Component for supercritical water oxidation plants, made of an austenitic stainless steel alloy

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.

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 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 U.S. Pat. No. 6,958,122 (Inconel and Hastelloy) and U.S. Pat. No. 5,804,066 (Inconel), while the use of the low-alloyed 316 L is mentioned in U.S. Pat. No. 5,823,220.

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 U.S. Pat. No. 5,358,645, U.S. Pat. No. 5,461,648, and U.S. Pat. No. 5,545,337. 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.

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 1194606 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.