ABSTRACT

In order to investigate corrosion issues of heat exchanger materials at intermediate temperatures in direct supercritical carbon dioxide power cycles, a series of autoclave exposure tests and electrochemical experiments have been conducted. Potential heat exchanger materials are austenitic stainless steels and ferritic-martensitic steels. The autoclave tests were performed in fluid mixtures containing CO2, O2, and H2O at 8 MPa and at two temperatures, 50 °C and 245 °C, to approximate heat exchanger operating conditions. Mass change measurements and characterization of the corroded surfaces using X-ray diffraction and scanning electron microscopy were performed to understand the corrosion behavior of steels. Additionally, electrochemical tests were performed in CO2-saturated H2O at ambient pressure and temperatures up to 50 °C to obtain kinetic data in terms of the corrosion rates of the alloys. The results demonstrate that significant corrosion occurs in a pressure / temperature region where H2O saturated with CO2 condenses on the coupons. Larger mass change was observed when oxygen was present in the supercritical CO2 environment. The ferritic-martensitic steel was found to be susceptible to corrosion at low temperatures in direct supercritical CO2 power cycle environments.

INTRODUCTION

The concept of using supercritical CO2 (s-CO2) as a working fluid in power cycles has gained interests over the last few years. Direct s-CO2 power cycles utilize oxy-fuel combustion exhaust as the working fluid. These power cycles have the advantages of higher thermodynamic efficiency, lower capital cost, and zero emission compared to steam cycles.1 The high-pressure combustion gas mixture expands in the turbine to generate power and passes through a series of heat exchangers to recover the useful heat.2 Heat exchangers are one of the components in which significant corrosion can take place due to condensation of corrosive fluid in a large gradient of temperature and pressure. Hence the selection of suitable alloys for heat exchangers is important to maintain a long component lifetime.

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