Patent ID: 12241626

DETAILED DESCRIPTION

For clarification of the inventive method reference is made to the device10according toFIG.1that comprises a combustion chamber11that is thermally insulated against heat losses and a heating chamber12from which useful heat can be extracted. The heat withdrawal from the heating chamber12can be executed by heat extraction, e.g. via a heating coil13in which a heat carrier fluid is heated or evaporated or by other technical means. For example, the heating chamber12can 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.

The combustion chamber11is supplied with fuel and air via a fuel line14and an air line15. 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 chamber11, 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 chamber11has been established.

Preferably, the wall of the combustion chamber11comprises a high heat resistance. For example, the combustion chamber11can be lined with ceramic or can consist of ceramic. In doing so a quick heating of the combustion chamber11and a quick attainment of an operation manner with flameless oxidation shall be allowed after the ignition of the fuel in the combustion chamber11.

The reaction gases created in the combustion chamber are introduced in the heating chamber12via a reaction gas passage16. In addition, air and/or fuel are introduced in the heating chamber12via a line17in order to mix with the hot reaction gases there and to effect a complete oxidation of the used fuel. The heating chamber12is preferably remarkably larger than the combustion chamber11, wherein an average temperature is obtained in the heating chamber12that is remarkably lower than in the combustion chamber11and that can be preferably also below the spontaneous ignition temperature of the used fuel. The created exhaust gases are discharged via a line18from the heating chamber12.

Preferably the combustion chamber11is 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 chamber11can 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 chamber11operates, for example (and preferably), in excess of air. In doing so, comparably small constructions of the combustion chamber11can be achieved. In addition, the impulse of secondary air that has to be supplied via line17and that is required for the complete oxidation is available for establishment of a large-scale and sufficiently quick recirculation flow in the downstream heating chamber12.

The temperature in the combustion chamber11depends 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 chamber11and secondary air nozzles in the heating chamber12. 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 chamber11. In addition, a temperature closed loop control can be established that influences the stoichiometry, i.e. the air ratio in the combustion chamber11, in order to maintain the temperature in the combustion chamber11within a desired range. This is particularly appropriate during use of lean gases with changing calorific value as fuel. Then the temperature in the combustion chamber11can be controlled in closed loop via the air ratio λ.

The control of the temperature in the combustion chamber11by appropriate definition of the stoichiometry (of the air ratio λ) can also be applied for the cold start such that the combustion chamber11can be quickly brought to the desired operation temperature of, e.g. 1000° C. For cold start the combustion chamber11can 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 chamber11, the combustion chamber11is 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 chamber11that support the formation of a recirculation flow.

Flameless operation can also be realized in the downstream heating chamber12, 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 coil13. For this reference is made toFIG.3in which a schematic longitudinal section of the combustion chamber11and the heating chamber12is illustrated. In the combustion chamber11a recirculation vortex19is established by means of a guide device, e.g. in the form of a hollow cylinder in which fuel is oxidized sub-stoichiometrically. The air20blown into the heating chamber12via the line17transmits its impulse on a gas jet21that consists of reaction gases discharged from the combustion chamber11. In this zone in the forming gas jet21a flameless oxidation of the fuels that are still present in the gas jet21can occur, whereby a further heat release occurs. However, the gas jet21mixes in the course of its recirculation in the heating chamber12with cooler rest gas that is present there and thus forms cooler zones such that the average temperature in the heating chamber12can 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 jet21.

The device10and 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 chamber11in flameless oxidation and preferably also the heating chamber12with flameless oxidation, the thermal NOx-formation can be nearly completely suppressed and thus values of below 10 mg/m3can be achieved. This applies independent from the temperature of the zones of the heating chamber12serving for heat withdrawal that can also be below the spontaneous ignition temperature of the used fuel, e.g. below 850° C.

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 chamber11with flameless oxidation. It is shown that the combustion chamber11can be adapted to different power ranges of 10 kW to some MW in a manner being identical in construction and cheap.

In the combustion chamber11operated without heat withdrawal a temperature can be substantially maintained constant, also during partial load, such that large control ratio is obtained without specific effort.

If the combustion chamber11is 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 chamber11can 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 chamber11as well as the heating chamber12.

FIG.2illustrates a system22based on the device10according toFIGS.1and3, for the description of which reference is made to the above description on the basis of the already introduced reference numerals. In the system22a heat exchanger23is connected to the exhaust gas line18that serves for air preheating and discharges cooled exhaust gas at an outlet24. The heat exchanger23heats fresh air supplied via a fresh air inlet25and discharges it to the lines15and/or17in a heated condition. In at least one of the lines15,17a flow rate regulating device26,27can 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 devices26,27are connected to a control28. This is in addition connected with a flow rate regulating device29that is arranged in the line14in order to regulate the fuel flow to the combustion chamber11. The flow rate regulating device29can also be a slider, a valve, a pump, a fan or the like.

The combustion chamber11can be provided with a temperature sensor30that is connected with control28. A task of the temperature sensor30is the monitoring of the operation of the combustion chamber11, 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.

In the system according toFIG.2, the air supplied to the combustion chamber11and/or the heating chamber12is preheated by heat exchanger23. The heat exchanger23is, however, optional and also embodiments are possible that do not require this heat exchanger and thus do not require air preheating for the heating chamber12and particularly also do not require air preheating for the combustion chamber11. The temperature required for the flameless oxidation in the combustion chamber11results then from the omitted useful heat withdrawal, i.e. the thermal isolation of the combustion chamber11.

The system22illustrated inFIG.2can operate, for example as follows:

First, a full load operation is illustrated. For this the control28adjusts the fuel flow according to the desired load by means of the flow rate regulating device29and then regulates the air flow by means of the flow rate regulating device26, i.e. the air ratio, such that the temperature in the combustion chamber11is 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 chamber11. By means of the temperature sensor30the control28detects the temperature and reduces the air flow in the air line15, 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 inFIG.4. The control28is thereby in addition configured to not exceed a limit value of λ1in this control range in order to avoid an excessive temperature increase and thus the creation of nitrogen oxide. The control28concurrently releases the air flow in the line17by means of the flow rate regulating device27such that the remaining fuel in the gas jet21completely oxides with air20in the heating chamber12. The oxidation occurs preferably flamelessly within the gas jet21. The latter heats the heating chamber12to a temperature below the spontaneous ignition temperature of the used fuel, although the gas jet21itself has a temperature above the spontaneous ignition temperature of the fuel. In doing so, also in the heating chamber12no thermal NOx-formation is noticed.

During transition into the partial load the control28reduces the fuel flow by means of the flow rate regulating device29to smaller values. The air ratio λ is thereby adjusted such that the spontaneous ignition temperature is reliably exceeded in the combustion chamber11. Potentially necessary air for the complete oxidation of still present remaining fuel in the heating chamber12is again supplied via line17.

During extreme low load the control can transition from the sub-stoichiometric operation of the combustion chamber11described so far to the super-stoichiometric operation thereof, as illustrated inFIG.4on the left part of the abscissa above the air ratio limit value λ2. 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 control28counters a too high temperature with an increase of the air ratio and a too low temperature of the combustion chamber11with a decrease of the air ratio.

The range between the two limit values λ1and λ2is avoided by control28at least as soon as the combustion chamber11is 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 chamber11above a critical limit value of, e.g. 1300° C. or 1400° C. and the accompanying thermal NOx-formation is avoided.

Numerous modifications can be made to the presented embodiments. For example, the air preheating by means of the heat exchanger23can be limited to the air supplied to the combustion chamber11via line15. As an alternative the air preheating can be limited to the air supplied to the heating chamber12via line17. It is also possible to supply a mixture of preheated and not preheated air via line15and/or line17. Further, also a preheating of the fuel supplied to the combustion chamber11via line14is basically possible. In addition, particularly for sub-stoichiometric operation of the combustion chamber11an additional fuel supply can be provided for the heating chamber12in order to effectuate an increased ratio of the heat creation in the heating chamber12. In all presented embodiments it is, however, considered to be advantageous to effectuate the majority of the heat creation in the combustion chamber11by means of flameless oxidation. The further oxidation in the heating chamber12can 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/m3.

In the inventive method for heating a heating chamber12with a temperature below the spontaneous ignition temperature of the used fuel a combustion chamber11is 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 chamber11is 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 chamber11. Thus, two reliable air ratio ranges are obtained, namely a first range between λminand λ1in the sub-stoichiometric operation and a second range λ2to λmaxin the super-stoichiometric operation of the combustion chamber11. The still reactive gases released from the combustion chamber11are brought to reaction with additional air and/or additional fuel in a zone of the heating chamber12, whereby it is preferably carried out with flameless oxidation. The named zone is particularly formed within the gas jet21. Due to the flameless oxidation, a thermal nitrogen oxide creation is avoided also in the heating chamber12.

LIST OF REFERENCE SIGNS

10device11combustion chamber12heating chamber13heating coil14fuel line15air line16reaction gas passage17line18exhaust gas19-aflow guide device19recirculation vortex20air21gas jet22system23heat exchanger24outlet25fresh air inlet26,27flow rate regulating device28controlλ air ratioλ1, λ2, λmin, λmax, λvair ratio limit values29flow rate regulating device30temperature sensor