ABSTRACT

Flue gas corrosion produces a severe attack in metal surfaces of stainless steel tubes in coke calciner heat recuperators. This corrosion is produced due to the presence of sulphur and vanadium in the calcining process which forms low melting and volatile corrosion products together with the reaction between the metal and the atmospheric oxygen at elevated temperatures. In an attempt to avoid this type of corrosion, advanced silica based coatings have been developed and applied over stainless steel samples and have been characterized and tested under laboratory conditions (microstructure, adherence and abrasion resistance, thermal resistance). Besides, a specific corrosion test has been designed in order to check chemical resistance to sodium vanadate salts at high temperature.

Due to the excellent behavior of the coating, showing a complete protection of the samples, several coated prototypes were prepared and installed in the hottest part of one coke calciner heat recuperator to certificate the protective performance of the coating under real conditions (T Fumes = 700 °C; T Steel = 580 °C). After 4 years of validation tests, 800 tubes of 310S stainless steel (63,50 mm OD × 2,41mm WT and 5000mm lenght) were coated to build a whole coke calciner heat recuperator unit, with an average coating thickness of 150 µm. In 2018, after 12 months in operation, visual inspection reports that ceramic coating remains in place and none of the tubes has been corroded.

INTRODUCTION

As refineries worldwide seek to operate more efficiently and extract more gasoline and other high value fuels from each barrel of crude oil, a solid carbon material known as coke is produced1. Petroleum coke calcining is a process whereby green or raw petroleum coke is thermally upgraded to remove associated moisture and volatile combustion matter and to improve critical physical properties like electrical conductivity, real density and oxidation characteristics, thus, the feedstock “Green Coke” is a by-product of the coker process in oil refineries. In addition, coke calcining process is used is to supply heat to waste heat boilers. The heat is produced by oxidizing coke in a sort of kiln, and channeling that heat through a series of integrated sections that feed a steam drum and produce steam to the facility. Calcined petroleum coke is an important industrial commodity linking the oil refining and metallurgical and chemical industries. For example, calcined coke is required in the aluminium industry to manufacture anodes for the aluminium potlines, in the steel industry to manufacture carbon and graphite electrodes or as carbon raiser and in the chemical industry as a feedstock for the production of titanium dioxide by the modern chloride process route2. Nevertheless, the coke calcining process implies a common problem known as hot ash corrosion, were the reason for this severe type of high-temperature corrosion is the presence of ash deposits containing vanadium oxides and sodium sulfates, which prevent the formation of protective oxide scales on the metal surface. At high temperatures, vanadates are among the most corrosive components. While part of their effect lies within lowering the viscosity of the deposits, another issue comes from the strong acidity of the complex oxide ions and their ability to dissolve the otherwise protective oxide scales formed on structural alloys.3 The combined effect of sodium sulfate-vanadate has been reported in several works.4,5,6 In many cases, corrosion is caused by a layer rich in sodium sulfate well adhered to the surface of the tubes, which reacts with the vanadium pentoxide present in the combustion gases, forming vanadate salts which lower the melting point of the metal to 500 °C. Besides, the resulting ash deposits reduces the heat transfer efficiency of the unit7. As a result, the tubes suffer from corrosion, melts and finally disappear in less than one year. Elemental sulfur is also an insidious form of attack that initiates pitting corrosion by creating small anodic with large cathodic areas where the passive/protective film formed on the metal surface is often broken8,9,10,11,12.

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