Figure 2. Schematic illustration of the highly exothermic reactor with a
catalytic single-channel reaction chamber. The thick arrows indicate the
direction of flow, whereas the thin arrows indicate the direction of
heat transfer. All external surfaces exposed to the surroundings are
subjected to convective and radiative heat losses. Symmetry allows the
simulation of only half of each system.
For the single-channel combustor, an orthogonal staggered grid is used
to perform these simulations, containing 200 and 80 points in the axial
and transverse directions, respectively. The grid is finer where the
gradients are steeper, at the entrance and in the vicinity of the active
catalytic surface. The solid wall is discretized such that the elements
at the fluid-solid interface have the same axial size as the
corresponding elements on the fluid sides, resulting in a 200 and 20
grid resolution in the axial and transverse directions, respectively.
For the heat-recirculating combustor, a non-uniform grid is used, with
elements near the catalytic wall being smaller than those at the
symmetry plane, in order to capture more accurately the sharp gradients
at the catalytic wall while minimizing the computational intensity. As a
result, the grid used contains a 260 and 160 grid resolution in the
axial and transverse directions, respectively. Prior to performing these
simulations, computations are performed using grids with varying nodal
densities to determine the optimum node spacing and density that would
give the desired accuracy and minimize computation time. The final grid
density is determined when the centerline profiles of temperature and
species concentration do not show obvious difference. Solutions obtained
with the above grids are reasonably accurate, and larger grid densities
yield no obvious advantage. The second-order upwind scheme is used to
discretize the mathematical model, and the semi-implicit method for
pressure-linked equations algorithm is employed to solve for the
pressure and velocity fields. The simulation convergence is judged upon
the residuals of all governing equations. Numerical convergence is in
general difficult. In order to assist convergence and compute extinction
points, natural parameter continuation is implemented.
The heat integrated reactor with multiple parallel channels is
illustrated schematically in Figure 3 for thermochemically producing
hydrogen from methanol by steam reforming. The heat integrated reactor
can be used to produce hydrogen for fuel cells for the production of
electricity. The reactor consists of two sets of flow channels: one
where the steam reforming reaction takes place and one where combustion
of the same fuel provides the heat necessary to carry out the reaction.
Heat is transferred across the channel walls, and the system results in
a compact configuration. The two sets of uniform parallel reaction
channels are separated by the walls that separate the combustion region
from the reforming region, and are in close thermal contact as to
facilitate the efficient transfer of heat from the combustion region to
the reforming region. In this manner, greater heat integration and
utilization is accomplished inside the reactor. The reactor operates in
the so-called co-current mode, namely combustion and reforming mixtures
flow in the same direction, and the combustion and reforming channels
are arranged alternately. Combustion takes place over a structured
catalyst comprising copper-oxide and zinc-oxide. Steam reforming is a
catalytic reaction and takes place over a structured catalyst comprising
copper and zinc-oxide. The washcoat for applying the combustion and
reforming catalysts is high surface area aluminum oxide, and it must be
in close contact with the channel walls to facilitate efficient heat
transfer. On account of the symmetry of the system, only two half
combustion and reforming channels associated with the surrounding walls
are modeled.