Abstract:
The invention relates to a method and devices for the chemical reaction of liquid, vapor or gaseous hydrocarbons with either water or water vapor and either air or oxygen, to produce a hydrogen-rich synthesized gas for use in fuel cells. In the process, the chemical reaction of the reactants takes place on the catalytically coated surface of a material that is electrically conductive, and as a consequence of the feeding an electrical voltage, directly heatable and consequently temperature-controllable.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a method of and an apparatus for the chemical conversion of liquid, vapor or gaseous hydrocarbons.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to a method of and an application for the chemical reaction of liquid, vapor, or gaseous hydrocarbons with either water or water vapor and/or either air or oxygen, into hydrogen-rich reaction gases for use in fuel cells, among other devices.  
           [0003]    Fuel cells, particularly PEM fuel cells, are an interesting option for the provision of a decentralized energy supply and for use in vehicles. In fuel cells, hydrogen is converted in an electrochemical process with a high degree of efficiency directly into electric current. For various reasons, hydrogen is often not used directly when operating fuel cells, but is produced from gaseous (e.g., natural gas) or liquid (e.g., methanol, benzine, diesel, propane/butane mixtures) hydrocarbons in a chemical reaction taking place prior to the electrochemical process. In the process, the hydrocarbons, with one of: water or water vapor (steam reforming); air or oxygen (partial oxidation); or a combination of these two processes (auto thermal reforming), are turned into hydrogen-rich gases.  
           [0004]    In terms of the hydrogen yield, which is important for the overall efficiency of a fuel cell, steam reforming is superior to the other, abovementioned methods. For instance, methanol is, in the steam reforming process, ideally completely turned into carbon dioxide and hydrogen in the presence of a catalytic converter (nickel or platinum), in accordance with the following overall reaction. 
           2 CH 3 OH+2H 2 O→2 CO 2 +6 H 2 . 
           [0005]    Steam reforming is endothermic, and for methanol, takes place at approx. 300° C. [ 1 ]. If natural gas is used, the temperatures are around 700 to 800° C.  
           [0006]    A considerable disadvantage of steam reforming is its inherent bad cold start performance and its sluggish transitional properties when there are load variations, since the energy required for the reaction, e.g., using a burner, must be supplied from the exterior. In the process, the catalytic converter and the housing must first be brought to the required operating temperature. The required time, during which the reformer delivers a product gas with a composition unacceptable for the electrochemical process, depends on the thermal capacity of the materials to be heated up. So-called cold spots are a further disadvantage, in which, because of the low temperatures present, there could be less conversion, and as a result, formation of increased quantities of carbon monoxide as well as soot formation.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore desirable to eliminate, to the greatest possible extent, the previously described disadvantages that may incidentally also appear in more or lesser form in the other mentioned methods of hydrogen generation.  
           [0008]    In the present invention, gas proof or porous silicon carbide SiC is used as a catalyst carrier. Its advantageous thermo mechanical and electrical properties are, among other things, described in [ 2 ], [ 3 ], and [ 4 ]. Its high temperature resistance (in reducing atmosphere, up to 2000° C.), paired with a good thermal and electrical conductivity, stands out, in particular. By applying an appropriate electrical voltage, the latter is used to always ensure optimal thermal conditions either in the reformed or on the catalytically coated surface, preferably in a honeycomb shape. Depending on the operating state, the heat necessary for the chemical reaction can be partially or completely covered by providing Joule heat, that is, from the conversion of electrical energy into heat within the SiC matrix. In the following, this device will be called an electrocatalytic reformer (ECR) for short.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a better understanding of the present invention and show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show preferred embodiments of the present invention and in which:  
         [0010]    [0010]FIG. 1 shows schematically an electrocatalytic reformer (ECR) in accordance with the first embodiment of the present invention, in partial section;  
         [0011]    [0011]FIG. 2 shows a second embodiment of the present invention including a plurality of electrocatalytic reformer elements arranged in a reaction vessel;  
         [0012]    [0012]FIG. 3 shows a variant of the embodiment of FIG. 2, showing an alternative flow configuration;  
         [0013]    [0013]FIG. 4 shows, schematically, a circuit, including an electrocatalytic reformer in accordance with the present invention and a fuel cell stack; and  
         [0014]    [0014]FIG. 5 shows a further embodiment of a circuit including an electrocatalytic reformer in accordance with the present invention and a fuel cell stack. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 shows one possible embodiment of an electrocatalytic reformer (ECR) element, in accordance with the present invention. Here, the ECR element is made of either silicon or silicon carbide and is preferably shaped like a pipe and has inner and outer pipes.  
         [0016]    The silicon carbide SiC is porous and has a surface catalytically coated with at least one Group VIII metals of the periodic table (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt), as well as Cu and Zn and their combinations to promote the endothermal reforming reactions. In the present case, the porous pipe is an inner pipe indicated at  10  and is enclosed by a non-porous outer pipe  12 , that is a gastight pipe made of metal or ceramic, which can be catalytically coated on its outer surface with the Group VIII metals of the periodic table (Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt), as well as Cu and Zn and their combinations, to promote any exothermal reactions of the anode exhaust gas with air or oxygen, which take place on the outer surface thereof, as detailed in FIG. 2.  
         [0017]    The inner and outer pipes  10 ,  12  together define an annular chamber  14 . One end of the inner pipe  10  is closed with a contact plug  16 , secured to the inner part  10  and forming an electrical contact therewith. The annular chamber  14  has an inlet provided adjacent and around the contact plug  16 . At the other end, the annular chamber  14  is closed, and the inner pipe  10  forms an outlet  18 .  
         [0018]    Consequently, in use, reactants, e.g. hydrocarbon and water, preferably in vapor state, flow into the inlet of the annular chamber  14  around the plug  16 . The gas and vapor then flow along the annular chamber  14 , and progressively pass through the porous inner pipe  10 , to the inside of the pipe  10 . The catalyst on the pipe  10  promotes reaction to generate hydrogen and carbon dioxide as a synthesized gas, as detailed above. This synthesized gas then leaves through the outlet  18 .  
         [0019]    The coating on the inner pipe  10  is advantageously provided on the surface, which will give optimum performance. For this purpose, it can be provided uniformly over the matrix of the porous inner pipe  10 , so that the incoming hydrocarbon and water vapor contact the catalyst as they pass through the porous inner pipe  10 , and are consequently heated.  
         [0020]    The heat necessary for the endothermal reaction of hydrocarbon with water can be supplied in a number of ways. It can be supplied either through simple heat emission from a gaseous or liquid heat transfer medium flowing around the outside of the outer pipe  12 , together, optionally, with exothermal reactions taking place on the outer surface of the outer pipe  12 . Additionally, the contacts provided by the contact plug  16  and the outlet  18  can be used to pass an electric current through the inner pipe  12 , to generate Joule heat, released in the SiC matrix of the pipe  12 .  
         [0021]    Reference will now be made to FIG. 2, which shows a second embodiment of the present invention, which provides a tube assembly including a number of the ECR elements of FIG. 1, the ECR elements being designated at  20  in FIG. 2.  
         [0022]    The ECR elements  20  are contained within a vessel  22 . At one end, the vessel  22  includes a plate  24  defining a first manifold  26 . A first port  28  is provided for reactants or reaction components, opening into the first manifold  26 .  
         [0023]    Correspondingly, at the other end, a second plate  30  defines a second manifold  32 , having a second port  34  for synthesized gas. Here, the port  28  and manifold  26  form an inlet port and an inlet manifold, while the second manifold  32  and the second port  34  form an outlet manifold and an outlet port.  
         [0024]    The ECR elements  20  are mounted in the plates  24 , 30  and hence a chamber  36  is defined around the ECR elements  20 . An exhaust gas inlet  38  and an exhaust gas outlet  40  are provided, for forcing a mixture of exhaust gas and air through the chamber  36 .  
         [0025]    In use, the individual ECR elements  20  function as described for FIG. 1. Thus, a mixture of hydrocarbon and water is delivered through the first or inlet port  28  and through the first or inlet manifold  26  into the annular chambers  14  of the various ECR elements  20  as shown by the arrows  42 . The reactants then flow through the porous inner tubes of the elements  20  and out of the elements  20  into the second or outlet manifold  32  (arrows  44 ), as synthesized gas, so the synthesized gas then exhausts from the vessel  22  through the second or outlet port  34 .  
         [0026]    At the same time, to provide additional heat to the ECR elements  20 , either a stream of gas comprising hydrogen containing anode exhaust gas and air, or a hot exhaust gas from a catalytic burner, is delivered to the exhaust gas inlet  38 . This gas flows around the exterior of the ECR elements  20  and out through the exhaust gas outlet  40 . In the case of the gas flow comprising a combination of hydrogen containing anode exhaust gas and air, the outside of the ECR elements  20  can be provided with the catalytic coating mentioned above, to promote reaction of remaining hydrogen with oxygen from the air, both to consume the hydrogen and to generate additional heat to heat the ECR elements  20 . Where the gas delivered to the inlet  30  is from a catalytic burner, then any excess hydrogen will already have been consumed and heat generated, so that the gas should be at a higher temperature, suitable for heating the ECR elements  20 . Where the gas mixture of air/oxygen and reactive substances is provided, the reactive substances can include hydrogen, carbon monoxide and hydrocarbons. By causing reaction of remaining reactive substances to occur on the outside of the ECR elements  20 , this optimizes heat transfer of the generated heat into the ECR elements  20 .  
         [0027]    It will also be understood that the necessary electrical contacts would be made to the contact plug  16  and outlets  18  of the ECR elements  20 .  
         [0028]    Reference will now be made to FIG. 3 which shows an apparatus similar to that of FIG. 2, and for simplicity and brevity, like components are given the same reference numeral and the description of these components is not repeated.  
         [0029]    In FIG. 3, the flow of the reaction substances or components is reversed. Thus, here the second inlet  34  and second manifold  32  receive the incoming reactive substances, as indicated by the arrows  48 . The incoming reaction components then flow along the inside of the tubes  10  of the ECR elements  20 , through the porous SiC matrix to the outer annular chambers of the ECR elements  20 . The components react as before on a catalyst coated on the surface of the inner pipes  10 , to form the synthesized gas. The synthesized gas then exits from the annular chambers to the first manifold  26  and out through the first port  28 , as indicated by the arrows  50 . Thus, the first manifold  26  and the first port  28  forms an outlet manifold and an outlet port in this embodiment.  
         [0030]    In this embodiment, the catalytic coating on the pipes  10  can again be located to optimize performance. For this purpose, as described above the coating is provided uniformly over the matrix of the inner pipes  10 , so that the reaction components are heated, by passage through the SiC matrix, during which they contact the catalyst . Additionally, exhaust gases, as detailed above for FIG. 2, can be passed around the outside of the ECR elements  20 , to heat the outside of the elements  20 . This provides radiant heat that serves to heat the outside of the pipes  10 , further promoting the endothermic reaction of the reaction components.  
         [0031]    It is also noted that the invention is applicable not just to the tubular bodies described as examples but basically to any shaped bodies, for examples, plates or blocks that the reactants pass over and/or flow thorough. In porous bodies, the catalytic coating may cover the entire porous surface.  
         [0032]    For reasons of efficiency, when using an ECR, it should be ensured that the heat needed for the chemical reaction is made available only as a small, additional portion by supplying Joule heat, i.e. from the conversion of electrical energy within the heat derived from the SiC matrix. This can be ensured through an optimized heat integration of the reformer into the entire process and the implementation of dynamic energy management.  
         [0033]    Reference is now being made to FIG. 4, which shows, schematically, a circuit incorporating a fuel cell stack and an electrocatalytic reformer in accordance with the present invention. The circuit or arrangement is indicated generally by the reference  60 , and includes an electrocatalytic reformer  62  and a fuel cell stack  64 . A process control computer  66  is connected, directly or indirectly, to various components of the circuit, for control thereof, and the function of the computer  66  is detailed below.  
         [0034]    A water supply  68  and a methanol supply  70  are connected to a pump  72  . In known manner, appropriate valves, pumps and the like would be provided to ensure delivery of required water and methanol, or other hydrocarbon, at suitable pressures and flow rates. The pump  72  is controlled by a load signal indicated at  74 . The water and methanol mixture then pass through a flow meter  76 , to a heat exchanger  78 , in which the water and methanol are heated.  
         [0035]    From the heat exchanger  78 , the water and methanol flow to the electrocatalytic reformer  62 , which is indicated schematically includes a plurality of ECR elements  20 . As detailed above, within the electrocatalytic reformer  62 , the water and methanol are reacted to form a synthesized gas comprising mainly hydrogen and carbon dioxide. The synthesized gas then passes to the anode of the fuel cell stack  64 . In known manner, an oxidant would be supplied to the cathode of the fuel cell stack  64 .  
         [0036]    Exhausted anode gas flows along a line  80  to a catalytic burner  82 . The burner  82  also has inlets  84  and  86  for air and methanol. Within the catalytic burner  82  any residual hydrogen, hydrocarbons and other combustible materials are consumed. Air is provided for this purpose, and additional methanol can be supplied, to ensure full consumption of all residual reactive components and generation of sufficient heat. The heated gas is then passed to the heat exchanger  78 , for heating the incoming methanol and water, and finally, the anode gas is exhausted as indicated at  88 .  
         [0037]    To monitor and control the system, a variety of sensors can be provided. Exemplary sensors are shown. Thus, temperature sensors are indicated at  90 , for monitoring the temperature of the incoming methanol and water mixture both upstream and downstream from the electrocatalytic reformer  62 , and also the temperature within the electrocatalytic reformer  62 . A pressure sensor is indicated at  92 , and a flow sensor  76 / 94 .  
         [0038]    A power source  96  is connected to the electrocatalytic reformer, for supplying power to the individual elements thereof, to generate heat, as described above.  
         [0039]    The various sensors  90 ,  92  and  94 , and also the power supply  96  are connected to the process control computer  66 . It will also be understood that the computer  66  could be connected to other sensors, and other control elements, for example, the load signal  74 .  
         [0040]    Accordingly, the process control computer  66  can constantly control the temperature in the electrocatalytic reformer  62 . In addition to the sensors shown, the specific electrical resistance of the reformer  62  can be measured as an indication of its temperature. In this way, the electrical energy supplied to the electrocatalytic reformer  62  can be adjusted such that the reaction temperature in the reformer  62  is maintained in the optimal range. More specifically, it is preferred to use “forward-looking” power regulation, i.e. anticipating future demand and adjusting the catalytic reformer  62  accordingly. This provides a dynamic operating characteristic, without having to accept the disadvantages with respect to the degree of conversion and the composition of the synthesized gas. In effect, this requires that, when the power demand from the fuel cell stack  64  increases, simultaneously, the heating power supplied to the electrocatalytic reformer  62  and the flow of methanol and water from the supplies  68 , 70  are all increased.  
         [0041]    It will also be understood that all the heating strategies for the electrocatalytic reformer  62 , as detailed at FIGS. 1, 2 and  3  can be employed simultaneously or in various combinations.  
         [0042]    Reference will now be made to FIG. 5, which shows a variant of the system or circuit of FIG. 4. Again, for simplicity and brevity, like components are given the same reference numeral and the description of these components is not repeated. The system of FIG. 5 is indicated generally by the reference  100 .  
         [0043]    In FIG. 5, the heat exchanger  78  has been replaced by a thermocatalytic reformer  102 . This provides for reformation of the methanol and water mixture, to generate hydrogen and carbon dioxide by use of catalysts and heat supplied from a catalytic burner  104 . As for the catalytic burner of FIG. 4, the anode exhaust line  80  is connected to the catalytic burner  104 , together with inlets  106  for air and  108  for methanol. Thus, again, residual hydrocarbons and the like, together with added methanol are consumed in the catalytic burner  104 , to generate carbon dioxide, water and heat. Resultant waste gas is exhausted at  110  and heat is transferred as indicated schematically, to the thermocatalytic reformer  102 .  
         [0044]    In this embodiment, the electrocatalytic reformer  62  then acts merely as a booster reformer, to carry out any residual reformation required. Its size and dimensions can be adjusted accordingly. Thus, the electrocatalytic reformer  62  provides two main functions here. Firstly, it provides a security function, to assure an optimal composition of the synthesized gas flowing to the fuel cell stack, on account of the ability to adjust its reaction conditions to an optimal state. Secondly, the “booster” function of the electrocatalytic reformer  62  is in a sense that, on account of its high internal dynamics, it can respond much faster to change in demand by the fuel stack  64  as compared to the thermocatalytic reformer  102 .  
         [0045]    The individual ECR elements  20  are preferably made of a ceramic, more preferably silicon or silicon carbide, which preferably is in the shape of a monolithic honeycomb structure. The elements  20  can also be formed from wire meshes of a suitable metal, for example, an aluminum-chrome-iron alloy, these meshes can be provided in the form of a rolled up sheet.  
         [0046]    Where the elements  20  are formed from silicon or silicon carbide, it is preferred for the material to have porosity in the range of 20 to 80%, more preferably in the range of 40 to 70%, with an electrical resistance in the range 0.001 Ωcm to 10 MΩcm.  
         [0047]    Advantageously, the ceramic, either silicon or silicon carbide, is formed as a porous, lateral, flowable pipe. In accordance with the present invention, one or both surfaces of the pipe are coated with a catalyst selected from the Group VIII metals of the periodic table (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt), Cu and Zn. It will be understood that the catalyst can comprise combinations of two or more of these metals, for promoting endothermal reforming reactions.  
         [0048]    Similarly, the outer pipe  12  can also be coated with a catalyst selected from the same group of metals in the same manner. It is preferably coated on the outside, to provide a catalyst for conversion of anode exhaust gases; it is also possible to consider coating the outer pipe  12  on the inside thereof, to enhance a catalytic reformation of the incoming reactive components.