulated behavior for the MPB. Thermal management of the MPB was found
to be superb and clearly superior to that of the TCC. Overall these results showed
the feasibility of improved thermal control and oxygen generation during H₂O₂
decomposition with a microchemical approach relative to conventionally
sized reactors.