Novel solution for low load parking and fast restart of combined cycles

A novel solution has been proposed for low load parking and fast restart of combined cycle plants. The solution was proposed at Power Gen International 2019 by S Can Gülen, Ilya Yarinovsky and Mark Boulden of Bechtel Infrastructure & Power, Inc.

The solution assumes a 2 X 1 configuration: two gas turbines, each with their own HRSG, but one bottoming cycle steam turbine.

The authors say fast start technology has problems. For instance, terminal attemperation may be theoretically sound but historically problematic and maintenance intensive. Water induction risk to the turbine must always be considered. Steam turbine stress controllers are necessary to have, but consideration must be given to the design basis assumptions. Change in operating tempo can result in unexpected stress accumulation, they add.

The solution they suggest has the following sequence.

• Operator selects Low Load Operation (LLO) mode
• GT2 shuts down normally and placed on turning gear (TG)
• GT1 is ramped down to its Minimum Emissions Compliance Load (MECL)
• ST is rolled down to low load commensurate with steamgeneration
• Dampers isolating the bypass duct at either end are opened
• A portion of the GT1 exhaust flow (x%) is diverted to the HRSG2
• Thus, the HRSG2 (including the associated steam pipes and valves)is kept warm even though its gas turbine, GT2, is offline
• The plant runs in this mode until the operator deselects LLO mode. Critical items to control would include prevention of reverse gas flow in the “idle” HRSG (i.e., towards the GT instead of the stack), uniform gas flow and temperature in the idle HRSG’s SCR cavity for emissions control, and avoidance of temperature excursion on the cold end of the idle HRSG.
• Upon detection of the temperature drop in the idle HRSG, the bypass damper on the hot jumper will open. Simultaneously (or with a short delay) the stack damper will open. The opening of the stack damper will initiate a cold turning gear flow as well as hot gas flow through the idle HRSG toward the stack. At a certain (optimal) position of the stack damper target temperature in the SCR cavity can be achieved.

There is no cold start when gas turbines restart. One gas turbine at MECL provides substantial ability for low load parking of the power block, but bleed can be used across the entire load range. Cross bleed maintains the offline HRSG(s) sufficiently hot to allow unlimited ramp-up to full load and keeps the SCR at operating temperature, reducing start-up emissions. The plant is faster to start, faster to full load, lower startup emissions and reduced use of steam attemperators.