Patent Publication Number: US-2010112392-A1

Title: Method for regenerating a reformer

Description:
The invention relates to a method for regenerating a reformer fed with a mixture of fuel and an oxidant having a mean air number λ 1  in continuous reformer operation, the air number being varied for the purpose of regenerating the reformer. 
     The invention relates furthermore to a system including a reformer and a controller. 
     Generic methods have a wealth of different applications, they serving in particular to feed a fuel cell with a gas mixture rich in hydrogen from which electrical energy can be generated on the basis of electrochemical reactions. Such fuel cells find application, for example, in motor vehicles as auxiliary power units (APUs). 
     The reforming process for converting the fuel and oxidant into the reformate can be done in accordance with various principles. For instance, catalytic reforming is known in which the fuel is oxidized in an exothermic reaction. The disadvantage in catalytic reforming is the high amount of heat it produces which can irreversibly ruin system components, particularly the catalyst. 
     Another possibility of generating a reformate from hydrocarbons is steam reforming in which hydrocarbons are converted with the aid of steam into hydrogen in an endothermic reaction. 
     A combination of these two principles, i.e. reforming on the basis of an exothermic reaction and generating hydrogen by an endothermic reaction in which the energy for reforming the steam is won from the combustion of the hydrocarbons is termed autothermal reforming. Here, however, additional disadvantages are met with in that means of feeding water need to be provided. High temperature gradients between the oxidation zone and the reforming zone pose further problems in the heat balance of the system as a whole. 
     In general, the reaction in which air and fuel are converted in a reformer into a hydrogen-rich gas mixture can be formulated as follows: 
     
       
         
           
             
               
                 
                   C 
                   n 
                 
                  
                 
                   H 
                   m 
                 
               
               + 
               
                 
                   n 
                   2 
                 
                  
                 
                   O 
                   2 
                 
               
             
             -&gt; 
             
               
                 
                   m 
                   2 
                 
                  
                 
                   H 
                   2 
                 
               
               + 
               
                 n 
                  
                 
                     
                 
                  
                 CO 
               
             
           
         
       
     
     Due to incomplete conversion of the hydrocarbons in this endothermic reaction—not reflected by the equation—side products such as remnant hydrocarbons or soot can materialize which are deposited at least in part on the reformer, resulting in deactivation of the catalyst provided in the reformer possibly to such an extent that the catalyst is almost totally sooted up. This increases the drop in pressure in the reformer, resulting in it being ruined or needing to be regenerated. 
     In accordance with prior art such a regeneration is implemented particularly by burning off the soot deposited in the reformer. This can produce high temperatures resulting in permanent, i.e. irreversible damage to the catalyst or substrate material. Apart from this, large temperature gradients hamper controlling the reformer when burning off the soot is started. Since with an excess of oxygen, oxygen can materialize at the output of the reformer during burn-off there is no possibility of using a reformer regenerated in this way in an SO fuel cell system. 
     Described in DE 101 52 083 A1 is a reformer fed with fuel, vapor and oxygen. The solution proposed in DE 101 52 083 A1 to avoid overheating is to implement regeneration pulsed by elevating the air number of feed mixture for limited time intervals. It is unavoidable that this has an effect on reforming operation, resulting in, for example, the electrical energy obtainable from the fuel cell system being diminished. 
     The invention is based on the object of achieving regeneration of a reformer in avoiding an effect on reforming operation. 
     This object is achieved by the features of the independent claims. 
     Advantageous embodiments of the invention read from the dependent claims. 
     In accordance with a first aspect the invention is based on the generic method in that regeneration in a shutoff phase of the reformer is achieved in that the reformer is operated during several successive time intervals with an air number λ 2  higher than in reformer operation (λ 2 &gt;λ 1 ). In normal operation the reformer receives a continual feed of fuel and air at temperatures in the region of 650° C. and above. The reformer works in thermal equilibrium so that in stationary operation no increase in temperature is to be reckoned with. The deposits, however, as described, result in the catalyst being deactivated by degrees. It is particularly in mobile applications, for instance in passenger cars or commercial vehicles, that a fuel cell system and thus also the reformer is regularly shut off at least when the vehicle is idle for a lengthy period. Since no further electrical energy can be generated during the shutoff phase in thus having a negligible effect on reformer operation as regards energy generation, the shutoff phase can be made use of to advantage for regeneration. However, it is to be noted that also during the shutoff phase with a long-term elevation of the air number—be it by reducing the fuel flow feed, by increasing the air flow feed, or by both—overheating is to be expected which can result in the catalyst or even the complete reformer being ruined. This is because the reaction in burning off the soot C+O 2 -&gt;CO 2  progresses exothermically. 
     Following complete burn-off of the catalyst oxygen is output at the end of the reformer which would result in the anode of a SO fuel cell being ruined. By the method in accordance with the invention it is now proposed to reduce the fuel feed pulsed during the shutoff phase, the individual pulses of which last for only a short time. Oxygen or air is applied to the soot deposit so that the oxidation process can commence, also resulting in an increase in temperature in the catalyst. But before the temperature is so high that the reformer could suffer damage, the fuel feed is again increased. Thus, at the end of a time interval with a reduced feed rate, part of the reformer is regenerated, i.e. rendered substantially free of soot or deposits. Reducing the air number results in the reformer cooling down to normal temperatures. This procedure can result in the reformer being partly regenerated or it can be repeated until the complete reformer is regenerated. Regeneration occurs zonally. Reducing the fuel feed pulsed now makes it possible that no oxygen gains access to the fuel cell anode which may prove problematic at fuel cell temperatures exceeding 500° C. since the anode material in the presence of oxygen can oxidate from Ni into NiO, resulting in the anode material being ruined and inhibiting the electrochemical reaction at the anode. 
     The invention is furthermore sophisticated to advantage in that the fuel feed rate amounts to zero during at least one of the successive time intervals. Due to the fuel feed being shut off completely during the successive time intervals, burn-off of the deposits is now more efficient. When the fuel feed is not completely shut off, water production in the reformer is increased. It is this water that is able to remove the soot and other deposits from the reformer in accordance with the equation C+H 2 O-&gt;CO+H 2 . 
     It may furthermore prove useful to measure the oxygen content in the substances leaving the reformer and the reformer translating into continuous operation when the oxygen content exceeds a threshold value. The oxygen content at the output of the reformer thus serves as an indicator of complete regeneration of the reformer. Keeping track of the oxygen content furthermore permits ensuring that no excess quantities of oxygen come into contact with the anode of the SO fuel cell. 
     In this context it is useful to measure the oxygen content with a lambda sensor. 
     It may likewise be provided for that the oxygen content is measured by a fuel cell. To save having to install a lambda sensor the electrical output values of the fuel cell can be used directly to detect an increase in the oxygen content. To determine the lambda value other sensing methods can, of course, be made use of, such as, for instance, infrared or CO sensing. 
     The method in accordance with the invention is particularly useful with a reformer having a dual fuel feed, when one of the fuel feeds works during regeneration with a feed rate which substantially corresponds to the feed rate in continuous operation. With a reformer having a dual fuel feed there is thus a greater possibility of varying the fuel feed rate. This particularly applies to the possibility of operating the reformer unchanged in part whilst in other portions of the reformer regeneration occurs by changing the function when this is desired during reformer operation, in other words, outside of the shutoff phase. 
     The method in accordance with the invention is in this context usefully sophisticated in that the reformer comprises am oxidation zone and a reforming zone, that the reforming zone is feedable with heat, that the oxidation zone is fed with a mixture of fuel and oxidant in using a first fuel feed, the mixture being feedable after oxidation of the fuel at least in part to the reforming zone at least in part, that the reforming zone is feedable with additional fuel by using a second fuel feed and that the second fuel feed works during the successive time intervals with a reduced feed rate. The additional fuel feed thus forms together with the waste gas from the oxidation zone the output mixture for the reforming process. By mixing the fuel with the waste gas a small A value is made available (for example λ=0.4) and in making use of heat an endothermic reforming reaction is achievable. As regards the regeneration in accordance with the invention it is to be noted that operation of the reformer in the oxidation zone can continue to run unchanged whilst only the second fuel feed is shut off or reduced. 
     It is particularly useful that the reforming zone can be provided with heat from the exothermic oxidation in the oxidation zone. The thermal energy resulting in the oxidation zone is thus converted in the scope of the reforming reaction so that the net heat produced by the process as a whole does not result in problems in managing the temperature of the reformer. 
     It is usefully provided for that the reforming zone comprises an oxidant feed via which additional oxidant is feedable, resulting in a further parameter being available for influencing reforming, in enabling it to be optimized. 
     The invention is particularly suitably sophisticated in that additional fuel is fed to an injection and mixing zone from which it can flow into the reforming zone. This injection and mixing zone is thus disposed upstream of the reforming zone so that the reforming zone makes a well mixed output gas available for the reforming reaction. 
     In this context it is particularly useful that the additional fuel is evaporated at least in part by the thermal energy of the gas mixture emerging from the oxidation zone, in thus enabling the reaction heat of oxidation to be also made use of to advantage for the fuel evaporation process. 
     It may furthermore prove useful in that the gas mixture generated in the oxidation zone is feedable to the reforming zone partly in bypassing the injection and mixing zone, in thus making available a further possibility of influencing the reforming process so that a further improvement of the reformate emerging from the reformer is achievable as regards its application. 
     It may be provided for that regeneration occurs during each shutoff phase of the reformer, in thus making an optimally prepared system available for the next reformer start. 
     In accordance with a second aspect the invention is based on the generic method in that regeneration occurs in a starting phase of the reformer in that the reformer is continually operated with an air number increased as compared to reformer operation λ 2 &gt;λ 1  until a critical temperature threshold is attained. During the starting phase, particularly on commencement thereof, the temperatures materializing in the reformer are uncritical, there thus being no need to select pulsed reformer operation for the purpose of regeneration. Instead, the reformer can be regenerated continually via the elevated air number. 
     It is particularly useful that the reformer can be operated in the starting phase with an air number λ≧1 in the reformer ultimately working as a burner, air numbers of λ&gt;1 being uncritical with the relatively low temperatures of a downstream fuel cell system. 
     For example, it may be provided for that the critical temperature threshold is defined in that the reformer or its components feature temperatures between 450 and 650° C. 
     It is likewise conceivable that the critical temperature threshold is defined in that a fuel cell stack or its components downstream of the reformer feature temperatures between 450 and 550° C. Terminating regeneration during the starting phase, for example, at a fuel cell stack temperature of 500° C. avoids damage on the part of the anode due to excess oxygen entering the fuel cell stack when the temperature thereof is further increased. 
     The invention is sophisticated to particular advantage in that the reformer is regenerated following its starting phase by the reformer being operated during several successive time intervals with an air number elevated as compared to that in reformer operation. Pulsed operation is appropriate following the starting phase to avoid overheating. 
     It is expediently provided for that the reformer is regenerated during each starting phase. Since operation of the reformer as a kind of burner can serve both preheating the system and regeneration, the system can be regenerated to advantage every time it is started. 
     The invention relates furthermore to a system comprising a reformer and a controller permitting regeneration of the reformer, the controller being adapted to control a method in accordance with the invention. 
    
    
     
       The invention will now be detailed by way of preferred example embodiments with reference to the attached drawings in which: 
         FIG. 1  is a flow diagram to assist in explaining a method in accordance with the invention; 
         FIG. 2  is a flow diagram to assist in explaining regeneration during reformer operation; and 
         FIG. 3  is a diagrammatic illustration of a reformer in accordance with the invention. 
     
    
    
     Referring now to  FIG. 1  there is illustrated a flow diagram to assist in explaining a method in accordance with the invention. Following the start of the reformer in step S 01 , the reformer is operated with an air number λ≧1, corresponding to operation as a burner. Burner operation serves regeneration by particularly removing carbon and its compounds and sulphur and its compounds from the reformer. Regeneration also has an effect on any other organic and inorganic compounds having become deposited in the reformer. This is followed in step S 03  by sensing whether a temperature T has already exceeded a critical value T K . This critical value can be established by the reformer itself determining, for example, the upper temperature value permissible for the catalyst in the reforming zone or also as dictated by the fuel cell stack downstream of the reformer. It is particularly at temperatures exceeding 500° C. that the fuel cell stack must not be charged with oxygen to thus avoid a heavy superstoichiometric inflow of oxygen into the reformer above one such critical temperature. As long as the critical temperature is not attained the reformer continues to be operated as a burner. But if the critical temperature is exceeded, the reformer enters into normal operation as a reformer as per step  04 . If need be, for further regeneration pulsed operation can be initiated as described with reference to  FIG. 3 . If, in step S 05 , the fuel cell stack is shut off, the involved shutoff phase of the reformer can be utilized for further regeneration in pulsed operation as per step S 06 . This is followed by operation of the reformer being terminated (step S 07 ). 
     Referring now to  FIG. 2  there is illustrated a flow diagram to assist in explaining regeneration during operation of the reformer. After starting regeneration of the reformer in step S 01  the fuel feed is shut off in step S 02 . Then in step S 03  a temperature in the reformer is sensed, in step S 04  it being determined whether the sensed temperature is higher than a predefined threshold value T S1 . If it is not, the temperature in the reformer is again sensed as per step S 03  with the fuel feed shut off. If it is sensed in step S 04  that the temperature exceeds the predefined threshold value T S1 , the fuel feed is returned ON in step S 05 . This is followed in step S 06  in that the temperature in the reformer again is sensed. In step S 07  it is determined whether this sensed temperature is lower than a predefined threshold value T S2 . If it is not, the temperature in the reformer is again sensed as per step S 06 , without shutting off the fuel feed. If it is sensed in step S 07  that the temperature is lower than the predefined threshold value T S2  the fuel feed is again shut off as per step S 02  so that the next time interval for reformer generation can commence. 
     Parallel to monitoring the temperature, oxygen breakthrough in the reformer is monitored in step S 08 . This serves to establish the end of regeneration. Thus, when an oxygen breakthrough occurs and the fuel feed is shut off, then in step S 09  the fuel feed is returned ON, after which regeneration ends with step S 10 . 
     Referring now to  FIG. 3  there is illustrated a diagrammatic illustration of a reformer in accordance with the invention. The invention is not restricted to the special configuration of the reformer as shown here. Instead, regeneration in accordance with the invention can take place in various types of reformer as long as it is possible to reduce or interrupt the fuel feed at short notice. The reformer  10  as shown here which is based on the principle of partial oxidation preferably without a steam feed can be fed with fuel  12  and oxidant  16  via respective feeds. A possible fuel  12  is for instance diesel, the oxidant  16  as a rule is air. The reaction heat resulting as soon as combustion commences can be partly removed in an optional cooling zone  36 . The mixture then enters the oxidation zone  24  which may be realized as a tube arranged within the reforming zone  26 . In alternative embodiments the oxidation zone is realized by a plurality of tubes or by a special tubing arrangement within the reforming zone  26 . In the oxidation zone the conversion of fuel and oxidant takes place in an exothermic reaction with λ≈1. The resulting gas mixture  32  then enters an injection and mixing zone  30  in which it is mixed with fuel  14 , whereby the thermal energy of the gas mixture  32  can support evaporation of the fuel  14 . It may be provided for in addition that the injection and mixing zone  30  is fed with an oxidant. The mixture formed in this way then enters the reforming zone  26  where it is converted in an endothermic reaction with e.g. λ≈0.4. The heat  28  needed for the endothermic reaction is taken from the oxidation zone  24 . To optimize the reforming process additional oxidant  18  can be fed into the reforming zone  26 . It is furthermore possible to feed part of the gas mixture  34  generated in the oxidation zone  24  directly to the reforming zone  26  in bypassing the injection and mixing zone  30 . The reformate  22  then flows from the reforming zone  26  and is available for further applications. 
     Assigned to the reformer is a controller  38  which, among other things, can control the primary fuel feed  12  as well as the secondary fuel feed  14 . 
     To undertake regeneration of the reforming zone  26  in the example embodiment as shown in  FIG. 3  it may be sufficient to shut off the fuel feed  14  pulsed whilst the fuel feed  12  for maintaining the oxidant in the reformer is operated with no change in the feed rate. The catalyst provided in the reforming zone  26  is then burnt off with waste combustion gases containing oxygen. 
     It is understood that the features of the invention as disclosed in the present description, drawings as well as in the claims may be essential to achieving the invention both singly and in any combination. 
     LIST OF REFERENCE NUMERALS 
     
         
           12  fuel 
           14  fuel 
           16  oxidant 
           18  oxidant 
           20  oxidant 
           22  reformate 
           24  oxidation zone 
           26  reforming zone 
           28  heat 
           30  injection and mixing zone 
           34  gas mixture 
           36  cooling zone 
           38  controller