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.