| Supercritical carbon dioxide
Above: STEP Demo main equipment and connecting pipework. Right, main compressor loop. Middle, high temperature recuperator. Left, turbine generator and bypass compressor
inventory management system, the cooling tower system and the main process cooler. The compressor performance was quantified across a range of operating conditions and speeds as part of this activity. In addition to being the world’s largest indirect-fired sCO2
power plant, the sCO2
turbine has the highest power density of any terrestrial turbine, at over 20 000 hp in a very small, compact package achieving approximately 100 hp/lb, which is similar to the NASA space shuttle fuel turbopump and 40+ times the power to weight ratio of a Formula One racing car engine. The turbine stop valve is the world’s largest high temperature Haynes 282 casting, at 9250 lb, and the high temperature recuperator is the world’s largest high temperature printed circuit heat exchanger (PCHE), with a 50 MWt capacity and weight of ~50 tons. The sCO2
at 93 MWt firing capacity, has the largest known Inconel 740H heater coil.
Technology challenges The challenges faced in the course of procurement, fabrication, and construction were significant however, and resulted in substantial cost and schedule impacts. The main challenges
encountered are attributed to the first of a kind nature of the required components and the readiness of the supply chain to deliver them. At 10 MWe, the facility is commercial in scale but almost none of the equipment could be purchased on typical commercial terms. Slips in schedule had the greatest impact on cost increases. The difficulties experienced by the project were exacerbated by the global pandemic, which impacted the overall supply of goods and services, although most STEP Demo suppliers remained in operation and the entire STEP Demo team remained in place and active, including in on-site construction.
heater,
The 93 MWt process heater is natural gas fired, with high temperature pressure parts fabricated out of Inconel 740H for the >700°C, 250 bar supercritical CO2
conditions. Its arrangement is
based upon a duct-fired HRSG (heat recovery steam generator). The heater was shipped to the site and installed in 2021. It was fired for the first time as part of acceptance testing in September 2023.
Weld cracking during heater fabrication was experienced with the alloy 740H main coil. Fabrication involved welding procedure
development, welder training, and the execution of over 1600 welds. Following post-weld heat treatment of the coil, around 3% of the tube- to-tube butt weld population failed inspection. A comprehensive failure investigation was conducted to determine the failure mechanism and highlight factors contributing to root cause. The failure mechanism was determined to be stress relaxation cracking during the post-weld heat treat (PWHT) cycle (also known as strain age cracking or stress relief cracking). Repairs were successfully completed and the application of 100% phased array ultrasonic testing provides the project with a baseline for further in-service inspection. Lessons learned were applied to the fabrication and installation of the 740H process piping.
Process piping was designed to ASME B31.1 and minor challenges were encountered with piping arrangement to accommodate thermal expansions, route around equipment, and match equipment load limits.
Welding 740H went better than expected and this is attributed to the prior experience gained by SwRI, good Industry guidance, and care taken to qualify the welders on the job. Overall, a large amount of time was spent on grinding each weld and the total weld time was long; however, there were no rejections in main bore welds. All welds were post-weld-heat treated and radiographically inspected.
The high temperature recuperator, which operates with a maximum hot inlet flow of up to 600°C from the turbine exit, is made of 316 stainless steel and weighs some 50 tons. The HTR will operate over a wide range of conditions and with the two different power cycle configurations. A controlled cold-side flow bypass aids flexibility. However, the thermal-mechanical stresses over the operating range challenged acceptable local stress limit loads and cyclic fatigue design life. Finite element modeling included elastic–plastic
Left: Gas fired heater. This employs 740H material for 715°C sCO2
www.modernpowersystems.com | November/December 2023 | 15
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