OTSG design details
Some of this flexibility must be attributed to the once-through heat-recovery units. As mentioned earlier, they can be started with no water in their tubes because they are certified for dry operation at up to 1,000F. Because the exhaust of the LM6000PDs is at a relatively cool 820F, the OTSGs can be run dry for an extended period of time. They can be ramped up to design steam conditions within 60 minutes of a gas turbine start—much quicker than traditional drum heat-recovery steam generators.
The most interesting OTSG start-up permissive is the unit’s stack gas temperature. Until this temperature reaches a certain level, warm water, rather than steam, will be flowing through the OTSG tubes. Because experience has shown that this transition temperature is reached 15 minutes after gas turbine synchronization, that’s where the permissive is set. After the OTSG starts up, the plant’s distributed control system ensures that feedwater is added to the unit’s high- and low-pressure sections at the proper rate, determined by a “feedwater forwarding curve.” During winter months, the low-pressure steam generated by the OTSGs is sent to the CUP to replace the steam generated by the CUP’s existing package boilers. This heat is used to provide space heating and energy savings with the reduction of natural gas consumption (Figure 4). During the summer, the low-pressure steam is sent to the CUP to power the steam turbine�driven centrifugal chillers, thereby reducing electrical consumption by up to 3 MW while still meeting the cooling requirements of the airport.
4. Steam in, cold water out. The Central Utilities Plant adjacent to the GTAA Cogeneration Complex has two 2,000-ton/hr steam turbine-driven chillers. They can be powered by steam extracted from the combined-cycle cogen plant or by steam from four dual-fuel-capable package boilers within the CUP. Courtesy: GTAA
The main steam supply to the CUP is managed at a 150-psig extraction port. The steam extracted from the GTAA’s steam turbine is given a few degrees of superheat to minimize moisture formation. The turbine’s throttle conditions are 750 psi and 150 to 170 degrees F of superheat. Activating the OTSGs’ duct burners will double each unit’s steam generation rate from the typical 100,000 lb/hr to 225,000 lb/hr. This design feature makes operation of the CUP as flexible as that of the cogen plant.
As you might expect, the treatment requirements for the feedwater used by the once-through heat-recovery units are strict, because any impurities can be scaled directly onto the inner-tube surfaces. To avoid this situation, makeup water (from Toronto’s public water system) is treated by mixed-bed polishers until it reaches a purity of 0.18 microSiemens.
On-time arrival
The two GE LM6000PD gas turbine-generators have operated extremely well since coming on-line in February 2006. Since then, each has accumulated approximately 4,300 operating hours and more than 500 starts.
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