3. Results and discussion
The steam molar fraction contour plots in the combined parallel plate
heat exchanger-reactor are illustrated in Figure 1 for hydrogen
production by steam-methanol reforming. As to reliability and cost,
conventional industrial catalytic steam reformers have at least two
major disadvantages with respect to fuel cell use. First, because
conventional industrial steam reformers operate at very high
temperatures and pressure differentials, the reformer tubes that contain
the catalyst must be constructed of rugged, thick-walled portions of
expensive materials [33, 34]. Additionally, conventional industrial
steam reformers also tend to be quite large, which again impacts
material costs [35, 36]. Smaller steam reformers are designed for
use in fuel cell system applications. Such steam reformers employ
single-tube and multiple-tube designs. The smaller steam reformer
designs have at least two major disadvantages in fuel cell system
applications. First, current steam reformer designs tend to lack quick
start-up capability. Lack of quick start-up capability can be
problematic in some fuel cell applications, particularly where the
reformer is expected to have a relatively short duty cycle [37, 38].
Some current steam reformer designs utilize a multi-element burner, but
these burners do not adequately provide for quick start-up and lack the
flexibility to efficiently operate on multiple fuels, including for
example, natural gas, fuel cell anode exhaust or pressure swing
absorption off-gas. For example, in a fuel cell power plant a steam
reformer may be used to convert natural gas into a hydrogen-rich fuel
stream, and it is desirable to have a burner capable of operating on
natural gas and air, a reformate stream and air, and the fuel cell anode
and cathode exhaust streams. Second, as part of fuel processing systems
in fuel cell-related applications or merchant hydrogen production, for
example, current steam reformer designs are less than cost-effective
[39, 40]. For example, high-pressure burners and reformer vessels
increase the parasitic load on the fuel processing system due to
associated compressors, thereby decreasing efficiency and increasing
cost and complexity. Conversely, in merchant hydrogen production
applications, a low-pressure steam reformer vessel increases the fuel
processing system parasitic load because of the associated process gas
or syngas compressor that is required. In addition, current steam
reformer designs tend to be relatively complex, resulting in increased
manufacturing costs and reliability concerns. It is desirable for a
steam reformer to be able to start up relatively quickly, and to be able
to operate efficiently without adding undue complexity or cost. At the
same time, it is desirable for a steam reformer to be low-cost,
scalable, and compatible with a variety of fuel processing systems. The
design consists of multiple packed tubes, of small diameter, being
placed in intimate contact with a heat generating flame. The arrangement
leads to improved heat transfer and therefore chemical conversion.
However, the packed tube results in a significant pressure drop and the
author states the process is still heat transfer limited. Therefore, a
reactor design which minimizes the process side pressure drop and does
not suffer from heat transfer limitation is required. The fuel and
oxidant manifolds and associated distribution tubes may use a
shell-and-tube construction, for example, for low-cost manufacturing.
The burner may comprise an array of distribution tubes, such as a
hexagonal array, for example. The steam reformer of the present combined
parallel plate heat exchanger-reactor also employs shell-and-tube
construction that is amenable to low-cost, high-volume manufacturing.
The reformer tubes may be arranged in an array having a high packing
density, such as a hexagonal array, for example, in order to reduce the
size and cost of the steam reformer.