Figure 5. Temperature contour maps in the integrated combustion-reforming reactor in which combustion chambers are in direct thermal contact to reaction chambers for an endothermic steam-methanol reforming reaction.
The steam mole fraction contour maps are illustrated in Figure 6 for the integrated combustion-reforming reactor in which combustion chambers are in direct thermal contact to reaction chambers for an endothermic steam reforming reaction. For efficient operation of the steam reforming reaction, large surface areas are required to transfer the heat from the combusted gases to the tubes [41, 42]. In reformers presently used for steam reforming small diameter reaction tubes are clustered closely together in the furnace so that heat transfer from the combusting gases in the reactor into the catalyst packed tubes is optimized [43, 44]. The use of a plurality of tubes to accomplish heat transfer contributes to the large size and high cost of the reformer. In fuel cell systems in which the reformer and the fuel cell are fully integrated, namely the combustion gases for the reforming reaction are derived from the fuel cell exhaust, the shell side heat transfer coefficient between the hot gas and the tube is characteristically low and hence, the rate of reaction is limited primarily by the rate of heat transfer. This problem is particularly severe at the reactor entrance as the rate of the endothermic reaction is very high, and thus, the amount of heat required is very high while the shell side heat transfer coefficient is often low as the mechanical design of typical reactors often allows the gases in the shell to be relatively stagnant near the tube entrances. This leads to a drop in the overall efficiency as a large portion of each reactor tube operates at an undesirably low temperature. Thus, in order to effect complete conversion, the reformer must be relatively large and expensive. It is the primary object of the present study to provide a novel process and apparatus for the production of hydrogen by steam reforming of methanol that can be accomplished with a thermally efficient reformer of reduced size and cost which can be integrated with a fuel cell power system or used as a standalone hydrogen generator. Production of hydrogen by steam reforming of methanol or other hydrocarbon fuels is accomplished in a reformer of substantially reduced size by superheating a gaseous mixture of water and methanol to a temperature of about 548 K and then passing the superheated gaseous mixture over a catalyst bed contained in a reformer. At least a substantial portion of the heat for the endothermic steam-methanol reforming reaction is provided by the sensible heat in the superheated steam-methanol stream augmented by heat transferred through the tube wall depending on the overall system considerations. The concept of providing a substantial portion of the heat for the endothermic reforming reaction by sensible heat in the superheated steam-methanol stream is referred to hence forth as direct heating. Direct heating is of considerable advantage as it largely overcomes the problems encountered with reaction rates being limited by the rate of heat transfer through the tube wall especially near the reformer entrance and thus, for a given conversion, the reactor may be smaller, more efficient and less expensive. High steam-to-methanol ratios are required for direct heated reformers. The relatively large amounts of steam passed through the bed continuously clean the catalyst by removing ethanol and suppressing production of carbon monoxide and retard catalyst poisoning thereby enhancing catalyst stability. Direct heated reformers are particularly suitable for integration with fuel cells as the heat and fuel values contained in exhaust stream from each component can be utilized in the other. Hydrogen contained in the exhaust gas from the fuel cell anode may be burned to superheat the methanol-water mixture being fed to the reformer. Once the reactor is warmed up, the entire fuel requirements for the system are provided by the methanol being fed to the reformer. In order to accommodate the endothermicity of the reforming reaction, at least a major portion of the heat required for reforming is provided to the reformer as sensible heat contained in the superheated gases. Thus, when methanol and steam vapors contact a catalyst such as a combination of zinc oxide and copper oxide at a temperature of about 548 K at atmospheric or higher pressure, methanol decomposes to carbon monoxide and hydrogen and the carbon monoxide and steam react according to the well-known water gas shift reaction to form carbon dioxide and hydrogen. High exhaust gas temperatures may be indicative of unreacted fuel entering the exhaust chamber while low exhaust gas temperatures are indicative of a low fuel feed rate, or an unreactive catalyst bed. The efficiency decreased because the thermal losses as a percent of the total amount of power fed to the device increases as the size is reduced. In reactor configurations comprising solid catalyst particles disposed as a bed outside a plurality of heat transfer tubes, the layout of such heat transfer tubes is of critical importance, since it would be desirable to achieve a uniform temperature distribution across the radial direction of the reactor.