EP1447622B1 - Pulverized fuel fired flame-tube boiler - Google Patents

Abstract

To fire a high power flue boiler, with a coal fuel dust, a high proportion of the fuel is burned in a pre-combustion chamber (1) with a supply of combustion air (L), and the emerging gas flame (6) is accelerated by at least 80% by a jet (2) to be blown into the boiler flue (7) . The gas flame is blown into the boiler flue coaxially, or at an angle of 8-14[deg] against the flue axis selected to ensure that the upper mantle line of the flame is parallel to the flue mantle line.

Description

The invention relates to a flame tube boiler for firing with fuel dust.

Known flame tube boilers designed for firing with oil or gas are characterized by the tightest possible flame tube dimensions and a temperature of the combustion exhaust gases at the outlet of the flame tube of 1,100 to 1,200 ° C. An oil or gas burner forms a short, bushy flame, which comes close to the peripheral wall of the flame tube and generates there large heat flow peaks of up to 400 kW / m ² . This results in a large temperature gradient in the pipe wall, which results in corresponding expansion and thus voltage differences within the pipe wall. These must be limited for reasons of the material, and the technical rules for steam boilers TRD 306, para. 11 therefore limit the wall thickness of the flame tube to 20 mm. Thus, the diameter of the flame tube is limited, because larger diameter require larger wall thicknesses.

At comparable power, a lignite-fired flame tube must be substantially larger in length and diameter than a flame tube fired with oil or gas. The reasons for this are, on the one hand, that brown coal dust requires about 2.5 to three times as large a volume for burning out against oil or gas, and, on the other hand, that the exhaust gas temperature must be lowered so that they are at a sufficient distance below the ash melting point from about 920 to 940 ° C, in order to prevent slagging of the flame tube and the downstream piping. In practice, the exhaust gas temperature of the fired with lignite dust flame tube should be in the range of 850 ° C, ie significantly below that of oil or gas-fired flame tubes.

Within the limited by the aforementioned technical rules for steam boiler wall thicknesses of 20 mm and the usual in this boiler range steam pressure of eg 18x10 ³ hPa (18 bar) corresponding to the standard pressure of 16x10 ³ hPA (16 bar) Flammrohrdurchmesser of at most 800 mm are possible, this for the burnout of lignite dust required flame tube volume can not reach.

If you place reinforcing rings on the flame tube, for example, at a distance of 1,000 mm, although an increase in the flame tube diameter to 1,300 mm is possible, but can thus achieve a firing capacity of only about 4.6 MW. However, the market requires firing capacities in the range of 20 MW. The cliff is the wall thickness limited by the technical rules mentioned above, which must be overcome if greater firing rates are to be achieved.

From the WO 92/03212 is already known a flame tube steam boiler for firing with dust-like fuels. The fire tube steam boiler has a pre-combustion chamber with an accelerating nozzle, a lance extending into the pre-combustion chamber, and a flame tube adjoining the accelerating nozzle.

From the US 4848666 A1 a burner is known in which a part of the fuel is supplied via a first outer annular channel and is accelerated at the outlet end to high speed, while another part of the fuel is supplied via a second, inner annular channel and braked at the outlet end to low speed, whereby a stabilization and control of the main flame is to be achieved by a permanent pilot flame. The burner is suitable for solid, liquid and gaseous fuels.

From the WO 83/02309 already known is a pulverized coal boiler, which has a burner adjacent to a burner, and in which connect to the firebox flue gas reins. The burner produces flame jet speeds of 50 to 60 m / s. The burner axis may be arranged at an angle of, for example, 17 ° with respect to the axis of the furnace.

From the DE 10055507 A1 already a boiler for firing with dust-like fuels is known. The boiler comprises a burner to which is supplied by means of air fluidized pulverized fuel. Within the burner, at least 30% of the fuel is burned, with the burning air / fuel mixture being blown into the combustion chamber to a flame jet at a speed of at least 40 m / s.

The invention has the object of developing a flame-retardant steam boiler for firing with dust-like fuel so that increased firing rates are achieved.

This object is achieved by a flame tube steam boiler according to claim 1.

The invention is based on the fact that larger wall thicknesses are permissible for the fire tube when reducing the heat flow density, and it is based on the consideration that one can achieve the reduction of the heat flux density, when producing a flame whose diameter is small in comparison is the diameter of the flame tube. The diameter of a flame of given heat output becomes smaller the higher its velocity. As a result, one achieves a larger flame tube diameter, if one increases the flame speed.

The high flame speed is achieved in that the highest possible proportion, preferably at least 30%, better 60%, of the fuel is burned in a pre-combustion chamber and accelerated from the pre-combustion chamber, burning flame gases are accelerated and only then blown into the flame tube.

According to the invention, the acceleration is at a speed of at least 80 m / s, preferably 100 m / s, and at this speed they are blown into the flame tube, preferably coaxially therewith. But you can blow the flame gases above the axis of the flame tube at an inclination of 8 to 14 ° C against the axis of the flame tube obliquely down in this. This achieves that impurities, such as ash, which collect at the bottom of the flame tube, in the direction of the Trigger the flame tube to be blown.

Although the Applicant has previously used flame-retardant nozzles along the way that has brought him to this invention. However, quite different purposes were pursued, and the flame jet speed was then only 50 to 60 m / s and was limited by the flame stability. The acceleration of the flame jet was formerly used solely to blow away ausfallendes combustion ash from the bottom of the flame tube, to which the flame jet was directed from above obliquely to the flame tube axis, and what the said flame jet velocity was sufficient. As has already been mentioned, advantageous features of the invention can also be utilized in the invention, but the further increase in the flame jet velocity pursues the already described other goal, which can not be achieved with the features of the prior art.

The pre-combustion chamber is included in the water cycle of the boiler and absorbs heat from the flame. The flame is already partially cooled when accelerated. The accelerated flame jet acts like an injector in the flame tube and, during the burnout of the remaining unburned fuel portion from the outer region of the flame tube, entrains and cools already cooled gases there, which further reduces the temperature of the flame.

The consequence of the measures according to the invention is a reduction of the peak value of the heat flow density from the initially mentioned 400 kW / m ² to about 150 to 160 kW / m ² . With the same material stress thus a proportion of the aforementioned heat flux densities proportional increase in the wall thickness of the flame tube from originally 20 mm corresponding to the context 20 x 400/160 to 50 mm now possible. At the given steam pressure of 18x10 ³ hPa, the flame tube diameter of 1,900 mm without reinforcing rings and 2,500 mm with reinforcing rings can be realized, which results in a firing capacity for lignite dust of 9 or 15 MW when using a single flame tube and 22 MW when using two flame tubes in the same Boiler corresponds. Theoretically, two 15-MW tubes could all be combined in a single boiler, but due to its dimensions this would no longer be transportable by road or rail. With the specified boiler capacity of 22 MW it is possible to reach the area of interest to customers of such boilers. As the energy costs of lignite dust are significantly lower than those of oil and gas, this is due to the larger boiler, the associated space requirements and the fuel preparation conditional additional investment compared to an oil- or gas-fired boiler quickly amortized.

Fig. 1

einen Längsschnitt durch einen Flammrohrkessel mit koaxial daran angesetztem Brenner zur Ausführung des Verfahrens, und

Fig. 2

einen Längsschnitt durch einen Flammrohrkessel mit einem außerhalb der Achse des Flammrohrs angesetzten, schräg nach unten gerichteten Brenner.

The invention will be explained in more detail with reference to two embodiments schematically illustrated in the drawings. It shows:

Fig. 1

a longitudinal section through a flame tube with coaxially attached burner for performing the method, and

Fig. 2

a longitudinal section through a flame tube boiler with an outside of the axis of the flame tube attached, obliquely downward burner.

FIG. 1 shows schematically as an exemplary embodiment of a flame tube steam boiler for carrying out the method, which in the example 9 MW has power and is fired with Rhenish brown coal dust. The lignite dust is burned in a pre-combustion chamber 1, which widens conically starting from an inlet end. At the extended end of the pre-combustion chamber 1 is followed by an acceleration nozzle 2, which narrows conically starting from the outlet diameter of the pre-combustion chamber 1 in the direction of an outlet end.

At the inlet end of the pre-combustion chamber 1 there is a collecting housing 3 with air guide vanes 4, which are capable of imparting a twist to a combustion air quantity L ₁ flowing in from the collecting housing 3 in the pre-combustion chamber 1.

Through the inlet end of the pre-combustion chamber 1, a lance 5 is guided concentrically with the pre-combustion chamber 1, which ends approximately at the location of the largest diameter of the pre-combustion chamber and carries there a deflection hood. The lance 5 is used to supply by means of a carrier gas, in particular air, outside of the arrangement shown in a known manner fluidized brown coal dust.

At the exit of the acceleration nozzle 2 is coaxially followed by a flame tube 7, at the opposite end of the acceleration nozzle 2 a turning chamber 8 is arranged, into which the flame tube 7 opens. Starting from the turning chamber extends parallel to the flame tube 7, a tube 9 from a plurality of mutually parallel tubes. The pre-combustion chamber 1 at least partially, the acceleration nozzle 2, the flame tube 7 and the tube 9 are in a boiler 10 partially filled with water to a level 11, wherein the tube 9 preferably extends below the flame tube 7.

In operation, combustion air L ₁ is injected into the collection chamber 3, and this is formed by the air guide vanes to toric flow, which flows near the wall of the pre-combustion chamber 1 in a spiral toward the end of larger diameter of the pre-combustion chamber 1. Due to physical conditions, a part of the combustion air flow in the region of the largest diameter of the pre-combustion chamber 1 reverses and flows centrally in the direction of the inlet end of the pre-combustion chamber 1. In this return flow of the fluidized lignite dust is blown by means of the lance 5. On its way inside the return flow of lignite dust is heated so that it spontaneously ignites when it comes into contact with the combustion air in the region of the inlet end of the pre-combustion chamber 1. The flame, which is not shown in the drawing within the pre-combustion chamber 1 and the acceleration nozzle 2, completely fills the pre-combustion chamber 1 and acceleration nozzle 2 except for a thin wall of cold air close to the wall. The flame jet 6 emerging from the accelerating nozzle 2 has a speed which is at least about 80 m / s, preferably about 100 m / s.

The exit diameter d _{1 of} the flame acceleration nozzle 2 in the example shown is 488 mm for a flame acceleration to 100 m / s or 545 mm for a flame acceleration to about 80 m / s, provided that the total amount of combustion air L ₁ passes through the pre-combustion chamber 1. The mentioned, near-wall cold air layer extends into the mouth of the flame acceleration nozzle 2, which is there in FIG. 1 is indicated accordingly.

The flame jet 6 is coaxially blown into the flame tube 7 of a diameter of D ₁ = 1800 mm and widens in the course of the burnout of the still unburned Brennstaubanteile from the original diameter, which is below the exit diameter of the flame acceleration nozzle 2, to a diameter D ₂ of about 700th up to 800 mm. By its momentum, the flame jet generates in a known manner a strong flue gas circulation in the flame tube 7, which has a corresponding heat transfer to the walls of the flame tube 7 by convection result, which adds to the heat transfer by flame radiation.

In contrast to conventional burners results due to the invention Combustion process a very uniform heat flux along the heating surface of the flame tube 7 of about 150 KW / m ² with a weak maximum of 170 KW / m ² in the region of the largest diameter D _{2 of} the flame jet 6 in the flame tube 7. Thus, the thermal conditions given for the desired larger wall thicknesses. The wall thickness may be 35 mm in the illustrated example, so that a dimension of 1730 mm remains for the inner diameter of the flame tube 7.

For other services, the inner diameter of the flame tube 7 is to be converted with the square root of the ratio of the services in a known manner. The same rule applies to the outlet diameter d _{1 of} the flame acceleration nozzle 2.

The length L _{1 of} the flame tube 7 is 5800 mm in the example shown and thus meets the requirements for sufficient burnout and adjustment of the NO equilibrium. With a boiler output of 3.5 MW, a length of 4800 mm is sufficient; for a boiler output of 13.5 MW, 7100 mm are required. For other services, interpolate linearly. The length measure is not particularly critical.

The flue gases developed by the flame jet leave the flame tube 7 at the opposite end of the flame acceleration nozzle 2 in the turning chamber 8, from where they are fed into the first pipe 9, which is arranged around the flame tube 7 in the lower part of the boiler. Fly ash settles in the turning chamber 8 and can be withdrawn from there.

The axial length of the turning chamber 8 is in the example 1250 mm and is to be converted for other boiler capacities proportional to the inner diameter of the flame tube 7.

FIG. 2 shows an embodiment in which the pre-combustion chamber 1 is arranged with the acceleration nozzle 2 above the axis of the flame tube 7 and obliquely to the axis of the flame tube 7, so that the emanating from the acceleration nozzle 2 flame jet 6 is directed obliquely downwards into the flame tube 7 in.

The angle of inclination α between the axis of the pre-combustion chamber 1 with flame acceleration nozzle 2 with respect to the axis of the flame tube 7 is preferably selected so that the distance A between the surface of the flame jet 6 and the flame tube 7 above the flame jet 6 over the length of the flame jet 6 is approximately constant , Stay that way the heat flow peaks unchanged. The most favorable angle α is between 7 and 10 °.

The success of this measure is that at the lowest point of the pre-combustion chamber 1 occasionally accumulating impurities, such as ash residues, etc., can be blown out more easily. The purging of these residues from the pre-combustion chamber 1 is favored when the pre-combustion chamber 1 is inclined in the manner described. The oblique course of the flame jet 6 also favors the removal of impurities from the flame tube 7.

If appropriate, the angle α can also be selected to be larger and extend into the range from 12 ° to 14 °, because the peak value of the heat flow density depends not only on the distance A but also on the diameter ratio D ₂ / D ₁ of the flame jet 6 and flame tube 7.

For types of carbon containing nitrogen contents of more than 0.3%, measures for NO _x alleviation may be necessary. For this purpose, it is effective when the pre-combustion chamber 1 is operated near the stoichiometric point or substoichiometrically and the required amount of residual air for combustion L ₂ through blowpipes 12, which opens at the end of the flame tube 7, where the accelerating nozzle 2 into the flame tube 7, attached are to be blown directly into the flame tube 7. These blowpipes 12 can also be advantageously used to blow out deposits of fly ash from the flame tube 7.

The remaining features of the illustrated embodiment correspond to those of the previously explained, so that can be dispensed with repetitions.

Claims (2)

1. Flame-tube steam boiler for firing with pulverized fuel, comprising:

   - a prefiring chamber (1) having an acceleration nozzle (2);

   - a lance (5) extending into the prefiring chamber (1); and

   - a flame tube (7) following the acceleration nozzle (2);

   wherein at least 30% of the pulverized fuel is burned in the firing of the flame-tube steam boiler, and the flame gases exiting the prefiring chamber are accelerated to at least 80 m/s; and
   wherein the wall thickness of the flame tube (7) is greater than 20 mm and not greater than 50 mm.
2. Flame-tube steam boiler according to claim 1, wherein reinforcement rings are arranged on the flame tube (7) in the flame region.

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