The apparatus according to the present invention, including its advantageous refinements, is explained in greater detail in the following on the basis of the drawing of different exemplary embodiments using multiple diagrams.
FIG. 1 schematically shows the apparatus according to the present invention having a methanization stage having four cooling zones in section;
FIG. 2 shows the temperature curve as a diagram plotted over the run length x within the methanization stage when one cooling zone is used (related art);
FIG. 3 shows the temperature curve as a diagram plotted over the run length x within the methanization stage when four cooling zones are used;
FIG. 4 shows the temperature curve as a diagram plotted over the run length x within the four cooling zones;
FIG. 5 schematically shows two further embodiments of the flow guiding housing on the methanization stage in section (summarized in one illustration for the sake of simplicity); and
FIG. 6 schematically shows a further embodiment of the flow guiding housing on the methanization stage in section.
FIG. 1 schematically shows the apparatus according to the present invention for producing hydrogen in section.
This comprises a reformer stage 1 for converting hydrocarbon gas and water into hydrogen and further reformer products. The reformer stage 1, which has a reformer catalyst, is preferably implemented, as shown, as a steam reformer stage heated using a burner 9, in particular a gas burner, i.e., in this stage, for example, CH4 and H2O are converted into CO, CO2, and H2 while heat is supplied (by the burner 9) (endothermic reaction). In order to ensure the most uniform possible temperature curve within the reformer stage 1 and thus optimum hydrogen production, the reformer stage 1 is preferably implemented as a hollow cylinder, as shown.
Furthermore, the apparatus according to the present invention comprises at least one catalyst stage 2, connected downstream from the reformer stage 1, for catalytic conversion of the carbon monoxide, i. e., this is at least partially converted into carbon dioxide, which is harmless to the fuel cell. As in the reformer stage 1, the catalyst stage 2 is advantageously also implemented as a hollow cylinder. This measure results in a more uniform temperature curve and thus in better carbon monoxide conversion within the catalyst stage 2.
Finally, the apparatus according to the present invention comprises a methanization stage 3 connected downstream from the catalyst stage 2, which has axial flow through it and which, as noted, is used for the purpose of methanizing as much as possible of the residual carbon monoxide contained in the reformate gas using hydrogen. For temperature control of the methanization stage 3, a flow guiding housing 4 for a coolant, which extends in the axial flow direction, is assigned thereto. The methanization stage 3 is preferably also implemented as a hollow cylinder, as shown.
In order to ensure flow through the individual stages of the apparatus according to the present invention which is as free of pressure loss as possible, it is also advantageously provided that the reformer stage 1, the catalyst stage 2, and the methanization stage 3 are situated one after another in the axial flow direction. With a hollow-cylindrical implementation, is also advantageous for the stages to be situated one after another defining a continuous annular chamber in the axial flow direction.
It is essential for the apparatus according to the present invention for producing hydrogen that the flow guiding housing 4 has at least two, preferably three or more cooling zones 5, 6, 7, 8 having different cooling effects situated one after another in the axial direction.
In the embodiment shown in FIG. 1, the flow guiding housing 4 is divided into four cooling zones 5, 6, 7, 8, to each of which the coolant may be supplied separately. In principle, however, two zones are already capable of achieving the object defined at the beginning. The more cooling zones are provided, the more precisely may the temperature curve within the methanization stage be fixed, but the outlay for apparatus also becomes greater. Four zones have been shown to be a favorable selection here.
With a hollow-cylindrical implementation of the methanization stage 3, it has also been shown to be advantageous for the cooling zones 5, 6, 7, 8 to be situated alternately inside and/or outside the methanization stage 3 (see FIG. 6). In this case, the cooling zones 5, 6, 7, 8 preferably enclose the methanization stage 3 like annular chambers situated axially one after another or, with a hollow-cylindrical implementation of the methanization stage 3, are enclosed thereby (see FIG. 6 again).
As schematically shown in FIG. 1, it is also advantageous for each cooling zone 5, 6, 7, 8 to have at least one coolant supply connection 10 and one coolant removal connection 11, each cooling zone 5, 6, 7, 8 additionally advantageously being able to have coolant flow through it alternately in parallel flow (not shown) or in counterflow to the methanization stage 3.
In order to also implement optimum cooling which is adapted to the requirements, it is advantageous for different coolants to be supplied to the cooling zones 5, 6, 7, 8.
Furthermore, it is advantageous for a coolant which is used to be supplied alternately at different temperatures to the individual zones 5, 6, 7, 8 or, if different coolants are used, for these to have different temperatures, for example, by using heat exchangers (not shown).
FIG. 2 shows a temperature curve over the run length x (see FIG. 1) within a methanization stage, which only has one cooling zone (related art). As noted, carbon monoxide and hydrogen is converted back into hydrocarbon gas (methane) in the methanization stage in order to reduce the carbon monoxide component in the reformate gas. Since the methanization is an exothermic procedure, the temperature first rises in the stage and then falls because of the cooling to a value just below the entry temperature. With a construction of this type, the carbon monoxide content is typically approximately 120 ppm, i.e., too much to conduct the reformate gas directly to the fuel cell. As noted, an “air bleed” is therefore typically connected downstream from the methanization stage in order to also remove this component of carbon monoxide.
The cause for the still comparatively high carbon monoxide component in the reformate gas after the methanization stage has been shown to be that retroshift reactions, in which carbon dioxide and hydrogen react to form carbon monoxide and water, occur again and again because of the quite high temperatures at the outlet of the stage.
According to the present invention, as described, the methanization stage is divided into multiple cooling zones in order to lower the temperature toward the outlet of the stage in a targeted way so that the undesired retroshift reactions no longer occur. A corresponding temperature curve is shown in FIG. 3, which may be implemented if the cooling zone distribution according to the present invention is used. The temperature in the methanization stage thus falls with this achievement of the object continuously from 240° C. to approximately 220° C., with the result that, in particular at the end of the methanization stage, retroshift reactions may no longer occur, since the temperatures are too low for this purpose in the area of this cooling zone. The reference numbers 5, 6, 7, 8 and the dotted lines in FIG. 3 are to illustrate the area where the cooling zones are situated.
FIG. 4 illustrates the temperature curve within the individual cooling zones. It is particularly noticeable that because of the cooling in counterflow, a type of sawtooth profile arises, but the temperature peaks always fall again toward the outlet of the stage, from which the desired, falling temperature curve within the methanization stage may necessarily be concluded.
According to the two further embodiments of the flow guiding housing 4 of the methanization stage illustrated in FIG. 5, it is provided as alternative to the achievement of the object shown in FIG. 1 that the cooling zones 5, 6, 7, 8 situated one behind another in the axial direction are directly hydraulically connected to one another, but have different flow cross-sections. According to the present invention, a direct hydraulic separation of the cooling zones 5, 6, 7, 8 is not required, rather the heat transmission in the individual areas of the methanization stage may also be influenced in a targeted way through suitable selection of the axial flow cross-sections. In this case, a large flow cross-section means a low flow speed and therefore relatively poor heat transmission, or a small cross-section means a high flow speed and therefore quite good heat transmission; all also as a function of temperature gradient between coolant and methanization stage, of course.
Finally, it is advantageously provided according to the upper illustration in FIG. 5 that the cooling zones 5, 6, 7, 8 have stepped flow cross-sections to one another in the axial direction. Alternatively (lower illustration) continuously changing flow cross-sections are also provided, in both cases the cooling zones 5, 6, 7, 8 alternately being able to have coolant flow through them in parallel flow or counterflow to the methanization stage 3.
List of Reference Numbers
1 reformer stage
2 catalyst stage
3 methanization stage
4 flow guiding housing
5 cooling zone
6 cooling zone
7 cooling zone
8 cooling zone
9 burner
10 coolant supply connection
11 coolant removal connection
Patent Application
20080019884
