Introduction

Research efforts devoted to supercritical carbon dioxide (S-CO2) corrosion, particularly at temperatures in excess of 500°C, are minimal at best, making subsequent data relatively sparse. With the advent of S-CO2 closed cycle gas turbines in the power generation industry, a lack of data poses a significant technical concern in the design of turbomachinery, heat exchangers, and piping. The development of a S-CO2 corrosion test rig would allow for long-term corrosion studies to be conducted in a laboratory environment and would establish the S-CO2 corrosion behaviour of candidate materials.

Operational Parameters and Design Description

The proposed S-CO2 corrosion test rig is arranged in an open-loop configuration, meaning that spent CO2 is exhausted to atmosphere rather than recycled. Assuming that the necessary filtration and piping were in place, this test rig could however be modified to operate in a closed-loop configuration. With the exception of electrical and compressed air sources, the test rig is intended to function as near an autonomous, stand alone unit as possible. Electrical requirements include standard 120 and 208/240 V service while compressed air must be delivered above a minimum set flowrate of 12 SCFM at 95 psig. Operational capabilities of the test rig include a maximum design temperature of 750°C at 20 MPa and a maximum design pressure of 25 MPa at 650°C. A working schematic of the corrosion test rig is seen in Figure 1 and preliminary design calculations for the pressure vessel are found in the attached appendix.

Gaseous CO2 is fed into a gas booster pump from one of two cylinder sources. The outlet pressure of one cylinder is to be set 5 to 10% lower than the other to serve as a mechanical source switchover. This allows for continual operation of the test rig as well as facilitates the removal and refilling of empty cylinders. Line heaters both upstream and downstream of the booster pump serve purpose to preheat the CO2 and ensure it remains in a supercritical state. In terms of safety, the corrosion test rig utilizes both a proportional relief valve and burst disc to protect against over pressurization, in addition to a bleed valve to allow for the quick evacuation of the system. Also, both CO2 and compressed air feed lines use solenoid valves for the automated shutdown of the booster pump if necessary.

S-CO2 enters a flow-through pressure vessel where it is heated by a gas tube furnace. Figures 2 through 4 depict preliminary working drawings of the pressure vessel design. An alumina specimen holder, slid into the middle of the pressure vessel from one of its ends, holds metal coupons for the duration of testing. Thermocouples monitor the temperature of the furnace itself as well as the ends of the pressure vessel and the S-CO2 passing over the coupons. Two methods to seal the ends of the pressure vessel have been proposed – (1) conical metal seals may be held against the inner walls of the vessel by spherical washers and large diameter bolts, or (2) a high temperature gasket may be compressed between mating flange faces. A very small flowrate on the order of 5 mL/min ensures that fresh S-CO2 circulates throughout the system.

Flowrate is controlled and monitored via a metering valve and flowmeter; both are located downstream of the pressure vessel. As seen in Figure 5, the whole unit, excluding the computer and control system, is intended to occupy a 30 in. by 72 in. footprint or the size of the stainless steel table it will sit on. As can be seen in Table 1, a preliminary estimate of the overall test rig cost will depend on a final pressure vessel selection.