Figure 3. Chemical enthalpy profiles along the length of 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 sensible enthalpy profiles along the length of the integrated combustion-reforming reactor are illustrated in Figure 4 in which combustion chambers are in direct thermal contact to reaction chambers for an endothermic steam reforming reaction. The present design can provide numerous advantages. First, light-weight and compact energy sources can be obtained [37, 38]. Further, the rapid heat and mass transfer in a small device can enable the use of extremely active catalysts, catalyst which are active at low temperature, and catalysts with high throughput per volume [39, 40]. It is also possible to control process conditions, such as operating temperature, very precisely, so that high performance can be attained. The fuel combustion and steam reforming processes can be stably and efficiently operated at lower temperatures, without the need for energy input to sustain or even to start the micro-combustion process. In some instances, the micro-combustor is started with hydrogen or vapors such as methanol. Heat losses can be effectively controlled and reduced. Another advantage is that the simplicity of the design and the materials used enable mass production at competitive costs. Advantages can also include higher conversions, lower carbon monoxide selectivity, and simplicity of design. Another advantage that results from small size is better control of heating and uniformity of temperature in a reaction zone. Another advantage is an extremely fast response time, that is, a change in fluid flow can result in a nearly instantaneous change in temperature. Further, the micro-combustor or micro-reformer can be part of an efficient integrated system, which can reform lower hydrocarbons and even higher hydrocarbons that require higher processing temperature, such as butane. Carbon dioxide selectivity over carbon monoxide, a poison to fuel cells, of the steam reforming process is high, so that it is possible to avoid or reduce requirements for removing carbon monoxide after reforming and before supplying the gas to the fuel cell, thereby greatly simplifying the overall system and reducing system size. Since catalytic combustion is used, stable low temperature performance is easily attained for the combustor to provide uninterrupted operational heat for vaporizers and steam reformer units so they may operate in a steady optimum manner. The low temperature operation and manufacturing made possible by the design allows a greater choice of insulating materials, enables greater use of materials with dissimilar thermal expansion coefficients, and enables manufacture on semiconductor chips. The combustors and reformers can be made from plastic. There are numerous advantages of manufacturing in plastic including low weight and less required insulation. Additional advantages include: more design options, cheap high-volume manufacturing, eliminating the need for expensive manufacturing machines compared to the equipment used in silicon processing, and inexpensive materials. A thermal cycle is heating a device up to operational temperature, operating the device at an operating temperature and observing the results, and cooling the device to about room temperature. Thermal efficiency is calculated by dividing the lower heating value of the hydrogen in the reformats stream by the total heating value of the methanol fed the reformer plus the heating value of the fuel fed to the combustor. Volume of a combustor, combustion chamber, reformer chamber or reformer, unless otherwise indicated, refers to the internal volume where reaction substantially occurs but not adjacent material. Where a catalyst is present, the volume includes at least the catalyst volume and catalyst void fraction. Volume of a device, unless otherwise indicated, refers to the combustor and reformer volume and the volume of any intervening and integral components such as heat exchangers, preheaters, vaporization chambers, and recuperators. By directing the flow in this manner, a temperature gradient is established between the center and the outer edges of the catalyst bed with the highest temperature at the center of the thermally conductive transverse separator plate located between the two reactor chambers; thus, minimizing heat loss through the reactor walls. That a fuel combusts on the combustion catalyst means contacting a fuel with a solid catalyst, including within a porous catalyst or over a catalyst coating. The efficiency could be substantially improved by removing the thermocouple and by use of improved insulator materials such as metallized polyimide, and it is believed that with these improvements the devices can be thermally efficient. The steam may be introduced by direct injection of steam and by saturation of the feedstock by contact of the latter with a stream of heated water. The amount of steam introduced is preferably such as to give a steam ratio in the range 2 to 3.5 moles of steam per gram atom of methanol carbon in the feedstock.