EP0937218A1

EP0937218A1 - Method applicable to a continuous steam generator, and the steam generator needed for applying same

Info

Publication number: EP0937218A1
Application number: EP97945787A
Authority: EP
Inventors: Wolfgang Kastner; Wolfgang Köhler; Eberhard Wittchow
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1996-11-06
Filing date: 1997-10-24
Publication date: 1999-08-25
Anticipated expiration: 2017-10-24
Also published as: DE59702415D1; EP0937218B1; JP2001503505A; KR20000053090A; CA2270596A1; DK0937218T3; CN1240020A; WO1998020280A1; ES2151295T3; US6250257B1; RU2181179C2; DE19645748C1

Abstract

The inventive continuous steam generator, comprising a combustion chamber the outer enclosure (4) of which consists of evaporating pipes (12) vertically welded to each other, is also designed to be able to reliably work within a pressure range of 200 to 221 bar, while guaranteeing high efficiency. Furthermore, the mass velocity (m) of the liquid (S) circulating in the evaporating pipes (12) can be expressed by the ratio m = 200 + 8,42 . 1012 . q3 . [d/(d - 2s)]s2 . T¿max?-5, where q represents the heat flux density affecting the evaporating pipes (12), T¿max? the permissible characteristic maximum temperature for the tubing, d the outer diameter of the evaporating pipes (12) and s the wall thickness of said pipes (12).

Description

description

Method for operating a once-through steam generator and once-through steam generator for carrying out the method

The invention relates to a method for operating a once-through steam generator with a combustion chamber, the peripheral wall of which is formed from vertically arranged evaporator tubes welded to one another in a gastight manner, a flow medium flowing through the evaporator tubes. It also relates to a once-through steam generator for carrying out the method.

Such a steam generator is known from the article "Evaporator Concepts for Benson Steam Generators" by J. Franke, W. Köhler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, pp. 352-360 In a continuous steam generator, the heating of the evaporator tubes forming the combustion chamber or the gas train leads - in contrast to a natural circulation or forced circulation steam generator with only partial evaporation of the water-steam mixture circulating - to an evaporation of the flow medium in the evaporator tubes in a single pass Evaporator tubes of the once-through steam generator can be arranged vertically or spirally and thus inclined.

In contrast to a natural circulation steam generator, a continuous steam generator is not subject to any pressure limitation, so that fresh steam pressures far above the critical pressure of water (P _n t ⁼ 221 bar) - where there is only a slight difference in density between liquid-like and steam-like medium - are possible. A high live steam pressure promotes high thermal efficiency and thus low C0 ₂ emissions from a fossil-fired power plant. A continuous steam generator, the throttle cable from vertically arranged steam pipes is constructed, is cheaper to produce than a spiral design. Continuous-flow steam generators with vertical pipes also have lower water vapor-side pressure losses than those with inclined or spirally rising evaporator tubes.

A continuous steam generator with a combustion chamber, the peripheral wall of which is formed from vertically arranged evaporator tubes welded together in a gastight manner, is known from DE 43 33 404 AI.

A particular problem is the design of the gas flue or combustion chamber wall of the continuous steam generator with regard to the pipe wall or material temperatures that occur there. In the subcritical pressure range up to about 200 bar, the temperature of the combustion chamber wall is essentially determined by the level of the saturation temperature of the water, if one Wetting of the heating surface in the evaporation area can be ensured. This is achieved, for example, by using evaporator tubes that have a surface structure on the inside. In addition, there are in particular male finned evaporator tubes whose use in continuous steam generators is known, for example, from European Patent 0 503 116. These so-called finned tubes, d. H. Pipes with a ribbed inner surface have a particularly good heat transfer from the inner pipe wall to the flow medium.

In the pressure range of approximately 200 to 221 bar, the heat transfer from the inner tube wall to the flow medium drops sharply, so that the mass flow density of the flow medium must be chosen to be sufficiently high to ensure adequate cooling of the evaporator tubes. For this purpose, the mass flow density must be present in the evaporator tubes of once-through steam generators, which are operated at pressures of approximately 200 bar and above higher than that of continuous steam generators that are operated with pressures below 200 bar. However, such an increased mass flow density also results in a higher friction pressure loss in the evaporator tubes. As a result of this higher loss of frictional pressure, the advantageous property of the vertical pipe is lost, particularly in the case of small internal pipe diameters, that the throughput also increases when an individual evaporator pipe is heated. However, since steam pressures above 200 bar are required for high thermal efficiency and low CO emissions from a power plant, it is necessary to ensure good heat transfer from the inner wall of the pipe to the flow medium even in this pressure range. Continuous-flow steam generators with vertical-walled combustion chamber walls are therefore usually operated with relatively high mass flow densities. For this purpose, in the publication "Thermal Engineering", IE Semenovker, Vol. 41, No. 8, 1994, pages 655 to 661, both for gas and for coal-fired once-through steam generators, a mass flow density at 100o load, with approximately 2000 kg / m ² s specified.

The invention has for its object to provide a method for operating a once-through steam generator of the type mentioned above, with which a particularly low loss of friction pressure and thus a particularly high efficiency can be achieved with safe and reliable cooling of the evaporator tubes. In addition, a continuous steam generator that is particularly suitable for carrying out this method is to be specified.

With regard to the method, this object is achieved according to the invention in that the mass flow density m of the flow medium m as a function of the heat flow density q acting on the evaporator tubes approximately to a manipulated value according to the relationship m = 200 + 8.42 ^• 10 ^{1 •} q ^{3 •} [d / ( d - 2s)] s ^{2 •} T _ma ^"5 is held. The heat flow density q is to be used on the outside of the pipe in kW / m ² in order to obtain the mass flow density mm kg / m ² • s. Furthermore, d means the outer diameter of the evaporator tubes m meters, s the tube wall thickness of the evaporator tubes m meters and

T _n _ the permissible maximum temperature m ° C that is characteristic of the pipe material.

The invention is based on the consideration that safe and reliable cooling of the evaporator tubes with a particularly low friction pressure loss is ensured when the continuous steam generator is operated by suitably fulfilling two conditions which contradict each other in principle. On the one hand, the average mass flow density m of the evaporator tubes should be chosen to be as low as possible. It can thereby be achieved that individual evaporator tubes, to which more heat is supplied than other evaporator tubes due to unavoidable heating differences, are flowed through by a higher mass flow than average-heated evaporator tubes. This natural circulation characteristic known from the drum boiler leads to a comparison of the vapor temperature and thus the tube wall temperatures at the outlet of the evaporator tubes.

On the other hand, the mass flow density m of the pipes is to be chosen so high that reliable cooling of the pipe wall is ensured and permissible material temperatures are not exceeded. In this way, high local overheating of the pipe material and the resulting damage (pipe breakers) are avoided. In addition to the temperature of the flow medium, the main influencing variables for the material temperature are the external heating of the pipe wall and the heat transfer from the inner pipe wall to the flow medium or fluid. There is a connection between the inner warm Transition, which is influenced by the mass flow density and the external heating of the pipe wall.

Taking these boundary conditions into account, the aforementioned relationship results in a particularly favorable mass flow density in the evaporator tubes, which ensures both a favorable flow characteristic (natural circulation characteristic) and reliable cooling of the evaporator tubes and thus compliance with the permissible material temperatures. The criterion for determining a particularly favorable mass flow density is that, in the case of a predefinable external heating of the pipe wall, the material temperature of the pipe wall should on the one hand only be minimal, but on the other hand it should certainly be below the permissible value. It is important to note the physical appearance, that in the critical

Pressure range from about 200 to 221 bar, the heat transfer from the inner tube wall to the flow medium is most unfavorable. The result of extensive investigations is that the highest material stress is reached when a relatively low mass flow density is combined with the largest occurring heat flow density in the evaporation area at about 200 to 221 bar. This is the case, for example, in the region of the combustion chamber in which the burners are arranged. When the evaporation has ended and the steam overheating begins, the material stress on the evaporator tubes of a combustion chamber wall drops again. The reason for this is that, with the usual burner arrangement and the usual combustion process, the heat flow density also decreases.

To determine a particularly favorable control value for the mass flow density m, it is expedient to use the permissible maximum temperature T _{rna ^ in} according to the relationship _πcL = T _{krl t} + 6σ / (ß ^• E) determined value used. T _{krιt is} the temperature of the flow _medium at critical pressure in ° C. Furthermore, σ means the permissible stress in N / mm ² , ß the coefficient of thermal expansion in 1 / K and E the modulus of elasticity in N / mm ^{2 of} the material of the evaporator tubes. When determining the permissible maximum temperature T ,, _^ it is assumed that the enclosure or combustion chamber wall of the continuous steam generator has an average temperature which is the average of the permissible maximum temperature T _ma> and the temperature of the flow medium at critical pressure T _ll , corresponds. From this, the maximum occurring thermal stress is calculated

Tmax - Tkπt σ _md , = ß ^• E.

When designing the continuous steam generator in accordance with the ASME code, this maximum occurring thermal stress should be three times the value permitted for the pipe material

Voltage σ are secured. This immediately results in the value to be used for the permissible maximum temperature T _ma .

It follows from these design principles that when a continuous steam generator is operated, the evaporator tubes of which are made of 13 CrMo44 material, a value of approximately T _{ma ^} = 515 ° C. is expediently used as the basis for the permissible maximum temperature T _ma . In contrast, when operating a continuous steam generator whose evaporator tubes are made of the material HCM12, a value of approximately T _ma = 590 ° C. is advantageously used as the permissible maximum temperature T _maz .

With regard to the once-through steam generator, which is particularly suitable for carrying out this method, the stated object is achieved by the once-through steam generator at one on the Evaporator tubes acting heat flow density q is designed for a mass flow density m according to the relationship m = 200 + 8.42 ^• 10 ¹ - ^• q ^{3 •} [d / (d - 2s)] s ² • T _max ^{~ 5} .

An embodiment of the invention is explained in more detail with reference to a drawing. In it show:

1 shows a simplified representation of a continuous steam generator with vertically arranged evaporator tubes, FIG. 2 shows a single evaporator tube in cross section, and FIG. 3 shows a diagram with characteristic curves A and B for the mass flow density as a function of the heat flow density for evaporator tubes.

Corresponding parts are provided with the same reference symbols in all figures.

In Figure 1, a continuous steam generator 2 with e.g. Rectangular cross section shown, the vertical throttle cable is surrounded by a surrounding wall 4 and forms a combustion chamber, which merges into a funnel-shaped bottom 6 at the lower end. The bottom 6 comprises a discharge opening 8 for ashes, not shown in detail.

In the lower region A of the throttle cable, a number of burners 10, only one of which is shown, are mounted in the peripheral wall of the combustion chamber formed from vertically arranged evaporator tubes 12. The burners 10 are designed for fossil fuel. The vertically arranged evaporator tubes 12 are welded together in area A via tube webs or fins 14 to form a gas-tight surrounding wall 4. The operation of the continuous steam generator 2 Evaporator tubes 12 flowed through from bottom to top form an evaporator heating surface 16 in area A.

When the once-through steam generator 2 is operating, the combustion chamber contains a fossil fuel

Fuel resulting flame body 17, so that the area A of the continuous steam generator 2 is characterized by a very high heat flow density q. The flame body 17 has a temperature profile which, starting from approximately the center of the combustion chamber, both in the vertical direction upwards and downwards and in the horizontal direction to the sides, i.e. H. towards the corners of the combustion chamber. Above the lower area A of the throttle cable there is a second area B remote from the flame, above which a third upper area C of the throttle cable is provided. Convection heating surfaces 18, 20 and 22 are arranged in regions B and C of the gas flue. Above area C of the gas flue there is a flue gas outlet channel 24, via which the flue gas RG generated by the combustion of the fossil fuel leaves the vertical gas flue. The relationships shown in FIG. 1 for a once-through steam generator 2 m infeed type also apply in a comparable manner to a once-through steam generator in two-pass type.

FIG. 2 shows an evaporator tube 12 provided on the inside with fins 26, which during operation of the continuous steam generator 2 on the outside inside the combustion chamber is exposed to heating with the heat flow density q and through which the flow medium S flows. Water or a water-steam mixture, for example, serves as the flow medium S.

At the critical point, ie at critical pressure p _lf of 221 bar, the temperature of the fluid or flow medium S in the evaporator _tube 12 is designated T _{> 13t} -. For the calculation voltage of the maximum heat stress σ _maλ the maximum allowable Mateπaltemperatur T _ma is the heated side of the pipe wall at the pipe apex inserted 28th

The inner diameter and the outer diameter of the evaporator tube 12 are denoted by d and d, respectively. In the case of a male finned evaporator tube 12, the equivalent inner diameter d _{x should} be used as the inner diameter, taking into account the influence of the fin heights and troughs. The equivalent inside diameter is the inside diameter that a smooth pipe with the same flow cross-section had. The pipe wall thickness is denoted by s.

The continuous-flow steam generator 2 is designed such that during its operation the mass flow density m of the flow medium S flowing through the evaporator tubes 12 is approximately at a manipulated value according to the relationship m = 200 + 8.42 ^• 10 ¹² • q ³ • [d / (d - 2s) ] s ² • T _max ^"5. The mass flow density mm kg / m ^{2 •} s and the permissible maximum temperature T _ma , in ° C, must be used.

The outer tube diameter d and the tube wall thickness sm meter must also be used. A value added with a safety margin is to be used as the heat flow density q on the outside of the pipe m kW / m ² . For this purpose, a value for an average heat flow density is first determined from the technical data of the once-through steam generator 2, such as, for example, cross section of the combustion chamber, combustion output, etc. A value for a maximum heat flow density is derived from the value for the mean heat flow density by multiplication by a safety factor. The safety factor in the case of hard coal firing lies in the interval from 1.4 to 1.6 and in the case of brown coal firing in the interval from 1.6 to 1.8. The value to be used for the heat flow density q is obtained by multiplying the maximum heat flow density by a further safety factor of 1.5 educated. In other words: the value to be used for the heat flow density 7 is 2.1 to 2.4 times for hard coal firing and 2.4 to 2.7 times for brown coal firing that can be determined from the technical data of the continuous steam generator 2 average heat flow density.

Depending on the heat flow density q, a design value for the once-through steam generator 2 results in a characteristic value for the mass flow density m, as shown graphically in FIG. 3 for different pipe geometries and different pipe materials. The characteristic curve A describes the mass flow density m kg / m ² s that is associated with a geometry parameter

[d / (d-2s)] s ² of 4 ^• 1CT ⁵ m ² for a permissible maximum temperature T _rna> of 590 ° C. The value of about 590 ° C. on which the maximum temperature T _{ma is} based is relevant for a once-through steam generator 2, the evaporator tubes 12 of which are made from the material HCM12. The characteristic curve B gives the particularly favorable mass flow density m as a function of the heat flow density q for a once-through steam generator 2, the evaporator tubes 12 of which have a geometry parameter td / (d-2s)] s ² of 10 ^"* m and a permissible maximum temperature T _ma> of about 515 C. The permissible maximum temperature T “ _d of 515 C is relevant for evaporator tubes 12 made of material 13 CrMo44.

In general, a value determined according to the relationship ma = _μιl , + 6σ / (ß ^• E) is used as the permissible maximum temperature T _ιra for any evaporator tube 12. The following are: T _rιt the temperature of the flow _medium S at critical pressure p _{i rι t} m ° C, σ the permissible tension of the material of the evaporator tube 12 m N / mm ² , ß the coefficient of thermal expansion of the material of the evaporator tube 12 in 1 / K and E the modulus of elasticity of the material of the evaporator tube 12 in N / mm ² .

Reference list

2 continuous steam generators

4 surrounding wall 6 floor

8 discharge opening

10 burners

12 evaporator tube

14 fins

16 evaporator heating surface

17 flame bodies

18, 20, 22 convection heating surfaces

24 flue gas outlet channel

26 ribs

28 pipe crests

ß thermal expansion coefficient σ allowable stress σ _{_ma;} Thermal stress

A lower area d. Throttle cable

B area away from the flame d. Throttle cable

C third upper area d. Throttle cable d, di outside and. Inner diameter of the evaporator tube m mass flow density q heat flow density s tube wall thickness

5 flow medium T _max maximum temperature

Claims

claims

1. Method for operating a once-through steam generator (2) with a combustion chamber, the peripheral wall (4) of which is formed from gas-tightly welded, vertically arranged evaporator tubes (12), in which a flow medium (S) flows through the evaporator tubes (12) , The mass flow density m of the flow medium (S) for evaporator tubes (12) with an outer tube diameter d and with a tube wall thickness s and with a characteristic maximum temperature characteristic of the tube material (T _ma m depending on the heat flow density q acting on the evaporator tubes (12) approximately is held at a manipulated value according to the relationship m = 200 + 8.42 ^• 10 ^{1 ^} • q ^{3 •} [d / (d - 2s)] s ^{2 •} T _ma ⁵ .

2. The method of claim 1, in which a value determined according to the relationship T _Ina = T _Ilt + 6σ / (ß ^• E) is used as the permissible maximum temperature (T _na ), where (T _kllt ) (° C) is the temperature of the Flow medium (S) at critical pressure

(Pi- _L it-), σ (N / mm ² ) is the permissible stress, ß (l / K) is the coefficient of thermal expansion and E (N / mm ² ) is the elasticity module of the material of the evaporator tubes (12).

3. The method according to claim 1 or 2, wherein the evaporator tubes (12) are made of the material 13 CrMo 44, and a value of about T _mα = 515 ° C is used as the permissible maximum temperature.

4. The method of claim 1 or 2, wherein the evaporator tubes (12) are made of the material HCM 12, wherein as permissible maximum temperature is based on a value of approximately T _max = 590 ° C.

5. Continuous-flow steam generator (2) with a combustion chamber, the surrounding wall (4) of which is made of vertically arranged evaporator tubes (12) welded together in a gas-tight manner, with an outer tube diameter α and with a tube wall thickness s and with a permissible maximum temperature (T _ma ) characteristic of the tube material. is formed, wherein the steam generator tube (12) can be flowed through by a flow medium (S) and have a surface structure on its inside, and the heat flow density q acting on the evaporator tubes (12) for a mass flow density m according to the relationship m = 200 + 8.42 • 10 ^{1 "} • q ³ • [d / (d - 2s)] s ² • T _max ^{~ 5} .

6. continuous steam generator (2) according to claim 5, the evaporator tubes (12) are made of the material 13CrMo 44, a value of 515 ° C being used as the basis for the permissible maximum temperature (T _ma ).

7. continuous steam generator (2) according to claim 5, the evaporator tubes (12) are made of the material HCM 12, whereby a value of 590 ° C is used as a basis for the permissible maximum temperature (T _max ).