Patent Publication Number: US-2009239181-A1

Title: Combustor

Description:
TECHNICAL FIELD 
     The present invention relates to a combustor for obtaining a high-temperature gas, e. g., for a fuel reforming apparatus in a compact fuel cell system for family use. 
     BACKGROUND ART 
     The combustion gas coming from a combustor of this type should desirably be free from pollutants, namely NOx and unburned fuel and be also the least in the amount of residual oxygen by complete combustion. 
     Conventional combustors of this type are known to include (1) a combustor in which air is mixed with a fuel in a concentration within its explosion mixture limits and a resultant mixed gas of air and fuel is ignited by an igniting means such as electric spark to form a flame, followed by continued combustion of the mixed gas; (2) a combustor in which a preheated mixed gas made by mixing a fuel into preheated air is combusted by means of a combustion catalyst; (3) a combustor in which a heating tube in a primary combustor is always externally heated by an auxiliary combustor to more than an ignition temperature of a fuel and a mixed gas of fuel and air is brought into contact with the heated heating tube in the primary combustor to continue combustion; and (4) a combustor in which a mixed gas of a fuel and air is brought into contact with a tube heated to more than an ignition temperature of the fuel and thereby combusted and the combustion gas is U-turned at an end of the tube while heated to continue combustion (JP H10-26309 A). 
     Any of the conventional combustors mentioned above cannot serve satisfactorily as a boiler&#39;s combustor used for a fuel reforming apparatus in compact fuel cell systems for family use. 
     The reason is because the combustor used for an apparatus of this type needs to be compact and small in volume of its combustion chamber and requires its surrounding wall surface temperature to be of a 110 to 180°, thus requiring a complete combustion with a reduced NOx concentration, though under very adversely limited conditions for combustion. 
     In addition, the fuel chamber small in volume means that it is liable to undergo a pressure fluctuation. In the case of flame combustion, a fluctuation in pressure likely causes the fuel to temporarily stop flowing, the flame to disappear and the combustion to stop continuing, so that vapor is no longer coming out. 
     Consequently, the S/C (steam/carbon) ratio in the area of a reforming catalyst becomes lower than an optimum value so that carbon is generated, causing the fuel reforming apparatus on the whole to cease operating. 
     Furthermore, the compact fuel cell system for family use calls for the use of both a gaseous fuel and a liquid fuel. While when it is started to operate, there is no choice but to use an auxiliary igniter, there must be adopted a method whereby the auxiliary igniter may thereafter be made unnecessary in a shortest possible time and moreover, even in the presence of a disturbance such as pressure fluctuation, the combustion is prevented from cessation. 
     DISCLOSURE OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a combustor which eliminates generation of NOx and unburned combustible, as a pollutant, which is compact and which makes it possible to obtain a combustion gas stably even if there is a bit of disturbance. 
     There is provided in accordance with the present invention in a first aspect thereof a combustor which comprises a fuel air mixer of venturi tube type having an igniter and a ceramic combustion tube having an oxidation catalyzing function positioned downstream of the said fuel air mixer. 
     In a second aspect thereof, the present invention provides a combustor in the first aspect which further comprises a vortex generator disposed at an extremity of the fuel air mixer of venturi tube type such that the air fuel mixture becomes a vortex in the said combustion tube. 
     In a third aspect thereof, the present invention provides a combustor in the first or second aspect which further comprises a top closed cylindrical cover member with which a downstream extremity of the ceramic combustion tube is covered to form a combustion chamber around the extremity of the ceramic combustion tube, the cover member having a cylindrical base end portion formed with an outlet for the combustion gas. 
     In a fourth aspect thereof, the present invention provides a combustor in the third aspect in which the cover member has an outer peripheral surface having a radiation promoting paint applied thereto. 
     In a fifth aspect thereof the present invention provides a combustor in the third or fourth aspect in which an oxidation catalyst layer is interposed between the ceramic combustion tube and the base end portion of the cover member. 
     According the first aspect of the invention, a combustor can be obtained that is suitable for use as a boiler&#39;s combustor for a fuel reforming apparatus in a compact fuel cell system for family use of which development is now in progress, that has a small combustion chamber and that is capable of combusting efficiently and stably. 
     According to the second aspect of the invention, a combustor can be obtained that is capable of combusting stably since an air fuel mixture is fully contacted by an oxidation catalyst. 
     According to the third aspect of the invention, a combustion gas flowing out of the ceramic combustion tube is allowed to promote its combustion further in the combustion chamber constituted with the cover member. 
     According to the fourth aspect of the invention, radiant heat can be radiated from an outer surface of the cover member onto a portion to be heated, thereby to promote heating the latter. 
     According to the fifth aspect of the invention, a combustion gas flowing out through a through hole from the combustion chamber in the cover member is allowed to pass through the catalyst layer having an oxidation function; hence an unburned combustible which there may remain in the combustion gas after passed to combust through the ceramic combustion tube can completely be burned while passing through the catalyst layer. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings: 
       FIG. 1  is a cross sectional view illustrating a combustor according to a first form of implementation of the present invention; 
       FIG. 2  is a cross sectional view illustrating a form of boiler in which the combustor as the first form of implementation of the invention; and 
       FIG. 3  is a cross sectional view illustrating a combustor according to a second form of implementation of the present invention and another form of boiler equipped therewith. 
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     An explanation is given of a first form of implementation of the present invention with reference to  FIGS. 1 and 2 .  FIG. 1  shows in a cross section the essential parts of a combustor of the present invention, and  FIG. 2  shows in a cross section a form of boiler in which the combustor of the invention is used. 
     In  FIG. 1 , a boiler main body  1  comprises an inner cylinder  2  and an outer cylinder  3  coaxially arranged. Mounted at the lower end of the boiler main body  1  is a combustor  4  according to the present invention. A high temperature combustion gas generated by the combustor  4  is supplied into the inner cylinder  2  of the boiler main body  1  along its inner surface to heat the inner surface of the inner cylinder  2  whereby water (evaporating medium) supplied in a heating medium jacket  5  formed between the inner and outer cylinders  2  and  3  coaxially arranged to generate a high temperature steam. A boiler lower flange  6  is coupled integrally with the lower end of the boiler main body  1  to close the heating medium jacket  5  for the evaporating medium, and a combustor flange  7  integrally attached to an outer periphery of the combustor  4  is coupled to the boiler lower flange  6  by means of a bolt  8  and a nut  9  so that the combustor  4  is coupled to the lower end of the boiler main body  1  in the state that a cover member  10  lying at the uppermost of the combustor  4  projects into the inner cylinder  2  of the boiler main body  1 . 
     The combustor  4  comprises a combustion section  11  including a ceramic combustion tube  14  through which the combustion gas spurts and a fuel supply section  12  for supplying the combustion section  11  with a fuel air mixture, namely a mixture of a fuel and air. 
     The combustion section  11  comprises a cover member  10  defining a combustion chamber  13  in its inside at the uppermost of the combustion section  11 , a ceramic combustion tube  14  disposed in an axial central area of the cover member  10  and opened at its top to the combustion section  11 , heat insulators  15   a,    15   b  and  15   c  composed of ceramic and disposed so as to surround outer lower sides of the ceramic combustion tube  14  and a frame  16  surrounding these heat insulators and securely connected to the combustor flange  7 . 
     The cover member  10  which is in the form of a bell (or canister) is mounted so as to cover the heat insulators  15   a,    15   b  and  15   c  and has its cylindrical base end portion formed with slit-like through holes  17  (an outlet for combustion gas) elongated vertically. And, the cover member  10  has in its side a catalyst layer  18  of small ceramic particles having an oxidizing function up to a height of the through holes  17 . The upper end of the ceramic combustion tube  14  is spaced by a distance L (spacing) from the top inner surface of the cover member  10  in the combustion chamber  13  and has a given height H from the upper surface of the catalyst layer  18  such that a combustion gas flowing out of the ceramic combustion tube  14  passes through the combustion chamber  13  and the catalyst layer  18 , then flowing through the through holes  17  into the inner cylinder  2  of the boiler main body  1 . 
     The fuel supply section  12  is securely connected to the lower end of the combustion section  11  by means of a fitting flange  21  which is disposed at its upper end and fastened by bolts  22  via a packing to the lower end of the combustion section  11 . 
     A mixer of venturi tube type  23  is fastened to the fitting flange  21  so as to pass therethrough. The exit portion of the mixer of venturi tube type  23  is concentric with the ceramic combustion tube  14  of the combustion section  11  and has an exit diameter substantially identical to the inner diameter of the ceramic combustion tube  14  and its exit end or extremity abutting on the lower end of the ceramic cylinder  14 . And, a region of the abutment is provided with a vortex generator  24 . The vortex generator  24  is configured so as to turn an air fuel mixture flow from the mixer of venturi tube type  23  circumferentially thereof, namely to generate a vortex such as flowing on and along the inner peripheral surface of the ceramic combustion tube  14 . 
     The throat portion of the mixer of venturi tube type  23  is formed on its inner peripheral surface with a plurality of fuel ejection ports  25 . The fuel ejection ports  25  are connected to a fuel supply port  26  via a fuel manifold  25   a  disposed in the throat portion of the mixer of venturi tube type  23 . 
     The mixer of venturi tube type  23  is provided along its axial center portion with an ignition rod guiding tube  27 . The ignition rod guiding tube  27  has its upper end portion fastened to the center of the vortex generator  24  and its lower end portion projecting downwards of the mixer of venturi tube type  23 . And, this ignition rod guiding tube  27  has an ignition rod  28  inserted therein so that its extremity protrudes from the ignition rod guiding tube  27 . 
     The mixer of venturi tube type  23  has its base end portion connected to a cross tube joint  29  in which the lower end portion of the ignition rod guiding tube  27  is positioned. And, one of two joining ports orthogonal to an extension of the port in which the mixer of venturi tube type  23  joins into this tube joint  29  constitutes an igniting power supply rod joining port, in which an igniting power supply rod  30  is inserted so that its forward end is electrically connected to the ignition rod in the tube joint  29 . Also, the other of the joining ports is connected to a port  31  for air supply for combustion. Further, a combustion monitoring port  32  is connected in a joining port on the extension of the port in which the mixer of venturi tube type  23  joins into the tube joint  29  to make the inside of the mixer of venturi tube  23  viewable. 
     Mention is made below of an operation of the combustor  4  for a boiler main body  1  as shown in  FIG. 1 . 
     Combustion air is supplied from the air supply port  31  in the fuel supply section  12  while a fuel supplied from the fuel supply port  26  is caused to spurt from the fuel ejection port  25  into the throat portion of the mixer of venturi tube type  23  whereby the air and fuel until they reach the exit of the mixer of venturi tube type  23  from that throat portion are mixed together into an fuel air mixture which then flows out of the exit of the mixer of venturi tube type  23  into the ceramic combustion tube  14 . The fuel air mixture then flowing into the ceramic combustion tube  14  has, owing to the vortex generator  24 , become a vortex flowing on and along the inner peripheral surface of the ceramic combustion tube  14 . And, it is in this state that the igniting power supply rod  30  is energized to create a spark at the tip of the ignition rod  28 , thereby igniting the fuel air mixture. 
     This commences combusting the fuel air mixture. And, after the ignition, though affected by the fuel air mixture proportion value, the combustion is allowed to continue if the ceramic combustion tube  14  is heated to more than a temperature at which it functions as an oxidation catalyst even if the sparking is ceased in two minutes or less. 
     In case a liquid fuel (e. g., kerosene) is used in the combustion, combustion air is supplied upon pre-heating to around 200 to 250°, although the fuel per se is supplied at the normal temperature. In this form of implementation, a gaseous fuel can also be used. 
     Where the ignited fuel air mixture commences combusting, the combustion gas flows through the inside of the ceramic combustion tube  14  upwards along its inner peripheral wall. As a result, the ceramic combustion tube  14  per se is heated up to more than a catalyst activation temperature. As a consequence, even if the ignition by the ignition rod  28  is ceased at the time the ceramic heat chamber  14  is heated up, bringing the fuel air mixture from the mixer of venturi tube type  23  into contact with the inner peripheral surface of the ceramic combustion tube  14  heated up to the catalyst activation temperature or more allows the oxidation reaction to be brought about immediately in the mixture, thus continuing the combustion. 
     Such combustion becomes stronger as the mixture runs downstream of the ceramic combustion tube  14 , so that at the upper end of the ceramic combustion tube  14  most of the mixture is combusted. Note here that the state of combustion then is affected by the length of the ceramic combustion tube  14 , from which the length of the ceramic combustion tube  14  is designed to be a length at which most of the mixture supplied from the fuel supply section  12  is combusted. 
     Since the fuel air mixture supplied into the ceramic heat chamber  14  has been mixed in a uniform concentration, the combustion reaction in the ceramic heat chamber  14  proceeds very rapidly. For example, a combustion speed of around 7/1000 second is shown when the fuel air mixture has a theoretical combustion temperature of around 1400 to 1600°. For this sake, the ceramic combustion tube  14  should preferably be composed of a material having properties as stated below. 
     To wit, the conditions necessary for the ceramic combustion tube  14  having the oxidation function mentioned above are: (1) high in resistance to thermal shock, (2) low in catalyst activation temperature, (3) high in thermal emissivity, (4) high in mechanical strength at high temperatures, and (5) high in thermal conductivity. 
     Suitable as ceramics having such properties are SiC, Si 3 N 4 , ZrO 2  and one having 5 to 20% of another metal oxide or oxides added to ZnO 2 . 
     Non-oxide ceramics of Si such as SiC and Si 3 N 4 , though they have catalyst activation temperatures as relatively high as 640 to 645°, can enough be used practically as they elsewhere meet the conditions mentioned above. While ZrO 2  simple exhibits a catalyst activation temperature of 465° C., ceramics having 5% to 20% of CoO, Cr 2 O 3 , MnO 2 , La 2 O 3 , SnO 2 , Y 2 O 3 , TbO 2  and MgO added thereto can have catalyst activation temperatures reduced to 330° F. to 497° F. (see “Catalytic Combustion by ZrO 2  compound oxide” read on the 9 th  Symposium on Catalytic Combustion (May 25, 1990)). 
     The reason for requirement for high resistance to thermal shock in (1) above is that rapid combustion speed is accompanied by large change in heat quantity which the ceramic combustion tube  14  must accept from its inside. To wit, large change in heat quantity which the ceramic combustion tube  14  receives means large temperature change of the ceramic combustion tube  14  itself. The thicker becomes the ceramic combustion tube  14 , the larger becomes the temperature difference in the ceramic combustion tube  14 , to an extent that the ceramic combustion tube  14  may be broken by thermal shock. To prevent this, the requirement also arises for high thermal conductivity in the condition (5) above. Also, the requirement for high thermal conductivity is suitable for transfer of the heat received in the downstream side to the upstream side. Also, the requirement for high thermal emissivity in (3) exhibits analogous operations and effects. 
     The combustion gas generated in the ceramic combustion tube  14  flows through the extremity of the ceramic combustion tube  14  into the combustion chamber  13  where it turns downwards along the inner surface of the cover member  10  constituting the combustion chamber  13 . The combustion gas then flows through the catalyst layer  18  and flows out into the inner cylinder  2  of the boiler main body  1  from the through holes  17 . 
     The combustion gas then for combustion, if the flow path through which it flows is small in volume and surrounded by a cold wall surface, is insufficiently combusted to the extent that unburned combustible may remain in the combustion gas. In this combustion section  11 , however, the combustion gas which flows from the ceramic combustion tube  14  into the combustion chamber  13  constituted by the cover member  10  and thereby formed with a large space continues to be fully combusted to a high temperature (1500 to 1700°) whereby the unburned combustible is burned substantially completely. 
     And, the combustion gas in the combustion chamber  13  turns downwards along the inner surface of the cover member  10  constituting the combustion chamber  13 . After passing through the catalyst layer  18 , it flows through the through holes  17  into the inner cylinder  2  of the boiler main body  1 , heating the inner surface of the inner cylinder  2 . And, the combustion gas then by contacting ceramic small particles in the catalyst layer  18  has its unburned components completely burned. And, the presence of this catalyst layer  18  makes it possible to prevent the combustion from being ceased by a pressure fluctuation of the fuel and combustion air. 
     It is advantageous if the ceramic small particles having the oxidation function and making up the catalyst layer  18  are Si hollow particles. 
     Concerning the combustor  4  shown in the above mentioned form of implementation, to mention the sizes of the components in an example of the combustor for steam generation in a fuel cell system for family use, the ceramic combustion tube  14  has an inner diameter of 20 mm, a thickness of 2.5 mm and a length of 40 mm and the cover member  10  has an inner diameter of 50 mm and H and L as indicated in  FIG. 1 , sized of 10 mm and 15 mm, respectively. 
     The combustor has hereinbefore been shown in  FIG. 1  and described, which is an example applied to the boiler main body  1  for steam generation in a fuel cell system for family use. Water filled in the heating medium jacket  5  of the boiler main body  1  is boiled to generate a steam which is guided out through a steam tube connected to an upper part of the heating medium jacket  5 . In this case, in order for the cover member  10  to be used over a prolonged time period, the heat received from the high-temperature gas need to be transferred as much as possible towards the heating medium jacket  5 . In this form of implementation, a radiation promoting paint, e. g., whose thermal emissivity is 92% or more, may be applied over the outer peripheral surface of the cover member  10  made of steel so that the wall portion of the cover member  10  can be held at not more than 950° F. if a combustion gas whose theoretical combustion temperature is 1600° is generated in the combustion section  11 . Thus, applying such a radiation promoting pair makes it possible for the cover member  10  to bear its services over a prolonged time period. 
       FIG. 2  shows a form of implementation of the boiler using the combustor  4  according to the present invention in the makeup in which a heat exchanger section  34  comprising a number of water pipes  33 ,  33 , , , , is disposed above the combustion section  11  within the inner cylinder  2  of the boiler main body  1 . Note here that an exhaust pipe  40  is connected to communicate with the inside of the inner cylinder  2 . 
     A water retaining tank  35  is disposed above the heat exchanger  34 , which has a water supply pipe  36  connected thereto. And, The water pipes  33  in the heat exchanger section  34  have their respective inlet ports  33   a  connected to the water retaining tank  35  and their respective outlet ports connected to communicate with an outlet chamber  37 . The outlet chamber  37  constitutes a space formed between an upper part of a supporting plate  39  supporting the heat exchanger section  34  and a bottom part of the water retaining tank  35 . The outlet chamber  37  serves to separate gas and liquid concurrently from both a gas liquid mixture in a vaporized state coming from the heating medium jacket  5  of the boiler main body  1  and a gas liquid mixture in a vaporized state coming from the outlet of the heat exchanger section  34 . And, since the outlet chamber  37  is in communication via a conduit  38  with a lower part of the heating medium jacket  50 , a separated liquid medium (hot water) is allowed to circulate, thus absorbing heat for vaporization. On the other hand, the gas liquid mixture in vaporized state from which the liquid is mostly removed by separation is moved up through a space between the inner cylinder  2  of the boiler main body  1  and an outer cylindrical portion of the water retaining tank  35  and is thereby separated into gas and liquid, with a separated vapor component taken out of a vapor outlet pipe  41  into the outside and with a separated hot water component collected in the water receiving tank  35 . 
     In such a boiler configured as mentioned above, the heat of a combustion gas flowing in the inner cylinder  2  of the boiler main body  1  from the combustion section  11  of the combustor  4  directly can act on the inner surface of the inner cylinder  2  and further on the heating medium jacket  5  while being heat-exchanged in the heat exchange section  34  to preheat the water to be supplied into the heating medium jacket  5 . Thus, the heat of a combustion gas generated in the combustion section  11  can thus be utilized effectively. 
     In a modification (to be described later) of the form of implementation mentioned above, the cover member  10  to constitute the combustion chamber  13  can be omitted on adjusting the length of the ceramic combustion tube  14  where an air fuel mixture supplied from the mixer of venturi tube type  23  in the fuel supply section  12  can be burned with the resultant ceramic combustion tube  14  to an extent that there is little unburned combustible. 
     Also, if the cover member  10  is yet used, the catalyst layer  18  incorporated so as to block the through holes  17  may be eliminated if the burning is effected in the combustion chamber constituted by the cover member  10  to an extent that unburned combustible in the combustion gas can completely be burned. 
     While in the form of implementation mentioned above, a cover member  10  is shown disposed on the exit side of the ceramic combustion tube  14 , such a cover member to constitute the combustion chamber  13  can be omitted on adjusting the length of the ceramic combustion tube  14  where an air fuel mixture supplied from the mixer of venturi tube type  23  in the fuel supply section  12  can be burned with the resultant ceramic combustion tube  14  to an extent that there is little unburned combustible. This modification is shown in  FIG. 3  to constitute a second form of implementation of the invention. In the second form of implementation shown in  FIG. 3 , the ceramic combustion tube  14  is closed at its exit side with a thermal insulator  15   d  having a number of guide passages  19  therein for guiding a combustion gas radially outwards therefrom. The combustion gas guided circumferentially outwards of the thermal insulator  15   d  in the process of being discharged upwards acts to heat the inner cylinder  2  of the boiler main body  1  having a number of vertically descending water pipes  42  disposed circumferentially spaced apart in a spaced between the inner cylinder  2  and the outer cylinder  3  and to heat the respective outer walls of a number of water pipes  33  disposed centrally of the boiler main body  1 . 
     In the second form of implementation shown in  FIG. 3 , note here that unlike in the first form of implementation shown in  FIG. 1 , a portion of the ceramic combustion tube  14  which is in an upper region of the combustion section  11  where the ceramic combustion tube  14  is not expectable to function as the oxidation catalyst is leaved out and substituted with a thermal insulator  15   a  forming a combustion passage whose inner diameter is identical to that of the ceramic combustion tube  14 . Such a structure saves the cost of material of the ceramic combustion tube and is economically advantageous. Also, the structure can be used for heating a reformer and for heating at the time of starting a fuel cell utilizing a solid oxide.