EP0937218B1 - Method applicable to a continuous steam generator, and the steam generator needed for applying same - Google Patents

Description

The invention relates to a method for operating of a once-through steam generator with a combustion chamber, the Boundary wall made of gas-tightly welded together, vertically arranged evaporator tubes are formed, the A flow medium flows through the evaporator tubes. It also relates to a once-through steam generator Execution of the procedure.

Such a steam generator is from the article "Evaporator concepts for Benson steam generators "by J. Franke, W. Köhler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), Issue 4, pp. 352-360. With a once-through steam generator performs the heating of the combustion chamber or the evaporator tubes forming the throttle cable - in contrast to one Natural circulation or forced circulation steam generator with only partial Evaporation of the circulating water-steam mixture - Evaporation of the flow medium in the evaporator tubes in a single pass. The evaporator tubes of the continuous steam generator can be vertical or be arranged in a spiral 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 _crit = 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 favors high thermal efficiency and thus low CO ₂ emissions from a fossil-fired power plant. A continuous steam generator, the gas train of which is made up of vertically arranged evaporator tubes, is less expensive to produce than a spiral version. 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 once-through steam generator with a combustion chamber, the surrounding wall made of gas-tight welded together vertically arranged evaporator tubes is formed from the DE 43 33 404 A1 known.

The design of the throttle cable or poses a particular problem Combustion chamber wall of the once-through steam generator with regard to the pipe wall or material temperatures occurring there in the subcritical pressure range up to about 200 bar the temperature of the combustion chamber wall essentially from that Height of the saturation temperature of the water determines if one Wetting of the heating surface in the evaporation area ensured can be. This is for example through the use of evaporator tubes, which have a Have surface structure. In addition, there are in particular ribs on the inside Evaporator tubes into consideration, their use in Continuous steam generators, for example from the European Patent specification 0 503 116 is known. These so-called finned tubes, d. H. Tubes with a finned inner surface, have a particularly good heat transfer from the inner tube wall to the flow medium.

In the pressure range of about 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 in the evaporator tubes of once-through steam generators, which are operated at pressures of approximately 200 bar and above, must be selected higher than in the case of once-through steam generators, which are operated at pressures below 200 bar. However, such an increased mass flow density also results in a higher loss of friction pressure in the evaporator tubes. As a result of this higher frictional pressure loss, 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 ₂ emissions from a power plant, it is necessary to ensure good heat transfer from the inner pipe wall to the flow medium even in this pressure range. Continuous-flow steam generators with a vertically tubular combustion chamber wall are therefore usually operated with relatively high mass flow densities. 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 100% load is stated uniformly at about 2000 kg / m ² s. Furthermore, from DE-A-20 32 891 a cross-drawn steam generator tube is known, which supports the maintenance of the core boiling of the working medium by the design of its inner wall and significantly increases the critical heat flow.

The invention has for its object a method for Operating a once-through steam generator of the above Specify the type with which safe and reliable cooling the evaporator tubes have a particularly low loss of friction pressure and thus a particularly high degree of efficiency can be achieved is. In addition, one should carry out this procedure particularly suitable continuous steam generator can be specified.

d der Außendurchmesser der Verdampferrohre in Metern,

s die Rohrwandstärke der Verdampferrohre in Metern und

T_max die für das Rohrmaterial charakteristische zulässige Maximaltemperatur in °C. und

q die Wärmestromdichte in kW/m² einzusetzen ist, wobei der einzusetzende Wert für die Wärmestromdichte das Produkt aus der mittleren Wärmestromdichte mit einem Sicherheitsfaktor ist.

With regard to the method, this object is achieved according to the invention in that the mass flow density m of the flow medium as a function of the heat flow density q acting on the evaporator tubes approximately corresponds to a manipulated value according to the relationship m = 200 + 8.42 • 10 12th • q 3rd • [d / (d - 2s)] s 2nd • T Max -5 is held. The heat flow density q on the outside of the pipe is to be used in kW / m ² in order to obtain the mass flow density m in kg / m ² • s. Furthermore mean:

d the outer diameter of the evaporator tubes in meters,

s the tube wall thickness of the evaporator tubes in meters and

T _{max is} the permissible maximum temperature in ° C that is characteristic of the pipe material. and

q the heat flow density is to be used in kW / m ² , the value to be used for the heat flow density being the product of the average heat flow density with a safety factor.

The invention is based on the consideration that during operation of the continuous steam generator a safe and reliable Cooling the evaporator tubes at a particularly low Frictional pressure loss is ensured by two yourself principally conflicting conditions in a suitable manner be fulfilled. On the one hand is the average mass flow density to be chosen as low as possible in the evaporator tubes. Thereby can be achieved that individual evaporator tubes, the more heat due to unavoidable heating differences is fed from a higher than other evaporator tubes Mass flow are flowed through, as an average heated Evaporator tubes. This natural circulation characteristic known from the drum kettle leads at the outlet of the evaporator tubes to an equalization of the steam temperature and thus the Pipe wall temperatures.

On the other hand, the mass flow density in the pipes is such high to choose that ensures safe cooling of the pipe wall and permissible material temperatures are not exceeded become. This will result in high local overheating of the pipe material and the resulting damage (Pipe ripper) avoided. Significant influencing factors for the Material temperature are in addition to the temperature of the flow medium the external heating of the pipe wall and the heat transfer from the inner tube wall to the flow medium or fluid. There is a connection between the internal heat transfer, which is influenced by the mass flow density, and the external heating of the pipe wall.

Taking these boundary conditions into account, the above results Relationship a particularly favorable mass flow density in the evaporator tubes, which both have a favorable flow characteristic (Natural circulation characteristics) as well as a safe one Cooling of the evaporator tubes and thus compliance with the permissible Guaranteed material temperatures. As a criterion when determining a particularly favorable mass flow density enters into a predetermined external heating the material temperature of the tube wall on the one hand only marginally, but certainly should be below the permissible value. Here is the physical appearance to note that in the critical Pressure range from about 200 to 221 bar the heat transfer from most unfavorable is. The result of extensive investigations is making sure that the highest material stress is then reached when in the evaporation area at about 200 to 221 bar a relatively low mass flow density with the largest occurring heat flow density is combined. This is for example in that area of the combustion chamber Case where the burners are arranged. If after that Vaporization has ended and steam superheating begins, the material stress on the evaporator tubes decreases Combustion chamber wall again. The reason for this is that with usual Burner arrangement and usual combustion process too the heat flow density decreases.

In order to determine a particularly favorable control value for the mass flow density m, it is expedient for the permissible maximum temperature T _max according to the relationship T Max = T crit + 6σ / (β • E) determined value used. T _{crit is} the temperature of the flow medium at critical pressure in ° C. Furthermore, σ mean the permissible stress in N / mm ² , β 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 _max , it is assumed that the enclosure or combustion chamber wall of the continuous steam generator has an average temperature which corresponds to the average of the permissible maximum temperature T _max and the temperature of the flow medium at critical pressure T _crit . The maximum thermal stress that occurs is calculated from this σ Max = T Max -T crit 2nd β • E.

When designing the continuous steam generator in accordance with the ASME code, this maximum occurring thermal stress should be secured with three times the value of the permissible stress σ for the pipe material. This immediately results in the value to be taken as the basis for the permissible maximum temperature T _max .

From these design principles it follows that when a continuous steam generator is operated, the evaporator tubes of which are made of 13 CrMo44 material, a value of approximately T _max = 515 ° C. is expediently used as the basis for the permissible maximum temperature T _max . When operating a continuous steam generator, on the other hand, whose evaporator tubes are made from the material HCM12, a value of approximately T _max = 590 ° C. is advantageously used as the permissible maximum temperature T _max .

d

der Außendurchmesser der Verdampferrohre in Metern,

s

die Rohrwandstärke der Verdampferrohre in Metern,

T_max

die für das Rohrmaterial charakteristische zulässige Maximaltemperatur in °C und

q

die Wärmestromdichte in kW/m² ist, wobei der einzusetzende Wert für die Wärmestromdichte das Produkt aus der mittleren Wärmestromdichte mit einem Sicherheitsfaktor.

With regard to the continuous steam generator which is particularly suitable for carrying out this method, the stated object is achieved by the continuous steam generator having a heat flow density q acting on the evaporator tubes for a mass flow density m in accordance with the relationship m = 200 + 8.42 • 10 12th • q 3rd • [d / (d - 2s)] s 2nd • T Max -5 is designed. Here is

d

the outer diameter of the evaporator tubes in meters,

s

the tube wall thickness of the evaporator tubes in meters,

T _max

the permissible maximum temperature in ° C and characteristic of the pipe material

q

is the heat flow density in kW / m ² , the value to be used for the heat flow density being the product of the average heat flow density with a safety factor.

Figur 1

in vereinfachter Darstellung einen Durchlaufdampferzeuger mit vertikal angeordneten Verdampferrohren,

Figur 2

im Querschnitt ein einzelnes Verdampferrohr, und

Figur 3

ein Diagramm mit Kennlinien A und B für die Massenstromdichte in Abhängigkeit von der Wärmestromdichte für Verdampferrohre.

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

Figure 1

in a simplified representation, a once-through steam generator with vertically arranged evaporator tubes,

Figure 2

in cross section a single evaporator tube, and

Figure 3

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 in all figures with the provided with the same reference numerals.

A continuous steam generator 2 is shown schematically in FIG e.g. shown rectangular cross section, its vertical Throttle cable is surrounded by a surrounding wall 4 and a combustion chamber forms the funnel-shaped at the lower end Floor 6 merges. The bottom 6 includes a not shown Discharge opening 8 for ashes.

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

When the continuous steam generator is in operation, it is located in the combustion chamber 2 one when burning a fossil Fuel arising flame body 17, so that the Area A of the continuous steam generator 2 by a very high Heat flux density q distinguishes. The flame body 17 has a temperature profile that, starting from about the middle the combustion chamber, both vertically and upwards down as well as horizontally to the sides, d. H. towards the corners of the combustion chamber. Above the lower one Area A of the throttle cable is a second flame away Area B, above which a third upper area C the throttle cable is provided. In areas B and C of the throttle cable convection heating surfaces 18, 20 and 22 are arranged. A flue gas outlet channel is located above area C of the gas flue 24, about which the burning of the fossil fuel flue gas RG generated the vertical gas flue leaves. The relationships shown in Figure 1 for a pass-through steam generator 2 in a pull-in design apply in a comparable way Way for a once-through steam generator in Two-pass design.

FIG. 2 shows a rib 26 on the inside Evaporator tube 12, which during the operation of the once-through steam generator 2 on the outside inside the combustion chamber exposed to heating with the heat flow density q and is flowed through by the flow medium S. As a flow medium S serves, for example, water or a water-steam mixture.

At the critical point, ie at critical pressure p _crit of 221 bar, the temperature of the fluid or flow medium S in the evaporator tube 12 is designated T _crit . For the calculation of the maximum thermal stress σ _max , the maximum permissible material temperature T _max at the pipe apex 28 of the heated side of the pipe wall is used.

The inner diameter and the outer diameter of the evaporator tube 12 are denoted by d _i and d, respectively. In the case of an internally finned evaporator tube 12, the equivalent inner diameter is to be used as the inner diameter d _i , which takes into account the influence of the fin heights and valleys. The equivalent inside diameter is the inside diameter that a smooth pipe with the same flow cross-section would have. 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 12th • q 3rd • [d / (d - 2s)] s 2nd • T Max -5 is held. The mass flow density m in kg / m ² • s and the permissible maximum temperature T _max in ° C must be used. The outer pipe diameter d and the pipe wall thickness s must also be used in meters. A value with a safety margin is to be used as the heat flow density q on the outside of the pipe in 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, firing capacity, 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 formed by multiplying the maximum heat flow density by a further safety factor of 1.5. 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 which can be determined from the technical data of the continuous steam generator 2 .

Depending on the heat flow density q, the design criterion for the continuous steam generator 2 is a characteristic value for the mass flow density m, as is shown graphically in FIG. 3 for different pipe geometries and different pipe materials. The characteristic curve A describes the mass flow density in kg / m ² s that is associated with a geometry parameter [d / (d-2s)] s 2nd from 4 • 10 -5 m 2nd for a permissible maximum temperature T _max of 590 ° C. The value of approximately 590 ° C. on which the maximum temperature T _{max 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 whose evaporator tubes 12 have a geometry parameter [d / (d-2s)] s 2nd of 10 -4 m 2nd and have a permissible maximum temperature T _max of about 515 C. The permissible maximum temperature T _max of 515 C is relevant for evaporator tubes 12 made of 13 CrMo44 material.

In general, for any evaporator tube 12, the maximum permissible temperature T _{max is} one according to the relationship T Max = T crit + 6σ / (β • E) determined value used. Here are: T _crit the temperature of the flow medium S at critical pressure p _crit in ° C, σ the permissible tension of the material of the evaporator tube 12 in N / mm ² , β the thermal expansion coefficient 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

2nd

Continuous steam generator

4th

Perimeter wall

6

ground

8th

Discharge opening

10th

burner

12th

Evaporator tube

14

Fins

16

Evaporator heating surface

17th

Flame body

18, 20, 22

Convection heating surfaces

24th

Flue gas outlet channel

26

Ribs

28

Pipe crest

β

coefficient of thermal expansion

σ

allowable voltage

σ _max

Thermal stress

A

lower area d. Throttle cable

B

area far from the flame d. Throttle cable

C.

third upper area d. Throttle cable

d, di

Foreign u. Inner diameter of the evaporator tube

m

Mass flow density

q

Heat flux density

s

Pipe wall thickness

s

Flow medium

T _max

Maximum temperature

Claims (7)

1. Method of operating a once-through steam generator (2) having a combustion chamber whose containing wall (4) is formed from vertically arranged evaporator tubes (12) welded to one another in a gas-tight manner, in which method a flow medium (S) flows through the evaporator tubes (12), in which case the mass flow density m of the flow medium (S) for evaporator tubes (12) having a tube outside diameter d and a tube wall thickness s and an admissible maximum temperature (T_max) characteristic of the tube material, as a function of the heat flow density q acting on the evaporator tubes (12), is kept approximately at a regulated value according to the equation m [kg/m2s] = 200 + 8.42 • 1012 • q3 • [d/(d - 2s)] • Tmax -5 where

   d

   is the outside diameter of the evaporator tubes in metres,

   s

   is the tube wall thickness of the evaporator tubes in metres,

   T_max

   is the admissible maximum temperature in °C characteristic of the tube material, and

   q

   is the heat flow density in kW/m²,

   where the value to be used for the heat flow density is the product of the maximum heat flow density and a safety factor.
2. Method according to Claim 1, in which a value determined according to the equation Tmax = Tcrit + 6σ(β • E) is taken as a basis for the admissible maximum temperature (T_max), where (T_crit) (°C) is the temperature of the flow medium (S) at critical pressure (p_crit), σ (N/mm²) is the admissible stress, β (1/K) is the coefficient of thermal expansion and E (N/mm²) is the modulus of elasticity of the material of the evaporator tubes (12).
3. Method according to Claim 1 or 2, the evaporator tubes (12) being made of the material 13 CrMo 44, a value of approximately T_max = 515° C being taken as a basis for the admissible maximum temperature.
4. Method according to Claim 1 or 2, the evaporator tubes (12) being made of the material HCM 12, a value of approximately T_max = 590° C being taken as a basis for the admissible maximum temperature.
5. Once-through steam generator (2) having a combustion chamber whose containing wall (4) is formed from vertically arranged evaporator tubes (12) welded to one another in a gas-tight manner and having a tube outside diameter d and a tube wall thickness s and an admissible maximum temperature (T_max) characteristic of the tube material, in which case a flow medium (S) can flow through the steam generator tubes (12), and the steam generator tubes (12) have a surface structure on their inside and, at a heat flow density q acting on the evaporator tubes (12), which generator is designed for a mass flow density m according to the equation m [kg/m2s] = 200 + 8.42 • 1012 • q3 • [d/(d - 2s)] • Tmax -5 where

   d

   is the outside diameter of the evaporator tubes in metres,

   s

   is the tube wall thickness of the evaporator tubes in metres,

   T_max

   is the admissible maximum temperature in °C characteristic of the tube material, and

   q

   is the heat flow density in kW/m²,

   where the value to be used for the heat flow density is the product of the maximum heat flow density and a safety factor.
6. Once-through steam generator (2) according to Claim 5, the evaporator tubes (12) of which are made of the material 13CrMo 44, a value of 515° C being taken as a basis for the admissible maximum temperature (T_max).
7. Once-through steam generator (2) according to Claim 5, the evaporator tubes (12) of which are made of the material HCM 12, a value of 590° C being taken as a basis for the admissible maximum temperature (T_max).

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