Patent Publication Number: US-2005142507-A1

Title: Hydrogen combustion heater

Description:
The present application is a continuation of U.S. application Ser. No. 09/924,941, filed Aug. 9, 2001, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a hydrogen combustion heater for heating a fluid (e.g., water) by a heat generated by a catalytic oxidation of hydrogen gas.  
      There are various combustion heaters in which a fluid is heated through a heat exchanger by a heat generated by a catalytic oxidation of a fuel gas. It is preferable to use a hydrogen combustion heater, for example, for heating of interior of an electric vehicle equipped with a hydrogen fuel cell, since a single fuel source (i.e., hydrogen gas) can be used for both of the heater and the fuel cell.  
      A hydrogen combustion heater is operated by bringing a mixture of hydrogen gas and air into contact with a catalyst to achieve a catalytic oxidation of hydrogen gas. It is necessary to get a suitable temperature for achieving the catalytic oxidation. Upon starting the operation of a hydrogen combustion heater, it is known to conduct a spark ignition of a mixture of hydrogen gas and air at a position upstream of the catalyst in the heater, thereby achieving a high temperature and the subsequent combustion of the hydrogen gas. Thus, it becomes possible to heat the catalyst to a temperature suitable for the catalytic oxidation by passing the resulting combustion gas of high temperature through the catalyst.  
      The above-mentioned spark ignition, however, can be achieved, if the mixing ratio of hydrogen and air is a value causing explosion or deflagration. Besides such explosion problem, the combustion temperature caused by the spark ignition may become too high. This may cause a high thermal stress on a heat exchanger of the heater due to a large temperature difference between the heat exchanger and its fluid. Furthermore, the formation of nitrogen oxides may become too much.  
      Japanese Patent Unexamined Publication JP-A-2000-291917 discloses a hydrogen combustion device. This device includes a separate heater for preheating a catalyst used in a catalytic oxidation of hydrogen gas.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a hydrogen combustion heater that is simple in construction and stable in heating capability.  
      According to the present invention, there is provided a hydrogen combustion heater comprising (a) a passage for allowing hydrogen gas and air to flow therethrough; (b) a first catalyst provided in said passage, said first catalyst being heated, when electricity is applied thereto, thereby starting a first combustion of a first mixture of said hydrogen gas and said air in said first catalyst; and (c) a heat exchanger provided downstream of said first catalyst in said passage, said heat exchanger being adapted to transfer heat generated by said first combustion to a heating medium of said heat exchanger.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic sectional view showing a first hydrogen combustion heater according to a first preferred embodiment of the present invention;  
       FIG. 2  is a perspective view showing a mixer used in a hydrogen combustion heater according to the present invention;  
       FIGS. 3A and 3B  are perspective views respectively showing a flat sheet and a corrugated sheet, which are used for preparing the mixer;  
       FIG. 4A  is a plan view of the flat sheet;  
       FIG. 4B  is a partial side view of the corrugated sheet;  
       FIG. 5  is a perspective view showing a first catalyst (electric heating catalyst) of the hydrogen combustion heater;  
       FIG. 6A  is a plan view showing a flat sheet of the electric heating catalyst;  
       FIG. 6B  is another perspective view of the electric heating catalyst;  
       FIG. 6C  is a partial side view of a corrugated sheet of the electric heating catalyst;  
       FIGS. 7A and 7B  are graphs showing temperature rising characteristics of the electric heating catalyst with heating time;  
       FIG. 8  is a graph showing temperature characteristics of the electric heating catalyst with and without holes;  
       FIG. 9  is a graph showing a hydrogen combustion characteristic;  
       FIG. 10  is a graph showing a relationship between the maximum combustion temperature and the mixing ratio of air to hydrogen gas;  
       FIGS. 11A and 11B  are perspective views each showing a part of a modification of the corrugated sheet of the mixer;  
       FIGS. 12A and 12B  are schematic views each showing a part of a modification of the sheet of the electric heating catalyst;  
       FIG. 13  is a schematic view showing an upstream part of a second hydrogen combustion heater according to a second preferred embodiment of the present invention;  
       FIG. 14  is a perspective view showing a first catalyst (electric heating catalyst) according to the second preferred embodiment;  
       FIG. 15  is a plan view showing a sheet for preparing the electric heating catalyst of  FIG. 14 ;  
       FIG. 16A  is an enlarged sectional view showing a discharge portion of a hydrogen introducing pipe of  FIG. 13 ;  
       FIG. 16B  is a sectional view taken along lines A-A of  FIG. 16A ;  
       FIG. 17A  is a view showing hydrogen concentration measurement points A-F in a passage of the second hydrogen combustion heater, sectioned at a position that is 30 mm downstream of the discharge portion of the hydrogen introducing pipe and is upstream of the electric heating catalyst;  
       FIG. 17B  is a graph showing a percentage of a deviation of hydrogen concentration at each point of  FIG. 17A  from the average hydrogen concentration;  
       FIG. 17C  is a graph similar to  FIG. 17B , but showing the results when the hydrogen and air flow rates in the case of  FIG. 17B  were each reduced by a factor of 6;  
       FIG. 18A  is an enlarged sectional view showing a first modification of the hydrogen introducing pipe of  FIG. 16 ;  
       FIG. 18B  is a graph similar to  FIG. 17B , but showing the results in the case of the first modification of  FIG. 18A ;  
       FIG. 19A  is an enlarged sectional view showing a second modification of the hydrogen introducing pipe of  FIG. 16 ;  
       FIG. 19B  is a graph similar to  FIG. 17B , but showing the results in the case of the second modification of  FIG. 19A ;  
       FIG. 20  is a perspective view showing a third hydrogen combustion heater according to a third preferred embodiment of the present invention;  
       FIG. 21A  is an enlarged sectional view showing a discharge portion of a hydrogen introducing pipe of  FIG. 20 ;  
       FIG. 21B  is a sectional view taken along lines A-A of  FIG. 21A ;  
       FIGS. 22A, 22B  and  22 C are plan views respectively showing first, second and third members of the mixer of  FIG. 20 ;  
       FIGS. 23A and 23B  are sectional views each showing schematic hydrogen gas flows from openings of the discharge portion of the hydrogen introducing pipe into the passage of the third hydrogen combustion heater;  
       FIG. 24A  is a partly sectional view showing the discharge portion of the hydrogen introducing pipe;  
       FIGS. 24B and 24C  are sectional views respectively taken along lines B-B and C—C of  FIG. 24A ;  
       FIG. 25A  is a sectional view of  FIG. 20  at a position of the downstream end of the first catalyst, showing numbers representing temperatures at their respective positions in the section, when the first to third members constitute the mixer (first mixer);  
       FIG. 25B  is a view similar to  FIG. 25A , but showing those numbers when the first to fourth members constitute the mixer (second mixer);  
       FIG. 25C  is a view similar to  FIG. 25A , but showing those numbers when only the second and third members constitute the mixer (third mixer);  
       FIG. 25D  is a graph showing the standard deviation and the difference between the maximum and minimum temperatures with respect to the temperatures of  FIGS. 25A, 25B  and  25 C; and  
       FIG. 26  is a view similar to  FIG. 20 , but showing a fourth hydrogen combustion heater according to a fourth preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The above-mentioned first combustion of the present invention can be limited to a mild oxidation, which is defined as being an oxidation free from firing of the hydrogen, by controlling the flow rate ratio of the air to the hydrogen. With this, it becomes possible to avoid explosion or deflagration and an excessively high combustion temperature. The flow rate ratio is preferably greater than 8:1, more preferably about 15.3:1, as will be described in detail hereinafter.  
       FIG. 1  shows a first hydrogen combustion heater according to a first preferred embodiment of the present invention. The first heater has a passage  13  for allowing hydrogen gas and air to flow therethrough. The passage  13  is formed at its upstream end with a blower  11 . Air is sucked by the blower  11  into the passage  13  through a filter  10  and a flow rate regulating valve  12 . The passage  13  (inner diameter: 58 mm) is formed on its sidewall with a hydrogen introducing pipe  16  (inner diameter: 8 mm), which is connected to a hydrogen reservoir (not shown) via a pressure reducing valve  15 . The hydrogen introducing pipe  16  has a first hydrogen passage  17  provided with a first valve  19  and a second hydrogen passage  18  provided with a second valve  20 . It is possible to adjust a hydrogen gas flow rate ratio of a first flow through the first hydrogen passage  17  to a second flow through the second hydrogen passage  18  to 1:9 by controlling the first and second valves  19  and  20 . In fact, it is possible to supply hydrogen gas at a flow rate of 5 liters/min to the passage  13  by reducing its pressure with the pressure reducing valve  15  and by opening only the first valve  19 . Then, it is possible to supply hydrogen gas at a total flow rate of 50 liters/min to the passage  13  by further opening the second valve  20 , too. In other words, it is possible to provide a hydrogen gas flow rate for the initial stage of the hydrogen combustion operation by opening only the first valve  19 . Furthermore, it is possible to provide a hydrogen gas flow rate for the steady state operation by opening both the first and second valves  19  and  20 . Thus, it is easy to control the fuel flow rate. The passage  13  is connected at its downstream end with a heater unit  30 .  
      In the first hydrogen combustion heater, a fuel supplying means for supplying or introducing hydrogen gas and air can be defined as being formed of the blower  11 , the flow rate regulating valve  12 , the pressure reducing valve  15 , the hydrogen introducing pipe  16 , the first and second valves  19  and  20 , and the passage  13 .  
      The heater unit  30  comprises a casing  31  that is made of stainless steel and covered with a heat insulating material  34 . This casing  31  constitutes a passage of the first hydrogen combustion heater and is formed of (a) a smaller diameter portion  32  having the same diameter as that of the passage  13  and (b) a larger diameter portion  33  (inner diameter: 102 mm). A mixer  40  and a first catalyst (electric heating catalyst)  50  are disposed in the smaller diameter portion  32 . In fact, the mixer is provided at a position upstream of the electric heating catalyst  50 . Therefore, hydrogen gas and air can sufficiently be mixed together before entering into the electric heating catalyst. With this, it is possible to prevent the heat spot generation in the electric heating catalyst, thereby improving durability of the first hydrogen combustion heater. A second catalyst (combustion catalyst)  60  and a heat exchanger  64  are disposed in the larger diameter portion  33 . In other words, the combustion catalyst  60  is larger in sectional area than the electric heating catalyst  50 . With this, the axial length of the heater unit  30  can be shortened, thereby making the hydrogen combustion heater compact in size.  
      The electric heating catalyst is smaller than the combustion catalyst in volume, as shown in  FIG. 1 . A first flow rate of a mixture of hydrogen gas and air for starting a first hydrogen combustion in the electric heating catalyst is adjusted to about Q 1  defined by the following expression: 
 
 Q   1   =Q   s   ×V   1 /( V   1   +V   2 ) 
 
 where Q s  is a flow rate of a mixture of hydrogen gas and air under the steady state operation of the hydrogen combustion heater for conducting a second combustion in both the electric heating catalyst and the combustion catalyst, V 1  is a volume of the electric heating catalyst, and V 2  is a volume of the combustion catalyst. At the initial stage of hydrogen combustion, a mixture of hydrogen gas and air is introduced at a relatively low flow rate (Q 1 ) that is enough for making an initial catalytic reaction (hydrogen combustion) possible only in the electric heating catalyst of a relatively small volume (V 1 ). With this, it is possible to start the initial catalytic reaction (i.e., a hydrogen combustion in the electric heating catalyst) in a relatively short time with a low electric power. Thus, it is also possible to start the steady state operation (i.e., a hydrogen combustion in the electric heating and combustion catalysts) in a relatively short time. 
 
      First and second terminals  55  and  56  extend outward from the electric heating catalyst through the casing  31 . The heat exchanger  64  communicates with a water introducing pipe  66 , which is connected with a water pump  65 , and a water discharge pipe  67 . The larger diameter portion  33  of the casing  31  is tapered at its downstream end into an outlet connected with a muffler  70 .  
      Air supplied into the passage  13  by the blower  11  and hydrogen gas from the hydrogen introducing pipe  16  are uniformly mixed together in the mixer  40  into a gas mixture. Then, the gas mixture is heated and subjected to a combustion in the electric heating catalyst  50 . Heat of this combustion can heat the combustion catalyst  60  up to a temperature enough for conducting a catalytic reaction in the combustion catalyst  60 . It is, however, possible to omit the combustion catalyst  60  in the invention, as will be described hereinafter in the fourth preferred embodiment shown in  FIG. 26 . The heated combustion gas (i.e., air) after the oxidation in the combustion catalyst  60  can heat a heating medium (e.g., pure water), and then is discharged from the muffler  70  to the outside.  
      There is a temperature sensor  73  in a space between the electric heating catalyst  50  and the combustion catalyst  60  for measuring temperature of the combustion gas coming from the electric heating catalyst  50 . Furthermore, there are a temperature sensor  74  for measuring temperature of the combustion catalyst  60 , and a temperature sensor  75  in a space between the combustion catalyst  60  and the heat exchanger  64  for measuring temperature of the combustion gas coming from the combustion catalyst  60 . Still furthermore, there is a temperature sensor  76  immediately downstream of the heat exchanger  70  for measuring temperature of the combustion gas after the heat exchange. There are a pressure sensor  77  at an inlet of the heat exchanger  64 , and a temperature sensor  78  for measuring temperature of water after the heat exchange, and a relief valve  79  at an outlet of the heat exchanger  64 . The blower  11 , the water pump  65 , the valves  12 ,  15 ,  19 ,  20  and  79 , and the sensors  73 - 78  are connected to and controlled by a controller (not shown).  
       FIG. 2  shows a structure of the mixer  40 , which is prepared by winding a laminate of a flat sheet  42  ( 41 ) with holes  44  (see  FIG. 3A ) and a corrugated sheet  43  with holes (see  FIG. 3B ). As shown in  FIG. 4A , the flat sheet  41  is an aluminum sheet (foil) having a width of 50 mm and a thickness of 50 μm. This sheet has on its entire surface a plurality of holes  44  (diameter: 1.0 mm) at a pitch of 2 mm in the orthogonal directions along its major sides. As shown in  FIG. 4B , the corrugated sheet  43  has a plurality of waves (height: 1.2 mm; pitch: 2.6 mm). In  FIGS. 2, 3A ,  3 B,  4 A and  4 B, the size and the pitch of the holes  44  are enlarged for easiness to understand the structure. A wound body of the flat sheet  42  and the corrugated sheet  43  is subjected to a vacuum heating treatment at 1,200° C. under a pressure of about IPa for about 20 min, thereby making a mixer with a high rigidity through diffusion bonding. Instead of the vacuum heating treatment, it is possible to easily make a mixer by conducting a spot welding during and after a winding of the flat and corrugated sheets  42  and  43 . The mixer  40  is disposed in the smaller diameter portion  32  of the heater unit  30  in a manner to align the axis of the mixer  40  with that of the smaller diameter portion  32 . The mixer  40  is provided with a plurality of cells each being defined between the flat sheet  42  and the corrugated sheet  43 . Each cell extends between the upstream and downstream ends of the mixer  40  in the axial direction of the smaller diameter portion  32 .  
       FIG. 5  shows an exemplary structure of the electric heating catalyst  50 . This catalyst  50  may be prepared by winding 20 times (20 windings) a laminate of a flat sheet  52  and a corrugated sheet  53 , then by brazing a plurality of spots of the wound body, and then pressing the wound body into an outer cylinder  54 . This brazing may be replaced with a bonding with a ceramic bond. Each of the flat and corrugated sheets  52  and  53  supports thereon a catalyst component containing 1% platinum and the remainder of alumina (Al 2 O 3 ). The electric heating catalyst  50  is electrically connected at its center and its periphery with first and second electrodes  55  and  56 , respectively. Such electric heating catalyst can easily be produced with a low cost. The electric heating catalyst is similar in structure to common automotive catalysts (catalytic converters) for purifying exhaust gas.  
      As shown in  FIG. 6A , a raw material (blank)  51  of the flat and corrugated sheets  52  and  53  is a stainless steel sheet containing 20% Cr, 5% Al and the remainder of Fe and having a width of 50 mm and a thickness of 50 μm. This stainless steel sheet is subjected to a surface oxidation by heating the same at 1,200° C. for about 20 min in the atmosphere. This stainless steel sheet has a plurality of holes  57  (diameter: 1.0 mm) to an extent corresponding to the outermost eight windings of the resulting electric heating catalyst  50  having 20 windings in total (see  FIG. 6B ). The holes  57  are arranged to have nineteen holes in a direction along the width at a pitch of 2.5 mm. The holes  57  are further arranged in the winding or longitudinal direction at a pitch of 5 mm. The corrugated sheet  53  is configured to have a wave height of 1.2 mm and a wave pitch of 2.6 mm. In  FIGS. 5, 6A  and  6 B, the size and the pitch of the holes  57  are enlarged for easiness to understand the structure.  
       FIG. 7A  is a graph showing temperature rising characteristics of the electric heating catalyst  50  (having 20 layers or windings in total) prepared by winding together the flat sheet  52  and the corrugated sheet  53  each provided at its outermost eight windings with the above-mentioned holes  57 . Due to the provision of the holes  57 , it is possible to increase the electric resistance of the outermost eight layers. Therefore, it is possible to increase heat generation and the temperature of the outermost eight layers by applying the electricity, even though the outer layers temperature tends to lower by heat radiation. With this, it is possible to suitably proceed the catalytic reaction (hydrogen combustion) in the electric heating catalyst and to start the steady state operation early. In fact, a power (voltage: 12.0V; current: 12.4A) of 149 W was applied to the electric heating catalyst  50  for a period of time shown in  FIG. 7A  for heating the same. The temperatures of the 5th, 10th and 15th layers (counted from the outermost layer) were measured during this heating.  
      In contrast with  FIG. 7A ,  FIG. 7B  is a graph showing temperature rising characteristics of a similar electric heating catalyst (having 20 layers or windings in total) prepared by winding together flat and corrugated sheets each provided with no holes. It is understood from  FIG. 7A  that the temperature of the 5th layer is not so low relative to those of the 10th and 15th layers, due to the provision of the holes  57 . In contrast, it is understood from  FIG. 7B  that the temperature of the 5th layer is quite low relative to those of the 10th and 15th layers, due to heat radiation. In this case, the catalytic reaction may become insufficient.  
       FIG. 8  also shows temperature characteristics of the electric heating catalyst  50  with the holes  57  and another catalyst with no holes, after the electricity was applied for 300 seconds. It is understood from  FIG. 8  that it is possible to increase the temperature of the 5th layer by about 60° C. by providing the holes  57 .  
      The combustion catalyst  60 , which is optionally provided in the invention, has a catalyst component (1% platinum and the remainder of alumina) in an amount of 200 g per liter of the catalyst. The combustion catalyst  60  (axial length: 120 mm) is disposed in the larger diameter portion  33  (inner diameter: 102 mm) of the heater unit  30 .  
      The heat exchanger  70  is a multi-pipe heat exchanger of stainless steel having a diameter of 102 mm, an axial length of 150 mm and a heat exchange capacity of 5 kW.  
       FIG. 9  shows a hydrogen combustion characteristic. It is understood from  FIG. 9  that hydrogen is subjected to a mild oxidation in a temperature region lower than a critical line G and that hydrogen is subjected to firing or ignition in a temperature region higher than the critical line G. This mild oxidation can be defined as being an oxidation free of firing or ignition of hydrogen. It is preferable to provide a temperature as high as possible for efficiently achieving hydrogen combustion. It is, however, understood from  FIG. 9  that hydrogen ignition, in place of its mild oxidation, occurs at a temperature higher than about 560° C. under atmospheric pressure (760 Torr). Therefore, it is possible to set the hydrogen combustion temperature of the invention at 500° C. in order to avoid hydrogen firing or ignition. It should be noted that hydrogen combustion in the electric heating catalyst or the combustion catalyst is conducted in the vicinity of atmospheric pressure (760 Torr).  
      As shown in  FIG. 10 , the maximum combustion temperature of a mixture of hydrogen and air changes depending on their mixing ratio. It is preferable in the invention to provide an air/hydrogen ratio greater than 8:1 in order to avoid hydrogen explosion and nitrogen oxides formation. Furthermore, it is understood from  FIG. 10  that, if a mixture of hydrogen and air is subjected to complete combustion, a combustion temperature of 500° C. can be obtained by an air/hydrogen ratio of about 15.3:1. The ratio may be varied to have a range of ±10% about 15.3:1. This range is also far from the hydrogen explosion region defined as being less than an air/hydrogen ratio of about 6:1. Thus, it is possible to maintain the combustion catalyst  60  at a temperature of 500° C. by adjusting the air/hydrogen ratio to about 15.3:1.  
      Operation of the hydrogen combustion heater will be described in the following (see  FIG. 1 ). At first, the water pump  65  is turned on to allow water to flow through the heat exchanger  64 , while electricity is applied to the electric heating catalyst  50 . Then, the blower  11  is energized to introduce air into the passage  13 , while the flow rate regulating valve  12  is controlled to have a predetermined air flow rate. Alternatively, the blower can directly be controlled to have a predetermined air flow rate. Furthermore, hydrogen gas is introduced into the passage  13  from the hydrogen introducing pipe  16  by opening the pressure reducing valve  15  and the first valve  19 . In fact, the hydrogen gas flow rate is set by the first valve  19  at 5 liters/min corresponding to {fraction (1/10)} of the flow rate under the steady state operation, and the air flow rate is set by the flow rate regulating valve  12  at about 76 liters/min. With this, the air/hydrogen flow rate ratio during the starting time of the hydrogen combustion heater operation becomes about 15.3:1.  
      Electricity is applied to the electric heating catalyst  50  for a predetermined time. When the electric heating catalyst  50  has a temperature of 200° C. or higher by applying electricity, the application of electricity is no longer necessary. In other words, the temperature of the electric heating catalyst  50  continues to rise until 500° C. by only the catalytic reaction (hydrogen combustion). It is possible to have electric heating catalyst temperatures of 200° C. and 300° C. by applying electricity for 60 seconds and 120 seconds, respectively. Therefore, it suffices to apply electricity for about 120 seconds.  
      The combustion catalyst  60  is heated by the combustion gas that has been passed through the electric heating catalyst  50  and has been heated until 500° C.  
      During the initial stage of the hydrogen combustion, the controller monitors the temperature of the combustion gas coming from the electric heating catalyst  50  by the temperature sensor  73 . In case that the combustion gas temperature exceeds a predetermined maximum temperature or does not rise during a predetermined time, the controller shuts down all the electric sources of the hydrogen combustion heater including the blower  11  and the water pump  65  since it is judged as being operational abnormality. Furthermore, the controller monitors temperatures in the vicinity of the combustion catalyst  60  by the temperature sensors  74  and  75 . In case that the monitored temperature exceeds a predetermined maximum temperature or does not rise during a predetermined time, the controller shuts down all the electric sources of the hydrogen combustion heater. Furthermore, in case that the pressure sensor  77  detects an abnormal pressure due to freezing or pipe clogging, the controller also shuts down all the electric sources of the hydrogen combustion heater.  
      When the temperature of the combustion catalyst  60 , which has been detected by the temperature sensor  74 , reaches 300° C., the second valve  20  is also opened to introduce hydrogen gas into the passage  13  from the hydrogen introducing pipe  16  at a total flow rate of 50 liters/min, and at the same time the air flow rate is adjusted to about 760 liters/min by the flow rate regulating valve  12 . With this, it is possible to have an air/hydrogen ratio of about 15.3:1. A part of the introduced hydrogen gas defined as flowing at a flow rate of 5 liters/min is subjected to an oxidation in the electric heating catalyst  50 , and the rest of that defined as flowing at a flow rate of 45 liters/min is subjected to an oxidation in the combustion catalyst  60 . The resulting combustion gas coming from the combustion catalyst  60  is heated to 500° C.  
      As stated above, each cell of the mixer  40  extends between the upstream and downstream ends of the mixer  40  in the axial direction of the smaller diameter portion  32 . Therefore, it becomes possible to have a smooth gas flow through each cell, thereby avoiding pressure loss. By the provision of the holes  44  in the mixer  40 , air and hydrogen gas can be uniformly mixed together. If this mixing is insufficient, a heat spot may be generated when the mixture is introduced into the electric heating catalyst  50 . However, according to the preferred embodiment of the invention, when hydrogen gas was allowed to flow from the hydrogen introducing pipe  16  at a flow rate of 50 liters/min and air was allowed to flow from the blower  11  at a flow rate of 760 liters/min, a heat spot was not generated. Therefore, it was possible to obtain a stable hydrogen combustion.  
      While the combustion gas, which has been heated to 500° C. in the combustion catalyst  60 , passes through the heat exchanger  64 , a heat exchange occurs between the combustion gas and pure water flowing at 6 liters/min by the water pump  65 . After that, the combustion gas (air) is discharged outside through the muffler  70 . The combustion gas that has passed through the heat exchanger  64  having a heat exchanging capacity of 5 kW still has a high temperature of about 200° C. or higher. Therefore, it is possible to prevent the occurrence of water condensation in the casing  31 .  
      During operation of the hydrogen combustion heater, when the temperature (detected by the temperature sensor  75 ) immediately downstream of the combustion catalyst  60  exceeds a predetermined temperature, the hydrogen gas flow rate is changed from 50 liters/min to 5 liters/min by closing the second valve  20 , and at the same time the air flow rate is also changed from about 760 liters/min to about 76 liters/min, thereby achieving a low-flow-rate combustion condition. Under this condition, the hydrogen combustion in the combustion catalyst  60  is stopped, and the combustion catalyst  60  is maintained in a heated condition by the combustion gas coming from the electric heating catalyst  50 . When it becomes possible to terminate the low-flow-rate combustion condition, the hydrogen and air flow rates are increased to the respective normal values (50 and about 760 liters/min). With this, the hydrogen combustion (oxidation) in the combustion catalyst  60  is immediately resumed. When the pressure sensor  77  detects an abnormal high pressure, the controller opens the relief valve  79 .  
      As stated above, the flow rate ratio of air to hydrogen gas is suitably controlled in the invention, thereby limiting the hydrogen combustion to a mild oxidation. This flow rate ratio of air to hydrogen gas is adjusted preferably to greater than 8:1, more preferably to about 15.3:1. Thus, it is possible to prevent firing or ignition of hydrogen gas. Furthermore, it is possible to avoid an excessive heating and thermal stress of the hydrogen combustion heater. Still furthermore, it is possible to suppress the formation of nitrogen oxides.  
      The mixer  40  of the invention does not have moving parts such as a screw. Therefore, it is improved in durability and lifetime.  
      The above-mentioned mixer  40  is prepared by winding a laminate of the flat and corrugated sheets  42  and  43  each having circular holes (diameter: 1.0 mm). This mixer  40  can be modified variously as long as the resulting cells are communicated with each other.  FIG. 11A  shows a modified sheet  41 ′ for making the mixer  40 . This sheet  41 ′ has a plurality of first and second cuts  48  and  49  that are alternately arranged in the direction along the width of the sheet  41 ′. Each first cut  48  is prepared by cutting a portion of a ridge  46  and then by folding the cut portion downward. Each second cut  49  is prepared by cutting a portion of a depression  47  and then by folding the cut portion upward. Thus, each of the first and second cuts has an opening for achieving a communication among the cells.  FIG. 11B  shows another modified sheet  41 ″ for making the mixer  40 . This sheet  41 ″ has triangular ridges and depressions  49 ′ and  48 ′ that are alternately arranged in the direction along the width of the sheet  41 ″.  
      Each of the above-mentioned flat and corrugated sheets  52  and  53  for preparing the electric heating catalyst  50  is formed with circular holes  57  (diameter: 1.0 mm). The shape of the holes is not particularly limited. For example, the holes may have a rectangular shape.  FIG. 12A  shows a sheet  51 ′ with rectangular holes  57 ′ each having a longitudinal axis arranged along the width of the sheet.  FIG. 12B  shows another sheet  51 ″ with rectangular holes  57 ″ each having a longitudinal axis arranged along the longitudinal direction of the sheet  51 ″.  
       FIG. 13  shows an upstream part of a second hydrogen combustion heater according to a second preferred embodiment of the present invention. This hydrogen combustion heater has an electric heating catalyst  80  that is provided with a function of the mixer. The electric heating catalyst  80  is disposed in a smaller diameter portion  32 ′ of a heater unit casing  31 ′.  
      As shown in  FIGS. 13 and 14 , the electric heating catalyst  80  is prepared by winding a laminate of flat and corrugated sheets  82  and  83 , then by brazing a plurality of spots of the wound body, and then pressing the wound body into an outer cylinder  84  of the smaller diameter portion  32 ′ (inner diameter: 58 mm). Each of the flat and corrugated sheets  82  and  83  is a stainless steel sheet (containing 20% Cr, 5% Al and the remainder of Fe) having a width of 80 mm and a thickness of 80 mm. The flat and corrugated sheets  82  and  83  each support thereon a catalyst component containing 1% Pt and the remainder of Al 2 O 3 . The electric heating catalyst  80  has a first electrode  85  passing through the center of the wound body and a second electrode  56  connected with the outer cylinder  84 . These electrodes  85  and  56  extend outward through an outer periphery of the casing  31 ′.  
      As shown in  FIG. 15 , a stainless steel sheet  81  used for the flat and corrugated sheets  82  and  83  is provided on its one side (length: 50 mm) with a perforated portion  86  having a plurality of circular holes (diameter: 1.0 mm) at a pitch of 2.5 mm in the direction along the width of the sheet  81  and at a pitch of 5 mm in the longitudinal direction of the sheet  81 . The corrugated sheet  83  has a pattern of waves having a height of 1.2 mm and a pitch of 2.6 mm. The resulting wound body has an electric resistance of about 0.8 Ω. The electric heating catalyst  80  is disposed in the smaller diameter portion  32 ′ in a manner to dispose the perforated portion  86  on the upstream side. Therefore, hydrogen gas and air introduced into the electric heating catalyst  80  are uniformly mixed together by a gas transportation or mixing among the cells of the electric heating catalyst  80 . The electric heating catalyst  80  is heated by applying electricity thereto during this mixing, too. Therefore, the catalytic reaction (hydrogen combustion) is assuredly accelerated. The electric heating catalyst  80  according to the second preferred embodiment is somewhat longer in the axial direction than the electric heating catalyst  50  according to the first preferred embodiment, since the former has the perforated portion  86 . It is, however, possible to shorten the length of the smaller diameter portion  32 ′ of the casing by the second preferred embodiment, as compared with the first preferred embodiment, since an independent mixer is not provided in the second preferred embodiment. Therefore, it becomes possible to make the size of the heater unit smaller in the second preferred embodiment. In  FIGS. 13-15 , the size and the pitch of the holes  87  are enlarged for easiness to understand the structure.  
      As shown in  FIG. 13 , a hydrogen introducing pipe  16 ′ (inner diameter: 8 mm) of the second hydrogen combustion heater passes through the passage  13 ′ (inner diameter: 58 mm) and is equipped with a hydrogen stopping valve  100 , in place of the pressure reducing valve, and is connected with a hydrogen reservoir (not shown). The hydrogen introducing pipe  16 ′ has a first hydrogen passage  17  with a first restrictor  102  and a second hydrogen passage  18  with a hydrogen stopping valve  104  and a second restrictor  106 . The first and second restrictors  102  and  106  are such that the ratio of the flow rate through the first restrictor  102  to that through the second restrictor  106  is 1:9. When only the hydrogen stopping valve  100  is opened, hydrogen gas passes through the first restrictor  102  and then is introduced into the passage  13 ′ at a flow rate of 5 liters/min. When the hydrogen stopping valve  104  is also opened, hydrogen gas passes through the first and second restrictors  102  and  106  and then is introduced into the passage  13 ′ at a total flow rate of 50 liters/min. The hydrogen stopping valves  100  and  104  are operated by gas. In fact, these valves are opened by adding thereto a pressure of a gas (air or inert gas) and maintained in a closed condition at a predetermined pressure or lower of the gas.  
      Gas supplying pipes  108  and  109  are respectively connected with the first and second hydrogen stopping valves  100  and  104  and are respectively formed with three-way solenoid valves  110  and  112 . These solenoid valves are each controlled by the controller. When the three-way solenoid valves  110  and  112  are opened, they add gas pressures to the hydrogen stopping valves  100  and  104 , respectively. When they are closed, they release pressures from the hydrogen stopping valves  100  and  104 , respectively. Therefore, the hydrogen stopping valves  100  and  104  are controlled to open or close by controlling the three-way solenoid valves  110  and  112 , respectively. As stated above, the hydrogen stopping valves  100  and  104  are operated by gas. Therefore, there is no danger of a contact between hydrogen gas and electric energy. Even if the operational fluid is leaked by an accident, the operational fluid is an inert gas or air. Therefore, there is no fear of having an accident, even if the operational gas is in contact with hydrogen gas.  
      As shown in  FIG. 1 , hydrogen gas is introduced into the passage  13  from a single opening of the hydrogen introducing pipe  16  in the first hydrogen combustion heater. In contrast, as shown in  FIG. 13 , hydrogen is introduced into the passage  13 ′ from a plurality of openings of the hydrogen introducing pipe  16 ′ in the second hydrogen combustion heater. This is preferable for achieving a uniform mixing of hydrogen gas and air. In fact, an end portion (hydrogen discharging portion) of the hydrogen introducing pipe  16 ′ passes through the passage  13 ′ upwardly, and this end portion has the above openings. The hydrogen introducing pipe  16 ′ is equipped with a filter  114  at a position immediately before the passage  13 ′ in order to separate water drops used for removing dust contained in hydrogen gas. The hydrogen introducing pipe  16 ′ is provided at its end with a pressure sensor  116  above the passage  13 ′. The pressure sensor  116  makes it possible to conduct a feedback control, thereby more highly precisely controlling the hydrogen gas flow rate. In case that the pressure sensor  116  is omitted, the hydrogen introducing pipe  16 ′ can have a closed end at a position near the after-mentioned opening  90  at the center of the section of the passage  13 ′.  
       FIGS. 16A and 16B  show a detailed arrangement of two openings  90  of the hydrogen introducing pipe  16 ′ for discharging hydrogen gas into the passage  13 ′. The upper opening  90  is directed upstream and positioned in the vicinity of the center of the passage  13 ′ in the section of  FIG. 16A . The lower opening  90  is also directed upstream and is at a position away from the bottom surface of the passage  13 ′ by a distance of ⅖ of the radius of the passage  13 ′. Such arrangement of the openings  90  makes it possible to start a mixing of hydrogen gas and air at the position of the openings  90  in the entire section of the passage  13 ′. Therefore, even if the distance between the end portion of the hydrogen introducing pipe  16 ′ and the electric heating catalyst  80  is short, it is possible to have a substantial degree of mixing of hydrogen gas and air when they reach the electric heating catalyst  80 . With this, it is possible to avoid the generation of heat spot at the upstream end surface of the electric heating catalyst  80 . The passage  13 ′ has an inner diameter of 58 mm, the hydrogen introducing pipe  16 ′ has an outer diameter of 10 mm and an inner diameter of 8 mm, and each of the openings  90  has a diameter of 1.5 mm.  
       FIG. 17B  shows hydrogen concentration changes or differences of respective positions (A-F) in the section (see  FIG. 17A ) of the passage  13 ′ at a position 30 mm downstream of the passage  13 ′ from the hydrogen introducing pipe  16 ′, from the average hydrogen concentration, when hydrogen gas was allowed to flow at 50 liters/min from the hydrogen introducing pipe  16 ′, and when air was allowed to flow at 760 liters/min from the blower. The inner diameter of the passage  13 ′ is represented by “d” in  FIG. 17A . It is understood from  FIG. 17B  that hydrogen concentration changes of the positions A-F were within a range of +1 to −1%. This means that a uniform mixing of hydrogen gas and air is already achieved at the upstream end of the electric heating catalyst  80 . Thus, it was possible to avoid the heat spot generation and to obtain a stable hydrogen combustion.  
       FIG. 17C  shows the results when the hydrogen and air flow rates in the case of  FIG. 17B  were each reduced by a factor of 6. It is understood from  FIG. 17C  that hydrogen concentration changes of the positions A-F were within a range of +2 to −2%.  
       FIG. 18A  shows a first modification  16 ″ of the hydrogen introducing pipe  16 ′ of  FIG. 16 . The first modification  16 ″ has three openings  90  for discharging hydrogen gas. The upper opening  90  is positioned at the center of the section of the passage  13 ′, and the middle and lower openings  90  are respectively at positions away from the bottom surface of the passage  13 ′ by distances of ⅔ and ⅓ of the radius of the passage  13 ′.  FIG. 18B  shows the results obtained under the same conditions as those for  FIG. 17B , except that the first modification  16 ″ was used for the hydrogen concentration determination. It is understood from  FIG. 18B  that hydrogen concentration changes of the positions A-F were within a range of +1 to −1%. This means that hydrogen gas and air were sufficiently mixed together.  
       FIG. 19A  shows a second modification of the hydrogen introducing pipe  16 ′ of  FIG. 16 . The second modification has three openings  90 A. The middle opening is positioned at the center of the section of the passage  13 ′, and the upper and lower openings are at symmetrical positions about the middle opening, as shown in  FIG. 19A .  FIG. 19B  shows the results obtained under the same conditions as those for  FIG. 17B , except that the second modification was used for the hydrogen concentration determination. It is understood from  FIG. 19B  that hydrogen concentration changes of the positions A-F had a greater variation (+5 to −4%) than those of  FIGS. 17B and 18B . This means that a heat spot tends to be generated and thereby it may be difficult to obtain a stable hydrogen combustion by the second modification.  
       FIG. 20  shows a third hydrogen combustion heater according to a third preferred embodiment of the present invention. This third heater is similar in construction to the first heater. Therefore, explanation of the same parts and the same functions may be omitted from the following description.  
      The third heater has a passage  130 , a mixer  140 , an electric heating catalyst  150 , a combustion catalyst  160 , and a heat exchanger  170 . This passage  130  corresponds to the passage  13  of  FIG. 3 . The mixer  140 , the electric heating catalyst  150  and the combustion catalyst  160  are disposed in their respective casings  132 ,  134  and  136 . Each of these casings has the same diameter as that of the passage  130  (inner diameter: 58 mm). The passage  130  and these casings  132 ,  134  and  136  are connected to constitute a single straight passage.  
      The electric heating catalyst  150  and the combustion catalyst  160  are respectively the same as those 50 and 60 of  FIG. 1 , except in that the catalysts  150  and  160  are the same in diameter.  
      The heat exchanger  170  is formed of flat sheets and corrugated sheets alternately laminated together. The heat exchanger  170  has a heat exchanging portion  171  having a shape of rectangular parallelepiped. The heat exchanging portion  171  is formed with first cells  172  and second cells  173  that are alternately laminated together at right angles relative to each other. In fact, the first and second cells  172  and  173  are formed by arranging adjacent corrugated sheets at right angles relative to each other. The heat exchanging portion  171  is disposed at a central portion of the casing  138 . The heat exchanger  170  is further formed at both side ends of the second cells  173  with first and second tanks  174  and  176  that communicates with each second cell  173 . A water introducing pipe  175  and a water discharging pipe  177  are respectively attached to the first and second tanks  174  and  176 . The combustion gas and water are respectively allowed to flow through the first and second cells  172  and  173  of the heat exchanging portion  171 . Thus, when water introduced into the first tank  174  from the water introducing pipe  175  passes through the second cells  173 , it is heated by the combustion gas flowing through the first cells  172 . The casing  138  has a rectangular section at the position of the heat exchanging portion  171 . This rectangular section gradually changes into a circular section toward an upstream position. Thus, the casing  138  is fitted at its upstream end with the casing  136  of the combustion catalyst  160 . Similarly, the rectangular section of the casing  138  gradually changes into a circular section toward a downstream position.  
      Although not shown in  FIG. 20 , the third hydrogen combustion heater has the temperature sensors  73 ,  74 ,  75  and  76 , the pressure sensor  77 , the temperature sensor  78 , and the relief valve  79  at positions corresponding to those of the first hydrogen combustion heater shown in  FIG. 1 .  
      As shown in  FIG. 21A , a hydrogen introducing pipe  100  extends in a horizontal direction and passes transversely through the passage  130  and has four openings  90   a - 90   d  for discharging hydrogen gas into the passage  130 . The hydrogen introducing pipe  100  has a filter  114  for separating water drops used for removing dust contained in hydrogen gas, immediately before the passage  130 . The hydrogen introducing pipe  100  is provided at its end with a pressure sensor  116  immediately after the passage  130 .  
      As is seen from  FIG. 21A , the opening  90   a  is disposed at the center of the section of the passage  130 , the openings  90   c  and  90   d  are disposed upstream of the opening  90   a , and the opening  90   b  is disposed downstream of the opening  90   a . For example, the passage  130  has an inner diameter of 58 mm; the hydrogen introducing pipe  100  has an outer diameter of 10 mm and an inner diameter of 8 mm; and each of the openings  90   a - 90   d  has a diameter of 1.5 mm. Each of the openings  90   b  and  90   d  are disposed at positions away from the opening  90   a  by a distance of 12 mm. It is preferable that the total opening area of the openings (the openings  90   c  and  90   d  in the case of  FIG. 21A ) positioned in the upstream half of the hydrogen introducing pipe in the section of the passage  130  is larger than that of the openings (the opening  90   b  in the case of  FIG. 21A ) positioned in the downstream half thereof. If each opening has the same opening area, the number of the openings positioned in the upstream half is preferably greater than that positioned in the downstream half. Furthermore, as shown in  FIG. 21B , the openings  90   a - 90   d  of the hydrogen introducing pipe  100  are each directed upstream of the passage  130  to be within 45 degrees down from horizontal (i.e., 0-45 degrees).  
      As shown in  FIG. 20 , the mixer  140  has first, second and third members  141 ,  142  and  143  that are spaced from each other and disposed in the casing  132 . As shown in  FIG. 22A , the first member  141  has a center hole  145  (diameter: 35 mm). The second member  142  has four holes  146  (diameter: 20 mm) equally spaced from each other. The third member  143  has 69 holes  147  (diameter: 6 mm) equally spaced from each other. The first and second members  141  and  142  are spaced from each other by a distance of 20 mm. The second and third members  142  and  143  are also spaced from each other by a distance of 20 mm. When the passage  130  and the electric heating catalyst  150  are connected with each other, the first member  141  and the hydrogen introducing pipe  10  are spaced from each other by a distance of 20 mm, and the third member  143  and the electric heating catalyst  150  are spaced from each other by a distance of 30 mm.  
      Operation of the third hydrogen combustion heater is substantially the same as that of the first hydrogen combustion heater. Therefore, the same descriptions are not repeated in is the following. Air and hydrogen gas are uniformly mixed together, when they pass through the first, second and third members  141 ,  142  and  143  of the mixer  140 .  
      As mentioned above, the hydrogen introducing pipe  100  is connected with the passage  130  in a manner to horizontally pass therethrough (see  FIG. 21A ). Furthermore, the openings are disposed such that the number of the openings in the upstream half is greater than that of the openings in the downstream half. Therefore, it became earlier to start a mixing of hydrogen gas and air, as compared with the case of providing a single opening to discharge hydrogen gas. Furthermore, it was possible to obtain a higher degree of mixing, even as compared with the case of providing a plurality of openings equally disposed about the center of the section of the passage  130 .  
       FIG. 23A  shows a hydrogen gas flow from the downstream and upstream openings  201  and  202  and the middle opening (no numeral) therebetween of the hydrogen introducing pipe  200  passing through the passage  130  in a horizontal direction. The openings  201  and  202  are equally spaced from the middle opening. In this case, as shown by the arrows of  FIG. 23A , the direction of the hydrogen flow from the upstream opening  202  is more inclined toward the direction of the hydrogen gas flow in the hydrogen introducing pipe  200 , as compared with that of the hydrogen flow from the downstream opening  201 . With this, there is a tendency to generate a low hydrogen concentration region R 1  near the upstream opening  202 . Furthermore, there is a tendency to generate a high hydrogen concentration region R 2  near the downstream opening  201 , due to the concentration of the hydrogen flows toward the region R 2 . In other words, there is a tendency to generate a hydrogen concentration imbalance in the case of  FIG. 23A . In contrast, according to the preferred embodiment shown in  FIG. 23B , the number of the openings  90   c  and  90   d  disposed in the upstream half is greater than that of the openings  90   b  disposed in the downstream half. Therefore, there is no tendency to generate the above-mentioned hydrogen concentration imbalance. This is advantageous to achieve a stable hydrogen combustion. In  FIG. 23B , the middle opening, the pressure sensor and the like are omitted for simplicity.  
      As stated above, the openings of the hydrogen introducing pipe  100  are directed upstream of the passage  130  to be within 45 degrees down from horizontal (0-45 degrees), as shown in  FIG. 21B . With this, it was possible to obtain a good mixing of hydrogen gas and air. In other words, the hydrogen concentration variation at a position 30 mm downstream of the passage from the hydrogen introducing pipe in a first case that the openings are directed upstream to be within 45 degrees down from horizontal was markedly less than that in a second case that the openings are directed upstream to be up from horizontal or in a third case that they are directed downstream. In the third case, it is assumed that hydrogen gas and air reach, the position 30 mm downstream of the passage from the hydrogen introducing pipe, while they are not sufficiently mixed together. In the second case, hydrogen tends to remain in an upper region of the passage  130  without a sufficient mixing with air, since hydrogen gas is lighter in weight than air.  
       FIG. 24A  shows an optional arrangement of the openings  90   a - 90   d  of the hydrogen introducing pipe  100 . In fact, only the opening  90   a  is directed upstream to be within 45 degrees down from horizontal (see  FIG. 24B ), and the openings  90   b - 90   d  are directed upstream to be horizontal (see  FIG. 24C ). This optional arrangement can also achieve a sufficient mixing of hydrogen gas and air.  
      The above-mentioned openings of the hydrogen introducing pipe according to the preferred embodiment of the invention have the same size and are arranged such that the number of the openings in the upstream half is greater than that of the openings in the downstream half. Alternatively, the openings in the upstream half can at least partly be made to have a size greater than those in the downstream half. In this case, the number of the former may be the same as that of the latter.  
      As stated above, the mixer  140  has first, second and third members  141 ,  142  and  143  (see  FIGS. 20 and 22 A- 22 C). In fact, the first, second and third members are disposed in this order toward a downstream position. In other words, the number of the holes  145 ,  146  and  147  is increased, and the size of these holes is reduced, as it passes from the first member  141  to the third member  143 . Therefore, hydrogen gas and air are sufficiently mixed together as they flow downstream from the first member  141  to the third member  143 .  
       FIG. 25A  shows numbers representing temperatures at their respective positions in the section of the hydrogen combustion heater at a position of the downstream end of the electric heating catalyst, when the first to third members  141 - 143  constitute the mixer (first mixer)  140 . In fact, the respective positions in the section were selected by dividing each of the vertical and horizontal diametral lines of the section into five sections.  FIG. 25B  is a slight modification of  FIG. 25A , showing those numbers when the first to third members  141 - 143  and a fourth member constitute the mixer (second mixer). The fourth member is disposed downstream of the third member  143  and has a plurality of holes that are greater in number than those 147 of the third member  143 .  FIG. 25C  is another slight modification of  FIG. 25A , showing those numbers when only the second and third members  142 - 143  constitute the mixer (third mixer). It is understood from  FIG. 25A  in the case of the first mixer that the maximum and minimum temperatures are respectively 654° C. and 500° C. It is understood from  FIG. 25B  in the case of the second mixer that they are respectively 694° C. and 555° C. It is understood from  FIG. 25C  in the case of the third mixer that they are respectively 750° C. and 500° C.  FIG. 25D  shows the standard deviation and the difference between the maximum and minimum temperatures with respect to the temperatures of  FIGS. 25A-25C . It is understood from  FIG. 25D  that the standard deviation and the difference between the maximum and minimum temperatures in the case of the first and second mixers are substantially different from those in the case of the third mixer. This means that the first and second mixers make it possible to achieve a more efficient mixing of hydrogen gas and air, as compared with the third mixer. Furthermore, it is understood from  FIG. 25D  that results of the second mixer are slightly better than those of the first mixer. It should be noted that the results of the first mixer are also enough and acceptable.  
      If a mixing of hydrogen gas and air is not sufficient, a heat spot may occur when they are introduced into the electric heating catalyst  150 . In contrast, according to the third hydrogen combustion heater of the invention, there was no heat spot and a stable hydrogen combustion when hydrogen gas was allowed to flow from the hydrogen introducing pipe  100  at a total flow rate of 50 liters/min and when air was allowed to flow from the blower  11  at a flow rate of 760 liters/min.  
      As shown in  FIG. 22C , the third member  143  of the mixer  140  disposed in the passage (inner diameter: 58 mm) have the holes  147  (diameter: 6 mm) that is 69 in number. It suffices that the third member  143  has a plurality of holes that is 10 or greater in number, which is about double of the second member, for obtaining a mixing that is in the same level as that of the third member  143  having 69 holes.  
       FIG. 26  shows a fourth hydrogen combustion heater according to a fourth preferred embodiment of the invention for a smaller output use, in which the combustion catalyst  160  of the third hydrogen combustion heater is omitted. Thus, a heat exchanger  170 ′ is disposed immediately downstream of the electric heating catalyst  150 .  
      The heat exchanger  170 ′ has a cylindrical shape. In fact, it has a cylindrical casing  138 ′ having a central bulge portion. First and second partition walls  180  and  182  are disposed in and water-tightly attached to the central bulge portion of the casing  138 ′. Each partition wall has a plurality of holes  184 , and each hole  184  of the first partition wall  180  is aligned with the corresponding hole  184  of the second partition wall  182 . A plurality of pipes  186  are water-tightly attached to the first and second partition walls  180  and  182  in a manner to connect each hole  184  of the first partition wall  180  with the corresponding hole  184  of the second partition wall  182 . In fact, the pipes  186  serve as passages of the combustion gas coming from the electric heating catalyst  150 . There is provided a room R 3  defined as being a space that is outside of the pipes  186 , is inside of the casing  138 ′ and is between the first and second partition walls  180  and  182 . Water introducing and discharging pipes  175  and  177  are connected to the room R 3  at its diametrical positions. Pure water is allowed to flow through the room R 3 . The upstream end of the casing  138 ′ has a diameter that is the same as that of the casing  134  of the electric heating catalyst  150 . The other structures of the fourth hydrogen combustion heater are the same as those of the third hydrogen combustion heater.  
      As stated above, the fourth hydrogen combustion heater has no combustion catalyst. After the electric heating catalyst  150  is heated to 200° C. or higher in this heater, the temperature continues to rise by the catalytic reaction to 500° C. Therefore, when the required quantity of heat is low (for example, when pure water is allowed to flow through the heat exchanger  170 ′ at a low flow rate) and when hydrogen gas for combustion is in a small amount, it becomes possible to achieve a sufficient combustion of a mixture of hydrogen gas and air only by the electric heating catalyst  150 . Thus, the fourth hydrogen combustion heater is preferably used for small output uses.  
      As compared with the third hydrogen combustion heater, the fourth heater can be compact in size, since the casing of the heat exchanger  170 ′ has a circular section, thereby making the fourth heater have a thin shape.  
      Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.  
      The entire disclosure of each of Japanese Patent Applications No. 2000-240816 filed on Aug. 9, 2000, No. 2001-177578 filed on Jun. 12, 2001, and No. 2001-178589 filed on Jun. 13, 2001, including specification, drawings, claims and summary, is incorporated herein by reference in its entirety.