BURNER SYSTEM AND METHOD FOR PROVIDING THERMAL ENERGY

Abstract

The present invention relates to a burner system for providing thermal energy comprising an evaporator device for evaporating a liquid alcohol fuel, a burner air supply means, a burner device for burning a fuel mixture comprising vaporized fuel and burner air to provide an exhaust gas stream, a functions device for controlling the thermal energy of the exhaust gas flow, wherein the burner device provides the thermal energy for evaporation in the evaporator device during operation.

Claims

1. A burner system for providing thermal energy, the burner system comprising: an evaporator device for vaporizing a liquid fuel and providing vaporized fuel, a burner air supply device for providing burner air, a surface burner for burning a fuel mixture comprising the vaporized fuel and burner air to provide an exhaust gas stream, a shielding plate with openings for passing and controlling the thermal energy of the exhaust gas flow of the surface burner, wherein the surface burner is coupled to the evaporator device via the shielding plate in such a way that, in operation, the exhaust gas flow of the surface burner provides the thermal energy and the evaporation temperature required for complete evaporation of the fuel in the evaporator device, and a tertiary air supply means for supplying tertiary air to the exhaust gas stream to adjust a temperature of the exhaust gas stream by mixing with the tertiary air to form a hot gas stream.

2. The burner system according to claim 1, wherein the thermal energy of the exhaust gas flow can be provided in the form of diluted exhaust gas, and this thermal energy is delivered in the form of tempered air directly or via a separately designed heat exchanger device to a device coupled to the burner system, this device being an internal combustion engine or a fuel cell stack or a battery.

3. The burner system according to claim 1, wherein the surface burner comprises a burner screen and a metal or ceramic fiber mesh connected together, a diffuser, an ignition device and preferably a flame monitoring device.

4. The burner system according to claim 1, wherein the burner air supply device is a primary/secondary air supply device which is provided for supplying primary air in order to form the fuel mixture, this being supplied to the fuel vapor prior to combustion, and/or which is provided for supplying secondary air for cooling the burner device, in particular the diffuser and/or the combustion chamber and/or for post-combustion.

5. The burner system according to claim 1, wherein that the shielding plate comprises openings for passing through and controlling the thermal energy of the exhaust gas flow and/or thermally shields the evaporator device and/or wherein the functions device is a component of the evaporator device.

6. The burner system according to claim 1, wherein the evaporator device is of closed design, wherein the evaporator device comprises an evaporator space for evaporating the liquid fuel, and wherein an outlet of the evaporator device is connected to an inlet of the burner device only via a conduit section, and the evaporator device has an inlet for supplying liquid fuel.

7. The burner system according to claim 1, wherein a 3/2-way valve is arranged in a line section which connects an outlet of the evaporator device with a fuel inlet of the burner device.

8. A method of providing thermal energy, the method comprising the following steps: heating an evaporator device with an electrical heating device during a start-up phase, complete evaporation of a liquid fuel in the evaporator device, feeding the fully vaporized fuel to a surface burner, and burning the vaporized fuel in the burner device to provide thermal energy in the form of an exhaust gas stream, using a portion of the thermal energy to vaporize the fuel in the evaporator device during operation so that the energy for vaporization in the evaporator is provided solely by the burner device.

9. The method according to claim 8, wherein a regulation or control of the thermal energy of the exhaust gas flow between the surface burner and the evaporator device takes place via openings formed in a shielding plate of a functions device.

10. The method according to claim 8, wherein the burner device comprises a surface burner for gaseous fuels, the fully vaporized fuel being burnt in the surface burner.

11. The method according to claim 8, wherein primary air is supplied to form a fuel mixture, the primary air being supplied to the fuel vapor prior to combustion and/or of secondary air provided for cooling a diffuser and/or a combustion chamber of the burner device and/or used for post-combustion, wherein the primary and secondary air is preferably supplied from a burner air supply means, referred to as primary/secondary air.

12. The method according to claim 8, wherein tertiary air is supplied in order to adjust a temperature of the thermal exhaust gas flow by mixing with the tertiary air so that a hot gas flow is formed therefrom, the output temperature of the hot gas flow in burner operation being controlled via the tertiary air independently of the combustion and the combustion air.

13. The method according to claim 8, wherein an outlet of the evaporator device is kept open via a 3/2-way valve during a starting phase (heating of the evaporator device) and/or in that methanol remaining in the evaporator device can be degassed via the 3/2-way valve when operation of the burner device ends, without a pressure increase occurring in the evaporator device, wherein an internal pressure of the evaporator device at the time of switching the valve to the burner device corresponds approximately to the ambient pressure, so that no pressure is discharged in the burner device and the latter is started up with a controlled fuel-air mixture.

14. The method according to claim 8, wherein liquid fuels with boiling temperatures of up to 350° C., in particular alcohols, such as methanol, ethanol, 2-propanol, 2-butanol, or petrol or petroleum, are provided as fuel.

Description

[0147] The invention is explained in more detail below with reference to the drawings. These show in:

[0148] FIG. 1 a schematic representation of a fuel cell system that can be combined with a burner system according to the invention,

[0149] FIG. 2 a perspective exploded view of the burner system according to the invention,

[0150] FIG. 3 a schematic representation of the media flows of the burner system,

[0151] FIG. 4 a side view of the burner system with media flows,

[0152] FIG. 5 a further side view of the burner system with media flows,

[0153] FIG. 6 a further side view of the burner system with media flows.

[0154] In the following, a fuel cell system 1 is first described which can be combined with a burner system 20 according to the invention in order to achieve the advantages described above (FIG. 1). This fuel cell system 1 corresponds essentially to the fuel cell systems described in WO 2010/066900 A1 and WO 2015/110545 A1, to which full reference is hereby made.

[0155] WO 2010/066900 A1 discloses a humidification unit for providing a carrier gas containing fuel and water vapor for supplying a fuel cell.

[0156] WO 2015/110545 A1 shows a fuel cell system for thermally coupled reforming with reformate preparation.

[0157] The fuel cell system 1 comprises a fuel cell stack 2, which is designed as an HT-PEMFC (high-temperature polymer electrolyte fuel cell). This high-temperature fuel cell operates in a temperature range of 160° C.-180° C. The required hydrogen is obtained from methanol by a reforming process and converted into electricity in the fuel cell 2.

[0158] A carrier gas flow saturated with water and methanol vapor, which is provided by a humidification unit 3, is used for cell supply. This enables water to be recovered from the exhaust gas by means of the water cycle. This eliminates the need for external addition of water and the use of premixes.

[0159] The components and the mode of operation of the fuel cell system 1 are explained by means of an anode circuit.

[0160] In the anode circuit, two metering pumps 4, 5 deliver methanol from a methanol storage tank 13 and water into the humidification unit (media unit) 3. In the humidification unit 3, a carrier gas flow is enriched with methanol. The methanol reservoir 13 can also be used to supply the burner system 20.

[0161] The carrier gas flow is conveyed by an anode or fuel pump 6 from an anode outlet to the humidification unit 3. The saturation of the carrier gas flow with water and methanol vapor takes place in a temperature range of about 80° C., the saturated gas then flows into the internal and external reformer units 7, 8.

[0162] Within the allothermal steam reforming process, water and methanol vapor decompose to hydrogen, carbon dioxide and carbon monoxide. The resulting reformate gas contains predominantly hydrogen and is passed on to the fuel cell stack 2.

[0163] After the hydrogen has been converted into electricity in fuel cell stack 2, the reformate gas still contains carbon dioxide for the most part. The depleted reformate gas flows out of the anode and the cycle starts again from the anode pump.

[0164] The fuel cell is supplied with oxygen (air) by a cathode blower 9. The air flow of the cathode blower 9 absorbs the water produced by the electricity generation, which is condensed out in a heat exchanger and phase separator 14. The recovered water is pumped back into the humidification unit 3 by the metering pump 5.

[0165] The heating of the humidification device 3 is realized by the hot exhaust gases of the reformer device 7, 8, a catalytic burner and the cathode waste heat. In order to keep the fuel cell stack 2 at the operating temperature, it is cooled by means of an air supply device 12. This air supply device 12 can be used as a tertiary air or burner air device of the burner system 20.

[0166] The burner system 20 according to the invention can be used as a heating system for fuel cell stacks 2 of fuel cell systems 1 and is described below (FIGS. 2 to 6).

[0167] Because a HT-PEMFC requires a certain starting temperature, this temperature of approx. 165° C. can be realized by heating the fuel cell stack 2 by means of the burner system 20.

[0168] So far, the fuel cell stack was heated by an electrical heating device. Heating plates were arranged around the fuel cell stack and insulated from the outside. These heating plates require electricity throughout the entire heating process. The heating process takes about one hour and requires a high electrical power. A connected battery is therefore first discharged in order to heat up the fuel cell. As soon as the fuel cell is at its temperature, it charges the battery and the electricity can be used.

[0169] A reduction of the power requirement through the burner system 20 is possible in the form of a non-electric power source.

[0170] The burner system 20 preferably uses methanol in conjunction with a burner device 21 designed as a line or surface burner. In this way, there are only low NOx generation and high exhaust gas temperatures.

[0171] The generated heat is dissipated to the fuel cell 2 via a tertiary air device 16 with a fan.

[0172] The burner system 20 according to the invention is described in detail below (FIGS. 2 to 6).

[0173] The burner system 20 has a housing device 23. The housing device 23 is formed by four interconnected plate modules, which are preferably made of vermiculite. These plate modules are connected to each other by threaded rods and are braced with foam. Vermiculite has a low thermal conductivity and may be formed as a non-combustible sheet material. This product is offered worldwide under the brand names “Fipro”, “Miprotec”, “bro-TECT”, “Vermilite 2000” and “Thermax”. Vermiculite has a high melting point of 1315° C. and is electrically non-conductive or insulating. Instead of vermiculite, Silcawool 115-36A board can be used. In general, it is also possible to separate the functions and produce a metal housing with extra insulation (e.g. high temperature glass wool). The housing device takes over the functions of insulation and media guidance in particular.

[0174] One of these plate modules is an end plate 24. An air inlet and a support surface for a diffuser 25 of the burner device 21 are formed in the end plate 24.

[0175] A rectangular recess for the passage of a methanol line and an air line for supplying the diffuser 25 is provided in a side edge or a top wall of the end plate 24, which is located at the top in the vertical direction.

[0176] A further plate module forming a center plate 26 is mounted on the end plate 24. A side wall of the center plate 26 facing the end plate 25 is provided for receiving a burner support 27 of the burner device 21 and the opposite side wall has a recess for receiving a baffle plate of an evaporator device 28. The baffle plate forms a functions device 29.

[0177] Like the end plate 24, this center plate 26 has a recess for the methanol and air line.

[0178] An installation space for the combustion device 21 is thus formed in the area between the end plate 24 and the center plate 26.

[0179] An outlet plate 30 is connected to the center plate 26. An installation space for the evaporator device 29 is provided between the center plate 26 and the outlet plate 30.

[0180] A front plate 31 is connected to the outlet plate 30.

[0181] The housing device 23 is approximately cuboid-shaped due to the plate-shaped construction. A top plate 33 is attached to a top wall of the housing device 23.

[0182] The top plate 33 serves to place the tertiary air device 16, and a burner air supply device 41, a solenoid valve and an air shaft. The burner air supply device 41 comprises a metering orifice, a corresponding hose system and an air preheater (heating metal comparable to a hot air dryer). Without the air preheater or diffuser preheater, the burner does not start or starts poorly because the methanol condenses in the diffuser. The heater itself is located in the diffuser.

[0183] Alternatively, the diffuser can also be preheated with a directly integrated heating cartridge. In this case, the burner system has two heating cartridges.

[0184] The hot exhaust gases are divided via flow channels 33 formed in the front panel 31 and directed to the fuel cell stack 2. The flow channels 33 of the front plate 33 are linked with the fuel cell stack 2 for optimum air guidance of the hot exhaust gas.

[0185] All panel modules are made of vermiculite and are braced with threaded rods using through holes on the sides of the panels.

[0186] Furthermore, the burner system comprises the evaporator device 28 and a heating cartridge. The methanol is vaporized in the evaporator device 28.

[0187] The evaporator device 28 comprises an inlet tube 42, a trough-like upper and lower unit interconnected by heat transfer component 45 having a heat exchanger structure disposed therebetween, and an outlet tube. The trough-like upper and lower members and the heat transfer device 45 arranged therebetween form an evaporator space.

[0188] The inlet pipe 42 for inserting the liquid methanol opens into the lower element 44 of the evaporator device 28. The heating cartridge with an output of 250 W is integrated into the lower wall. A fuel pump 47 is provided for this purpose, which delivers methanol from a fuel storage tank 48.

[0189] When operating the evaporator device 48 with the waste heat or the exhaust gas flow of the burner device 21, the heat transfer device 45 ensures a good heat transfer for the evaporation of the methanol. The evaporated methanol is concentrated at the top and is conducted to the diffuser through a welded pipe via a solenoid valve. To increase the surface area of the phase interface, the heat transfer device 45 is filled with fine metal wool, preferably stainless steel wool. An upper trough (upper element) of the evaporator device 28 is also filled with stainless steel wool, a lower trough (lower element) into which the methanol first flows is filled with a solid metal foam, which also includes the heating cartridge and ensures or improves the heat conduction from the heating cartridge to the evaporator device 28.

[0190] Alternatively, the lower trough can also be made of a solid stainless steel material and the heating cartridge is located in an extra hole that has no fluid connection to the methanol distribution chamber. The dispersion chamber is preferably equipped with metal foam.

[0191] The burner device 21 comprises, among other things, the burner screen 27, the diffuser 25 and electrodes.

[0192] The burner port 27 is made of a perforated stainless steel sheet with a metal fiber coating. Holes arranged in the edge area of the burner screen 27 are provided for a secondary air flow 36. In addition, fastening holes are provided for fastening electrodes.

[0193] The fuel mixture flows through the primary holes formed in a bulge of the burner screen 27.

[0194] A metal fiber mesh is welded or a porous ceramic structure is provided on the side of the burner screen 27 facing the center plate 26. This consists of a heat-resistant Fe—Cr—Al alloy that provides protection against oxidation and high temperatures. This provides effective protection against overheating and also prevents deformation and breakage of the burner.

[0195] When the metal fiber is operated at low heat output, combustion occurs on the metal surface and the fibers glow brightly. The metal fiber is now operating in radiation mode.

[0196] The use of a metal fiber offers the following advantages: [0197] Precise combustion with significant noise reduction [0198] High flame stability without flashback [0199] Dimensional stability/deformation resistance

[0200] The mixing of the gaseous fuel and the air by the starting burner fan is realized by a diffuser positioned in the end plate.

[0201] On an upper side of the diffuser 27 as well as on the end plate there are inlet openings for the fuel and the burner air supply 34.

[0202] The evaporated methanol is dosed directly into the spiral of the diffuser 27. The spiral forms a mixing device 17. The air is passed through a non-return membrane and then divided into primary air 35 and secondary air 36. The primary air 35 is mixed with the methanol vapor in the spiral and the secondary air 37 flows around the diffuser 25 into the secondary holes. The fuel mixture of gas and air accumulates in a pre-combustion chamber and flows through the primary holes of the burner screen 27.

[0203] The primary air supply is adjustable by means of a stainless steel cover on the spiral.

[0204] Furthermore, two electrodes (HighVoltage (HV) and flame monitoring (FW)) are screwed to the diffuser via the burner screen and extend in the combustion chamber over the primary area of the burner screen. The ignition spark is positioned between the ignition electrode (HV) and the burner screen, which are on electrical ground to obtain a better flame monitoring signal.

[0205] Furthermore, the burner system 20 comprises the tertiary air device 16 and a metering orifice.

[0206] The tertiary air device 16 comprises a PWM (pulse width modulation)-capable radial fan. The PWM enables precise adjustment of the fan speed and thus a controlled flow of tertiary air. A sensor line provides information about the current fan speed and can indicate a fan failure by means of a consistency check.

[0207] In addition to cooling the fuel cell 2, the tertiary fan 16 heats the fuel cell 2 by the hot exhaust air (hot gas stream 39 comprising exhaust gas stream 38 and tertiary air 37) of the burner device 21. The fan allows sufficient cooling of a fuel cell system 1 at any power level.

[0208] A further fan forms a burner air device 41 for supplying burner air 34 (primary 35 and secondary air 36). The burner air device 42, like the tertiary air device 16, is a radial blower that conveys air to the diffuser 25 via a flow metering device. The three-pin connection makes it possible to regulate the speed via a corresponding control device.

[0209] An automatic burner control system is used to provide the ignition spark and detect the flame. This automatic burner control system detects a flame failure in a maximum of 0.8 seconds. A flame failure leads to an internal error that also closes a solenoid valve for the fuel supply. The ignition periods are integrated into an electronic system of the automatic burner control. For the starting system, an ignition period of approximately 4 to 20 seconds is used to ensure safe ignition of the fuel mixture. If the burner unit 21 has not been started within the ignition phase, the control device outputs an error.

[0210] The 3/2-way-solenoid valve 19 is used to open and close the gas pipe of the burner and is located between the evaporator device 28 and the diffuser 25. In case of malfunctions, the valve closes within a very short time and stops the fuel supply to the burner device. This ensures a safe shutdown of the burner in case of malfunctions. The valve is in the open state as soon as the automatic burner control unit operates the ignition process and is closed in the basic state.

[0211] Another safety device is a safety temperature limiter. This is arranged in a pre-combustion chamber, with the measuring sensor fixed to the diffuser. The function is based on a temperature sensor filled with a liquid. If the set temperature range is exceeded, the liquid expands from the sensor via the transmission mechanism to the snap-action switch and the circuit is opened, the solenoid valve closes. Influences of the ambient temperature are compensated by a bimetal disc. The advantage of the component is its electromechanical design, which allows temperature measurement without additional auxiliary energy. To open the circuit again, the snap-action switch must be reset manually.

[0212] Alternatively, a thermo fuse can be used. This is mounted on the back of the diffuser in a provided recess for this purpose. If the permissible temperature is exceeded in the event of a flashback, it opens the circuit to the valve, which then closes. To close the circuit again, the fuse must be replaced. A corresponding fuse wire of the thermal fuse melts irreversibly at approx. 200° C.

[0213] The burner device is spatially separated from the evaporator device by a functions device that is designed as a baffle plate with openings.

[0214] The basic concept of the burner system 20 is thus based on a surface burner with a fan, which is operated with methanol vapor. In addition, the system contains an evaporator to evaporate the liquid methanol. A radial fan with a high throughput is used to dissipate the hot or cold air.

[0215] The housing device 23 is made of four vermiculite plates that are braced together by means of a bracing system. The vermiculite plates are already milled out for the internal components and provide two exhaust ducts for the outlet of the hot gas. The evaporator device, the baffle plate of the functions device 29 and the burner device 21 comprising the diffuser 25 and the burner screen 27 are positioned in the housing. In addition, a mounting device for the burner air device 41 with solenoid valve is located on the top of the starting system.

[0216] The fresh air supply is provided on the one hand by the burner air device (primary air 35 and secondary air 36) and on the other hand by the tertiary air device 16 (tertiary air 37). The burner air 34 flows over the diffuser 25 and is divided there into primary air 35 and secondary air 36. The primary air 35 is mixed with the gas flow of the evaporator device 28 and flows through a central area of the burner screen. The secondary air is directed through the outer holes of the burner screen 27 and serves to cool and additionally oxygenate the combustion process.

[0217] A method for providing thermal energy using the burner system described above is explained below (FIG. 3).

[0218] In the start-up process, liquid methanol is provided from the tank through a pump and a 3/2-way valve 19 into the evaporator device 28. In the evaporator device 28, the liquid methanol is first evaporated by an electrically operated heating cartridge, passed through a solenoid valve and mixed by means of a spiral with primary air 35 provided by the primary/secondary air device or the burner air device 41.

[0219] The ignitable fuel-air mixture is then ignited in the combustion chamber and additionally supplied with oxygen by the secondary air 36.

[0220] An evaporator outlet is open in the heating phase and thus pressureless, therefore no check valve is necessary as pump protection. As soon as the 3/2-way valve is actuated, a connection between the evaporator device and the diffuser is opened while the connection to the outlet is closed. Methanol that escapes from the evaporator during the heating phase is returned to the system in the same way as the overflow from the methanol reservoir is fed into a phase separator cathode condensate. From there, it reaches the humidification unit and is converted into electricity.

[0221] Regarding the design positioning, the arrangement and the further advantages of the 3/2-way valve, reference is made to the corresponding paragraphs in the introduction to the description.

[0222] Subsequently, the hot exhaust gas 38 passes through the baffle plate of the functions device 29 and is here mixed with the tertiary air in order to reduce its temperature. The tertiary air flow is thus first mixed with the exhaust gas and then the hot gas or useful gas flow 39 formed from it flows through the evaporator device 29 and then on to the fuel cell stack. Otherwise, the evaporator could overheat. In this way, the evaporator device 28 no longer needs to be heated electrically. In particular, the hot gas flow heats a heat transfer structure of the fuel cell stack and heats it to operating temperature. As soon as the fuel cell stack has reached the desired operating temperature, a cooling process is initiated by means of the burner system.

[0223] The pump 6 or 47 and the primary/secondary air device 41 gradually shut down and the tertiary air device 16 cools the fuel cell stack 2 to its desired operating temperature.

[0224] The burner system 20 is operated with a liquid fuel, such as methanol. Thereby a phase transition of the liquid component into the gas phase must be ensured. The phase transition is necessary to ensure mixing of the energy carrier with the oxidant.

[0225] This process is called mixture preparation and leads to the reduction of pollutant emissions and takes first place in the evaporator device and in a downstream mixing device. The mixing device is designed in the form of a spiral and is integrally formed on a rear wall of a diffuser of the burner device. The diffuser disperses the mixed fuel gas over the burner cross-section so that it can flow homogenous through a primary region of the burner screen. The burner screen also takes part in the homogenous dispersion of the fuel gas over the burner cross-section due to the pressure loss when flowing through them.

[0226] Four sub-steps are necessary for the thermal combustion of liquid fuels. [0227] Fuel preparation [0228] Evaporation of the fuel [0229] Mixture preparation [0230] Combustion (heat release) of the mixture

[0231] In the first sub-step, the liquid energy carrier is processed by surface enlargement. This process is operated out by using atomization or dispersion systems. Atomizing or dispersing the liquid fuel serves on the one hand for fluid transport and on the other hand to increase the specific surface area. Because the evaporation of a liquid energy carrier takes place at the phase boundary surfaces, an increase of the reaction rate and thus a minimization of the process time can be achieved here.

[0232] An evaporation of a liquid medium into a gaseous component is called a phase transition. In the case of vaporization in an air environment, the fuel is mixed with air and afterwards vaporized. A direct vaporization, first represents a vaporization of the fuel, which can thereby be mixed with air in gaseous form. In the case of vaporization of methanol, the methanol is added in the evaporator and then the vaporized methanol is mixed with air in the mixing device. Due to the spatial separation of vaporization and mixing, an ignitable mixture of fuel and air is only present after the mixture is formed.

[0233] The energy for igniting a fuel mixture is provided by the ignition control.

[0234] In particular, the evaporator device 28 is designed in such a way that the evaporated fuel is discharged with a homogenous or continuous volume flow and not in a pulsating manner.

[0235] The burner system 20 according to the invention prevents pulsating combustion or pulsating evaporation leading thereto by 1. preventing condensation of the fuel in burner components due to overheating of the burner vapor. 2. by compensating the pulsation of the pump by sponge-like structures in the evaporator device (porous metal foam), which cause an increase in surface area due to their low surface tension or capillary forces. Due to a pump stroke the fuel overflow can be temporarily stored in the inner structure and continuously released as steam. In this way, the fuel can also be supplied to the evaporator device 28 by means of a simply constructed pump with an unsteady delivery rate.

[0236] Furthermore, according to the invention, it is provided that a temperature of the thermal energy of the hot gas stream emitted by the burner system is adjustable. Because the hot gas temperature to which, for example, a fuel cell 2 can be subjected is limited, cool ambient air (tertiary air 37) is supplied to the exhaust gas stream 38 via the tertiary air device 16 in order to set or regulate it to a predetermined temperature. The output temperature of the hot gas stream 39 can be controlled or adjusted very precisely (+/−2° C.) by controlling the fan speed of the fan of the tertiary air device 16 accordingly.

[0237] The burner device 21 is designed in such a way that its thermal output (via temperature or volume flow) is adjustable or controllable. Depending on the application of the burner system, the required heating power can vary. In order to avoid an unfavorable cycling of the burner device (on/off) of the burner system 20 with all its disadvantages (cooling losses, electrical losses due to preheating, poorer exhaust gas values during start-up), the power of the burner device can be widely modulated independently of the output temperature. This can be done either on a small scale by adjusting a combustion lambda at constant burner air flow or over a wide range by keeping the burner lambda constant while varying the burner air supply. Nevertheless, the output temperature can be kept constant within the scope of the delivery rate of the fan of the tertiary air device, because the outgoing gas mass flow varies depending on the desired temperature level, but not the heat output.

[0238] The thermal power for starting the burner system 20 before it goes into operation and the evaporator device 28 is heated via the burner device 21 is provided by means of the heating cartridge. This is a simple and inexpensive solution.

[0239] In addition, the temperature in the evaporator device 28 can be detected via a thermocouple integrated in the heating cartridge.

[0240] This is provided to avoid overheating of the heating element and thus damage to it, and to reduce the risk of pyrolysis of the fuel. In the start phase, the heating cartridge is regulated to a maximum temperature that is below the pyrolysis temperature of the fuel used. For this purpose, the current supply of the heating cartridge is clocked.

[0241] The evaporator device can switch off an electric heater in stages by means of empirical values (temperature detection is also possible). How such empirical values can be found is described below.

[0242] Preheat: [0243] The heating cartridge for electrical preheating of the evaporator can be controlled via a pulse width modulation (PWM) control, e.g. with an interval of 50 ms. [0244] A regulation in e.g. 10% steps is intended. [0245] In order to prolong the life of the heating cartridge, a maximum heating power of e.g. 80% is used, as this puts less strain on the heating conductor due to the currentless phases. [0246] Furthermore, the temperature of the heating cartridge is detected by a thermal sensor integrated in the cartridge and limited to a maximum of 500° C. with the help of the control system. If the limit temperature is reached or exceeded, the heating power of the cartridge is gradually reduced until the constant limit temperature is reached. [0247] One of the indicators for burner ignition is that the cartridge heater temperature must be above 450° C. for more than 5 seconds (the others are preheating of the diffuser to min. 70° C. and at least 4 minutes heating time). [0248] The temperature limitation to a maximum of 500° C. is not only used to protect the heating cartridge but also to prevent pyrolysis of the methanol. [0249] The temperature limits are stored in the software and can be variably adjusted depending on the fuel used.

[0250] Fading out the heater: [0251] After successful ignition of the burner (fuel valve open for more than 20 seconds and an output temperature of more than 150° C.), the heating is initially reduced to 30% power. [0252] Then the heating power is reduced step by step (1% increments) until the heating is completely switched off. 32 system software clocks (500 ms) are counted per step. [0253] Whereby an active power adjustment only takes place in, for example, 10% steps, the remaining steps in between only used because of the simpler control strategy. Accordingly, a control value of 39% is started, which corresponds to only 30% of the real heating output. [0254] The number of required system cycles per step can be individually adapted to the fuel used. [0255] In addition, the integrated temperature detection integrated in the heating cartridge can be used to ensure that the temperature in the evaporator does not fall below the boiling temperature of the fuel used. This means that the shut-off steps of the heater can also be delayed or reversed if necessary.

LIST OF REFERENCE SIGNS

[0256]

TABLE-US-00001  1 Fuel cell system 35 Primary air  2 Fuel cell stack 36 Secondary air  3 Humidification unit 37 Tertiary air  4 Methanol dosing pump 38 Exhaust gas flow  5 Dosing pump air 39 Hot gas flow  6 Anode pump 40 Methanol stream  7 Internal reformer 41 Burner air supply unit  8 External reformer 42 Inlet pipe  9 Cathode blower 43 To element 10 Catalytic burner 44 Sub-element 11 Cooling fins 45 Heat transfer device 12 Air supply unit 46 Evaporator compartment 13 Methanol storage tank 47 Fuel pump 14 Heat exchanger/ 48 Methanol storage tank phase separator 15 16 Tertiary air device 17 Mixing device 18 19 3/2-way valve 20 Burner system 21 Burner equipment 22 Tertiary air device 23 Housing device 24 End plate 25 Diffuser 26 Centre plate 27 Burner screen 28 Evaporator device 29 Functions device 30 Outlet plate 31 Front panel 32 Flow channel 33 Top panel 34 Burner air