EP0258348B1 - Method and device for the post combustion of process exhaust gasses - Google Patents

Abstract

Method and device used for the thermal combustion of oxidizable gas components in process gasses. The gasses are conducted through a post combustion device (10) comprising amongst other elements a combustion chamber (18) and a process gas outlet (24). The cleaned exhaust gasses discharged through the process gas outlet (24) are mixed with gasses used in the process in order to maintain constant their concentration.

Description

The invention relates to a method for the controlled thermal afterburning of process exhaust gas containing oxidizable constituents and to an apparatus for carrying out the method according to the preambles of claims 1 and 4.

A device for thermal post-combustion of combustible substances in a process exhaust gas such. B. Hydrocarbons can be found in EP-B1-0 040 690. Here, the process exhaust gas preheated in heat exchanger tubes is fed to a burner, the heat output of which is to be adjusted at every moment to the fluctuating amount of the components to be burned and the inconsistent amount of the process air flow.

US-A-2 905 523 shows a method for treating exhaust gas which is used for the catalytic combustion of soot and combustible dusts together with gaseous constituents. For the purpose of raising the temperature of the process gas which is too cold, this method uses the recirculation and mixing of part of the burned hot gas into the cold gas as an alternative for the otherwise conventional recuperative heat exchange and for starting the system in a circuit. This feedback ensures the ignition level, ie the maintenance of the minimum bed temperature in the catalytic converter. In addition, the method knows the feeding of air into the main and a by-pass stream of the unpurified exhaust gas, for the purpose of oxygen enrichment in the event of its lack, or also for the purpose of dilution when the flammable substance is too high. The latter is used to protect the catalyst, which should not be heated above 1600 ° F. Both functions, the recirculation of hot exhaust gas and the addition of air are procedurally completely separate functions and serve different purposes. The return of hot air is only used to maintain the process. In the case of recuperative preheating of the process exhaust gas, there is no recirculation. In the case in which the feed is air dilution, and not the 0 ₂ -Beimengung, it satisfies only to protect the catalyst from overheating the purpose. Through the US-A-2,905,523 a method is thus described, in which is allowed to move, the combustion chamber with catalytic converter and downstream components in the temperature range between 570 ⁰ F and 1600 ° F (573 K to 1143 K), without this influencing of burning takes place.

It would be desirable if the combustion chamber temperature were kept as constant as possible, since otherwise excessive material stress and fatigue due to high temperature would result in changing speeds.

It is known practice of thermal post-combustion to allow the temperature of the combustion chamber to fluctuate within a 'tolerance range' up to a value which is just below the safety cut-off limit set until the process-related temperature peaks subside again. Occasionally, however, the peaks are so high that the switch-off temperature is reached and normal operation must be interrupted. One then speaks of overtemperature shutdowns. Both the overshoots and these interruptions have a negative impact on the service life of the parts that are subject to greater stress. The latter usually also automatically interrupts the production process in the networking of production and exhaust air purification required today and thus leads to high production losses.

Added to this is the fact that temperature sensors such as thermocouples are inserted in protective tubes in a practical design and thus temperature peaks are delayed, reduced or not registered at all. This fact also does not promote the lifespan of post-combustion devices. Smaller volumetric flow fluctuations - as they can occur due to the process - usually also have an adverse effect on the combustion chamber temperature. The effects of these fluctuations can be compared with those resulting from a fluctuating input of flammable substances.

The temperature fluctuations discussed so far are unavoidable in the prior art if an incineration plant is operated in the border area of its thermal capacity and impurity capacity, and if measures to remove excess energy have not been taken.

However, if the input of thermal energy increases significantly more than the burner of the afterburning system has reserves to readjust, then the system must inevitably be switched off (by triggering the overtemperature switch) if this system does not have a secondary system to reduce the temperature in the combustion chamber total heat input is equipped.

The 'total heat quantity' is to be understood as the enthalpy of the process gas to be cleaned, including the heat quantities introduced by the combustible substance and still supplied by the burner in the minimum position.

Since high energy prices nowadays force the process exhaust air to have a high degree of preheating, the enthalpy of the air preheated in the heat exchanger also becomes the limiting variable.

As already mentioned, this is determined by a high degree of preheating, but also by the temperature of the exhaust air brought in from the production process. With increasing exhaust air temperature from production, the preheating temperature also rises further, so that the total absorption capacity for combustible substances decreases.

As a proportion of the design capacity, this loss of capacity caused by increased exhaust gas temperature can even be very considerable, but especially if the burner is only operated with smaller partial volume flows, the minimum output of the burner - which is a constant size - is already a large part of the Capacity for combustible substance consumes.

The conventional technology therefore uses - in order to reduce the degree of preheating of the exhaust air - the principle of bypassing one or both sides of the mostly recuperative heat exchangers, each with part quantities of exhaust air volume flows, i.e. by-pass technology.

This partial bypassing of the heat exchanger requires integrated or external ducts or pipes, control and heat-compatible valve and flap technology, compensation elements for thermal expansion and suitable mixing techniques for remixing with the main air flows after passing through and bypassing the heat exchanger. There is also an increased need for insulation.

Bypass or by-pass techniques in post-combustion devices always have the property that bypassing on one side (hot side or cold side) that the mass of the heat exchanger - due to the regulation of the by-pass - must constantly find a new heat balance; in other words: the mass of the heat exchanger is moved back and forth in its temperature level. If a heat exchanger is partially circumnavigated on the hot gas side, this has the consequence that the change in the preheating temperature can only be achieved by changing the thermal equilibrium of the entire mass of the heat exchanger, i.e. only by means of a very slow process. The latter is therefore not suitable as a spontaneous regulator and is therefore less common.

If you drive around alone on the cold gas side, the control speed must be mentioned spontaneously, but as the volume flow in the heat exchanger decreases, the reduced amount of air still flowing there is preheated, and the higher the by-pass extraction, the higher. This property sometimes results in extreme burn-up of the combustible substance in the heat exchanger. It turns this heat exchanger, which is usually unsuitable for the combustion of the oxidizable substances, into a combustion chamber stage and this, together with all the negative effects.

In addition, there is the general increase in the temperature level of this exchanger, a process that is slow due to its mostly large mass.

Although the solution of the one-sided bypass the heat exchanger within the meaning of Praktikabil ⁱ ty only the cold bypass out of the question, but it has nevertheless other important limitations and negative consequences: it forces to a very good mixing of the cold, not gated heated by-pass flow rate in and with the preheated very hot air. This constraint is due to the fact that temperature differences in the combustion chamber flow cross sections of 15 K can mean inadequate burnout and high CO values. The result of this is that the temperature of this combustion chamber has to be raised by as much as 15 K. In the higher temperature range of modern systems with a low burner base load and the very high final cleanliness requirements, another 15 K may mean a greater technological obligation.

The high demands on burnout while avoiding higher values for CO and NOx force good mixing technology and combustion chamber technology. The demand for spontaneous adaptation of the combustion technology to the ever faster and more responsive production processes, the safety requirements and the desire for great availability and durability often only allow energy control systems with conventional technology that consist of the heat exchanger being bypassed on both sides. Compared to the one-sided (cold) bypass, bilateral by-pass systems also compensate for much larger differences in concentrations of oxidizing substance. So when it comes to large capacity fluctuations and higher quality requirements for process engineering, then solid technology often only allows bypasses on both sides. This is particularly true where the combustible substance has a low ignition temperature, e.g. B. mineral oils and petrol. The additional temperature increase of the heat exchanger resulting solely from a cold bypass could have inadmissible consequences for the CO generation in the heat exchanger and also for the steels; because it is generally known that CO is a carbon supplier and can lead to embrittlement of steels in the higher temperature range, but also to faster scaling.

High CO generation should be avoided as far as possible. However, high CO production is virtually linked to by-pass technology: the higher the concentration of the combustible substance, the longer the residence time in the heat exchanger, the higher the CO generation. By-pass operation is a further amplifier of these relationships.

By-pass techniques are usually technically complex, expensive and require a high degree of regulation and monitoring. If the heat exchanger is bypassed on both sides, the volume flows in each control moment must be as large as possible and the control elements must always run in parallel.

The by-pass systems are also complex in terms of construction, detailed technology, assembly and commissioning. They require an increased amount of service during operation.

The object of the present invention is to develop the method of the type described in the introduction such that a continuous adjustment of the heat output of the burner as a result of concentration fluctuations of the oxidizable constituents in the process exhaust gas does not have to take place, so that in particular temperature peaks are avoided, at the same time being ensured It is intended that a rise in the impurity concentration of the process exhaust gas to be fed to the combustion device, which exceeds the usual values, can be coped with without problems, and in particular that material stresses and fatigue caused by high temperature change rates are avoided.

This object is achieved according to the invention in that the concentration of the oxidizable constituents in the combustion chamber is kept at an adjustable value and that the inlet temperature of the gas mixture to be cleaned consisting of process exhaust gas, purified process exhaust gas and fresh air to be supplied to a gas mixture adjustable value is kept. In other words, with increasing concentration of combustible substance from the moment the burner has reached its control minimum (its base load), the amount increases with the amount and in a controlled manner grow the concentration of combustible substance, cleaned process exhaust gas mixed in with fresh air. The admixture is carried out at any time in the amount required to maintain the temperature in the combustion chamber in accordance with its setpoint. The burner itself remains at a minimum during the mixing operation and no longer intervenes in the process. The production of the mixed air temperature is the responsibility of a second control circuit, by means of which it is decided whether more or less warm cleaned exhaust gas or cold fresh air is added. The measure for this control task is the respective deviation of the actual exhaust gas temperature from its target temperature. In other words, the inlet temperature of the gas mixture to be supplied to the afterburning device and consisting of process exhaust gas to be cleaned, purified process exhaust gas and fresh air is kept at an adjustable value. According to the invention, it is therefore proposed that an adequate amount of air mixture of more or less already cleaned exhaust air and less or more fresh air is added to the process exhaust gas, which is to be rich in combustible substance, before entering the afterburning device and before its heat exchanger, and in just that amount, which is necessary to keep the combustion chamber temperature constant at a minimum control of the burner by dilution. In other words, with the burner moving at a constant minimum, the combustion chamber temperature is regulated in a precisely constant manner and, at the same time, the concentration of the combustible substance in the exhaust gas is also almost constant.

This results in Advantages that are characterized by the fact that the combustion chamber temperature is always adjusted to the desired level and cannot overshoot under the same conditions, that the heat exchanger always maintains the same temperature level, regardless of the concentration of impurities and the degree of excess energy control, that the length of time that is to be heated Medium in the heat exchanger does not increase but increases with increasing energy control, so that the CO generation does not increase, but rather decreases, so that the heat exchanger does not become a pre-combustion zone to an increased extent, but rather less that the preheating temperature does not fluctuate, but remains constant that temperature equilibria remain constant, that there are further advantages associated with this technology, such as a standstill or warm-up operation that takes place at a constantly warm temperature, a cheaper start-up of the entire system, a shorter start-up of the entire system, a lengthening rn the service life of the device by reducing almost all major temperature peaks and harmonics, reducing the carbon diffusion in the steels by lowering the CO level and thus maintaining the properties of these steels for a longer time, avoiding environmental shocks by switching process air and caftar air, superfast React to procedural changes as the burner can (or even faster), a lower CO level due to less self-generation, a lower NOx level by avoiding an increased combustion chamber temperature and countermeasures against an excessively high exhaust air temperature if the concentration increases combustible substance is already too high for burner control.

According to the invention, the concentration of the oxidizable constituents is always adjusted in a constant manner after the burner minimum has been reached so that the amount of heat released from the combustion of the oxidizable constituents keeps the combustion chamber temperature exactly at the desired level, that is to say it does not drop or rise.

The following property is also associated with the solution according to the invention: the constancy of the outlet temperature of the cleaned and again cooled exhaust gas from the afterburning device. While conventional by-pass systems cause fluctuations of up to 150 K (= 270 ° F), in the method according to the invention the control process takes place at an almost constant temperature. This consistency not only has the positive effects already made on the device itself, but also on all subsequent equipment: all subsequent technology must be designed and manufactured solely for the low standard temperature level. This applies to the fireplace.

A future-oriented and essential property of the system is its safe suitability for the use of highly preheating heat exchangers. Where conventional, by-pass systems have to put an end to preheating due to the CO problem - mentioned and documented in the literature are max. 550 ° C (1022 ° F) - the system according to the invention is far from over: the preheating can be operated up to 650 ° C (1202 ° F) and, as mentioned, almost without fluctuations.

The measure of the admixture of air to the unpurified process air is then the excess amount of combustible substance above the maximum possible capacity with a burner base load.

Another variable defines the mixture of more or less warm air and cold air in metering mode: the level of the process air temperature. If this temperature is also above the nominal value, fresh air will only flow in first when mixed air is requested and warm air will only flow in after the nominal temperature has been reached.

However, if the temperature is unacceptably low, only warm air will flow if necessary. This means that the system maintains the normal temperature level at all times and at all points, a) for the medium, b) for the device. In contrast, by-pass systems are subject to huge fluctuations. In the method according to the invention, there is therefore no need for the components to be pulled back and forth. Everything is warm and stays warm or is hot and stays hot. The company approaches and achieves the ideal company: the complete constant running of all links for a long time.

On the other hand, some of the properties specified above are also achieved by the fact that in the event of a process air flow failure (process-related and fault-related), a small amount of equally mixed hot air regulated to the normal process air temperature continues to operate in the most economical manner, and thereby the complete equality of the Maintains orders of magnitude of all temperatures with the normal process operation at each point of the system and ensures them for later operation with process exhaust gas.

A device according to the preamble of claim 4 is characterized in that there is a connection between the device and the gas supply, via which process exhaust gas cleaned to the desired extent within the device can be circulated in that the regulation of the temperature of the to be cleaned Process exhaust gas to be admixed cleaned process exhaust gas or the fresh air via control elements such as flaps, the controlled variable of which can be determined by the temperature which the gas mixture consisting of exhaust gas to be cleaned and cleaned exhaust gas and / or fresh air has on the pressure side of the blower, and that the heat exchanger tubes are bent outwards at their cold ends and the process exhaust gas can flow around them. The connection preferably runs between the process exhaust gas outlet and the feed. This gives the possibility of using structurally simple means without them running inside the device and z. B. have flap mechanisms, the process exhaust gas to be cleaned to the extent required to supply cleaned process exhaust gas and / or air in order to have the proportion of the oxidizable constituents at a constant value and to correct the temperature of the process gas.

Accordingly, combustion devices are designed in such a way that a connection is made between the process exhaust gas outlet and the process exhaust gas supply, which allows exhaust gas which has been cleaned to the desired extent to be circulated or recirculated, always with the same, more or less fresh air mixed.

The mixed air thus generated is admixed with the process exhaust gas near the suction side of the process exhaust gas blower.

The warm air is recirculated externally and using simple design means. The dosing of the warm air and the cold air each take over an independent control body, i.e. Flaps or valves.

A temperature controller, which monitors the temperature of the mixed process air conveyed to the afterburning device, takes over the determination of the respective hot and caft air quantity.

The temperature controller, which is responsible for the constancy of the combustion chamber temperature, determines the total amount of air to be conveyed.

• Fig. 1 eine Prinzipdarstellung eines Nachverbrennungs-Prozesses von Prozeß-Abgas enthaltend oxidierbare Bestandteile mit 'By-pässen' zum Zwecke der Energieregelung,
• Fig. 2 einen erfindungsgemäß ablaufenden Prozeß und
• Fig. 3 eine den erfindungsgemäßen Prozeß realisierende Nachverbrennungs-Vorrichtung.

Further details, advantages and features of the invention result not only from the claims, the features to be extracted from them - individually and / or in combination - but also from the following description of a preferred exemplary embodiment shown in the drawing. Show it:

• 1 is a schematic diagram of a post-combustion process of process exhaust gas containing oxidizable components with 'by-pass' for the purpose of energy control,
• Fig. 2 shows a process running according to the invention and
• 3 shows an afterburning device implementing the process according to the invention.

A conventional excess energy control is to be illustrated with the aid of FIG. 1, the essential elements of the afterburning device (10) being shown purely schematically. The process gas to be cleaned is brought to the afterburning device via a blower (12) and the process gas or process exhaust gas or carrier gas supply (14). Then the process gas to be cleaned flows through a heat exchanger (16) in order to reach a combustion chamber (18) in which the oxidizable components are burned, provided that they have not already been burned in the heat exchanger part. The combustion chamber (18) can emanate from a burner (20) via a high-speed pipe (not shown), the fuel supply of which can be adjusted via a control valve (22). From the combustion chamber (18) the Purified exhaust gas passes again through the heat exchanger (16) to preheat in this still to be cleaned process gas recuperator t ⁱ v.

The cleaned exhaust gas is then discharged via a line (24). If there are major fluctuations in the process gas with regard to the concentration of the constituents to be oxidized - that is, in the line (14) - bypasses (26) and (28) are provided, which counteract the increase in temperature in the combustion chamber (18). that by partially bypassing the heat exchanger (16) they lower the level of the preheating as much as the increase (fluctuation) in the concentration of combustible substance requires. The burner (22) fires in its control minimum as long as the excessive supply of combustible substance continues.

In this process, the by-pass control (26) is designed as a connection for cold gases and the by-pass control (28) for hot gases. Each by-pass control (26) or (28) has a line (30) or (32) running in / or around the device (10), which has control mechanisms such as valves (34.1) or (36.1) in order to drive the bypass in a modulating manner or to put it out of operation. The by-pass arrangement (26) between the cold process gas flowing in the line (14) and the burner antechamber - in the schematic representation the line opens into the combustion chamber 18). The by-pass arrangement (28) established a connection between the combustion chamber (18) and the exhaust gas outlet (24). Since a by-pass can only increase its flow rate as long as the remaining amount flowing in the heat exchanger experiences greater flow resistance than the amount flowing in the by-pass, the ability to regulate is quickly exhausted unless a second control device throttles the main side and thus the by-pass Funding increases continuously. These organs are labeled (34.2) and (36.2).

The devices downstream of the device (10) for utilizing residual heat in the cleaned exhaust air are shown in FIG. 1 in the form of a hot water / air heat exchanger. The device comprises a heat exchanger (65), the by-pass control element represented by flaps (63.1) and (63.2) to increase or decrease the heat to be changed, the by-pass line (62) and the reunification line (64 ), and from the water circuit (61) with its consumers (67) and its circuit pump (66).

After leaving the heat exchanger (65) or partially or completely bypassing it, the further cooled amount of exhaust air flows to the room (68).

All elements of the device (10) and the exhaust gas line (33) must be designed for the maximum temperature that can be generated.

The method according to the invention for the controlled afterburning of oxidizable constituents in the process exhaust gas (exhaust air, carrier gas) can be seen in FIG. 2. Elements which correspond to those in FIG. 1 are provided with the same reference symbols.

The process gas to be cleaned is fed to the heat exchanger (16) and then to the combustion chamber (18) via a feed line (14) in which a process exhaust gas blower (38) with volume flow control (shown here as a speed change) is arranged. After it has been preheated in the heat exchanger (16), the process gas to be cleaned is passed into the immediate area of the burner (20) in order to get from there to the actual main combustion chamber (18) via a high-speed pipe (not shown). The burner (20) is supplied with the amount of fuel required at any moment by means of a control valve (22). After the combustion chamber (18), the now cleaned exhaust gas reaches the outlet (24) via the hot gas side of the heat exchanger (16). If the concentration of the exhaust gases to be cleaned increases beyond the control capacity of the burner, it is proposed according to the invention that by adding already cleaned exhaust gas, mixed with fresh air, the concentration is corrected so that only such an exhaust gas is added to the device (10) is carried out, the proportion of oxidizable substance (such as solvents) is consistently high. This ensures that the burner (20) can always be operated with the same minimum regulation (basic load). Since the specific proportion of the substance to be burned remains the same, the constancy of the temperatures inside the device (10) is ensured, so that its structural elements, in particular also the tubes of the heat exchanger (16), are not subjected to expansion fluctuations and voltage fluctuations. This extends the life of the heat exchanger.

As mentioned, the regulation takes place as a function of the temperature (actual temperature) determined in the combustion chamber via a thermocouple (49), which is compared in a temperature controller (49.1) with a target temperature. Depending on the deviation between the actual and the target temperature, the fuel supply is first regulated via the valve (22) in such a way that the burner (20) runs at minimum load. This is indicated by a minimum switch (22.1). Subsequently, actuators (46.1) and (46.2) for the admixture of fresh air and / or cleaned process exhaust gas are influenced to the process exhaust gas to be cleaned, which is guided in line (14), by the temperature in the combustion chamber (18) on the Setpoint to stick.

The cleaned and cooled in the heat exchanger (16) exhaust air is tapped at the exhaust outlet (24) - illustrated by the connection point (42), from where it flows in the line (44) to the union (47), which can have mixing properties. The amount of cleaned air required or requested is provided by means of a control flap (46.1). The adequate amount of fresh air flows through the control system like the control flap (46.2) to the mixing point (47). Both quantities - now as a mixed air quantity - are drawn in by means of negative pressure in the line (48). The line (48) opens into the process exhaust air line (14), in which this vacuum or suction pressure is kept constant.

The mixture of process exhaust air and added air is then conveyed from the blower (38) via line (14.1) to the heat exchanger (16).

The preheating does not change, nor does the combustion chamber temperature. The burner burns in the control minimum, because the responsibility for the complete constancy has taken over the control described here immediately when the control minimum of the burner is reached and also keeps this responsibility until the amount of combustible substance in the exhaust gas drops again enough that the metering operation finished and the burner can take over the control task again.

• Die Praxis zeigt, daß meist gleichzeitig mit dem Eintreten höherer Konzentrationen brennbarer Substanz auch die Temperatur der Prozeß-Abluft ansteigt. Oft ist die höhere Prozeß-Temperatur die Voraussetzung für das Freiwerden der Substanzen, wie z. B. von Lösemitteln aus Farben und Lacken.

It has now been adequately demonstrated that - and how - excessive levels of combustible substance are depressed to a lower specific size and how they are maintained there. And it was explained why the burner burns with a minimum flame. The role of temperature control according to the invention is to be explained below:

• Practice shows that the temperature of the process exhaust air usually rises with the occurrence of higher concentrations of combustible substance. The higher process temperature is often the prerequisite for the release of the substances, such as. B. of solvents from paints and varnishes.

Now the effect of higher temperature of the process exhaust gas is also the raising of the preheating temperature. This means that the higher preheating of the air reduces the temperature difference between the constant high combustion temperature in the combustion chamber and the preheating temperature of the air. However, since the burner - even if it withdraws to its minimum control - claims a certain proportion of it, less and less is left for the thermal conversion of the oxidizable substance in the process exhaust air. The higher the process air temperature, the higher the preheating in the heat exchanger, and the lower the acceptable concentration of oxidizable substance in the waste air (this behaves like a second fuel source, and is also one).

• Erreicht eine Anlage ihre "erste Kapazitätsgrenze" durch die Brenner-Minimum-Stellung, dann entscheidet die Regelung anhand der nach Gebläse (38) mittels Thermoelement (15) gemessenen und mit einem Sollwert am Temperaturregler (15.1) verglichenen Wert, ob zuerst mehr oder weniger kalte Luft zugegeben werden muß und ab wann Warmluft gleichlaufend mit hinzugezogen wird. Auf diese Weise wird auch die Temperatur der Vorheizung auf die Normalhöhe zurückgeführt und die Verarbeitungskapazität für die brennbare Substanz wird erhöht. Die Gesamtanlage kehrt so auch in den Bereich ihrer spezifischen Parameter zurück.

The temperature control of the device according to the invention counteracts this behavior:

• If a system reaches its "first capacity limit" due to the minimum burner position, the control system decides whether more or less first, based on the value measured by the blower (38) using a thermocouple (15) and compared with a setpoint on the temperature controller (15.1) cold air must be added and when warm air is drawn in at the same time. In this way, the temperature of the preheating is brought back to the normal level and the processing capacity for the combustible substance is increased. The entire system thus returns to the area of its specific parameters.

If, however, the rarer case occurs that the concentration of oxidizable substance is combined with a lower than the desired exhaust air temperature, then the control system corrects itself automatically by raising the exhaust gas temperature by preferably supplying hot air. This also prevents the formation of condensate in the pipeline and in the inlet area of the combustion device. I.e. If the risk of condensate is particularly high, namely at high concentrations of condensable components and at low temperature, the described control reacts to the tendency towards condensation.

According to the invention, all operating cases which usually run with cold air run in warm operation. What is meant is warming in the event of an interruption and starting or warming up the still cold system.

The first-mentioned case represents an economy mode with a very small warm air volume flow. The warm air temperature corresponds exactly to the nominal process gas temperature. The temperature controller (15.1) produces the mixture temperature exactly.

As a result of the warm interruption operation, all parts of the afterburning device keep their usual temperature level. The start-up mode using warm air allows a faster and more economical start-up than with caftar air. In addition, the areas between the blower (38) and the heat exchanger (16) are successively brought to higher temperatures until the system is ready for operation at a level at which the risk of condensate in the hazardous areas is switched off when switching to process conditions.

• a) Durch den warmen Unterbrechungsbetrieb herrschen auch bei kleinsten Volumenströmen noch deutlich bessere thermodynamische Verhältnisse in der gesamten Nachverbrennungsvorrichtung, so daß der zum Unterbrechungsbetrieb erforderliche Mindestluftstrom um bis zu 35 % gesenkt werden konnte. Entsprechend konnten die Kosten für den Unterbrechungsbetrieb gesenkt werden. Hinzu kommen die Kostensenkungen durch Warmluftbetrieb generell, die dieser Betriebsweise innewohnen.
• b) Das Verfahren regelt in Sekundenschnelle und ist somit der Brennerregelung mindestens ebenbürtig, aber übertrifft die By-pass-Systeme bei weitem. Es gestattet jetzt auch den Einsatz superschneller Thermoelemente.
• c) Im Unterbrechungs- bzw. Warmhaftebetrieb bleibt jetzt die Temperatur auch am Austritt der Nachverbrennungsvorrichtung konstant. Das hat nicht nur die bekannten positiven Folgen für die nachfolgende Peripherie (wie für Warmwasser-Wärmetauscher), sondern weitere: solche mit sog. 'kalten Flächen' fahrende Wärmeaustauscher werden bei Kaltluft-Betrieb der Verbrennungsvorrichtung stark abgekühlt und gelangen so in die Kondensationszone. Um dies zu vermeiden, darf die Wärmerückgewinnung nicht zu weit getrieben werden. Erfindungsgemäß wird dies vermieden. Die Wärmerückgewinnung kann deutlich und ohne Gefahr gesteigert werden. Der Gesamtprozeß wird wirtschaftlicher.
• d) Druckschwankungen, hervorgerufen durch das Arbeiten von nachgeschalteter Verfahrenstechnik wirken sich nicht auf die Menge der Warmluftrückführung aus, da die Temperaturregelung Priorität hat.
• e) Durch die Ausschaltung jeglicher Kondensatgefahr im Bereich des Eintritts der Nachverbrennungsvorrichtung wird eine Brandgefahr grundsätzlich ausgeschaltet.
• f) Neueste Produktionstechniken beinhalten heute auch schon Schnellreinigungs-Systeme, wie z.B. der Rotations-Offsetdruck. In Sekundenschnelle und für kurze Zeit nur werden hier große Mengen von Lösemitteln in den Abgasstrom eingebracht. Die Konzentration brennbarer Substanz steigt dann plötzlich und stark an. Das erfindungsgemä-Be Verfahren reagiert auf diese Spitzen sofort und schützt die Nachverbrennungs-Anlage vor Übertemperatur.

The large-scale testing of the process has shown a number of other properties that were unpredictable and therefore particularly surprising. In detail these are:

• a) Because of the warm interruption operation, even with the smallest volume flows, there are still significantly better thermodynamic conditions in the entire afterburning device, so that the minimum air flow required for interruption operation could be reduced by up to 35%. The costs for interruption operations were reduced accordingly. Added to this are the general cost reductions due to warm air operation, which are inherent in this mode of operation.
• b) The process regulates in seconds and is therefore at least on a par with the burner control, but far exceeds the by-pass systems. It now also allows the use of super-fast thermocouples.
• c) In the interruption or warm operation, the temperature at the outlet of the afterburning device now remains constant. This not only has the known positive consequences for the downstream periphery (such as for hot water heat exchangers), but also more: heat exchangers with so-called 'cold surfaces' are strongly cooled when the combustion device is operated with cold air and thus reach the condensation zone. To avoid this, the heat recovery must not be pushed too far. This is avoided according to the invention. The heat recovery can be increased significantly and without danger. The overall process is becoming more economical.
• d) Pressure fluctuations caused by the work of downstream process technology do not affect the amount of warm air recirculation, since the temperature control has priority.
• e) By switching off any risk of condensate in the area of the entry of the afterburning device, a fire risk is basically eliminated.
• f) The latest production techniques already include rapid cleaning systems, such as rotary offset printing. In a matter of seconds and only for a short time, large amounts of solvents are introduced into the exhaust gas flow. The concentration of flammable substance then suddenly and sharply increases. The method according to the invention reacts immediately to these peaks and protects the afterburning system from excess temperature.

3 shows a basic illustration of an afterburning device on the basis of which the teaching according to the invention can be implemented.

• (52), der durch Stirnwände (54) und (56) begrenzt ist. Im Bereich der Stirnwand (56) ist konzentrisch zur Achse
• (58) des Mantels (52) ein Brenner (60) angeordnet, der in ein Hochgeschwindigkeitsmischrohr (62) mündet, weiches wiederum zum Hauptbrennraum (64), der durch die äußere Stirnwand (54) begrenzt wird, verbindet. Dabei ist es jedoch nicht notwendigerweise erforderlich, daß - wie in der Zeichnung dargestellt - das Hochgeschwindigkeitsmischrohr (62) in den Hauptbrennraum (64) hineinragt.

The afterburning device (50), shown lying here, comprises a cylindrical outer jacket

• (52), which is bounded by end walls (54) and (56). In the area of the end wall (56) is concentric with the axis
• (58) of the jacket (52) a burner (60) is arranged, which opens into a high-speed mixing tube (62), which in turn connects to the main combustion chamber (64), which is delimited by the outer end wall (54). However, it is not necessarily necessary for the high-speed mixing tube (62) to protrude into the main combustion chamber (64), as shown in the drawing.

An internal annular space (66) runs concentrically to the high-speed mixing tube (62) and merges into the space (68) in which the heat exchanger tubes (70) are arranged concentrically to the longitudinal axis (58). The heat exchanger tubes (70) themselves open into an outer annular space (72) adjoining the outer wall (52), which passes into the inlet (74). An annular chamber (76) is also provided, which merges into the outlet (78).

The ends (80) of the heat exchanger tubes (70) are bent outwards in the area of the outlet (78), that is to say towards the wall (52), so as to open almost perpendicularly into the wall (82) of the outer annular space (72) . The other ends (84) of the heat exchanger tubes (70) open into a tube plate (86) which separates a pre-combustion chamber (88) surrounding the burner (60) from the chamber (68).

The burner (60) is continued by means of a burner stem (90), which usually widens conically in the direction of the high-speed tube (62) and has recesses such as holes (92) on the peripheral surface. The high-speed pipe (62) forms a Coanda nozzle (in the area (98) to (94)) at its inflow cone (96) together with the burner stem (90). This forms a concentric ring around the burner , doing part of the work of supplying and disposing of the burner with air

A connection (100) or the outlet (78) is connected to a mixing device, not shown, which corresponds to the mixing device (46) and (47) shown in FIG. 2.

The process gas to be combusted by the device according to the invention is fed via the inlet (74) with the annular space (72) in order to in via the heat exchanger tubes (70), the burner stem (90), the Coanda nozzle (96), the high-speed tube (62) the main combustion chamber (64) to be directed. The cleaned exhaust gas can then be discharged to the outlet (78) via the ring channel (66) and the space (68) in which the heat exchanger tubes (70) run.

So that the burner (60) can work in the minimum regulation (base load), even though the amount of combustible substance increases, cleaned exhaust gas is led via a connection (100) to the mixing device designated in (2) with (46) and (47), in which more or less fresh air is added for the purpose of achieving a desired mixing temperature. The resulting mixture of warm air reaches, according to FIG. 2, via line (48) to line (14), where it meets and is mixed with the unpurified process exhaust gas with increasing or increased impurity concentration. Mixed air is mixed in to such an extent that it is necessary to keep the concentration of combustible substance constant and to keep the combustion chamber temperature constant, as well as to achieve the required or desired temperature in front of the incineration plant.

Since the concentration is now constant, temperature fluctuations in the individual areas of the system, in particular in the area of the heat exchanger tubes (70), are fundamentally no longer or only very slightly, so that even larger critical expansion fluctuations are excluded.

All negative influences resulting from high pre-burning are avoided. Since the connection (100), from which the cleaned exhaust gas for mixing with process gas still to be cleaned is removed, is not within the device (10), the mixing proposed according to the invention is consequently possible without any design effort on the device (10), thus increasing the concentration to keep the oxidizable constituents at the tolerance level. As a result, the device (50) according to the invention is easy to maintain and ensures a high degree of functional reliability.

The following tables 1 to 3 are intended to illustrate once again that a post-combustion device operated according to the method according to the invention creates optimal conditions for the thermal combustion and thus for the device itself in a self-regulating manner.

The thermal afterburning system considered here is designed for a maximum of 15,000 m ³ _o / h and is equipped with a heat exchanger efficiency of 76%. The nominal exhaust gas temperature is 160 ° C in the example, but it effectively deviates from it. The combustion chamber temperature must be kept constant at 760 ° C. The system presented is equipped with a special burner which takes the oxygen it needs for combustion from the exhaust gas (secondary air burner; combustor burner). The minimum output of the burner (- lower end of the control range) is 67.8 KWh / h.

The system is fed from various individual sources. Depending on the source and the number of sources, the volume flows are different and the exhaust gas temperature and above all the amount and concentration of combustible substances in the exhaust gas vary. The flammable substances are mineral oils. Three different operating conditions are examined. The results are shown in a table.

• Aufgabenstellung und Leistungsvermögen der Nachverbrennungs-Vorrichtung ohne Energieüberschuß-Regelung.
  
  Kommentar: Bei den Betriebsfällen 1 und 2 besteht ein beträchtlicher Überschuß von Wärme aus oxidierbarer Substanz, bezogen auf die obige Abgasmenge V. Das heißt, in diesen beiden Fällen greift die erfindungsgemäße Regelung ein, nachdem der Brenner am unteren Ende seines Regelbereichs (8O Regelungs-Minimum = Grundlast) angekommen ist, und zwar bei dem Versuch, für die angewachsene Menge oxidierbarer Substanz Platz zu machen. In beiden Fällen ist auch die nominale Abgastemperatur (hier 160°C) deutlich überfahren, so daß das System korrigierend eingreift.

Illustration 1:

• Task and performance of the afterburning device without excess energy control.
  
  Comment: In operating cases 1 and 2, there is a considerable excess of heat from oxidizable substance, based on the above exhaust gas quantity V. That is, in these two cases the control according to the invention intervenes after the burner has reached the lower end of its control range (8O control minimum = base load), trying to make room for the increased amount of oxidizable substance. In both cases, the nominal exhaust gas temperature (here 160 ° C) is clearly exceeded, so that the system intervenes to correct it.

In operating case 3, the concentration of the oxidizable components in the exhaust gas is lower than the capacity of the system with this volume flow would allow. Therefore, the burner regulates the missing amount of energy exactly through its modulating throughput of fuel, without the control according to the invention having to be used.

• Bewältigung der Aufgabe durch das erfindungsgemäße System für die Betriebsfälle 1, 2 und 3 nach Abbildung 1.
  
  Würde die thermische Nachverbrennung mit einer aus dem Stand der Technik bekannten By-pass-Anlage durchgeführt werden, so würden die Austrittstemperaturen für die Betriebsfälle 1, 2 und 3 442°C bzw. 399°C bzw. 310°C betragen.

Figure 2:

• Coping with the task by the system according to the invention for operating cases 1, 2 and 3 according to Figure 1.
  
  If the thermal afterburning were carried out with a by-pass system known from the prior art, the outlet temperatures for operating cases 1, 2 and 3 would be 442 ° C or 399 ° C or 310 ° C.

Claims (6)

1. Process for the controllable thermal afterburning of process exhaust gas containing oxidizable constituents, which is led through an afterburning apparatus in which it is fed via a gas inlet, heat exchanger, burner and combustion space and from there via the heat exchanger, purified, to a gas outlet, and process exhaust gas purified to the desired extent, together with fresh air, is mixed with the process exhaust gas to be fed to the afterburning apparatus, characterized by the process measures:

(a) the concentration of the constituents oxidizable in the combustion space is maintained at an adjustable value and

(b) the inlet temperature of the gas mixture to be fed to the afterburning apparatus, consisting of process exhaust gas to be purified, purified process exhaust gas and fresh air, is maintained at an adjustable value.

2. Process according to Claim 1, characterized in that the burner is run at the control minimum (base load).

3. Process according to Claim 1, characterized in that purified process exhaust gas is mixed with the process exhaust gas to be purified after the former has flowed round the heat exchanger.

4. Apparatus for the controlled thermal afterburning of process exhaust gas containing oxidizable constituents, comprising a gas inlet, a burner with following high-velocity mixing tube, a combustion space, a heat exchanger with heat exchanger tubes arranged concentrically with the high-velocity mixing tube and bent at one end as well as a gas outlet, for carrying out the process according to Claim 1, characterized in that between the apparatus (10, 50) and the gas feed line (14,48, 74) there is a connection (44) via which process exhaust gas purified to the desired extent within the apparatus (10, 50) can be recycled; the temperature of the purified process exhaust gas to be mixed with the process exhaust gas to be purified and that of the fresh air are controlled via control devices such as butterfly valves (46.1; 46.2) whose controlled condition can be determined by the temperature shown on the pressure side of a blower (38) by the gas mixture of the exhaust gas to be purified and purified exhaust gas and/or fresh air; and the heat exchanger tubes (70) are bent outwards at their cold ends (80) and purified process exhaust gas can flow round them.

5. Apparatus according to Claim 4, characterized in that on the suction side of the blower (38) a reduced pressure can be generated through which process exhaust gas purified to the desired extent and fresh air can be fed to the process exhaust gas to be purified.

6. Process according to Claim 4, characterized in that the control of the concentration in the combustion space (18, 64, 94) of the oxidizable constituents of the process exhaust gas which are to be thermally burnt is carried out as a function of the temperature in the combustion space with burner (60) run at the control minimum.

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