CFD Simulations of Coupled, Countercurrent Combustor/Reformer Microdevices for Hydrogen Production

Abstract

Two-dimensional computational fluid dynamics (CFD) simulations are used to study spatially segregated, multifunctional, microchemical devices for hydrogen production. In particular, coupling between homogeneous propane combustion and catalytic ammonia decomposition on a Ru catalyst is studied in a microdevice consisting of alternating combustion and decomposition channels as a function of flow rate and materials conductivity in the countercurrent flow configuration. It is found that the high temperatures generated via homogeneous combustion lead to high conversions in short contact times and thus to compact devices. Different performance measures are evaluated to assess the operability of the device. Sufficiently high ammonia flow rates serve a dual purpose by lowering device temperatures and enabling the production of larger flow rates of hydrogen. Finally, it is shown that device operation is limited only to high-conductivity materials and fast ammonia flows.

Millisecond Methane Steam Reforming Via Process and Catalyst Intensification

“…The materials stability limit is defined as a reasonable, though somewhat arbitrary, upper wall temperature bound. In accordance with our previously published work [11,21], this is set to 1500 K. The breakthrough point is defined as 99% methane conversion. Fig.…”

Effect of flow configuration on the operation of coupled combustor/reformer microdevices for hydrogen production

“…An adaptive mesh with up to 200,000 nodes is used to obtain accurate solutions with minimal computational effort. Simulation details along with the reaction rate expressions are given in our previous work (Deshmukh and Vlachos, 2005).…”

“…A propane/air inlet flow velocity of 0.5 m/s (corresponding to a residence time of 20 ms, at room temperature, for a 1 cm long device; in reality the contact time is shorter due to gas expansion). This flow velocity represents a stable flame close to the inlet, and was chosen inline with our previous work on counter-currently coupled microdevices (Deshmukh and Vlachos, 2005). Obviously, the propane/air flow velocity is a key parameter in the stability of multifunctional devices and its effect is discussed later (see Section 7).…”

“…The coupling of endothermic and exothermic reactions can be achieved in a number of ways (see, for example, Deshmukh and Vlachos, 2005;Glöckler et al, 2004;Kolios et al, 2000 and the references therein). Traditionally, the counter-current mode of heat exchange has a better performance (Kern, 1950).…”

“…In our recent work, a spatially segregated mode of coupling was investigated in a counter-currently operating parallel plate microdevice using two-dimensional (2D) computational fluid dynamics (CFD) (Deshmukh and Vlachos, 2005), where ammonia decomposition on Ru was coupled with homogeneous propane combustion. The latter was chosen to enable high temperatures and complete conversions within short contact times while eliminating problems associated with catalyst deactivation.…”