EP0405087B1 - Low nox fossil-fuel-fired steam generator - Google Patents

Description

The invention relates to a steam boiler according to the preamble of claim 1.

The nitrogen oxides (NO _x ) generated during the combustion of fossil fuels are harmful to health. Your emissions should therefore be reduced as much as possible.

• Senkung der Verbrennungstemperatur und
• Erzeugung von Sauerstoffmangel bei der Verbrennung.

The two largest sources of NO _x are thermal NO _x and fuel NO _x . Measures to reduce NO _x formation in the combustion process are:

• Lowering the combustion temperature and
• Generation of oxygen deficiency during combustion.

• Einsatz von NO_x-armen Brennern,
• Einrichtung von Oberluft (Over-Fire-Air, d.h. OFA) und
• sogenannte In-Furnace-NO_x-Reduction (IFNR), wobei Zusatzbrennstoff hinter der Hauptverbrennungszone zugeführt wird.

These measures are implemented, for example, by

• Use of low NO _x burners,
• Installation of over-air (OFA) and
• So-called in-furnace NO _x reduction (IFNR), with additional fuel being fed in behind the main combustion zone.

In the process described in (not yet published) German patent application P 38 25 291, the temperature profile in the combustion chamber is reduced by reducing the NO _x , in particular the thermal NO _x , by influencing the combustion process by gradually supplying fuel and / or flue gas in the main combustion zone smoothed.

However, there are limits to these NO _x- lowering measures if the ignition and basic stability of the flame and the complete burnout can no longer be guaranteed. It is therefore possible that, despite exhausting all measures possible while ensuring the combustion, the maximum temperatures are still so high that considerable NO _x formation takes place. This is particularly the case with flame-retardant and combustible fuels, such as heavy oils or fiber-rich coals.

In addition to the NO _x reducing measures which influence the combustion process by varying the air, fuel and flue gas supply, the installation of central walls is conceivable for lowering the temperature in the combustion chamber. Such middle walls are known in a closed and open design. They divide up a combustion chamber and primarily serve to reduce the boiler height.

Even in the middle walls, which allow pressure equalization in the flue gas, there is no temperature equalization, so that undesirable temperature differences can occur in the divided combustion chamber. In addition, the size of the heating surface is determined by the boiler width and is therefore not very adaptable for targeted temperature reduction. In particular, the temperature is evenly reduced in the vertical direction by means of central walls, although a temperature reduction is only sensible in the area of the maximum temperature.

DE-A-2 527 482 describes the installation of partition elements in the combustion chamber as a temperature-reducing measure for NO _x reduction. In this steam boiler, narrow strips of wall, which are located between two adjacent rows of burners, protrude from the surrounding wall into the combustion chamber. Wall strips of this type are exposed to the flow of flue gas during operation and require a high degree of stiffening because of the possibility of fastening on one side only. Since in normal operation of a combustion system with low-NO _x burners, the temperature profile of the flue gas in the horizontal direction also has a maximum in the middle and falls towards the surrounding walls, such narrow wall strips reinforce the difference between the maximum and minimum flue gas temperatures. It cannot effectively reduce the NO _x emission.

A steam boiler is known from DE-A-2 021 815, the combustion chamber of which is divided in the central plane by a side wall that is not supported. The intermediate wall extends over the entire height of the combustion chamber and has pressure equalization openings of small diameter above the upper burner level. The intermediate wall of the known boiler serves to reduce the construction volume with the same boiler output.

The object of the invention is to design the steam boiler according to the preamble of claim 1 such that the formation of NO _x in the combustion chamber is effectively reduced by the installation of an intermediate wall. This object is achieved by the features of the characterizing part of patent claim 1.

The intermediate wall in the form of a tubular band, which extends in a vertical central plane between the burner wall and the rear wall and is arranged between the lower burner level and a distance from the burner level above the upper burner level. causes a targeted lowering of the flue gas temperature in the area of the maximum temperature in the middle of the combustion chamber. There is only a very slight drop in temperature in the areas of low flue gas temperature, such as in the vicinity of the combustion chamber walls, the combustion chamber floor, and above the burner levels. This effectively reduces the formation of NO _x . This is especially true for the formation of thermal NO _x , the formation rate of which increases disproportionately with temperature at higher temperatures.

Sufficient vibration rigidity is guaranteed by attaching the pipe band to the burner and rear wall.

The horizontal passages ensure complete pressure and temperature compensation, which is required for trouble-free and effective operation of the furnace.

Another advantage of this measure for reducing NO _x emissions is that it is suitable for retrofitting old boilers as well as for new ones. The furnace band can be used as the only NO _x -measuring measure or in addition to the known measures (eg OFA, IFNR). It is particularly advantageous in old systems that, for reasons of heat balance, the additional heating surfaces in the combustion chamber make it possible to recirculate flue gas or to increase the amount of recirculated flue gas already present.

It is particularly advantageous to install the pipe band on boilers that are fueled with flame-retardant and combustible fuels, such as. B. heavy fuel oil, because with these boilers the previously known NO _x -measuring measures cannot reduce the NO _x emissions sufficiently.

The feature of claim 2 is particularly advantageous for boilers that are already provided with a flue gas recirculation. The construction of a second, parallel flue gas recirculation can be carried out while the plant is in operation.

The shape and arrangement of the tubular band described in claim 3 maintains the flame structure and thus the mutual support effect of the individual flames with one another in the horizontal direction, which is particularly important when igniting the burners.

Another advantage of the feature of claim 3 is that the flames of the burners arranged next to the tube band spread essentially in the free areas, the passages, of the zigzag band. This prevents the tubes from coming into direct contact with the flame.

The guidance of the supply pipes through one of the combustion chamber walls in the lower third of the combustion chamber according to claim 4 simplifies the arrangement of the pipe band in the main combustion zone. It is particularly convenient to run the supply pipes between the first and second burner levels because i.a. The high temperatures can only be found in the combustion chamber from this height onwards (cf.

The routing of the discharge pipes through one of the combustion chamber walls in the upper half of the combustion chamber according to claim 5 enables the arrangement of the pipe band in the main combustion zone in a simple form. It is particularly convenient to run the discharge pipes by a half to one burner level spacing above the top burner levels because i.a. From this height, the temperatures in the combustion chamber drop again (see Fig. 9).

The feature of claim 6 is particularly suitable for forced circulation and once-through boilers, for example Benson boilers. It enables easy connection of the pipe belt to an existing boiler.

The feature of claim 7 is also particularly suitable for forced circulation and once-through boilers.

The features of claims 8 and 9 are particularly suitable for natural circulation boilers.

An advantage of the feature of claim 10 is that the tips of the tubular band can be easily attached to the firebox walls at their central sections. Another advantage is that the distances between the pipe bends prevent expansion stresses.

An advantage of the feature of claim 11 is that the middle sections can be easily manufactured.

The arrangement of the tubes in the central region of the tips on the combustion chamber wall is advantageous because additional heating surfaces and thus an additional temperature reduction in the vicinity of the combustion chamber walls are avoided (cf. FIG. 10).

The feature of claim 13 describes an advantageous embodiment and attachment of the band for a steam boiler with four burner levels and that of claim 14 for a steam boiler with three burner levels.

The drawing serves to explain the invention using examples.

FIG. 1 shows a schematic representation of a Benson boiler (example 1) with a subsequently installed, welded tubular band. In Figure 2 is a corresponding plan view from above of the firebox of the boiler and in Figure 3 is an enlarged section with implementation of the Drainage pipes of the pipe band can be seen through a combustion chamber wall. A circuit diagram of Example 1 is shown in Figure 4.

FIGS. 5 to 8 show two exemplary embodiments of tips of the tubular band, front plan views in FIGS. 5 and 7 and corresponding cross sections in FIGS. 6 and 8.

The diagrams in FIGS. 9 and 10 show the temperature profiles of the flue gas as a function of the boiler height (FIG. 9) and the boiler width (FIG. 10) without a pipe band (dashed line) and with a pipe band (solid line).

FIG. 11 shows a schematic illustration of a natural circulation boiler (example 2) with a built-in tube band, FIG. 12 shows a corresponding top view of the combustion chamber and FIG. 13 shows a corresponding circuit diagram.

Example 1 (retrofitted Benson boiler):

Four burner levels 2, 3, 4 and 5 are arranged vertically one above the other in a combustion chamber 1 of a two-pass Benson boiler. The distance between the first, lowest burner level 2 and the combustion chamber floor 6 is approximately half the distance between the burner levels. There are air openings 7 at approximately 70% of the total height of the combustion chamber 1, for example by a burner level distance above the fourth burner level 5. Each of the four burner levels 2 to 5 is equipped with four horizontally aligned burners 10 with flue gas recirculation 11. The four burners 10 of one level are arranged side by side on a burner wall 12. The burner wall 12 and an opposite rear wall 13 form the longer sides in the rectangular cross section of the combustion chamber 1. The air openings 7 are located on both side walls 14, 15 of the combustion chamber 1 perpendicular to the burner wall 12, three air openings 7 being arranged next to one another.

In a central plane of the combustion chamber 1 which is perpendicular to the burner wall 12 and the rear wall 13 and parallel to the side walls 14 and 15, there is a tube band 16 made of tubes 17 welded to one another. The tube band 16 is in a region which is between the first and second burner levels 2, 3 begins and ends between fourth burner level 5 and air openings 7, is arranged and extends in a vertical direction in a zigzag shape between rear wall 13 and burner wall 12.

It has four straight sections running between the combustion chamber walls 12 and 13, which are connected by flattened tips 18, 19 and 20. The three tips 18, 19 and 20 are attached to the combustion chamber walls 12, 13 at the level of the three upper burner levels 3, 4 and 5. Supply pipes 21, discharge pipes 22 and the middle tip 19 of the tube band 16 are located on the side of the rear wall 13, the lower and the upper tips 18, 20 extend to the burner wall 12, the middle tip 19 to the rear wall 13.

As shown in FIG. 1, the shape of the tubular band 16 resembles two superimposed Vs rotated through 90 ° in the clockwise direction, the tips of which are flattened. The level of Burner levels 3, 4, 5 between the straight sections of the tube band 16 have free, horizontal passages, corresponding to the shape of the tube band 16, the shape of triangles with flattened tips, ie trapezoids.

Except for the tips 18, 19, 20 of the tube band 16 and the parts of the supply pipes 21 and the discharge pipes 22 located within the combustion chamber 1, the areas of the combustion chamber 1 in the vicinity of the combustion chamber walls 12, 13, 14, 15 are free of heating surfaces. Likewise, the firebox 1 between the firebox floor 6 and the first burner level 2 is free of heating surfaces.

A supply pipe 21 opens into each pipe 17 of the pipe band 16. The supply pipes branch off from a lower header 23 and are guided alternately around a pulling band 24 attached to the center of the rear wall 13 through the rear wall 13. Each pipe 17 also has a discharge pipe 22. The discharge pipes 22 are routed in the same way as the supply pipes 21, alternating left and right around the drawstring 24 to an upper collector 25. These implementations are shown in Figure 3.

A second flue gas duct 26 is connected to the firebox ceiling 8 and the wall superheater 9 on the rear wall 13 side. A flue gas duct 27 branches off from the second flue gas duct 26 near the ground and leads, for example, to a flue gas cleaning system via an air preheater. Two flue gas lines 28, 29 branch off from this flue gas duct 27, the second of which was subsequently installed together with the pipe band 16. The flue gas lines 28, 29 are each provided with a blower 30, 31. The flue gas line 28 is connected to the flue gas return lines 11 of the burners 10 and the flue gas line 29 to the secondary air of the burners 10.

An economizer 32 is arranged in the lower part of the second flue gas duct 26, approximately at the level of the second burner level 3. A pipeline leading from the economizer 32 to an evaporator 33 before the installation of the tube band 16 is interrupted at a height above the lower header 23; its pipe section 34 leading away from the economizer 32 is connected to the lower collector 23 via a feed line 35 and its pipe section 36 leading to the evaporator 33 is connected to the upper collector 25 of the pipe band 16 via a discharge pipe 37.

By connecting the supply line 35 to the economizer 32 and the discharge line 37 to the evaporator 33, the tube band 16 is connected between the economizer 32 and the evaporator 33. In the flow direction of the water or steam, steam coolers 38 and superheaters 39 are arranged behind the evaporator 33 and wall superheater 9 (FIG. 4). The economizer 32 is connected to a feed pump 41 by a feed line 40. An injection water line 42 branches off from line 40 and leads to the steam coolers 38.

The tube band 16 is formed from fifty directly welded tubes 17, only a few being shown in the drawing (FIGS. 5 and 6). The tubes 17 consist of heat-resistant or high-temperature steel and have, for example, an outer diameter of 57 mm and a wall thickness of 5.6 mm. The entire width of the tube band 16 is approximately 3 m. In the area in which the tube band 16 is arranged, it covers approximately 40% of the central area, and accordingly approximately 60% of the area remains free through the passages. As a result, the tube band 16 forms approximately 20% of the heating surface of the combustion chamber 1 in this area.

The flattened tips 18, 19, 20 each consist of three sections, a middle section 43 and two outer sections 44. The tubes 17 are arranged straight, vertically next to one another in the middle section 43 and welded to one another via (about 10 mm wide) webs 45. In the outer sections 44 adjoining it at the top and bottom, the tubes 17 have tube bends 46 which run at a distance from one another. In the lower outer section 47 of the middle tip 19, the tubular band 16 has a fold. The positions of the tubes 17 are exchanged so that the outer tube 48 in the lower "V" lies in the upper "V" inside. The tubes 17 are all of the same length due to the folding.

In the middle sections 43 of the tips 18, 19, 20, the belt 16 is fastened to the combustion chamber walls 12, 13 in the case of forced-circulation and forced-flow boilers via holding irons for load transfer. (FIGS. 5 to 8 show an exemplary embodiment for fastening the tube band 16 by means of full welding to the tube webs of the natural circulation boiler; the fastening described above with holding irons is not shown.)

To further explain the invention, the influence of the tube band 16 on the temperature profile of the flue gas in the combustion chamber 1 was calculated and represented using a furnace zone model as a function of the boiler height (FIG. 9) and as a function of the boiler width (FIG. 10). The temperature profile of the flue gas after installation of the tube band 16 (solid line) shows a significant reduction in the temperature in the maximum compared to the temperature profile before installation of the tube band 16 (dashed line).

The lowering of the flue gas temperatures in the combustion chamber is achieved on the one hand by the additional heating surface of the pipe belt and on the other hand by doubling the amount of flue gas returned. This increase in the quantity of flue gas returned to the combustion chamber 1 also serves to regulate the heat balance of the boiler, which is changed by an enlarged heating surface.

In the operation of this converted Benson boiler, which is fired with heavy fuel oil or gas, the additional combustion chamber heating surface of the belt 16 and the doubling of the recirculated flue gas amount achieve a NO _x reduction of 25 to 35%.

• Das Rohrband 16 besteht aus fünfundvierzig über Stege 49 verschweißten Rohren 17. Seine Breite beträgt dabei 3,15 m.
• Die Rohre 17 sind in den mittleren Abschnitten 43 der Spitzen 18, 19, 20 senkrecht zur Mittelebene, d.h. parallel zu den Feuerraumwänden 12, 13, nebeneinander angeordnet. Diese Anordnung der Rohre 17 ist aufwendiger. Sie verhindert jedoch eine Temperaturabsenkung in der Nähe der Feuerraumwände 12, 13, durch Heizflächen des Rohrbandes 16.

Another exemplary embodiment of the tube band 16 (FIGS. 7 and 8) differs from the tube band 16 described above as follows:

• The tube band 16 consists of forty-five tubes 17 welded via webs 49. Its width is 3.15 m.
• The tubes 17 are arranged in the middle sections 43 of the tips 18, 19, 20 perpendicular to the central plane, ie parallel to the combustion chamber walls 12, 13. This arrangement of the tubes 17 is more complex. However, it prevents a temperature drop in the vicinity of the combustion chamber walls 12, 13 by heating surfaces of the tubular band 16.

Example 2 (natural circulation boiler)

• Im Feuerraum sind nur drei Brennerebenen 2, 3, 4 übereinander angeordnet, wobei der Abstand der ersten Brennerebene 2 zum Feuerraumboden 6 etwa ein Drittel des Brennerebenenabstandes beträgt.
• Der Bereich, in dem das Rohrband 16 angeordnet ist, beginnt zwischen erster und zweiter Brennerebene 2, 3 und endet um etwa einen halben Brennerebenenabstand oberhalb der dritten Brennerebene 4.
• Untere Spitze 18 und obere Spitze 20 des mit drei Spitzen 18, 19, 20 versehenen, zickzackförmigen Rohrbandes 16 sind in Höhe der zweiten und dritten Brennerebene 3, 4 an der Rückwand 13 befestigt. Die mittlere, zwischen zweiter und dritter Brennerebene 3, 4 angeordnete Spitze 19 sowie die Zuleitungsrohre 21 und Ableitungsrohre 22 befinden sich auf der Seite der Brennerwand 12.
• Das Rohrband 16 ist gemäß Figur 13 parallel zum Verdampfer 33 geschaltet. Dabei ist der untere Sammler 23 über die Zuleitung 35 mit einem von einer Verdampfertrommel 50 abzweigenden Fallrohr 51 verbunden und der obere Sammler 25 über die Ableitung 37 an die Verdampfertrommel 50 angeschlossen.
• Das Rohrband 16 ist durch Schweißverbindungen zwischen mittleren Abschnitten 43 der Spitzen 18, 19, 20 mit den Feuerraumwänden 12, 13 an diesen befestigt (vgl. Figuren 5 bis 8).
• Der Steigungswinkel der Rohre 17 des Rohrbandes 16 im Feuerraum 1 beträgt mindestens 10 %.

The natural circulation boiler of example 2 differs from the boiler of example 1 in addition to the known, fundamental differences in the following points:

• Only three burner levels 2, 3, 4 are arranged one above the other in the combustion chamber, the distance between the first burner level 2 and the combustion chamber floor 6 being approximately one third of the burner plane distance.
• The area in which the tube band 16 is arranged begins between the first and second burner levels 2, 3 and ends by approximately half a burner level distance above the third burner level 4.
• Lower tip 18 and upper tip 20 of the zigzag-shaped tubular band 16 provided with three tips 18, 19, 20 are attached to the rear wall 13 at the level of the second and third burner levels 3, 4. The middle tip 19 arranged between the second and third burner levels 3, 4 and the feed pipes 21 and drain pipes 22 are located on the burner wall 12 side.
• The tube band 16 is connected in parallel to the evaporator 33 according to FIG. The lower collector 23 is connected via the feed line 35 to a down pipe 51 branching off from an evaporator drum 50 and the upper collector 25 is connected to the evaporator drum 50 via the discharge line 37.
• The tubular band 16 is fastened to the combustion chamber walls 12, 13 by welded connections between middle sections 43 of the tips 18, 19, 20 (cf. FIGS. 5 to 8).
• The pitch angle of the tubes 17 of the tube band 16 in the combustion chamber 1 is at least 10%.

In old systems in which the arrangement of the supply pipes 21 and the discharge pipes 22, e.g. for reasons of space, neither through the rear wall 13 nor through the burner wall 12, the supply pipes 21 (not shown) are guided through the combustion chamber floor 6 and the discharge pipes 22 through the combustion chamber ceiling 8.

Claims (14)

1. Boiler with a combustion chamber (1), which is provided with burner planes (2, 3, 4, 5) arranged one above the other, with burners (10) arranged beside one another on at least one burner wall (12) and with a heating surface formed by a pipe band (16), wherein the pipe band (16) is arranged in a vertical centre plane of the combustion chamber (1) and provided with horizontal passages, characterised thereby that the pipe band (16) extends from the burner wall (12) as far as the oppositely disposed back wall (13), is arranged between lower burner plane (2) and above, by one burner plane spacing, the upper burner plane (5, 4) and is fastened to the burner wall (12) and the back wall (13), and that the horizontal passages are disposed at the level of the burner planes (3, 4, 5).
2. Boiler according to claim 1, characterised by two flue gas ducts (28, 29), which are connected with the combustion chamber (1).
3. Boiler according to claim 1 or 2, characterised thereby that the pipe band (16) is executed to be zigzag-shaped in vertical direction and is fastened to the burner wall (12) or the back wall (13) at at least one of its points (18, 19, 20) at the level of the burner planes (3, 4, 5).
4. Boiler according to one of claims 1 to 3, characterised thereby that feed pipes (21) of the pipe band (16) are led through the back wall (13) or the burner wall (12), in particular between the first and second burner plane (2, 3), in the lower third of the combustion chamber (1).
5. Boiler according to one of claims 1 to 4, characterised thereby that the discharge pipes (22) of the pipe band (16) are fed through the back wall (13) or the burner wall (12) in the upper half of the combustion chamber (1), in particular above the uppermost burner plane (5, 4) by one half to one full burner plane spacing.
6. Boiler according to one of claims 1 to 5, characterised thereby that the feed pipes (21) of the pipe band (16) are connected to an economiser (32) and its discharge pipes (22) to an evaporator (33).
7. Boiler according to one of claims 1 to 6, characterised thereby that the length of the pipes (17) is through a folding of the pipe band (16), equal in dimension.
8. Boiler according to one of claims 1 to 5, characterised thereby that the feed pipes (21) and the discharge pipes (22) of the pipe band (16) are connected to an evaporator drum (50).
9. Boiler according to claim 8, characterised thereby that the angle of inclination of the pipes (17) of the pipe band (16) in the combustion chamber (1) amounts to at least 10°.
10. Boiler according to one of claims 3 to 9, characterised thereby that the points (18, 19, 20) of the pipe band (16) are flattened off and consist each time of three sections (43, 44), wherein the pipes (17) in the middle section (43) are arranged to be vertical and are welded together and in the two upper and lower sections (44) adjoining thereat are constructed as pipe bends (46), which are arranged at a spacing relative to one another.
11. Boiler according to claim 10, characterised thereby that the pipes (17) in the middle section (43) of the points (18, 19, 20) are arranged beside one another in the centre plane and are welded together by way of webs (45).
12. Boiler according to claim 10, characterised thereby that the pipes (17) in the middle section (43) of the points (18, 19, 20) are arranged beside one another and perpendicularly to the centre plane, i.e. parallel to the burner wall (12) and to the back wall (13).
13. Boiler according to one of claims 3 to 12 with four burner planes, characterised thereby that the pipe band (16) has three points (18, 19, 20) which are fastened at the level of the upper three burner planes (3, 4, 5).
14. Boiler according to one of claims 3 to 12 with three burner planes, characterised thereby that the pipe band (16) has three points (18, 19, 20), wherein the lower point (20) is fastened at the level of the middle burner plane (3) and the upper point (18) at the level of the upper burner plane (4).

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