EP0663563A1 - Combustion gas guidance - Google Patents

Abstract

The boiler (1) is fired by a burner (2) for liquid or gas fuels. The escaping flue gases pass from the combustion chamber (5) between the walls of outer and inner water-cooled heat exchangers (6, 12), thence past the outside of the combustion chamber. Before they reach the regions between the heat exchangers, the flue gases first circulate in another heat-exchanging space (15) inside the inner exchanger and then, a portion of the flue gases is returned to the burner flame roots through openings (4) in the flame tube (3). The heat-exchanging spaces have devices to increase turbulence like seams, rings, disks, etc.

Description

The invention relates to a method for operating the combustion of fuels in combustion plants with a boiler fired by a burner, in which the thermal energy of the fuel is transferred to a heat transfer medium, and to an apparatus for carrying out such a method.

The design of the boiler or the heat generator is of great importance for the transfer of the thermal energy of a fuel to a heat transfer medium, and thus for the overall efficiency of a combustion system. The design of the boiler is essentially influenced by the geometry of the combustion chamber and by the guidance of the combustion gases generated during the combustion. Boiler constructions are designed in such a way that as much heat as possible, which arises during the chemical conversion of the fuel, is completely transferred to the heat transfer medium without loss. In addition to water, low or high pressure steam and organic liquids can also be used as heat carriers.

Depending on the type of heat transfer, the known boiler designs can be divided into two construction elements: the combustion chamber, in which the combustion process takes place and the thermal energy is essentially transmitted by radiation or heat radiation; and downstream cooled heat exchange surfaces through which the combustion gases release their enthalpy by convection to the heat transfer medium.

According to the prior art, so-called three-pass boilers are preferably used, particularly in a medium power range. These are usually equipped with a cylindrical firebox, with the "hot" combustion gases at the end of the firebox being led via a deflection chamber into the secondary heat exchange surfaces, consisting of steel pipes.

Another well-known boiler design is the boiler with a reverse flame in the combustion chamber. The flame burns in a cylindrical combustion chamber closed at the rear and the combustion gases are redirected in the cylindrical combustion chamber and returned to the boiler entrance. The combustion gases at the front of the boiler are then fed to the convective secondary heat exchange surfaces via an appropriate deflection chamber at the boiler inlet.

In the known boiler designs, particular care must be taken that the geometry of the combustion chamber and the discharge of the combustion gases are precisely matched to the flame geometry and thus to the performance of the burner. In addition to avoiding convection and radiation losses, this applies in particular to avoiding the cooling of the combustion gases within the boiler, for example if the combustion chamber is oversized. If the combustion gases cool down to a temperature below the water or acid dew point on the way out of the combustion chamber, the exhaust gases condense, which in the long term causes corrosion to form in the boiler and in the exhaust pipes. Boiler corrosion is mainly due to the corrosive effects of sulfuric acid. But also a condensate failure, for example ash parts of fuel oil, soot, etc. leads to an attack on the boiler materials and eventually to their destruction.

In order to largely avoid the formation of corrosion, in particular boiler corrosion, different boiler sizes are also required in the known boiler designs for different burner output ranges, in order to avoid a decrease in the exhaust gas temperatures in the boiler by adapting the combustion chamber geometry to the burner output.

Compared to the aforementioned prior art, the invention aims to provide a firing system and a method for operating the same, which is designed for a large burner output range.

This object is achieved by the subject matter of claims 1 and 10.

Thereafter, the combustion gases which are produced when the fuel is burned in the boiler are at least partially removed from the boiler between a "hot" heat-radiating surface and a convective heat exchanger surface cooled by the heat transfer medium. This means that the combustion gases flow through an intermediate space - hereinafter referred to as the heat exchange space - between the convective heat exchanger surface of an infinitely large heat exchanger or heat bath to which the combustion gases release their enthalpy by convection and a "hot" radiation surface, the heat energy essentially by radiation to the Transfers combustion gases. As a result, the combustion gases flowing through the heat exchange space on the one hand emit thermal energy to the convective heat exchanger surface and on the other hand absorb heat from the radiation surface at the same time.

With a large burner output, the heat given off by the combustion gases to the heat exchanger surface - and thus to the heat transfer medium - is greater than the heat absorption from the radiation surface, so that the combustion gases are finally cooled. With a low burner output, the heat emission and heat absorption of the combustion gases are essentially in balance, so that the combustion gas temperature is kept constant in this output range. The value of the lower limit temperature of the combustion gases depends on the temperature and geometry of the thermal contact surfaces. Depending on the design, in particular the heat radiation surfaces, the limit temperature can be set to a certain value - at least above the critical dew point temperatures - so that the risk of corrosion and other condensate failures is excluded. Because of the exhaust gas routing in the boiler according to the invention, such combustion systems are suitable for a large output range of the burner - preferably from approximately 30% to 100%.

In a preferred exemplary embodiment of the invention, the combustion gases are conducted in a heat exchange space between the uncooled heat-radiating wall of the combustion chamber and an outer heat transfer jacket, in particular a water jacket, surrounding the combustion chamber wall (claims 2 and 11). If the heat-radiating surfaces of the glowing combustion chamber wall and the convective heat exchanger surfaces of the outer heat transfer jacket extend over the entire circumference and the axial height of the combustion chamber, a particularly effective heat transfer from the combustion chamber wall to the combustion gases and further to the heat transfer medium is guaranteed.

The combustion gases deflected in a boiler with a reverse flame in the combustion chamber are preferred through a heat exchange chamber between a flame tube of the burner projecting into the combustion chamber and an inner flame tube dissipating surrounding heat transfer jacket (claims 3 and 12). The flame burns in a closed combustion chamber at its downstream end, and the combustion gases are redirected by appropriate shaping of the combustion chamber floor in the combustion chamber and discharged via the above-mentioned heat exchange chamber between the flame tube and the inner heat transfer jacket. By reversing the combustion gases over the flame, a complete and perfect burnout of the fuel is first guaranteed. The combustion gases then enter the above-mentioned heat exchange chamber, which prevents the combustion gases from cooling below critical dew point temperatures, especially when the burner output is low.

The combustion gases are preferably also conducted in a region between the outer and the inner heat transfer jacket, in particular water jacket (claims 4 and 13). If both heat transfer jackets extend over the entire axial length of the combustion chamber, this achieves effective cooling of the combustion gases, which release a large part of their thermal energy by convection to the heat transfer medium. If the inner heat transfer jacket on the inside of the combustion chamber wall extends around the flame as a flame cooling cylinder, part of the thermal energy of the combustion gases is also transferred directly from the hot flame zone to the heat transfer medium and, at the same time, a reduction of the NO _x by cooling the combustion chamber Education reached.

Another preferred embodiment of the invention is that the combustion gases deflected in the combustion chamber first flow through the heat exchange space between the flame tube and the inner heat transfer jacket, then redirect again and are guided through the purely convective heat exchange space between the outer and inner heat transfer jacket and finally the heat exchange space between the combustion chamber wall and Flow through the outer heat transfer jacket (Claims 5 and 14). Such exhaust gas routing has an effect both on the stabilization of the lower exhaust gas limit temperature at low burner output and on the most effective possible heat transfer to the heat transfer medium at high burner output. The deflected combustion gases first flow through a heat exchange space determined by radiation and convection, then through a pure convective heat exchange space and finally again through a radiation and convection heat exchange space.

In order to increase the effective heat contact area, outer and inner heat transfer jackets are equipped with heat exchange fins (claim 15). For the same reason, the outside of the combustion chamber wall is provided with appropriate heat exchange fins (claim 16). This ensures that the combustion gases can transfer as much as possible of the amount generated during combustion in the combustion chamber to the heat exchange surfaces and, at the same time, can absorb as much heat as possible from the heat-radiating surfaces, particularly the combustion chamber and flame tube walls.

In order to ensure good heat exchange even at low exhaust gas temperatures, the above Heat exchange rooms turbulence-promoting devices, in particular beads, rings, disks and twisted sheet metal strips, arranged (claim 17). This also leads to an increase in the heat contact area and thus to an improved heat transfer.

According to a further preferred exemplary embodiment, part of the deflected combustion gases is returned to the flame root area via recesses in the flame tube of the burner, in particular after the combustion gases have passed through the heat exchange space between the flame tube and the inner heat transfer jacket (claims 6 and 18). Due to the internal exhaust gas recirculation - especially with the previous one Cooling of the combustion gases - an effective reduction in flame temperatures is made possible, which further leads to a reduction in NO _x formation.

The combustion system according to the invention and the method for operating the combustion system in connection with a modulating or controllable burner have proven to be particularly advantageous (claims 8, 19). Such a burner contains a continuously controllable control valve and a correspondingly controllable fan for regulating the amount of combustion air supplied. As a result, the burner output can be changed continuously over a larger control range. In connection with the exhaust gas routing in the boiler according to the connection, it is ensured at all times, both with high burner output and with low burner output, that the exhaust gas temperature does not fall below the critical dew point values. The boiler construction according to the invention is therefore suitable for all output ranges of a controllable burner.

Preferably, the burner output is automatically changed depending on the heat output of the heat transfer medium, so that between the individual load points - i.e. minimum and maximum combustion heat output - the burner can automatically adjust at each load point, so that the fuel and combustion air supply corresponds to the heat output of the heat transfer medium . On the basis of the exhaust gas routing according to the invention, the burner output is regulated with the aid of such a modulating burner in such a way that the temperature of the heat transfer medium in the boiler, in particular the boiler water in the boiler wall, remains constant regardless of the load on the combustion system (claim 9).

This also makes the temperature of the heat exchange jackets or the convective heat exchange surfaces in the boiler, at which the combustion gases flow past, particularly advantageous. kept at a constant value. This has the consequence that the temperature of the combustion gases emerging from the boiler is also kept at a defined value in order to counteract the condensation of the exhaust gases.

The boiler according to the invention is manufactured particularly cheaply from simple steel and / or cast iron (claim 20). Due to the guidance of the combustion gases according to the invention, expensive corrosion-resistant boiler materials can advantageously be dispensed with.

Fig. 1

ein Ausführungsbeispiel der erfindungsgemäßen Kesselkonstruktion im Längsschnitt;

Fig. 2

einen Querschnitt entlang der Linie A-A' in Fig. 1; und

Fig. 3

ein Beispiel für die erfindungsgemäße Feuerungsanlage mit Kessel-, Speicher- und Versorgungskreislauf.

Further advantages and refinements of the invention also result from the following description of preferred exemplary embodiments. Therein, reference is made to the attached schematic drawing. The drawing shows:

Fig. 1

an embodiment of the boiler construction according to the invention in longitudinal section;

Fig. 2

a cross section along the line AA 'in Fig. 1; and

Fig. 3

an example of the furnace system according to the invention with boiler, storage and supply circuit.

Fig. 1 shows a longitudinal section through a boiler 1, which is fired by a burner 2, preferably a modulating burner. The burner 2 opens with its flame tube 3 into a combustion chamber 5 of the boiler 1. The combustion air is fed to the burner 2 in the direction of arrow I.

In the flame tube 3, downstream of a baffle plate of the burner 2 (not shown in FIG. 1), horizontal slots 4 are formed, distributed uniformly over the circumference of the flame tube 3, for the internal return of the combustion gases into the root region of the flame.

The boiler 1 contains an outer water jacket 6 fed by a boiler circuit, which extends essentially over the entire axial length of the boiler and completely surrounds the combustion chamber wall 8 at a distance. The outer water jacket 6 fulfills the function of a heat exchanger or heating bath and is equipped on the inside with heat exchanger ribs 10, which extend radially inward almost over the entire axial length of the water jacket 6. Furthermore, an inner water jacket 12 is arranged between the outer water jacket 6 and the flame tube 3 such that it completely surrounds the flame tube 3 at a distance. The inner water jacket 12 is also equipped with convective heat exchanger fins 14 directed towards the boiler axis.

The combustion chamber 5 is closed at its rear end, so that the combustion gases in the combustion chamber 5 are deflected and discharged in the upstream direction. Another deflection area forms the inner wall of the boiler at the front end of the boiler 1, so that the combustion gases flowing in from the combustion chamber 5 are redirected and flow in the direction downstream between the outer water jacket 6 and the inner water jacket 12 and the combustion chamber wall 8 until they finally pass through the boiler 1 leave the combustion gas outlet 13.

The result is a combustion gas flow, whereby the combustion gases flow through three ring-shaped heat exchange chambers in succession: a first heat exchange chamber 15 between the flame tube 3 and the inner water jacket 12, in which the combustion gases absorb heat energy from the "glowing" flame tube and at the same time heat by convection via the heat exchanger fins Deliver 14 to the inner water jacket 12; a second purely convective heat exchange space 16 between the inner 12 and the outer water jacket 6; and finally a third heat exchange space 17 between the outer water jacket 16 and the uncooled heat-radiating combustion chamber wall 8, in which heat exchange by radiation and convection also takes place with the combustion gases.

Due to the inventive design of the outer 6 and inner water jacket 12 around the heat radiating surfaces of the combustion chamber wall 8 or the flame tube 3, the heat output of the combustion gas flow to the water jackets is greater than the heat absorption from the heat radiating surfaces with a large burner output, so that the combustion gases cool down he follows. At low burner output, however, the heat emission and heat absorption of the combustion gases are in equilibrium, so that the combustion gas temperature remains essentially constant in this output range. In particular, it is kept above the critical dew point temperatures by the heat absorption from the flame tube 3 or from the combustion chamber wall 8. Due to the pronounced heat contact surfaces in the heat exchange rooms 15 and 17, the limit temperature of the combustion gases is well above a critical value of approx. T = 110 ° C even with the lowest burner output. There is therefore no risk of corrosion.

In the case of a modulating furnace, as is preferably used in the furnace according to the invention, the combustion gases are cooled in the heat exchange rooms 15 and 17, depending on the load of the burner, or - ideally - are kept at a constant value when the burner output is low.

Furthermore, modulating firing saves frequent burner starts depending on the load on the firing system, which avoids high starting emissions and the formation of corrosion when the burner is started.

The sectional view in Fig. 2 along the line AA 'of the boiler structure in Fig. 1 illustrates that the outer 6 and the inner water jacket 12 are conductively connected to one another and are jointly supplied by the boiler circuit. 2 shows the radially inwardly directed heat exchanger fins 10 and 14 of the water jackets 6 and 12, respectively.

Finally, FIG. 3 shows a combustion system according to the invention with a boiler 1, in the combustion chamber of which a modulating burner 2 projects. The boiler 1 is connected via a storage circuit 18 to a hot water tank 20 which is fed with hot water by the boiler 1. A boiler circuit 22 is arranged parallel to the storage circuit 18 and in turn is coupled to a consumer circuit 26 via a heat exchanger 24.

3 with a thermostat T, the burner output of the modulating burner 2 is controlled as a function of the heat output removed from the supply circuit 26 such that the temperature of the (boiler) water in the boiler circuit is at a constant value, for example to T _K = 60 ° C, remains. In connection with the exhaust gas routing according to the invention in the boiler 1, the temperature of the combustion gases emerging from the boiler 1 is thereby fixed to a certain value independently of the load on the supply circuit 26.

If, furthermore, the flow temperature in the consumer circuit is kept at a constant value, e.g. at T _v = 30 ° C, depending on the load on the consumer 28, hot water with T _K = 60 ° from the boiler circuit can be fed in in defined quantities into a mixing device 30 the consumer circuit 26 are mixed.

Claims (20)

Method for operating the combustion in combustion plants with a boiler (1) fired by a burner (2), in which the thermal energy of the fuel is transferred to a heat transfer medium, the combustion gases in the boiler (1) at least partially between a heat radiating surface and a convective one Heat exchanger surface are performed. Method according to claim 1, characterized in that the combustion gases are conducted in a heat exchange space (17) between an outer heat transfer jacket (6), in particular water jacket, and a wall (8) of a combustion chamber (5) of the boiler (1). Method according to claim 1 or 2, characterized in that the combustion gases deflected in the combustion chamber (5) are passed through a heat exchange chamber (15) between a flame tube (3) projecting into the combustion chamber (5) and an inner heat transfer jacket (12). A method according to claim 2 or 3, characterized in that the combustion gases in a heat exchange room (16) between the outer (6) and inner heat transfer jacket (12) are performed. Method according to claims 2, 3 and 4, characterized in that the combustion gases deflected in the combustion chamber (5) first flow upstream through the heat exchange chamber (15), then are deflected and successively passed downstream through the heat exchange chambers (12) and (17). Method according to one of the preceding claims, characterized in that some of the combustion gases are returned to the flame root of the burner flame via openings (4) in the flame tube (3). Method according to claim 6, characterized in that the combustion gases are passed through the heat exchange chamber (15) before being returned to the flame root area. Method according to one of the preceding claims, characterized by the use of a controllable or modulating burner (2). Method according to claim 8, characterized in that the temperature of the heat transfer medium in the boiler circuit (22) is kept constant. Furnace system, in particular for carrying out a method according to one of the preceding claims, with a boiler (1) which is fired by a burner (2), the flame tube (3) of which opens into a combustion chamber (5) of the boiler (1), a heat transfer jacket (6; 12), in particular a water jacket, surrounds a heat-radiating surface in the boiler (1) such that the combustion gases in a heat exchange space (15; 17) can be dissipated between a convective heat exchanger surface and the heat radiating surface. Furnace system according to claim 10, characterized in that an outer heat transfer jacket (6) surrounds the wall (8) of the combustion chamber (5) at a distance (heat exchange chamber (17)). Firing system according to claim 10 or 11, characterized in that the combustion chamber (5) for deflecting the combustion gases has an end at the outlet and an inner heat transfer jacket (12) surrounds the flame tube (3) at a distance (heat exchange chamber (15)). Furnace system according to claim 11 or 12, characterized in that the outer heat transfer jacket (6) surrounds the inner heat transfer jacket (12) at a distance (heat exchange chamber 16). Furnace system according to claim 11, 12 and 13, characterized in that a deflection area is provided on the inner wall of the boiler so that the combustion gases are deflected on the inner wall of the boiler after passing through the heat exchange area (15) and successively through the convective heat exchange space (16) and the heat exchange area ( 17) stream. Firing system according to one of Claims 10 to 14, characterized in that the inner (6) and / or outer heat exchange jacket (12) is / is equipped with heat exchange ribs (10; 14). Furnace system according to one of claims 10 to 15, characterized in that heat exchange fins are provided on the outside of the combustion chamber wall (8). Furnace system according to one of Claims 10 to 16, characterized in that turbulence-increasing devices, in particular beads, rings, disks and twisted sheet metal strips, are arranged in the heat exchange space (15; 16; 17). Furnace system according to one of claims 13 to 17, characterized by one or more openings (4) in the flame tube (3) for internal exhaust gas recirculation into the flame root area, in particular after the combustion gases have passed through the heat exchange chamber (15). Furnace system according to one of claims 10 to 18, characterized by a modulating or controllable burner (2). Furnace system according to one of claims 10 to 19, characterized in that the boiler (1) is made of steel and / or cast iron.

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