ABSTRACT

The corrosion behavior of mild steel in a gas reforming CO2-H2O-CHOOH environment at high temperature and pressure (210°C; 3500KPa) was studied using polarization and mass loss tests as well as online corrosion monitoring techniques. Corrosion control measures in this environment including the use of stainless steels rather than carbon steel, chemical conditioning of the environment through pH stabilization, dosing of amines and organic surfactants were investigated. It was found that siderite precipitated on the steel which should provide effective corrosion protection under non-turbulent flow conditions. The ex situ measurement of corrosion rates of the steel in the water using polarization and mass loss experiments differed significantly with higher corrosion rates predicted by the electrochemical compared to mass loss measurements. The electrochemical measurements also predicted lower corrosion rates at higher flow rates and with longer exposure times. The corrosion rates measured in the presence of a filming amine were variable but more positive for the organic surfactant indicating reduced corrosion rates in all the tests. Stainless steels were more corrosion resistant in the water but application would only make sense in turbulent flow where a protective siderite layer would not form on steel. The installed online electrochemical monitoring system functioned well and confirmed the beneficial effect of the organic surfactant addition on the corrosion of the steel. The layer of siderite that precipitated on the probes would, however, reduce the response time and sensitivity of the system to process upsets and would have to be moved to a more critical location in the system where the monitoring of the corrosion induced by turbulently flowing water on the steel is more relevant from a process control point of view.

INTRODUCTION

In the gas reforming plant under study, auto-thermal gas reformers use steam and oxygen at high temperatures (˜900°C) to reform methane-rich gas to syngas (mixture of carbon monoxide and hydrogen gas). The syngas is then counter-currently washed with water in a cooling column to cool and wash the syngas of any solids using a recirculating water system (GRCW) which is also used to saturate the methane gas feed with steam. This circulation water system is constructed of steel and suffers from corrosion and fouling, posing significant challenges in terms of direct costs (sectional repairs, sectional replacements, downtime production losses, etc.) and indirect costs (safety, environmental, impact on downstream processes, etc.). The present investigation was aimed at developing an understanding of the key processes underlying the corrosion and fouling mechanism involved and to develop value-evaluated mitigation measures.

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