EP1559158A2

EP1559158A2 - Postcombustion system and method for operating a postcombustion system

Info

Publication number: EP1559158A2
Application number: EP03750312A
Authority: EP
Inventors: Guenter Hoenig; Frank Miller
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2002-10-29
Filing date: 2003-09-03
Publication date: 2005-08-03
Also published as: WO2004040683A2; DE10250360A1; JP4546831B2; US7762806B2; US20060153756A1; JP2006503787A; WO2004040683A3

Abstract

.The invention concerns a postcombustion system (1) and a method for operating a postcombustion system (1), in particular for a chemical reformer to obtain hydrogen, so as to provide available heat from residual fuels and/or gases derived from a reforming process and/or a process in a fuel cell. Said method consists in controlled transfer of the heat derived from fuel gases recycled to a first housing (5) and/or to a combustion chamber (8) which is filled at least partly with a heat-resistant alveolar ceramic (4) with open pores located in said first housing. The control is for example carried out based on the temperature detected in the combustion chamber by means of infrared light measurement.

Description

Afterburner and method for operating a

postcombustion

State of the art

The invention is based on an afterburner according to the preamble of claim 7 or on a method for operating an afterburner according to the preamble of claim 1.

In the case of transport systems supported by fuel cells, so-called chemical reformers are used to obtain the required hydrogen from hydrocarbon-containing fuels.

The optimal operating temperature of a chemical reformer is usually far above its ambient temperature. This leads to problems in particular for vehicles for private transport. The numerous standstill phases of the vehicle lead to a large number of cold start phases, in which the chemical reformer in particular does not work optimally. At very low loads, the reformer may not reach the optimum operating temperature due to the heat it contains, or may lose it during operation. Particularly in the case of fuel cell-assisted drive systems with a chemical reformer, it is therefore advantageous to use afterburning devices which, in particular, have the task of using the heat generated by them to bring the chemical reformer quickly to the operating temperature and / or to thermally utilize any residual gases.

The afterburner burns the combustible residual gases, for example residual hydrogen, with flame formation and / or at least partially catalytically and is thermally coupled to the chemical reformer. Usually, however, the thermal energy of the flammable residual gases alone is not sufficient to provide a sufficiently large thermal output. For this reason, fuel is usually measured additionally or solely in the afterburner. The fuel, which is preferably in liquid form, is injected into a combustion chamber in finely divided form as a droplet cloud with the smallest possible droplet diameter, using complex and error-prone devices. The small droplet diameter is necessary in order to bring the fuel into contact with oxygen and heat over as large a surface as possible and so as to complete the combustion process as completely as possible.

The disadvantage here is that metering devices for generating a droplet cloud with a small droplet diameter are very complex, cost-intensive and prone to errors. The necessary small droplet diameter can often only be achieved by using high fuel pressure, the generation of high pressure taking up a relatively large amount of power and in particular the installation for generating the pressure taking up a lot of space. In addition, such metering devices usually have very small metering openings which, due to combustion residues or deposits, change the metering behavior of the metering device in an impermissible and difficult to control manner. As an alternative or in support of the use of high fuel pressure, solutions with air support are known for the fine atomization of the fuel, the fuel or the Residual gas is swirled with air for a sufficiently long time before combustion. The disadvantages here are the relatively large space requirement, the complex and prone to failure regulation of the air metering and the additional energy requirement.

Finally, there is a risk of unforeseen flame extinction of the open, continuously burning flame in the combustion chamber, particularly at low power. The thermal output of the afterburner is therefore severely restricted downwards. Furthermore, a certain amount of time is always required to switch off the fuel supply or to re-ignite the flame. During this time, fuel or residual gas can accumulate in the combustion chamber. This has a negative impact on the re-ignition, a catalyst which may be present can be damaged and unburned fuel or residual gas can escape into the atmosphere. Despite all of the measures mentioned, unburned or incompletely burned portions remain in the exhaust gas of the afterburner, which are sometimes toxic or chemically aggressive. This leads to increased environmental pollution and material pollution, and the calorific value of the fuel or residual gas is only incompletely used.

Advantages of the invention

The method according to the invention and the afterburner device according to the invention with the characterizing features of the independent claims have the advantage, in contrast, that by metering fuel onto or into the open-pore heat-resistant foam ceramic without the use of complex atomizing devices to produce the finest fuel drops, a very good fuel distribution in the combustion chamber or in the ceramic foam. The associated relatively large contact area with atmospheric oxygen leads to an almost complete combustion of the supplied fuel and residual gas and thus to an excellent efficiency and very low pollutant emissions. -The requirements on the The metering device or the fuel nozzle, which measures the fuel in the combustion chamber or on or in the foam ceramic, are very small, since the fuel is distributed within the foam ceramic.

Due to the low 'heat capacity of the foam ceramic and the evenly distributed combustion process in the foam ceramic, the foam ceramic heats up very quickly, so that after a short period of operation and a possible brief interruption in the fuel supply, spark ignition by spark plugs, for example, is not necessary when the fuel supply is restarted , The use of the exhaust gas heat by recirculating the exhaust gases resulting from the combustion via a return line and a heat exchange duct, which heats the supplied air and / or the combustion chamber or the foam ceramic, especially in cold start operation, with exhaust gas heat leads to a shortened cold start phase and thus to a further reduction in pollutant emissions and a further improvement in fuel implementation. By detecting the rate of combustion, it is possible to regulate the amount of heat returned. This makes it possible to return a maximum amount of heat in the cold start phase without generating unfavorable temperatures for the afterburner or its operation as the combustion speed increases.

It is also advantageous that the foam ceramic initially absorbs part of the metered fuel without it being ignited immediately. Rather, some of the fuel is distributed in the foam ceramic before it is ignited on its surface. The foam ceramic is therefore able to initially store a certain amount of fuel. This property is advantageous, for example, when starting the afterburner from the cold state with insufficient spark ignition, for example by a filament, since the fuel cannot escape unburned immediately through the combustion chamber. Rather, it is stored in the foam ceramic and is still available for combustion. Deflagration processes in the combustion chamber or an enrichment of the fuel-air mixture beyond the ignitability are thus largely prevented.

It is also very advantageous that the distribution of the fuel takes place automatically, largely independently of the geometric shape of the foam ceramic. This allows a very adaptable placement of the foam ceramic in the combustion chamber or in the afterburner, for example to improve the thermal coupling between the foam ceramic and the combustion chamber, or with other elements of the afterburner.

In addition, the afterburner device according to the invention has a very large heat output range, which is particularly due to the possibility of setting very small heat outputs. These adjustable, very low heat outputs or combustion outputs make it possible to avoid pollutant-intensive, material-burdensome and efficiency-reducing, post-combustion processes of the afterburner, particularly in the case of load change processes typical of private automobile traffic.

Advantageous developments of the afterburning device according to the invention and of the method according to the invention for operating an afterburning device can be found in the respective subclaims.

In a first advantageous development of the method according to the invention, the combustion rate is determined on the basis of a temperature measurement. This can be done particularly advantageously by means of a contactless and thus largely wear-free infrared light measurement. In a further advantageous development, the quantity of the recirculated combustion gases is regulated on the basis of the determined combustion rate.

The method according to the invention is advantageously further developed by a further method step which, depending on the detected combustion rate, regulates the supply of air, fuel and / or residual gas. In a further development, the supply of air to the combustion chamber or the proportion of air in the fuel / gas-air mixture is increased in order to lower the temperature in the combustion chamber or in the foam ceramic.

If the method also includes a process step in which the combustion chamber or the foam ceramic is electrically heated, the method is also advantageously further developed. As a result, the combustion chamber or the foam ceramic can be heated, for example, even before the start of the cold start phase, which further shortens the cold start phase of the afterburner. Likewise, the ignition energy required in each case can be provided or a necessary ignition temperature can be generated.

The afterburner can advantageously be further developed in that the foam ceramic consists at least partially of silicon carbide. Silicon carbide is extremely heat-resistant, an excellent heat conductor and also gives the foam ceramic good mechanical rigidity with a relatively low density. Silicon carbide also conducts electrical current relatively well. The good electrical conductivity can be used for measurement purposes, for example to determine the temperature via the electrical resistance derived from current and voltage, or the combustion process can be influenced, controlled or, for example in catalytic combustion, completely achieved by the thermal effect of the electrical current be, for example in part-load operation. It is also advantageous if the foam ceramic is made open-pored by so-called reticulation, which can be carried out, for example, thermally or chemically. This enables a very high degree of open porosity to be achieved and, moreover, the pore size can be set very easily, preferably in the range from 0.05 mm to 5 mm, in the production of the foam ceramic.

Advantageously, the foam ceramic is in good heat-conducting contact with at least part of the wall of the combustion chamber or the first housing, since the heat can thereby be released quickly and efficiently to, for example, the reformer or a fuel cell. Likewise, the combustion chamber or the foam ceramic can be heated from the outside by this wall of the combustion chamber or the first housing, for example by a recirculated exhaust gas flow, without exhaust gas getting into the combustion chamber or the foam ceramic.

By the arrangement of heat-conducting elements within the first housing, in particular also within the ceramic foam, it is advantageously possible to conduct heat from a relatively hot area to a to ^'relatively cool region, in particular in the area of the air supply or in the area in which Fuel or the residual gases are measured. This compensates for the cooling effect of the reactants supplied and increases the reaction rate, especially in cold start phases, in the areas mentioned. As a result, the reaction rates run uniformly in all areas of the combustion chamber or the foam ceramic. It is particularly advantageous if the heat-conducting elements consist of metal or a metal-containing alloy, since metals are particularly good heat conductors and also have good mechanical and chemical properties.

It is also advantageous that the thermal energy of the hot exhaust gases through a return line and one Heat exchange channel of the foam ceramic or the combustion chamber and / or the supplied air, and thus the combustion reaction itself. As a result, the low-quality exhaust gas heat is used to quickly increase the combustion speed, especially in cold start phases, and to warm the reactants or the combustion chamber.

It is also advantageous if a controller regulates or controls the recirculation of the exhaust gases. It is thereby advantageously possible to recirculate hot exhaust gases in a metered manner and to adapt the recirculated heat quantity to the heat requirement. In particular, this prevents overheating of the afterburner and keeps the exhaust gas back pressure as low as possible.

It is also advantageous to manufacture the heat exchange channels from cylindrical tubes, since these are inexpensive and easy to process.

drawing

Embodiments of the invention are shown in simplified form in the drawing and explained in more detail in the following description. It showed:

1 is a schematic representation of a first embodiment of an afterburner according to the invention as a schematic diagram,

2 shows an excerpted schematic representation of a second exemplary embodiment according to the invention in the area of the combustion chamber,

3 is a partial section through the open-pore foam ceramic as a schematic diagram,

4 shows a schematic illustration of a third exemplary embodiment according to the invention, 5 shows a schematic sectional illustration of the third exemplary embodiment according to the invention in a top view,

Fig. 6 is a schematic representation of a fourth embodiment of the invention and

Fig. 7 is a schematic representation of a fifth embodiment of the invention.

Description of the embodiments

Exemplary embodiments of the invention are described below by way of example. The method according to the invention can be used particularly advantageously in these exemplary embodiments. In the figures, the same components are each provided with the same reference numerals. Arrows symbolize the fuel and gas flows.

An embodiment of an afterburner 1 according to the invention shown in FIG. 1 has a tubular cylindrical second housing 14 closed at the ends, a nozzle 2, a regulator 17 and a return line 16. The nozzle 2 engages in the upper end face of the second housing 14 and is arranged axially in the center of an axis 22, which in this exemplary embodiment is identical to the axis of symmetry of the second housing 14. The top end of the second housing 14 also has an air supply 3, which is realized in this embodiment as a mere opening. An outlet opening 7, which opens into an outlet pipe 15, is located laterally near the lower end of the second housing 14. The outlet pipe 15, a first exhaust line 20 and the return line 16 open into the controller 17. The return line 16 leads from the controller 17 to the upper end face of the second housing 14 and opens there into the second housing 14. Inside the The second housing 14 contains, among other things, a combustion chamber 8, which is not shown in FIG. 1.

It works as follows:

The nozzle 2 either only fuels in preferably liquid form, only residual gas from e.g. a reforming process or fuel cell process or a mixture of these two combustible substances in the combustion chamber 8 not shown in FIG. 1 in the second housing 14. The air required for combustion is sucked in by the air supply 3. A forced supply of air or other oxygen-containing substances is, however, conceivable.

The exhaust gases emerging at the outlet opening 7 flow through the outlet pipe 15 into the regulator 17 and are at least partially returned to the second housing 14 via the return line 16. The recirculated exhaust gases emit thermal energy in the interior of the second housing 14 without the recirculated exhaust gases mixing with the fuel, the residual gases or the air, and are released into the environment or into another via a first exhaust gas line 19 (not shown in FIG. 1) Trial convicted. The exhaust gases not recirculated by the controller 17 are conducted by the controller 17 through a second exhaust pipe 20 into the environment or another process.

In this exemplary embodiment, the instantaneous temperature or the instantaneous temperature distribution in the interior of the second housing 14 or the combustion chamber 8, not shown, is measured by infrared sensors (not shown). In particular, the instantaneous combustion speed in the combustion chamber 8 can thereby be determined. Depending on the combustion speed, the amount of exhaust gases returned to the second housing 14 is regulated in this exemplary embodiment. The respective proportions and amounts of air, fuel and residual gases that pass through the nozzle 2 or through the air supply 3 into the second housing 14 arrive, are regulated depending on the combustion rate according to the invention. It is also conceivable to change the respective proportions and amounts of air, fuel and residual gases in a time-controlled manner. For example, at the start of the cold start, fewer reactants are added and the proportion of fuel is increased, with the air proportion being increased at a later point in time and the total amount of reactants being increased. If the temperature is too high, the air supply or the air fraction in the combustion chamber 8 is increased according to the invention.

In addition, a lambda sensor can also be present.

FIG. 2 shows a partially schematic representation of a second exemplary embodiment according to the invention in the area of the combustion chamber 8, which is arranged in the second housing 14, which is not shown in this figure. The combustion chamber 8 is delimited laterally by a tubular cylindrical first housing 5, at the top by an upper ring 9 and at the bottom by a lower ring 10 in the housing 5. The upper ring 9 delimits the combustion chamber 8 from a nozzle 2 and the lower ring 11 from an outlet chamber 11. In this exemplary embodiment, the combustion chamber 8 is completely filled with a foam ceramic 4. The pores of the foam ceramic are connected to each other in the transverse and longitudinal directions and in particular allow an excellent flow and almost complete combustion. In this exemplary embodiment, the surface of the foam ceramic 4 is completely covered with a catalytic layer consisting of CuO.

An excerpted section is shown as a basic sketch in FIG. 3. The pores 13 embedded in the carrier foam 12 can be seen.

The foam ceramic is e.g. B. by reticulating the carrier foam 12, such as polyurethane foam, and subsequent treatment with a Silicon carbide suspension, for example ceramic powder made of silicon carbide suspended in water, can be produced.

The nozzle 2 receives fuel, residual gas, air or a mixture of these components at its axial end facing away from the foam ceramic 4 and measures it at its lower axial end, which faces the foam ceramic 4, through an opening (not shown) into the foam ceramic 4 , Air is also fed to the combustion chamber 8 or the combustion via an air supply 3. The introduction of a residual gas-air or residual gas-oxygen mixture is also possible via the air supply 3. Fuel, residual gas or a mixture of these components ignites with air and / or oxygen or reacts chemically during operation on the hot surface of the foam ceramic.

The combustion process can also be started or maintained by ignition devices, not shown. Such ignition devices are installed, for example, as an electrical glow plug or filament (not shown) between the nozzle 2 and the ceramic foam 4. It is also possible to mount the ignition device in the foam ceramic 4. It is also conceivable to design the ignition device in such a way that the entire foam ceramic 4, or at least part of it, is electrically heated such that an ignition device is formed. Finally, the ceramic foam 4 can also be heated from the outside or by the implementation of wires. This enables operation of the afterburner 1 according to the invention.

After the oxidation of the fuel and / or the residual gases has taken place, the combustion gases escape downward through the lower ring 10 into the outlet space 11, in order to then escape through outlet openings 7 here.

The first housing 5 is in good heat-conducting contact over a large area with heat exchange channels 18, not shown in this figure. Strip-shaped heat-conducting elements 23 run inside the foam ceramic 4. For example, they can also be tubular or cylindrical-tubular. In this exemplary embodiment, the heat-conducting elements 23 run from top to bottom, parallel to the axis 22. They serve to transport heat into areas within the first housing 5, which, for example, only slowly in a cold start phase, relative to other areas within the first housing 5 heat. For example, heat can be conducted from an area near the lower ring 10 into an area near the upper ring 9. At least some of the heat-conducting elements 23 can also reach through the upper ring 9 and thus, for example, heat the air supplied by the air supply 3, and they can also reach through the lower ring 10 in order to derive thermal energy from the combustion gases. The heat-conducting elements 23 are to be arranged in such a way that they are not acted upon directly by the fuels metered through the nozzle 2.

FIG. 4 shows a third exemplary embodiment according to the invention similar to the exemplary embodiment from FIG. 2. However, this exemplary embodiment additionally has the return line 16, which conduct the combustion gases via the outlet opening 7, the outlet pipe 15 and the regulator 17 into the lower end of the heat exchange channels 18. As can be seen in FIG. 5, the heat exchange channels 18, in the exemplary embodiment of FIGS. 4 and 5, run in one half of a hollow cylindrical tube 21 closed at one end. The tubes 21 extend from below upwards along the side wall of the first housing 5 and are thermally coupled to the combustion chamber 8 or the first housing 5. The pipe 21 is divided into two halves by a pipe wall 24 dividing the pipe cross section, the half facing the first housing 5 representing the heat exchange channel 18 and the opposite half being a first exhaust pipe 19. The pipe wall 24 extends until shortly before the closed end of the pipe 24 to establish a connection between to create the heat exchange channel 18 and the first exhaust pipe 19. Otherwise, it hermetically separates the two halves of the tube 21. The tubes 21 are distributed radially around the first housing 5 at regular intervals. The tubes 21 and the first housing 5 are surrounded by the second housing 14, the side walls of the second housing 14 acting in particular in a heat-insulating manner.

The controller 17 determines the amount of the recirculated combustion gases and directs them via the return line

16 in the lower end of the tube 21 in the heat exchange channels

18. For example, in a cold start phase, the in the

Combustion gases contain heat supplied to the first housing 5 and thus to the combustion chamber 8 and the space located above the upper ring. The combustion speed can thereby be accelerated in a cold start phase and the cold start phase shortened as a result. In particular, the added fuel can evaporate more easily and quickly due to the heat supplied. The combustion gases then exit via the first exhaust line 19

Afterburner 1. The non-recirculated

Combustion gases are also removed from the afterburner 1 via a second exhaust line 20.

FIG. 6 shows a fourth embodiment similar to the third embodiment shown in FIGS. 4 and 5. The return line 16, however, divides the returned combustion gases into heat exchange channels 18 which run through the combustion chamber 8 or the foam ceramic 4. The heat exchange channels 18 are of tubular cylindrical shape and run through the side walls of the first housing 5. A second housing 14 is not present.

FIG. 7 shows a fifth exemplary embodiment according to the invention with the first housing 5 arranged in the second housing 14. The recirculated combustion gases are conducted via the return line 16 through a first opening 25 arranged in the second housing 14 into the interior of the second housing 14 through the heat exchange channel 18 formed between the two housings to a second opening 26 of the second housing 14. There, the combustion gases leave the afterburner 1 via the first exhaust line 19. The first housing 5 is hermetically sealed against the returned combustion gases and, for example, absorbs heat from the returned combustion gases in a cold start phase. As a result, the combustion chamber 8 or the foam ceramic 4, which are arranged in the interior of the first housing, are heated.

Claims

Expectations

1. A method for operating an afterburner (1), in particular for chemical reformers for the production of hydrogen, for providing heat from fuels and / or residual gases from a reforming and / or from a fuel cell process, with a nozzle (2) for measuring fuel and / or residual gases and / or air in a combustion chamber (8) at least partially filled with a foam ceramic (4) and an outlet opening (7) for discharging the combustion gases, with the following method steps: - detecting the combustion speed in the combustion chamber (8) and / or the foam ceramic (4),

Returning at least some of the combustion gases to a heat exchange channel (18) thermally coupled to the combustion chamber (8) and / or the foam ceramic (4) and - regulating the proportion of the returned combustion gases by changing the amount of the returned combustion gases.

2. The method according to claim 1, characterized in that the method comprises a method step by which the combustion speed in the combustion chamber (8) and / or in the foam ceramic (4) by a Temperature measurement, in particular by infrared light measurement, is detected.

3. The method according to claim 1 or 2, characterized in that the amount of the returned combustion gases is regulated on the basis of the determined combustion speed in the combustion chamber (8) and / or the foam ceramic (4).

4. The method according to any one of claims 1 to 3, characterized in that the method comprises a procedural step through which the supply of fuel, residual gas and / or air is regulated depending on the detected combustion rate.

5. The method step according to claim 4, characterized in that the supply of air through the air supply (3) or the air portion in the combustion chamber (8) is increased at too high a temperature or too high a combustion rate.

6. The method step according to one of claims 1 to 5, characterized in that the method comprises a procedural step through which the combustion chamber (8) and / or the foam ceramic (4) are electrically heated.

7. afterburner (1); in particular for chemical reformers for the production of hydrogen, for the provision of heat from fuels and / or residual gases from a reforming and / or from a fuel cell process with at least one nozzle (2) for metering fuel and residual gases into one arranged in a first housing (5) Combustion chamber (8) and at least one air supply (3), characterized in that that the combustion chamber (8) is at least partially filled with a heat-resistant open-pore foam ceramic (4) which is at least partially covered with a catalytic material.

8. Afterburner according to claim 7, characterized in that the catalytic material consists of ZnCuO and / or CuO.

9. Afterburner according to claim 7 or 8, characterized in that the ceramic foam (4) consists at least partially of silicon carbide.

10. Afterburner according to one of claims 7 to 9, characterized in that the ceramic foam (4) is made open-pore by reticulation.

11. Afterburner according to one of claims 7 to 10, characterized in that the ceramic foam (4) is electrically heated.

12. Afterburner according to one of claims 7 to 11, characterized in that the foam ceramic (4) with at least part of the first housing (5) is in good heat-conducting contact.

13. Afterburner according to one of claims 7 to 12, characterized in that heat-conducting elements (23) run within the first housing (5).

14. Afterburner according to claim 13, characterized in that the heat-conducting elements (23) consist of metal or a metal alloy and run in the foam ceramic (4) or in the area of the air supply (3).

15. Afterburner according to one of claims 7 to 14, characterized in that the afterburner (1) at least one return line (16) for returning the at

Combustion gases in at least one combustion

Has heat exchange channel (18) which with the combustion chamber

(8) or the foam ceramic (4) is thermally coupled and conducts the exhaust gas heat into the combustion chamber (8), into the foam ceramic (4) and / or into the area of the air supply (3).

16. Afterburner according to claim 15, characterized in that the afterburner (1) has a controller (17) which regulates or controls the recirculation of the combustion gases formed during the combustion in the at least one heat exchange channel (18).

17. Afterburner according to claim 15 or 16, characterized in that the at least one heat exchange channel (18) consists of tubes (21), in particular in a hollow cylindrical shape.

18. Afterburner according to one of claims 15 to 17, characterized in that at least part of the heat exchange channels (18) is arranged radially around the combustion chamber (8) parallel to an axis (22).

19. Afterburner according to claim 15 to 17, characterized in that at least some of the heat exchange channels (18) through the combustion chamber (8) or the foam ceramic (4).

20. Afterburner according to claim 15 to 17, characterized in that the at least one heat exchange channel (18) conducts the exhaust gas flow onto and / or around the first housing (5).