Metal dusting problems experienced around one of the world's largest oxygen-blown secondary methane reformers in a gas-to-liquids synthetic fuel plant are reviewed.
Practical plant exposure trials of various materials were conducted in the reformer inlet area, the waste heat boiler tubesheet in the gas inlet chamber, and the waste heat boiler by- pass valve in the gas outlet chamber. Alloy UNS N08810 was found to greatly out-perform the high nickel alloy UNS N06600 in the reformer gas inlet stream but was found to be extremely vulnerable to metal dusting attack in the reformer gas outlet stream. Improved performance was experienced in the by-pass valve with 310 stainless steel which had been treated by a proprietary process to diffuse aluminium into the surface, but the most resistant materials were found to be alloy UNS N06601 and 50/50 Cr-Ni. The latter material appeared to be immune to metal dusting attack after 17 months exposure.
The waste heat boiler inlet tubesheet was also successfully protected from metal dusting attack by a thermal spray coating of 50/50 Cr-Ni. The high velocity oxy-fuel (HVOF) application process was found to achieve superior coating integrity to the arc spray process.
Mossgas is the world's first commercial scale gas-to-liquids synthetic fuels plant and is located on the Southern Cape coast of South Africa. The plant was commissioned in 1992 to process natural gas and associated condensate piped from an offshore platform 85km south of Mossel Bay.
The front end of the plant layout incorporates a gas reforming unit, which supplies the feed for a downstream Fischer-Tropsch synthesis plant. This consists of three identical trains designed on the principle of a combined reforming process, and includes both primary tubular steam reforming and oxygen-blown secondary reforming in what are believed to be the world's largest secondary reformers of this type (1). Half the natural gas feed stock is fed directly to the inlet of the secondary reformer along with the primary reformer effluent, and a recycle stream from the downstream Fischer-Tropsch synthesis unit.
The secondary reformer is a vertical refractory-lined cylindrical fixed-bed reactor having a conical upper section to accommodate the reactor burner. The shell is 70mm thick 1 Cr 1/2 Mo steel lined with high alumina refractory. The feedgas enters the burner from the side via the refractory-lined transfer line, and steam and oxygen enter centrally through the single top-mounted burner. Reformed gas exits at the bottom of the reformer where waste heat is recovered via a pair of waste heat boilers and a superheater.
The purpose of this paper is to highlight the metal dusting problems encountered in the secondary reformers and associated waste heat boilers, and to review the successful solutions that have been developed to counter them.
ZONES SUSCEPTIBLE TO METAL DUSTING DAMAGE
A schematic outline of the Secondary Reformer train is shown in FIG.I. The zones where severe metal dusting damage has been encountered are the reformer inlet area, the waste heat boiler tubesheet in the gas inlet chamber, and the waste heat boiler by-pass valve in the gas outlet chamber. The gas compositions and temperatures in these zones are shown in TABLE 1.
Reformer Inlet
During December 1993 a 200mm-diameter hole was burnt through the conical shell section of train one Secondary Reformer. The shock wave and ensuing fire was fortunately directed upwards and away from the ground causing only minimal damage to adjacent equipment.
Subsequent investigation revealed that severe metal dusting of the UNS N06600 burner liner in the neck of
