KR20090005145A - A component for supercritical water oxidation plants, made of an austenitic stainless steel alloy - Google Patents

Abstract

The present invention relates to a component with improved corrosion resistance for use in supercritical water oxidation plants. The component is made of an austenitic stainless steel alloy comprising 15-30 % Cr and 20-35 % Ni.

Description

Components for Supercritical Water Oxidation Plants Made of Austenitic Stainless Steel Alloys {A COMPONENT FOR SUPERCRITICAL WATER OXIDATION PLANTS, MADE OF AN AUSTENITIC STAINLESS STEEL ALLOY}

The present invention relates to components of austenitic stainless steel alloys for plants configured to effect hydrothermal oxidation, more particularly supercritical water oxidation (SCWO). This oxidation has received a lot of potential attention as a method of removing a large number of wastes, especially for environmental reasons.

When water is pressurized to at least 221 bar and heated to a temperature above 374 ° C., as long as the organics and gases therein are completely dissolved, their physical properties change drastically and enter a supercritical state that eliminates mass transfer constraints. When oxygen is added to the organic constituents under these conditions, very rapid and efficient destruction reactions are carried out. In fact, 99.999% destruction efficiency can be achieved in a matter of seconds, regardless of the nature of the species of the organic matter.

Various plants have been developed in recent decades to make the SCWO process industrially feasible. Although it is possible to change the design of such a plant, the process in the plant is basically carried out by the step of introducing waste water from the storage tank, ie water sludge containing organic and inorganic components, into the reaction vessel by means of a high pressure pump, The water pressure in the reaction vessel is elevated to, for example, 250 bar, or at least at a critical level of 221 bar. The water is also preheated by a heater and a coal mill prior to entering the reaction vessel, and more particularly preheated above a critical temperature of about 400 ° C, ie 374 ° C. Oxygen from the other tank is pumped through the evaporator into the reaction vessel so that oxidation takes place directly in the reaction vessel. After that, the organic substance is dissolved while generating heat by an automatic heat process and further raising the temperature to 550 to 600 ° C. This process is carried out even if the organic content in the wastewater is low (3-6%), which means that extra heat is always available to heat the wastewater passing through the coal mill to the reactor. The plant also includes devices for treating the treated water phase exiting the reactor, for example steam boilers, coolers, pressure reducing devices, gas / liquid separators and the like. All of these components are interconnected by various piping and are also controlled by other components such as valves, accumulators, pressure reducing devices, fluid vibrators, injectors, nozzles, filters, traps and the like.

Indeed, the SCWO process can be used to neutralize almost unlimited waste. Thus, municipal sludge can be treated while completely destroying the organic matter therein. Thereafter, the inorganic substances, which are non-hazardous substances in the discharge, can be buried or used as raw materials for regeneration. Wastes containing valuable inorganic materials may be used differently. Organic contaminants are completely destroyed, leaving behind a pure mineral phase that can be recycled. An example of such an application is to de-inking paper sludge while regenerating paper fillers and also to treat waste catalysts while regenerating precious metals. It is also possible to remove the active aid additives from the waste water, and the halogenated waste is destroyed so as not to form dangerous byproducts such as dioxins. As an alternative to incineration, the SCWO process is of particular interest because it can destroy wastes containing nitrogen without forming NOx.

In any SCWO plant, the environment is generally very strict. In particular, the environment in the reaction vessel and the piping to which it is connected can be corrosive, and many species are very aggressive against materials of different components. For example, there are acids such as nitric acid, sulfuric acid and hydrochloric acid, and these acids have strong corrosiveness even in the range of 270 to 380 ° C. Thus, any surface in contact with aggressive and corrosive species is at risk of being corroded or otherwise degraded in a short time.

In order to solve this problem caused by aggressive conditions in SCWO plants, the choice of materials for the different components of the plant is important but difficult. Currently, the common constituent material is Alloy 625, which is used in plant devices such as heaters, coal mills, reactors, steam boilers, and evaporators, as well as single components such as tubes and plates. The main reasons for using Alloy 625 are to withstand high pressures (250 bar) and high temperatures (600 ° C.), and also to have adequate corrosion resistance of the process fluid in the temperature range of, for example, 270 to 380 ° C., which is such devices and components. Means that an acceptable lifetime can be obtained. However, a serious disadvantage of high alloyed nickel-based grades such as Alloy 625 is that they are very expensive due to the high content of nickel and molybdenum, resulting in high investment costs for plant installation. Another austenitic stainless steel alloy similar to Alloy 625 from the high nickel content is C 276. Both of these grades contain at least 60% nickel.

To reduce the cost of the constituent materials, we have attempted to use low alloy grades, such as 304 L and 316 L, which contain very suitable amounts of nickel (8-15%) and chromium (18-20%) in SCWO plants. As a result, it was cheaper than the high alloy grade. However, it was found that both 304L and 316L did not withstand the high corrosiveness of the fluid in the region just below the high pressure (221 bar) and the supercritical temperature (374 ° C).

In this patent document, the use of nickel-based and high-alloy stainless steel alloys in SCWO plants is described, for example, in US Pat. Nos. 6,958,122 (Inconel and Hastelloy) and US Pat. No. 5,804,066 (Inconel). The use of alloy 316 L is described in US Pat. No. 5,823,220.

For the sake of completeness, the use of ceramics and / or cermets as a lining and coating of components in general in SCWO plants is also proposed in US Pat. Nos. 5,358,645, 5,461,648, and 5,545,337. Although ceramics have considerable corrosion resistance, they limit the degree of freedom of construction design to a great extent, which means that the various structures of the plant cannot be best implemented, for example in mechanical strength, weldability, functionality, and the like.

Despite the fact that supercritical water oxidation is in many respects, for example, environmentally superior and the ability to safely handle hazardous wastes, the disadvantages of the known constituent materials described above are attractive for traditional methods of waste destruction such as incineration. It would hinder the improvement and commercialization of the SCWO technique as an alternative. Thus, there is still a need for construction materials that are reasonably inexpensive and nevertheless are suitable for corrosion resistance, mechanical strength, temperature strength, weldability and machinability.

25 의 등급 형태의 오스테나이트계 스테인리스 강을 제공한다. 그리하여, SANDVIK SANICRO

25 는 아주 적절한 함량의 고가 성분 (무엇보다도 니켈) 을 포함하여 Alloy 625 및 Alloy C 276 보다 생산 비용이 적다는 사실에도 불구하고, 특히 상기 특허문헌에 기재된 합금은 큰 기계적 부하에 성공적으로 대처할 뿐만 아니라 SCWO 환경에서 수용가능한 내부식성을 제공하는 것으로 판명났다.The present invention meets the above requirements and comes in direct contact with a supercritical or near supercritical solution, ie the trade name SANDVIK SANICRO described in EP 1194606.

It provides an austenitic stainless steel in grade 25. Thus, SANDVIK SANICRO

In spite of the fact that 25 is less expensive to produce than Alloy 625 and Alloy C 276, including a very high content of expensive components (most of all nickel), the alloys described in this patent document not only successfully cope with large mechanical loads, It has been found to provide acceptable corrosion resistance in SCWO environments.

25 는, 내부식성 및 수명에 있어서 고합금 등급 A 625 만큼 적어도 양호하며 어떤 면에서는 보다 훨씬 낫다.According to reports of subsequent corrosion tests, SANDVIK SANICRO

25 is at least as good as high alloy grade A 625 in corrosion resistance and service life and in some ways even better.

Austenitic stainless steel alloys according to the present disclosure comprise 20 to 35 weight percent nickel (Ni) and 15 to 30 weight percent chromium (Cr).

In a more preferred embodiment, the alloy comprises 20 to 35 weight percent nickel (Ni); 15-30% chromium (Cr); And 0.5-6.0 wt% copper (Cu).

Indeed, alloys according to the present disclosure may comprise 20 to 35 weight percent nickel (Ni); 15-30% chromium (Cr); 0.5-6.0 wt.% Copper (Cu); 0.01-0.10 weight percent carbon (C); 0.20 to 0.60 weight percent niobium (Nb), 0.4 to 4.0 weight percent tungsten (W); 0.10 to 0.30 wt% nitrogen (N); 0.5 to 3.0 weight percent cobalt (Co); 0.02-0.10 weight percent titanium (Ti); Up to 4.0% molybdenum (Mo); 0.4 wt% or less of silicon (Si); And up to 0.6% by weight manganese (Mn), with the balance being iron and common steelmaking impurities.

1 shows the weight change of two alloys and Alloy 625 according to the present application when exposed to a simulated SCWO environment at 350 ° C. for 125 hours, and

FIG. 2 shows the weight change of two alloys and Alloy 625 according to the present application when exposed to a simulated SCWO environment at 600 ° C. for 125 hours.

The components of the alloy used according to the preferred embodiment of the present application are as follows (all% are by weight).

nickel

Nickel is a major component to ensure a stable austenite structure. The tissue stability depends on the relative amounts of ferrite stabilizers such as chromium, silicon, tungsten, titanium and niobium on the one hand and austenite stabilizers such as nickel, carbon and nitrogen on the other. After prolonged periods at elevated temperature, the nickel content is at least 20%, preferably in order to suppress the formation of sigma phases, especially when high amounts of chromium, tungsten and niobium are required to ensure high temperature corrosion resistance and high creep rupture strength. Should be at least 22.5%. Nickel may also be at least 25%. At certain chromium levels, the nickel content is increased to inhibit oxide growth and also to improve the tendency to form a continuous chromium oxide layer. However, in order to keep material costs at a reasonable level, the nickel content should not exceed 35%, preferably not 32%. In view of this, the nickel content in the alloy is limited to the 20 to 35% range.

chrome

Chromium is effective in improving general corrosion and oxidation resistance. In order to obtain sufficient corrosion and oxidation resistance in this respect, the chromium content is set at least 15%. Preferably at least 20% chromium may be added. However, if the chromium content exceeds 30% and reaches 30%, the nickel content must be further increased to produce a stable austenite structure. Chromium content above 30% requires increasing the nickel content too much (35% or more), which does not guarantee a cost effective composition. For this reason, the chromium content is limited to the range of 15 to 30%, preferably 20 to 27%.

Copper

Copper is added to create fine and uniformly precipitated copper rich phases in the matrix while contributing to improving creep rupture strength. This effect requires at least 0.5% content of copper and significantly improves to about 2% content. Copper is also added to improve the general corrosion resistance to sulfuric acid. However, excessive amounts of copper (6% or more) will reduce workability. In addition, for economic reasons, the Cu content should be kept at an appropriate level, for example 3.5%. In view of this, the copper content is limited to the range of 0.5 to 6%, preferably 2 to 3.5%.

carbon

Carbon is an effective component for providing the proper tensile strength and creep rupture strength required for high temperature steels. However, if too much carbon is added, the toughness of the alloy may be reduced and the weldability may be degraded. In addition, too high a carbon content reduces corrosion resistance in the SCWO environment. For this reason, the carbon content is limited to a maximum of 0.1%. Preferably the carbon content may be 0.04 to 0.08%.

Niobium

Niobium is generally believed to contribute to the improvement of creep rupture strength by precipitation of carbonitrides and nitrides. However, excessive amounts of niobium can reduce weldability and processability. In view of this, the niobium content is limited to the range of 0.20 to 0.60%. Preferably, the niobium content should be 0.33 to 0.50%.

Tungsten and molybdenum

Tungsten is added mainly to improve the high temperature strength by solid solution hardening, and at least 0.4% is required to obtain this effect. Tungsten and molybdenum also contribute to the general corrosion resistance in SCWO environments. However, molybdenum and tungsten promote the formation of sigma phases. In improving the strength, tungsten is believed to be more effective than molybdenum. For these and economic reasons, the molybdenum content is kept low, below 0.5%, preferably below 0.02%. In order to maintain sufficient processability, the tungsten content should not exceed 4%, so that the tungsten content is defined in the range of 0.4-4%, preferably 1.8-3.5%.

nitrogen

It is known that not only carbon but also nitrogen improves strength and creep rupture strength at elevated temperatures, for example 500 ° C. or higher, and also stabilizes the austenite phase. However, excessive addition of nitrogen reduces the toughness and ductility of the alloy. For this reason, the content of nitrogen is limited to the range of 0.10 to 0.30%, preferably 0.20 to 0.25%.

cobalt

Cobalt is an austenite stabilizing element. By adding cobalt, it is possible to suppress the formation of sigma phase after prolonged exposure at elevated temperature and to improve the high temperature strength by strengthening the solid solution. However, in order to keep manufacturing costs at a reasonable level, the cobalt content, if added, should be in the range of 0.5-3.0%.

titanium

Titanium is added to improve creep rupture strength by precipitation of carbonitrides, carbides and nitrides. However, excessive amounts of titanium can reduce weldability and processability. For this reason, the content of titanium, if added, is limited to the range of 0.02 to 0.10%.

As an example of a component or structural member which is made of the steel alloy according to the present application in direct contact with a supercritical or near supercritical solution, there are tubes, plates, bars, rods, strips, foils, linings, blocks, sleeves, Wires, beams, girders, pillars, and webs. All of these components are also (individually or in combination) reactors, oxygen tanks, sludge water tanks, evaporators, coal mills, steam boilers, coolers, gas / liquid separators, as well as various valves, accumulators, pressure reducers, fluid vibrators, injectors. It can be used to construct various instruments and devices included in the finished SCWO plant, such as nozzles, filters and traps. Tubes and plates are simple to manufacture from the aforementioned steel alloys.

With respect to the necessary equipment upstream and downstream of the reactor of the SCWO plant, by using the components according to the invention, i.e. the components made of steel alloys as described above, the cost of the high alloy grades such as Alloy 625, It is expected to reduce the cost of materials associated with installation by approximately 25-40%. Therefore, the present invention makes a positive contribution to the use of the SCWO technique as a method of eliminating organic waste in order to be improved later and not harmful to the environment.

Test report

Three different superalloys were tested to determine corrosion resistance under near critical and supercritical solution conditions. The superalloy was exposed for 125 hours at a pressure of 29 MPa and at temperatures of 350 ° C. and 600 ° C., respectively. In order to simulate a harsh SCWO environment, the solution contained chloride ions and oxygen. The compositions of the tested alloys are listed in Table 1 and the experimental conditions are summarized in Table 2. The two runs differ only in the temperature at which they are applied, and so also in the density of the fluid.

Square coupons were cut out of the alloy, then ground (80-100 mesh) and ground (9-0.25 μm diamond). This coupon was weighed before and after exposure to the experimental conditions described above. The surface layer formed during the experiment was examined by field emission scanning electron microscopy (SEM) and X-ray microanalysis (EDX). Other tests of coupons were made with an optical microscope. Corrosion attack was evaluated through microscopic observation.

Results for 350 ℃

The coupons tested were washed with rectified water and acetone and finally blow dried to perform later back-weighing and other testing steps. This test resulted in significant material loss resulting from weight change. The weight and dimensions before exposure and the weight change after execution at 350 ° C. are shown in Table 3. The average value is shown in FIG.

The coupon was photographed by scanning electron microscopy (SEM), and the elemental composition of the surface layer was analyzed by energy dispersive X-ray spectroscopy (EDX). As a result of micro analysis of the surface, the composition of the scale was mainly composed of an oxide and a small amount of chlorine, and the results are shown in Table 4.

Results for 600 ℃

No significant corrosion attack was observed in the sample after exposure. Only a thin layer of oxide remained on the surface of the specimen and showed a slight weight increase. The weight and dimensions before exposure and the weight change after running at 600 ° C. are shown in Table 5. The average value is shown in FIG.

The coupon was photographed by scanning electron microscopy (SEM), and the elemental composition of the surface layer was analyzed by energy dispersive X-ray spectroscopy (EDX). EDX analysis confirmed that a thin layer of oxidizing composition was formed. EDX results are summarized in Table 6.

The component according to the present invention provides mechanical strength comparable to the constituent materials of SCWO plants that are commonly used, with improved or comparable corrosion resistance when the component is in direct contact with a supercritical or near supercritical solution.

Claims (14)

A component for a supercritical water oxide plant, made of an austenitic stainless steel alloy, wherein the austenitic stainless steel alloy is 15 to 30 wt% chromium (Cr); 20 to 35 weight percent nickel (Ni), The component is in direct contact with the supercritical or near supercritical solution. The method of claim 1, wherein the alloy, 15 to 30 wt% chromium (Cr), 20 to 35 weight percent nickel (Ni), and Component comprising 0.5 to 6.0 wt% copper (Cu). The component of claim 1, wherein the alloy further comprises 0.01 to 0.10 wt% carbon (C). The component of claim 1, wherein the alloy further comprises up to 4.0 wt% molybdenum (Mo). The component of claim 1, wherein the alloy further comprises 0.2-0.6 weight percent niobium (Nb). The component of claim 1, wherein the alloy comprises 0.4 to 4.0 wt% tungsten (W). The component of claim 1, wherein the alloy comprises 0.10 to 0.30 wt% nitrogen (N). The component of claim 1, wherein the alloy further comprises 0.5 to 3 weight percent cobalt (Co). The component of claim 1, wherein the alloy comprises 0.02 to 0.10 wt% titanium (Ti). The component according to claim 1, in the form of plates, tubes, bars, rods, strips, foils, linings, sleeves, blocks, wires, beams, girders, pillars and webs. The method of claim 10, wherein the reactor, oxygen tank, sludge water tank, evaporator, coal mill, steam boiler, cooler, gas / liquid separator, valve, accumulator, pressure reducing device, fluid vibrator, separator, nozzle, filter and trap Components that are at least part of them. Use of austenitic stainless steel alloys containing 15 to 30 wt% Cr and 20 to 35 wt% Ni in a supercritical water oxidation plant. 13. The use of an austenitic stainless steel alloy as claimed in claim 12, wherein the austenitic stainless steel alloy is in direct contact with a supercritical or near supercritical solution. The austenitic stainless steel alloy according to claim 12 or 13, 20 to 35 weight percent nickel (Ni); 15-30% chromium (Cr); 0.5-6.0 wt.% Copper (Cu); 0.01-0.10 weight percent carbon (C); 0.20 to 0.60 weight percent niobium (Nb), 0.4 to 4.0 weight percent tungsten (W); 0.10 to 0.30 wt% nitrogen (N); 0.5 to 3.0 weight percent cobalt (Co); 0.02-0.10 weight percent titanium (Ti); Up to 4.0% molybdenum (Mo); 0.4 wt% or less of silicon (Si); And up to 0.6% by weight manganese (Mn), with the balance being iron and common steelmaking impurities.

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