A tale of three plants: Issues facing power producers Down Under and how they are coping

System chemistry and plant layup were significant topics at this Fifth Annual Meeting of the Australasian HRSG Users Group, which began with three case studies that follow.

1. Osborne Cogen. Commissioned for cogeneration service at the end of 1998, ATCO/Origin Energy Ltd’s Osborne Power Station, located near Adelaide, South Australia, had been in 180-MW baseload operation (120 MW of gas turbine power, 60 MW of steam) until recently.

Chemistry had been optimized towards the recommended IAPWS (International Association for the Properties of Water and Steam) program consistent with its operating profile. But recently Osborne has gone through a further change to more flexible operation and more startups. This has required even more chemistry changes. Current plans call for dehumidifiers and a nitrogen generator for layup and storage. A gas bypass stack also is being considered for long-term flexibility.

2. Tallawarra Power Station. Designed for cycling, EnergyAustralia’s Tallawarra station in New South Wales (260 MW gas, 160 MW steam), commissioned in January 2009, has experienced blowdown and carryover issues. The three-pressure HRSG has been operating on trisodium phosphate (TSP) treatment; feedwater is AVT(O), all-volatile treatment with a few parts-per-million (ppm) of oxygen present.

The unit has experienced low dissolved oxygen and high iron transport. Although some of these issues have been resolved, Tallawarra also is facing an uncertain future. A new operating philosophy is being considered. Modifications include nitrogen capping of the feedwater tank for wet layup, degassed cation conductivity to improve steam-turbine startup, and water-treatment plant modifications to permit continuous reliable operation when HRSG demand is zero.

3. Darling Downs. Originally designed for baseload service (3 × 120 MW gas, 270 MW steam with bypass stacks), Origin Energy’s Darling Downs Power Station in Queensland had many operating issues after commissioning in July 2010. These included LP drum feedforward stripping ammonia and oxygen from the HP drum, poor-quality recycled blowdown condensate (corrosion products), and superheater exfoliation. Air-cooled-condenser corrosion control was difficult because of high pH (about 10).

Plant operation is now two-shifting, with one or two GTs taken out of service. Layup and storage systems are under review. Considerations include nitrogen capping, dehumidified air, chemical dosing before layup, and drainage system changes. Operators predict more steam/water-cycle corrosion and know that more shutdowns will provoke more exfoliation.

The three case studies showed, to varying degrees, the impact of modified operations. Comments and discussions on correct oxygen levels followed, including specific drum-level control experiences and the need for proper testing in accordance with the Technical Guidance Documents published by IAPWS. This set the stage for the presentations that followed.

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