By Brad Buecker, ChemTreat, and Dan Dixon, Lincoln Electric System

As coal-fired energy crops are being retired on account of environmental and financial elements, the first bridge expertise to fill energy era voids has been mixed cycle energy manufacturing. 

For these new to the trade, the combustion generators of a mixed cycle plant perform on the Brayton thermodynamic cycle, whereas the warmth restoration steam turbines (with steam produced by exhaust warmth from the fuel turbine) function on the traditional Rankine cycle.  Net efficiencies of contemporary mixed cycle models now attain or exceed 60 %, which is considerably larger than even essentially the most superior supercritical coal models.  Logic means that crops with such excessive efficiencies would usually be base loaded, however that’s typically not the case.  Load swings related to renewable expertise operation require most mixed cycle energy crops to cycle frequently in load.

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Any time a unit comes offline, the potential for air ingress might exist on the small openings throughout the water/steam community.  Oxygen can then assault in localized areas and trigger severe harm to tubing, piping, and different elements.  Oxygen corrosion could also be notably problematic if a unit cools considerably earlier than the following working part.  Compounding this situation, corrosion merchandise can dislodge upon startup and journey to the steam generator, the place they precipitate on boiler tubes.  Porous iron oxide product deposition reduces warmth switch and might set up websites for under-deposit corrosion.  Unfortunately, plant administration at each devoted energy crops and co-generation amenities usually underestimate the severity of off-line corrosion and the necessity for well-designed layup procedures. 

From an early stage, the employees at Lincoln Electric System’s (LES) Terry Bundy mixed cycle plant in Lincoln, Nebraska acknowledged that correct layup was vital to keep up the integrity of the dual mixed cycle models.  This article outlines the layup strategies the employees carried out, and the superb safety the applied sciences have supplied during the last decade-plus.  These classes will even be relevant for hydrogen-fueled mixed cycle crops, whose emergence provides attention-grabbing promise.

Introductory Details

Power on the Terry Bundy plant is produced by two GE LM 6000 combustion generators (47 MW most output per CT) and two Nooter-Eriksen dual-pressure HRSGs (no reheat) that feed a 26-MW steam turbine.  The plant additionally features a stand-alone easy cycle turbine, additionally with a 47-MW output capability.  These models frequently cycle on and off, regularly each day, throughout the summer season.  At different instances of the 12 months, the models could also be off for a number of weeks however must be obtainable for dispatch on brief discover. 

Shortly after the models had been commissioned within the mid-2000s, the plant employees transformed the HRSG feedwater chemistry program from all-volatile remedy decreasing [AVT(R)] to all-volatile remedy oxidizing [AVT(O)] to reduce the potential for flow-accelerated corrosion (FAC) within the condensate/feedwater community.  Per AVT(O) pointers, decreasing agent/oxygen scavenger feed is eradicated and ammonia (or probably an ammonia/amine mix) is utilized for feedwater remedy to keep up pH inside a mid- to upper-9 vary.  The small quantity of dissolved oxygen that (usually) enters the feedwater through condenser air in-leakage is allowed to stay, as this chemistry in high-purity feedwater (≤zero.2 mS/cm cation conductivity) induces formation of a decent a-hematite boundary layer on carbon metal.

Figure 1.  The deep pink a-hematite layer on the internals of the Terry Bundy HRSGs.

When plant personnel switched feedwater chemistry from AVT(R) to…

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