Combined cycles: Looking back, looking ahead - II

(This is Part 2 of a two-part series. Click here for Part 1)

Wolverine Electric’s Burnips unit, according to GE records, was the first of 2 STAG units of 21 and 11 MW CC output that went operational in 1967. Both are still running. 

The 11 MW unit operates for the City of Ottawa at 1600°F firing temperature and 895°F exhaust to produce 35.2 %. Station chief Randy Boyles provided the following information: 

In the last decade there have been significant equipment repairs and updates to Unit #6 at the Burnips generation plant for the purpose of extending its life. Above and beyond the work performed on the equipment, new operating procedures and methods of performing maintenance have been implemented to reduce equipment failure and extend its lifespan. Currently, the unit has run over 145,000 hours since its installation in 1967. The STAG was base loaded for many years then went through a period of non-operation for economical reasons. It now operates as a peaking generator.

Since 1999, the facility has undergone several upgrades. The original cooling tower was undersized by 40% due to economic reasons. New towers to design specs raised unit output on a 90°F day from 18 MW to 20.5 MW. The steam turbine rotor was sent out for grit blasting and magnetic particle testing. The rotor was found to be in good condition with minimal repairs needed. It was also balanced at that time. Steam turbine diaphragm inspection showed eight needing light weld repairs. All bearings were replaced in the steam turbine during reassembly.

The generator rotor was also removed and disassembled. Retaining rings subjected to DPT, UT scans and Brinell hardness tests. The rotor was balanced and new bearings were installed during reassembly. The four generator coolers had new tube bundles installed.

The GT at this site supplies 70% of the power. An exhaust temperature of 925°F is used for steam generation in the HRSG which has five tube sections for condensate heating reheat and superheat incorporated in a low pressure (LP) and high pressure (HP) configuration. The LP produces steam at 85 psig 388°F and the HP 400 psig at 806°F. There are no steam control valves and the pressure varies according to exhaust heat flow (Figure 2). The fuel control to the GT is the basic governor. The generator voltage of 13.8kv is boosted in a two-winding transformer to 44kV and 69kV respectively.

“The improvements made to Unit #6 over the last 10 years have helped to extend the life of the unit and it is in better condition now than it was in 2000,” said Boyles. “With the continuation of preventative maintenance and any required corrective maintenance that might come up, the expected life of Unit #6 should easily extend beyond 10 years”.

At full load, the net plant heat rate is 9,700 Btu/kW/hr with an over all efficiency of 35.2 %. A conventional steam plant of that era would have an efficiency of 27.0% .Prior to STAG, units of 150 MW would be required to achieve comparable efficiency.

Figure 3: Energy loss breakdown in combined cycle gas turbines 

Moving forward

Trends for CC efficiency increased over the next 4 decades for the STAG systems. It was not until the mid 70s that pressure and steam temperatures increased to 600 psig and a popular 850 psig with a few at 1,250 psig.

During the four-decade lifespan of the STAG, 141 gas turbines and 78 steam turbines were installed for an accumulated 16.239 GW of generating capacity. The conclusion is that the 65% goal could be a reality in the coming years (Figure 4). This should help younger engineers to realize that there are endless possibilities for innovation by taking into consideration the history of combined cycle technology development.  

Acknowledgements:

Randy Boyles of Wolverine, John Sanders and his associates from GE, Dr. Hans Wettstein of ETH and Ivan Rice

Author: Septimus van der Linden, BRULIN Associates LLC.