A COMPONENT FOR SUPERCRITICAL WATER OXIDATION PLANTS, MADE OF AN AUSTENITIC STAINLESS STEEL ALLOY.

Technical Field of the Invention This invention relates to a component of an austenitic stainless steel alloy, for plants designed to carry out hydrothermal oxidation, more specifically Supercritical Water Oxidation (SCWO). Such oxidation is of great potential interest as a method of disposing a number of waste products, especially for environmental reasons.

Background and Prior Art

When water is pressurized to at least 221 bar and heated to a temperature above 374 ^° C, it enters a supercritical state in which the physical properties thereof change dramatically inasmuch as organics and gases become completely soluble therein, thus eliminating mass transfer constraints. If oxygen is added to organic constituents under these conditions, a very rapid and efficient destruction reaction takes place. As a matter of fact a destruction efficiency of 99,999 % can be achieved in the course of seconds, regardless of the nature of the organic species.

In order to make the SCWO-process industrially practicable, a variety of plants have been developed during the recent decades. Though the design of those plants may vary, the process therein is basically carried out by feeding waste water, i.e. a water sludge containing organic constituents as well as inorganic constituents, from a storage tank to a reactor vessel by means of a high pressure pump in which the water pressure is raised to e.g. 250 bar, or at least above the critical level 221 bar. Before entering the reactor vessel the water is also preheated by means of a heater and an economizer, more specifically to about 400 ^° C, i.e. well above the critical temperature of 374 ^° C. From another tank oxygen is pumped through a vaporizer to the reactor vessel, in which oxidation immediately takes places. Then the organics are dissolved, while generating heat in an autothermal process by which the temperature is further raised to 550 to 600 ^° C. This process occurs even if the content of organics in the waste water is low (3 to 6 %), meaning that surplus heat always becomes available for heating the waste water passing the economizer towards the reactor. Furthermore the plant includes apparatus for treating the processed water phase leaving the

reactor, such as a steam boiler, a cooler, pressure reduction devices, a gas/liquid separator, etcetera. All of these components are interconnected by various tubings and controlled by other components, such as valves, accumulators, pressure reduction devices, fluid oscillators, injectors, nozzles, filters, traps etc.

In practice the SCWO-process may be used to neutralize well-nigh an infinite number of waste products. Thus municipal water sludge may be treated while completely destructing the organics thereof. Inorganic material in the effluent can thereafter be landfilled as a non-hazardous material or used as a raw material for recovery purposes. Another application is waste products containing valuable inorganic materials. The organic contaminants are completely destroyed, leaving a purely inorganic phase that can be recycled. Examples of this application are the de-inking of paper sludge while recovering paper fillers, and treatment of spent catalysts while recovering precious metals. Furthermore active pharmaceutical ingredients can be eliminated from waste water, and halogenated waste be destroyed without forming hazardous by-products, such as dioxins. Of special interest is the SCWO-process as an alternative to incineration, since waste products containing nitrogen can be destroyed without forming NOx.

The environment in any SCWO-plant is generally very harsh. Especially the environment within the reactor vessel and the tubing connected thereto may be corrosive, a number of species being very aggressive relative to the material of the different components. E.g. there are acids, such as nitric acid, sulphuric acid and hydrochloric acid, which are strongly corrosive in the range of 270 to 380 ^° C. Any surface in contact with the aggressive and corrosive species therefore runs the risk of corroding or otherwise deteriorate in a short period of time.

In order to master the problems caused by the aggressive conditions in SCWO- plants, the choice of material for the different components of the plant is complicated, though crucial. Currently a common constructing material is

Alloy 625, which is used in the apparatus of the plants, such as the heater, the economizer, the reactor, the steam boiler, and the vaporizer, as well as in single components, such as tubes and plates. The main reason for using Alloy 625 is that it withstands high pressures (250 bar) and high temperatures (600 ^° ), and is fairly

resistant to the corrosivity of the process fluid, e.g. in the temperature range of 270 to 380 ^° , meaning that the apparatus and components get an acceptable service life. A severe disadvantage of high-alloyed nickel-based grades, such as Alloy 625, is, however, that they are very expensive due to the high contents of nickel and molybdenum, resulting in heavy investment costs for erecting the plants. Another austenitic stainless steel alloy being similar to Alloy 625 in respect of high contents of nickel, is C 276. Both of these grades contains 60 % nickel or more.

Aiming at reducing the costs of the constructing materials, attempts have also been made to use, in SCWO-plants, low-alloyed grades such as 304 L and 316 L, which contain quite moderate amounts of nickel (8 to 15 %) and chromium (18 to 20 %), and therefore are inexpensive in comparison with the high-alloyed grades. It has, however, turned out that 304 L and 316 L neither resist the high pressure (221 bar) nor the high corrosivity of the fluid in the region immediately below the supercritical temperature point (374 ^° C).

In the patent literature the use of nickel-based and high-alloyed stainless steel alloys in SCWO-plants is disclosed e.g. in US 6958122 (Inconel and Hastelloy) and US 5804066 (Inconel), while the use of the low-alloyed 316 L is mentioned in US 5823220.

For the sake of completeness it may also be mentioned that the use of ceramics and/or cermets, chiefly as linings and coatings of constructing components in SCWO-plants, has been suggested by US 5358645, US 5461648, and US 5545337. Though ceramics are quite resistant to corrosion, they do, however, limit the freedom of constructional design to an exorbitant extent, meaning that the various structures of the plant cannot be carried out in the best manner regarding e.g. mechanical strength, weldability, functioning, etcetera.

The shortcomings of the known constructing materials accounted for above hamper the development and the commercialization of the SCWO technique into an attractive alternative to the traditional methods of waste destruction, such as incineration, in spite of the fact that supercritical water oxidation is superior in many respects, e.g. environmentally, and as regards the ability of taking care of

hazardous waste products in a safety manner. Therefore a need still exists for a constructing material, which is reasonably inexpensive and nevertheless adapted to its purpose as to resistance to corrosion, mechanical strength, temperature strength, weldability, and machinability.

Summary of the invention

The present invention provides an austenitic stainless steel, intended to be in direct contact with supercritical or near supercritical solution, that meets the above-mentioned need, viz. in the form a grade named SANDVIK SANICRO ^® 25 being disclosed e.g. in EP 1 194606 B1. Thus it has turned out that an alloy essentially designed as specified in said patent document does not only successfully cope with high mechanical loads, but also provides an acceptable corrosion protection in SCWO environments, notwithstanding the fact that SANDVIK SANICRO ^® 25 contains quite moderate contents of expensive constituents (above all nickel) and therefore is less expensive to produce than Alloy 625 and Alloy C 276.

As will be evident from the subsequent report on a corrosion test, SANDVIK SANICRO ^® 25 is at least as good as, and in certain respects even better than, the high-alloyed grade A 625 as regards corrosion resistance and service life.

An austenitic stainless steel alloy according to the present invention comprises (by weight) 20 to 35 % nickel(Ni), and 15 to 30 % chromium (Cr).

In a more preferred embodiment the alloy comprises 20 to 35 % nickel (Ni); 15 to 30 % chromium (Cr); and 0,5 to 6,0 % copper (Cu).

In practice the alloy according to the invention may advantageously comprise (by weight): 20 to 35 % nickel (Ni); 15 to 30 % chromium (Cr); 0,5 to 6,0 % copper (Cu); 0,01 to 0,10 % carbon (C); 0,20 to 0,60 % niobium (Nb), 0,4 to 4,0 % tungsten (W); 0,10 to 0,30 % nitrogen (N); 0,5 to 3,0 % cobalt (Co); 0,02 to 0,10 % titanium (Ti); not more than 4,0 % molybdenum (Mo); not more than 0,4 % silicon (Si); and not more than 0,6 % manganese (Mn), the balance being iron and normal steelmaking impurities.

Brief description of the drawings

Figure 1 illustrates the weight change of two alloys according to the invention and Alloy 625 when exposed to a simulated SCWO environment at 350 °C for 125 hours.

Figure 2 illustrates the weight change of two alloys according to the invention and Alloy 625 when exposed to a simulated SCWO environment at 600 ⁰ C for 125 hours.

Detailed description of the invention

The constituents of the alloy utilized according to a preferred embodiment of the invention are discussed below (all percentages being by weight).

Nickel

Nickel is an essential constituent for the purpose of ensuring a stable austenitic structure. The structural stability is depending on the relative amounts of, on one hand, the ferrite stabilizers, such as chromium, silicon, tungsten, titanium and niobium, and, on the other hand, the austenite stabilizers, such as nickel, carbon and nitrogen. In order to suppress the formation of sigma phases after a long time at elevated temperatures, particularly when using a high content of chromium, tungsten and niobium needed to ensure a high temperature corrosion resistance and a high creep rupture strength, the nickel content should be at least 20 %, and preferably at least 22,5 %. It may also be 25 % or higher. At a specific chromium level, an increased nickel content suppresses the oxide growth rate and improves the tendency to form a continuous chromium oxide layer. However, in order to keep the material cost at a reasonable level, the nickel content should not exceed 35 %, and preferably not 32 %. In view of the above considerations, the nickel content of the alloy is restricted to the range of 20 to 35 %.

Chromium

Chromium is effective of improving the general corrosion resistance and the oxidation resistance. In order to achieve a sufficient resistance in these respects, a chromium content of at least 15 % is prescribed. Preferably 20 %, or more,

chromium may be added. If, however, the chromium content would exceed 27 % and approach 30 %, the nickel content must be further increased in order to produce a stable austenitic structure. A content of chromium exceeding 30% would necessitate an increase of the content of nickel to a level being too high (above 35%) to ensure a cost-efficient composition. For these reasons the chromium content is restricted to the range of 15 to 30 %, preferably 20 to 27 %.

Copper

Copper is added in order to produce a copper-enriched phase, finely and uniformly precipitated in the matrix, which contributes to an improvement of the creep rupture strength. Such an effect calls for an amount of at least 0,5 % copper, a marked improvement being achieved of about 2 %. Copper is also added for improving the general corrosion resistance against sulphuric acid. However, an excessive amount of copper (6 % or more) would result in a reduced workability. Also for economical reasons the Cu-content should be kept moderate, e.g. at 3,5 %. In view of these considerations the copper content is restricted to the range of 0,5 to 6 %, preferably 2 to 3,5 %.

Carbon Carbon is a constituent effective to provide adequate tensile strength and creep rupture strength required for high temperature steel. If, however, too much carbon is added, the toughness of the alloy is reduced and the weldability may deteriorate. Furthermore a carbon content being too high would reduce the corrosion resistance in SCWO- environments. For these reasons, the carbon content is restricted to maximally 0,1 %. Preferably it may amount to at least 0,04 % and at most 0,08 %.

Niobium

Niobium is generally accepted to contribute to the improvement of the creep rupture strength by the precipitation of carbonitrides and nitrides. However, an excessive amount of niobium may decrease the weldability and the workability. In view of these considerations, the niobium content is restricted to a range of 0,20 to 0,60 %. Preferably the niobium content should be at least 0,33 % and at most 0,50 %.

Tungsten and Molybdenum

Tungsten is added to improve the high temperature strength, mainly by solid solution hardening, and a minimum of 0,4 % is being needed to achieve this effect. Tungsten and molybdenum are also contributing to the general corrosion resistance in SCWO-environments. However, both molybdenum and tungsten promote the formation of the sigma phase. Tungsten is considered to be more effective than molybdenum in improving the strength. For this reason and for economical reasons, the molybdenum content is held low, not more than 0,5 %, preferably lower than 0,02 %. In order to maintain a sufficient workability the tungsten content should not exceed 4 %, and therefore the tungsten content is restricted to a range of 0,4 to 4 %, preferably 1 ,8 to 3,5 %.

Nitrogen Nitrogen, as well as carbon, is known to improve the strength at elevated temperatures, e.g. above 500 °C, and the creep rupture strength, as well as to stabilize the austenite phase. However, if nitrogen is added in excess, the toughness the and ductility of the alloy are reduced. For these reasons, the content of nitrogen is defined to the range of 0,10 to 0,30 %, preferably 0,20- 0,25 %.

Cobalt

Cobalt is an austenite-stabilizing element. The addition of cobalt may improve the high temperature strength by solid solution strengthening and suppression of sigma phase formation after long exposure times at elevated temperatures. However, in order to keep the production cost at a reasonable level, the cobalt content should be in the range 0,5 to 3,0 %, if added.

Titanium Titanium may be added for the purpose of improving the creep rupture strength by the precipitation of carbonitrides, carbides and nitrides. However, an excessive amount of titanium can decrease the weldability and the workability. For these reasons, the content of titanium is defined to a range of 0,02 to 0,10 %, if added.

As examples of components or structural members, intended to be in direct contact with supercritical or near supercritical solution, made from the steel alloy according the invention, the following ones may be mentioned: Tubes, plates, bars, rods, strips, foils, linings, blocks, sleeves, wires, beams, girders, pillars and webs. All of these components may in turn be used (indivually or in combination) to design the various apparatus and devices included in a complete SCWO-plant, such as a reactor, an oxygen tank, a sludge water tank, a vaporizer, an economizer, a steam boiler, a cooler, a gas/liquid separator, as well as various valves, accumulators, pressure reduction devices, fluid oscillators, injectors, nozzles, filters and traps. Tubes and plates are simple to produce from the steel alloy described above.

By using components according to the present invention, i.e. components consisting of a steel alloy as specified above, it is expected that the material costs in connection with the erection of SCWO-plants, will be reduced by roughly 25 to 40 % in comparison with the costs for high-alloyed grades, such as Alloy 625, as regards the vital equipment upstream and downstream the reactor of the plant. Accordingly the invention will contribute positively to the future development and utilization of the SCWO-technique as a method of disposing organic waste products in a manner being harmless to the environment.

Test report

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

Table 1 .

Table 2.

Rectangular cupons were cut out of the alloys and thereafter ground (80 to 1000 mesh) and polished (9 to 0,25 μm diamond). The coupons were weighted before and after exposure to the above identified experimental conditions. The surface layers formed during the experiments were investigated by field emission- scanning electron microscopy (SEM) and X-ray microanalysis (EDX). Further examination of the coupons was made by optical microscopy. The corrosion attack was evaluated by microscopic observation.

Results for 350 °C

The coupons from the test were cleaned with distilled water and acetone, and final blown-dried for back-weighing and to perform further examination steps. The test resulted in a considerable loss of material, indicated by the weight change. Weight and dimension before exposure and amount of weight change after the run in 350 °C is given in Table 3. The mean values are depicted in Figure 1.

Table 3.

Table 4.

The coupons were imaged by scanning electron microscopy (SEM) and subsequently, the elemental composition of the surface layers was analyzed by energy dispersive X-ray spectrometry [EDX]. The microanalytical results of the surfaces revealed a composition of the scale mainly of oxides and small amount of chlorine, the results are listed in Table 4.

Results for 600 °C

No remarkable corrosion attack was observed on the samples after exposure. Only a thin oxide layer remained on the specimen surface, indicated by a small weight gain. Weight and dimension before exposure and amount of weight change after the run at 600°C is given in Table 5. The mean values are depicted in Figure 2.

The coupons were imaged by scanning electron microscopy (SEM) and subsequently, the elemental composition of the surface layers was analyzed by energy dispersive X-ray spectrometry [EDX]. The EDX analysis confirmed an oxidic composition of the thin layers formed. A summary of the EDX results is listed in Table 6.

Table 5.

Table 6.

The component, according to the present invention, provides mechanical strength comparable to commonly used construction materials in SCWO-plants, combined with improved or comparable resistance to corrosion when the component is in direct contact with supercritical or near supercritical solutions.