WO2016116227A1

WO2016116227A1 - Tubular element consisting of austenitic steel, and solar receiver

Info

Publication number: WO2016116227A1
Application number: PCT/EP2015/079841
Authority: WO
Inventors: Meike SCHMIDT; Markus Arntzen; Sebastian Dreyer; Thomas Kuckelkorn
Original assignee: Schott Ag
Priority date: 2015-01-21
Filing date: 2015-12-15
Publication date: 2016-07-28
Also published as: DE102015200881A1

Abstract

The invention relates to a tubular element consisting of austenitic steel for molten salt, in particular a solar absorber tube of a solar receiver containing molten salt as a heat transfer medium, or another conduit for conveying molten salt. The invention also relates to a solar receiver of this type.

Description

Tubular austenitic steel body and solar receiver

description

The present invention relates to a tubular body made of steel, in particular austenitic steel, for a molten salt, in particular an absorber tube of a solar receiver with a molten salt as heat transfer medium or another pipeline for conveying a molten salt, and such a solar receiver.

The power generation from solar radiation is u.a. possible with concentrating solar thermal power plants. Here, a distinction is made between centrally concentrating systems such as, for example, tower power plants or linearly concentrating systems based, for example, on Fresnel technology or parabolic trough technology.

Solar receivers, for example for parabolic trough solar receiver systems, for example for solar thermal power plants, include, inter alia, a steel tube as absorber tube with a radiation-selective absorber layer and a glass tube as a cladding tube.

Pipelines through which a molten salt flows are to be used, for example, in solar power plants, in particular parabolic trough or Fresnel solar power plants.

In such a solar power plant, the radiant energy of the sun is concentrated on the receiver with the help of parabolic mirrors or Fresnel mirrors. A number of receivers are connected in series. The absorbed by the absorber tube of the receiver radiation energy is converted into heat, on a heat transfer fluid in the absorber tube and transported by the heat transfer fluid to a steam generator. Currently, synthetic oils, in particular a biphenyl / diphenyl ether mixture, are used as heat transfer fluid. The decomposition temperature of this

However, mixture of 400 ° C gives the maximum operating temperature of the

Power plant before.

In order to make higher operating temperatures possible, which allow a higher efficiency of the solar thermal power plant, other heat transfer fluids are required.

It would be advantageous, a temperature of about 535 ° C at the turbine of

It would be possible to connect the solar thermal technology to the components and systems used today in conventional power plants. Due to the losses in the heat exchanger for a temperature of the heat transfer fluid of about 550 ° C is required. Due to the temperature gradients in the wall of the absorber tube of the receiver, a temperature of approx. 580 ° C on the surface of the absorber tube is required.

There are two possible approaches to this, firstly the direct evaporation of water in the receiver, the so-called direct evaporation technology, and secondly the so-called Molten Salt technology. In the latter technology, molten salts, in particular nitrate-based molten salts, e.g. a mixture of sodium nitrate and potassium nitrate in the ratio of 60 to 40, used.

Molten Salt technology has the advantage over direct evaporation technology that, although salt melts have not yet been widely used as a heat transfer medium, they are already used as a heat storage medium, for example in combination with solar thermal power plants with oil

Heat transfer fluid. The Molten-Salt technology, in which heat transfer fluid and heat storage medium are identical, has the advantage over the technology with oil as heat transfer fluid and salt as heat storage medium that no lossy heat transfer between heat transfer fluid and

Heat storage medium must be made.

The hitherto for absorber tubes in solar receivers and / or for other

Pipes used for solar thermal power plants are, for example, the steels with the material numbers 1 .4404, 1 .4571 and

1 .4541, but also 1 .4301. These are austenitic stainless steels, which were developed primarily with the aim of achieving a high

Have corrosion resistance.

As in solar thermal power plants, the materials are mechanically and thermally stressed over a very long period of time, is at

Operating temperatures above 400 ° C, in addition to consider the creep behavior of the steels for a design period of 200,000 hours.

At higher temperatures, in particular, there is insufficient creep rupture strength in the steels mentioned above, since the steel in the

Creeping occurs because a austenitic structure consolidating moment is missing. Heat-resistant austenitic steels such as 1 .4941 and 1 .4910 have

In contrast, a sufficient creep behavior.

A disadvantage of all the steels mentioned above is that they are not sufficiently resistant to sensitization to intergranular corrosion. Only when used below 400 ° C, these steels do not change within 100,000 hours so that sensitivity to intercrystalline corrosion is detected when tested according to EN ISO 3651-2.

According to DIN EN 10216-5, a steel with the material number 1 .4910 has the following composition on a weight basis:

up to 0.04% C; 0.10% to 0.18% N; 2.00% to 3.00% Mo; up to 0.75% Si; up to 2.00% Mn; up to 0.035% P; up to 0.015% S; 16.00% to 18.00% Cr; 12.00% to 14.00% Ni; 0.0015% to 0.0050% B. According to DIN EN 10216-5, a steel with the material number 1 .4941 has the following composition on a weight basis:

0.04% to 0.08% C; up to 1, 00% Si; up to 2.00% Mn; up to 0.035% P; up to 0.015% S; 17.00% to 19.00% Cr; 9.00% to 12.00% Ni; From 0.0015% to 0.0050% B; with a Ti content of 5 × C to 0.80.

From US 2012/0279607 A, a steel of a broad base composition is known, although it is used for high-temperature applications but has poor toughness and, when an absorber layer is applied to it, rapidly degrades it at high temperature.

The invention is therefore based on the object, tubular body

To provide materials which are suitable for receiving and transporting hot and oxidizing liquids and in use

molten salts required as heat transfer fluids high

Have temperature resistance. Make the tubular body

Absorber tubes for solar receivers, they should also allow good adhesion and resistance to degradation of the absorber layer.

Such a requirement profile also includes sufficient

Creep behavior at the corresponding temperatures; So must at temperatures up to 580 ° C sufficient creep over a

Guaranteed period of 200,000 hours.

In addition, the steel used for the tubular body should be inexpensive and have high strength to allow a small wall thickness and thus low weight of the tubular body.

This object is achieved with the tubular body according to the main claim.

It is made of austenitic steel. Austenitic steels are non-magnetic steels, because of their

Alloy components chromium, manganese and / or nickel also at

Room temperature, the cubic-face centered space grid of austenite maintained.

The steel composition of the tubular body according to the invention comprises on a weight basis

0% to 0.025% C, preferably 0.0095% to 0.024% C;

0.05% to 0.16% N;

2.4% to 2.6% Mo;

0.4% to 0.7% Si;

0.5% to 1, 63% Mn;

0% to 0.0375% P;

0% to 0.0024% S;

17.15% to 18.0% Cr;

12.0% to 12.74% Ni;

0.0025% to 0.0045% B

and contains as balance Fe and possibly common impurities.

The contents of the possible impurities Ti, Cu, Al, Nb, Co are, if present at all, preferably not more than

up to 0.03% Ti;

to 0.35% Cu;

up to 0.007% AI;

to 0.0164% Nb;

to 0.164% Co.

A preferred steel composition of the tubular body comprises on a weight basis

0.015% to 0.024% C, preferably 0.020% C;

0.05% to 0.16% N; 2.4% to 2.6% Mo;

0.43% to 0.48% Si;

1, 55% to 1, 61% Mn,

0.025% to 0.0375% P;

0.00095% to 0.0024% S, preferably 0.00095% to 0.0014% S;

17.15% to 18.0% Cr;

12.0% to 12.74% Ni;

From 0.0025% to 0.0045% B;

0% to 0.03% Ti;

0% to 0.35% Cu, preferably 0% to <0.34% Cu;

0% to 0.007% Al, preferably 0% to <0.006 Al;

0.0155% to 0.0164% Nb, preferably 0.0155% to <0.0164% Nb;

0.155% to 0.164% Co, preferably 0.155% to <0.164% Co;

the remainder being Fe and optionally further customary impurities.

Another preferred steel composition of the tubular body comprises on a weight basis

0.01% to 0.02% C;

0.12% to 0.16% N;

2.4% to 2.6% Mo;

0.4% to 0.7% Si;

0.5% to 1.0% Mn;

From 0.03% to 0.035% P;

0% to 0.0020% S;

17.5% to 18.0% Cr;

12.0% to 12.3% Ni;

From 0.0025% to 0.0045% B;

0% to 0.03% Ti;

the remainder being Fe and optionally further customary impurities. In a preferred variant within this composition range, the Mn content is 0.8% to 1.0%. This content is preferably combined with a low C content, in particular of about 0.01%, for example from 0.010% to 0.01 1%, and / or a high N content, in particular of about 0.16%, for example from 0.155% to 0.160%, and / or a high Cr content, in particular from about 18%, for example from 17.9% to 18.0%.

In another preferred variant within this

Composition range, the Mn content is even lower, namely 0.5 to 0.8%. Also, this content is preferably combined with a low C content, in particular about 0.01%, e.g. from 0.010% to 0.01 1%, and / or a high N content, especially about 0.16%, e.g. from 0.155% to 0.160%, and / or a high Cr content, especially about 18%, e.g. from 17.9% to 18.0%. The lower the Mn content, the more favorable this is for the aging resistance of the absorber layer, in particular if it is a sputtered absorber layer.

Carbon is a strong Austenitbildner and increases with increasing content strength. The content of carbon, if any, should be low, as indicated by the upper limit and preferred upper limits, to minimize the risk of intergranular corrosion sensitization due to chromium carbides.

The presence of nitrogen in the amounts mentioned and in the amounts mentioned as preferred advantageously increases the creep resistance, serves

Improvement of the creep rupture strength and improves the resistance to intergranular corrosion sensitization.

The silicon content in the above ranges improves the

Corrosion resistance to molten salts. Manganese is an austenite former. The content of manganese should be low, and less, and in preferred embodiments, significantly lower than the standard composition of 1 .4910 permits. Thus, premature degradation of the absorber layer on the absorber tube is counteracted.

The manganese content and the boron content in said range contribute to

Solidification by hindering the formation of precipitates at the grain boundaries and increasing the creep rupture strength.

Boron in the stated minimum amount not only increases the creep rupture strength but also the heat resistance. However, the content of boron is limited to the stated upper limit, since at higher contents the corrosion resistance would be reduced.

The content of the Austenitbildners chrome should be relatively high, namely at the upper limit of the standard composition of .49 0, since it improves the general corrosion resistance and in particular the steel is more resistant to Nitratsalzschmelzen.

The presence of molybdenum advantageously increases creep rupture strength and corrosion resistance. However, the molybdenum content should remain low as indicated by the upper limit to ensure a cost-effective steel. The molybdenum content therefore varies within the composition range according to the invention only within very narrow limits.

Also, the content of the austenite former nickel should remain low, as indicated by the upper limit and the preferred upper limit

to ensure cost-effective steel.

Phosphorus and sulfur may be present. Their salary is limited to the mentioned upper limits and preferred upper limits to the negative Prevent influence of these alloying elements on the ductility of the steel as far as possible.

This alloyed with molybdenum and nitrogen steel of the composition of the invention has proved to be very resistant in molten salt and shows in particular compared to non-molybdenum and nitrogen alloyed steels a lower corrosion.

Thus, a piece of pipe of the below-mentioned composition A1 with a wall thickness of 2.0 mm after aging in molten sodium potassium nitrate salts at 550 ° C after 4,000 hours in turbulent flow a Wandstärkenabtrag of 10 μιτι, which extrapolated to 200,000 hours a Wandstärkenabtrag of 0 , 5 mm corresponds.

In contrast, showed a piece of pipe of the non-inventive

composition

0.043% C;

0.55% Si;

0.82% Mn;

0.022% P;

0.001% S;

17.01% Cr;

9% Ni;

0.0029% B;

0.34% Ti;

Balance Fe and possibly other common impurities;

So a composition of the material number .494, with a

Wall thickness of 2.0 mm after the same treatment after 4,000 hours a wall thickness of 15 μιτι, which extrapolated to 200,000 hours corresponds to a wall thickness of 0.75 mm. A piece of pipe made of steel according to the invention thus shows a 33% increase

lower wall thickness loss than a pipe section from 1 .4941.

Also because of the also better creep at 580 ° C after

200,000 hours of the steel according to the invention of 151 MPa compared to 1 10 MPa of 1 .4941, the wall thickness of the tubular body according to the invention can be designed to be 30% lower than that of a comparable tubular body from 1 .4941. This brings not only cost, but also weight advantages.

This austenitic steel composition, the components of which vary only within very narrow limits, combines in a particularly advantageous manner

mechanical properties of a heat-resistant steel. At the same time it meets the requirements, resistant to molten nitrate salts and external

To be weathering.

The invention particularly modifies and further develops the composition of 1 .4910 in that, in addition to the properties known to it, excellent resistance to salt melts has been demonstrated. In the swelling test in the liquid salt at 550 ° C., the steel according to the invention showed no susceptibility after 4,000 hours

Sensitization, whereas the Type 1 .4941 steel was already sensitized after 2,000 hours.

Possible impurities depend on those in steelmaking

used starting materials. Common contaminants are

One skilled in the art and may be beyond the explicitly mentioned minor constituents such as P, S, Al, Co, Cu, Ti and Nb addition, for example, V, Na, and Mg.

The person skilled in the art knows how to produce a steel of the stated composition range in the usual way and to process it into a steel tube. Usually, after the processing and processing steps

Melting of a steel with a composition in said

Composition range and from this first by hot rolling, then by cold rolling production of coils in the desired thickness band or sheet formed cold and then by fusion welding for

welded final pipe. After this processing step, a solution annealing is carried out in order to at least partially reverse the microstructure change, in particular chromium carbide formation, during cold working and welding. Alternatively, a seamless tube, e.g. be produced by extrusion. Even a seamless tube is solution annealed, for example, to undo the above-mentioned chromium carbide formation at least partially.

The invention will be explained by way of example with reference to the following figures. Show it

Figure 1 is a solar receiver end with absorber tube

Figure 2 is a pipeline

In detail:

In Fig. 1, one end of a solar receiver 1 is shown schematically in section. The solar receiver 1 has an existing cladding tube 2 and an arranged in the cladding tube 2 absorber tube 3, which is coated on its outside with radiation-selective coating for absorbing solar radiation. The receiver has a stretch compensation device in the form of a bellows 4. The inner end of the bellows 4 is over

Connection element 5 with the steel tube 2 and the outer end of the bellows 4 via a glass transition element 6 connected to the glass tube 2. The absorber tube 3 consists of a composition of the

claimed composition range. Connecting element 5 is made of stainless steel, preferably of the same steel as the absorber tube 3. The glass-metal transition element 6 is made of Kovar, and the bellows 4 is made of stainless steel. Connection element and inventive steel tube are gas-tight welded together.

Of course, the invention is not limited to this specific embodiment of the transition from the steel tube to the glass tube. Arrangements and connections of the tubes in other shapes and with other materials are possible.

Conventional highly efficient absorber coatings, as known, for example, from DE 10 2006 056 536 B3 or DE 10 2008 010 199 A1, adhere well to the austenitic steel pipe according to the invention. Internal coatings, such as chromium oxide, are possible. Due to the property of the austenitic steel to be non-magnetic, coatings can be well applied by sputtering because the electromagnetic field in the

Sputtering chamber is not affected by the steel. Thus, particularly homogeneous layers are obtained.

FIG. 2 shows the longitudinal section of a pipeline section. Shown is in addition to the tube 7 and the molten salt 8 an Innenheizleiter 9. There are also corresponding pipes with Außenheizleitern instead of a

Inner heating conductor possible.

The invention will be further illustrated by the following embodiment.

A steel tube with the composition A1 on a weight basis

0.02% C; 0.056% N;

0.48% Si;

1, 61% Mn;

0.037% P;

0.001% S;

17.33% Cr;

12.18% Ni;

0.0031% B;

0.001% Ti;

0.34% Cu;

0.006% AI;

2.42% Mo;

0.016% Nb;

0.16% Co;

the remainder being Fe and possibly other common impurities, has the following properties:

_Yield strength at RT: R _p0 , ₂ = 367 MPa; R _p1 , ₀ = 393 MPa

Tensile strength R _m = 620 MPa

Elongation at RT: along 49.7%

The steel according to embodiment A1 showed in the test with moving sample in Nitratsalzschmelze (simulated turbulent flow) at 550 ° C less than 10 μητι material removal in 4000 test hours.

The special requirements for receiver tubes, namely

in terms of their inside

- resistance to molten salts

- Formation of a protective, homogeneous oxide layer

- Resistance to abrasion

and in relation to its outside: - good surface finish, a good adhesion and durable

Apply absorber layer,

are met by the tubular bodies according to the invention.

Thus, the tubular body according to the invention for use as absorber tubes of a solar receiver with a molten salt as heat transfer medium or as other pipelines for promoting a molten salt, so as pressure-carrying pipelines, are outstandingly suitable.

In particular, they withstand the following stresses without damage:

Continuous use at temperatures up to 580 ° C,

cyclic temperature stress,

Compression,

chemical stress due to molten salts,

Outdoor weathering, which means humidity, wetness, heat, cold, salty air,

mechanical load.

They have the following values for the characteristic quantities of the mechanical properties:

_Minimum values of the yield strength at RT: R _p0.2 = 260 MPa, R _p , ₀ = 300 MPa _Minimum values of the yield strength at 400 ° C: R _p0 , 2 = 134 MPa, R _{p | 0} = 164 MPa Minimum values of the yield strength at 550 ° C C: R _p0 , 2 = 124 MPa, R _{p | 0} = 154 MPa Tensile strength: R _m = 550 - 750 MPa

Minimum elongation at RT: longitudinal 35%, transverse 30%

Minimum values of impact energy at RT: longitudinal 120 J, transverse 80 J

Claims

claims

Austenitic steel tubular body for a molten salt, in particular absorber tube of a solar receiver with a molten salt as a heat transfer medium or other pipeline to promote a

Molten salt, with a steel composition comprising by weight:

0% to 0.025% C, preferably 0.0095% to 0.024% C;

0.05% to 0.16% N;

2.4% to 2.6% Mo;

0.4% to 0.7% Si;

0.5% to 1, 63% Mn;

0% to 0.0375% P;

0% to 0.0024% S;

17.15% to 18.0% Cr;

12.0% to 12.74% Ni;

From 0.0025% to 0.0045% B;

and the remainder being Fe and possibly conventional impurities. Tubular body according to claim 1

with a steel composition in addition to weight

0% to 0.03% Ti;

0% to 0.35% Cu;

0% to 0.007% AI;

0% to 0.0164% Nb;

0% to 0.164% Co

includes.

Tubular body according to claim 1 or 2

with a steel composition comprising by weight:

0.015% to 0.024% C, preferably 0.020% C

0.05% to 0.16% N, 2.4% to 2.6% Mo;

0.43% to 0.48% Si;

1, 55% to 1, 61% Mn;

0.025% to 0.0375% P;

0.00095% to 0.0024% S, preferably 0.00095% to 0.0014% S; 17.15% to 18.0% Cr;

12.0% to 12.74% Ni,

From 0.0025% to 0.0045% B;

0% to 0.03% Ti;

0% to 0.35% Cu, preferably 0% to <0.34% Cu;

0% to 0.007% Al, preferably 0% to <0.006 Al; ₇

0.0155% to 0.0164% Nb, preferably 0.0155% to <0.0164% Nb;

0.155% to 0.164% Co, preferably 0.155% to <0.164% Co;

and the remainder being Fe and possibly other common impurities.

4. Tubular body according to claim 1 or 2

with a steel composition comprising by weight:

0.01% to 0.02% C;

0.12% to 0.16% N;

2.4% to 2.6% Mo;

0.4% to 0.7% Si;

0.5% to 1.0% Mn;

From 0.03% to 0.035% P;

0% to 0.0020% S;

17.5% to 18.0% Cr;

12.0% to 12.3% Ni;

From 0.0025% to 0.0045% B;

0% to 0.03% Ti;

the remainder being Fe and optionally further customary impurities.

5. tubular body according to claim 4 characterized in that (on a weight basis)

the Mn content is 0.8% to 1.0%.

Tubular body according to claim 4

characterized in that (on a weight basis)

the Mn content is 0.5% to 0.8%.

Solar receiver with a molten salt as heat carrier with a

Absorber tube of a tubular body according to one of claims 1 to 6.