Method and Device for Flameless Stepwise Combustion

Abstract

A method for heating a heating chamber to a temperature below the spontaneous ignition temperature of the fuel that is used, wherein fuel and air are reacted in flameless oxidation in a non-stoichiometric mixture ratio in a combustion chamber. The air ratio λ is at least lower than the stoichiometric ratio λ=1 such that the temperature in the combustion chamber does not exceed the temperature at which thermal nitrous oxide generation begins. Otherwise, λ is established such that the spontaneous ignition temperature of the fuel is exceeded. This results in two permissible air ratio ranges, between λ.sub.min and λ.sub.1 in sub-stoichiometric operation, and λ.sub.2 to λ.sub.max in superstoichiometric operation of the combustion chamber. The still-reactive gases released from the combustion chamber are made to react in the heating chamber, preferably by flameless oxidation. This avoids thermal nitrous oxide generation in the heating chamber.

Claims

1. A method for heating a heating chamber (12), comprising: heating the chamber to a temperature below a spontaneous ignition temperature of a fuel used to heat the chamber; reacting fuel and air in flameless oxidation in a nonstoichiometric ratio in a combustion chamber (11); and supplying the fuel and air in the heating chamber (12) as a reaction gas without heat withdrawal from the combustion chamber (11) apart from non-avoidable thermal losses; wherein the reaction gas is completely oxidized in the heating chamber (12), by adding air and/or fuel as necessary.

2. The method according to claim 1, wherein the oxidation in the heating chamber (12) is carried out with flame formation.

3. The method according to claim 1, wherein the oxidation in the heating chamber (12) is carried out without flame creation by gas guidance with large-scale recirculation.

4. The method according to claim 1, wherein a non-stoichiometric fuel/air ratio is adjusted in the combustion chamber (11) such that a temperature of 1400° C. is not exceeded in the combustion chamber (11).

5. The method according to claim 1, wherein a temperature between 1000° C. and 1300° C. is achieved in the combustion chamber (11) by adjustment or closed loop control of a fuel/air ratio.

6. The method according to claim 1, wherein the combustion chamber (11) is sub-stoichiometrically (λ<1) operated during full load operation and is super-stoichiometrically (λ>1) operated in a partial load operation.

7. The method according to claim 6, wherein a range (λ.sub.1, λ.sub.2) of non-used fuel/air ratios is defined between a fuel/air ratio in the super-stoichiometric operation and a fuel/air ratio in the sub-stoichiometric operation.

8. The method according to claim 1, further comprising extracting heat from the heating chamber (12).

9. A device (10) for heating a heating chamber (12), comprising: an adiabatic combustion chamber (11) that comprises at least one air supply device (15), at least one fuel supply device (14), and at least one discharge (16) wherein the adiabatic combustion chamber (11) is configured for flameless oxidation of fuel in a non-stoichiometric fuel/air ratio; and a heating chamber (12) in which the at least one discharge (16) of the combustion chamber (11) opens, wherein the heating chamber (12) includes a supply device for supplying air and/or fuel.

10. The device according to claim 9, wherein the adiabatic combustion chamber (11) has a volume that is smaller than a volume of the heating chamber (12).

11. The device according to claim 9, wherein the combustion chamber (11) is free of flame-maintaining structures.

12. The device according to claim 9, wherein the heating chamber (12) includes a heat withdrawal device (13).

13. The device according to claim 9, further comprising a heat exchanger (23) upstream of the combustion chamber (11) and/or the heating chamber (12).

14. The device according to claim 9, further comprising a regulating device for regulation of fuel and/or air supply to the combustion chamber (11) and/or heating chamber (12) that is configured such that a predefined upper temperature limit is not exceeded in the combustion chamber and/or the heating chamber (12).

15. The device according to claim 14, wherein the regulating device is configured to influence a fuel/air ratio for temperature regulation.

Description

[0026] In the drawings embodiments of an inventive device are illustrated. The drawings show:

[0027] FIG. 1 an inventive device in schematic illustration,

[0028] FIG. 2 a modified inventive device in schematic illustration,

[0029] FIG. 3 a combustion chamber and a heating chamber of an inventive device in schematic illustration in longitudinal section,

[0030] FIG. 4 a diagram for clarification of the load closed loop control of the device according to FIGS. 1 to 3.

[0031] For clarification of the inventive method reference is made to the device 10 according to FIG. 1 that comprises a combustion chamber 11 that is thermally insulated against heat losses and a heating chamber 12 from which useful heat can be extracted. The heat withdrawal from the heating chamber 12 can be executed by heat extraction, e.g. via a heating coil 13 in which a heat carrier fluid is heated or evaporated or by other technical means. For example, the heating chamber 12 can be used for drying of products, for heating of products, e.g. for soldering or for other purposes that require a heating of fluids or objects on moderate temperatures that can be below the spontaneous ignition temperature of a used fuel, e.g. 850° C.

[0032] The combustion chamber 11 is supplied with fuel and air via a fuel line 14 and an air line 15. The ratio of fuel and air is thereby defined such that the air ratio is λ=1. Preferably λ is remarkably smaller than 1, i.e. the operation is executed in excess of fuel. For initiating the oxidation in the combustion chamber 11, it is preferably provided with a not further illustrated ignition device, e.g. with a spark ignition device or a pilot burner. It can be operated in continuous operation or can also be turned off after the flameless oxidation in the combustion chamber 11 has been established.

[0033] Preferably, the wall of the combustion chamber 11 comprises a high heat resistance. For example, the combustion chamber 11 can be lined with ceramic or can consist of ceramic. In doing so a quick heating of the combustion chamber 11 and a quick attainment of an operation manner with flameless oxidation shall be allowed after the ignition of the fuel in the combustion chamber 11.

[0034] The reaction gases created in the combustion chamber are introduced in the heating chamber 12 via a reaction gas passage 16. In addition, air and/or fuel are introduced in the heating chamber 12 via a line 17 in order to mix with the hot reaction gases there and to effect a complete oxidation of the used fuel. The heating chamber 12 is preferably remarkably larger than the combustion chamber 11, wherein an average temperature is obtained in the heating chamber 12 that is remarkably lower than in the combustion chamber 11 and that can be preferably also below the spontaneous ignition temperature of the used fuel. The created exhaust gases are discharged via a line 18 from the heating chamber 12.

[0035] Preferably the combustion chamber 11 is operated in a temperature range that is at least such high that the spontaneous ignition temperature of the used fuel is exceeded, whereby it is however concurrently so low that the nitrogen oxide formation is nearly completely suppressed. The useable temperature range in the combustion chamber 11 can be, for example, defined such that the lower temperature limit is between 800° C. and 1100° C., preferably 850° C. and 1100° C., whereas the upper temperature limit is, for example, between 1100° C. and 1400° C., preferably 1100° C. and 1300° C. and has, for example, an amount of 1200° C. The desired temperature range is preferably adjusted by a respective definition or regulation of the air ratio λ. Thereby the combustion chamber 11 operates, for example (and preferably), in excess of air. In doing so, comparably small constructions of the combustion chamber 11 can be achieved. In addition, the impulse of secondary air that has to be supplied via line 17 and that is required for the complete oxidation is available for establishment of a large-scale and sufficiently quick recirculation flow in the downstream heating chamber 12.

[0036] The temperature in the combustion chamber 11 depends during adiabatic operation only from the fuel/air ratio, i.e. the air ratio, and thus from the cross-section ratio of the air inlet nozzles of the combustion chamber 11 and secondary air nozzles in the heating chamber 12. In case of a ratio of, for example, 1:1, which corresponds to an air shortage of about 50%, a temperature of about 1100° C. is achieved with natural gas as fuel in the nearly adiabatic combustion chamber 11. In addition, a temperature closed loop control can be established that influences the stoichiometry, i.e. the air ratio in the combustion chamber 11, in order to maintain the temperature in the combustion chamber 11 within a desired range. This is particularly appropriate during use of lean gases with changing calorific value as fuel. Then the temperature in the combustion chamber 11 can be controlled in closed loop via the air ratio λ.

[0037] The control of the temperature in the combustion chamber 11 by appropriate definition of the stoichiometry (of the air ratio λ) can also be applied for the cold start such that the combustion chamber 11 can be quickly brought to the desired operation temperature of, e.g. 1000° C. For cold start the combustion chamber 11 can be, for example, operated in stoichiometric operation (λ=1), until the desired temperature is reached, after which the operation is continued in an sub-stoichiometric manner. In order to allow the desired flameless operation in the combustion chamber 11, the combustion chamber 11 is configured for creation of a large-scale recirculation vortex. Flame-holding structures are, however, not present. For this suitable flow guide devices can be arranged in the combustion chamber 11 that support the formation of a recirculation flow.

[0038] Flameless operation can also be realized in the downstream heating chamber 12, if required, although the temperature thereof is less than the spontaneous ignition temperature of the used fuel in the area of the heat withdrawing structures, e.g. the heating coil 13. For this reference is made to FIG. 3 in which a schematic longitudinal section of the combustion chamber 11 and the heating chamber 12 is illustrated. In the combustion chamber 11 a recirculation vortex 19 is established by means of a guide device, e.g. in the form of a hollow cylinder in which fuel is oxidized sub-stoichiometrically. The air 20 blown into the heating chamber 12 via the line 17 transmits its impulse on a gas jet 21 that consists of reaction gases discharged from the combustion chamber 11. In this zone in the forming gas jet 21 a flameless oxidation of the fuels that are still present in the gas jet 21 can occur, whereby a further heat release occurs. However, the gas jet 21 mixes in the course of its recirculation in the heating chamber 12 with cooler rest gas that is present there and thus forms cooler zones such that the average temperature in the heating chamber 12 can be below the spontaneous ignition temperature of the used fuel, e.g. below 850° C., in spite of the additional energy release in the gas jet 21.

[0039] The device 10 and the method explained based thereon have numerous advantages compared with conventional heating devices, particularly those based on the operation with flame. Due to the operation of the combustion chamber 11 in flameless oxidation and preferably also the heating chamber 12 with flameless oxidation, the thermal NOx-formation can be nearly completely suppressed and thus values of below 10 mg/m.sup.3 can be achieved. This applies independent from the temperature of the zones of the heating chamber 12 serving for heat withdrawal that can also be below the spontaneous ignition temperature of the used fuel, e.g. below 850° C.

[0040] In the case of use of lean gases with changing calorific value, problems of flame stabilization that are otherwise present can be avoided by operation of the combustion chamber 11 with flameless oxidation. It is shown that the combustion chamber 11 can be adapted to different power ranges of 10 kW to some MW in a manner being identical in construction and cheap.

[0041] In the combustion chamber 11 operated without heat withdrawal a temperature can be substantially maintained constant, also during partial load, such that large control ratio is obtained without specific effort.

[0042] If the combustion chamber 11 is provided with a heat transition impeding lining, e.g. a ceramic lining, or consists itself of ceramic or another material with a high thermal resistance, the combustion chamber 11 can also operate flamelessly during cold start. With the technique of flameless oxidation local temperature peaks in flames are avoided, which has a material-conserving effect on the combustion chamber 11 as well as the heating chamber 12.

[0043] FIG. 2 illustrates a system 22 based on the device 10 according to FIGS. 1 and 3, for the description of which reference is made to the above description on the basis of the already introduced reference numerals. In the system 22 a heat exchanger 23 is connected to the exhaust gas line 18 that serves for air preheating and discharges cooled exhaust gas at an outlet 24. The heat exchanger 23 heats fresh air supplied via a fresh air inlet 25 and discharges it to the lines 15 and/or 17 in a heated condition. In at least one of the lines 15, 17 a flow rate regulating device 26, 27 can be provided, e.g. in the form of a slider, a valve, a fan or similar means that influences the flow velocity. The flow rate regulating devices 26, 27 are connected to a control 28. This is in addition connected with a flow rate regulating device 29 that is arranged in the line 14 in order to regulate the fuel flow to the combustion chamber 11. The flow rate regulating device 29 can also be a slider, a valve, a pump, a fan or the like.

[0044] The combustion chamber 11 can be provided with a temperature sensor 30 that is connected with control 28. A task of the temperature sensor 30 is the monitoring of the operation of the combustion chamber 11, where usual flame sensors cannot be used due to lack of flames in the flameless oxidation. Preferably the temperature sensor is a “quick” sensor, that means it comprises a remarkably small thermal inertia.

[0045] In the system according to FIG. 2, the air supplied to the combustion chamber 11 and/or the heating chamber 12 is preheated by heat exchanger 23. The heat exchanger 23 is, however, optional and also embodiments are possible that do not require this heat exchanger and thus do not require air preheating for the heating chamber 12 and particularly also do not require air preheating for the combustion chamber 11. The temperature required for the flameless oxidation in the combustion chamber 11 results then from the omitted useful heat withdrawal, i.e. the thermal isolation of the combustion chamber 11.

[0046] The system 22 illustrated in FIG. 2 can operate, for example as follows:

[0047] First, a full load operation is illustrated. For this the control 28 adjusts the fuel flow according to the desired load by means of the flow rate regulating device 29 and then regulates the air flow by means of the flow rate regulating device 26, i.e. the air ratio, such that the temperature in the combustion chamber 11 is in a desired operation range, e.g. between 850° C. and 1300° C., for example at about 1100° C. Thereby a flameless oxidation is achieved in the combustion chamber 11. By means of the temperature sensor 30 the control 28 detects the temperature and reduces the air flow in the air line 15, if the temperature increases above a desired amount and increases the air flow, if the temperature decreases too far. The temperature closed loop control is thus carried out by means of a variation of the air ratio λ in the sub-stoichiometric range. This is apparent from the right section of the abscissa in FIG. 4. The control 28 is thereby in addition configured to not exceed a limit value of λ.sub.1 in this control range in order to avoid an excessive temperature increase and thus the creation of nitrogen oxide. The control 28 concurrently releases the air flow in the line 17 by means of the flow rate regulating device 27 such that the remaining fuel in the gas jet 21 completely oxides with air 20 in the heating chamber 12. The oxidation occurs preferably flamelessly within the gas jet 21. The latter heats the heating chamber 12 to a temperature below the spontaneous ignition temperature of the used fuel, although the gas jet 21 itself has a temperature above the spontaneous ignition temperature of the fuel. In doing so, also in the heating chamber 12 no thermal NOx-formation is noticed.

[0048] During transition into the partial load the control 28 reduces the fuel flow by means of the flow rate regulating device 29 to smaller values. The air ratio λ is thereby adjusted such that the spontaneous ignition temperature is reliably exceeded in the combustion chamber 11. Potentially necessary air for the complete oxidation of still present remaining fuel in the heating chamber 12 is again supplied via line 17.

[0049] During extreme low load the control can transition from the sub-stoichiometric operation of the combustion chamber 11 described so far to the super-stoichiometric operation thereof, as illustrated in FIG. 4 on the left part of the abscissa above the air ratio limit value λ.sub.2. While during sub-stoichiometric operation with reduction of the air ratio λ a temperature decrease has been achieved, now during super-stoichiometric operation a temperature decrease is achieved by increase of the air ratio λ. In this range control 28 counters a too high temperature with an increase of the air ratio and a too low temperature of the combustion chamber 11 with a decrease of the air ratio.

[0050] The range between the two limit values λ.sub.1 and λ.sub.2 is avoided by control 28 at least as soon as the combustion chamber 11 is heated in that this range is passed in a short period during switching from sub-stoichiometric operation to super-stoichiometric operation or is switched without transition from sub-stoichiometric operation to super-stoichiometric operation (and vice versa). In doing so, the increase of the temperature in the combustion chamber 11 above a critical limit value of, e.g. 1300° C. or 1400° C. and the accompanying thermal NOx-formation is avoided.

[0051] Numerous modifications can be made to the presented embodiments. For example, the air preheating by means of the heat exchanger 23 can be limited to the air supplied to the combustion chamber 11 via line 15. As an alternative the air preheating can be limited to the air supplied to the heating chamber 12 via line 17. It is also possible to supply a mixture of preheated and not preheated air via line 15 and/or line 17. Further, also a preheating of the fuel supplied to the combustion chamber 11 via line 14 is basically possible. In addition, particularly for sub-stoichiometric operation of the combustion chamber 11 an additional fuel supply can be provided for the heating chamber 12 in order to effectuate an increased ratio of the heat creation in the heating chamber 12. In all presented embodiments it is, however, considered to be advantageous to effectuate the majority of the heat creation in the combustion chamber 11 by means of flameless oxidation. The further oxidation in the heating chamber 12 can be carried out with or without flame, wherein the flameless oxidation allows the decrease of the nitrogen oxide creation down to values below 10 mg/m.sup.3.

[0052] In the inventive method for heating a heating chamber 12 with a temperature below the spontaneous ignition temperature of the used fuel a combustion chamber 11 is provided in which fuel and air are brought to reaction with one another in flameless oxidation in a non-stoichiometric mixture ratio. The air ratio λ is thereby at least so far away from the stoichiometric ratio λ=1 that a temperature in the combustion chamber 11 is not exceeded beginning with which a thermal nitrogen oxide creation starts. This temperature is, e.g. 1300° C. to 1400° C. On the other hand the air ratio λ is defined such that the spontaneous ignition temperature of the used fuel is reliably exceeded in the combustion chamber 11. Thus, two reliable air ratio ranges are obtained, namely a first range between λ.sub.min and λ.sub.1 in the sub-stoichiometric operation and a second range λ.sub.2 to λ.sub.max in the super-stoichiometric operation of the combustion chamber 11. The still reactive gases released from the combustion chamber 11 are brought to reaction with additional air and/or additional fuel in a zone of the heating chamber 12, whereby it is preferably carried out with flameless oxidation. The named zone is particularly formed within the gas jet 21. Due to the flameless oxidation, a thermal nitrogen oxide creation is avoided also in the heating chamber 12.

LIST OF REFERENCE SIGNS

[0053] 10 device [0054] 11 combustion chamber [0055] 12 heating chamber [0056] 13 heating coil [0057] 14 fuel line [0058] 15 air line [0059] 16 reaction gas passage [0060] 17 line [0061] 18 exhaust gas [0062] 19-a flow guide device [0063] 19 recirculation vortex [0064] 20 air [0065] 21 gas jet [0066] 22 system [0067] 23 heat exchanger [0068] 24 outlet [0069] 25 fresh air inlet [0070] 26, 27 flow rate regulating device [0071] 28 control [0072] λ air ratio [0073] λ.sub.1, λ.sub.2, λ.sub.min, λ.sub.max, λ.sub.v air ratio limit values [0074] 29 flow rate regulating device [0075] 30 temperature sensor