WO1994010501A1

WO1994010501A1 - Steam generator

Info

Publication number: WO1994010501A1
Application number: PCT/DE1993/000998
Authority: WO
Inventors: Rudolf Kral; Eberhard Wittchow
Original assignee: Siemens Aktiengesellschaft
Priority date: 1992-11-02
Filing date: 1993-10-20
Publication date: 1994-05-11
Also published as: CN1086592A; DE4236835A1

Abstract

The invention pertains to a fossil-fuel fired steam generator (2) with a vertical gas flue whose gas-tight tube wall (4) in the area (C) near the flames is formed by evaporator tubes (10) that run diagonal to the horizontal (H) at an angle of inclination a and have an inside ribbing, wherein the angle of inclination a is a function a(P) of the amount of live steam generated per unit of time when operating at full load, i.e. steam generator capacity P. The paired values P, a fall within a range (Z) the lower limit of which is defined by one of the paired values 500 t/h; 16° and 3000 t/h; first straight line (F) at 42°, and the upper limit of which is defined by one of the paired values 500 t/h; 25° and 2500 t/h; second straight line (E) at 70°. As a result, high steam temperatures can be attained even with a minimum load of the steam generator (2) below 30 % of the full load, while ensuring both a safe cooling of the evaportor tubes at low load and an economical frictional pressure loss in the evaporator.

Description

Steam generator

The invention relates to a fossil-fired steam generator with a vertical gas flue, the gas-tight tube wall of which is formed in the area near the flame by evaporator tubes arranged obliquely at an angle of inclination a with respect to the horizontal and having internal ribs.

In order to achieve a high power plant efficiency, fossil-fired steam power plants of the new generation with continuous steam generators are designed for high steam temperatures up to about 600 "C and high live steam pressures from 250 to 300 bar. The wall thicknesses of superheater outlet collectors and steam lines between the

The steam generator and the steam turbine, as well as the inlet parts of the steam turbine, should be chosen so large that only very low temperature change rates of 1 to 2 K / min are permissible. For this reason, power plants of this type can only be started up slowly, the start-up times being so long and the start-up losses incurred being so high that a large part of the high power plant efficiency is lost again when starting up frequently.

Since a fluctuating demand for electrical energy often requires a drastic reduction in the power plant output and the shutdown and start-up is extremely uneconomical, such steam power plants with high steam temperatures of, for example, 600 ° C. also have to be less than less at low loads 30% of the full load (100% load) can be operated in order to prevent cooling and reheating of thick-walled components. However, with the previous steam generators, the steam temperature drops sharply at a partial load below 35 to 40% of the full load. In order to be able to operate previous steam generators below the partial load mentioned, a circulating stream is usually superimposed on the evaporator below this partial load of about 35 to 40% of the full load by means of a circulating pump, so that at a correspondingly high flow rate or mass flow density Evaporator tubes are safely cooled. The minimum mass flow density required for this is about 800 kg / m ² s. In evaporator tubes with a smooth inner surface (smooth tubes) at full load, this results in a mass flow density of 2000 to 2300 kg / ms and an acceptable pressure loss of approximately 10 to 12 bar. In such a circulating operation, however, the evaporator heating surface is increased and thereby the superheater heating surface is reduced, because a water / steam separating device marking the end of the evaporator heating surface in circulating operation is already arranged in the area of superheated steam in continuous operation. As a result, the temperature at the outlet of the superheater drops accordingly. Such a partial-load mode of operation in recirculation mode is therefore unusable for steam power plants with high live steam conditions.

In principle, it is conceivable to operate the evaporator of previous steam generators even in continuous operation without a circulation pump with a partial load of approximately 20% of the full load.

In this case too, the minimum mass flow density required in the evaporator tubes must also be approximately 800 kg / m ² s - corresponding to approximately 4000 kg / m ² s at full load. The resulting pressure loss for the evaporator alone from 35 to 45 bar at full load is extremely uneconomical due to a substantial reduction in efficiency due to the high power requirement of a feed water pump and the large wall thicknesses of system parts carrying feed water. There is also the risk of material being removed in the evaporator tubes by erosion corrosion at high loads. The invention is therefore based on the object of specifying a steam generator with which high steam temperatures can be achieved even with a minimum load of the steam generator below 30% of the full load. This should ensure safe cooling of the evaporator tubes at low load as well as an economical loss of friction pressure of the evaporator at full load (100% load).

This object is achieved in that the pitch angle α of the obliquely rising evaporator tubes is a function of the live steam quantity P generated at full load per unit time, the pairs of values P; a are in a range that is limited by a value of 500 t / h; 16 "and 3000 t / h; 42 ° assigned first straight line, and the upper limit is limited by the value pairs 500 t / h; 25" and 2500 t / h; 70 "assigned second straight line.

The invention is based on the consideration that with an inclined position of the evaporator tubes and the use of internally finned tubes with a to the steam generator output, i.e. the amount of live steam generated per unit of time, adjusted pitch angle α, the large number of influencing factors to be taken into account when designing such a steam generator are reduced to only a few. These influencing variables are e.g. the mass flow density in the evaporator tubes, the heat flow density as a measure of the heating, the tube inner diameter, the steam generator output, the minimum partial load and the width of the webs provided between the tubes.

The pitch angle α is expediently greater than 20 °. In addition, the pitch angle is advantageously less than 70 °, preferably less than 65 °. In addition, the pitch angle α is adapted in an advantageous embodiment to the selected outer diameter d of the evaporator tubes. The preferred selected standardized outer diameters d of 38.1 mm or 35.0 mm or 31.8 mm are the value pairs Pn: 'a' n in the subclaims 4, 5 and 6 ang 3eg3

Full load per unit of time generated live steam quantity P and the angle of inclination a.

In order to keep the friction pressure loss in the tubes of the steam generator particularly low, the obliquely rising evaporator tubes merge into a vertical arrangement in a region in which the heat flow density has exceeded its maximum value. The tube wall is therefore formed in this area remote from the flame, in which the heat flow density decreases, by tubes arranged vertically and parallel to one another. The number of vertically extending tubes is greater than that of the evaporator tubes, which run at an incline. As a result, a particularly low mass flow density in the tubes is achieved with a simultaneously short tube length. Pipes with a smooth inner surface (smooth pipes) can also be used in the area remote from the flame. In order to improve the flow distribution in the parallel pipe system, a pressure compensation collector should be arranged at the transition point from the oblique to the vertical pipe.

The steam generator according to the invention can also be operated in continuous operation at a low partial load below 30% of the full load, an increase in the evaporator throughput by means of a circulation pump no longer being necessary. A circulating pump with associated pipelines, which is nevertheless expediently assigned to the evaporator tubes in terms of flow, is therefore only functional during start-up operation. This results in a minimum throughput through the evaporator and possibly through one of these upstream economizer secured. So that permissible temperature changes during start-up can be as large as possible even with a complicated housing of the circulating pump, the circulating pump and the associated pipes are dimensioned in terms of strength only for a pressure of at most 150 bar. As a result, the wall thicknesses of these plant parts can be kept particularly small.

The advantages achieved with the invention are, in particular, that a steam generator output, i.e. the fresh steam quantity P generated at full load per unit time P of the steam generator can also be operated at a minimum load below 30% of the full load in the pass and thus at high steam temperatures. This ensures reliable cooling of the obliquely arranged evaporator tubes at minimum load as well as a low loss of frictional pressure of the evaporator at full load.

Embodiments of the invention are explained in more detail with reference to a drawing; show in it:

FIG. 1 shows a simplified representation of a steam generator with a vertical throttle cable that is partially touched at an angle,

Figure 2 in a diagram of steam generator power P and pitch angle a three characteristics for three different pipe outside diameter d, and

Figure 3 greatly simplified a starting system for a steam generator according to Figure 1 with a circulation pump.

The vertical throttle cable of the steam generator 2 according to FIG. 1 with a rectangular cross section is formed by a tube wall 4 which merges into a funnel-shaped bottom 6 at the lower end of the gas cable. The bottom 6 comprises a discharge opening 8 for ashes, not shown. A number of burners (not shown) for a fossil fuel are attached in the lower wall 4 near the flame or near the burner for a fossil fuel in the surrounding wall 4 formed from pipes 10. The tubes 10 are arranged in a spiral shape in this area C and form an evaporator heating surface 12.

Above area C of the throttle cable is a flame D or burner-remote area D of the gas cable. In area D of the gas flue, in which convection heating surfaces 14, 16 and 18 are also arranged, the pipes 10 'run vertically. Above area D of the gas flue there is a smoke outlet channel 20, via which the flue gas RG generated by burning a fossil fuel leaves the vertical gas flue.

The area near the flame C is characterized by a high heat flow density, which decreases towards the top in the area D remote from the flame. Therefore, the transition from the spiral arrangement of the evaporator tubes 10 to the vertical tube arrangement expediently lies in a region 22 in which the heat flow density has exceeded its maximum value. The number of vertically extending tubes 10 ′ is greater than that of the obliquely extending evaporator tubes 10.

While the vertically extending tubes 10 'are expediently so-called smooth tubes, ie have a smooth inner surface, the obliquely extending evaporator tubes 10 advantageously have ribs forming a multi-start thread on their inside. The evaporator tubes 10 form the gas-tight tube wall 4 in the area near the flame C in a so-called tube-web-tube construction. The evaporator tubes 10 are arranged at an inclination angle a with respect to the horizontal H. According to FIG. 1, the pitch angle a is defined as the angle function sin a from the quotient G / L. G is the vertical distance between the transition from the funnel-shaped bottom 6 to the vertical throttle cable and the upper end of the helical winding in the transition region 22. L is the length of an evaporator tube 10 rising with a constant inclination over the vertical distance or the height G.

In order to be able to operate the steam generator 2 even at a minimum load below 30% of the full load in a continuous manner and thus at high steam temperatures, and in order to ensure both reliable cooling of the evaporator tubes 10 at a minimum load and a low friction pressure loss of the evaporator 12 To ensure at full load, the pitch angle α of the obliquely arranged evaporator tubes 10 is adapted to the steam generator power P. The dependence of the pitch angle a on the steam generator power P, i.e. the amount of live steam generated at full load per unit time is shown in FIG.

In FIG. 2, a Pa diagram, in which the steam generator power P is plotted on the abscissa and the gradient angle a on the ordinate, shows an area Z which is delimited by two straight lines E and F upwards and downwards, in which the value pairs P; a lie. The straight line E runs through the points or pairs of values: (500 t / H; 25 °) and (2500 t / h; 70 °). The straight line F runs through the points or pairs of values: (500 t / h; 16 °) and (3000 t / h; 42 °).

Within area Z there are three different outer diameters d, lines A, B and C assigned to the evaporator tubes 10. The straight line A corresponds to a standardized outer diameter d- of 38.1 mm. The straight line A the value pairs are ^_'(λ P - 900 t / h; _{j ^} a = 21.8 °)

(P ₂ = 1500 t / h; a ₂ = 30.3 °), (P ₃ = 2100 t / h; a ₃ = 38.8 ") and (P. = 3000 t / h; a, = 51, 5 "assigned.

The straight line B corresponds to a standardized outer diameter of 35.0 mm, this straight line B having the value pairs P; a _n : (P ₅ = 900 t / h; a ₅ = 24.6 °), (P _g = 1500 t / h; a ₆ = 34.5 °), (P ₇ = 2100 t / h; a _? = 44.7 °) and (Pg = 3000 t / h; a ₈ = 61.4 ").

The straight line C corresponds to a standardized outer diameter d of 31.8 mm, this straight line C being the value pairs P; a _n : (P ₉ - 900 t / h; a ₉ = 28.0 °), (P _1Q = 1500 t / h; a _χo = 39.7 °) and (P = 2100 t / h; a - 52 , 4 °) are assigned.

The gradient angles a corresponding to the straight lines A, B and C are mean values. A particularly favorable gradient angle a, which takes into account all requirements and influencing variables, can deviate from the mean value if, for example the web width between adjacent evaporator tubes 10, the wall thickness of the tubes 10, 10 'and the maximum local heat flow density must be adapted directly to the respective application. With lines A, B or C, a deviation of the pitch angle a by 25%, preferably 20%, is permissible.

The slope angle α can be linearly extrapolated for values of the steam generator output P above 3000 t / h or below 900 t / h. Tube outer diameters d other than the outer diameters d,, d ₂ and d, can also be linearly interpolated or extrapolated between or outside the straight lines A, B and C, respectively.

Figure 3 shows schematically and in detail the water-steam circuit 30 of a (not shown) steam turbine of a steam power plant. The evaporator 12 and the con- Vection heating surfaces 14, 16, 18 of the steam generator 2 according to FIG. 1 as well as an Econo izer 32 and a superheater 34 form a high-pressure system of the water-steam circuit 30.

Between the evaporator 12 and the superheater 34, a 2 water-steam drum 36 is connected, which is connected via a water reservoir 38 and a circulation pump 40 to the outlet of a steam-covered preheater 42 and to the inlet of the economizer 32. Protection against an impermissible excess pressure is carried out by means of a

Safety valve 44. The circulation pump 40 and the water reservoir 38 as well as associated pipelines 46 form a start-up system 48 of the steam generator 2.

The start-up system 48 only functions when the steam generator 2 starts up. In this case, the preheated feed water conveyed by means of a feed water pump 50 and flowing through the economizer 32 and the evaporator 12 and the superheater 34 is superimposed behind the preheater 42 by a circulating flow conveyed via the circulating pump 40.

A minimum throughput through the evaporator 12 is ensured by the circulation pump 40 associated with the flow to the evaporator 12 when the steam generator 2 starts up. In order to allow the greatest possible temperature changes, the circulating pump 40 and the associated pipelines 46 are only designed for a pressure of 150 bar and therefore only have small wall thicknesses.

Claims

1. Fossil-fired steam generator (2) with a vertical gas flue, the gas-tight tube wall (4) of which in the region 5 near the flame (C) through evaporator tubes arranged obliquely at an angle of inclination a with respect to the horizontal (H) and having internal ribs ( 10) is formed, characterized in that the

10 pitch angle a is a function a (P) of the live steam quantity P generated at full load operation per time unit, the pairs of values P, a lying in a range (Z) which is limited at the bottom by one of the pairs of values 500 t / h; 16 ° and 3000 t / h; 42 "assigned first straight line (F), and

15 which is limited by one of the value pairs 500 t / h; 25 ° and 2500t / h; 70 ° assigned second straight line (E).

2. Steam generator according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the 20 pitch angle a> 20 °.

3. Steam generator according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the pitch angle a <70 °, preferably <65 °.

25

4. Steam generator according to one of claims 1 to 3, d a d u r c h g e k e n n z e i c h n e t that with an outer diameter d, the evaporator tubes (10) of about 38 + 2 mm, the pairs of values P; a:

30 P ₁ = 900 t / h; a ₁ = 21.8 °

P ₂ = 1500 t / h; a ₂ = 30.3 ° P ₃ = 2100 t / h; a ₃ = 38.8 ° P ₄ = 3000 t / h; a = 51.5 ° are assigned to a straight line A, a relative - * ⁵⁵ deviation of the gradient angle a from the straight line (A) of 25%, preferably 20%, being permissible. 5. Steam generator according to one of claims 1 to 3, characterized in that with an outer diameter d _{2 of} the evaporator tubes (10) of about 35 + 2 mm, the pairs of values P; a: P ₅ = 900 t / h; a ₅ = 24.6 °

P ₆ = 1500 t / h; a ₆ = 34,

5 "P _y = 2100 t / h; a _? = 44.7 ° P _g = 3000 t / h; a _g = 61.4 ° are assigned to a straight line B, with a relative deviation of the gradient angle a from the straight line ( B) of 25%, preferably of 20%, is permissible.

6. Steam generator according to one of claims 1 to 3, characterized in that with an outer diameter d _{3 of} the evaporator tubes (10) of approximately 31 + 2 mm, the pairs of values P; a:

P ₉ = 900 t / h; a ₉ = 28.0 ° P ₁₀ = 1500 t / h; a _1Q = 39.7 ° P = 2100 t / h; a _n = 52.4 ° are assigned to a straight line C, a relative deviation of the gradient angle a from the straight line (C) of approximately 25%, preferably 20%, being permissible.

7. Steam generator according to one of claims 1 to 6, characterized in that the tube wall (4) in the flame-distant area (D) is formed by vertically and mutually parallel tubes (10 '), the number of vertically extending tubes (10' ) is larger than that of the evaporator tubes (10), which run at an incline.

8. Steam generator according to claim 7, characterized in that the tubes (10 ') in the flame-distant area (D) have a smooth inner surface.

9. Steam generator according to one of claims 1 to 8, d a d u r c h g e k e n n z e i c h n e t that the evaporator tubes (10) flow-associated circulation pump (40) is designed for a pressure less than or equal to 150 bar.